line coding.ppt
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Aug 23, 2023
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About This Presentation
line coding
Slide Content
CS6551 COMPUTER NETWORKS
UNIT I
FUNDAMENTALS AND LINK LAYER
UNIT I
FUNDAMENTALS AND LINK LAYER
INTRODUCTION
IntroductiontoNetwork:
1.Anetworkisaninterconnectedsetofautonomouscomputers.
2.Devicesoftenreferredtoasnodescanbeacomputer,printer,oranyotherdevicescapableof
sending/receivingdata.
DataCommunications:
DataCommunicationsaretheexchangeofdatabetweentwodevicesviasomeformoftransmission
mediumsuchaswirecable.
FundamentalCharacteristicsofDCS
1.Delivery(Thesystemmustdeliverdatatothecorrectdestination)
2.Accuracy(Thesystemmustdeliverthedataaccurately)
3.Timeliness(Thesystemmustdeliverdatainatimelymanner)
4.Jitter(variationinthepacketarrivaltime)
IntroductiontoNetwork:
1.Anetworkisaninterconnectedsetofautonomouscomputers.
2.Devicesoftenreferredtoasnodescanbeacomputer,printer,oranyotherdevicescapableof
sending/receivingdata.
DataCommunications:
DataCommunicationsaretheexchangeofdatabetweentwodevicesviasomeformoftransmission
mediumsuchaswirecable.
FundamentalCharacteristicsofDCS
1.Delivery(Thesystemmustdeliverdatatothecorrectdestination)
2.Accuracy(Thesystemmustdeliverthedataaccurately)
3.Timeliness(Thesystemmustdeliverdatainatimelymanner)
4.Jitter(variationinthepacketarrivaltime)
Components of Data Communication
Datacommunicationsystemsaremadeupoffivecomponents.
1.Message
2.sender
3.Receiver
4.Medium
5.Protocol
Sender Receiver
Step 1:
Step 2:
Step 3:
…………
…………
Protocol Protocol
Medium
message
Fig.1.1Fivecomponentsofdatacommunication
Step 1:
Step 2:
Step 3:
…………
…………
Datacommunicationsystemsaremadeupoffivecomponents.
1.Message
2.sender
3.Receiver
4.Medium
5.Protocol
Sender Receiver
Step 1:
Step 2:
Step 3:
…………
…………
Protocol Protocol
Medium
message
Fig.1.1Fivecomponentsofdatacommunication
Step 1:
Step 2:
Step 3:
…………
…………
1.Message:
Thisistheinformationtobecommunicated.Itincludestext,
numbers,pictures,audioandvideo.
2.Sender:
Itisthedevicethatsendsthedatamessage.Itcanbeacomputer,
workstation,telephonehandset,videocameraandsoon.
3.Receiver:
Itisthedevicethatreceivesthemessage.Itcanbeacomputer,
workstation,telephonehandset,televisionandsoon.
4.Medium:
Itisthephysicalpathbywhichamessagetravelsfromsenderto
receiver.Someexamplesoftransmissionmediaincludetwistedpair
wire,coaxialcable,fiberopticcable,laserorradiowaves.
5.Protocol:
Itisasetofrulesthatgoverndatacommunications.Itrepresentsan
agreementbetweenthecommunicationdevices.
Thisistheinformationtobecommunicated.Itincludestext,
numbers,pictures,audioandvideo.
2.Sender:
Itisthedevicethatsendsthedatamessage.Itcanbeacomputer,
workstation,telephonehandset,videocameraandsoon.
3.Receiver:
Itisthedevicethatreceivesthemessage.Itcanbeacomputer,
workstation,telephonehandset,televisionandsoon.
4.Medium:
Itisthephysicalpathbywhichamessagetravelsfromsenderto
receiver.Someexamplesoftransmissionmediaincludetwistedpair
wire,coaxialcable,fiberopticcable,laserorradiowaves.
5.Protocol:
Itisasetofrulesthatgoverndatacommunications.Itrepresentsan
agreementbetweenthecommunicationdevices.
Data Representation
Information today comes in different forms.
They are
–Text
–Numbers
–Images
–Audio
–Video
Data Flow:
Communication between two devices can be
–simplex
–half-duplex
–full-duplex
Information today comes in different forms.
They are
–Text
–Numbers
–Images
–Audio
–Video
Data Flow:
Communication between two devices can be
–simplex
–half-duplex
–full-duplex
1.Simplex
•Insimplexmode,thecommunicationisunidirectional,asonaone-way
street.
•Onlyoneofthetwodevicesonalinkcantransmit;theothercanonly
receive.
•Keyboardsandtraditionalmonitorsareexamplesofsimplexdevices.
Fig.Simplex
2.Half-Duplex
•Inhalf-duplexmode,eachstationcanbothtransmitandreceive,butnot
atthesametime.
•Whenonedeviceissending,theothercanonlyreceive,andviceversa.
•Walkie-talkiesandCB(citizensband)radiosarebothhalf-duplexsystems.
Mainf
rame
Direction of data
Monitor
•Insimplexmode,thecommunicationisunidirectional,asonaone-way
street.
•Onlyoneofthetwodevicesonalinkcantransmit;theothercanonly
receive.
•Keyboardsandtraditionalmonitorsareexamplesofsimplexdevices.
Fig.Simplex
2.Half-Duplex
•Inhalf-duplexmode,eachstationcanbothtransmitandreceive,butnot
atthesametime.
•Whenonedeviceissending,theothercanonlyreceive,andviceversa.
•Walkie-talkiesandCB(citizensband)radiosarebothhalf-duplexsystems.
Mainf
rame
Direction of data
Monitor
Fig.Half-Duplex
3.Full-Duplex
•Infull-duplex,bothstationscantransmitandreceivesimultaneously.
•Onecommonexampleoffull-duplexcommunicationisthetelephone
network.
•Whentwopeoplearecommunicatingbyatelephoneline,bothcantalk
andlistenatthesametime.
Fig.Full-Duplex
station station
Direction of data at time 2
Direction of data at time 1
station station
Direction of data all the time
3.Full-Duplex
•Infull-duplex,bothstationscantransmitandreceivesimultaneously.
•Onecommonexampleoffull-duplexcommunicationisthetelephone
network.
•Whentwopeoplearecommunicatingbyatelephoneline,bothcantalk
andlistenatthesametime.
Fig.Full-Duplex
station station
Direction of data at time 2
Direction of data at time 1
station station
Direction of data all the time
NETWORKS
•Anetworkisasetofdevices(oftenreferredtoasnodes)
connectedbycommunicationlinks.
•Anodecanbeacomputer,printer,oranyotherdevice
capableofsendingand/orreceivingdatageneratedby
othernodesonthenetwork.
Network Criteria:
A network must be able to meet a certain number of
criteria.
The most important of these are
–Performance
–Reliability
–Security
•Anetworkisasetofdevices(oftenreferredtoasnodes)
connectedbycommunicationlinks.
•Anodecanbeacomputer,printer,oranyotherdevice
capableofsendingand/orreceivingdatageneratedby
othernodesonthenetwork.
Network Criteria:
A network must be able to meet a certain number of
criteria.
The most important of these are
–Performance
–Reliability
–Security
1.Performance
•Performancecanbemeasuredinmanyways,including
transittimeandresponsetime.
–Transittimeistheamountoftimerequiredforamessageto
travelfromonedevicetoanother.
–Responsetimeistheelapsedtimebetweenaninquiryanda
response.
•Theperformanceofanetworkdependsonanumberof
factors,theyarenumberofusers
–typeoftransmissionmedium
–capabilitiesoftheconnectedhardware
–efficiencyofthesoftware
•Performanceisoftenevaluatedbytwonetworkingmetrics
–throughput
–delay
•Performancecanbemeasuredinmanyways,including
transittimeandresponsetime.
–Transittimeistheamountoftimerequiredforamessageto
travelfromonedevicetoanother.
–Responsetimeistheelapsedtimebetweenaninquiryanda
response.
•Theperformanceofanetworkdependsonanumberof
factors,theyarenumberofusers
–typeoftransmissionmedium
–capabilitiesoftheconnectedhardware
–efficiencyofthesoftware
•Performanceisoftenevaluatedbytwonetworkingmetrics
–throughput
–delay
2. Reliability
Network reliability is measured by
–frequency of failure
–time it takes a link to recover from a failure
–network's robustness in a catastrophe
3. Security
Network security issues include
–protecting data from unauthorized access
–protecting data from damage and development
–implementing policies and procedures for recovery
from breaches and data losses
Network reliability is measured by
–frequency of failure
–time it takes a link to recover from a failure
–network's robustness in a catastrophe
3. Security
Network security issues include
–protecting data from unauthorized access
–protecting data from damage and development
–implementing policies and procedures for recovery
from breaches and data losses
Type of Connection
•Anetworkhastwoormoredevicesconnectedthroughlinks.
•Alinkisacommunicationspathwaythattransfersdatafrom
onedevicetoanotherasinfig.1.3(a).
Therearetwopossibletypesofconnections:
1.point-to-point:
Apoint-to-pointconnectionprovidesadedicatedlink
betweentwodevices.
Figure 1.3(a)Types of connections: point-to-point
station station
Link
•Anetworkhastwoormoredevicesconnectedthroughlinks.
•Alinkisacommunicationspathwaythattransfersdatafrom
onedevicetoanotherasinfig.1.3(a).
Therearetwopossibletypesofconnections:
1.point-to-point:
Apoint-to-pointconnectionprovidesadedicatedlink
betweentwodevices.
Figure 1.3(a)Types of connections: point-to-point
station station
Link
2.multipoint:
Amultipoint(alsocalledmultidrop)connectionisonein
whichmorethantwospecificdevicesshareasinglelinkasin
fig.1.3(b).
Thecapacityofthechannelisshared,eitherspatiallyor
temporary.
Figure1.3(b)Typesofconnections:multipoint
Mainframe
Link
station
station
station
Amultipoint(alsocalledmultidrop)connectionisonein
whichmorethantwospecificdevicesshareasinglelinkasin
fig.1.3(b).
Thecapacityofthechannelisshared,eitherspatiallyor
temporary.
Figure1.3(b)Typesofconnections:multipoint
Mainframe
Link
station
station
station
Physical Topology
•PhysicalTopologyreferstothewayinwhichanetworkislaid
outeitherphysicallyorlogically.
•Twoormoredevicesconnecttoalink;twoormorelinksform
atopology.
•Thetopologyofanetworkisthegeometricrepresentationof
therelationshipofallthelinksandlinkingdevicestoone
another.
•Therearefourbasictopologiespossible:
─Mesh
─Star
─Bus
─Ring
•PhysicalTopologyreferstothewayinwhichanetworkislaid
outeitherphysicallyorlogically.
•Twoormoredevicesconnecttoalink;twoormorelinksform
atopology.
•Thetopologyofanetworkisthegeometricrepresentationof
therelationshipofallthelinksandlinkingdevicestoone
another.
•Therearefourbasictopologiespossible:
─Mesh
─Star
─Bus
─Ring
Mesh Topology
•Everydevicehasadedicatedpoint-to-pointlinktoeveryother
deviceasinfig.1.4.
•Inameshtopology,weneedn(n-1)/2duplex-modelinks.
•Toaccommodatethatmanylinks,everydeviceonthe
networkmusthaven-1input/output(I/O)ports.
Fig.1.4afullyconnectedmeshtopology
station
station
station
station station
•Everydevicehasadedicatedpoint-to-pointlinktoeveryother
deviceasinfig.1.4.
•Inameshtopology,weneedn(n-1)/2duplex-modelinks.
•Toaccommodatethatmanylinks,everydeviceonthe
networkmusthaven-1input/output(I/O)ports.
Fig.1.4afullyconnectedmeshtopology
station
station
station
station station
Advantages:
•Theuseofdedicatedlinksguaranteesthateachconnectioncancarryits
owndataload.Sotrafficproblemcanbeavoided.
•Itisrobust.Ifonelinkbecomesunusable,itdoesnotaffecttheentire
system.
•Itgivesprivacyandsecurity.
•Faultidentificationandfaultisolationareeasy.
Disadvantages:
•Since every device must be connected to every other device, installation
and reconnection are difficult.
•The bulk of the wiring is greater than the available space.
•Hardware required to connect each link can be prohibitively expensive
•Theuseofdedicatedlinksguaranteesthateachconnectioncancarryits
owndataload.Sotrafficproblemcanbeavoided.
•Itisrobust.Ifonelinkbecomesunusable,itdoesnotaffecttheentire
system.
•Itgivesprivacyandsecurity.
•Faultidentificationandfaultisolationareeasy.
Disadvantages:
•Since every device must be connected to every other device, installation
and reconnection are difficult.
•The bulk of the wiring is greater than the available space.
•Hardware required to connect each link can be prohibitively expensive
Example:
•Onepracticalexampleofameshtopologyistheconnectionof
telephoneregionalofficesinwhicheachregionalofficeneeds
tobeconnectedtoeveryotherregionaloffice.
•Ameshnetworkhas8devices.Calculatetotalnumberof
cablelinksandIOportsneeded.
Solution:
Numberofdevices =8
Numberoflinks =n(n-1)/2
=8(8-1)/2
=28
Numberofport/device=n-1
=8-1=7
•Onepracticalexampleofameshtopologyistheconnectionof
telephoneregionalofficesinwhicheachregionalofficeneeds
tobeconnectedtoeveryotherregionaloffice.
•Ameshnetworkhas8devices.Calculatetotalnumberof
cablelinksandIOportsneeded.
Solution:
Numberofdevices =8
Numberoflinks =n(n-1)/2
=8(8-1)/2
=28
Numberofport/device=n-1
=8-1=7
2. Star topology
•Inastartopologyasinfig.1.5,eachdevicehasadedicated
point-to-pointlinkonlytoacentralcontroller,usuallycalleda
hub.
•Thedevicesarenotdirectlylinkedtooneanother.
•Unlikeameshtopology,astartopologydoesnotallowdirect
trafficbetweendevices.
Fig.1.55Astartopologyconnectingfourstations
stationstationstationstation
Hub
•Inastartopologyasinfig.1.5,eachdevicehasadedicated
point-to-pointlinkonlytoacentralcontroller,usuallycalleda
hub.
•Thedevicesarenotdirectlylinkedtooneanother.
•Unlikeameshtopology,astartopologydoesnotallowdirect
trafficbetweendevices.
Fig.1.55Astartopologyconnectingfourstations
stationstationstationstation
Hub
Advantages:
•Lessexpensivethanmeshsinceeachdeviceisconnectedonly
tothehub.
•Installationandreconfigurationareeasy.
•Lesscablingisneededthanmesh.
•Thisincludesrobustness.Ifonelinkfails,onlythatlinkis
affected.Allotherlinksremainactive.Thisfactoralsolends
itselftoeasytofaultidentification&isolation.
Disadvantages:
•Ifthehubgoesdown,thewholesystemisdead.
•Lessexpensivethanmeshsinceeachdeviceisconnectedonly
tothehub.
•Installationandreconfigurationareeasy.
•Lesscablingisneededthanmesh.
•Thisincludesrobustness.Ifonelinkfails,onlythatlinkis
affected.Allotherlinksremainactive.Thisfactoralsolends
itselftoeasytofaultidentification&isolation.
Disadvantages:
•Ifthehubgoesdown,thewholesystemisdead.
3. Bus Topology
•Abustopologyismultipoint.Onelongcableactsasabackbonetolinkall
thedevicesinanetwork.
•Nodesareconnectedtothebuscablebydroplinesandtaps.Adroplineis
aconnectionbetweenthedeviceandthemaincable.
•Atapisaconnectorthateithersplicesintothemaincable,orpunctures
thesheathingofacabletocreateacontactwiththemetalliccoreas
showninfig.1.6.
Figure1.6Abustopologyconnectingthreestations
station station station
Tap Tap Tap
Drop Line
Drop Line Drop Line
Cable end Cable end
•Abustopologyismultipoint.Onelongcableactsasabackbonetolinkall
thedevicesinanetwork.
•Nodesareconnectedtothebuscablebydroplinesandtaps.Adroplineis
aconnectionbetweenthedeviceandthemaincable.
•Atapisaconnectorthateithersplicesintothemaincable,orpunctures
thesheathingofacabletocreateacontactwiththemetalliccoreas
showninfig.1.6.
Figure1.6Abustopologyconnectingthreestations
station station station
Tap Tap Tap
Drop Line
Drop Line Drop Line
Cable end Cable end
Advantages:
•Easeofinstallation
•Lesscabling
Disadvantages:
•Difficultreconnectionandfaultisolation.
•Difficulttoaddnewdevices.
•Signalreflectionatthetapscancausedegradationinquality.
•Ifanyfaultinthebuscablestopsalltransmission.
•Easeofinstallation
•Lesscabling
Disadvantages:
•Difficultreconnectionandfaultisolation.
•Difficulttoaddnewdevices.
•Signalreflectionatthetapscancausedegradationinquality.
•Ifanyfaultinthebuscablestopsalltransmission.
4. Ring Topology
•Eachdevicehasadedicatedpoint-to-pointconnectionwith
onlythetwodevicesoneithersideofitasinfig.1.7.
•Asignalispassedalongtheringinonedirection,fromdevice
todevice,untilitreachesitsdestination.
•Eachdeviceincorporatesarepeater(regeneratesthesignal
energy).
Figure1.7Aringtopologyconnectingsixstations
•Eachdevicehasadedicatedpoint-to-pointconnectionwith
onlythetwodevicesoneithersideofitasinfig.1.7.
•Asignalispassedalongtheringinonedirection,fromdevice
todevice,untilitreachesitsdestination.
•Eachdeviceincorporatesarepeater(regeneratesthesignal
energy).
Figure1.7Aringtopologyconnectingsixstations
Advantages:
•Easytoinstallandreconfigure.
•Faultidentificationiseasy.
Disadvantages:
•Unidirectionaltraffic.
•Abreakinthesimpleringcandisabletheentirenetwork.
CategoriesofNetworks:
Therearethreeprimarycategories,theyare
–Localareanetwork
–Metropolitanareanetwork
–Wideareanetwork
•Easytoinstallandreconfigure.
•Faultidentificationiseasy.
Disadvantages:
•Unidirectionaltraffic.
•Abreakinthesimpleringcandisabletheentirenetwork.
CategoriesofNetworks:
Therearethreeprimarycategories,theyare
–Localareanetwork
–Metropolitanareanetwork
–Wideareanetwork
1. Local Area Network
•Alocalareanetwork(LAN)isusuallyprivatelyownedandlinks
thedevicesinasingleoffice,building,orcampusasshownin
fig.1.8.
•Currently,LANsizeislimitedtoafewkilometers.
•LANsaredesignedtoallowresourcestobesharedbetween
personalcomputersorworkstations.
•Theresourcestobesharedcanincludehardware(e.g.,a
printer),software(e.g.,anapplicationprogram),ordata.
•ThemostcommonLANtopologiesarebus,ring,andstar.
EarlyLANshaddataratesinthe4to16megabitspersecond
(Mbps)range.Today,however,speedsarenormally100or
1000Mbps.
•Alocalareanetwork(LAN)isusuallyprivatelyownedandlinks
thedevicesinasingleoffice,building,orcampusasshownin
fig.1.8.
•Currently,LANsizeislimitedtoafewkilometers.
•LANsaredesignedtoallowresourcestobesharedbetween
personalcomputersorworkstations.
•Theresourcestobesharedcanincludehardware(e.g.,a
printer),software(e.g.,anapplicationprogram),ordata.
•ThemostcommonLANtopologiesarebus,ring,andstar.
EarlyLANshaddataratesinthe4to16megabitspersecond
(Mbps)range.Today,however,speedsarenormally100or
1000Mbps.
Figure 1.8 An isolated LAN connecting 12 computers to a hub in a closet
2. Metropolitan Area Network
•Ametropolitanareanetwork(MAN)isanetworkwithasize
betweenaLANandaWAN.
•Itnormallycoverstheareainsideatownoracity.
•Itisdesignedforcustomerswhoneedahigh-speed
connectivity,andhaveendpointsspreadoveracityorpartof
city.
•AgoodexampleofaMANisthepartofthetelephone
companynetworkthatcanprovideahigh-speedDSLlineto
thecustomer.
•AnotherexampleisthecableTVnetwork.
•Ametropolitanareanetwork(MAN)isanetworkwithasize
betweenaLANandaWAN.
•Itnormallycoverstheareainsideatownoracity.
•Itisdesignedforcustomerswhoneedahigh-speed
connectivity,andhaveendpointsspreadoveracityorpartof
city.
•AgoodexampleofaMANisthepartofthetelephone
companynetworkthatcanprovideahigh-speedDSLlineto
thecustomer.
•AnotherexampleisthecableTVnetwork.
Fig .MAN
3. Wide Area Network
•Awideareanetwork(WAN)provideslong-distancetransmissionofdata,image,
audio,andvideoinformationoverlargegeographicareas(likecountry,continent
oreventhewholeworld).
•TheswitchedWANconnectstheendsystems,whichusuallycomprisearouter
(internetworkingconnectingdevice)thatconnectstoanotherLANorWANasin
fig.1.9.a.
•Thepoint-to-pointWANisnormallyaline,leasedfromatelephoneorcableTV
providerthatconnectsahomecomputer,orasmallLANtoanInternetservice
provider(ISP)asshowninfig.1.9.b.ThistypeofWANisoftenusedtoprovide
Internetaccess.
•AnearlyexampleofaswitchedWANisX.25,anetworkdesignedtoprovide
connectivitybetweenendusers.
•AgoodexampleofaswitchedWANistheasynchronoustransfermode(ATM)
network,whichisanetworkwithfixed-sizedataunitpacketscalledcells.
•Awideareanetwork(WAN)provideslong-distancetransmissionofdata,image,
audio,andvideoinformationoverlargegeographicareas(likecountry,continent
oreventhewholeworld).
•TheswitchedWANconnectstheendsystems,whichusuallycomprisearouter
(internetworkingconnectingdevice)thatconnectstoanotherLANorWANasin
fig.1.9.a.
•Thepoint-to-pointWANisnormallyaline,leasedfromatelephoneorcableTV
providerthatconnectsahomecomputer,orasmallLANtoanInternetservice
provider(ISP)asshowninfig.1.9.b.ThistypeofWANisoftenusedtoprovide
Internetaccess.
•AnearlyexampleofaswitchedWANisX.25,anetworkdesignedtoprovide
connectivitybetweenendusers.
•AgoodexampleofaswitchedWANistheasynchronoustransfermode(ATM)
network,whichisanetworkwithfixed-sizedataunitpacketscalledcells.
Figure 1.9 WANs: a switched WAN and point-to-point WAN
Internetwork
InterconnectionofNetworks:Internetwork
•Whentwoormorenetworksareconnected,theybecomean
internetwork,orinternetasinfig.1.10.
•Asanexample,assumethatanorganizationhastwooffices,oneon
theeastcoastandtheotheronthewestcoast.
•TheestablishedofficeonthewestcoasthasabustopologyLAN;
thenewlyopenedofficeontheeastcoasthasastartopologyLAN.
•Thepresidentofthecompanylivessomewhereinthemiddleand
needstohavecontroloverthecompanyfromherhome.
•TocreateabackboneWANforconnectingthesethreeentities(two
LANsandthepresident'scomputer),aswitchedWAN(operatedby
aserviceprovidersuchasatelecomcompany)hasbeenleased.
•ToconnecttheLANstothisswitchedWAN,however,threepoint-
to-pointWANsarerequired.
InterconnectionofNetworks:Internetwork
•Whentwoormorenetworksareconnected,theybecomean
internetwork,orinternetasinfig.1.10.
•Asanexample,assumethatanorganizationhastwooffices,oneon
theeastcoastandtheotheronthewestcoast.
•TheestablishedofficeonthewestcoasthasabustopologyLAN;
thenewlyopenedofficeontheeastcoasthasastartopologyLAN.
•Thepresidentofthecompanylivessomewhereinthemiddleand
needstohavecontroloverthecompanyfromherhome.
•TocreateabackboneWANforconnectingthesethreeentities(two
LANsandthepresident'scomputer),aswitchedWAN(operatedby
aserviceprovidersuchasatelecomcompany)hasbeenleased.
•ToconnecttheLANstothisswitchedWAN,however,threepoint-
to-pointWANsarerequired.
Figure 1.10 A heterogeneous network made off our WANs and two LANs
Internet
•TheInternetisastructured,organizedsystem.
•Privateindividualsaswellasvariousorganizationssuchas
governmentagencies,schools,researchfacilities,corporations,
andlibrariesinmorethan100countriesusetheInternet.Millions
ofpeopleareusers.
•TodaymostenduserswhowantInternetconnectionusethe
servicesofInternetserviceproviders(ISPs).
•Thereareinternationalserviceproviders,nationalservice
providers,regionalserviceproviders,andlocalserviceproviders.
•TheInternettodayisrunbyprivatecompanies,notthe
government.
•TheInternetisastructured,organizedsystem.
•Privateindividualsaswellasvariousorganizationssuchas
governmentagencies,schools,researchfacilities,corporations,
andlibrariesinmorethan100countriesusetheInternet.Millions
ofpeopleareusers.
•TodaymostenduserswhowantInternetconnectionusethe
servicesofInternetserviceproviders(ISPs).
•Thereareinternationalserviceproviders,nationalservice
providers,regionalserviceproviders,andlocalserviceproviders.
•TheInternettodayisrunbyprivatecompanies,notthe
government.
Protocols and Standards
•Incomputernetworks,communicationoccursbetweenentriesindifferent
systems.
•Anentityisanythingcapableofsendingorreceivinginformation.
However,twoentitiescannotsimplysendbitstreamstoeachotherand
expecttobeunderstood.
•Forcommunicationoccur,theentitiesmustagreeonaprotocol.
Protocols
•Aprotocolisasetofrulesthatgoverndatacommunications.
•Aprotocoldefineswhatiscommunicated,howitiscommunicated,and
whenitiscommunicated.
•Thekeyelementsofaprotocolare
1.Syntax
2.Semantics
3.Timing
•Incomputernetworks,communicationoccursbetweenentriesindifferent
systems.
•Anentityisanythingcapableofsendingorreceivinginformation.
However,twoentitiescannotsimplysendbitstreamstoeachotherand
expecttobeunderstood.
•Forcommunicationoccur,theentitiesmustagreeonaprotocol.
Protocols
•Aprotocolisasetofrulesthatgoverndatacommunications.
•Aprotocoldefineswhatiscommunicated,howitiscommunicated,and
whenitiscommunicated.
•Thekeyelementsofaprotocolare
1.Syntax
2.Semantics
3.Timing
1.Syntax:
Syntaxreferstothestructureorformatofthedata,meaningtheorderin
whichtheyarepresented.
2.Semantics:
─Semanticsreferstothemeaningofeachsectionofbits.
─Itrefersthathowisaparticularpatterntobeinterpreted
─whatactionistobetakenbasedonthatinterpretation?
3.Timing:
Timingreferstotwocharacteristics.Theyare,
─Whendatashouldbesent
─Howfasttheycanbesent
Syntaxreferstothestructureorformatofthedata,meaningtheorderin
whichtheyarepresented.
2.Semantics:
─Semanticsreferstothemeaningofeachsectionofbits.
─Itrefersthathowisaparticularpatterntobeinterpreted
─whatactionistobetakenbasedonthatinterpretation?
3.Timing:
Timingreferstotwocharacteristics.Theyare,
─Whendatashouldbesent
─Howfasttheycanbesent
Standards
-Standardsprovideguidelinestomanufacturers,vendors,government
agencies,andotherserviceproviders
Datacommunicationstandardsfallintotwocategories:
1.defacto(meaning"byfact"or"byconvention")
2.dejure(meaning"bylaw"or"byregulation")
1.Defacto:
–Standardsthathavenotbeenapprovedbyanorganizedbodybuthave
beenadoptedasstandardsthroughwidespreadusearedefacto
standards.
–Defactostandardsareoftenestablishedoriginallybymanufacturerswho
seektodefinethefunctionalityofanewproductortechnology.
2.Dejure:
–Thosestandardsthathavebeenlegislatedbyanofficiallyrecognizedbody
are,dejurestandards.
-Standardsprovideguidelinestomanufacturers,vendors,government
agencies,andotherserviceproviders
Datacommunicationstandardsfallintotwocategories:
1.defacto(meaning"byfact"or"byconvention")
2.dejure(meaning"bylaw"or"byregulation")
1.Defacto:
–Standardsthathavenotbeenapprovedbyanorganizedbodybuthave
beenadoptedasstandardsthroughwidespreadusearedefacto
standards.
–Defactostandardsareoftenestablishedoriginallybymanufacturerswho
seektodefinethefunctionalityofanewproductortechnology.
2.Dejure:
–Thosestandardsthathavebeenlegislatedbyanofficiallyrecognizedbody
are,dejurestandards.
NETWORK MODELS
NetworkModels:
•Computernetworksarecreatedbydifferententities.
•Standardsareneededsothattheseheterogeneousnetworkscancommunicatewithoneanother.
•Twostandardsare
–OSI(OpenSystemsInterconnection)Model->Itdefinesaseven-layernetwork.
–InternetModel->Itdefinesafive-layernetwork.
LayeredTasks:
•Weusetheconceptoflayersinourdailylife.
•Asshowninfig.1.11,letusconsidertwofriendswhocommunicatethroughpostalmail.
•Theprocessofsendingalettertoafriendwouldbecomplexiftherewerenoservicesavailable
fromthepostoffice.
Hierarchy:
•Accordingtoouranalysis,therearethreedifferentactivitiesatthesendersiteandanotherthree
activitiesatthereceiversite.
•Thetaskoftransportingtheletterbetweenthesenderandthereceiverisdonebythe
carrier.
•Atthesendersite,thelettermustbewrittenanddroppedinthemailboxbeforebeing
pickedupbythelettercarrieranddeliveredtothepostoffice.
•Atthereceiversite,thelettermustbedroppedintherecipientmailboxbeforebeingpicked
upandreadbytherecipient.
NetworkModels:
•Computernetworksarecreatedbydifferententities.
•Standardsareneededsothattheseheterogeneousnetworkscancommunicatewithoneanother.
•Twostandardsare
–OSI(OpenSystemsInterconnection)Model->Itdefinesaseven-layernetwork.
–InternetModel->Itdefinesafive-layernetwork.
LayeredTasks:
•Weusetheconceptoflayersinourdailylife.
•Asshowninfig.1.11,letusconsidertwofriendswhocommunicatethroughpostalmail.
•Theprocessofsendingalettertoafriendwouldbecomplexiftherewerenoservicesavailable
fromthepostoffice.
Hierarchy:
•Accordingtoouranalysis,therearethreedifferentactivitiesatthesendersiteandanotherthree
activitiesatthereceiversite.
•Thetaskoftransportingtheletterbetweenthesenderandthereceiverisdonebythe
carrier.
•Atthesendersite,thelettermustbewrittenanddroppedinthemailboxbeforebeing
pickedupbythelettercarrieranddeliveredtothepostoffice.
•Atthereceiversite,thelettermustbedroppedintherecipientmailboxbeforebeingpicked
upandreadbytherecipient.
Higher Layers
Middle Layers
Lower Layers
Figure 1.11 Tasks involved in sending a letter
Middle Layers
Lower Layers
Figure 1.11 Tasks involved in sending a letter
Services:
•Eachlayeratthesendingsiteusestheservicesofthelayer
immediatelybelowit.Thesenderatthehigherlayerusesthe
servicesofthemiddlelayer.
•Themiddlelayerusestheservicesofthelowerlayer.Thelower
layerusestheservicesofthecarrier.
•Thelayeredmodelthatdominateddatacommunicationsand
networkingliteraturebefore1990wastheOpenSystems
Interconnection(OSI)model.
•EveryonebelievedthattheOSImodelwouldbecometheultimate
standardfordatacommunications,butthisdidnothappen.
•TheTCP/IPprotocolsuitebecamethedominantcommercial
architecturebecauseitwasusedandtestedextensivelyinthe
Internet;theOSImodelwasneverfullyimplemented.
•Eachlayeratthesendingsiteusestheservicesofthelayer
immediatelybelowit.Thesenderatthehigherlayerusesthe
servicesofthemiddlelayer.
•Themiddlelayerusestheservicesofthelowerlayer.Thelower
layerusestheservicesofthecarrier.
•Thelayeredmodelthatdominateddatacommunicationsand
networkingliteraturebefore1990wastheOpenSystems
Interconnection(OSI)model.
•EveryonebelievedthattheOSImodelwouldbecometheultimate
standardfordatacommunications,butthisdidnothappen.
•TheTCP/IPprotocolsuitebecamethedominantcommercial
architecturebecauseitwasusedandtestedextensivelyinthe
Internet;theOSImodelwasneverfullyimplemented.
OSI MODEL
•Establishedin1947,theInternationalStandardsOrganization(ISO)isa
multinationalbodydedicatedtoworldwideagreementoninternational
standards.
•AnISOstandardthatcoversallaspectsofnetworkcommunicationsisthe
OpenSystemsInterconnectionmodel.Itwasfirstintroducedinthelate
1970s.
•Anopensystemisasetofprotocolsthatallowsanytwodifferentsystems
tocommunicateregardlessoftheirunderlyingarchitecture.
•ThepurposeoftheOSImodelistoshowhowtomakeeasy
communicationbetweendifferentsystemswithoutrequiringchangesto
thelogicoftheunderlyinghardwareandsoftware.
•TheOSImodelisnotaprotocol;itisamodelforunderstandingand
designinganetworkarchitecturethatisflexible,robust,and
interoperable.ISOistheorganization.OSIisthemodel.
•TheOSImodelisalayeredframeworkforthedesignofnetworksystems
thatallowscommunicationbetweenalltypesofcomputersystems.
•Itconsistsofsevenseparatebutrelatedlayers,eachofwhichdefinesa
partoftheprocessofmovinginformationacrossanetwork.
•Establishedin1947,theInternationalStandardsOrganization(ISO)isa
multinationalbodydedicatedtoworldwideagreementoninternational
standards.
•AnISOstandardthatcoversallaspectsofnetworkcommunicationsisthe
OpenSystemsInterconnectionmodel.Itwasfirstintroducedinthelate
1970s.
•Anopensystemisasetofprotocolsthatallowsanytwodifferentsystems
tocommunicateregardlessoftheirunderlyingarchitecture.
•ThepurposeoftheOSImodelistoshowhowtomakeeasy
communicationbetweendifferentsystemswithoutrequiringchangesto
thelogicoftheunderlyinghardwareandsoftware.
•TheOSImodelisnotaprotocol;itisamodelforunderstandingand
designinganetworkarchitecturethatisflexible,robust,and
interoperable.ISOistheorganization.OSIisthemodel.
•TheOSImodelisalayeredframeworkforthedesignofnetworksystems
thatallowscommunicationbetweenalltypesofcomputersystems.
•Itconsistsofsevenseparatebutrelatedlayers,eachofwhichdefinesa
partoftheprocessofmovinginformationacrossanetwork.
Layered Architecture
WhyLayeredarchitecture?
•Tomakethedesignprocesseasybybreaking
unmanageabletasksintoseveralsmallerand
manageabletasks(bydivide-and-conquerapproach).
•Modularityandclearinterfaces,soastoprovide
comparabilitybetweenthedifferentproviders'
components.
•Ensureindependenceoflayers,sothat
implementationofeachlayercanbechangedor
modifiedwithoutaffectingotherlayers.
•Eachlayercanbeanalyzedandtestedindependently
ofallotherlayers.
WhyLayeredarchitecture?
•Tomakethedesignprocesseasybybreaking
unmanageabletasksintoseveralsmallerand
manageabletasks(bydivide-and-conquerapproach).
•Modularityandclearinterfaces,soastoprovide
comparabilitybetweenthedifferentproviders'
components.
•Ensureindependenceoflayers,sothat
implementationofeachlayercanbechangedor
modifiedwithoutaffectingotherlayers.
•Eachlayercanbeanalyzedandtestedindependently
ofallotherlayers.
•TheOSImodeliscomposedofsevenorderedlayers:theyare
–Physicallayer
–Datalinklayer
–Networklayer
–Transportlayer
–Sessionlayer
–Presentationlayer
–Applicationlayer
•Figure1.12showsthelayersinvolvedwhenamessageissentfromdeviceAtodeviceB.
•AsthemessagetravelsfromAtoB,itmaypassthroughmanyintermediatenodes.These
intermediatenodesusuallyinvolveonlythefirstthreelayersoftheOSImodel.
•Withinasinglemachine,eachlayercallsupontheservicesofthelayerjustbelowit.
•Layer3,forexample,usestheservicesprovidedbylayer2andprovidesservicesforlayer
4.
•Betweenmachines,layerxononemachinecommunicateswithlayerxonanother
machine.Thiscommunicationisgovernedbyanagreed-uponseriesofrulesand
conventionscalledprotocols.
•Theprocessesoneachmachinethatcommunicateatagivenlayerarecalledpeer-to-peer
processes.
•Communicationbetweenmachinesisthereforeapeer-to-peerprocessusingthe
protocolsappropriatetoagivenlayer.
–Physicallayer
–Datalinklayer
–Networklayer
–Transportlayer
–Sessionlayer
–Presentationlayer
–Applicationlayer
•Figure1.12showsthelayersinvolvedwhenamessageissentfromdeviceAtodeviceB.
•AsthemessagetravelsfromAtoB,itmaypassthroughmanyintermediatenodes.These
intermediatenodesusuallyinvolveonlythefirstthreelayersoftheOSImodel.
•Withinasinglemachine,eachlayercallsupontheservicesofthelayerjustbelowit.
•Layer3,forexample,usestheservicesprovidedbylayer2andprovidesservicesforlayer
4.
•Betweenmachines,layerxononemachinecommunicateswithlayerxonanother
machine.Thiscommunicationisgovernedbyanagreed-uponseriesofrulesand
conventionscalledprotocols.
•Theprocessesoneachmachinethatcommunicateatagivenlayerarecalledpeer-to-peer
processes.
•Communicationbetweenmachinesisthereforeapeer-to-peerprocessusingthe
protocolsappropriatetoagivenlayer.
Figure 1.12 The interaction between layers in the OSI model
Peer-to-Peer Processes
•Atthephysicallayer,communicationisdirect.Atthehigherlayers,however,
communicationmustmovedownthroughthelayersondeviceAbackupthroughthe
layers.
•Eachlayerinthesendingdeviceaddsitsowninformationtothemessageitreceives
fromthelayerjustaboveitandpassesthewholepackagetothelayerjustbelowit.
•Entirepackageisconvertedtoaformthatcanbetransmittedtothereceivingdevice.
Atthereceivingmachine,themessageisunwrappedlayerbylayer,witheachprocess
receivingandremovingthedatameantforit.
•Forexample,layer2removesthedatameantforit,andthenpassestheresttolayer
3.Layer3thenremovesthedatameantforitandpassestheresttolayer4,andso
on.
InterfacesbetweenLayers:
•Thepassingofthedataandnetworkinformationdownthroughthelayersofthe
sendingdeviceandbackupthroughthelayersofthereceivingdeviceismade
possiblebyaninterfacebetweeneachpairofadjacentlayers.
•Eachinterfacedefinestheinformationandservicesalayermustprovideforthelayer
aboveit.
•Well-definedinterfacesandlayerfunctionsprovidemodularitytoanetwork
•Atthephysicallayer,communicationisdirect.Atthehigherlayers,however,
communicationmustmovedownthroughthelayersondeviceAbackupthroughthe
layers.
•Eachlayerinthesendingdeviceaddsitsowninformationtothemessageitreceives
fromthelayerjustaboveitandpassesthewholepackagetothelayerjustbelowit.
•Entirepackageisconvertedtoaformthatcanbetransmittedtothereceivingdevice.
Atthereceivingmachine,themessageisunwrappedlayerbylayer,witheachprocess
receivingandremovingthedatameantforit.
•Forexample,layer2removesthedatameantforit,andthenpassestheresttolayer
3.Layer3thenremovesthedatameantforitandpassestheresttolayer4,andso
on.
InterfacesbetweenLayers:
•Thepassingofthedataandnetworkinformationdownthroughthelayersofthe
sendingdeviceandbackupthroughthelayersofthereceivingdeviceismade
possiblebyaninterfacebetweeneachpairofadjacentlayers.
•Eachinterfacedefinestheinformationandservicesalayermustprovideforthelayer
aboveit.
•Well-definedinterfacesandlayerfunctionsprovidemodularitytoanetwork
OrganizationoftheLayers:
Thesevenlayersarearrangedbythreesubgroups.
–NetworkSupportLayers
–UserSupportLayers
–IntermediateLayer
1.NetworkSupportLayers
–Physical,DatalinkandNetworklayerscomeunderthegroup.
–Theydealwiththephysicalaspectsofthedatasuchaselectrical
specifications,physicalconnections,physicaladdressing,and
transporttimingandreliability.
2.UserSupportLayers
–Session,PresentationandApplicationlayerscomesunderthe
group.
–Theydealwiththeinteroperabilitybetweenthesoftwaresystems.
3.IntermediateLayer
Thetransportlayeristheintermediatelayerbetweenthenetwork
supportandtheusersupportlayers.
Thesevenlayersarearrangedbythreesubgroups.
–NetworkSupportLayers
–UserSupportLayers
–IntermediateLayer
1.NetworkSupportLayers
–Physical,DatalinkandNetworklayerscomeunderthegroup.
–Theydealwiththephysicalaspectsofthedatasuchaselectrical
specifications,physicalconnections,physicaladdressing,and
transporttimingandreliability.
2.UserSupportLayers
–Session,PresentationandApplicationlayerscomesunderthe
group.
–Theydealwiththeinteroperabilitybetweenthesoftwaresystems.
3.IntermediateLayer
Thetransportlayeristheintermediatelayerbetweenthenetwork
supportandtheusersupportlayers.
Figure 1.13 An exchange using the OSI Model
•TheupperOSIlayersarealmostalwaysimplementedinsoftware;lowerlayersarea
combinationofhardwareandsoftware,exceptforthephysicallayer,whichismostly
hardware.
AnexchangeusingtheOSIModel
•Infig.1.13,D7meansthedataunitatlayer7,D6meansthedataunitatlayer6,andsoon.
•Theprocessstartsatlayer7(theapplicationlayer),thenmovesfromlayertolayerin
descending,sequentialorder.
•Ateachlayer,aheader,orpossiblyatrailer,canbeaddedtothedataunit.
•Commonly,thetrailerisaddedonlyatlayer2.Whentheformatteddataunitpassesthrough
thephysicallayer(layer1),itischangedintoanelectromagneticsignalandtransportedalong
aphysicallink.
•Uponreachingitsdestination,thesignalpassesintolayer1andistransformedbackinto
digitalform.ThedataunitsthenmovebackupthroughtheOSIlayers.
•Aseachblockofdatareachesthenexthigherlayer,theheadersandtrailersattachedtoitat
thecorrespondingsendinglayerareremoved,andactionsappropriatetothatlayerare
taken.
•Bythetimeitreacheslayer7,themessageisagaininaformappropriatetotheapplication
andismadeavailabletotherecipient.
Encapsulation:
•Apacket(headeranddata)atlevel7isencapsulatedinapacketatlevel6.Thewholepacket
atlevel6isencapsulatedinapacketatlevel5,andsoon.
•Inotherwords,thedataportionofapacketatlevelN-1carriesthewholepacket(dataand
headerandmaybetrailer)fromlevelN.Theconceptiscalledencapsulation.
combinationofhardwareandsoftware,exceptforthephysicallayer,whichismostly
hardware.
AnexchangeusingtheOSIModel
•Infig.1.13,D7meansthedataunitatlayer7,D6meansthedataunitatlayer6,andsoon.
•Theprocessstartsatlayer7(theapplicationlayer),thenmovesfromlayertolayerin
descending,sequentialorder.
•Ateachlayer,aheader,orpossiblyatrailer,canbeaddedtothedataunit.
•Commonly,thetrailerisaddedonlyatlayer2.Whentheformatteddataunitpassesthrough
thephysicallayer(layer1),itischangedintoanelectromagneticsignalandtransportedalong
aphysicallink.
•Uponreachingitsdestination,thesignalpassesintolayer1andistransformedbackinto
digitalform.ThedataunitsthenmovebackupthroughtheOSIlayers.
•Aseachblockofdatareachesthenexthigherlayer,theheadersandtrailersattachedtoitat
thecorrespondingsendinglayerareremoved,andactionsappropriatetothatlayerare
taken.
•Bythetimeitreacheslayer7,themessageisagaininaformappropriatetotheapplication
andismadeavailabletotherecipient.
Encapsulation:
•Apacket(headeranddata)atlevel7isencapsulatedinapacketatlevel6.Thewholepacket
atlevel6isencapsulatedinapacketatlevel5,andsoon.
•Inotherwords,thedataportionofapacketatlevelN-1carriesthewholepacket(dataand
headerandmaybetrailer)fromlevelN.Theconceptiscalledencapsulation.
LAYERS IN THE OSI MODEL
1.PhysicalLayer
Thephysicallayerisresponsiblefor
movementsofindividualbitsfromonehop(node)to
thenext.
Figure 1.14 Physical layer
1.PhysicalLayer
Thephysicallayerisresponsiblefor
movementsofindividualbitsfromonehop(node)to
thenext.
Figure 1.14 Physical layer
Otherresponsibilitiesare,
Physicalcharacteristicsofinterfacesandmedium:
•Itdefinesthecharacteristicsoftheinterfacebetweenthedevicesandthe
transmissionmediumasinfig.1.14.
•Italsodefinesthetypeoftransmissionmedium.
RepresentationofBits:
•Totransmitthestreamofbits,theymustbeencodedintosignals
(electricaloroptical).
•Itdefinesthetypeofencoding.
Fig. Physical layer
Physicalcharacteristicsofinterfacesandmedium:
•Itdefinesthecharacteristicsoftheinterfacebetweenthedevicesandthe
transmissionmediumasinfig.1.14.
•Italsodefinesthetypeoftransmissionmedium.
RepresentationofBits:
•Totransmitthestreamofbits,theymustbeencodedintosignals
(electricaloroptical).
•Itdefinesthetypeofencoding.
Fig. Physical layer
DataRate:
•Itdefinesthetransmissionratei.e.thenumberof
bitssentpersecond(physicallayerdefinesthe
durationofabit,whichishowlongitlasts)
SynchronizationofBits:
•Thesenderandreceivermustbesynchronizedat
thebitlevel.
•Inotherwords,thesenderandthereceiver
clocksmustbesynchronized.
.
•Itdefinesthetransmissionratei.e.thenumberof
bitssentpersecond(physicallayerdefinesthe
durationofabit,whichishowlongitlasts)
SynchronizationofBits:
•Thesenderandreceivermustbesynchronizedat
thebitlevel.
•Inotherwords,thesenderandthereceiver
clocksmustbesynchronized.
.
Line Configuration:
It defines the type of connection between the devices. Two types of
connection are,
–point to point
–multipoint
Physical Topology:
It defines how devices are connected to make a network.
Devices can be connected by
–mesh
–star
–bus
–ring
–hybrid
Transmission Mode
It defines the direction of transmission between devices: simplex,
half duplex and full duplex
It defines the type of connection between the devices. Two types of
connection are,
–point to point
–multipoint
Physical Topology:
It defines how devices are connected to make a network.
Devices can be connected by
–mesh
–star
–bus
–ring
–hybrid
Transmission Mode
It defines the direction of transmission between devices: simplex,
half duplex and full duplex
2. Data link Layer
Thedatalinklayerisresponsiblefor
movingframesfromonehop(node)tothe
next.
Figure 1.15 Data link layer
Thedatalinklayerisresponsiblefor
movingframesfromonehop(node)tothe
next.
Figure 1.15 Data link layer
Data Link Layer Example
Otherresponsibilitiesare,
Framing
•Itdividesthestreamofbitsreceivedfromnetworklayerintomanageabledataunitscalled
framesasinfig.1.15.
PhysicalAddressing
•Ifframesaretobedistributedtodifferentsystemsonthenetwork,thedatalinklayeraddsa
headertotheframetodefinethesenderand/orreceiveroftheframe.
•Iftheframeisintendedforasystemoutsidethesender'snetwork,thereceiveraddressis
theaddressofthedevicethatconnectsthenetworktothenextone.
Flowcontrol
•Iftherateatwhichthedataareabsorbedbythereceiverislessthantherateatwhichdata
areproducedinthesender,thedatalinklayerimposesaflowcontrol
mechanismtoavoidoverwhelmingthereceiver.
Errorcontrol
•Thedatalinklayeraddsreliabilitytothephysicallayerbyaddingmechanismstodetectand
retransmitdamagedorlostframes.
•Italsousesamechanismtorecognizeduplicateframes.Errorcontrolisnormallyachieved
throughatraileraddedtotheendoftheframe.
Accesscontrol
•Whentwoormoredevicesareconnectedtothesamelink,datalinklayerprotocolsare
necessarytodeterminewhichdevicehascontroloverthelinkatanygiventime.
Framing
•Itdividesthestreamofbitsreceivedfromnetworklayerintomanageabledataunitscalled
framesasinfig.1.15.
PhysicalAddressing
•Ifframesaretobedistributedtodifferentsystemsonthenetwork,thedatalinklayeraddsa
headertotheframetodefinethesenderand/orreceiveroftheframe.
•Iftheframeisintendedforasystemoutsidethesender'snetwork,thereceiveraddressis
theaddressofthedevicethatconnectsthenetworktothenextone.
Flowcontrol
•Iftherateatwhichthedataareabsorbedbythereceiverislessthantherateatwhichdata
areproducedinthesender,thedatalinklayerimposesaflowcontrol
mechanismtoavoidoverwhelmingthereceiver.
Errorcontrol
•Thedatalinklayeraddsreliabilitytothephysicallayerbyaddingmechanismstodetectand
retransmitdamagedorlostframes.
•Italsousesamechanismtorecognizeduplicateframes.Errorcontrolisnormallyachieved
throughatraileraddedtotheendoftheframe.
Accesscontrol
•Whentwoormoredevicesareconnectedtothesamelink,datalinklayerprotocolsare
necessarytodeterminewhichdevicehascontroloverthelinkatanygiventime.
3. Network Layer
•Thenetworklayerisresponsibleforthe
deliveryofindividualpacketsasinfig.1.16,
fromthesourcehosttothedestinationhost.
•Iftwosystemsareconnectedtothesamelink,
thereisusuallynoneedforanetworklayer.
•However,ifthetwosystemsareattachedto
differentnetworks(links)withconnecting
devicesbetweenthenetworks(links),thereis
oftenaneedforthenetworklayerto
accomplishsource-to-destinationdelivery.
•Thenetworklayerisresponsibleforthe
deliveryofindividualpacketsasinfig.1.16,
fromthesourcehosttothedestinationhost.
•Iftwosystemsareconnectedtothesamelink,
thereisusuallynoneedforanetworklayer.
•However,ifthetwosystemsareattachedto
differentnetworks(links)withconnecting
devicesbetweenthenetworks(links),thereis
oftenaneedforthenetworklayerto
accomplishsource-to-destinationdelivery.
Figure 1.16 Network layer
Theresponsibilitiesare,
LogicalAddressing
─Ifapacketpassesthenetworkboundary,weneedanotheraddressingsystem
tohelpdistinguishthesourceanddestinationsystems.
─Thenetworklayeraddsaheadertothepacketcomingfromtheupperlayer
that,amongotherthings,includesthelogicaladdressesofthesenderand
receiver.
Routing
─Whenindependentnetworksorlinksareconnectedtocreatealargenetwork,
theconnectingdevices(calledrouterorswitches)routeorswitchthepacketsto
theirfinaldestination.
Theresponsibilitiesare,
LogicalAddressing
─Ifapacketpassesthenetworkboundary,weneedanotheraddressingsystem
tohelpdistinguishthesourceanddestinationsystems.
─Thenetworklayeraddsaheadertothepacketcomingfromtheupperlayer
that,amongotherthings,includesthelogicaladdressesofthesenderand
receiver.
Routing
─Whenindependentnetworksorlinksareconnectedtocreatealargenetwork,
theconnectingdevices(calledrouterorswitches)routeorswitchthepacketsto
theirfinaldestination.
4. Transport Layer
•Thetransportlayerisresponsibleforprocesstoprocessdeliveryof
theentiremessage.
•Aprocessisanapplicationprogramrunningonahost.
•Itensuresthatthewholemessagearrivesinorderandintact.It
ensuresboththeerrorcontrolandflowcontrolatsourceto
destinationlevel.
Figure 1.17 Transport Layer
•Thetransportlayerisresponsibleforprocesstoprocessdeliveryof
theentiremessage.
•Aprocessisanapplicationprogramrunningonahost.
•Itensuresthatthewholemessagearrivesinorderandintact.It
ensuresboththeerrorcontrolandflowcontrolatsourceto
destinationlevel.
Figure 1.17 Transport Layer
Theresponsibilitiesare,
ServicepointAddressing
•Computersoftenrunseveralprogramsatthesametime.
•Thetransportlayerheadermustthereforeincludeatypeofaddresscalledaservice-
pointaddress(orportaddress).
•Thetransportlayergetseachpackettothecorrectcomputer;thetransportlayergets
theentiremessagetothecorrectprocessonthatcomputer.
SegmentationandReassembly
•Amessageisdividedintotransmittablesegments,witheachsegmentcontaininga
sequencenumberasshowninfig.1.17.
•Thesenumbersenablethetransportlayertoreassemblethemessagecorrectlyupon
arrivingatthedestinationandtoidentifyandreplacepacketsthatwerelostin
transmission.
Connectioncontrol
•Thetransportlayercanbeeitherconnectionlessorconnection-oriented.
•Aconnectionlesstransportlayertreatseachsegmentasanindependentpacketand
deliversittothetransportlayeratthedestinationmachine.
•Aconnection-orientedtransportlayermakesaconnectionwiththetransportlayer
atthedestinationmachinefirstbeforedeliveringthepackets.Afterallthedataare
transferred,theconnectionisterminated.
ServicepointAddressing
•Computersoftenrunseveralprogramsatthesametime.
•Thetransportlayerheadermustthereforeincludeatypeofaddresscalledaservice-
pointaddress(orportaddress).
•Thetransportlayergetseachpackettothecorrectcomputer;thetransportlayergets
theentiremessagetothecorrectprocessonthatcomputer.
SegmentationandReassembly
•Amessageisdividedintotransmittablesegments,witheachsegmentcontaininga
sequencenumberasshowninfig.1.17.
•Thesenumbersenablethetransportlayertoreassemblethemessagecorrectlyupon
arrivingatthedestinationandtoidentifyandreplacepacketsthatwerelostin
transmission.
Connectioncontrol
•Thetransportlayercanbeeitherconnectionlessorconnection-oriented.
•Aconnectionlesstransportlayertreatseachsegmentasanindependentpacketand
deliversittothetransportlayeratthedestinationmachine.
•Aconnection-orientedtransportlayermakesaconnectionwiththetransportlayer
atthedestinationmachinefirstbeforedeliveringthepackets.Afterallthedataare
transferred,theconnectionisterminated.
Flowcontrol
•Likethedatalinklayer,thetransportlayerisresponsiblefor
flowcontrol.However,flowcontrolatthislayerisperformed
endtoendratherthanacrossasinglelink.
Errorcontrol
•Likethedatalinklayer,thetransportlayerisresponsiblefor
errorcontrol.However,errorcontrolatthislayerisperformed
process-to-processratherthanacrossasinglelink.
•Thesendingtransportlayermakessurethattheentire
messagearrivesatthereceivingtransportlayerwithouterror
(damage,loss,orduplication).
•Errorcorrectionisusuallyachievedthroughretransmission.
•Likethedatalinklayer,thetransportlayerisresponsiblefor
flowcontrol.However,flowcontrolatthislayerisperformed
endtoendratherthanacrossasinglelink.
Errorcontrol
•Likethedatalinklayer,thetransportlayerisresponsiblefor
errorcontrol.However,errorcontrolatthislayerisperformed
process-to-processratherthanacrossasinglelink.
•Thesendingtransportlayermakessurethattheentire
messagearrivesatthereceivingtransportlayerwithouterror
(damage,loss,orduplication).
•Errorcorrectionisusuallyachievedthroughretransmission.
5. Session Layer
•Itisthenetworkdialogcontroller.
•Itestablishes,maintainsandsynchronizesthe
interactionbetweenthecommunicating
systems.
Figure 1.18 Session Layer
•Itisthenetworkdialogcontroller.
•Itestablishes,maintainsandsynchronizesthe
interactionbetweenthecommunicating
systems.
Figure 1.18 Session Layer
Specificresponsibilitiesare,
DialogControl
•Thesessionlayerallowstwosystemstoenterintoa
dialog.
•Itallowsthecommunicationbetweentwoprocessesto
takeplaceineitherhalf-duplex(onewayatatime)or
full-duplex(twowaysatatime)mode.
Synchronization
•Itallowsaprocesstoaddcheckpoints,or
synchronizationpointasinfig.1.18,toastreamofdata.
DialogControl
•Thesessionlayerallowstwosystemstoenterintoa
dialog.
•Itallowsthecommunicationbetweentwoprocessesto
takeplaceineitherhalf-duplex(onewayatatime)or
full-duplex(twowaysatatime)mode.
Synchronization
•Itallowsaprocesstoaddcheckpoints,or
synchronizationpointasinfig.1.18,toastreamofdata.
6. Presentation Layer
•Thepresentationlayerisresponsibleforthesyntaxand
semanticsoftheinformationexchangedbetweentwo
systemsasshowninfig.1.19.
Specificresponsibilitiesare,
Translation
•Theprocesses(runningprograms)intwosystemsare
usuallyexchanginginformationintheformofcharacter
strings,numbers,andsoon.
•Theinformationmustbechangedtobitstreamsbefore
beingtransmitted.Becausedifferentcomputersuse
differentencodingsystems.
•Thepresentationlayerisresponsibleforinteroperability
betweenthesedifferentencodingmethods.
•Thepresentationlayerisresponsibleforthesyntaxand
semanticsoftheinformationexchangedbetweentwo
systemsasshowninfig.1.19.
Specificresponsibilitiesare,
Translation
•Theprocesses(runningprograms)intwosystemsare
usuallyexchanginginformationintheformofcharacter
strings,numbers,andsoon.
•Theinformationmustbechangedtobitstreamsbefore
beingtransmitted.Becausedifferentcomputersuse
differentencodingsystems.
•Thepresentationlayerisresponsibleforinteroperability
betweenthesedifferentencodingmethods.
Encryption
•Tocarrysensitiveinformation,asystemmustbeabletoensureprivacy.
•Encryptionmeansthatthesendertransformstheoriginalinformationtoanother
formandsendstheresultingmessageoutoverthenetwork.
Decryption
•Itreversestheoriginalprocesstotransformthemessagebacktoitsoriginalform.
Compression
•Datacompressionreducesthenumberofbitscontainedintheinformation.
•Datacompressionbecomesparticularlyimportantinthetransmissionof
multimediasuchastext,audio,andvideo.
Figure 1.19 Presentation Layer
•Tocarrysensitiveinformation,asystemmustbeabletoensureprivacy.
•Encryptionmeansthatthesendertransformstheoriginalinformationtoanother
formandsendstheresultingmessageoutoverthenetwork.
Decryption
•Itreversestheoriginalprocesstotransformthemessagebacktoitsoriginalform.
Compression
•Datacompressionreducesthenumberofbitscontainedintheinformation.
•Datacompressionbecomesparticularlyimportantinthetransmissionof
multimediasuchastext,audio,andvideo.
Figure 1.19 Presentation Layer
7. Application Layer
•The application layer enables the user, whether human or software,
to access the network as in fig.1.20.
•It provides user interfaces and support for services such as
electronic mail, remote file access and transfer, shared database
management, and other types of distributed information services.
•X.400 (message-handling services), X.500 (directory services), and
file transfer, access, and management (FTAM).
Figure 1.20 Application Layer
•The application layer enables the user, whether human or software,
to access the network as in fig.1.20.
•It provides user interfaces and support for services such as
electronic mail, remote file access and transfer, shared database
management, and other types of distributed information services.
•X.400 (message-handling services), X.500 (directory services), and
file transfer, access, and management (FTAM).
Figure 1.20 Application Layer
Specificresponsibilitiesare,
NetworkVirtualTerminal(NVT)
•Itisasoftwareversionofaphysicalterminal,andit
allowsausertologontoaremotehost.
FileTransfer,Access,andManagement(FTAM)
•Itallowsausertoaccessfiles,toretrievefilesfrom
remotehost,andtomanageorcontrolfilesina
remotecomputerlocally.
MailServices
•Itprovidesthebasisfore-mailforwardingandstorage.
DirectoryServices
•Itprovidesdistributeddatabasesourcesandaccessfor
globalinformationaboutvariousobjectsandservices.
NetworkVirtualTerminal(NVT)
•Itisasoftwareversionofaphysicalterminal,andit
allowsausertologontoaremotehost.
FileTransfer,Access,andManagement(FTAM)
•Itallowsausertoaccessfiles,toretrievefilesfrom
remotehost,andtomanageorcontrolfilesina
remotecomputerlocally.
MailServices
•Itprovidesthebasisfore-mailforwardingandstorage.
DirectoryServices
•Itprovidesdistributeddatabasesourcesandaccessfor
globalinformationaboutvariousobjectsandservices.
TCP/IP PROTOCOL SUITE
•TheTCP/IPprotocolsuitewasdevelopedpriortotheOSImodel.
Therefore,thelayersintheTCP/IPprotocolsuitedonotexactlymatch
thoseintheOSImodel.
•TheoriginalTCP/IPprotocolsuitewasdefinedashavingfourlayers:
–host-to-network
–internet
–transport
–application
•TCP/IPprotocolsuiteismadeoffivelayers:
–physical
–datalink
–network
–transport
–application
•Thefirstfourlayersprovidephysicalstandards,networkinterfaces,
internetworking,andtransportfunctionsthatcorrespondtothefirstfour
layersoftheOSImodelasinfig.1.21.
•ThethreetopmostlayersintheOSImodelarerepresentedinTCP/IPbya
singlelayercalledtheapplicationlayer.
•TheTCP/IPprotocolsuitewasdevelopedpriortotheOSImodel.
Therefore,thelayersintheTCP/IPprotocolsuitedonotexactlymatch
thoseintheOSImodel.
•TheoriginalTCP/IPprotocolsuitewasdefinedashavingfourlayers:
–host-to-network
–internet
–transport
–application
•TCP/IPprotocolsuiteismadeoffivelayers:
–physical
–datalink
–network
–transport
–application
•Thefirstfourlayersprovidephysicalstandards,networkinterfaces,
internetworking,andtransportfunctionsthatcorrespondtothefirstfour
layersoftheOSImodelasinfig.1.21.
•ThethreetopmostlayersintheOSImodelarerepresentedinTCP/IPbya
singlelayercalledtheapplicationlayer.
Figure 1.21 TCP/IP and OSI model
•TCP/IPisahierarchicalprotocolmadeupofinteractivemodules.Theterm
hierarchicalmeansthateachupper-levelprotocolissupportedbyoneormore
lower-levelprotocols.
•Atthetransportlayer,TCP/IPdefinesthreeprotocols:TransmissionControl
Protocol(TCP),UserDatagramProtocol(UDP),andStreamControlTransmission
Protocol(SCTP).
•Atthenetworklayer,themainprotocoldefinedbyTCP/IPistheInternetworking
Protocol(IP).
•Atthephysicalanddatalinklayers,TCP/IPdoesnotdefineanyspecificprotocol.
Itsupportsallthestandardandproprietaryprotocols.
•AnetworkinaTCP/IPinternetworkcanbealocal-areanetworkorawide-area
network.
•ThetransportlayerwasrepresentedinTCP/IPbytwoprotocols:TCPandUDP.
•IPisahost-to-hostprotocol,meaningthatitcandeliverapacketfromone
physicaldevicetoanother.
•UDPandTCParetransportlevelprotocolsresponsiblefordeliveryofamessage
fromaprocess(runningprogram)toanotherprocess.
•Anewtransportlayerprotocol,SCTP(StreamControlTransmissionProtocol)has
beendevisedtomeettheneedsofsomenewerapplications.
•TheapplicationlayerinTCP/IPisequivalenttothecombinedsession,presentation,
andapplicationlayersintheOSImodel.Manyprotocolsaredefinedatthislayer.
hierarchicalmeansthateachupper-levelprotocolissupportedbyoneormore
lower-levelprotocols.
•Atthetransportlayer,TCP/IPdefinesthreeprotocols:TransmissionControl
Protocol(TCP),UserDatagramProtocol(UDP),andStreamControlTransmission
Protocol(SCTP).
•Atthenetworklayer,themainprotocoldefinedbyTCP/IPistheInternetworking
Protocol(IP).
•Atthephysicalanddatalinklayers,TCP/IPdoesnotdefineanyspecificprotocol.
Itsupportsallthestandardandproprietaryprotocols.
•AnetworkinaTCP/IPinternetworkcanbealocal-areanetworkorawide-area
network.
•ThetransportlayerwasrepresentedinTCP/IPbytwoprotocols:TCPandUDP.
•IPisahost-to-hostprotocol,meaningthatitcandeliverapacketfromone
physicaldevicetoanother.
•UDPandTCParetransportlevelprotocolsresponsiblefordeliveryofamessage
fromaprocess(runningprogram)toanotherprocess.
•Anewtransportlayerprotocol,SCTP(StreamControlTransmissionProtocol)has
beendevisedtomeettheneedsofsomenewerapplications.
•TheapplicationlayerinTCP/IPisequivalenttothecombinedsession,presentation,
andapplicationlayersintheOSImodel.Manyprotocolsaredefinedatthislayer.
ADDRESSING
Fourlevelsofaddressesareusedinan
internetemployingtheTCP/IPprotocols:
–Physical(link)addresses
–Logical(IP)addresses
–Portaddresses
–Specificaddresses
Fourlevelsofaddressesareusedinan
internetemployingtheTCP/IPprotocols:
–Physical(link)addresses
–Logical(IP)addresses
–Portaddresses
–Specificaddresses
1.Physicaladdress(MACaddress)
•Infig.1.22,thePhysicaladdresses,alsoknownas
thelinkaddress,istheaddressofanodeas
definedbyitsLANorWAN.
•Forexample,Ethernetusesa6-byte(48-bit)
physicaladdressthatisimprintedontheNetwork
interfacecard(NIC).
Figure1.22Physicaladdresses
•Infig.1.22,thePhysicaladdresses,alsoknownas
thelinkaddress,istheaddressofanodeas
definedbyitsLANorWAN.
•Forexample,Ethernetusesa6-byte(48-bit)
physicaladdressthatisimprintedontheNetwork
interfacecard(NIC).
Figure1.22Physicaladdresses
MAC address
•ThefirsthalfofaMACaddresscontainstheID
numberoftheadaptermanufacturer.
•ThesecondhalfofaMACaddressrepresents
theserialnumberassignedtotheadapterby
themanufacturer.
Intheexample,
00:A0:C9:14:C8:29
Theprefix00A0C9indicatesthemanufacturer
isIntelCorporation.
•ThefirsthalfofaMACaddresscontainstheID
numberoftheadaptermanufacturer.
•ThesecondhalfofaMACaddressrepresents
theserialnumberassignedtotheadapterby
themanufacturer.
Intheexample,
00:A0:C9:14:C8:29
Theprefix00A0C9indicatesthemanufacturer
isIntelCorporation.
2.Logicaladdresses(IPaddresses)
•Logicaladdressesarenecessaryforuniversal
communicationsthatareindependentof
underlyingphysicalnetworksasinfig.1.23.
•AlogicaladdressintheInternetiscurrentlya32-
bitaddressthatcanuniquelydefineahost
connectedtotheInternet.
•Notwopubliclyaddressedandvisiblehostson
theInternetcanhavethesameIPaddress.
•Thenumbersareseparatedbyadot.132.24.75.9
isanexampleofsuchanIPaddress.
•Logicaladdressesarenecessaryforuniversal
communicationsthatareindependentof
underlyingphysicalnetworksasinfig.1.23.
•AlogicaladdressintheInternetiscurrentlya32-
bitaddressthatcanuniquelydefineahost
connectedtotheInternet.
•Notwopubliclyaddressedandvisiblehostson
theInternetcanhavethesameIPaddress.
•Thenumbersareseparatedbyadot.132.24.75.9
isanexampleofsuchanIPaddress.
Fig 1.23 IP addresses
3.Portaddresses
•Today,computersaredevicesthatcanrun
multipleprocessesatthesametime.
•TheendobjectiveofInternetcommunicationisa
processcommunicatingwithanotherprocess.
•Fortheseprocessestoreceivedata
simultaneously,weneedamethodtolabelthe
differentprocesses.
•IntheTCP/IParchitecture,thelabelassignedto
aprocessiscalledaportaddressasinfig.1.24.A
portaddressinTCP/IPis16bitsinlength.
•Today,computersaredevicesthatcanrun
multipleprocessesatthesametime.
•TheendobjectiveofInternetcommunicationisa
processcommunicatingwithanotherprocess.
•Fortheseprocessestoreceivedata
simultaneously,weneedamethodtolabelthe
differentprocesses.
•IntheTCP/IParchitecture,thelabelassignedto
aprocessiscalledaportaddressasinfig.1.24.A
portaddressinTCP/IPis16bitsinlength.
Fig 1.24 Port addresses
4. Specific Addresses
•Some applications have user-friendly addresses that are designed for that specific address.
•Examples include the e-mail address (for example, [email protected]) and the Universal
Resource Locator (URL) (for example, www.mhhe.com).
•The first defines the recipient of an e-mail the second is used to find a document on the World
Wide Web.
4. Specific Addresses
•Some applications have user-friendly addresses that are designed for that specific address.
•Examples include the e-mail address (for example, [email protected]) and the Universal
Resource Locator (URL) (for example, www.mhhe.com).
•The first defines the recipient of an e-mail the second is used to find a document on the World
Wide Web.
Performane
Physical Links
Transmissionmediacanbedefinedasphysicalpathbetweentransmitterand
receiverinadatatransmissionsystem.
1.Guided:
̶Transmissioncapacitydependsonthemediumandthelength.
̶Alsoitisbasedonwhetherthemediumispoint-to-pointormultipoint(e.g.
LAN).
̶Examplesareco-axialcable,twistedpair,andopticalfiber.
2.Unguided:
̶providesameansfortransmittingelectro-magneticsignalsbutdonotguide
them.Examplewirelesstransmission.
–Characteristicsandqualityofdatatransmissionaredeterminedbymediumand
signalcharacteristics.
–Forguidedmedia,themediumismoreimportantindeterminingthelimitationsof
transmission.
–Forunguidedmedia,thebandwidthofthesignalproducedbythetransmitting
antennaandthesizeoftheantennaismoreimportantthanthemedium.
–Signalsatlowerfrequenciesareomni-directional(propagateinalldirections).
–Forhigherfrequencies,focusingthesignalsintoadirectionalbeamispossible.
–Thesepropertiesdeterminewhatkindofmediaoneshoulduseinaparticular
application.
Transmissionmediacanbedefinedasphysicalpathbetweentransmitterand
receiverinadatatransmissionsystem.
1.Guided:
̶Transmissioncapacitydependsonthemediumandthelength.
̶Alsoitisbasedonwhetherthemediumispoint-to-pointormultipoint(e.g.
LAN).
̶Examplesareco-axialcable,twistedpair,andopticalfiber.
2.Unguided:
̶providesameansfortransmittingelectro-magneticsignalsbutdonotguide
them.Examplewirelesstransmission.
–Characteristicsandqualityofdatatransmissionaredeterminedbymediumand
signalcharacteristics.
–Forguidedmedia,themediumismoreimportantindeterminingthelimitationsof
transmission.
–Forunguidedmedia,thebandwidthofthesignalproducedbythetransmitting
antennaandthesizeoftheantennaismoreimportantthanthemedium.
–Signalsatlowerfrequenciesareomni-directional(propagateinalldirections).
–Forhigherfrequencies,focusingthesignalsintoadirectionalbeamispossible.
–Thesepropertiesdeterminewhatkindofmediaoneshoulduseinaparticular
application.
Figure 1.19 Classification of the transmission media
Guided transmission media
The most commonly used guided transmission media such as
–Twisted-pair of cable
–Coaxial cable
–Optical fiber
v
Guided transmission media
The most commonly used guided transmission media such as
–Twisted-pair of cable
–Coaxial cable
–Optical fiber
v
1.TwistedPairCable
̶Thetwowiresaretypically``twisted''togethertoreduceinterference(likecrosstalk)
betweenthetwoconductorsasshowninFig.1.20.
̶Typically,anumberofpairsarebundledtogetherintoacablebywrappingthemina
toughprotectivesheath.
Figure1.20CAT5cable(twistedcable)
–DataratesofseveralMbpscommon.
–Spansdistancesofseveralkilometers.
–Dataratedeterminedbywirethicknessandlength.
–Inaddition,shieldingtoeliminateinterferencefromotherwiresimpactssignal-to-noise
ratio,andultimately,thedatarate.
–Good,low-costcommunication.Inreality,manysitesalreadyhavetwistedpairinstalled
inoffices--existingphonelines!
̶Thetwowiresaretypically``twisted''togethertoreduceinterference(likecrosstalk)
betweenthetwoconductorsasshowninFig.1.20.
̶Typically,anumberofpairsarebundledtogetherintoacablebywrappingthemina
toughprotectivesheath.
Figure1.20CAT5cable(twistedcable)
–DataratesofseveralMbpscommon.
–Spansdistancesofseveralkilometers.
–Dataratedeterminedbywirethicknessandlength.
–Inaddition,shieldingtoeliminateinterferencefromotherwiresimpactssignal-to-noise
ratio,andultimately,thedatarate.
–Good,low-costcommunication.Inreality,manysitesalreadyhavetwistedpairinstalled
inoffices--existingphonelines!
Typicalcharacteristics:
1.Twisted-paircanbeusedforbothanaloganddigitalcommunication.
2.Thedataratethatcanbesupportedoveratwisted-pairisinverselyproportional
tothesquareofthelinelength.
3.Maximumtransmissiondistanceof1Kmcanbeachievedfordataratesupto1
Mb/s.
4.Foranalogvoicesignals,amplifiersarerequiredaboutevery6Kmandfordigital
signals,repeatersareneededforabout2Km.
5.Toreduceinterference,thetwistedpaircanbeshieldedwithmetallicbraid.
ThistypeofwireisknownasShieldedTwisted-Pair(STP)andtheotherformis
knownasUnshieldedTwisted-Pair(UTP).
Uses:
1.Theoldestandthemostpopularuseoftwistedpairareintelephony.(Category6
cable-6
th
generationoftwistedpaircable)
2.InLANitiscommonlyusedforpoint-to-pointshortdistancecommunication(say,
100m)withinabuildingoraroom.
1.Twisted-paircanbeusedforbothanaloganddigitalcommunication.
2.Thedataratethatcanbesupportedoveratwisted-pairisinverselyproportional
tothesquareofthelinelength.
3.Maximumtransmissiondistanceof1Kmcanbeachievedfordataratesupto1
Mb/s.
4.Foranalogvoicesignals,amplifiersarerequiredaboutevery6Kmandfordigital
signals,repeatersareneededforabout2Km.
5.Toreduceinterference,thetwistedpaircanbeshieldedwithmetallicbraid.
ThistypeofwireisknownasShieldedTwisted-Pair(STP)andtheotherformis
knownasUnshieldedTwisted-Pair(UTP).
Uses:
1.Theoldestandthemostpopularuseoftwistedpairareintelephony.(Category6
cable-6
th
generationoftwistedpaircable)
2.InLANitiscommonlyusedforpoint-to-pointshortdistancecommunication(say,
100m)withinabuildingoraroom.
2.CoaxialCable
2.1BasebandCoaxial
1.With``coax'',themediumconsistsofacoppercoresurroundedbyinsulating
materialandabraidedouterconductorasshowninFig.1.21.
2.Thetermbasebandindicatesdigitaltransmission(asopposedtobroadband
analog).
Figure1.21Co-axialcable
3.Physicalconnectionconsistsofmetalpintouchingthecoppercore.
4.Therearetwocommonwaystoconnecttoacoaxialcable:
–Withvampiretaps,ametalpinisinsertedintothecoppercore.Aspecialtooldrillsaholeinto
thecable,removingasmallsectionoftheinsulation,andaspecialconnectorisscrewedinto
thehole.Thetapmakescontactwiththecoppercore.
–WithaT-junction,thecableiscutinhalf,andbothhalvesconnecttotheT-junction.AT-
connectorisanalogoustothesignalsplittersusedtohookupmultipleTVstothesamecable
wire.
2.1BasebandCoaxial
1.With``coax'',themediumconsistsofacoppercoresurroundedbyinsulating
materialandabraidedouterconductorasshowninFig.1.21.
2.Thetermbasebandindicatesdigitaltransmission(asopposedtobroadband
analog).
Figure1.21Co-axialcable
3.Physicalconnectionconsistsofmetalpintouchingthecoppercore.
4.Therearetwocommonwaystoconnecttoacoaxialcable:
–Withvampiretaps,ametalpinisinsertedintothecoppercore.Aspecialtooldrillsaholeinto
thecable,removingasmallsectionoftheinsulation,andaspecialconnectorisscrewedinto
thehole.Thetapmakescontactwiththecoppercore.
–WithaT-junction,thecableiscutinhalf,andbothhalvesconnecttotheT-junction.AT-
connectorisanalogoustothesignalsplittersusedtohookupmultipleTVstothesamecable
wire.
Characteristics:
1.Co-axialcablecanbeusedforbothanaloganddigitalsignaling.
2.InbasebandLAN,thedataratesliesintherangeof1KHzto20MHzoveradistanceinthe
rangeof1Km.
3.Co-axialcablestypicallyhaveadiameterof3/8".
4.Coaxialcablesareusedbothforbasebandandbroadbandcommunication.
5.Inbroadbandsignaling,signalpropagatesonlyinonedirection,incontrasttopropagationin
bothdirectionsinbasebandsignaling.
6.Broadbandcablinguseseitherdual-cableschemeorsingle-cableschemewithaheadend
tofacilitateflowofsignalinonedirection.Becauseoftheshielded,concentricconstruction,
co-axialcableislesssusceptibletointerferenceandcrosstalkthanthetwisted-pair.
7.Forlongdistancecommunication,repeatersareneededforeverykilometer.
8.Dataratedependsonphysicalpropertiesofcable,but10Mbpsistypical.
Uses:
1.Oneofthemostpopularusesofco-axialcableisincableTV(CATV)forthedistributionof
TVsignals.RG-6iscabletelevision(CATV)distributioncoaxcable.RG59isgoodforlower
frequencysignals(anythingunderabout50MHz).Thatmakesitagoodchoiceforaclosed
circuittelevision(CCTV)videosurveillancesystem
2.Anotherimportanceuseofco-axialcableisinLAN.
1.Co-axialcablecanbeusedforbothanaloganddigitalsignaling.
2.InbasebandLAN,thedataratesliesintherangeof1KHzto20MHzoveradistanceinthe
rangeof1Km.
3.Co-axialcablestypicallyhaveadiameterof3/8".
4.Coaxialcablesareusedbothforbasebandandbroadbandcommunication.
5.Inbroadbandsignaling,signalpropagatesonlyinonedirection,incontrasttopropagationin
bothdirectionsinbasebandsignaling.
6.Broadbandcablinguseseitherdual-cableschemeorsingle-cableschemewithaheadend
tofacilitateflowofsignalinonedirection.Becauseoftheshielded,concentricconstruction,
co-axialcableislesssusceptibletointerferenceandcrosstalkthanthetwisted-pair.
7.Forlongdistancecommunication,repeatersareneededforeverykilometer.
8.Dataratedependsonphysicalpropertiesofcable,but10Mbpsistypical.
Uses:
1.Oneofthemostpopularusesofco-axialcableisincableTV(CATV)forthedistributionof
TVsignals.RG-6iscabletelevision(CATV)distributioncoaxcable.RG59isgoodforlower
frequencysignals(anythingunderabout50MHz).Thatmakesitagoodchoiceforaclosed
circuittelevision(CCTV)videosurveillancesystem
2.Anotherimportanceuseofco-axialcableisinLAN.
2.2BroadbandCoaxial
1.Thetermbroadbandreferstoanalog
transmissionovercoaxialcable.
2.Thetechnology:
–Typicallybandwidthof300MHz,totaldatarateof
about150Mbps.
–Operatesatdistancesupto100km(metropolitan
area!).
–Usesanalogsignaling.
–Requiresamplifierstoboostsignalstrength;because
amplifiersareoneway,dataflowsinonlyone
direction.
1.Thetermbroadbandreferstoanalog
transmissionovercoaxialcable.
2.Thetechnology:
–Typicallybandwidthof300MHz,totaldatarateof
about150Mbps.
–Operatesatdistancesupto100km(metropolitan
area!).
–Usesanalogsignaling.
–Requiresamplifierstoboostsignalstrength;because
amplifiersareoneway,dataflowsinonlyone
direction.
3. Fiber Optics
1.Infiberoptictechnology,themediumconsistsofahair-widthstrand(filament)ofsiliconorglass,andthe
signalconsistsofpulsesoflight.
2.Forinstance,apulseoflightmeans``1'',lackofpulsemeans``0''.
3.Ithasacylindricalshapeandconsistsofthreeconcentricsections:thecore,thecladding,andthejacketas
showninFig.1.22.
Figure1.22OpticalFiber
4.Thecore,innermostsectionconsistsofasinglesoliddielectriccylinderofdiameterd
1andofrefractiveindex
n
1.
5.Thecoreissurroundedbyasoliddielectriccladdingofrefractiveindexn
2thatislessthann
1.
6.Asaconsequence,thelightispropagatedthroughmultipletotalinternalreflections.
7.Thecorematerialisusuallymadeofultrapurefusedsilicaorglassandthecladdingiseithermadeofglassor
plastic.
8.Thecladdingissurroundedbyajacketmadeofplastic.Thejacketisusedtoprotectagainstmoisture,
abrasion,crushingandotherenvironmentalhazards.
1.Infiberoptictechnology,themediumconsistsofahair-widthstrand(filament)ofsiliconorglass,andthe
signalconsistsofpulsesoflight.
2.Forinstance,apulseoflightmeans``1'',lackofpulsemeans``0''.
3.Ithasacylindricalshapeandconsistsofthreeconcentricsections:thecore,thecladding,andthejacketas
showninFig.1.22.
Figure1.22OpticalFiber
4.Thecore,innermostsectionconsistsofasinglesoliddielectriccylinderofdiameterd
1andofrefractiveindex
n
1.
5.Thecoreissurroundedbyasoliddielectriccladdingofrefractiveindexn
2thatislessthann
1.
6.Asaconsequence,thelightispropagatedthroughmultipletotalinternalreflections.
7.Thecorematerialisusuallymadeofultrapurefusedsilicaorglassandthecladdingiseithermadeofglassor
plastic.
8.Thecladdingissurroundedbyajacketmadeofplastic.Thejacketisusedtoprotectagainstmoisture,
abrasion,crushingandotherenvironmentalhazards.
Threecomponentsarerequired:
1.Fibermedium:Currenttechnologycarrieslightpulsesfortremendousdistances(e.g.,100sof
kilometers)withvirtuallynosignalloss.
2.Lightsource:typicallyaLightEmittingDiode(LED)orlaserdiode.Runningcurrentthrough
thematerialgeneratesapulseoflight.
3.Aphotodiodelightdetector,whichconvertslightpulsesintoelectricalsignals.
Advantages:
1.Veryhighdatarate,lowerrorrate.
2.Difficulttotap,whichmakesithardforunauthorizedtapsaswell?Thisisresponsiblefor
higherreliabilityofthismedium.
3.Muchthinner(perlogicalphoneline)thanexistingcoppercircuits.
4.Notsusceptibletoelectricalinterference(lightning)orcorrosion(rust).
5.Greaterrepeaterdistancethancoax.
Disadvantages:
1.Expensive.
2.One-waychannel.Twofibersneededtogetfullduplex(bothways)communication.
1.Fibermedium:Currenttechnologycarrieslightpulsesfortremendousdistances(e.g.,100sof
kilometers)withvirtuallynosignalloss.
2.Lightsource:typicallyaLightEmittingDiode(LED)orlaserdiode.Runningcurrentthrough
thematerialgeneratesapulseoflight.
3.Aphotodiodelightdetector,whichconvertslightpulsesintoelectricalsignals.
Advantages:
1.Veryhighdatarate,lowerrorrate.
2.Difficulttotap,whichmakesithardforunauthorizedtapsaswell?Thisisresponsiblefor
higherreliabilityofthismedium.
3.Muchthinner(perlogicalphoneline)thanexistingcoppercircuits.
4.Notsusceptibletoelectricalinterference(lightning)orcorrosion(rust).
5.Greaterrepeaterdistancethancoax.
Disadvantages:
1.Expensive.
2.One-waychannel.Twofibersneededtogetfullduplex(bothways)communication.
OpticalFiberworksindifferenttypesofmodes:
Theyare
1.Multi-ModeFiber(MMF)-thecoreandcladdingdiameterliesintherange50-
200μm(1mm=1000μm)and125-400μm,respectively.
2.Single-ModeFiber(SMF)-thecoreandcladdingdiameterslieintherange8-12μmand125μm,
respectively.Single-modefibersarealsoknownasMono-ModeFiber.
Moreover,bothsingle-modeandmulti-modefiberscanhavetwotypes;
1.Stepindex-therefractiveindexofthecoreisuniformthroughoutandatthecorecladding
boundarythereisanabruptchangeinrefractiveindex
2.Gradedindex-therefractiveindexofthecorevariesradicallyfromthecentretothecore-
claddingboundaryfromn
1ton
2inalinearmanner.Fig1.23showstheopticalfibertransmission
modes.
Theyare
1.Multi-ModeFiber(MMF)-thecoreandcladdingdiameterliesintherange50-
200μm(1mm=1000μm)and125-400μm,respectively.
2.Single-ModeFiber(SMF)-thecoreandcladdingdiameterslieintherange8-12μmand125μm,
respectively.Single-modefibersarealsoknownasMono-ModeFiber.
Moreover,bothsingle-modeandmulti-modefiberscanhavetwotypes;
1.Stepindex-therefractiveindexofthecoreisuniformthroughoutandatthecorecladding
boundarythereisanabruptchangeinrefractiveindex
2.Gradedindex-therefractiveindexofthecorevariesradicallyfromthecentretothecore-
claddingboundaryfromn
1ton
2inalinearmanner.Fig1.23showstheopticalfibertransmission
modes.
Characteristics:
1.Whenlightisappliedatoneendoftheopticalfibercore,itreachestheotherend
bymeansoftotalinternalreflectionbecauseofthechoiceofrefractiveindexof
coreandcladdingmaterial(n
1>n
2).
2.Thelightsourcecanbeeitherlightemittingdiode(LED)orinjectionlaserdiode
(ILD).Thesesemiconductordevicesemitabeamoflightwhenavoltageisapplied
acrossthedevice.
3.Atthereceivingend,aphotodiodecanbeusedtodetectthesignal-encodedlight.
4.Single-modefiberallowslongerdistanceswithoutrepeater.
5.Formulti-modefiber,thetypicalmaximumlengthofthecablewithoutarepeater
is2km,whereasforsingle-modefiberitis20km.
FiberUses:
1.Ithassmallerdiameter,lighterweight,lowattenuation,immunityto
electromagneticinterference.
2.usefulforlong-distancetelecommunications.
3.Fiberopticcablesarealsousedinhigh-speedLANapplications.
4.Multi-modefiberiscommonlyusedinLAN.
1.Whenlightisappliedatoneendoftheopticalfibercore,itreachestheotherend
bymeansoftotalinternalreflectionbecauseofthechoiceofrefractiveindexof
coreandcladdingmaterial(n
1>n
2).
2.Thelightsourcecanbeeitherlightemittingdiode(LED)orinjectionlaserdiode
(ILD).Thesesemiconductordevicesemitabeamoflightwhenavoltageisapplied
acrossthedevice.
3.Atthereceivingend,aphotodiodecanbeusedtodetectthesignal-encodedlight.
4.Single-modefiberallowslongerdistanceswithoutrepeater.
5.Formulti-modefiber,thetypicalmaximumlengthofthecablewithoutarepeater
is2km,whereasforsingle-modefiberitis20km.
FiberUses:
1.Ithassmallerdiameter,lighterweight,lowattenuation,immunityto
electromagneticinterference.
2.usefulforlong-distancetelecommunications.
3.Fiberopticcablesarealsousedinhigh-speedLANapplications.
4.Multi-modefiberiscommonlyusedinLAN.
UnguidedTransmission
1.Unguidedtransmissionisusedwhenrunningaphysicalcable(eitherfiberorcopper)betweentwoend
pointsisnotpossible.
2.Radiofrequency(RF)isarateofoscillationintherangeofaround3kHzto300GHz,whichcorresponds
tothefrequencyofradiowaves.
3.Infraredsignalstypicallyusedforshortdistances(frequencyrange430THzto300GHz)
4.Microwavesignals-anelectromagneticwaveintherangebetween300MHz(0.3GHz)and300GHz,
shorterthanthatofanormalradiowavebutlongerthanthoseofinfraredradiation.
– SenderandreceiverusesomesortofdishantennaasshowninFig.1.24.
– Microwavesareusedinradar,incommunications,andforheatinginmicrowaveovensandinvarious
industrialprocesses.
Figure1.24CommunicationusingTerrestrialMicrowave
Difficulties:
1.Weatherinterfereswithsignals.Forinstance,clouds,rain,lightning,etc.mayadverselyaffect
communication.
2.Radiotransmissionseasytotap.
1.Unguidedtransmissionisusedwhenrunningaphysicalcable(eitherfiberorcopper)betweentwoend
pointsisnotpossible.
2.Radiofrequency(RF)isarateofoscillationintherangeofaround3kHzto300GHz,whichcorresponds
tothefrequencyofradiowaves.
3.Infraredsignalstypicallyusedforshortdistances(frequencyrange430THzto300GHz)
4.Microwavesignals-anelectromagneticwaveintherangebetween300MHz(0.3GHz)and300GHz,
shorterthanthatofanormalradiowavebutlongerthanthoseofinfraredradiation.
– SenderandreceiverusesomesortofdishantennaasshowninFig.1.24.
– Microwavesareusedinradar,incommunications,andforheatinginmicrowaveovensandinvarious
industrialprocesses.
Figure1.24CommunicationusingTerrestrialMicrowave
Difficulties:
1.Weatherinterfereswithsignals.Forinstance,clouds,rain,lightning,etc.mayadverselyaffect
communication.
2.Radiotransmissionseasytotap.
•Theatmosphereis divided into five layers. It is thickest near the surface and thins out with height until it
eventually merges with space.
•1) Thetroposphereis the first layer above the surface and contains half of the Earth's
atmosphere.Weatheroccurs in this layer.
2) Many jet aircrafts fly in thestratospherebecause it is very stable. Also, the ozone layer absorbs harmful
rays from the Sun.
3) Meteors or rock fragments burn up in themesosphere.
4) Thethermosphereis a layer with auroras. It is also where the space shuttle orbits.
5) The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our
atmosphere.
eventually merges with space.
•1) Thetroposphereis the first layer above the surface and contains half of the Earth's
atmosphere.Weatheroccurs in this layer.
2) Many jet aircrafts fly in thestratospherebecause it is very stable. Also, the ozone layer absorbs harmful
rays from the Sun.
3) Meteors or rock fragments burn up in themesosphere.
4) Thethermosphereis a layer with auroras. It is also where the space shuttle orbits.
5) The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our
atmosphere.
SatelliteCommunication
1.Acommunicationsatelliteisessentiallyabig
microwaverepeaterorrelaystationinthesky.
2.Microwavesignalsfromagroundstationispickedup
byatransponder(deviceattachedtoasatellite),
amplifiesthesignalandrebroadcastsitinanother
frequency,whichcanbereceivedbygroundstations
atlongdistances.
3.Tokeepthesatellitestationarywithrespecttothe
groundbasedstations,thesatelliteisplacedina
geostationaryorbitabovetheequatoratanaltitude
ofabout36,000km.
4.Asatellitecanbeusedforpoint-to-point
communicationbetweentwoground-basedstations
asshowninFig.1.25.
1.Acommunicationsatelliteisessentiallyabig
microwaverepeaterorrelaystationinthesky.
2.Microwavesignalsfromagroundstationispickedup
byatransponder(deviceattachedtoasatellite),
amplifiesthesignalandrebroadcastsitinanother
frequency,whichcanbereceivedbygroundstations
atlongdistances.
3.Tokeepthesatellitestationarywithrespecttothe
groundbasedstations,thesatelliteisplacedina
geostationaryorbitabovetheequatoratanaltitude
ofabout36,000km.
4.Asatellitecanbeusedforpoint-to-point
communicationbetweentwoground-basedstations
asshowninFig.1.25.
Figure 1.25 Satellite Microwave Communication: point –to-point
Figure 1.26 Satellite Microwave Communication: Broadcast links
Figure 1.26 Satellite Microwave Communication: Broadcast links
Characteristics:
1.Optimumfrequencyrangeforsatellitecommunicationis1to10
GHz.
2.Themostpopularfrequencybandis3.7to4.2GHzfordownlink
and5.925to6.425foruplinktransmissions.
3.Themostimportantisthelongcommunicationdelayforthe
roundtrip(about270ms)becauseofthelongdistance(about
72,000km)thesignalhastotravelbetweentwoearthstations.
4.Allstationsunderthedownwardbeamcanreceivethe
transmission.
5.Itmaybenecessarytosendencrypteddatatoprotectagainst
piracy.
Uses:
1.Now-a-dayscommunicationsatellitesarenotonlyusedtohandle
telephone,telexandtelevisiontrafficoverlongdistances,butare
usedtosupportvariousinternetbasedservicessuchase-mail,
FTP,WorldWideWeb(WWW),etc.
2.Newtypesofservices,basedoncommunicationsatellites,are
emerging.
1.Optimumfrequencyrangeforsatellitecommunicationis1to10
GHz.
2.Themostpopularfrequencybandis3.7to4.2GHzfordownlink
and5.925to6.425foruplinktransmissions.
3.Themostimportantisthelongcommunicationdelayforthe
roundtrip(about270ms)becauseofthelongdistance(about
72,000km)thesignalhastotravelbetweentwoearthstations.
4.Allstationsunderthedownwardbeamcanreceivethe
transmission.
5.Itmaybenecessarytosendencrypteddatatoprotectagainst
piracy.
Uses:
1.Now-a-dayscommunicationsatellitesarenotonlyusedtohandle
telephone,telexandtelevisiontrafficoverlongdistances,butare
usedtosupportvariousinternetbasedservicessuchase-mail,
FTP,WorldWideWeb(WWW),etc.
2.Newtypesofservices,basedoncommunicationsatellites,are
emerging.
Channel Access on links
MultipleAccessTechniques
Variousmultipleaccesstechniquesare
1.FrequencyDivisionMultipleAccess(FDMA)
2.TimeDivisionMultipleAccess(TDMA)
3.CodeDivisionMultipleAccess(CDMA)
1.FrequencyDivisionMultipleAccess
Infrequency-divisionmultipleaccess(FDMA),theavailablebandwidthisdividedinto
frequencybands.
Eachstationisallocatedabandtosenditsdata.
Inthismethodwhenanyonefrequencyleveliskeptidleandanotherisusedfrequently
leadstoinefficiency.
Figure1.27FrequencyDivisionMultipleAccess
f
f5
f4
f3
f2
f1
t
MultipleAccessTechniques
Variousmultipleaccesstechniquesare
1.FrequencyDivisionMultipleAccess(FDMA)
2.TimeDivisionMultipleAccess(TDMA)
3.CodeDivisionMultipleAccess(CDMA)
1.FrequencyDivisionMultipleAccess
Infrequency-divisionmultipleaccess(FDMA),theavailablebandwidthisdividedinto
frequencybands.
Eachstationisallocatedabandtosenditsdata.
Inthismethodwhenanyonefrequencyleveliskeptidleandanotherisusedfrequently
leadstoinefficiency.
Figure1.27FrequencyDivisionMultipleAccess
f
f5
f4
f3
f2
f1
t
2.TimeDivisionMultipleAccess
Intime-divisionmultipleaccess(TDMA),thestationssharethe
bandwidthofthechannelintime.
Eachstationisallocatedatimeslotduringwhichitcansenddata.
ThemainproblemwithTDMAliesinachievingsynchronizationbetween
thedifferentstations.
Eachstationneedstoknowthebeginningofitsslotandthelocationofits
slot.
Figure1.28TimeDivisionMultipleAccess
f
f1 f2 f3 f4 f5 f6
t
Intime-divisionmultipleaccess(TDMA),thestationssharethe
bandwidthofthechannelintime.
Eachstationisallocatedatimeslotduringwhichitcansenddata.
ThemainproblemwithTDMAliesinachievingsynchronizationbetween
thedifferentstations.
Eachstationneedstoknowthebeginningofitsslotandthelocationofits
slot.
Figure1.28TimeDivisionMultipleAccess
f
f1 f2 f3 f4 f5 f6
t
3. Code Division Multiple Access
CDMAdiffersfromFDMAbecauseonlyonechanneloccupiestheentire
bandwidthofthelink(transmission)atthesametime.
ItalsodiffersfromTDMAbecauseallstationscansenddataatthesametime
withouttimesharing.
CDMAsimplymeanscommunicationwithdifferentcodes.
CDMAisbasedoncodingtheory.Eachstationisassignedacode,whichisa
sequenceofnumberscalledchips.
Chipswillbeaddedwiththeoriginaldataanditcanbetransmittedthrough
samemedium.
Figure 1.29 Code Division Multiple Access
Code c
C5
c4
c3
c2
c1
Frequency f
CDMAdiffersfromFDMAbecauseonlyonechanneloccupiestheentire
bandwidthofthelink(transmission)atthesametime.
ItalsodiffersfromTDMAbecauseallstationscansenddataatthesametime
withouttimesharing.
CDMAsimplymeanscommunicationwithdifferentcodes.
CDMAisbasedoncodingtheory.Eachstationisassignedacode,whichisa
sequenceofnumberscalledchips.
Chipswillbeaddedwiththeoriginaldataanditcanbetransmittedthrough
samemedium.
Figure 1.29 Code Division Multiple Access
Code c
C5
c4
c3
c2
c1
Frequency f
Issues in the data link layer
Framing
1.Itseparatesamessagefromonesourcetoadestination.Totransmitframesover
thenodeitisnecessarytomentionstartandendofeachframe.
2.Framescanbeoffixedorvariablesize.
3.Infixed-sizeframing,thereisnoneedfordefiningtheboundariesofframes;in
variable-sizeframing,weneedadelimiter(flag)todefinetheboundaryoftwo
frames.
4.Variable-sizeframingusestwocategoriesofprotocols:
̶Byte-oriented(orcharacteroriented)
̶Bit-oriented
5.Inabyte-orientedprotocol,thedatasectionofaframeisasequenceofbytes;in
abit-orientedprotocol,thedatasectionofaframeisasequenceofbits.
6.Therearethreetechniquestosolvethisframe
1.Byte-OrientedProtocols(BISYNC,PPP,DDCMP)
2.Bit-OrientedProtocols(HDLC)
3.Clock-BasedFraming(SONET)
Framing
1.Itseparatesamessagefromonesourcetoadestination.Totransmitframesover
thenodeitisnecessarytomentionstartandendofeachframe.
2.Framescanbeoffixedorvariablesize.
3.Infixed-sizeframing,thereisnoneedfordefiningtheboundariesofframes;in
variable-sizeframing,weneedadelimiter(flag)todefinetheboundaryoftwo
frames.
4.Variable-sizeframingusestwocategoriesofprotocols:
̶Byte-oriented(orcharacteroriented)
̶Bit-oriented
5.Inabyte-orientedprotocol,thedatasectionofaframeisasequenceofbytes;in
abit-orientedprotocol,thedatasectionofaframeisasequenceofbits.
6.Therearethreetechniquestosolvethisframe
1.Byte-OrientedProtocols(BISYNC,PPP,DDCMP)
2.Bit-OrientedProtocols(HDLC)
3.Clock-BasedFraming(SONET)
1. Byte Oriented protocols
1.Thisistovieweachframeasacollectionofbytes(characters)
ratherthanacollectionofbits.
2.Suchabyte-orientedapproachisexemplifiedbytheBISYNC
(BinarySynchronousCommunication)protocolandtheDDCMP
(DigitalDataCommunicationMessageProtocol).
3.Thesetwoprotocolsareexamplesoftwodifferentframing
techniques,thesentinelapproachandthebyte-counting
approach.
SentinelApproach
TheBISYNCprotocolillustratesthesentinelapproachtoframing;its
frameformatisshowninfig.1.23.
Figure1.30BISYNCFrameformat
1.Thisistovieweachframeasacollectionofbytes(characters)
ratherthanacollectionofbits.
2.Suchabyte-orientedapproachisexemplifiedbytheBISYNC
(BinarySynchronousCommunication)protocolandtheDDCMP
(DigitalDataCommunicationMessageProtocol).
3.Thesetwoprotocolsareexamplesoftwodifferentframing
techniques,thesentinelapproachandthebyte-counting
approach.
SentinelApproach
TheBISYNCprotocolillustratesthesentinelapproachtoframing;its
frameformatisshowninfig.1.23.
Figure1.30BISYNCFrameformat
1.ThebeginningofaframeisdenotedbysendingaspecialSYN
(synchronization)character.
2.Thedataportionoftheframeisthencontainedbetweenspecial
sentinelcharacters:STX(startoftext)andETX(endoftext).
3.TheSOH(startofheader)fieldservesmuchthesamepurposeas
theSTXfield.
4.TheframeformatalsoincludesafieldlabeledCRC(cyclic
redundancycheck)thatisusedtodetecttransmissionerrors.
5.TheproblemwiththesentinelapproachisthattheETXcharacter
mightappearinthedataportionoftheframe.
6.BISYNCovercomesthisproblemby“escaping”theETXcharacter
byprecedingitwithaDLE(data-link-escape)characterwhenever
itappearsinthebodyofaframe;theDLEcharacterisalso
escapedintheframebody.Thisapproachiscalledcharacter
stuffing.
(synchronization)character.
2.Thedataportionoftheframeisthencontainedbetweenspecial
sentinelcharacters:STX(startoftext)andETX(endoftext).
3.TheSOH(startofheader)fieldservesmuchthesamepurposeas
theSTXfield.
4.TheframeformatalsoincludesafieldlabeledCRC(cyclic
redundancycheck)thatisusedtodetecttransmissionerrors.
5.TheproblemwiththesentinelapproachisthattheETXcharacter
mightappearinthedataportionoftheframe.
6.BISYNCovercomesthisproblemby“escaping”theETXcharacter
byprecedingitwithaDLE(data-link-escape)characterwhenever
itappearsinthebodyofaframe;theDLEcharacterisalso
escapedintheframebody.Thisapproachiscalledcharacter
stuffing.
Point-to-PointProtocol(PPP)
ThemorerecentPoint-to-PointProtocol(PPP),whichiscommonlyrun
overdialupmodemlinks.TheformatofPPPframeis
Figure1.31PPPFrameFormat
1.TheFlagfieldhas01111110asstartingsequence.
2.TheAddressandControlfieldsusuallycontaindefaultvalues.
3.TheProtocolfieldisusedfordemultiplexing.
4.Theframepayloadsizecanhenegotiated,butitis1500bytesbydefault.
5.ThePPPframeformatisunusualinthatseveralofthefieldsizesare
negotiatedratherthanfixed.
6.NegotiationisconductedbyaprotocolcalledLCP(LinkControlProtocol).
7.LCPsendscontrolmessagesencapsulatedinPPPframes—suchmessages
aredenotedbyanLCPidentifierinthePPPProtocol.
ThemorerecentPoint-to-PointProtocol(PPP),whichiscommonlyrun
overdialupmodemlinks.TheformatofPPPframeis
Figure1.31PPPFrameFormat
1.TheFlagfieldhas01111110asstartingsequence.
2.TheAddressandControlfieldsusuallycontaindefaultvalues.
3.TheProtocolfieldisusedfordemultiplexing.
4.Theframepayloadsizecanhenegotiated,butitis1500bytesbydefault.
5.ThePPPframeformatisunusualinthatseveralofthefieldsizesare
negotiatedratherthanfixed.
6.NegotiationisconductedbyaprotocolcalledLCP(LinkControlProtocol).
7.LCPsendscontrolmessagesencapsulatedinPPPframes—suchmessages
aredenotedbyanLCPidentifierinthePPPProtocol.
Byte-CountingApproach
Figure1.32DDCMPframeformat
1.Thenumberofbytescontainedinaframecanbeincludedasafieldin
theframeheader.DDCMPprotocolusesthisapproachasshownin
fig.1.32.
2.COUNTFieldspecifieshowmanybytesarecontainedintheframe’s
body.
3.AtransmissionerrorcouldcorrupttheCOUNTfield,inwhichcasethe
endoftheframewouldnotbecorrectlydetected.
4.HencethereceiverwillaccumulateasmanybytesasthebadCOUNT
fieldindicatesandthenusetheerrordetectionfieldtodeterminethat
theframeisbad.Thisissometimescalledaframingerror.
5.ThereceiverwillthenwaituntilitseesthenextSYNcharactertostart
collectingthebytesthatmakeupthenextframe.
Figure1.32DDCMPframeformat
1.Thenumberofbytescontainedinaframecanbeincludedasafieldin
theframeheader.DDCMPprotocolusesthisapproachasshownin
fig.1.32.
2.COUNTFieldspecifieshowmanybytesarecontainedintheframe’s
body.
3.AtransmissionerrorcouldcorrupttheCOUNTfield,inwhichcasethe
endoftheframewouldnotbecorrectlydetected.
4.HencethereceiverwillaccumulateasmanybytesasthebadCOUNT
fieldindicatesandthenusetheerrordetectionfieldtodeterminethat
theframeisbad.Thisissometimescalledaframingerror.
5.ThereceiverwillthenwaituntilitseesthenextSYNcharactertostart
collectingthebytesthatmakeupthenextframe.
2. Bit-Oriented Protocols (HDLC)
1.Inthis,framesareviewedasacollectionofbits.
2.TheSynchronousDataLinkControl(SDLC)protocolisanexampleofabit-orientedprotocol;SDLCwaslater
standardizedbytheISOastheHigh-LevelDataLinkControl(HDLC)protocol.
Figure1.33HDLCFrameFormat
3.HDLCdenotesboththebeginningandtheendofaframewiththedistinguishedbitsequence01111110.
4.Thissequencemightappearanywhereinthebodyoftheframe,itcanbeavoidedbybitstuffing.
5.Onthesendingside,anytimefiveconsecutive1’shavebeentransmittedfromthebodyofthemessage
(i.e.,excludingwhenthesenderistryingtotransmitthedistinguished01111110sequence),thesender
insertsa0beforetransmittingthenextbit.
6.Bylookingatthenextbit,thereceivercandistinguishbetweenthesetwocases:
Ifitseesa0(i.e.,thelasteightbitsithaslookedatare01111110),thenitistheend-of-frame
marker.
Ifitseesa1(i.e.,thelasteightbitsithaslookedatare01111111),thentheremusthavebeenan
errorandthewholeframeisdiscarded.
1.Inthis,framesareviewedasacollectionofbits.
2.TheSynchronousDataLinkControl(SDLC)protocolisanexampleofabit-orientedprotocol;SDLCwaslater
standardizedbytheISOastheHigh-LevelDataLinkControl(HDLC)protocol.
Figure1.33HDLCFrameFormat
3.HDLCdenotesboththebeginningandtheendofaframewiththedistinguishedbitsequence01111110.
4.Thissequencemightappearanywhereinthebodyoftheframe,itcanbeavoidedbybitstuffing.
5.Onthesendingside,anytimefiveconsecutive1’shavebeentransmittedfromthebodyofthemessage
(i.e.,excludingwhenthesenderistryingtotransmitthedistinguished01111110sequence),thesender
insertsa0beforetransmittingthenextbit.
6.Bylookingatthenextbit,thereceivercandistinguishbetweenthesetwocases:
Ifitseesa0(i.e.,thelasteightbitsithaslookedatare01111110),thenitistheend-of-frame
marker.
Ifitseesa1(i.e.,thelasteightbitsithaslookedatare01111111),thentheremusthavebeenan
errorandthewholeframeisdiscarded.
Byte stuffing and unstuffing
Bytestuffingistheprocessofadding1extrabytewheneverthereisaflagor
escapecharacterinthetext.
Bytestuffingistheprocessofadding1extrabytewheneverthereisaflagor
escapecharacterinthetext.
A frame in a bit-oriented protocol
Bitstuffingistheprocessofaddingoneextra0wheneverfiveconsecutive1s
followa0inthedata,sothatthereceiverdoesnotmistakethepattern
0111110foraflag.
Bitstuffingistheprocessofaddingoneextra0wheneverfiveconsecutive1s
followa0inthedata,sothatthereceiverdoesnotmistakethepattern
0111110foraflag.
3. Clock-Based Framing (SONET)
1.SynchronousOpticalNetworkStandardisusedforlongdistancetransmissionofdata
overopticalnetwork.
2.Itsupportsmultiplexingofseverallowspeedlinksintoonehighspeedlinks.
3.AnSTS-1frameisusedinthismethod.
Figure1.34ASONETSTS-1frame
4.Itisarrangedasninerowsof90byteseach,andthefirst3bytesofeachroware
overhead,withtherestbeingavailablefordata.
5.Thefirst2bytesoftheframecontainaspecialbitpattern,anditisthesebytesthat
enablethereceivertodeterminewheretheframestarts.
6.Thereceiverlooksforthespecialbitpatternconsistently,onceinevery810bytes,since
eachframeis9x90=810byteslong.
1.SynchronousOpticalNetworkStandardisusedforlongdistancetransmissionofdata
overopticalnetwork.
2.Itsupportsmultiplexingofseverallowspeedlinksintoonehighspeedlinks.
3.AnSTS-1frameisusedinthismethod.
Figure1.34ASONETSTS-1frame
4.Itisarrangedasninerowsof90byteseach,andthefirst3bytesofeachroware
overhead,withtherestbeingavailablefordata.
5.Thefirst2bytesoftheframecontainaspecialbitpattern,anditisthesebytesthat
enablethereceivertodeterminewheretheframestarts.
6.Thereceiverlooksforthespecialbitpatternconsistently,onceinevery810bytes,since
eachframeis9x90=810byteslong.
Figure1.35ThreeSTS-1framesmultiplexedontooneSTS-3cframe.
1.TheSTS-NframecanhethoughtofasconsistingofNSTS-1frames,wherethe
bytesfromtheseframesareinterleaved;thatis,abytefromthefirstframeis
transmitted,thenabytefromthesecondframeistransmitted,andsoon.
2.PayloadfromtheseSTS-1framescanhelinkedtogethertoformalargerSTS-N
payload,suchalinkisdenotedSTS-Nc.Oneofthebitsinoverheadisusedforthis
purpose.
1.TheSTS-NframecanhethoughtofasconsistingofNSTS-1frames,wherethe
bytesfromtheseframesareinterleaved;thatis,abytefromthefirstframeis
transmitted,thenabytefromthesecondframeistransmitted,andsoon.
2.PayloadfromtheseSTS-1framescanhelinkedtogethertoformalargerSTS-N
payload,suchalinkisdenotedSTS-Nc.Oneofthebitsinoverheadisusedforthis
purpose.
Error Detection and Correction
1.Networks must be able to transfer data from one device to another with complete
accuracy.
2.Data can be corrupted during transmission.
3.For reliable communication, errors must be detected and corrected.
4.Error detection and correctionare implemented either at the data link layeror
the transport layerof the OSImodel.
5.The number of bits position in which code words differ is called the Hamming
distance.
Typesoferrors
Theseerrorscanbedividedintotwotypes:
–Single-biterror
–Bursterror
1.Networks must be able to transfer data from one device to another with complete
accuracy.
2.Data can be corrupted during transmission.
3.For reliable communication, errors must be detected and corrected.
4.Error detection and correctionare implemented either at the data link layeror
the transport layerof the OSImodel.
5.The number of bits position in which code words differ is called the Hamming
distance.
Typesoferrors
Theseerrorscanbedividedintotwotypes:
–Single-biterror
–Bursterror
Single-bitError
Thetermsingle-biterrormeansthatonlyone
bitofgivendataunit(suchasabyte,
character,ordataunit)ischangedfrom1to0
orfrom0to1asshowninFig.1.36
Figure1.36Singlebiterror
Thetermsingle-biterrormeansthatonlyone
bitofgivendataunit(suchasabyte,
character,ordataunit)ischangedfrom1to0
orfrom0to1asshowninFig.1.36
Figure1.36Singlebiterror
2.BurstError
1.Thetermbursterrormeansthattwoormorebitsinthedataunithavechangedfrom0to1or
vice-versa.
2.Bursterrorsaremostlylikelytohappeninserialtransmission.
3.Thelengthofthebursterrorismeasuredfromthefirstcorruptedbittothelastcorruptedbit.
4.Somebitsinbetweenmaynotbecorrupted.
Figure1.37BurstError
Figure1.38BurstError
1.Thetermbursterrormeansthattwoormorebitsinthedataunithavechangedfrom0to1or
vice-versa.
2.Bursterrorsaremostlylikelytohappeninserialtransmission.
3.Thelengthofthebursterrorismeasuredfromthefirstcorruptedbittothelastcorruptedbit.
4.Somebitsinbetweenmaynotbecorrupted.
Figure1.37BurstError
Figure1.38BurstError
Error Detecting Codes
•Errordetectionmeanstodecidewhetherthereceiveddataiscorrector
notwithouthavingacopyoftheoriginalmessage.
•Errordetectionusestheconceptofredundancy,whichmeansadding
extrabitsfordetectingerrorsatthedestination.
conceptofredundancy
•Errordetectionmeanstodecidewhetherthereceiveddataiscorrector
notwithouthavingacopyoftheoriginalmessage.
•Errordetectionusestheconceptofredundancy,whichmeansadding
extrabitsfordetectingerrorsatthedestination.
conceptofredundancy
Populartechniquesare:
–SimpleParitycheckorVerticalRedundancyCheck(VRC)
–Two-dimensionalParitycheck(LRC)
–Checksum
–Cyclicredundancycheck(CRC)
–SimpleParitycheckorVerticalRedundancyCheck(VRC)
–Two-dimensionalParitycheck(LRC)
–Checksum
–Cyclicredundancycheck(CRC)
SimpleParityCheckingorOne-dimensionParityCheck(VRC)
1.Themostcommonandleastexpensivemechanismforerror-detectionisthesimple
paritycheck.
2.Inthistechnique,aredundantbitcalledparitybit,isappendedtoeverydataunitso
thatthenumberof1sintheunit(includingtheparitybecomeseven).
3.BlocksofdatafromthesourcearesubjectedtoacheckbitorParitybitgeneratorform,
whereaparityof1isaddedtotheblockifitcontainsanoddnumberof1’s(ONbits)
and0isaddedifitcontainsanevennumberof1’s.
4.Atthereceivingendtheparitybitiscomputedfromthereceiveddatabitsand
comparedwiththereceivedparitybit,asshowninFig.1.38.
5.Thisschememakesthetotalnumberof1’seven,thatiswhyitiscalledevenparity
checking.
Figure1.38Even-paritycheckingscheme
1.Themostcommonandleastexpensivemechanismforerror-detectionisthesimple
paritycheck.
2.Inthistechnique,aredundantbitcalledparitybit,isappendedtoeverydataunitso
thatthenumberof1sintheunit(includingtheparitybecomeseven).
3.BlocksofdatafromthesourcearesubjectedtoacheckbitorParitybitgeneratorform,
whereaparityof1isaddedtotheblockifitcontainsanoddnumberof1’s(ONbits)
and0isaddedifitcontainsanevennumberof1’s.
4.Atthereceivingendtheparitybitiscomputedfromthereceiveddatabitsand
comparedwiththereceivedparitybit,asshowninFig.1.38.
5.Thisschememakesthetotalnumberof1’seven,thatiswhyitiscalledevenparity
checking.
Figure1.38Even-paritycheckingscheme
Winter 2005
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Computer Interfacing and Protocols
111
Example for Vertical Redundancy Check (VRC)
–Append a single bit at the end of data block such that the number of
ones is even
Even Parity (odd parity is similar)
0110011 01100110
0110001 01100011
–VRC is also known as Parity Check
–It can detect single bit error
–It can detect burst errors only if the total number of errors is odd.
ECE
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Computer Interfacing and Protocols
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Example for Vertical Redundancy Check (VRC)
–Append a single bit at the end of data block such that the number of
ones is even
Even Parity (odd parity is similar)
0110011 01100110
0110001 01100011
–VRC is also known as Parity Check
–It can detect single bit error
–It can detect burst errors only if the total number of errors is odd.
Decimalvalue DataBlock Paritybit Codeword
0 0000 0 00000
1 0001 1 00011
2 0010 1 00101
3 0011 0 00110
4 0100 1 01001
5 0101 0 01010
6 0110 0 01100
7 0111 1 01111
8 1000 1 10001
9 1001 0 10010
10 1010 0 10100
11 1011 1 10111
12 1100 0 11000
13 1101 1 11011
14 1110 1 11101
15 1111 0 11110
Table 1.1 Possible 4-bit data words and corresponding code words
0 0000 0 00000
1 0001 1 00011
2 0010 1 00101
3 0011 0 00110
4 0100 1 01001
5 0101 0 01010
6 0110 0 01100
7 0111 1 01111
8 1000 1 10001
9 1001 0 10010
10 1010 0 10100
11 1011 1 10111
12 1100 0 11000
13 1101 1 11011
14 1110 1 11101
15 1111 0 11110
Table 1.1 Possible 4-bit data words and corresponding code words
Two-dimensionParityCheck(LRC)
1.Performancecanbeimprovedbyusingtwo-dimensionalparitycheck,whichorganizestheblockofbits
intheformofatable.
2.Paritycheckbitsarecalculatedforeachrow,whichisequivalenttoasimpleparitycheckbit.
3.Paritycheckbitsarealsocalculatedforallcolumnsthenbotharesentalongwiththedata.
4.Atthereceivingendthesearecomparedwiththeparitybitscalculatedonthereceiveddata.
5.Organizedataintoatableandcreateaparityforeachcolumn.
Figure1.39Two-dimensionParityChecking
1.Performancecanbeimprovedbyusingtwo-dimensionalparitycheck,whichorganizestheblockofbits
intheformofatable.
2.Paritycheckbitsarecalculatedforeachrow,whichisequivalenttoasimpleparitycheckbit.
3.Paritycheckbitsarealsocalculatedforallcolumnsthenbotharesentalongwiththedata.
4.Atthereceivingendthesearecomparedwiththeparitybitscalculatedonthereceiveddata.
5.Organizedataintoatableandcreateaparityforeachcolumn.
Figure1.39Two-dimensionParityChecking
LRCincreasesthelikelihoodofdetecting
bursterrors.
Iftwobitsinonedataunitsaredamagedand
twobitsinexactlythesamepositionsin
anotherdataunitarealsodamaged,theLRC
checkerwillnotdetectanerror.
bursterrors.
Iftwobitsinonedataunitsaredamagedand
twobitsinexactlythesamepositionsin
anotherdataunitarealsodamaged,theLRC
checkerwillnotdetectanerror.
Checksum
1.Inchecksumerrordetectionscheme,thedataisdividedintoksegmentseachofmbits.
2.Inthesender’sendthesegmentsareaddedusing1’scomplementarithmetictogetthesum.
3.Thesumiscomplementedtogetthechecksum.Thechecksumsegmentissentalongwiththedata
segments(10110011,10101011,01011010,11010101)asshowninFig.1.40(a).
4.Atthereceiver’send,allreceivedsegmentsareaddedusing1’scomplementarithmetictogetthesum.
5.Thesumiscomplemented.Iftheresultiszero,thereceiveddataisaccepted;otherwisediscarded,as
showninFig.1.40(b).
Figure1.40(a)Sender’sendforthecalculationofthechecksum,(b)Receivingendforcheckingthechecksum
1.Inchecksumerrordetectionscheme,thedataisdividedintoksegmentseachofmbits.
2.Inthesender’sendthesegmentsareaddedusing1’scomplementarithmetictogetthesum.
3.Thesumiscomplementedtogetthechecksum.Thechecksumsegmentissentalongwiththedata
segments(10110011,10101011,01011010,11010101)asshowninFig.1.40(a).
4.Atthereceiver’send,allreceivedsegmentsareaddedusing1’scomplementarithmetictogetthesum.
5.Thesumiscomplemented.Iftheresultiszero,thereceiveddataisaccepted;otherwisediscarded,as
showninFig.1.40(b).
Figure1.40(a)Sender’sendforthecalculationofthechecksum,(b)Receivingendforcheckingthechecksum
At the sender
The unit is divided into ksections, each of n
bits.
All sections are added together using one’s
complement to get the sum.
The sum is complemented and becomes the
checksum.
The checksum is sent with the data
The unit is divided into ksections, each of n
bits.
All sections are added together using one’s
complement to get the sum.
The sum is complemented and becomes the
checksum.
The checksum is sent with the data
At the receiver
The unit is divided into ksections, each of n
bits.
All sections are added together using one’s
complement to get the sum.
The sum is complemented.
If the result is zero, the data are accepted:
otherwise, they are rejected.
The unit is divided into ksections, each of n
bits.
All sections are added together using one’s
complement to get the sum.
The sum is complemented.
If the result is zero, the data are accepted:
otherwise, they are rejected.
CyclicRedundancyChecks(CRC)
1.ThisCyclicRedundancyCheckisthemostpowerfulandeasytoimplement
technique.CRCisbasedonbinarydivision.
2.InCRC,asequenceofredundantbits,calledcyclicredundancycheckbits,
areappendedtotheendofdataunitsothattheresultingdataunitbecomes
exactlydivisiblebyasecond,predeterminedbinarynumber.
3.Atthedestination,theincomingdataunitisdividedbythesamenumber.Ifthere
isnoremainder,thedataunitisassumedtobecorrectandisthereforeaccepted.
4.Aremainderindicatesthatthedataunithasbeendamagedintransitand
thereforemustberejected.
Figure1.41BasicschemeforCyclicRedundancyChecking
1.ThisCyclicRedundancyCheckisthemostpowerfulandeasytoimplement
technique.CRCisbasedonbinarydivision.
2.InCRC,asequenceofredundantbits,calledcyclicredundancycheckbits,
areappendedtotheendofdataunitsothattheresultingdataunitbecomes
exactlydivisiblebyasecond,predeterminedbinarynumber.
3.Atthedestination,theincomingdataunitisdividedbythesamenumber.Ifthere
isnoremainder,thedataunitisassumedtobecorrectandisthereforeaccepted.
4.Aremainderindicatesthatthedataunithasbeendamagedintransitand
thereforemustberejected.
Figure1.41BasicschemeforCyclicRedundancyChecking
1.Considerthecasewherek=1101asinfig.1.42.Hencewehavetodivide1101000(i.e.k
appendedby3zeros)by1011,whichproducestheremainderr=001,sothatthebitframe
(k+r)=1101001isactuallybeingtransmittedthroughthecommunicationchannel.
2.Atthereceivingend,ifthereceivednumber,i.e.1101001isdividedbythesamegenerator
polynomial1011togettheremainderas000,itcanbeassumedthatthedataisfreeof
errors.
Figure1.42CyclicRedundancyChecks(CRC)
1.Apolynomialisselectedtohaveatleastthefollowingproperties:
ItshouldnotbedivisiblebyX.
Itshouldnotbedivisibleby(X+1)
2.Thefirstconditionguaranteesthatallbursterrorsofalengthequaltothedegreeof
polynomialaredetected.
3.Thesecondconditionguaranteesthatallbursterrorsaffectinganoddnumberofbitsare
detected.
appendedby3zeros)by1011,whichproducestheremainderr=001,sothatthebitframe
(k+r)=1101001isactuallybeingtransmittedthroughthecommunicationchannel.
2.Atthereceivingend,ifthereceivednumber,i.e.1101001isdividedbythesamegenerator
polynomial1011togettheremainderas000,itcanbeassumedthatthedataisfreeof
errors.
Figure1.42CyclicRedundancyChecks(CRC)
1.Apolynomialisselectedtohaveatleastthefollowingproperties:
ItshouldnotbedivisiblebyX.
Itshouldnotbedivisibleby(X+1)
2.Thefirstconditionguaranteesthatallbursterrorsofalengthequaltothedegreeof
polynomialaredetected.
3.Thesecondconditionguaranteesthatallbursterrorsaffectinganoddnumberofbitsare
detected.
Division in CRC encoder
Division in the CRC decoder for two cases
Error Correcting Codes
1.ErrorCorrectioncanbehandledintwoways.
2.Oneiswhenanerrorisdiscovered;thereceivercanhavethesenderretransmittheentiredata
unit.Thisisknownasbackwarderrorcorrection.
3.Intheother,receivercanuseanerror-correctingcode,whichautomaticallycorrectscertain
errors.Thisisknownasforwarderrorcorrection.
Single-biterrorcorrection
1.Asingleadditionalbitcandetecterror,butit’snotsufficientenoughtocorrectthaterrortoo.
2.Forcorrectinganerroronehastoknowtheexactpositionoferror,i.e.exactlywhichbitisin
error.
3.Forexample,tocorrectasingle-biterrorinanASCIIcharacter,theerrorcorrectionmust
determinewhichoneofthesevenbitsisinerror.
4.Tothis,wehavetoaddsomeadditionalredundantbits.
5.Tocalculatethenumbersofredundantbits(r)requiredtocorrectddatabits,letusfindoutthe
relationshipbetweenthetwo.
1.ErrorCorrectioncanbehandledintwoways.
2.Oneiswhenanerrorisdiscovered;thereceivercanhavethesenderretransmittheentiredata
unit.Thisisknownasbackwarderrorcorrection.
3.Intheother,receivercanuseanerror-correctingcode,whichautomaticallycorrectscertain
errors.Thisisknownasforwarderrorcorrection.
Single-biterrorcorrection
1.Asingleadditionalbitcandetecterror,butit’snotsufficientenoughtocorrectthaterrortoo.
2.Forcorrectinganerroronehastoknowtheexactpositionoferror,i.e.exactlywhichbitisin
error.
3.Forexample,tocorrectasingle-biterrorinanASCIIcharacter,theerrorcorrectionmust
determinewhichoneofthesevenbitsisinerror.
4.Tothis,wehavetoaddsomeadditionalredundantbits.
5.Tocalculatethenumbersofredundantbits(r)requiredtocorrectddatabits,letusfindoutthe
relationshipbetweenthetwo.
1.Sowehave(d+r)asthetotalnumberofbits,whicharetobetransmitted;thenr
mustbeabletoindicateatleastd+r+1differentvalue.
2.Onevaluemeansnoerror,andremainingd+rvaluesindicateerrorlocationof
errorineachofd+rlocations.So,d+r+1statemustbedistinguishablebyrbits,
andrbitscanindicate2
r
states.Hence,2
r
mustbegreaterthand+r+1.i.e
2
r
>=d+r+1
1.Thevalueofrmustbedeterminedbyputtinginthevalueofdintherelation.
2.Forexample,ifdis7,thenthesmallestvalueofrthatsatisfiestheaboverelation
is4.Sothetotalbits,whicharetobetransmittedis11bits(d+r=7+4=11).
3.AtechniquedevelopedbyR.W.Hammingprovidesapracticalsolution.The
solutionorcodingschemehedevelopediscommonlyknownasHammingCode.
4.Hammingcodecanbeappliedtodataunitsofanylengthandusesthe
relationshipbetweenthedatabitsandredundantbits.
mustbeabletoindicateatleastd+r+1differentvalue.
2.Onevaluemeansnoerror,andremainingd+rvaluesindicateerrorlocationof
errorineachofd+rlocations.So,d+r+1statemustbedistinguishablebyrbits,
andrbitscanindicate2
r
states.Hence,2
r
mustbegreaterthand+r+1.i.e
2
r
>=d+r+1
1.Thevalueofrmustbedeterminedbyputtinginthevalueofdintherelation.
2.Forexample,ifdis7,thenthesmallestvalueofrthatsatisfiestheaboverelation
is4.Sothetotalbits,whicharetobetransmittedis11bits(d+r=7+4=11).
3.AtechniquedevelopedbyR.W.Hammingprovidesapracticalsolution.The
solutionorcodingschemehedevelopediscommonlyknownasHammingCode.
4.Hammingcodecanbeappliedtodataunitsofanylengthandusesthe
relationshipbetweenthedatabitsandredundantbits.
Number of Data Bits
(d)
Number of
Redundancy Bits
(r)
Total Bits
(d+r)
1
2
3
4
5
6
7
2
3
3
3
4
4
4
3
5
6
7
9
10
11
(d)
Number of
Redundancy Bits
(r)
Total Bits
(d+r)
1
2
3
4
5
6
7
2
3
3
3
4
4
4
3
5
6
7
9
10
11
BasicapproachforerrordetectionbyusingHammingcodeisasfollows:
1.Toeachgroupofminformationbitskparitybitsareaddedtoform(m+k)bitcodeasshown
inFig.1.43
2.Locationofeachofthe(m+k)digitsisassignedadecimalvalue.
3.Thekparitybitsareplacedinpositions1,2…2
k-1
positions.–Kparitychecksareperformedon
selecteddigitsofeachcodeword.
4.Atthereceivingendtheparitybitsarerecalculated.Thedecimalvalueofthekparitybits
providesthebit-positioninerror,ifany
Figure 1.44 Use of Hamming code for error correction for a 4-bit data
1.Toeachgroupofminformationbitskparitybitsareaddedtoform(m+k)bitcodeasshown
inFig.1.43
2.Locationofeachofthe(m+k)digitsisassignedadecimalvalue.
3.Thekparitybitsareplacedinpositions1,2…2
k-1
positions.–Kparitychecksareperformedon
selecteddigitsofeachcodeword.
4.Atthereceivingendtheparitybitsarerecalculated.Thedecimalvalueofthekparitybits
providesthebit-positioninerror,ifany
Figure 1.44 Use of Hamming code for error correction for a 4-bit data
1.Figure1.44showshowhammingcodeisusedforcorrectionfor4-bitnumbers
(d4d3d2d1)withthehelpofthreeredundantbits(r3r2r1).
2.Fortheexampledata1010,firstr1(0)iscalculatedconsideringtheparityofthe
bitpositions,1,3,5and7.
3.Thentheparitybitsr2iscalculatedconsideringbitpositions2,3,6and7.
4.Finally,theparitybitsr4iscalculatedconsideringbitpositions4,5,6and7as
shown.
5.Ifanycorruptionoccursinanyofthetransmittedcode1010010,thebitposition
inerrorcanbefoundoutbycalculatingr3r2r1atthereceivingend.
6.Forexample,ifthereceivedcodewordis1110010,therecalculatedvalueof
r3r2r1is110,whichindicatesthatbitpositioninerroris6,thedecimalvalueof
110.
(d4d3d2d1)withthehelpofthreeredundantbits(r3r2r1).
2.Fortheexampledata1010,firstr1(0)iscalculatedconsideringtheparityofthe
bitpositions,1,3,5and7.
3.Thentheparitybitsr2iscalculatedconsideringbitpositions2,3,6and7.
4.Finally,theparitybitsr4iscalculatedconsideringbitpositions4,5,6and7as
shown.
5.Ifanycorruptionoccursinanyofthetransmittedcode1010010,thebitposition
inerrorcanbefoundoutbycalculatingr3r2r1atthereceivingend.
6.Forexample,ifthereceivedcodewordis1110010,therecalculatedvalueof
r3r2r1is110,whichindicatesthatbitpositioninerroris6,thedecimalvalueof
110.
Single-bit error correction
To correct an error, the receiver reverses the value
of the altered bit. To do so, it must know which
bit is in error.
Number of redundancy bits needed
•Let data bits = m
•Redundancy bits =r
Total message sent =m+r
The value of r must satisfy the following relation:
2
r
≥ m+r+1
To correct an error, the receiver reverses the value
of the altered bit. To do so, it must know which
bit is in error.
Number of redundancy bits needed
•Let data bits = m
•Redundancy bits =r
Total message sent =m+r
The value of r must satisfy the following relation:
2
r
≥ m+r+1
Error Correction
Hamming Code
Hamming Code
Hamming Code
Example of Hamming Code
Single-bit error
Error
Detection
Detection
Link-level Flow Control
1.Flowcontrolensuresthatatransmittingstationdoesnotoverwhelmareceivingstation
withlesserprocessingcapability.
2.ErrorControlinvolvesbotherrordetectionanderrorcorrection.
3.Whenanerrorisdetected,thereceivercanhavethespecifiedframeretransmittedby
thesender.ThisprocessiscommonlyknownasAutomaticRepeatRequest(ARQ).
FlowControl
1.FlowControlisasetofproceduresthattellsthesenderhowmuchdataitcantransmit
beforeitmustwaitforanacknowledgmentfromthereceiver.
2.Theflowofdatashouldnotbeallowedtooverwhelmthereceiver.
3.Therearetwomethodsdevelopedforflowcontrol.Theyare
̶Stop-and-wait(Request/reply)
̶Sliding-window
1.Flowcontrolensuresthatatransmittingstationdoesnotoverwhelmareceivingstation
withlesserprocessingcapability.
2.ErrorControlinvolvesbotherrordetectionanderrorcorrection.
3.Whenanerrorisdetected,thereceivercanhavethespecifiedframeretransmittedby
thesender.ThisprocessiscommonlyknownasAutomaticRepeatRequest(ARQ).
FlowControl
1.FlowControlisasetofproceduresthattellsthesenderhowmuchdataitcantransmit
beforeitmustwaitforanacknowledgmentfromthereceiver.
2.Theflowofdatashouldnotbeallowedtooverwhelmthereceiver.
3.Therearetwomethodsdevelopedforflowcontrol.Theyare
̶Stop-and-wait(Request/reply)
̶Sliding-window
1.Stop-and-wait
1.Thisisthesimplestformofflowcontrolwhereasendertransmitsadataframe.
2.Afterreceivingtheframe,thereceiverindicatesitswillingnesstoacceptanotherframebysendingback
anACKframeacknowledgingtheframejustreceived.
3.ThesendermustwaituntilitreceivestheACKframebeforesendingthenextdataframe.Thisis
sometimesreferredtoasping-pongbehavior(Piggybacking).
4.Request/replyissimpletounderstandandeasytoimplement,butnotveryefficient.
5.Figure1.45illustratestheoperationofthestop-and-waitprotocol.Theprotocolreliesontwo-way
transmissiontoallowthereceiverattheremotenodetoreturnframesacknowledgingthesuccessful
transmission.
6.Asmallprocessingdelaymaybeintroduced.
7.MajordrawbackofStop-and-WaitFlowControlisthatonlyoneframecanbeintransmissionatatime,
thisleadstoinefficiencyifpropagationdelayismuchlongerthanthetransmissiondelay.
Figure1.45Stop-andWaitprotocol
1.Thisisthesimplestformofflowcontrolwhereasendertransmitsadataframe.
2.Afterreceivingtheframe,thereceiverindicatesitswillingnesstoacceptanotherframebysendingback
anACKframeacknowledgingtheframejustreceived.
3.ThesendermustwaituntilitreceivestheACKframebeforesendingthenextdataframe.Thisis
sometimesreferredtoasping-pongbehavior(Piggybacking).
4.Request/replyissimpletounderstandandeasytoimplement,butnotveryefficient.
5.Figure1.45illustratestheoperationofthestop-and-waitprotocol.Theprotocolreliesontwo-way
transmissiontoallowthereceiverattheremotenodetoreturnframesacknowledgingthesuccessful
transmission.
6.Asmallprocessingdelaymaybeintroduced.
7.MajordrawbackofStop-and-WaitFlowControlisthatonlyoneframecanbeintransmissionatatime,
thisleadstoinefficiencyifpropagationdelayismuchlongerthanthetransmissiondelay.
Figure1.45Stop-andWaitprotocol
LinkUtilizationinStop-and-Wait
Letusassumethefollowing:
1. Transmissiontime:Thetimeittakesforastationtotransmitaframe(normalizedtoavalueof1).
2. Propagationdelay:Thetimeittakesforabittotravelfromsendertoreceiver(expressedasa).
3. a<1:Theframeissufficientlylongsuchthatthefirstbitsoftheframearriveatthedestinationbeforethesourcehas
completedtransmissionoftheframe.
4. a>1:Sendercompletestransmissionoftheentireframebeforetheleadingbitsoftheframearriveatthereceiver.
5. ThelinkutilizationU=1/(1+2a),a=Propagationtime/transmissiontime
6. Whenthepropagationtimeissmall,asincaseofLANenvironment,thelinkutilizationisgood.But,incaseoflong
propagationdelays,asincaseofsatellitecommunication,theutilizationcanbeverypoor.
7. Toimprovethelinkutilization,wecanusesliding-windowprotocolinsteadofusingstop-and-waitprotocol.
2.SlidingWindow
1.Slidingwindowisanabstractconceptthatdefinestherangeofsequencenumbersthatistheconcernofthesenderand
receiver.(Bothneedtodealwithpartofthepossiblesequencenumbers)
2.Instop-and-waitflowcontrol,onlyoneframeatatimecanbeintransit.
3.Efficiencycanbegreatlyimprovedbyallowingmultipleframestobeintransitatthesametime.
4.Tokeeptrackoftheframes,senderstationsendssequentiallynumberedframes.
Letusassumethefollowing:
1. Transmissiontime:Thetimeittakesforastationtotransmitaframe(normalizedtoavalueof1).
2. Propagationdelay:Thetimeittakesforabittotravelfromsendertoreceiver(expressedasa).
3. a<1:Theframeissufficientlylongsuchthatthefirstbitsoftheframearriveatthedestinationbeforethesourcehas
completedtransmissionoftheframe.
4. a>1:Sendercompletestransmissionoftheentireframebeforetheleadingbitsoftheframearriveatthereceiver.
5. ThelinkutilizationU=1/(1+2a),a=Propagationtime/transmissiontime
6. Whenthepropagationtimeissmall,asincaseofLANenvironment,thelinkutilizationisgood.But,incaseoflong
propagationdelays,asincaseofsatellitecommunication,theutilizationcanbeverypoor.
7. Toimprovethelinkutilization,wecanusesliding-windowprotocolinsteadofusingstop-and-waitprotocol.
2.SlidingWindow
1.Slidingwindowisanabstractconceptthatdefinestherangeofsequencenumbersthatistheconcernofthesenderand
receiver.(Bothneedtodealwithpartofthepossiblesequencenumbers)
2.Instop-and-waitflowcontrol,onlyoneframeatatimecanbeintransit.
3.Efficiencycanbegreatlyimprovedbyallowingmultipleframestobeintransitatthesametime.
4.Tokeeptrackoftheframes,senderstationsendssequentiallynumberedframes.
1. The receiver acknowledges a frame by sending an ACK frame that includes the sequence number of the next frame
expected.
2. This also explicitly announces that it is prepared to receive the next N frames, beginning with the number specified. This
scheme can be used to acknowledge multiple frames.
3. It could receive frames 2, 3, 4 but withhold ACK until frame 4 has arrived. By returning an ACK with sequence number 5, it
acknowledges frames 2, 3, 4 in one go.
4. The receiver needs a buffer of size 1.Sliding window algorithm is a method of flow control for network data transfers.
5. TCP, the Internet's stream transfer protocol, uses a sliding window algorithm.
Figure 1.46 Buffer in sliding window
6. A sliding window algorithm places a buffer between the application program and the network data flow.
7. For TCP, the buffer is typically in the operating system kernel, but this is more of an implementation detail than a hard-
and-fast requirement.
expected.
2. This also explicitly announces that it is prepared to receive the next N frames, beginning with the number specified. This
scheme can be used to acknowledge multiple frames.
3. It could receive frames 2, 3, 4 but withhold ACK until frame 4 has arrived. By returning an ACK with sequence number 5, it
acknowledges frames 2, 3, 4 in one go.
4. The receiver needs a buffer of size 1.Sliding window algorithm is a method of flow control for network data transfers.
5. TCP, the Internet's stream transfer protocol, uses a sliding window algorithm.
Figure 1.46 Buffer in sliding window
6. A sliding window algorithm places a buffer between the application program and the network data flow.
7. For TCP, the buffer is typically in the operating system kernel, but this is more of an implementation detail than a hard-
and-fast requirement.
SenderslidingWindow:Atanyinstant,thesenderispermittedtosendframeswithsequence
numbersinacertainrange(thesendingwindow)asshowninFig.1.47
Figure1.47Sender’swindow
1.Iftheheaderoftheframeallowskbits,thesequencenumbersrangefrom0to2
k
–1.
2.Sendermaintainsalistofsequencenumbersthatitisallowedtosend(senderwindow).
Thesizeofthesender’swindowisatmost2
k
–1.
3.Thesenderisprovidedwithabufferequaltothewindowsize.Receiveralsomaintainsa
windowofsize2
k
–1.
numbersinacertainrange(thesendingwindow)asshowninFig.1.47
Figure1.47Sender’swindow
1.Iftheheaderoftheframeallowskbits,thesequencenumbersrangefrom0to2
k
–1.
2.Sendermaintainsalistofsequencenumbersthatitisallowedtosend(senderwindow).
Thesizeofthesender’swindowisatmost2
k
–1.
3.Thesenderisprovidedwithabufferequaltothewindowsize.Receiveralsomaintainsa
windowofsize2
k
–1.
Figure 1.48 Receiver window
ReceiverslidingWindow:
1.Thereceiveralwaysmaintainswindowofsize1asshowninFig.1.48.Itlooksforaspecificframe(frame4
asshowninthefigure)toarriveinaspecificorder.
2.Ifitreceivesanyotherframe(outoforder),itisdiscardedanditneedstoberesent.However,thereceiver
windowalsoslidesbyoneasthespecificframeisreceivedandacceptedasshowninthefigure.
3.ThereceiveracknowledgesaframebysendinganACKframethatincludesthesequencenumberofthe
nextframeexpected.
4.ThisalsoexplicitlyannouncesthatitispreparedtoreceivethenextNframes,beginningwiththenumber
specified.Thisschemecanbeusedtoacknowledgemultipleframes.
5.Itcouldreceiveframes2,3,4butwithholdACKuntilframe4hasarrived.
6.ByreturninganACKwithsequencenumber5,itacknowledgesframes2,3,4atonetime.Thereceiver
needsabufferofsize1.
ReceiverslidingWindow:
1.Thereceiveralwaysmaintainswindowofsize1asshowninFig.1.48.Itlooksforaspecificframe(frame4
asshowninthefigure)toarriveinaspecificorder.
2.Ifitreceivesanyotherframe(outoforder),itisdiscardedanditneedstoberesent.However,thereceiver
windowalsoslidesbyoneasthespecificframeisreceivedandacceptedasshowninthefigure.
3.ThereceiveracknowledgesaframebysendinganACKframethatincludesthesequencenumberofthe
nextframeexpected.
4.ThisalsoexplicitlyannouncesthatitispreparedtoreceivethenextNframes,beginningwiththenumber
specified.Thisschemecanbeusedtoacknowledgemultipleframes.
5.Itcouldreceiveframes2,3,4butwithholdACKuntilframe4hasarrived.
6.ByreturninganACKwithsequencenumber5,itacknowledgesframes2,3,4atonetime.Thereceiver
needsabufferofsize1.
1.If the window size is larger than the packet size, then multiple packets can be outstanding
in the network, since the sender knows that buffer space is available on the receiver to
hold all of them.
2.Ideally, a steady-state condition can be reached where a series of packets (in the forward
direction) and window announcements (in the reverse direction) are constantly in transit.
3.As each new window announcement is received by the sender, more data packets are
transmitted.
Sliding Window Flow Control
1.Allows transmission of multiple frames
2.Assigns each frame a k-bit sequence number
3.Range of sequence number is [0…2
k
-1], i.e., frames are counted modulo 2k.
4.The link utilization in case of Sliding Window Protocol
U = 1, for N > 2a + 1
N/(1+2a), for N < 2a + 1
Where N = the window size, and a = Propagation time / transmission time
in the network, since the sender knows that buffer space is available on the receiver to
hold all of them.
2.Ideally, a steady-state condition can be reached where a series of packets (in the forward
direction) and window announcements (in the reverse direction) are constantly in transit.
3.As each new window announcement is received by the sender, more data packets are
transmitted.
Sliding Window Flow Control
1.Allows transmission of multiple frames
2.Assigns each frame a k-bit sequence number
3.Range of sequence number is [0…2
k
-1], i.e., frames are counted modulo 2k.
4.The link utilization in case of Sliding Window Protocol
U = 1, for N > 2a + 1
N/(1+2a), for N < 2a + 1
Where N = the window size, and a = Propagation time / transmission time
Error Control Techniques
1.Whenanerrorisdetectedinamessage,thereceiversendsarequesttothe
transmittertoretransmitthemessageorpacket.
2.ThemostpopularretransmissionschemeisknownasAutomatic-Repeat-Request
(ARQ).Suchschemes,wherereceiveraskstransmittertore-transmitifitdetects
anerror,areknownasreverseerrorcorrectiontechniques.
3.ThereexistthreepopularARQtechniques,asshowninFig.1.49.
Figure1.49Errorcontroltechniques
1.Whenanerrorisdetectedinamessage,thereceiversendsarequesttothe
transmittertoretransmitthemessageorpacket.
2.ThemostpopularretransmissionschemeisknownasAutomatic-Repeat-Request
(ARQ).Suchschemes,wherereceiveraskstransmittertore-transmitifitdetects
anerror,areknownasreverseerrorcorrectiontechniques.
3.ThereexistthreepopularARQtechniques,asshowninFig.1.49.
Figure1.49Errorcontroltechniques
1.Stop-and-WaitARQ
1.InStop-and-WaitARQ,whichissimplestamongallprotocols,thesender(say
stationA)transmitsaframeandthenwaitstillitreceivespositive
acknowledgement(ACK)ornegativeacknowledgement(NACK)fromthe
receiver(saystationB).
2.StationBsendsanACKiftheframeisreceivedcorrectly,otherwiseitsends
NACK.
3.StationAsendsanewframeafterreceivingACK;otherwiseitretransmitsthe
oldframe,ifitreceivesaNACK.ThisisillustratedinFig1.50
Figure1.50Stop-And-WaitARQtechnique
1.InStop-and-WaitARQ,whichissimplestamongallprotocols,thesender(say
stationA)transmitsaframeandthenwaitstillitreceivespositive
acknowledgement(ACK)ornegativeacknowledgement(NACK)fromthe
receiver(saystationB).
2.StationBsendsanACKiftheframeisreceivedcorrectly,otherwiseitsends
NACK.
3.StationAsendsanewframeafterreceivingACK;otherwiseitretransmitsthe
oldframe,ifitreceivesaNACK.ThisisillustratedinFig1.50
Figure1.50Stop-And-WaitARQtechnique
1.Totackletheproblemofalostordamagedframe,thesenderisequipped
withatimer.
2.IncaseofalostACK,thesendertransmitstheoldframe.IntheFig.1.51,the
secondPDUofDataislostduringtransmission.
3.Thesenderisunawareofthisloss,butstartsatimeraftersendingeachPDU.
4.NormallyanACKPDUisreceivedbeforethetimerexpires.InthiscasenoACK
isreceived,andthetimercountsdowntozeroandtriggersretransmissionof
thesamePDUbythesender.
5.Thesenderalwaysstartsatimerfollowingtransmission,butinthesecond
transmissionreceivesanACKPDUbeforethetimerexpires,finallyindicating
thatthedatahasnowbeenreceivedbytheremotenode.
Figure1.51Retransmissionduetolostframe
withatimer.
2.IncaseofalostACK,thesendertransmitstheoldframe.IntheFig.1.51,the
secondPDUofDataislostduringtransmission.
3.Thesenderisunawareofthisloss,butstartsatimeraftersendingeachPDU.
4.NormallyanACKPDUisreceivedbeforethetimerexpires.InthiscasenoACK
isreceived,andthetimercountsdowntozeroandtriggersretransmissionof
thesamePDUbythesender.
5.Thesenderalwaysstartsatimerfollowingtransmission,butinthesecond
transmissionreceivesanACKPDUbeforethetimerexpires,finallyindicating
thatthedatahasnowbeenreceivedbytheremotenode.
Figure1.51Retransmissionduetolostframe
Retransmission due to lost frame
1.Thereceivernowcanidentifythatithasreceivedaduplicateframefrom
thelabeloftheframeanditisdiscarded.
2.Totackletheproblemofdamagedframes,thereisaconceptofNACK
frames,i.e.NegativeAcknowledgeframes.
3.ReceivertransmitsaNACKframetothesenderifitfoundsthereceived
frametobecorrupted.
4.WhenaNACKisreceivedbyatransmitterbeforethetime-out,theold
frameissentagainasshowninFig.1.52.
Figure1.52Retransmissionduetodamagedframe
thelabeloftheframeanditisdiscarded.
2.Totackletheproblemofdamagedframes,thereisaconceptofNACK
frames,i.e.NegativeAcknowledgeframes.
3.ReceivertransmitsaNACKframetothesenderifitfoundsthereceived
frametobecorrupted.
4.WhenaNACKisreceivedbyatransmitterbeforethetime-out,theold
frameissentagainasshowninFig.1.52.
Figure1.52Retransmissionduetodamagedframe
Retransmission due to damaged frame
The main advantage of stop-and-wait ARQ is
•simplicity
•minimum buffer size.
The main advantage of stop-and-wait ARQ is
•simplicity
•minimum buffer size.
2.SlidingWindowARQ
2.1Go-back-NARQ
1.ThemostpopularARQprotocolisthego-back-NARQ,wherethesendersendstheframes
continuouslywithoutwaitingforacknowledgement.
2.Asthereceiverreceivestheframes,itkeepsonsendingACKsoraNACK,incaseaframeis
incorrectlyreceived.
3.WhenthesenderreceivesaNACK,itretransmitstheframeinerrorplusallthesucceeding
framesasshowninFig.1.53.Hence,thenameoftheprotocolisgo-back-NARQ.
4.Ifaframeislost,thereceiversendsNAKafterreceivingthenextframeasshownin
Fig.1.54.
5.IncasethereislongdelaybeforesendingtheNAK,thesenderwillresendthelostframe
afteritstimertimesout.
6.IftheACKframesentbythereceiverislost,thesenderresendstheframesafteritstimer
timesoutasshowninFig.1.55.
7.Assumingfull-duplextransmission,thereceivingendsendspiggybackedacknowledgement
byusingsomenumberintheACKfieldofitsdataframe.
8.Letusassumethata3-bitsequencenumberisusedandsupposethatastationsends
frame0andgetsbackanRR1,andthensendsframes1,2,3,4,5,6,7,0andgetsanother
RR1.
9.ThismighteithermeanthatRR1isacumulativeACKorall8framesweredamaged.This
ambiguitycanbeovercomeifthemaximumwindowsizeislimitedto7,i.e.forak-bit
sequencenumberfielditislimitedto2
k
-1.
10.ThenumberN(=2
k
-1)specifieshowmanyframescanbesentwithoutreceiving
acknowledgement.
2.1Go-back-NARQ
1.ThemostpopularARQprotocolisthego-back-NARQ,wherethesendersendstheframes
continuouslywithoutwaitingforacknowledgement.
2.Asthereceiverreceivestheframes,itkeepsonsendingACKsoraNACK,incaseaframeis
incorrectlyreceived.
3.WhenthesenderreceivesaNACK,itretransmitstheframeinerrorplusallthesucceeding
framesasshowninFig.1.53.Hence,thenameoftheprotocolisgo-back-NARQ.
4.Ifaframeislost,thereceiversendsNAKafterreceivingthenextframeasshownin
Fig.1.54.
5.IncasethereislongdelaybeforesendingtheNAK,thesenderwillresendthelostframe
afteritstimertimesout.
6.IftheACKframesentbythereceiverislost,thesenderresendstheframesafteritstimer
timesoutasshowninFig.1.55.
7.Assumingfull-duplextransmission,thereceivingendsendspiggybackedacknowledgement
byusingsomenumberintheACKfieldofitsdataframe.
8.Letusassumethata3-bitsequencenumberisusedandsupposethatastationsends
frame0andgetsbackanRR1,andthensendsframes1,2,3,4,5,6,7,0andgetsanother
RR1.
9.ThismighteithermeanthatRR1isacumulativeACKorall8framesweredamaged.This
ambiguitycanbeovercomeifthemaximumwindowsizeislimitedto7,i.e.forak-bit
sequencenumberfielditislimitedto2
k
-1.
10.ThenumberN(=2
k
-1)specifieshowmanyframescanbesentwithoutreceiving
acknowledgement.
Figure 1.53 Frames in error in go-Back-N ARQ
Figure 1.54 Lost Frames in Go-Back-N ARQ
Figure1.55LostACKinGo-Back-NARQ
1.IfnoacknowledgementisreceivedaftersendingNframes,thesendertakesthehelpofatimer.
2.Afterthetime-out,itresumesretransmission.Thego-back-Nprotocolalsotakescareofdamagedframes
anddamagedACKs.Thisschemeislittlemorecomplexthanthepreviousonebutgivesmuchhigher
throughput.
3.Assumingfull-duplextransmission,thereceivingendsendspiggybackedacknowledgementbyusingsome
numberintheACKfieldofitsdataframe.
4.Letusassumethata3-bitsequencenumberisusedandsupposethatastationsendsframe0andgets
backanRR1,andthensendsframes1,2,3,4,5,6,7,0andgetsanotherRR1.
5.ThismighteithermeanthatRR1isacumulativeACKorall8framesweredamaged.Thisambiguitycanbe
overcomeifthemaximumwindowsizeislimitedto7,i.e.forak-bitsequencenumberfielditislimitedto
2
k
-1.
6.ThenumberN(=2
k
-1)specifieshowmanyframescanbesentwithoutreceivingacknowledgement.
7.IfnoacknowledgementisreceivedaftersendingNframes,thesendertakesthehelpofatimer.
1.IfnoacknowledgementisreceivedaftersendingNframes,thesendertakesthehelpofatimer.
2.Afterthetime-out,itresumesretransmission.Thego-back-Nprotocolalsotakescareofdamagedframes
anddamagedACKs.Thisschemeislittlemorecomplexthanthepreviousonebutgivesmuchhigher
throughput.
3.Assumingfull-duplextransmission,thereceivingendsendspiggybackedacknowledgementbyusingsome
numberintheACKfieldofitsdataframe.
4.Letusassumethata3-bitsequencenumberisusedandsupposethatastationsendsframe0andgets
backanRR1,andthensendsframes1,2,3,4,5,6,7,0andgetsanotherRR1.
5.ThismighteithermeanthatRR1isacumulativeACKorall8framesweredamaged.Thisambiguitycanbe
overcomeifthemaximumwindowsizeislimitedto7,i.e.forak-bitsequencenumberfielditislimitedto
2
k
-1.
6.ThenumberN(=2
k
-1)specifieshowmanyframescanbesentwithoutreceivingacknowledgement.
7.IfnoacknowledgementisreceivedaftersendingNframes,thesendertakesthehelpofatimer.
2.2Selective-RepeatARQ
1.Theselective-repetitiveARQschemeretransmitsonlythoseforwhich
NAKsarereceivedorforwhichtimerhasexpired,thisisshowninthe
Fig.1.56.
2.ThisisthemostefficientamongtheARQschemes,butthesendermust
bemorecomplexsothatitcansendout-of-orderframes.
Figure1.56Selective-repeatReject
3.Thereceiveralsomusthavestoragespacetostorethepost-NAKframes
andprocessingpowertoreinsertframesinpropersequence
1.Theselective-repetitiveARQschemeretransmitsonlythoseforwhich
NAKsarereceivedorforwhichtimerhasexpired,thisisshowninthe
Fig.1.56.
2.ThisisthemostefficientamongtheARQschemes,butthesendermust
bemorecomplexsothatitcansendout-of-orderframes.
Figure1.56Selective-repeatReject
3.Thereceiveralsomusthavestoragespacetostorethepost-NAKframes
andprocessingpowertoreinsertframesinpropersequence
Selective-repeat Reject
LINE CODING
(Digital to Digital Conversion)
(Digital to Digital Conversion)
Linecoding
•Convertingastringof1’sand0’s(digitaldata)intoa
sequenceofsignalsthatdenotethe1’sand0’s.
•Forexampleahighvoltagelevel(+V)couldrepresenta“1”
andalowvoltagelevel(0or-V)couldrepresenta“0”.
•Itconvertsdigitaldatatodigitalsignal,knownasline
coding,asshowninFig.1.58.
Figure.1.58Linecodingtoconvertdigitaldatatodigitalsignal
•Convertingastringof1’sand0’s(digitaldata)intoa
sequenceofsignalsthatdenotethe1’sand0’s.
•Forexampleahighvoltagelevel(+V)couldrepresenta“1”
andalowvoltagelevel(0or-V)couldrepresenta“0”.
•Itconvertsdigitaldatatodigitalsignal,knownasline
coding,asshowninFig.1.58.
Figure.1.58Linecodingtoconvertdigitaldatatodigitalsignal
Line coding Characteristics
•Noofsignallevels:
–Thisreferstothenumbervaluesallowedinasignal,knownassignallevels,to
representdata.Fig.1.59(a)showstwosignallevels,whereasFig1.59(b)shows
threesignallevelstorepresentbinarydata.
–Adataelementisthesmallestentitythatcanrepresentapieceofinformation
(abit).Asignalelementistheshortestunitofadigitalsignal.
–Dataelementsarewhatweneedtosend;signalelementsarewhatwecan
send.Dataelementsarebeingcarried;signalelementsarethecarriers."
•BitrateversusBaudrate:
–Thebitraterepresentsthenumberofbitssentpersecond.
–Thebaudratedefinesthenumberofsignalelementspersecondinthesignal.
Figure .1.59 (a) Signal with two voltage levels, (b) Signal with three voltage levels
•Noofsignallevels:
–Thisreferstothenumbervaluesallowedinasignal,knownassignallevels,to
representdata.Fig.1.59(a)showstwosignallevels,whereasFig1.59(b)shows
threesignallevelstorepresentbinarydata.
–Adataelementisthesmallestentitythatcanrepresentapieceofinformation
(abit).Asignalelementistheshortestunitofadigitalsignal.
–Dataelementsarewhatweneedtosend;signalelementsarewhatwecan
send.Dataelementsarebeingcarried;signalelementsarethecarriers."
•BitrateversusBaudrate:
–Thebitraterepresentsthenumberofbitssentpersecond.
–Thebaudratedefinesthenumberofsignalelementspersecondinthesignal.
Figure .1.59 (a) Signal with two voltage levels, (b) Signal with three voltage levels
•SignalSpectrum:
–Differentencodingofdataleadstodifferentspectrumofthesignal.
–Itisnecessarytousesuitableencodingtechniquetomatchwiththemediumsothatthe
signalsuffersminimumattenuationanddistortionasitistransmittedthrougha
medium.
•Synchronization:
–Tointerpretthereceivedsignalcorrectly,thebitintervalofthereceivershouldbe
exactlysameorwithincertainlimitofthatofthetransmitter.
–Anymismatchbetweenthetwomayleadwronginterpretationofthereceivedsignal.
Usually,clockisgeneratedandsynchronizedfromthereceivedsignalwiththehelpofa
specialhardwareknownasPhaseLockLoop(PLL).
•DCcomponents:
–Afterlinecoding,thesignalmayhavezerofrequencycomponentinthespectrumofthe
signal,whichisknownasthedirect-current(DC)component.
–DCcomponentinasignalisnotdesirablebecausetheDCcomponentdoesnotpass
throughsomecomponentsofacommunicationsystemsuchasatransformer.
–Thisleadstodistortionofthesignalandmaycreateerrorattheoutput.TheDC
componentalsoresultsinunwantedenergylossontheline.
–Whenthevoltagelevelinadigitalsignalisconstantforawhile,thespectrumcreates
verylowfrequencies,calledDCcomponents,thatpresentproblemsforasystemthat
cannotpasslowfrequencies.
–Differentencodingofdataleadstodifferentspectrumofthesignal.
–Itisnecessarytousesuitableencodingtechniquetomatchwiththemediumsothatthe
signalsuffersminimumattenuationanddistortionasitistransmittedthrougha
medium.
•Synchronization:
–Tointerpretthereceivedsignalcorrectly,thebitintervalofthereceivershouldbe
exactlysameorwithincertainlimitofthatofthetransmitter.
–Anymismatchbetweenthetwomayleadwronginterpretationofthereceivedsignal.
Usually,clockisgeneratedandsynchronizedfromthereceivedsignalwiththehelpofa
specialhardwareknownasPhaseLockLoop(PLL).
•DCcomponents:
–Afterlinecoding,thesignalmayhavezerofrequencycomponentinthespectrumofthe
signal,whichisknownasthedirect-current(DC)component.
–DCcomponentinasignalisnotdesirablebecausetheDCcomponentdoesnotpass
throughsomecomponentsofacommunicationsystemsuchasatransformer.
–Thisleadstodistortionofthesignalandmaycreateerrorattheoutput.TheDC
componentalsoresultsinunwantedenergylossontheline.
–Whenthevoltagelevelinadigitalsignalisconstantforawhile,thespectrumcreates
verylowfrequencies,calledDCcomponents,thatpresentproblemsforasystemthat
cannotpasslowfrequencies.
Effect of lack of Synchronization
LineCodingTechniques
•Linecodingtechniquescanbebroadlydividedintothreebroadcategories:Unipolar,PolarandBipolar,as
showninFig.1.60.
Figure.1.60Threebasiccategoriesoflinecodingtechniques
1.Unipolar:
•Inunipolarencodingtechnique,onlytwovoltagelevelsareused.Itusesonlyonepolarityofvoltagelevel
asshowninFig.1.61.
•Inthisencodingapproach,thebitratesameasdatarate.
•Unfortunately,DCcomponentpresentintheencodedsignalandthereislossofsynchronizationforlong
sequencesof0’sand1’s.Itissimplebutobsolete.
Figure.1.61Unipolarencodingwithtwovoltagelevels
•Linecodingtechniquescanbebroadlydividedintothreebroadcategories:Unipolar,PolarandBipolar,as
showninFig.1.60.
Figure.1.60Threebasiccategoriesoflinecodingtechniques
1.Unipolar:
•Inunipolarencodingtechnique,onlytwovoltagelevelsareused.Itusesonlyonepolarityofvoltagelevel
asshowninFig.1.61.
•Inthisencodingapproach,thebitratesameasdatarate.
•Unfortunately,DCcomponentpresentintheencodedsignalandthereislossofsynchronizationforlong
sequencesof0’sand1’s.Itissimplebutobsolete.
Figure.1.61Unipolarencodingwithtwovoltagelevels
2.Polar:
•Polarencodingtechniqueusestwovoltagelevels–onepositiveandtheotherone
negative.
Figure.1.62EncodingSchemesunderpolarcategory
2.1NonReturntozero(NRZ):
•Themostcommonandeasiestwaytotransmitdigitalsignalsistousetwo
differentvoltagelevelsforthetwobinarydigits.
•Usuallyanegativevoltageisusedtorepresentonebinaryvalueandapositive
voltagetorepresenttheother.
•Thedataisencodedasthepresenceorabsenceofasignaltransitionatthe
beginningofthebittime.
•Asshowninthefigurebelow,inNRZencoding,thesignallevelremainssame
throughoutthebit-period.TherearetwoencodingschemesinNRZ:NRZ-Land
NRZ-I,asshowninFig.1.63.
•Polarencodingtechniqueusestwovoltagelevels–onepositiveandtheotherone
negative.
Figure.1.62EncodingSchemesunderpolarcategory
2.1NonReturntozero(NRZ):
•Themostcommonandeasiestwaytotransmitdigitalsignalsistousetwo
differentvoltagelevelsforthetwobinarydigits.
•Usuallyanegativevoltageisusedtorepresentonebinaryvalueandapositive
voltagetorepresenttheother.
•Thedataisencodedasthepresenceorabsenceofasignaltransitionatthe
beginningofthebittime.
•Asshowninthefigurebelow,inNRZencoding,thesignallevelremainssame
throughoutthebit-period.TherearetwoencodingschemesinNRZ:NRZ-Land
NRZ-I,asshowninFig.1.63.
Figure1.63NRZencodingscheme
InNRZ-Lthelevelofthevoltagedeterminesthevalueofthebit.InNRZ-Itheinversion
orthelackofinversiondeterminesthevalueofthebit.
InNRZ-Lthelevelofthevoltagedeterminesthevalueofthebit.InNRZ-Itheinversion
orthelackofinversiondeterminesthevalueofthebit.
2.2 Return to Zero RZ:
•To ensure synchronization, there must be a signal transition in each bit as shown in
Fig. 1.64.
Key characteristics of the RZ coding are:
•Three levels
•Bit rate is double than that of data rate
•No dc component
•Good synchronization
•Main limitation is the increase in bandwidth
Figure.1.64RZencodingtechnique
•To ensure synchronization, there must be a signal transition in each bit as shown in
Fig. 1.64.
Key characteristics of the RZ coding are:
•Three levels
•Bit rate is double than that of data rate
•No dc component
•Good synchronization
•Main limitation is the increase in bandwidth
Figure.1.64RZencodingtechnique
2.3Biphase:
•ToovercomethelimitationsofNRZencoding,biphaseencodingtechniquescanbeadopted.
ManchesteranddifferentialManchesterCodingarethetwocommonBiphasetechniquesin
use,asshowninFig.1.65.
•InthestandardManchestercodingthereisatransitionatthemiddleofeachbitperiod.A
binary1correspondstoalow-to-hightransitionandabinary0toahigh-to-lowtransition
inthemiddle.
•InDifferentialManchester,inversioninthemiddleofeachbitisusedforsynchronization.
•Theencodingofa0isrepresentedbythepresenceofatransitionbothatthebeginningand
atthemiddleand1isrepresentedbyatransitiononlyinthemiddleofthebitperiod.
Figure.1.65Manchesterencodingschemes
•ToovercomethelimitationsofNRZencoding,biphaseencodingtechniquescanbeadopted.
ManchesteranddifferentialManchesterCodingarethetwocommonBiphasetechniquesin
use,asshowninFig.1.65.
•InthestandardManchestercodingthereisatransitionatthemiddleofeachbitperiod.A
binary1correspondstoalow-to-hightransitionandabinary0toahigh-to-lowtransition
inthemiddle.
•InDifferentialManchester,inversioninthemiddleofeachbitisusedforsynchronization.
•Theencodingofa0isrepresentedbythepresenceofatransitionbothatthebeginningand
atthemiddleand1isrepresentedbyatransitiononlyinthemiddleofthebitperiod.
Figure.1.65Manchesterencodingschemes
In Manchester and differential Manchester encoding, the transition
at the middle of the bit is used for synchronization.
at the middle of the bit is used for synchronization.
3.BipolarEncoding:
•BipolarAMIusesthreevoltagelevels.
•UnlikeRZ,thezerolevelisusedtorepresenta
0andabinary1’sarerepresentedby
alternatingpositiveandnegativevoltage,as
showninfig.1.66.
Figure.1.66BipolarAMIsignal
•BipolarAMIusesthreevoltagelevels.
•UnlikeRZ,thezerolevelisusedtorepresenta
0andabinary1’sarerepresentedby
alternatingpositiveandnegativevoltage,as
showninfig.1.66.
Figure.1.66BipolarAMIsignal
Bipolar with 8-zero substitution (B8ZS):
•The limitation of bipolar AMI is overcome in B8ZS, which is used in North America.
A sequence of eight zero’s is preplaced by the following encoding:
•A sequence of eight 0’s is replaced by 000+-0+-, if the previous pulse was positive.
•A sequence of eight 0’s is replaced by 000-+0+-, if the previous pulse was negative.
High Density Bipolar-3 Zeros: Another alternative, which is used in Europe and Japan
is HDB3.It replaces a sequence of 4 zeros by a code as per the rule given in the
following table. The encoded signals are shown in Fig.1.67.
Figure.1.67B8ZS and HDB3 encoding techniques
•The limitation of bipolar AMI is overcome in B8ZS, which is used in North America.
A sequence of eight zero’s is preplaced by the following encoding:
•A sequence of eight 0’s is replaced by 000+-0+-, if the previous pulse was positive.
•A sequence of eight 0’s is replaced by 000-+0+-, if the previous pulse was negative.
High Density Bipolar-3 Zeros: Another alternative, which is used in Europe and Japan
is HDB3.It replaces a sequence of 4 zeros by a code as per the rule given in the
following table. The encoded signals are shown in Fig.1.67.
Figure.1.67B8ZS and HDB3 encoding techniques
Comparision between 1G,2G,3G and 4G
Generation
(1G,2G,3G,
4G,5G)
Definition Throughput/
Speed
Technology Time period Features
1G Analog 14.4 Kbps (peak)AMPS,NMT,TACS 1970 –1980 During 1G Wireless phones
are used forvoice only.
2G Digital Narrow
band circuit
data
9.6/14.4 Kbps TDMA,CDMA 1990 to 2000 2G capabilities are achieved
by allowingmultipleusers on
a single channel via
multiplexing.During2G
Cellular phones are used
fordata also along with
voice.
2.5G Packet Data171.2 Kbps(peak)
20-40 Kbps
GPRS 2001-2004 In 2.5G theinternetbecomes
popular and data becomes
more
relevant.2.5GMultimedia
servicesand streaming starts
to show growth.Phonesstart
supportingweb
browsingthoughlimited and
very few phones have that.
Generation
(1G,2G,3G,
4G,5G)
Definition Throughput/
Speed
Technology Time period Features
1G Analog 14.4 Kbps (peak)AMPS,NMT,TACS 1970 –1980 During 1G Wireless phones
are used forvoice only.
2G Digital Narrow
band circuit
data
9.6/14.4 Kbps TDMA,CDMA 1990 to 2000 2G capabilities are achieved
by allowingmultipleusers on
a single channel via
multiplexing.During2G
Cellular phones are used
fordata also along with
voice.
2.5G Packet Data171.2 Kbps(peak)
20-40 Kbps
GPRS 2001-2004 In 2.5G theinternetbecomes
popular and data becomes
more
relevant.2.5GMultimedia
servicesand streaming starts
to show growth.Phonesstart
supportingweb
browsingthoughlimited and
very few phones have that.
3G Digital
Broadband
Packet Data
3.1 Mbps (peak)
500-700 Kbps
CDMA 2000
(1xRTT, EVDO)
UMTS, EDGE
2004-2005 3G hasMultimedia
services supportalongwith
streaming are more
popular.In3G,Universal
accessandportabilityacros
s different device types are
made possible. (Telephones,
PDA’s, etc.)
3.5G Packet Data 14.4 Mbps (peak)
1-3 Mbps
HSPA 2006 –2010 3.5G supportshigher
throughput and speedsto
support higher data needs of
the consumers.
4G Digital
Broadband
Packet
All IP
Very high
throughput
100-300 Mbps
(peak)
3-5 Mbps
100 Mbps (Wi-Fi)
WiMax
LTE
Wi-Fi
Now (Read more
onTransitioningto 4G)
Speedsfor 4G are further
increased to keep up with
data access demand used
by various services.High
definition streamingis now
supported in 4G. New
phones with HD capabilities
surface. It gets pretty cool.In
4G,Portabilityis increased
further.World-wide
roamingis not a distant
dream.
5G Not Yet Probably gigabitsNot Yet Soon (probably 2020)
Update:Samsung
conducts tests on 5G
Currently there is no 5G
technology deployed.
When this becomes
available it will provide very
high speeds to the
consumers. It would also
provide efficient use of
available bandwidth as has
been seen through
development of each new
technology.
Broadband
Packet Data
3.1 Mbps (peak)
500-700 Kbps
CDMA 2000
(1xRTT, EVDO)
UMTS, EDGE
2004-2005 3G hasMultimedia
services supportalongwith
streaming are more
popular.In3G,Universal
accessandportabilityacros
s different device types are
made possible. (Telephones,
PDA’s, etc.)
3.5G Packet Data 14.4 Mbps (peak)
1-3 Mbps
HSPA 2006 –2010 3.5G supportshigher
throughput and speedsto
support higher data needs of
the consumers.
4G Digital
Broadband
Packet
All IP
Very high
throughput
100-300 Mbps
(peak)
3-5 Mbps
100 Mbps (Wi-Fi)
WiMax
LTE
Wi-Fi
Now (Read more
onTransitioningto 4G)
Speedsfor 4G are further
increased to keep up with
data access demand used
by various services.High
definition streamingis now
supported in 4G. New
phones with HD capabilities
surface. It gets pretty cool.In
4G,Portabilityis increased
further.World-wide
roamingis not a distant
dream.
5G Not Yet Probably gigabitsNot Yet Soon (probably 2020)
Update:Samsung
conducts tests on 5G
Currently there is no 5G
technology deployed.
When this becomes
available it will provide very
high speeds to the
consumers. It would also
provide efficient use of
available bandwidth as has
been seen through
development of each new
technology.
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Categories
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