Unit 2.1 - OSI Model.ppt Physical Layer

Description of OSI model layers

1. 1.1
Chapter 2
Physical Layer
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2. 2.2
Network Models

3. 2.3
2-1 LAYERED TASKS
Weusetheconceptoflayersinourdailylife.As
anexample,letusconsidertwofriendswho
communicatethroughpostalmail.Theprocessof
sendingalettertoafriendwouldbecomplexif
therewerenoservicesavailablefromthepost
office.
Sender, Receiver, and Carrier
Hierarchy
Topics discussed in this section:

4. 2.4
Figure 2.1 Tasks involved in sending a letter

5. 2.5
2-2 THE OSI MODEL
Establishedin1947,theInternationalStandards
Organization(ISO)isamultinationalbody
dedicatedtoworldwide agreement on
internationalstandards.AnISOstandardthat
coversallaspectsofnetworkcommunicationsis
theOpenSystemsInterconnection(OSI)model.It
wasfirstintroducedinthelate1970s.
Layered Architecture
Peer-to-Peer Processes
Encapsulation
Topics discussed in this section:

6. 2.6
ISO is the organization.
OSI is the model.
Note

7. 2.7
Figure 2.2 Seven layers of the OSI (Open Systems Interconnection)
model

8. 2.8
Figure 2.3 The interaction between layers in the OSI model

9. 2.9
Figure 2.4 An exchange using the OSI model

10. 2.10
2-3 LAYERS IN THE OSI MODEL
Inthissectionwebrieflydescribethefunctions
ofeachlayerintheOSImodel.
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
Topics discussed in this section:

11. 2.11
Figure 2.5 Physical layer

12. 2.12
PHYSICAL LAYER
•Responsibleformovementsofindividualbitsfromonehop(node)tothe
next.
PhysicalCharacteristicsofInterfacesandmedium–
•Thephysicallayerdefinesthecharacteristicsoftheinterfacebetweenthe
devicesandthetransmissionmedium.
•Italsodefinesthetypeoftransmissionmedium.
Representationofbits
•Thephysicallayerdataconsistsofastreamofbits(sequenceof0sor1s)
withnointerpretation.
•Tobetransmitted,bitsmustbeencodedintosignals,electricaloroptical.
•Thephysicallayerdefinesthetypeofencoding(how0’sand1’sarechanged
tosignals).

13. 2.13
PHYSICAL LAYER
Synchronizationofbits
•Thesenderandreceivernotonlymustusethesamebitratebutalsomustbe
synchronizedatthebitlevel.
•Inotherwords,thesenderandthereceiverclocksmustbesynchronized
Lineconfiguration
•Thephysicallayerisconcernedwiththeconnectionofdevicestothemedia.
•Inapoint-to-pointconfiguration,twodevicesareconnectedthrougha
dedicatedlink.
•Inamultipointconfiguration,alinkissharedamongseveraldevices.
Datarate
•Thetransmissionrate-thenumberofbitssenteachsecondisalsodefined
bythephysicallayer.
•Inotherwords,thephysicallayerdefinesthedurationofabit,whichishow
longitlasts

14. 2.14
PHYSICAL LAYER
Physicaltopology
•defineshowdevicesareconnectedtomakeanetwork.
•Devicescanbeconnectedbyusinga
•meshtopology(everydeviceisconnectedtoeveryotherdevice),
•startopology(devicesareconnectedthroughacentraldevice),
•ringtopology(eachdeviceisconnectedtothenext,formingaring),
•bustopology(everydeviceisonacommonlink),or
•hybridtopology(thisisacombinationoftwoormoretopologies).
Transmissionmode
•definesthedirectionoftransmissionbetweentwodevices:simplex,half-
duplex,orfull-duplex.
•Insimplexmode,onlyonedevicecansend;theothercanonlyreceive.The
simplexmodeisaone-waycommunication.
•Inhalf-duplexmode,twodevicescansendandreceive,butnotatthesame
time.
•Infull-duplex(orsimplyduplex)mode,twodevicescansendandreceiveat
thesametime.

15. 2.15
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

16. 2.16
Figure 2.6 Data link layer

17. 2.17
DATA LINK LAYER
•Responsibleformovingframesfromonehop(node)tothenext
•Framing-The data link layer divides the stream of bits received from the network
layer into manageable data units called frames
•Physical addressing -If frames are to be distributed to different systems on the
network, the data link layer adds a header to the frame to define the sender and/or
receiver of the frame.
•Ifthe frame is intended for a system outside the sender’s network, the receiver
address is the address of the device that connects the network to the next one.

18. 2.18
DATA LINK LAYER
•Flow control -If the rate at which the data are absorbed by the receiver is less than
the rate at which data are produced in the sender, the data link layer imposes a flow
control mechanism to avoid overwhelming the receiver.
•Error control -The data link layer adds reliability to the physical layer by adding
mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism
to recognize duplicate frames. Error control is normally achieved through a
trailer added to the end of the frame.
•Access control -When two or more devices are connected to the same link, data
link layer protocols are necessary to determine which device has control over the
link at any given time.

19. 2.19
Figure 2.7 Hop-to-hop delivery

20. 2.20
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

21. 2.21
Figure 2.8 Network layer

22. 2.22
NETWORK LAYER
Responsiblefordeliveryofindividualpacketsfromsourcehosttothedestination
host
Logicaladdressing
•Thephysicaladdressingimplementedbythedatalinklayerhandlesthe
addressingproblemlocally.
•Ifapacketpassesthenetworkboundary,weneedanotheraddressing
systemtohelpdistinguishthesourceanddestinationsystems.
•Thenetworklayeraddsaheadertothepacketcomingfromtheupperlayer
that,amongotherthings,includesthelogicaladdressesofthesenderand
receiver
Routing
•Whenindependentnetworksorlinksareconnectedtocreateinternetworks
(networkofnetworks)oralargenetwork,theconnectingdevices(called
routers

23. 2.23
Figure 2.9 Source-to-destination delivery

24. 2.24
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

25. 2.25
Questions
1)Thephysicallayersinvolve________.
a)Optical,electricalandmechanicalproperties
b)Voltagelevels,timingandfrequency
c)Physicalconnections
d)Alloftheabove
2)WhodevelopedstandardsforOSIreferencemodel?
a)ANSI-AmericanNationalStandardsInstitute
b)ISO-InternationalStandardsOrganization
c)IEEE-InstituteofElectricalandElectronicsEngineers
d)ACM-AssociationforComputingMachinery
3)WhichisthelayerofOSImodelthatemployserrordetection?
a)Session
b)Network
c)Physical
d)DataLink

26. 2.26
Figure 2.10 Transport layer

27. 2.27
TRANSPORT LAYER
Responsiblefordeliveryofmessagefromoneprocesstoanother
Service-pointaddressing(Portaddress)
•Computersoftenrunseveralprogramsatthesametime.So,source-to-
destinationdeliverymeansdeliverynotonlyfromonecomputertothenext
butalsofromaspecificprocess(runningprogram)ononecomputertoa
specificprocess(runningprogram)ontheother.
•Thetransportlayerheadermustthereforeincludeatypeofaddresscalleda
service-pointaddress(orportaddress).
•Thenetworklayergetseachpackettothecorrectcomputer;thetransport
layergetstheentiremessagetothecorrectprocessonthatcomputer.
Segmentationandreassembly
•Amessageisdividedintotransmittablesegments,witheachsegment
containingasequencenumber.
•Thesenumbersenablethetransportlayertoreassemblethemessage
correctlyuponarrivingatthedestinationandtoidentifyandreplacepackets
thatwerelostintransmission

28. 2.28
TRANSPORT LAYER
Connectioncontrol
•Thetransportlayercanbeeitherconnectionlessorconnection-oriented.
•Aconnectionlesstransportlayertreatseachsegmentasanindependent
packetanddeliversittothetransportlayeratthedestinationmachine.
•Aconnectionorientedtransportlayermakesaconnectionwiththetransport
layeratthedestinationmachinefirstbeforedeliveringthepackets.
•Afterallthedataaretransferred,theconnectionisterminated.
Flowcontrol
•Likethedatalinklayer,thetransportlayerisresponsibleforflowcontrol.
•However,flowcontrolatthislayerisperformedendtoendratherthan
acrossasinglelink.
Errorcontrol
•Likedatalinklayer,transportlayerisresponsibleforerrorcontrol.However,
errorcontrolatthislayerisperformedprocess-to-processratherthan
acrossasinglelink.
•Thesendingtransportlayermakessurethattheentiremessagearrivesat
thereceivingtransportlayerwithouterror(damage,loss,orduplication).
•Errorcorrectionisusuallyachievedthroughretransmission.

29. 2.29
Figure 2.11 Reliable process-to-process delivery of a message

30. 2.30
The transport layer is responsible for the delivery
of a message from one process to another.
Note

31. 2.31
Figure 2.12 Session layer

32. 2.32
SESSION LAYER
Responsiblefordialogcontrolandsynchronization
DialogControl
•Thesessionlayerallowstwosystemstoenterintoadialog.
•Itallowsthecommunicationbetweentwoprocessestotakeplaceineither
halfduplexorfull-duplexmode
Synchronization
•Thesessionlayerallowsaprocesstoaddcheckpoints,orsynchronization
points,toastreamofdata.
•Forexample,ifasystemissendingafileof2000pages,itisadvisableto
insertcheckpointsafterevery100pagestoensurethateach100-pageunitis
receivedandacknowledgedindependently.
•Inthiscase,ifacrashhappensduringthetransmissionofpage523,theonly
pagesthatneedtoberesentaftersystemrecoveryarepages501to523.
Pagespreviousto501neednotberesent.

33. 2.33
The session layer is responsible for dialog
control and synchronization.
Note

34. 2.34
Figure 2.13 Presentation layer

35. 2.35
PRESENTATION LAYER
Itisconcernedwithsyntaxandsemanticsofinformationexchangedbetweentwo
systems.
Itisresponsiblefortranslation,compression,andencryption
Translation
•Processes(runningprograms)intwosystemsareusuallyexchanginginformationin
theformofcharacterstrings,numbers,andsoon.
•Informationmustbechangedtobitstreamsbeforebeingtransmitted.Because
differentcomputersusedifferentencodingsystems,presentationlayeris
responsibleforinteroperabilitybetweenthesedifferentencodingmethods.
•Presentationlayeratthesenderchangestheinformationfromitssender-dependent
formatintoacommonformat.
•Presentationlayeratthereceivingmachinechangesthecommonformatintoits
receiver-dependentformat.

36. 2.36
PRESENTATION LAYER
Encryption
•Tocarrysensitiveinformation,asystemmustbeabletoensureprivacy.
•Encryptionmeansthatthesendertransformstheoriginalinformationtoanother
formandsendstheresultingmessageoutoverthenetwork.
•Decryptionreversestheoriginalprocesstotransformthemessagebacktoits
originalform
Compression
•Datacompressionreducesthenumberofbitscontainedintheinformation.
•Datacompressionbecomesparticularlyimportantinthetransmissionof
multimediasuchastext,audio,andvideo

37. 2.37
The presentation layer is responsible for translation,
compression, and encryption.
Note

38. 2.38
APPLICATION LAYER
Responsibleforprovidingservicestotheuser
•Ofthemanyapplicationservicesavailable,thefigure2.14showsonlythree:
•XAOO(message-handlingservices),
•X.500(directoryservices),and
•Filetransfer,access,andmanagement(FTAM).
•TheuserinthisexampleemploysXAOOtosendane-mailmessage
Networkvirtualterminal
•Anetworkvirtualterminalisasoftwareversionofaphysicalterminal
•Itallowsausertologontoaremotehost.
•Todoso,theapplicationcreatesasoftwareemulationofaterminalattheremote
host.Theuser'scomputertalkstothesoftwareterminalwhich,inturn,talkstothe
hostandviceversa.
•Theremotehostbelievesitiscommunicatingwithoneofitsownterminalsand
allowstheusertologon.

39. 2.39
APPLICATION LAYER
Filetransfer,access,andmanagement
•Thisapplicationallowsauser
•toaccessfilesinaremotehost(tomakechangesorreaddata),
•toretrievefilesfromaremotecomputerforuseinthelocalcomputer,and
•tomanageorcontrolfilesinaremotecomputerlocally.
Mailservices
•Thisapplicationprovidesthebasisfore-mailforwardingandstorage.
Directoryservices.
•Thisapplicationprovidesdistributeddatabasesourcesandaccessforglobal
informationaboutvariousobjectsandservices

40. 2.40
Figure 2.14 Application layer

41. 2.41
The application layer is responsible for
providing services to the user.
Note

42. 2.42
Figure 2.15 Summary of layers