Framming data link layer
PREMALGAJJAR
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May 26, 2014
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About This Presentation
Data Link Control
FRAMING
The data link layer needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of framing. The simple act of inserting a letter into an envelope separates one piece of information from another; the envelope serves...
Slide Content
11.1
Data Link Control
Data Link Control
11.2
11-1 FRAMING
Thedatalinklayerneedstopackbitsintoframes,so
thateachframeisdistinguishablefromanother.Our
postalsystempracticesatypeofframing.Thesimple
actofinsertingaletterintoanenvelopeseparatesone
pieceofinformationfromanother;theenvelopeserves
asthedelimiter.
Fixed-Size Framing
Variable-Size Framing
Topics discussed in this section:
11-1 FRAMING
Thedatalinklayerneedstopackbitsintoframes,so
thateachframeisdistinguishablefromanother.Our
postalsystempracticesatypeofframing.Thesimple
actofinsertingaletterintoanenvelopeseparatesone
pieceofinformationfromanother;theenvelopeserves
asthedelimiter.
Fixed-Size Framing
Variable-Size Framing
Topics discussed in this section:
11.3
Figure 11.1 A frame in a character-oriented protocol
Figure 11.1 A frame in a character-oriented protocol
11.4
Figure 11.2 Byte stuffing and unstuffing
Figure 11.2 Byte stuffing and unstuffing
11.5
Byte stuffing is the process of adding 1
extra byte whenever there is a flag or
escape character in the text.
Note
Byte stuffing is the process of adding 1
extra byte whenever there is a flag or
escape character in the text.
Note
11.6
Figure 11.3 A frame in a bit-oriented protocol
Figure 11.3 A frame in a bit-oriented protocol
11.7
Bit stuffing is the process of adding one
extra 0 whenever five consecutive 1s
follow a 0 in the data, so that the
receiver does not mistake
the pattern 0111110 for a flag.
Note
Bit stuffing is the process of adding one
extra 0 whenever five consecutive 1s
follow a 0 in the data, so that the
receiver does not mistake
the pattern 0111110 for a flag.
Note
11.8
Figure 11.4 Bit stuffing and unstuffing
Figure 11.4 Bit stuffing and unstuffing
11.9
11-2 FLOW AND ERROR CONTROL
Themostimportantresponsibilitiesofthedatalink
layerareflowcontrolanderrorcontrol.Collectively,
thesefunctionsareknownasdatalinkcontrol.
Flow Control
Error Control
Topics discussed in this section:
11-2 FLOW AND ERROR CONTROL
Themostimportantresponsibilitiesofthedatalink
layerareflowcontrolanderrorcontrol.Collectively,
thesefunctionsareknownasdatalinkcontrol.
Flow Control
Error Control
Topics discussed in this section:
11.10
Flow control refers to a set of procedures
used to restrict the amount of data
that the sender can send before
waiting for acknowledgment.
Note
Flow control refers to a set of procedures
used to restrict the amount of data
that the sender can send before
waiting for acknowledgment.
Note
11.11
Error control in the data link layer is
based on automatic repeat request,
which is the retransmission of data.
Note
Error control in the data link layer is
based on automatic repeat request,
which is the retransmission of data.
Note
11.12
11-3 PROTOCOLS
Nowletusseehowthedatalinklayercancombine
framing,flowcontrol,anderrorcontroltoachievethe
deliveryofdatafromonenodetoanother.The
protocolsarenormallyimplementedinsoftwareby
usingoneofthecommonprogramminglanguages.To
makeourdiscussionslanguage-free,wehavewritten
inpseudocodeaversionofeachprotocolthat
concentratesmostlyontheprocedureinsteadof
delvingintothedetailsoflanguagerules.
11-3 PROTOCOLS
Nowletusseehowthedatalinklayercancombine
framing,flowcontrol,anderrorcontroltoachievethe
deliveryofdatafromonenodetoanother.The
protocolsarenormallyimplementedinsoftwareby
usingoneofthecommonprogramminglanguages.To
makeourdiscussionslanguage-free,wehavewritten
inpseudocodeaversionofeachprotocolthat
concentratesmostlyontheprocedureinsteadof
delvingintothedetailsoflanguagerules.
11.13
Figure 11.5 Taxonomy of protocols discussed in this chapter
Figure 11.5 Taxonomy of protocols discussed in this chapter
11.14
11-4 NOISELESS CHANNELS
Letusfirstassumewehaveanidealchannelinwhich
noframesarelost,duplicated,orcorrupted.We
introducetwoprotocolsforthistypeofchannel.
Simplest Protocol
Stop-and-Wait Protocol
Topics discussed in this section:
11-4 NOISELESS CHANNELS
Letusfirstassumewehaveanidealchannelinwhich
noframesarelost,duplicated,orcorrupted.We
introducetwoprotocolsforthistypeofchannel.
Simplest Protocol
Stop-and-Wait Protocol
Topics discussed in this section:
11.15
Figure 11.6 The design of the simplest protocol with no flow or error control
Figure 11.6 The design of the simplest protocol with no flow or error control
11.16
Figure11.7showsanexampleofcommunicationusing
thisprotocol.Itisverysimple.Thesendersendsa
sequenceofframeswithouteventhinkingaboutthe
receiver.Tosendthreeframes,threeeventsoccuratthe
sendersiteandthreeeventsatthereceiversite.Notethat
thedataframesareshownbytiltedboxes;theheightof
theboxdefinesthetransmissiontimedifferencebetween
thefirstbitandthelastbitintheframe.
Example 11.1
Figure11.7showsanexampleofcommunicationusing
thisprotocol.Itisverysimple.Thesendersendsa
sequenceofframeswithouteventhinkingaboutthe
receiver.Tosendthreeframes,threeeventsoccuratthe
sendersiteandthreeeventsatthereceiversite.Notethat
thedataframesareshownbytiltedboxes;theheightof
theboxdefinesthetransmissiontimedifferencebetween
thefirstbitandthelastbitintheframe.
Example 11.1
11.17
Figure 11.7 Flow diagram for Example 11.1
Figure 11.7 Flow diagram for Example 11.1
11.18
Figure 11.8 Design of Stop-and-Wait Protocol
Figure 11.8 Design of Stop-and-Wait Protocol
11.19
Figure11.9showsanexampleofcommunicationusing
thisprotocol.Itisstillverysimple.Thesendersendsone
frameandwaitsforfeedbackfromthereceiver.Whenthe
ACKarrives,thesendersendsthenextframe.Notethat
sendingtwoframesintheprotocolinvolvesthesenderin
foureventsandthereceiverintwoevents.
Example 11.2
Figure11.9showsanexampleofcommunicationusing
thisprotocol.Itisstillverysimple.Thesendersendsone
frameandwaitsforfeedbackfromthereceiver.Whenthe
ACKarrives,thesendersendsthenextframe.Notethat
sendingtwoframesintheprotocolinvolvesthesenderin
foureventsandthereceiverintwoevents.
Example 11.2
11.20
Figure 11.9 Flow diagram for Example 11.2
Figure 11.9 Flow diagram for Example 11.2
11.21
11-5 NOISY CHANNELS
AlthoughtheStop-and-WaitProtocolgivesusanidea
ofhowtoaddflowcontroltoitspredecessor,noiseless
channelsarenonexistent.Wediscussthreeprotocols
inthissectionthatuseerrorcontrol.
Stop-and-Wait Automatic Repeat Request
Go-Back-N Automatic Repeat Request
Selective Repeat Automatic Repeat Request
Topics discussed in this section:
11-5 NOISY CHANNELS
AlthoughtheStop-and-WaitProtocolgivesusanidea
ofhowtoaddflowcontroltoitspredecessor,noiseless
channelsarenonexistent.Wediscussthreeprotocols
inthissectionthatuseerrorcontrol.
Stop-and-Wait Automatic Repeat Request
Go-Back-N Automatic Repeat Request
Selective Repeat Automatic Repeat Request
Topics discussed in this section:
11.22
Error correction in Stop-and-Wait ARQ is
done by keeping a copy of the sent
frame and retransmitting of the frame
when the timer expires.
Note
Error correction in Stop-and-Wait ARQ is
done by keeping a copy of the sent
frame and retransmitting of the frame
when the timer expires.
Note
11.23
In Stop-and-Wait ARQ, we use sequence
numbers to number the frames.
The sequence numbers are based on
modulo-2 arithmetic.
Note
In Stop-and-Wait ARQ, we use sequence
numbers to number the frames.
The sequence numbers are based on
modulo-2 arithmetic.
Note
11.24
In Stop-and-Wait ARQ, the
acknowledgment number always
announces in modulo-2 arithmetic the
sequence number of the next frame
expected.
Note
In Stop-and-Wait ARQ, the
acknowledgment number always
announces in modulo-2 arithmetic the
sequence number of the next frame
expected.
Note
11.25
Figure 11.10 Design of the Stop-and-Wait ARQ Protocol
Figure 11.10 Design of the Stop-and-Wait ARQ Protocol
11.26
Figure11.11showsanexampleofStop-and-WaitARQ.
Frame0issentandacknowledged.Frame1islostand
resentafterthetime-out.Theresentframe1is
acknowledgedandthetimerstops.Frame0issentand
acknowledged,buttheacknowledgmentislost.The
senderhasnoideaiftheframeortheacknowledgment
islost,soafterthetime-out,itresendsframe0,whichis
acknowledged.
Example 11.3
Figure11.11showsanexampleofStop-and-WaitARQ.
Frame0issentandacknowledged.Frame1islostand
resentafterthetime-out.Theresentframe1is
acknowledgedandthetimerstops.Frame0issentand
acknowledged,buttheacknowledgmentislost.The
senderhasnoideaiftheframeortheacknowledgment
islost,soafterthetime-out,itresendsframe0,whichis
acknowledged.
Example 11.3
11.27
Figure 11.11 Flow diagram for Example 11.3
Figure 11.11 Flow diagram for Example 11.3
11.28
Assumethat,inaStop-and-WaitARQsystem,the
bandwidthofthelineis1Mbps,and1bittakes20msto
makearoundtrip.Whatisthebandwidth-delayproduct?
Ifthesystemdataframesare1000bitsinlength,whatis
theutilizationpercentageofthelink?
Solution
The bandwidth-delay product is
Example 11.4
Assumethat,inaStop-and-WaitARQsystem,the
bandwidthofthelineis1Mbps,and1bittakes20msto
makearoundtrip.Whatisthebandwidth-delayproduct?
Ifthesystemdataframesare1000bitsinlength,whatis
theutilizationpercentageofthelink?
Solution
The bandwidth-delay product is
Example 11.4
11.29
Thesystemcansend20,000bitsduringthetimeittakes
forthedatatogofromthesendertothereceiverandthen
backagain.However,thesystemsendsonly1000bits.We
cansaythatthelinkutilizationisonly1000/20,000,or5
percent.Forthisreason,foralinkwithahighbandwidth
orlongdelay,theuseofStop-and-WaitARQwastesthe
capacityofthelink.
Example 11.4 (continued)
Thesystemcansend20,000bitsduringthetimeittakes
forthedatatogofromthesendertothereceiverandthen
backagain.However,thesystemsendsonly1000bits.We
cansaythatthelinkutilizationisonly1000/20,000,or5
percent.Forthisreason,foralinkwithahighbandwidth
orlongdelay,theuseofStop-and-WaitARQwastesthe
capacityofthelink.
Example 11.4 (continued)
11.30
Whatistheutilizationpercentageofthelinkin
Example11.4ifwehaveaprotocolthatcansendupto
15framesbeforestoppingandworryingaboutthe
acknowledgments?
Solution
Thebandwidth-delayproductisstill20,000bits.The
systemcansendupto15framesor15,000bitsduringa
roundtrip.Thismeanstheutilizationis15,000/20,000,or
75percent.Ofcourse,iftherearedamagedframes,the
utilizationpercentageismuchlessbecauseframeshave
toberesent.
Example 11.5
Whatistheutilizationpercentageofthelinkin
Example11.4ifwehaveaprotocolthatcansendupto
15framesbeforestoppingandworryingaboutthe
acknowledgments?
Solution
Thebandwidth-delayproductisstill20,000bits.The
systemcansendupto15framesor15,000bitsduringa
roundtrip.Thismeanstheutilizationis15,000/20,000,or
75percent.Ofcourse,iftherearedamagedframes,the
utilizationpercentageismuchlessbecauseframeshave
toberesent.
Example 11.5
11.31
In the Go-Back-N Protocol, the sequence
numbers are modulo 2
m
,
where m is the size of the sequence
number field in bits.
Note
In the Go-Back-N Protocol, the sequence
numbers are modulo 2
m
,
where m is the size of the sequence
number field in bits.
Note
11.32
Figure 11.12 Send window for Go-Back-N ARQ
Figure 11.12 Send window for Go-Back-N ARQ
11.33
The send window is an abstract concept
defining an imaginary box of size 2
m
− 1
with three variables: S
f, S
n, and Ssize.
Note
The send window is an abstract concept
defining an imaginary box of size 2
m
− 1
with three variables: S
f, S
n, and Ssize.
Note
11.34
The send window can slide one
or more slots when a valid
acknowledgment arrives.
Note
The send window can slide one
or more slots when a valid
acknowledgment arrives.
Note
11.35
Figure 11.13 Receive window for Go-Back-N ARQ
Figure 11.13 Receive window for Go-Back-N ARQ
11.36
The receive window is an abstract
concept defining an imaginary box
of size 1 with one single variable Rn.
The window slides
when a correct frame has arrived;
sliding occurs one slot at a time.
Note
The receive window is an abstract
concept defining an imaginary box
of size 1 with one single variable Rn.
The window slides
when a correct frame has arrived;
sliding occurs one slot at a time.
Note
11.37
Figure 11.14 Design of Go-Back-N ARQ
Figure 11.14 Design of Go-Back-N ARQ
11.38
Figure 11.15 Window size for Go-Back-N ARQ
Figure 11.15 Window size for Go-Back-N ARQ
11.39
In Go-Back-N ARQ, the size of the send
window must be less than 2
m
;
the size of the receiver window
is always 1.
Note
In Go-Back-N ARQ, the size of the send
window must be less than 2
m
;
the size of the receiver window
is always 1.
Note
11.40
Example 11.6
Figure11.16showsanexampleofGo-Back-N.Thisisan
exampleofacasewheretheforwardchannelisreliable,
butthereverseisnot.Nodataframesarelost,butsome
ACKsaredelayedandoneislost.Theexamplealso
showshowcumulativeacknowledgmentscanhelpif
acknowledgmentsaredelayedorlost.Afterinitialization,
therearesevensenderevents.Requesteventsare
triggeredbydatafromthenetworklayer;arrivalevents
aretriggeredbyacknowledgmentsfromthephysical
layer.Thereisnotime-outeventherebecauseall
outstandingframesareacknowledgedbeforethetimer
expires.NotethatalthoughACK2islost,ACK3servesas
bothACK2andACK3.
Example 11.6
Figure11.16showsanexampleofGo-Back-N.Thisisan
exampleofacasewheretheforwardchannelisreliable,
butthereverseisnot.Nodataframesarelost,butsome
ACKsaredelayedandoneislost.Theexamplealso
showshowcumulativeacknowledgmentscanhelpif
acknowledgmentsaredelayedorlost.Afterinitialization,
therearesevensenderevents.Requesteventsare
triggeredbydatafromthenetworklayer;arrivalevents
aretriggeredbyacknowledgmentsfromthephysical
layer.Thereisnotime-outeventherebecauseall
outstandingframesareacknowledgedbeforethetimer
expires.NotethatalthoughACK2islost,ACK3servesas
bothACK2andACK3.
11.41
Figure 11.16 Flow diagram for Example 11.6
Figure 11.16 Flow diagram for Example 11.6
11.42
Figure11.17showswhathappenswhenaframeislost.
Frames0,1,2,and3aresent.However,frame1islost.
Thereceiverreceivesframes2and3,buttheyare
discardedbecausetheyarereceivedoutoforder.The
senderreceivesnoacknowledgmentaboutframes1,2,or
3.Itstimerfinallyexpires.Thesendersendsall
outstandingframes(1,2,and3)becauseitdoesnotknow
whatiswrong.Notethattheresendingofframes1,2,and
3istheresponsetoonesingleevent.Whenthesenderis
respondingtothisevent,itcannotacceptthetriggeringof
otherevents.ThismeansthatwhenACK2arrives,the
senderisstillbusywithsendingframe3.
Example 11.7
Figure11.17showswhathappenswhenaframeislost.
Frames0,1,2,and3aresent.However,frame1islost.
Thereceiverreceivesframes2and3,buttheyare
discardedbecausetheyarereceivedoutoforder.The
senderreceivesnoacknowledgmentaboutframes1,2,or
3.Itstimerfinallyexpires.Thesendersendsall
outstandingframes(1,2,and3)becauseitdoesnotknow
whatiswrong.Notethattheresendingofframes1,2,and
3istheresponsetoonesingleevent.Whenthesenderis
respondingtothisevent,itcannotacceptthetriggeringof
otherevents.ThismeansthatwhenACK2arrives,the
senderisstillbusywithsendingframe3.
Example 11.7
11.43
Thephysicallayermustwaituntilthiseventiscompleted
andthedatalinklayergoesbacktoitssleepingstate.We
haveshownaverticallinetoindicatethedelay.Itisthe
samestorywithACK3;butwhenACK3arrives,the
senderisbusyrespondingtoACK2.Ithappensagain
whenACK4arrives.Notethatbeforethesecondtimer
expires,alloutstandingframeshavebeensentandthe
timerisstopped.
Example 11.7 (continued)
Thephysicallayermustwaituntilthiseventiscompleted
andthedatalinklayergoesbacktoitssleepingstate.We
haveshownaverticallinetoindicatethedelay.Itisthe
samestorywithACK3;butwhenACK3arrives,the
senderisbusyrespondingtoACK2.Ithappensagain
whenACK4arrives.Notethatbeforethesecondtimer
expires,alloutstandingframeshavebeensentandthe
timerisstopped.
Example 11.7 (continued)
11.44
Figure 11.17 Flow diagram for Example 11.7
Figure 11.17 Flow diagram for Example 11.7
11.45
Stop-and-Wait ARQ is a special case of
Go-Back-N ARQ in which the size of the
send window is 1.
Note
Stop-and-Wait ARQ is a special case of
Go-Back-N ARQ in which the size of the
send window is 1.
Note
11.46
Figure 11.18 Send window for Selective Repeat ARQ
Figure 11.18 Send window for Selective Repeat ARQ
11.47
Figure 11.19 Receive window for Selective Repeat ARQ
Figure 11.19 Receive window for Selective Repeat ARQ
11.48
Figure 11.20 Design of Selective Repeat ARQ
Figure 11.20 Design of Selective Repeat ARQ
11.49
Figure 11.21 Selective Repeat ARQ, window size
Figure 11.21 Selective Repeat ARQ, window size
11.50
In Selective Repeat ARQ, the size of the
sender and receiver window
must be at most one-half of 2
m
.
Note
In Selective Repeat ARQ, the size of the
sender and receiver window
must be at most one-half of 2
m
.
Note
11.51
Figure 11.22 Delivery of data in Selective Repeat ARQ
Figure 11.22 Delivery of data in Selective Repeat ARQ
11.52
ThisexampleissimilartoExample11.3inwhichframe1
islost.WeshowhowSelectiveRepeatbehavesinthis
case.Figure11.23showsthesituation.Onemain
differenceisthenumberoftimers.Here,eachframesent
orresentneedsatimer,whichmeansthatthetimersneed
tobenumbered(0,1,2,and3).Thetimerforframe0
startsatthefirstrequest,butstopswhentheACKforthis
framearrives.Thetimerforframe1startsatthesecond
request,restartswhenaNAKarrives,andfinallystops
whenthelastACKarrives.Theothertwotimersstart
whenthecorrespondingframesaresentandstopatthe
lastarrivalevent.
Example 11.8
ThisexampleissimilartoExample11.3inwhichframe1
islost.WeshowhowSelectiveRepeatbehavesinthis
case.Figure11.23showsthesituation.Onemain
differenceisthenumberoftimers.Here,eachframesent
orresentneedsatimer,whichmeansthatthetimersneed
tobenumbered(0,1,2,and3).Thetimerforframe0
startsatthefirstrequest,butstopswhentheACKforthis
framearrives.Thetimerforframe1startsatthesecond
request,restartswhenaNAKarrives,andfinallystops
whenthelastACKarrives.Theothertwotimersstart
whenthecorrespondingframesaresentandstopatthe
lastarrivalevent.
Example 11.8
11.53
Atthereceiversiteweneedtodistinguishbetweenthe
acceptanceofaframeanditsdeliverytothenetwork
layer.Atthesecondarrival,frame2arrivesandisstored
andmarked,butitcannotbedeliveredbecauseframe1is
missing.Atthenextarrival,frame3arrivesandis
markedandstored,butstillnoneoftheframescanbe
delivered.Onlyatthelastarrival,whenfinallyacopyof
frame1arrives,canframes1,2,and3bedeliveredtothe
networklayer.Therearetwoconditionsforthedeliveryof
framestothenetworklayer:First,asetofconsecutive
framesmusthavearrived.Second,thesetstartsfromthe
beginningofthewindow.
Example 11.8 (continued)
Atthereceiversiteweneedtodistinguishbetweenthe
acceptanceofaframeanditsdeliverytothenetwork
layer.Atthesecondarrival,frame2arrivesandisstored
andmarked,butitcannotbedeliveredbecauseframe1is
missing.Atthenextarrival,frame3arrivesandis
markedandstored,butstillnoneoftheframescanbe
delivered.Onlyatthelastarrival,whenfinallyacopyof
frame1arrives,canframes1,2,and3bedeliveredtothe
networklayer.Therearetwoconditionsforthedeliveryof
framestothenetworklayer:First,asetofconsecutive
framesmusthavearrived.Second,thesetstartsfromthe
beginningofthewindow.
Example 11.8 (continued)
11.54
AnotherimportantpointisthataNAKissentafterthe
secondarrival,butnotafterthethird,althoughboth
situationslookthesame.Thereasonisthattheprotocol
doesnotwanttocrowdthenetworkwithunnecessary
NAKsandunnecessaryresentframes.ThesecondNAK
wouldstillbeNAK1toinformthesendertoresendframe
1again;thishasalreadybeendone.ThefirstNAKsentis
remembered(usingthenakSentvariable)andisnotsent
againuntiltheframeslides.ANAKissentonceforeach
windowpositionanddefinesthefirstslotinthewindow.
Example 11.8 (continued)
AnotherimportantpointisthataNAKissentafterthe
secondarrival,butnotafterthethird,althoughboth
situationslookthesame.Thereasonisthattheprotocol
doesnotwanttocrowdthenetworkwithunnecessary
NAKsandunnecessaryresentframes.ThesecondNAK
wouldstillbeNAK1toinformthesendertoresendframe
1again;thishasalreadybeendone.ThefirstNAKsentis
remembered(usingthenakSentvariable)andisnotsent
againuntiltheframeslides.ANAKissentonceforeach
windowpositionanddefinesthefirstslotinthewindow.
Example 11.8 (continued)
11.55
ThenextpointisabouttheACKs.Noticethatonlytwo
ACKsaresenthere.Thefirstoneacknowledgesonlythe
firstframe;thesecondoneacknowledgesthreeframes.In
SelectiveRepeat,ACKsaresentwhendataaredeliveredto
thenetworklayer.Ifthedatabelongingtonframesare
deliveredinoneshot,onlyoneACKissentforallofthem.
Example 11.8 (continued)
ThenextpointisabouttheACKs.Noticethatonlytwo
ACKsaresenthere.Thefirstoneacknowledgesonlythe
firstframe;thesecondoneacknowledgesthreeframes.In
SelectiveRepeat,ACKsaresentwhendataaredeliveredto
thenetworklayer.Ifthedatabelongingtonframesare
deliveredinoneshot,onlyoneACKissentforallofthem.
Example 11.8 (continued)
11.56
Figure 11.23 Flow diagram for Example 11.8
Figure 11.23 Flow diagram for Example 11.8
11.57
Figure 11.24 Design of piggybacking in Go-Back-N ARQ
Figure 11.24 Design of piggybacking in Go-Back-N ARQ
11.58
11-6 HDLC
High-levelDataLinkControl(HDLC)isabit-oriented
protocolforcommunicationoverpoint-to-pointand
multipointlinks.ItimplementstheARQmechanisms
wediscussedinthischapter.
Configurations and Transfer Modes
Frames
Control Field
Topics discussed in this section:
11-6 HDLC
High-levelDataLinkControl(HDLC)isabit-oriented
protocolforcommunicationoverpoint-to-pointand
multipointlinks.ItimplementstheARQmechanisms
wediscussedinthischapter.
Configurations and Transfer Modes
Frames
Control Field
Topics discussed in this section:
11.59
Figure 11.25 Normal response mode
Figure 11.25 Normal response mode
11.60
Figure 11.26 Asynchronous balanced mode
Figure 11.26 Asynchronous balanced mode
11.61
Figure 11.27 HDLC frames
Figure 11.27 HDLC frames
11.62
Figure 11.28 Control field format for the different frame types
Figure 11.28 Control field format for the different frame types
11.63
Table 11.1 U-frame control command and response
Table 11.1 U-frame control command and response
11.64
Figure11.29showshowU-framescanbeusedfor
connectionestablishmentandconnectionrelease.NodeA
asksforaconnectionwithasetasynchronousbalanced
mode(SABM)frame;nodeBgivesapositiveresponse
withanunnumberedacknowledgment(UA)frame.After
thesetwoexchanges,datacanbetransferredbetweenthe
twonodes(notshowninthefigure).Afterdatatransfer,
nodeAsendsaDISC(disconnect)frametoreleasethe
connection;itisconfirmedbynodeBrespondingwitha
UA(unnumberedacknowledgment).
Example 11.9
Figure11.29showshowU-framescanbeusedfor
connectionestablishmentandconnectionrelease.NodeA
asksforaconnectionwithasetasynchronousbalanced
mode(SABM)frame;nodeBgivesapositiveresponse
withanunnumberedacknowledgment(UA)frame.After
thesetwoexchanges,datacanbetransferredbetweenthe
twonodes(notshowninthefigure).Afterdatatransfer,
nodeAsendsaDISC(disconnect)frametoreleasethe
connection;itisconfirmedbynodeBrespondingwitha
UA(unnumberedacknowledgment).
Example 11.9
11.65
Figure 11.29 Example of connection and disconnection
Figure 11.29 Example of connection and disconnection
11.66
Figure11.30showsanexchangeusingpiggybacking.
NodeAbeginstheexchangeofinformationwithan
I-framenumbered0followedbyanotherI-frame
numbered1.NodeBpiggybacksitsacknowledgmentof
bothframesontoanI-frameofitsown.NodeB’sfirst
I-frameisalsonumbered0[N(S)field]andcontainsa2
initsN(R)field,acknowledgingthereceiptofA’sframes
1and0andindicatingthatitexpectsframe2toarrive
next.NodeBtransmitsitssecondandthirdI-frames
(numbered1and2)beforeacceptingfurtherframesfrom
nodeA.
Example 11.10
Figure11.30showsanexchangeusingpiggybacking.
NodeAbeginstheexchangeofinformationwithan
I-framenumbered0followedbyanotherI-frame
numbered1.NodeBpiggybacksitsacknowledgmentof
bothframesontoanI-frameofitsown.NodeB’sfirst
I-frameisalsonumbered0[N(S)field]andcontainsa2
initsN(R)field,acknowledgingthereceiptofA’sframes
1and0andindicatingthatitexpectsframe2toarrive
next.NodeBtransmitsitssecondandthirdI-frames
(numbered1and2)beforeacceptingfurtherframesfrom
nodeA.
Example 11.10
11.67
ItsN(R)information,therefore,hasnotchanged:B
frames1and2indicatethatnodeBisstillexpectingA’s
frame2toarrivenext.NodeAhassentallitsdata.
Therefore,itcannotpiggybackanacknowledgmentonto
anI-frameandsendsanS-frameinstead.TheRRcode
indicatesthatAisstillreadytoreceive.Thenumber3in
theN(R)fieldtellsBthatframes0,1,and2haveallbeen
acceptedandthatAisnowexpectingframenumber3.
Example 11.10 (continued)
ItsN(R)information,therefore,hasnotchanged:B
frames1and2indicatethatnodeBisstillexpectingA’s
frame2toarrivenext.NodeAhassentallitsdata.
Therefore,itcannotpiggybackanacknowledgmentonto
anI-frameandsendsanS-frameinstead.TheRRcode
indicatesthatAisstillreadytoreceive.Thenumber3in
theN(R)fieldtellsBthatframes0,1,and2haveallbeen
acceptedandthatAisnowexpectingframenumber3.
Example 11.10 (continued)
11.68
Figure 11.30 Example of piggybacking without error
Figure 11.30 Example of piggybacking without error
11.69
Figure11.31showsanexchangeinwhichaframeislost.
NodeBsendsthreedataframes(0,1,and2),butframe1
islost.WhennodeAreceivesframe2,itdiscardsitand
sendsaREJframeforframe1.Notethattheprotocol
beingusedisGo-Back-NwiththespecialuseofanREJ
frameasaNAKframe.TheNAKframedoestwothings
here:Itconfirmsthereceiptofframe0anddeclaresthat
frame1andanyfollowingframesmustberesent.Node
B,afterreceivingtheREJframe,resendsframes1and2.
NodeAacknowledgesthereceiptbysendinganRRframe
(ACK)withacknowledgmentnumber3.
Example 11.11
Figure11.31showsanexchangeinwhichaframeislost.
NodeBsendsthreedataframes(0,1,and2),butframe1
islost.WhennodeAreceivesframe2,itdiscardsitand
sendsaREJframeforframe1.Notethattheprotocol
beingusedisGo-Back-NwiththespecialuseofanREJ
frameasaNAKframe.TheNAKframedoestwothings
here:Itconfirmsthereceiptofframe0anddeclaresthat
frame1andanyfollowingframesmustberesent.Node
B,afterreceivingtheREJframe,resendsframes1and2.
NodeAacknowledgesthereceiptbysendinganRRframe
(ACK)withacknowledgmentnumber3.
Example 11.11
11.70
Figure 11.31 Example of piggybacking with error
Figure 11.31 Example of piggybacking with error
11.71
11-7 POINT-TO-POINT PROTOCOL
AlthoughHDLCisageneralprotocolthatcanbeused
forbothpoint-to-pointandmultipointconfigurations,
oneofthemostcommonprotocolsforpoint-to-point
accessisthePoint-to-PointProtocol(PPP).PPPisa
byte-orientedprotocol.
Framing
Transition Phases
Multiplexing
Multilink PPP
Topics discussed in this section:
11-7 POINT-TO-POINT PROTOCOL
AlthoughHDLCisageneralprotocolthatcanbeused
forbothpoint-to-pointandmultipointconfigurations,
oneofthemostcommonprotocolsforpoint-to-point
accessisthePoint-to-PointProtocol(PPP).PPPisa
byte-orientedprotocol.
Framing
Transition Phases
Multiplexing
Multilink PPP
Topics discussed in this section:
11.72
Figure 11.32 PPP frame format
Figure 11.32 PPP frame format
11.73
PPP is a byte-oriented protocol using
byte stuffing with the escape byte
01111101.
Note
PPP is a byte-oriented protocol using
byte stuffing with the escape byte
01111101.
Note
11.74
Figure 11.33 Transition phases
Figure 11.33 Transition phases
11.75
Figure 11.34 Multiplexing in PPP
Figure 11.34 Multiplexing in PPP
11.76
Figure 11.35 LCP packet encapsulated in a frame
Figure 11.35 LCP packet encapsulated in a frame
11.77
Table 11.2 LCP packets
Table 11.2 LCP packets
11.78
Table 11.3 Common options
Table 11.3 Common options
11.79
Figure 11.36 PAP packets encapsulated in a PPP frame
Figure 11.36 PAP packets encapsulated in a PPP frame
11.80
Figure 11.37 CHAP packets encapsulated in a PPP frame
Figure 11.37 CHAP packets encapsulated in a PPP frame
11.81
Figure 11.38 IPCP packet encapsulated in PPP frame
Figure 11.38 IPCP packet encapsulated in PPP frame
11.82
Table 11.4 Code value for IPCP packets
Table 11.4 Code value for IPCP packets
11.83
Figure 11.39 IP datagram encapsulated in a PPP frame
Figure 11.39 IP datagram encapsulated in a PPP frame
11.84
Figure 11.40 Multilink PPP
Figure 11.40 Multilink PPP
11.85
Letusgothroughthephasesfollowedbyanetworklayer
packetasitistransmittedthroughaPPPconnection.
Figure11.41showsthesteps.Forsimplicity,weassume
unidirectionalmovementofdatafromtheusersitetothe
systemsite(suchassendingane-mailthroughanISP).
Thefirsttwoframesshowlinkestablishment.Wehave
chosentwooptions(notshowninthefigure):usingPAP
forauthenticationandsuppressingtheaddresscontrol
fields.Frames3and4areforauthentication.Frames5
and6establishthenetworklayerconnectionusingIPCP.
Example 11.12
Letusgothroughthephasesfollowedbyanetworklayer
packetasitistransmittedthroughaPPPconnection.
Figure11.41showsthesteps.Forsimplicity,weassume
unidirectionalmovementofdatafromtheusersitetothe
systemsite(suchassendingane-mailthroughanISP).
Thefirsttwoframesshowlinkestablishment.Wehave
chosentwooptions(notshowninthefigure):usingPAP
forauthenticationandsuppressingtheaddresscontrol
fields.Frames3and4areforauthentication.Frames5
and6establishthenetworklayerconnectionusingIPCP.
Example 11.12
11.86
ThenextseveralframesshowthatsomeIPpacketsare
encapsulatedinthePPPframe.Thesystem(receiver)
mayhavebeenrunningseveralnetworklayerprotocols,
butitknowsthattheincomingdatamustbedeliveredto
theIPprotocolbecausetheNCPprotocolusedbeforethe
datatransferwasIPCP.
Afterdatatransfer,theuserthenterminatesthedatalink
connection,whichisacknowledgedbythesystem.Of
coursetheuserorthesystemcouldhavechosento
terminatethenetworklayerIPCPandkeepthedatalink
layerrunningifitwantedtorunanotherNCPprotocol.
Example 11.12 (continued)
ThenextseveralframesshowthatsomeIPpacketsare
encapsulatedinthePPPframe.Thesystem(receiver)
mayhavebeenrunningseveralnetworklayerprotocols,
butitknowsthattheincomingdatamustbedeliveredto
theIPprotocolbecausetheNCPprotocolusedbeforethe
datatransferwasIPCP.
Afterdatatransfer,theuserthenterminatesthedatalink
connection,whichisacknowledgedbythesystem.Of
coursetheuserorthesystemcouldhavechosento
terminatethenetworklayerIPCPandkeepthedatalink
layerrunningifitwantedtorunanotherNCPprotocol.
Example 11.12 (continued)
11.87
Figure 11.41 An example
Figure 11.41 An example
11.88
Figure 11.41 An example (continued)
Figure 11.41 An example (continued)
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