datakommunikasjon høsten 2002

Datakom høsten 2002 1

Datakommunikasjon høsten 2002

Forelesning nr 7, mandag 30. septemberChapter 5, Link layer and Local Area Networks


ØvingsoppgaverOppgave 1Gitt M=11010101 P=10011 Regn ut FCS

Oppgave 2Gitt M=11110001 P=10011 Regn ut FCS

Oppgave 3Hvis hvordan mottaker kan sjekke at mottatt bitsekvens er riktig for

oppgave 1 og 2.

Oppgave 4P=10011 M=10110101

M+FCS som mottas er 101101011100 Er melding overført uten feil? Hvis hvordan du kan sjekke dette.


Flow ControlEnsuring the sending entity does not

overwhelm the receiving entity Preventing buffer overflow

Transmission time Time taken to emit all bits into medium

Propagation time Time for a bit to traverse the link


Model of Frame Transmission


Stop and WaitSource transmits frameDestination receives frame and replies

with acknowledgementSource waits for ACK before sending next

frameDestination can stop flow by not send ACKWorks well for a few large frames


Sliding Windows Flow ControlAllow multiple frames to be in transitReceiver has buffer W longTransmitter can send up to W frames

without ACKEach frame is numberedACK includes number of next frame

expectedSequence number bounded by size of field

(k) Frames are numbered modulo 2k


Sliding Window Diagram


Example Sliding Window


Sliding Window EnhancementsReceiver can acknowledge frames without

permitting further transmission RNR (Receive Not Ready)

Must send a normal acknowledge to resume

If duplex, use piggybacking If no data to send, use acknowledgement

frame If data but no acknowledgement to send, send

last acknowledgement number again, or have ACK valid flag (TCP)


Error DetectionAdditional bits added by transmitter for

error detection codeParity

Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones

Even number of bit errors goes undetected

Eksempel, One-bit even parity 0111000110101011 => 9 enere dvs. odde tall Parity bit =1


Cyclic Redundancy CheckFor a block of k bits transmitter generates

n bit sequenceTransmit k+n bits which is exactly

divisible by some numberReceive divides frame by that number

If no remainder, assume no error


Error detection


CRC Cyclic Redundancy CheckM= 10110101, melding som skal overføres

CRC generatorpolynom P= x4+x+1=10011

FCS (Frame Check Sequence) er rest av divisjonen

2n * M : P n=4

101101010000 : 10011 = 10101010

Gir rest på 1110, dvs FCS = 1110

Sender M + FCS = 101101011110


101101010000:10011=1010101010011 01011 00000 10110 10011 01011 00000 10110 10011 01010 00000 10100 10011 01110 00000 1110

FCS = 1110

Utregning

Overfører:

101101010000 (2nM)

+ 1110 (FCS)=101101011110


CRC polynomer

CRC-12 x12

+x11

+x3 +x

3+x+1

CRC-16CRC-CCITTCRC-32


Error ControlDetection and correction of errorsLost framesDamaged framesSamlebegrep: Automatic repeat

request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and

retransmission


Automatic Repeat Request (ARQ)Stop and waitGo back NSelective reject (selective retransmission)


Stop and WaitSource transmits single frameWait for ACKIf received frame damaged, discard it

Transmitter has timeout If no ACK within timeout, retransmit

If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1


Stop and Wait -Diagram

Benytter ACK0 og ACK1 som i sliding window


Stop and Wait - Pros and ConsSimpleInefficient


Go Back N Based on sliding windowIf no error, ACK as usual with next frame

expectedUse window to control number of

outstanding framesIf error, reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames


Go Back N - Damaged FrameReceiver detects error in frame iReceiver sends rejection-iTransmitter gets rejection-iTransmitter retransmits frame i and all

subsequent


Go Back N - Lost Frame (1)Frame i lostTransmitter sends i+1Receiver gets frame i+1 out of sequenceReceiver send reject iTransmitter goes back to frame i and

retransmits


Go Back N - Lost Frame (2)Frame i lost and no additional frame sentReceiver gets nothing and returns neither

acknowledgement nor rejectionTransmitter times out and sends

acknowledgement frame with P bit set to 1(P=1 betyr kommando)

Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )

Transmitter then retransmits frame i


Go Back N - Damaged AcknowledgementReceiver gets frame i and send

acknowledgement (i+1) which is lostAcknowledgements are cumulative, so

next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out, it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated


Go Back N - Diagram

RR=Receive ReadyREJ = Reject


Selective RejectAlso called selective retransmissionOnly rejected frames are retransmittedSubsequent frames are accepted by the

receiver and bufferedMinimizes retransmissionReceiver must maintain large enough

bufferMore complex logic in transmitter


Selective Reject -Diagram

RR = Receive ReadyREJ = RejectSREJ = Selective reject


High Level Data Link ControlHDLCISO 33009, ISO 4335


HDLC Station TypesPrimary station

Controls operation of link Frames issued are called commands Maintains separate logical link to each

secondary station

Secondary station Under control of primary station Frames issued called responses

Combined station May issue commands and responses


HDLC Link ConfigurationsUnbalanced

One primary and one or more secondary stations

Supports full duplex and half duplex

Balanced Two combined stations Supports full duplex and half duplex


HDLC Transfer Modes (1)Normal Response Mode (NRM)

Unbalanced configuration Primary initiates transfer to secondary Secondary may only transmit data in response

to command from primary Used on multi-drop lines Host computer as primary Terminals as secondary


HDLC Transfer Modes (2)Asynchronous Balanced Mode (ABM)

Balanced configuration Either station may initiate transmission

without receiving permission Most widely used No polling overhead


HDLC Transfer Modes (3)Asynchronous Response Mode (ARM)

Unbalanced configuration Secondary may initiate transmission without

permission form primary Primary responsible for line rarely used


Frame Structure Diagram

Start Stopp


Flag FieldsDelimit frame at both endsFlagg = 01111110May close one frame and open anotherReceiver hunts for flag sequence to

synchronize


Bitstuffing - transparens

Brukes i HDLC for å ungå problemer med like datafelt og flagg.

Mellom sending av start og stop flagg, senderen vil alltid sette inn en 0er etter fem 1ere: Etter å ha detektert start flagget gransker den bit strømmen: når det kommer et mønster på fem 1ere, ser den på det

6. er dette en 0er, slettes det er dette en 1er og det syvende en 0er, er dette et

godkjent flagg. dersom både 6 og 7 er 1ere, et abort signal sendt.


Bit Stuffing Example with

possible errors


Address Field Identifies secondary station that sent or will receive

frameUsually 8 bits longMay be extended to multiples of 7 bits

LSB of each octet indicates that it is the last octet (1) or not (0)

All ones (11111111) is broadcast


Control FieldDifferent for different frame type

Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames

Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

First one or two bits of control field identify frame type


Control Field Diagram


Poll/Final BitUse depends on contextCommand frame

P bit 1 to solicit (poll) response from peer

Response frame F bit 1 indicates response to soliciting command


Information FieldOnly in information and some

unnumbered framesMust contain integral number of octetsVariable length


Frame Check Sequence FieldFCSError detection16 bit CRCOptional 32 bit CRC


HDLC OperationExchange of information, supervisory and

unnumbered framesThree phases

Initialization Data transfer Disconnect


Examples of Operation (1)


Examples of Operation (2)

N(S), N(R)


Other DLC Protocols (LAPB,LAPD)LAPB - Link Access Procedure, Balanced

Part of X.25 (ITU-T) Subset of HDLC - ABM Point to point link between system and packet

switching network node (mellom DTE og DCE)LAPD - Link Access Procedure, D-Channel

Beskrevet i Q.921 spesifikasjonen ABM Always 7-bit sequence numbers 16 bit address field contains two sub-

addressesOne for device and one for user (next layer up)


ISDN – datalinklaget (lag 2)

FCS = Frame Check Sequence


LAPD commands and responses

I - Information

RR – Receive ReadyRNR – Receive Not ReadyREJ – Reject

SABME – Set Asynchronous Balanced ModeDM – Disconnect ModeUI – Unnumbered InformationDISC – DisconnectUA – Unnumbered acknowledgeFRMR – Frame RejectXID – Exchange Identification


Other DLC Protocols (LLC)Logical Link Control (LLC)

IEEE 802 Different frame format Link control split between medium access layer

(MAC) and LLC (on top of MAC) No primary and secondary - all stations are peers Two addresses needed

Sender and receiver

Error detection at MAC layer32 bit CRC

Destination and source access points (DSAP, SSAP)


Logical Link Control/Medium Access Control


Eksempel på LAPD

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