Link Layer Protocols

Martin Weiss

Slide 2

Objectives of this Meeting

Explain what a protocol is Compare connection-oriented with

connectionless protocol types Describe some protocol

mechanisms Describe different link layer

protocols

Slide 3

Review of the Last Meeting

Error detection Flow control Performance of flow control

techniques

Slide 4

Data Link Layer Issues

Type Error detection and control Flow control

Slide 5

OSI Reference ModelEnd User Application Process

Application Services: FTAM, MHS, EDI

Data Representation, Transfer Syntax, Data TransformationDialog Synchronization and Control

End-to-End Message TransferNetwork Routing and Addressing

Data Link Control, Error Checking, FramingMechanical and Electrical Interface

Data Communications NetworkPhysical Connection

Network Independent Message Interchange

Syntax-Independent Message Interchange

Distributed Information ServicesApplication

PresentationSession

TransportNetworkLinkPhysical

OSI Layer

Slide 6

Structure of Protocols

Source AP Desintation AP

Application Process (AP) to AP Messages

Source LinkLayer

Desintation LinkLayer

L_DATA.request L_DATA.indicationLink Layer PDU’s

Physical Transmission Medium

Slide 7

Connectionless PrimitivesDTE A DTE B

Link Layer

DATA.request

DATA.indication

DATA.indication

DATA.request

Link LayerApp. Process App. Process

Slide 8

Connection-Oriented Primitives

DTE A DTE B

Link Layer

CON.request

CON.confirm

CON.indication

Link LayerApp. Process App. Process

DATA.request

DATA.indication

DATA.indication

DATA.request

Slide 9

Kermit Protocol Structure

SOH LEN SEQ TYPE data BCC CR

Slide 10

Kermit Details SOH = Start of Header LEN = Number of Bytes in the

Frame SEQ = Sequence Number of the

Current Frame

Slide 11

Kermit Details TYPE = Type of Frame

– S: Send Initiation Parameters– F: Filename– D: File Data– Z: End of File– B: End of Transaction– Y: ACK– N: NAK– E: Error (Fatal)

Slide 12

Kermit Details Data = Contents of Frame BCC = Block Check Character CR = End of Block Marker (ASCII

CR Character)

Slide 13

Operation of Kermit

Kermit

Time

KermitUser User

Kermit KermitConnect Connect

Receive

Send (File)

Data Block [1]Sent

Call Setup

V(S) = 0 V(R) = 0

V(S) = 1I[0,S,Init]

ACK[0,Y,Init]

I[1,F,Filename]ACK[1,Y]

V(R) = 1

V(R) = 2V(S) = 2

I[2,D,data]

ACK[2,Y]V(R) = 3

V(S) = 3 Data Block [1]Received

Slide 14

Operation of Kermit (Error)

Kermit

Time

KermitUser User

Data Block [1]Sent

V(R) = 2V(S) = 2 I[2,D,data]

ACK[2,Y]V(R) = 3

V(S) = 3

Data Block [2]Sent

I[3,D,data]

NACK[3,N]V(R) = 3

V(S) = 4

I[3,D,data]ACK[3,Y]

V(R) = 4

Data Block [1]Received

Data Block [2]Received

Slide 15

Operation of Kermit (Lost Frame)

Kermit

Time

KermitUser User

Data Block [1]Sent

V(R) = 2V(S) = 2 I[2,D,data]

ACK[2,Y]V(R) = 3

V(S) = 3

Data Block [2]Sent

I[3,D,data]V(S) = 4

I[3,D,data]ACK[3,Y]

V(R) = 4

Timeout

Data Block [1]Received

Data Block [2]Received

Slide 16

Operation of Kermit (Lost ACK)

Kermit

Time

KermitUser User

Data Block [1]Sent

V(R) = 2V(S) = 2 I[2,D,data]

ACK[2,Y]V(R) = 3

V(S) = 3

Data Block [2]Sent

I[3,D,data]V(S) = 4

I[3,D,data]

ACK[3,Y] V(R) = 4

Timeout

ACK[3,Y]V(R) = 4

Data Block [1]Received

Data Block [2]Received

Slide 17

Operation of Kermit

Destination Computer

Kermit

Time

KermitUser User

Data Block [n*]Sent

EOFExit

Data Block [n*]Received

EOF

EOTExit

V(S) = n* I[n*,D,data]ACK[n*,Y]

V(R) = n*I[(n+1)*,Z]

ACK[(n+1)*,Y]

I[(n+2)*,B]

ACK[(n+2)*,Y]

Slide 18

Structure of Bisync Information Frames

SYN SYN SOH Identifier Address STX Text ETX BCCSingle Block Message

SYN SYN SOH Identifier STX Text ETB BCCIntermediate of Multiblock Message

SYN = SynchronizationSOH = Start of HeaderSTX = Start of TextETX = End of TextETB = End of Text BlockIdentifier = Sequence Number of Block

First of Multiblock MessageSYN SYN SOH Identifier Address STX Text ETB BCC

End of Multiblock MessageSYN SYN SOH Identifier STX Text ETX BCC

Slide 20

HDLC Frame Format

Flag Address Control Information FCS Flag8 8/16 8/16 0 to N 16/32 8

Frame Format

Control Field for Normal Information Frame

0 N(S) N(R)P/F

Control Field for Normal Supervisory Frame

0 N(R)P/F1 S

Control Field for Normal Information Frame

MP/FM1 0

P/F = Poll/Final BitN(S) = Send Sequence NumberN(R) = Receive Sequence Number

S: RR RNR REJ SREJ

M: Defines 9 Commands and 4 Unnumbered Responses

Slide 21

Unidirectional Data (ACK and NAK)

Sender ReceiverUnACK’edList

V(S) V(R) UnACK’edList

V(S)I(0,0)

0 1 00 0 0

N(S) = V(R) => Frame OKRR(1)-

I(1,0)

I(2,0)

2 0

3 0N(S) = V(R) => Frame OK

N(S) = V(R) => Frame OK

1

1,2

RR(2)

RR(3)

2

-

V(R)0

0 1

0 2

0 3

(N(S), N(R))

Slide 22

Unidirectional Data (With Error)

Sender ReceiverUnACK’ed V(S) V(R) UnACK’ed

ListV(S)

I(3,0)3 4 0

3 0 0

I(4,0)5 0

N(S) = V(R) => Frame OK

3,4

3,4 REJ(3)

RR(5)-

V(R)3

0 4

(N(S), N(R))

N(S) = 4 V(R) = 3 ERROR!

I(3,0)

I(4,0)N(S) = V(R) => Frame OK

0 5

-

Slide 23

Example 2: Bidirectional Data

Sender ReceiverUnACK’edList

V(S) V(R) UnACK’edList

V(S)I(0,0)

0 1 00 0 0

I(0,1)-I(1,1)

I(2,1)2 1

3 1

1

1,2

I(1,3)-

V(R)0

0 1

1 2

2 3

(N(S), N(R))

1 1 01 1

-

1 3 -

13 2

RR(2) 2 3 -

Slide 24

Modems Error Correction (LAPM)

DTE

Modem (DCE)

User InterfacePart(UIP)

ErrorCorr-ectingPart(ECP) Public

TelephoneNetwork

Modem (DCE)

DTE

User InterfacePart(UIP)

ErrorCorr-ectingPart(ECP)