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

Link Management

Connection-Oriented Connectionless

Slide 8

Review of Protocol Types

Protocol Environments Byte oriented protocol Bit oriented protocol

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

Sliding Window Protocols

Frequently assumes a peer to peer protocol

Acknowledgements can be “piggybacked”

Slide 19

Bit-Oriented Protocols IBM’s Synchronous Data Link Control

(SDLC) ISO’s High-Level Data Link Control

(HDLC)– ITU’s Link Access Protocol-Balanced (LAP-B)– ITU’s Link Access Protocol for the ISDN D

channel (LAP-D)– ITU’s Link Access Protocol for Modems

(LAP-M)

Slide 20

HDLC

Bit oriented protocol Can operate in several modes

– Normal Response Mode (NRM)– Asynchronous Response Mode (ARM)– Asynchronous Balanced Mode (ABM)

Slide 21

HDLC Frame Types

Unnumbered frames– Used for connection establishment

and disconnection– Do not contain acknowledgements

Information frames– Carry information between nodes– Can use piggyback

acknowledgements in ARM and ABM

Slide 22

HDLC Frame Types

Supervisory frames– Used for error and flow control– Contain sequence numbers

Slide 23

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 24

Operation of HDLC Normal mode Components

– Received frame counter (V(R))– Sent frame counter (V(S))– Contents of frame

»V(R), S(R)»Frame type»Data

– Unacknowledged list

Slide 25

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 26

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 27

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 28

Modems Error Correction (LAPM)

DTE

Modem (DCE)

User InterfacePart(UIP)

ErrorCorr-ectingPart(ECP) Public

TelephoneNetwork

Modem (DCE)

DTE

User InterfacePart(UIP)

ErrorCorr-ectingPart(ECP)

Slide 29

Point to Point Protocol (PPP)

Used as the link layer in dialup Internet access

Slide 30

Summary

Description of a protocol Types of protocols Protocol primitives and

mechanisms Examples