data link layer data link control protocolscis.bentley.edu/jgorgone/cs340/b/pdf/datalink.pdf ·...

Datalink8 Copyright 1998, Professor John T. Gorgone 1

DATA LINK LAYER

DATA LINK CONTROL PROTOCOLS


GOALS OF THE DATA LINK CONTROL LAYER

n To provide the basic communication function of moving information blocks over an unreliable communication channel and detecting bit errors introduced by the channel.

n To mange the orderly transmission and reception on channel with multiple attached transmitters and/or receivers.

n To be efficient, high performing and utilize the maximum available bandwidth of the communication channel.

n To be easy to implement, within hardware and by small microcomputers.


DATA LINK PROTOCOLS

Grammar of data communication.

Rules and procedures for control of data communication links.

Etiquette of data exchange on a communications link.


ELEMENTS OF NETWORK PROTOCOL

1. SyntaxCode and character setBlock structureMessage length and typeFramingCommand format

2. SemanticsHandshakingIdentificationInformation TransferTransparency ControlError controlTermination

3. TimingSynchronizationPrecedenceOrder of transmissionSequence controlFlow controlTime-out control


Requirements for communications between directly connected stations.

• Frame synchronization• Variety of line configurations

• Flow control

• Error control• Addressing

• Link management

• Control and data on same link


The data link control layer/protocol provides an envelope for the transmission of message blocks over a communication channel.It detects bit error introduced by the communication channel anddelivers only good blocks (those without errors).Basic functions performed by data link protocols are:

Framing of informationBit error detection

DATA LINK CONTROL FUNCTIONS

DataLink

Protocol

MessageBlocks

MessageBlocks

Communication Channel

DataLink

Protocol


ADDITIONAL DATA LINK CONTROL FUNCTIONS

n Error Correction

n Sequencing

n Receipt and Flow Control

– The correction of information blocks with bit errors and delivery of all blocks.

– The delivery of all blocks in proper sequence, without missing blocks or duplicate blocks, even when correcting for bit errors.

– The notification to the sender of the proper delivery of information blocks and control by the destination on the rate of information flow.


CONTROL FUNCTIONS (Continued)

n Addressing

n Pipelining and Performance

n Priority

n Security

– The addressing of an information block to one, some, or all possible destinations.

– The ability to send multiple blocks through the communication channel simultaneously and the ability of the link control layer to specify performance characteristics (delay, throughput).

– An interface to queue and order messages for transmission.

– The protection of data being sent over the channel.


DATA LINK CONTROL INFORMATION RESTRICTIONS

n Block Size

n Information representation

n Physical Channel Requirements

n Topology Constraints

– The maximum and minimum size data blocks accepted by the data link control mechanism.

– The codes sets, byte sizes and information formats accepted by the mechanism.

– The required characteristics of the physical media.

– Restriction on the paths of data flow due to the location of users and their connected channels.


STAND ALONE USE

Data link control protocols can also be used by themselves (outside network architectures) to create a single error free sequential path between two computers and/or devices.

DataLink

ProtocolUnreliable Channel

DataLink

Protocol

Reliable Sequential Path


RELATIONSHIP TO NETWORK ARCHITECTURE

Link control protocols (Modules) can be thought of as black box creating a sequential error - free data link from a channel that has some probability of introducing errors.

They are used within network architecture structures to create error-free paths between nodes connected by data channels. These point to point paths are used by the higher level layer to create end to end paths.

SwitchSwitchPT TO PT PATH PT TO PT PATHNode Node

PT TO PT PATH

End to End Path


Topology of a Data Link refers to the Physical Arrangement of stations in a link.

TOPOLOGY

Computer(Primary)

(1.) Point-to-Point

Computer

(2.) Multipoint or Multidrop

Secondary Device

SS

SSSS

SSS S S S


Duplexity of a link refers to the direction and timing of the signal flow.

DUPLEXITY

Simplex One direction

Half-Duplex Two-way alternate

Full-Duplex Two-way simultaneous


DATA LINK CONFIGURATIONS

P S

P transmits while S receives

P S

P receives while S transmits

P S

Both stations can transmit while they receive

(a) Half-Duplex (b) Full DuplexPoint-to-Point Point-to-Point


DATA LINK CONFIGURATIONS (Continued)

(c) Multi-multipoint (e) Multipoint Duplex

P S1

S2

S3

P S1

S2

S3

FDX FDXFDXHDX


DATA LINK CONFIGURATIONS (Continued)

(d) Multi-point half-duplex

P S1

S2

S3

P S1

S2

S3

HDX HDX HDX HDX


THREE PHASES OFCOMMUNICATION CONTROL

n Establishment

n Data Transfer

n Termination


Station 1 Station 2 Station 2 Station 1

Establishment

Data Transfer

Termination

ENQ

NAK ACK

NAK ACKInvalid orno reply

Invalid orno reply

Error Recovery

Error Recovery

Frame

EOTPoint-to-Point HDX


POLLING = PERMISSION

Primary Secondary Primary Secondary

Poll Poll

NAKData

ACK

Polled terminal hasnothing to send

Polled terminal hasdata to send

Multipoint link with one station as a primary station


Primary Secondary

Select

ACK

Data

ACK

Select

Primary Secondary

Select+

Data

ACK

Fast select

SELECTION = TO SEND DATA


HUB POLLING

Secondary

Primary

SecondarySecondary

Output Line

Input Line

A B C

An improvement over polling (Roll-call) each station.

NOTE: Only one poll issued by primary.


CONTENTION

n Contention is another form of line discipline.

n No primary station.

n All stations are peer (equal) stations.

n Any station may transmit if the line if free.

n Both transmitter and receiver stations must be identified (an address is needed for each).


ADDRESS IMPLICATIONSFOR LINE DISCIPLINE

Point-to-Point:No address is needed

Primary-Secondary Multipoint:One address is needed to identify the secondary station

Peer Multipoint:Two addresses are needed

Primary (Sender or/Transmitter)Secondary (Receiver(s))


FLOW CONTROL

Assures that a transmitting station does not overwhelm a receiving station with data.

Stop-and-Wait

n Each frame of data is acknowledgedn Only one frame outstanding

Sliding Window

n Station may send frames from an allowable "Window"n Multiple frames outstandingn Numbered frames

1.

2.


t0

+at0

+1t0

+1+a

+1+2at0

t0

+1t0

+at0

+1+at0

+1+2at0

t0


The bit length (distance to travel) is less than the frame length (message) or the frame length is greater than the bit length. Example: LANS

The frame length (message) is less than the bit length (distance to travel) or the bit length is greater than the frame length. Example: Satellite


ExampleStop-and-wait Protocol1000-bit frame1 megabit satellite channel

Time(Millisecond) Event

0 A starts sending frame1 Last bit sent; A waits

270 1st bit arrives at B271 Last bit arrives at B271 B send ACK541 ACK arrives at A

1. How long does station A wait ? ms?2. What is the channel utilization = ? %?


ExampleStop-and-wait1000-bit frame1 megabit satellite channel

Time(Millisecond) Event

0 A starts sending frame1 Last bit sent; A waits

270 1st bit arrives at B271 Last bit arrives at B271 B send ACK541 ACK arrives at A

1. How long does station A wait = 541-1 = 540ms 2. What is the channel utilization =1/541 or <1%


Sliding Window Protocol

• With stop & wait protocol serious inefficiencies of link utilization will occur if the bit length of the link is greater than the frame length.

• To improve link utilization multiple frames must be in transit at one time.

• This technique is known as the sliding window protocol.


How does this work for two stations, A & B?

• Assume full-duplex link.• B needs buffer space for n frames.• A can send n frames without waiting for an ACK.

• To keep track of acknowledged frames, each has a sequence number.

• B acknowledges a frame by sending an ACK with a sequence number of the NEXT frame EXPECTED. This announces that B is prepared to receive the next n frames beginning with the number specified.


Sliding Window continued

• Multiple frames can be acknowledged with this scheme.

• A keeps a list of sequence numbers it is allowed to send.

• B keeps a list of sequence numbers it is prepared to receive.

• These lists are thought of as a window of frames, hence the name: Sliding Window Flow Control.


A Restriction

• The sequence number to be used occupies a field in the frame and it is bounded by size.

• For a n-bit field, the sequence number can range from 0 to (2n-1). For example, a 3-bit sequence number field will range from 0 to 7. And after 7 is zero again.

• The next example assumes a 3-bit sequence field and a maximum window size of seven.


A may send 7 of 8 frames.Frame #s 0,1,2,3,4,5,6,7

Sending Machine A Sending Machine B

A shrinks its list witheach transmission. A can still send 4 more frames,#s 3,4,5,6.

A expands its counter witheach acknowledgment andmay send 7 frames starting with #s 3,4,5,6,7,0,1. A sends #s 3 thru 0 and shrinks its list to one frame, sequence #1. A can send only one more frame.

In the mean time, A receives ACK4 and expands its counter.

A may now only send 2 more frames.

B is prepared to receive7 frames. Frames #s 0,1,2,3,4,5,6

B acknowledges all3 frames (ACK3) and adjustsits list to receive 7more. Frame #s 3,4,5,6,7,0,1.

Ns = Sequence No. of frame sent

Nr = Sequence No. of next frame Expected

Nr = 4

Ns = 7

Ns = 0

Ns = 3

Ns = 4

Ns = 5

Ns = 6

Nr = 3

Ns = 0

Ns = 2

Ns = 1

Ns = 2

Ns = 1

SLIDING WINDOW FLOW CONTROL

ACK3

ACK4

B decides to limit the flow of frames and sends an ACK4.

It adjusts its list to receive 7 frames, #s 4,5,6,7,0,1,2


1. Sender transmits 8 frames, frame #s 0 - 7.2. All 8 frames arrive and are ACKnowledged.3. All ACKs for the 8 frames are lost.4. Sender must Time Out and Retransmit frame #s 0 - 7.5. Receiver unknowingly accepts the 8 DUPLICATE frames!

Why can you only send 7 of 8 frames at any one time?

Example


1. Sender transmits 8 frames, 0 - 7.2. All 8 frames arrive and are ACKnowledged.3. All 8 ACKs are lost.4. Sender must Time Out and Retransmit frames #s 0 - 7.5. Receiver unknowingly accepts the 8 DUPLICATE frames!

Why can you only send 7 of 8 frames at any one time?

Example

Solution: Restrict the window size to 2 - 1 or 7 outstanding frames.

2 - 1 XXX = 3 position field son = 3

2 - 1 = 2 - 1= (2 X 2 X 2) - 1= 8 - 1= 7

n

n

n

3


To determine the maximum window size use 2 - 1.

For a 3-bit position field it is 7.

For a 6-bit position field:

Max. window size: 2 - 164 - 163

For a 7-bit position field it would be: 2 - 1128 - 1127

n

6

7


ERROR CONTROL

Automatic Repeat Request (ARQ)

n Positive Acknowledgment (ACK)

n Negative Acknowledgment (NAK)

n Time-out - Retransmit Frame

n Frame Labeling


ERROR CONTROL TECHNIQUES

nStop-and-Wait ARQnFrames numbered 0,1

nContinuous ARQ

Go-Back-NSelective Repeat


1

1

1

0

0

0

A B

A B

A B

1

1

NAK1

Frames queued for transmission

1

1

1

Frames awaitingACK/NAK

Frames intransit

Frames Retained

Frame 1retransmitted

Frame 1in error,

NAK1 sent

(a) Stop-and-Wait ARQ


1

1

0

0

1

0

A B

A B

A B

1

0

ACK0


0

1

1


Frames intransit

Frames Retained

Receiving Frame 0

Frame 1is ok.,

ACK0 sent

(a) Stop-and-Wait ARQ


7

6

5

43

2

1

7

654

3

2

7

654

3

2

A B 1

ACK 2

4 3 2

A B 1

NAK 2

5 4 3

A B 1

ACK 5

4 3 2

Retransmit

Frame 2 in error,NAK2 is sent and

Frame 3, 4 and5 are discarded

(b) Go-Back-N Continuous ARQ



Frames intransit

Frames Retained


0 1 2 3 4 5 32 4 65 7 0

0 1 32 4 65E D D D

ACK 1 ACK 2 NAK 2 ACK 3 ACK 4 ACK 5 ACK 6

Error DiscardedFrames


7

6

5

43

2

1

7

654

3

2

3

2

76

5

4

A BACK 2

4 3 2

A BACK 6

7 6 2

A BNAK 2

5 4 3

(c) Selective-repeat Continuous ARQ



Frames intransit

Frames Retained

543

1

1

1

Frame 1 is valid,ACK 2 sent

Frame 2 is in error, NAK2is sent

Frame 3,4 and 5retained until 2arrives correctlyand ACK6 willacknowledge2, 3, 4 and 5


SELECTIVE REPEAT ARQ

0 1 2 3 4 5 2 6 7 0

0 1 3 4 5 2 6E

ACK 1 ACK 2 NAK 2 ACK 6

Error Buffered byReceiver

Frames 2-5released

NOTE: CUMULATIVE ACKs are not used becauseframes may be accepted out of order.


WINDOW SIZE FOR SELECTIVE REPEAT Example

1. Sender transmit frame #s 0 thru 6.2. Receiver ACKs all seven frames.3. All ACKs are lost.4. Sender T.O. and retransmit frame #s 0 thru 6.5. Receiver ready to accept new frame #s 7, 0, 1, 2, 3,

4, 5 (sequence numbers 7 - 13).6. Receiver assumes that frame #s 0 thru 5 are new

frames when in fact they are duplicates.


SOLUTION

Restrictive window size:

Maximum window size should be NO MORE THAT 1/2 THE RANGE OF THE SEQUENCE

NUMBERS.

If range is 0 - 7, then (1/2 * 8/1) only 4 frames can be outstanding.


Restrictive Window Size For Selective Repeat

Example

1. Sender transmit frame #s 0 thru 3.2. Receiver ACKs all four frames.3. All ACKs are lost.4. Sender T.O. and retransmit frame #s 0 thru 3.5. Receiver ready to accept new frames #s 4,5,6,7

(sequence numbers 4 - 7).6. Receiver discards frame #s 0 thru 0 as duplicates

since it is waiting on frame #4.7. Receiver sends ACK4.


BIT-ORIENTED LINK CONTROL PROTOCOLS

n HDLC = High-level Data Link Control (ISO 3309, ISO 4335)

n ADCCP = Advanced Data Communication Control Procedures.(ANSI X3.66), -- Same as HDLCAdopted by U.S. National Bureau of Standards(FIPS PUB 71-1) and by Federal TelecommunicationsStandards Committee (FED-STD-1003A)

n LAP-B = Link Access Procedure, Balanced(ITU-T/CCITT) -- Subset of HDLC and is part of the X.25packet-switched network standard.

n SDLC = Synchronous Data Link Control(IBM) -- Subset of HDLC but is not a standard


EXAMPLES OF CHARACTER ORIENTED PROTOCOLS

ANSI-DLC (x3.28)

ECMA-16, and ISO IS1745

IBM BSC

IATA SLC

DEC DDCMP

Basic mode control procedures for data communication systems

Binary Synchronous Communication

Synchronous Link Control

Digital Data Communications Message Protocol (Byte count oriented)

1.

2.

3.

4.

5.


ANSI X34-1976COMMUNICATIONSCONTROLCHARACTERS

CHARACTER MEANING CODE(H…I)

SOHSTXETXEOTENQACKDLENAKSYNETB

Start of Heading Start of TextEnd of TextEnd of TransmissionEnquiryAcknowledgment Data Link Escape Negative ACK Synchronous IdleEnd of Transmission Block

Block

00000010000010000001100001000000101000011000100000010101001011000101110010111


OTHER COMMUNICATIONCHARACTERS

DLE< Reverse InterruptDLE; WACK (Wait Positive Acknowledgment)DLE= Start of Transmission Block

DC1 XOnDC3 XOffDC2 Enable PeripheralDC4 Disable Peripheral


HDLC Basic Characteristics

n Three station types:n Primary Station, Secondary Station, and

Combined Stationn Two link configurations:

n Unbalanced: for point-to-point & multipointoperations

n Balanced: used only in point-to-point operations

n Three data transfer modes of operation:n NRM, ABM, and ARM


HDLC LINK CONFIGURATIONS

(a) Unbalanced configuration - Multipoint or Point-to-Point

(b) Balanced configuration - only used in Point-to-Point (FDX)

Responses

Commands

Responses

Commands

~ ~Primary

SecondarySecondary

Combined Combined


HDLC MODES (DATA TRANSFER MODES)

n Normal Response Mode (NRM)– Unbalanced configuration– Primary initiates all exchanges (Polling & Selecting)– Secondary may only respond to a poll from the secondary – Used on Multipoint lines and on Point-to-Point links (host to terminal)

n Asynchronous Balanced Mode (ABM)– Balanced configurations– Combined stations (Peer), either may initiate transmission– Full-Duplex Point-to-Point links

n Asynchronous Response Mode (ARM)– Unbalanced configurations– Secondary may initiate an exchange (Hub Polling, Other)


HDLC FRAME FORMAT

Flag Address Control Data FCS Flag

Header Information Trailer

8 8 or 16 Variable 816 or 32

BitsFlag - 8 bits.Address - 8 extendible bits, one or more octets.Control - 8 or 16 bits.Data - Variable bits, 0 to infinity, usually in multiples of 8 bits.FCS - 16 or 32 bits. The frame check sequence is 16 bit CRC or 32 bit CRC.Flag - 8 bits.

8…n


ADDRESS FIELD

Normal - 8 bits long.

Extended - address length in multiples of seven bits.

All-stations address.

All 1s = Primary sending to all secondary stations.

1st bit

8 bits

1st bit

- - - - - - - -

1 1 1 1 1 1 1 1

1 = Last Octet ofaddress field

0 = Count Octet as partof address field

0 0 07 Bits 7 Bits 7 Bits


Three Types of HDLC Frames

n Information Frame or I-Frame

n Supervisory Frame or S-Frame

n Unnumbered Frame or U-Frame

n Identification of the frame is determined in the Control Field.


CONTROL FIELD

I: Information

S: Supervisory

U: Unnumbered

3-Frame Types1 2 3 4 5 6 7 8

0

1

11

0

N(S)

S

M M

N(R)

N(R)

P/F

P/F

P/F

N(S) = Send sequence numberN(R) = Receive sequence numberS = Supervisory function bitsM = Unnumbered function bitsP/F = Poll/final bitP = Poll (primary)F = Final (secondary)


CONTROL FIELD FORMAT

1 2 3 4 5 6 7 80

1

1 1

0

N (S)

S

M

P/F

P/F

P/F

N (R)

N (R)

M

3 TypesI: Information (User Data)

S: Supervisory

U: Unnumbered

Identify frame types

N(S) = Send sequence numberN(R) = Receive sequence numberS = Supervisory function bitsM = Unnumbered function bitsP/F = Poll/final bitP = Poll (Primary)F = Final (secondary)


CONTROL FIELD FORMAT - EXTENDED

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Identify frame types

0

1 0 0 0 0S P/F

P/F

N (R)

N (R) Extended Information

Extended Supervisory

N(S) = Send sequence numberN(R) = Receive sequence numberS = Supervisory function bitsM = Unnumbered function bitsP/F = Poll/final bitP = Poll (Primary)F = Final (secondary)


Flag Fields: BIT STUFFING

Flag Pattern: 01111110 - Unique Pattern- Used to Syn

Original Stream:

. . . 111111111111011111101111110. . .Transmitter will insert an extra 0 bit after sending five 1's (except for flag fields)

HDLC allows arbitrary bit patterns. So, the flag pattern may appear inside a frame, thus destroying frame-levelsynchronization. To avoid this problem, bit stuffing is used.


BIT STUFFING (Continued)

After Bit-Stuffing: Inserted extra 0 bit

. . .11111011111011011111010111110. . . .

Receiver detects 5 1's and examines 6th bitIf 6th bit = 0 then delete,If 6th bit = 1 and 7th bit = 0 then a flag,If 6th bit = 1 and 7th bit = 1 then send abort condition

With the use of bit stuffing, arbitrary bit patterns can be used in the data field--this is known as data transparency.


Examples

Flag Flag

Flag FlagFlag

Flag Flag

Flag FlagFlag

(a) An inverted bit splits a frame in two

(b) An inverted bit merges two frames

Transmitted frame

bit inverted

received frame

Transmitted frame

bit inverted

received frame

Two pitfalls of bit stuffing

HDLC COMMANDS AND RESPONSES

NAME FUNCTION DESCRIPTION

Information (I) C/R Exchange user dataSupervisory (S)

Receive Ready (RR) C/R Positive acknowledgment; ready to receive I-frameReceive Not Ready (RNR) C/R Positive acknowledgment; not ready to receive

Reject (REJ) C/R Negative acknowledgment; go back NSelective Reject (SREJ) C/R Negative acknowledgment; selective repeat

Unnumbered (U)Set Normal Response/

Extended mode (SNRM/SNRME)C Set mode; extended = two octet control field

Set Asynchronous Response/Extended mode (SARM/SARME)

C Set mode; extended = two octet control field

Set Asynchronous Balanced/Extended Mode (SABM/SABME)

C Set mode; extended = two octet control field

Set Initialization Mode (SIM) C Initialize link control functions in addressed stationDisconnect (DISC) C Terminate logical link connection

Unnumbered Acknowledgment (UA) R Acknowledges acceptance of one of the above set-mode commands

Disconnected Mode (DM) R Secondary is logically disconnectedRequest Disconnect (RD) R Request for DISC command

Request Initialization Mode (RIM) R Initialization needed; request for SIM commandUnnumbered Information (UI) C/R Used to exchange control information

Unnumbered Poll (UP) C Used to solicit control informationReset (RSET) C Used for recovery; resets N(R), N(S)

Exchange Identification (XID) C/R Used to request/report identity and statusTest (TEST) C/R Exchange identical information fields for testing

Frame Reject (FRMR) R Reports receipt of unacceptable frame


TABLE 2HDLC Command/Response Repertoire

FormatControl Field

Bit Encoding**1 2 3 4 5 6 7 8

Commands Responses

0 - N(S) - * - N(R) - I - Information

1 0 0 0 * - N(R) -1 0 0 1 * - N(R) -1 0 1 0 * - N(R) -1 0 1 1 * - N(R) -

RR - Receive ReadyREJ - RejectRNR - Receive Not ReadySREJ - Selective Reject

1 1 0 0 * 0 0 0 1 1 0 0 * 0 0 11 1 0 0 * 0 1 01 1 0 0 * 1 0 01 1 0 0 * 1 1 01 1 0 0 * 1 1 11 1 0 1 * 0 0 0 1 1 0 1 * 0 0 11 1 0 1 * 0 1 01 1 0 1 * 0 1 11 1 1 0 * 0 0 01 1 1 0 * 0 0 11 1 1 1 * 0 0 01 1 1 1 * 0 0 11 1 1 1 * 0 1 01 1 1 1 * 0 1 11 1 1 1 * 1 0 01 1 1 1 * 1 0 11 1 1 1 * 1 1 0

UI - Unnumbered Information

RD - Request Disconnect

UA - Unnumbered AcknowledgeTEST - Data Link TestNon Reserved 0Non Reserved 1Non Reserved 2Non Reserved 3RIM Request Initialization ModeFRMR - Frame RejectDM - DISC. Mode

XID - Exchange Identification

NOTE: - * Bit 5 represents the position of the poll/final bit.** The bit order is shown as presented in the text of HDLC.

Bit 1 is the low-order bit and is transmitted first.

Unnumbered

Supervisory

Information I - Information

UI - Unnumbered InformationSNRM - Set Normal Response ModeDISC - DisconnectUP - Unnumbered Poll

TEST - Data Link TestNon Reserved 0Non Reserved 1Non Reserved 2Non Reserved 3SIM - Set Initialization Mode

SARM - Set ASYNC Response ModeRSET - ResetSARME - Set Arm Extended ModeSNRME - Set NRM Extended ModeSABM - Set ASYNC Balanced ModeXID - Exchange IdentificationSABME - Set ABM Extended Mode

RR - Receive ReadyREJ - RejectRNR - Receive Not ReadySREJ - Selective Reject


PIGGYBACKING OR ACKNOWLEDGMENTS

Station A

4 FramesTransmitted

No Frames Received

Station B

3 FramesTransmitted

2 Frames Received

Ns=4

Nr=1

Ns=3

Nr=1

Fr.4 Fr.3

Nr=3

Ns=1

Nr=3

Ns=3

Nr=3

Ns=2

Fr.1 Fr.3Fr.2


PIGGYBACKED ACKNOWLEDGMENT

0,0

1,0

2,0

3,0

4,1

5,2

6,2

7,2 2,7

0,3

1,4

N(S),N(R)

All Information Frames


SUPERVISORY FRAMES (S-FRAME)

1. Receive Ready = Positive acknowledgment (RR)

2. Receive Not Ready = Positive acknowledgment (RNR) and Send no more I-frames until subsequent RR is sent

3. Reject = Go-Back-N ARQ (REJ)

4. Selective Reject = Selective-repeat ARQ (SREJ)

S-Frame is used for Flow and Error Control:


UNNUMBERED FRAMES

1. Mode-setting Commands and Responses

2. Information transfer Commands and Responses

3. Recovery Commands and Responses

4. Miscellaneous Command and Responses


LINK SETUP AND DISCONNECT EXAMPLE

B,SABME

B,SABME,P

A,UA,F

Data Transfer

A,UA

B,DISC

Station A Station B

T.O.


TWO WAY DATA EXCHANGE EXAMPLE

B,1,0,0Station A Station B

A,1,0,1

B,1,1,1

B,1,2,1

A,1,1,3

B,1,3,2

A,1,2,4

B,RR,4

A,1,3,4


BUSY CONDITION EXAMPLE

Station A Station BA,1,4,0

B,RNR,4

A,RR,0,P

B,RNR,4,F

B,RR,4

A,1,4,0


TIME-OUT RECOVERY EXAMPLE

Station A Station BB,1,2,0

A,RR,3

B,1,3,0

B,RR,0,P

A,RR,3,F

B.1.3.0.P

T.O.

A,RR,4,F


KEY

SABM = Set Asynchronous Balanced Mode

UA = Unnumbered Acknowledge

DISC = DISConnect


HDLC MODES (DATA TRANSFER MODES)

n Normal Response Mode (NRM)– Unbalanced configuration– Primary initiates all exchanges (Polling and Selecting)– Used on Multipoint lines and Point-to-Point links (host to terminal)

n Asynchronous Balanced Mode (ABM)– Combined stations– Balanced configurations– Full-Duplex Point-to-Point links

n Asynchronous Response Mode (ARM)– Secondary may initiate an exchange (Hub Polling, Other)– Primary responsibility for link maintenance


LAYER INTERACTIONS PROTOCOL ACTIONS LAYER INTERACTIONS

System A

Network LinkLayer Layer

System B

Link Network Layer Layer

Connect Request

Connect Confirm

Data Request

Data Confirm

Disconnect Request

Disconnect Confirm

SABM

UA

Information Frame (s)

Acknowledge Frame (s)

DISC

UA

Connect Indicate

Data Indicate

Disconnect Indicate

EXAMPLE


SYSTEM ASYSTEM B

Data Message

EOT

ENQ

ACK0

Wish to Send

Okay

ACK0

Data Message

ACK1

Received OK

AcknowledgeData Message

ACK0

Received OK

AcknowledgeData Message

WACK

Received OK but not ready next msge

Acknowledge wit waitENQ

WACK

Received OK still busy

Acknowledge wit waitENQ

ACK1

Received OK

Okay to send

Received OK

AcknowledgeFinished transmitting

EXAMPLE


Typical Message Traffic Controlled by Data Link Control Protocol

Computer System 1 Computer System 2

Hello, I wish to connectSTARTUP

ACK

OkayENQI wish to send data

Okay

ACK

Data Message 1Sending first message

ACK

Data Message 2 It is okay

*Sending second message

NAK

Second message had CRC errorData Message 2Sending second message again

ACK

Your message was not correctEOT

I am finished transmittingACK

OkayACK

ACK

Data Message 1 I am here, too

I am here


Feature Comparison of Data Link Control Procedures

Feature BSC (IBM) DDCMP (DEC) HDLCTransmission Technique

Async/sync Async/sync Sync

Transmission Mode Half duplex Half/full duplex Half/full duplex

FramingStartStop

SYNSYNCharacters

SYNSYNCount

FlagFlag

Frame Formats Numerous 1 (three types) 1 (three types)Control Information Header, control

framesHeader (8 bytes) Header (1-2

bytes)Addressing Contention or polling Header HeaderError Detection LRC, CRC-16, CR-12 CRC-16 CRC-CCITTError Checking Transmission block

onlyHeader and data field

Entire frame

Error Control Stop and wait Go-Back-N Go-Back-N or selective repeat

Flow Control WACK Sliding window Sliding windowWindow Size 1 255 7 or 127Transparency DLE Count Bit stuffing

Character-Oriented Link Control

data link layer data link control protocolscis.bentley.edu/jgorgone/cs340/b/pdf/datalink.pdf ·...

Documents