Data Link Layer Protocols

transmitted in a retransmission list using FIFO S returns an ACK for each correctly received I‐frame Each I‐frame contains a unique identifier which is returned is the On receipt of the

Data Communication

and Networ

1

Cont ent

 Chapter 3: Data Link

2

Framing

 The data link layer needs to

pack bits into frames, that each frame is s o

Flow and Error

Control  responsibilities of the data link The most important

layer are flow control and error

control Collectively,

these functions are known as data link control.

procedures used to restrict the amount of data that the sender can send before

waiting for acknowledgment.

based on automatic repeat request, which is the retransmission of data.

can combine framing,

flow control, and error control to

achieve the delivery of data

from one node to another The

protocols are normally

4

Error

Control  Forward Error Control (FEC)Usually used in real‐time

application (e.g voice, video)

 Error Detection + ARQ (Automatic Retransmission

transmission

email, file

transmission)

– Go‐back N: e.g HDLC, V.42 – Selective‐Repeat: TCP, Service Specific Connection Oriented

Request)

5

I ‐ Notations

fram e

I frame: data/Information frame

I(N): Data frame with sequence N

sendi





response/reply sent from S back to

P, confirm receiving a good I‐frame

NAK (Negative Acknowledge) frame: response/reply sent

6

7

8

Idle RQ

 Advantages :

(Stop and Wait)

9

U

= Tt T1+2Tix +2Tp

p /T ix 1+2a

 Total average delay

per frame?Telecomm

1 0

RQ

order to transmit 1 I‐

frame successfully, sender needs to

Probability that i transmission are

needed to deliver frame

successfully ( i‐1 transmission

in error and the

1 2

Idle‐RQ:

Example  be transmitted using A series of 1000‐bit frames is to

an idle RQ protocol Determine the link utilization for

the following types of data link



Solutio

send and acknowledge data

 Piggy back methods:

header of information frames

from a data frame and an ACKframe (addresses, CRC, etc) can be

1 4

us RQ

 Continuous RQ improves the link utilization at the expense of

increased buffer storage requirements.

transmitted in a retransmission list (using FIFO)

S returns an ACK for each correctly received I‐frame

Each I‐frame contains a unique identifier which is returned is the

On receipt of the next in‐sequence I‐frame expected, S delivers the

information content within the frame to the upper layer

1 5

1 6

Continuous

RQ  utilization of In the absent of errors, the link

continuous RQ is approximately

100%

 When errors occur:

 S detects and requests the retransmission of just those

frames in the sequence that are corrupted: Selective

Repeat

 S detects the receipt of an out‐of‐sequence I‐frame

and requests P to retransmit all outstanding unacknowledged

Continuous

RQ  implemented in 2 ways: Selective Repeat: can be

 S acknowledges correctly received frames and P

determines from the sequence of ACK‐frames

received that a frame has been lost: implicit

retransmission

for a frameseque that is missing from the

nce: expli cit requ est

1 8

Continuous

RQ





Assume I‐frame N+1 is corrupted

S returns a ACK for each correctly received I‐frame as before

S returns ACK‐frame for I‐frames N,N+2,N+3,…

On receipt of ACK for I‐frame N+2, P detects that frame N+1 has not been acknowledged

To allow for the possibility of more than one I‐frame being corrupted,

on detecting an unacknowledged frame P enter the retransmission

state.

When in this state, the transmission of the new frames is suspended until all unacknowledged frames have been retransmitted

P removes I‐frame N+2 from the retransmission list and retransmits I‐

frame N+1 before transmitting frame N+5

On receipt of I‐frame N+1, the contents of the queued frames in the link receive list are delivered by S

2 0

ves the frame from the retransmission list.

2 2

 An ACK acknowledges all frames in the

retransmission list up to and

including the I‐frame with the sequence number the ACK contains

Assume I‐frame N+1 is corrupted

S returns an ACK for I‐frame N

When S receives I‐frame N+2 it detects I‐frame N+1 is missing and hence

returns a NAK containing the identifier of the

missing I‐frame N+1

On receipt of NAK N+1, P interprets this as S is still awaiting I‐frame N+1

and hence retransmits it

When S returns a NAK, it enters the

2 3

2 4

2 5

In many application, frames must be

delivered in order Hence,

2 6

Continuous

RQ

 Go‐back‐N: the secondary detects an out‐of‐order

sequence frame, it informs

the primary to start to retransmit frames from a

specified frame number It

does this be returning a special NAK frame known

Assume I‐frame N+1 is corrupted

S receives I‐frame N+2 out of sequence

On receipt of I‐frame N+2, S returns NAK N+1

informing P to go back and

start retransmit from I‐frame N+1

On receipt of NAK N+1, P enters the

retransmission state

When in this state, it suspends sending new

frames and starts to retransmit

the frames waiting acknowledgement in the

retransmission list

S discards frames until it receives I‐frame N+1

On receipt of I‐frame N+1, S resumes accepting frames and returning

acknowledgements

A timeout is applied to NAK frames by S and a

second NAK is returned if

2 7

2 8

2 9

Continuous

RQ  Link Utilization: In general, the link utilization U

for a send window K:

 Using Selective Repeat:

3 0

•

Example of sliding window

 A series of 1000‐bit frames is to

be transmitted using

a continuous RQ protocol

Determine the link

efficiency for the following type of data link if the

velocity of propagation is 2.108

m/s and the bit error

rates of the links are all negligibly low:

3 2

Example of link utilization

 A series of 1000‐bit frames is to

Time out

expires, the frame is retransmitted

 The timeout interval must be greater than the worst‐case

propagation delay between transmitting a frame and receiving the associated ACK

 S may receive multiple copies due to corrupted ACK‐frames

• With Go‐back‐N: this is not the problem The N(S) in

the duplicate frames will not equal to the current V(R) held by S and will be discarded

• With Selective Repeat: S retains a ordered list of the

last N correctly received I‐frames In this way, S can check if a received frame is a duplicate of an already correctly received frame or a new frame.

3 4

Timeout

 Control the data transmission rates of characters or Flow control

frames on a link so that the receiver always has

sufficient buffer storage resources to accept them prior

to processing

 Two kinds: X‐ON/X‐OFF and Sliding Window

 When the receive buffer at S is overflowed, S sends X‐OFF

back to P, P stops the data

transmission

 When S escape the overload state, it will return X‐ON to P

to inform its ready state for receiving data P will continue

3 6

 Sliding Flow control

window: Send window size

(K): define

the maximum number

of frames can be

transmitted Has UWE and LWE

to control the window

length Receive window: the maximum number

of frame buffers

3 7

Sequence

Number  All previous example: assuming that the next frame sequence

is simply equal to the last sequence + 1: sequence number go

to infinitive

Need to define the maximum limit on

the number of I‐frames

being transferred across a link: limit the size of the link

retransmission and receive list, and limit the range of

sequence numbers to define frames

uniquely

 Idle RQ: both send and receive window are

1 Hence 2 identifiers are required to allow S to determine the last I‐

frame and the new I‐frame.

 Go‐back‐N: send window is K: the identifier

must be K+1.

 Selective Repeat: send and receive windows

are K Hence the

identifiers must not be less than 2K

3 8

P times‐out each I‐frame

and retransmits them

S discards each duplicate

I‐frame and returns a

NAK

indicating I(3) is the next

frame

Since the sequence

number in the NAK is

equal to V(S), P takes this

3 9

 In practice, since the

identifier of a frame is in

binary form, a set number of

binary digits must be

reserved for its use.

For example, with a send

window of 7 and Go‐back‐N

is

used, 2 binary digits are

required for the send and

receive sequence numbers

yielding 8 possible

identifiers:

0 7 The send and receive

sequence variables are then

4 0

Sequence

wind ow edge

Window

wind ow edge

tx 1 frame

receive ACK0

max window size

max

window

4 2

Data Link Control Protocols  The data link control layer is concerned with the transfer of data

over a serial data link

The link can be:

 point‐to‐point physical circuit (twisted‐pair

wire, coaxial cable or optical fiber),

 radio‐based channel (satellite, or logical

channels over physical circuits)

Data Link Protocol

Application DTE DTE DTE: Data Terminal EquipmentDLP: Data Link Protocol

DCE: Data Circuit Terminating Equipment

: Communication Subsystem

PSTN

Modem Modem

4 4

DLP

Data Link Protocol

DLP

Share bus

DLP

transmission:•• Half‐duplex protocol: BSC

Full‐duplex protocol: APRANET IMP‐to‐IMP protocol

 High‐level Data Link Control: single link procedure

 Link access procedure version B (LAPB): extension of LAPA, used in

X.25 networks

 Multilink procedure: extension of LAPB

 Link access procedure for modems (LAPM): used in error

correction modems (e.g V32)

 Link access procedure for D‐channel (LAPD): used in ISDN networks

4 6

Binary Synchronous

Communication

 BSC Protocol:

 Character‐oriented

 Error control: Idle RQ

 Synchronous transmission control

 Connection‐oriented

 Half‐duplex

 Topology: Poin‐to‐point, Multipoint, Multi‐drop

network

(DLE/STX,

 Frame formats

DLE/ETX)

4 8

structure of

Multi-frame

Data

Last frame Data

SOH – Start of Header STX – Start of Text ETX – End of Text

ETB – End of Tranmission Block

BCC – Block (sum) Check Character

ETX BCC SYN SYN SOH Header STX

ETB BCC SYN SYN SOH Header STX

ETX BCC IBT BCC STX

SYN SYN SOH Header STX

ETX BCC SYN SYN SOH Header

ETX BCC SYN SYN STX

one or other is returned to

a previously transmitted data block and hence

contains an sequence number.

AS a response to

a select

control message:

an ACK indicates that the selected

station is able to receive a

data block whereas a NAK

5 0

Binary Synchronous

Communication

both Poll and Select

control frames The address of the

polled or selected slave

station is followed by either a P (for

Poll) or an S( for Select)

character, which is in turn followed by the ENQ character

message exchangesequence and clear the logical link between the 2

communicating parties

• To provides

amea

ns of resetting the link to the idle state

5 2

performance: Low link utilization (Idle RQ),

used with multi‐

Exam ple

 A BSC protocol is to be used to control the flow of messages between a

computer (Master station) and 10 block‐mode

terminals (secondaries) over

a multipoint data link The link data rate, R, is 10

kbps and the average

length of a message, N i, is 1000 bits If a poll message and its associated ACK is 30 bits and the total time to process these messages is 1ms,

determine the average time each terminal will be polled if the average rate

at which messages are generated is:

 1 message per minute

 6 message per second

The bit error rate and signal propagation delay

times of the link can be

5 4

 High‐level Data Link HDLC

Variations: LAPB, LAPD,LAPM

Used in Frame Relay, PPP

 Controlled by Primary station

 Send the response frames

Combined station (Both Primary

5 5

by the master (primary) station.

The link may be point‐to‐point, multipoint (only 1 primary allowed)

•

 Asynchronous

Response Mode (ARM)•• Used in unbalanced configuration

Allow a secondary to initiate a transmission without receiving permission from the primary Normally used in point‐to‐point configuration and duplex links

•

 Asynchronous Balanced Mode (ABM)

• Mainly used on duplex point‐to‐point links

• Each station has an equal status and performs both primary and

5 6

HD LC

 Frame formats: both data and control messages are

carried in a standard format

determine the beginning and

the end of a frame

Receiver must hunt this value

If the next bit is bit 1, and 7th bit is bit 1: continue

to count number of bits 1–– If number of bits 1<15:

receiver stops

If number of bits 1>=15:

HDLC

 Address Field:

NRM mode, multidropline: each station has 1 unique

address If the primary

wants to connect with the slave, it will put the slave

address in this address field Certain

point) Instead, it is used

to indicate the direction of commands and their associated

responses

octet will have the first bit equal to 1

5 8

HDLC

 Control field: There are 3

frame types in HDLC: Unnumbered frames (U—frame): used for such functions as link setup

and disconnection They do not contain any

acknowledgement

information

Information frame (I‐frame): carry the actual

information I‐frames can

be used to piggyback acknowledgement

information if the operational

mode is ABM or ARM

Supervisory frame (S‐frame): are used for

error and flow control and

hence contain send and receive sequence

5 9

HD LC

 P/F: Poll or Select

depending on• • Command: bit P, request the response from a secondaryResponse: bit F, indicating this is the the context:

response to a command

0 0 1 0 0 1 1 1

6 0

HDLC

6 2

6 4

HD LC

6 6

frames onto an I‐frame of its own

Node B’s first I‐

frame is also numbered 0 [N(S)

field] and contains a

2 in its N(R) field, acknowledging

the receipt of A’s

frames 1 and 0 and indicating that

it expects frame 2

to arrive next Node B transmits its

second and third

I‐frames (numbered 1 and 2)

indicate that node B is still

expecting A’s frame 2 to

arrive next Node A has sent all its

data Therefore, it

cannot piggyback an

acknowledgment onto an I‐

frame and sends an S‐frame

instead The RR code

indicates that A is still ready to

receive The number

3 in the N(R) field tells B that

frames 0, 1, and 2 have

all been accepted and that A is

6 7

HD LC

errors: Example: an exchange in which a frame is

lost Node B sends three data

frames (0, 1,

and 2), but frame 1 is lost.

When node A receives frame

2, it discards it

and sends a REJ frame for

frame 1 Note that

the protocol being used is

Go‐Back‐N with

the special use of an REJ

frame as a NAK

frame The NAK frame does

two things here:

It confirms the receipt of

frame 0 and

declares that frame 1 and any

following

frames must be resent.

Node B, after receiving the

6 8

 So HDLC

me other cases: