Data Link Control and Protocols Chapter 11

7/29/2019 Data Link Control and Protocols Chapter 11

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Data Link Control and

Protocols


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Data communication system always have a

sending and a Receiving device Flow control A set of procedures that tells the sender how much data

it can transmit before receiving an acknowledgmentfrom the receiver.

The flow control must not overwhelm the receiver

Receiver have slower processing speed than incomingTx Data, and limited memory(buffer) for storing the

incoming transmission data. Once the buffer filled the sender should be informed to

halt transmission until it is once again able to receive.


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Data Link Control and Protocols ErrorControl A mechanism for error detection and error correction

Inform receiver about any lost or damaged receivedframe

Error correction is the retransmission of specifiedframes

Error detection + retransmission is called

Automatic Repeat Request or ARQ.


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DLL is responsible for flow control and

error control or

Flow control + error control is collectively

know as Data link Control


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Flow and error control protocols

Stop and-Wait ARQ

Go-Back-N ARQ

Selective Repeat ARQ


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Stop and-Wait ARQ Keep a copy of last transmitted frame for retransmission

For identification purpose both DATA & ACK frames arenumbered alternately 0 & 1.

In case of a damaged frame is detected the receiverdiscard the frame and send no Acknowledgment.

If the receiver receives an out of order frame, it knowsthat a frame is lost it simply discard the out-of-order

received frame The sender has a Control Variable S that holds the

number of the recently send frame (0 or 1).

The Receiver has a Control Variable R that holds the

number of the next frame expected (0 or 1).


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Stop and-Wait ARQ The sender starts a timer when its send a frame to

receive an Acknowledgment with in an allotted time

period. The receiver sends only positive Acknowledgment for

frames receive safe and sound.

The Acknowledgment number always define thenumber of next expected frame.

If frame 0 is received, ACK1 is sent, if frame 1 isreceived ACK0 is sent.


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Stop and-Wait ARQ

A B

Sender Receiver

Frame 0

Frame 1

ACK 1

ACK 0

S = 0

S = 0

S = 1

R = 0

R = 1

Time Time


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In transmission of a frame we have foursituations

Normal Operation

The frame is lost

The acknowledgment is lost

The acknowledgment is delayed.

Stop and-Wait ARQ


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Lost or Damaged Frame

A B

Sender Receiver

Frame 0

Frame 1

ACK 1

ACK 0

S = 0

S = 0

S = 1

R = 0

R = 1

Time Time

lost

Frame 1S = 1

R = 1

R = 0

Time out

Sender send anothercopy of frame whentime expires

The The receiverremains silent abouta lost frame andkeeps its value of R


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LostAcknowledgment

A B

Sender Receiver

Frame 0

Frame 1

ACK 1

ACK 0

S = 0

S = 0

S = 1

R = 0

R = 1

Time Time

lostFrame 1

S = 1

R = 0Time out

ACK 0

Expecting frame 0,frame 1 discarded


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Delayed Acknowledgment

A B

Sender Receiver

Frame 0

Frame 0ACK 1

ACK 0

S = 0

S = 0

S = 1

R = 0

R = 1

Time Time

Frame 1S = 1

R = 1

ACK 1

Frame 1

Expecting Frame1,

frame 0 discarded

Time out

Time out

Discarded


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Bi-directional Transmission

The stop and wait mechanism is unidirectional

It can be bi-directional if

Both parties have two separate channels for full duplextransmission or

Share the same channel for half-duplex Transmission

Both parties will need S & R Variables to track frame sent

and expect.


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PiggybackingA method to combine a Data frame with an Acknowledgment. SaveBandwidth

A B

Sender Receiver

Frame 0, ACK0

Frame 1, ACK 1

Frame 0, ACK 1

Frame 1, ACK 0

S = 0

S = 0

S = 1

R = 0

R = 1

Time Time


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GO-BACK-N

ARQ In Stop-and-Wait ARQ the sender send one frame and

waiting for the ACK. This is wastage of bandwidth

For better efficiency, multiple frames should be intransition while waiting for Acknowledgment

Go-Back-N ARQ send up to WFrames before worryingabout ACK

like Stop-and-Wait ARQ it keeps the copy of theseFrames until the ACK arrive.

Some extra features are added to Stop-and-Wait ARQ


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GO-BACK-N

ARQ Each transmitted frame have a sequence No.

If the header of the frame allows m bits,

then sender can transmit 2()sequentially I.e 0-7

The sequence can be repeated but will be

considered the next cycle. The sender sets a timer for each frame sent.

The receiver has no timer


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Sliding WindowThe size of the window at the receiver site in this

protocol is always 1

6 7 0 1 2 3 4 5 6 7

6 7 0 1 2 3 4 5 6 7

a. Before Sliding

b. After Sliding

.

.


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Sliding Window Control Variables

The sender has three variables S, SF, and SL

S holds the sequence number of recently sent frame SF holds the sequence number of the first frame in the window

SL holds the sequence number of the last frame in the window

The size of the window is W = SLSF + 1

The receiver has only one variable , R that holds the sequence

No of the frame it expects to receive.


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GO-BACK-N

ARQAcknowledgment The receiver send only positive ACK for the frame

arrived safe and sound

If a frame lost or damaged the receiver discard allthe subsequent frame

In case of no ACK and from receiver the sender goback and resend all frames, beginning with the one

with the expired timer Receiver can also send one cumulative ACK for

several frames


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A B

Sender Receiver

Frame 0

Frame 1

ACK 2

ACK 3

Time Time

Frame 2

0 1 2 3 0 1 20 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

S

R

R

R

R

S

S

S

Frame 3

SF SL

Normal Operation


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Damaged or Lost Frame

A B

Sender Receiver

Frame 0

Frame 1

ACK 2

ACK 3

Time Time

Frame 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

R

R

R

RS

Frame 3

SF SL

Frame 2

0 1 2 3 0 1 2

R

Frame 3

Frame 3 discardedResent

Resent

0 1 2 3 0 1 2

0 1 2 3 0 1 2

S

S

S

S

S

Lost

Time out


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Selective Repeat ARQ GO-BACK-N ARQ is inefficient over noisy link

Resending of multiple frames for one damaged or lostframe

Selective Repeat ARQ is a new mechanism and does notuse resending of multiple frame for one damaged orlost frame

Processing at receiver side is now more complex

Now the size of window should be at most one-half ofthe value 2^m

The receiver and sender now have same size windows


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Selective Repeat ARQ The receiver is now looking for a range of sequence

numbers

The receiver now have two variables RF and RL

Now receiver can send a Negative Acknowledgment

(NAK) for a specific lost or damaged frame to the sender

Sender also sets a timer for each frame sent

The size of the sender and receiver window must be atmost one-half of 2^m or 2^m/2.


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A B

Frame 0

Frame 1

ACK 2

NAK 2

Time Time

Frame 3

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

0 1 2 3 0 1 2

S

Frame 2

0 1 2 3 0 1 2Frame 2

0 1 2 3 0 1 2

Lost

0 1 2 3 0 1 2

S

S

S

S

Resent


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High-level Data Link Control(HDLC)

An actual protocol designed to support both Half-duplex and full-duplex communication over point-to-

point and multi link It implement the ARQ mechanism

HDLC provides the following modes of Transmission

Normal Response Mode Asynchronous Balanced Mode


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Normal Response Mode-NRM

A B

A CB

Primary

Point-to-Point

Command

Secondary

Primary

Response

Response

Command

Response

Secondary

Secondary

Primary

Unbalanced configuration, one primary and multiple secondary stations

A primary station can send commands, a Secondary station can only response

Multi-point


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Asynchronous Balanced ModeThe configurations is balanced

The link is point-to-point and each station can function as a primary and a secondary

A B

Combined

Point-to-Point

Command/Response

Combined

Command/Response


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HDLC defines three types of frames

Information Frames (I-frame)used to transport user data and control informationrelating to user data (piggybacking)

Supervisory frames (S-frames)used to transport Control information

Unnumbered Frames (U-frames)are reserved for system management (for link itself)e.g. connects and disconnects connections betweensenders and receivers.


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Flag Address Control Information FCS Flag

8-bit 1-or 2-byte 2-or 4-byte

Only in I- and U-frames

8-bit

Flag field with a 01111110 pattern identifies both start and end o f aframe and synchronization

The control field is used for flow and error control

FCS is for HDLC,s error detection field

The information field contain the user,s data

Address field contains a from address if originated by secondarystation (final). it contain a to address if originated by primary station(poll).

Those networks that do not use primary secondary configuration(Ethernet) use two address fields I.e source and destination addresses

1-or multi-byte


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HDLC frame type


Flag Address Control FCS Flag


I-Frame

S-Frame

U-Frame


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HDLC frame type


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Poll=2ndry station originate

Final=primary station originate


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Pi b ki ith t & With E


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Piggybacking without & With Error

I - Frame

0 1

FLag

FCS

Data

FLag

B

A B

I - Frame

0 0

FLag

FCS

Data

FLag

AFLag

FCS

Data

FLag

B

S - Frame

3 RR

FL

ag

F

CS

FL

agB

DATA

Acknowledgment

I - Frame

0 2

DataAFLag

FCS

FLag

I - Frame

2 2

DataAFLag

FCS

FLag

I - Frame

1 2

A B


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A B

DataAFLag

FCS

FLag

S - Frame

1 REJ

FLag

FCS

FLag

B

DATA

Reject

I - Frame

0 2

DataAFLag

FCS

FLag

I - Frame

2 2

DataAFLag

FCS

FLag

I - Frame

1 2Error

S - Frame

3 RR

FL

ag

F

CS

FL

agB

Acknowledgment

DATA

DataAFLa

g

FC

S

FLa

g

I - Frame

1 2

DataAFLag

FCS

FLag

I - Frame

2 2


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Ethernet Frame


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Thanks a lot

Documents

Data Link Control and Protocols Chapter 11