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Data LinkControl

andProtocols

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Flow and Error ControlFlow and Error Control

Flow Control

Error Control

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Flow control refers to a set of Flow control refers to a set of procedures used to restrict the amount procedures used to restrict the amount of data that the sender can send before of data that the sender can send before

waiting for acknowledgment.waiting for acknowledgment.

NoteNote::

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Error control in the data link layer is Error control in the data link layer is based on automatic repeat request, based on automatic repeat request,

which is the retransmission of data. which is the retransmission of data.

NoteNote::

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Flow Control Protocols

Stop and Wait Protocol Sliding Window Protocol

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Stop and Wait Protocol

Data transmission is in one direction at a time (half-duplex)

Sender sends a frame and waits for an acknowledgement

Avoids flooding frames at receiving end

Receiver holds the data in buffer Once complete frame is received, then

the acknowledgement is sent i.e. it is ready to receive another frame

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Stop and Wait Protocol

Only one frame is transmitted at a time Time taken by a station to transmit a

frame is ‘transmission time’ Time taken by a bit to reach destination

from source is ‘propagation time’ Capacity of medium: Length of link in Bits Calculated as: B=R x (d / v)

where, R is Data rate of Link in bps d is length of link in meters v is velocity of propagation

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Two Cases of Stop and Wait Protocol Case 1: When transmission time

(t=1) is greater than propagation time

Case 2: When propagation time (a) is greater than transmission line

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Case 1: a<1

i.e. propagation time is less than transmission time

Frame is so long that, the first bit of frame reaches the destination before the source completes transmitting the whole frame

Case when the sender and receiver are at very short distance

Eg: LAN Communication

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Numerical Example for a<1 Medium capacity (B)=200 bits( R=1Gbps, d=40m,

v=2x108m/s) Transmission time=1 second Frame size (L)=800 bits Propagation time (a)= B/L=200/800=0.25= 250 ms At t0, transmission starts After 250 ms, first 200 bits are on transmission line After 1000 ms, final bit of frame is emitted to medium After 1250 ms, final bit reaches the receiver If Ack takes 250 ms to reach sender Total Transmission Time= (1250 +250)=1500ms The Actual Transmission Time for first frame=

1000ms Total Transmission Time =1500ms

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Case 1: a>1

i.e. propagation time is greater than transmission time

Sender finishes transmitting complete frame before the first bit of frame could reach the destination

The medium is under utilized as the medium is never fully occupied during the frame transmission

Case when the distance between sender and receiver is very long

Eg: Satellite Communication

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Numerical Example: a>1 Medium capacity (B)=10000 bits ( R=1Mbps,

d=30000m, v=3x106m/s) Transmission time=1 second Frame size (L)=800 bits Propagation time (a)= B/L=10000/800=25 seconds At t0, transmission starts After 1s, final bit of frame is emitted to medium After 25seconds, first bit of frame reaches the

receiver After 26seconds, final bit reaches the receiver If Ack takes 25s to reach sender Total Transmission Time= (26 +25)=51 seconds The Actual Transmission Time = 25seconds Total Transmission Time =51seconds>>25seconds

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So concluding stop and wait... Medium Capacity increases if data rate

increases as well as if distance increases

If capacity increases, propagation time will also slowly increase keeping other values constant

Which slowly degrades the efficiency More the propagation delay is, more

inefficient is the stop and wait protocol

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Sliding Window Protocol

Introduced to overcome the limitation of stop and wait protocol of sending only one frame at a time

Sends multiple frames say W frames (window-size) Receiver will have buffer size for W frames Sender is allowed to send W number of frames

without waiting for an acknowledgement To keep track of frames acknowledged, sequence

number is used The number of bits used in sequence number

determines the value of W Sender’s window will constitute of frames sent but

not acknowledged Receiver’s window will constitute of frames

expected

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Sliding Window Protocol

Whenever receiver receives a frame correctly, it sends a acknowledgement for that frame, and then slides the window next expected frame adding new expected frame to the window

Whenever sender receives an acknowledgement for a frame it sends next frame to receiver and slides the window one frame maintaining window size

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How to number frames?

Frames are numbered using sequence number

If there is 3 bit sequence number, then the maximum possible sequence number is 7

And maximum window size:0-7, i.e 8 But not always max. window size is the

required window size The window size of the frames depend

upon protocols used

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Piggybacking

Until now only unidirectional data flow is discussed But what if bidirectional data flow is required What if both station A and B needs to send data At such case, it would be inefficient use of medium

to send acknowledgement and data separately Since Acknowledgement is a small piece of

information, it would be efficient to append the acknowledgement information with next out-going data

This technique of combining a data frame with an acknowledgement in bidirectional transmission system to make the optimum use of the medium is called piggy-backing

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Different Sliding Window Protocols for Error Control One Bit Sliding Window Protocol Go-Back N ARQ (Automatic Repeat

reQuest) Selective Repeat ARQ

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One-Bit Sliding Protocol

Also known as Stop and Wait ARQ Follows the principle of stop and wait

protocol The window size at receiver and

sender is 1 frame

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One-Bit Sliding Protocol (Features) Sender keeps a copy of frame sent until the

acknowledgement for that frame is received Data frames and acknowledgement frames are identified

with two values 0 and 1 Data frame 0 is acknowledged by ACK 1 and vice versa If receiver receives damaged frame, then it simply discards

the frame and doesn’t send an acknowledgement If receiver receives out of order frame, then also it discards

the frame and sends no acknowledgement Sender starts timer as soon as it sends a frame and if it

doesn’t receive acknowledgement before the timer goes off, it retransmits the frame

In case of unidirectional transmission, sender has control variable S to store the number of recently sent frame (0 or 1) and receiver has control variable R to store the number of next frame expected

In bidirectional transmission, each station have both control variables

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Normal operation of One Bit Sliding Protocol

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Stop-and-Wait ARQ, lost frame

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Stop-and-Wait ARQ, lost ACK frame

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In Stop-and-Wait ARQ, numbering In Stop-and-Wait ARQ, numbering frames prevents the retaining of frames prevents the retaining of

duplicate frames.duplicate frames.

NoteNote::

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Stop-and-Wait ARQ, delayed ACK

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Numbered acknowledgments are Numbered acknowledgments are needed if an acknowledgment is needed if an acknowledgment is

delayed and the next frame is lost. delayed and the next frame is lost.

NoteNote::

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Piggybacking in Stop and Wait ARQ

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Go-Back-N ARQ

Multiple frames can be sent without waiting for the acknowledgments

If m bits are used for sequence number, the maximum possible window size that can be used by sender is 2m-1

The receiver window size is always 1 frame window

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Sender sliding window in Go-Back-N ARQ

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Receiver sliding window in Go-Back-N ARQ

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Control variables in Go-Back-N ARQ

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Go-Back-N ARQ (Operation) Multiple frames are sent in this protocol by sender If the receiver doesn’t receive the frame or the

frame is damaged then, the acknowledgment is not sent

Also the receiver discards all the subsequent frames followed by erroneous frame

Receiver also discards all the out of order frames When sender doesn’t get an acknowledgment before

the timer goes off, it retransmits all the frames all over again starting from frame whose acknowledgement is not received

Acknowledgment can be sent by receiver in the cumulative form as well

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Go-Back-N ARQ, normal operation

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Go-Back-N ARQ, lost frame

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Go-Back-N ARQ: sender window size

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11.4 Selective-Repeat ARQ11.4 Selective-Repeat ARQ

Sender and Receiver Windows

Operation

Sender Window Size

Bidirectional Transmission

Pipelining

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Selective Repeat ARQ

Go-Back-N ARQ would not be efficient protocol with noisy channel where the tendency of error is high resulting high number of retransmissions

So for such scenario, there should be mechanism to not resend all N frames but just to send the frame which is damaged

Selective Repeat ARQ provides such mechanism

Sender and Receiver have same window size which is equal to at most 2m/2

There is use of NAK (Negative ACK) for damaged frames

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11.12 Selective Repeat ARQ, sender and receiver windows

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Selective Repeat ARQ(Operation) In selective repeat, if receiver

encounters a damaged frame it sends NAK

When sender receives a NAK, it resends the frame for which the NAK was sent by receiver

Timers are used at both ends to tackle the problem when the frames, ACKS and NAKS are delayed or lost

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11.13 Selective Repeat ARQ, lost frame

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In Selective Repeat ARQ, the size of In Selective Repeat ARQ, the size of the sender and receiver window must the sender and receiver window must

be at most one-half of 2be at most one-half of 2mm. .

NoteNote::

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11.14 Selective Repeat ARQ, sender window size