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Data Link Layer - Flow Control

1 Data & ComputerCommunications

William Stallings Ch. 7

2 Data Communications &Networking

B. A. Forouzan Ch. 11

3 Computer Networks A. S. Tanenbaum Ch. 3

Data Link Control Protocols

need layer of logic above Physical

to manage exchange of data over a link

frame synchronization

flow control

error control

addressing

control and data

link management

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Model of Frame Transmission

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Flow Control

Flow control refers to a set of procedures used to restrict the amount of data that the

sender can send

before waiting for acknowledgment.

ensures sending entity does not overwhelmreceiving entity by preventing buffer overflow

influenced by: transmission time

time taken to emit all bits into medium

propagation time

time for a bit to traverse the link

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Error & Flow Control Protocols

There are two main types of flow control protocols: Stop-and-Wait

Sliding Window

There are three main types of error control protocols(ARQ-based): Stop-and-Wait ARQ

Sliding Window ARQ

Go-back-N ARQ

Selective-Reject ARQ

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Categories of Flow Control

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Simplest protocol flow diagram

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Simplest protocol with no flow or error control

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Sender-site algorithm for the simplest protocol

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Receiver-site algorithm for the simplest protocol

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Stop and Wait

source transmits frame destination receives frame and replies with

acknowledgement (ACK)

source waits for ACK before sending next frame

destination can stop flow

by not sending ACK

works well for a few large frames

Stop and wait becomes inadequate

if large block of data is split into small frames

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Design of stop & wait

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Sender-site algorithm for Stop-and-Wait Protocol

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Receiver-site algorithm for Stop-and-Wait Protocol

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Stop & wait flow diagram

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Stop & Wait: Drawbacks

Inefficient link utilization because only one frame issent at a time

Transmitter/sender must wait for ACK

During this waiting period no frames are sent

Inefficiency gets even worse when

Transmission speed is high

Transmitter quickly sends frame then sits idle Propagation distance is high

It takes longer for the frame and its ACK to reach destination

Both cases leave the Transmitter waiting idle for longertimes

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Stop & Wait: normal operation

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Alternatives - Stop & wait

How to improve the efficiency?

To improve the efficiency ACK should be sent aftermultiple frames

Sliding Window protocols

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Fragmentation

Large block of data may be split into smallframes.

Why? Limited buffer size at receiver

Errors can be detected sooner

Retransmission of smaller frames is needed in case oferror

Prevents one station occupying medium for longperiods (LAN)

However, stop and wait is inadequate to cater forlong-delay links

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Sliding Window Flow Control

allows multiple numbered frames to be in transit

receiver has buffer W long

transmitter sends up to W frames without ACK

ACK includes number of next frame expected

Sequence number is bounded by size of field (k-bit) frames are numbered modulo 2k

giving max window size of up to 2k - 1

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Sliding Window Diagram

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Sliding Window Diagram

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Sliding Window Example

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Sliding Window Enhancements

Receiver can acknowledge frames without permittingfurther transmission using Receive Not Ready or RNR frame

must send a normal acknowledge to resume (Receive Ready orRR frame)

If no data to send, simple ACK is sent

Use piggybacking When need to ACK and send data, use same data frame to carry

ACK number

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Piggybacking

Piggybacking is a method that combines the data and theACK in one frame

It is useful in bidirectional communications

Stations can send their data along-with ACK to datapreviously received

Piggybacking is faster and saves bandwidth

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Stop-and-Wait ARQ Piggybacking

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Sliding Window Protocol - Piggybacking

When using sliding window protocol in full duplexconnections:

Node A maintains its own transmit window

Node B maintains its own transmit window

A frame contains: data field + ACK field

There is a sequence number for the data field, and asequence number for the ACK field

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Assignment

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