Flow Control is another important function of data link layer.

Flow Control refers to a set of procedure that tells the sender how much data it can transmit before waiting for acknowledgement from receiver.

Purpose of Flow Control:

§  Any receiving device has a limited speed at which it can process incoming data and also a limited amount of memory to store incoming data.

§  If the source is sending the data at faster rate than the capacity of receiver, there is a possibility of receiver being swamped.

§  The receiver will keep losing some of the frames simply because they are arriving too quickly.

§  Therefore, receiving device must have some mechanism to inform the sender to send fewer frames or stop transmission temporally.

§  In this way, flow control will control the rate of frame transmission.

Methods of Flow Control:

There are two methods or protocols that are used to implement flow control.

§  Stop and Wait Protocol or Method

§  Sliding Window Protocol or Method

Stop and Wait Protocol or Method

§  In this method of flow control, the sender sends a single frame to receiver and waits for an acknowledgement

§  The next frame is sent by the sender only when acknowledgement of previous frame is received.

§  This process of sending a frame and waiting for an acknowledgement continuous as long as the sender has data to send.

§  To end up the transmission sender transmits End of Transmission (EOT).

§  The main advantage of stop and wait protocol, is its accuracy.

§  Next frame is transmitted only when the first frame is acknowledged.

§  So, there is no chance of frame being lot.

§  The main disadvantage of this method is that it is inefficient.

§  It makes the transmission process slow.

§  In this method frame travels from source to destination and single acknowledgement travels from destination to source.

§  As a result, each frame send and receive uses the entire transmission time.

Sliding Window Protocol or Method

§  This method over comes the limitation of stop and wait mechanism.

§  It is universally adopted method.

§  In this mechanism, the frames are transmitted one after the another without waiting for acknowledgement

§  It contains a sliding window refers to an imaginary boxes that holds the frames on both sender and receiver side.

§  Sliding Window has the limit on maximum how many frames can be transmitted.

§  It contains number system telling up to which number of frames are received properly.

§  Each data frame is identified by a sequence number.

§  The window has a specific size in which the frames are number.

§  If the size of window is n then the frames are numbers from 0 to n-1.

§  The number of frames which can stay at a time in window is known as size of window.

§  Its typical value is 7 that most of the windows have their size 7 and 7 frames may stay in it.

§  But its size can be less or more depending on the frame size of sender.

§  It means if the size is 7 then maximum of 7 frames can be sent before an acknowledgement.

§  The sliding window of the sender shrinks from left when frames of data are sent.

§  The sliding window of sender expands to right when acknowledgement received.

§  The sliding window of the receiver shrinks from left when frames of data are received and expands to right when acknowledgement is sent.


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