Flow Control in Data Link Layer

Flow control is design issue at Data Link Layer. It is technique that generally observes proper flow of data from sender to receiver. It is very essential because it is possible for sender to transmit data or information at very fast rate and hence receiver can receive this information and process it. This can happen only if receiver has very high load of traffic as compared to sender, or if receiver has power of processing less as compared to sender.

Flow control is basically technique that gives permission to two of stations that are working and processing at different speeds to just communicate with one another. Flow control in Data Link Layer simply restricts and coordinates number of frames or amount of data sender can send just before it waits for an acknowledgment from receiver. Flow control is actually set of procedures that explains sender about how much data or frames it can transfer or transmit before data overwhelms receiver.

The receiving device also contains only limited amount of speed and memory to store data. This is why receiving device should be able to tell or inform the sender about stopping the transmission or transferring of data on temporary basis before it reaches limit. It also needs buffer, large block of memory for just storing data or frames until they are processed.

Approaches to Flow Control :
Flow Control is classified into two categories –




  • Feedback – based Flow Control :
    In this control technique, sender simply transmits data or information or frame to receiver, then receiver transmits data back to sender and also allows sender to transmit more amount of data or tell sender about how receiver is processing or doing. This simply means that sender transmits data or frames after it has received acknowledgments from user.



  • Rate – based Flow Control :
    In this control technique, usually when sender sends or transfer data at faster speed to receiver and receiver is not being able to receive data at the speed, then mechanism known as built-in mechanism in protocol will just limit or restricts overall rate at which data or information is being transferred or transmitted by sender without any feedback or acknowledgment from receiver.



Techniques of Flow Control in Data Link Layer :

There are basically two types of techniques being developed to control the flow of data –





1. Stop-and-Wait Flow Control :
This method is the easiest and simplest form of flow control. In this method, basically message or data is broken down into various multiple frames, and then receiver indicates its readiness to receive frame of data. When acknowledgment is received, then only sender will send or transfer the next frame.

This process is continued until sender transmits EOT (End of Transmission) frame. In this method, only one of frames can be in transmission at a time. It leads to inefficiency i.e. less productivity if propagation delay is very much longer than the transmission delay.

Advantages –

  • This method is very easiest and simple and each of the frames is checked and acknowledged well.
  • It can also be used for noisy channels.
  • This method is also very accurate.

Disadvantages –

  • This method is fairly slow.
  • In this, only one packet or frame can be sent at a time.
  • It is very inefficient and makes the transmission process very slow.



2. Sliding Window Flow Control :
This method is required where reliable in-order delivery of packets or frames is very much needed like in data link layer. It is point to point protocol that assumes that none of the other entity tries to communicate until current data or frame transfer gets completed. In this method, sender transmits or sends various frames or packets before receiving any acknowledgment.

In this method, both the sender and receiver agree upon total number of data frames after which acknowledgment is needed to be transmitted. Data Link Layer requires and uses this method that simply allows sender to have more than one unacknowledged packet “in-flight” at a time. This increases and improves network throughput.

Advantages –

  • It performs much better than stop-and-wait flow control.
  • This method increases efficiency.
  • Multiples frames can be sent one after another.

Disadvantages –

  • The main issue is complexity at the sender and receiver due to the transferring of multiple frames.
  • The receiver might receive data frames or packets out the sequence.

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