Flow Control and Error Control – Scaler Blog (2024)

Data Link Layer is responsible for reliablepoint-to-point datatransfer over a physical medium. To implement this data link layer provides three functions :

• Line Discipline:
  Line discipline is the functionality used to establish coordination between link systems. It decides which device sends data and when.
• Flow Control:
  Flow control is an essential function that coordinates the amount of data the sender can send before waiting for acknowledgment from the receiver.
• Error Control:
  Error control is functionality used to detect erroneous transmissions in data frames and retransmit them.

What is Flow Control in the Data Link Layer?

Flow controlis a set of procedures that restrict the amount of data a sender should send before it waits for some acknowledgment from the receiver.

• Flow Control is an essential function of the data link layer.
• It determines the amount of data that a sender can send.
• It makes the sender wait until an acknowledgment is received from the receiver’s end.
• Methods of Flow Control areStop-and-wait, andSliding window.

Purpose of Flow Control

The device on the receiving end has a limitedamount of memory(to store incoming data) andlimited speed(to process incoming data). The receiver might get overwhelmed if the rate at which the sender sends data is faster or the amount of data sent is more than its capacity.

Buffers are blocks in the memory that store data until it is processed. If the buffer is overloaded and there is more incoming data, then the receiver will start losing frames.

Theflow control mechanismwas devised to avoid this loss and wastage of frames. Following this mechanism, the receiver, as per its capacity, sends an acknowledgment to send fewer frames or temporarily halt the transmission until it can receive again.

Thus,flow controlis the method of controlling the rate of transmission of data to a value that the receiver can handle.

Methods to Control the Flow of Data

Stop-and-wait Protocol

Stop-and-wait protocolworks under the assumption that the communication channel isnoiselessand transmissions areerror-free.

Working:

• The sender sends data to the receiver.
• The sender stops and waits for the acknowledgment.
• The receiver receives the data and processes it.
• The receiver sends an acknowledgment for the above data to the sender.
• The sender sends data to the receiver after receiving the acknowledgment of previously sent data.
• The process is unidirectional and continues until the sender sends theEnd of Transmission (EoT)frame.

Sliding Window Protocol

Thesliding window protocolis theflow control protocolfor noisy channels that allows the sender to send multiple frames even before acknowledgments are received. It is called aSliding windowbecause the sender slides its window upon receiving the acknowledgments for the sent frames.

Working:

• The sender and receiver have a“window”of frames. A window is a space that consists of multiple bytes. The size of the window on the receiver side is always1.
• Each frame is sequentially numbered from0ton – 1, wherenis the window size at the sender side.
• The sender sends as many frames as would fit in a window.
• After receiving the desired number of frames, the receiver sends an acknowledgment. The acknowledgment (ACK) includes the number of the next expected frame.

Example:

1. The sender sends the frames0and1from the first window (because the window size is2).
2. The receiver after receiving the sent frames, sends an acknowledgment for frame2(as frame2is the next expected frame).
3. The sender then sends frames2and3. Since frame2is lost on the way, the receiver sends back a“NAK”signal (a non-acknowledgment) to inform the sender that frame2has been lost. So, the sender retransmits frame2.

What is Error Control in the Data Link Layer?

Error Controlis a combination of both error detection and error correction. It ensures that the data received at the receiver end is the same as the one sent by the sender.

Error detectionis the process by which the receiver informs the sender about any erroneous frame (damaged or lost) sent during transmission.

Error correctionrefers to the retransmission of those frames by the sender.

Purpose of Error Control

Error controlis a vital function of the data link layer that detects errors in transmitted frames and retransmits all the erroneous frames. Error discovery and amendment deal with data frames damaged or lost in transit and the acknowledgment frames lost during transmission. The method used in noisy channels to control these errors isARQorAutomatic Repeat Request.

Categories of Error Control

Stop-and-wait ARQ

• In the case ofstop-and-wait ARQafter the frame is sent, the sender maintains a timeout counter.
• If acknowledgment of the frame comes in time, the sender transmits the next frame in the queue.
• Else, the sender retransmits the frame and starts the timeout counter.
• In case the receiver receives a negative acknowledgment, the sender retransmits the frame.

Sliding Window ARQ

To deal with the retransmission of lost or damaged frames, a few changes are made to the sliding window mechanism used in flow control.

Go-Back-N ARQ :

InGo-Back-N ARQ, if the sent frames are suspected or damaged, all the frames are re-transmitted from the lost packet to the last packet transmitted.

Selective Repeat ARQ:

Selective repeat ARQ/ Selective Reject ARQis a type of Sliding Window ARQ in which only the suspected or damaged frames are re-transmitted.

Differences between Flow Control and Error Control

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Conclusion

• Data framesare transmitted from the sender to the receiver.
• For the transmission to bereliable,error-free, andefficient flow controlanderror controltechniques are implemented.
• Both these techniques are implemented in theData Link Layer.
• Flow Controlis used to maintain the proper flow of data from the sender to the receiver.
• Error Controlis used to find whether the data delivered to the receiver is error-free and reliable.