Flow and Error Control

In this tutorial, we will be covering the concept of Flow and Error Control in the Data Link layer.

Flow control and Error control are the two main responsibilities of the Data link layer. Let us understand what these two terms specify. For the node-to-node delivery of the data, the flow and error control are done at the data link layer.

Flow Control mainly coordinates with the amount of data that can be sent before receiving an acknowledgment from the receiver and it is one of the major duties of the data link layer.

• For most of the protocols, flow control is a set of procedures that mainly tells the sender how much data the sender can send before it must wait for an acknowledgment from the receiver.

• The data flow must not be allowed to overwhelm the receiver; because any receiving device has a very limited speed at which the device can process the incoming data and the limited amount of memory to store the incoming data.

• The processing rate is slower than the transmission rate; due to this reason each receiving device has a block of memory that is commonly known as buffer, that is used to store the incoming data until this data will be processed. In case the buffer begins to fillup then the receiver must be able to tell the sender to halt the transmission until once again the receiver become able to receive.

Thus the flow control makes the sender; wait for the acknowledgment from the receiver before the continuation to send more data to the receiver.

Some of the common flow control techniques are: Stop-and-Wait and sliding window technique.

Error Control contains both error detection and error correction. It mainly allows the receiver to inform the sender about any damaged or lost frames during the transmission and then it coordinates with the retransmission of those frames by the sender.

The term Error control in the data link layer mainly refers to the methods of error detection and retransmission. Error control is mainly implemented in a simple way and that is whenever there is an error detected during the exchange, then specified frames are retransmitted and this process is also referred to as Automatic Repeat request(ARQ).

Protocols

The implementation of protocols is mainly implemented in the software by using one of the common programming languages. The classification of the protocols can be mainly done on the basis of where they are being used.

Protocols can be used for noiseless channels(that is error-free) and also used for noisy channels(that is error-creating). The protocols used for noiseless channels mainly cannot be used in real-life and are mainly used to serve as the basis for the protocols used for noisy channels.

All the above-given protocols are unidirectional in the sense that the data frames travel from one node i.e Sender to the other node i.e receiver.

The special frames called acknowledgment (ACK) and negative acknowledgment (NAK) both can flow in opposite direction for flow and error control purposes and the data can flow in only one direction.

But in the real-life network, the protocols of the data link layer are implemented as bidirectional which means the flow of the data is in both directions. And in these protocols, the flow control and error control information such as ACKs and NAKs are included in the data frames in a technique that is commonly known as piggybacking.

Also, bidirectional protocols are more complex than the unidirectional protocol.

In our further tutorials we will be covering the above mentioned protocols in detail.