Automatic repeat request (ARQ) is a mechanism used in data link control and transport protocols and relies on the use of an error detection code, such as the cyclic redundancy check (CRC). Automatic repeat request is also known as Automatic Repeat Query (ARQ).

ARQ is a group of error-control protocols to achieve reliable data transmission over an unreliable source or service. These protocols reside in Transport Layer and Data Link Layer of the OSI(Open System Interconnection) model . These protocols are responsible for automatic retransmission of packets that are found to be corrupted or lost during the transmission process.

Fig: Automatic Repeat Request (ARR):

Working principle of ARQ: The main function of these protocols is, the sender receives an acknowledgement from the receiver end implying that the frame or packet is received correctly before a timeout occurs, timeout is a specific time period within which the acknowledgement has to be sent by the receiver to the sender. If a timeout occurs: the sender does not receive the acknowledgement before the specified time, it is implied that the frame or packet has been corrupt or lost during the transmission. Accordingly, the sender retransmits the packet and these protocols ensure that this process is repeated until the correct packet is transmitted.

• Advantages of ARQ: 1) The Error-detection and correction mechanisms are quite simple compared to the other techniques.

2) A much simpler decoding equipment can be put to use compared to the other techniques.

• Disadvantages of ARQ: 1) A medium or a channel with a high error rate might cause too much transmission of the frames or packets of information.

2)The high error rate in the channel might also lead to loss of information, therefore reducing the efficiency or the productivity of the system.