What is ARQ (Automatic Repeat Request)?

ARQ stands for Automatic Repeat Request also known as Automatic Repeat Query. ARQ is an error-control strategy used in a two-way communication system. It 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.

Working Principle of ARQ

The main function of these protocols is, the sender receives an acknowledgment 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 acknowledgment has to be sent by the receiver to the sender. If a timeout occurs: the sender does not receive the acknowledgment 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.


ARQ protocols have a wide range of applications as they provide reliable transmissions over unreliable upper sources. These protocols are mainly functional on shortwave radio to ensure reliable delivery of signals.
For the same function of ARQ, there are various applications:

  1. Transmission Control Protocol (TCP)
  2. Specific Service Orientation Protocol: Error-correction of message signals in ATM networks.
  3. High-Level Data Link protocol.
  4. IBM Binary synchronous Communications Protocol.
  5. Xmodem : modem file transfer protocol.


There are several types of ways in which these protocols function in the data link layer :

Advantages of ARQ

Disadvantages of ARQ

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