What is ARQ and HARQ?

What is ARQ and HARQ?

ARQ stands for Automatic Repeat Request. This is the protocol used at data link layer. it is an error-control strategy that is used in a two-way communication system.  It is used to achieve reliable data transmission over an unreliable source or service.

 It uses CRC(cyclic redundancy check) to determine, whether the packet received is correct or not. If the packet is received correctly at receiver side, receiver send ACK to the transmitter, but in case if the packet is not received correctly at receiver side, receiver send NACK to the transmitter. And then after receiving NACK from receiver, transmitter re-transmits the same packet again.

 

HARQ does not retransmit packet/PDU as it is; as done by ARQ technique. HARQ modifies certain physical parameters before retransmission.

  

The HARQ is a technique when the receiver gets a new data with some errors then it tries to make correction if the error is minor, but if the error is not minor then it sends re-transmission request to the sender. After getting the data again, it combines the new received data with previous erroneous data.

If some packets passed from HARQ to upper layer with a little bit errors which might be acceptable for some applications, but in any case there is one more mechanism which is ARQ or Automatic Repeat Request. The ARQ mechanism takes care of residual errors which passed from HARQ. If there is an error then it discards the packets and a new re-transmission is requested from the sender. ARQ is an error control protocol.

HARQ:
1. It works at Physical layer but controlled by MAC layer.
2. If the received data has an error then the receiver buffers the data and requests a re-transmission from the transmitter

3. HARQ  works for both UM and AM mode.

4. HARQ provides very fast retransmission which is suitable for high speeds (eg voice call).

ARQ: 
1. It works at RLC layer for RLC AM MODE ONLY.
2. If the received data has an error which is passed through HARQ then it is discarded, and a new re-transmission is requested from the transmitter.

3. ARQ is responsible for reliability.