In this chapter we shall understand what is ARQ and HARQ.
ARQ:
ARQ stands for Automatic Repeat Request.
It is used in Data Link Layer.
It is used to increase the data reliability.
It is based on ACK/NACK mechanism.
If the packet is received correctly at the receiver side, then it will send ACK to the sender/transmitter.
If the packet is not received correctly at the receiver side, then it will send NACK to the sender/transmitter.
Once the receiver gets a NACK, then it will re-transmit the same packet again.
This is called as CRC (Cyclic Redundancy Check).
Note:
1. ARQ will work on RLC AM mode only.
2. If there is a data that has an error and is passed through HARQ, then that data will be discarded, and a new packet/data will be transmitted.
3. ARQ is used for reliability.
HARQ:
HARQ stands for Hybrid Automatic Repeat Request.
It is a combination of High-Rate Forward error Correction and Automatic Repeat Request.
In HARQ, it will not re-transmit packet, it will try to modify some PHY parameters.
When a data is received with error, then if the error is minor, then it will try to correct it.
else it sends re-transmit request to transmitter, after getting the new packet, it will combine the new data with the error data and fix it.
