If the erroneous packets are received by eNodeB or UE in radio interface, what kind of action will be taken? In intelligent technology there should be a mechanism to solve this issue. And there is, which is Hybrid Automatic Repeat Request or with a shortened version HARQ. 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.
ARQ:
1. It works at RLC layer.
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.