HARQ operation performed in the MAC layer. This is the key difference from UMTS, where
the HARQ reordering is performed in the MAC layer. The advantage of HARQ reordering in
RLC is that no additional SN and reception buffer are needed for HARQ reordering. In LTE,
the RLC SN and RLC reception buffer are used for both HARQ reordering and RLC-level
ARQ related operations.
When the receiving side of the AM RLC entity receives a poll from the peer transmitting
side, it checks the reception buffer status and transmits a status report at the earliest
The receiving side can also generate a status report of its own accord if it detects a
reception failure of an AM RLC PDU. For the detection of a reception failure, a similar
mechanism is used as in the case of UM RLC in relation to the HARQ reordering delay.
In AM RLC, however, the detection of a reception failure triggers a status report instead of
considering the relevant RLC PDUs as permanently lost.