Reordering: When the receiving RLC entity receives RLC PDUs, it first reorders them if they are received out of sequence. Out-of sequence reception is unavoidable due to the fact that the HARQ operation in the MAC layer uses multiple HARQ processes. Any RLC PDUs received out of sequence
are stored in the reception buffer until all the previous RLC PDUs are received and delivered to the upper layer.
During the reordering process, any duplicate RLC PDUs received are detected by checking the SN's and discarded. This ensures that the upper layer receives upper layer PDUs only once.
To detect reception failures and avoid excessive reordering delays, a reordering timer is used in the receiving RLC entity to set the maximum time to wait for the reception of RLC PDUs that have not been received in sequence. The receiving RLC entity starts the reordering timer when a missing RLC PDU is detected, and it waits for the missing RLC PDUs until the timer expires. When the timer expires, the receiving UM RLC entity declares the missing RLC PDUs as lost and starts to reassemble the next available RLC SDU's from the RLC PDUs stored in the reception buffer.
The reordering is not applicable to RLC entities using the Multicast Control CHannel (MCCH) or Multicast Traffic CHannel (MTCH). This is because the HARQ operation in the MAC layer is not used for these channels.
