SN = Sequence Number
P = Poll bit

The physical layer combines the transport blocks together, interleaves them and then transmits them via two radio frames.
Thus, RLC PDUs in the receiver cannot be recovered until all of the data from that 20 ms TTI has been received.

SN=12, Poll bit is set.
At receiving side, it took 20ms for processing. Then send STATUS PDUs.
After 20ms, the missing SN=6, 8 will be retransmitted.

1. By the receiver to inform the sender about missing and received AMD PDUs.

2. By the sender to request the receiver to move the reception window

3. By the receiver to acknowledge the sender about the reception of the request to move the window

4. By the receiver to inform the sender of the size of the transmission window when the receiver needs to update the sender’s window

1. AM data (AMD) PDUs: user data
2. AM control PDUs

AM control PDUs are further classified into:

1. STATUS PDU
2. Piggybacked STATUS PDU
3. RESET and RESET ACK PDU