There are 4 use cases for STATUS PDU:
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 [...]
There are two types of PDUs in RLC/AM:
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
A given layer receives service data units (SDUs) containing the message from its upper layer, process them adding the header and control elements and eventually delivers them in the form of Protocol Data Units (PDUs) to its lower layer.
In summary,
SDU –> Protocol Layer –> PDU
So,
RLC PDU = MAC SDU
