LTE: What is the use of PDCP status report message ?

Question

In 3GPP 36.323 specification mentioned that the size of PDCP SDU is 8188 Octets. From where this number comes ?

Answer 1

Due to re-establishment at lower layers. Some PDCP packet can be lost . So those bearers which are mapped to RLC AM, receiver entity sends PDCP Control PDU for PDCP status report so transmitter can re-transmit those missing packets towards receiver.

PDCP Control PDU for PCDP Status report has mainly "FMS field". FMS stands for first missing PDCP SN. And a bitmap which indicates that what are the packets need to re-transmit and what are not.

Answer 2

The PDCP Control PDU is generated only by the radio bearer (RB) of the U-plane, and there are two types: (1) a PDCP status report message (i.e. PDCP Status Report) used to inform the transmitting side about the PDCP entity reception buffer status and
(2) a Header Compression Feedback Packet used to inform about the header decompressor state to the header compressor of the transmitting side.
The PDCP status report message (PDCP Status Report message) is transmitted from the receiver PDCP to the transmitter PDCP in order to inform the transmitter PDCP about the PDCP PDUs that were received or not received by the receiver PDCP, such that non-received PDCP SDUs can be retransmitted received PDCP SDUs need not be retransmitted. This PDCP status report message can be send in the form of a PDCP Status PDU,

Comments

Also,
1. During UL Data forwarding:-
UE on receiving the PDCP status report may re-transmit based on the bitmap where in 0 in bitmap stands for packets the need to be re-transmitted by the UE and 1 stands for packets that need not be re-transmitted by the UE.
In few cases, even if the bitmap for packet N says "0"(meaning to be re-transmitted) the UE may not have the copy of that packet at all as the packet loss might be during RLC-Retransmissions in eNodeb itself.
Note the bitmap filling is always byte aligned
Apart from Handover, this logic holds good during RRC-Restablishment of UE to the source cell again.
 
2. DL Data forwarding:-
enodeb simple drops the packets during RLC Re-establishment and maintains the dl_count