LTE: Does discard timer applicable for UM mapped DRB's or both (UM and AM) ?

Question

In PDCP layer, there is functionality of timer based discard. Can anyone please elaborate on this functionality of PDCP layer.

Answer 1

Discard timer is configured per bearer level(if configured). discard timer value may vary bearer to bearer. Based on the QoS being served by a bearer, value of discard timer is configured by RRC layer at pdcp layer while establishing/modifying a bearer. I have not any statement which tells that discard timer is applicable only for UM or RLC mode bearer. So I think it is applicable for both type of data bearers. Please correct me if I am wrong. Since basic objective of discard timer is to keep the PDCP SDU for a configured time (discard timer value) . Minimum value of discard timer is 50 millisecond. It means if anything wrong happened in the system, re-transmission of PDCP PDU can be done if required or if discard timer gets expired and there is no acknowledgement from lower layer for corresponding PDCP PDU, discard the stored PDCP SDU since it is loo late for PDCP layer. Now re-transmission would be at application level.

Comments

Yes discard timer is applicable for both for UM & AM mapped DRB's , as the discard timer is started as a SDU from upper layer reaches PDCP .