LTE: How does integrity verification is performed by receiving PDCP entity ?

Question

What are the steps involved to do integrity verification ?

Answer 1

PDCP layer performs integrity protection and verification based on the role. I mean when PDCP entity works as a transmitter, it does integrity protection of upper layer SDU coming over SRB1 and SRB2 of an UE. When PDCP transmitting entity receives a packet, it prepares PDCP Control plane data pdu. For the format of PDCP control plane data pdu, please check 36.323 spec. PDCP layer calculates 32 bits long MAC-I from RRC integrity key and values of variables COUNT, BEARER ID and , DIRECTION.

At the receiver side, PDCP layer calculates X-MAC based on input parameters i.e RRC Integrity key, COUNT, BEARER ID and DIRECTION. If the calculated X-MAC corresponds to the received MAC-I, Integrity protection is verified successfully.

Comments

Thanks for your quick reply. But I have a related question. How PDCP receiver entity knows that for which of the bearer (SRB1 or SRB2) data is received. Since SRB1 and SRB2 both would have different bearer Id and it is one of the necessary parameter to do integrity verification.