How computation of MAC-I happens in eNodeB PDCP when re-establishment?

Question

As UE has valid C-RNTI , can eNodeB confirms same previous RNTI to UE on occasion of successful re-establishment?

Answer 1

I worked in a LTE stack software and in that software, PDCP layer was generating MAC-I for the RRC message that was being sent from eNodeB to UE. This MAC-I appended with the RRC message and after that encryption is applied.

In case of rrc re-establishment procedure, RRC Re-establishment Request message carries short-MAC-I information. Since PDCP layer already has both algorithms to perform integrity and ciphering of RRC messages, MAC-I can be calculated at PDCP layer and get back at the RRC layer. RRC layer compares the short-MAC-I of the RRC re-establishment request message with the 16 least significant bits of MAC-I, just to make sure that this is valid RRC re-establishment request message.

In short, for the validation of short-MAC-I, RRC layer prepares and send the message to PDCP. After getting the response from PDCP layer, it compares. This is one kind of implementation in which I worked. It is completely dependent on vendor specific implementation.

Comments

Hi vikram,

Thanks for your detailed explanation. i am convinced with your answer. I have few doubts please clarify those.

1. Can RNTI of UE changed by eNodeB or not, before and after re-establishment?

2. At time of Re-establishment UE sends MAC CRNTI Control Element or not to covey its existing RNTI to UE when the time of Re-establishment?

3. PDCP should generate MAC-I for every PDCP PDU ?

Please help ...