msg3 buffer in LTE

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What is msg3?

Message transmitted on UL-SCH containing a C-RNTI MAC CE or CCCH SDU, submitted from upper layer and associated with the UE Contention Resolution Identity, as part of a random access procedure.

What is msg3 buffer?

When msg3 is transmitted during initial attach it will be moved to HARQ buffer to handle the retransmission. So msg3 buffer is used to store when UE has msg3.
Also MSG3 buffers is sent on SRB0 in initial time.

Detailed explanation for understanding:

UE can perform RACH either to get sync or initial access.
If it is an initial attach with network, UE will send RRC connection request in msg3. If it is a synch RACH then UE will send MAC_CRNTI CE to delete the new RA-RNTI.
During Data throughput UE may synch loss then UE will send RRC re-establishment message in msg3.
Initial attach synch loss is either TA timer expires or SR count reached MAX.
UE synch loss is while in data pumping if RLC retx reached MAX, due to poor coverage, due to interference, etc.


  1. What will happen if UE sending some UL data and between them handover got triggered

    1. The data will be buffered at source eNB and UE will latch with the new eNB. In the meantime, a tunnel will be established between source and target eNB, and all the buffered data will be sent to target eNB along with end marker. Now, the target eNB will send those packets to UE.

  2. "If it is a synch RACH then UE will send MAC_CRNTI CE to delete the new RA-RNTI"
    Can you please explain what this means ??


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