Need clarification on Piggybacking NAS Messages?

Question

As per 36.331 Section 4.2.2: Signalling Radio Bearers

In downlink piggybacking of NAS messages is used only for one dependant (i.e. with joint success/ failure) procedure: bearer establishment/ modification/ release.

Can you explain elaborately what the above statement tells.

Answer 1

When an UE make radio connection with eNodeB, two signalling radio bearers (SRBs) are established. One radio bearer is known as SRB1 and the second one is known as SRB2. SRB1 mainly used to exchange RRC messages while SRB2 is dedicated to carry only NAS messages.
When a procedure requires some actions need to be taken at core network as well as at radio network, MME sends NAS message to UE as piggybacked in S1-AP message. After receiving the S1-AP signalling message, eNodeB takes appropriate actions at radio layer and sends RRC message to UE. eNodeB includes received NAS message into the RRC message.
In other cases, where MME and UE only involve NAS messages are exchanged between them. ENodeB does not involve.

Comments

Thanks Harshitha.
In downlink piggybacking of NAS messages is used only for one dependant (i.e. with joint success/ failure) procedure.What is the Exact meaning of this Statement.
Could you explain.