Why NAS-PDU is piggybacked within E-RAB setup request message over the S1-AP interface?

Question

I found two MME UE S1-AP IDs i.e. MME UE S1AP ID (Mandatory) and MME UE S1AP ID 2 (optional).
I got why mandatory one is present but what's use of optional one.

Can someone please explain it ?

Answer 1

NAS communication is always between MME and UE and between MME and eNB S1AP communication is there. When MME has sent E-RAB setup message to eNB, in same message piggybacked NAS-PDU, so extra signalling is not required for the NAS messages and NAS-PDU is always transparent for eNB. Extra signalling is always increase latency for established dedicated bearer.
As per my knowledge it is just a optimization of signalling by using Piggybacked.

Answer 2

I am not sure if I have understood your question correctly. If you mean to ask "what info is passed in the piggybacked NAS pdu?" then the below answer may help you.

As part of dedicated bearer establishment, MME sends the E-RAB setup request to the enodeB & piggybacks a NAS pdu inside this msg. This NAS pdu is nothing but the "activate dedicated EPS bearer context request'.

It contains the EPS bearer Id, Linked EPS bearer Id, QOS info, Traffic Flow Template (TFT), etc.

This info is passed to the UE while RRC Reconfiguration. In response to this NAS msg, the UE sends an activate dedicated EPS bearer context accept message to acknowledge the activation of a dedicated EPS bearer context associated with the same PDN address and APN as an already active EPS bearer context.

After receiving the response from the UE, the MME knows that the dedicated bearer establishment procedure was successful.

Hope this helps!

Comments

Hi Ankush, just to add to your answer this procedure is also applicable for default bearer. I mean to say when UE explicitly ask to network for a new PDN connection.

Answer 3

Each E-RAB is created either based on UE request or network takes decision to create a new bearer for an UE. In both cases, when UE triggers or network triggers it is a NAS related message.

Most of the lte procedure for an UE has two portion. One part of signaling is done at core network and another part of signaling is done towards access side. For example: An attach procedure for an UE is divided into two parts: one is known as RRC signalling and rest of signaling messages towards core network.

Likewise, UE may request to MME network to create a new bearer by using "Bearer Resource Allocation Request" NAS message. Once MME finishes core network signalling to establish E-RAB, it prepares NAS message i.e. known as "Activate dedicated EPS bearer context Request" message to UE by using S1-AP "E-RAB setup request" message.

As part of this bearer setup procedure, eNodeB configures it's own layers for this new bearer. Finally, eNodeB prepares and sends RRC connection reconfiguration message to UE. The "Activate dedicated EPS bearer context request" message s is piggybacked within the RRC-connection reconfiguration message.