When enodeB sends RRC-Connection-Reconfiguration message?

Question

There are couple of procedures are defined which consist of NAS as well as RRC signalling messages exchange.
Some of the NAS message sends to an UE through RRC Connection Reconfiguration message as piggybacked and some as DL Info transfer.

I want to know, Is there any procedure exists in lte when an UE receives RRC Connection Reconfiguration along with NAS message and responds back to network with NAS message first and then Reconfiguration Complete to eNodeB.

Usually, I saw UE sends RRC Connection Reconfiguration Complete message first then NAS response message.

Answer 1

RRC Reconfiguration messages is triggered by the ENBs in different cases according to different requirements Following are the some cases :-

1. When ENB wants neighbour cell information so he will send measurement configuration in RRC Reconfig message so that ENB can get measurement report.
2. In case of Handover happening from one ENB to other ENB UE need target ENBs data (New Identity, Target PCI, RACH Configuration ) so the Source ENB get these info from Target ENB in Handover ACK message and after this Source ENB sends these info to UE so that UE can aware of target ENB.So RRC Reconfig is used to send mobility control info.
3. If ENB forwards the NAS messages to UE in Dedicated NAS info in RRC Reconfg message.this message is used as transparent container for ENB.
4. If ENB wants add/delete/modify a bearer , modifying configuarations like mac ,phy , sps config he sends these info as part of radio resource info in RRC Reconfig.
5. This message is also used for sharing security related info during handover case as part of Security config in RRC Reconfig message.

Most of the time this message flow happens at any time ENB wants to change configurations related to these cases.

Answer 2

Intra-frequency neighbours can be defined automatically as part of the normal intra-frequency handover procedure. The addition of a neighbour increases the handover signalling requirement but once added, the neighbour can be used by all other UE.

The intra-frequency handover procedure starts when the serving eNode B sends an RRC Connection Reconfiguration message which instructs the UE to start intra-frequency measurements.

The UE searches for neighbouring cells, identifies their Physical Layer Cell Identities (PCI) and measures their RSRP (and/or RSRQ). The UE then follows the instructions of the eNode B in terms of reporting, e.g. when a reporting event is triggered
The eNode B receives the neighbour cell measurements and decides whether or not the UE should complete a handover. If a handover is to be completed, the eNode B checks whether or not the reported PCI is included within its existing neighbour database.

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