LTE: What is the use of RRC Connection Reconfiguration message ?

Question

During attach procedure, UE sends "Attach Request" along with "RRC Connection Setup Complete", eNodeB sends "Attach Accept" along with "RRC Connection Reconfiguration then why UE does not send "Attach Complete" along with "RRC Connection Reconfiguration Complete" message ? What could be the reason for defining the messages so ?

Answer 1

The UE receiving RRC Connection Reconfiguration message -with any related action, for e.x. mobility, measurements, physical config, radio config etc.- will validate a set of necessary conditions. If these conditions are not met then UE will trigger Re-establishment Procedure. There are couple of scenarios where the UE can behave in a different way and are stated below:

if the UE is unable to comply with (part of) the configuration included in the RRCConnectionReconfiguration message then roll back to the previous configurations
if security has not been activated and UE receives RRCConnectionReconfiguration message then proceed to release the RRC Connection with release cause 'other'

Answer 2

RRC CONNECTION RECONFIGURATION message is the command to modify an RRC connection. Its prime purpose is
- To establish/modify/release Radio Bearers
- To perform Handover
- To setup/modify/release Measurements
- To add/modify/release SCells
- Dedicated NAS Information might also be transferred from eNodeB to UE

Differences with UMTS:
- In LTE, RRC CONNECTION RECONFIGURATION is the only message used to perform all logical, transport, and physical channel configurations where as in case UMTS, there exists Transport Channel, Physical Channel and Radio Bearer Reconfiguration messages
- In LTE, RRC CONNECTION RECONFIGURATION can also be used to send NASdedicated signalling to the UE to reduce the latency whereas this option is not available in UMTS
- LTE has only one RRC connected mode state (RRC_CONNECTED), so it has only one Temporary Radio Network Temporary Identity (RNTI) i.e. C-RNTI
- In UMTS, MEASUREMENT CONTROL is used to setup/modify/release measurements where as in LTE, RRC CONNECTION RECONFIGURATION message is the only message for this purpose