RRC Connection Request

RRC Connection Request In LTE

RRC connection establishment is used to make the transition from RRC Idle mode to RRC Connected mode. UE must make the move to RRC Connected mode before transferring any application data, or completing any signalling procedures.

UE.................................................................................................EnodeB

The RRC connection establishment procedure is always initiated by the UE.
But this procedure can be triggered by both UE and Network.

In the case of LTE, the initial Non-Access Stratum (NAS) message is transferred as part of the RRC connection establishment procedure.

The approach used by LTE helps to reduce connection establishment delay

RRC connection establishment configures Signalling Radio Bearer (SRB) 1 and allows subsequent signalling to use the Dedicated Control Channel (DCCH) rather than the Common Control Channel (CCCH) used by SRB 0.

A 3-way handshake is used to move the UE into RRC connected mode.

The RRC Connection Request message is sent as part of the Random Access procedure. It is transferred using SRB 0 on the Common Control Channel (CCCH) because neither SRB 1 nor a Dedicated Control Channel (DCCH) have been setup at this point.

The uplink Resource Block allocation for the RRC Connection Request message is signalled within the Random Access Response message

Content Of The RRC Connection Request Message:

The content of the RRC Connection Request message includes a UE identity and an establishment cause.

UE identity is signalled using the SAE Temporary Mobile Subscriber Identity (S-TMSI) if the UE is registered with the Tracking Area to which the current cell belongs. Otherwise, the UE selects a random number in the range from 0 to 240 - 1 to represent the UE identity.




Relationship between higher layer establishment cause and RRC establishment cause:



 

•  The UE starts the T300 timer after transmitting the RRC Connection Request message.  The value of T300 is broadcast within SIB 2.
• LTE does not have an N300 parameter and the RRC Connection Request message is sent only once per establishment procedure.
• LTE uses the T300 timer to define how long the UE waits for a response to the RRC Connection Request message.     The establishment procedure fails if T300 expires before receiving an RRC Connection Setup message.