What is establishment cause in LTE?

An UE requests an RRC connection by sending an RRC Connection Request (Establishment Cause=“Mobile Originating Signaling”) message to an eNB. The “Mobile Originating Signaling” is a value used in the Establishment Cause field when a UE requests Attach, Detach or TAU (Tracking Area Update).

What are SRBs and DRBs?

The UE requires (dedicated) SRBs and DRBs to exchange data/messages with the network. In particular, for transfer of NAS messages, SRBs are established. They carry registration and signaling for user-plane setup. The user-plane application data is transported on DRBs.

Why is SRB2 needed?

SRB2 used to transfer RRC messages which use DCCH channel and encapsulates a NAS message. SRB1 is also used to encapsulate NAS message if SRB2 has not been configured. SRB2 has lower priority then SRB1 and it is always configured after security activation.

When does RRC establishment procedure fail on LTE?

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.

When to initiate RRC connection re-establishment procedure?

1. Upon re-entry into service area after radio link failure 2. Handover failure 3. When lower layers detect problems as specified in TS 36.322 7. RRC connection reconfiguration failure UE in RRC Connected state can initiate RRC Connection Re-Establishment procedure in order to keep the RRC connection while security is activated.

How does SRB1 work in LTE RRC connection?

After confirmation SRB1 operation is resumed. Once this is completed all the subsequent messages are send ciphered and integrity protected using previously configured (respective) algorithms and newly derived keys KRRCenc and KRRCint respectively. This message is used to indicate the rejection of an RRC connection reestablishment request.

How is the NAS message transferred in LTE?

The flat network architecture for LTE removes the requirement for these signalling procedures  In the case of LTE, the initial Non-Access Stratum (NAS) message is transferred as part of the RRC connection establishment procedure. In the case of UMTS, the initial NAS message is transferred after the RRC connection establishment procedure.