In LTE, Complete connection is divided into two steps.
1. UE and ENB connection.
2. UE and MME connection.
1.While RRC connection setup procedure, SRB1 is established, which is also known as DCCH (Dedicated control channel). It is used to carry RRC messages. It might be used to carry NAS message as a piggy backing.
Please check RRC Connection Reconfiguration message which contains NAS info as a piggybacking.
- While UE and MME signalling, which is known as NAS signaling. MME sends Initial conetext setup Req message to establish data radio bearer only. it is up to eNB that it will establish SRB2 and use to send NAS messages only or it will use SRB1.
SRB1 has higher priority over SRB2 if SRB2 is also established for an UE.