After the RRC connection, the Initial network attach allows to establish all the bearers to carry the data from the UE to the gateway.
- After the RRC connection, Signaling Radio Bearers (SRBs) are established.
- An SRB is a Radio Bearer that only carries the signaling:
- SRB1 carries the RRC signaling.
- SBR2 carries the NAS signaling, i.e. between the Core Network and the UE.
- An MME is selected.
- The UE is authenticated.
- An IP address is allocated to the UE.
- S-GW and P-GW are selected.
- Bearers are established on the S1-U, S5/S8 and on the air interface.
- The RRC connection is reconfigured to allow user data traffic.
- At the end of the Initial Attach, the UE is able to reach external networks.
The RRC Connection is basically made up of 2 steps: