When the UE is powered up, it has to be RRC connected to be able to exchange data and signaling with the network.
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.
During the Initial Attach:
- 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:
- Contention Based Random Access.
- Exchange of Signaling to establish the connection.
- When a UE requests a connection, it has no dedicated resources to reach the eNodeB. It uses an uplink common channel which is able to manage the collision between 2 UEs requesting an access at the same time.