Step of RRC Connection Setup in LTE

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.

Steps of RRC Connection Setup in LTE

RRC (Radio Resource Control) connection setup in LTE is the process that establishes communication between your device (UE) and the network. This process ensures that your device is properly connected, authenticated, and ready to send and receive data. Here’s how it works step by step:

1. RRC Connection Request

The process starts when your device needs to connect to the network. It sends an RRC Connection Request message to the eNB (evolved NodeB). This request includes information like the device’s identity and type of service it needs (e.g., voice, video, data).

2. RRC Connection Setup

In response, the eNB sends an RRC Connection Setup message to the device. This message includes the necessary configuration parameters, such as encryption and security settings, to prepare the device for communication with the network.

3. RRC Connection Setup Complete

Once the device receives the connection setup message, it responds with an RRC Connection Setup Complete message. This confirms that the device has successfully received the setup information and is now ready to communicate with the network.

4. Security Mode Command

To ensure secure communication, the network sends a Security Mode Command message. The device then uses this command to activate encryption and integrity protection for data exchange, ensuring privacy and data integrity.

5. Security Mode Complete

After the device applies the security settings, it sends a Security Mode Complete message back to the network, confirming that all security measures are active.

6. RRC Connection Established

Finally, the network sends an RRC Connection Established message to the device, confirming that the connection is fully set up, and communication can begin. At this point, your device is connected to the network and ready to send/receive data.

Why This Matters

These steps ensure a reliable and secure connection between your device and the LTE network. From security to data routing, RRC setup plays a crucial role in providing you with smooth mobile services.