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LTE Attach and Default Bearer Setup Sequence Diagram

LTE Attach and Default Bearer Setup

Click to open the PDF sequence diagram describing LTE attach and default bearer setup

This call flow describes the signaling involved in LTE attach and default bearer setup. The sequence diagram covers the following phases:

  1. Random Access Procedure: The UE uses a slotted aloha procedure to access the eNodeB. The UE sends a preamble on a random access channel, the network replies with corrected timing and resource assignments.
  2. RRC Connection Establishment: The UE and the eNodeB then interact to establish a Signaling Radio Bearer (SRB). The SRB will be used for the signaling between the eNodeB and the UE.
  3. Attach and Authentication: The UE now registers with the Core Network. Session contexts are setup at the MME and the Serving Gateway. This step also results in the authentication of the UE and the Network.
  4. Default Radio Bearer Setup: Finally, the default bearer for data transfer is established. Default bearer session is established at the UE, eNodeB, MME, Serving GW and PDN Gateway. User data sessions is exchanged once the default bearer is setup.

The sequence diagram also contains links to access further details about the messages. Click on the messages with blue title for details about the message.

RRC Connection, LTE Attach and Default Radio Bearer Setup

 

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LTE RRC Connection Setup

lte-rrc-connection

Establishing an RRC connection in LTE typically involves the following steps:

  1. UE initiates a session to the eNodeB using a random RA-RNTI.
  2. eNodeB assigns a C-RNTI and sends timing adjustment to the UE.
  3. UE adjusts the the timing and requests an RRC Connection Setup via the UL-SCH channel.
  4. The eNodeB sends the RRC Connection Setup to the UE on the DL-SCH channel. This message sets up the SRB. DRB may also be established at this point.
  5. Finally, the UE acknowledges the message with RRC Connection Complete message.

The messages in the RRC Connection Setup are described in the following presentation.

LTE RRC Connection Setup Messaging