A Routing Area Update (RAU) is a procedure in mobile communication networks, specifically in GPRS (General Packet Radio Service) and later technologies like LTE, to maintain the user equipment (UE) registration status as it moves between different routing areas within the same or different serving GPRS support nodes (SGSN).
Functions of RAU
- Mobility Management: Ensures that the network knows the current location of the UE so that it can efficiently route data to it.
- Session Continuity: Keeps ongoing sessions active as the UE moves between routing areas.
- Resource Optimization: Helps in the efficient utilization of network resources by updating the location information of the UE.
Routing Area Update Procedure (GMM).
- Initiated by UE whenever a change of Routing Area is detected.
- Very similar to Location Area Update procedure.
The Routing Area Update procedure is initiated by the UE whenever a change of Routing Area is detected. This procedure is very similar to the Location Area Update procedure. The first NAS message sent in the uplink is Routing Area Update Request. This will trigger the SGSN to verify the UE subscription with the HLR/VLR. Again optional procedures are available regarding authentication and ciphering.
The procedure is finalised with the Routing Area Update Accept message and the optional Routing Area Update Complete. Sometimes the Routing Area Update is rejected and a Routing Area Update Reject message is sent in the downlink. Reasons could be illegal UE, or GPRS services not allowed.
Types of Routing Area Updates
- Periodic Update: Occurs at regular intervals to confirm that the UE is still present in the network.
- Normal Update: Triggered when the UE moves to a different routing area.
- Combined RA/LA Update: Updates both the Routing Area and Location Area simultaneously, typically used in networks supporting both GPRS and circuit-switched services.
RAU Procedure
- RAU Request: The UE detects a change in the routing area and sends an RAU Request message to the network. This message is sent to the new SGSN if the UE has moved to a new routing area managed by a different SGSN.
- Identity Check: The new SGSN may perform an identity check and retrieve the UE’s context from the old SGSN if necessary.
- Authentication and Security: The network may authenticate the UE to ensure secure communication.
- Update SGSN Context: The new SGSN updates its records and may establish new tunnels for data transmission.
- RAU Accept: The new SGSN sends an RAU Accept message to the UE, confirming the update.
- RAU Complete: The UE responds with an RAU Complete message to acknowledge the update.
RAU Messages
- RAU Request: Sent by the UE to initiate the update.
- RAU Accept: Sent by the SGSN to acknowledge the update.
- RAU Complete: Sent by the UE to confirm the update.
- RAU Reject: Sent by the SGSN if the update request cannot be accepted.
Importance of RAU
- Ensures Network Connectivity: Keeps the network updated with the UE’s location, ensuring seamless connectivity.
- Maintains Session Continuity: Helps in maintaining ongoing sessions without interruption as the UE moves.
- Efficient Resource Use: Optimizes network resources by updating the location information of the UE.
Example Scenario
- UE Moves to New Routing Area: A UE moves from Routing Area 1 to Routing Area 2.
- RAU Request Sent: The UE detects the change and sends an RAU Request to the SGSN managing Routing Area 2.
- SGSN Context Update: The new SGSN may contact the old SGSN to retrieve the UE’s context and update its records.
- RAU Accept Sent: The new SGSN sends an RAU Accept message to the UE.
- RAU Complete Sent: The UE sends an RAU Complete message to confirm the procedure.
Conclusion
The Routing Area Update (RAU) procedure is crucial for maintaining the UE’s registration status and ensuring seamless connectivity as it moves between different routing areas. It helps manage mobility, maintain session continuity, and optimize network resources, making it an essential process in mobile communication networks.