Routing Area Update ‘RAU’ in GPRS

Thr Routing Area Update Procedure occurs when the MS moves between different Routing Areas (RAs).
The MS will trigger this procedure when it detects that the RAI sent from the network is different than the RAI exists in its own memory.


Types of Routing Area Update 'RAU'

Routing Area Update 'RAU' types
Routing Area Update 'RAU' types

Intra-SGSN RAU

The Intra SGSN RAU occurs when the MS moves from one RA to another RA, and both RAs exist under the same SGSN.

Inter-SGSN RAU

The Inter-SGSN RAU occurs when the MS moves from one RA to another RA, and both RAs exist under different SGSNs.

Inter-RAT RAU

The Inter-RAT RAU occures when the MS moves from 2G to 3G or vice versa.

Routing Area Update 'RAU' signaling

Part 1 - Identifying and authenticating the MS

RAU signaling - part 1
RAU signaling - part 1

1. 'RAU request' (MS --> SGSN)

  • The MS sends 'RAU request' to the new SGSN

2. 'SGSN context' (new SGSN <--> old SGSN)

  • The 'SGSN context' is the 'MM context' and 'PDP context' of an MS.
  • The New SGSN can derive the old SGSN from the P-TMSI and old RAI sent from the MS in the RAU request.
  • Therefore, the new SGSN sends 'SGSN context request' requesting the SGSN context from old SGSN.
  • Then, the old SGSN replies with 'SGSN context response'.
  • Before sending the SGSN context from the old SGSN to the new SGSN, the old SGSN can authenticate the MS using the P-TMSI and the P-TMSI signature exist in the MM context.
  • If the authentication failed on the old SGSN, the new SGSN will trigger authentication procedure for this MS with the HLR.

3. MS authentication (MS <--> SGSN <--> HLR)

  • In case the MS authentication failed on the old SGSN, the new SGSN will trigger authentication procedure with the HLR.
  • The new SGSN will request the HLR to send the MS authentication info , and will ask the MS to authenticate itself.

4. 'SGSN context ack' (new SGSN <--> old SGSN)

  • The new SGSN will acknowledge the old SGSN with the receive of the SGSN context.

5. 'Forward packets' (old SGSN --> new SGSN)

  • Now, if the MS is having an active data session, the old SGSN will start forwarding the data packets received from the GGSN to the new SGSN. This will remain till the link is establishment completes between the new SGSN and the GGSN.
  • This is done in order not to lose in data during the RAU procedure.

6. 'Update PDP context' (SGSN <--> GGSN)

  • The new SGSN will start establishing the link between the new SGSN and the GGSN.
  • The new SGSN achieve this by sending 'Update PDP context request' to the GGSN, telling the GGSN with the address of the new SGSN.
  • Then, the GGSN will send 'Update PDP context response' to the new SGSN.
  • Now, the link establishment completed between the new SGSN and the GGSN.

Part 2 - Updating the Location and get subscription data

RAU signaling - part 2
RAU signaling - part 2

7. Update Location and get subscription data (SGSN <--> HLR)

The new SGSN will inform the HLR with its own address, and will ask the HLR to provide the subscription data of the MS.

a. the new SGSN will send 'Update Location' to the HLR, telling it with the address of the new SGSN

b. The HLR will send 'Cancel Location' to the old SGSN asking old SGSN to delete the MM context of the MS. Then, the old SGSN will send 'Cancel Location Ack' to the HLR acknowledging it with the deletion of the MM context of the MS.

c. The HLR will send the MS subscription information to the new SGSN through sending 'insert subscriber data' to it.

d. the new SGSN will acknowledge the receive of 'insert subscriber data'

e. the HLR will acknowlege the receive of 'Update Location'.

8. 'RAU accept' (SGSN --> MS)

  • The new SGSN will send 'RAU accept' to the MS including the P-TMSI and the P-TMSI signature.

9. 'RAU complete' (MS --> SGSN)

  • Then, the MS will confirm the receive of 'RAU accept' by sending 'RAU complete' to the new SGSN.