'Combined Attach' means the MS is making an attach to the CS and PS domains in the same time. And this will save the radio resources of the network.
This will be available if the Gs interface exists between the CS domain and the PS domain. In addition, the MS should support working in both CS and PS domains.
Combined Attach signaling
Steps (from 4 to 11) showing the SGSN-MSC/VLR association establishment
- The MS sends 'Attach request' to the new SGSN, with attach type set as 'combined attach'.
- Identification and authentication occurs between new SGSN , old SGSN, MS, and HLR as exist in the ‘GPRS Attach procedure’.
- 'Update Location' and 'insert subscriber data' occurs between new SGSN, HLR, old SGSN as exist in 'GPRS Attach procedure'.
- The SGSN derive the new MSC/VLR address from the RAI sent from the MS. the SGSN sends 'Location Update Request' to the MSC/VLR including the (new LAI, IMSI, SGSN Number).
- If the new MSC/VLR is different than the old MSC/VLR, the new MSC will send 'Update Location' to the HLR telling it about the new MSC/VLR number serving this MS.
- The HLR sends 'Cancel location' to the old MSC/VLR
- Then, the old MSC/VLR replies with 'Cancel Location Ack'.
- The HLR sends 'Insert subscriber data' to the new MSC/VLR, telling it about the subscription data of the MS.
- Then, the new MSC/VLR replies with 'insert subscriber data Ack'.
- And, the HLR replies with 'Update Location Ack'.
- Now, the new MSC/VLR replies the new SGSN with 'Location Update accept'.
- Then, the new SGSN replies the MS with 'Attach accept'. And here, the SGSN will add an information element, which is the 'VLR TMSI', which is a temporary ID generated from the MSC/VLR for this MS.
- The MS will reply the new SGSN with 'Attach complete'.