6.2.2 Handling of USSD request at MSC

23.0903GPPRelease 17Stage 2TSUnstructured Supplementary Service Data (USSD)

When an MSC receives a USSD request containing an HPLMN service code, it shall set up a transaction to the VLR and forward the request unchanged. If this forwarding fails, an error shall be returned to the MS. The MSC shall be transparent to any further requests or responses (in either direction) for that transaction, passing them between the MS and VLR without taking any action. When one transaction is released (MS-MSC or MSC-VLR), the MSC shall release the other.

If an HPLMN service code is not included, the MSC shall process the request locally (see clause 6.2.5).

If the MSC does not support the alphabet used in a USSD request, it shall set up a transaction to the VLR and forward the request unchanged, in the same way as when a HPLMN service code is received.