In the GSM and similar systems, USSD (Unstructured Supplementary Service
Data) transfer takes place on a fast channel if a mobile station is
involved in that call, and otherwise on a slow channel. The USSD transfer
can be switched to the fast channel by determining the amount of data to
be transmitted. If the amount of data to be transmitted is likely to
exceed a predetermined threshold, and if the mobile station is not
involved in a call, a call attempt is initiated for switching the USSD
transfer to the fast channel.
