Supplementary Services
Supplementary services includes the following operations:
registerSS eraseSS activateSS deactivateSS interrogateSS registerPassword getPassword
In addition to these supplementary services, the following operations are considered unstructured supplementary services:
The following section introduces the unstructured supplementary services (USSs) concept and discusses operations.
Unstructured Supplementary Services (USSs)
GSM 02.04 defines supplementary services. In addition to supplementary services, GSM has defined the concept of USSs. USSs allow PLMN operators to define operator-specific supplementary services and to deliver them to market quickly. The final three operations listed at the beginning of this chapter are used in USS implementation. USS allows the MS (subscriber) and the PLMN operator-defined application to communicate in a way that is transparent to the MS and intermediate network entities.
The communication is carried out using Unstructured supplementary service data (USSD) data packets, which have a length of 80 octets (91 ASCII characters coded, using seven bits) and are carried within the MAP operation. USSD uses the dialogue facility (which is connection oriented) of TCAP and is specified in GSM 02.90 (USSD Stage 1) and GSM 03.90 (USSD Stage 2). Unlike SMS, which is based on a store and forward mechanism, USSD is session oriented and, therefore, has a faster turnaround and response time than SMS, which is particularly beneficial for interactive applications. USSD can carry out the same two-way transaction up to seven times more quickly than SMS can.
The wireless application protocol (WAP) supports USSD as a bearer; the mobile chatting service relies on USSD transport for the text, and most, if not all, prepay roaming solutions are implemented using USSD. With such prepay applications, the subscriber indicates to the network from a menu on the MS the desire to place a roaming call. The serving MSC connects to the subscriber's HLR, which sends the request to a USSD gateway, which, in turn, sends the request to a prepay application server. The server checks the balance and then issues call handling instructions back to the MSC in the visited network. USS is still likely to find applications even in 3G networks.
Operations
The following bullets describe the operations for supplementary services and unstructured supplementary services:
The registerSS operation is used to register a supplementary service for a particular subscriber. The supplementary service (such as call forwarding) is often automatically activated at the same time.
EraseSS is used to delete a supplementary service that was entered for a particular subscriber using registerSS.
ActivateSS is used to activate a supplementary service for a particular subscriber. Example supplementary services include CLIP/CLIR.
This operation switches off a supplementary service for a particular subscriber; it is the reverse of activateSS.
InterogateSS allows the state of a single supplementary service to be queried for a particular subscriber in the HLR.
This operation is used to create or change a password for a supplementary service. When the HLR receives this message, it responds with a getPassword message to request the old password, the new password, and a verification of the new password. If the old password is entered incorrectly three consecutive times, this operation is blocked.
The HLR sends this message if the subscriber wants to change his current password or modify or activate a supplementary service. See also registerPassword. This operation is blocked if the old password is entered incorrectly three consecutive times.
This message is used to provide a means to support non-GSM standardized supplementary services. Both the MS and the addressed NSS network entity use it, only if the MS initiated the transaction.
Same as processUnstructuredSS-Request, except that both the MS and the addressed NSS network entity use it, only if the NSS entity initiated the transaction.
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