GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features. A partial listing of supplementary services follows.
• call forwarding - This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.
• barring of outgoing calls - This service makes it possible for a mobile subscriber to prevent all outgoing calls.
• barring of incoming calls - This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: bar ing of all incoming calls and barring of incoming calls when roaming outside the home PLMN.
• advice of charge (AoC) - The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.
• call hold - This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.
• call waiting - This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscr iber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.
• multiparty service - The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscr ibers. This service is only applicable to normal telephony.
• calling line identification presentation/ restriction - These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.
• closed user groups (CUGs ) - CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.
