ISO/IEC 21991:2002 specifies signalling interworking between "QSIG" and "H.323" in support of call completion supplementary services within a Corporate telecommunication Network (CN).

"QSIG" is a signalling protocol that operates at the Q reference point between Private Integrated services Network eXchanges (PINX) within a Private Integrated Services Network (PISN). The Q reference point is defined in ISO/IEC 11579-1. A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other Standards, in particular ISO/IEC 11572 (call control in support of basic services), ISO/IEC 11582 (generic functional protocol for the support of supplementary services) and a number of standards specifying individual supplementary services. ISO/IEC 13870 specifies the QSIG protocol in support of call completion services.

"H.323" is a set of signalling protocols for the support of voice or multimedia communication within a packet network, in particular a packet network that uses the Internet Protocol (IP) as its network layer protocol (IP network). H.323 signalling protocols operate between endpoints in an IP network, either indirectly via one or more gatekeepers, or directly. An endpoint can be a terminal or a gateway to another network. H.323 is an "umbrella" recommendation referring to various ITU-T recommendations, in particular Recommendations H.225.0 and H.245 (basic communication capabilities) and Recommendation H.450.1 (generic functional protocol for the support of supplementary services). Recommendation H.450.9 specifies the H.323 protocol in support of call completion services.

NOTE H.450.9 applies only to the 1998 version of H.323 (also known as H.323 version 2) and to later versions.

In both ISO/IEC 13870 (QSIG) and ITU-Recommendation H.450.9 (H.323), the call completion supplementary services are Completion of Calls to Busy Subscribers (SS-CCBS) and Completion of Calls on No Reply (SS-CCNR). These supplementary services apply after a call establishment attempt has failed because the called user was busy or not available, and provide means to re-establish the call when the called user becomes available.

Interworking between QSIG and H.323 permits a call originating at a user of a PISN to terminate at a user of a private IP network, or a call originating at a user of a private IP network to terminate at a user of a PISN. In such a scenario, ISO/IEC 21991:2002 allows the completion of calls when the called user becomes available after having been busy (SS-CCBS), or having not answered the original call (SS-CCNR).

Interworking between a PISN employing QSIG and a public IP network employing H.323 is outside the scope of ISO/IEC 21991:2002. However, the functionality specified in ISO/IEC 21991:2002 is in principle applicable to such a scenario when deployed in conjunction with other relevant functionality (e.g., number translation, security functions, etc.).

ISO/IEC 21991:2002 is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG and a private IP network employing H.323.