scope:

The present document specifies signalling interworking between "QSIG" and "H.323" in support of call diversion supplementary services within a Corporate telecommunication Network (CN).

"QSIG" is a signalling protocol that operates at the Q reference point between Private Integrated Services eXchanges (PINX) within a Private Integrated Services Network (PISN). The Q reference point is defined in ECMA-133 [1]. A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other ECMA Standards, in particular ECMA-143 [2] (call control in support of basic services), ECMA-165 [3] (generic functional protocol for the support of supplementary services) and a number of standards specifying individual supplementary services. ECMA-174 [4] specifies the QSIG protocol in support of call diversion 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 H.225.0 [6] and H.245 [7] (basic communication capabilities) and Recommendation H.450.1 [9] (generic functional protocol for the support of supplementary services). ITU-T Recommendation H.450.3 [10] specifies the H.323 protocol in support of call diversion services.

NOTE: H.450.3 applies only to the 1998 version of ITU-T Recommendation H.323 [8] (also known as H.323 version 2) and to later versions.

In both ECMA-174 [4] (QSIG) and ITU-Recommendation H.450.3 [10] (H.323), the call diversion supplementary services are Call Forwarding Unconditional (SS-CFU), Call Forwarding Busy (SS-CFB), Call Forwarding No Reply (SS-CFNR) and Call Deflection (SS-CD). These supplementary services apply during call establishment and provide diversion of an incoming call to another destination.

Interworking between QSIG and H.323 permits a call originating at a user of a PISN to terminate at a user of an IP network, or a call originating at a user of an IP network to terminate at a user of a PISN. The present document provides the following additional capabilities:

- a call originating from a PISN and destined for a user of an H.323 network to be diverted by the H.323 network to an alternative destination;

- a call originating from an H.323 network and destined for a user of a PISN to be diverted by the PISN to an alternative destination;

- a call destined for a user of a PISN to be diverted to an alternative destination where that alternative destination is in an H.323 network;

- a call destined for a user of an H.323 network to be diverted to an alternative destination where that alternative destination is in a PISN.

The present document is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG and an IP network employing H.323.