scope:

The present document specifies signalling interworking between "QSIG" and "H.323" in support of the call transfer 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 [2]. A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other ECMA Standards, in particular ECMA-143 [3] (call control in support of basic services), ECMA-165 [4] (generic functional protocol for the support of supplementary services) and a number of standards specifying individual supplementary services. ECMA-178 [5] specifies the QSIG protocol in support of call transfer by consultation and ECMA-300 [6] specifies the QSIG protocol in support of single step call transfer.

"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 [8] and H.245 [9] (basic communication capabilities) and Recommendation H.450.1 [11] (generic functional protocol for the support of supplementary services). Recommendation H.450.2 [12] specifies the H.323 protocol in support of call transfer.

NOTE: H.450.2 applies only to the 1998 version of ITU-T Recommendation H.323 [1] (also known as H.323 version 2) and to later versions.

Call transfer by consultation, as supported by ECMA-178 [5], is a supplementary service that enables a user (user A) to transform two of that user's calls (at least one of which must be answered) into a new call between the two other users in the two calls (users B and C).

Single step call transfer, as supported by ECMA-300 [6], is a supplementary service that enables a user (user A) to transform an existing call between user A and user B into a new call between user B and user C without user A needing to establish a call with user C prior to transfer.

Call transfer, as supported by H.450.2 [12], is a supplementary service that enables the served user (user A) to transform an existing call with a second user (user B) into a new call between user B and a third user (user C) selected by user A. User A may or may not have a call established with user C prior to call transfer. If a call is already established between user A and user C, this is known as transfer by consultation and is equivalent to call transfer as supported by ECMA-178 [5]. If a call is not already established between user A and user C, this is known as single step call transfer and is equivalent to call transfer as supported by ECMA-300 [6].

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 PISN user with two calls established, at least one of which being to or from a user in an IP network, to be able to transform those two calls into a new call between the other two users involved;

- a PISN user with a call established to or from a user in an IP network to be able to transform that call into a new call between the IP network user and a third user selected by the PISN user, that third user being either in the IP network or in the PISN; - a PISN user with a call established to a second PISN user to be able to transfer that call into a new call between the second user and a third user selected by the first user, that third user being in an IP network;

-an IP network user with two calls established, at least one of which being to or from a user in a PISN, to be able to transform those two calls into a new call between the other two users involved;

- an IP network user with a call established to or from a user in a PISN to be able to transform that call into a new call between the PISN user and a third user selected by the IP network user, that third user being either in the IP network or in the PISN; and

- an IP network user with a call established to a second user in the IP network to be able to transfer that call into a new call between the second user and a third user selected by the first user, that third user being 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.