This Recommendation, which defines protocol stacks for terminals and MCUs, specifies
network-specific aspects of the T.120 protocol suite in the form of profiles for each network
identified. Each profile specifies a set of protocols that extend to layer 4 of the OSI reference
model.
The rationale for this Recommendation is as follows: audiographic and video conferencing are
intended to form part of the repertoire of ISDN services. Teleconferencing via ISDN involves the
integration of multiple media (audio, video and data) in a connection, which may be the aggregate
of a number of physical channels. The provision of these services is not, however, limited to the
ISDN, and a range of other network scenarios is identified. For instance, CSDN may provide a
similar, though less flexible, service to that of ISDN, and PSTN may provide a service that, though
limited in performance, is more readily available. Teleconferencing may also be extended over
PSDN and B-ISDN. LANs may provide conferencing locally within an enterprise or a means of
access to wide area networks.
1.1 Networks identified
Network-specific profiles are defined for ISDN, CSDN, PSDN and PSTN, as required by
[ITU-T F.702]. The scope of this Recommendation also includes B-ISDN and LAN.
1.2 Audio and video signals
The handling of audio and video signals in a multimedia conference is not part of this
Recommendation, other than the possibility of their multiplexed transport over the same connection
in some cases.
1.3 ISDN call set-up
Examples of ISDN call set-up procedures for the audiographic teleconference are given in
Appendix I. These procedures illustrate:
a) the use of ISDN information elements;
b) coordination of the D-channel and the B-channel;
c) the phases of connection establishment;
d) interworking with telephone services.