■■ So-called Web Conferencing in which a browser window is used to pre-
sent conferencing information such as a presentation or slide show, the
participant roster, and so on.
The main issues we have with the H.3xx type of conferencing products is the
way they are architected, based on ITU network models, and their technology,
which makes them a poor fit for the Internet and the World Wide Web. Thus,
integration with other communication and Web services is difficult, and there
are a number of divergent approaches (such as for security and scalability).
Considerable effort is expended in both the ITU and IETF on interoperabil-
ity between SIP and H.323 signaling, but this work is mainly for telephony. A
summary on H.323-SIP internetworking aspects is provided in Schulzrinne
and Agboh [2].
Web conferencing services are currently limited in that they require a full-
featured Web browser, often with specialized plug-ins that must be installed
prior to using the Web conferencing service. Smaller mobile devices are not
able to be participants in Web conferences, even though they have a simple
Web browser built in. Despite the use of the Web, Web conferencing systems
actually represent yet another proprietary signaling channel that fails to inte-
grate all media types and devices.
Most commercial telecom conferencing services are based on a central mul-
tipoint control unit (MCU) that serves both as the central signaling control
point and media mixer. Rosenberg provides a detailed review of SIP confer-
encing models in the SIP Conferencing Framework [3]. The requirements for
SIP conferencing are described in [4]. The Best Current Practice (BCP) docu-
ment for SIP conferencing is defined in [5], which utilized the isfocusfeature
tag defined in RFC 3840 [6].
We will present in this chapter the Internet conferencing models and ser-
vices based on SIP that include the MCU model but also have other flexible
approaches for various other conferencing models.SIP Conferencing Models
IP conferences may differ in many respects, depending on the signaling to set
up the conference and the way media is transported and mixed for the confer-
ence participants. Table 14.1 shows the main conference models possible. The
models are roughly ordered by the possible scale of the conference. We will use
the generic term “conference bridge” in the table, since the conferencing net-
work element is sometimes a SIP server only, and sometimes a SIP server com-
bined with an RTP media mixer. Telephony conferencing network elements
are sometimes called multipoint controller units to emphasize the control
aspects for certain telephony conference types.246 Chapter 14