Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Each conference model shown in Table 14.1 differs from the other models by

one or more of the following:

■■ Scale of the conference

■■ Call flows for users to join the conference

■■ How and where the media is sent and mixed

■■ Location of the service logic (in endpoints or in servers)

The conference models apply equally well for both audio-only (as in tele-

phony conferences) and for mixed-media conferences (for audio, video, and

text). Depending on the quality of video that users may send and receive

because of bandwidth limitations, several IP addresses may be required for

layered video codecs. Users with the lowest bandwidth may send and receive

only the basic video layer, suitable for small images only. We will focus, for

simplicity, on audio conference examples in the following.

Small-scale conferences do not require any support from network servers,

since a few RTP streams may be mixed in one of the endpoints that originates

the conference. Such a small-scale conference model is shown in the first row

of Table 14.1.

In the second row of Table 14.1, a pure SIP service is shown, with no media

mixing provided in a server. A SIP server for conferences can support confer-

ences by setting up a full mesh of RTP streams between participants. Each par-

ticipant mixes all incoming streams for individual use. Since it is unlikely to

experience more than one or two speakers at the same time, the required RTP

processing in the user endpoints is quite modest.

Telephony-style conferencing is shown in the third row in Table 14.1. The

conferencing bridge(see Figure 14.1) is a conceptually simple device, consisting

of a SIP user agent to handle signaling, an RTP mixer to handle the media

streams, and a conference application layer for the authentication, authoriza-

tion, and accounting (AAA) service, and possible conference control functions,

as shown in Table 14.1. The RTP mixer will send out to each participant the mix

of media streams from all other participants.

Figure 14.1 Generic conferencing bridge

ENDPOINT INITIATES

CONFERENCE AND ACTS

AS MEDIA MIXER

GATEWAY

PSTN

248 Chapter 14