create them or to terminate them. Accordingly, a SIP INVITE for a multicast conference is used only to INVITE users and not to establish the conference session. SIP BYEs are also not used when a user exits the conference. Note that each participant typically has a SIP signalling rela- tionship with just one other participant, not with all the participants in the conference. Multicast conferences work well for prearranged events with a large number of participants. However, for smaller events or for ad-hoc confer- ences, obtaining multicast addresses to deliver the media is not worthwhile. Hence, SIP supports other conferencing models that don't scale as well but provide other advantages. End User Mixing Model It is common for a two-party session to add participants as time goes by. The most straightforward way to conference in a new participant is for a current member of the session to INVITE him or her. The member who issues the Chapter 7 INVITE will receive media from both the previous member and the new member. He or she will then have to mix the media and send the result to both of them. This mechanism, depicted in Figure 7-17, works when the user perform- ing the mixing is also the last person to leave the conference. But as the number of participants grows, the processing power required to mix the media from all participants will quickly exceed the user's equipment capa- bilities. So the end user mixing model is great for small ad-hoc conferences, but it cannot successfully be extended to larger sessions. Other conference models supported by SIP, notably those using a Multipoint Control Unit (MCU), are preferable. Multipoint Control Unit (MCU) MCUs are used for both prearranged and ad-hoc conferences. In a pre- arranged conference, users INVITE the MCU and the MCU mixes the media and sends it to all of them, establishing a signalling relationship between the MCU and each participant. Figure 7-17 End user mixing conference model. Signalling Media Signalling Media Signalling Media Building Applications with t