he SIP Toolkit Alternatively, instead of having users invite the MCU, the MCU can INVITE participants to join a conference-a tactic particularly useful for prearranged events, where the MCU can INVITE all the users at once. In either case, the MCU handles both signalling and media for the conference (Figure 7-18). An MCU-based conference scales better than an end user mixing confer- ence. MCUs are prepared to handle media mixing and boast high process- ing capacity. However, an end user mixing conference can be converted into an MCU conference by means of the REFER method. In order to migrate from one model to the other, users are instructed to INVITE the MCU. Decentralized Multipoint Conference When media is sent in a decentralized fashion, it constitutes a special case of using a central signalling point such as an MCU. The conference unit handles signalling only when users send media using unicast or multicast (Figure 7-19). When multicast is used, the conference unit returns an SDP Figure 7-18 Conference with an MCU. Signalling Signalling Media Media Signalling Signalling Media Media Chapter 7 Figure 7-19 Decentralized multipoint Media conference model. Signalling Signalling Media Media Signalling Signalling Media description to each participant with the multicast address that will be used. In the case of unicast, the conference unit adds a new m line though a re- INVITE every time a new participant joins the conference to trigger all cur- rent users to send media to the new participant as well. This model simplifies the building of conference units because it obviates the need to handle media. In the case of multicast media, this model scales very well and a conference can handle many signalling relationships. When unicast media is used, though, the end systems have to send the same media in parallel to several different locations. End systems with low pro- cessing power might experience difficulty as the number of participants grows and choose to migrate to an MCU that handles media as well. Building Applic