lling to receive voice calls, but Building Applications with the SIP Toolkit Presence Bob's Bob's Figure 7-4 Laura server laptop workstation Bob has two PUAs that send presence (1) REGISTER information to the Unavailable (2)200 OK presence server. (3) REGISTER Available (4)200 OK (5) SUBSCRIBE Bob's presence info (6)202 Accepted (7) NOTIFY Bob is available on his worksation (8)200 OK want to receive instant messages. Or Bob could even be unwilling to receive voice calls from his boss, but happy to receive them from Laura. The SIP REGISTER method and the SUBSCRIBE/NOTIFY framework provide SIP with enough flexibility to implement this type of service in some granularity. Instant Messaging Although presence information can be used to provide a wide range of ser- vices, instant messaging is the one that has been most closely associated with presence. Simple presence systems that just provide available or unavailable status have typically been used to tell users who is or isn't receiving messages. We've stipulated that the MESSAGE [draft-rosenberg-impp-im] method can be used to send instant messages in SIP (Figure 7-5). The advantage of using SIP is that it does more than just enable presence information and instant messaging to be combined. SIP also lets presence information be utilized in establishing any kind of session, including instant messaging and multimedia sessions. Therefore, for applications that offer combined services, SIP is definitively the choice as a presence and instant messaging protocol. Chapter 7 Figure 7-5 How do you spell the Bob sends an instant name of the place we are message to Laura. going on vacation? Laura (1) INVITE (2) 200 OK (3) ACK (4) MESSAGE "Costa del Sol" (5) 200 OK PacketCable PacketCable is a project carried out by Cable Television Laboratories and its member companies. Its purpose is to offer audio, video, and multimedia services through the cable access network. It's envisioned that cable TV subscribers will be able to use their cable modem to make phone calls, for instance. 