Figure 13.4 Commercial presence and IM products: (a) Microsoft Messenger integrated
with Microsoft Office 2003 and (b) the CounterPath eyeBeam SIP UA
SIP Event-Based Communications and Applications
SIP presence is a form of a SIP-specific event notification [2] also called “SIP
Event Package.” SIP events can support the many types of SIP services and
their integration with applications. Examples of notification-based services
include automatic callback based on the presence of the called party, message
waiting indicators, and alerts caused by communication events (such as alert-
ing a supervisor when a customer is calling an agent in a contact center).
Not all applications are good candidates for SIP events. Frequent notifica-
tions about the geographic location provided by a GPS location service for a
moving person would probably overload the SIP system and be of little value.
The operation for SIPNOTIFYflow is shown in Figure 13.5.
As shown in Figure 13.5, the subscriber sends a SUBSCRIBEmessage to the
notifier to request a subscription to an application state. The notifier will
acknowledge the request with a 200 OKmessage and also send a NOTIFYmes-
sage with the complete current state. As soon as there is some change in state,
the notifier will send either updates for the new state or complete new state
information to the subscriber. Sending only updates or all the new state infor-
mation depends on the type of application.
Client-Server Presence Peer-to-Peer Presencea bPresence and Instant Messaging 229