CHAPTER 4 Basic "Presence" Concepts 4.1 Introduction This chapter describes, in a general manner, presence concepts and services. Presence and Instant Messag- ing have recently emerged as a new medium of communications over enterprise networks, extranets, and the Internet. Presence is a means for finding, retrieving, and subscribing to changes in the presence information (e.g., "online" or "offline") of other users, typically in the context of messaging (e-mail), Unified Messaging, and/or VoIP. It can be defined as the willingness and ability of a user to communicate with other users on the network [ROS200401] . Historically, presence has been limited to "on-line" and "off-line" indicators. Instant messaging is a means for sending small, simple messages that are delivered immediately to online users [AGG200001] . The notion of presence is now taking on a broader definition/role. VoIP can be used in support of, or in conjunction with, presence: it is expected that the sophisticated voice environments made possible by VoIP, including Unified Messaging, will make have use of presence capabilities for one-to-one conversations, roaming applications, and voice/video-conferencing. Specifically, the anticipation is that presence concepts and services will play a key