Presence service is a network service

Network service

Network services are the foundation of a networked computing environment. Generally network services are installed on one or more servers to provide shared resources to client computers.- Network services in LAN :...

 which accepts, stores and distributes presence information

Presence information

In computer and telecommunications networks, presence information is a status indicator that conveys ability and willingness of a potential communication partner—for example a user--to communicate...

.

Presence service may be implemented as a single server or have an internal structure involving multiple servers and proxies. There may be complex patterns of redirection and proxying while retaining logical connectivity to a single presence service. Also presence service may be implemented as direct communication among presentity

Presentity

Presentity is an term used to describe an entity that provides presence information to help other entities, such as individuals, know whether it is acceptable to initiate a communications session, such as a phone call, a videoconference or an instant message session...

 and watchers

Watcher (presence)

Presence information watcher is an entity that requests presence information about a presentity from a presence service. Usually in order to get presence information a watcher have to subscribe for it to a presence server. When subscribed, the watcher receives event notifications as presence...

, i.e. server is not required.

Presence server

The Presence Server, that reside in the presentity's home network, is able to receive and manage presence information. It is a SIP Application Server and is located using SIP URLs.