Presence is the ability of endpoints to provide information to other users about their current
status - such as whether they are offline, online, or in a call. Any entity which provides presence
information, or about whom presence information can be requested, is known as a presentity.
Presentities publish information about their own presence status, and also subscribe to the
information being published by other presentities and FindMe users.
Endpoints that support presence, such as Movi™ v2.0 (or later) clients, can publish their own
status information. The VCS can also provide basic presence information on behalf of endpoints
that do not support presence, including H.323 endpoints, as long as they have registered with an
alias in the form of a URI.
If FindMe is enabled, the VCS can also provide presence information about FindMe users by
aggregating the information provided by each presentity configured for that FindMe user.
The Presence application on the VCS supports the SIP-based SIMPLE standard and is made up of
two separate services. These are the

Presence Server

and the

Presence User Agent

(PUA). These

services can be

enabled and disabled

separately.

The

Presence status pages

provide information about the presentities who are providing presence

information and the users who are requesting presence information on others.

Presence is supported by clustering. For specific information about how Presence
information is managed across peers in a cluster, see the

Clustering and Presence

section.

Presence Server

The Presence Server application on the VCS is responsible for managing the presence information
for all presentities in the SIP domain(s) for which the VCS is authoritative (refer to the

SIP domains

section for more information). The Presence Server can manage the presence information for
locally registered endpoints and presentities whose information has been received via a SIP proxy
(e.g. another VCS Control or Expressway).
The Presence Server is made up of the following services, all of which are enabled (or disabled)
simultaneously when the Presence Server is enabled (or disabled):

•

Publication Manager: receives PUBLISH messages, which contain the status information about
a presentity, and writes this information to the Presence Database. PUBLISH messages are
generated by presence-enabled endpoints and by the

Presence User Agent

(PUA).

•

Subscription Manager: handles SUBSCRIBE messages, which request information about the
status of a presentity. Upon receipt of a SUBSCRIBE message, the Subscription Manager
sends a request to the Presentity Manager for information about that presentity, and forwards
the information that is returned to the subscriber. The Subscription Manager also receives
notifications from the Presentity Manager when a presentity’s status has changed, and send this
information to all subscribers.

•

Presentity Manager: an interface to the Presence Database. It is used to support VCS features
such as FindMe and the PUA, where the presence information provided by a number of different
devices must be aggregated in order to provide an overall presence status for one particular
presentity.
When the Presentity Manager receives a request from the subscription manager for information
on a presentity, it queries the Presence Database for all information available on all the
endpoints associated with that particular presentity. The Presentity Manager then aggregates
this information to determine the presentity’s current status, and returns this to the Subscription
Manager.

•

Presence Database: stores current presence information received in the form of PUBLISH
messages. Also sends NOTIFY messages to the Presentity Manager to inform it of any changes.