Question

Explain the procedure and significance of the presence service elements with an appropriate usage case.

Answer 1

Presence is
   defined as the willingness and ability of a user to communicate with
   other users on the network.  Historically, presence has been limited
   to "on-line" and "off-line" indicators; the notion of presence here
   is broader.  Subscriptions and notifications of presence are
   supported by defining an event package within the general SIP event
   notification framework.  This protocol is also compliant with the
   Common Presence Profile (CPP) framework.Presence, also known as presence information, conveys the ability and willingness of a user to communicate across a set of devices.  RFC
   2778 [10] defines a model and terminology for describing systems that
   provide presence information.  In that model, a presence service is a
   system that accepts, stores, and distributes presence information to
   interested parties, called watchers.  A presence protocol is a
   protocol for providing a presence service over the Internet or any IP
   network.

Watcher             Server                 PUA
      | F1 SUBSCRIBE      |                    |
      |------------------>|                    |
      | F2 200 OK         |                    |
      |<------------------|                    |
      | F3 NOTIFY         |                    |
      |<------------------|                    |
      | F4 200 OK         |                    |
      |------------------>|                    |
      |                   |                    |
      |                   |   Update presence  |
      |                   |<------------------ |
      |                   |                    |
      | F5 NOTIFY         |                    |
      |<------------------|                    |
      | F6 200 OK         |                    |
      |------------------>|                    |