IMS - RCS

Presence Notify

Presence in the context of RCS indicates 'a User's willingness and abiligy to communicate other users on the network'. It means 'I am here on the online and I can/want to communicate with anybody who likes to talk to me".  

There are several components that are involved in Presence Subscription and Notification. Following is overall interaction between these entities. (I reillustrated based on Reference [1]).

Even with UE that does not support RCS, the procedure (1),(2),(3) always happens. It may require UE to report its RCS related capability in (1) REGISTER message as shown in this example.

If UE is capable of RCS, it perform step (5) to report its capability to Presence Agent as shown in this example.

When a user tries some RCS operation on UE (e.g, try to send message using 1-to-1 chat function) and UE does not have any presence information of the other party, it would go through procedure (8) and (9) as in this example. If it does not get proper presence information of the other party via (9), it would fall back to non-RCS function or popup 'Failure' message.

Reference :

[1] Presence Tutorial - Columbia University

[2] RFC 3856 : A Presence Event Package for the Session Initiation Protocol (SIP)

[3] RFC 3265 : Session Initiation Protocol (SIP)-Specific Event Notification

[4] RFC 3857 : A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP)