Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

(Steven Felgate) #1
Table 2.4 Examples of Rich Presence Information

On the phone
Away
Appointment
Holiday
Meal
Meeting
Driving
In transit
Travel
Vacation
Busy
Permanent absence

Rich presence information as described in Chapter 13 is not only very valu-
able for communications between humans but can also support the under-
standing of presence by machines.
Presence requires the messages SUBSCRIBEand NOTIFYbetween the par-
ties that display the presence (the “presentity”) and the party that is interested
in this information (the “watcher”) SIP presence can also be explained in a nut-
shell with the help of the message flow shown in Figure 2.1 by noting the
analogies between the SIP and presence messages shown in Table 2.5.
The SUBSCRIBEand NOTIFYmessages can be extended from human users
to any type of applications, and this can support the integration of communi-
cations and applications, as we will be described in Chapter 13, “Presence and
Instant Messaging.”

Table 2.5 SIP Messages for Presence and IM vs. Voice and Video
PRESENCE AND IM VOICE/VIDEO
SUBSCRIBE INVITE
200 OK 200 OK
NOTIFY ACK
MESSAGE RTP Media

22 Chapter 2

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