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

IP communications, by contrast, consider the intelligence, and control

resides primarily in user devices. As a consequence, dynamic user preferences

can be fully enabled on a scalable basis, no matter how many users are on a

network. User preferences are well documented for SIP [1]. The methods of

specifying caller preferences are documented in [2].

Following are some examples of caller and called party preferences:

■■ Call someone, but speak only to voicemail, so as to shorten the call as

much as possible.

■■ Receive calls only from certain parties at certain times (such as accept-

ing calls during lunch hour only from the spouse or the boss) and send-

ing all other calls to voicemail.

■■ Specify certain times/dates to be accessible only on the mobile phone

or at hotel phone numbers when traveling.

■■ Specify instant text messaging only when in a meeting or at the theater.

There are three parties to user preferences, each having very different roles

and using different technologies:

1. Caller preferences—Since the caller is the active party, it can express
   clearly the preferences for the call at call setup by three SIP headers, as
   will be shown later.


2. Called party preferences—The called party is passive, since it has to wait
   for incoming calls and it cannot anticipate all possible preferences of
   callers, but can formulate clear rules how incoming calls should be han-
   dled. Such rules can be expressed in CPL scripts that reside either in the
   designated proxy server that handles the calls or in the user agent.


3. Server support for user preferences—SIP servers can be designed to under-
   stand and process caller preferences, and also to execute scripts with
   rules for incoming calls. SIP servers can, however, also enforce policy
   rules for communications on behalf of the network administrator.

Preferences of Caller

The caller can request servers proxy or redirect a call, and also specify how

to search for the destination. The instructions are carried in the Request-

Dispositionheader. For example:

Request-Disposition: proxy, parallel, queue

154 Chapter 8