Chapter 17 - Call Control

Abstract

This chapter focuses on call control and the various tools and approaches used for implementing it. In the remit of session initiation protocol (SIP), the term call control is used to refer to the manipulation of SIP session dialogs or SIP conference media policy, to cause participants in a conversation to perceive specific media relationships. To manipulate SIP dialogs for causing changes in the conversation space, SIP signaling is used. There are various approaches for this manipulation of SIP dialogs where each of them uses a number of call control primitives to achieve the change in the conversation space. A possible approach is to use third party call control (3PCC), which relies only on the three basic primitives that are part of the core SIP specification that include creating a new dialog (INVITE), modifying a dialog (re-INVITE), and destroying a dialog (BYE). User agents in this approach have to support the basic SIP functionality only. Another approach to call control is the peer-to-peer approach, which does not require the intervention of a central controller but does require support of additional call control primitives in the user agents. Some of these primitives include replacing an existing dialog, joining a new dialog with an existing dialog, and asking another user agent to send a request on one's behalf. SIP can be used for remote call control where the manipulation of conversations is done by a user agent that is not directly involved in those conversations.