user who is Figure 7.4 Third Party Call Control NETWORK Application Server The Telecom Handbook << 163 CONVERGENCE OF VOICE & DATA part of a group using a Local Area Network. Third party call control is not limited to the LAN, but may operate a more widespread network, not restricted to one physical location. The application runs in a shared server. There may not be a direct physical connection between the user's computer and the user's telephone line, for example, in the case of a traditional PBX. Instead there is a logical connection. The PC application talks to the server, which in turn controls the com- munications server or PBX. The server is acting on behalf of the user. "Make a call for extension 123" or "Answer the call for extension 205" are examples of call control commands available. The shared server can offer both personal and group services such as directories, organizers, etc. The server provides a coordination point for all calls be- ing handled in the group. The central server-based application can handle the distribution of all calls to the members of the group, including activities like Call Screening or Back Up Call Answering. One standard for third party call control is TSAPI (Telephony Services Application Programmer's Interface) which was devel- oped jointly by Novell and AT&T. TSAPI is an Applications Programming Interface (API) writ- ten in C programming language. As an alternative to APIs, modern communications servers use SIP and markup such as CCXML for controlling a call. CCXML is what is called a "mark up" pro- gramming language for describing how an application should handle a call. People are beginning to buy servers to develop their own localized (within a department) applications using SIP (Session Initiation Protocol). SIP is particularly well suited to third party call control. As an example of First VS. Third Party call control, if some- one calls into a Call Center and the software in the Telephone System (Communications Server) recognizes the caller's tele- phone number and 