5.1. Application Terminology
Many PBX vendors refer to their telephony functions using words like actions or routes or forwarding plans . Others, the author included, prefer another word: applications . This word is appropriate for a modern, soft-driven telephony world. The words function and action invoke an algorithmic meaning, while application is much more broad and programmatic. Traditional TDM systems and VoIP systems have a similar distinction: TDM is monolithic, rigid; VoIP is programmable, flexible.
But whatever the semantics, things the network can do with a phone call are referred to in this book as applications. They've been organized here in five groups: basic call handling, administrative, messaging, advanced call handling, and CTI. What's covered in this chapter is by no means an exhaustive study of telephony apps. Indeed, while I was busy writing this book, new apps were being developed.
Applications themselves are invented out of necessity and driven by market forces more than by the creativity of technologists. When the market demand for certain functionality reaches a boiling point, a vendor or a group of vendors produce a solution to the demand. Sometimes the solution is submitted to the ITU, ANSI, or IETF for recommendation as a standard.
Not all applications are standards-based. Not every feature in a shiny new Avaya Communication Manager PBX is enabled by SIP or some other open IETF or ITU recommendation. In fact, most PBX vendors, even the ones making commercial-grade VoIP servers, deliver a majority of features using their own proprietary methods . Telephony equipment manufacturers are constantly competing on the basis of features, and often the open standards don't support the features the market is looking for. The standards have to be augmented, and this takes time. The 802.3af standard (power over Ethernet) was ratified in 2003, several years after Cisco had begun shipping its own proprietary solution to the need it addressed. The standards bodies may not have a published recommendation for a certain feature until years after the feature itself is made available in proprietary form.
So, while Cisco, Avaya, and Nortel all have rudimentary SIP call-signaling support functions , they all offer a much broader selection of telephony applications through their own closed, proprietary, or "flavored standard" signaling protocols and software. At the same time, no telephony platform supports 100% of the features of SIP or H.323.