chapter  3
28 Pages

Background: Providing Telephony Service

The principles of the public switched telephone network (PSTN) and the Internet are diametrically opposite each other. The former is a specialpurpose network built to transport one communication aspect: voice. The endpoints are simple (a phone with a 12-button keypad and limited display capabilities, if any) and the intelligence associated with routing voice circuits and executing services is concentrated in the core of the network, where expensive computers called switches reside. The Internet, on the other hand, is a general-purpose network built to transport any media — voice, video, data, — using a best-effort delivery mechanism. The core of the Internet is fairly simple and consists of special-purpose computers called routers that receive and forward a packet toward the next router, and so on, until the packet reaches its intended destination. The intelligence in the Internet is concentrated at the edges in the form of powerful desktop and laptop computers and personal digital assistants.