Overview

Sending voice over data networks is a process of sending digitized telephone signals over a network that was designed primarily for data communications. If the telephone signal is in analog form (voice or fax, the signal is first converted to a digital form. Packet routing information is then added to the digital voice signal so it can be routed through the data network.

Modern private telephone systems use digital telephony to connect the handset to the local switching system. The analog signal is converted to digital form in the telephone set. By using digital information to represent analog signals, the digital communication system can integrate digital voice information along with advanced signal processing control messages.

Figure 10.1 shows how a standard telephone can call a standard telephone through the Internet. In this example, the calling phone dials a local telephone number of a voice gateway. The telephone call is routed through the local exchange company to gateway. When the gateway automatically answers the call, the gatekeeper will receive the dialed digits and it s database will convert the telephone number to the Internet protocol (IP) address of a destination gateway that is close to the receiving telephone. The gatekeeper sends a service request message to the receiving gatekeeper asking if it is willing and able to complete the telephone call. If the receiving gatekeeper is willing and able to complete the call, the call setup information is exchanged between the gateways (e.g. call and feature preferences). This setup information allows...