VoIP Voice and Fax Signal Processing

1.3 DATA SERVICES ON TELEPHONE LINES

Telephone, fax, and modem use voice band frequencies (300 to 3400 Hz) on the telephone wires. Fax and modem are data terminals. Fax is an independent device that sends data modulated into voice band. A dial-up modem also modulates data into voice band. A dial-up modem is used for Internet connectivity and data transfer. Dial-up modems provide data connectivity up to 56 kbps. Some more details on dial-up modems are given in Chapter 15. Telephones, fax machines, and dial-up modems use telephone lines one at a time, but they can work simultaneously with a broadband service like xDSL.

For VoIP, Internet connectivity is essential. A dial-up modem is one of the basic and most popular options for Internet connectivity. It can provide data connectivity of up to 56 kbps, which is sufficient only with a higher compression VoIP voice call. DSL can provide much higher bandwidth compared with the dial-up modems, and hence, it is used in several deployments for Internet connectivity. In theory, both dial-up and DSL can be used together, but when DSL service is used, a dial-up modem may not be required.

1.3.1 DSL Basics

DSL service is provided through the existing analog telephone two-wire interface or on four-wire ISDN lines. Voice, fax, and modem signals occupy a 0- to 4-kHz band. Telephone wires are capable of carrying wider bandwidth signals to a distance of several thousand feet. DSL technology is based on discrete multi-tone (DMT) modulation using orthogonal ...