When first designed, landline phone service was intended to carry sound signals, and its uses as a carrier of data were years away from realization. It's ironic that the technology that predated the telephone was itself a data transport technology: the telegraph. This device carried encoded messages from terminal to terminal across the 19th-century equivalent of a peer-to-peer network.

A lifetime later, in the 1960s, sound-encoding devices emerged, and, very soon, computers were able to send data, represented as sound, across the telephone network. Those devices were modems, and later fax machines-the descendants of the telegraph. Modems, fax machines, voice mail systems, emergency 911 service, and a slew of other messaging tools evolved around the international telephone network. Today voice and data networks converge and VoIP begins to replace Bell's brainchild. IP telephony has the same fundamental goal as legacy telephony: facilitate human interaction at a distance. But, since IP telephony goes about this goal differently, not all of the specialized devices that evolved around the old system work with the new one. Fax machines, modems, and voice mail systems aren't necessarily compatible with VoIP, because they grew into a mold that was shaped by the old network. In this chapter, we'll cover some of the great legacy technologies we've come to rely on and discuss ways of migrating their functionality to the converged network.

Fax and Modems

Fax machines and modems encode digital data into analog sound signals for transport across the PSTN. At the local CO, these analog sound signals are digitized into PCM digital signals at 64 kbps. After being transported over the network, the CO at the opposite end of the link decodes the PCM signal and plays it back for the analog receiver, whose job it is to reassemble it into a facsimile of the original digital data. Since the maximum rate of transmission on a POTS line is 64 kbps, faxes and modems can't transmit data very quickly (one twenty-fourth the speed of a T1). Even with compression techniques, which introduce signaling overhead, most modems will never transmit data faster than about 54 kbps. They are limited by the digital resolution of the sound pathway they use.

The challenge posed by VoIP is this: codecs like G.729A distort the sound signal by compressing it using lossy vocoder algorithms such as CELP. When a modem or fax's analog sound signal is encoded and decoded using vocoders, it becomes distorted, such that the device on the receiving end of the transmission is receiving a different analog signal than the one that was sent. The side effect of compressed VoIP codecs is that faxes and modems simply don't work. So devices that rely on modems, such as some burglary alarm systems, TiVo consoles, and maybe that old-fashioned Amiga 1000 computer, have a hard time getting along in a VoIP network. (Series 2 TiVo devices can work using the Internet instead of a modem-check out http://www.tivo.com/adapters.)