Introduction

In the world of telecommunications, modem is the traditional name for an apparatus that modulates and demodulates a signal to transfer digital information over a public network infrastructure such as the Public Switched Telephone Network (PSTN). For many years, the word modem was synonymous with voiceband modem, a modem designed for transmission of data within the narrow frequency band that is normally used to carry voice conversations; in recent years, the word has commonly been used to designate broadband access devices (e.g., cable modems, DSL modems).

There are, however, a number of important differences between voiceband modems and broadband access devices.

• Voiceband modems really only care about getting the bits onto the telephone wire. Broadband access devices, on the other hand, will often provide some form of data encapsulation, because the terminals will generate data in discrete packets, whereas the medium expects a steady flow of bits or bytes. The encapsulation is necessary to fill up the gaps between the packets (e.g., idle cells in ATM or flag bytes in HDLC) and to indicate to the other end where packets begin and end, a task performed by end-terminal software in the case of voice-band modems (PPP, SLIP). These functions were discussed in more detail in Chapter 3 for the point-to-point copper PHYs.

• The owner of a voiceband modem can use it to directly call any other compatible modem; the analog signal of the modem is switched through the PSTN from one modem to the other,...