Telephone Modems

Telephone modems plug into an ordinary telephone jack using ordinary telephone cord. They can then dial telephone numbers just like a telephone, but in most cases it's expected that they' ll connect to another telephone modem on the other end. The two devices go through a handshake period, in which each device communicates its capabilities, and they settle on a set of protocols to govern their communications. Thereafter, a data link exists, and the data one computer sends to its modem appears as input to the other computer. Figure 18.1 illustrates this arrangement.

Figure 18.1

A modem uses the telephone company's wires as a networking medium.

Figure 18.1

A modem uses the telephone company's wires as a networking medium.

The fact that telephone modems work over ordinary telephone lines makes them an extremely useful tool for networking, because there's no need to lay specialized network cables. On the

Chapter 18

other hand, modems are slow as networking technologies go. In early 2000, the fastest analog modems use the v.90 protocol, which permits a theoretical maximum speed of 56Kbps downstream—substantially less than competing technologies allow. (Even this theoretical maximum speed isn't attainable in practice, for reasons I describe later in this chapter.) If you spend a lot of time online, a telephone modem also poses the problem that you can't use the phone line for voice calls while using the modem. Many businesses and even home users therefore obtain a phone line that's dedicated to modem use, which raises the cost of Internet access.

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