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This chapter is from the book

Modems

Modem is a contraction of the terms modulator and demodulator. Modems perform a simple function: They translate digital signals from a computer into analog signals that can travel across conventional phone lines. The modem modulates the signal at the sending end and demodulates at the receiving end.

Modems provide a relatively slow method of communication. In fact, the fastest modem available on the market today has a maximum speed of 56Kbps. Compare that to the speed of a 10Mbps network connection, and you’ll find that the modem is approximately 180 times slower. That makes modems okay for browsing web pages or occasionally downloading small files but wholly unsuitable for downloading large files. As a result, many people prefer to use other remote access methods, including ISDN (which is discussed later in this chapter, in the section "ISDN Terminal Adapters") and cable/DSL access.

Modems are available as internal devices that plug into expansion slots in a system; external devices that plug into serial or USB ports; PCMCIA cards designed for use in laptops; and specialized devices designed for use in systems such as handheld computers. In addition, many laptops now come with integrated modems. For large-scale modem implementations, such as at an ISP, rack-mounted modems are also available. Figure 3.17 shows an internal modem and a PCMCIA modem.

Figure 3.17

Figure 3.17 An internal modem (left) and a PCMCIA modem (right).

Modems are controlled through a series of commands known as the Hayes AT command set. Hayes was a company that, for many years, led the field in the development of modems and modem technology. The AT commands allow you to control a modem as well as configure and diagnose it. Table 3.1 lists some of the most commonly used AT commands.

Table 3.1 Commonly Used AT Modem Commands

Command

Result

ATA

Answers an incoming call

ATH

Hangs up the current connection

ATZ

Resets the modem

ATI3

Displays modem identification information


Modem Connection Speeds

The actual speed you obtain on a modem connection depends on a variety of factors, including the quality of the line you are using and the speed of the modem. For example, you might find (as we often do) that even with a 56Kbps modem, the most you can get on a certain connection is 49Kbps. If you try the same connection again on a different phone line, you might get a higher or lower rate. Quality of the connection aside, two factors govern the maximum speed attainable by your modem: the speed of the Universal Asynchronous Receiver/Transmitter (UART) chip in your system (which controls the serial ports) and the speed of the modem itself.

In older systems, the UART chips were capable of only slow speeds, making them unable to keep up with fast modems. Today, most systems have UART chips capable of speeds well in excess of those offered by modems. Now the modem, not the UART chip, is the bottleneck. Table 3.2 lists the types of commonly used UART chips and their associated speeds.

Table 3.2 UART Chips and Their Associated Speeds

UART Chip

Speed (bps)

8250

9,600

16450

115,200

16550

115,200

16650

430,800

16750

921,600

16950

921,600


Modem speeds can be expressed in either baud rate or bits per second (bps). The baud rate refers to the number of times a signal changes in each second, and the bps rate is the number of bits of data that can be sent or received in a second. Although the figures are identical in some modems, in others the bps rate is higher than the baud rate. The baud rate is actually not as important, and the higher the bps figure, the better. Most modern modems offer bps rates far greater than the baud rate.

To make it easier to compare modems, standards have been created that define the data throughput of the modem and what features it provides. These are sometimes referred to as the V standards, and you can use them when buying a modem to determine the modem’s capabilities.

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