Modems

Modems connect remote computers with their resources using regular telephone lines. Since modems (short for modulator-demodulator) are in widespread use, we need to discuss some of the technical details. Most of the time, communication between computers or peripheral devices involves an exchange of letters and numbers as they appear in a typewritten test. The American Standard Committee for Information Interchange (ASCII) has devised a standard for the sequence of bits for such alphanumerically characters. In this standard, each character is represented by a specific eight-bit code of 0's and 1's. Using specialized electrical circuits acting as an intermediary called an interface, these character codes can be transferred to and from other computers and peripherals. There are two basic methods whereby these eight-bit character codes can move across the interface: either one bit at a time or all eight bits at the same time. The first mode is referred to as serial communication, and the second as parallel communication.

Modems typically connect to a computer using the serial interface. You will often hear the term "RS-232 revision C," which is the Electronics Industries Association standard for serial communications. This standard describes in terms of hardware how characters are transferred, one bit at the time, along the serial communication interface how two different devices exchange information.

Now, how do modems work? Simply put, a modem converts the data from a computer into audible tones and sends these tones over dial-up phone lines. Conversely, a modem converts the tones it receives from the phone line into data for the computer. A more "techy" definition would be that the series of 0's and 1's (bits) from the computer are translated into an analog signal and these signals are encoded onto another analog signal: the carrier frequency. The receiving modem converts these frequencies back into a digital signal. The speed at which a modem transfers data (in bits of data per second, BPS) is often referred to as the baud rate. For example, a 9600-baud modem is capable of sending and receiving 9600 bits of information per second. (Although many use the baud rate to refer to the speed of data transfer, the baud rate actually refers to the number of state changes per second on the serial communication line.)

When shopping for a modem, one will see an alphabet soup of abbreviations and numbers. As rule of thumb, get the fastest modem that fits your pocketbook. Built-in error correction techniques and data compression allow modems to communicate faster and more reliably. Look for the MNP, V.32 bis and V.42 bis type modems.

In addition to the hardware, communication by the modem also requires special software packages that enable micro-, mini-, and mainframe computers to communicate with each other. These software packages manage the communication between computer and modem and between modems. They also allow the user to transfer information between computers using well-defined protocols. Common protocols include X-MODEM or KERMIT, with Z-Modem being the most popular.

Contents

Prev. Page