Technology Reference Guide

	Technology Reference Guide 2. Hardware M. Modems-CSU/DSU

What is it?
Modems are devices used to transmit data across wide-area public carrier services. At the low-end, the public carrier may be a regular voice-grade phone line. At the high-end are special, leased high-speed data lines. These data lines vary in cost and in the maximum data speed they can handle. The most common modem specification has to do with their maximum rate of data transfer, known as baud rate. Baud rate translates loosely into the number of bits/second that the modem can handle. Baud rates are typically multiples of 1200, with 9600, 14,400, and 28,800 being the most common today. A voice-grade line may have difficulty fully handling 28,800 baud, while a dedicated data line may handle speeds well in excess of 1 million bits/second. A CSU/DSU is a special type of modem used on high-speed data lines. The designation stands for channel service unit/data service unit. In contrast to a regular modem, which converts data to an analog (varying, wave-like) signal, a CSU/DSU uses a digital (pulses) signal.

What does it do?
The basic work of a modem is to take the bits and bytes that come into it from the serial port to which it is attached, and convert them into a signal that can be transmitted to another modem at the other end of the phone or data line. The other modem takes the signal from the phone line, and reconverts it back to a signal that feeds into the serial port on the receiving system. This process is called MOdulation and DEModulation, hence the name. Modems may also perform data compression, which allows them to transmit data at higher speed.

How is it used?
Modems are commonly used to provide system or network access to remote users and remote networks. A modem can be used by a remote user to dial into his desktop system, the network or directly into a mainframe. Modems can also be used with a remote bridge to provide a wide-area link between a small remote LAN and a larger home office network.

Where do I get more information?
Contact your local CBV Office.