Part15: Modems

Introduction
This section briefly discusses modems, what they are, what they are used and a summary of the standards associated with them.

Modems
Modems are devices that allow digital data signals to be transmitted across an analogue link. As has already been discussed, the connections provided by telephone companies for the use of speech via dial up telephones is analogue based. However, in recent times, with the development of on-line information boards (bulletin boards) and the Internet, it is now standard practice to want to use the dial-up telephone connection to access these services.

As the computer uses digital information, and the telephone line uses analogue signals, a device is needed which converts the digital data from the computer and converts it to analogue tones (within the voice channel range of 300Hz to 3400Hz) so that the signals can travel across the dial up speech connection. At the other end, the signals are converted back to digital. The device that converts digital signals to analogue for transmission across a dial-up telephone connection, and then converts them back again, is a modem.

Modem stands for modulator demodulator. A modem changes the digital signal to an analogue frequency, and sends this tone across the analogue link. At the other end, another modem receives the signal and converts it back to digital.

Smart Modems
This is a standard modem with a micro-processor to provide both data communications and automatic dialing in the one unit. These modems offer a range of features,

Modem Commands
Modems can receive and act on commands from the computer terminal. Commands begin with the symbol AT and are terminated with a carriage return.

The range of commands is called the AT Command Set. This is detailed in the reference listed above. Depending upon how the modem is configured (whether to echo results of commands), the modem will return the symbol OK in response to a command request if it is performed.

The AT Command Set
Hayes Corporation developed a smart modem that accepted AT type commands. This is now a widely accepted standard. The following is a brief list of the AT command set.

ITU V.XX Modem Standards
The International Telegraphic Union (ITU) is an international organization responsible for the establishment of the world wide V standards.

V Series Standard	Details
V.22	1200bps full duplex 2 wire dial-up for use in general telephone networks
V.22 bis	2400bps dial-up or 2 wire leased line
V.23	600/1200bps dial-up for use in general telephone networks
V.26 bis	2400/1200bps dial-up for use in general telephone networks
V.27	4800bps leased line
V.27 bis	4800/2400bps leased line with automatic adaptive equaliser
V.27 ter	4800/2400bps dial-up for use in general telephone networks
V.29	9600bps leased line
V.32	9600bps dial-up for use in general telephone networks
V.32 bis	14400bps dial-up for use in general telephone networks or leased line, synchronous or asynchronous full-duplex
V.33	14000bps over 4 wire leased line, synchronous full-duplex
V.34	28800bps dial-up for use in general telephone networks or leased line, synchronous or asynchronous full-duplex
V.34 Enhanced	33600bps dial-up for use in general telephone networks or leased line, synchronous or asynchronous full-duplex
V.35	48Kbps using 60-108Khz group-band circuits

Summary
A modem is required to send digital data over an analogue connection such as a dial up telephone line. Modems change the digital data to analogue tones that fit within the frequency range of the voice channel. A modem can also convert these analogue tones back to the original digital data.

Nowadays, it is common for the home user to access the Internet via a dial up telephone connection using a modem. Typically, these modems run at 28.8Kbps or 33.3Kbps, although 56.6Kbps modems are now becoming available.