3.7 Modems


A modem converts (modulates) digital signals from a computer to analogue signals for transmission over a telephone line and demodulates analogue signals back to digital signals at the receiving end.

Bandwidth varies depending on the type of encoding used and whether full of half duplex is being used. Most modern modems have a much higher bit rate (bps) than baud rate. The voice channel of a basic telephone line was designed to handle analogue voice in the range 300Hz to 3,400Hz, giving a bandwidth of only 3,100Hz.

For telephone cables, the limiting factor in speed is the number of line changes per second. A line change is defined as a switch from one state to another, for instance, switching from 0 to 1.

Modem Standards

Most modem standards are based on the International Telecommunications Union (ITU-T) standards. The ITU standards for telecommunications are denoted by a series of letters:



G series

Used for digital networks (Digital exchanges)

I series

Used for ISDN

V series

Used with telephone circuits (Analogue)

X series

Used for data networks (X25)

The current voiceband standard is V.90, which support speeds of 56,000 bits per second for downloads and 33,600 for uploading.

The V.92 standard is available but Internet Service Providers (ISP) have been slow to update to this because of the prevalence of new technologies, such as broadband. V.92 is still a 56k technology but it supports faster uploads, therefore, the maximum speed for voiceband modems is 56k.