A modem is the peripheral used to transfer information between several computers over a wire transmission medium (e.g. telephone lines). Computers operate digitally using binary language (a series of zeros and ones), but modems are analogue. The digital signals pass from one value to another. There is no middle or half-way point. It is "All or Nothing" (one or zero). On the other hand, analogue signals do not move "in steps", but rather continuously.
For example, a piano works more or less digitally because there are no "steps" between notes. Conversely, a violin can modulate its notes to pass through all possible frequencies.
A computer operates like a piano and a modem like a violin. The modem converts binary information from the computer into analogue information in order to modulate it over the telephone line. You can hear bizarre noises if you turn up the sound from the modem.
Thus, a modem modulates digital information on analogue waves. In the opposite direction, it demodulates analogue data in order to convert them into digital data. The word "modem" is an acronym for "MOdulate/DEModulate".
A modem's transmission speed is generally expressed in bauds, in tribute to Emile Baudot (11 September 1845 - 28 March 1903), a famous French engineer who worked in the area of telecommunications. This unit of transmission speed characterises the frequency of (de)modulation, i.e. the number of times the modem makes the signal change status per second. Thus, the bandwidth in bauds is not quite equal to the bandwidth in bits per second because a signal status change may be necessary to encode a bit.
As modems proliferated, the need increased for standardized protocols for communication by modem, so that all the protocols would use a common language. This is why two organizations developed communication standards:
The goal of the ITU is to define international communications standards. Modem standards can be divided into 3 categories:
Here is a list of the main modem standards:
|Modulation Standard||Theoretical Bandwidth||Mode||Description|
|Bell 103||300 bps||Full duplex||An American and Canadian standard that uses audio frequency-shift keying to encode data. This allows one bit to be sent per baud.|
|CCITT V.21||300 bps||Full duplex||An international standard similar to the Bell 103 standard.|
|Bell 212A||1,200 bps||Full duplex||An American and Canadian standard that uses differential phase-shift keying to encode data. This standard allows 2 bits to be sent per baud.|
|ITU V.22||1,200 bps||Half duplex||An international standard close to the Bell 212A standard.|
|ITU V.22bis||2,400 bps||Full duplex||An international standard that is an improved version of the V.22 standard (thus the name V.22bis).|
|ITU V.23||1,200 bps||Half duplex||An international standard that transmits data in half-duplex mode, i.e. data is transmitted in just one direction at a time. Optional 75 baud reverse channel.|
|ITU V.23||1,200 bps/75 bps||Full duplex||An international standard giving asymmetrical full duplex, i.e. it allows data transmission in one direction at 1,200 bps and at 75 bps in the other direction.|
|ITU V.29||9,600 bps||Half duplex||An international standard that transmits data in half-duplex mode, i.e. data is transmitted in just one direction at a time. This standard was developed especially for fax machines.|
|ITU V.32||9,600 bps||Full duplex||An international standard that transmits in full-duplex mode and incorporates error correction standards. Data transmission takes place according to an error correction technique called quadrature amplitude trellis-coded modulation. This technique consists in sending an additional bit for each group of 4 bits that are sent on the transmission line.|
|ITU V.32bis||14,400 bps||Full duplex||An international standard that improves on the v.32 standard by allowing 6 bits per baud to be sent and a transmission speed of up to 14,400 bps.|
|ITU V.32fast||28,800 bps||Full duplex||An international standard sometimes called V.FC (Fast Class) that allows data transmission at a speed of 28,800 bps.|
|ITU V.34||28,800 bps||Full duplex||An international standard that allows data transfer at 28,800 bps. Thanks to a DSP processor (Digital Signal Processor), modems using this standard can attain a speed of up to 33,600 bps.|
|ITU V.90||56,000 bps||Full duplex||An international standard that allows transmission speeds of up to 56,000 bps.|