The Bell 103 modem or Bell 103 dataset was the second commercial modem

Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

 for computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s, released by AT&T

AT&T

AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...

 in 1962. It allowed digital

Digital

A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

 data

Data

The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

 to be transmitted over regular unconditioned telephone line

Telephone line

A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

s at a speed of 300 bits per second. It followed the introduction of the 110 baud Bell 101

Bell 101

The Bell 101 dataset or Bell 101 modem was the first commercial modem for computers, released by AT&T in 1958 for use by SAGE and in 1959 made commercial shortly after AT&T's Bell Labs announced their 110 baud modulation frequencies...

 dataset in 1958.

The Bell 103 modem used audio frequency-shift keying to encode data. Different pairs of audio frequencies were used by each station:

• The originating station used a mark
  Mark
  Mark may refer to:* Mark , a male given name* Mark -Business:* Service mark, trademark used to identify a service rather than a product...
  
   tone of 1,270 Hz
  Hertz
  The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
  
   and a space tone of 1,070 Hz.
• The answering station used a mark
  Mark
  Mark may refer to:* Mark , a male given name* Mark -Business:* Service mark, trademark used to identify a service rather than a product...
  
   tone of 2,225 Hz and a space tone of 2,025 Hz.

Although original Bell 103 modems are no longer in common use, this encoding scheme is referred to generically as "Bell 103 modulation", and any device employing it as "Bell 103-compatible" or "a Bell 103 modem".

For many years, higher-speed modems retained the ability to emulate the Bell 103, allowing a fallback method for data to be communicated at low speed if channel conditions deteriorated.

Applications

Bell 103 modulation is still in use today, in shortwave radio, amateur radio

Amateur radio

Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

, and some commercial applications. Its low signalling speed

Data signaling rate

In telecommunication, data signaling rate , also known as gross bit rate, is the aggregate rate at which data pass a point in the transmission path of a data transmission system.Notes:#The DSR is usually expressed in bits per second....

 and use of audio frequencies makes it suitable for noisy or unreliable narrowband links.

For example, the CHU shortwave station in Ottawa

Ottawa

Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 transmits a Bell 103-compatible digital time code every minute. Bell 103 modulation is also the standard for amateur packet radio

Packet radio

Packet radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated...

 in the HF

High frequency

High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...

 (shortwave) bands.

Related technology

The ITU-T

ITU-T

The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

 V.21 communications standard defines a very similar modulation scheme. Commercial 300 baud modems typically support both formats.