For many years, the Bell System (AT&T) maintained a monopoly on the use of its phone lines, and what devices could be connected to its lines. However, the seminal Hush-a-Phone v. FCC case of 1956 concluded that it was within the FCC's jurisdiction to regulate the operation of the System. Subsequently, the FCC examiner found that as long as the device was not electrically attached it would not threaten to degenerate the system. This led to a number of devices that mechanically connected to the phone, through a standard handset. Since most handsets were supplied from Western Electric, it was relatively easy to build such an acoustic coupler, and this style of connection was used for many devices like answering machines.
Acoustically coupled Bell 103A-compatible 300 bit/s modems became common during the 1970s, with well-known models including the Novation CAT and the Anderson-Jacobson, the later spun off from an in-house project at Stanford Research Institute (now SRI International). An even lower-cost option was the Pennywhistle modem, designed to be built using parts found at electronics scrap and surplus stores.
In December 1972, Vadic introduced the VA3400, which was notable because it provided full duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit and receive. In November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in design to Vadic's model, but used the lower frequency set for transmission. One could also use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency assignments made the 212 intentionally incompatible with acoustic coupling, thereby locking out many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.
Carterfone and direct connection[edit source | editbeta]
The Hush-a-Phone decision applied only to mechanical collections, but the Carterfone decision of 1968 led to the FCC introducing a rule setting stringent AT&T-designed tests for electronically coupling a device to the phone lines. AT&T's tests were complex, making electronically coupled modems expensive,[citation needed] so acoustically coupled modems remained common into the early 1980s.
However, the rapidly falling prices of electronics in the late 1970s led to an increasing number of direct-connect models around 1980. In spite of being directly connected, these modems were generally operated like their earlier acoustic versions - dialling and other phone-control operations were completed by hand, using an attached handset. A small number of modems added the ability to automatically answer incoming calls, or automatically place an outgoing call to a single number, but even these limited features were relatively rare or limited to special models in a lineup. When more flexible solutions were needed, 3rd party "diallers" were used to automate calling, normally using a separate serial port.
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The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300-bit/s modem, but it was attached to a small microcontroller that let the computer send it commands. The command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls. This eliminated the need for any manual operation, a handset, or a dialler. Terminal programs that maintained lists of phone numbers and sent the dialing commands became common. The basic Hayes command set remains the basis for computer control of most modern modems.
The Smartmodem and its clones also aided the spread of bulletin board systems (BBSs) because it was the first low-cost modem that could answer calls. Modems had previously been typically either the call-only, acoustically coupled models used on the client side, or the much more expensive, answer-only models used on the server side. These were fine for large computer installations, but useless for the hobbiest who wanted to run a BBS but then periodically use the same telephone line to call other systems. The first hobby BBS system, CBBS, started as an experiment in ways to better use the Smartmodem.
Almost all modern modems can inter-operate with fax machines. Digital faxes, introduced in the 1980s, are simply an image format sent over a high-speed (commonly 14.4 kbit/s) modem. Software running on the host computer can convert any image into fax format, which can then be sent using the modem. Such software was at one time an add-on, but has since become largely universal.
1200 and 2400 bps[edit source | editbeta]
The 300 bit/s modems used audio frequency-shift keying to send data. In this system the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines. In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system and not harmonics of each other.
In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase. Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid-1980s. A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe. The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still only 600 baud (symbols per second). The bit rate increases were achieved by defining 4 or 8 distinct symbols, which allowed the encoding of 2 or 3 bits per symbol instead of only 1. The use of smaller shifts had the drawback of making each symbols more vulnerable to interference, but improvements in phone line quality at the same time helped compensate for this. By the late 1980s, most modems could support all of these standards and 2,400-bit/s operation was becoming common.
Acoustically coupled Bell 103A-compatible 300 bit/s modems became common during the 1970s, with well-known models including the Novation CAT and the Anderson-Jacobson, the later spun off from an in-house project at Stanford Research Institute (now SRI International). An even lower-cost option was the Pennywhistle modem, designed to be built using parts found at electronics scrap and surplus stores.
In December 1972, Vadic introduced the VA3400, which was notable because it provided full duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit and receive. In November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in design to Vadic's model, but used the lower frequency set for transmission. One could also use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency assignments made the 212 intentionally incompatible with acoustic coupling, thereby locking out many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.
Carterfone and direct connection[edit source | editbeta]
The Hush-a-Phone decision applied only to mechanical collections, but the Carterfone decision of 1968 led to the FCC introducing a rule setting stringent AT&T-designed tests for electronically coupling a device to the phone lines. AT&T's tests were complex, making electronically coupled modems expensive,[citation needed] so acoustically coupled modems remained common into the early 1980s.
However, the rapidly falling prices of electronics in the late 1970s led to an increasing number of direct-connect models around 1980. In spite of being directly connected, these modems were generally operated like their earlier acoustic versions - dialling and other phone-control operations were completed by hand, using an attached handset. A small number of modems added the ability to automatically answer incoming calls, or automatically place an outgoing call to a single number, but even these limited features were relatively rare or limited to special models in a lineup. When more flexible solutions were needed, 3rd party "diallers" were used to automate calling, normally using a separate serial port.
//
The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300-bit/s modem, but it was attached to a small microcontroller that let the computer send it commands. The command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls. This eliminated the need for any manual operation, a handset, or a dialler. Terminal programs that maintained lists of phone numbers and sent the dialing commands became common. The basic Hayes command set remains the basis for computer control of most modern modems.
The Smartmodem and its clones also aided the spread of bulletin board systems (BBSs) because it was the first low-cost modem that could answer calls. Modems had previously been typically either the call-only, acoustically coupled models used on the client side, or the much more expensive, answer-only models used on the server side. These were fine for large computer installations, but useless for the hobbiest who wanted to run a BBS but then periodically use the same telephone line to call other systems. The first hobby BBS system, CBBS, started as an experiment in ways to better use the Smartmodem.
Almost all modern modems can inter-operate with fax machines. Digital faxes, introduced in the 1980s, are simply an image format sent over a high-speed (commonly 14.4 kbit/s) modem. Software running on the host computer can convert any image into fax format, which can then be sent using the modem. Such software was at one time an add-on, but has since become largely universal.
1200 and 2400 bps[edit source | editbeta]
The 300 bit/s modems used audio frequency-shift keying to send data. In this system the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines. In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system and not harmonics of each other.
In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase. Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid-1980s. A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe. The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still only 600 baud (symbols per second). The bit rate increases were achieved by defining 4 or 8 distinct symbols, which allowed the encoding of 2 or 3 bits per symbol instead of only 1. The use of smaller shifts had the drawback of making each symbols more vulnerable to interference, but improvements in phone line quality at the same time helped compensate for this. By the late 1980s, most modems could support all of these standards and 2,400-bit/s operation was becoming common.
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