A modem (modulator-demodulator) 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. Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.
Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps), or bytes per second (B/s). Modems can alternatively be classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.
News wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems.
Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs. In 1942, IBM adapted this technology to their unit record equipment and were able to transmit punched cards at 25 bits/second.[citation needed]
Mass-produced modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used[1]), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.
In summer 1960[citation needed], the name Data-Phone was introduced to replace the earlier term digital subset. The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960[citation needed]. In 1962[citation needed], the 201A and 201B Data-Phones were introduced. They were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels each running on their own set of two wires.
The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.
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.
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.
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.
Proprietary standards[edit source | editbeta]
Many other standards were also introduced for special purposes, commonly using a high-speed channel for receiving, and a lower-speed channel for sending. One typical example was used in the French Minitel system, in which the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1,200 bit/s for reception, and 75 bit/s for sending commands back to the servers.
Three U.S. companies became famous for high-speed versions of the same concept. Telebit introduced its Trailblazer modem in 1984, which used a large number of 36 bit/s channels to send data one-way at rates up to 18,432 bit/s. A single additional channel in the reverse direction allowed the two modems to communicate how much data was waiting at either end of the link, and the modems could change direction on the fly. The Trailblazer modems also supported a feature that allowed them to spoof the UUCP g protocol, commonly used on Unix systems to send e-mail, and thereby speed UUCP up by a tremendous amount. Trailblazers thus became extremely common on Unix systems, and maintained their dominance in this market well into the 1990s.
USRobotics (USR) introduced a similar system, known as HST, although this supplied only 9,600 bit/s (in early versions at least) and provided for a larger backchannel. Rather than offer spoofing, USR instead created a large market among Fidonet users by offering its modems to BBS sysops at a much lower price, resulting in sales to end users who wanted faster file transfers. Hayes was forced to compete, and introduced its own 9,600-bit/s standard, Express 96 (also known as Ping-Pong), which was generally similar to Telebit's PEP. Hayes, however, offered neither protocol spoofing nor sysop discounts, and its high-speed modems remained rare.
Echo cancellation was the next major advance in modem design.
Local telephone lines use the same wires to send and receive, which results in a small amount of the outgoing signal being reflected back. This is useful for people talking on the phone, as it provides a signal to the speaker that their voice is making it through the system. However, this reflected signal causes problems for the modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal. This was why earlier modems split the signal frequencies into "answer" and "originate"; the modem could then ignore any signals in the frequency range it was using for transmission. Even with improvements to the phone system allowing higher speeds, this splitting of available phone signal bandwidth still imposed a half-speed limit on modems.
Echo cancellation eliminated this problem. Measuring the echo delays and magnitudes allowed the modem to tell if the received signal was from itself or the remote modem, and create an equal and opposite signal to cancel its own. Modems were then able to send over the whole frequency spectrum in both directions at the same time, leading to the development of 4,800 and 9,600 bit/s modems.
Increases in speed have used increasingly complicated communications theory. Twelve hundred and 2,400 bit/s modems used the phase shift key (PSK) concept. This could transmit two or three bits per symbol. The next major advance encoded four bits into a combination of amplitude and phase, known as Quadrature Amplitude Modulation (QAM).
The new V.27ter and V.32 standards were able to transmit 4 bits per symbol, at a rate of 1,200 or 2,400 baud, giving an effective bit rate of 4,800 or 9,600 bit/s. The carrier frequency was 1,650 Hz. For many years, most engineers considered this rate to be the limit of data communications over telephone networks.
Error correction and compression[edit source | editbeta]
Operations at these speeds pushed the limits of the phone lines, resulting in high error rates. This led to the introduction of error-correction systems built into the modems, made most famous with Microcom's MNP systems. A string of MNP standards came out in the 1980s, each increasing the effective data rate by minimizing overhead, from about 75% theoretical maximum in MNP 1, to 95% in MNP 4. The new method called MNP 5 added data compression to the system, thereby increasing overall throughput above the modem's rating. Generally the user could expect an MNP5 modem to transfer at about 130% the normal data rate of the modem. Details of MNP were later released and became popular on a series of 2,400-bit/s modems, and ultimately led to the development of V.42 and V.42bis ITU standards. V.42 and V.42bis were non-compatible with MNP but were similar in concept because they featured error correction and compression.
Another common feature of these high-speed modems was the concept of fallback, or speed hunting, allowing them to communicate with less-capable modems. During the call initiation, the modem would transmit a series of signals and wait for the remote modem to respond. They would start at high speeds and get progressively slower until there was a response. Thus, two USR modems would be able to connect at 9,600 bit/s, but, when a user with a 2,400-bit/s modem called in, the USR would fall back to the common 2,400-bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at 2,400 bit/s. The same applies to V.32bis and 14,400 bit/s HST modem, which would still be able to communicate with each other at 2,400 bit/s.
[read more..]
Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps), or bytes per second (B/s). Modems can alternatively be classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.
News wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems.
Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs. In 1942, IBM adapted this technology to their unit record equipment and were able to transmit punched cards at 25 bits/second.[citation needed]
Mass-produced modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used[1]), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.
In summer 1960[citation needed], the name Data-Phone was introduced to replace the earlier term digital subset. The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960[citation needed]. In 1962[citation needed], the 201A and 201B Data-Phones were introduced. They were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels each running on their own set of two wires.
The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.
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.
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.
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.
Proprietary standards[edit source | editbeta]
Many other standards were also introduced for special purposes, commonly using a high-speed channel for receiving, and a lower-speed channel for sending. One typical example was used in the French Minitel system, in which the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1,200 bit/s for reception, and 75 bit/s for sending commands back to the servers.
Three U.S. companies became famous for high-speed versions of the same concept. Telebit introduced its Trailblazer modem in 1984, which used a large number of 36 bit/s channels to send data one-way at rates up to 18,432 bit/s. A single additional channel in the reverse direction allowed the two modems to communicate how much data was waiting at either end of the link, and the modems could change direction on the fly. The Trailblazer modems also supported a feature that allowed them to spoof the UUCP g protocol, commonly used on Unix systems to send e-mail, and thereby speed UUCP up by a tremendous amount. Trailblazers thus became extremely common on Unix systems, and maintained their dominance in this market well into the 1990s.
USRobotics (USR) introduced a similar system, known as HST, although this supplied only 9,600 bit/s (in early versions at least) and provided for a larger backchannel. Rather than offer spoofing, USR instead created a large market among Fidonet users by offering its modems to BBS sysops at a much lower price, resulting in sales to end users who wanted faster file transfers. Hayes was forced to compete, and introduced its own 9,600-bit/s standard, Express 96 (also known as Ping-Pong), which was generally similar to Telebit's PEP. Hayes, however, offered neither protocol spoofing nor sysop discounts, and its high-speed modems remained rare.
Echo cancellation was the next major advance in modem design.
Local telephone lines use the same wires to send and receive, which results in a small amount of the outgoing signal being reflected back. This is useful for people talking on the phone, as it provides a signal to the speaker that their voice is making it through the system. However, this reflected signal causes problems for the modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal. This was why earlier modems split the signal frequencies into "answer" and "originate"; the modem could then ignore any signals in the frequency range it was using for transmission. Even with improvements to the phone system allowing higher speeds, this splitting of available phone signal bandwidth still imposed a half-speed limit on modems.
Echo cancellation eliminated this problem. Measuring the echo delays and magnitudes allowed the modem to tell if the received signal was from itself or the remote modem, and create an equal and opposite signal to cancel its own. Modems were then able to send over the whole frequency spectrum in both directions at the same time, leading to the development of 4,800 and 9,600 bit/s modems.
Increases in speed have used increasingly complicated communications theory. Twelve hundred and 2,400 bit/s modems used the phase shift key (PSK) concept. This could transmit two or three bits per symbol. The next major advance encoded four bits into a combination of amplitude and phase, known as Quadrature Amplitude Modulation (QAM).
The new V.27ter and V.32 standards were able to transmit 4 bits per symbol, at a rate of 1,200 or 2,400 baud, giving an effective bit rate of 4,800 or 9,600 bit/s. The carrier frequency was 1,650 Hz. For many years, most engineers considered this rate to be the limit of data communications over telephone networks.
Error correction and compression[edit source | editbeta]
Operations at these speeds pushed the limits of the phone lines, resulting in high error rates. This led to the introduction of error-correction systems built into the modems, made most famous with Microcom's MNP systems. A string of MNP standards came out in the 1980s, each increasing the effective data rate by minimizing overhead, from about 75% theoretical maximum in MNP 1, to 95% in MNP 4. The new method called MNP 5 added data compression to the system, thereby increasing overall throughput above the modem's rating. Generally the user could expect an MNP5 modem to transfer at about 130% the normal data rate of the modem. Details of MNP were later released and became popular on a series of 2,400-bit/s modems, and ultimately led to the development of V.42 and V.42bis ITU standards. V.42 and V.42bis were non-compatible with MNP but were similar in concept because they featured error correction and compression.
Another common feature of these high-speed modems was the concept of fallback, or speed hunting, allowing them to communicate with less-capable modems. During the call initiation, the modem would transmit a series of signals and wait for the remote modem to respond. They would start at high speeds and get progressively slower until there was a response. Thus, two USR modems would be able to connect at 9,600 bit/s, but, when a user with a 2,400-bit/s modem called in, the USR would fall back to the common 2,400-bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at 2,400 bit/s. The same applies to V.32bis and 14,400 bit/s HST modem, which would still be able to communicate with each other at 2,400 bit/s.
