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News wire services in the 1920s used multiplex equipment that met the definition, but the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems. George Stibitz connected a New Hampshire teletype to a computer in New York City by phone lines in 1940, but the word "modem" had not yet been invented, so this too doesn't count.
Modems in the United States were part of the SAGE air-defense system in the 1950s, 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 ran on dedicated communications lines, but the devices at each end were otherwise similar in concept to today's modems. IBM was the primary contractor for both the computers and the modems used in the SAGE system.
A few years later, a chance meeting between the CEO of American Airlines and a regional manager of IBM led to development of a "mini-SAGE" as an automated airline ticketing system. The terminals were at ticketing offices, tied to a central computer that managed availability and scheduling. The system, known as SABRE, is the ancestor of today's Sabre system.
AT&T monopoly in the United States
For many years, AT&T maintained a monopoly in the United States on the use of its phone lines, allowing only AT&T-supplied devices to be attached to its network. For the growing group of computer users, AT&T introduced two digital sub-sets in 1958. One is the wideband device shown in the picture to the left. The other was a low-speed modem, which ran at 200 baud.
In the summer of 1960, 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. In 1962, 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 2000 bit/s over normal phone lines, while the 201B provided full duplex 2400 bit/s service on four-wire leased lines, the send and receive channels running on their own set of two wires each. The famous 103A was also introduced in 1962. It provided full-duplex service at up to 300 baud over normal phone lines. Frequency-shift keying (FSK) was used with the call originator transmitting at 1070 or 1270 Hz and the answering modem transmitting at 2025 or 2225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the KSR33, the ASR33, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.
The Carterfone decision
Before 1968, AT&T maintained a monopoly on what devices could be electrically connected to its phone lines. This led to a market for 103A-compatible modems that were mechanically connected to the phone, through the handset, known as acoustically coupled modems. Particularly common models from the 1970s were the Novation CAT (shown in the image) and the Anderson-Jacobson, spun off from an in-house project at the Lawrence Livermore National Laboratory.
In 1967, the U.S. Supreme Court broke AT&T's monopoly on the lines in the landmark Carterfone decision. Now, the lines were open to anyone, as long as they passed a stringent set of AT&T-designed tests. AT&T made these tests complex and expensive, so acoustically coupled modems remained common into the early 1980s.
In December 1972, Vadic introduced the VA3400. This device was remarkable because it provided full duplex operation at 1200 bit/s over the dial network, using methods similar to those of the 103A in that 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 transmit from originating modem. It was also possible to 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 1200-bit/s mode, AT&T's 212A mode, and 103A operation.
The Smartmodem
The next major advance in modems was the Smartmodem, introduced in 1981 by Hayes Communications. The Smartmodem was an otherwise standard 103A 300-bit/s modem, but was attached to a small controller that let the computer send commands to it and enable it to operate the phone line. The command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls. The basic Hayes command set remains the basis for computer control of most modern modems.
Prior to the Hayes Smartmodem, modems almost universally required a two-step process to activate a connection: first, the user had to manually dial the remote number on a standard phone handset, and then secondly, plug the handset into an acoustic coupler. Hardware add-ons, known simply as dialers, were used in special circumstances, and generally operated by emulating someone dialing a handset.
With the Smartmodem, the computer could dial the phone directly by sending the modem a command, thus eliminating the need for an associated phone for dialing and the need for an acoustic coupler. The Smartmodem instead plugged directly into the phone line. This greatly simplified setup and operation. Terminal programs that maintained lists of phone numbers and sent the dialing commands became common. The Smartmodem and its clones also aided the spread of bulletin-board systems (BBSs). 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. The Smartmodem could operate in either mode depending on the commands sent from the computer. There was now a low-cost server-side modem on the market, and the BBSs flourished.
Increasing speeds
Voiceband modems generally remained at 300 and 1200 bit/s into the mid 1980s, although, over this period, the acoustic coupler disappeared, seemingly overnight, as Smartmodem-compatible modems flooded the market.
A 2400-bit/s system similar in concept to the 1200-bit/s Bell 212 signalling was introduced in the U.S., and a slightly different, and incompatible, one in Europe. By the late 1980s, most modems could support all of these standards, and 2400-bit/s operation was becoming common. 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 1200 bit/s for reception, and 75 bit/s for sending commands back to the servers. Such solutions were useful in many circumstances in which one side would be sending more data than the other. In addition to a number of "medium-speed" standards, like Minitel, four 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 low-speed channels to send data one-way at rates up to 19,200 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 switch which side had the high-speed channels 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. U.S. Robotics (USR) introduced a similar system, known as HST, although this supplied only 9600 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 9600-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.
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 slowing the effective data rate by a smaller amount each time, from about 25% in MNP 1, to 5% in MNP 4. MNP 5 took this a step further, adding data compression to the system, thereby actually increasing the data rate: generally, the user could expect an MNP modem to transfer at about 1.3 times the normal data rate of the modem. MNP was later "opened" and became popular on a series of 2400-bit/s modems, although it was never widespread.
Another common feature of these high-speed modems was the concept of fallback, allowing them to talk to less-capable modems. During the call initiation the modem would play a series of signals into the line and wait for the remote modem to "answer" them. They would start at high speeds and progressively get slower and slower until they heard an answer. Thus, two USR modems would be able to connect at 9600 bit/s, but, when a user with a 2400-bit/s modem called in, the USR would "fall back" to the common 2400-bit/s speed. Without such a system, the operator would be forced to have multiple phone lines for high- and low-speed use.
V.32
Echo cancellation was the next major advance in modem design. Local telephone lines use the same wires to send and receive, while longer distances use separate wires for the two directions. A small amount of the outgoing signal bounces back. This signal can confuse the modem: is the signal it is "hearing" from the remote modem, or its own transmission bouncing back? This was why earlier modems split the signal frequencies into answer and originate; each modem simply didn't listen to its own transmitting frequencies. 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 got around 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 at "full speed" in both directions at the same time, leading to the development of the 9600 bit/s V.32 standard. Starting in the late 1980s a number of companies started introducing V.32 modems, most of them also using the newly opened MNP standards for error correction and compression. These earlier systems were not very popular due to their price, but by the early 1990s the prices started falling. The "tipping point" occurred with the introduction of the SupraFax 14400 in 1991. Rockwell had introduced a new chipset supporting not only V.32 and MNP, but the newer 14,400 bit/s V.32bis and the higher-compression V.42bis as well, and even included 9600 bit/s fax capability. Supra, then known primarily for their hard drive systems for the Atari ST, used this chip set to build a low-priced 14,400 bit/s modem which cost the same as a 2400 bit/s modem from a year or two earlier (about 300 USD). The product was a runaway best-seller, and it was months before the company could keep up with demand. The SupraFax was so successful that a huge number of companies joined the fray, and by the next year 14.4 modems from a wide variety of companies were available. The Rockwell chipset, while not terribly reliable, became extremely common, but Texas Instruments and AT&T Paradyne quickly responded with similar chipsets of their own. V.32bis was so successful that the older high-speed standards had little to recommend them. USR fought back with a 16,800 bit/s version of HST, but this small increase in performance did little to keep HST interesting. AT&T introduced a one-off 19,200 bit/s "standard" they referred to as V.32ter (also known as V.32 terbo), but this also did little to increase demand, and typically this mode came into use only when two users with AT&T-based modems just happened to call each other. Motorola also introduced another, incompatible, 19.2 standard, but charged very high prices for their modems, which they had previously sold into commercial settings only.
V.34
Any interest in these systems was destroyed during the lengthy introduction of the 28,800 bit/s V.34 standard. While waiting, several companies decided to "jump the gun" and introduced modems they referred to as "V.FAST". In order to guarantee compatibility with V.34 modems once the standard was ratified (which happened in 1994), the manufacturers were forced to use more "flexible" parts, generally a DSP and microcontroller, as opposed to purpose-designed "modem chips". A good example of this was USR, which changed their modems to use a DSP from Texas Instruments, and introduced a top-of-the-line Courier product, the V.everything. As the name implied, the new model supported practically every standard on the market, including all of the HST modes, V.32bis, V.FAST and, later, V.34. Rockwell also introduced a V.FAST chipset in late 1993, which they referred to as V.FC (for "Fast Class"). Rapid commoditization in 1994 forced almost all vendors out of the market; Motorola gave up and disappeared without a trace, AT&T throwing in the towel soon after. Their attempts to introduce their own standards were failures in both technical and business senses.
V.70
In 1995, the first DSVD (Digital Simultaneous Voice and Data) modems became available to consumers, and the standard was ratified as V.70 by the ITU in 1996. Two DSVD modems can establish a completely digital link between each other over standard phone lines. Sometimes referred to as "the poor man's ISDN", and employing a similar technology, V.70 compatible modems allow for a maximum speed of 33.6 kbit/s between peers. By using a majority of the bandwidth for data and reserving part for voice transmission, DSVD modems allow users (often while playing video games) to pick up a telephone handset interfaced with the modem, and initiate a call to the other peer. Advocates of DSVD envisioned whiteboard sharing and other practical applications for the standard, however, with advent of cheaper 56kbit/s analog modems intended for internet connectivity, peer-to-peer data transmission over the PSTN became quickly irrelevant. Today, Multi-Tech is the only known company to continue to support a V.70 compatible modem. While their device also offers V.92 at 56kbit/s, it remains significantly more expensive than comparable modems without V.70 support.
V.90
With the rapid introduction of all-digital phone systems in the 1990s, it became possible to use much greater bandwidth on the assumption that users would generally be based on digital lines – if not immediately, then in the near future. Digital lines are based on a standard using 8 bits of data for every voice sample, sampled 8000 times a second, for a total data rate of 64 kbit/s. However, many systems use in-band signaling for command data, "robbing" one bit of command data per six bytes of signal, thereby reducing real throughput to 56k. In 1996, modems came to market that took advantage of the widespread use of digital phone systems at ISPs in order to provide download speeds up to 56kbit/s. Originally, there were two available protocols for achieving such speeds, K56flex, designed and promoted by Rockwell and X2, designed and promoted by U.S. Robotics. The already widespread use of the Rockwell chip set made K56flex more popular. A standardization effort started around 1996 working towards a single standard for 56k modems that would replace K56flex and X2. Originally known as V.pcm (PCM referring to the pulse code modulation used in digital telephony), it became the V.90 protocol when finalized in 1998. There are certain special requirements and restrictions associated with V.90 modems. In order for users to obtain up to 56k upload speeds from their ISP, the telephone line had to be completely digital between the ISP and the telephone company central office (CO) of the user. From there the signal could be converted from digital to analog but only at this point. If there was a second conversion anywhere along the line 56k speeds were impossible. Also, the line quality of the user's telephone line could affect the speed of the 56k connection with line noise causing slow downs, sometimes to the point of only being marginally faster than a 33.6 kbit/s connection. An important restriction with V.90 is that while V.90 modems can obtain up to 56kbit/s download speeds, they are limited to 33.6 kbit/s upload speeds. Only in the rarest of circumstances would a user actually see 56kbit/s speeds, with 48kbit/s to 50kbit/s being the most likely on a clean line. Prior to the adoption of the V.90 protocol in 1998, users were slow to adopt K56flex and X2 based 56k modems, many simply waited for V.90 to arrive. Some modem manufacturers promised and later offered firmware or driver updates for their modems so that users could add V.90 functionality. As ISPs slowly adopted either the K56flex or X2 (or both) protocols on their dial up lines, users would typically choose an ISP based on which modem type they had. However, when the V.90 spec was finally ratified and V.90 modems started to appear, they were backward compatible with the K56flex or X2 protocols, thus allowing ISPs (and users) to continue to connect to K56flex or X2 dial in lines, even if the ISP did not support V.90 directly. Following the adoption of V.90, there was an attempt to adopt a protocol that would define a standard to allow all-digital communications (i.e where both the ISP and the user had digital connections to the telephone network). It was to be known as V.91 but the process essentially died with the advent of rapid introduction of short-haul high-speed solutions like ADSL and cable modems which offer much higher speeds from the user's local machine onto the Internet. With the exception of rural areas, the need for point-to-point calls has generally disappeared as a result, as the bandwidth and responsiveness of the Internet has improved greatly. It appears that V.90 will be the last analog modem standard to see widespread use.
V.92
V.92 is the standard that followed V.90. While it provides no increase in download bit rate (56kbit/s appears to be the maximum speed for voiceband modems), it does have slightly better compression and allows upload speeds up to 48kbit/s provided both ISP and caller have V.92 compatible modems. It also adds two features. The first is the ability for users who have call waiting to put their dial-up Internet connection on hold for extended periods of time while they answer a call. The second feature is the ability to "quick connect" to one's ISP. This is achieved by remembering key information about the telephone line one is using, and using this saved information to help speed up future calls made from the line to the ISP. ISPs have been slow to adopt V.92 due to the high cost of upgrading their equipment and the lack of demand from their customers. With the rise in broadband take-up that has led to declining numbers of dial-up users, some ISPs have decided not to bother ever upgrading to V.92.
Long haul modems
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