Modem (from modulate and demodulate) 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 over any means of transmitting analog signals, from driven diodes to radio.
The most familiar example is a voiceband modem that turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those 1s and 0s back into a form used by a USB, Serial, or Network connection. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". They can also be classified by Baud, the number of distinct symbols transmitted per second; these numbers are directly connected, but not necessarily in linear fashion (as discussed under Baud.)
Faster modems are used by Internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optical fibers. Most intercontinental data links now use optical modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x10) bits per second. One kilobit per second (kbit/s or kb/s or kbps) as used in this article means 1000 bits per second and not 1024 bits per second. For example, a 56k modem can transfer data at up to 56,000 bits per second over the phone line.
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Telephone network modems history
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 right. 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.
AT&T monopoly in the United States
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.
Hush-a-Phone v. FCC was a seminal ruling in United States telecommunications law decided by the DC Circuit Court of Appeals on November 8, 1956. The District Court found that it was within the FCC's authority to regulate the terms of use of AT&T's equipment. Subsequently, the FCC examiner found that as long as the device was physically attached it would not threaten to degenerate the system. Later, in the Carterfone decision, the FCC passed a rule setting stringent AT&T-designed tests for electronically coupling a device to the phone lines. 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 Carterfone decision
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.
The Smartmodem
Voiceband modems generally remained at 300 and 1200 bit/s (V.21 and V.22) into the mid 1980s, although, over this period, the acoustic coupler disappeared, seemingly overnight, as Smartmodem-compatible modems flooded the market.
A V.22bis 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 18,400 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.
Increasing speeds (V.21 V.22 V.22bis)
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.
Error correction and compression
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.32Any 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.
Eventually the V.34 standard was expanded to the 33.6 kbit/s V.34bis standard which was and still is the fastest speed using analog technology. Later modems achieved faster rates via using digital encoding (PCM).
V.34/V.34bis
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 56 kbit/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 56 kbit/s, it remains significantly more expensive than comparable modems without V.70 support.
V.70
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 56 kbit/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 download 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 56 kbit/s download speeds, they are limited to 33.6 kbit/s upload speeds. Only in the rarest of circumstances would a user actually see 56 kbit/s speeds, with 48 kbit/s to 50 kbit/s being the most likely on a clean line. In addition, the FCC has limited the power output from modems to prevent crosstalk between bundled phone lines; thus even on a clean line 56K modems are limited to approximately 53 kbit/s to stay compliant with the regulation.
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.90
V.92 is the standard that followed V.90. While it provides no increase in download bit rate (56 kbit/s appears to be the maximum speed for voiceband modems), it does have slightly better compression and allows upload speeds up to 48 kbit/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.
V.92
In the 1960s, Bell began to digitize the telephone system, and developed early high-speed radio modems for this purpose. Once digital long-haul networks were in place, they were leased for every other purpose.
Optic fiber manufacturing was mastered in the 1980s, and optic modems were first invented for these early systems. The first systems simply used light-emitting diodes and PIN diodes. Faster modulation was quickly adopted for long-haul networks. In the 1990s, multispectral optical modems were adopted as well.
Long haul modems
A standard modem of today is what would have been called a "smart modem" in the 1980s. It contains two functional parts: an analog section for generating the signals and operating the phone, and a digital section for setup and control. This functionality is actually incorporated into a single chip, but the division remains in theory.
In operation the modem can be in one of two "modes", data mode in which data is sent to and from the computer over the phone lines, and command mode in which the modem listens to the data from the computer for commands, and carries them out. A typical session consists of powering up the modem (often inside the computer itself) which automatically assumes command mode, then sending it the command for dialing a number. After the connection is established to the remote modem, the modem automatically goes into data mode, and the user can send and receive data. When the user is finished, the escape sequence, "+++" followed by a pause of about a second, is sent to the modem to return it to command mode, and the command to hang up the phone is sent. One problem with this method of operation is that it is not really possible for the modem to know if a string is a command or data. When the modem misinterprets a string, it generally causes odd things to happen.
The commands themselves are typically from the Hayes command set, although that term is somewhat misleading. The original Hayes commands were useful for 300 bit/s operation only, and then extended for their 1200 bit/s modems. Hayes was much slower upgrading to faster speeds however, leading to a proliferation of command sets in the early 1990s as each of the high-speed vendors introduced their own command styles. Things became considerably more standardized in the second half of the 1990s, when most modems were built from one of a very small number of "chip sets", invariably supporting a rapidly converging command set. We call this the Hayes command set even today, although it has three or four times the numbers of commands as the actual standard.
The 300 bit/s modems used 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 1070 Hz tone, and 1s at 1270 Hz, with the answering modem putting its 0s on 2025 Hz and 1s on 2225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system, and also are not harmonics of each other. For the 103F leased line version, internal strapping selected originate or answer operation. For dial models, the selection was determined by which modem originated the call. Modulation was so slow and simple that some people were able to learn how to whistle short bits of data into the phone with some accuracy.
In the 1200 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 the similar frequencies as in the 300 bit/s systems, but slightly out of phase. By comparing the phase of the two signals, 1s and 0s could be pulled back out, for instance if the signals were 90 degrees out of phase, this represented two digits, "1,0", at 180 degrees it was "1,1". In this way each cycle of the signal represents two digits instead of one, 1200 bit/s modems were, in effect, 600 bit/s modems with "tricky" signaling.
It was at this point that the difference between baud and bit per second became real. Baud refers to the signaling rate of a system, in a 300 bit/s modem the signals sent one bit per signal, so the data rate and signalling rate was the same. In the 1200 bit/s systems this was no longer true since the modems were actually 600 baud. This led to a series of flame wars on the BBSes of the 80s.
Increases in speed have since used increasingly complicated communications theory. The Milgo 4500 introduced the 8 phase shift key concept. This could transmit three bits per signaling instance (baud). The next major advance was introduced by the Codex Corporation in the late 1960s. Here the bits were encoded into a combination of amplitude and phase, known as Quadrature Amplitude Modulation. Best visualized as a two-dimensional or constellation diagram, the bits are mapped onto points on a graph with the x (real) and y (quadrature) coordinates transmitted over a single carrier. This technique became very effective and was incorporated into an international standard named V.29, by the CCITT (now ITU) arm of the United Nations. The standard was able to transmit 4 bits per baud, at a rate of 2400baud, giving an effective bit rate of 9600 bits per second (9.6k). The carrier frequency was 1650 Hz.
For many years, most engineers considered this rate to be the limit of data communications over telephone networks.
Narrowband/various encoding methods
In 1980 Godfried Ungerboeck from IBM applied powerful channel coding techniques to search for new ways to increase the speed of modems. His results were astonishing but only conveyed to a few colleagues. Finally in 1982, he agreed to publish what is now a landmark paper in the theory of information coding. By applying powerful parity check coding to the bits in each symbol, and mapping the encoded bits into a two dimensional "diamond pattern", Ungerboeck showed that it was possible to increase the speed by a factor of two with the same error rate. The new technique was called mapping by set partitions (now known as trellis modulation). This new view was an extension of the "penny packing" problem and the related and more general problem of how to pack points into an N-dimension sphere such that they are far away from their neighbors. When bit sequences differ a lot, one faulty bit can still be corrected.
The industry was galvanized into new research and development. More powerful coding techniques were developed, commercial firms rolled out new product lines, and the standards organizations rapidly adopted to new technology. Today the ITU standard V.34 represents the culmination of the joint efforts. It employs the most powerful coding techniques including channel encoding and shape encoding. From the mere 4 bits per symbol (9.6 kbit/s), the new standards used the equivalent of 6 to 10 bits per symbol and very sophisticated algorithms to create 14.4k, 28.8, and 33.6 kbit/s modems. This rate is near the theoretical Shannon limit. When calculated, the Shannon capacity of a narrowband line is Bandwidth * log_2 (1 + Pu/Pn), with Pu/Pn the signal-to-noise ratio. Narrowband phone lines have a bandwidth from 300-3100 Hz, so using Pu/Pn=100,000: capacity is approximately 35 kbit/s.
Breaking the 9.6k barrier
In the late 1990s Rockwell and U.S. Robotics introduced new technology based upon the digital transmission used in modern telephony networks. The standard digital transmission in modern networks is 64 kbit/s but some networks use a part of the bandwidth for remote office signaling (eg to hang up the phone), limiting the effective rate to 56 kbit/s DS0. This new technology was adopted into ITU standards V.90 and is common in modern computers.
The 56 kbit/s rate is only possible from the central office to the user site (downlink) and in the United States, government regulation limits the maximum power output to only 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34 technology at 33.6k. Later in V.92, the PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s. If it becomes possible to approach more closely the DS0 rate of 64 kbit/s, it may be due more to improvements in the underlying phone system (increasing signal-to-noise ratio) than anything in the technology of the modems themselves.
Using digital lines and PCM
Software is as important to the operation of the modem today as the hardware. Even with the improvements in the performance of the phone system, modems still lose a considerable amount of data due to noise on the line. The MNP standards were originally created to automatically fix these errors, and later expanded to compress the data at the same time. Today's V.42 and V.42bis fill these roles in the vast majority of modems, and although later MNP standards were released, they are not common.
With such systems it is possible for the modem to transmit data faster than its basic rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3*6 == 320 kbit/s. In reality, the data rate rarely exceeds 3:1 compression. One problem is that the compression tends to get better and worse over time due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video). At some points the modem will be sending compressed files at 53 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between. (See http://www.digit-life.com/articles/compressv44vsv42bis/ for actual compression results.)
In such situations a small amount of memory in the modem, a buffer, is used to hold the data while it is being compressed and sent across the phone line, but in order to prevent overflow of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream. This is accomplished through hardware flow control using extra pins on the modem–computer connection. The computer is then set to supply the modem at some higher rate, such as 320 kbit/s, and the modem will tell the computer when to start or stop sending data.
Using compression to exceed 56k
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