A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information and demodulates the 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 type is a voice band modem that turns the digital data of a 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 also 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), transmitted 1,200 bits by sending 600 symbols per second (600 baud) using phase shift keying.
History
TeleGuide terminal
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.
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), 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, 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 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.
Acoustic couplers
The Novation CAT acoustically coupled modem
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 latter 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
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, 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 Smartmodem and the rise of BBSs
The original model 300-baud Smartmodem
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 hobbyist who wanted to run a BBS but then to 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
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 suffers minimum distortion on the phone system and are 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 symbol 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
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, 9600 and 14,400
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. 1,200 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
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.
Breaking the 9.6k barrier
In 1980, Gottfried Ungerboeck from IBM Zurich Research Laboratory applied 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. In 1982, he agreed to publish what is now a landmark paper in the theory of information coding. By applying 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.
Error correcting codes, which encode code words (sets of bits) in such a way that they are far from each other, so that in case of error they are still closest to the original word (and not confused with another) can be thought of as analogous to sphere packing or packing pennies on a surface: the further two bit sequences are from one another, the easier it is to correct minor errors.
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, while AT&T introduced a one-off 19,200 bit/s method they referred to as V.32ter, but neither non-standard modem sold well.
V.34/28.8k and 33.6k
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 release hardware and introduced modems they referred to as V.FAST. In order to guarantee compatibility with V.34 modems once the standard was ratified (1994), the manufacturers were forced to use more flexible parts, generally a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips.
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 functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 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 + P u / P n ) {\displaystyle {\text{bandwidth}}\times \log _{2}(1+P_{u}/P_{n})} 
, with P u / P n {\displaystyle P_{u}/P_{n}} the (linear) signal-to-noise ratio. Narrowband phone lines have a bandwidth of 3000 Hz so using P u / P n = 1000 {\displaystyle P_{u}/P_{n}=1000} (SNR = 24 dB), the capacity is approximately 24 kbit/s.
Without the discovery and eventual application of trellis modulation, maximum telephone rates using voice-bandwidth channels would have been limited to 3,429 baud × 4 bit/symbol = approximately 14 kbit/s using traditional QAM.[original research?]
V.61/V.70 Analog/Digital Simultaneous Voice and Data
The V.61 Standard introduced Analog Simultaneous Voice and Data (ASVD). This technology allowed users of v.61 modems to engage in point-to-point voice conversations with each other while their respective modems communicated.
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 International Telecommunication Union (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 to pick up a telephone handset interfaced with the modem, and initiate a call to the other peer.
One practical use for this technology was realized by early two-player video gamers, who could hold voice communication with each other over the phone while playing.
Using digital lines and PCM (V.90/92)
In the late 1990s Rockwell/Lucent and USRobotics introduced new competing technologies 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 (e.g. 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). In the United States, government regulation limits the maximum power output, resulting in a maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34 technology at 33.6 kbit/s.
Later in V.92, the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s, but at the expense of download rates. A 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo on the telephone line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a 33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher. V.92 also adds two other 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 quickly connect to one's ISP. This is achieved by remembering the analog and digital characteristics of the telephone line, and using this saved information when reconnecting.
Using compression to exceed 56k
Today's V.42, V.42bis and V.44 standards allow 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 using pure text. However, the compression ratio tends to vary 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 approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.
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 lines 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.
Compression by the ISP
As telephone-based 56k modems began losing popularity, some Internet service providers such as Netzero/Juno, Netscape, and others started using pre-compression to increase the throughput and maintain their customer base. The server-side compression operates much more efficiently than the on-the-fly compression done by modems due to the fact these compression techniques are application-specific (JPEG, text, EXE, etc.). The website text, images, and Flash executables are compacted to approximately 4%, 12%, and 30%, respectively. The drawback of this approach is a loss in quality, which causes image content to become pixelated and smeared. ISPs employing this approach often advertise it as "accelerated dial-up."
These accelerated downloads are now integrated into the Opera and Amazon Silk web browsers, using their own server-side text and image compression.
Softmodem
A PCI Winmodem/softmodem (on the left) next to a traditional ISA modem (on the right).
Main article: Softmodem
A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally handled in hardware with software. In this case the modem is a simple interface designed to act as a digital-to-analog and an analog-to-digital converter. Softmodems are cheaper than traditional modems because they have fewer hardware components. However, the software generating and interpreting the modem tones to be sent to the softmodem uses many system resources. For online gaming, this can be a real concern. Another problem is the lack of cross-platform compatibility, meaning that non-Windows operating systems (such as Linux) often do not have an equivalent driver to operate the modem.
List of dialup speeds
These values are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines). For a complete list see the companion article list of device bandwidths. A baud is one symbol per second; each symbol may encode one or more data bits.
Popularity
A CEA study in 2006 found that dial-up Internet access is declining in the U.S. In 2000, dial-up Internet connections accounted for 74% of all U.S. residential Internet connections. The US demographic pattern for dial-up modem users per capita has been more or less mirrored in Canada and Australia for the past 20 years.
Dial-up modem use in the US had dropped to 60% by 2003, and in 2006 stood at 36%. Voiceband modems were once the most popular means of Internet access in the U.S., but with the advent of new ways of accessing the Internet, the traditional 56K modem is losing popularity. The dial up modem is still widely used by customers in rural areas, where DSL, Cable or Fiber Optic Service is not available, or they are unwilling to pay what these companies charge. AOL in its 2012 annual report showed it still collects around $700 million in fees from dial-up users; about 3 million people.
Radio modems
Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fibre is not economical.
Even where a cable is installed, it is often possible to get better performance or make other parts of the system simpler by using radio frequencies and modulation techniques through a cable. Coaxial cable has a very large bandwidth, however signal attenuation becomes a major problem at high data rates if a baseband digital signal is used. By using a modem, a much larger amount of digital data can be transmitted through a single wire. Digital cable television and cable Internet services use radio frequency modems to provide the increasing bandwidth needs of modern households. Using a modem also allows for frequency-division multiple access to be used, making full-duplex digital communication with many users possible using a single wire.
Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies.
Transparent modems operate in a manner similar to their phone line modem cousins. Typically, they were half duplex, meaning that they could not send and receive data at the same time. Typically transparent modems are polled in a round robin manner to collect small amounts of data from scattered locations that do not have easy access to wired infrastructure. Transparent modems are most commonly used by utility companies for data collection.
Smart modems come with media access controllers inside, which prevents random data from colliding and resends data that is not correctly received. Smart modems typically require more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE 802.11 standard defines a short range modulation scheme that is used on a large scale throughout the world.
WiFi and WiMax
The WiFi and WiMax standards use wireless mobile broadband modems operating at microwave frequencies.
Mobile broadband modems
Huawei CDMA2000 Evolution-Data Optimized USB wireless modem
Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax, etc.), are known as mobile broadband modems (sometimes also called wireless modems). Wireless modems can be embedded inside a laptop or appliance, or be external to it. External wireless modems are connect cards, USB modems for mobile broadband and cellular routers. A connect card is a PC Card or ExpressCard which slides into a PCMCIA/PC card/ExpressCard slot on a computer. USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot. A USB modem used for mobile broadband Internet is also sometimes referred to as a dongle. A cellular router may have an external datacard (AirCard) that slides into it. Most cellular routers do allow such datacards or USB modems. Cellular routers may not be modems by definition, but they contain modems or allow modems to be slid into them. The difference between a cellular router and a wireless modem is that a cellular router normally allows multiple people to connect to it (since it can route data or support multipoint to multipoint connections), while a modem is designed for one connection.
Most of GSM wireless modems come with an integrated SIM cardholder (i.e., Huawei E220, Sierra 881, etc.) and some models are also provided with a microSD memory slot and/or jack for additional external antenna such as Huawei E1762 and Sierra Wireless Compass 885. The CDMA (EVDO) versions do not use R-UIM cards, but use Electronic Serial Number (ESN) instead.
The cost of using a wireless modem varies from country to country. Some carriers implement flat rate plans for unlimited data transfers. Some have caps (or maximum limits) on the amount of data that can be transferred per month. Other countries have plans that charge a fixed rate per data transferred—per megabyte or even kilobyte of data downloaded; this tends to add up quickly in today's content-filled world, which is why many people are pushing for flat data rates.
The faster data rates of the newest wireless modem technologies (UMTS, HSPA, EVDO, WiMax) are also considered to be broadband wireless modems and compete with other broadband modems below.
Until the end of April 2011, worldwide shipments of USB modems surpassed embedded 3G and 4G modules by 3:1 because USB modems can be easily discarded, but embedded modems could start to gain popularity as tablet sales grow and as the incremental cost of the modems shrinks, so by 2016 the ratio may change to 1:1.
Like mobile phones, mobile broadband modems can be SIM locked to a particular network provider. Unlocking a modem is achieved the same way as unlocking a phone, by using an 'unlock code'
Broadband
ADSL (asymmetric digital subscriber line) modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Early proprietary ADSL modems used carrierless amplitude phase (CAP) modulation. All standardized asymmetric DSL variants, including ANSI T1.413 Issue 2, G.DMT, ADSL2, ADSL2+, VDSL2, and G.fast, use discrete multi-tone (DMT) modulation, also called (coded) orthogonal frequency-division multiplexing (OFDM or COFDM).
Standard twisted-pair telephone cable can, for short distances, carry signals with much higher frequencies than the cable's maximum frequency rating. ADSL broadband takes advantage of this capability. However, ADSL's performance gradually declines as the telephone cable's length increases. This limits ADSL broadband service to subscribers within a relatively short distance from the telephone exchange.
Cable modems use a range of radio frequencies originally intended to carry television signals. A single cable can carry radio and television signals at the same time as broadband internet service without interference. Multiple cable modems attached to a single cable can use the same frequency band by employing a low-level media access protocol to avoid conflicts. In the prevalent DOCSIS system, frequency-division duplexing (FDD) separates uplink and downlink signals. For a single-cable distribution system, the return signals from customers require bidirectional amplifiers or reverse path amplifiers that send specific customer frequency bands upstream to the cable plant amongst the downstream frequency bands.
Newer types of broadband modems are available, including satellite and power line modems.
Most consumers did not know about networking and routers when broadband became available. However, many people knew that a modem connected a computer to the Internet over a telephone line. To take advantage of consumers' familiarity with modems, companies called these devices broadband modems rather use less familiar terms such as adapter, interface, transceiver, or bridge. In fact, broadband modems fit the definition of modem because they use complex waveforms to carry digital data. They use more advanced technology than dial-up modems: typically they can modulate and demodulate hundreds of channels simultaneously or use much wider channels than dial-up modems.
Residential Gateways
Some devices referred to as "broadband modems" are residential gateways, integrating the functions of a modem, network address translation (NAT) router, Ethernet switch, WiFi access point, DHCP server, firewall, among others. Some residential gateway offer a so-called "bridged mode", which disables the built-in routing function and makes the device function similarly to a layer 1/2 modem. This bridged mode is separate from RFC 1483 bridging.
Home networking
Although the name modem is seldom used in this case, modems are also used for high-speed home networking applications, especially those using existing home wiring. One example is the G.hn standard, developed by ITU-T, which provides a high-speed (up to 1 Gbit/s) Local area network using existing home wiring (power lines, phone lines and coaxial cables). G.hn devices use orthogonal frequency-division multiplexing (OFDM) to modulate a digital signal for transmission over the wire.
The phrase "null modem" was used to describe attaching a specially wired cable between the serial ports of two personal computers. Basically, the transmit output of one computer was wired to the receive input of the other; this was true for both computers. The same software used with modems (such as Procomm or Minicom) could be used with the null modem connection.
