# Modem

(Redirected from Modems)

A modem – a portmanteau of "modulator-demodulator" – is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telephone lines or radio. A modem modulates one or more carrier wave signals to encode digital information for transmission, and demodulates signals to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded reliably to reproduce the original digital data.

Modems can be used with almost any means of transmitting analog signals, from light-emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into a modulated electrical signal for transmission over telephone lines, to be demodulated by another modem at the receiver side to recover the digital data.

## Speeds

Modems are frequently classified by the maximum amount of data they can send in a given unit of time, usually expressed in bits per second (symbol bit/s, sometimes abbreviated "bps") or rarely in bytes per second (symbol B/s). Modern broadband modems are typically described in megabits.

Historically, modems were often 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.

Many modems are variable-rate, permitting them to be used over a medium with less than ideal characteristics, such as a telephone line that is of poor quality or is too long. This capability is often adaptive so that a modem can discover the maximum practical transmission rate during the connect phase, or during operation.

Collection of modems once used in Australia, including dial-up, DSL, and cable modems.

## Dial-up modem

A dial-up modem transmits computer data over an ordinary telephone line that has not been designed for data use. Other modem technologies (such as leased line modems) operate over lines that are often provided by a telephone company, but are intended for data use and do not impose the same signaling constraints.

The modulated data must fit the frequency constraints of a normal voice audio signal, and the modem must be able to perform the actions needed to connect a call through a telephone exchange, namely: picking up the line, dialing, understanding signals sent back by phone company equipment (dialtone, ringing, busy signal,) and on the far end of the call, the second modem in the connection must be able to recognize the incoming ring signal and answer the line.

Dial-up modems have been made in a wide variety of speeds and capabilities, with many capable of testing the line they're calling over and selecting the most advanced signaling mode that the line can support. Generally speaking, the fastest dialup modems ever available to consumers never exceeded 56 KiB/s, and never achieved that speed in both directions.

The dial-up modem was once a widely known technology, since it was mass-marketed to consumers in many countries for dial-up internet access. In the 90s, tens of millions of people in the United States used dial-up modems for internet access.[1]

Dial-up service has since been largely supplanted by broadband internet[2], which typically still uses a modem, but of a very different type which may still operate over a normal phone line, but with substantially relaxed constraints.

### History

TeleGuide terminal

News wire services in the 1920s used multiplex devices that satisfied the definition of a modem.[3] 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 1941, the Allies developed a voice encryption system called SIGSALY which used a vocoder to digitize speech, then encrypted the speech with one-time pad and encoded the digital data as tones using frequency shift keying.

Mass production of modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used[4]), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the United States 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.

The advent of metal–oxide–semiconductor field-effect transistor (MOSFET) technology, integrated circuit chips and shift register MOS memory led to the application of solid-state microelectronic semiconductor technology and digital circuitry to the implementation of digital multi-channel modems in the 1960s. This enabled advantages such as a very small size, light weight, stability, high reliability, low cost, and freedom from adjustment.[5]

The 201A and 201B Data-Phones 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 Bell 103A dataset standard was also introduced by AT&T in 1962.[6] 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

While Bell (AT&T) provided modems that attached via direct wire connection to the phone network as early as 1958, their regulations at that time did not permit the direct electrical connection of any non-Bell device to a telephone line. However, the Hush-a-Phone ruling allowed customers to attach any device to a telephone set as long as it did not interfere with its functionality. This allowed third-party (non-Bell) manufacturers to sell modems utilizing an acoustic coupler.

With an acoustic coupler, an ordinary telephone handset was placed in a cradle containing a speaker and microphone positioned to match up with those on the handset. The tones used by the modem were transmitted and received into the handset, which then relayed them to the phone line.

Because the modem was not electrically connected, it was incapable of picking up, hanging up or dialing, all of which required direct control of the line. Touch-tone dialing would have been possible, but touch-tone was not universally available at this time. Consequently, the dialing process was executed by the user lifting the handset, dialing, then placing the handset on the coupler.

Throughout the 1970s, acoustically coupled modems compatible with the Bell 103A de-facto standard were commonplace. Well-known models included the Novation CAT and the Anderson-Jacobson. A lower-cost option was the Pennywhistle modem, designed to be built using parts from electronics scrap and surplus stores.

In December 1972, Vadic introduced the VA3400, notable for 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, 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.

#### Directly connected modems

The Hush-a-Phone decision applied only to mechanical connections, but the Carterfone decision of 1968 permitted customers to attach devices directly to a telephone line, as long as they followed stringent Bell-defined standards for non-interference with the phone network. This opened the door to direct-connect modems that plugged directly into the phone line rather than via a coupler. However, the cost of passing the tests was considerable, and acoustically coupled modems remained common into the early 1980s.

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 – dialing 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, third party "dialers" were used to automate calling, normally using a separate serial port to communicate with the dialer, which would then control the modem through a private electrical connection.

The introduction of microcomputer systems with internal expansion slots made the first software-controllable modems common. Slot connections gave the computer complete access to the modem's memory or input/output (I/O) channels, which allowed software to send commands to the modem, not just data. This led to a series of popular modems for the S-100 bus and Apple II computers that could directly dial the phone, answer incoming calls, and hang up the phone, the basic requirements of a bulletin board system (BBS). The seminal CBBS for instance was created on an S-100 machine with a Hayes internal modem, and a number of similar systems followed.

#### The Hayes Smartmodem

The original model 300-baud Hayes Smartmodem
In later years, USRobotics Courier modems were common at many BBSes because they had better compatibility with more brands of modems

The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300 bit/s direct-connect modem, but it included a microcontroller that watched the data stream for certain character strings representing commands, which it used to control functions like dialing and answering.

The Hayes command set used by this device became a de-facto standard supported by devices from many other manufacturers. It included instructions for picking up and hanging up the line, dialing and answering calls, among others. This was similar to the commands offered by internal modems, but unlike those, the Smartmodem could be connected to any computer with an RS-232 port, which was practically every microcomputer built.

The introduction of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features.

Through the 1980s, a number of new higher-speed modems, first 1,200 and then 2,400 bit/s, greatly improved the responsiveness of online systems, and made file transfer practical. This led to rapid growth of online services with their large file libraries, which in turn gave more reason to own a modem. The rapid update of modems led to a similar rapid increase in BBS use, which was helped by the fact that BBSs could control the modem simply by sending strings, rather than talking to a device driver that was different for every direct-connect modem.

#### Increasing speed - 1200 and 2400 bit/s

Higher speed modems began to reach the market eventually, at speeds of 1,200 and 2,400 bit/s. These used new signaling techniques.

Earlier 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.

For instance, 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 four or eight distinct symbols, which allowed the encoding of two or three 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 around 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 the early versions) 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.

A 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 with standards supporting high speeds and progressively fall back to slower standards 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.

#### Echo cancellation - 9,600 bit/s

USRobotics Sportster 14,400 Fax modem (1994)

Echo cancellation was the next major advance in modem design.

Local telephone lines use the same wires to send and receive data, 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 a modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal.

For this reason, earlier modems split the signal frequencies, so that the devices on either end used different tones, allowing each one to 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 limits on modems.

Echo cancellation eliminated this problem. During the call setup and negotiation period, both modems send a series of unique tones and then listen for them to return through the phone system. They measure the total delay time and then set up a local delay loop to the same time. Once the connection is completed, they send their signals into the phone lines as normal, but also into a delay line. When their signal is reflected back, it is mixed with the inverted signal from the delay line, which cancels out the echo. This allowed both modems to use the full spectrum available, doubling the possible speed.

Additional improvements were introduced via the quadrature amplitude modulation (QAM) encoding system. Previous systems using phase shift keying (PSK) encoded two bits (or sometimes three) per symbol by slightly delaying or advancing the signal's phase relative to a set carrier tone. QAM used a combination of phase shift and amplitude to encode four bits per symbol.

Transmitting at 1,200 baud produced the 4,800 bit/s V.27ter standard, and at 2,400 baud the 9,600 bit/s V.32. The carrier frequency was 1,650 Hz in both systems. For many years, most engineers considered this rate to be the limit of data communications over telephone networks.

The introduction of these higher-speed systems also led to the development of the digital fax machine during the 1980s. While early fax technology also used modulated signals on a phone line, digital fax used the now-standard digital encoding used by computer modems. This eventually allowed computers to send and receive fax images.

### Voice/fax modems

"Voice" and "fax" are terms added to describe any dial modem that is capable of recording/playing audio or transmitting/receiving faxes. Some modems are capable of all three functions.[28]

Voice modems are used for computer telephony integration applications as simple as placing/receiving calls directly through a computer with a headset, and as complex as fully automated robocalling systems.

Fax modems can be used for computer-based faxing, in which faxes are sent and received without inbound or outbound faxes ever needing to ever be printed on paper. This differs from efax, in which faxing occurs over the internet, in some cases involving no phone lines whatsoever.

## Leased-Line Modem

A leased line modem also uses ordinary phone wiring, like dial-up and DSL, but does not use the same network topology. While dial-up uses a normal phone line and connects through the telephone switching system, and DSL uses a normal phone line but connects to equipment at the telco central office, leased lines do not terminate at the telco.

Leased lines are pairs of telephone wire that have been connected together at one or more telco central offices so that they form a continuous circuit between two subscriber locations, such as a business' headquarters and a satellite office. They provide no power or dialtone - they are simply a pair of wires connected at two distant locations.

A dialup modem will not function across this type of line, because it does not provide the power, dialtone and switching that those modems require. However, a modem with leased-line capability can operate over such a line, and in fact can have greater performance because the line is not passing through the telco switching equipment, the signal is not filtered, and therefore higher analog bandwidth is available.

Leased-line modems can operate in 2-wire or 4-wire mode. The former uses a single pair of wires and can only transmit in one direction at a time, while the latter uses two pairs of wires and can transmit in both directions simultaneously. When two pairs are available, bandwidth can be as high as 1.5mbps, a full data T1 circuit.[29]

Cable modem

The term broadband gained widespread adoption in the late 90s to describe internet access technology exceeding the 56 kilobit/s maximum of dialup. There are many broadband technologies, such as various DSL (Digital Subscriber Line) technologies and cable broadband.

DSL technologies such as ADSL, HDSL and VDSL use telephone lines (wires that were installed by a telephone company and originally intended for use by a telephone subscriber) but do not utilize most of the rest of the telephone system. Their signals are not sent through ordinary phone exchanges, but are instead received by special equipment (a DSLAM) at the telephone company central office.

Because the signal does not pass through the telephone exchange, no "dialing" is required, and the bandwidth constraints of an ordinary voice call are not imposed. This allows much higher frequencies, and therefore much faster speeds. ADSL in particular is designed to permit voice calls and data usage over the same line simultaneously.

Similarly, cable modems use infrastructure originally intended to carry television signals, and like DSL, typically permit receiving television signals at the same time as broadband internet service.

Other broadband modems include satellite modems and power line modems.

### Terminology

Different terms are used for broadband modems, because they are frequently contain more than just a modulation/demodulation component.

Because high speed connections are frequently used by multiple computers at once, many broadband modems do not have direct (e.g. USB) PC connections, but connect over a network such as Ethernet or Wi-Fi. Early broadband modems offered Ethernet handoff allowing the use of one or more public IP addresses, but no other services such as NAT and DHCP that would allow multiple computers to share one connection. This led to many consumers purchasing separate "broadband routers," placed between the modem and their network, to perform these functions.

Eventually, ISPs began providing residential gateways which combined the modem and broadband router into a single package that provided routing, NAT, security features, and even Wi-Fi access in addition to modem functionality, so that subscribers could connect their entire household without purchasing any extra equipment. Even later, these devices were extended to provide "triple play" features such as telephony and television service. Nonetheless, these devices are still often referred to simply as "modems" by service providers and manufacturers.

Consequently the terms "modem," "router" and "gateway" are now used interchangeably in casual speech, but in a technical context "modem" may carry a specific connotation of basic functionality with no routing or other features, while the other terms describe a device with features such as NAT.

Broadband modems may also handle authentication such as PPPoE. While it is often possible to authenticate a broadband connection from a users PC, as was the case with dial-up internet service, moving this task to the broadband modem allows it to establish and maintain the connection itself, which makes sharing access between PCs easier since each one does not have to authenticate separately. Broadband modems typically remain authenticated to the ISP as long as they are powered on.

Any communication technology sending digital data wirelessly involves a modem. This includes direct broadcast satellite, WiFi, WiMax, mobile phones, GPS, Bluetooth and NFC.

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 fiber optic is not economical.

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 wireless communication links to work simultaneously on different frequencies.[relevant? ]

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.

Huawei HSPA+ Evolution-Data Optimized (EVDO) USB wireless modem from Movistar Colombia
Huawei 4G+ Dual Band 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, mobile phone or other device, or be connected externally. External wireless modems include connect cards, USB modems, and cellular routers.

Most GSM wireless modems come with an integrated SIM cardholder (i.e. Huawei E220, Sierra 881.) Some models are also provided with a microSD memory slot and/or jack for additional external antenna, (Huawei E1762, Sierra Compass 885.)[30][31]

The CDMA (EVDO) versions do not typically use R-UIM cards, but use Electronic Serial Number (ESN) instead.

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. Embedded modems may overtake separate modems as tablet sales grow and the incremental cost of the modems shrinks, so by 2016, the ratio may change to 1:1.[32]

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'.[citation needed]

## Optical modem

A modem that connects to a fiber optic network is known as an optical network terminal (ONT) or optical network unit (ONU). These are commonly used in fiber to the home installations, installed inside or outside a house to convert the optical medium to a copper Ethernet interface, after which a router or gateway is often installed to perform authentication, routing, NAT, and other typical consumer internet functions, in addition to "triple play" features such as telephony and television service.

Fiber optic systems can use quadrature amplitude modulation to maximize throughput. 16QAM uses a 16-point constellation to send four bits per symbol, with speeds on the order of 200 or 400 gigabits per second.[33][34] 64QAM uses a 64-point constellation to send six bits per symbol, with speeds up to 65 terabits per second. Although this technology has been announced, it may not yet be commonly used.[35][36][37]

## Home networking

Although the name modem is seldom used, some high-speed home networking applications do use modems, such as powerline ethernet. The G.hn standard for instance, developed by ITU-T, 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.

As described above, technologies like Wi-Fi and Bluetooth also use modems to communicate over radio at short distances.

## "Null Modem"

The phrase "null modem" was common parlance in the heydey of RS-232 communication between computers, but as the name suggests, it in fact describes the absence of a modem.

A null modem cable is a specially wired cable connected between the serial ports of two personal computers, with the transmit and receive lines reversed. The name referred to the use of this type of cable to connect two computers for applications where a modem would typically be used, and the same software used with modems (such as Procomm or Minicom) could be used with this type of connection.

## Short-haul modem

A "short haul modem" is a device that bridges the gap between leased-line and dial-up modems. Like a leased-line modem, they transmit over "bare" lines with no power or telco switching equipment, but are not intended for the same distances that leased lines can achieve. Ranges up to several miles are possible, but significantly, short-haul modems can be used for medium distances, greater than the maximum length of a basic serial cable but still relatively short, such as within a single building or campus. This allows a serial connection to be extended for perhaps only several hundred to several thousand feet, a case where obtaining an entire telephone or leased line would be overkill.

While some short-haul modems do in fact use modulation, low-end devices (for reasons of cost or power consumption) are simple "line drivers" that increase the level of the digital signal but do not modulate it. These are not technically modems, but the same terminology is used for them.[38]

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