Modem

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A modem (modem, acronym for modulator demodulator; pl. modems) is a device that converts digital signals into analog signals (modulation) and vice versa (demodulation), and thus enables communication between computers over the telephone line or cable modem. It is used to send the modulating signal by means of another signal called carrier.

Modems have been used since the 1960s because direct transmission of intelligible electronic signals over long distances is not efficient; for example, transmitting audio signals over the air would require large antennas (of the order of hundreds of meters) for proper reception. It is common to find in many switched network modems the automatic answering and dialing facility, which allows them to connect when they receive a call from the PSTN (Public Switched Telephone Network) and proceed to dial any number previously recorded by the user. Thanks to these functions, all communication establishment operations can be performed automatically.

History

Cable news wire services in the 1920s used multiplex devices that met the definition of a modem. However, the modem function was incidental to the multiplexing function, so they are generally not included in the history of modems.

Modems arose from the need to connect teletypes over regular telephone lines rather than over the more expensive leased lines that had previously been used for current loop-based teletypes and automated telegraphs.
In 1941, the Allies developed a voice encryption system called SIGSALY that used a vocoder to digitize speech, then encrypted the speech with a one-time keypad and encoded the digital data as tones using frequency shifting.

Widespread production of modems in the United States began in 1958 (the year the word “modem” was first used) as a device part of the SAGE air defense system, connecting terminals at various air bases, radar sites, and command and control centers to SAGE director centers scattered across the United States and Canada. AT&T’s Bell Labs described the SAGE modems as conforming to its then recently released Bell 101 data set standard. While they operated on dedicated telephone lines, the devices at each end were no different from commercial Bell 101 modems, 110 baud modems.
The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2000 bit/s on normal telephone lines, while the 201B provided full-duplex service at 2400 bit/s on four-wire leased lines, with the send and receive channels being run on their own two-wire set.

AT&T also introduced the famous Bell 103A data set standard in 1962. It provided full duplex service at 300 bit/s on normal telephone lines. Phase-shift modulation was used, with the call originator transmitting at 1070 or 1270 Hz and the responding modem transmitting at 2025 or 2225 Hz. The readily available 103A2 gave a major boost to the use of low-speed remote terminals such as the Teletype Model 33 ASR and KSR, and the IBM 2741. AT&T reduced modem costs by introducing the 113D source-only and 113B/C reply-only modems.
For many years, the Bell system (AT&T) maintained a monopoly on the use of its telephone lines and the devices that could be connected to them. However, in the seminal 1968 FCC Carterfone Decision, the FCC concluded that electronic devices could be connected to the telephone system as long as they used an acoustic coupler. Since most telephones were supplied by Western Electric, and thus had a standard design, acoustic couplers were relatively easy to build. Acoustically coupled 300 bit/s Bell 103A-compatible modems were common during the 1970s. The best known models were the Novation CAT and the Anderson-Jacobson, the latter derived from an in-house project at Stanford Research Institute (now SRI International). An even cheaper option was the Pennywhistle modem, designed to be built from electronic scrap and surplus store parts.
In December 1972, Vadic released the VA3400, remarkable for full duplex operation at 1200 bit/s over the telephone network. Like the 103A, it used different frequency bands for transmitting and receiving. In November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in design, but used the lower set of frequencies for transmission. One could also use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency assignments intentionally made the 212 incompatible with acoustic coupling, thus locking out many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple modem, a reply-only modem sold to computer center operators that supported Vadic’s 1200 bit/s mode, AT&T’s 212A mode, and 103A operation.

How it works

The modulator emits a signal called a carrier. This is usually a simple sinusoidal electrical signal of much higher frequency than the modulating signal. The modulating signal is the information that is prepared for transmission (a modem prepares the information for transmission, but does not actually transmit it). The modulator modifies some characteristic of the carrier (which is the action of modulating), so that a signal is obtained, which includes the information from the modulator. Thus the demodulator can recover the original modulating signal, removing the carrier. The characteristics that can be modified of the carrier signal are:
A combination of modulations or more complex modulations, such as quadrature amplitude modulation, is also possible.

PC modems

The most common distinction is usually made between internal modems and external modems, although modems called software modems, better known as winmodems or linuxmodems, have appeared and have made the picture more complex. There are also modems for XDSL, ISDN, and those used to connect via 75 ohm coaxial cable (cable modems).

Telephone modems

Their most common and well-known use is in telephone data transmissions.

Computers process data digitally; however, the basic network telephone lines only transmit analog signals.

Modulation methods and other characteristics of telephone modems are standardized by ITU-T (formerly CCITT) in the “V” series of Recommendations. These Recommendations also determine the transmission rate. They stand out:

There are also DSL (Digital Subscriber Line) modems, which use a frequency spectrum above the voice band (300-3400 Hz) on telephone lines or above the 80 kHz occupied on ISDN lines, and allow much higher speeds than a conventional telephone modem. They also have other qualities, such as the possibility of establishing voice telephone communication at the same time as sending and receiving data.
Extending this historical evolution a little more, we can make an important difference if we take as a starting point the first modems that operated analogically, being able to transmit at very low speed, reaching through the V.34 standard to about 34 kbps maximum and there reaches its limit.

In a very short time began to be implemented at certain ends, the first packet switching networks (university, research, military), thereby gaining a lot in the signal to noise ratio and avoiding a second digital analog conversion, the V.92 standard was its maximum exponent exceeding 64 kbps.

The concrete fact that starts this new change is the implementation of “Digital Hierarchies”, initially Plesiochronous with PDH and today Synchronous with SDH, through these new technologies the voice, complying with the three steps (sampling, quantification and coding) is transmitted digitally, occupying 64 Kbps channels in basic (BRI = 128 kbps) and primary (PRI = 2 Mbps with E1 frames) accesses.
ISDN (Integrated Services Digital Network) technology appears, which is quickly surpassed by xDSL (x Digital Subscriber Line), which will be discussed here.
These xDSL services are based above all on new forms of modulation (combining above all phase and amplitude) through bit “constellations”, based on the capacity of several carriers associated with the signal-to-noise ratio of this “last mile” mentioned above; for this reason xDSL is very dependent on the distance and quality of the copper pair that reaches the home, the better the signal-to-noise ratio, the more bits can be transmitted over this copper pair and therefore the greater the bandwidth that can be offered. These xDSL technologies are a family (HDSL, VDSL, ADSL, etc…), of which ADSL (asynchronous DSL) is the most widely used in home telephony networks. The concept of “asynchronous or asymmetric” is given by virtue of the fact that two channels are used for data (and a third, independent channel for voice). Of the two data channels, one is used for “downstream” information, which is usually of higher capacity, and the other for “upstream” information, which is usually significantly lower. The technical specifications of this technology can be found in ITU-T recommendation G.992.1 (G.dmt) and G992.2 (G.lite) and in ANSI standard T1.413-1998. An image of its operation is shown below:
A fax modem allows the computer to transmit and receive documents as faxes on a telephone line. A fax modem is like a data modem, but is designed to transmit and receive documents from a fax machine or other fax modem. Some, but not all, also function as data modems. Like other modems, these can be internal or external. The internal type are often called fax cards.

In the early 1990s, small businesses commonly used computers with fax/modem cards and fax software (typically WinFax Pro). With the turn of the century, fax/modem was almost completely replaced by email. If it is necessary to send a fax from a PC, there are virtual fax alternatives on the Internet. Similarly, fax machines have also been supplanted by e-mail.

Medical systems continue to use this technology in the 21st century, but many ordinary users and businesses prefer the use of high-speed Internet.

Note that the values shown are maximum values and actual values may be slower under certain conditions (e.g. noisy telephone lines). A baud is one symbol per second, and each symbol can encode one or more bits of data.

Depending on whether the modem is digital or analog, a modulation of the same nature is used. For digital modulation, for example, the following types of modulation are used:
However, there are also disturbances in the telephone channel that the modem must face in order to transmit information. These disturbances can be listed as: distortions, distortions and echoes; random and impulsive noises; and finally, interference.

For analog modulation, for example, the following types of modulation exist:

Each modem uses a series of common and specific “AT” commands. For this reason, the manuals that come with the modem should be used to configure it properly.

The registers or S-registers are portions of memory where parameters that define the modem profile (profiles) can be permanently stored. In addition to the “AT” commands, this series of registers allows the user to modify other characteristics of its operation. As with the “AT” commands, there are common registers and others specific to the modem. The most common ones are listed below.

There are three types of profile for modem operation:

These profiles are stored in their NVRAM, and the factory profile is stored in ROM.

There are two options or memory locations where profiles can be saved:

These commands are sent before the modem is turned off, so that it will load them at its next power up.
When the “AT” commands are written, depending on the modem buffer size, they can be concatenated without the need to write the “AT” prefix for each of them. In this way, for example, when a program asks for a modem initialization sequence, all the commands needed to configure the modem can be included together in a single line.

In summary, the steps for establishing a connection are:

Test on Hayes modems

The tests allow you to verify the local modem, the local terminal, the remote modem and the communications line. The S18 modem register indicates the duration of the tests. If its content is 0, there is no time limit and it is up to the user to end the tests with the AT&T0 command. When the modem is switched on, it performs a series of internal tests. If any error occurs, it will be indicated to the DTE in a timely manner.

The tests that can be performed are:

Error checking protocols

Error checking are various techniques by which the reliability of data blocks or characters is checked.

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