| A modem (from modulate and demodulate) | | | | unfamiliar to consumers. However, many |
| is a device that modulates an analogue | | | | people knew what a modem was as most |
| carrier signal to encode digital | | | | internet access was through dialup. Due |
| information, and also demodulates such a | | | | to this familiarity, companies started |
| carrier signal to decode the transmitted | | | | selling broadband modems using the |
| information. The goal is to produce a | | | | familiar term "modem" rather than vaguer |
| signal that can be transmitted easily | | | | ones like "adapter" or "transceiver". |
| and decoded to reproduce the original | | | | Direct broadcast satellite, WiFi, and |
| digital data. Modems can be used over | | | | mobile phones all use modems to |
| any means of transmitting analog | | | | communicate, as do most other wireless |
| signals, from driven diodes to radio. | | | | services today. Modern |
| Experiments have even been performed in | | | | telecommunications and data networks |
| the use of modems over the medium of two | | | | also make extensive use of radio modems |
| cans connected by a string.[citation | | | | where long distance data links are |
| needed] | | | | required. Such systems are an important |
| The most familiar example is a voiceband | | | | part of the PSTN, and are also in common |
| modem that turns the digital '1s and 0s' | | | | use for high-speed computer network |
| of a personal computer into sounds that | | | | links to outlying areas where fibre is |
| can be transmitted over the telephone | | | | not economical. |
| lines of Plain Old Telephone Systems | | | | Even where a cable is installed, it is |
| (POTS), and once received on the other | | | | often possible to get better performance |
| side, converts those sounds back into 1s | | | | or make other parts of the system |
| and 0s. Modems are generally classified | | | | simpler by using radio frequencies and |
| by the amount of data they can send in a | | | | modulation techniques through a cable. |
| given time, normally measured in bits | | | | Coaxial cable has a very large |
| per second, or "bps". | | | | bandwidth, however signal attenuation |
| Faster modems are used by Internet users | | | | becomes a major problem at high data |
| every day, notably cable modems and ADSL | | | | rates if a digital signal is used. By |
| modems. In telecommunications, "radio | | | | using a modem, a much larger amount of |
| modems" transmit repeating frames of | | | | digital data can be transmitted through |
| data at very high data rates over | | | | a single piece of wire. Digital cable |
| microwave radio links. Some microwave | | | | television and cable Internet services |
| modems transmit more than a hundred | | | | use radio frequency modems to provide |
| million bits per second. Optical modems | | | | the increasing bandwidth needs of modern |
| transmit data over optic fibers. Most | | | | households. Using a modem also allows |
| intercontinental data links now use | | | | for frequency-division multiple access |
| optic modems transmitting over undersea | | | | to be used, making full-duplex digital |
| optical fibers. Optical modems routinely | | | | communication with many users possible |
| have data rates in excess of a billion | | | | using a single wire. |
| (1x109) bits per second. | | | | Wireless modems come in a variety of |
| ADSL modems, a more recent development, | | | | types, bandwidths, and speeds. Wireless |
| are not limited to the telephone's | | | | modems are often referred to as |
| "voiceband" audio frequencies. Some ADSL | | | | transparent or smart. They transmit |
| modems use coded orthogonal frequency | | | | information that is modulated onto a |
| division modulation (DMT). | | | | carrier frequency to allow many |
| Cable modems use a range of frequencies | | | | simultaneous wireless communication |
| originally intended to carry RF | | | | links to work simultaneously on |
| television channels. Multiple cable | | | | different frequencies. |
| modems attached to a single cable can | | | | Transparent modems operate in a manner |
| use the same frequency band, using a | | | | similar to their phone line modem |
| low-level media access protocol to allow | | | | cousins. Typically, they were half |
| them to work together within the same | | | | duplex, meaning that they could not send |
| channel. Typically, 'up' and 'down' | | | | and receive data at the same time. |
| signals are kept separate using | | | | Typically transparent modems are polled |
| frequency division multiple access. | | | | in a round robin manner to collect small |
| New types of broadband modems are | | | | amounts of data from scattered locations |
| beginning to appear, such as doubleway | | | | that do not have easy access to wired |
| satellite and powerline modems. | | | | infrastructure. Transparent modems are |
| Broadband modems should still be classed | | | | most commonly used by utility companies |
| as modems, since they do utilise analog | | | | for data collection. |
| digital conversion. They are more | | | | Smart modems come with a media access |
| advanced devices than traditional | | | | controller inside which prevents random |
| telephone modems as they are capable of | | | | data from colliding and resends data |
| modulating/demodulating hundreds of | | | | that is not correctly received. Smart |
| channels simultaneously. | | | | modems typically require more bandwidth |
| Many broadband "modems" include the | | | | than transparent modems, and typically |
| functions of a router (with Ethernet and | | | | achieve higher data rates. The IEEE |
| WiFi ports) and other features such as | | | | 802.11 standard defines a short range |
| DHCP, NAT and firewall features. | | | | modulation scheme that is used on a |
| When broadband technology was | | | | large scale throughout the world. |
| introduced, networking and routers were | | | | |