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