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