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{{Modulation techniques}}
A '''modulator-demodulator''' or most commonly referred to as '''modem''' is a [[computer hardware]] device that converts [[Digital data|data from a digital format]] into a format suitable for an analog [[transmission medium]] such as telephone or radio. A modem transmits data by [[Modulation#Digital modulation methods|modulating]] one or more [[carrier wave]] signals to encode [[digital information]], while the receiver [[Demodulation|demodulates]] the signal to recreate the original digital information. The goal is to produce a [[Signal (electronics)|signal]] that can be transmitted easily and decoded reliably. Modems can be used with almost any means of transmitting analog signals, from [[light-emitting diode]]s to [[radio]].
Early modems were devices that used audible sounds suitable for transmission over traditional [[telephone]] systems and [[leased line]]s. These generally operated at 110 or 300 bits per second (bit/s), and the connection between devices was normally manual, using an attached [[telephone handset]]. By the 1970s, higher speeds of 1,200 and 2,400 bit/s for asynchronous dial connections, 4,800 bit/s for synchronous leased line connections and 35 kbit/s for synchronous conditioned leased lines were available. By the 1980s, less expensive 1,200 and 2,400 bit/s dialup modems were being released, and modems working on radio and other systems were available. As device sophistication grew rapidly in the late 1990s, telephone-based modems quickly exhausted the available [[Bandwidth (computing)|bandwidth]], reaching 56 kbit/s.
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Commodore's 1982 ''VicModem'' for the [[VIC-20]] was the first modem to be sold under $100, and the first modem to sell a million units.<ref>{{Cite journal |last=Herzog |first=Marty |date=January 1988 |title=Neil Harris |journal=Fictioneer Books. |volume=Comics Interview |issue= 54 |pages=41–51}}</ref>
In 1984, [[ITU-T V.22bis|V.22bis]] was created, a 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212. This bit rate
Increasing modem speed greatly improved the responsiveness of online systems and made [[file transfer]] practical. This led to rapid growth of [[online service]]s with 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.
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| 2,400 bit/s (1200 baud) ([[ITU-T V.26bis|V.26bis]]) || PSK || 2.4 ||
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| 4,800 bit/s (1600 baud) ([[ITU-T V.27ter|V.27ter]]) || PSK || 4.8 || 1976<!--Phase=PSK?--><ref>{{cite web |url=http://www.garretwilson.com/essays/computers/group3fax.html |title=Group 3 Facsimile Communication |publisher=garretwilson.com |date=2013-09-20 |access-date=2014-02-10 |url-status=live |archive-url=https://web.archive.org/web/20140203210353/http://www.garretwilson.com/essays/computers/group3fax.html |archive-date=2014-02-03 }}</ref><ref>{{cite book|title=INTERNATIONAL TELECOMMUNICATION UNION - CCITT - VOLUME VIII - FASCICLE Vlll/I - DATA COMMUNICATION OVER THE TELEPHONE NETWORK - RECOMMENDATIONS OF THE V SERIES|url=https://search.itu.int/history/HistoryDigitalCollectionDocLibrary/4.258.43.en.1025.pdf|section=Recommendation V.27 ter|section-url=https://search.itu.int/history/HistoryDigitalCollectionDocLibrary/4.258.43.en.1025.pdf#page=157|series=YELLOW BOOK|pages=148-160|publisher=[[International Telecommunication Union]] - [[ITU-T|International Telegraph and Telephone Consultative Committee (CCITT]])}}</ref>
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| 4,800 bit/s (1600 baud, Bell 208B) || DPSK || 4.8 ||
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Different terms are used for broadband modems, because they 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. Rather they 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.<ref>{{Cite web|title=What's the Difference between a Modem and Router?|url=https://www.lifewire.com/difference-between-modem-and-router-4159854|access-date=2021-11-23|website=Lifewire|language=en}}</ref><ref>{{Cite web|date=2021-04-07|title=Modem vs. router: The differences between the pieces of hardware that connect you to the internet, explained|url=https://www.businessinsider.com
Eventually, ISPs began providing [[residential gateway]]s which combined the modem and broadband router into a single package that provided routing, [[Network address translation|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 (telecommunications)|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.<ref>hp.com/us-en/shop/tech-takes/modem-vs-router</ref>
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