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{{Short description|Device that modulates an analog carrier signal to encode digital information}}
{{Redirect-distinguish|Computer modem|Computer Modern
{{
[[File:Analogue modem - acoustic coupler.jpg|thumb|[[Acoustic coupler]] modems used a telephone handset as the audio medium, with the user dialing the desired number and then pressing the handset into the modem to complete the connection. These systems generally operated at a speed of 300 bits per second.]]
{{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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Many modems are variable-rate, permitting them to be used over a medium with less than ideal characteristics, such as a telephone line that is of poor quality or is too long. This capability is often adaptive so that a modem can discover the maximum practical transmission rate during the connect phase, or during operation.
[[File:Australian-modems.jpg|center|thumb|800x800px|Collection of modems once used in Australia, including dial-up, DSL, and cable modems
== Overall history ==
Modems grew out of the need to connect [[teleprinter]]s over ordinary phone lines instead of the more expensive leased lines which had previously been used for [[current loop]]–based teleprinters and automated [[telegraph]]s. The earliest devices
In 1941, the [[Allies of World War II|Allies]] developed a voice [[encryption]] system called [[SIGSALY]] which used a [[vocoder]] to digitize speech, then encrypted the speech with one-time pad and encoded the digital data as tones using frequency shift keying. This was also a digital modulation technique, making this an early modem.<ref>{{Cite web|title=National Security Agency Central Security Service > About Us > Cryptologic Heritage > Historical Figures and Publications > Publications > WWII > Sigsaly – The Start of the Digital Revolution|url=https://www.nsa.gov/about/cryptologic-heritage/historical-figures-publications/publications/wwii/sigsaly-start-digital/|access-date=2020-08-13|website=NSA.gov}}</ref>
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==Dial-up==
A dial-up modem transmits computer data over an ordinary [[Public switched telephone network|switched]] telephone line that has not been designed for data use. It was once a widely known technology,
Dial-up service has since been largely
=== History ===
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A significant advance in modems was the [[Hayes Microcomputer Products#The Smartmodem|Hayes Smartmodem]], introduced in 1981. The Smartmodem was an otherwise standard 103A 300 bit/s direct-connect modem, but it introduced a command language which allowed the computer to make control requests, such as commands to dial or answer calls, over the same RS-232 interface used for the data connection.<ref>{{Cite book|last=Enterprise|first=I. D. G.|url=https://books.google.com/books?id=KbM9-s49yCMC&q=computerworld%20hayes%20smartmodem%201981&pg=RA1-PA42|title=Computerworld|date=1981-04-27|publisher=IDG Enterprise|language=en}}</ref> The command set used by this device became a de facto standard, the [[Hayes command set]], which was integrated into devices from many other manufacturers.
Automatic dialing was not a new capability
The introduction of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features.<ref>{{Cite book|last=Enterprise|first=I. D. G.|url=https://books.google.com/books?id=hCqdKWvxFT4C&q=%22computerworld%22%20%22hayes%20compatible%22&pg=PA61-IA6|title=Computerworld|date=1987-03-30|publisher=IDG Enterprise|language=en}}</ref> This eventually led to legal action over use of the patented Hayes command language.<ref>{{Cite book|last=Enterprise|first=I. D. G.|url=https://books.google.com/books?id=mUSIMiurpfYC&q=%22computerworld%22%20%22hayes%20compatible%22&pg=PP137|title=Computerworld|year=1987|publisher=IDG Enterprise|language=en}}</ref>
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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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Additional improvements were introduced by [[quadrature amplitude modulation]] (QAM) encoding, which increased the number of bits per symbol to four through a combination of phase shift and amplitude.
Transmitting at 1,200 baud produced the 4,800 bit/s [[V.27ter]] standard, and at 2,400 baud the 9,600 bit/s [[ITU-T V.32|V.32]]. The [[carrier frequency]] was 1,650 Hz in both systems.
The introduction of these higher-speed systems also led to the development of the digital [[fax]] machine during the 1980s. While early fax technology also used modulated signals on a phone line, digital fax used the now-standard digital encoding used by computer modems. This eventually allowed computers to send and receive fax images.
==== 1990s ====
[[Image:fax modem antigo.jpg|thumb|[[USRobotics]] Sportster 14,400 Fax
In the early 1990s, V.32 modems operating at 9,600 bit/s were introduced, but were expensive and were only starting to enter the market when V.32bis was standardized, which operated at 14,400 bit/s.
[[Rockwell International]]'s chip division developed a new driver chip set incorporating the [[V.32bis]] standard and aggressively priced it. [[Supra, Inc.]] arranged a short-term exclusivity arrangement with Rockwell, and developed the [[SupraFAXModem 14400]] based on it. Introduced in January 1992 at {{US$|long=no|399}} (or less), it was half the price of the slower V.32 modems already on the market. This led to a price war, and by the end of the year V.32 was dead, never having been really established, and V.32bis modems were widely available for {{US$|long=no|250}}.
V.32bis was so successful that the older high-speed standards had little advantages. USRobotics (USR) fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off 19,200 bit/s method they referred to as ''V.32ter'', but neither non-standard modem sold well.
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}}
Consumer interest in these proprietary improvements waned during the lengthy introduction of the {{val|28800|u=bit/s|fmt=commas}} [[ITU-T V.34|V.34]] standard. While waiting, several companies decided to release hardware and introduced modems they referred to as [[V.Fast]].
In order to guarantee compatibility with V.34 modems once a standard was ratified (1994), manufacturers used more flexible components, generally a [[digital signal processor|DSP]] and [[microcontroller]], as opposed to purpose-designed [[Application-specific integrated circuit|ASIC]] modem chips. This would allow later firmware updates to conform with the standards once ratified.
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====== {{Anchor|X2|K56flex}} Early 56k dial-up products ======
The first 56k (56
At that time, USRobotics held a 40% share of the retail modem market, while Rockwell International held an 80% share of the modem [[chipset]] market. Concerned with being shut out, Rockwell began work on a rival 56k technology. They joined with [[Lucent]] and [[Motorola]] to develop what they called "K56Flex" or just "Flex".
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The [[ITU-T V.92]] standard was approved by ITU in November 2000<ref>{{Cite web |title=V.92: Enhancements to Recommendation V.90 |url=https://www.itu.int/rec/T-REC-V.92-200011-I/en |access-date=2020-06-29 |website=www.itu.int}}</ref> and utilized digital [[Pulse-code modulation|PCM]] technology to increase the upload speed to a maximum of {{val|48|u=kbit/s}}.
The high upload speed was a tradeoff. Use of the {{val|48|u=kbit/s}} upstream rate would reduce the downstream as low as {{val|40|u=kbit/s}} due to echo effects on the line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a plain 33.{{val|6|u=kbit/s}} analog connection in order to maintain a high digital downstream of {{val|50|u=kbit/s}} or higher.<ref>{{cite web |url=http://www.modemsite.com/56k/v92s.asp |title=V.92 – News & Updates |access-date=17 September 2012 |work=November and October 2000 updates |url-status=live |archive-url=https://web.archive.org/web/20120920104645/http://modemsite.com/56k/v92s.asp |archive-date=20 September 2012}}</ref>
V.92 also added two other features. The first is the ability for users who have call waiting to put their [[dial-up Internet]] connection on hold for extended periods of time while they answer a call. The second feature is the ability to quickly connect to one's ISP, achieved by remembering the analog and digital characteristics of the telephone line and using this saved information when reconnecting.
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| 300 baud ([[Bell 103]] or [[ITU-T V.21|V.21]]) || FSK || 0.3 || 1962
|-
| 1,200 bit/s (1200 baud) ([[Bell 202]])
|-
| 1,200 bit/s
|-
| 2,000 bit/s (1000 baud) (Bell 201A) || PSK || 2.0 || 1962
|-
| 2,400 bit/s (600 baud) ([[ITU-T V.22bis|V.22bis]])
|-
| 2,400 bit/s (1200 baud) ([[ITU-T V.26bis|V.26bis]])
|-
| 4,800 bit/s (1600 baud) ([[ITU-T V.27ter|V.27ter]])
|-
| 4,800 bit/s (1600 baud, Bell 208B)
|-
| 9,600 bit/s (2400 baud) ([[ITU-T V.32|V.32]]) || [[Trellis coded
|-
| 14.4
|-
| 19.2
|-
| 28.8
|-
| 33.6
|-
| 56
|-
| 56
|-
| Bonding modem (two 56k modems) ([[ITU-T V.92|V.92]])<ref>{{cite web|url=http://www.56k.com/reports/bonding.shtml|archive-url=https://web.archive.org/web/19971210202848/http://www.56k.com/reports/bonding.shtml|url-status=dead|archive-date=1997-12-10|title=Bonding: 112K, 168K, and beyond|last=Jones|first=Les|publisher=56K.COM}}</ref> || || 112.0/96.0 ||
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| Hardware compression (variable) ([[ITU-T V.90|V.90]]/[[ITU-T V.42bis|V.42bis]]) || || 56.0–220.0 ||
|-
| Hardware compression (variable) ([[ITU-T V.92|V.92]]/[[ITU-T V.44|V.44]])
|-
| Server-side web compression (variable) ([[Netscape#Netscape Internet Service|Netscape ISP]]) || || 100.0–1,000.0 ||
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Many dial-up modems implement standards for [[data compression]] to achieve higher effective throughput for the same bitrate. [[ITU-T V.44|V.44]]<ref>{{Cite web |title=V.44 |url=http://www.linfo.org/v_44.html |access-date=2023-04-06 |website=www.linfo.org}}</ref> is an example used in conjunction with [[ITU-T V.92|V.92]] to achieve speeds greater than 56k over ordinary phone lines.<ref>{{Cite web |title=V.92 |url=https://www.techopedia.com/definition/3018/v-92 |access-date=2023-04-06 |website=Techopedia |language=en-US}}</ref>
As telephone-based 56k modems began losing popularity, some Internet service providers such as [[NetZero|Netzero]]/Juno, [[Netscape]], and others started using pre-compression to increase apparent throughput.
These accelerated downloads are integrated into the [[Opera (web browser)|Opera]]<ref>{{Cite web |last=Holcombe |first=Jeremy |date=2019-05-01 |title=How To Enable Turbo Mode in Opera |url=https://www.greengeeks.com/tutorials/how-to-enable-turbo-mode-in-opera/ |access-date=2023-04-06 |website=GreenGeeks |language=en-US}}</ref> and [[Amazon Silk]]<ref name=":1">{{Cite web |last=Shimpi |first=Anand Lal |title=Amazon's Silk Browser Acceleration Tested: Less Bandwidth Consumed, But Slower Performance |url=https://www.anandtech.com/show/5139/amazons-silk-browser-tested-less-bandwidth-consumed-but-slower-performance |access-date=2023-04-06 |website=www.anandtech.com}}</ref> web browsers, using their own server-side text and image compression requiring all data to pass through their own servers before reaching the user.<ref name=":1" />
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A dialup modem will not function across this type of line, because it does not provide the power, dialtone and switching that those modems require. However, a modem with leased-line capability can operate over such a line, and in fact can have greater performance because the line is not passing through the telco switching equipment, the signal is not filtered, and therefore greater bandwidth is available.
Leased-line modems can operate in 2-wire or 4-wire mode. The former uses a single pair of wires and can only transmit in one direction at a time, while the latter uses two pairs of wires and can transmit in both directions simultaneously. When two pairs are available, bandwidth can be as high as 1.5
While the slower leased line modems used, e.g., [[RS-232]], interfaces, the faster wideband modems used, e.g., [[List of ITU-T V-series recommendations#Wideband modems|V.35]].
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| section = 2.2.3 Data Products
| section-url = http://bitsavers.org/communications/westernElectric/books/Engineering_and_Operations_in_the_Bell_System_2ed_1984.pdf#page=60
| quote = Speeds on broadband private-line channels range from 19.2 to 230.4
| url = http://bitsavers.org/communications/westernElectric/books/Engineering_and_Operations_in_the_Bell_System_2ed_1984.pdf
| publisher = AT&T Bell Laboratories
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| section = 6.2.1 Basic Concept
| section-url = http://bitsavers.org/communications/westernElectric/books/Engineering_and_Operations_in_the_Bell_System_2ed_1984.pdf#page=215
| quote = Analog channels can be further characterized by bandwidth:
| url = http://bitsavers.org/communications/westernElectric/books/Engineering_and_Operations_in_the_Bell_System_2ed_1984.pdf
| publisher = AT&T Bell Laboratories
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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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{{See also|Mobile broadband|Mobile broadband modem}}
[[File:Huawei E173 3G HSPA+ Modem USB Movistar Colombia (6).jpg|thumb|[[Huawei]] [[HSPA+]]
[[File:Huawei 4G+ Modem.jpg|thumb|Huawei 4G+ Dual Band Modem|alt=|left|127x127px]]
Modems which use a mobile telephone system ([[GPRS]], [[UMTS]], [[High Speed Packet Access|HSPA]], [[Evolution-Data Optimized|EVDO]], [[WiMAX|WiMax]], [[5G]] etc.), are known as mobile broadband modems (sometimes also called wireless modems). Wireless modems can be embedded inside a [[laptop]], mobile phone or other device, or be connected externally. External wireless modems include [[connect card]]s, USB modems, and [[cellular router]]s.
Most [[GSM]] wireless modems come with an integrated [[SIM card]]holder (i.e. [[Huawei E220]], Sierra 881.) Some models are also provided with a microSD memory slot and/or jack for additional external antenna, (Huawei E1762, Sierra Compass 885.)<ref>{{cite web |url=http://www.3gmodem.com.hk/Huawei/E1762.html |title=HUAWEI E1762, HSPA/UMTS 900/2100 Support 2Mbps (5.76Mbps ready) HSUPA and 7.2Mbps HSDPA services |publisher=3gmodem.com.hk |access-date=2013-04-22 |url-status=live |archive-url=https://web.archive.org/web/20130510031830/http://www.3gmodem.com.hk/Huawei/E1762.html |archive-date=2013-05-10 }}</ref><ref>{{cite web |url=http://www.reghardware.com/2008/07/16/review_sierra_compass_885/ |title=Sierra Wireless Compass 885 HSUPA 3G modem |publisher=The Register |access-date=2014-02-10 |url-status=live |archive-url=https://web.archive.org/web/20130104002944/http://www.reghardware.com/2008/07/16/review_sierra_compass_885/ |archive-date=2013-01-04 }}</ref>
The CDMA (EVDO) versions do not typically use [[Removable User Identity Module|R-UIM]] cards, but use [[Electronic Serial Number]] (ESN) instead.
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==Optical modem==
[[File:Tellabs ONT611 inside.jpeg|left|thumb|An ONT providing data, telephone and television service]]
A
Fiber optic systems can use quadrature amplitude modulation to maximize throughput. 16QAM uses a 16-point constellation to send four bits per symbol, with speeds on the order of 200 or 400 gigabits per second.<ref>{{cite web
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|archive-date = November 9, 2016
}}</ref><ref>{{cite journal
|title = 112
|journal = Optics Express
|volume = 19
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==See also==
{{Columns-list|colwidth=31em|
* [[56
* [[Handshake (computing)|Handshaking]]
* [[Raytheon BBN]]
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* [https://www.bugswave.com/difference-between-modems-and-routers/ Difference between Modems and Routers] – Bugswave
* [http://bitsavers.org/communications/westernElectric/books/Telecommunications_Transmission_Engineering_Vol_2_Facilities_1ed_1977.pdf Telecommunications Transmission_Engineering Volume 2 Facilities] - AT&T
* [https://proxy-hobbit-poland.com/en Polish proxies made on 4G modems]
{{Modem standards}}
{{Internet access}}
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