Definition
Modem or Modulator and Demodulator as the name says modulates and demodulates the signal. Now what does this means, in simple words it converts the digital signals from the computer to the analog signals which can be transmitted through a traditional phone wire or any medium which can transmit the analog signals as desired. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data.
How does it works?
A computer modem works by using phone tones as substitutes for binary code, and the modem receiving these signals translates them back into a usable form. Discover how DSL modems work differently than dial-up modems with help from an IT professional in this free video on computer modems.
300-bps Modems
We'll use 300-bps modems as a starting point because they are extremely easy to understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits (see How Guitars Work for a discussion of tones and frequencies).
When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare.
Let's say that two 300-bps modems are connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary (see How Bits and Bytes Work for details on binary). A device inside the terminal called a UART (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out one at a time through the terminal's RS-232 port (also known as a serial port). The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its job is to send them over the phone line.
We'll use 300-bps modems as a starting point because they are extremely easy to understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits (see How Guitars Work for a discussion of tones and frequencies).
When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare.
Let's say that two 300-bps modems are connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary (see How Bits and Bytes Work for details on binary). A device inside the terminal called a UART (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out one at a time through the terminal's RS-232 port (also known as a serial port). The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its job is to send them over the phone line.
Faster Modems
In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these techniques (see the links at the end of this article for more information).
All of these high-speed modems incorporate a concept of gradual degradation, meaning they can test the phone line and fall back to slower speeds if the line cannot handle the modem's fastest speed.
The next step in the evolution of the modem was asymmetric digital subscriber line (ADSL) modems. The word asymmetric is used because these modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your phone's voice channel. If both the phone company's central office and your house are equipped with an ADSL modem on your line, then the section of copper wire between your house and the phone company can act as a purely digital high-speed transmission channel. The capacity is something like 1 million bits per second (Mbps) between the home and the phone company (upstream) and 8 Mbps between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data.
The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe.
In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these techniques (see the links at the end of this article for more information).
All of these high-speed modems incorporate a concept of gradual degradation, meaning they can test the phone line and fall back to slower speeds if the line cannot handle the modem's fastest speed.
The next step in the evolution of the modem was asymmetric digital subscriber line (ADSL) modems. The word asymmetric is used because these modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your phone's voice channel. If both the phone company's central office and your house are equipped with an ADSL modem on your line, then the section of copper wire between your house and the phone company can act as a purely digital high-speed transmission channel. The capacity is something like 1 million bits per second (Mbps) between the home and the phone company (upstream) and 8 Mbps between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data.
The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe.
Point-to-Point Protocol
Today, no one uses dumb terminals or terminal emulators to connect to an individual computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see How Web Servers and the Internet Work for details). Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP.
The standard technique for routing these packets through your modem is called the Point-to-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same process occurs to get data from the ISP to your computer.
Today, no one uses dumb terminals or terminal emulators to connect to an individual computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see How Web Servers and the Internet Work for details). Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP.
The standard technique for routing these packets through your modem is called the Point-to-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same process occurs to get data from the ISP to your computer.
History of the Modem
Now, we know about the modem lets see how it has evolved inthe past few decades and what it has become.
1943: The birth of Modems
Modems grew out of the need to connect teletype machines over ordinary telephone lines instead of more expensive leased lines which were previously used for current based teleprinters and automated telegraphs.In 1943, IBM adapted this technology to their unit record equipment and were able to transmit punched cards at 25 bitss per second.
1948: Early teletype Modems
During the late 1940's, the United States military expressed a need to transmit hundreds of radar images to command centres during the Cold War, they turned to the telephone system as a solution, using early Teletype modems to transmit images across the country. This would eventually lead to the AT&T Digital Subset in the late 1950's.
1958: AT&T Digital subset
By 1958, AT&T had developed the first commercial, mass produced computer modems, called a Digital Subset, to link SAGE computers amass the United States and Canada. The AT&T modem could communicate at 110 bits per second. At 110 bits per second, an average modern webpage (500kb) would take approximately 15 hours to fully load. Even sending a short email at this speed would take around 20 minutes.
1962:The Bell 103 Data Phone
In 1962, AT&T introduced the Bell 103 Data Phone, which set the standard for 300 bits per second full duplex modems. It allowed digital data to be transmitted aver regular unconditioned telephone lines and used audio frequency-shift keying to encode data.
1977: The Hayes 80-130A
In 1977, Dale Heatherington and Dennis Hayes created the world's first personal computer modem; the Hayes 80-130A. This modem was hugely popular in the US, as it offered all the right features at the right price point, and allowed a direct connection to the phone, something that users had not had the luxury of experienced until this point. The release of the Hayes 80-130A modem coincided with the release of the Apple II in 1977, helping the vintage Apple personal computer to become one of the most popular brands in the world.
1981: Hayes Smartmodem
In 1981 the Hayes Smartmodem was introduced, delivering the first 303bps modem to integrate its own command set This allowed users to initialise, hang up, autodial and more. Prior to this, most users had to dial a phone number manually and hookup the modem once they heard an answer. The basic com mand set remains the basis for computer control of most moderns even today. The Hayerns smartmodem also delivered a 1200bps version, representing a step forward in internet speeds.
Even at faster speeds of 1.2Kbps, downloading and emailing would still take a fair amount of time:
• Average mp3 (4mb): 11:06:40
• Average webpage (500kb): 1:23:30
• Average email (10kb): 1:40
In 1981 the Hayes Smartmodem was introduced, delivering the first 303bps modem to integrate its own command set This allowed users to initialise, hang up, autodial and more. Prior to this, most users had to dial a phone number manually and hookup the modem once they heard an answer. The basic com mand set remains the basis for computer control of most moderns even today. The Hayerns smartmodem also delivered a 1200bps version, representing a step forward in internet speeds.
Even at faster speeds of 1.2Kbps, downloading and emailing would still take a fair amount of time:
• Average mp3 (4mb): 11:06:40
• Average webpage (500kb): 1:23:30
• Average email (10kb): 1:40
The MID 80's: Increasing speeds
During this period, IBM PCclones dominated the PC market, leading to a new era of internal ISA (and later PCI) modem cards designed for PC compatibles. External serial modems held on strong however, and during this time speedy 2400bps modems emerged on the market. Technology pushed this limit further over the years, first to 4800bps, then 9600bps, 14400bps, 28800bps and beyond. Costs of the IBM ISA 2400hps modem in`88.— $1000
1998: 56K Modem
In 1996, Brent Townsend came up with the technolcgy for the 56K =elem.Two years later; in 1998, the first widely available 56K modem was introduced to the market.
The 00's:ISDN, ADSL &Cable
Having reached the limits of analogue technology, companies tried various new approaches to On faster modem speeds. The first alternative was all-digital phone lines (ISDN), though the cost limited their popularity. In the early 2000's, modems that worked over cable TV won a following. Phone companies also figured out how to deliver digital data more economical ly through ADSL lines, ma king them a hugely popular choice among the consumers.The faster speeds offered by ISDN lines and other developments increased the issue of online piracy, as online users could download an mp3 in around 6 minutes, and a whole film in around 26 hours.
2002: The birth of 3G
The first commercial launch of 3G was around 2002, offering application services including wide-area wireless voice telephone, mobile internet access, video calks and N, all in a mobile environment To meet the IMT-2000 standards, a system is required to provide peak data rates of at least 200kbit per second. With the speed of 3G making the iPhone the most popular smartphone on the market, iTunes took back some of the legitimate download market share, allowing users to download legal, fully licensed mp3's on the move. A single mp3 could be downloaded in an average time of just 3 minutes.
2009:4G introduced
4G was introduced to the mass market during 2W9, representing the fourth generation of cellular wireless standards. The requirements for 46 standards have set peak speed requirements for 4G service at 100Mbit per second for high mobility communication (such as trains and cars) and 1G bit per second for low mobility communication (such as pedestrians and stationary users). At maximum 4G speeds, the time it takes to download an mp3 will be around 1 second, and a film around 12 seconds.
Broadband
ADSL (asymmetric digital subscriber line) modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Early proprietary ADSL modems used carrierless amplitude phase (CAP) modulation. All standardized asymmetric DSL variants, including ANSI T1.413 Issue 2, G.dmt, ADSL2, ADSL2+, VDSL2, and G.fast, use discrete multi-tone (DMT) modulation, also called (coded) orthogonal frequency-division multiplexing (OFDM or COFDM).
Standard twisted-pair telephone cable can, for short distances, carry signals with much higher frequencies than the cable's maximum frequency rating. ADSL broadband takes advantage of this capability. However, ADSL's performance gradually declines as the telephone cable's length increases. This limits ADSL broadband service to subscribers within a relatively short distance from the telephone exchange.
Cable modems use a range of radio frequencies originally intended to carry television signals. A single cable can carry radio and television signals at the same time as broadband internet service without interference. Multiple cable modems attached to a single cable can use the same frequency band by employing a low-level media access protocol to avoid conflicts. In the prevalent DOCSIS system, frequency-division duplexing (FDD) separates uplink and downlink signals. For a single-cable distribution system, the return signals from customers require bidirectional amplifiers or reverse path amplifiers that send specific customer frequency bands upstream to the cable plant amongst the downstream frequency bands.
Newer types of broadband modems are available, including satellite and power line modems.
Most consumers did not know about networking and routers when broadband became available. However, many people knew that a modem connected a computer to the Internet over a telephone line. To take advantage of consumers' familiarity with modems, companies called these devices broadband modems rather than using less familiar terms such as adapter, interface, transceiver, or bridge. In fact, broadband modems fit the definition of modem because they use complex waveforms to carry digital data. They use more advanced technology than dial-up modems: typically they can modulate and demodulate hundreds of channels simultaneously or use much wider channels than dial-up modems.
ADSL (asymmetric digital subscriber line) modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Early proprietary ADSL modems used carrierless amplitude phase (CAP) modulation. All standardized asymmetric DSL variants, including ANSI T1.413 Issue 2, G.dmt, ADSL2, ADSL2+, VDSL2, and G.fast, use discrete multi-tone (DMT) modulation, also called (coded) orthogonal frequency-division multiplexing (OFDM or COFDM).
Standard twisted-pair telephone cable can, for short distances, carry signals with much higher frequencies than the cable's maximum frequency rating. ADSL broadband takes advantage of this capability. However, ADSL's performance gradually declines as the telephone cable's length increases. This limits ADSL broadband service to subscribers within a relatively short distance from the telephone exchange.
Cable modems use a range of radio frequencies originally intended to carry television signals. A single cable can carry radio and television signals at the same time as broadband internet service without interference. Multiple cable modems attached to a single cable can use the same frequency band by employing a low-level media access protocol to avoid conflicts. In the prevalent DOCSIS system, frequency-division duplexing (FDD) separates uplink and downlink signals. For a single-cable distribution system, the return signals from customers require bidirectional amplifiers or reverse path amplifiers that send specific customer frequency bands upstream to the cable plant amongst the downstream frequency bands.
Newer types of broadband modems are available, including satellite and power line modems.
Most consumers did not know about networking and routers when broadband became available. However, many people knew that a modem connected a computer to the Internet over a telephone line. To take advantage of consumers' familiarity with modems, companies called these devices broadband modems rather than using less familiar terms such as adapter, interface, transceiver, or bridge. In fact, broadband modems fit the definition of modem because they use complex waveforms to carry digital data. They use more advanced technology than dial-up modems: typically they can modulate and demodulate hundreds of channels simultaneously or use much wider channels than dial-up modems.
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