The Origin of Modems The Origin of Modems

ModemsModems

The word "modem" is a contraction of the words The word "modem" is a contraction of the words modulator-demodulatormodulator-demodulator. A modem is typically . A modem is typically used to send used to send digital datadigital data over a over a phone linephone line. .

The sending modem The sending modem modulatesmodulates the data into a the data into a signal that is compatible with the phone line, and signal that is compatible with the phone line, and the receiving modem the receiving modem demodulatesdemodulates the signal the signal back into digital data. back into digital data. Wireless modemsWireless modems convert digital data into convert digital data into radio signalsradio signals and back. and back.

Modems came into existence in the 1960s as a Modems came into existence in the 1960s as a way to allow terminals to connect to computers way to allow terminals to connect to computers over the phone lines. over the phone lines.

ModemsModems

A typical A typical arrangement is arrangement is shown below: shown below:

ModemsModems

In a configuration like this, a In a configuration like this, a dumb dumb terminalterminal at an off-site office or store at an off-site office or store could "dial in" to a large, central computer. could "dial in" to a large, central computer.

The 1960s were the age of The 1960s were the age of time-sharedtime-shared computers, so a business would often buy computers, so a business would often buy computer time from a time-share facility computer time from a time-share facility and connect to it via a 300-bit-per-second and connect to it via a 300-bit-per-second (bps) modem. (bps) modem.

ModemsModems

When When personal computerspersonal computers started appearing started appearing in the late 1970s, in the late 1970s, bulletin board systemsbulletin board systems (BBS) became the rage. A person would set (BBS) became the rage. A person would set up a computer with a modem or two and up a computer with a modem or two and some BBS software, and other people would some BBS software, and other people would dial in to connect to the dial in to connect to the bulletin boardbulletin board. .

The users would run The users would run terminal emulatorsterminal emulators on their computers to emulate a dumb on their computers to emulate a dumb terminal. terminal.

ModemsModems

People got along at 300 bps for quite a People got along at 300 bps for quite a while. The reason this speed was tolerable while. The reason this speed was tolerable was because 300 bps represents about 30 was because 300 bps represents about 30 characters per second, which is a lot more characters per second, which is a lot more characters per second than a person can characters per second than a person can type or read. type or read.

Once people started transferring large Once people started transferring large programs and images to and from bulletin programs and images to and from bulletin board systems, however, 300 bps became board systems, however, 300 bps became intolerable. intolerable.

ModemsModems

Modem speeds went through a series of steps at Modem speeds went through a series of steps at approximately two-year intervals: approximately two-year intervals: 300 bps - 1960s through 1983 or so 300 bps - 1960s through 1983 or so 1200 bps - Gained popularity in 1984 and 1985 1200 bps - Gained popularity in 1984 and 1985 2400 bps 2400 bps 9600 bps - First appeared in late 1990 and early 1991 9600 bps - First appeared in late 1990 and early 1991 19.2 kilobits per second (Kbps) 19.2 kilobits per second (Kbps) 28.8 Kbps 28.8 Kbps 33.6 Kbps 33.6 Kbps 56 Kbps - Became the standard in 1998 56 Kbps - Became the standard in 1998 ADSL, with theoretical maximum of up to 8 megabits per ADSL, with theoretical maximum of up to 8 megabits per

second (Mbps) - Gained popularity in 1999 second (Mbps) - Gained popularity in 1999

300-bps Modems 300-bps Modems

We'll use 300-bps modems as a starting We'll use 300-bps modems as a starting point because they are extremely easy to point because they are extremely easy to understand. understand.

A 300-bps modem is a device that uses A 300-bps modem is a device that uses frequency shift keyingfrequency shift keying (FSK) to transmit (FSK) to transmit digital information over a telephone line. digital information over a telephone line. In frequency shift keying, a different tone In frequency shift keying, a different tone (frequency) is used for the different bits (frequency) is used for the different bits

300-bps Modems 300-bps Modems

When a terminal's modem dials a computer's When a terminal's modem dials a computer's modem, the terminal's modem is called the modem, the terminal's modem is called the originateoriginate modem. It transmits a 1,070-hertz tone modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. for a 0 and a 1,270-hertz tone for a 1.

The computer's modem is called the The computer's modem is called the answeranswer modem, and it transmits a 2,025-hertz tone for a modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. 0 and a 2,225-hertz tone for a 1.

Because the originate and answer modems Because the originate and answer modems transmit different tones, they can use the line transmit different tones, they can use the line simultaneously. This is known as simultaneously. This is known as full-duplexfull-duplex operation. Modems that can transmit in only one operation. Modems that can transmit in only one direction at a time are known as direction at a time are known as half-duplexhalf-duplex modems, and they are rare. modems, and they are rare.

300-bps Modems 300-bps Modems

Let's say that two 300-bps modems are Let's say that two 300-bps modems are connected, and the user at the terminal types the connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary decimal or 01100001 binary

A device inside the terminal called a UART A device inside the terminal called a UART (universal asynchronous receiver/transmitter) (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out converts the byte into its bits and sends them out one at a time through the terminal's one at a time through the terminal's RS-232 portRS-232 port (also known as a (also known as a serial portserial port). ).

The terminal's modem is connected to the RS-232 The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its port, so it receives the bits one at a time and its job is to send them over the phone line job is to send them over the phone line

Faster Modems Faster Modems

In order to create faster modems, modem In order to create faster modems, modem designers had to use techniques far more designers had to use techniques far more sophisticated than frequency-shift keying. sophisticated than frequency-shift keying.

First they moved to First they moved to phase-shift keyingphase-shift keying (PSK), and (PSK), and then then quadrature amplitude modulationquadrature amplitude modulation (QAM). (QAM).

These techniques allow an incredible amount of These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these perfect lines, are about the limit of these techniques .techniques .

Faster Modems Faster Modems

Here's a look inside Here's a look inside a typical 56K a typical 56K modem modem

Faster Modems Faster Modems

All of these high-speed modems All of these high-speed modems incorporate a concept of incorporate a concept of gradual gradual degradationdegradation, meaning they can test , meaning they can test the phone line and fall back to slower the phone line and fall back to slower speeds if the line cannot handle the speeds if the line cannot handle the modem's fastest speed modem's fastest speed

Faster Modems Faster Modems

The next step in the evolution of the modem was The next step in the evolution of the modem was asymmetric digital subscriber lineasymmetric digital subscriber line ( (ADSLADSL) ) modems. modems.

The word The word asymmetricasymmetric is used because these is used because these modems send data faster in one direction than modems send data faster in one direction than they do in another. they do in another.

An ADSL modem takes advantage of the fact that An ADSL modem takes advantage of the fact that any normal home, apartment or office has a any normal home, apartment or office has a dedicated copper wirededicated copper wire running between it and running between it and phone company's nearest mux or central office. phone company's nearest mux or central office.

This dedicated copper wire can carry far more This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your data than the 3,000-hertz signal needed for your phone's voice channel. phone's voice channel.

Faster Modems Faster Modems

If both the phone company's central office and your If both the phone company's central office and your house are equipped with an ADSL modem on your house are equipped with an ADSL modem on your line, then the section of copper wire between your line, then the section of copper wire between your house and the phone company can act as a purely house and the phone company can act as a purely digital high-speed transmission channel. digital high-speed transmission channel.

The capacity is something like 1 million bits per The capacity is something like 1 million bits per second (Mbps) between the home and the phone second (Mbps) between the home and the phone company (company (upstreamupstream) and 8 Mbps between the ) and 8 Mbps between the phone company and the home (phone company and the home (downstreamdownstream) under ) under ideal conditions. The same line can transmit both a ideal conditions. The same line can transmit both a phone conversation phone conversation andand the digital data. the digital data.

Faster Modems Faster Modems

The approach an ADSL modem takes is very The approach an ADSL modem takes is very simple in principle. The phone line's simple in principle. The phone line's bandwidth between 24,000 hertz and bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz 1,100,000 hertz is divided into 4,000-hertz bands, and a bands, and a virtual modemvirtual modem is assigned to is assigned to each band. each band.

Each of these 249 virtual modems tests its Each of these 249 virtual modems tests its band and does the best it can with the slice band and does the best it can with the slice of bandwidth it is allocated. The aggregate of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the 249 virtual modems is the total speed of the pipe. of the pipe.

Point-to-Point ProtocolPoint-to-Point Protocol

Today, no one uses dumb terminals or Today, no one uses dumb terminals or terminal emulators to connect to an terminal emulators to connect to an individual computer. Instead, we use our individual computer. Instead, we use our modems to connect to an modems to connect to an Internet service Internet service providerprovider (ISP), and the ISP connects us into (ISP), and the ISP connects us into the Internet. the Internet.

Because of the relationship between your Because of the relationship between your computer, the ISP and the Internet, it is no computer, the ISP and the Internet, it is no longer appropriate to send individual longer appropriate to send individual characters. Instead, your modem is routing characters. Instead, your modem is routing TCP/IP packets between you and your ISP. TCP/IP packets between you and your ISP.

Point-to-Point ProtocolPoint-to-Point Protocol

The standard technique for The standard technique for routingrouting these these packets through your modem is called the packets through your modem is called the Point-to-Point ProtocolPoint-to-Point Protocol ( (PPPPPP). ).

The basic idea is simple -- your computer's The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are normally, but then the datagrams are handed to the modem for transmission. handed to the modem for transmission.

The ISP receives each datagram and The ISP receives each datagram and routes it appropriately onto the Internet. routes it appropriately onto the Internet. The same process occurs to get data from The same process occurs to get data from the ISP to your computer. the ISP to your computer.

How Cable Modems Work How Cable Modems Work

For millions of people, For millions of people, televisiontelevision brings brings news, entertainment and educational news, entertainment and educational programs into their homes. Many people get programs into their homes. Many people get their TV signal from their TV signal from cable televisioncable television (CATV) (CATV) because cable TV provides a clearer picture because cable TV provides a clearer picture and more channels. and more channels.

Many people who have cable TV can now Many people who have cable TV can now get a high-speed connection to the Internet get a high-speed connection to the Internet from their cable provider. Cable modems from their cable provider. Cable modems compete with technologies like compete with technologies like asymmetrical digital subscriber linesasymmetrical digital subscriber lines (ADSL). (ADSL).

We'll look at how a cable modem We'll look at how a cable modem works and see how 100 cable works and see how 100 cable television channels and any Web site television channels and any Web site out there can flow over a single out there can flow over a single coaxial cable into your home. coaxial cable into your home.

Extra Space Extra Space

You might think that a television channel You might think that a television channel would take up quite a bit of electrical "space," would take up quite a bit of electrical "space," or or bandwidthbandwidth, on a cable. , on a cable.

In reality, each television signal is given a 6-In reality, each television signal is given a 6-megahertz (MHz, millions of cycles per megahertz (MHz, millions of cycles per second) channel on the cable. second) channel on the cable.

The The coaxial cablecoaxial cable used to carry cable used to carry cable television can carry hundreds of megahertz of television can carry hundreds of megahertz of signals -- all the channels you could want to signals -- all the channels you could want to watch and more watch and more

Extra Space Extra Space

In a cable TV system, signals from the In a cable TV system, signals from the various channels are each given a 6-MHz various channels are each given a 6-MHz slice of the cable's available bandwidth and slice of the cable's available bandwidth and then sent down the cable to your house. then sent down the cable to your house.

In some systems, coaxial cable is the only In some systems, coaxial cable is the only medium used for distributing signals. In medium used for distributing signals. In other systems, other systems, fiber-optic cablefiber-optic cable goes from goes from the cable company to different the cable company to different neighborhoods or areas. neighborhoods or areas.

Extra Space Extra Space

Then the fiber is terminated and the Then the fiber is terminated and the signals move onto coaxial cable for signals move onto coaxial cable for distribution to individual houses distribution to individual houses

Extra Space Extra Space

When a cable company offers Internet When a cable company offers Internet access over the cable, Internet information access over the cable, Internet information can use the same cables because the cable can use the same cables because the cable modem system puts modem system puts downstreamdownstream data -- data -- data sent from the Internet to an individual data sent from the Internet to an individual computer -- into a 6-MHz channel.computer -- into a 6-MHz channel.

On the cable, the data looks just like a TV On the cable, the data looks just like a TV channel. So Internet downstream data takes channel. So Internet downstream data takes up the same amount of cable space as any up the same amount of cable space as any single channel of programming. single channel of programming.

Extra Space Extra Space

UpstreamUpstream data -- information sent from an data -- information sent from an individual back to the Internet -- requires even individual back to the Internet -- requires even less of the cable's bandwidth, just 2 MHz, since less of the cable's bandwidth, just 2 MHz, since the assumption is that most people download far the assumption is that most people download far more information than they upload.more information than they upload.

Putting both upstream and downstream data on Putting both upstream and downstream data on the cable television system requires two types of the cable television system requires two types of equipment: a equipment: a cable modemcable modem on the customer end on the customer end and a and a cable modem termination systemcable modem termination system (CMTS) at the cable provider's end. (CMTS) at the cable provider's end.

Between these two types of equipment, all the Between these two types of equipment, all the computer networking, security and management computer networking, security and management of Internet access over cable television is put into of Internet access over cable television is put into place place

Inside the Cable Modem Inside the Cable Modem

Cable modems can be either internal or Cable modems can be either internal or external to the external to the computercomputer..

In some cases, the cable modem can be In some cases, the cable modem can be part of a set-top cable box, requiring that part of a set-top cable box, requiring that only a only a keyboardkeyboard and and mousemouse be added for be added for Internet access. Internet access.

In fact, if your cable system has upgraded In fact, if your cable system has upgraded to digital cable, the new set-top box the to digital cable, the new set-top box the cable company provides will be capable of cable company provides will be capable of connecting to the Internet, whether or not connecting to the Internet, whether or not you receive Internet access through your you receive Internet access through your CATV connection. CATV connection.

Inside the Cable Modem Inside the Cable Modem

Regardless of their outward Regardless of their outward appearance, all cable modems appearance, all cable modems contain certain key components: contain certain key components: A A tunertuner A A demodulatordemodulator A A modulatormodulator A A media access control (MAC) media access control (MAC)

devicedevice A A microprocessormicroprocessor

Inside the Cable Modem Inside the Cable Modem

TunerTuner

The tuner connects to the cable outlet, The tuner connects to the cable outlet, sometimes with the addition of a sometimes with the addition of a splittersplitter that separates the Internet data channel that separates the Internet data channel from normal CATV programming. from normal CATV programming.

Since the Internet data comes through an Since the Internet data comes through an otherwise unused cable channel, the otherwise unused cable channel, the tuner simply receives the modulated tuner simply receives the modulated digital signal and passes it to the digital signal and passes it to the demodulator. demodulator.

DemodulatorDemodulator

The most common demodulators have The most common demodulators have four functions. A quadrature amplitude four functions. A quadrature amplitude modulation (QAM) demodulator takes a modulation (QAM) demodulator takes a radio-frequencyradio-frequency signal that has had signal that has had information encoded in it by varying information encoded in it by varying both the amplitude and phase of the both the amplitude and phase of the wave, and turns it into a simple signal wave, and turns it into a simple signal that can be processed by the analog-that can be processed by the analog-to-digital (A/D) converter. to-digital (A/D) converter.

DemodulatorDemodulator

The A/D converter takes the signal, which The A/D converter takes the signal, which varies in voltage, and turns it into a series of varies in voltage, and turns it into a series of digital 1s and 0s. An error correction module digital 1s and 0s. An error correction module then checks the received information then checks the received information against a known standard, so that problems against a known standard, so that problems in transmission can be found and fixed. in transmission can be found and fixed.

In most cases, the network In most cases, the network framesframes, or , or groups of data, are in groups of data, are in MPEG formatMPEG format, so an , so an MPEG synchronizer is used to make sure the MPEG synchronizer is used to make sure the data groups stay in line and in order. data groups stay in line and in order.

ModulatorModulator

In cable modems that use the cable system for In cable modems that use the cable system for upstream traffic, a modulator is used to convert upstream traffic, a modulator is used to convert the digital computer network data into radio-the digital computer network data into radio-frequency signals for transmission. This frequency signals for transmission. This component is sometimes called a component is sometimes called a burst burst modulatormodulator, because of the irregular nature of , because of the irregular nature of most traffic between a user and the Internet, and most traffic between a user and the Internet, and consists of three parts: consists of three parts: A section to insert information used for error correction A section to insert information used for error correction

on the receiving end on the receiving end A QAM modulator A QAM modulator A digital-to-analog (D/A) converter A digital-to-analog (D/A) converter

Media Access Control (MAC) Media Access Control (MAC)

The MAC sits between the upstream and The MAC sits between the upstream and downstream portions of the cable modem, and acts downstream portions of the cable modem, and acts as the interface between the hardware and software as the interface between the hardware and software portions of the various network portions of the various network protocolsprotocols. .

All computer network devices have MACs, but in the All computer network devices have MACs, but in the case of a cable modem the tasks are more complex case of a cable modem the tasks are more complex than those of a normal network interface card. than those of a normal network interface card.

For this reason, in most cases, some of the MAC For this reason, in most cases, some of the MAC functions will be assigned to a central processing functions will be assigned to a central processing unit (CPU) -- either the CPU in the cable modem or unit (CPU) -- either the CPU in the cable modem or the CPU of the user's system. the CPU of the user's system.

MicroprocessorMicroprocessor

The microprocessor's job depends somewhat on The microprocessor's job depends somewhat on whether the cable modem is designed to be part whether the cable modem is designed to be part of a larger computer system or to provide of a larger computer system or to provide Internet access with no additional computer Internet access with no additional computer support. support.

In situations calling for an attached computer, the In situations calling for an attached computer, the internal microprocessor still picks up much of the internal microprocessor still picks up much of the MAC function from the dedicated MAC module. MAC function from the dedicated MAC module.

In systems where the cable modem is the sole In systems where the cable modem is the sole unit required for Internet access, the unit required for Internet access, the microprocessor picks up MAC slack and much microprocessor picks up MAC slack and much more. more.

Cable Modem Termination Cable Modem Termination System System

At the cable provider's head-end, the CMTS provides At the cable provider's head-end, the CMTS provides many of the same functions provided by the DSLAM in a many of the same functions provided by the DSLAM in a DSL system. DSL system.

The CMTS takes the traffic coming in from a group of The CMTS takes the traffic coming in from a group of customers on a single channel and routes it to an Internet customers on a single channel and routes it to an Internet service provider (ISP) for connection to the Internet. service provider (ISP) for connection to the Internet.

At the head-end, the cable providers will have, or lease At the head-end, the cable providers will have, or lease space for a third-party ISP to have, servers for accounting space for a third-party ISP to have, servers for accounting and logging, Dynamic Host Configuration Protocol (DHCP) and logging, Dynamic Host Configuration Protocol (DHCP) for assigning and administering the IP addresses of all the for assigning and administering the IP addresses of all the cable system's users, and control servers for a protocol cable system's users, and control servers for a protocol called CableLabs Certified Cable Modems -- formerly Data called CableLabs Certified Cable Modems -- formerly Data Over Cable Service Interface Specifications (DOCSIS), the Over Cable Service Interface Specifications (DOCSIS), the major standard used by U.S. cable systems in providing major standard used by U.S. cable systems in providing Internet access to users. Internet access to users.

Cable Modem Termination Cable Modem Termination System System