chapter 2 the physical layer ( 物理层)

CHAPTER 2 THE PHYSICAL LAYER ( 物理层 )

The Theoretical Basis for Data CommunicationThe Theoretical Basis for Data Communication

Guided Transmission Media (Terrestrial)Guided Transmission Media (Terrestrial) Wireless Transmission (Terrestrial)Wireless Transmission (Terrestrial) Communication Satellites (Celestial)Communication Satellites (Celestial)

The Public Switched Telephone Network (Stationary)The Public Switched Telephone Network (Stationary) The Mobile Telephone System (Mobile)The Mobile Telephone System (Mobile) Cable Television (Stationary)Cable Television (Stationary)

THE THEORETICAL BASIS FOR DATA COMMUNICATION ( 数据通信的基本理论 )

Fourier analysisFourier analysis （（傅立叶分析）傅立叶分析）

Bandwidth-limited signalsBandwidth-limited signals （（有限带宽信号）有限带宽信号）

Maximum data rate of a channelMaximum data rate of a channel （（信道的最大信道的最大数据传送速率）数据传送速率）

The theoretical basis for data communication Fourier AnalysisFourier Analysis

Any reasonably behaved periodic functionAny reasonably behaved periodic function （（周期函周期函数）数） can be reconstructed by summing a (possibly can be reconstructed by summing a (possibly infinite) number of sins and cosines. infinite) number of sins and cosines.

Such a decomposition is called Fourier series(Such a decomposition is called Fourier series( 傅立叶级傅立叶级数数 ).).

A data signal that has a finite duration (which all of them A data signal that has a finite duration (which all of them do) can be handled by just imagining that it repeats the do) can be handled by just imagining that it repeats the entire pattern over and over forever. entire pattern over and over forever.

The theoretical basis for data communication Bandwidth-Limited SignalsBandwidth-Limited Signals

Input Input System (transmission, filtering) System (transmission, filtering) output output Bandwidth (Bandwidth ( 带宽带宽 ) for systems) for systems

The range of frequencies transmitted without being The range of frequencies transmitted without being strongly attenuatedstrongly attenuated

The cutoff frequency is not really sharp, is defined The cutoff frequency is not really sharp, is defined as the frequency at which half the power gets as the frequency at which half the power gets through.through.

Any signal (bandwidth-limited or unlimited) Any signal (bandwidth-limited or unlimited) system (filtering) system (filtering) bandwidth-limited signal bandwidth-limited signal

(in time domain, convolution; (in time domain, convolution; In frequency domain, multiplication)In frequency domain, multiplication)


A binary signal and its root-mean-square Fourier A binary signal and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to amplitudes. (b) – (c) Successive approximations to the original signal.the original signal.


(d) – (e) Successive approximations to the original (d) – (e) Successive approximations to the original signal.signal.


The time required to transmit the character depends on both The time required to transmit the character depends on both the encoding method and the signaling speed. The number of the encoding method and the signaling speed. The number of changes per second is measured in baud(changes per second is measured in baud( 波特波特 ). ).

Given a bit rate b bits/sec, Given a bit rate b bits/sec, the time to send 1 bit is 1/b, the time to send 1 bit is 1/b, the time required to send 8 bits 1 bit at a time is 8/b sec, so the time required to send 8 bits 1 bit at a time is 8/b sec, so the frequency of the first harmonic (the frequency of the first harmonic ( 一次谐波一次谐波 ) ) is b/8.is b/8.

An ordinary telephone line (often called as a voice-grade An ordinary telephone line (often called as a voice-grade line(line( 话音级线路话音级线路 )) )) has an artificially introduced cutoff has an artificially introduced cutoff frequency near 3000Hz. This restriction means that the frequency near 3000Hz. This restriction means that the number of the highest harmonic passed through is 3000/(b/8) number of the highest harmonic passed through is 3000/(b/8) or 24000/b. or 24000/b.


Relation between data rate and harmonics for the voice-grade lineRelation between data rate and harmonics for the voice-grade line

Conclusion: the data rate is limited by the Conclusion: the data rate is limited by the bandwidth of transmission system or media. bandwidth of transmission system or media.

The theoretical basis for data communication The maximum data rate of a channelThe maximum data rate of a channel

In 1924, Henry Nyquist (AT&T) derived an equation In 1924, Henry Nyquist (AT&T) derived an equation expressing the maximum data rate for a finite expressing the maximum data rate for a finite bandwidth noiseless channel.bandwidth noiseless channel. maximum number of bits/sec = maximum number of bits/sec = 2 H log_2 V 2 H log_2 V where where HH is the bandwidth, is the bandwidth, VV is the discrete levels. is the discrete levels.

In 1948, Claude Shannon carried Nyquist‘s work In 1948, Claude Shannon carried Nyquist‘s work further and extended it to the case of a channel further and extended it to the case of a channel subject to random noise. subject to random noise.

maximum number of bits/sec = maximum number of bits/sec = H log_2 (1+S/N)H log_2 (1+S/N)

where where HH is the bandwidth, is the bandwidth, S/NS/N ( ( 信噪比信噪比 ) is the ) is the signal-to-noise ratio.signal-to-noise ratio.

GUIDED TRANSMISSION MEDIA （有线数据传）

Magnetic mediaMagnetic media （（磁介质）磁介质）

Twisted pairTwisted pair （（双绞线）双绞线）

Coaxial cableCoaxial cable （（同轴电缆）同轴电缆）

Fiber opticsFiber optics （（光纤）光纤）

Guided Transmission Media: Magnetic media One of the most common ways to transport data form one One of the most common ways to transport data form one

computer to another is computer to another is to write them onto magnetic tape or removable media,to write them onto magnetic tape or removable media, To transport the tape or disks to the destination To transport the tape or disks to the destination

machine, machine, and to read them back in again.and to read them back in again.

Bandwidth: Bandwidth: 200GB/tape * 1000 tapes/box / (24*60*60s) = 19Gbps200GB/tape * 1000 tapes/box / (24*60*60s) = 19Gbps

Cost: Cost: (200GB/tape * 1000 tapes/box) / 5000$ = 40GB/$(200GB/tape * 1000 tapes/box) / 5000$ = 40GB/$ Conclusion:Conclusion: Never underestimate the bandwidth of Never underestimate the bandwidth of

station wagon full of tapes hurtling down the highwaystation wagon full of tapes hurtling down the highway Why computer network? Delay and so on. Why computer network? Delay and so on.

Guided Transmission Media: Twisted pair A twisted pair consists of two insulated copper wires, A twisted pair consists of two insulated copper wires,

typically about 1 mm thick. The wires are twisted typically about 1 mm thick. The wires are twisted together in a helical form, just like a DNA molecule. together in a helical form, just like a DNA molecule.

Twisted pairs can run several kilometers without Twisted pairs can run several kilometers without amplification, but for longer distances, repeaters are amplification, but for longer distances, repeaters are needed. needed.

Twisted pairs can be used for transmitting either analog Twisted pairs can be used for transmitting either analog or digital signals.or digital signals.

TypesTypes Category 3, 5 (16MHz, 100MHz)Category 3, 5 (16MHz, 100MHz) Category 6,7 (250MHz, 600MHz)Category 6,7 (250MHz, 600MHz)

Guided Transmission Media: Twisted pair

((a) Category 3 UTP.a) Category 3 UTP.(b) Category 5 UTP.(b) Category 5 UTP.

Guided Transmission Media: Coaxial cable A coaxial cable consists of a stiff copper wire as the core, A coaxial cable consists of a stiff copper wire as the core,

surrounded by an insulating material. The insulator is surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely-encased by a cylindrical conductor, often as a closely-woven breaded mesh. The outer conductor is covered in a woven breaded mesh. The outer conductor is covered in a protective plastic sheath.protective plastic sheath.

Two types: 50-ohm (digital), 75-ohm (analog and digital)Two types: 50-ohm (digital), 75-ohm (analog and digital) High bandwidth and excellent noise immunityHigh bandwidth and excellent noise immunity Used to be widely used for long-distance lines.Used to be widely used for long-distance lines.

Guided Transmission Media: Coaxial cable• Baseband Baseband （（基带基带）） coaxial cablecoaxial cable• Broadband Broadband （（宽带宽带）） coaxial cablecoaxial cable

• Dual cableDual cable• Single cableSingle cable

Guided Transmission Media: Fiber optics In the race between computing and communication, In the race between computing and communication,

communication woncommunication won IBM PC at 4.77 MHz in 1981 IBM PC at 4.77 MHz in 1981 2GHz 2GHz

(20 per decade) and physical limits(20 per decade) and physical limits 56kpbs (ARPANET) 56kpbs (ARPANET) 1Gbps (optical communication) 1Gbps (optical communication)

125 per decade and error rate almost zero and almost no 125 per decade and error rate almost zero and almost no limitslimits

The new conventional wisdom should be that:The new conventional wisdom should be that:all computers are hopeless slow and all computers are hopeless slow and networks should try to avoid computation at all costs, networks should try to avoid computation at all costs, no matter how much bandwidth that wastes.no matter how much bandwidth that wastes.

An optical transmission system: the light source An optical transmission system: the light source the the transmission medium transmission medium the detector. the detector.

Guided Transmission Media: Fiber optics

((a) Three examples of a light ray from inside a silica fiber a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.impinging on the air/silica boundary at different angles.

(b) Light trapped by total internal reflection (no refraction).(b) Light trapped by total internal reflection (no refraction).

Two types of fiber opticsTwo types of fiber opticsmultimode: may different rays multimode: may different rays Unimode: single rays, longer distanceUnimode: single rays, longer distance

Guided Transmission Media: Fiber optics Transmission of light through fiberTransmission of light through fiber

Attenuation (Attenuation ( 衰减衰减 )) of light through fiber in the infrared region.of light through fiber in the infrared region. Three wavelength bands are used for optical communication. Three wavelength bands are used for optical communication.

They are centered at 0.85, 1.30, and 1.55 micros, respectively. They are centered at 0.85, 1.30, and 1.55 micros, respectively.


Fiber cablesFiber cables Fiber optic cables are similar to coax, except without Fiber optic cables are similar to coax, except without

the braid. the braid. ((a) Side view of a single fiber.a) Side view of a single fiber.(b) End view of a sheath with three fibers.(b) End view of a sheath with three fibers.

Guided Transmission Media: Fiber optics Fiber cablesFiber cables

DeploymentDeploymentLaid in the ground within a meter of the surfaceLaid in the ground within a meter of the surfaceburied in trenches near the shoreburied in trenches near the shoreLie on the bottom in deep waterLie on the bottom in deep water

ConnectionConnectionFiber socketsFiber socketsSpliced mechanicallySpliced mechanicallyFusion spliceFusion splice

DetectionDetectionPhotodiode. The response time is 1ns, which Photodiode. The response time is 1ns, which

limits data rates to about 1Gbps. limits data rates to about 1Gbps.

Guided Transmission Media: Fiber optics Fiber cablesFiber cables

Light sourcesLight sourcesLED LED Semiconductor laserSemiconductor laser


Fiber optics networkFiber optics network A fiber optic ring with active repeaters A fiber optic ring with active repeaters （（有源中继有源中继

器）器） ..

Guided Transmission Media: Fiber optics Fiber optics networkFiber optics network

A passive (A passive ( 无源无源 ) ) star connection in a fiber optics star connection in a fiber optics network.network.

Guided Transmission Media: Fiber optics Comparison of fiber optics and copper wireComparison of fiber optics and copper wire

AdvantagesAdvantagesHigher bandwidths and Low attenuation.Higher bandwidths and Low attenuation.Not being affected by power surges, electromagnetic Not being affected by power surges, electromagnetic

interference, or power failures. interference, or power failures. Not affected by corrosive chemicals in the air.Not affected by corrosive chemicals in the air.Thin and lightweight.Thin and lightweight.Fibers do not leak light and quite difficult to tap.Fibers do not leak light and quite difficult to tap.

DisadvantagesDisadvantagesLess familiar technology.Less familiar technology.Fiber interfaces more expensive.Fiber interfaces more expensive.

Conclusion: Conclusion: For new routes (longer ones), fiber wins!For new routes (longer ones), fiber wins!

WIRELESS TRANSMISSION （无线传输）

The electromagnetic spectrumThe electromagnetic spectrum （（电磁波谱）电磁波谱）

Radio transmissionRadio transmission （（无线电传输）无线电传输）

Microwave transmissionMicrowave transmission （（微波传输）微波传输）

Infrared and millimeter wavesInfrared and millimeter waves （（红外线和毫米波）红外线和毫米波）

Lightwave transmissionLightwave transmission （（光波传输）光波传输）

Wireless Transmission: The Electromagnetic Spectrum

The electromagnetic spectrum and its uses for The electromagnetic spectrum and its uses for communication.communication.

Wireless Transmission: The Electromagnetic Spectrum LF(Low), MF(Medium), HF(High), VHF(Very), LF(Low), MF(Medium), HF(High), VHF(Very),

UHF(Ultra), SHF(Super), EHT(Extremely), UHF(Ultra), SHF(Super), EHT(Extremely), THF(Tremendously), (THF(Tremendously), (IHF(Incredibly), IHF(Incredibly), AHF(Astonishingly), PHF(Prodigiously)AHF(Astonishingly), PHF(Prodigiously) ））

The wider the band, the higher the data rate. The wider the band, the higher the data rate. To prevent total chaos, there are national and international To prevent total chaos, there are national and international

agreements about who gets to use which frequencies. agreements about who gets to use which frequencies. Most transmissions use a Most transmissions use a narrow frequency bandnarrow frequency band. (GSM). (GSM) Some spread its frequency over a Some spread its frequency over a wide frequency bandwide frequency band

(spread spectrum(spread spectrum ，，散布频谱散布频谱 ). (). (CDMA)CDMA) frequency hopping spread spectrum frequency hopping spread spectrum (military, 802.11, Bluetooth)(military, 802.11, Bluetooth)

direct sequence spread spectrum (2G mobile phones)direct sequence spread spectrum (2G mobile phones)

Wireless Transmission: Radio transmission Radio waves are easy to generate, can travel long Radio waves are easy to generate, can travel long

distances, and can penetrate buildings easily, so they are distances, and can penetrate buildings easily, so they are widely used for communicationwidely used for communication

Radio waves also are omnidirectional, meaning that Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the they travel in all directions from the source, so the transmitter and receiver do not have to carefully aligned transmitter and receiver do not have to carefully aligned physically. physically.

Due to radio’s ability to travel long distances, Due to radio’s ability to travel long distances, interference between users is a problem. For this reason, interference between users is a problem. For this reason, all governments tightly license the use of radio all governments tightly license the use of radio transmitters. transmitters.

Wireless Transmission: Radio transmission

((a) In the VLF, LF, and MF bands, radio waves follow the a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.curvature of the earth.

(b) In the HF band, they bounce off the ionosphere(b) In the HF band, they bounce off the ionosphere （（电离电离层层）） ..

Wireless Transmission: Microwave transmission Above 100MHz, the waves travel in straight lines and Above 100MHz, the waves travel in straight lines and

can therefore be narrowly focused. can therefore be narrowly focused. The higher the transmission towers are, the further apart The higher the transmission towers are, the further apart

they can be. For 100-m high towers, repeaters can be they can be. For 100-m high towers, repeaters can be spaced 80km apart. spaced 80km apart.

Microwave can cause Microwave can cause multipath fadingmultipath fading or be or be absorbed absorbed by rainby rain. .

Microwave communication is so widely used for long-Microwave communication is so widely used for long-distance telephone communication, cellular telephones, distance telephone communication, cellular telephones, television distribution, and other uses, that a severe television distribution, and other uses, that a severe shortage of spectrum has developed. shortage of spectrum has developed.

Microwave has some advantages over fiber: no right of Microwave has some advantages over fiber: no right of way problem, being inexpensive. way problem, being inexpensive.

Wireless Transmission: Microwave transmission

Who gets to use which frequenciesWho gets to use which frequencies ITU-RITU-R FCC, FCC, 无线电管理委员会无线电管理委员会 ??

Three algorithms for deciding which company or other Three algorithms for deciding which company or other unit is allowed to which frequenciesunit is allowed to which frequencies Beauty contestBeauty contest Holding a lottery among the interested companiesHolding a lottery among the interested companies Auctioning off the band width to the highest bidderAuctioning off the band width to the highest bidder

The location of the ISM (Industrial, Scientific, Medical) The location of the ISM (Industrial, Scientific, Medical) band varies from country to country. band varies from country to country.

Wireless Transmission: Microwave transmission

The ISM bands in the United States.The ISM bands in the United States.

Wireless Transmission: Infrared and millimeter waves

Infrared and millimeter waves can not pass through Infrared and millimeter waves can not pass through solid objects solid objects

Infrared can be used for indoor wireless LANs, remote Infrared can be used for indoor wireless LANs, remote controllers. controllers.

Infrared can not be used outdoors for the sun shines as Infrared can not be used outdoors for the sun shines as brightly in the infrared as in the visible spectrum. brightly in the infrared as in the visible spectrum.

AdvantagesAdvantages The infrared controller cannot control the TV of your The infrared controller cannot control the TV of your

neighbor. neighbor. More secure. More secure.

Wireless Transmission: Lightwave transmissionConvection currents (Convection currents ( 气流气流 ) ) can interfere with laser can interfere with laser

communication systems. communication systems. A bidirectional system with two lasers is pictured here.A bidirectional system with two lasers is pictured here.

COMMUNICATION SATELLITES （通讯卫星）

Geostationary Satellites Geostationary Satellites （（地球同步卫地球同步卫星）星）

Medium-Earth Orbit Satellites Medium-Earth Orbit Satellites （（中轨道卫中轨道卫星）星）

Low-Earth Orbit Satellites Low-Earth Orbit Satellites （（低轨道卫星）低轨道卫星）

Satellites versus FiberSatellites versus Fiber

Communication satellites In the 1950s and early 1960s, people tried to set up In the 1950s and early 1960s, people tried to set up

communication systems by bouncing signals off communication systems by bouncing signals off metallized weather balloons.metallized weather balloons.

The U.S. Navy built an operational system for ship-to-The U.S. Navy built an operational system for ship-to-shore communication by bouncing signals off the moon.shore communication by bouncing signals off the moon.

Communication satellite is the real celestial Communication satellite is the real celestial communication player.communication player. A big microwave repeaterA big microwave repeater Many transponders (Many transponders ( 异频收发机异频收发机 ))

The higher the satellite, the longer the period. The higher the satellite, the longer the period. There are two Van Allen belts, layers of highly charged There are two Van Allen belts, layers of highly charged

particles trapped by the earth’s magnetic field. particles trapped by the earth’s magnetic field.

Communication Satellites

Communication satellites and some of their properties, Communication satellites and some of their properties, including altitude above the earth, round-trip delay time including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.and number of satellites needed for global coverage.

Communication Satellites: Geostationary satellites Orbit slots: to avoid interference, geostationary satellites Orbit slots: to avoid interference, geostationary satellites

should be placed 2 degrees or so in the 360 degree should be placed 2 degrees or so in the 360 degree equatorial plane. equatorial plane.

Principal satellite bands: Principal satellite bands: C C L,SL,S Ku, KaKu, Ka

Communication Satellites: Geostationary satellites

Wide beam and spot beam:Wide beam and spot beam: Wide beam: covers large area ( as large as 1/3 of the Wide beam: covers large area ( as large as 1/3 of the

earth’s surface)earth’s surface) Spot beam: covers small area (a few hundred km in Spot beam: covers small area (a few hundred km in

diameter)diameter) Satellites has many transponders.Satellites has many transponders.

Every transponder can use Every transponder can use FDM (the bandwidth is simply split up into fixed FDM (the bandwidth is simply split up into fixed

frequency bands) or frequency bands) or TDM (divided into time slots) techniques.TDM (divided into time slots) techniques.

Communication Satellites: Geostationary satellites

VSAT (Very Small Aperture Terminals)VSAT (Very Small Aperture Terminals) Many small VSATs Many small VSATs Satellite Satellite Many small VSATs Many small VSATs VSAT hub VSAT hub satellite satellite

Communication Satellites: Geostationary satellites Discussion of propertiesDiscussion of properties

Longer delay (270ms) (3us/km for terrestrial Longer delay (270ms) (3us/km for terrestrial microwave, 5us/km for coaxial cable or fiber optic microwave, 5us/km for coaxial cable or fiber optic links)links)

Inherently broadcast Inherently broadcast Good from broadcasting sportsGood from broadcasting sportsSecurity is an issue. Security is an issue.

The cost of transmitting a message is independent of The cost of transmitting a message is independent of the distance traversed.the distance traversed.

Satellites also have excellent error rates.Satellites also have excellent error rates. Satellites can be deployed almost instantly, a major Satellites can be deployed almost instantly, a major

consideration for military communication. consideration for military communication.

Communication Satellites: Medium-earth orbit satellites

MEO satellitesMEO satellites Lower than GEOs. Between the two Van Allen beltsLower than GEOs. Between the two Van Allen belts Smaller footprint on the ground. Smaller footprint on the ground. Requires less powerful transmitters to reach them.Requires less powerful transmitters to reach them. Examples: 24 GPS satellites.Examples: 24 GPS satellites. They are not used for telecommunication.They are not used for telecommunication.

Communication Satellites: Low-earth orbit satellites

LEO satellitesLEO satellites Lower than MEOs. Lower than MEOs. Much smaller footprint on the ground Much smaller footprint on the ground Requires far less powerful transmitters to reach them Requires far less powerful transmitters to reach them The round-trip delay is only a few millisecondsThe round-trip delay is only a few milliseconds Used for voice communication and Internet services.Used for voice communication and Internet services.


Iridium project (Iridium project ( 铱星铱星 )) In 1990 Motorola applied the permission to launch In 1990 Motorola applied the permission to launch

77 low-orbit satellites for the Iridium project 77 low-orbit satellites for the Iridium project (77(7766).66).

In 1997 Motorola and its partners launched the In 1997 Motorola and its partners launched the Iridium satellites. Iridium satellites.

In November 1998 Communication service began.In November 1998 Communication service began. In August 1999, Iridium was not profitable and was In August 1999, Iridium was not profitable and was

forced into bankruptcy ($5 billion forced into bankruptcy ($5 billion $25million). $25million). In March 2001, the Iridium service was restarted. In March 2001, the Iridium service was restarted.


Iridium projectIridium project((a)a) The Iridium satellites form six necklaces around the earth. The Iridium satellites form six necklaces around the earth.

(b)(b) 1628 moving cells cover the earth. 1628 moving cells cover the earth.


GlobalstarGlobalstar 48 LEOs48 LEOs Relaying on the groundRelaying on the ground

TeledesicTeledesic Conceived in 1990 by mobile phone pioneer Craig Conceived in 1990 by mobile phone pioneer Craig

McCaw and Microsoft founder Bill gatesMcCaw and Microsoft founder Bill gates Scheduled to go live in 2005 if all goes as planned. Scheduled to go live in 2005 if all goes as planned. 30 LEOs30 LEOs

Communication Satellites: Satellites (celestial) versus Fiber (terrestrial)

High bandwidth: With satellites, it is practical for a user High bandwidth: With satellites, it is practical for a user to erect an antenna on the roof of the building and to erect an antenna on the roof of the building and completely bypass the telephone system to get high completely bypass the telephone system to get high bandwidth. Teledesic is based on this idea. bandwidth. Teledesic is based on this idea.

Mobile communicationMobile communication Good for broadcastingGood for broadcasting For communication in places with hostile terrain or a For communication in places with hostile terrain or a

poorly developed terrestrial infrastructurepoorly developed terrestrial infrastructure No right of way problemNo right of way problem Rapid developmentRapid development

PUBLIC SWITCHED TELEPHONE SYSTEM公用交换电话系统

Structure of the Telephone SystemStructure of the Telephone System

The Politics of TelephonesThe Politics of Telephones

The Local Loop: Modems, ADSL, and WirelessThe Local Loop: Modems, ADSL, and Wireless

Trunks and MultiplexingTrunks and Multiplexing

SwitchingSwitching

Public Switched Telephone System How to connect computers:How to connect computers:

For a small number of computers and a local area, LAN For a small number of computers and a local area, LAN can be used. can be used.

For a large number of computers or a wide area, or For a large number of computers or a wide area, or lacking right of way, LAN can not be used and we have lacking right of way, LAN can not be used and we have to rely upon the existing communication facilities such to rely upon the existing communication facilities such as PSTNas PSTN. .

The PSTN is suitable for transmitting the human voice in The PSTN is suitable for transmitting the human voice in a more or less recognizable form. a more or less recognizable form.

The suitability for use in computer-computer networking The suitability for use in computer-computer networking is often marginal at best although the situation is rapidly is often marginal at best although the situation is rapidly changing with the introduction of fiber optics and digital changing with the introduction of fiber optics and digital technology. technology.

Public Switched Telephone System A cable running between two computers can transfer data A cable running between two computers can transfer data

at 10^9 bps or more. A dial-up line has maximum date at 10^9 bps or more. A dial-up line has maximum date date of 56 kbps, date of 56 kbps, a difference of a factor of almost 20,000a difference of a factor of almost 20,000. . That is the difference between a duck waddling leisurely That is the difference between a duck waddling leisurely though the grass and a rocket to the moon.though the grass and a rocket to the moon.

The combined bit rate times error performance of a The combined bit rate times error performance of a computer system cable is computer system cable is 11 orders of magnitude better11 orders of magnitude better than a voice-grade telephone line. (the ratio of the cost of than a voice-grade telephone line. (the ratio of the cost of the entire Apollo project to the cost of a bus ride the entire Apollo project to the cost of a bus ride downtown is about 11 orders of magnitude (in 1965 downtown is about 11 orders of magnitude (in 1965 dollars: 40 billion$ to 0.40$)) dollars: 40 billion$ to 0.40$))

Computer system designers have devoted much time and Computer system designers have devoted much time and effort to figure out how to use it efficiently. effort to figure out how to use it efficiently.

Public Switched Telephone System: Structure

In 1876, Alexander Graham Bell patented the telephone In 1876, Alexander Graham Bell patented the telephone (just a few hours ahead of his rival, Elisha Gray). (just a few hours ahead of his rival, Elisha Gray). The initial market was for the sale of telephones, The initial market was for the sale of telephones,

which came in pairs.which came in pairs. If a telephone owner wanted to talk to If a telephone owner wanted to talk to nn other other

telephone owners, separate wires had to strung to all telephone owners, separate wires had to strung to all nn houses. houses.

Enormous demand Enormous demand Within a year, the cities were Within a year, the cities were covered with wires passing over houses and trees in a covered with wires passing over houses and trees in a wild jumble. wild jumble.

1878, Bell formed the Bell Telephone Company, which 1878, Bell formed the Bell Telephone Company, which opened its first switching office. opened its first switching office.

Public Switched Telephone System: Structure To connect every switching office to every other To connect every switching office to every other

switching office by means of a wire between them switching office by means of a wire between them quickly became unmanageable, so second-level switching quickly became unmanageable, so second-level switching offices were invented. (See the next slide)offices were invented. (See the next slide)

After a while, multiple second-level offices were needed. After a while, multiple second-level offices were needed. Eventually, the hierarchy grew to five levels: end office Eventually, the hierarchy grew to five levels: end office

(( 本地局本地局 ), toll office(), toll office( 长途局长途局 ), primary office(), primary office( 初级初级局局 ), sectional office (), sectional office ( 地区局地区局 ), and regional office(), and regional office( 区域区域局局 ).).

By 1890, the three major parts of the telephone system By 1890, the three major parts of the telephone system were in places: the switching offices, the wires between were in places: the switching offices, the wires between the customers and the switching offices, the long-distance the customers and the switching offices, the long-distance connections between the switching offices. connections between the switching offices.

Public Switched Telephone System: Structure

((a) Fully-interconnected network.a) Fully-interconnected network.

(b) Centralized switch.(b) Centralized switch.

(c) Two-level hierarchy => … => Five-level hierarchy .(c) Two-level hierarchy => … => Five-level hierarchy .

Public Switched Telephone System: Structure The basic Bell system model has remained essentially The basic Bell system model has remained essentially

intact for over 100 years. intact for over 100 years. Major Components of the Telephone SystemMajor Components of the Telephone System

Local loopsLocal loops （（本地回路）本地回路） : : Analog twisted pairs Analog twisted pairs going to houses and businesses (not open-loop).going to houses and businesses (not open-loop).

TrunksTrunks （（干线）干线） : : Digital fiber optics connecting Digital fiber optics connecting the switching offices (used to be coaxial cables).the switching offices (used to be coaxial cables).

Switching officesSwitching offices （（交换局）交换局） : : to switch calls from to switch calls from one trunk to another (manual operators + jumpers one trunk to another (manual operators + jumpers automatic switching computer).automatic switching computer).

Public Switched Telephone System: Structure If a subscriber attached to a given end office calls another If a subscriber attached to a given end office calls another

subscriber attached to the same end office, the switching subscriber attached to the same end office, the switching mechanism within the office sets up a direct electrical mechanism within the office sets up a direct electrical connection between the two local loops. connection between the two local loops.

The call between the subscribers attached to different end The call between the subscribers attached to different end offices via tolloffices via toll, , primaryprimary, , sectionalsectional, , and regional offices. and regional offices.

A typical circuit route for a medium-distance call.A typical circuit route for a medium-distance call.

Public Switched Telephone System: Politics In the 1970s, the U.S. Federal Government sued AT&T In the 1970s, the U.S. Federal Government sued AT&T

for telephone monopoly.for telephone monopoly. On January 1, 1984, AT&T was broken up into AT&T On January 1, 1984, AT&T was broken up into AT&T

Long lines, 23 BOCs (Bell Operating Companies), and Long lines, 23 BOCs (Bell Operating Companies), and a few other pieces. a few other pieces.

LATA: The United States was divided up into 164 LATA: The United States was divided up into 164 LATA (Local Access and Transport Areas).LATA (Local Access and Transport Areas).

LEC: Within a LATA, there was one LEC (Local LEC: Within a LATA, there was one LEC (Local Exchange Carrier) that had a monopoly on traditional Exchange Carrier) that had a monopoly on traditional telephone service within its area. telephone service within its area.

IXC: All inter-LATA traffic was handled by a different IXC: All inter-LATA traffic was handled by a different kind of company, and IXC (InterXchange Carrier)kind of company, and IXC (InterXchange Carrier)

Public Switched Telephone System: Politics LATA(Local Access and Transport AreaLATA(Local Access and Transport Area ，，本地访问和传本地访问和传

输区域输区域 )) LECLEC （（ Local Exchange CarrierLocal Exchange Carrier ，，本地交换电信公司）本地交换电信公司） IXC(IntereXchange CarrierIXC(IntereXchange Carrier ，，外部交换电信公司外部交换电信公司 ))

Public Switched Telephone System: Politics Before February 1996, as part of the MFJ (Modified Final Before February 1996, as part of the MFJ (Modified Final

Judgment), the IXCs were forbidden to offer local Judgment), the IXCs were forbidden to offer local telephone service and the LECs were forbidden to offer telephone service and the LECs were forbidden to offer inter-LATA telephone service. inter-LATA telephone service.

As cable television went from one way to two way and As cable television went from one way to two way and mobile phones exploded in popularity, both LECs and mobile phones exploded in popularity, both LECs and IXCs began buying up or merging with cable and mobile IXCs began buying up or merging with cable and mobile operators. operators.

By 1995, Congress saw that trying to maintain a By 1995, Congress saw that trying to maintain a distinction between the various kinds of companies was distinction between the various kinds of companies was no longer tenable and drafted a bill to allow cable TV no longer tenable and drafted a bill to allow cable TV companies, local telephone companies, long-distance companies, local telephone companies, long-distance carriers, and mobile operators to enter one another’s carriers, and mobile operators to enter one another’s business. business.

Public Switched Telephone System: Local loop

The use of both analog and digital transmissions for a computer to The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modemscomputer call. Conversion is done by the modems （（调制解调调制解调器）器） and codecsand codecs （（编码解码器）编码解码器） ..

Public Switched Telephone System: Local loop

A computer A computer PSTN PSTN A computerA computer The data must be converted to analog form for transmission over The data must be converted to analog form for transmission over

the local loop.the local loop. Between switching centers, digital transmission is used. Between switching centers, digital transmission is used. For ISP1, the digital data must be converted back to analog For ISP1, the digital data must be converted back to analog

form.form.

Transmission lines suffer from three major problemsTransmission lines suffer from three major problems AttenuationAttenuation is the loss of the energy as the signal is the loss of the energy as the signal

propagates outward. propagates outward. Delay distortionDelay distortion is caused by the fact that different is caused by the fact that different

Fourier components travel at different speeds. Fourier components travel at different speeds. NoiseNoise is unwanted energy from sources other than the is unwanted energy from sources other than the

transmitter. transmitter.

Public Switched Telephone System: Local loop: Dialup modems Both attenuation and propagation speed are frequency Both attenuation and propagation speed are frequency

dependent, it is undesirable to have a wide range of dependent, it is undesirable to have a wide range of frequencies in the signal. Unfortunately, digital signals frequencies in the signal. Unfortunately, digital signals have a wide spectrum and thus are subject to strong have a wide spectrum and thus are subject to strong attenuation and delay distortion. attenuation and delay distortion.

To get around the problems associated with DC To get around the problems associated with DC signaling (DCsignaling (DC 信号信号 ), ), especially on telephone lines, AC especially on telephone lines, AC signaling (ACsignaling (AC 信号信号 )) is used. is used.

To convert between DC signaling and AC To convert between DC signaling and AC signalingsignaling ，， a modem (modulator-demodulatora modem (modulator-demodulator ，，调制调制解调器解调器 ) is used. ) is used.

Public Switched Telephone System: Local loop: Dialup modems

((a) A binary signala) A binary signal

(b) Amplitude (b) Amplitude modulationmodulation

((c) Frequency c) Frequency modulationmodulation

(d) Phase (d) Phase modulationmodulation

Public Switched Telephone System: Local loop: Dialup modems To go to higher and higher speeds, it is not possible to To go to higher and higher speeds, it is not possible to

just keep increasing the sampling rate. (The Nyquist just keep increasing the sampling rate. (The Nyquist theorem says that even with a perfect 3000-Hz line, theorem says that even with a perfect 3000-Hz line, there is no point in sampling faster than 6000Hz.) In there is no point in sampling faster than 6000Hz.) In practice, most modems sample 2400 times/sec and practice, most modems sample 2400 times/sec and focus on getting more bits per samplefocus on getting more bits per sample

The number of samples per second is measured in baud. The number of samples per second is measured in baud. During each baud, one symbol is sent. Thus, an During each baud, one symbol is sent. Thus, an nn-baud -baud line transmits line transmits nn symbols/sec. symbols/sec.

The modulation technique determines the number of The modulation technique determines the number of bits/symbol.bits/symbol.


Constellation diagrams (Constellation diagrams ( 星座图星座图 ))

((a) QPSK. (Quadrature Phase Shift Keying)a) QPSK. (Quadrature Phase Shift Keying)

(b) QAM-16. (Quadrature Amplitude Modulation)(b) QAM-16. (Quadrature Amplitude Modulation)

(c) QAM-64.(c) QAM-64.


((a) V.32 for 9600 bps.a) V.32 for 9600 bps.(b) V32 bis for 14,400 bps.(b) V32 bis for 14,400 bps.


V.34 for 28800 bpsV.34 for 28800 bps

V.90 33.6kbps upstream and 56kbps downstreamV.90 33.6kbps upstream and 56kbps downstream

V.92 48 kbps upstream and 56kbps downstreamV.92 48 kbps upstream and 56kbps downstream

Dialup model Dialup model ISDN, ADSL, … ISDN, ADSL, …

Public Switched Telephone System: Local loop: ADSL modems The date rate by various industriesThe date rate by various industries

56 kpbs reached by the telephone industry56 kpbs reached by the telephone industry 10Mbps offered by the cable TV industry10Mbps offered by the cable TV industry 50Mbps planned by the satellite companies50Mbps planned by the satellite companies

Dial-up modem (56kbps) Dial-up modem (56kbps) xDSL (Digital Subscriber xDSL (Digital Subscriber Line)Line) ADSL (Asymmetric DSL)ADSL (Asymmetric DSL)

For xDSL, the bandwidth is wider.For xDSL, the bandwidth is wider.

Public Switched Telephone System: Local loop: ADSL modems

Bandwidth versus distanced over category 3 UTP for DSL.Bandwidth versus distanced over category 3 UTP for DSL.


Design goals for xDSLDesign goals for xDSL The services must work over the existing category 3 The services must work over the existing category 3

twisted pair local loops.twisted pair local loops. They must not affect customer’s existing telephones They must not affect customer’s existing telephones

and fax machines. and fax machines. They must be much faster than 56kbps. They must be much faster than 56kbps. They should be always on, with just a monthly They should be always on, with just a monthly

charge but no per-minute charge.charge but no per-minute charge.


Operation of ADSL using discrete multitone Operation of ADSL using discrete multitone modulation.modulation.

Public Switched Telephone System: Local loop: ADSL modemsA typical ADSL equipment configurationA typical ADSL equipment configuration

NID (Network Interface Device)NID (Network Interface Device) DSLAM: (DSL Access Multiplexor)DSLAM: (DSL Access Multiplexor)

Public Switched Telephone System: Local loop: Wireless local loop

Architecture of an LMDS (Local Multipoint Architecture of an LMDS (Local Multipoint Distribution Service) system.Distribution Service) system.

Public Switched Telephone System: Trunks

Frequency Division MultiplexingFrequency Division Multiplexing （（频分多路复用）频分多路复用）

Wavelength Division MultiplexingWavelength Division Multiplexing （（波分多路复用）波分多路复用）

Time Division MultiplexingTime Division Multiplexing （（时分多路复用）时分多路复用）

SONETSONET （（同步光纤网络）同步光纤网络） //SDHSDH （（同步数字分级结同步数字分级结构）构）

Public Switched Telephone System: Trunks Economies of scale play an important role in the Economies of scale play an important role in the

telephone system. It costs essentially the same amount telephone system. It costs essentially the same amount of money to install and maintain a high-bandwidth of money to install and maintain a high-bandwidth trunk as a low-bandwidth trunk between two switching trunk as a low-bandwidth trunk between two switching offices (i.e. the costs come from having to dig the trench offices (i.e. the costs come from having to dig the trench and not from the copper wire or optical fiber). and not from the copper wire or optical fiber). Consequently, telephone companies have developed Consequently, telephone companies have developed elaborate schemes for multiplexing many conversions elaborate schemes for multiplexing many conversions over a single physical trunk. over a single physical trunk.

There are two basic categories of multiplexing: There are two basic categories of multiplexing: FDM (Frequency Division Multiplexing) FDM (Frequency Division Multiplexing) TDM (Time Division Multiplexing) TDM (Time Division Multiplexing)

Example of FDM and TDM: AM radio broadcasting Example of FDM and TDM: AM radio broadcasting

Public Switched Telephone System: Trunks: FDM

((a) The original bandwidths.a) The original bandwidths.(b) The bandwidths raised in frequency.(b) The bandwidths raised in frequency.(b) The multiplexed channel.(b) The multiplexed channel.

Public Switched Telephone System: Trunks: WDMWavelength division multiplexing.Wavelength division multiplexing.

Passive and reliable (compared with electrical FDM)Passive and reliable (compared with electrical FDM)

Public Switched Telephone System: Trunks: WDM

Wavelength division multiplexing.Wavelength division multiplexing.

Public Switched Telephone System: Trunks: WDM

Date ratesDate rates In 1990, 8 channels x 2.5Gbps per channelIn 1990, 8 channels x 2.5Gbps per channel In 1998, 40 channels x 2.5Gbps per channelIn 1998, 40 channels x 2.5Gbps per channel In 2001, 96 channels x 10 Gbps per channelIn 2001, 96 channels x 10 Gbps per channel The limit is 2500 channels x 10 Gbps per channelThe limit is 2500 channels x 10 Gbps per channel

Optical amplifiersOptical amplifiers Previously, for every 100km, it was necessary to do Previously, for every 100km, it was necessary to do

opto-electrical conversion and amplification.opto-electrical conversion and amplification. All optical amplifiers can regenerate the entire signal All optical amplifiers can regenerate the entire signal

once every 1000 km without the need for multiple once every 1000 km without the need for multiple opto-electrical conversion.opto-electrical conversion.

Public Switched Telephone System: Trunks: TDM

TDM is amenable to being done by a computer. TDM is amenable to being done by a computer. The analog signals are digitized in the end office by a The analog signals are digitized in the end office by a

device called a codec(coder-decoder), producing a 7- or device called a codec(coder-decoder), producing a 7- or 8-bit number. The codec makes 8000 samples per 8-bit number. The codec makes 8000 samples per second (125usec/sample). This technique is called PCM second (125usec/sample). This technique is called PCM (Pulse Code Modulation(Pulse Code Modulation ，，脉冲编码调制脉冲编码调制 ). ).

PCM forms the heart of the modern telephone system. PCM forms the heart of the modern telephone system. As a consequence, virtually all time intervals within the As a consequence, virtually all time intervals within the telephone systems are multiples of 125usec. telephone systems are multiples of 125usec.


The T1 carrier (1.544 Mbps).The T1 carrier (1.544 Mbps).

Public Switched Telephone System: Trunks: TDM TDM compaction methods: The signal changes TDM compaction methods: The signal changes

relatively slowly compared to the sampling frequency. relatively slowly compared to the sampling frequency. Differential pulse code modulation (Differential pulse code modulation ( 差分脉冲编码差分脉冲编码

调制调制 ) ) consists of outputting not the digitized consists of outputting not the digitized amplitude, but the difference between the current amplitude, but the difference between the current value and the previous one. value and the previous one.

Delta modulation(Delta modulation( 增量调制增量调制 ) ) requires each requires each sampled value to differ from its predecessor by sampled value to differ from its predecessor by either +1 or -1. (See the next slide)either +1 or -1. (See the next slide)

Predictive encodingPredictive encoding （（预测编码）预测编码） is to extrapolate is to extrapolate the previous few values to predict the next value and the previous few values to predict the next value and then to encode the difference between the actual then to encode the difference between the actual signal and the predicted one. signal and the predicted one.


Delta modulation.(Delta modulation.( 增量调制增量调制 ))


Multiplexing T1 streams into higher carriers.Multiplexing T1 streams into higher carriers.

Public Switched Telephone System: Trunks: SONET/SDH SONETSONET （（ Synchronous Optical NETwork, Synchronous Optical NETwork, 同步光纤网同步光纤网

络）络） //SDHSDH （（ Synchronous Digital Hierarchy, Synchronous Digital Hierarchy, 同步数字同步数字分级结构）分级结构） goalsgoals To interwork different carriers, to define a common To interwork different carriers, to define a common

signaling standard with respect to wavelength, timing, signaling standard with respect to wavelength, timing, framing structure, and other issues. framing structure, and other issues.

To unify the U.S., European, and Japanese digital To unify the U.S., European, and Japanese digital systems, to combine 64kbps PCM channels in a systems, to combine 64kbps PCM channels in a compatible way. compatible way.

To provide a way to multiplex multiple digital To provide a way to multiplex multiple digital channels together: T1, T2, T3, T4, …channels together: T1, T2, T3, T4, …

To provide support for operations, administration, and To provide support for operations, administration, and maintenance(OAM).maintenance(OAM).

Public Switched Telephone System: Trunks: SONET/SDH SONET/SDH transfer modesSONET/SDH transfer modes

Synchronous transfer mode: An early decision was to make Synchronous transfer mode: An early decision was to make SONET a traditional TDM system, with the entire bandwidth of SONET a traditional TDM system, with the entire bandwidth of the fiber devoted to one channel containing time slots for the the fiber devoted to one channel containing time slots for the various subchannels. As such, SONET is a synchronous system. various subchannels. As such, SONET is a synchronous system. It is controlled by a master clock with an accuracy of about 1 It is controlled by a master clock with an accuracy of about 1 part in 10^9. Bits on a SONET line are sent out at extremely part in 10^9. Bits on a SONET line are sent out at extremely precise intervals, controlled by the master clock. precise intervals, controlled by the master clock.

Asynchronous transfer mode: When cell switching was later Asynchronous transfer mode: When cell switching was later proposed to be the basis of broadband ISDN, the fact that it proposed to be the basis of broadband ISDN, the fact that it permitted irregular cell arrivals got it labeled as asynchronous permitted irregular cell arrivals got it labeled as asynchronous transfer mode(i.e., ATM) to contrast it to the synchronous transfer mode(i.e., ATM) to contrast it to the synchronous operation of SONET. operation of SONET.

Public Switched Telephone System: Trunks: SONET/SDH

A SONET system consists of switches, multiplexers, and A SONET system consists of switches, multiplexers, and repeaters, all connected by fiber. repeaters, all connected by fiber.


Two back-to-back SONET frames.Two back-to-back SONET frames.


SONET and SDH multiplex rates.SONET and SDH multiplex rates.

Public Switched Telephone System: Trunks: Switching

((a) Circuit switching.a) Circuit switching.(b) Packet switching.(b) Packet switching.


((a)Circuit switching (b)Message switching (c)Packet switchinga)Circuit switching (b)Message switching (c)Packet switching


A comparison of circuit switched andA comparison of circuit switched and

packet-switched networks.packet-switched networks.


Switching for PSTNSwitching for PSTN Circuit switchingCircuit switching （（电路交换）电路交换） The switch hierarchy The switch hierarchy SwitchesSwitches

Crossbar switchesCrossbar switches （（交叉点交换机）交叉点交换机） Space divsion switchesSpace divsion switches （（空间分隔交换机）空间分隔交换机） Time division switchesTime division switches （（时分交换机）时分交换机）

THE MOBILE TELEPHONE SYSTEM移动电话系统 First-generation Mobile Phones: First-generation Mobile Phones:

Analog VoiceAnalog Voice

Second-generation Mobile Phones: Second-generation Mobile Phones: Digital VoiceDigital Voice

Third-generation Mobile phones:Third-generation Mobile phones:digital voice and datadigital voice and data

The Mobile Telephone System In USA, In USA, 1G (success) 1G (success) 2G (fiasco) 2G (fiasco) In Europe, 1G (fiasco) In Europe, 1G (fiasco) 2G (success) 2G (success) The reasonThe reason

Free competition or government interferenceFree competition or government interference Phone numbersPhone numbers

Mobile phone numbers are the same as fixed Mobile phone numbers are the same as fixed phone numbersphone numbers

Different for GSMDifferent for GSM Marketing decisionsMarketing decisions

Prepaid mobile phonesPrepaid mobile phones

The Mobile Telephone System: 1G During the early decades of the 20th century, mobile During the early decades of the 20th century, mobile

radiotelephones were used sporadically for maritime radiotelephones were used sporadically for maritime and military communicationand military communication

In 1946, the first system car-based telephones was In 1946, the first system car-based telephones was setup. Push-to-talk systems (single frequency).setup. Push-to-talk systems (single frequency).

In the 1960s, IMTS (Improved Mobile Telephone In the 1960s, IMTS (Improved Mobile Telephone System). Two frequencies. System). Two frequencies. Due to the large power of the hilltop transmitter, Due to the large power of the hilltop transmitter,

adjacent systems had to be several hundred adjacent systems had to be several hundred kilometers apart to avoid interference.kilometers apart to avoid interference.

In 1982, AMPS (Advanced Mobile Phone System) by In 1982, AMPS (Advanced Mobile Phone System) by AT&T.AT&T.

The Mobile Telephone System: 1G

((a) Frequencies are not reused in adjacent cells.a) Frequencies are not reused in adjacent cells.

(b) To add more users, smaller cells can be used.(b) To add more users, smaller cells can be used.

The Mobile Telephone System: 1G The structureThe structure

Mobile phones or cell phonesMobile phones or cell phones Base stationBase station MSC (Mobile switching center) or MTSO (Mobile MSC (Mobile switching center) or MTSO (Mobile

Telephone Switching Office)Telephone Switching Office) The 832 channels are divided into four categories The 832 channels are divided into four categories

(transmission channels from 824 to 849 MHz and (transmission channels from 824 to 849 MHz and receiving channels from 869 to 894MHz):receiving channels from 869 to 894MHz): Control (base to mobile) to manage the systemControl (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for themPaging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel Access (bidirectional) for call setup and channel

assignmentassignment Data (bidirectional) for voice, fax, or data. Data (bidirectional) for voice, fax, or data.

The Mobile Telephone System: 1G RegisteringRegistering

When a phone is switched on, it scans a When a phone is switched on, it scans a preprogrammed list of 21 control channels to find the preprogrammed list of 21 control channels to find the most powerful signalmost powerful signal

Then the phone broadcasts its 32-bit serial number Then the phone broadcasts its 32-bit serial number and 34-bit telephone numberand 34-bit telephone number

When the base station hears the announcement, it When the base station hears the announcement, it tells the MSC, which records the existence of its new tells the MSC, which records the existence of its new customer and also informs the customer’s home customer and also informs the customer’s home MSC of his current location.MSC of his current location.

During normal operation, the mobile telephone During normal operation, the mobile telephone registers about once every 15 minutes. registers about once every 15 minutes.

The Mobile Telephone System: 1G How to make a callHow to make a call

A mobile phone transmits the number to be called A mobile phone transmits the number to be called and its own identity on the access channel. If a and its own identity on the access channel. If a collision occurs there, it tries again.collision occurs there, it tries again.

When the base station gets the request, it informs the When the base station gets the request, it informs the MSC. MSC.

The MSC looks for an idle channel for the call and The MSC looks for an idle channel for the call and then send the found channel number back on the then send the found channel number back on the control channel. control channel.

The mobile phone then automatically switches on to The mobile phone then automatically switches on to the selected voice channel and waits until the called the selected voice channel and waits until the called party picks up the phone. party picks up the phone.

The Mobile Telephone System: 1G How to handle incoming callsHow to handle incoming calls

When a call is placed to a mobile phone (either from When a call is placed to a mobile phone (either from a fixed phone or another mobile phone), a packet is a fixed phone or another mobile phone), a packet is sent to the callee’s home MSC to find out where it is. sent to the callee’s home MSC to find out where it is.

Then a packet is sent to the base station in ins current Then a packet is sent to the base station in ins current cell.cell.

The base station (on the paging channel): “Unit 14, The base station (on the paging channel): “Unit 14, are you there?”are you there?”

The called phone : “Yes” on the access channel. The called phone : “Yes” on the access channel. Then the base station: “Unit 14, call for you on Then the base station: “Unit 14, call for you on

channel 3.”channel 3.” The called phone then switches to channel 3 and The called phone then switches to channel 3 and

starts making ringing sounds. starts making ringing sounds.

The Mobile Telephone System: 2G D-AMPS: Digital Advanced Mobile Phone SystemD-AMPS: Digital Advanced Mobile Phone System

USUS GSM: Global System for Mobile CommunicationsGSM: Global System for Mobile Communications

Europe, ChinaEurope, China CDMA: Code Division Multiple AccessCDMA: Code Division Multiple Access

US, ChinaUS, China

The Mobile Telephone System: 2G: D-AMPS D-AMPSD-AMPS

Fully digital, Fully digital, To coexist with AMPSTo coexist with AMPS Two frequency bands: 850MHz and 1900MHz bandsTwo frequency bands: 850MHz and 1900MHz bands Improved handoff (MAHO: Mobile Assisted Improved handoff (MAHO: Mobile Assisted

HandOff)HandOff) Better encoding scheme than delta modulation and Better encoding scheme than delta modulation and

predictive encoding. Done by a circuit called vocoder predictive encoding. Done by a circuit called vocoder (( 声音合成机声音合成机 ) inside a mobile phone. ) inside a mobile phone.

The Mobile Telephone System: 2G: D-AMPS

((a) A D-AMPS channel with three users.a) A D-AMPS channel with three users.(b) A D-AMPS channel with six users.(b) A D-AMPS channel with six users.

The Mobile Telephone System: 2G: GSM

GSM uses 124 frequency channels, each of which uses GSM uses 124 frequency channels, each of which uses an eight-slot TDM systeman eight-slot TDM system

The Mobile Telephone System: 2G: GSM

A portion of the GSM framing structure.A portion of the GSM framing structure.

The Mobile Telephone System: 2G: CDMA An analogy: an airport lounge with many pairs of An analogy: an airport lounge with many pairs of

people conversing. people conversing. TDM is comparable to all the people being in the TDM is comparable to all the people being in the

middle of the room but taking turns speaking. middle of the room but taking turns speaking. FDM is comparable to the people being in widely FDM is comparable to the people being in widely

separated clumps, each clump holding its own separated clumps, each clump holding its own conversation at the same time as, but still conversation at the same time as, but still independent of, the others. independent of, the others.

CDMA is comparable to every body being in the CDMA is comparable to every body being in the middle of the room talking at once, but with each middle of the room talking at once, but with each pair in a different language. pair in a different language.

The Mobile Telephone System: 2G: CDMA In CDMA, each bit time is subdivided into m short In CDMA, each bit time is subdivided into m short

intervals called chips. intervals called chips. Each station has a chip sequence.Each station has a chip sequence.

To transmit a 1 bit, a station sends its chip sequence. To transmit a 1 bit, a station sends its chip sequence. (00011011)(00011011)

To transmit a 0 bit , it sends the one’s complement of To transmit a 0 bit , it sends the one’s complement of its chip squence. (11100100)its chip squence. (11100100)

For pedagogical purpose, it’s more convenient to use a For pedagogical purpose, it’s more convenient to use a bipolar notation with binary 0 being –1 and binary 1 bipolar notation with binary 0 being –1 and binary 1 being +1.being +1.

All chip sequences are pairwise orthogonal.All chip sequences are pairwise orthogonal.

The Mobile Telephone System: 2G: CDMA((a) Binary chip sequences for four stationsa) Binary chip sequences for four stations(b) Bipolar chip sequences (b) Bipolar chip sequences (c) Six examples of transmissions(c) Six examples of transmissions(d) Recovery of station C’s signal(d) Recovery of station C’s signal

The Mobile Telephone System: 3G Basic services an IMT-2000 network should provideBasic services an IMT-2000 network should provide

High-quality voice transmissionHigh-quality voice transmission Messaging (replace e-mail, fax, SMS, chat, etc.)Messaging (replace e-mail, fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.)Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.)Internet access (web surfing, w/multimedia.)

W-CDMA (Wideband CDMA) (Ericsson)W-CDMA (Wideband CDMA) (Ericsson) CDMA2000 (Qualcomm)CDMA2000 (Qualcomm) Other variationsOther variations

2.5G: EDGE (Enhanced Data rates for GSM Evolution), 2.5G: EDGE (Enhanced Data rates for GSM Evolution), GPRS (General Packet Radio Service)GPRS (General Packet Radio Service)

4G4G 802.11, 802.16802.11, 802.16

CABLE TELEVISION ( 有线电视 )

Community antenna televisionCommunity antenna television

Internet over cableInternet over cable

Spectrum allocationSpectrum allocation

Cable modemsCable modems

ADSL versus cableADSL versus cable

Cable televisionAn early cable television system.An early cable television system.

A big antenna on top of a hill to pluck the television A big antenna on top of a hill to pluck the television signal out of the airsignal out of the air

An amplifier, called the headend (An amplifier, called the headend ( 数据转发器数据转发器 ), to ), to strengthen it, strengthen it,

And a coaxial cable to deliver it to people’s houses. And a coaxial cable to deliver it to people’s houses.

Cable Television: Internet over Cable Cable television (HFC, Hybrid Fiber Coax)Cable television (HFC, Hybrid Fiber Coax)

Optical fibers, fiber nodesOptical fibers, fiber nodes

Cable Television: Internet over Cable

The fixed telephone system.The fixed telephone system.

Cable Television: Internet over Cable HFC and PSTNHFC and PSTN

Local loopLocal loopCable shared by many housesCable shared by many housesEvery house has its own private loop.Every house has its own private loop.

BandwidthBandwidthThe bandwidth of coax is much higher than that of The bandwidth of coax is much higher than that of

twisted pairs.twisted pairs. The cable industry splits up long cables and connect The cable industry splits up long cables and connect

each directly to a fiber node. each directly to a fiber node.

Cable Television: Spectrum allocation

Frequency allocation in a typical cable TV system Frequency allocation in a typical cable TV system used for Internet accessused for Internet access

Cable Television: Cable modems

Typical details of the upstream and downstream Typical details of the upstream and downstream channels in North America.channels in North America.

Cable Television: ADSL versus Cable Both use fiber in the backbone, but they differ on the Both use fiber in the backbone, but they differ on the

edge. edge. Effective capacity. Effective capacity. Cable is more unpredictable.Cable is more unpredictable. AvailabilityAvailability SecuritySecurity ADSL and cable are much more alike than they are ADSL and cable are much more alike than they are

different. different.

Homework

2.2.44 2.62.6 2.92.9 2.202.20 2.222.22 2.262.26 2.292.29 2.332.33 2.392.39

chapter 2 the physical layer ( 物理层)

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