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CHAPTER 2 THE PHYSICAL LAYERCHAPTER 2 THE PHYSICAL LAYER (物理层)

• The Theoretical Basis for Data Communication

• Guided Transmission Media (Terrestrial 大地的)• Wireless Transmission (Terrestrial 大地的)• Communication Satellites (Celestial 天体的) *

• Digital Modulation and Multiplexing

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

CHAPTER 2 THE PHYSICAL LAYERCHAPTER 2 THE PHYSICAL LAYER (物理层)

• The Theoretical Basis for Data Communication

• Guided Transmission Media (Terrestrial 大地的)• Wireless Transmission (Terrestrial 大地的)

• Digital Modulation and Multiplexing

• The Public Switched Telephone Network (Stationary)• Communication Satellites (Celestial 天体的) *• The Mobile Telephone System (Mobile) *• Cable Television (Stationary)*

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

• Fourier analysis (傅立叶分析)y (傅叶分析)

d id h li i d i l (有限带宽信号)• Bandwidth-limited signals (有限带宽信号)

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

The Theoretical Basis For Data Communication: F i A l iCommunication: Fourier Analysis

• We model the variation of voltage or current with th ti l f timathematical function

• Any reasonably behaved periodic function g(t) with period T can be constructed as the sum of a (possiblyperiod T, can be constructed as the sum of a (possibly infinite) number of sines and cosines

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

The Theoretical Basis For Data CommunicationThe Theoretical Basis For Data Communication• Bandwidth-Limited Signals

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

• The range of frequencies transmitted without beingThe range of frequencies transmitted without being strongly attenuated

• The cutoff frequency is not really sharp, is defined q y y pas the frequency at which half the power gets through.

A i l (b d idth li it d li it d) (i t)– Any signal (bandwidth-limited or unlimited) (input) system (filtering) b d idth li it d i l ( t t) bandwidth-limited signal (output)

(in time domain, convolution; I f d i lti li ti )In frequency domain, multiplication)


– A binary signal and its root-mean-square Fourier amplitudes (These squares are proportional t th t itt d t th dito the energy transmitted at the corresponding frequency)


– Successive approximations to the original signal.


– Given a bit rate b bits/sec, the time to send 1 bit is 1/b, t e t e to se d b t s /b,the time required to send 8 bits 1 bit at a time is 8/bsecsec, the frequency of the first harmonic (一次谐波) is b/8.

di l h li ( f ll d i d– An ordinary telephone line (often called as a voice-grade line (话音级线路)) has an artificially introduced cutoff f hi i i h hfrequency near 3000Hz. This restriction means that the number of the highest harmonic passed through is 3000/(b/8) 2 000/b3000/(b/8) or 24000/b.

The Theoretical Basis For Data CommunicationThe Theoretical Basis For Data Communication• Bandwidth-Limited Signals: Relation between data rate

d h i f h i d li (3000H )and harmonics for the voice-grade line (3000Hz)

Conclusion: the data rate is limited by theConclusion: the data rate is limited by theConclusion: 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

h d f h l• The max data rate of a channel– In 1924, Henry Nyquist derived an equation

expressing the max data rate for a finite bandwidth noiseless channel:M d t t 2H l V bit /Max data rate = 2H log 2 V bits/secwhere H is the bandwith, V is the discrete levelsI 1943 Cl d Sh t d it t h l ith– In 1943, Claude Shannon extend it to channel with random noiseMax data rate = B log (1+S/N) bits/secMax data rate = B log 2 (1+S/N) bits/secwhere S/N is the signal to noise ratio

GUIDED TRANSMISSION MEDIAGUIDED TRANSMISSION MEDIA(有线数据传输)

M ti di (磁介质)• Magnetic media (磁介质)• Twisted pair (双绞线)• Coaxial cable (同轴电缆)• Power Lines (电力线)owe es (电力线)• Fiber optics (光纤)

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

t t th icomputer to another is – To write them onto magnetic tape or removable media,

To transport the tape or disks to the destination machine and– To transport the tape or disks to the destination machine, and – To read them back in again.

• Bandwidth:• Bandwidth: 800GB/tape * 1000 tapes/box / (24*60*60s) = 70+Gbps

C• Cost: (800GB/tape * 1000 tapes/box) / 5000$ = 160GB/$

• Conclusion:Never underestimate the bandwidth of station wagon full

of tapes hurtling down the highway.• Why computer network?

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

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

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

Guided Transmission Media: Twisted pairGuided Transmission Media: Twisted pair

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

Guided Transmission Media: Twisted pairGuided Transmission Media: Twisted pair• Twisted pairs can be used for transmitting either analog

or digital signals.• Full-Duplex, Half-Duplex, Simplex• Types

– Category 5 (100Mbps 1Gbps)Category 5 (100Mbps, 1Gbps)– Category 6 (10Gbps)

( hi ld d i d i )– Category 7 (Shielded Twisted Pair)

Guided Transmission Media: Coaxial cableA i l bl i f• A coaxial cable consists of – a stiff copper wire as the core,

surrounded by an insulating material– surrounded by an insulating material. – The insulator is encased by a cylindrical conductor,

often as a closely-woven breaded mesh. y– The outer conductor is covered in a protective plastic

sheath.

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

Guided Transmission Media: Power LinesGuided Transmission Media: Power Lines• Power lines have been used by electricity companies for

low-rate communication such as remote metering for many years, as well in the home to control devices.

• In recent years, there has been renewed interest in high-rate communication over these lines, both inside the home as a LAN and outside the home for broadband Internet access.

Guided Transmission Media: Power LinesGuided Transmission Media: Power Lines• Now, many products use various proprietary standards

for power-line networking, so international standards are actively under development.

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

communication woncommunication won– Original IBM PC at 4.77 MHz in 1981 four-core

CPU at 3GHz (16 per decade) and physical limitsC U at 3G ( 6 pe decade) a d p ys ca ts– 45Mpbs (T3 line) 100Gbps (optical communication) 16 per decade and error rate almost zero and almost no limits

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

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

Guided Transmission Media: Fiber opticsGuided Transmission Media: Fiber optics(a) Three examples of a light ray from inside a silica fiber

impinging on the air/silica boundary at different anglesimpinging on the air/silica boundary at different angles.(b) Light trapped by total internal reflection (反射) (no

refraction (折射) )refraction (折射) ).

Two types of fiber opticsMultimode (多模): many different raysMultimode (多模): many different rays Unimode (单模): single rays, longer distance

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

– Attenuation (衰减) of light through fiber in the infrared (衰减) g gregion.

– Three wavelength bands are used for optical communication. g pThey are centered at 0.85, 1.30, and 1.55 micros, respectively.

Guided Transmission Media: Fiber opticsGuided Transmission Media: Fiber optics

• Fiber cables– Fiber optic cables are similar to coax, except without

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

Guided Transmission Media: Fiber opticsGuided Transmission Media: Fiber optics• Fiber cables

D l t– Deployment• Laid in the ground within a meter of the surface• B i d i t h th h• Buried in trenches near the shore• Lie on the bottom in deep water

C ti– Connection• Fiber sockets• Spliced mechanicall• Spliced mechanically• Fusion splice

Detection– Detection• Photodiode. The response time is 1ns, which

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

Guided Transmission Media: Fiber opticsGuided Transmission Media: Fiber optics• Fiber cables

– Light sources• LED • Semiconductor laser

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

– Advantages• Higher bandwidths and Low attenuation.• Not being affected by power surges, electromagnetic

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

– DisadvantagesDisadvantages• Less familiar technology.• Fiber interfaces more expensive• Fiber interfaces more expensive.

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

WIRELESS TRANSMISSION (无线传输)

• The electromagnetic spectrum (电磁波谱)

• Radio transmission (无线电传输)( )

• Microwave transmission (微波传输)Microwave transmission (微波传输)

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

Li ht t i i (光波传输)• Lightwave transmission (光波传输)

Wireless Transmission: The Electromagnetic SpectrumThe Electromagnetic Spectrum

The electromagnetic spectrum and its uses for communication.

Wireless Transmission:Wireless Transmission: The Electromagnetic Spectrum

LF(L ) MF(M di ) HF(Hi h)• LF(Low), MF(Medium), HF(High), VHF(Very), UHF(Ultra), SHF(Super), EHT(Extremely) THF(Tremendously)EHT(Extremely), THF(Tremendously), (IHF(Incredibly), AHF(Astonishingly), PHF(Prodigiously))PHF(Prodigiously))

• 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. • Most transmissions use a narrow frequency band (GSM)• Most transmissions use a narrow frequency band. (GSM)

Wireless Transmission: The Electromagnetic Spectrum

• Some spread its frequency over a wide frequency bandSome spread its frequency over a wide frequency band(spread spectrum, 散布频谱). – Frequency hopping spread spectrum (military, 802.11,Frequency hopping spread spectrum (military, 802.11,

Bluetooth)– Direct sequence spread spectrum (2G mobile phones)Direct sequence spread spectrum (2G mobile phones)– UWB (UltraWideBand)

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

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

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

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

Wireless Transmission: Radio transmissionWireless Transmission: Radio transmission

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

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

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

th f b l f dcan therefore be narrowly focused. • The higher the transmission towers are, the further apart

th b F 100 hi h t t bthey can be. For 100-m high towers, repeaters can be spaced 80km apart.

• Mi lti th f di b b b d• Microwave can cause multipath fading or be absorbed by rain.

• Micro a e comm nication is so idel sed for long• Microwave communication is so widely used for long-distance telephone communication, cellular telephones, television distribution and other uses that a severetelevision distribution, and other uses, that a severe shortage of spectrum has developed.

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

Wireless Transmission: Microwave transmissionWireless Transmission: Microwave transmission• Who allocates frequencies?

– ITU: recommendation– 无线电管理委员会 (CN)无线电管理委员会 (CN)– FCC (Federal Communication Commission) (US)

• Wh t f i ?• Who gets frequencies?– Beauty contest– Holding a lottery among the interested companies– Auctioning off the band width to the highest bidderg g

Wireless Transmission: Microwave transmission• Free frequencies:

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

– ISM and U-NII (Unlicensed National Information Infrastructure) bands used in the United States byInfrastructure) bands used in the United States by wireless devices.

Wireless Transmission:Wireless Transmission: Infrared and millimeter waves

• Infrared and millimeter waves can not pass through solid objectssolid objects.

• Infrared can be used for indoor wireless LANs, remote controllerscontrollers.

• Infrared can not be used outdoors for the sun shines as b i h l i h i f d i h i iblbrightly in the infrared as in the visible spectrum.

• Advantages– The infrared controller cannot control the TV of your

neighbor. g– More secure.

Wireless Transmission: Lightwave transmissionConvection currents (气流) can interfere with laser commConvection currents (气流) can interfere with laser commsystems. A bidirectional system with two lasers is pictured hereA bidirectional system with two lasers is pictured here.

DIGITAL MODULATION AND MULTIPLEXINGDIGITAL MODULATION AND MULTIPLEXING

B b d T i i (基带传输)• Baseband Transmission (基带传输)• Passband Transmission (通带传输)• Frequency Division Multiplexing (频分复用)• Time Division Multiplexing (时分复用)Time Division Multiplexing (时分复用)• Code Division Multiplexing (码分复用)

Di it l M d l ti d M lti l iDigital Modulation and Multiplexing

• To send digital information we must devise analog signals to represent bits.

• The process of converting between bits and signals that represent them is called Digital Modulation (数字调g制).

• Channels are often shared by multiple signals. Thi ki d f h i i ll d M lti l i (复用)• This kind of sharing is called Multiplexing (复用)

Digital Modulation and Multiplexing:Baseband TransmissionBaseband Transmission

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Baseband Transmission

• Clock Recovery– For all schemes that encode bits into symbols, the y

receiver must know when one symbol ends and the next symbol begins to correctly decode the bits.

– With NRZ (Non-Return-to-Zero,不归零制), in which the symbols are simply voltage levels, a long run ofthe symbols are simply voltage levels, a long run of 0s or 1s leaves the signal unchanged. After a while it is hard to tell the bits apart, as 15 zeros look much s a d to te t e b ts apa t, as 5 e os oo uclike 16 zeros unless you have a very accurate clock.

– Accurate clocks?Accurate clocks?


Cl k R• Clock Recovery– One strategy is to send a separate clock signal to the

ireceiver. – A clever trick is to mix the clock signal with the data

i l b XORi h h h lisignal by XORing them together so that no extra line is needed. Manchester EncodingA i h i h di i i lif h– As a step in the right direction, we can simplify the situation by encoding a 1 as a transition and a 0 as no transition or vice versa NRZI (Non Return to Zerotransition, or vice versa. NRZI (Non-Return-to-Zero Inverted,不归零倒相制) (Used in USB)4B/5B– 4B/5B


• Clock RecoveryClock Recovery– 4B/5B: The five bit patterns are chosen so that there

will never be a run of more than three consecutive 0swill never be a run of more than three consecutive 0s.


• Bandwidth Efficiency– Data rate = symbol rate * bits per symbolData rate symbol rate bits per symbol– Symbol rate == baud rate

• Balanced Signals– Capacitive couplingCapacitive coupling– Bipolar encoding

Digital Modulation and Multiplexing:Passband Transmission

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Passband Transmission

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Frequency Division Multiplexing

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Frequency Division Multiplexing

• Orthogonal frequency division multiplexing (OFDM).

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Time Division Multiplexing

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Code Division Multiplexing

(a) Chip sequences for four stations. (b) Signals the sequences represent( ) g

Digital Modulation and Multiplexing:Digital Modulation and Multiplexing:Code Division Multiplexing

(c) Six examples of transmissions. (d) Recovery of station C’s( ) y

PUBLIC SWITCHED TELEPHONE SYSTEMPUBLIC SWITCHED TELEPHONE SYSTEM公用交换电话系统

• Structure of the Telephone System

• The Politics of Telephones

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

• Trunks and Multiplexing

• Switching

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

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

– For a large number of computers or a wide area, or lacking right of way, LAN can not be used and we have t l th i ti i ti f iliti hto rely upon the existing communication facilities such as PSTN.

• The PSTN is s itable for transmitting the h man oice in• The PSTN is suitable for transmitting the human voice in 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 changing with the introduction of fiber optics and digitalchanging with the introduction of fiber optics and digital technology.

Public Switched Telephone SystemPublic Switched Telephone System• How hopeless is the PSTN?

A bl i b t t t t f d t– A cable running between two computers can transfer data at 109 bps or more. A dial-up line has maximum date dateof 56 kbps, a difference of a factor of almost 20,000. o 56 bps, a d e e ce o a acto o a ost 0,000.That is the difference between a duck waddling leisurely though the grass and a rocket to the moon.

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

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

Public Switched Telephone System: StructurePublic Switched Telephone System: Structure• In 1876, Alexander Graham Bell patented the telephone , p p

(just a few hours ahead of his rival, Elisha Gray). – The initial market was for the sale of telephones, p ,

which came in pairs.– If a telephone owner wanted to talk to n other p

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

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

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

Public Switched Telephone System: StructureT t it hi ffi t th it hi• To connect every switching office to every other switching office by means of a wire between them quickly became unmanageable, so second-level switching offices wereunmanageable, so second level switching offices were invented. (See the next slide)

• After a while, multiple second-level offices were needed. p• Eventually, the hierarchy grew to five levels: end office (本地局), toll office (长途局), primary office (初级局),

ti l ffi (地区局) d i l ffi (区域局)sectional office (地区局), and regional office (区域局).• By 1890, the three major parts of the telephone system

were in places:were in places: – Local loops: the wires between the customers and the

switching offices,switching offices, – Trunks: the long-distance connections between the

switching offices, – Switching offices.

Public Switched Telephone System: StructurePublic Switched Telephone System: Structure

(a) Fully-interconnected network.(b) Centralized switch(b) Centralized switch.(c) Two-level hierarchy => … => Five-level hierarchy .

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

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

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

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

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

Public Switched Telephone System: StructurePublic Switched Telephone System: Structure• If a subscriber (用户) attached to a given end office calls

th b ib tt h d t th d ffi thanother subscriber attached to the same end office, the switching mechanism within the office sets up a direct electrical connection between the two local loopselectrical connection between the two local loops.

• The call between the subscribers attached to different end offices via toll primary sectional and regional officesoffices via toll, primary, sectional, and regional offices.

A typical circuit route for a medium-distance call

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

t l h ltelephone monopoly.• On January 1, 1984, AT&T was broken up into AT&T Long

li 23 BOC (B ll O ti C i ) d flines, 23 BOCs (Bell Operating Companies), and a few other pieces.

• LATA Th U it d St t di id d i t 164 LATA• LATA: The United States was divided up into 164 LATA (Local Access and Transport Areas, 本地访问和传输区域).

• LEC: Within a LATA there as one LEC (Local E change• LEC: Within a LATA, there was one LEC (Local Exchange Carrier, 市话电信局) that had a monopoly on traditional telephone service within its areatelephone service within its area.

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

Public Switched Telephone System: PoliticsPublic Switched Telephone System: Politics• LATA (Local Access and Transport Area, 本地访问和传输区域)传输区域)

• LEC (Local Exchange Carrier, 本地交换电信公司)外部交换电信公司• IXC (IntereXchange Carrier, 外部交换电信公司)

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

J d t) th IXC (长话公司) f bidd t ffJudgment), the IXCs (长话公司) were forbidden to offer local telephone service and the LECs (市话公司) were forbidden to offer inter-LATA telephone serviceforbidden to offer inter-LATA telephone service.

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

• By 1995, Congress saw that trying to maintain a distinction between the various kinds of companies was pno longer tenable and drafted a bill to allow cable TV companies, local telephone companies, long-distance carriers, and mobile operators to enter one another’s business.

Public Switched Telephone System: Local loopPublic Switched Telephone System: Local loopThe use of both analog and digital transmissions for a

computer to computer call Conversion is done by thecomputer to computer call. Conversion is done by the modems (modulator demodulator, 调制解调器) and codecs (coder decoder, 编码解码器）.

Public Switched Telephone System: Local loopA S A• A computer PSTN A computer– The data must be converted to analog form for

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

analog formanalog form.• Transmission lines suffer from three major problems

A i i h l f h h i l– Attenuation is the loss of the energy as the signal propagates outward.

– Delay distortion is caused by the fact that different Fourier components travel at different speeds.

– Noise is unwanted energy from sources other than the transmitter.

Public Switched Telephone System: Local loop: Dialup modems

• Both attenuation and propagation speed are frequencyBoth attenuation and propagation speed are frequency dependent, it is undesirable to have a wide range of frequencies in the signal Unfortunately digital signalsfrequencies in the signal. Unfortunately, digital signals have a wide spectrum and thus are subject to strong attenuation and delay distortionattenuation and delay distortion.

• To get around the problems associated with DC signaling (DC信号) especially on telephone lines ACsignaling (DC信号), especially on telephone lines, AC signaling (AC信号) is used. T t b t DC i li d AC i li• To convert between DC signaling and AC signaling，amodem (modulator-demodulator, 调制解调器) is used.


• To go to higher and higher speeds it is not possible toTo go to higher and higher speeds, it is not possible to just keep increasing the sampling rate. (The Nyquisttheorem says that even with a perfect 3000-Hz line theretheorem says that even with a perfect 3000 Hz line, there is no point in sampling faster than 6000Hz.) In practice, most modems sample 2400 symbols/sec and focus onmost modems sample 2400 symbols/sec and focus on getting more bits per symbol.

• The number of samples per second is measured in• The number of samples per second is measured in symbol/sec or baud. Thus, an n-baud line transmits nsymbols/sec Symbol rate is preferred than baudsymbols/sec. Symbol rate is preferred than baud.

• The modulation technique determines the number of bit / b lbits/symbol.

• Data rate = symbol rate * bits/symbol


Constellation diagrams (星座图)g ( )(a) QPSK. (Quadrature Phase Shift Keying)(b) QAM 16 (Quadrature Amplitude Modulation)(b) QAM-16. (Quadrature Amplitude Modulation)(c) QAM-64.


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


V.34 for 28800 bps

V.90 33.6kbps upstream and 56kbps downstream

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

Di l d l ISDN ADSLDialup model ISDN, ADSL, …

Public Switched Telephone System: Local loop: ADSL modems

• The data rate by various industriesThe data rate by various industries– 56 kpbs reached by the telephone industry

10Mb ff d b h bl TV i d– 10Mbps offered by the cable TV industry– 50Mbps planned by the satellite companies

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

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


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


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

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

d f hiand fax machines. – They must be much faster than 56kbps. – They should be always on, with just a monthly

charge but no per-minute charge. g p g


Operation of ADSL using discrete multitoned l timodulation.

Public Switched Telephone System: Local loop: ADSL modemsmodems

A typical ADSL equipment configuration– NID (Network Interface Device)NID (Network Interface Device)– DSLAM: (DSL Access Multiplexor)

Public Switched Telephone System: TrunksPublic Switched Telephone System: Trunks

• Time Division Multiplexing (时分多路复用)

• SONET (同步光纤网络)/SDH(同步数字分级结构)

• Frequency Division Multiplexing (频分多路复用)

• Wavelength Division Multiplexing (波分多路复用)g p g (波分多路复用)

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

t l h t It t ti ll th ttelephone system. It costs essentially the same amount of money to install and maintain a high-bandwidth trunk as a low-bandwidth trunk between two switching officesas a low-bandwidth trunk between two switching offices. Telephone companies have developed elaborate schemes for multiplexing many conversions over a p g ysingle physical trunk.

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

• Example of TDM and FDM: AM radio broadcasting

Public Switched Telephone System: Trunks: TDMPublic Switched Telephone System: Trunks: TDM• TDM is amenable to being done by a computer. • The analog signals are digitized in the end office by a

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

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

Public Switched Telephone System: Trunks: TDMPublic Switched Telephone System: Trunks: TDM

The T1 carrier (1.544 Mbps).

Public Switched Telephone System: Trunks: TDMPublic Switched Telephone System: Trunks: TDM

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

Public Switched Telephone System: Trunks: p ySONET/SDH

• SONET (Synchronous Optical NETwork 同步光纤网SONET (Synchronous Optical NETwork, 同步光纤网络)/SDH (Synchronous Digital Hierarchy, 同步数字分级结构) goals– To interwork different carriers, to define a common

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

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

– To provide a way to multiplex multiple digital p y p p gchannels together: T1, T2, T3, T4, …

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

Public Switched Telephone System: Trunks: SONET/SDHSONET/SDH

Two back-to-back SONET frames.

Public Switched Telephone System: Trunks: p ySONET/SDH

SONET and SDH multiplex rates.

Public Switched Telephone System: Trunks: WDMPublic Switched Telephone System: Trunks: WDM

Wavelength division multiplexingWavelength division multiplexing.

Public Switched Telephone System: Trunks: WDM• Data rates

– In 1990, 8 channels x 2.5Gbps per channel, p p– In 1998, 40 channels x 2.5Gbps per channel– In 2001 96 channels x 10 Gbps per channel– In 2001, 96 channels x 10 Gbps per channel– In 2006, 192 channels x 10 Gbps per channel

Th li it i 2500 h l 10 Gb h l– The limit is 2500 channels x 10 Gbps per channel• Optical amplifiers

– Previously, for every 100km, it was necessary to do opto-electrical conversion and amplification.

– All optical amplifiers can regenerate the entire signal once every 1000 km without the need for multiple opto-electrical conversion.

– And optical processing devices!

Public Switched Telephone System: Trunks: Switching(a) Circuit switching.(b) Packet switching.

Public Switched Telephone System: Trunks: Switching

(a)Circuit switching (b)Packet switching

Public Switched Telephone System: Trunks: SwitchingA i f i it it h d dA comparison of circuit switched and

packet-switched networks.

COMMUNICATION SATELLITES*COMMUNICATION SATELLITES(通讯卫星*)

• Geostationary Satellites (地球同步卫星)y ( )

• Medium Earth Orbit Satellites (中轨道卫星)• Medium-Earth Orbit Satellites (中轨道卫星)

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

• Satellites versus Fiber

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

i ti t b b i i l ff t lli dcommunication systems by bouncing signals off metallizedweather balloons.

• Th U S N b ilt ti l t f hi t h• The U.S. Navy built an operational system for ship-to-shore communication by bouncing signals off the moon.

• Comm nication satellite is the real celestial• Communication satellite is the real celestial communication player.

A big micro a e repeater– A big microwave repeater– Many transponders (异频收发机)h hi h h lli h l h i d• The higher the satellite, the longer the period.

• There are two Van Allen belts, layers of highly charged i l d b h h’ i fi ldparticles trapped by the earth’s magnetic field.

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

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

h ld b l d 2 d i th 360 dshould be placed 2 degrees or so in the 360 degree equatorial plane.

• P i i l t llit b d• Principal satellite bands: – C – L,S– Ku, Ka

Communication Satellites: Geostationary satellitesCommunication Satellites: Geostationary satellites• Wide beam and spot beam:

– Wide beam: covers large area (as large as 1/3 of the earth’s surface)

– Spot beam: covers small area (a few hundred km in diameter))

• Satellites has many transponders.E er transponder can se– Every transponder can use • FDM (the bandwidth is simply split up into fixed

f b d )frequency bands) or • TDM (divided into time slots) techniques.

Communication Satellites: Geostationary satellitesCommunication Satellites: Geostationary satellites• VSAT (Very Small Aperture Terminals)

– Many small VSATs Satellite– Many small VSATs VSAT hub satellitea y s a VS s VS ub sate te

Communication Satellites: Geostationary satellitesCommunication Satellites: Geostationary satellites• Discussion of properties

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

– Inherently broadcast • Good from broadcasting sports• Security is an issue.

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

– Satellites also have excellent error rates.– Satellites can be deployed almost instantly, a major p y y j

consideration for military communication.

Communication Satellites:Communication Satellites: Medium-earth orbit satellites

• MEO satellitesLower than GEOs Between the two Van Allen belts– Lower than GEOs. Between the two Van Allen belts

– Smaller footprint on the ground. – Requires less powerful transmitters to reach them.– Examples: 24 GPS satellites.p– They are not used for telecommunication.

Communication Satellites:Communication Satellites: Low-earth orbit satellites

• LEO satellitesLower than MEOs– Lower than MEOs.

– Much smaller footprint on the ground – Requires far less powerful transmitters to reach them – The round-trip delay is only a few millisecondsp y y– Used for voice communication and Internet services.


• Iridium project (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 (7766)(7766).

– In 1997 Motorola and its partners launched the I idi lliIridium satellites.

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

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


• Iridium project(a) The Iridium satellites form six necklaces around the earth(a) The Iridium satellites form six necklaces around the earth.(b) 1628 moving cells cover the earth.

(a) (b)


• Relaying in space


• Relaying on the ground

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

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

• Mobile communicationG d f b d i• Good for broadcasting

• For communication in places with hostile terrain or a poorly developed terrestrial infrastructure

• No right of way problemg y p• Rapid development

THE MOBILE TELEPHONE SYSTEMTHE MOBILE TELEPHONE SYSTEM移动电话系统

• First-generation Mobile Phones: Analog Voice

• Second-generation Mobile Phones: Digital VoiceDigital Voice

Thi d ti M bil h• Third-generation Mobile phones:digital voice and data

The Mobile Telephone SystemThe Mobile Telephone System• In USA, 1G (success) 2G (fiasco) • In Europe, 1G (fiasco) 2G (success)• The reasone easo

– Free competition or government interferencePh b– Phone numbers• Mobile phone numbers are the same as fixed

h bphone numbers• Different for GSM

– Marketing decisions• Prepaid mobile phonesPrepaid mobile phones

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

di t l h d di ll f itiradiotelephones were used sporadically for maritime and military communication

• I 1946 th fi t t b d t l h• In 1946, the first system car-based telephones was setup. Push-to-talk systems (single frequency).

• I th 1960 IMTS (I d M bil T l h• In the 1960s, IMTS (Improved Mobile Telephone System). Two frequencies.

D e to the large po er of the hilltop transmitter– Due to the large power of the hilltop transmitter, adjacent systems had to be several hundred kilometers apart to avoid interferencekilometers apart to avoid interference.

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

The Mobile Telephone System: 1GThe 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.

The Mobile Telephone System: 1GThe Mobile Telephone System: 1G• The structure

M bil h ll h– Mobile phones or cell phones– Base station

MSC (M bil it hi t ) MTSO (M bil– MSC (Mobile switching center) or MTSO (Mobile Telephone Switching Office)

• The 832 channels are divided into four categoriesThe 832 channels are divided into four categories (transmission channels from 824 to 849 MHz and receiving channels from 869 to 894MHz):– Control (base to mobile) to manage the system– Paging (base to mobile) to alert users to calls for them– Access (bidirectional) for call setup and channel

assignment(bidi i l) f i f d– Data (bidirectional) for voice, fax, or data.

The Mobile Telephone System: 1GThe Mobile Telephone System: 1G• Registering

– When a phone is switched on, it scans a preprogrammed list of 21 control channels to find the

t f l i lmost powerful signal– Then the phone broadcasts its 32-bit serial number

d 34 bit t l h band 34-bit telephone number– When the base station hears the announcement, it

tells the MSC hich records the e istence of its netells the MSC, which records the existence of its new customer and also informs the customer’s home MSC of his current locationMSC of his current location.

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

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

– A mobile phone transmits the number to be called and its own identity on the access channel. If a

lli i th it t i icollision occurs there, it tries again.– When the base station gets the request, it informs the

MSCMSC. – The MSC looks for an idle channel for the call and

then send the fo nd channel n mber back on thethen send the found channel number back on the 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 party picks up the phoneparty picks up the phone.

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

Wh ll i l d t bil h ( ith f– When a call is placed to a mobile phone (either from a fixed phone or another mobile phone), a packet is sent to the callee’s home MSC to find out where it is. se t to t e ca ee s o e SC to d out w e e t s.

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

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

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

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

starts making ringing soundsstarts making ringing sounds.

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

– US• GSM: Global System for Mobile CommunicationsGS : G oba Syste o ob e Co u cat o s

– Europe, China• CDMA C d Di i i M lti l A• CDMA: Code Division Multiple Access

– US, China

The Mobile Telephone System: 2G: D-AMPSThe Mobile Telephone System: 2G: D AMPS• D-AMPS

– Fully digital, – To coexist with AMPSo coe st w t S– Two frequency bands: 850MHz and 1900MHz bands

I d h d ff (MAHO M bil A i t d– Improved handoff (MAHO: Mobile Assisted HandOff)

di h h d l d l i d– Better encoding scheme than delta modulation and predictive encoding. Done by a circuit called vocoder (声音合成机) i id bil h(声音合成机) inside a mobile phone.

The Mobile Telephone System: 2G: GSMThe Mobile Telephone System: 2G: GSMGSM uses 124 frequency channels, each of which

i ht l t TDM tuses an eight-slot TDM system

The Mobile Telephone System: 2G: GSMThe Mobile Telephone System: 2G: GSM

A portion of the GSM framing structure.p g

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

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

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

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

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

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

– High-quality voice transmission– 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.)

( id b d ) ( i )• W-CDMA (Wideband CDMA) (Ericsson)• CDMA2000 (Qualcomm)• Other variations

– 2 5G: EDGE (Enhanced Data rates for GSM2.5G: EDGE (Enhanced Data rates for GSM Evolution), GPRS (General Packet Radio Service)4G– 4G

– 802.11, 802.16

CABLE TELEVISION (有线电视)CABLE TELEVISION (有线电视)

• Community antenna television

• Internet over cable

• Spectrum allocation

• Cable modems

• ADSL versus cable• ADSL versus cable

Cable televisionCable televisionAn early cable television system.

A bi f hill l k h l i i– A big antenna on top of a hill to pluck the television signal out of the airAn amplifier called the headend (数据转发器) to– An amplifier, called the headend (数据转发器), to strengthen it,

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

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

– Optical fibers, fiber nodes

Cable Television: Internet over CableCable Television: Internet over Cable

The fixed telephone systemThe fixed telephone system.

Cable Television: Internet over CableCable Television: Internet over Cable• HFC and PSTN

– Local loop• Cable shared by many housesCab e s a ed by a y ouses• Every house has its own private loop.

B d idth– Bandwidth• The bandwidth of coax is much higher than that of

i d itwisted pairs.• The cable industry splits up long cables and connect

each directly to a fiber node.

Cable Television: Spectrum allocationCable Television: Spectrum allocation

Frequency allocation in a typical cable TV system q y yp yused for Internet access

Cable Television: Cable modemsCable Television: Cable modems

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

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

the edge. • Effective capacity• Effective capacity. • Cable is more unpredictable.• Availability• Security• Security• ADSL and cable are much more alike than

they are different.

Homework

15. What is the minimum bandwidth needed to achieve a data rate of B bits/sec if the signal is transmitted using NRZ and Manchester encoding? Explain your answer.

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