现代计算机网络讲义2(英语)+物理层

1、5:24:4412. The Physical LayernDefines the mechanical, electrical, functional and procedural (timing) interfacenTransmission medianExamples of communication systems5:24:4422.1 The Theoretical Basis for Data CommunicationnFourier AnalysisnBandwidth-Limited SignalsnMaximum Data Rate of a Channel5:24:44

2、32.1.1 Fourier AnalysisnAmplitudesnFrequencynA data signal can be imagined that it repeats the entire pattern over and over forever )2cos()2sin(21)(11nftbnftactgnnnnnAny periodic function g(t) with period T5:24:4442.1.2 Bandwidth-Limited Signals(1)5:24:445 2.1.2 Bandwidth-Limited Signals(2)n 5:24:44

3、62.1.2 Bandwidth-Limited Signals(3)nRelation between data rate and harmonics on voice-grade line (3000Hz bandwidth)nBandwidth limited data rate5:24:4472.1.3 The Maximum Data Rate of a channelnNyquists theoremmaximum data rate = 2H log2 V bits/secnShannons theoremmaximum data rate = Hlog2 (1+S/N) bit

4、s/secnExample (Modem,Telephone Line):S/N = 1000, maximum data rate 7H = 28kbps5:24:4482.2 Guided Transmission MedianMagnetic MedianTwisted Pairnthe twisted wire radiates less effectively nCoaxial CablenFiber Optics5:24:449Fiber OpticsnThree components:nLight source,transmission medium,detectornA pul

5、se of light indicates a 1 bit and the absence of light indicates a 0 bit nLight through fibernMultimode & single mode fiber5:24:4410Attenuation of light through fiber in the infrared region 5:24:4411Information Wave CarriednFrequency and wave length f = cnto finite differences n1.30-micron band,

6、 = 1.3 x 10-6n= 0.17 x 10-6nf is about 3*1013Hz = 30THzn8 bits/Hz, we get 240 Tbps. nA few bits per Hertz at low frequenciesnOften 8 bits at high frequencies5:24:4412Fiber Optic Networks (1)*nA fiber optic ring with active repeaters5:24:4413Fiber Optic Networks (2)*nWDM (Wavelength Division Multiple

7、xing )nDWDM (Dense Wavelength Division Multiplexing )nOXC (Optical Cross-Connects)nOptical and Electrical transition nFiber Cross-Connect , Automatic layoutnWavelength Selective Cross-ConnectnOptical Storage ?5:24:44142.3 Wireless TransmissionnThe Electromagnetic SpectrumnRadio TransmissionnMicrowav

8、e TransmissionnInfrared WavesnLightwave Transmission5:24:44152.3.1 The Electromagnitic Spectrum 5:24:4516Frequencies and regulationsEuropeUSAJapanMobilephonesNMT 453-457MHz,463-467 MHz;GSM 890-915 MHz,935-960 MHz;1710-1785 MHz,1805-1880 MHzAMPS, TDMA, CDMA824-849 MHz,869-894 MHz;TDMA, CDMA, GSM1850-

9、1910 MHz,1930-1990 MHz;PDC810-826 MHz,940-956 MHz;1429-1465 MHz,1477-1513 MHzCordlesstelephonesCT1+ 885-887 MHz,930-932 MHz;CT2864-868 MHzDECT1880-1900 MHzPACS 1850-1910 MHz,1930-1990 MHzPACS-UB 1910-1930 MHzPHS1895-1918 MHzJCT254-380 MHzWirelessLANsIEEE 802.112400-2483 MHzHIPERLAN 15176-5270 MHzIEE

10、E 802.112400-2483 MHzIEEE 802.112471-2497 MHz5:24:45172.4 Communication Satellites*nAmplifies incoming signal, rebroadcasts at another frequencynGeostationary Satellites (GEO)nMedium-Earth Orbit Satellites (MEO)nLow-Earth Orbit Satellites (LEO)5:24:4518Satellites and properties*nAltitude above the e

11、arthnRound-trip delay timenNumber of satellites for global coverage.5:24:4519VSAT (Very Small Aperture Terminal)* n1-meter or smaller antennasn1 watt of powernThe uplink is 19.2 kbps, the downlink is 512 kbps or more.nHub with a large antenna to relay traffic between VSATs. 5:24:4520Satellites versu

12、s Fiber nFibersnwithin the telephone system to handle long distance calls. nSatellitesnHigher bandwidth. nMobile communication nBroadcasting nMilitary communication nDifficult to lay fiber5:24:45212.5 The Public Switch Telephone Network (PSTN)nStructure of the Telephone SystemnThe Local Loop: Modems

13、, ADSL and WirelessnTrunks and MultiplexingnSwitching5:24:45222.5.1 Structure of the Telephone SystemnPoint to Pointn(a) Fully-interconnected network.n(b) Centralized switch.n(c) Two-level hierarchy5:24:4523A typical circuit routenLocal loops (analog twisted pairs )nTrunks (digital fiber optics)nSwi

14、tching officesnAnalog, low bandwidth5:24:45242.5.3 The Local Loop: Modems,ADSL,and wirelessnThe analog transmissions, modemnThe digital transmissions, codecs5:24:4525Influence on signal transfer nAttenuationnLoss of energynDepends on frequency and distance nDistortionnDifferent Fourier component at

15、different speednNoisenThermal noise, random , unavoidablenCrosstalknImpulse 5:24:4526Some conceptsnBandwidth, the range of frequency nSymbol, one piece info., a samplenBaud rate, number of sample/sec, symbol ratenBit rate, number of symbol/sec times number of bits/symbol 5:24:4527Modems (1)nA binary

16、 signalnAmplitude modulationnFrequency modulationnPhase modulation5:24:4528Modems (2)n(a) QPSK.n(b) QAM-16.n(c) QAM-645:24:4529Modems (3)n(b) V.32 for 9600 bps,2400Baud*(5bit-1)n(c) V32 bis for 14,400 bps, 2400Baud*(7bit-1)nV.90 provides a 33.6-kbps upstream channel (user to ISP), but a 56 kbps down

17、stream channel (ISP to user), according the limits of Shannon and Nyquist theorem .5:24:4530Digital Subscriber Lines (1)*nMaking the entire capacity of the local loop availablenFactors: length, thickness, general qualitynBandwidth versus distanced over category 3 UTP for DSL.5:24:4531Digital Subscri

18、ber Lines (2)*nOver category 3 UTPnDont affect telephonesnMuch fast than 56kbpsnAlways OnnADSL using discrete multitone modulation: nfrequency division multiplexing5:24:4532A typical ADSL equipment configuration *nNID :Network interface devicenDSLAM:Digital subscriber line access multiplexer5:24:453

19、3Wireless Local Loops: WLL*nFixed wireless, 2-5kmnArchitecture of an LMDS systemnLocal Multipoint Distribution ServicenAsymmetric bandwidth allocation. 36 Gbps downstream and 1 Mbps upstream nIEEE 802.16,wireless MAN, WiMAX(World Interoperability for Microwave Access )5:24:45342.5.4 Trunks and Multi

20、plexingnHigh-bandwidth TrunknMultiplexing over a physical trunknFDMnWDMnTDM (PCM)nSONET/SDH5:24:4535Frequency Division Multiplexing(a) The original bandwidths.(b) The bandwidths raised in frequency.(c) The multiplexed channel 5:24:4536Wavelength Division MultiplexingnJust FDM at very high frequencie

21、snFor fiber opticnDWDM, wavelengths are spaced close together, 0.1nm5:24:4637Time Division Multiplexing(1)nCopper line in the telephone systemnDigital data transmissionnPCM, Pulse Code ModulationnNyquist theorem: Making 2N samples per second from N hz bandwidth is sufficient to capture all the infor

22、mation nTelephone system: 8000 samples from 4khz channel, 125us/samplen7 or 8 bit/samplenTransmission and recover5:24:4638Time Division Multiplexing(2)nSignaling and datanFraming codenThe T1 carrier (24 channels,1.544 Mbps)nThe E1 carrier (32 channels,2.048Mbps)5:24:4639Time Division Multiplexing(3)

23、*nTo reduce the number of bits needed per channel nDifferential pulse code modulation, 5 bit is sufficient nDelta modulation, 1 bit is sufficientnPredictive encodingnInfluence when packet lost5:24:4640Time Division Multiplexing(4)nMultiplexing T1 streams into higher-order carriers bit for bit5:24:46

24、41SONET/SDH(1)*nOptical TDM systemnSynchronous operation, sender / receiver have common clockn810 bytes every 125 us, 51.84MbpsnSection: device to devicenLine: Multiplexer to MultiplexernPath: Source to destination5:24:4642SONET/SDH(2)*nTwo back-to-back SONET frames5:24:4643SONET/SDH(3)*nSONET and S

25、DH multiplex rates5:24:46442.5.5 Switching(a) Circuit switching. (b) Packet switching.5:24:4645Timing of events(a)Circuit switching(b)Message switching(c)Packet switching5:24:4646ComparisonA comparison of circuit switched and packet-switched networks.5:24:46472.6 The Mobile Telephone SystemnCordless

26、 phone and mobile phonenFirst-Generation Mobile Phones: Analog VoicenSecond-Generation Mobile Phones: Digital VoicenThird-Generation Mobile Phones: Digital Voice and DatanThe Fourth-Generation ?5:24:46482.6.1 First-Generation: Advanced Mobile Phone SystemnPush-to-Talk system: Single ChannelnIMTS: Im

27、proved Mobile Telephone SystemnTwo frequency: Sending and Receivingn23 Channels, spread out from 150Mhz to 450MhznAMPS: Advanced Mobile Phone SystemnCell, 10 to 20 km acrossnBase StationnAdjacent cells use different frequenciesnReuse of frequencies in nearby cellsnMTSO: Mobile Telephone Switching Of

28、ficenHandoff5:24:4649Frequencies Reuse(a) Frequencies are not reused in adjacent cells.(b) To add more users, smaller cells can be used5:24:4650Channel CategoriesnFDM n824 to 849 MHz : 832 simplex transmission channels n869 to 894 MHz : 832 simplex receive channels nEach channels is 30 kHz wide. nTh

29、e 832 channels are divided into four categories:nControl (base to mobile) to manage the systemnPaging (base to mobile) to alert users to calls for themnAccess (bidirectional) for call setup and channel assignmentnData (bidirectional) for voice, fax, or data5:24:4651Call Management*nWhen a phone is s

30、witched on, it broadcasts its 32-bit serial number and 34-bit telephone number. nBS heared the announcement tells the MTSO, and informs the customers home MTSO of his current location.nIdle phones listen to the paging channel to detect messages. nCall: phone transmits the number to be called. nMTSO

31、looks for an idle voice channel for the call.nThe phone switches to the selected voice channel and waits until the called party picks up the phone.nMTSO to find out where the callees home is and where the callees current cell is, sends a packet to the BS.nBS sends a broadcast on the paging channel t

32、o check the exist of the called phone.nThe called phone responds on the access channel nBS then informs the called phone to switches to a channel and starts making ringing sounds.5:24:46522.6.2 Second-Generation Mobile phonesnD-AMPSnGSMnCDMA5:24:4653D-AMPS Digital Advanced Mobile Phone SystemnThree

33、users share a single frequency (30kHz)pair using time division multiplexing nEach user has 8kbps for speech data5:24:4654Global System for Mobile Communications - GSM (1)nGSM uses 124 frequency channels, each of which uses an eight-slot TDM systemnTransmission and receiving does not happen in the sa

34、me time slot5:24:4655GSM (2)nA portion of the GSM framing structure.5:24:4756GSM (3)nEach TDM slot consists of a 148-bit data framenEight data frame make up a TDM framen26 TDM frames make up a 120-msec multiframenSeveral control channelnBroadcast control channelBS ID. and channel statusnDedicated co

35、ntrol channel Location updating / Registration / Call setup nCommon control channel paging, user request, assigned slot announce 5:24:4757GSM ArchitectureBSS Base Station SystemBTS Base Transceiver StationBSC Base Station Controller MS Mobile StationNSS Network Sub-SystemSS7BTSBSCMSCVLRHLRAuCGMSCBSS

36、PSTNNSSAECDPSTNAbisBHMSMSC Mobile-service Switching ControllerVLR Visitor Location RegisterHLR Home Location RegisterAuC Authentication ServerGMSC Gateway MSC5:24:4758CDMA (1)Code Division Multiple AccessnTalking in a different language 5:24:4759Courtesy of Suresh Goyal & Rich Howard5:24:4760Cou

37、rtesy of Suresh Goyal & Rich Howard5:24:4761CDMA (2)Code Division Multiple AccessnConcept nChip, interval in each bit timenChip sequence, a unique m-bit code assigned to each station nChip sequence and complement, S and S, express the value 1 or 0nOrthogonalnIf S and T are two distinct chip sequence, then S ? S = 1 , S ? S = 0 , S ? T = 0 , S ? T = 0 5:24:4762CDMA (3)nBinary chip sequences for four stationsnBipolar chip sequences nSix examples of tra