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1、RoadmapIntroductionPhysical LayerData Link LayerTransport LayerNetwork LayerMedium Access SublayerApplication Layer1The Physical LayerChapter 22What You Will Learn In This Chapter1.Theoretical Basis For Data Communication How much data can be put on a wire? What are the limits imposed by a medium?2.
2、Three kinds of transmission mediaguided (copper wire and fiber optics)wireless (terrestrial radio)satellite. 3Data Communication TheoryThis is Physics or Electrical Engineering stuff.Its how we understand what is actually happening on a wire.2.1 Basic Idea of the Physical Layer2.2 Theoretical Basis
3、For Data Communication2.3 Transmission Media under the Physical Layer42.1 Basic Idea of the Physical LayerlProvides mechanical, electrical, functional, and procedural characteristics to activate, maintain, and deactivate physical links that transparently send the bit stream.5lMechanical characterist
4、ic: specifying connectors shape,size and pins number.lElectrical characteristic: specifying the range of voltage on the each pin.lFunctional characteristic: specifying the meaning of voltage value .lProcedural characteristic: specifying the sequence of all kinds of possible events. 62.2 Theoretical
5、Basis For Data Communication System Model of Data CommunicationModemPCPSTNModemDigital bit streamAnalog signalTextData Communication SystemPCAnalog signalDigital bit stream7Some Terms and ConceptslData The entity carring information.lSignal The electrical or magnetic representation of data. lAnalogo
6、us values are continuous.lDigital values are discrete.lModulation The procedure of converting digital signal into analogous signal.lDemudulation The procedure of converting analogous signal into digital signal. 8Analog and Digital data、signal analog dataanalog signalAmplifierModulatoranalog datadigi
7、tal signal PCMencoderdigital dataanalog signalModulatordigital datadigital signalDigitalEncoder9Some Terms and ConceptslBaseband signal -directly representing digital signal 1 or 0 by two different voltages, then sending out for transmission. lBroadband signal -frequency-multipexing signal after mod
8、ulating baseband signal . 10Some Terms and ConceptslSimplex communication can be carried out in one direction at any time.lDuplex both terminals can send information ,but not simutaneously.lFull duplex both terminal can send and receive information simutaneously. 11lBandwidthUnfortunately, life isnt
9、 perfect. All frequencies arent possible in a transmission medium. Usually, the amplitudes are transmitted undiminished from 0 up to some frequency fc measured in cycles/sec or Hertz (Hz) with all frequencies above this cutoff frequency attenuated. The range of frequencies transmitted without being
10、strongly attenuated is called the bandwidth. Signals can also be intentionally distorted or constrained. Perhaps only a certain range of frequencies is allowed for each channel. This allows multiple channels per medium.Bandwidth-limited Signals 12Digital signal through a real channellNot serious dis
11、tortionlSerious distortion Real channel（band-limited ,noisy,interference and distortion）Input signal waveOutput signal waveReal channel（band-limited ,noisy,interference and distortion）Output signal waveInput signal wave13The Nyquist equationlThe maximum symbol rate of ideal low-pass channel (pre hz)
12、 is two symbol per second. lBaud is the unit of symbol transmission .The maximum symbol rate of ideal low-pass channel = 2W Baud W is the bandwidth of ideal low-pass channel, its unit is Hz can not passcan pass0frequecy(Hz)W (Hz) 14The Nyquist equationanother formlThe maximum symbol rate of ideal ba
13、nd-pass channel (pre hz) is one symbol per second. The maximum symbol rate of ideal band-pass channel = W Baud W is the bandwidth of ideal band-pass channel , its unit is Hz. can not passcan pass0frequency(Hz)W (Hz) can not pass 15Baud rate and Bit rateBaud - The number of changes in the signal per
14、second. A b baud line does not necessarily transmit b bits/second - each signal may convey several bits - for example if 8 voltages are possible per signal, then 3 bits are sent on every signal. If the signal is BINARY (only two voltage levels), then the bit rate is equal to the baud rate.16The maxi
15、mum bit rate in a real channell Applying information theory ,Shannon infered the limted and non-errorous information transmission rate of band-limited and Gaussian white noisy channel.lThe limited information transmission rate of channel can be represented:l C = W log2(1+S/N) b/s lW is the bandwidth
16、 of channel （its unit is Hz );lS is the average power of signal transmitted in channel;lN is the Gaussian noisy power in channel ;lS/N:Signal to noise ratio 172.3 Physical Media under Physical LayerlGuided media: lsignals propagate in solid media: copper, fiber, coaxlUnguided media: lsignals propaga
17、te freely, e.g., radio18The Electromagnetic SpectrumThe electromagnetic spectrum and its uses for communication.Ultraviolet 19Guided Transmission MedialTwisted PairlSTP (Shielded Twisted Pair)lUTP (Unshielded Twisted Pair) lCoaxial Cablel50 l75 lFiber Cable20Twisted Pair(a) Category 3 UTP: 16 MHz (b
18、) Category 5 UTP: 100 MHz two insulated copper wires21UTP & STPUTP & STP UTP STP22Coaxial CableA coaxial cable.Two type50-ohm: digital data75-ohm analog TVBandwidth: 1 GHz 2324The refraction of light in the fiber refracton angle incident angle cladding with lowerindex of refraction cladding with low
19、er index of refraction core with higher index of refraction claddingcore25The transmission mechanism of the fibercorecladding withlower index of refacon total internal reflection 26input pulseoutput pulsesingle-mode fiber Multimode fiber and single-mode fiberinput pulse output pulse multimode fiber2
20、7Fiber Optics(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.(b) Light trapped by total internal reflection.28Multimode and Single-modeldifferent rays will be bouncing around at different angles. Each ray is said to have a differe
21、nt mode, so a fiber having this property is called a multimode fiber .lif the fibers diameter is reduced to a few wavelengths of light, the fiber acts like a wave guide, and the light can propagate only in a straight line, without bouncing, yielding a single-mode fiber. 29Transmission of Light throu
22、gh FiberAttenuation of light through fiber in the infrared region.All three bands are 25,000 to 30,000 GHz wide 30Chromatic Dispersion lChromatic dispersion occurs in both single mode and multimode optical fibers. Chromatic dispersion occurs because different colors of light travel through the fiber
23、 at different speeds. Since the different colors of light have different velocities, some colors arrive at the fiber end before others. This delay difference is called the differential group delay per unit length. This differential group delay leads to pulse broadening.31Fiber Cables(a) Side view of
24、 a single fiber.(b) End view of a sheath with three fibers. Diameter:50um of Multimode 10um of singlemode32Unguided medialWireless transmisson used wide frequency range。lShort wave communication applies reflection of the ionosphere ,but with poor quality.lMicrowave can propagate in nearly straight l
25、iens in the space . lthe relay communication of ground microvave station.lsatellite communication.33Modems (a) A binary signal (b) Amplitude modulation(c) Frequency modulation(d) Phase modulation34Multiple bits/baud modulationlThe 3 kHz phone line can only be sampled at 6 kHz. So it doesnt do any go
26、od to sample more - instead try to get in more bits per sample. For example, the Figure shows a combination of phase and amplitude modulation leading to multiple bits/baud.constellation diagrams 35Multiple bits/baud modulation(a) QPSK ( Quadrature Phase Shift Keying )(b) QAM-16 ( Quadrature Amplitud
27、e Modulation). (c) QAM-6436Modems (3)(a) V.32 for 9600 bps.(b) V32 bis for 14,400 bps.(a)(b)37Modems In addition, modems use compression and error correction to increase the effective bits per second.lFull Duplex - Able to transmit in both directions on a wire at the same time( by using different fr
28、equencies for different directions ).lHalf duplexlSimplex38Trunks and MultiplexinglThe cost of a wire is pretty much constant, independent of the bandwidth of that wire - costs come from installation and maintenance of the physical space (digging, etc.), not from the media or the electrical support
29、structure. So, how can we stuff more through that medium?39Frequency Division Multiplexing(a) The original bandwidths.(b) The bandwidths raised in frequency.(b) The multiplexed channel.40Wavelength Division MultiplexingWavelength division multiplexing.41What is Digital Voice: PCM SignalBACDAABBCCDDAnalog Analog voicevoiceclockclockPAM signalPAM signalPCM SignalPCM SignalSampleQuantify Digital Digital VoiceVoice42Time Division MultiplexingThe T1 carrier (1.544 Mbps).43Time Division Multiplexing (2)Delta modulation.44Time Division Multiplexing (3)Multiplexing T1 streams into higher carriers.45
