通信網(wǎng)絡(luò)技術(shù)課件：chapter 2 The Physical Layer (2)

1、RoadmapIntroductionPhysical LayerData Link LayerTransport LayerNetwork LayerMedium Access SublayerApplication Layer1The Physical LayerChapter 22What You Will Learn In This ChapterTheoretical Basis For Data Communication How much data can be put on a wire? What are the limits imposed by a medium?Thre

2、e 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 For

3、Data Communication2.3 Transmission Media under the Physical Layer42.1 Basic Idea of the Physical LayerProvides mechanical, electrical, functional, and procedural characteristics to activate, maintain, and deactivate physical links that transparently send the bit stream.5Mechanical characteristic: sp

4、ecifying connectors shape,size and pins number.Electrical characteristic: specifying the range of voltage on the each pin.Functional characteristic: specifying the meaning of voltage value .Procedural characteristic: specifying the sequence of all kinds of possible events. 62.2 Theoretical Basis For

5、 Data Communication System Model of Data CommunicationModemPCPSTNModemDigital bit streamAnalog signalTextData Communication SystemPCAnalog signalDigital bit stream7Some Terms and ConceptsData The entity carring information.Signal The electrical or magnetic representation of data. Analogous values ar

6、e continuous.Digital values are discrete.Modulation The procedure of converting digital signal into analogous signal.Demudulation The procedure of converting analogous signal into digital signal. 8Analog and Digital data、signal analog dataanalog signalAmplifierModulatoranalog datadigital signal PCMe

7、ncoderdigital dataanalog signalModulatordigital datadigital signalDigitalEncoder9Some Terms and ConceptsBaseband signal -directly representing digital signal 1 or 0 by two different voltages, then sending out for transmission. Broadband signal -frequency-multipexing signal after modulating baseband

8、signal . 10Some Terms and ConceptsSimplex communication can be carried out in one direction at any time.Duplex both terminals can send information ,but not simutaneously.Full duplex both terminal can send and receive information simutaneously. 11BandwidthUnfortunately, life isnt perfect. All frequen

9、cies 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 strongly attenuated i

10、s 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 channelNot serious distortionSerious distort

11、ion 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 equationThe maximum symbol rate of ideal low-pass channel (pre hz) is two symbol per secon

12、d. Baud 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 formThe maximum symbol rate of ideal band-pass channel (pre hz) i

13、s 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 second. A b baud line does

14、 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 maximum bit rate in a real cha

15、nnel Applying information theory ,Shannon infered the limted and non-errorous information transmission rate of band-limited and Gaussian white noisy channel.The limited information transmission rate of channel can be represented: C = W log2(1+S/N) b/s W is the bandwidth of channel （its unit is Hz );

16、S is the average power of signal transmitted in channel;N is the Gaussian noisy power in channel ;S/N:Signal to noise ratio 172.3 Physical Media under Physical LayerGuided media: signals propagate in solid media: copper, fiber, coaxUnguided media: signals propagate freely, e.g., radio18The Electroma

17、gnetic SpectrumThe electromagnetic spectrum and its uses for communication.Ultraviolet 19Guided Transmission MediaTwisted PairSTP (Shielded Twisted Pair)UTP (Unshielded Twisted Pair) Coaxial Cable50 75 Fiber Cable20Twisted Pair(a) Category 3 UTP: 16 MHz (b) Category 5 UTP: 100 MHz two insulated copp

18、er wires21UTP & 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 lower index of refraction core with higher index of refr

19、action 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 fiber27Fiber Optics(a) Three examples of a light ray from i

20、nside a silica fiber impinging on the air/silica boundary at different angles.(b) Light trapped by total internal reflection.28Multimode and Single-modedifferent rays will be bouncing around at different angles. Each ray is said to have a different mode, so a fiber having this property is called a m

21、ultimode fiber .if 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 through FiberAttenuation of light through fiber in the infra

22、red region.All three bands are 25,000 to 30,000 GHz wide 30Chromatic Dispersion Chromatic dispersion occurs in both single mode and multimode optical fibers. Chromatic dispersion occurs because different colors of light travel through the fiber at different speeds. Since the different colors of ligh

23、t 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 a single fiber.(b) End view of a sheath with three fibe

24、rs. Diameter:50um of Multimode 10um of singlemode32Unguided mediaWireless transmisson used wide frequency range。Short wave communication applies reflection of the ionosphere ,but with poor quality.Microwave can propagate in nearly straight liens in the space . the relay communication of ground micro

25、vave station.satellite communication.33Modems (a) A binary signal (b) Amplitude modulation(c) Frequency modulation(d) Phase modulation34Multiple bits/baud modulationThe 3 kHz phone line can only be sampled at 6 kHz. So it doesnt do any good to sample more - instead try to get in more bits per sample

26、. 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 Amplitude 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.Full Duplex - Able to transmit in both directions on