# William Stallings Data and Computer Communications Chapter 3 Data Transmission.

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1 William Stallings Data and Computer Communications Chapter 3 Data Transmission

2 Terminology (1) zTransmitter zReceiver zMedium yGuided medium xe.g. twisted pair, optical fiber yUnguided medium xe.g. air, water, vacuum

3 Terminology (2) zDirect link yNo intermediate devices zPoint-to-point yDirect link yOnly 2 devices share link zMulti-point yMore than two devices share the link zPoint-to-multipoint yOne hub and many spokes

4 Terminology (3) zSimplex yOne direction xe.g. Television zHalf duplex yEither direction, but only one way at a time xe.g. police radio zFull duplex yBoth directions at the same time xe.g. telephone

5 Frequency, Spectrum and Bandwidth zTime domain concepts yContinuous signal xVarious in a smooth way over time yDiscrete signal xMaintains a constant level then changes to another constant level yPeriodic signal xPattern repeated over time yAperiodic signal xPattern not repeated over time

6 Continuous & Discrete Signals

7 Periodic Signals

8 Sine Wave zPeak Amplitude (A) ymaximum strength of signal yvolts zFrequency (f) yRate of change of signal yHertz (Hz) or cycles per second yPeriod = time for one repetition (T) yT = 1/f zPhase (  ) yRelative position in time

9 Varying Sine Waves

10 Wavelength zDistance occupied by one cycle zDistance between two points of corresponding phase in two consecutive cycles z zAssuming signal velocity v y = vT y f = v yc = 3*10 8 ms -1 (speed of light in free space)

11 Frequency Domain Concepts zSignal usually made up of many frequencies zComponents are sine waves zCan be shown (Fourier analysis) that any signal is made up of component sine waves zCan plot frequency domain functions

13 Frequency Domain

14 Spectrum & Bandwidth zSpectrum yrange of frequencies contained in signal zAbsolute bandwidth ywidth of spectrum zEffective bandwidth yOften just bandwidth yNarrow band of frequencies containing most of the energy zDC Component yComponent of zero frequency

15 Signal with DC Component

16 Data Rate and Bandwidth zAny transmission system has a limited band of frequencies zThis limits the data rate that can be carried

17 Analog and Digital Data Transmission zData yEntities that convey meaning zSignals yElectric or electromagnetic representations of data zTransmission yCommunication of data by propagation and processing of signals

18 Data zAnalog yContinuous values within some interval ye.g. sound, video zDigital yDiscrete values ye.g. text, integers

19 Acoustic Spectrum (Analog)

20 Signals zMeans by which data are propagated zAnalog yContinuously variable yVarious media xwire, fiber optic, space ySpeech bandwidth 100Hz to 7kHz yTelephone bandwidth 300Hz to 3400Hz yVideo bandwidth 4MHz zDigital yUse two DC components

21 Data and Signals zUsually use digital signals for digital data and analog signals for analog data zCan use analog signal to carry digital data yModem zCan use digital signal to carry analog data yCompact Disc audio

22 Analog Signals Carrying Analog and Digital Data

23 Digital Signals Carrying Analog and Digital Data

24 Analog Transmission zAnalog signal transmitted without regard to content zMay be analog or digital data zAttenuated over distance zUse amplifiers to boost signal zAlso amplifies noise

25 Digital Transmission zConcerned with content zIntegrity endangered by noise, attenuation etc. zRepeaters used zRepeater receives signal zExtracts bit pattern zRetransmits zAttenuation is overcome zNoise is not amplified

26 Advantages of Digital Transmission zDigital technology yLow cost LSI/VLSI technology zData integrity yLonger distances over lower quality lines zCapacity utilization yHigh bandwidth links economical yHigh degree of multiplexing easier with digital techniques zSecurity & Privacy yEncryption zIntegration yCan treat analog and digital data similarly

27 Transmission Impairments zSignal received may differ from signal transmitted zAnalog - degradation of signal quality zDigital - bit errors zCaused by yAttenuation and attenuation distortion yDelay distortion yNoise

28 Attenuation zSignal strength falls off with distance zDepends on medium zReceived signal strength: ymust be enough to be detected ymust be sufficiently higher than noise to be received without error zAttenuation is an increasing function of frequency

29 Delay Distortion zOnly in guided media zPropagation velocity varies with frequency

30 Noise (1) zAdditional signals inserted between transmitter and receiver zThermal yDue to thermal agitation of electrons yUniformly distributed yWhite noise zIntermodulation ySignals that are the sum and difference of original frequencies sharing a medium

31 Noise (2) zCrosstalk yA signal from one line is picked up by another zImpulse yIrregular pulses or spikes ye.g. External electromagnetic interference yShort duration yHigh amplitude

32 Channel Capacity zData rate yIn bits per second yRate at which data can be communicated zBandwidth yIn cycles per second of Hertz yConstrained by transmitter and medium

33 Required Reading zStallings chapter 3

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