Chaiporn Jaikaeo Department of Computer Engineering Kasetsart University 01204325 Data Communications and Computer Networks Based on lecture materials from Data Communications and Networking 5 ID: 1018398
Download Presentation The PPT/PDF document "Signal Transmission and Impairments" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
1. Signal Transmission and ImpairmentsChaiporn JaikaeoDepartment of Computer EngineeringKasetsart University01204325 Data Communications and Computer NetworksBased on lecture materials from Data Communications and Networking, 5th ed.,Behrouz A. Forouzan, McGraw Hill, 2012.Revised 2019-07-22
2. OutlineAnalog and digital data/signalsTime and frequency domain views of signalsBandwidth and bit rateTransmitting digital signals as analogTheoretical data rateSignal impairments
3. Physical LayerFramefrom Data Linkto Data LinkFrame0100101101001011Transmission medium(bits)
4. Analog vs. Digital DataAnalog dataData take on continuous valuesE.g., human voice, temperature readingDigital dataData take on discrete valuesE.g., text, integersCliparts are taken from http://openclipart.org
5. Analog vs. Digital SignalsAnalog signalshave an infinite number of values in a rangeDigital signalsHave a limited number of valid valuesvaluetimevaluetimeTo be transmitted, data must be transformed to physical signals
6. Data and SignalsTelephoneAnalog DataAnalog SignalModemDigital DataAnalog SignalCodecAnalog DataDigital SignalDigitaltransmitterDigital DataDigital SignalTelephoneModemCodecDigitalreceiverTransmission Medium(Channel)Analog DataDigital DataAnalog DataDigital Data
7. How channel affects signal?Channeltt???
8. Simplest form of periodic signalGeneral form: periodT = 1/fpeakamplitudetimesignal strengthSine Wavesphase / phase shiftDemo: Sine Wave
9. Time vs. Frequency DomainsConsider the signal+=Demo: desmos
10. Time vs. Frequency Domains01-124timesignal strength01-124signal strengthfrequencyTime Domain Representation plots amplitude as a function of timeFrequency Domain Representation plots each sine wave’s peak amplitude against its frequencyDemo: Equalizer
11. Fourier AnalysisAny periodic signal can be represented as a sum of sinusoidsknown as a Fourier SeriesE.g., a square wave:++++ …=Joseph Fourier(1768-1830)Demo: Fourier Series
12. Fourier AnalysisEvery periodic signal consists ofDC componentAC componentsFundamental frequency (f0)Harmonics (multiples of f0)DC componentAC componentsfundamentalfrequency3rd harmonic5th harmonic…
13. Fourier Series: RepresentationsMagnitude-phase formSine-cosine (in-phase/quadrature) formComplex exponential form (Euler's formula)Notes:cn are complex numbers DemoDemoDemo
14. Frequency SpectrumThe frequency spectrum of a signal describes the distribution of signal's power into frequency components
15. Bandwidth of Signal and ChannelSignal bandwidth<highest freq of signal> – <lowest freq of signal>Channel (medium) bandwidth<highest freq allowed> – <lowest freq allowed>
16. ExampleWhat is the bandwidth of this signal?A channel allows frequencies from 4000 to 7000 Hz to pass. Can the above signal pass through?
17. Low-Pass and Band-Pass ChannelsLow-pass channelBand-pass channelfrequencyf1f2gainfrequencyf1gain
18. Baseband vs. BroadbandIn baseband transmission, a digital signal is transmitted over a channel directlyA low-pass channel is required
19. Baseband vs. BroadbandIn broadband transmission, a digital signal gets modulated into an analog signalThe signal can pass through a band-pass channel
20. 01001011tamplitude1 secProperties of Digital SignalsBit rate – number of bits per secondSymbol rate – number of signal level changes per secondSymbol interval – time duration of one symbol01001011tamplitude1 secOne bit per symbol#symbols = 2Bit rate = 8 bpsSymbol rate = 8 symbols/s (baud)Symbol interval = 1/8 sTwo bits per symbol#symbols = 4Bit rate = 8 bpsSymbol rate = 4 symbols/s (baud)Symbol interval = 1/4 s
21. Digital vs. Analog BandwidthDigital bandwidthExpressed in bits per second (bps)Analog bandwidthExpressed in Hertz (Hz)Bit rate and bandwidth are proportional to each other
22. 101010fmax = 3 HzDigital vs. Analog BandwidthAllowing one harmonic to pass1111111 secBit rate = 6Digital101010Bit rate = 6111111f = 0 HzAnalog
23. Bit Rate: Noiseless ChannelsNyquist TheoremBit rate in bps (i.e., digital bandwidth)Bandwidth in Hz (i.e., analog bandwidth)L – number of signal levelsHarry Nyquist(1889-1976)
24. Example: Nyquist TheoremWe need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz. How many signal levels do we need?Solution: From Nyquist TheoremSince this result is not a power of 2, we need to either increase the number of levels or reduce the bit rate.If we have 128 levels, the bit rate is 280 kbps.If we have 64 levels, the bit rate is 240 kbps.
25. Transmission ImpairmentsAttenuationSignal strength falls off with distanceThe higher the frequency, the higher the attenuationDistortionNoiseThermal, crosstalk, impulseChannel
26. Relative Signal StrengthMeasured in Decibel (dB)P1 and P2 are signal powers at points 1 and 2, respectivelyPositive dB signal is amplified (gains strength)Negative dB signal is attenuated (loses strength)Point 1Point 2
27. Example – dBm Power UnitdBm (decibel-milliwatts) is the power ratio in decibels compared to 1 milliwatt of powerCalculate the power of a signal with -30 dBm power levelSolution:
28. Link BudgetAccounting of all gains and losses of signal power throughout the signal's path CableSenderReceiverTx PowerCable lossRx PowerTXAmplifierRXAmplifierSenderReceiverTx PowerRx PowerTx ampgainRx ampgainTx antenna gainRx antenna gainpath loss
29. Example: Cable LossThe loss in a cable is usually defined in decibels per kilometer (dB/km)If the signal at the beginning of a cable with −0.3 dB/km has a power of 2 mW, what is the power of the signal at 5 km?Solution: The loss in the cable in decibels is 5(−0.3) = −1.5 dB. We can calculate the power as
30. Signal-to-Noise RatioA measurement of signal reception's quality
31. Example – SNR The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB ?Solution: the values of SNR and SNRdB can be calculated as follows
32. Data Rate: Noisy ChannelsShannon's CapacityCapacity (maximum bit rate) in bpsBandwidth in HzSNR – Signal-to-Noise RatioClaude Elwood Shannon(1916-2001)
33. Example – Shannon's CapacityA telephone line normally has a bandwidth of 3000. The signal-to-noise ratio is usually 3162. Calculate the theoretical highest bit rate of a regular telephone line.Solution:
34. Example – Shannon + NyquistWe have a channel with a 1-MHz bandwidth. The SNR for this channel is 63. What are the appropriate bit rate and signal level?SolutionFirst, use the Shannon capacityfollowed by the Nyquist formula The Shannon capacity gives us the upper limit; the Nyquist formula tells us how many signal levels we need.
35. Network PerformanceBandwidthAnalog – Hertz Digital – Bits per second (bps)ThroughputActual data rateLatency (delay)Time it takes for an entire message to completely arrive at the destination
36. LatencyComposed ofPropagation timeTransmission timeQueuing timeProcessing timeEntiremessagetransmissiontimepropagationtime
37. Transmission LatencyTimeTimeSenderReceiverLast bit leavesFirst bit leavesLast bit arrivesFirst bit arrivesPropagation timeTransmission timeData bits
38. SummaryData need to take form of signal to be transmittedFrequency domain representation of signal allows easier analysisFourier analysisMedium's bandwidth limits certain frequencies to passBit rate is proportional to bandwidthSignals get impaired by attenuation, distortion, and noise