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- 1. Data and Computer CommunicationsChapter 3 – Data Transmission Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011
- 2. Data Transmission What weve got here is failure to communicate.Paul Newman in Cool Hand Luke
- 3. Data Transmission • quality of the signal being transmittedThe successfultransmission of • characteristics of thedata depends on transmission mediumtwo factors:
- 4. Transmission TerminologyData transmission occurs between transmitterand receiver over some transmission medium.Communication Unguidedis in the form of Guided mediaelectromagnetic media (wireless) waves. twisted pair, air, vacuum, coaxial cable, seawater optical fiber
- 5. TransmissionTerminology
- 6. TransmissionTerminology Simplex signals transmitted in one direction • eg. Television Half duplex both stations transmit, but only one at a time • eg. police radio Full duplex simultaneous transmissions • eg. telephone
- 7. Frequency, Spectrum and Bandwidth Time Domain Concepts analog signal • signal intensity varies smoothly with no breaks digital signal • signal intensity maintains a constant level and then abruptly changes to another level periodic signal • signal pattern repeats over time aperiodic signal • pattern not repeated over time
- 8. Analog and Digital Signals
- 9. PeriodicSignals
- 10. Sine Wave (periodic continuous signal) peak amplitude (A) maximum strength of signal typically measured in volts frequency (f) rate at which the signal repeats Hertz (Hz) or cycles per second period (T) is the amount of time for one repetition T = 1/f phase () relative position in time within a single period of signal
- 11. Varying Sine Waves s(t) = A sin(2ft +)
- 12. Wavelength () the wavelength of a signal is the distance occupied by a single cyclecan also be stated as the especially when v=c distance between two points of corresponding • c = 3*108 ms-1phase of two consecutive (speed of light in cycles free space) assuming signal or velocity v, then the wavelength is related equivalently to the period as = vT f = v
- 13. Frequency Domain Concepts signals are made up of many frequencies components are sine waves Fourier analysis can show that any signal is made up of components at various frequencies, in which each component is a sinusoid can plot frequency domain functions
- 14. Addition of FrequencyComponents (T=1/f)c is sum of f & 3f
- 15. Frequency DomainRepresentations frequency domain function of Fig 3.4c frequency domain function of single square pulse
- 16. Spectrum & Bandwidth
- 17. Signal with dc Component
- 18. Data Rate and Bandwidth any transmission this limits the data system has a rate that can be limited band of carried on the frequencies transmission medium square waves limiting most energy in have infinite bandwidth first few components and creates components hence an infinite distortions bandwidthThere is a direct relationship between data rate and bandwidth.
- 19. Analog and Digital Data Transmission data entities that convey information signals electric or electromagnetic representations of data signaling physically propagates along a medium transmission communication of data by propagation and processing of signals
- 20. Acoustic Spectrum (Analog)
- 21. Analog and Digital Transmission
- 22. Digital Data Examples:IRA Text Character strings
- 23. Advantages & Disadvantages of Digital Signals
- 24. Audio Signals frequency range of typical speech is 100Hz-7kHz easily converted into electromagnetic signals varying volume converted to varying voltage can limit frequency range for voice channel to 300-3400Hz
- 25. to produce a video signal a TV camera is used USA standard is 483 lines per Video frame, at a rate of 30 completeSignals frames per second actual standard is 525 lines but 42 lost during vertical retrace horizontal scanning frequency is 525 lines x 30 scans = 15750 lines per second max frequency if line alternates black and white max frequency of 4.2MHz
- 26. Conversion of PC Input to Digital Signal
- 27. Analog Signals
- 28. Digital Signals
- 29. Analog and DigitalTransmission
- 30. Transmission Impairments signal received may differ from signal transmitted causing: analog - degradation of signal quality digital - bit errors most significant impairments are attenuation and attenuation distortion delay distortion noise
- 31. Equalize Received signal attenuation across strength must be: the band of •strong enough to be frequencies used detected by using loading •sufficiently higher than noise to be received coils or amplifiers. without error Strength can be increased using amplifiers or repeaters. ATTENUATION signal strength falls off with distance over anytransmission medium varies with frequency
- 32. Attenuation Distortion
- 33. Delay Distortion occurs because propagation velocity of a signal through a guided medium varies with frequency various frequency components arrive at different times resulting in phase shifts between the frequencies particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference
- 34. Noiseunwanted signalsinserted betweentransmitter andreceiver is the major limiting factor in communications system performance
- 35. Categories of Noise Intermodulation noise • produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium • effect is to produce signals at a frequency that is the sum or difference of the two original frequencies
- 36. Categories of Noise Crosstalk: a signal from one line is picked up by another can occur by electrical coupling between nearby twisted pairs or whenImpulse Noise: microwave antennas pick up unwanted signals caused by external electromagnetic interferences noncontinuous, consisting of irregular pulses or spikes short duration and high amplitude minor annoyance for analog signals but a major source of error in digital data
- 37. Channel CapacityMaximum rate at which data can be transmitted over agiven communications channel under given conditions bandwidth main noise error ratedata rate limitations constraint in cycles due to on average rate of physical achievingin bits per per noise level corrupted properties efficiency second second or over path bits is noise Hertz
- 38. Nyquist BandwidthIn the case of a channel that is noise free: if rate of signal transmission is 2B then can carry signal with frequencies no greater than B given bandwidth B, highest signal rate is 2B for binary signals, 2B bps needs bandwidth B Hz can increase rate by using M signal levels Nyquist Formula is: C = 2B log2M data rate can be increased by increasing signals however this increases burden on receiver noise & other impairments limit the value of M
- 39. Shannon Capacity Formula considering the relation of data rate, noise and error rate: faster data rate shortens each bit so bursts of noise corrupts more bits given noise level, higher rates mean higher errors Shannon developed formula relating these to signal to noise ratio (in decibels) SNRdb=10 log10 (signal/noise) capacity C = B log2(1+SNR) theoretical maximumcapacity get much lower rates in practice
- 40. Summary transmission concepts and terminology guided/unguided media frequency, spectrum and bandwidth analog vs. digital signals data rate and bandwidth relationship transmission impairments attenuation/delay distortion/noise channel capacity Nyquist/Shannon

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