Data and Computer Communications

1. Data and Computer Communications Data Transmission

2. Data Transmission What we've got here is failure to communicate. Paul Newman in Cool Hand Luke

3. Data Transmission The successful transmission of data depends on two factors: • quality of the signal being transmitted • characteristics of the transmission medium

4. Transmission Terminology

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 Wavess(t) = A sin(2ft +)

12. Wavelength ()

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. FrequencyDomainRepresentations • frequency domain function of Fig 3.4c • frequency domain function of single square pulse

16. Spectrum & Bandwidth

17. Data Rate and Bandwidth There is a direct relationship between data rate and bandwidth.

18. 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

19. Acoustic Spectrum (Analog)

20. Digital Data

21. Advantages & Disadvantages of Digital Signals

22. 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

23. Analog Signals

24. Digital Signals

25. Analog and Digital Transmission

26. 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

27. ATTENUATION • signal strength falls off with distance over any transmission medium • varies with frequency

28. 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

29. Noise

30. Categories of Noise

31. Categories of Noise Impulse Noise: 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 Crosstalk: • a signal from one line is picked up by another • can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals

32. Channel Capacity

33. Nyquist Bandwidth In 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

34. 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

35. Classifications of Transmission Media • Transmission Medium • Physical path between transmitter and receiver • Guided Media • Waves are guided along a solid medium • E.g., copper twisted pair, copper coaxial cable, optical fiber • Unguided Media • Provides means of transmission but does not guide electromagnetic signals • Usually referred to as wireless transmission • E.g., atmosphere, outer space

36. Unguided Media • Transmission and reception are achieved by means of an antenna • Configurations for wireless transmission • Directional • Omnidirectional

37. General Frequency Ranges • Microwave frequency range • 1 GHz to 40 GHz • Directional beams possible • Suitable for point-to-point transmission • Used for satellite communications • Radio frequency range • 30 MHz to 1 GHz • Suitable for omnidirectional applications • Infrared frequency range • Roughly, 3x1011 to 2x1014 Hz • Useful in local point-to-point multipoint applications within confined areas

38. Terrestrial Microwave • Description of common microwave antenna • Parabolic "dish", 3 m in diameter • Fixed rigidly and focuses a narrow beam • Achieves line-of-sight transmission to receiving antenna • Located at substantial heights above ground level • Applications • Long haul telecommunications service • Short point-to-point links between buildings

39. Satellite Microwave • Description of communication satellite • Microwave relay station • Used to link two or more ground-based microwave transmitter/receivers • Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) • Applications • Television distribution • Long-distance telephone transmission • Private business networks

40. Broadcast Radio • Description of broadcast radio antennas • Omnidirectional • Antennas not required to be dish-shaped • Antennas need not be rigidly mounted to a precise alignment • Applications • Broadcast radio • VHF and part of the UHF band; 30 MHZ to 1GHz • Covers FM radio and UHF and VHF television

41. Multiplexing • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing - carrying multiple signals on a single medium • More efficient use of transmission medium

42. Multiplexing

43. Reasons for Widespread Use of Multiplexing • Cost per kbps of transmission facility declines with an increase in the data rate • Cost of transmission and receiving equipment declines with increased data rate • Most individual data communicating devices require relatively modest data rate support

44. Multiplexing Techniques • Frequency-division multiplexing (FDM) • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal • Time-division multiplexing (TDM) • Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal

45. Frequency-division Multiplexing

46. Time-division Multiplexing

47. 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