Data and Computer Communications

1. Data and Computer Communications Chapter 3 Data Transmission Required Reading: Stallings chapter 3

2. Source node Destination node Application Application Presentation Presentation Session Session Intermediate node transport transport Packets Network Network Network Frames Data link Data link Data link Bits Physical Physical Physical Signals Physical Layer

3. Physical / Data Link Layer Interface Sender Receiver NL HDR DLL Frame ACK PL HDR Transmitted Bits

4. Physical Layer • Communications and Information Theory are topics of whole courses • We’ll cover some theoretical basics regarding communications over a physical channel • We discover that there are physical limitations to communications over a given channel • We’ll cover some fundamental theorems

5. Terminology (1) • Transmitter • Receiver • Medium • Guided medium • e.g. twisted pair, optical fiber • Unguided medium • e.g. air, water, vacuum

6. Terminology (2) • Direct link • No intermediate devices • Point-to-point • Direct link • Only 2 devices share link • Multi-point • More than two devices share the link

7. Terminology (3) • Simplex • One direction (but in Europe means half duplex) • e.g. Television • Half duplex • Either direction, but only one way at a time • e.g. police radio • Full duplex • Both directions at the same time • e.g. telephone

8. Electromagnetic Signals • Function of time • Analog (varies smoothly over time) • Digital (constant level over time, followed by a change to another level) • Function of frequency • Spectrum (range of frequencies) • Bandwidth (width of the spectrum)

9. Frequency, Spectrum and Bandwidth • Time domain concepts • Continuous signal • Varies in a smooth way over time • Discrete signal • Maintains a constant level then changes to another constant level • Periodic signal • Pattern repeated over time • Aperiodic signal • Pattern not repeated over time

10. Periodic Signal Characteristics • Amplitude (A): signal value, measured in volts • Frequency (f ): repetition rate, cycles per second or Hertz • Period (T): amount of time it takes for one repetition, T=1/f • Phase (Φ): relative position in time, measured in degrees or radians

11. Analog Signaling • represented by sine waves 1 cycle amplitude (volts) phase difference time (sec) frequency (hertz) = cycles per second

12. Digital Signaling • represented by square waves or pulses 1 cycle amplitude (volts) time (sec) frequency (hertz) = cycles per second

13. Continuous & Discrete Signals

14. PeriodicSignals

15. Sine Wave • Peak Amplitude (A) • maximum strength of signal • volts • Frequency (f) • Rate of change of signal • Hertz (Hz) or cycles per second • Period = time for one repetition (T) • T = 1/f • Phase () • Relative position in time

16. Varying Sine Waves Sin2πt 0.5Sin2πt Phase Shift in radians Sin4πt or Phase Shift in seconds

17. Wavelength () • Distance occupied by one cycle • Distance between two points of corresponding phase in two consecutive cycles • Assuming signal velocity in space is equal to v •  = vT or • f = v • Here, V=c = 3*108 ms-1 (speed of light in free space)

18. Frequency Domain Concepts • A Signal is usually made up of many frequencies • Components are sine waves • It Can be shown (Fourier analysis) that any signal is made up of component sine waves • One can plot frequency domain functions instead of/in addition to time domain functions

19. Addition of FrequencyComponents (a) Sin(2πft) (b) (1/3)Sin(2π(3f)t) (c) (4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]

20. FrequencyDomain Note: For square waves, only odd harmonics exist (plus the fundamental component of course). Figure a is discrete because the time domain function is periodic. Figure b is continuous because the time domain function is aperiodic. (a) Frequency domain function for s(t)=(4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)] See Figure 3.16 Page 103. Note that s(f) is of the form (b) Frequency domain function for a single square pulse s(t)=1 for -X/2<t<X/2

21. Period = T g(t) = (1/2)c + S an sin(2pnft) + S bn cos(2pnft) n=1 n=1 f = 1/T is fundamental frequency a & b coefficients are the amplitude of the nth harmonic This is a Fourier Series Communications Basics • Represent a signal as a single-valued function of time, g(t), to model behavior of a signal (may be voltage, current or other change) • Jean-Baptiste Fourier showed we can represent a periodic signal (given some conditions) as the sum of a possibly infinite number of sines and cosines

22. Time -> Original Harmonic spectrum As we add more harmonics the signal reproduces the original more closely

23. Signal Transmission • No transmission facility can transmit signals without losing some power • Usually this attenuation is frequency dependent so the signal becomes distorted • Generally signal is completely attenuated above some max frequency (due to medium characteristics or intentional filtering) • The signal is bandwidth limited

24. Signal Transmission • Time T necessary to transmit a character depends on coding method and signalling speed • Signaling speed = number of times per second the signal changes value and is measured in baud • Note that baud rate is not necessarily the same as the bit rate • By limiting the bandwidth of the signal we also limit the data rate even if a channel is perfect • Overcome this by encoding schemes

25. Spectrum & Bandwidth • Spectrum • range of frequencies contained in signal • Absolute bandwidth • width of spectrum • Effective bandwidth • Often just bandwidth • Narrow band of frequencies containing most of the energy • DC Component • Component of zero frequency

26. Signal with DC Component (a) s(t)=1+(4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]

27. Data Rate and Bandwidth • Any transmission system has a limited band of frequencies • This limits the data rate that can be carried See Figure 3.8 Page 79

28. Bandwidth • Width of the spectrum of frequencies that can be transmitted • if spectrum=300 to 3400Hz, bandwidth=3100Hz • Greater bandwidth leads to greater costs • Limited bandwidth leads to distortion • Analog measured in Hertz, digital measured in baud

29. BPS vs. Baud • BPS=bits per second • Baud=# of signal changes per second • Each signal change can represent more than one bit, through variations on amplitude, frequency, and/or phase

30. Analog and Digital Data Transmission • Data • Entities that convey meaning • Signals • Electric or electromagnetic representations of data • Transmission • Communication of data by propagation and processing of signals

31. Data • Analog • Continuous values within some interval • e.g. sound, video • Digital • Discrete values • e.g. text, integers

32. Acoustic Spectrum (Analog)

33. Signals • Means by which data are propagated • Analog • Continuously variable • Various media • wire, fiber optic, space • Speech bandwidth 100Hz to 7kHz • Telephone bandwidth 300Hz to 3400Hz • Video bandwidth 4MHz • Digital • Use two DC components

34. Digital Text Signaling • Transmission of electronic pulses representing the binary digits 1 and 0 • How do we represent letters, numbers, characters in binary form? • Earliest example: Morse code (dots and dashes) • Most common current form: ASCII

35. ASCII Character Codes • Use 8 bits of data (1 byte) to transmit one character • 8 binary bits has 256 possible outcomes (0 to 255) • Represents alphanumeric characters, as well as “special” characters

36. Digital Image Signaling • Pixelization and binary representation Code: 00000000 00111100 01110110 01111110 01111000 01111110 00111100 00000000

37. Data and Signals • Usually use digital signals for digital data and analog signals for analog data • Can use analog signal to carry digital data • Modem • Can use digital signal to carry analog data • Compact Disc audio

38. Why Study Analog? • Telephone system is primarily analog rather than digital (designed to carry voice signals) • Low-cost, transmission medium (present almost at all places at all times • If we can convert digital information (1s and 0s) to analog form (audible tone), it can be transmitted inexpensively

39. Voice Signals • Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage • Human voice has frequency components ranging from 20Hz to 20kHz • For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz

40. Analog Signals Carrying Analog and Digital Data

41. Digital Signals Carrying Analog and Digital Data

42. Analog Transmission • Analog signal transmitted without regard to content • May be analog or digital data • Attenuated over distance • Use amplifiers to boost signal • Also amplifies noise

43. Digital Transmission • Concerned with content • Integrity endangered by noise, attenuation etc. • Repeaters used • Repeater receives signal • Extracts bit pattern • Retransmits • Attenuation is overcome • Noise is not amplified

44. Advantages of Digital Transmission • Digital technology • Low cost LSI/VLSI technology • Data integrity • Longer distances over lower quality lines • Capacity utilization • Economical high bandwidth links • High degree of multiplexing easier with digital techniques • Security & Privacy • Encryption • Integration • Can treat analog and digital data similarly

45. Transmission Media • the physical path between transmitter and receiver • design factors • bandwidth • attenuation: weakening of signal over distances • interference • number of receivers

46. Impairments and Capacity • Impairments exist in all forms of data transmission • Analog signal impairments result in random modifications that impair signal quality • Digital signal impairments result in bit errors (1s and 0s transposed)

47. Transmission Impairments • Signal received may differ from signal transmitted • Analog - degradation of signal quality • Digital - bit errors • Caused by • Attenuation and attenuation distortion • Delay distortion • Noise

48. Transmission Impairments • Attenuation • loss of signal strength over distance • Attenuation Distortion • different losses at different frequencies • Delay Distortion • different speeds for different frequencies • Noise

49. Attenuation P2 watts P1 watts receiver transmitter 10 log10 (P1/P2) dB Attenuation 10 log10 (P2/P1) dB Amplification

50. Attenuation • Signal strength falls off with distance • Depends on medium • Received signal strength: • must be enough to be detected • must be sufficiently higher than noise to be received without error • Attenuation is an increasing function of frequency