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COMP 421 /CMPET 401

COMP 421 /CMPET 401

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COMP 421 /CMPET 401

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  1. COMP 421 /CMPET 401 COMMUNICATIONS and NETWORKING Chapter 3 Data Transmission

  2. Review Connection/Connectionless Service Example Reliable Message Stream Sequence of Pages Reliable byte stream Remote logon Unreliable connection Digitized Voice Unreliable datagram Electronic Junk Mail Acknowledged Datagram Registered mail Request-reply Database Query { Connection- oriented { Connection- less

  3. Review Connection/Connectionless • Connection-oriented service is modeled after the Telephone Company • Connectionless Service is modeled after the Postal System PRIMITIVE MEANING Request A Entity wants the service to do something Indication A Entity is informed about an event Response An Entity wants to respond to an event Confirm The response to an earlier request has come back

  4. A Sample Connection Oriented Service CONNECT.request Request a connection CONNECT.indication Signal the called Party CONNECT.response Callee accepts or rejects call CONNECT.confirm Tell Caller whether call was accepted DATA.request Request that data be sent DATA.indication Signal the arrival of data DATA.response Request that connection be released DATA.confirm Signal peer about request Layer N+1 1 5 7 Computer 1 Layer N 4 6 1 2 3 4 5 6 7 8 9 10 Time Layer N+1 3 5 Computer 2 Layer N 2 6 8

  5. LAST WEEK - OSI • We Spoke about the OSI/ISO and TCP/IP Models • NEITHER the OSI model and its Protocols nor • the TCP/IP models and its protocols are perfect • Bad Timing • Bad Technology • Bad Implementations • Bad Politics. • OSI Model is • Printed Standards almost a meter thick • The standards are difficult to implement • The stands are inefficient in operation

  6. LAST WEEK - TCP/IP • The TCP/IP Model is • The first implementation of TCP/IP was part • of Berkeley UNIX and was good • The model does not clearly distinguish the concept of • Service • Interface • Protocol • The TCP/IP model is NOT general and is poorly • suited for describing any protocol other than TCP/IP • The TCP/IP model does not distinguish between the • Physical and Data Link Layers, which are completely different • While the TCP and IP stack are well thought out and • implemented, many of the other protocols were Ad Hoc, generally • produced by a couple of Grad Students hacking away until they got tired

  7. DECIBELS • Decibels are often used in communications when: • Talking about signal strength • Talking about the net gain or loss of a cascaded transmission path • A Decibel is a measure of the ratio between two signal levels • N = 10logP2/P1 N = number of decibels • P1=input power level • P2=output power level • dBW (decibel-watt) is the absolute power level • Power = 10log Power (watts)/1(watt) • 1mW = -30dBW • 1 W = 0 dBW • 1000W = 30dBW

  8. This Week: The 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

  9. 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

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

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

  12. Transmission 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

  13. Transmission 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

  14. Frequency, Spectrum, and Bandwidth • Electromagnetic signal are used to transmit data • This transmitted signal is a function of Time • Time-Domain • This transmitted signal can also be a function of Frequency • Frequency-Domain • The Frequency domain is more important in understanding • data transmission

  15. 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)

  16. Time domain concepts • A Continuous signal • Varies in a smooth way over time • A Discrete signal • Maintains a constant level then changes to another constant level • A Periodic signal • Pattern repeated over time • An Aperiodic signal • Pattern not repeated over time

  17. 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

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

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

  20. 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

  21. Continuous & Discrete Signals

  22. PeriodicSignals

  23. 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

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

  25. 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) • Remember T=1/ f

  26. 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

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

  28. 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

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

  30. 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

  31. Signal Transmission • Time T necessary to transmit a character depends on coding method and signaling 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

  32. 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

  33. Signal with DC Component

  34. Data Rate and Bandwidth • Any transmission system has a limited band of frequencies • This in turn limits the data rate that can be carried

  35. 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 or Bps

  36. 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

  37. Voice Grade Line • For a given Bit Rate of b bits/sec the time required to send 8 bits is b/8 Hz. • For a voice Grade Line has a cutoff frequency near 3000Hz • This restriction means that the number of the highest harmonic passed through is 3000/(b/8) or 24000/b

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

  39. Acoustic Spectrum (Analog)

  40. 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

  41. 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

  42. 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

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

  44. Bit rate and Baud rate • Bit rate number of bits that are transmitted in a second • Baud rate number of line signal changes (variations) per second If a modem transmits 1 bit for every signal change bit rate = baud rate If a signal change represents 2 or more or n bits bit rate = baud rate *n

  45. 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

  46. 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

  47. 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

  48. Analog Signals Carrying Analog and Digital Data

  49. QAM • QAM - Quadrature Amplitude Modulation • Diagrams that show legal combinations of amplitude and phase • are called CONSTELLATION PATTERNS 2 bits/Baud 8 Valid combinations 4800bps 4 bits/Baud 16 valid combinations 9600bps ITU V.32 modem standard • The next step after 9600bps is 14400bps and is called V.32 bis (transmits 6 bits) • This is followed by V.34 running at 28,800bps with 128 bit constellation

  50. Digital Signals Carrying Analog and Digital Data