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ITC242 – Introduction to Data Communications Week 9 Topic 14 Data Transmission Topic 15 Data Communication Fundamentals Reading 3. Last Week. Reading 2 – Wide Area and Large-Scale Networks Describe the basic concepts associated with wide area networks

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Last Week

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  1. ITC242 – Introduction to Data CommunicationsWeek 9Topic 14 Data TransmissionTopic 15 Data Communication FundamentalsReading 3

  2. Last Week Reading 2 – Wide Area and Large-Scale Networks • Describe the basic concepts associated with wide area networks • Identify the uses, benefits, and drawbacks of WAN technologies such as ATM, FDDI, SONET, SMDS

  3. Topic 14 – Data Transmission Learning Objectives • Describe the difference between analogue and digital signals • Discuss the various transmission impairments that affect signal quality and transfer rate • Use Shannon’s formula to calculate the capacity of a channel

  4. Electromagnetic Signals • Analog Signal • signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal • Digital Signal • signal intensity maintains a constant level for some period of time and then changes to another constant level

  5. Analog Sine Wave

  6. Digital Square Wave

  7. Signal Characteristics • Peak Amplitude (A) • Maximum signal value, measured in volts • Frequency (f) • Repetition rate • Measured in cycles per second or Hertz (Hz) • Period (T) • Amount of time it takes for one repetition, T=1/f • Phase () • Relative position in time, measured in degrees

  8. Frequency Domain Concepts • Spectrum of a signal is the range of frequencies that it contains • Absolute bandwidth of a signal is the width of the spectrum • Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found • The greater the bandwidth, the higher the information-carrying capacity of the signal

  9. 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

  10. Voice/Audio Analog 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 • Actual bandwidth used is 4kHz to isolate audio signal from adjacent bandwidths.

  11. Voice Bandwidth

  12. Digital Text Signals • 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 forms: ASCII, UTF

  13. e.g. ASCII Character Set Control Characters

  14. e.g. ASCII Character Set Printable Characters

  15. Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate • bandwidth • physical environment • number of receivers • impairments

  16. 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)

  17. Transmission Impairments:Guided Media • Attenuation • loss of signal strength over distance • Attenuation Distortion • different losses at different frequencies • Delay Distortion • different speeds for different frequencies • Noise • distortions of signal caused by interference

  18. Transmission Impairments:Unguided (Wireless) Media • Free-Space Loss • Signals disperse with distance • Atmospheric Absorption • Water vapor and oxygen contribute to signal loss • Multipath • Obstacles reflect signal creating multiple copies • Refraction • Thermal Noise

  19. Types of Noise • Thermal (aka “white noise”) • Uniformly distributed, cannot be eliminated • Intermodulation • When different frequencies collide (creating “harmonics”) • Crosstalk • Overlap of signals • Impulse noise • Irregular spikes, less predictable

  20. Channel Capacity • The rate at which data can be transmitted over a given path, under given conditions • Four concepts • Data rate • Bandwidth • Noise • Error rate

  21. Shannon’s Equation • The signal to noise ratio (SNR) sets the upper bound on the achievable data rate. • Shannon’s equation provides a theoretical maximum channel capacity given SNR. • C = B log2 (1 + SNR) • B = Bandwidth of channel in Hertz • C= Channel capacity in bps • SNR = Signal-to-noise ratio

  22. Shannon’s Equation • What is the capacity of a signal operating at 5kHz with a SNR of 3? C = B log2 (1 + SNR) C = 5000 Hz x log2(1+3) C = 5000 Hz x log24 C = 5000 Hz x 2 C = 10000 bps

  23. Review • Describe the difference between analogue and digital signals • Transmission impairments – attenuation and noise affect signal quality • Shannon’s formula provides a theoretical estimate of maximum channel capacity

  24. Topic 15 – Data Communication Fundamentals Learning Objectives • Explain the difference between analogue and digital transmission • Describe digital and analogue encoding techniques for the transmission of data • Explain the difference between asynchronous and synchronous transmission and when each technique is used • Describe the process of error detection

  25. Data Communication Components • Data • Analog: Continuous value data (sound, light, temperature) • Digital: Discrete value (text, integers, symbols) • Signal • Analog: Continuously varying electromagnetic wave • Digital: Series of voltage pulses (square wave) • Transmission • Analog: Works the same for analog or digital signals • Digital: Used only with digital signals

  26. Analog DataSignal Options • Analog data to analog signal • Inexpensive, easy conversion (eg telephone) • Data may be shifted to a different part of the available spectrum (multiplexing) • Used in traditional analog telephony • Analog data to digital signal • Requires a codec (encoder/decoder) • Provides benefits of digital transmission (e.g Audio CD)

  27. Digital DataSignal Options • Digital data to analog signal • Requires modem (modulator/demodulator) • Allows use of PSTN to send data • Necessary when analog transmission is used • Digital data to digital signal • Requires CSU/DSU (channel service unit/data service unit) • Less expensive when large amounts of data are involved • More reliable because no conversion is involved

  28. Transmission Choices • Analog transmission • only transmits analog signals, without regard to data content • attenuation overcome with amplifiers • signal is not evaluated or regenerated • Digital transmission • transmits analog or digital signals • uses repeaters rather than amplifiers • switching equipment evaluates and regenerates signal

  29. Advantages of Digital Transmission • The signal is exact • Signals can be checked for errors • Noise/interference are easily filtered out • A variety of services can be offered over one line • Higher bandwidth is possible with data compression

  30. Why Use Analog Transmission? • Already in place • Significantly less expensive • Lower attenuation rates • Sufficient for transmission of voice signals

  31. Analog Encoding of Digital Data • Data encoding and decoding technique to represent data using the properties of analog waves • Modulation: the conversion of digital signals to analog form • Demodulation: the conversion of analog data signals back to digital form

  32. Methods of Modulation • Amplitude modulation (AM) or amplitude shift keying (ASK) • Frequency modulation (FM) or frequency shift keying (FSK) • Phase modulation or phase shift keying (PSK)

  33. Amplitude Shift Keying (ASK) • In radio transmission, known as amplitude modulation (AM) • The amplitude (or height) of the sine wave varies to transmit the ones and zeros • Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

  34. ASK Illustration 1 0 0 1

  35. Frequency Shift Keying (FSK) • In radio transmission, known as frequency modulation (FM) • Frequency of the carrier wave varies in accordance with the signal to be sent • Signal transmitted at constant amplitude • More resistant to noise than ASK • Less attractive because it requires more analog bandwidth than ASK

  36. FSK Illustration 1 1 0 1

  37. Phase Shift Keying (PSK) • Also known as phase modulation (PM) • Frequency and amplitude of the carrier signal are kept constant • The carrier signal is shifted in phase according to the input data stream • Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

  38. PSK Illustration 0 0 1 1

  39. Modulation Example Asymmetric Digital Subscriber Line (ADSL) • Telephone exchange can provide support for a number of ISPs, • At the exchange a combined data/voice signal is transmitted over a subscriber line • At subscriber’s site, twisted pair is split and routed to both a PC and a telephone • At the PC, an ADSL modem demodulates the data signal for the PC. • At the telephone, a microfilter passes the 4-kHz voice signal. • The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques

  40. DSL Modem Layout

  41. Digital Encoding of Analog Data • Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form • Best-known technique for voice digitization is pulse-code modulation (PCM) • The sampling theorem: Exact reconstruction of a continuous-time baseband signal from its samples is possible if the signal is bandlimited and the sampling frequency is greater than twice the signal bandwidth • Good-quality voice transmission can be achieved with a data rate of 8 kbps • Some videoconference products support data rates as low as 64 kbps

  42. Converting Samples to Bits • Quantizing • Similar concept to pixelization • Breaks wave into pieces, assigns a value in a particular range • 8-bit range allows for 256 possible sample levels • More bits means greater detail, fewer bits means less detail

  43. Converting Samples to Bits

  44. Codec • Coder/Decoder • Converts analog signals into a digital form and converts it back to analog signals • Where do we find codecs? • Sound cards • Scanners • Voice mail • Video capture/conferencing

  45. Digital Encodingof Digital Data • Most common, easiest method is different voltage levels for the two binary digits • Typically, negative=1 and positive=0 • Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the value during a bit time is a level voltage

  46. Differential NRZ • Differential version is NRZI (NRZ, invert on ones) • A bit time: a constant-voltage pulse • A signal transition at the beginning of the bit time: Change=1, no change=0 • Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

  47. Problems With NRZ • Difficult to determine where one bit ends and the next begins • In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses • Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

  48. Biphase Alternatives to NRZ • Require at least one transition per bit time, and may even have two • Modulation rate is greater, so bandwidth requirements are higher • Advantages • Synchronization due to predictable transitions • Error detection based on absence of a transition

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