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TEL205 Week 14

TEL205 Week 14. Stallings Chapter 15: Data Transmission. 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

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TEL205 Week 14

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  1. TEL205 Week 14 Stallings Chapter 15:Data Transmission

  2. 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

  3. Analog Sine Wave

  4. Digital Square Wave

  5. Periodic 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

  6. s(t) = (4/)  (sin (2ft) + (1/3) sin (2(3f)t))

  7. 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

  8. 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

  9. Analog Signaling

  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

  11. Image/Video: Analog Data to Analog Signals • Image is scanned in lines; each line is displayed with varying levels of intensity • Requires approximately 4Mhz of analog bandwidth • Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal

  12. Digital Signaling

  13. 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

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

  15. 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)

  16. 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

  17. 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

  18. 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

  19. 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

  20. Shannon Equation • C = B log2 (1 + SNR) • B = Bandwidth • C= Channel • SNR = Signal-to-noise ratio

  21. StallingsChapter 16:Data Communication Fundamentals

  22. 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

  23. 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) • Allows use of digital telephony, voice mail

  24. 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

  25. Transmission Choices • Analog transmission • only transmits analog signals, without regard for 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

  26. Data, Signal, and Transmission Matrix A Data D D Transmission System A A D Signal

  27. 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

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

  29. 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

  30. Modem • An acronym for modulator-demodulator • Uses a constant-frequency signal known as a carrier signal • Converts a series of binary voltage pulses into an analog signal by modulating the carrier signal • The receiving modem translates the analog signal back into digital data

  31. 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)

  32. 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

  33. ASK Illustration 1 0 0 1

  34. 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

  35. FSK Illustration 1 1 0 1

  36. 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)

  37. PSK Illustration 0 0 1 1

  38. Differential Phase Shift Keying (DPSK) 0 0 1 1

  39. Cable Modems • Permits Internet access over cable television networks. • ISP is at or linked by high-speed line to central cable office • Cables used for television delivery can also be used to deliver data between subscriber and central location • Upstream and downstream channels are shared among multiple subscribers, time-division multiplexing technique • Splitter is used to direct TV signals to a TV and the data channel to a cable modem

  40. Cable Modem Layout

  41. Asymmetric DigitalSubscriber Line (ADSL) • New modem technology for high-speed digital transmission over ordinary telephone wire. • Telephone central office can provide support for a number of ISPs, • At central office, 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 (Chapter 17)

  42. DSL Modem Layout

  43. 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: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal. • 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

  44. 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

  45. 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

  46. 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 voltage during a bit transmission is level

  47. Differential NRZ • Differential version is NRZI (NRZ, invert on ones) • 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

  48. 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

  49. 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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