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Module 1.9: Data Transmission

Module 1.9: Data Transmission

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Module 1.9: Data Transmission

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  1. Module 1.9: Data Transmission • Terminology and fundamental concepts • Harmonics • Bandwidth • Transmission impairments • Channel Capacity and Nyquist’s Theorem K. Salah

  2. Terminology • Medium • Guided medium • e.g. twisted pair, optical fiber • Unguided medium • e.g. air, water, vacuum • Simplex • One direction • 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 K. Salah

  3. Fundamental Concepts • Time domain concepts • Continuous signal (Analog) • Various in a smooth way over time • e.g. sound, video • Discrete signal (Digital) • Maintains a constant level then changes to another constant level • e.g., text, integers • Periodic signal • Pattern repeated over time • Aperiodic signal • Pattern not repeated over time K. Salah

  4. Sine Wave (Harmonic) Components • 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 K. Salah

  5. Bandwidth • Bandwidth (in analog communications) • The total capacity of communications channel measured in hertz(Hz). • It is the difference between the highest and lowest frequencies capable of being carried over a channel. • Any transmission system has a limited band of frequencies • This limits the data rate that can be carried • Bandwidth (in digital communications and networking) • The theoretical capacity of communications channel expressed in bits per second (bps), which is called data rate. K. Salah

  6. Analog and Digital Data Transmission • Signals • Electric or electromagnetic or light representations of data. Means of propagating Data. • Analog Signal • Represent data with continuously varying sine wave. • Continuously variable. Various media, e.g. wire, fiber optic, space • Speech bandwidth 100Hz to 7kHz, Telephone bandwidth 300Hz to 3400Hz, Video bandwidth 4MHz • Digital Signal • Represent data with sequence of voltage pulses • Use two DC components and vary it over time. Component of zero frequency. K. Salah

  7. Digital Signal • In RS-232, voltage that varies between -15 and -5 volts is interpreted as a binary 0, and voltage that varies between +5 and +15 volts is interpreted as a binary 1. Voltage that varies between -5 and +5 volts is ignored; that is, no interpretation is made. K. Salah

  8. Analog Signal • A single frequency with fixed A and  has 0 bandwidth. It carries no information. However, when it is combined with other frequencies or its components become variable, the bandwidth gets changed. Amplitude Change Frequency Change K. Salah

  9. Analog Signal Phase Change K. Salah

  10. Modem Modulation Techniques • Usually use digital signals for digital data and analog signals for analog data • Can use analog signal to carry digital data • Modem • Digital signals have a wide spectrum and thus are subject to strong attenuation and delay distortion. DC is good for short distances. • Analog signals are used instead with different modulation of components. K. Salah

  11. Analog vs. Digital Transmission • Transmission • Communication of data by propagation and processing of signals • 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 • 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 K. Salah

  12. 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 K. Salah

  13. Attenuation • 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 • Unlike FM, low frequency (AM) travels very far K. Salah

  14. Delay Distortion • Delay Distortion: • Only in guided media • Propagation velocity varies with frequency • Called also “Intersymbol Interference”. Due to delay distortions, some of the signal components of one bit position will spill over into other bit positions. K. Salah

  15. Noise • Additional signals inserted between transmitter and receiver • Thermal • Due to thermal agitation of electrons • Uniformly distributed • White noise • Crosstalk • A signal from one line is picked up by another • Impulse • Irregular pulses or spikes • Short duration • High amplitude • e.g. External electromagnetic interference such as lightning or cut in cable or fault in communication equipment. K. Salah

  16. Channel Capacity • Data rate = Capacity • In bits per second • Rate at which data can be communicated • Nyquist’s Theorem • In 1920, Henry Nyquist discovered that sampling rate must be at least twice the highest frequency, in order to construct the original signal. • Called also Sampling Rate theorem. • See nice animation at http://www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/cs/exploratories/applets/nyquist/nyquist_limit_java_browser.html • Also, maximum data rate (how much data you can represent in one signal) of any signal of frequency w is • MDR (noiseless) = 2 wlog2 M • M is the values used per signaling state • Shannon’s formula (only thermal noise) • MDR = wlog2 (1 + SNR) K. Salah