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Transcript

Module 3.0: Data Transmission • • • • • Terminology and fundamental concepts Harmonics Bandwidth Transmission impairments Channel Capacity and Nyquist’s Theorem K. Salah 1 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 2 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 3 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 4 Wavelength • • • • • Length of wave or Distance occupied by one cycle Meaning the distance a simple signal can travel in one period Distance between two points of corresponding phase in two consecutive cycles Assuming signal velocity v – = vT – f = v – c = 3*108 m/s (speed of light in free space) or = 3*108 meters. 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 Propagation Delay • Propagation time measures the time required for a signal (or a bit) to travel from one point of the transmission medium to anoterh. K. Salah 7 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 8 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 9 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 10 Analog Signal Phase Change K. Salah 11 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 12 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 13 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 14 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 K. Salah 15 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 16 Noise • • Additional signals inserted between transmitter and receiver • Crosstalk – A signal from one line is picked up by another • Thermal – Due to thermal agitation of electrons – Uniformly distributed – White noise 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 17 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 w log2 M » M is the values used per signaling state – Shannon’s formula (only thermal noise) MDR = w log2 (1 + SNR) K. Salah 18