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Unformatted text preview: Data and Computer Communications Communications
Chapter 3 – Data Transmission Chapter Eighth Edition by William Stallings Lecture slides by Lawrie Brown Data Transmission Data Toto, I've got a feeling we're not in Kansas Toto, anymore. Judy Garland in The Wizard of anymore Judy Oz Oz Transmission Terminology Transmission Terminology data transmission occurs between a data transmitter & receiver via some medium transmitter guided medium eg. twisted pair, coaxial cable, optical fiber eg. air, water, vacuum unguided / wireless medium Transmission Terminology Transmission Terminology direct link no intermediate devices direct link direct only 2 devices share link more than two devices share the link pointtopoint multipoint Transmission Terminology Transmission Terminology simplex one direction
• eg. television half duplex either direction, but only one way at a time
• eg. police radio full duplex both directions at the same time
• eg. telephone Frequency, Spectrum and Bandwidth Bandwidth time domain concepts analog signal
• various in a smooth way over time digital signal
• maintains a constant level then changes to another maintains constant level constant periodic signal
• pattern repeated over time aperiodic signal
• pattern not repeated over time Analogue & Digital Signals Analogue Periodic Periodic Signals Sine Wave Sine peak amplitude (A) maximum strength of signal volts rate of change of signal Hertz (Hz) or cycles per second period = time for one repetition (T) T = 1/f relative position in time frequency (f) phase (φ ) Varying Sine Waves Varying
s(t) = A sin(2π ft +Φ ) Wavelength (λ ) Wavelength is distance occupied by one cycle between two points of corresponding between phase in two consecutive cycles phase assuming signal velocity v have λ = vT assuming or equivalently λ f = v or especially when v=c especially v=c c = 3*108 ms1 (speed of light in free space) ms 1 (speed Frequency Domain Concepts Frequency signal are made up of many frequencies components are sine waves Fourier analysis can shown that any signal Fourier is made up of component sine waves is can plot frequency domain functions Addition of Frequency Frequency Components (T=1/f) c is sum of f & 3f is 3f Frequency Frequency Domain Representations Representations freq domain func of freq Fig 3.4c Fig freq domain func of freq single square pulse single Spectrum & Bandwidth Spectrum spectrum range of frequencies contained in signal width of spectrum absolute bandwidth effective bandwidth often just bandwidth often bandwidth narrow band of frequencies containing most energy component of zero frequency DC Component Data Rate and Bandwidth Data any transmission system has a limited band of any frequencies frequencies this limits the data rate that can be carried square have infinite components and hence square bandwidth bandwidth but most energy in first few components limited bandwidth increases distortion have a direct relationship between data rate & have bandwidth bandwidth Analog and Digital Data Transmission Transmission data data entities that convey meaning electric or electromagnetic representations of electric data, physically propagates along medium data, communication of data by propagation and communication processing of signals processing signals & signalling transmission Acoustic Spectrum (Analog) Acoustic Audio Signals Audio freq range 20Hz20kHz (speech 100Hz7kHz) easily converted into electromagnetic signals varying volume converted to varying voltage can limit frequency range for voice channel to can 3003400Hz 3003400Hz Video Signals Video USA  483 lines per frame, at frames per sec have 525 lines but 42 lost during vertical retrace 63.5µ s per line 11µ s for retrace, so 52.5 µ s per video line for 525 lines x 30 scans = 15750 lines per sec max frequency if line alternates black and white horizontal resolution is about 450 lines giving horizontal 225 cycles of wave in 52.5 µ s max frequency of 4.2MHz Digital Data Digital as generated by computers etc. has two dc components bandwidth depends on data rate Analog Signals Analog Digital Signals Digital Advantages & Disadvantages of Digital Signals of cheaper less susceptible to noise but greater attenuation digital now preferred choice Transmission Impairments Transmission signal received may differ from signal signal transmitted causing: transmitted analog  degradation of signal quality digital  bit errors attenuation and attenuation distortion delay distortion noise most significant impairments are Attenuation Attenuation where signal strength falls off with distance depends on medium received signal strength must be: strong enough to be detected sufficiently higher than noise to receive without error so increase strength using amplifiers/repeaters is also an increasing function of frequency so equalize attenuation across band of so frequencies used frequencies eg. using loading coils or amplifiers Delay Distortion Delay only occurs in guided media propagation velocity varies with frequency hence various frequency components hence arrive at different times arrive particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference Noise Noise additional signals inserted between additional transmitter and receiver transmitter thermal due to thermal agitation of electrons uniformly distributed white noise signals that are the sum and difference of signals original frequencies sharing a medium original intermodulation Noise Noise crosstalk a signal from one line is picked up by another irregular pulses or spikes
• eg. external electromagnetic interference impulse short duration high amplitude a minor annoyance for analog signals but a major source of error in digital data
• a noise spike could corrupt many bits Channel Capacity Channel max possible data rate on comms channel max is a function of data rate  in bits per second bandwidth  in cycles per second or Hertz noise  on comms link error rate  of corrupted bits limitations due to physical properties want most efficient use of capacity Nyquist Bandwidth Nyquist consider noise free channels iif rate of signal transmission is 2B then can carry f signal with frequencies no greater than B ie. given bandwidth B, highest signal rate is 2B for binary signals, 2B bps needs bandwidth B Hz can increase rate by using M signal levels Nyquist Formula is: C = 2B log2M so increase rate by increasing signals at cost of receiver complexity limited by noise & other impairments Shannon Capacity Formula Shannon consider relation of data rate, noise & error rate faster data rate shortens each bit so bursts of noise faster affects more bits affects given noise level, higher rates means higher errors Shannon developed formula relating these to Shannon signal to noise ratio (in decibels) signal SNRdb=10 log10 (signal/noise) 10 (signal/noise) Capacity C=B log2(1+SNR) theoretical maximum capacity capacity get lower in practise Summary Summary looked at data transmission issues frequency, spectrum & bandwidth analog vs digital signals transmission impairments ...
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 Spring '09
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