Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 1 Continuous & Discrete Signals

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 2 Example of Periodic Signals Frequency, Spectrum and Bandwidth

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 3 Varying Sine Waves

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 4 Frequency Domain Concepts Signal usually made up of many frequencies Components are sine waves Can be shown (Fourier analysis) that any signal is made up of component sine waves Can plot frequency domain functions

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 5 Addition of Frequency Components

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 6 Frequency Domain

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 7 Spectrum & Bandwidth Spectrum –range of frequencies contained in signal Absolute bandwidth –width of spectrum Effective bandwidth –Often just bandwidth –Narrow band of frequencies containing most of the energy DC Component –Component of zero frequency

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 8 Signal with DC Component

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 9 Data Rate and Bandwidth Any transmission system has a limited band of freq This limits the data rate that can be carried Data - entities that convey meaning Signals - Electric or electromagnetic rep of data Transmission - Communication of data by propagation and processing of signals

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 10 Relationship between Data Rate and BW any waveform may contain frequencies over a broad spectrum digital waveform has an infinite range of frequencies if the data rate is W bps, then a very good representation of the data can be achieved using a signal with bandwidth >2W Hz the greater the bandwidth of a transmission system, the higher is the cost; and the higher the data rate which can be transmitted over that system

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 11

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 12

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 13 Signals Means by which data are propagated Analog –Continuously variable –Various media: wire, fiber optic, space –Speech bandwidth 100Hz to 7kHz –Telephone bandwidth 300Hz to 3400Hz –Video bandwidth 4MHz Digital –Use two DC components

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 14 Analog Signals Carrying Analog and Digital Data

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 15 Digital Signals Carrying Analog and Digital Data

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 16 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

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 17 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

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 18 Advantages of Digital Transmission Digital technology - low cost LSI/VLSI Data integrity - longer dist over lower quality lines Capacity utilization –High bandwidth and cheaper links –High degree of multiplexing with digital techniques Security & Privacy - easier encryption Integration –Can treat analog and digital data similarly

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 19 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

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 20 Signal Strength as signal propagates along a transmission path, signal strength is reduced Why dB? signal strength often falls off logarithmically, hence it is easier to express the strength in dB. Gains and losses in a cascaded path can be calculated with addition or subtraction of the numerical values.

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 21

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 22 Attenuation: signal amplitude decreases along the transmission medium for guided media, attenuation is generally logarithmic for unguided media, attenuation is more complex specified max medium distance, use repeaters or amplifiers received signal strength must be enough to be detected, and sufficiently higher than noise to be received without error attenuation increases as a function of freq, use equalizers to equalizing attenuation across a band of freq Delay Distortion: vel of propag of a signal through a guided medium varies with freq for a bandlimited signal, velocity is highest at the centre freq and velocity will be reduced at the higher and the lower ends with digital signal, different freq components will arrive at the receiver at different time, this gives rise to intersymbol interference, a major limitation to max bit rate

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 23 Noise: thermal noise is a function of temperature and uniformly distributed across freq effect of intermodulation is to produce unwanted signal at a freq that is the sum and difference of original freqs sharing a medium crosstalk is the unwanted coupling between signal paths impulse noise is noncontinuous, irregular high amplitude pulses of short duration, primary source of errors in digital data Channel Capacity: 4 related concepts: data rate - in bps bandwidth - bandwidth of the transmitted signal constrained by the transmitter or the transmission medium, in cycles/s or Hz noise - average level of noise error rate - rate at which errors occur

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 24 for digital data, we would like to get as high a data rate as possible at a particular limit of error rate for a given bandwidth Nyquist Rate: for noise free channel, given a channel with bandwidth W Hz, the highest signal rate that can be carried is 2W bps, with multilevel signalling: Shannon's equation - the max channel capacity in a noisy channel: where S/N is the signal-to-noise ratio of the channel Shannon's capacity is the theoretical maximum, it does NOT account for impulse noise, attenuation or delay distortion

Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Transmission Slide 25