Part II Data Communications Chapter 3 Data Transmission

Part II Data Communications Chapter 3 Data Transmission • Concept & Terminology • Signal : Time Domain & Frequency Domain Concepts • Signal & Data • Analog and Digital Data Transmission • Transmission Impairments & Attenuation • Nyquist Signaling Rate & Shannon Channel Capacity 12/4/2020 Spring, 2003 EE 4272

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Concept & Terminology • Direct link: No intermediate devices (except amplifiers, or repeaters) • Medium Guided medium: e. g. twisted pair, coaxial cable , optical fiber Ø Unguided medium: e. g. air, water, vacuum Ø • Simplex Ø One direction: e. g. Television • Half duplex Ø Either direction, but only one way at a time • Full duplex Ø Both directions at the same time: e. g. telephone 12/4/2020 Spring, 2003 EE 4272

Electromagnetic Signal : Frequency, Spectrum and Bandwidth 1. Time domain concepts Continuous signal: Various in a smooth way over time 2. Discrete signal 1. - Maintains a constant level then changes to another constant level Periodic signal: Pattern repeated over time 4. Aperiodic signal: Pattern not repeated over time 3. 12/4/2020 Spring, 2003 EE 4272

Periodic Signals Sine Wave • Peak Amplitude (A) Ø maximum strength of signal • Frequency (f) Rate of change of signal Ø Hertz (Hz) or cycles per second Ø Period = time for one repetition (T) Ø T = 1/f Ø 12/4/2020 Spring, 2003 EE 4272

Frequency Domain Concepts • The electromagnetic 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 • Addition of Frequency Components • Can plot frequency domain functions 12/4/2020 Spring, 2003 EE 4272

Frequency Domain • Time-domain function: s(t) • Frequency-domain function: S(f) 12/4/2020 Spring, 2003 EE 4272

Spectrum & Bandwidth • Spectrum: range of frequencies contained in signal • Absolute bandwidth: width of spectrum • Effective bandwidth Ø Narrow band of frequencies containing most of the energy • DC Component: Component of zero frequency 12/4/2020 Spring, 2003 EE 4272

Data Rate, Bandwidth & Data Transmission Data Rate and Bandwidth • Any transmission system has a limited band of frequencies -> bandwidth; This limits the data rate that can be carried Data Transmission • Data: Entities that convey meaning Analog: Continuous values within some interval. e. g. sound, video Ø Digital: Discrete values. e. g. text, integers Ø • Signals: Electric or electromagnetic representations of data Analog Signal: continuously varying electromagnetic wave via various media, such as wire, fiber optic, space Ø Digital Signal: Use two DC components Ø • Transmission: Communication of data by propagation and processing of signals 12/4/2020 Spring, 2003 EE 4272

More on Signals & Data • In a comm. sys. , data are propagated from one point to another by means of electric signal. • Usually use digital signals for digital data and analog signals for analog data transmission • Can use analog signal to carry digital data: Modem • Can use digital signal to carry analog data: Compact Disc audio Reading assignment: p 86 -89 12/4/2020 Spring, 2003 EE 4272

Analog Signals Carrying Analog & Digital Data 12/4/2020 Spring, 2003 EE 4272

Digital Signals Carrying Analog and Digital Data (e. g. Compact Disc Audio) (e. g. DSL) 12/4/2020 Spring, 2003 EE 4272

Analog & Digital Transmission • Analog Transmission Analog signal transmitted without regard to content Ø May be analog or digital data Ø Attenuated over distance Ø Use amplifiers to boost signal; Also amplifies noise->distort Ø • Digital Transmission Concerned with content Ø Integrity endangered by noise, attenuation etc. Ø Repeaters used : Repeater receives signal -> Extracts bit pattern -> Retransmits Ø Distortion is overcome since noise is not amplified Ø 12/4/2020 Spring, 2003 EE 4272

Advantages of Digital Transmission • Digital technology Ø Low cost LSI/VLSI technology • Data integrity: use repeater rather than amplifier Ø Longer distances while maintain data integrity • Capacity utilization High bandwidth links are economical Ø High degree of multiplexing easier with digital techniques Ø • Security & Privacy Ø Encryption • Integration: e. g. , voice, video, and digital data Ø Can treat analog and digital data similarly 12/4/2020 Spring, 2003 EE 4272

Transmission Impairments & Attenuation Transmission Impairments • Signal received may differ from signal transmitted Analog - degradation of signal quality Ø Digital - bit errors Ø • Caused by Attenuation and attenuation distortion Ø Delay distortion: the velocity of the propagation of a signal through a guided medium varies with frequency Ø Noise Ø Transmission Attenuation • Signal strength falls off with distance • Depends on medium • Received signal strength: must be enough to be detected Ø must be sufficiently higher than noise to be received without error Ø 12/4/2020 Spring, 2003 EE 4272

Noise Additional signals inserted between transmitter and receiver • Thermal Due to thermal agitation of electrons Ø Uniformly distributed across the frequency spectrum Ø Refer as White noise Ø • Intermodulation Ø Signals that are the sum or difference of original frequencies of the signals sharing a medium • Crosstalk: A signal from one line is picked up by another Ø due to electrical coupling between nearby twisted pairs, etc. • Impulse: Short duration; High amplitude Ø Irregular pulses or spikes: e. g. External electromagnetic interference 12/4/2020 Spring, 2003 EE 4272

Nyquist Signaling Rate & Shannon Channel Capacity • Channel Capacity: Maximum rate at which data can be transmitted over a path/channel, under given conditions • Data rate: Rate (in bps) at which data can be communicated • Bandwidth B: bandwidth of the transmitted signal is Ø Constrained by transmitter and medium (in Hertz) • Nyquist Signaling Rate: The fastest rate at which pulses can be transmitted into the channel (refer to “digital communication” ? ) For binary signaling: r = 2 B Pulse/second Ø For multilevel signaling: r = 2 B log 2 M Pulse/second Ø • Shannon Channel Capacity : maximum achievable bit rate at which reliable comm. is possible over a channel of bandwidth B & of a given SNR Ø C = B log 2 (1+SNR) 12/4/2020 Spring, 2003 refer to “Information Theory” EE 4272