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

EE 4272Spring, 2003

EE 4272Spring, 2003 Concept & Terminology Direct link: No intermediate devices (except amplifiers, or repeaters) Medium  Guided medium: e.g. twisted pair, coaxial cable, 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 Full duplex  Both directions at the same time: e.g. telephone

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

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

EE 4272Spring, 2003 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 Frequency Domain Concepts

EE 4272Spring, 2003 Frequency Domain Time-domain function: s ( t ) Frequency-domain function: S ( f )

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

EE 4272Spring, 2003 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  Digital: Discrete values. e.g. text, integers Signals: Electric or electromagnetic representations of data  Analog Signal: continuously varying electromagnetic wave via various media, such as wire, fiber optic, space  Digital Signal: Use two DC components Transmission:Communication of data by propagation and processing of signals Data Rate, Bandwidth & Data Transmission

EE 4272Spring, 2003 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: p86-89

EE 4272Spring, 2003 Analog Signals Carrying Analog & Digital Data

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

EE 4272Spring, 2003 Analog & Digital Transmission 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->distort Digital Transmission  Concerned with content  Integrity endangered by noise, attenuation etc.  Repeaters used : Repeater receives signal -> Extracts bit pattern -> Retransmits  Distortion is overcome since noise is not amplified

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

EE 4272Spring, 2003 Transmission Impairments & Attenuation 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: the velocity of the propagation of a signal through a guided medium varies with frequency  Noise Transmission 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

EE 4272Spring, 2003 Noise Additional signals inserted between transmitter and receiver Thermal  Due to thermal agitation of electrons  Uniformly distributed across the frequency spectrum  Refer as White noise Intermodulation  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  due to electrical coupling between nearby twisted pairs, etc. Impulse: Short duration; High amplitude  Irregular pulses or spikes: e.g. External electromagnetic interference

EE 4272Spring, 2003 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  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 = 2B Pulse/second  For multilevel signaling: r = 2B log 2 M Pulse/second Shannon Channel Capacity : maximum achievable bit rate at which reliable comm. is possible over a channel of bandwidth B & of a given SNR  C = B log 2 (1+SNR) refer to “Information Theory”