William Stallings Data and Computer Communications Chapter 3

William Stallings Data and Computer Communications Chapter 3 Data Transmission

Terminology (1) z Transmitter z Receiver z Medium y. Guided medium xe. g. twisted pair, optical fiber y. Unguided medium xe. g. air, water, vacuum

Terminology (2) z Direct link y. No intermediate devices z Point-to-point y. Direct link y. Only 2 devices share link z Multi-point y. More than two devices share the link

Terminology (3) z Simplex y. One direction xe. g. Television z Half duplex y. Either direction, but only one way at a time xe. g. police radio z Full duplex y. Both directions at the same time xe. g. telephone

Frequency, Spectrum and Bandwidth z Time domain concepts y. Continuous signal x. Various in a smooth way over time y. Discrete signal x. Maintains a constant level then changes to another constant level y. Periodic signal x. Pattern repeated over time y. Aperiodic signal x. Pattern not repeated over time

Continuous & Discrete Signals

Periodic Signals

Sine Wave z Peak Amplitude (A) ymaximum strength of signal yvolts z Frequency (f) y. Rate of change of signal y. Hertz (Hz) or cycles per second y. Period = time for one repetition (T) y. T = 1/f z Phase ( ) y. Relative position in time

Varying Sine Waves

Wavelength z Distance occupied by one cycle z Distance between two points of corresponding phase in two consecutive cycles z z Assuming signal velocity v y = v. T y f = v yc = 3*108 ms-1 (speed of light in free space)

Frequency Domain Concepts z Signal usually made up of many frequencies z Components are sine waves z Can be shown (Fourier analysis) that any signal is made up of component sine waves z Can plot frequency domain functions

Addition of Frequency Components

Frequency Domain

Spectrum & Bandwidth z Spectrum yrange of frequencies contained in signal z Absolute bandwidth ywidth of spectrum z Effective bandwidth y. Often just bandwidth y. Narrow band of frequencies containing most of the energy z DC Component y. Component of zero frequency

Signal with DC Component

Data Rate and Bandwidth z Any transmission system has a limited band of frequencies z This limits the data rate that can be carried

Analog and Digital Data Transmission z Data y. Entities that convey meaning z Signals y. Electric or electromagnetic representations of data z Transmission y. Communication of data by propagation and processing of signals

Data z Analog y. Continuous values within some interval ye. g. sound, video z Digital y. Discrete values ye. g. text, integers

Acoustic Spectrum (Analog)

Signals z Means by which data are propagated z Analog y. Continuously variable y. Various media xwire, fiber optic, space y. Speech bandwidth 100 Hz to 7 k. Hz y. Telephone bandwidth 300 Hz to 3400 Hz y. Video bandwidth 4 MHz z Digital y. Use two DC components

Data and Signals z Usually use digital signals for digital data and analog signals for analog data z Can use analog signal to carry digital data y. Modem z Can use digital signal to carry analog data y. Compact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Analog Transmission z Analog signal transmitted without regard to content z May be analog or digital data z Attenuated over distance z Use amplifiers to boost signal z Also amplifies noise

Digital Transmission z Concerned with content z Integrity endangered by noise, attenuation etc. z Repeaters used z Repeater receives signal z Extracts bit pattern z Retransmits z Attenuation is overcome z Noise is not amplified

Advantages of Digital Transmission z Digital technology y. Low cost LSI/VLSI technology z Data integrity y. Longer distances over lower quality lines z Capacity utilization y. High bandwidth links economical y. High degree of multiplexing easier with digital techniques z Security & Privacy y. Encryption z Integration y. Can treat analog and digital data similarly

Transmission Impairments z Signal received may differ from signal transmitted z Analog - degradation of signal quality z Digital - bit errors z Caused by y. Attenuation and attenuation distortion y. Delay distortion y. Noise

Attenuation z Signal strength falls off with distance z Depends on medium z Received signal strength: ymust be enough to be detected ymust be sufficiently higher than noise to be received without error z Attenuation is an increasing function of frequency

Delay Distortion z Only in guided media z Propagation velocity varies with frequency

Noise (1) z Additional signals inserted between transmitter and receiver z Thermal y. Due to thermal agitation of electrons y. Uniformly distributed y. White noise z Intermodulation y. Signals that are the sum and difference of original frequencies sharing a medium

Noise (2) z Crosstalk y. A signal from one line is picked up by another z Impulse y. Irregular pulses or spikes ye. g. External electromagnetic interference y. Short duration y. High amplitude

Channel Capacity z Data rate y. In bits per second y. Rate at which data can be communicated z Bandwidth y. In cycles per second of Hertz y. Constrained by transmitter and medium

Required Reading z Stallings chapter 3