CS553_ST7_Ch03-DataTransmission.ppt
- 1. William Stallings Data and Computer Communications 7th Edition Chapter 3 Data Transmission
- 2. Terminology (1) • Transmitter • Receiver • Medium —Guided medium • e.g. twisted pair, optical fiber —Unguided medium • e.g. air, water, vacuum
- 3. Terminology (2) • Direct link —No intermediate devices • Point-to-point —Direct link —Only 2 devices share link • Multi-point —More than two devices share the link
- 4. Terminology (3) • Simplex —One direction • e.g. Television • Half duplex —Either direction, but only one way at a time • e.g. police radio • Full duplex —Both directions at the same time • e.g. telephone
- 5. Frequency, Spectrum and Bandwidth • Time domain concepts —Analog signal • Various in a smooth way over time —Digital signal • Maintains a constant level then changes to another constant level —Periodic signal • Pattern repeated over time —Aperiodic signal • Pattern not repeated over time
- 6. Analogue & Digital Signals
- 8. Sine Wave • Peak Amplitude (A) —maximum strength of signal —volts • Frequency (f) —Rate of change of signal —Hertz (Hz) or cycles per second —Period = time for one repetition (T) —T = 1/f • Phase () —Relative position in time
- 9. Varying Sine Waves s(t) = A sin(2ft +)
- 10. Wavelength • Distance occupied by one cycle • Distance between two points of corresponding phase in two consecutive cycles • • Assuming signal velocity v — = vT —f = v —c = 3*108 ms-1 (speed of light in free space)
- 11. 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
- 14. 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
- 15. Signal with DC Component
- 16. Data Rate and Bandwidth • Any transmission system has a limited band of frequencies • This limits the data rate that can be carried
- 17. Analog and Digital Data Transmission • Data —Entities that convey meaning • Signals —Electric or electromagnetic representations of data • Transmission —Communication of data by propagation and processing of signals
- 18. Analog and Digital Data • Analog —Continuous values within some interval —e.g. sound, video • Digital —Discrete values —e.g. text, integers
- 20. Analog and Digital 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
- 21. Advantages & Disadvantages of Digital • Cheaper • Less susceptible to noise • Greater attenuation —Pulses become rounded and smaller —Leads to loss of information
- 22. Attenuation of Digital Signals
- 23. Components of Speech • Frequency range (of hearing) 20Hz-20kHz —Speech 100Hz-7kHz • Easily converted into electromagnetic signal for transmission • Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage • Limit frequency range for voice channel —300-3400Hz
- 24. Conversion of Voice Input into Analog Signal
- 25. Video Components • USA - 483 lines scanned per frame at 30 frames per second — 525 lines but 42 lost during vertical retrace • So 525 lines x 30 scans = 15750 lines per second — 63.5s per line — 11s for retrace, so 52.5 s per video line • Max frequency if line alternates black and white • Horizontal resolution is about 450 lines giving 225 cycles of wave in 52.5 s • Max frequency of 4.2MHz
- 26. Binary Digital Data • From computer terminals etc. • Two dc components • Bandwidth depends on data rate
- 27. Conversion of PC Input to Digital Signal
- 28. Data and Signals • Usually use digital signals for digital data and analog signals for analog data • Can use analog signal to carry digital data —Modem • Can use digital signal to carry analog data —Compact Disc audio
- 29. Analog Signals Carrying Analog and Digital Data
- 30. Digital Signals Carrying Analog and Digital Data
- 31. 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
- 32. 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
- 33. Advantages of Digital Transmission • Digital technology — Low cost LSI/VLSI technology • Data integrity — Longer distances over lower quality lines • Capacity utilization — High bandwidth links economical — High degree of multiplexing easier with digital techniques • Security & Privacy — Encryption • Integration — Can treat analog and digital data similarly
- 34. 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
- 35. 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 • Attenuation is an increasing function of frequency
- 36. Delay Distortion • Only in guided media • Propagation velocity varies with frequency
- 37. Noise (1) • Additional signals inserted between transmitter and receiver • Thermal —Due to thermal agitation of electrons —Uniformly distributed —White noise • Intermodulation —Signals that are the sum and difference of original frequencies sharing a medium
- 38. Noise (2) • Crosstalk —A signal from one line is picked up by another • Impulse —Irregular pulses or spikes —e.g. External electromagnetic interference —Short duration —High amplitude
- 39. Channel Capacity • Data rate —In bits per second —Rate at which data can be communicated • Bandwidth —In cycles per second of Hertz —Constrained by transmitter and medium
- 40. Nyquist Bandwidth • If rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rate • Given bandwidth B, highest signal rate is 2B • Given binary signal, data rate supported by B Hz is 2B bps • Can be increased by using M signal levels • C= 2B log2M
- 41. Shannon Capacity Formula • Consider data rate,noise and error rate • Faster data rate shortens each bit so burst of noise affects more bits —At given noise level, high data rate means higher error rate • Signal to noise ration (in decibels) • SNRdb =10 log10 (signal/noise) • Capacity C=B log2(1+SNR) • This is error free capacity
- 42. Required Reading • Stallings chapter 3