Communications - PowerPoint PPT Presentation

communications n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Communications PowerPoint Presentation
Download Presentation
Communications

play fullscreen
1 / 90
Communications
132 Views
Download Presentation
akira
Download Presentation

Communications

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Communications • Getting information from one endpoint to another across a channel is communication • Type of data and medium affect coding of data • Modulation is the process of converting raw data to into signal for transmission • Demodulation is the process of decoding the signal back to raw data ICSS420/740 - Communication Theory

  2. Types of Modulation • The types of modulation can be roughly categorized by the type of data and signaling capabilities of the line • Analog Data, Analog Channel • Digital Data, Analog Channel • Digital Data, Digital Channel • Analog Data, Digtial Channel ICSS420/740 - Communication Theory

  3. Analog Data, Analog Channel • Communication on analog channel usually done with the help of a carrier signal • Carrier signal is a signal whose properties are know to the sender are receiver • Information is transmitted with deviations from this carrier signal(modulations) • Receiver knows what the carrier signal is supposed to look like, so it can detect changes ICSS420/740 - Communication Theory

  4. A-Data, A-Channel (cont’d) • Want to keep the bandwidth of the modulated signal small. By doing so, we can share bandwidth without interference. • Carriers are usually sinusoidal waves, which are ‘pure’ signals on a line. • Every sinusoid can have the following properties changed: amplitude, frequency, phase. ICSS420/740 - Communication Theory

  5. Bandwidth • Bandwidth: the information carrying capability of a channel. • The bandwidth for a given transmission medium is fixed. • Different mediums have different bandwidths. • “never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.” ICSS420/740 - Communication Theory

  6. Bandwidth LOW SPEED HIGH SPEED Ball size represents the Data Rate Box size represents the Bandwidth ICSS420/740 - Communication Theory

  7. Bandwidth ICSS420/740 - Communication Theory

  8. Bandwidth • bandwidth = highest frequency that can be transmitted • Voice line bandwidth (cut off frequency) = 3000 Hz • The bandwidth determines the max data rate • Nyquist’s Theorem for a noiseless channel • Shannon’s Theorem for a noisy channel • Generally, the data rate for a noisy channel is less than the data rate for a noiseless channel ICSS420/740 - Communication Theory

  9. Maximum Data Rate of a Channel • THM (Nyquist 1924) Noiseless Channels • If an arbitrary signal is run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by taking only 2H samples per second • Max data rate = 2H log2V bits/sec, where there are V values (levels) of the signal and log2V bits per sample • EX: H = 3000 Hz, V = 2 values (0 and 21-1=1) • Max data rate = 2 (3000) log22 = 6000 bits/sec • EX: H = 3000 Hz, V = 26 = 64 values (0,1,2,3,…,26-1=63) • Max data rate = 2 (3000) log264 = 36000 bits/se ICSS420/740 - Communication Theory

  10. Amplitude Modulation • Simplest form of analog modulation • Amplitude of the carrier signal is varied to represent data. • Sine wave is carrier, value of analog data is used as a gain factor on the amplitude. • Used for AM radio, but has largely been replaced due to susceptibility to noise. ICSS420/740 - Communication Theory

  11. Amplitude Modulation • Simplest form of analog modulation • Amplitude of the carrier signal is varied to represent data. • Sine wave is carrier, value of analog data is used as a gain factor on the amplitude. ICSS420/740 - Communication Theory

  12. ICSS420/740 - Communication Theory

  13. Dual Sideband / SSB • The picture previously duplicated the signal above and below the carrier signal, resulting in double bandwidth (Dual Sideband) • Single Sideband results by ignoring one of these, but then you lose your carrier signal as well ICSS420/740 - Communication Theory

  14. Frequency Modulation (FM) • Carrier’s signal frequency is modulated with respect to the data • Analog 0 usually means sinusoid at same frequency as carrier • Negative values compress carrier • Positive values expand carrier ICSS420/740 - Communication Theory

  15. ICSS420/740 - Communication Theory

  16. FM • Less susceptible to noise than AM, because noise that results in an increase in signal will not affect the decoded signal • Total bandwidth is greater than that of AM though. ICSS420/740 - Communication Theory

  17. Phase Modulation • Phase modulation involves changing the final property of the carrier, phase, or when it starts. • No practical in analog signals, due to subtleties in data with continuous communications (analog) • Works similar to FM ICSS420/740 - Communication Theory

  18. Periodic Waves Amplitude Cycle Cycle Cycle • The frequency of a periodic wave, measured in hertz (Hz), is the number of cycles that occur per second ICSS420/740 - Communication Theory

  19. Phase • Two identical waves that begin at different points in time are said to differ in their phase. ICSS420/740 - Communication Theory

  20. Digital Data, Analog Channel • Clearly needed for modern communications • Very large bandwidth – due to abrupt edges from signal pules. • Can share medium with multiple channels ICSS420/740 - Communication Theory

  21. Analog Transmission • Analog transmission has dominated all communication for the past 100 years. • Even though long-distance trunks are now digital, the local loop is still analog and will probably stay that way for a long time. • So when a computer uses a telephone to send data, the data must be converted to analog form for transmission. ICSS420/740 - Communication Theory

  22. Modems • A device that converts digital data into a modulated analog carrier signal that can be sent over analog transmission lines is called a modem. • MODEM stands for MOdulator/DEmodulator. • Modulation refers to the process of superimposing digital data onto an analog carrier signal. • Demodulation refers to the process of recovering the digital data from the modulated carrier. ICSS420/740 - Communication Theory

  23. THE DTE/DCE Interface DCE DTE ICSS420/740 - Communication Theory

  24. The DTE/DCE Interface • DTE - Data Terminal Equipment • typically an end-user device • supports end-user applications • DCE - Data Communications Equipment • connects the DTE into the communications circuit • The data communications path is the physical path between the DCEs. ICSS420/740 - Communication Theory

  25. Fourier Analysis • Any reasonably behaved periodic function, g(t), with period T can be constructed by summing a (possibly infinite) number of sines and cosines: Where f=1/T is the fundamental frequency and an and bn are the sine and cosine amplitudes of the nth harmonics (terms) ICSS420/740 - Communication Theory

  26. Fourier in Practice • Say you wanted to send the following signal 1 0 1 1 0 0 0 1 0 1 ICSS420/740 - Communication Theory

  27. Components of a Digital Signal ICSS420/740 - Communication Theory

  28. Components of a Digital Signal ICSS420/740 - Communication Theory

  29. Components of a Digital Signal ICSS420/740 - Communication Theory

  30. Components of a Digital Signal ICSS420/740 - Communication Theory

  31. Components of a Digital Signal ICSS420/740 - Communication Theory

  32. How does this affect communication? • No transmission facility can transmit signals without loosing some power in the process. • If all Fourier components were equally diminished, the resulting signal would be reduced in amplitude but not distorted. • Usually, the amplitudes are transmitted undiminished from 0 up to some frequency fc with all frequencies above the cutoff strongly attenuated. ICSS420/740 - Communication Theory

  33. Communication. • Additionally, if we want to have more signal changes, we have less room for harmonics, due to the signal being attenuated. • The maximum analog bandwidth is the number of changes we can get while still distinguishing the data. ICSS420/740 - Communication Theory

  34. Communication over a telephone • An ordinary telephone line has an artificially introduced cutoff frequency near 3000Hz. • If we transmit digital data at a rate of b bits/sec, the time required to send 8 bits is 8/b seconds, so the frequency of the first harmonic is b/8Hz. • This means on a telephone line the number of the highest harmonic passed through is 3000/(b/8) or 24,000/b. ICSS420/740 - Communication Theory

  35. Data Rates and Harmonics ICSS420/740 - Communication Theory

  36. Multi-Level Encoding • Multi-level encoding sends several bits in a single signal unit. Red 00 Green 01 Blue 10 White 11 ICSS420/740 - Communication Theory

  37. Baud Rate • The speed at which analog transmissions take place is usually measured in terms of BAUD. • The BAUD rate is the rate of signaling changes per second on a channel. The BAUD rate does not have to equal the bit rate. • Based on the Fourier analysis of a voice grade telephone line the highest BAUD rate that can be used is 2400 BAUD. • data rate = rate at which the data are sent, i.e., bits/sec • baud rate = rate at which the signal changes its value, • i.e., number of changes/sec ICSS420/740 - Communication Theory

  38. Amplitude Shift Keying (ASK) • Limited form of AM • Amplitude is max for a digital 1 • Amplitude is 0 (or very low) for digital 0 • Sometimes called On-Off-Keying (OOK) ICSS420/740 - Communication Theory

  39. ICSS420/740 - Communication Theory

  40. Multi-Level Encoding ICSS420/740 - Communication Theory

  41. M-ASK • If we want more then 2 leveled, we can! • M-ASK has M states • 8-ASK has 8 levels, and can encode 8 bits • 8-ASK has 4 levels and can encode 4 bits • The more levels you have, to more noise affect it ICSS420/740 - Communication Theory

  42. Frequency Shift Keying • Build upon FM • Can also do M-FSK where M is the number of frequencies we allow ICSS420/740 - Communication Theory

  43. ICSS420/740 - Communication Theory

  44. Phase Shift Keying • One of the most important modulation techniques • Allows for high data throughput with low channel bandwidth • Phase of the carrier sinusoid is shifted for each symbol ICSS420/740 - Communication Theory

  45. BPSK (PSK) • Single bit • Phase shifts are pi/2 and –pi/2 or • 0 and Pi ICSS420/740 - Communication Theory

  46. ICSS420/740 - Communication Theory

  47. What if we have 4 states? • Quadrature PSK (QPSK) • Smaller increments • Fewer abrupt signaling changes • Data throughput is double BPSK ICSS420/740 - Communication Theory

  48. Phase Modulation • Phase modulation is used almost exclusively on high speed modems • The phase modulation method is also called phase shift keying (PSK). • PSK can be used to provide multi-level encoding: ICSS420/740 - Communication Theory

  49. PSK 90 90 135 45 135 45 0 0 180 180 225 315 225 315 270 270 4PSK 4800bps 4PSK 4800 bps 8PSK 7200bps ICSS420/740 - Communication Theory

  50. M-PSK • We use constellation patterns to show represent the M-PSK • Angular component on the polar plane give us the phase offset • Radius give us the Amplitude (which is fixed in a PSK scheme) • Error rate beyond QPSK is high. ICSS420/740 - Communication Theory