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Communications

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.

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Communications

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  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

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