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CSIS 625 Week 2

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  1. CSIS 625 Week 2 Encoding and Transmission of Data Copyright 2001, 2002 - Dan Oelke Portions Copyright 2000 - Dmitry Gringauz For use by students of CSIS 625 for purposes of this class only. CSIS 625

  2. Overview • Analog and Digital Signals • Vocabulary • Analog Signals • Digital Signals • Encoding and Modulation • Digital to Digital Conversion • Analog to Digital Conversion • Digital To Analog Conversion • Analog to Analog Conversion CSIS 625

  3. Analog and Digital Signals • Signal - an electromagnetic wave that transfers information • Analog Signal - Continuous set of data • Real Numbers • Digital Signals - Discrete set of data • Integer Numbers • Often binary (1 or 0 only) Digital Signal Analog Signal CSIS 625

  4. Periodic vs Aperiodic Signals • Periodic Signal • A signal that completes a pattern in a measureable time frame • Aperiodic Signal • A signal that does not exhibit a pattern • All aperiodic signals can be shown to be a combination of periodic signals APeriodic Signal Periodic Signal CSIS 625

  5. Signal definitions • Amplitude - The “height” of a signal. Measured in Volts, Amps, Watts, etc. • Period - The amount of time to complete one cycle • Frequency - The number of periods per second. Measured in Hertz (Hz) Amplitude Period CSIS 625

  6. Phase • The position of a sine wave relative to time zero. Measured in degrees. 0 Degrees 90 Degrees 1/4 Cycle 180 Degrees 1/2 Cycle 270 Degrees 3/4 Cycle CSIS 625

  7. Bandwidth • Bandwidth - A range of frequencies • Analog - measured in Hz • Bandwidth = High-Freq – Low-Freq • Spectrum - synonym - used only in analog measurements. • Bandwidth in digital realm - often used to refer to bits-per-second CSIS 625

  8. Bit Rate • Most digital signals are aperiodic • Period and frequency are not appropriate to describe digital signals • Bit Interval - time to send one bit • Bit rate - number of bits send in a second. Measured in bits per second • bps - Bits Per Second • Do NOT use Hz when you mean bps or vice-versa CSIS 625

  9. Decomposing a digital signal • A digital signal can be decomposed into an infinite number of simple sine waves • It is not practical or necessary to send all of these components • Significant Bandwidth - Those frequencies necessary to recreate a digital bit pattern • Significant Bandwidth is related to bit rate • Greater bit rate = Greater significant bandwidth CSIS 625

  10. Medium Bandwidth and Significant Bandwidth • All transmission mediums have limited bandwidth • The significant bandwidth of a digital bit rate must fit within the limited bandwidth of the medium that carries it. CSIS 625

  11. Encoding • Information must often be encoded before being sent over a medium • Four basic types of encoding • Digital to Digital • Analog to Digital • Digital to Analog • Analog to Analog • Encoding schemes may be stacked • Voice to digital data to radio waves CSIS 625

  12. Digital to Digital Encoding • Using a digital signal to represent digital data • Binary data is translated to different voltage, current, or light pulses that can be transported over the medium. • Types • Unipolar - uses 1 signal level • Polar - uses 2 signal levels • Bipolar - uses 2 signal levels and 0 CSIS 625

  13. Digital signal encoding formats 0 1 0 0 1 1 0 0 0 1 Unipolar NRZL NRZI RZ Manchester Differential Manchester Bipolar-AMI Pseudoternary CSIS 625

  14. Unipolar Encoding • Simplest scheme • Uses two signal levels • 1’s are encoded with signal present • 0’s are encoded by absence of a signal • (Sometimes inverse of the above) • Long run of 0s or 1s can’t be handled by some mediums CSIS 625

  15. Unipolar encoding - synchronization • When a signal isn’t varying, receiver can’t determine beginning and ending of each bit • Solutions: • A separate line with a clock signal • Asynchronzous Serial lines wrap each byte with start and stop bit • Scrambling of data to ensure enough transitions • Use of additional coding schemes like 8b10b CSIS 625

  16. Polar Encoding • Uses a positive and a negative signal • but not a zero level • Several types of Polar encoding • NRZ - Non-Return to Zero • RZ - Return to Zero • Biphase CSIS 625

  17. Non-Return to Zero - Level • NRZL - Non-Return to Zero - Level • Simple - exactly like Polar, except • 1’s are encoded with positive signal • 0’s are encoded with negative signal • (Sometimes inverse of the above) • Same synchronization problems and solutions CSIS 625

  18. Non-Return to Zero - Invert on Ones • NRZI - Non-Return to Zero - Invert on Ones • A change in voltage level indicates a 1 • No change in voltage level indicates a 0 • Synchronization less of a problem • Every 1 bit causes a signal change • A string of 0’s still causes problems • Same synchronization solutions CSIS 625

  19. Return to Zero • RZ - Return to Zero • Not strictly polar - uses 0 in addition to positive and negative • Works like NRZL, except it goes to zero between each bit. • Transition to/from zero provides for synchronization • Because there are more transisitions (2 per bit time) it has a higher significant bandwidth than NRZ CSIS 625

  20. Manchester Coding • A biphase mechanism • Inversion of signal in middle of each bit • low to high transition is 1 • high to low transition is 0 • Mid-bit inversion provides for both data and synchronization information • May have transition between bits so that right transition can be made in middle of a bit CSIS 625

  21. Differential Manchester • A biphase mechanism • Always has a mid-bit inversion to provide timing information • Inversion at beginning of bit time provides data • Presence of inversion means 0 • No inversion means 1 CSIS 625

  22. Bipolar AMI • Bipolar Alternate Mark Inversion • Mark comes from old telegraphy - means 1 • Encoding • 0 = lack of signal (0) • 1 = positive or negative values alternating for successive ones CSIS 625

  23. Pseudoternary • Same as Bipolar AMI, but inverts 1s and 0s • Encoding • 0 = positive or negative values alternating for successive zeros • 1 = lack of signal (0) CSIS 625

  24. 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 Binary-AMI V V B8ZS V = Bipolar AMI Violation B8ZS • Bipolar 8-Zero Substitution • A modification of Bipolar AMI to solve the synchronization problem that occurs when a long string of 0s occurs • Substitutes 8 consecutive 0s with fixed pattern that contains 2 AMI violations CSIS 625

  25. 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 Binary-AMI V V V HDB3 V = Bipolar AMI Violation HDB3 • High Density Bipolar - 3 Zeros • Similar to B8ZS • Substitutes 4 zeros with a pattern that contains 1 AMI violation CSIS 625

  26. Analog to Digital Encoding • Digitizing - analog to digital conversion • Approximate analog information with a digital signal • Reduces infinite number of analog values to a finite number of digital values. • Codec - Coder-Decoder • Analog to digital converter CSIS 625

  27. Pulse Amplitude Modulation (PAM) • First step to analog to digital encoding • Sample analog amplitude information at equal intervals • PAM alone not useful as measurements are still analog values CSIS 625

  28. Pulse Code Modulation (PCM) • Modifies PAM output to create completely digital signal • PCM quantizes Take the samples from PAM and assigns digital values to each measurement. • Nyquist theorem - To ensure accurate reproduction of a signal, the sample rate must be twice the highest frequency of the original signal CSIS 625

  29. PCM & Telephony • Telephony system uses 8 bits (256 levels) when quantizing • A non-linear set of quantizing levels is used so that quiet sounds are accurately reproduced • 300-3300Hz is voice range. • 8kHz sample rate is used to cover this range • 8kHz * 8 bits/sample = 64,000 bps CSIS 625

  30. DM - Delta Modulation • Analog data is approximated using a staircase function that moves up or down by one level each sampling time. • Digital data is a stream of 1s and 0s that specify the up and down steps. • Can be implemented using simple components. • Not as good quality as PCM • Quantizing noise when slope changes slowly • Slope overload noise when slope changes fast CSIS 625

  31. Digital to Analog Conversion • ASK - Amplitude Shift Keying • FSK - Frequency Shift Keying • PSK - Phase Shift Keying • QAM - Quadrature Amplitude Modulation • combination of ASK & PSK CSIS 625

  32. Bit rate vs. Baud Rate & Carrier Signal • Bit rate is Bits per Second • Baud Rate is number of signal units per second • Baud rate is less than or equal bit rate • Don’t mix them up! • Carrier Signal • high frequency signal that is modified to carry digital signal CSIS 625

  33. ASK - Amplitude Shift Keying • Amplitude of signal varied for 1 or 0 • Frequency and phase remain constant • Very susceptible to noise • On-Off-Keying - signal and no-signal • Example: 1 BIT 0 1 BIT 1 1 BIT 0 1 BIT 1 CSIS 625

  34. FSK- Frequency Shift Keying • Frequency of the carrier signal is varied to represent a 1or 0. • Avoids many of the noise problems of Amplitude Shift keying • Example: 1 BIT 0 1 BIT 1 1 BIT 0 1 BIT 1 CSIS 625

  35. PSK - Phase Shift Keying • The phase of the carrier signal is varied to represent a 1 or 0. • Avoids noise problems of ASK • Uses less bandwidth than FSK • Example: 1 BIT 0 1 BIT 1 1 BIT 0 1 BIT 1 CSIS 625

  36. QPSK - Quadrature PSK • A type of PSK that uses 90° shifts instead of 180° shifts. • Allows for 2 bits per baud to be encoded. CSIS 625

  37. DPSK - Differential PSK • The bit pattern defines the phase change, instead of the current phase • V.22bis standard at 1200 bps uses: • 00  90 Degree phase change • 01  0 Degree phase change • 10  180 Degree phase change • 11  270 Degree phase change CSIS 625

  38. Quadrature Amplitude Modulation • The phase and amplitude of the carrier signal is varied to give several bits per baud • Number of different phases is greater than number of amplitudes • Example: 2 amplitudes & 4 phases 3 BITS 000 3 BITS 010 3 BITS 001 3 BITS 111 CSIS 625

  39. Trellis Coded Modulation • Uses QAM, but includes extra data • Trellis coding is a specific type of convolutional encoding • Viterbi Decoder - a specific algorithm for decoding convolutionally encoded data. • Convolutional codes add redundancy to the data, which makes it more resistant to noise. • Resistance to noise is more important as data rates get higher. CSIS 625

  40. Constellation diagrams • Constellation diagram shows relationship between amplitude and phase of different signal levels • polar diagram, • amplitude shown as distance from center • phase shown as degrees around circle 011 010 0 1 0 1 101 100 000 001 110 ASK PSK 111 8-QAM 16-QAM CSIS 625

  41. Bandwidth required • Amplitude Shift Keying • bandwidth = baud rate * (1 + noise factor) • noise factor is 0 in ideal world • Frequency Shift Keying • bandwidth = (fc1 - fc0) + baud rate • Phase Shift Keying & QAM • bandwidth = baud rate * (1 + noise factor) • but bit rate is higher because more than one bit per baud CSIS 625

  42. Analog to Analog Encoding • AM - Amplitude Modulation • The amplitude of the carrier is modified • Bandwidth = 2x Bandwidth of modulating signal • FM- Frequency Modulation • The frequency of the carrier is modified • Bandwidth = 10x Bandwidth of modulation signal CSIS 625

  43. Analog to Analog Encoding • Phase Modulation • The phase of the carrier is modified • Phase Modulation and FM are a special case of Angle modulation • Observing the signal, it is impossible to tell apart FM and phase modulation CSIS 625

  44. Parallel/Serial Transmission of Data • Transmission of Digital Data • Serial & Parallel transmission • Serial interfaces - DTE & DCE - Modems CSIS 625

  45. Parallel Transmission of Data • Send several bits of data at the same time, each one over a separate media link. • Typically 8 bits of data sent over 8 wires • Examples: Printer cables, SCSI, PCI bus • Allows faster transmission of data, but at the cost of multiple wires, multiple transmitters, and multiple receivers • Must keep all bits in sync • Typically uses a separate clock line CSIS 625

  46. Serial Transmission of Data • Sends all bits from node to node over a single media link. • Bits are sent one after another - or “serially” • May or may not have additional media links for clock, frame, or flow control. • Need some method of keeping track of when a byte starts and ends. • Asynchronous or Synchronous CSIS 625

  47. Serial - Asynchronous transmission • Bits are grouped together into characters • Start and stop bits frame the data bits • A start bit is sent first • Followed by the data bits • Followed by a stop bit or bits • Variable number of idle bits between characters CSIS 625

  48. Start Start Data Data Data Data Data Data Data Data Data Data Data Data Data Data Data Data Stop Stop Serial - Asynchronous transmission • At best - 80% efficient • 8 data bits • 1 start bit • 1 stop bit • Allows for about a lot of timing error • Example: CSIS 625

  49. Serial - Synchronous transmission • Each byte of data is sent with no extra gaps between bytes. • Data is grouped into frames • Frame contains • Between frames, special idle patterns used • Much less overhead that asynchronous • Can achieve faster bit rates than asynchronous • Requires synchronization method CSIS 625

  50. Data transparency on serial links • Data transparency - the ability of a link to send any data pattern • Bit stuffing - insertion of extra bits to change a flag pattern so that data transparency is achieved • Byte stuffing - insertion of extra bytes to change a flag pattern so that data transparency is achieved • Flag character - special bit pattern to show start or end of a frame CSIS 625