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Data Communication Digital Transmition

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  1. Data CommunicationDigital Transmition Behrouz A. Forouzan Data Communication - Digital Transmition

  2. Index • Digital to Digital Conversion • Analog to Digital Conversion • Transmission Modes Data Communication - Digital Transmition

  3. Digital to Digital Conversion • Techniques • line coding • Always needed • block coding • May or may not needed • Scrambling • May or may not needed Data Communication - Digital Transmition

  4. Digital to Digital ConversionLine Coding • At sender, digital data are encoded into digital signal • At receiver, digital data are recreated by decoding the digital signal Data Communication - Digital Transmition

  5. Digital to Digital ConversionLine Coding Characteristics • Signal Element Versus Data Element • Data Rate Versus Signal Rate • Required Bandwidth • Baseline Wandering • DC Components • Self-synchronization • Built-in Error Detection • Immunity to Noise and Interference • Complexity Data Communication - Digital Transmition

  6. Digital to Digital ConversionData element Versus Signal Element • Data Element • smallest entity that can represent a piece of information (Bit) • Carried • Signal Element • the shortest unit of a digital signal • Carrier • r : number of data elements carried by each signal element Data Communication - Digital Transmition

  7. Digital to Digital ConversionData element Versus Signal Element Data Communication - Digital Transmition

  8. Digital to Digital ConversionData Rate Versus Signal Rate • Data Rate • number of data bits sent in 1 Sec. (bps) • Speed of transmition • Signal Rate • number of signal elements sent in 1 Sec. (baud) • Also called pulse rate or modulation rate • More signal rate more bandwidth requirement • Interest to increase the data rate while decreasing the signal rate Data Communication - Digital Transmition

  9. Digital to Digital ConversionData Rate Versus Signal Rate • Relationship depend on: • data stream (all 0, all 1 or alternate 0 1) • r (data element / signal element) • Relationship Formula: • define three cases: the worst (maximum signal rate), best (minimum signal rate ), and average • N = data rate (bps); • c is the case factor • S is signal rate • r previously defined Data Communication - Digital Transmition

  10. Digital to Digital ConversionRequired Bandwidth • actual bandwidth of digital signal is infinite but effective bandwidth is finite • baud rate, not bit rate, determines required bandwidth for a digital signal. • More changes means more baud rate means more frequency range means more required bandwidth • Minimum bandwidth for a given data rate: • Maximum data rate for a given Bandwidth: • . Data Communication - Digital Transmition

  11. Digital to Digital ConversionRequired Bandwidth • Compare Nyquist formula with previous formula • c = ½ (average case) and L = 2 , Then two formulas are the same Data Communication - Digital Transmition

  12. Digital to Digital ConversionBaseline Wandering • In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline. • The incoming signal power is evaluated against this baseline to determine the value of the data element • A long string of 0s or 1s can cause a drift in the baseline (baseline wandering) • Baseline wandering make it difficult for the receiver to decode correctly • A good line coding scheme needs to prevent baseline wandering. Data Communication - Digital Transmition

  13. Digital to Digital ConversionDC components • When voltage level in a digital signal is constant for a while, spectrum creates very low frequencies (around zero), called DC components • Creates problems for a system that cannot pass low frequencies • a telephone line cannot pass frequencies below 200 Hz. Data Communication - Digital Transmition

  14. Digital to Digital ConversionSelf-synchronization • receiver's bit intervals must correspond exactly to the sender's bit intervals • self-synchronizing digital signal includes timing information in the data being transmitted. • This achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. Data Communication - Digital Transmition

  15. Digital to Digital ConversionBuilt-in Error Detection • built-in error-detecting capability in the generated code to detect some of or all the errors that occurred during transmission • Differentiated coding Data Communication - Digital Transmition

  16. Digital to Digital ConversionImmunity to Noise and Interference • Good coding that is immune to noise and other interferences • Lower levels Data Communication - Digital Transmition

  17. Digital to Digital ConversionComplexity • complex scheme is more costly to implement than a simple one • Lower levels , lower signal change Data Communication - Digital Transmition

  18. Digital to Digital ConversionUnipolar NRZ • Unipolar Scheme • NRZ (Non-Return-to-Zero) • Polar Schemes • NRZ-L • NRZ-I • Return to Zero (RZ) • Biphase • Manchester • Differential Manchester • Bipolar Schemes (multilevel binary) • AMI • Pseudoternary • Multilevel Schemes • 2B/IQ, 8B/6T, and 4U-PAM5 • Multitransition • Multiline Transmission MLT-3 Data Communication - Digital Transmition

  19. Digital to Digital Conversionunipolar - NRZ • In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below • non-return-to-zero (NRZ) • Bit 0: zero voltage • Bit 1: positive voltage • NRZ means the signal does not return to zero at the middle of the bit Data Communication - Digital Transmition

  20. Digital to Digital ConversionLine Coding Schemes • Very costly because normalized power (power needed to send 1 bit) is double that for polar NRZ. • this scheme is normally not used in data communications. Data Communication - Digital Transmition

  21. Digital to Digital ConversionPolar / NRZ-L, NRZ-I • In polar schemes, the voltages are on the both sides of the time axis • NRZ-Level • level of the voltage determines the value of the bit • Bit 0: positive voltage • Bit 1: negative voltage • NRZ-Invert • change or lack of voltage change determines value of the bit • Bit 0: there is no change • Bit 1: there is a change Data Communication - Digital Transmition

  22. Digital to Digital ConversionPolar / NRZ-L, NRZ-I Bit Stream: 01001110 Data Communication - Digital Transmition

  23. Digital to Digital ConversionPolar / NRZ-L, NRZ-I Data Communication - Digital Transmition

  24. Digital to Digital ConversionPolar / NRZ-L, NRZ-I Characteristic • sudden change of polarity resulting in all 0s interpreted as 1s and all 0s interpreted as 1s • Required Bandwidth • Average: N/2 • Baseline Wandering • Yes • twice as severe in NRZ-L (long sequence of 0s or 1s) compare to NRZ-I (long sequence of 0s) • DC Components • Yes • value of power density is very high around frequencies close to zero • Self-synchronization • NO • more serious in NRZ-L Data Communication - Digital Transmition

  25. Digital to Digital ConversionPolar / Return to Zero (RZ) • Solution to synchronization problem in NRZ methods • uses three values: positive, negative, and zero • signal goes to 0 in the middle of each bit. • Advantages: • There is no DC component problem • DisAdvantages: • it requires two signal changes to encode a bit and occupies greater bandwidth • a sudden change of sudden change of polarity resulting in all 0s interpreted as 1s and all 0s interpreted as 1s • complexity • three levels of voltage, which is more complex to create and discern Data Communication - Digital Transmition

  26. Digital to Digital ConversionPolar / Return to Zero (RZ) Data Communication - Digital Transmition

  27. Digital to Digital ConversionManchester, Differentiated Manchester • Manchester : • idea of RZ and idea of NRZ-L are combined • always a transition at the middle of the bit, • Voltage level is determined by bit value like NRZ-L • Bit 0: positive voltage • Bit 1: negative voltage • Manchester : • combines the ideas of RZ and NRZ-I • always a transition at the middle of the bit, • bit values are determined at the beginning of the bit • Bit 0:transition • Bit 1: no transition Data Communication - Digital Transmition

  28. Digital to Digital Conversion Manchester, Differentiated Manchester Data Communication - Digital Transmition

  29. Digital to Digital Conversion Manchester, Differentiated Manchester • Advantage: • Self- synchronization • no baseline wandering • no DC component • Drawback: • signal rate is double that for NRZ • minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ Data Communication - Digital Transmition

  30. Digital to Digital Conversion Bipolar / AMI and Pseudoternary • there are three voltage levels: • positive, negative, and zero. • Alternate mark inversion (AMI): • Mark means 1 • Bit 0: zero voltage • Bit 1: alternating positive and negative voltages. • pseudoternary • Bit 0: alternating positive and negative voltages. • Bit 1: zero voltage Data Communication - Digital Transmition

  31. Digital to Digital Conversion Bipolar / AMI and Pseudoternary Data Communication - Digital Transmition

  32. Digital to Digital Conversion Bipolar / AMI and Pseudoternary • Alternative to NRZ but better • Advantage: • same signal rate as NRZ, but no DC component (why?) • For a long sequence of 0s, voltage remains constant, but its amplitude is zero, which is the same as having no DC component • We can prove it by using the Fourier transform • energy in bipolar encoding is around frequency N/2 but in NRZ energy was around zero which unsuitable for transmission over channels with poor performance around this frequency • commonly used for long-distance communication • Drawback: • Synchronization problem when a long sequence of 0s • scrambling technique can solve this problem (learn later) Data Communication - Digital Transmition

  33. Digital to Digital Conversion Multilevel Schemes • In mBnL schemes, a pattern of m data elements can be encoded as a pattern of n signal elements with L Levels in which ≤ • If < data patterns occupy only a subset of signal patterns. The subset can be carefully designed to prevent baseline wandering, to provide synchronization, and to detect errors that occurred during data transmission • B (binary data) for • L (Level) • (binary) for L =2, T (ternary) for L =3, Q (quaternary) for L =4. Data Communication - Digital Transmition

  34. Digital to Digital Conversion Multilevel Schemes / 2B1Q • 2BIQ: two binary, one quaternary • used in DSL • encodes the 2-bit data patterns as one signal element belonging to a four-level signal • Bit 00: +1 volt • Bit 01: +3 volt • Bit 10: -1 volt • Bit 11: - 3 volt • Advantages: • average signal rate of 2BlQ is S =N/4. • Drawbacks: • receiver has to discern four different thresholds (no noise immunity) Data Communication - Digital Transmition

  35. Digital to Digital Conversion Multilevel Schemes / 8B6T • 8B6T: eight binary, six ternary • Used in 100BASE-4T cable • 222 redundant signal elements that provide synchronization and error detection and DC balance • DC balance: • To make the whole stream Dc-balanced, the sender keeps track of the weight. • Each signal pattern has a weight of 0 or +1 DC value • If two groups of weight 1 are encountered one after another, the first one is sent as is, while the next one is totally inverted to give a weight of -1 Data Communication - Digital Transmition

  36. Digital to Digital Conversion Multilevel Schemes / 8B6T • Example: • The first 8-bit pattern 00010001 is encoded as the signal pattern -0-0++ with weight 0 • the second 8-bit pattern 010 10011 is encoded as - + - + + 0 with weight +1. • The third bit pattern should be encoded as + - - + 0 + with weight +1 • To create DC balance, the sender inverts the actual signal. The receiver can easily recognize that this is an inverted pattern because the weight is -1 • Theory: Savg = ½ *6/8 *N • Practice: Savg = 6/8 *N Data Communication - Digital Transmition

  37. Digital to Digital Conversion Multilevel Schemes / 4D-PAMS • 4D-PAMS: four dimensional five-level pulse amplitude modulation (4D-PAM5) • 4D: data is sent over four wires at the same time • It uses five voltage levels, such as -2, -1, 0, 1, and 2 • level 0, is used only for forward error detection • If we assume that the code is just one-dimensional, the four levels create something similar to 8B4Q. • Signal rate is 4N/8 = N/2 • With four channels (4 wires), signal rate can be reduced to N/8 • Gigabit LANs use this technique to send 1-Gbps data over four copper cables with 125 Mbaud (4 * 250Mbps = 1Gbps) • a lot of redundancy in the signal pattern Data Communication - Digital Transmition

  38. Digital to Digital Conversion Multiline Transmission: MLT-3 • NRZ-I and differential Manchester are differential encoding method • with two transition rules (no inversion, inversion). • signal with more than two levels, • Then differential encoding with more than two transition rules. Data Communication - Digital Transmition

  39. Digital to Digital Conversion Multiline Transmission: MLT-3 • three level (MLT-3) scheme uses three levels (+V, 0, and - V) and three transition rules to move between the levels • Bit 0: no transition • Bit 1 and current level not 0 : level 0 • Bit 1 and current level 0: opposite of last non-zero level Data Communication - Digital Transmition

  40. Digital to Digital Conversion Multiline Transmission: MLT-3 Data Communication - Digital Transmition

  41. Digital to Digital Conversion Multiline Transmission: MLT-3 • 1 bit for 1 Signal element So • Signal rate = NRZ-I but greater complexity why choose this method? • worst-case scenario: • A sequence of Is. • signal element pattern is +VO - VO is repeated every 4 bits. • A nonperiodic signal has changed to a periodic signal with the period equal to 4 times the bit duration. • This worst-case situation can be simulated as an analog signal with a frequency one-fourth of the bit rate. • signal rate for MLT-3 is one-fourth the bit rate • MLT-3 a suitable choice when we need to send 100 Mbps on a copper wire that cannot support more than 32 MHz Data Communication - Digital Transmition

  42. Digital to Digital Conversion Summary of Line Coding Schemes Data Communication - Digital Transmition

  43. Digital to Digital Conversion Block Coding • Block coding gives redundancy to ensure synchronization and to provide error detecting. • Block coding (mBlnB coding) replaces each m-bit group with an n-bit group, (n is larger than m) • division • substitution • combination • Methods: • 4B/5B • 8B/10B Data Communication - Digital Transmition

  44. Digital to Digital Conversion Block Coding 4B/5B • designed to be used with NRZ-I which has a good signal rate, but synchronization problem • Solution: 4B/5B Block Coding • no more than one leading zero (left bit) and no more than two trailing zeros (right bits) Data Communication - Digital Transmition

  45. Digital to Digital Conversion Block Coding 4B/5B • 4 bits  16 different combinations • 5 bits  32 different combinations. • there are 16 groups that are not used for 4B/5B encoding and used for • control purposes • (unused) error detection • If a 5-bit group arrives that belongs to the unused portion of the table, the receiver knows that there is an error in the transmission Data Communication - Digital Transmition

  46. Digital to Digital Conversion Block Coding 4B/5B Data Communication - Digital Transmition

  47. Digital to Digital Conversion Block Coding 4B/5B • add 20 percent more baud rate • Still, signal rate is less than biphase (2-times of NRZ-I) • don't solve DC component problem of NRZ-I • If a DC component is unacceptable, use biphase or bipolar encoding Data Communication - Digital Transmition

  48. Digital to Digital Conversion Block Coding 4B/5B • Example: • We need to send data at a 1-Mbps rate. What is the minimum required bandwidth, using a combination of 4B/5B and NRZ-I or Manchester coding? • 4B/5B : • increases bit rate to 1.25 Mbps. minimum bandwidth using NRZ-I is NI2 or 625 kHz. • DC Problem • Manchester: • needs a minimum bandwidth of 1 MHz. • No DC problem Data Communication - Digital Transmition

  49. Digital to Digital Conversion Block Coding 8B/10B • group of 8 bits data is substituted by a 10 bit • 768 redundant groups • Better built-in error-checking capability and better synchronization than 4B/5B • a combination of 5B/6B and 3B/4B encoding, Data Communication - Digital Transmition

  50. Digital to Digital Conversion Scrambling • Biphase (used in LAN ) are not suitable for long-distance communication because of their wide bandwidth requirement • block coding with NRZ-I is not suitable for long-distance, because of the DC component • Bipolar AMI has narrow bandwidth and does not create a DC component However, a long sequence of 0s upsets the synchronization Data Communication - Digital Transmition