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Data Communication Digital Transmition

Data Communication Digital Transmition. Behrouz A. Forouzan. Index. Digital to Digital Conversion Analog to Digital Conversion Transmission Modes. Digital to Digital Conversion. Techniques line coding Always needed block coding May or may not needed Scrambling May or may not needed.

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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

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