Chap4
- 1. Chapter 4. Digital Transmission 1. Digital-to-Digital Conversion 2. Analog-to-Digital Conversion 3. Transmission Mode 4-1
- 2. Digital-to-Digital ConversionDigital-to-Digital Conversion 4-2 • Involves three techniques: – Line coding (always needed), block coding, and scrambling • Line coding: the process of converting digital data to digital signals 3-2
- 3. Signal Element and Data ElementSignal Element and Data Element 4-3 • Data elements are what we need to send; signal elements are what we can send 3-3
- 4. Data Rate Versus Signal RateData Rate Versus Signal Rate 4-4 • Data rate defines the number of data elements (bits) sent in 1s: bps • Signal rate is the number of signal elements sent in 1s: baud • Data rate = bit rate, signal rate = pulse rate, modulation rate, baud rate • S = c x N x 1/r, where N is the date rate; c is the case factor, S is the number of signal elements; r is the number of data elements carried by each signal element • Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite • The bandwidth is proportional to the signal rate (baud rate) • The minimum bandwidth: Bmin = c x N x 1/r • The maximum data rate: Nmax = 1/c x B x r 3-4
- 5. Design Consideration for Line CodingDesign Consideration for Line Coding SchemeScheme Baseline wandering Long string of 0s and 1s can cause a drift in the baseline DC components DC or low frequencies cannot pass a transformer or telephone line (below 200 Hz) Self-synchronization Built-in error detection Immunity to noise and interference Complexity 4-5 3-5
- 6. Lack of SynchronizationLack of Synchronization 4-6 3-6
- 7. Line Coding SchemesLine Coding Schemes 4-7 3-7
- 8. Unipolar SchemeUnipolar Scheme One polarity: one level of signal voltage Unipolar NRZ (None-Return-to-Zero) is simple, but DC component : Cannot travel through microwave or transformer Synchronization : Consecutive 0’s and 1’s are hard to be synchronized → Separate line for a clock pulse Normalized power is double that for polar NRZ 4-8 3-8
- 9. Polar SchemePolar Scheme Two polarity: two levels of voltage Problem of DC component is alleviated (NRZ,RZ) or eliminated (Biphaze) 4-9 3-9
- 10. Polar NRZPolar NRZ NRZ-L (Non Return to Zero-Level) Level of the voltage determines the value of the bit NRZ-I (Non Return to Zero-Invert) Inversion or the lack of inversion determines the value of the bit 4-10 3-10
- 11. Polar NRZ: NRZ-L and NRZ-IPolar NRZ: NRZ-L and NRZ-I Baseline wandering problem Both, but NRZ-L is twice severe Synchronization Problem Both, but NRZ-L is more serious NRZ-L and NRZ-I both have an average signal rate of N/2 Bd Both have a DC component problem 4-11 3-11
- 12. RZRZ Provides synchronization for consecutive 0s/1s Signal changes during each bit Three values (+, -, 0) are used Bit 1: positive-to-zero transition, bit 0: negative-to-zero transition 4-12 3-12
- 13. BiphaseBiphase Combination of RZ and NRZ-L ideas Signal transition at the middle of the bit is used for synchronization Manchester Used for Ethernet LAN Bit 1: negative-to-positive transition Bit 0: positive-to-negative transition Differential Manchester Used for Token-ring LAN Bit 1: no transition at the beginning of a bit Bit 0: transition at the beginning of a bit 4-13 3-13
- 14. Polar BiphasePolar Biphase 4-14 • Minimum bandwidth is 2 times that of NRZ 3-14
- 15. Bipolar SchemeBipolar Scheme Three levels of voltage, called “multilevel binary” Bit 0: zero voltage, bit 1: alternating +1/-1 (Note) In RZ, zero voltage has no meaning AMI (Alternate Mark Inversion) and pseudoternary Alternative to NRZ with the same signal rate and no DC component problem 4-15 3-15
- 16. Multilevel SchemeMultilevel Scheme To increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln 2B1Q (two binary, one quaternary) 8B6T (eight binary, six ternary) 4D-PAM5 (four-dimensional five-level pulse amplitude modulation) 4-16 3-16
- 17. 2B1Q: for DSL2B1Q: for DSL 4-17 3-17
- 18. 8B6T8B6T Used with 100Base-4T cable Encode a pattern of 8 bits as a pattern of 6 (three-levels) signal elements 222 redundant signal element = 36 (478 among 729) - 28 (256) The average signal rate is theoretically, Save = 1/2 x N x 6/8; in practice the minimum bandwidth is very close to 6N/8 4-18 3-18
- 19. 4D-PAM5: for Gigabit LAN4D-PAM5: for Gigabit LAN 4-19 3-19
- 20. Multiline Transmission: MLT-3Multiline Transmission: MLT-3 The signal rate for MLT-3 is one-fourth the bit rate MLT-3 when we need to send 100Mbps on a copper wire that cannot support more than 32MHz The MLT-3 uses three different voltage levels (+v, 0, -v) and three transition rules to move between the levels: If the next bit is 0, there is no transition If the next bit is 1 and the current level is not 0, the next level is 0. If the next bit is 1 and the current level is 0, the next level is the opposite of last nonzero level. 4-20 3-20
- 21. Multiline Transmission: MLT-3Multiline Transmission: MLT-3 4-21 3-21
- 22. Summary of Line CodingSummary of Line Coding SchemesSchemes 4-22 3-22
- 23. Block CodingBlock Coding Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group 4-23 3-23
- 24. 4B/5B4B/5B Solve the synchronization problem of NRZ-I 20% increase the signal rate of NRZ-I (Biphase scheme has the signal rate of 2 times that of NRZ-I Still DC component problem 4-24 3-24
- 25. 4B/5B Mapping Codes4B/5B Mapping Codes 4-25 3-25
- 26. 8B/10B8B/10B 210 – 28 = 768 redundant groups used for disparity checking and error detection 4-26 3-26
- 27. ScramblingScrambling Biphase : not suitable for long distance communication due to its wide bandwidth requirement Combination of block coding and NRZ: not suitable for long distance encoding due to its DC component problem Bipolar AMI: synchronization problem Scrambling 4-27 3-27
- 28. B8ZSB8ZS Commonly used in North America Updated version of AMI with synchronization Substitutes eight consecutive zeros with 000VB0VB V denotes “violation”, B denotes “bipolar” 4-28 3-28
- 29. HDB3HDB3 High-density bipolar 3-zero Commonly used outside of North America HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution 4-29 3-29
- 30. Sampling: Analog-to-DigitalSampling: Analog-to-Digital ConversionConversion Analog information (e.g., voice) → digital signal (e.g., 10001011…) Codec(Coder/Decoder): A/D converter 3-30
- 31. PCMPCM Pulse Code Modulation Three processes The analog signal is sampled The sampled signal is quantized The quantized values are encoded as streams of bits Sampling: PAM (Pulse amplitude Modulation) According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal. 4-31 3-31
- 32. Components of PCM EncoderComponents of PCM Encoder 4-32 3-32
- 33. Different Sampling Methods forDifferent Sampling Methods for PCMPCM 4-33 3-33
- 34. Nyquist Sampling RateNyquist Sampling Rate 4-34 3-34
- 36. QuantizationQuantization 4-36 3-36 Dr. Md. Abdur Razzaque Green Networking Research Group Dept. of Computer Science and Engineering, University of Dhaka
- 37. QuantizationQuantization Quantization level (L) Quantization error : depending on L (or nb) SNRdB = 6.02nb + 1.76 dB Nonuniform quantization: Companding and expanding Effectively reduce the SNRdB 4-37 3-37
- 38. Original Signal Recovery: PCMOriginal Signal Recovery: PCM DecoderDecoder 4-38 3-38
- 39. PCM BandwidthPCM Bandwidth The min. bandwidth of a line-encoded signal Bmin = c x N x 1/r = c x nb x fs x 1/r = c x nb x 2 x Banalog x 1/r = nb x Banalog where 1/r = 1, c = 1/2 Max. data rate of a channel Nmax = 2 x B x log2L bps Min. required bandwidth Bmin = N/(2 x log2L) Hz 4-39 3-39
- 40. Delta ModulationDelta Modulation To reduce the complexity of PCM 4-40 3-40
- 41. Delta Modulation ComponentsDelta Modulation Components 4-41 3-41
- 42. Delta Demodulation ComponentsDelta Demodulation Components 4-42 3-42
- 44. Parallel TransmissionParallel Transmission Use n wires to send n bits at one time synchronously Advantage: speed Disadvantage: cost ⇒ Limited to short distances 4-44 3-44
- 45. Serial TransmissionSerial Transmission On communication channel Advantage: reduced cost Parallel/serial converter is required Three ways: asynchronous, synchronous, or isochronous 4-45 3-45
- 46. Asynchronous TransmissionAsynchronous Transmission Use start bit (0) and stop bits (1s) A gap between two bytes: idle state or stop bits It means asynchronous at byte level Must still be synchronized at bit level Good for low-speed communications (terminal) 4-46 3-46
- 47. Synchronous TransmissionSynchronous Transmission Bit stream is combined into “frames” Special sequence of 1/0 between frames: No gap Timing is important in midstream Byte synchronization in the data link layer Advantage: speed ⇒ high-speed transmission 4-47 3-47