Chap4 d t-d conversion
- 1. Chapter 4. Digital Transmission 1. Digital-to-Digital Conversion 2. Analog-to-Digital Conversion 3. Transmission Mode
- 2. Digital-to-Digital Conversion • Involves three techniques: – Line coding (always needed), block coding, and scrambling • Line coding: the process of converting digital data to digital signals
- 3. Signal Element and Data Element • Data elements are what we need to send; signal elements are what we can send • r is number of data elements carried by each signal element
- 4. Data Rate Versus Signal Rate • 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 • Goal of DC is to increase data rate while decreasing 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
- 5. Design Consideration for Line Coding Scheme • Baseline – Running average of received signal power calculated by receiver • Baseline wandering – Long string of 0s and 1s can cause a drift in the baseline so prevent it • 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
- 8. Unipolar 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
- 9. Polar Scheme • Two polarity: two levels of voltage • Problem of DC component is alleviated (NRZ,RZ) or eliminated (Biphaze)
- 10. Polar 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
- 11. Polar 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
- 12. RZ • 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
- 13. Biphase • 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
- 14. Polar Biphase • Minimum bandwidth is 2 times that of NRZ
- 15. Bipolar 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
- 16. Multilevel 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) • Note L=2 Binary, L=3 Ternary, L=4 Quaternary
- 17. 2B1Q: for DSL
- 18. Multiline 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
- 19. Summary of Line Coding Schemes
- 20. Block Coding • Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group
- 21. 4B/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
- 23. Scrambling • 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
- 24. B8ZS • Commonly used in North America • Updated version of AMI with synchronization • Substitutes eight consecutive zeros with 000VB0VB • V denotes “violation”, B denotes “bipolar”
- 25. B8ZS Encoding
- 26. HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution. If # of non zero pulses is even the substitution is B00V to make total # of non zero pulse even. If # of non zero pulses is odd the substitution is 000V to make total # of non zero pulses even.
- 27. Figure 5-12 HDB3 Encoding
- 28. HDB3 • 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
- 29. Sampling: Analog-to-Digital Conversion • Analog information (e.g., voice) → digital signal (e.g., 10001011…) • Codec(Coder/Decoder): A/D converter
- 30. PCM • 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.
- 31. Components of PCM Encoder
- 32. Different Sampling Methods for PCM
- 34. Sampling Rate
- 35. Quantization
- 36. Quantization • 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
- 37. Original Signal Recovery: PCM Decoder
- 38. 4-3 TRANSMISSION MODES The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous. 38
- 40. Parallel Transmission • Use n wires to send n bits at one time synchronously • Advantage: speed • Disadvantage: cost ⇒ Limited to short distances
- 41. Serial Transmission • On communication channel • Advantage: reduced cost • Parallel/serial converter is required • Three ways: asynchronous, synchronous, or isochronous
- 42. Asynchronous 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)
- 43. Synchronous 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