Digital Transmission Lecture Review

Digital Transmission Notes

Chapter Overview

  • Instructor: Dr. Rami Alazrai

  • Institution: German-Jordanian University

  • Main Topics Covered:

    • Line Coding

    • Characteristics of Line Coding

    • Line Coding Schemes

    • Block Coding

    • Transformation

    • Common Block Codes

    • Sampling

    • Pulse Amplitude Modulation

    • Pulse Code Modulation

    • Sampling Rate: Nyquist Theorem

    • Bit Rate

    • Transmission Mode

    • Parallel Transmission

    • Serial Transmission

Line Coding

  • Definition: Line coding is the process of converting sequences of binary data (bits) into digital signals.

Characteristics of Line Coding

  • Signal Level vs. Data Level:

    • Signal Levels: Number of values used to represent a signal.

    • Data Levels: Number of values used to represent data.

  • Pulse Rate vs. Bit Rate:

    • Pulse Rate (Baud): Number of pulses transmitted per second.

    • Bit Rate: Number of bits transmitted per second.

  • DC Components: Excess energy in the line due to unbalanced data levels; does not carry information.

  • Self-Synchronization: Maintaining synchronization between sender and receiver without an external clock.

Signal Level versus Data Level

  • Examples:

    • 1st Diagram: 2 signal levels, 2 data levels (0 for 0, polarity for 1).

    • 2nd Diagram: 3 signal levels, 2 data levels (0 for 0, change in polarity for 1).

Pulse Rate versus Bit Rate

  • Pulse: Minimum time to transmit a symbol.

  • Formula: ext{Bit Rate} = ext{Pulse Rate} imes ext{Log}_2(L) where L is the number of data levels.

  • Example Calculation:

    • Signal with a pulse duration of 1 ms:

    • ext{Pulse Rate} = rac{1}{10^{-3}} = 1000 ext{ pulses/s} (Baud).

    • If L = 2:

      • ext{Bit Rate} = 1000 imes ext{log}_2(2) = 1000 ext{ bps}

    • If L = 4:

      • ext{Bit Rate} = 1000 imes ext{log}_2(4) = 2000 ext{ bps}

DC Component

  • Excess energy caused by unbalanced data levels; leads to distortion and energy loss without information transmission.

Synchronization

  • Importance: The receiver must be synchronized with the sender to interpret data correctly.

    • Mismatched bit intervals result in erroneous interpretation of signals.

Self-Synchronization

  • Self-synchronizing signals include timing information within the signal, allowing the receiver to reset the clock using signal transitions.

Example Synchronization Scenario

  • If the receiver clock is 0.1% faster than the sender's clock:

    • Data Rate = 1 Kbps → 1 extra bit received per second.

    • Data Rate = 1 Mbps → 1000 extra bits received per second.

  • If sender transmits at 10 Kbps for 1 second and a 1% slower receiver is used:

    • Sender transmits 10,000 bits, receiver reads 9,900 bits, resulting in errors.

Line Coding Schemes

  1. Unipolar:

    • One voltage level (positive); polarity for 1, no polarity for 0.

  2. Polar Encoding:

    • Two voltage levels (positive and negative).

    • Types:

      • Non-Return to Zero (NRZ)

      • Return to Zero (RZ)

      • Manchester

      • Differential Manchester

  3. Bipolar Encoding:

    • Three levels (positive, negative, zero).

    • Types:

      • Alternate Mark Inversion (AMI)

      • Bipolar N-Zero Substitution (BnZS).

Unipolar Encoding

  • Advantages: Simplicity and low cost.

  • Disadvantages: DC component, transformer issues, and lack of synchronization during long sequences of identical bits.

Polar Encoding Types

  1. NRZ-L: Level represents the state of the bit (e.g., +v = 0, -v = 1).

  2. NRZ-I: Inversion of level for 1, no change for 0.

  • Disadvantage: Synchronization issues with long sequences of identical bits.

  1. Return to Zero (RZ): Uses three voltage levels; signal returns to zero within each bit interval, ensuring synchronization but requiring more bandwidth.

  2. Manchester Encoding: Two voltage levels, transitions in the middle of each bit for synchronization.

  3. Differential Manchester: Similar to Manchester but uses inversion presence or absence to indicate bits.

Bipolar Encoding Schemes

  • AMI: Alternating positive and negative voltages for bits; 0s represented by zero voltage.

  • BnZS: Substitutes 0s for voltage levels after a certain number of consecutive 0s to reset clocks.

Other Line Coding Schemes

  1. 2B1Q Encoding:

    • Four voltage levels representing two bits each (higher bit rate).

  2. MLT-3 Encoding:

    • Three voltage levels and similar to NRZ-I with transitions.

Block Encoding

  • Purpose: Add redundancy for synchronization and error detection.

  • Steps:

    1. Division: Bitstream divided into m-bit blocks.

    2. Substitution: m-bit blocks replaced with n-bit codes (where n > m) to ensure synchronization and error detection.

    3. Line Coding: Simple line coding converts new bitstream to signals.

Common Block Codes

  1. 4B/5B Code:

    • Every 4-bit block substituted with a 5-bit code; limits sequential zeros.

  2. 8B/10B Code:

    • Same concept with different block sizes for better error detection.

  3. 8B/6T Code:

    • Each 8-bit block substituted with a ternary 6-symbol code.

Sampling

  • Definition: Process of converting analog signals to digital by sampling at uniform intervals.

  • Originated to maintain integrity over long distances by addressing noise.

Pulse Amplitude Modulation (PAM)

  • Definition: Sampling of an analog signal to create a series of pulses using a sample-and-hold technique.

  • PAM itself remains analog and is foundational for Pulse Code Modulation (PCM).

Quantization

  • Process of converting continuous PAM signal into digital by assigning discrete values to samples.

Pulse Code Modulation (PCM)

  • Converts binary digits into a digital signal using line coding.

  • Involves generating a sequence of energy signals based on quantized binary data.

Sampling Rate: Nyquist Theorem

  • Theorem Statement: The sampling rate must be at least twice the highest frequency component in the original signal to reproduce it accurately.

Example Calculations Using Nyquist Theorem

  1. Voltage Signal Range: -5 to +5 Volt, precision of 12 levels:

    • ext{Bits per sample} = ext{log}_2(12) = 4 ext{ bits}.

  2. Sampling Human Voice: 0 to 4000 Hz:

    • Sampling Rate = 8000 ext{ samples/s} → 64 Kbps for 8 bits/sample.

Data Transmission Types

  1. Parallel Transmission:

    • Transmits a group of bits simultaneously (e.g., 8, 16 bits); faster but costlier.

  2. Serial Transmission:

    • Single bit transmitted at a time; slower, costs less, and supports longer distances.

    • Types: Synchronous (continuous) and Asynchronous (start/stop frame bits).

Differences Between Asynchronous and Synchronous Transmission

  • Asynchronous: Start (0) and stop bits (1); gaps may occur between bytes, allowing timing adjustments.

  • Synchronous: Continuous stream without gaps; relies on the receiver to group bits correctly.