Chapter 4 Digital Transmission

Chapter 4 of Data Communications and Networking by Behrouz A. Forouzan, titled "Digital Transmission", explains how both digital and analog data are transmitted as digital signals. Here are detailed notes for this chapter:

1. Digital-to-Digital Conversion

• Definition: This process involves converting digital data into digital signals. The conversion involves three techniques:

  1. Line Coding: Converts digital data into digital signals.

  2. Block Coding: Adds redundancy to ensure synchronization and error detection.

  3. Scrambling: Replaces long sequences of zeros with predefined patterns to maintain signal energy and synchronization.

• Line Coding Characteristics:

  1. Signal Element vs. Data Element: Data elements represent information (bits), while signal elements are used to transmit these bits. The ratio of data elements to signal elements is crucial for performance.

  2. DC Component: Some coding schemes produce a direct current (DC) component that could create problems in transmission.

  3. Baseline Wandering: Long sequences of similar bits can cause the baseline of the signal to drift, complicating decoding.

  4. Self-Synchronization: Signals need to include synchronization data to ensure the receiver can decode the message accurately.

  5. Immunity to Noise and Errors: Coding schemes need to minimize errors and resist noise.

  6. Complexity: Different coding schemes vary in complexity and efficiency.

• Popular Line Coding Schemes:

  1. Unipolar Scheme: Simplest scheme; uses one voltage level for binary '1' and zero voltage for binary '0'.

  2. Polar Scheme: Uses positive and negative voltage levels for binary data.

  3. Bipolar Scheme: Combines positive, negative, and zero voltages for encoding.

  4. Multilevel Schemes: Use more than two voltage levels to encode multiple bits per signal element.

2. Block Coding

• Purpose: Ensures synchronization and error detection by adding redundant bits to the data stream. It breaks down data into smaller blocks and encodes them.

• Types of Block Codes:

  • 4B/5B: Encodes 4-bit data into 5-bit blocks, ensuring no long sequences of zeros.

  • 8B/10B: Converts 8-bit blocks into 10-bit blocks, widely used in high-speed LANs.

3. Scrambling

• Purpose: Scrambling is used to ensure that long sequences of zeros (which would disrupt synchronization) are replaced with predefined patterns.

• Techniques:

  • B8ZS (Bipolar 8-Zero Substitution): Used to avoid long sequences of zeros in bipolar signals.

  • HDB3 (High-Density Bipolar 3-Zero): Similar to B8ZS, but is used in European networks.

4. Analog-to-Digital Conversion

• Pulse Code Modulation (PCM):

  • PCM is the most common method to digitize analog data. It involves three steps:

    1. Sampling: Measuring the amplitude of the analog signal at regular intervals.

    2. Quantization: Approximating each sampled amplitude to the nearest quantization level.

    3. Encoding: Converting the quantized values into a binary form.

  • Nyquist Theorem: The sampling rate must be at least twice the highest frequency of the analog signal to accurately reproduce the original signal.

• Delta Modulation (DM): A simpler form of PCM where only the change between the current and previous sample is encoded.

5. Transmission Modes

• Parallel Transmission: Sends multiple bits simultaneously across multiple channels (e.g., data buses inside computers). It is faster but only works for short distances.

• Serial Transmission: Sends data one bit at a time over a single channel. This mode is used for most data communication.

  1. Asynchronous Transmission: Data is sent one byte at a time, with start and stop bits to synchronize.

  2. Synchronous Transmission: Data is sent as a continuous stream without start and stop bits. Synchronization is maintained by the sender and receiver clocks.

  3. Isochronous Transmission: Ensures that data is delivered at fixed intervals, ideal for real-time applications like video or audio streaming.

6. Summary

• The chapter covers techniques for converting data into digital signals for transmission. Digital-to-digital conversion techniques (line coding, block coding, and scrambling) and analog-to-digital conversion (PCM and DM) are essential for transmitting data efficiently across networks. Various transmission modes (parallel, serial, asynchronous, synchronous, and isochronous) are discussed, with different applications depending on speed, distance, and synchronization needs.

These detailed concepts form the basis for understanding digital transmission in data communication networks .