Chapter 6 Bandwidth Utilization: Multiplexing and Spreading

Chapter 6 of Data Communications and Networking by Behrouz A. Forouzan is titled "Bandwidth Utilization: Multiplexing and Spreading". It explains how to use the available bandwidth efficiently. Below are detailed notes from the chapter:

1. Bandwidth Utilization

• Definition: Bandwidth utilization is the wise use of available bandwidth to achieve efficiency or specific goals.

• There are two main techniques for bandwidth utilization:

  • Multiplexing: Combining multiple signals for transmission over a single medium, which focuses on efficiency.

  • Spreading: Expanding the bandwidth of a signal to achieve privacy and antijamming.

2. Multiplexing

• Definition: Multiplexing allows the simultaneous transmission of multiple signals across a single data link.

• Types of Multiplexing:

  1. Frequency-Division Multiplexing (FDM): The bandwidth is divided into frequency bands, with each signal occupying a different band. Used for analog signals, such as radio and TV broadcasting.

  2. Wavelength-Division Multiplexing (WDM): A variant of FDM designed for fiber-optic channels. Different light wavelengths are used to carry multiple signals.

  3. Time-Division Multiplexing (TDM): The channel is divided into time slots, with each signal using the channel for a short duration in sequence. This is typically used for digital signals.

     • Synchronous TDM: Assigns fixed time slots to each signal, even when there is no data to send.

     • Statistical TDM: Dynamically allocates time slots based on demand, which improves efficiency.

3. Spreading

• Definition: Spreading techniques are used to increase the bandwidth of a signal for purposes such as privacy and antijamming.

• Types of Spreading:

  1. Frequency Hopping Spread Spectrum (FHSS): The signal frequency hops between different channels in a pseudorandom pattern, which makes the signal more resistant to interference and eavesdropping.

  2. Direct Sequence Spread Spectrum (DSSS): Each data bit is replaced with a sequence of bits (called chips) to spread the signal over a wider bandwidth. This method also provides some privacy and resistance to interference.

4. Efficiency and Goals

• Efficiency: Achieved primarily through multiplexing by combining multiple signals to share a single channel. This reduces the waste of bandwidth.

• Privacy and Antijamming: Achieved through spreading techniques, which make signals harder to intercept and jam.

5. Applications of Multiplexing

• AM/FM Radio Broadcasting: Uses FDM, where multiple radio stations use different frequency bands.

• Television Broadcasting: Each TV channel has a specific frequency bandwidth.

• First-Generation Cellular Phones: FDM was used to allocate two frequency bands for each user (one for transmission and one for reception).

6. Applications of Spreading

• Wireless LANs: DSSS is often used in wireless LANs for privacy and to reduce interference.

• Military Communication: FHSS is widely used to prevent signal interception and jamming.

7. Summary

• Multiplexing and spreading are key techniques for efficient and secure communication. Multiplexing improves the efficiency of bandwidth usage, while spreading enhances privacy and makes communication more resistant to interference.

These techniques form the foundation of modern communication systems and are critical for maximizing the capacity and security of data transmission over shared media .