Communications Fundamentals FAQ

Chapter 7 FAQ

1. 7.11.1 What Is Multiplexing and Why Is It Needed
   1. What Is Multiplexing?
   2. Why Is Multiplexing Necessary?
   3. What Is a Communication Channel?
   4. How Does Multiplexing Work?
   5. What Is a Multiplexer?
   6. What Is a Demultiplexer?
   7. What Happens Without Multiplexing?
   8. Is Multiplexing Used Only in Telephone Networks?
   9. What Resources Can Be Shared?
   10. What Are the Main Types of Multiplexing?
   11. Does Multiplexing Increase Channel Capacity?
   12. How Is Multiplexing Different from Switching?
   13. How Has Multiplexing Evolved?
   14. Why Is Multiplexing Important for Optical Fiber?
   15. Why Is Multiplexing Important for Satellite Communications?
   16. Is Multiplexing Used Together with Other Techniques?
   17. Why Is Multiplexing One of the Foundations of Telecommunications?
2. 7.11.2 What Is the Difference Between Multiplexing and Multiple Access
   1. What Is Multiplexing?
   2. What Is Multiple Access?
   3. What Is the Fundamental Difference?
   4. Can One Exist Without the Other?
   5. How Are Multiplexing and Multiple Access Related?
   6. What Are Common Multiplexing Techniques?
   7. What Are Common Multiple-Access Techniques?
   8. Why Do the Names Sound Similar?
   9. How Does a Cellular Network Use Both?
   10. How Does a Satellite System Use Both?
   11. How Does Wi-Fi Use Both?
   12. Why Is This Distinction Important?
   13. Do Modern Networks Combine Many Techniques?
   14. Which Concept Came First?
   15. Why Is It Helpful to Learn Both Concepts Together?
3. 7.11.3 What Is Frequency-Division Multiplexing (FDM)
   1. What Is Frequency-Division Multiplexing?
   2. Why Is It Called Frequency-Division Multiplexing?
   3. How Does FDM Work?
   4. What Is a Subcarrier?
   5. Why Must the Frequency Bands Be Separated?
   6. What Are Guard Bands?
   7. Why Is Single-Sideband Often Used with FDM?
   8. Where Was FDM First Used?
   9. How Was FDM Used in Analog Telephone Systems?
   10. How Is FDM Used in Radio Broadcasting?
   11. Is Television Also an Example of FDM?
   12. How Was FDM Used in Satellites?
   13. Does FDM Require Synchronization?
   14. What Are the Advantages of FDM?
   15. What Are the Disadvantages of FDM?
   16. Why Did Telephone Networks Move Away from FDM?
   17. Is FDM Still Used Today?
   18. Why Is FDM Important?
4. 7.11.4 What Is Time-Division Multiplexing (TDM)
   1. What Is Time-Division Multiplexing?
   2. Why Is It Called Time-Division Multiplexing?
   3. How Does TDM Work?
   4. What Is a Time Slot?
   5. What Is a Frame?
   6. Why Does TDM Work So Well?
   7. How Is TDM Different from FDM?
   8. Why Did TDM Become Popular?
   9. How Was TDM Used in Telephone Networks?
   10. Does Each User Receive the Same Bandwidth?
   11. What Happens If One User Has Nothing to Send?
   12. Why Is Synchronization Important?
   13. What Are the Advantages of TDM?
   14. What Are the Disadvantages of TDM?
   15. Is TDM Still Used Today?
   16. Can TDM Be Combined with Other Techniques?
   17. Why Is TDM Important?
5. 7.11.5 Why Is Synchronization So Important in TDM Systems
   1. What Is Synchronization?
   2. Why Is Synchronization Necessary?
   3. What Is Clock Synchronization?
   4. Why Can't the Receiver Simply Use Its Own Clock?
   5. What Is Bit Synchronization?
   6. What Is Frame Synchronization?
   7. How Is Frame Synchronization Achieved?
   8. What Happens If Synchronization Is Lost?
   9. What Is Clock Recovery?
   10. Why Is Line Coding Important?
   11. What Is Timing Jitter?
   12. What Is Wander?
   13. How Do Large Networks Maintain Synchronization?
   14. Why Is Synchronization Especially Important in Optical Networks?
   15. Is Synchronization Important Outside TDM?
   16. What Are the Advantages of Good Synchronization?
   17. Why Is Synchronization One of the Foundations of Digital Communications?
6. 7.11.6 What Is Statistical Time-Division Multiplexing (STDM)
   1. What Is Statistical Time-Division Multiplexing?
   2. Why Is It Called "Statistical"?
   3. Why Is Conventional TDM Inefficient for Data?
   4. How Does STDM Work?
   5. What Is a Buffer?
   6. Why Are Buffers Necessary?
   7. How Does the Receiver Know Which Data Belong to Which User?
   8. Does STDM Introduce Additional Overhead?
   9. What Is the Main Advantage of STDM?
   10. Can Too Many Users Share the Same Link?
   11. What Is Queueing?
   12. What Types of Traffic Benefit Most from STDM?
   13. How Does STDM Differ from Conventional TDM?
   14. Is STDM a Form of Packet Switching?
   15. Is STDM Still Used Today?
   16. What Are the Advantages of STDM?
   17. What Are the Disadvantages?
   18. Why Is STDM Important?
7. 7.11.7 What Are SONET and SDH, and Why Were They Developed
   1. What Are SONET and SDH?
   2. Why Were New Standards Needed?
   3. What Problems Did PDH Create?
   4. What Does "Synchronous" Mean?
   5. How Does Synchronization Improve Multiplexing?
   6. What Is the Basic SONET Hierarchy?
   7. What Is the SDH Hierarchy?
   8. Are SONET and SDH Compatible?
   9. What Is a SONET or SDH Frame?
   10. Why Is the Overhead Important?
   11. What Are Add-Drop Multiplexers?
   12. Why Was This Such an Improvement?
   13. Why Are SONET and SDH Well Suited to Optical Fiber?
   14. Were SONET and SDH Used Only for Telephone Calls?
   15. Are SONET and SDH Still Used Today?
   16. Did SONET and SDH Influence Modern Networks?
   17. Why Are SONET and SDH Important?
8. 7.11.8 What Is Wavelength-Division Multiplexing (WDM)
   1. What Is Wavelength-Division Multiplexing?
   2. Why Is It Called Wavelength-Division Multiplexing?
   3. Is WDM Really Just FDM for Light?
   4. How Does WDM Work?
   5. What Is an Optical Multiplexer?
   6. What Is an Optical Demultiplexer?
   7. Why Is WDM So Important?
   8. What Is CWDM?
   9. What Is DWDM?
   10. How Many Wavelengths Can One Fiber Carry?
   11. Which Wavelengths Are Normally Used?
   12. What Is an Optical Amplifier?
   13. Why Doesn't One Wavelength Interfere with Another?
   14. Are There Any Practical Limitations?
   15. Where Is WDM Used?
   16. How Does WDM Work with Other Multiplexing Techniques?
   17. Is WDM Still Evolving?
   18. Why Is WDM Important?
9. 7.11.9 What Is Orthogonal Frequency-Division Multiplexing (OFDM)
   1. What Is Orthogonal Frequency-Division Multiplexing?
   2. Why Is It Called Orthogonal Frequency-Division Multiplexing?
   3. What Does Orthogonal Mean?
   4. How Does OFDM Work?
   5. Why Use Many Slow Subcarriers Instead of One Fast Carrier?
   6. What Is Multipath Propagation?
   7. What Is Inter-Symbol Interference?
   8. What Is the Cyclic Prefix?
   9. Why Can OFDM Subcarriers Overlap?
   10. How Is OFDM Generated?
   11. What Modulation Is Used on Each Subcarrier?
   12. What Is Adaptive Modulation?
   13. Where Is OFDM Used?
   14. What Are the Advantages of OFDM?
   15. Does OFDM Have Any Disadvantages?
   16. Is OFDM Used Together with Other Technologies?
   17. Why Is OFDM Important?
10. 7.11.10 What Is Spatial Division Multiplexing (SDM)
    1. What Is Spatial Division Multiplexing?
    2. Why Is It Called Spatial Division Multiplexing?
    3. Why Is SDM Becoming So Important?
    4. How Does Frequency Reuse Demonstrate SDM?
    5. How Do Satellites Use SDM?
    6. What Is Beamforming?
    7. What Is MIMO?
    8. How Can Several Data Streams Occupy the Same Frequency?
    9. What Is Spatial Multiplexing?
    10. Does MIMO Always Increase Capacity?
    11. What Is Massive MIMO?
    12. Is SDM Used in Optical Fibers?
    13. What Is a Multicore Fiber?
    14. What Are the Advantages of SDM?
    15. What Are the Challenges?
    16. Where Is SDM Used?
    17. Can SDM Be Combined with Other Multiplexing Techniques?
    18. Why Is SDM Important?
11. 7.11.11 Why Did Telephone Networks Change from FDM to TDM
    1. Why Was FDM Used First?
    2. How Successful Was FDM?
    3. How Did Analog Telephone Hierarchies Work?
    4. What Were the Limitations of FDM?
    5. What Changed During the 1960s?
    6. What Is Pulse-Code Modulation?
    7. Why Does Digital Speech Favour TDM?
    8. Why Is TDM Simpler?
    9. What Happened to Noise?
    10. Why Was This So Important?
    11. How Did Digital Switching Change Networks?
    12. Why Was TDM Better Suited to Optical Fiber?
    13. Did Digital Systems Use Less Bandwidth?
    14. Why Was Network Management Easier?
    15. What Role Did SONET and SDH Play?
    16. Did FDM Disappear Completely?
    17. Why Was the Transition So Successful?
    18. What Can Engineers Learn from This Transition?
    19. Why Is This Transition Important?
12. 7.11.12 Can Different Multiplexing Techniques Be Used Together
    1. Why Combine Multiplexing Techniques?
    2. Isn't One Technique Enough?
    3. How Does a Modern Optical Network Combine Multiplexing?
    4. How Does a Cellular Network Combine Techniques?
    5. How Does Wi-Fi Combine Multiplexing?
    6. How Do Satellites Combine Multiplexing?
    7. How Does Television Broadcasting Use Multiple Techniques?
    8. Can One Signal Be Multiplexed Several Times?
    9. How Does the Internet Use Multiplexing?
    10. Does Combining Techniques Increase Capacity?
    11. Are There Practical Limits?
    12. Why Doesn't Every System Use Every Technique?
    13. Will Future Systems Combine Even More Techniques?
    14. Why Is This Layered Approach Important?