Chapter 5 – Multiplexing (Forouzan, Data Communications & Networking 4e)

Bandwidth Utilization & Multiplexing

  • Wise sharing of available bandwidth ➔ multiplexing (efficiency) / spreading (privacy, anti-jamming)
  • Path = physical link; Channel = portion of the path
  • In a multiplexed link: n devices share one medium via MUX/DEMUX

Types of Multiplexing

  • Frequency-Division Multiplexing (FDM) – analog
  • Wavelength-Division Multiplexing (WDM) – analog (optical)
  • Time-Division Multiplexing (TDM)
    • Synchronous TDM
    • Statistical (Asynchronous) TDM

Frequency-Division Multiplexing (FDM)

  • Each source modulates a unique carrier frequency ➔ composite signal
  • Guard bands prevent overlap
  • Bandwidth requirement: B{link}=\sum{i=1}^{n}Bi + (n-1)\times B{guard}
  • Applications: radio (AM 10\,\text{kHz} / FM 200\,\text{kHz}), cable TV (≈6\,\text{MHz}/channel)

Wavelength-Division Multiplexing (WDM)

  • Optical analogue of FDM; channels = light wavelengths (colors)
  • Implemented with prisms, fiber couplers; used in SONET/SDH

Time-Division Multiplexing (TDM)

Synchronous TDM

  • Link rate = n × input rate; frame = fixed, cyclic slots
  • Slot / frame durations:
    • Input slot: t{i}=1/R{i}
    • Output slot: t{o}=t{i}/n
    • Frame: Tf=n\times t{o}=t_{i}
  • Bit/byte interleaving; framing bit(s) for sync
  • Data-rate mismatch strategies:
    • Multilevel multiplexing
    • Multiple-slot allocation
    • Pulse stuffing

Key Equations & Examples

  • Example bandwidth with guards: 5 channels 100\,\text{kHz} + 4 guards 10\,\text{kHz} ⇒ 540\,\text{kHz}
  • TDM link bit rate: R_{link}=\text{frame rate}\times\text{bits per frame}

Statistical TDM

  • Slots assigned on demand; #slots < #lines
  • No sync bits; each slot carries address + data
  • Link capacity < Σ channel capacities; delays during peak load

Applications Summary

  • FDM: broadcasting (AM/FM, cable), legacy telephony
  • WDM: high-speed fiber (SONET/SDH)
  • Synchronous TDM: T/E carrier systems, 2G cellular (30 kHz/6 users)
  • Statistical TDM: digital TV, LAN/WAN data services

Quick Facts

  • FDM/WDM ➔ analog; TDM ➔ digital
  • WDM uses wavelengths instead of frequencies
  • Synchronous TDM may create empty slots; Statistical TDM improves efficiency by dynamic slot use