7.8 CHAPTER SUMMARY

Multiplexing addresses the practical problem of enabling multiple independent information streams to share a common transmission medium. Whereas modulation determines how a single signal is represented on a carrier, multiplexing determines how many signals coexist within the same physical infrastructure.

Frequency-division multiplexing (FDM) partitions the available spectrum into distinct sub-bands, historically forming the backbone of analog carrier telephony. Time-division multiplexing (TDM) partitions access in time and became dominant with the transition to digital transmission, forming the basis of PCM hierarchies, PDH, and later SONET/SDH. Wavelength-division multiplexing (WDM) extends the frequency-partitioning principle into the optical domain, enabling massive capacity growth in fiber systems. Orthogonal frequency-division multiplexing (OFDM) improves spectral efficiency through mathematically orthogonal subcarriers, combining frequency partitioning with digital signal processing to combat inter-symbol interference. Spatial division multiplexing (SDM) exploits the spatial dimension, increasing throughput without additional spectrum.

Taken together, these techniques illustrate a central theme of communications engineering: capacity growth is achieved by exploiting every available physical degree of freedom—frequency, time, wavelength, and space.

Multiplexing, however, assumes that the participating channels are already defined and coordinated. It answers the question: How do multiple signals share a medium? It does not yet answer the related but distinct question: Who is permitted to transmit, and under what rules?

That question leads naturally to the next stage of system organization.