Multiplexing enables two or more transmission sources to share a common circuit. The two most common forms are frequency division multiplexing (FDM) and time division multiplexing (TDM). FDM involves the simultaneous transmission of analog signals modulated to different carrier frequencies, while TDM involves interleaving digital signals in pre-assigned time slots.

1. Multiplexing
• Enable two or more transmission sources to
share a common circuit
• Most common forms – FDM and TDM
• FDM – associated with analog signal,
simultaneous transmission
• TDM – associated with digital signal (could also
be analog, but single frequency) with time slices
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2. Multiplexing
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3. Frequency Division Multiplexing
• FDM
• Each signal is modulated to a different carrier
frequency
• Carrier frequencies separated so signals do not
overlap (guard bands)
• e.g. broadcast radio
• Channel allocated even if no data
• Broadband
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4. Frequency Division Multiplexing
Diagram
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5. Wavelength Division
Multiplexing
• Multiple beams of light at different frequency
• Carried by optical fiber
• A form of FDM
• Each colour of light (wavelength) carries separate data
channel
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6. WDM Operation
• Same general architecture as other FDM
• Number of sources generating laser beams at
different frequencies
• Multiplexer consolidates sources for transmission
over single fiber
• Optical amplifiers amplify all wavelengths
—Typically tens of km apart
• Demux separates channels at the destination
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7. Synchronous Time Division
Multiplexing
• Multiple digital signals interleaved in time
• May interleave bits, so not necessarily
synchronous transmission
• Time slots pre-assigned to sources and fixed
• Time slots allocated even if no data
• Time slots do not have to be evenly distributed
amongst sources
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8. Time Division Multiplexing
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