Multiplexing
Concept
Multiplexing lets many signals share one medium. A multiplexer selects one of 2^n inputs using n select lines; a demultiplexer routes the result back to the right output. Time-division multiplexing gives each channel a slot in a repeating frame and needs synchronisation and framing bits so the receiver knows where a frame starts. Frequency-division multiplexing places channels in separate bands with guard bands; wavelength-division multiplexing does the same for optical carriers.
In aircraft systems
Data concentrators multiplex many analogue and discrete inputs onto a digital bus. ARINC 629 uses time-division multiple access, where each terminal transmits in a defined slot; AFDX uses virtual links over a switched network; ARINC 429 is inherently single-source. The same principle appears in display systems, where several sources are selected and addressed to different windows, and in lighting or probe-heat control, where outputs are addressed and latched.
Traps
Mixing up TDM and FDM; forgetting that a multiplexed link carries only one channel at a time, which limits bandwidth per channel; believing multiplexing removes the need for addressing; ignoring contention, where two transmitters on a shared medium corrupt data unless the protocol arbitrates.
Worked example
A 16-channel TDM system with 8 data bits per channel and one synchronisation bit per frame: each frame is 16 x 8 + 1 = 129 bits. Repeated 1000 times per second, the line rate is 129 000 bit/s and each channel gets 8000 bit/s. Questions often ask for the channel rate, not the line rate.