Discussion
The following are the components in optical communication :
(i) Modulator
(ii) Wavelength Division Multiplexer
(iii) Repeaters
(iv) Time Division Multiplexer
The correct sequence of their placement from transmission side is :
Answer: D
The official key gives the order modulator, time division multiplexer, wavelength division multiplexer, repeaters, option D.
On the transmitting side each message signal is first put into a form suitable for the link by a modulator (for digital links, PCM or another modulator), several such channels are then combined in time by the TDM multiplexer, several TDM streams are placed on separate optical wavelengths by the WDM multiplexer, and repeaters along the fibre restore the signal strength afterwards. Repeaters always come last, which already excludes options B and C.
What could not be confirmed: a real system could also place the TDM before the modulator, and no source for this exact question was found. Option A puts the repeaters before the WDM, and option C puts the TDM before the modulator and the repeaters after the WDM. The official key is kept.
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