In PON technologies a downstream flow is split among endusers using passive optical splitters (I suppose it's some kind of prism), but for a upstream the Time-Division Multiple Access method is used. And here the main question arises: Why the TDMA strategy is used instead of merging the upstream multicolored signals by prism in reverse course? Is it even technically feasible to combine upstream beams of different colors into singular ray? Moreover, why TDMA is superior to such approach?

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It's absolutely technically feasible, but it's all about cost and budget. Wavelength-division multiplex in the upstream would be more expensive to deploy and maintain - if budget didn't matter you would use P2P and no PON.

In detail, WDM would require a dedicated wavelength-specific transceiver per subscriber. The maximum split ration of 1:64 requires DWDM which is about 10x the cost of CWDM or uniform wavelength. Tunable transceivers are even more expensive. TDMA uses universal transceivers, and time slots are assigned by the COE.

Also, in a TDMA-based PON there's no lambda-specific passive infrastructure. You can upgrade it with a new protocol at any time, without digging up the multiplexers.

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