Assuming that for some reason (either deliberately or accidentally) one of
the ONU transmitters deviates from the allocated channel, it can degrade not only its
but also the adjacent channels. To prevent such a situation from occurring, it has been
suggested that all optical sources should be placed in the OLT, so that the ONUs would
only modulate the optical carrier delivered in the downstream direction. In certain
solutions, the downstream and upstream channels operate at the same wavelength,
where the downstream data stream is composed of the OLT data and unmodulated
optical carrier slots, which are used by the ONU in accordance with the DBA mechanism
to deliver the buffered data towards the OLT module. Such a solution constitutes
therefore the so-called shared-source system. Two types of modulators are generally
considered as fit for this particular system structure, either an external modulator or
a reflective semiconductor optical amplifier (so-called R-SOA).
Raw Data-rate Upgrade
Migration to higher data rates to increase the available bandwidth is perhaps the
most straightforward way to increase the supported subscriber population or provide
more bandwidth per customer.
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