In this system, all downstream wavelengths, ?»1 ??¦ ?»n,
reach each ONU in the system, and thus the ONU receiver must be broadband and
must employ a form of tuneable filtering. The disadvantages of this scenario include
a high splitting loss in the downstream channel; in a double 1 ?— 32 port PSC system,
a total of 1024 ports are available, resulting in 10 log 1024 = 30.1 dB of splitting loss
(ideal situation) increased by the excess losses in the PSC modules and coupling between
the PSC modules and the fiber. Taking the coupling losses as well as fiber attenuation
of 0.2 dB/km for standard G.652 SMF with 20 km physical reach (4 dB in total),
a total power loss exceeding 35 dB is obtained. Such a power loss level is acceptable
in the downstream and results from the inherent PON broadcast policy. It is, however,
unacceptable in the upstream direction, where the power loss should be minimized to
avoid application of expensive optical amplifiers.
A number of WDM PON proposals are also focused on the elimination of optical
sources from the ONU module altogether, since it is expensive and risky (in terms of
network security) to let the subscriber modules manage the upstream transmission
wavelength.
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