To deal with the jitter accumulation problems and tighter jitter
transfer function requirements of GPON, large analog filters with low-time constants
are needed, the system must deal with baseline wander due to an unbalanced line code,
and more expensive DC-coupled optical receivers are needed. Additionally, the shorter
upstream burst overhead of GPON requires faster CDR and gain adjustment than does
EPON, which translates into a more expensive OLT receiver, while the ONUs must
include much faster lasers with significantly decreased laseron/laseroff periods (approximately
50 times shorter than the ones adopted for EPON grade ONU equipment).
Lastly, the requirement for adjustable ONU laser power levels increases the relative
cost and complexity of the GPON ONU. At the protocol layer, GPON uses fixed framing,
and packets are fragmented at frame boundaries; hence, bidirectional SAR functions are
needed for every flow. This adds considerable complexity and cost (in the form of buffering)
to the system, especially at the OLT, which may need to support as many as 4000 flows
simultaneously. In short, any business case that selects GPON must be able to tolerate
significantly higher equipment prices than would be needed for an EPON deployment.
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