Rather, it is best thought of as a broad category of proprietary systems that
vary markedly in terms of basic technology. As a rule, they are ????ยป-per-customer??? systems,
meaning there is a laser and a receiver in the OLT for every customer, instead of a single
laser and a single receiver for the entire PON. This additional cost, along with the need
for much more sophisticated WDM functionality in the system, leads to significantly
higher per-subscriber costs when compared to either IEEE or ITU-T solutions.
The ITU-T systems, specified by the ITU-T G.983.x and G.984.x series, were originally
designed as the access portion of an end-to-end multiservice ATM network, and this
Passive Optical Networks (PONs) 193
heritage permeates these platforms. At the physical layer, ITU-T PON systems use
SONET/SDH requirements for timing, scrambling, and so on, which leads to intrinsically
higher per-subscriber costs when compared to EPON. For example, the SONET
scrambler must be able to tolerate up to 71 consecutive, identical digits, which presents
a more difficult clock and data recovery (CDR) problem than that found in EPON,
which uses 8B/10B.
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