In fact, many OADM filters directly incorporate manual or
software-selectable VOA control.
In terms of survivability, fixed OADM rings are most amenable to unidirectional pathswitched
ring (UPSR) protection, also termed dedicated protection ring (OCh-DPRING)
[3, 4]. This robust scheme is basically an optical adaptation of SONET/SDH UPSR [5]
and features simplified and extremely fast per-wavelength recovery (under 10 ms).
Nearly all OADM vendors support this capability, which uses two counter-propagating
fibers (working, protection) to implement dedicated channel protection via head-end
splitting and receive-end switching (see Figure 8.5). Again, this is a hardware-based,
nonsignaled recovery approach in which the receiver simply selects the better of two
bridged signals. Although more selective than span/fiber protection, associated perchannel
hardware cost/complexities limit the scalability of UPSR in handling fiber cuts.
Moreover, splitting the signal at the source also lowers achievable ring diameters. In
general, UPSR rings have been widely deployed in many metro-area domains and can
achieve very high ???five nines??? reliability.
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