Many OTM systems also feature a wide range of laser and amplifier combinations to
handle different span lengths and device losses. For example, DFB lasers are sufficient
for SMF spans less than 60 km and bit rates up to OC-48/STM-16 (2.5 Gbps). However,
for increased 10 Gbps speeds, more powerful externally modulated lasers and EDFA
devices are necessary. In fact, larger spans may even mandate dispersion compensation
fiber (DCF) coil placements. An alternate means for boosting reach for higher data
rates is via forward error correction (FEC), though this adds cost and compromises
service transparency (see the ITU-T ???digital wrappers??? approach detailed in Optical
Network Management).
Given the massive terabit capacity of a single fiber strand, most OTM systems implement
some type of fiber/span protection. The most common scheme is dedicated 1+1
protection, which uses passive splitters to bridge/switch all client traffic onto separate
working and protection fibers, as shown in Figure 8.5. This simple setup is purely
hardware-based and precludes any ???end-to-end??? span signaling as it splits and sends
TABLE 8.2 Summary of DWDM Optical Network Element (ONE) Designs
ONE Type Cost Topologies Survivability Applications
Optical terminal multiplexer
(OTM)
Low Point-to-point, linear 1+1, 1:1, 1:N Fiber-relief on
congested spans
Static optical add-drop
multiplexer (SOADM)
Medium Linear, ring 1+1 UPSR, 1+1
span
Metro and access
add-drop
Reconfigurable optical adddrop
multiplexer (ROADM)
Medium Linear, ring 1+1 UPSR, OCh/
OMS-SPRING
Metro-core/regional
IOF add-drop
All-optical cross-connect
switch (OXC)
High Mesh, interconnected
rings
Mesh protection,
restoration
Long-haul backbone
Optical+digital cross-connect
switch (OXC+DCS/MSTP)
High Mesh, interconnected
rings
Mesh, ring
protection
Traffic add/drop, 3R
regeneration
204 Chapter 8
Figure 8.
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