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Figure 84 Serial and parallel filtering designs for DWDM transport stages
Fiber Span Protection (1+1) Working 2:1 OTM splitter Protection OADM Fiber Span Protection (1:1) Working 2x1 OTM switch Protection 1x2 switch Span signaling 1x2 switch
Dedicated Protection Ring Unidirectional Path-Switched Ring (UPSR) Source bridging Working (inner fiber)
Two-fiber dedicated channel protection
Protection (outer fiber) Mesh Recovery OXC
Shared Protection Ring Bidirectional Line Switched Ring (BLSR) Bidirectional connection ROADM Two-fiber dedicated channel protection
Working lightpath Protection lightpath
Preemptable, shared lightpath Preemptable bidirectional connection Receiver switching
Figure 85 Fiber and WDM layer survivability schemes
Fiber and WDM
two copies of each signal As such, 1+1 protection doubles fiber requirements but halves power budgets (distance reach) Alternatively, 1:1 or 1:N shared protection can improve fiber efficiency and span reach These setups use active switching and rapid protection signaling and allow for lower priority users to share idle protection fibers However, there are no standards for optical fiber/span protection and most offerings are vendorproprietary Albeit nonselective at the service layer, fiber protection can significantly lower higher-layer protection costs As point-to-point DWDM systems proliferated, the next logical step for carriers was the extension of wavelength channels across fiber rings, ie, secondgeneration DWDM  In essence, the goal was to leverage entrenched ring-fiber plants in incumbent carrier networks This evolution yielded transparent optical add-drop multiplexer (OADM) designs, as shown in Figure 83, which implemented all-optical wavelength bypass at intermediate ring sites, creating multihop lightpath connections OADM designs proved much more cost-effective than back-to-back OTM configurations, as they obviated the need for service-specific electronics to retransmit bypass channels With add-drop traffic averaging almost 25 percent per site these transponderless designs enable sizeable CAPEX reduction, particularly at higher 10 Gbps speeds Static OADM nodes augment basic OTM designs by adding wavelength/wavelength band bypass-and-add-drop filters (see Figure 83) These designs lower insertion losses for transit channels (by about 2 dB per node) and deliver commensurate increases in ring diameters Most OADM designs are also complemented with pre- and post-amplifiers in order to handle transmission and nodal losses, respectively Nevertheless, fixed OADM rings have sizeable manual overheads (OPEX) and require skilled technical staff Careful preplanning is required to ensure wavelength connectivity for all node demands, ie, static routing and wavelength assignment (RWA)  In addition, complex amplifier preengineering is needed to maintain lightpath signal-to-noise ratios (SNR) Finally, careful power-balancing is also required between bypass and add-drop channels within an OADM to ensure proper EDFA operation This is commonly done using advanced EDFA gain equalization features and placing a variable optical attenuator (VOA) along channel paths 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  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 85) 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
Fixed Add-Drop Rings
Nevertheless, fixed OADM rings are generally best-suited for static, long-standing service profiles, eg, weeks long or months long holding times Moreover these setups mandate careful demand projections, and inaccurate estimates can result in significant stranded capacity To mitigate operational complexity, many OADM vendors offer detailed software planning tools These packages allow carriers to input their connection demands and fiber routes/characteristics and then compute the required system configurations at all nodes, for example, wavelength assignments, VOA settings, amplifier locations, and so on Many such tools also provide automated order placement for required modules As traffic dynamics increase, static rings become less efficient due to excessive preplanning and manual provisioning requirements Moreover, larger IOF rings need improved scalability and dynamic on-demand provisioning, particularly for meshed demands These contingencies, coupled with advances in soft optics switching/tunable technologies, have led to the emergence of third-generation DWDM systems  A key example here is the reconfigurable OADM (ROADM) node, which allows carriers to add-drop wavelength circuits dynamically at a given node, in other words dynamic online RWA This ONE design vastly accelerates service delivery times (from days/weeks to minutes/hours) and lowers manual operational costs Akin to its static counterpart, a ROADM also features transport, amplification, and (dynamic) add-drop stages (see Figure 83) Initial ROADM designs were opaque and used opto-electronic transponders and SONET/SDH fabrics to implement add-drop functions Although these systems provided subrate TDM grooming and client-side hair-pinning capabilities, service transparency was eliminated Overall, opaque ROADM nodes proved too expensive for most carriers, as large transponder arrays were needed to terminate/launch all wavelength channels Additionally, related OPEX costs footprint and power consumption were also very significant As a result, new advances have shifted carrier interests toward transparent all-optical ROADM designs Today the ROADM market represents one of the fastest-growing and most promising sectors in DWDM space  In fact, related price-points for ROADM systems are even becoming competitive with static OADM systems (owing to technological innovations, market competition, and intense pricing pressures from carriers) Commercial ROADM systems can provide remote automated add-drop of up to 40 wavelengths (any-wavelength-any-node) and use various technologies  For example, some vendors deploy wavelength selective switch (WSS) fabrics whereas others use broadcast and select designs In addition, tunable filters can also be used on input trunks to drop selected channels, for example, fiber Bragg gratings Nevertheless alloptical ring transmission is quite challenging and requires some very specialized provisions Foremost is the need for rapid AGC to stabilize wavelength powers across all links  This is a crucial requirement as individual lightpath connections can experience sizeable power fluctuations during transient events such as connection setup/ takedown or faults AGC is achieved by coupling EDFA amplifier designs with attenuators and modern subsystems to provide good gain flatness over wide input ranges with millisecond timings