Libro para traducirMetro Edge SolutionsSimilar to traditional taxonomies, the metro edge will continue to representa merging between the core interoffice and the client-access spaces. Here it isbecoming increasingly evident that SONET/SDH is not the best unifying layer. Conversely, since DWDM is bandwidth-inefficient for subgigabit linerates,advanced electronic multiplexing technologies are needed to aggregatediverse end-user protocols onto large-granularity optical (DWDM) tributaries. Dense IC technologies are finding particular favor here, helping collapselegacy multiplexing hierarchies (i.e., "system-on-a-shelf/card" ) and furtherblurring traditional access boundaries. Many new metro edge solutions,broadly termed as optical edge devices (OED) , have been proposed, includingDWDM edge rings, next-generation SONET/multiservice provisioning,and IP routing/packet rings. Some details are now presented.6.1 DWDM EDGE RINGSIn Section 5.2.1 it was stated that low-cost passive optical rings are generallywell-suited for hubbed traffic patterns and can serve as metro edge solutions,i.e., metro DWDM access [64, 65]. However, a key issue is mapping clientprotocols onto the underlying wavelengths, and several solutions are possible (seeFig. 8.15). A straightforward, transparent approach is to assign a completewavelength to each client signal, regardless of bit rate (termed "protocolper-lambda"). This works well for low demand/node counts and sufficientwavelength channels. For example,  proposes to back-haul individual subrateclient tributaries (DS1, DS3, OC-3) across a dual-homed DWDM (access)ring to a CO for aggregation/termination. Nevertheless, given the propensityof metro-edge DS1/DS3 subrate tributaries, this approach cannot scale inwavelengths and is very cost/capacity inefficient for rates below the "breakeven"OC-12/STM-4 value, see Section 5.4. Clearly, edge aggregation must becoupled with static DWDM rings in order to improve efficiency and reducewavelength port requirements, as shown in Fig. 8.4.A variety of subrate "circuit" multiplexing schemes can be implemented.For legacy TDM support, dense IC chipsets can reduce many multiplexinghierarchies onto Hne cards, e.g., 4:1 OC-3 to OC-12, and even OC-12to OC-48 (Fig. 8.15). These "integrated SONET/DWDM" interfaces ,also termed "thin mux" , can combine the benefits of both technologiesand further improve cost effectiveness. For example, in , integration ofSONET/SDH line termination functionality with DWDM transport is foundto yield significant savings in electronic protection overheads. The availability ofnewer software-programmable SONET/SDH transceivers (OC-3/12/48)
further improves flexibility, as line rates can be adjusted per demand, eliminatingthe need for constant line card upgrades. In some cases, a complete subrateDCS switching unit can also be added to aggregate multiple flows. However,SONET/SDH multiplexing is only amenable for legacy voice/leased-line trafficand not native packet interfaces (e.g., 10/100 Mb/s, 1 Gb/s Ethernet). The latterrequire expensive "telecom adapter" (mapping) interfaces, more than quadruplingoverall interface costs (electronics, labor) and yielding high bandwidthinefficiencies [46, 133], see also Section 6.2.More flexible TDM multiplexing techniques can also be used for metroedge-aggregation. For example, proprietary "asynchronous" multiplexingcan combine multiple subrate circuits onto a higher-bit-rate time-divisioncarrier. The resultant protocol concurrencies can be much more efficientand can include a mixture of SONET/SDH and other alternate data tributaries (e.g.,155 Mb/s OC-3, 200 Mb/s ESCON, 1 Gb/s Ethernet). Recent developments indigital wrapper standards [12,13] will also facilitate more flexible TDM multiplexingschemes. Digital wrappers define client-independentoverheads for transport and management of payload bit streams across opticaldomains, for example, bytes for management, monitoring, protectionsignaling, even FEC (about 6% FEC overhead ). These overheads areprocessed at "electronic" (opaque) monitoring points, such as boundaries betweenaccess/core rings/domains. Currently, several "wavelength" rates are defined,namely 2.5, 10, and 40Gb/s, albeit subrate multiplexing hierarchies are notdefined [12, 145]. Conceivably, a full variety of protocols can be multiplexedinto the payload section, unlike rigid SONET hierarchies, although there canbe FEC implications (see ). Nevertheless, digital wrappers will inevitablyentail similar overhead processing complexity as SONET/SDH and relatedchipset costs will likely relegate this technology to long-haul/regional transportand/or for larger (metro) interdomain interfacing functionality for thenear/medium term.Optical frequency division multiplexing (O-FDM) has also been proposedfor edge multiplexing, using a single laser to modulate "subrate" carriers(i.e., client channels) within a spectral band. Specifically, all signals undergoquadrature amplitude modulation (QAM) and are subsequentlyfrequencymultiplexed onto a fiber/wavelength (i.e., O-FDM/DWDM ringcombination).O-FDM is genuinely bit rate transparent and can improve spectral efficiencyover on-off keying (OOK) modulation schemes used in SONET/SDH or GigabitEthernet encoding (between 20-50%, 20 Gb/s per wavelength possible).O-FDM transmission is also more dispersion tolerant, and can work wellon older fibers, for example, high-PMD types, unlike 10- or 40-Gb/s TDM. Additionally, related electronic costs are lower, since speeds need only
match slower subcarrier channels. Studies for moderate demand scenariosshow O-FDM to be more fiber efficient than OC-48 and more capacity efficientthan OC-192 . Nevertheless, DWDM-induced transmission impairmentsfor O-FDM transmission may be problematic, and this needs propercharacterization. Note that only DWDM technology can transport O-FDM signalsin their native formats.2 NEXTGENERATIONSONET/MULTISERVICEPROVISIONING PARADIGMSEven though legacy TDM technology has many shortcomings (Section 3.4),it will continue to play a significant role in the convergence of data and opticalnetworks at the metro edge. Demand for short-haul SONET/SDH gear isstill high and may continue to grow for the next several years [63, 65]. A largepart of this market comprises larger OC-48/STM-16 and OC-192/STM-64 systems,although smaller OC-3/STM-1 and OC-12/STM-4 systems will also seeincreased deployments (see ). Moreover, many existing routers/switcheshave SONET/SDH interfaces (e.g., POS, AAL5), and recent efforts to define abroader generic framing protocol (GFP)  (for mapping "nonstandard"data protocols) may further propagate the ubiquity of such framing .In light of this, many proposals have sought to "enhance" SONET/SDHparadigms to better suit data traffic needs [130, 131, 133-135, 141-144].Although these proposals have appeared under different names (e.g., "superSONET" , "data-aware SONET" ), herein the term "next-generationSONET" (NGS) is chosen (Fig. 8.4). Overall, all these solutions share twomain features, namely efficient data tributary mappings and integrated higherlayer (two/three) protocol functionalities, as shown in Fig. 8.16. Concurrently,these solutions also leverage ubiquitous SONET/SDH performance monitoring,protection switching, and network management capabilities. Some detailsare presented.SONET/SDH mapping of smaller packet interfaces (10, lOOMb/s Ethernet) isusually done in "coarse" STM-1 increments and the resultant bit-rateincongruencies usually yield large amounts of stranded bandwidth [129, 144](e.g., 10-Mb/s Ethernet allocated a full STS-1, 80% unused capacity). Burstydata profiles can further exacerbate bandwidth inefficiencies. Nevertheless,advanced IC technologies are permitting high-density switching fabrics withmuch finer TDM granularities, particularly at smaller/fractional VT1.5 levels[141, 158]. By combining finer tributaries, for example, virtual concatenation (wideband packet-over-SONET ), native packet interface rates can now bematched much more closely (e.g., 10-Mb/s Ethernet via sevenVT1.5s). Multiple "matched" tributaries can then be more efficiently packedinto existing standardized tributaries, and this will help collapse multiplexing
(equipment) hierarchies. Switching designs can also use multilevel DCS fabrics toassign capacities in both VT1.5 and larger STM-1 increments to betterscale electronic complexities  (Fig. 8.16). Overall, advanced DCS designswill extend ubiquitous TDM tributary add-drop/switching/protection functions tocover a full range of combined streams, in addition to interworkingwith legacy streams (i.e., from ADM, W-DCS, B-DCS gears). Furthermore,more advanced renditions are possible that dynamically adjust allocations to"match" bursty loads on incoming interfaces (albeit layer two/threebuffering/processing and end-to-end signaling are required here). Along theselines,there have been notable developments in the link capacity adjustment scheme(LCAS)  mechanism. LCAS defines a control protocol that allows for"hitlessly" increasing/decreasing the number of "trails" (e.g., STS-1 circuits)assigned to a connection. Moreover, each circuit trail can be diversely routedto improve resiliency and failed trails can be removed altogether. Additionally,connection asymmetry also can be achieved by assigning a different numberof trail counts to a given connection direction. Overall, LCAS defines a verypowerful new capability for exploiting virtual concatenation techniques andimproving capacity utilization (see  for more details).To further improve data efficiency/scalability, NGS designs intend to provide a fullrange of higher-layer "non-SONET/SDH" protocol functionalities(i.e., "data-aware" TDM interfaces). Examples include IP routing, ATMswitching, LAN switching, and even frame-relay aggregation, see [4, 130,133, 135, 142, 144]. Namely, intelligent layer two/three cell/packet processing(e.g., buffering, scheduling, switching, routing, Fig. 8.16) capabilities areused to increase capacity oversubscription ratios (e.g., statistical multiplexinggains) between multiple customer ports , a step beyond "circuit"aggregation. Many designs also provide direct data (Ethernet) packet interfaces,eliminating the need for more expensive "telecom adapter" private-lineinterfaces at client switches/routers. Additionally, line-termination capabilitiescan also be added to extract and process payloads from existing privateline datainterfaces . This essentially "decouples" link interfaces fromtheir associated data payload/protocols, an important step in extending thebenefits of oversubscription to private-line traffic (Fig. 8.16). Traffic multiplexingcoupled with tributary concatenation achieves aggregation closerto the edge, leaving more free capacity inside the ring and yielding verygood bandwidth efficiencies. For example, three 10-Mb/s Ethernet streamsaveraging 3Mb/s can be edge-buffered and packed into six VT1.5 circuitsversus three OC-1 legacy interfaces, a bandwidth savings of 94%. Notethat edge-multiplexing of multiple packet interfaces also reduces port countsand the need for complex, costly centralized back-hauling setups .
Overall, integrating formerly distinct packet/cell and SONET/SDH protocols onto acommon platform removes multiple subtending aggregationgears (routers, switches, ADMs) and their associated management systems.This reduction can yield significant operational cost/provisioning complexityimprovements and reduce footprint space/complex cabling considerably.Moreover, the emerging generalized MPLS framework (Section 7.1) presentsa comprehensive control setup for NGS systems, i.e., edge equivalencemappings between circuit and packet labels and more recently, even provisioningof concatenated SONET/SDH tributaries (see ). As an aside,note that some earlier schemes proposed using ATM as the primary multiserviceSONET/SDH aggregation layer [4, 144]. However, these designssuffered from high bandwidth inefficiency (about 20% ) and hardwarescalability/cost concerns, and have been largely usurped by improving IPparadigms .Despite its capacity improvements, NGS still reuses rigid, synchronizedelectronic payload framing/encapsulation formats. As a result, this solutionis not truly capacity scalable (i.e., electronic bit rate and cost limitations) andis much better suited to improving time-slot packing on existing rings (OC-3,OC-12) and/or for areas with limited demand growth or high fiber count .SONET/SDH framing again precludes transparency, making it difficult tosupport data protocols such as Fibre Channel, ESCON, or FICON withoutproprietary handling [51, 141] (until the formalization of GFP at least ).Moreover, the associated functionalities of such protocols may be too specialized,still mandating the use of subtending gear. A more ominous concern withNGS is that its associated packet/cell functionalities/features likely may notmatch those of "best-in-class" solutions offered by specialized router/switchvendors . Here, many operators already have (or plan to deploy) separate"best-in-class" gear/management systems and will be unwilling to acceptsingle-vendor solutions. Consequently, more generalized multiservice provisioningplatforms (MSPP) attempt to address these limitations by furtherintegrating DWDM functionality to boost transparency/scalability. In essence,MSPP solutions combine NGS with DWDM (ring) technology (Section 6.1),and have also been more aptly termed as integrated metro DWDM [64,65]. Anoverview of an MSPP node is given in Fig. 8.16, where added passive DWDMtransport/ring functions are shown. To lower costs and increase flexibility,many MSPP designs intend to add "optical" functionalities (multiplexing,transport, filtering) in a modular fashion via line-card additions. Again, "allin-one"MSPP solutions may be overly expensive and impractical, especiallyif the existing base of legacy TDM and/or "best-in-class" routing/switchinginfrastructures is large (see also Section 6.4).
On the subject of SONET/SDH enhancements, recent advances have proposedincreasing SONET/SDH line rates to 40Gb/s (OC-768/STM-256),further propagating existing TDM-paradigms. Currently, 40-Gb/s transmission isusually done by optically interleaving  four 10-Gb/s streams (channelized), ascommercial availability of electronic SONET/SDH overheadprocessing/clock recovery circuitry at direct 40-Gb/s rates is still a ways ofF(i.e.,OC-768c concatenated interfaces). Regardless of the interface type, dispersion(slope) effects at these bit rates will restrict transmission distances significantly(e.g., chromatic dispersion at 40 Gb/s is 16 times larger than at 10 Gb/s, yielding adispersion limit of about 25 km [106, 112]). This will hinder applicabilitybeyond small metro domains, and usually extensive dispersion compensationand fiber characterization considerations will be necessary (as used in moststudies, see ). Moreover, bandwidth scalability at these increased bit ratesstill falls well short of those yielded by DWDM. Furthermore, mapping OC-768/STM-256 tributaries onto wavelengths (e.g., for transmission across coremetro rings) will likely require larger 100-Ghz spacings (and not 50Ghz) dueto interchannel crosstalk limitations. To an extent, this mitigates the gains ofincreasing the channel bit rate. Overall, 40-Gb/s OC-768/STM-256 solutionshave yet to be deployed, and related technical and cost concerns will adverselyaffect or delay their applicability in the highly cost-sensitive metro edge .When they do emerge, such large TDM interfaces will likely interface withlarger metro core wavelength routing gears. Moreover, it is also conceivablethat cheaper multiplexed 40-Gb/s Ethernet router interfaces will emerge first.Namely, these interfaces will simply multiplex four 10-Gb/s Ethernet streamstogether, thereby avoiding many complexities associated with genuine 40-Gb/sTDM clock recovery and/or header processing.6.3 PACKET-BASED SOLUTIONSAlthough DWDM rings provide significant improvements over TDM rings,as discussed previously, they still embody a circuit-switching paradigm. Itis well known that circuit switching is generally less bandwidth efficient thanpacket switching , and bandwidth utilization on circuit-multiplexed DWDMrings can be very low for bursty data profiles [60, 166]. Nevertheless, theemergence of "next-generation" packet-switching devices (Ethernet switches,IP routers) is helping resolve many of these data inefficiencies. Specifically,advanced hardware-based packet filtering  and switching technologies can now support line-rate input/output switching at full "wavelength"tributary rates (OC-48c/192c, 10-Gb/s Ethernet) (e.g., via custom high-speedASIC solutions or even generalized network-processor chips). Hence thesenodes can serve as direct (POP) aggregation boxes for metro edge, even core,DWDM rings, completely collapsing inefficient, legacy "leased-line" datahierarchies (e.g., "evolutionary delayering," see Fig. 8.18). There have also
been significant improvements in the overall IP routing framework to support"TDM-style" guarantees (bandwidth, delay, loss, etc.), namely the multiprotocol-label switching (MPLS) framework and its associated resource reservationprotocol (RSVP). Overall, this emergent framework can support very finequality of service (QOS) levels via the integrated services model (Intserv) or morecoarse (scalable) class of service (COS) levels via the differentiatedservices (Diffserv) model (see [154, 156] and related references). Thesecapabilities provide "soft circuit" setups that achieve high statistical multiplexinggains and can vary bandwidth allocations per any given criterion (e.g., perport,client group, application, etc.). However, various provisioning concernsstill need to be addressed before "carrier-class" services can be offered (e.g.,hardware/control scalability, service survivability). In light of these, morespecialized packet-switching schemes are being developed.Recently, the concept of "packet rings" has been proposed, aiming tocombine the salient features of "TDM-origin" ring topologies (i.e., simpleconnectivity, high resiliency) with the advantages of packet switching (statisticalmultiplexing, finer QOS), namely resilient packet rings (RPR, IEEE 802.17)[162, 163]. Specifically, a new Ethernet-layer media access control (MAC)protocol is defined to statistically multiplex multiple IP packets onto Ethernetpackets (i.e., layer-two). The MAC protocol itself is "media-independent," andwill be capable of running over various underlying networking infrastructures,including SONET/SDH, DWDM, or dark fiber. RPR designs can provide separate"layer-two" packet bypass capabilities at coarser granularities, and thiswill relieve packet loads at the IP (layer-three) routing level and improve QoSprovisioning . For example, sample RPR node design in Fig. 8.17 shows twodata priorities, or COS categories. Additionally, packet rings will alsoprovide a rapid "layer-two" protection signaling protocol, designed to matchthe 50-ms timescales yielded by SONET/SDH . The current RPR frameworkfocuses on two (i.e., dual) counterpropagating "rings" that can bothcarry working traffic (i.e., no reserved protection bandwidth for bandwidthefficiency). All control messages are carried "in-stream," making the controlstrictly in-band. Additionally, (layer-two) destination stripping is performedfor unicast flows, unlike earlier source-stripping FDDI rings, permitting spatial reuseof bandwidth (note that multicast and broadcast still require sourcestripping, however). Collectively, the above features significantly improve ringcapacity utilization/throughput (i.e., bandwidth multiplication, see  fordetails). Currently, a spatial reuse protocol (SRP) framework has been tabledfor standardization and is commercially available (amongst others), aiming toprovide all RPR features (e.g., protection switching, topology discovery, bandwidthfairness, etc.). In particular, the related protection switching protocol,termed the intelligent protection switching (IPS) protocol, is an architectural
counterpart to the SONET/SDH K1-K2 byte protocol.Nevertheless, since packet rings have emerged from enterprise LAN requirements,they clearly cannot support legacy TDM traffic (without proprietarymappings). Moreover, since RPR nodes must perform "electronic" packetprocessing operations, realistically, their scalability to high speeds (lOGb/s andbeyond) and large node counts needs to be proven . As such, they are mostsuitable for new IP-based carriers , at least initially, providing very low-costmetro solutions. Most likely, initial deployments will run over smaller-scalemetro edge rings, and here, packet ring/optical ring interworking will becomean important issue (see  for early discussions on this topic). Nevertheless,it is likely that future advances in optical packet switching (Section 8) will beleveraged to design substantially faster terabit packet rings. Overall, this is anevolving area, and more work will emerge (i.e., standardization, design, andperformance evaluation).6.4 MIGRATION STRATEGIESMetro edge evolution will likely exhibit high variability due to the diversityof subrate client protocols and available solutions. Ultimately, any chosensolution will depend very much upon existing infrastructures, economicconsiderations, and client/operational needs. Many metro edge networks still runat lower TDM bit rates (OC-3/STM-1, OC-12/STM-4), and therefore, relatively largecapacity expansions can be cost effectively achieved by simplyupgrading to higher bit rate TDM systems . This is especially true formoderate demand growth and smaller tributary rates (DS3, OC-3/STM-1)and/or fiber-rich scenarios. Meanwhile, newer operators with little/no existinggear and more constrained fiber counts may prefer compact "data-efficient"NGS/MSPP platforms to rapidly provision a full range of services (TDM and layertwo/three). Furthermore, those incumbents with costly "best-in-class"routing/switching gear may adopt a more cautious strategy towards NGS,choosing to deploy full solutions only for highly compelling price/performancealternatives. Still other incumbents may prefer NGS, given its strong originsfrom existing paradigms and finer-capacity allocation capabilities. Meanwhile, fornative Ethernet traffic, clearly packet-ring technology will bemuch more cost-effective than solutions using "telecom adapter" interfaces(SONET/SDH, NGS). Moving forward, this will likely be the solution ofchoice for newer data-centric operators without legacy clients. For example,packet rings will offer very low-cost aggregation between residential (Internet)cable and DSL hubs.Although the above alternatives may prevent immediate deployment ofDWDM technology in the metro edge, in the longer term it remains themost scalable and complementary solution [63, 64]. Namely, DWDM rings
can agnostically support all other solutions (e.g., by reserving different wavelengthsets for SONET/SDH, NGS, and IP routing solutions) and will clearlydecouple operators from continuing fluctuations in technology directions.More importantly, DWDM rings will allow operators to easily expand serviceofferings (e.g., legacy TDM voice/private line to data or vice versa).Most likely, many larger operators with existing legacy gear and diverse,specialized "higher-layer" systems (IP routers, ATM switches. Fibre Channel hubs,telephony switches) will deploy passive DWDM rings with flexibleedge aggregation interfaces to consolidate their architectures . This willensure abundant capacities for any future demand "spikes," and also addressthe growing "wavelength services" market (gigabits to customer edge [62,64]). Other operators who choose NGS gear may also move to modularlyadd DWDM capabilities in the future (e.g., flexible MSPP solutions). Akey planning/costing activity will be choosing when to cross over to opticalring architectures (see also Section 5.4). For example, some operators maymove from OC-48/STM-4 rings to DWDM rings rather than upgrade to lessscalableOC-192/STM-16 systems. Clearly, metro edge evolution requires moredefining studies, see [63-65] for market-related considerations.7. Network StandardsInteroperable standards are a key factor in ensuring the success and adoption ofnext-generation metro optical networking solutions. Standards helpto properly formalize both features and functionality, and will also help insulateoperators from single-vendor solutions. The key components of opticalinteroperability are now beginning to emerge. At the physical and link layers,many interface standards are well-defined, for example, SONET/SDHconcatenated formats, ITU-T wavelength grids, IEEE Ethernet interfaces, OIFinterfaces, etc. Increasingly, higher-layer control and architectural issues are nowbeing considered, and the ITU-T optical transport network (OTN) architecturedefines three layers of transport (channel, multiplex, transport) [12,145]. Meanwhile the IETF and OIF are beginning to tackle more detailed networksignaling/protocols issues  and the ANSI TlXl is studying opticalring frameworks. Perhaps the most notable development is the multiprotocollambda switching (MPXS) [148, 149, 157] framework, superseded recentlyby the more generalized (emerging) multiprotocol label switching (GMPLS)framework [150, 154]. GMPLS represents a strong push to increase horizontalcontrol plane integration (data and optical) by extending/reusing existingdata networking concepts/protocols. The overall aim is to replace the featuresof multiple protocol layers in traditional multilayered models (e.g., separateaddressing schemes, SONET/SDH protection, ATM traffic engineering) witha more unified solution, as shown in Fig. 8.18. A brief summary is presentedhere (refer to related references for details).
7.1 CHANNEL PROVISIONINGThere are several major required components for dynamic channel provisioningand advanced SLA management in metro optical networks, namelysetup signaling, resource discovery, and constraint-based routing . GMPLSimplements all of these requirements by extending MPLS signaling and resourcediscovery protocols and defining multiple link-specific abstractionsof the original MPLS label-swapping paradigm (i.e., "implicit labels" for timeslots,wavelengths, and fibers), see [148,149,153,154]. These definitions can befurther coupled with hierarchical label-stacking schemes to exploit scalability(e.g., packet labels into TDM circuit labels into lambda labels). In particular,this ubiquity make GMPLS an ideal control framework for multiservice metroedge platforms (Sections 6.1 and 6.2).First, optical channel setup signaling is accomplished by extensions toMPLS signaling protocols, namely RSVP-TE (RSVP traffic engineering) andCR-LDP (constraint-routing label distribution protocol) (see [148, 154] andreferences therein). Here, the explicit-routing (ER) capability  is usedto indicate the channel route and reserve resources. Meanwhile, actual routecomputation (i.e., RWA, Section 5.2.3) is done via constrained routing/pathcomputation (i.e., constraint-based routing (CBR) ). Moreover, CBR canalso incorporate advanced traffic/resource engineering algorithms for dynamicring/mesh networks. Finally, route computation requires networktopological/resource information (i.e., self-inventory capability), and this ispropagatedvia extensions to pertinent routing protocols, namely open-shortest path first(OSPF) and intermediate-system to intermediate-system (IS-IS) (see  forfull details). Examples include fiber-types, wavelength counts, wavelengthconversion resources, and possibly even analog metrics. More recently, resourcediversity information has also been proposed to explicitly capture risk associations(physical, logical) [106, 155], and this can help channel-routingalgorithms improve the "disjointedness" between working/protection paths.A key concern is provisioning architectures, namely centralized or distributedarchitectures . Data routing traditionally uses distributed control(signaling, routing), whereas optical ring/mesh routing is much more amenableto centralized implementations [7, 106, 155]. For example, many shared protectionschemes (Sections 5.2.2 and 5.3) require advanced optical ring/meshRWA algorithms with global per-connection state. Distributing/flooding suchinformation to all nodes is clearly unscalable. In other cases, if transmissionimpairments are incorporated, the resulting computations themselves are lessamenable to distributed renditions. Nevertheless, many distributed shortestpathheuristic RWA algorithms are still possible, and possible future advances(components, algorithms) may permit more feasible distributed renditions (see
[1,7,128]). Regardless, the GMPLS framework can be applied for either model(e.g., appropriate LSA extensions (distributed) and/or policy/route servers(centralized) ).7.2 PROTECTION SIGNALINGDynamic optical rings, and likely even hybrid/mesh architectures(Sections 5.2.2 and 5.3), must provide fast optical protection signaling protocols inorder to match the capabilities of SONET/SDH APS (i.e., 50-ms recovery).Moreover, these protocols are necessary to implement advancedservice definitions (e.g., multilevel resource sharing. Section 5.2.2). Variousstandardization efforts for optical protection are underway [15, 75, 95], but nosignaling standards currently exist. However, early proposals for fast opticalprotection signaling in GMPLS have appeared [75, 95]. For example, [75,94] discusses extending existing MPLS LSP protection signaling or defining analtogether new optical APS protocol. Initially, APS protocol(s) canbe defined for rings (i.e., leveraging on SONET/SDH concepts), but subsequentgeneralizations to hybrid ring-mesh networks can also be considered.Meanwhile,  presents a new "lightweight" restoration signaling protocol in lieuof RSVP/CR-LDP signaling. In general, until such standards aredefined, metro operators will continue to rely upon SONET/SDH protection,ultimately delaying the introduction of dynamic optical services provisioning.Assuming that fast optical protection signaling schemes will emerge, interlayerprotection coordination becomes an issue. Many metro-area protocolshave their own recovery mechanisms, operating across multiple domains (e.g.,optical channel protection, SONET APS, MPLS LSP protection switching,IP flow rerouting), and the simultaneous interference of such functionalitiescan be very detrimental. Specifically, problems can include reduced resourceutilization, increased recovery times, or routing instabilities [96, 100, 102, 103](e.g., prolonged SONET/SDH recovery times. Section 5.2.2). DWDM canalso compromise higher-layer survivability, as the high degree of multiplexing canlower higher-layer connectedness without proper preplanning .Additionally, replicated (excessive) protection functionality across layers canbe very inefficient . To date, no standards exist for multilayer protectioninterworking, and this is largely done via careful preplanning, see  for adetailed study. Moving forward, more formalized mechanisms are needed forcoordinating interlayer recovery actions between the packet/wavelength/fiberlevels, termed escalation strategies [94, 100, 103]. Various escalation strategiesare possible, such as bottom-up/top-down [7, 100] or serial/parallel ,and these will require complex interlayer signaling and hold-off timer mechanisms.In particular, metro edge (NGS, MSPP) platforms handling manyprotocols and their associated control/monitoring functions present some veryunique protection coordination possibilities. For example, routing diversity
information can be used to ensure higher-layer working/protection resourceseparation. Overall, this is a complex area that requires much more work .73 DOMAIN INTERFACINGEdge clients will need intelligent interfaces in order to automaticallyrequest/release "optical" bandwidth (i.e.,"bandwidth-on-demand" applications.Here, several interworking models have been defined to propagaterouting/connectivity information between the data (IP) and optical routingdomains, namely overlay, peer, and integrated models [154, 155]. The overlaymodel achieves maximum separation using separate routing/signaling protocols ineach domain and defining an intermediate optical user networkinterface (O-UNI) [146, 147, 151]. Conversely, the peer model achieves maximumintegration running the same (extended) routing/signaling protocols inboth domains. However, this proves overly complex/cumbersome, requiringrouters (optical nodes) to maintain/process optical (data) routing information. Theintegrated model strikes a balance between the above two schemes,running different instances of the same protocols (e.g., signaling, routing withextensions) and using gateway protocols for end-point exchange (see for full details). In the near term, however, the overlay model will see mostfavor since related UNI standards are available and proprietary optical controlprotocols can be accommodated. Moreover, this model provides bettermultiservice support, not just IP, and thus is well-suited for the metro space.At the core of "optical" channel provisioning is the concept of a service definition,as extended via an O-UNI or element/network management system(EMS/NMS) interface. The O-UNI is intended improve vertical integrationbetween layers by allowing automated service discovery along with bandwidthsignaling functions (e.g., request/release/modify operations). In addition, a setof generic signaled attributes are defined that can be mapped to subsequentchannel requests (e.g., RSVP/CR-LDP, Section 7.1; framing type; bit rate;protection type; priority; etc.) [147, 151, 157]. These mappings can cover abroad range of underlying capabilities (e.g., ring protection, mesh restoration, etc.). Signaled attributes will help facilitate multiple service levelsfor differing customer requirements, a necessary requirement in metro networks.Overall, O-UNIs are very germane to metro edge platforms, and evenmetro core nodes with direct wavelength interfaces. Several O-UNI definitions havebeen tabled for standardization of which both the ODSI interface and OIF standard  have been completed. Meanwhile, interdomainchannel routing and protection coordination between operator networks willrequire (optical) network-to-network interface (O-NNI) definitions and earlyconsiderations are also underway here [152, 155].7.4 NETWORK MANAGEMENT
As metro network elements continue to integrate many more diverseinterfaces/capabilities, especially at the metro edge, integrated networkmanagement is obviously a major requirement . Network management is a verylarge focus area in its own right, and here only a brief discussion is provideddue to scope limitations. In general, the well-accepted telecommunicationnetwork management (TMN) framework defines a hierarchical managementmodel comprising vendor element management systems (EMS) entities interfacingwith multivendor network management systems (NMS). Althoughmost early metro (DWDM) systems only provided proprietary EMS supportfor point-to-point transport nodes , more advanced solutions are now beingoffered. Optical channel (and link) visibility is of particular concernhere, and is complicated by the fact that there are still no standards for relatedparameters. As an interim, detailed SONET/SDH Bl/JO overhead byte monitoring(or digital wrappers equivalent) can be used at "opaque" points tomeasure bit-level performance (e.g., errored/severely-errored seconds, etc.).These "opaque" points can either be inside opaque nodes and/or at "edge"interfaces in transparent networks. Note that some vendors are also beginningto offer various (proprietary) sets of optical monitoring parameters, such aslaser powers/current/temperature, amplifier power, etc. .Meanwhile, with metro networks supporting many more protocols,advanced NMS solutions will be the key enablers for "end-to-end" servicesmanagement operating across multiple vendors equipment . Ideally, NMSsolutions should provide operators with a full range of functionalities thatare derived across multiple protocol domains, such as remote configuration,performance monitoring, rapid fault detection/alarm processing, failure isolation,diagnostics testing, and comprehensive logging/reporting, well-definedgraphical interfaces, etc. . However, genuine multivendor servicesmanagement requires widescale adoption of standardized managementframeworks,and overall this area is still in its infancy. Going forward, the commonapproach here will likely be to adapt TMN concepts and develop appropriatemanagement information models between EMS and NMS systems .8. Future DirectionsAs data traffic volumes continue to increase, packet-switching paradigms willgain increasing favor, owing to their inherent statistical efficiencies [7, 165-168]. Particularly, optical packet switching (OPS) designs are under study,utilizing optical techniques to perform as much of the packet-routing operations aspossible (e.g., switching, buffering, even processing, i.e., "fourthgeneration" optical networks). On the transport level, data packets aresent directly over wavelength channels (i.e., "packet-over-lightwave" (POL),Fig. 8.1). OPS nodes intend to achieve ultra-high packet throughputs, in the
multiterabits range, largely surpassing current gigabit router designs. Eventhough all packet-switching/routing functions are difficult to perform optically (andmay remain so for the foreseeable future), various multifacetedopto-electronic designs are being studied, and inevitably this work will leadto significant improvements in packet-routing performance. In particular, thethree main functionalities pertaining to OPS are buffering, switching, andheader processing (i.e., label lookup) [2, 168]. Some brief details are reviewed,and readers are referred to related references for more complete treatments.By and large, OPS nodes have the same architecture as electronic packetswitches (i.e., input buffering, space switching, output buffering [168,169], seeFig. 8.19). In packet switching, contention can occur if multiple packets are routedto the same output port , and resolution is commonly achieved viabuffering. Most OPS designs use fiber delay line buffers, and recently, the useof fast tunable lasers/converters has also been proposed to exploit the wavelengthdimension to store multiple (wavelength) packets in a given delay line. However, fiber delay line buffers constrain packets to multiples of a fixedlength, as there is no means to retrieve packets before minimum buffer delays.Furthermore, large buffer sizes become costly/bulky (requiring complex sharingsetups), and additionally, fiber attenuation concerns will limit the numberof "circulations," (usually under 100 ). Hence, as a tradeoff, a mixtureof electronic memory and delay line buffering can be utilized . Note thatthere are also very interesting, early developments in all-optical memories (e.g.,molecular transistors, see references in ). OPS switching fabrics, meanwhile,can also exploit the wavelength dimension to reduce contention andboost throughputs by orders of magnitude. However, nanosecond timings areneeded for packet transfers between switch ports, and this can be problematic forMEMS technology. SOA gate technology has been considered here,although careful design is necessary to control crosstalk . Another switchingsetup couples ultra-fast tunable lasers and wavelength converters withpassive wavelength routing devices (e.g., AWG) . As component tunabilityperformances improve and integration technologies mature, this approach maybecome very feasible. Finally, carefully note that unlike circuit switching,OPS is still linked to the bit rate of the client signal, at least the header. Specifically,even though payload flow is optically transparent, electronic headerprocessing/synchronization (and subsequent control of switching/bufferingresources) must be done electronically, as "all-optical" processing is not currentlyfeasible (see ). Clearly, electronic cost/scalability concerns can arisefor high-wavelength/fiber count systems and/or very small packet sizes, andthis may limit the complexity/range of label processing/packet filtering operationsperformed. Consequently, various bit-serial packet-coding techniques
and/or guard-band schemes have been proposed to reduce electronic processingbottlenecks . Note also that transparent payload sections will sufferfrom multi-hop optical degradations (loss, crosstalk), and this may requireall-optical regeneration to maintain transmission distances.Overall, OPS will provide a good match for limited metro distances andcan reuse much of the existing packet-switching (MPLS, DiflHServ) protocolsuites [156, 166, 167]. Moreover, metro OPS will prove highly complementary toemerging packet-based PON access solutions. Most likely, futuremetro packet switching solutions will evolve towards hybrid opto-electronicswitching architectures (Fig. 8.19). Specifically, electronic switching/bufferingwill be utilized for finer-granularity/more complex label-processing "edge"operations, whereas OPS will implement less complex label-swappingfunctionalities, achieving higher throughputs for more "aggregated" packet flows(e.g., lOTb/s stated in ). This delineation potentially lends well to"optical" packet rings, where more "coarse" stages (layer two CoS) can beimplemented using OPS. Even more germane to the metro arena, opticalpacket- and circuit-switching paradigms can be integrated onto a commonplatform, as the packet switches are still optically transparent to data payloads. Namely, lightpaths can be switched over the same OPS switchingfabric by simply "decoupling" the switching state from header-processing controllogic (i.e., bypass control logic and apply zero delay lines. Fig. 8.19). Thisoptical circuit bypassing will permit transparent legacy support (in exactly thesame manner as current DWDM systems. Section 5), and when applied tothe packet domain, will further improve scalability (i.e., eliminate per-nodeprocessing of large transit/labeled packet flows). Overall, OPS is an excitingnew frontier, and future advances in optical processing/buffering will undoubtedlyyield fundamental conceptual evolutions in the metro domain. Note alsothat slotted DWDM rings have also been proposed for access/metro data transport,utilizing fast tunable transmitters and/or receiver devices and fixed slottimings [170, 171]. Here, no optical buffering is performed inside the ring, andinstead, advanced multiaccess (MAC) protocols are used to arbitrate DWDMchannel slots in a fair manner between multiple users/traffic classes. Theserings require edge packet buffering but can yield improved efficiencies versuscircuit-provisioned rings, as the wavelengths are shared between multiplesource/destination points. A sample, advanced design using subcarrier sensingtechniques is studied in . However, fixed slot timings are very inefficient forvariable-length IP packets, and proposed protocol amendments (e.g., multiple slotsizes, backoff schemes ) entail further design/protocol complexity.Moreover, access-control protocol timings may adversely affect scalability overlarger metro core distances.9. Conclusions
Metropolitan networks occupy a strategic place in the overall network hierarchy,bridging end-users with abundant long-haul capacities. Traditionally,hierarchical SONET/SDH architectures have dominated the metro landscape,with slower speed access rings interconnecting to larger, faster metro core rings.However, as metro operators look to the future, many foresee surging bandwidthdemands, primarily driven by data traffic, and a plethora of diverseclients with differing protocols and service requirements. As competitionintensifies, legacy multilayered architectures are proving overly sluggish andunscalable in meeting complex, stringent service requirements. Clearly, metrooperators are in urgent need of scalable, flexible, multiservice bandwidthprovisioning solutions that allow for achieving a high level of service differentiation.DWDM technology provides many benefits in the metro arena, includingscalable capacity, transparency, and survivability. Moreover, manytechnoeconomic studies have confirmed the cost-effectiveness of DWDM for bitratesbeyond OC-12/STM-4, bolstered further by falling component pricepoints. Asa result, DWDM technology has gained strong favor as a metro core solution,and various architectures are possible (ranging from simpler point-to-pointtransmission systems to dynamic wavelength-routing ring and mesh networks).Nevertheless, given the large existing base of (SONET/SDH) fiber rings in themetro area, network migration is a key issue. Very likely, the first step in thismigration will be a move to point-to-point DWDM "fiber-relief" applications,and then onwards to more advanced optical ring/hybrid architectures. Meanwhile,metro edge networks are evolving to represent a merging of the opticaland electronic domains, aggregating many user protocols onto large metrocore wavelength tributaries. Many metro edge solutions have been proposed,ranging from DWDM edge rings, next-generation SONET/SDH, and multiserviceprovisioning platforms, to "IP-based" packet rings. The choice of edgesolution will clearly depend upon an individual operators needs, but over time,those incorporating DWDM technology will be most beneficial and hence willlikely gain prominence.