Design of Ring and Mesh based WDM Transport Networks - CiteSeerX

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Design of Ring and Mesh based WDM Transport Networks P. ARIJS B. VAN CAENEGEM P. DEMEESTER P. LAGASSE Ghent University – IMEC Department of Information Technology Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium

W. VAN PARYS P. ACHTEN Comsof N.V. Ketelvest 16, B-9000 Gent, Belgium

ABSTRACT The advent of reconfigurable nodal elements, such as Optical Add/Drop Multiplexers (OADM) and CrossConnects (OXC) for Wavelength Division Multiplexed (WDM) networks, has led to a myriad of possible network architectures for the optical layer. In this paper we describe different architectures for resilient ring and mesh based WDM networks, and compare these architectures from a conceptual viewpoint. The network design process is heavily influenced by the choice of the network architecture, because the different architectures have very distinct design issues. We describe the different design issues for mesh and ring based networks and give an overview of possible solution methods for the specific design problems. Some of this theory is put in practice in a case study, considering the design of realistic networks based on different network architectures. The results of this case study confirm the conceptual statements that we made.

1 - INTRODUCTION

© 2000 SPIE/Baltzer Science Publishers 1388 6916/2000 $15.00

Wavelength Division Multiplexing (WDM), which allows to multiplex several optical channels on a single strand of fiber, has become the preferred transmission technology in transport networks of long distance operators. WDM was initially deployed in point-to-point configurations to offer capacity relief on congested links suffering from fiber exhaustion. At the same time, the combination of several wavelengths on a single fiber allows for sharing the same amplifier among the wavelength channels, resulting in considerable cost savings. With WDM being increasingly deployed throughout the network, the need for advanced networking requirements at the optical layer has also intensified. As the amount of traffic passing through the nodes is ever increasing, routing functionality at the optical layer provides clear benefits. Optical Add/Drop Multiplexers (OADMs) and Optical Cross-Connects (OXCs) allow to terminate individual wavelength channels in a node, while transparently passing through the remaining wavelengths. This enables to set-up end-to-end wavelength paths (so-called lightpaths), which alleviates the need for expensive opto-electronic conversions and electrical processing equipment. Systems and subsystems that allow such all-optical networking functions are underway from research labs to commercial availability. Meanwhile standardization bodies are making efforts to render the optical network a common platform with full transport network functions, including network management and recovery schemes [1]. Providing recovery at the optical layers becomes inherently attractive as the network throughput increases [2]. With the advent of reconfigurable OADMs and OXCs, recovery actions (such as protection switching) can indeed be performed in the optical domain. A first step towards a survivable optical layer is seen through the use of WDM rings [3]. Since components and management for rings are simpler than for meshed networks, OADMs for ring architectures are expected to be earlier commercially available than OXCs. In fact, fixed OADMs (capable of adding and dropping a fixed subset of the wavelengths) are already available from some vendors today. More flexible July 2000 OPTICAL NETWORKS MAGAZINE 25

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OADMs enabling full connectivity, are expected in the near future. At a later stage, OXCs can be configured in mesh topologies to offer an even larger flexibility and make better use of spare capacity in the optical network. This roadmap towards all-optical networking, evolving from point-to-point systems to more complex systems with OADMs and OXCs is depicted in Figure 1 [4]. Ring and mesh topologies are very different in many aspects. Consequently, also the design approach is very different. In section 2 and 3 of this paper we describe the different WDM ring and mesh architectures and section 4 highlights the conceptual differences between these different architectures. In section 5 we focus on the different design issues and solution methods for ring and mesh networks. Finally, in section 6, a case study will be carried out, comparing the design of a realistic network based on ring and mesh topologies. This comparison will focus on some key performance indicators, such as cost and network availability. A similar comparison of network architectures has been done in [5] [6] [7]. Different architectures based on ring and meshed topologies for transport networks in SDH (Synchronous Digital Hierarchy) or WDM technology have been subject to comparison before in [8], [9], [10], [11]. Technology evolution

Interconnected rings and mesh topologies

OXC OXC

Figure 2b: OCh-DPRing after failure. OXC

OXC

OXC

Reconfigurable shared and dedicated WDM rings with full connectivity OADM Dedicated protection WDM rings with fixed OADM node adressing

OADM OADM

OADM OADM OADM OADM

OADM OADM

WDM transmission with fixed add/drop OADM

OADM

WDM transmission 1996

Year 1998

Figure 2a: OCh-DPRing.

2000

2002

Figure 1: WDM network roadmap with time indication of availability.

2 - RING NETWORK ARCHITECTURE The basic building blocks for WDM rings are the Optical Add/Drop Multiplexers (OADMs). Such OADMs terminate two fiber pairs and allow to extract and insert one or more wavelength channels from the WDM comb, while (transparently) passing through the remaining wavelengths. Automatic ring protection mechanisms on the channel or multiplex level, well known from the SDH/SONET technology, can be implemented analogously for WDM-based rings. As such two main types of WDM rings can be distinguished: dedicated protection rings and shared protection rings.

2.1 - Dedicated protection rings The dedicated protection ring consists of two counter-rotating fibers using WDM on each of the fibers. Each wavelength demand is protected using a main path along one side of the ring and a backup path along the other side of the ring. As the channels are protected on a per wavelength basis, this ring is also referred to as optical channel dedicated protection ring (OCh-DPRing).When a link or node failure occurs within the ring, the affected traffic is switched over to the protection 26 OPTICAL NETWORKS MAGAZINE July 2000

path, as illustrated in Figure 2. The advantage of this ring type is its relative simplicity: when based on 1+1 protection, the signal is split at the source node and the switching action is initiated by a selector at the receiver side, based on the monitoring information of the optical channel. This approach is simple and robust because no complicated signaling protocol is required. Alternatively, 1:1 protection can be used, requiring dual-ended switching. In the latter case, the protection path can also be used for transporting low priority traffic, as long as no failures occur on the working path. As such the redundant protection capacity can be used to generate revenue. However, such 1:1 protection does require a signaling protocol to co-ordinate the switching actions at both sides. A more near term implementation of this ring type, could be achieved with fixed OADMs and protection switching in the client layer. In this case, WDM is just used to lower the overall electronic cost.The colored section ring [12], for example, uses the mature and well-standardized linear protection switching in the SDH/SONET multiplex section layer (1+1 MSP), with the SDH/SONET multiplex sections carried by two wavelength paths routed on diverse sides of the ring. The main drawback of the OCh-DPRing is that it uses capacity rather inefficiently because more than 100% spare capacity is required, since a protected demand consumes capacity on the entire ring. As such, the required ring capacity is determined by the total amount of protected demand to be carried by the ring and independent of the individual nodeto-node demand distribution. On the other hand, this makes the OCh-DPRing architecture flexible with respect to unpredictable demand dispersions.

2.2 - Shared protection rings In the shared protection ring, 50% of the ring capacity is dedicated for protection purposes, which allows to share this pool of protection capacity amongst different wavelength

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Figure 3a: OMS-SPRing/OCh-SPRing. demands routed on the ring (see Figure 3a). The protection switching can occur on a wavelength basis, in a so-called optical channel shared protection ring (OCh-SPRing) or on a multiplex section level, in a so-called optical multiplex section shared protection ring (OMS-SPRing). There are two variants of the OMS-SPRing protection concept: 2-fiber OMS-SPRing and 4-fiber OMS-SPRing. The 2-fiber OMS-SPRing scheme only requires two fibers between each pair of adjacent OADMs. Half of the wavelengths on each fiber are reserved as protection channels. To accommodate transmission demands, optical paths are routed on the other half of the wavelengths (i.e., the working channels). The working channels in one fiber are protected by the protection channels in the other fiber, traveling in the other direction around the ring. In the event of a failure condition, the OADMs adjacent to the failure will loop back the affected lightpaths on the protection channels of the ring (see Figure 3b). It is interesting to note that bi-directional traffic has to use different wavelengths for both directions [3], otherwise wavelength conversion is required when traffic on a working channel is switched to a protection channel in the opposite direction (see also Figure 3b).

Figure 3b: OMS-SPRing after failure.

Figure 3c: OCh-SPRing after failure.

A 4-fiber OMS-SPRing requires four fibers between adjacent OADMs. Working and protection channels are now carried over different fibers, which enables to assign both directions of each working path to the same wavelength. The 4fiber arrangement combines both ring protection and span protection on the same architecture. If only the OMS in the working fiber of the ring is affected (e.g., failure of the inline amplifier), the parallel protection fiber can be addressed by a simple span switch and no loop back occurs. Certain multiple failures can as such be fully protected. The OCh-SPRing uses the same configuration as the OMS-SPRing, and the same advantages in terms of capacity sharing, only the protection switching is different.When a failure occurs, the affected connections are individually switched at the terminating OADMs to the other side of the ring, using the protection capacity on the other fiber (Figure 3c). An important advantage of the OCh-SPRing over the OMSSPRing is the reduced length of the protection path, an important consideration in optical networks, in which longer paths suffer from more attenuation and signal distortion [13]. For most traffic patterns (particularly uniform and adjacent node patterns [14]) the capacity utilization of the shared protection ring is higher than that of the dedicated protection ring (which inherently prohibits any sharing of protection capacity). On the other hand the implementation and management of OADMs for a shared protection ring is more complex – and potentially more expensive – than for a simple dedicated protection ring, e.g. since the development of suitable protection switching protocols is required.

2.3 - Ring interconnection strategies Optical rings are typically restricted in terms of fiber length and number of nodes allowed on the ring, to avoid transmission impairments and to minimize the protection switching delay, or to guarantee a high ring availability. Therefore large-scale ring-based networks are typically covered with multiple rings. While initially overlapping rings can be created such that no interconnection between rings is required [15], ring interconnection is a long-term requirement for an economical, reliable and manageable large-scale ring network. The rings can be interconnected logically in two ways, either by defining a hierarchy [16], or in a flat way without defining a hierarchy [17]. In the former case nodes of the rings are selected to be combined in a hierarchically higher ring.The difference between both approaches is in the routing of the inter-ring traffic, which is straightforward in a hierarchical set-up, but more complex in a flat interconnected setup.The hierarchical interconnected ring networks have therefore a simpler routing and management model. On the other hand, the routing inflexibility in hierarchical networks renders such networks less capacity efficient compared to flat ring networks. On the physical level, there are three options for ring interconnection, similar to SDH/SONET ring interconnection [18]. The simplest option is to directly interconnect the wavelength dropped at the tributary side of the OADM in one ring to the tributary side of the OADM in the other ring. Typically, both OADMs are co-located in the same building in a so-called ‘back-to-back’ configuration. Such a hard-wired interconnection (e.g. using a fiber distribution frame) does not offer any flexibility and is therefore only appropriate for static traffic. A second option, adding more flexibility, can be July 2000 OPTICAL NETWORKS MAGAZINE 27

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achieved by inserting an OXC between both OADMs. In that case, also the add/drop traffic can be terminated in the OXC. Finally, a third and most flexible option integrates both OADMs and OXC in a single OXC, providing both ring passthrough, add/drop and interconnection functionality. Such OXCs should also support the protection switching protocols of the ring. Ring interconnection allows for protection independence (i.e. a failure occurring in one ring does not trigger protection switching in the other ring), which allows surviving from multiple failures taking place in distinct rings. Failures of the ring interconnection gateways can be survived from by using dual node interconnection strategies as drop & continue [19]. The difference with considerations in SDH/SONET and WDM technology is the analogue medium of the latter, which imposes physical impairments such as noise accumulation and dispersion. In standardization bodies many contributions have suggested the use of transponders (3R regenerators) at subnetwork interfaces (SNI). At sub-network and administrative domain boundaries, transponders effect a clean hand-off of signals between domains and also provide OCh performance monitoring. In the interconnected ring network, the interconnection points between rings are natural places to put such transponders.

Figure 5a: Link restoration/protection.

3 - MESH NETWORK ARCHITECTURE Unlike ring networks, where 2 fiber pairs are interconnected by OADMs featuring local add-drop and pass-through functionality, meshed networks rely on OXCs capable of interconnecting multiple fiber pairs. In the OXC, wavelengths can be routed between incoming, outgoing and local fibers by appropriately configuring the space switch in the OXC (Figure 4). If there are no wavelength conversion facilities in the OXC, we talk about a Wavelength Routing OXC (WR-OXC). Networks featuring WR-OXCs are denoted as Wavelength Path (WP) networks, because a path is characterized by a unique wavelength. In that case a wavelength channel from an incoming fiber can only be routed to an outgoing fiber if this wavelength is not yet occupied on that fiber. Otherwise the route cannot be continued in this fiber, although other wavelengths may be still available, which is then called ‘wavelength blocking’. To overcome wavelength blocking, the incoming wavelength can be converted to a free wavelength in the outgoing fiber through the use of wavelength converters. OXCs featuring such wavelength conversion are called Wavelength space switch λ-demux λ-mux

λ1 λ3

input fibers

output fibers

λ2 fiber

WR-OXC local ports (a)

(b)

Figure 4: A wavelength path network requiring WR-OXCs (a) and representation of an OXC (b). 28 OPTICAL NETWORKS MAGAZINE July 2000

Figure 5b: Path restoration/protection. Translating OXCs (WT-OXC). Networks featuring WTOXCs are denoted as Virtual Wavelength Path (VWP) networks, because a path is not longer characterized by a unique wavelength but can have different wavelengths on subsequent links.WT-OXCs render a higher routing flexibility, yielding an increased throughput, however at the expense of costly wavelength conversion elements and higher internal node connectivity. Indeed, the complexity of the OXC depends on the use of wavelength converters: while the WR-OXC only requires connectivity between corresponding wavelengths, the WTOXC requires full internal connectivity. Intermediate forms, featuring only a limited amount of wavelength converters can also exist, in many different ways [20]. A more elaborated discussion about node architectures and wavelength conversion in optical networks is held in [21]. Optical cross-connects do not only provide a flexible means of routing wavelengths, but also allow to reroute traffic in spare channel resources in case of failures. The rerouting can occur using protection or restoration. Protection reroutes the traffic on pre-assigned spare resources that have been provisioned for a pre-determined failure or set of failures (which makes the rerouting 100% predictable). Restoration, on the other hand, makes use of a pool of spare resources in the network and for each failure, a restoration algorithm computes a restoration path within these spare resources. As such, restoration can make better use of the shared spare resources. Furthermore, the optical signals can be protected/restored end-to-end (on the path level) or by their constituting links (see Figure 5).

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3.1 - Restoration Restoration strategies can be classified according to three main criteria: • Link or path restoration • Pre-computed or real-time restoration • Centralized or distributed restoration Figure 5a shows the principle of link restoration. In this case, the traffic is rerouted between the end nodes of the failed link (provided that there are spare channels).The channels are either kept together or rerouted separately. This strategy typically consumes a lot of spare capacity in the vicinity of the failure. Yet, different links can be protected sharing the same spare capacity. In some cases backhauling may occur, i.e. the final route runs up and down in the same link as can be seen in the example of Figure 5a. Figure 5b shows end-to-end path restoration. In case of a link or node failure, all affected lightpaths are individually restored. Thus much more switching actions are required compared to link restoration, and the restoration time could be slower. On the other hand, path restoration works at a smaller granularity and can use capacity more globally, such that is makes better use of spare capacity in the network. The restoration route can coincide partly with the original route or a completely disjoint restoration route can be preferred. In case of WP networks, continuity of the wavelength along the restoration route is also necessary. This wavelength can however be different from the wavelength of the working route if the transmitter and receiver are tunable to another wavelength. The restoration routes can be pre-calculated (before the failure happens) or determined in real-time (after the failure happens). The speed and optimality requirements for the restoration algorithms to compute the alternative routes, have a different priority in both cases. Pre-calculation requires to calculate and store the restoration routes each time the network status changes or at regular time intervals.The algorithm then has knowledge of the entire network status, and the restoration routes for a set of pre-determined failure scenarios can be stored in a database. Upon a failure, a capacity reservation protocol then verifies the proposed restoration routes. Pre-calculation gives room for optimization, multiple failures are not considered to occur in a short timeframe and thus the optimization can be done off-line. Real-time restoration on the other hand requires, upon a failure, to figure out the current network status and to propose adequate restoration routes. While real-time restoration can typically survive from more unexpected failures than pre-calculated restoration, it is also slower.The restoration process can be speeded up however, by sacrificing on the optimality of the restoration algorithm. In centralized restoration, computation of restoration routes is done in a centralized network controller, where all necessary and up-to-date network information is available. After computation, the routes are downloaded into the databases of the nodes. Real-time centralized restoration is based on alarm messages to identify the failure and obtain topology information, which is typically a slow process. Pre-calculated centralized restoration is much faster, but requires frequent communications between the centralized controller and network elements to acquire up-to-date topology information, which might not be scalable for large size networks. Distributed restoration can be real-time, based on flooding messages, sent out by the terminating nodes of a failed link, that search for

alternative routes. Although simple, this mechanism is fairly slow and more importantly it has not yet shown to scale beyond single link failures. Therefore pre-computed distributed restoration seems like a more viable alternative. To restrict the amount of restoration routes to be stored in memory, it is best to use failure independent restoration paths. Although some vendors claim restoration within 50 ms., it is believed that in general this kind of restoration requires more time and that unnoticeable interruption for the service layer is not achievable, since a large sequence of events needs to be executed upon a failure. After the detection and isolation of the failure, a decision has to be taken (centrally or in a distributed way) on the actions to be taken to resolve the failure, the involved cross-connects need to be informed and reconfiguration of the cross-connects has to be effectively carried out. To achieve fast recovery in a mesh network, protection architectures are a better alternative, because less coordination between network elements is required.

3.2 - Protection Path protection, reserves for each working path a dedicated end-to-end protection path and can use 1+1 or 1:1 protection switching. The working and protection path can be link disjoint if the protection mechanism only has to protect link failures or it can also be node disjoint if the protection mechanism also has to protect node failures. Alternatively, link protection or optical multiplex section (OMS) protection, switches an entire multiplex section, either to a dedicated parallel fiber, or to a disjoint fiber route in the network. Link protection can also be applied in a shared fashion, e.g. in a M:1 configuration: on the link there are M working fibers, protected by 1 redundant fiber. Such an approach can be used to cope with equipment failures (e.g. failure of one of the in-line amplifiers) or for maintenance of one of the working fibers, during which the signal on one of these fibers can be shifted to the protection fiber. For path protection, spare channels are dedicated for protection and are embedded in the fibers together with working channels. These spare channels are made available in the OXC in the same way as the working channels and they therefore contribute to the dimension of the switch matrix. In case of OMS protection, the access to the dedicated protection resources can be arranged semi-statically or with simple switches outside the cross-connect, making the nodes less complex.

3.3 - Transparent versus opaque Since signal quality monitoring and all-optical regeneration are currently unresolved issues in the optical network, so-called 'opaque' network architectures come to the forefront [22] [23]. In opaque networks, channels are regenerated through the electrical domain after every link or node passage to get rid of signal degradations and to allow performance monitoring. This electrical regeneration imposes restrictions on the signal format and bitrate and therefore limits the transparency. This is however not felt as a severe limitation in today’s transport networks, which are based on a limited set of digital client layer signals. As transponders are still fairly expensive devices, an opaque network is a costly approach. Alternatively, transponders can be allocated only in these places where it is required with regards to the signal deterioration. In that case, planning of the locations where such regeneration should take place is also part of the network design process. July 2000 OPTICAL NETWORKS MAGAZINE 29

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4 - COMPARISON OF RECOVERY SCHEMES A first measure to compare the different recovery schemes is the installation cost. While a more detailed case study will be worked out in section 6, we can already make some preliminary statements regarding the cost of the different schemes (Table 1). The link cost is highest for the dedicated protected schemes, because no protection capacity can be shared. The meshed link protection scheme has the highest link cost if this scheme requires a diverse protection route per protected link, because each protection route typically spans a multitude of links on which capacity needs to be nailed up. Contrary, in the meshed path protection scheme only one protection path is required between the end points of each connection. As such the protection works at a lower granularity and the protection is more global, such that the amount of capacity required (i.e. the amount of links spanned) by working and protection path is more balanced and redundant capacity can be better spread out over the network, as shown in [24]. In the dedicated protection ring a diverse protection path is provided within each ring that the working path traverses. As such the link cost of the dedicated ring protection scheme typically lies between the dedicated link and path protection scheme. With respect to the capacity requirement, protection requires substantial more redundant capacity than restoration. On top of the working capacity, 100 to 200% redundant capacity is required for protection, while only 50–100% for restoration. Therefore the lowest link cost can be expected for the shared protection and restoration schemes: shared protection rings, but even more for shared restoration in a mesh because spare capacity can be used more globally [6]. Also for mesh restoration, path based schemes have shown to use spare capacity more efficiently than link based schemes [25]. Concerning the node cost, OADMs are expected to be considerably cheaper than comparative OXCs. Consequently, the ring based protection schemes should have the lowest node cost if the amount of required OADMs does not considerably exceed the equivalent amount of OXCs.When interconnecting a large amount of rings, all OADMs of the different rings have to be interconnected, which becomes highly complex. In this case, the use of a more expensive OXC for ring interconnection can be considered to improve the scalability. In addition, the use of drop&continue to interconnect rings, can increase the node cost for ring based schemes. Although mesh based schemes relying on OXCs are expected to be more expensive than rings with respect to node cost, dedicated link protection schemes can also rely on external fiber switches to reduce the switching matrix dimension and thus the node cost. As restoration schemes require less spare capacity compared to the path protection scheme, the amount of traffic to be cross-connected for restoration schemes is also

Ded. prot. ring Shared prot. ring Mesh path prot. Mesh link prot Mesh path res. Mesh link res.

Link cost Higher Low High Highest Lowest Low

Node cost Lowest Lowest High High/Mid Mid/Low Mid

Table 1: Comparison of network architectures. 30 OPTICAL NETWORKS MAGAZINE July 2000

lower, which in turn also contributes to a lower node cost. The management cost is lowest for the protection schemes, since they require no signaling protocol (as long as 1+1 protection is used). For the shared protection ring, an APS protocol is required, which adds up to the management cost. Also for restoration schemes in meshed networks, signaling is required (either centralized or distributed). This signaling is even more complicated, since multiple possible routes for the restoration path exist and may need to be evaluated (in a dynamic restoration scheme). In addition, the restoration path has to be set up by reconfiguring all the OXCs on its route, while the shared protection ring only needs to switch and bridge at two ADMs to perform the switching. Therefore mesh-based restoration comprises a higher management cost than shared protection rings. Flexibility involves the ability to cope with unpredicted traffic patterns and failures (e.g. double link failures) and the scalability of the network. The ring based schemes offer the lowest flexibility because they do not fully exploit the connectivity of the network and the route of the protection paths is fixed. Rings are also more difficult to upgrade compared to meshed networks, because the capacity should be upgraded along the entire ring or a whole ring system must be added at once, while in meshed networks upgrades can be made per line system or per OXC. The flexibility with regard to unpredicted demand dispersions is higher for the dedicated protection ring than for the shared protection ring, because its sizing only depends on the amount of traffic and not on the pattern. Meshed based schemes, relying on OXCs, are most flexible as they can be configured to use any potentially free route in the network. In addition, real-time restoration is also very flexible with regard to unpredicted network failures. The availability of a path/network is the proportion of time for which the path/network is expected to be available relative to the total time. Interconnected ring networks offer the highest availability, as multiple failures occurring in different rings can be recovered from (protection independence). Ring interconnection gateways can be protected using drop&continue, resulting in a very secure architecture, consisting of survivable subnetworks. Long paths running over several rings can therefore recover from several failures along its track. Mesh path protection schemes protect the paths endto-end and if both working and protection path have failed the connection can not be restored. Restoration schemes are more flexible and can react adequately for several failure scenarios. However, it depends on the amount of spare resources in the network, whether failures can be survived from or not. The availability depends in this case on the overdimensioning for spare resources and on the behavior of the restoration scheme. The link based protection/restoration schemes cannot recover from node failures, but they can potentially recover better

Manag. cost Low Mid Low Low Higher Higher

Flexibility Mid/Low Lower Mid Mid High High

Availability High High Mid Mid Mid/High Mid/High

Recovery time Fast Fast Fast Fast Slowest Slower

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from multiple link failures. Overall availability thus depends on the ratio of availability of nodes and links and the amount of spare resources. Ring and path based 1+1 protection schemes should allow to invoke the protection switching in less than 50 ms as their SDH/SONET counterparts (or even outpacing SDH/SONET), because no complex signaling is required. Also for shared protection rings fast recovery times have been demonstrated [26]. Restoration schemes, either real-time restoration (requiring extensive messaging) or pre-planned restoration are expected to achieve slower recovery times than protection. However, recent advances might allow more competitive recovery times [27]. So far, the network architectures and their availability were described based on one type of recovery scheme. Normally the required availability is imposed by the traffic demand. When considering different traffic types and some types require a higher availability than others, protection selectivity is required. For example, traffic that has been protected in the client layer should not be protected again in the WDM layer. Also for the support of a low priority IP network, protection in the server layer may not be required, since IP routers can effectively reroute traffic from broken links through other links. However restoration in the server layer may still be preferred as a second-line of protection or in order to avoid congestion in the IP network. Protection schemes in the client layer can also coexist with WDM recovery schemes. This allows to protect the network against outages in the client layers, which WDM is unable to protect. On the other hand this also adds an additional cost and interworking complexities to the network [28]. The existing myriad of services has very different availability requirements from a server WDM network point of view. In a mesh network the different kinds of service can be easily combined. In rings, this could be achieved through the support of non-preemptible unprotected traffic and preemptible extra traffic in the ring. To combine the advantages of both architectures, one can think about alternative architectures based on ring and mesh influences. In [29] a hybrid ring mesh network is proposed. Traffic is first routed in a mesh architecture and afterwards rings are inserted where traffic well matches the traffic conditions to a ring architecture. Also in [30] hybrid ring mesh schemes have been investigated.

5 - NETWORK DESIGN Given a set of points of presence (POP), which the network needs to serve, and an estimated traffic forecast for the next couple of years between these POPs, a cost-effective network architecture and dimensioning needs to be determined to transport this traffic. In this paper we focus on the design of optical transport networks and we thus consider POPs with concentrated traffic, expressed as wavelengths, covering large geographical areas (e.g. Europe or the US). Figure 6 shows the major blocks for the network planning process. Before the actual design, a strategic decision on the network and node architecture and on the recovery strategy must be made. This confines the equipment that will be used in the network, such as OADMs or OXCs, the type of protection rings, etc. Once this is chosen, the actual network design can be conducted, which tries to dimension the network such that it is able to route all the traffic between the nodes, taking

Strategic planning decisions

Network design: Dimensioning of mesh or ring architecture

Post-planning evaluation

• Network architecture • Node architecture • Recovery strategy • ..........

• Traffic • Costs • Node architecture • Re-routing strategy • Failure scenarios • Physical limitations • ..........

Figure 6: Network design phases. into account certain constraints, such as guarding physical transmission quality, and ensuring that the considered failure scenarios can be survived from. Often the design phase focussed on a limited set of failure scenarios (e.g. only single link failures), however survivability for more failure scenarios is desired and appreciated. In the design phase, the major objective is to minimize the cost (although the overall planning objective can be much more complicated [31]). The cost structure of the network and appropriate cost values are thus required as input parameters for the design process. Most of the encountered problems in network design are optimization problems for which techniques from operations research can be used, such as Linear Programming (LP) [32] or heuristic search techniques (e.g. Simulated Annealing, Genetic Algorithms or Tabu Search). Alternatively, constructive heuristics that build up a solution based on common sense considerations may result in good solutions as well. Depending on the problem complexity and the required optimality, the different planning tasks can be performed in consecutive steps, or in an integrated approach. While a sequential approach can handle the problem within acceptable computational effort, the integrated approach can yield a better overall result. An integrated approach can be implemented by solving as much as possible sub-problems together in an integrated algorithm or by using an algorithm that internally still makes a decomposition in multiple phases but with strong feedback and interaction, solved by iteration, so to end up with an optimized solution for all phases. In the post-planning evaluation phase, a more extensive evaluation of the designed network can be performed such as an analysis of the network behavior for unexpected failure scenarios and dynamic traffic conditions [33]. As such, we can measure the sensitivity of the network design to fluctuations in the input parameters. This evaluation phase can in turn give feedback to the design process to adapt the network slightly, such that it will better perform in the next iteration. In this paper, we compare ring and mesh architectures and different recovery architectures. The minimization of the network cost will always be our main goal when designing a WDM network, with the chosen network architecture, provided that the traffic demand can be routed and that the network is survivable in case of the considered failures.These networks are then compared with respect to cost and availability in order to draw conclusions on the network architectures and their applicability. Since mesh design and interconnected ring design are very different, their design issues and an overview of solution methods are described separately in the following sections. July 2000 OPTICAL NETWORKS MAGAZINE 31

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5.1 - Mesh network architecture

Topology design

Traffic routing and capacity allocation

Spare capacity allocation

Mesh network design

Once a mesh architecture is adopted, the network design must decide on the amount of resources required in the network to carry the traffic. This includes determining the size of the OXCs in the different nodes, and the amount of fibers and wavelengths required between the nodes. Figure 7 shows a decomposition of the network design into three major tasks: topology design (which decides on the links to use), traffic routing and capacity allocation (that allows the transport of traffic between the demand node pairs), and the spare capacity allocation (to make the network survivable in case of failures).

• Traffic • Costs • Node architecture • Re-routing strategy • Failure scenarios • Physical limitations • ..........

Figure 7: Mesh network design phases. The topology design phase starts from a set of candidate links between the nodes of the transport network, and decides which links to install. The choice of the candidate links can be based on availability in the field, traffic forecast considerations, architectural constraints or topology connectivity considerations. Some candidate links can be left unused if it appears to be more cost-effective [34]. Indeed, if the cable installation cost is dominant, we will end up with a very sparse network, while if the capacity installation cost is dominant we will end up with a much denser network. Traffic routing and dimensioning can be tackled as one problem or as a sequence of sub-problems, e.g. first solving the routing in an uncapacitated network, followed by the dimensioning of the equipment, based on this routing. For routing problems one often relies on principles from graph theory. Dijkstra’s algorithm [35] finds the shortest routes, other algorithms look for the shortest pair of disjoint routes [36] or the K shortest routes between two nodes [37]. In case of K eligible routes, an optimization algorithm can pick out the best one considering the overall design objective. In case of a WP network, the wavelength continuity of the paths is important. The wavelength assignment is then a subproblem that can be done after routing, or in combination with the routing. It can involve dimensioning at the same time to get all traffic routed or be performed in a dimensioned network. In the latter case, the throughput can be maximized but routing of 100% of the traffic cannot be reassured. The wavelength assignment problem can be considered assuming that the number of wavelengths per fiber is unlimited [38]. The aim of the study is then to minimize the number of required wavelengths and the routing objective is therefore to spread out the paths as smoothly as possible over the links in order to minimize the load on the most loaded link. A more realistic formulation of the wavelength assignment problem considers a particular WDM line system to be adopted throughout the network. The problem is then reformulated as to minimize the number of WDM line systems in the network and it comprises routing and dimensioning at the 32 OPTICAL NETWORKS MAGAZINE July 2000

same time [25], [39]. The complexity of the problem depends on the amount of wavelengths per line system versus the total traffic demand. If the amount of wavelengths is small compared to the traffic demand, the problem is easier to solve. In the extreme case of one wavelength per fiber, no wavelength assignment problems exist. The result of the routing and dimensioning depends heavily on the cost structure of the equipment. WDM line systems can only be added in discrete steps with a fixed number of wavelengths (e.g. 16, 32, …).The ratio between the fixed installation cost of a WDM line system and the routing cost per wavelength is very important for the routing [25]. If the routing cost per wavelength is high, the goal is to minimize the total routing cost, and thus shortest path routing is preferred. If the fixed line system cost is dominant, the aim is to minimize the amount of line systems, and thus using the line systems as efficiently as possible, which requires a different routing scheme. When the routing of traffic and wavelength assignment is considered in an already dimensioned network, the routing objective is to maximize the network throughput for a given traffic matrix. Since several traffic demands compete for the same link capacity resources, the routing of the traffic demands must be dealt with simultaneously in order to achieve the optimal result. This routing problem is also called ‘the multi-commodity flow problem’ [40]. Integer Linear Programming has been used successfully for this problem, while well-performing heuristics exist as well and can be applied for larger problem instances [41], [42], [43]. Such heuristics can be based on common sense rules, e.g. assigning first the wavelength of the most difficult multiple hop connections, balancing the wavelength usage with a kind of ‘leastused’ approach favoring the wavelengths that are least-used so far. In case of dynamic traffic, the objective is to minimize the blocking probability and similar routing heuristics can be used [33], [44], [45]. The wavelength requirement for VWP networks was found to be only slightly lower than for WP networks and it was therefore questioned whether wavelength conversion is really required in the network [41], [46], [47], [48], [49]. Apart from the presence of wavelength converters in the node, it has also impact on the node architecture. However also other considerations about wavelength conversion should be made, such as implications for the network management, network transparency, signal quality, etc. [50]. Since it was found that complete provisioning of wavelength converters resulted in only moderate performance improvements, attention was paid to network and node architectures that employ a limited set of wavelength converters [20], [51], [52], [53]. In [21] an overview is given of these network and node architectures. For the allocation of spare capacity, the survivability strategy that is selected is important since every strategy requires a specific design approach. For link protection, the fibers in the links should be doubled and it has to be assured that they follow a cable diverse route. For path protection, a second disjoint path is required between the source and destination node. For restoration or shared protection, the spare capacity should be minimized while optimizing the sharing. The amount of failure scenarios considered has an influence on the amount of spare capacity that is required. The spare capacity allocation problem can be expressed in a linear model that can be solved to optimality with Integer Linear Programming

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techniques [54] or heuristic techniques [55]. While many papers in literature refer to general meshed networks, in WDM networks a slightly different problem formulation is required [25], [56]. The node functional restrictions need to be taken into account, such as the wavelength continuity requirement of the restoration route in WP networks. If the transmitters and receivers are not wavelength tunable, the restoration route must use the same wavelength, requiring somewhat more spare capacity than with tunable transmitters and receivers [25]. Another general conclusion is that path restoration allows for a more distributed spreading of spare resources in the network than link restoration, thereby requiring less spare capacity. Apart from functional restrictions also physical restrictions must be considered in WDM networks which may impose limitations on the length of the restoration routes due to transmission problems.

5.2 - Ring network architecture

Ring identification and interconnection

Inter- and intra-ring routing and ring dimensioning

Ring network design

Once the decision has been made about which ring type to use, and which interconnection strategy between rings, the process of designing an interconnected ring network can be considered, which typically consists of 2 main sub-problems (Figure 8): 1. Ring identification (greenfield or not, hierarchical or not) 2. Ring dimensioning based on inter- and intra-ring routing (and wavelength assignment). No specific spare capacity planning phase (as in meshed networks) is required for interconnected rings, because the spare capacity is embedded in the rings. While rings are conceptually easy, and single ring design issues have been well understood [14], building a network based on a set of interconnected rings is far from a straightforward task.The problem consists of finding a minimum cost combination of interconnected rings, such that all demands in the network can be routed and protected. The problem can be stated in a greenfield scenario (i.e. no cables installed yet) or in an existing network topology. As there exist a large number of potential ring positions (even in an existing network topology) this is a very complicated problem. Hierarchical ring network design was already considered by [57] for SDH/SONET rings, using a clustering heuristic that starts from the lowest level in the hierarchy and subsequently works its way up. A somehow similar but elaborated approach has been taken by [16] and [58] for WDM rings. The non-hierarchical ring network design problem has received most attention in literature, mainly for SDH/SONET rings. Because of the complexity of the problem, most authors used a heuristic optimization method. A first kind of heuristic solution method adds rings to the network in a sequential fashion (based on certain traffic metrics), until all traffic can

Figure 8: Ring network design phases.

• Traffic • Costs • Node architecture • Re-routing strategy • Failure scenarios • Physical limitations • ..........

be routed.This approach was taken in [59] and [60]. A second kind of heuristic consists of several phases in which potential rings are generated, and several suitable ring combinations are evaluated by routing traffic on the selected rings [61]. Other authors have used heuristic search techniques, such as simulated annealing [62] or genetic algorithms [63] to find suitable ring combinations. Also integer linear programming techniques (ILP) [32] have been used, but mostly make some simplifying assumptions to facilitate the problem complexity. E.g. [64], make more or less abstraction of the routing problem. In [65] an optimal routing is used, but only considering a very limited subset of possible rings. In [66] on the other hand, the problem is split up in two phases (ring assignment and routing) and solved both using ILP. In the ring planning process, two important single ring design problems come forward: ring loading (which considers which way around the ring the traffic must be routed) and wavelength assignment: (which considers which particular wavelength will be use for each path on the ring). The objective of ring loading is to accommodate all nodeto-node optical demands on the ring while utilizing as few wavelengths as possible. For an OCh-DPRing this is trivial, since the main and protection lightpaths that are set up to serve a particular demand use both sides of the ring. As such, every demand is carried over every section of the optical ring. On a shared protection ring, each demand can be routed either clockwise or counter-clockwise in the working channels of the ring. For calculating the required number of wavelengths on a shared protection ring, the link of the ring (i.e., optical multiplex section) carrying the most traffic is determinative. Hence, the routing strategy that minimizes the utilization of the most loaded link results in a minimal number of required wavelengths. The same problem appears for routing unprotected demands on an OCh-DPRing. This optimization problem has been discussed before in the literature in the context of SDH/SONET ring planning. It has been shown that this problem is NP complete when it is not permitted to split a transmission demand on the ring (i.e., each node-to-node demand between two nodes has to be routed either clockwise or counter-clockwise in its entirety). Solution models, based on integer linear programming and heuristics are presented in [67], [68], [69], [14]. The ring loading problem does not consider to which particular wavelength each lightpath should be assigned on each section of the ring. When the nodes are able to perform wavelength conversion, the wavelength assignment problem is trivial to solve, since we can change the wavelength of each lightpath in intermediate nodes to avoid wavelength conflicts. When wavelength conversion is undesirable, each lightpath has to be assigned a unique wavelength on all the sections it crosses. In case of OCh-DPRings, the wavelength assignment problem is straightforward to solve, since working and backup path ‘use up’ the same wavelength along the ring. Consequently, it is impossible to share a wavelength amongst multiple demands (no potential for wavelength conflicts). In shared protection rings, optimal wavelength assignment algorithms have been devised for certain specific demand patterns (e.g., uniform pattern) as in [70], [71]. In [14] a model is presented for more general demand patterns. It has been shown that a certain routing on the ring (e.g. the outcome of the ring loading problem) hardly poses any waveJuly 2000 OPTICAL NETWORKS MAGAZINE 33

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length conflicts (that is, the amount of wavelengths resulting from the loading problem generally allows a unique wavelength assignment). This shows that wavelength conversion does generally not reduce the wavelength requirement on optical rings.

After having discussed the conceptual differences between ring and mesh based networks and the different design issues, we now seek to confirm some of these considerations in a comparison based on a realistic case study. We present results for 2 sample networks: a 16-node/21-link and a 32-node/42- link pan-European optical network. We considered a static bi-directional traffic matrix with the demand between two nodes varying between 0 and 2 wavelengths. The network has been designed for meshed path protection (using WR- or WT-OXCs), meshed link and path restoration (using WT-OXCs) and interconnected OCh-DPRings (using OADMs and hardwired back-to-back ring interconnection) and to cope with single link failures. For meshed path protection, the network was designed using a heuristic based on the shortest pair of disjoint paths [36] and the wavelength assignment in case of WP networks was done using a heuristic. For meshed restoration the working capacity was assigned using a heuristic based on the shortest path algorithm of Dijkstra [35] and the spare capacity was assigned using integer linear programming based on the K-shortest paths [25]. For interconnected OCh-DPRings, the rings were identified based on tabu search, and using a heuristic for the routing of traffic on the rings [17]. In the 16-node network, 6 rings were used, while in the 32-node network, 11 rings were used. Comparisons will be made based on link and node cost, and on the network availability. The used cost values and failure rates for the 16 wavelength WDM equipment we considered, are presented in Table 2 and Table 3 respectively.

OXC OXC OXC OXC

8x8 fiber 16x16 fiber 32x32 fiber 64x64 fiber

OADM and OXC In-line amplifier

MTTR 24 h 6h 24 h

Table 3: Failure rates.

6 - CASE STUDY

WDM terminal multiplexer In-line amplifier OADM Ring interconnection cost

MTBF 1 failure per year per 300 km 10 5 h 5.10 5 h

Line system

Installation cost 100 + 10 per wavelength 40 50 10 per wavelength interconnected between 2 rings 640 1600 3840 8960

tion) and to reflect the potential use of transponders at ring interconnection points. The failure rates have been expressed as mean time between failures (MTBF) and mean time to repair (MTTR) a failure. The unavailability of the network element is thus: MTTR/(MTBF+MTTR).

6.1 - Cost comparison First we compare the link and node cost of the different recovery schemes.These costs are presented in Figure 9 for the 16-node network and in Figure 10 for the 32-node network. Link cost: As can be seen, the network design based on interconnected OCh-DPRings results in the highest link cost. For the 32-node network, the use of drop&continue requires an additional link cost of about 5%, because some connections have to take longer routes (compared to the case without drop&continue) to ensure that all rings on the route have dual node interconnectivity. The link cost of interconnected OChDPRings is about 20–25% more expensive than that of path protection in a meshed network. The difference in link cost between the WP and VWP path protection scheme is not that high. The WP scheme requires about 15% more fibers compared to VWP to resolve wavelength conflicts in the 16-node network. In the 32-node network this is only 5%, because a lot more fibers per link are required in the 32-node network, making the wavelength assignment easier. When wavelength tunability at transmitter and receiver is provided, enabling a different wavelength for working and protection path, the WP scheme requires less than 5% extra fibers compared to VWP for both networks. As the fixed link cost is not that high compared to cost per wavelength, the WP scheme results in a link cost increase of only 2–5% without tunability at transmitter and receiver and 1–2% with tunability.

60000 Node cost 50000

The link cost comprises the cost of the WDM terminal multiplexers and the in-line amplifiers on the link. We do not consider cable installation costs, as we assume a cable topology with sufficient fibers already exists.The cost per wavelength for the WDM terminal multiplexer reflects the modularity of the equipment.The OADM and OXC cost depends mostly on the size of the switching matrix. Both OXCs and OADMs are assumed 100% non-blocking. The OXC cost has been estimated by the amount of basic switch elements required to realize a WR-OXC. The ring interconnection cost has been added to make the comparison with the mesh design more fair (since no cross-connects are assumed for ring interconnec-

40000

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Cost

Table 2: Cost values.

Link cost

30000

20000

10000

0 WP protection

WP protection +tunability

VWP VWP link VWP path protection restoration restoration

Figure 9: Cost of different recovery schemes for 16-node network.

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400000 Node cost

350000

Link cost

300000

Cost

250000 200000

and the node cost of path restoration is about 30% lower that that of link restoration, but still much higher than interconnected OCh-DPRings. Because of the low link cost, path restoration is the cheapest overall solution: about 10–20% cheaper than link restoration, 30% cheaper than interconnected OChDPRings and 40–45% cheaper than path protection.

6.2 - Availability comparison

150000 100000 50000 0 WP protection

WP VWP VWP link VWP path protection protection restoration restoration +tunability

Figure 10: Cost of different recovery schemes for 32-node network. VWP link restoration yields a link cost about 25% lower than VWP path protection. The link cost of path restoration compared to link restoration is even 15% lower, making this the cheapest scheme with regard to link cost.When comparing VWP path protection with VWP path restoration, 35% of the link cost can be saved through sharing of protection capacity. Node cost: The node cost is lowest for the interconnected OChDPRing due to the relatively cheap OADMs (note that the cost of terminal multiplexers is included in the link cost). The use of drop&continue requires a 10–15% higher node cost, because traffic is exchanged at two nodes between two rings. The node cost for mesh path protection is the highest, due to the expensive cross-connects, making this the most expensive option overall. The difference in node cost between WP (both with and without tunability at transmitter and receiver) and VWP protection is small (less than 5%), which is partly due to the discrete node cost used. For VWP only a few nodes tend to be smaller hence only a limited node cost reduction is observed. As also the difference in link cost is small between WP and VWP protection, the overall cost difference is also small. Note however that in this comparison the same cost was used for a WR- and WT- OXC, while it is clear that the WTOXC installation cost will be larger (because wavelength converters and more internal connectivity is required), thus favoring the WP solution. On the other hand, the total cost of ownership, including controlling and management could be lower for a WT-OXC since the link capacity can be controlled and assigned independently, no wavelength assignment protocols and algorithm are required. Due to a lack of cost figures for these aspects, the comparison is limited to installation cost. The difference in total cost between VWP meshed path protection and interconnected OCh-DPRings is about 20%. The cost of the OXCs (as in Table 2) should decrease by about 50% to make the total cost of the path protection approach competitive with interconnected OCh-DPRings. Link restoration has a node cost, about 50% less than path protection but still more than double than that of interconnected OCh-DPRings. Path restoration has about the same node cost as link restoration for the 16-node network, because due to the discrete sizes of the OXCs, no OXCs can be reduced in size. For the 32-node network some OXCs can be reduced in size

A second means for comparing the different recovery schemes is the availability of the network. As a comparative measure, we use the expected loss of traffic (ELT), which is the amount of traffic that the network is expected to lose per year due to failures [72]. We express ELT in STM-16 hours per year (h/y), assuming each wavelength transports one STM-16 signal (=2.5 Gb/s). We have calculated the ELT both for link failures only and for link and node failures. The ELT figures versus the cost of the ring and meshed protection schemes is depicted for both networks in Figure 11 and Figure 12 (for only link failures) and in Figure 13 and Figure 14 (for both node and link failures). We did not calculate the ELT values for restoration, because it requires extensive simulations considering manifold failure scenarios and the result depends too much on the restoration algorithm assumed and the amount of spare capacity that is available. When only considering link failures, the meshed path protection scheme has the worst availability. Especially in the 32-node network, the ELT is almost doubled compared to the interconnected OCh-DPRings, because long paths are protected end-to-end in contrast to the interconnected OChDPRings, where sub-paths per ring are individually protected. As such, interconnected OCh-DPRings (even without drop&continue) allow to survive from multiple link failures occurring in different rings and are therefore more reliable with regard to link failures. With drop&continue, the interconnected OCh-DPRings can survive from slightly more link failures (e.g. double link failures in one ring, affecting a link between both gateway nodes and a different link), but the difference in ELT is not that large. When considering both node and link failures, the ELT values are considerably larger (although the unavailability of the nodes is not that high). The main reason for this is that when a node failure occurs, all traffic terminating in that node is irrevocably lost, no matter what recovery scheme is used. In this case, the interconnected OCh-DPRing scheme without drop&continue has the worst unavailability. This is because 25 Mesh protection

20 OCh-DPRing ELT (STM-16 h/y)

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10

5

0 40000

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Figure 11: ELT versus cost for 16-node network (link failures only). July 2000 OPTICAL NETWORKS MAGAZINE 35

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OXC can be implemented within the appropriate cost range. By making sensitivity analysis on the obtained results, we can obtain boundaries on the OXC cost range, to render the meshed architectures cheaper than the ring architecture, as shown in [73].

180 160 Mesh protection 120

OCh-DPRing

100

7 - CONCLUSION

80 OCh-DPRing + D&C 60 40 20 0 250000

275000

300000

325000

350000

Total cost

Figure 12: ELT versus cost for 32-node network (link failures only). traffic routed over multiple rings is unable to recover from failures of the OADMs in which ring interconnection occurs. Therefore it certainly makes sense to use drop&continue when node failures come into play. Indeed, the difference in ELT between interconnected OCh-DPRings with and without drop&continue is significantly larger in case both node and link failures are considered. Meshed path protection can also survive from single node failures, because main and protection path are considered to be node disjoint. Consequently, meshed path protection has a higher availability than interconnected OCh-DPRings without drop&continue in case link and node failures are considered. Meshed path protection has only slightly higher ELT values than interconnected OChDPRings with drop&continue for the 16-node network. This is because most connections in the 16-node network are routed over a single ring, and thus the influence of drop&continue is not that large. In the 32-node network, where more connections are routed over multiple rings, the difference in ELT between meshed path protection and interconnected OChDPRings with drop&continue is already more significant. Overall, the interconnected OCh-DPRings with drop&continue result in the best combination of cost and availability. Considering the restoration schemes, link restoration might be able to recover from multiple link failures, but it can not recover from node failures. Therefore path restoration is expected to have a better availability. In combination with its low cost, path restoration is thus an attractive alternative if fast recovery times can be achieved and scalable large-size

ELT (STM-16 h/y)

250

200 OCh-DPRing 150 Mesh protection

OCh-DPRing + D&C 100

50

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Figure 13: ELT versus cost for 16-node network (link and node failures). 36 OPTICAL NETWORKS MAGAZINE July 2000

This paper discussed several architectures for WDM networks, featuring routing and recovery schemes at the optical layer. Ring networks, based on relatively simple OADMs, are easiest to implement in the near term. Dedicated protection rings (OCh-DPRing) based on 1+1 protection have low management overhead, and are available today. Such rings however require more than 100% spare capacity, which is not the case for shared protection rings. Therefore, shared protection rings are an attractive alternative for reducing the installation cost, but still require some management issues to be resolved. Besides ring networks, also meshed networks based on OXCs can be considered. Resilience can be provided through protection or restoration, both at the path or link level.While protection is easiest to implement, restoration requires less capacity because the restoration capacity can be shared amongst different connections. In general, path based recovery schemes require less spare capacity than link based schemes. 1400 1200 ELT (STM-16 h/y)

ELT (STM-16 h/y)

140

OCh-DPRing 1000 800 OCh-DPRing + D&C

Mesh protection

600 400 200 0 250000

275000

300000

325000

350000

Total cost

Figure 14: ELT versus cost for 32-node network (link and node failures). When comparing the different network architectures, several criteria can be used. Ring based networks have the lowest node cost, while meshed restoration schemes have the lowest link cost. On the other hand restoration requires a higher management cost, which is less the case for the more simple mesh and ring based protection schemes. The flexibility is highest for the mesh based schemes, relying on OXCs, and more in particular for restoration schemes, which can adequately react to unexpected failure conditions. Therefore these restoration schemes also have a high availability (although this also depends on the amount of spare capacity and the restoration algorithm). Interconnected rings, featuring drop&continue, also have a high availability because multiple failures in different rings can be independently survived from. Protection switching in rings can happen in a short timeframe as in SDH/SONET rings. This is equally the case for end-to-end protection in a mesh, while mesh restoration is expected to take more time. Equally important as selecting the right network architecture is a proper planning of the network. Due to their concep-

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tual differences, ring and mesh based networks require different design methods. Many of the problems encountered in WDM network design can build on experiences from graph theory and operations research. However, the problem is further complicated because one has to take into account the node architecture and physical transmission issues. Meshed network design consists of well-understood subproblems as topology design, routing and dimensioning and spare capacity allocation. In case of WP networks also wavelength assignment is part of the design problem.The main difficulty is solving the total problem in such a way that the interdependencies between the different sub-problems are taken into account as much as possible, by providing feedback between the different phases or by solving as much sub-problems at once in an integrated manner. Single ring design issues (such as intra-ring routing or wavelength assignment) have been well understood and pose no intractable problems. Interconnected ring design problems (such as ring selection in a mesh network or inter-ring routing between identified rings) are much harder to solve and require appropriate solution methods, often based on heuristics. Finally, a case study based on realistic network examples has been worked out, in which the different architectures are compared both from a cost and availability point of view. Interconnected OCh-DPRings turned out to be about 20% cheaper than path protection schemes, due to the low-cost OADMs. In addition, OCh-DPRings using drop&continue, provide a better availability than path protection. The restoration schemes are the most cost-efficient architectures, because both link and node cost can be significantly reduced through the sharing of the restoration capacity. Path restoration yields the lowest overall design cost, about 30% lower than interconnected OCh-DPRings.

ACKNOWLEDGMENT Bart Van Caenegem thanks the Fund for Scientific Research - Flanders (Belgium) (FWO-V) for a research fellowship. This work was supported by the Flemisch Government through the ITA-II project.

REFERENCES [1] A. McGuire, P. Bonenfant. “Standards: The Blueprints for Optical Networking”, IEEE Communications Magazine, vol. 36, no. 2, pp. 68-78, Feb. 1998. [2] O. Gerstel, “Opportunities for Optical Protection and Restoration”, Proc. of OFC'98 (San Jose, CA), Session ThD, pp. 269-270, Feb. 1998. [3] P. Bonenfant, “Protection and Restoration in Optical Networks”, Proc. of OFC'99 (San Diego, CA), Tutorial Sessions, pp. 199-214, Feb. 1999. [4] P. Lagasse et al., “Photonic Technologies in Europe”, ACTS Horizon-Infowin, Telenor AS R&D, 1998. [5] P. Batchelor et al., “Ultra High Capacity Optical Transmission Networks”, Final report of action COST 239, 1999. [6] M. Garnot et al., “Dimensioning and Optimization of the Wavelength-Division-Multiplexed Optical Layer of Future Transport Networks”, Proc. of ICC'98 (Atlanta, GA), pp. 202-206, June 1998.

[7] J. Späth, H. Weißschuh, “Investigation of Protection Strategies: Problem Complexity and Specific Aspects for WDM Networks”, Proc. of NOC'99 (Delft, Holland), Vol. 2, pp. 68-75, June 1999. [8] B. Van Caenegem, W. Van Parys, P. Demeester, “Restoration in Optical WDM Networks”, Proc. of DRCN’98 (Brugge, Belgium, May 1998), paper O19. [9] N. Parnis, E. Limal, D. R. Hjelme, W. Van Parys, E. V. Jones, “WDM Networking on a European Scale”, Proc. of ECOC'98 (Madrid, Spain), Sept. 1998. [10] D. Johnson, P. Veitch, N. Hayman, “Core Transport Network Redesign”, Proc. of DRCN’98 (Brugge, Belgium), paper O2, May 1998. [11] J.D. Allen, S. Nathan, J. Huang, “Rings in a HighlyConnected Network – an Economic Comparison”, Proc. of DRCN’98 (Brugge, Belgium), paper O42, May 1998. [12] A. Hamel, V. Tholey, A. Sutter, L. Blain, F. Chatter, “Increased Capacity in an MS Protection Ring using WDM Technique and OADM: The 'Colored Section' Ring”, IEE Electronic Letters, vol. 32, no. 3, pp. 234235, Feb. 1996. [13] N. Henmi, S.-Y. Nakamura, S. Cortez, S. Hasegawa, “Multiple WDM Ring-based Transport Network Architecture for Future Broadband-Data Services, Proc. NFOEC'99 (Chicago, IL), Sept. 1999. [14] P. Arijs, M. Gryseels, P. Demeester, “Planning of WDM Ring Networks”, Journal of Photonic Network Communications, Special Issue on WDM Transport Networks, Jan.-March 2000. [15] L. Wuttisittikulkij, M.J. O'Mahony, “Design of a WDM Network using a Multiple Ring Approach”, Proc. of Globecom'97 (Phoenix, AZ), pp. 551-555, Nov. 1997. [16] B. Van Caenegem, P. Demeester, “Design of Interconnected WDM Ring Networks”, Proc. of ONDM'99 (Paris, France), pp. 61-68, Feb. 1999. [17] P. Arijs, P. Demeester, “Dimensioning of NonHierarchical Interconnected WDM Ring Networks”, Proc. of ONDM2000 (Athens, Greece), pp. 147-159, Feb. 2000. [18] T.-H. Wu, “Fiber Network Service Survivability”, Artech House, Boston/London, 1992. [19] M. Sexton, A. Reid, “Broadband Networking: ATM, SDH and SONET”, Artech House, Boston/London, 1997. [20] W. Van Parys, B. Van Caenegem, B. Vandenberghe, P. Demeester, “Meshed Wavelength Division Multiplexed Networks partially equipped with Wavelength Converters”, Proc. of OFC’98 (San Jose, CA), session ThU, pp. 359-360, Feb. 1998. [21] J. M. Yates, M.P. Rumsewicz, and J.P.R. Lacey, “Wavelength Converters in Dynamically-Reconfigurable WDM Networks”, IEEE Communications Surveys, vol. 2, no. 2, second quarter 1999. [22] W. Van Parys, B. Van Caenegem, P. Demeester, E. Iannone and F. Bentivoglio, “Evolution towards WDM Transport Networks”, Proc. of Networks'98 (Sorrento, Italy), pp. 365-370, Oct. 1998. [23] E. Goldstein, J. Nagel, J. Strand and R. Tkach, “National-Scale Networks likely to be Opaque”, Lightwave Xtra!, Feb. 1998.

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Peter Arijs Ghent University - IMEC Department of Information Technology St-Pietersnieuwstraat 41, B-9000, Gent, Belgium [email protected] Peter Arijs received his M.Sc. degree in electrical engineering in 1996 at the Ghent University, Belgium. Afterwards he joined the Department of Information Technology (INTEC) at the Ghent University, where he is finalizing his Ph.D. in the Broadband Communication Networks group headed by Prof. Piet Demeester. He worked on several projects, studying technologies and architectures for SDH and WDM transport networks and optical access networks, and developing planning and evaluation methods for these networks. Wim Van Parys Comsof N.V. Ketelvest 16, B-9000 Gent, Belgium [email protected] Wim Van Parys received his M.Sc. degree in electrical engineering in 1995 at the Ghent University, Belgium. He joined the research group on broadband communication networks headed by Prof. Demeester, where he was involved in the ACTS projects OPEN and HORIZON on the design and evolution of optical networks. In 1999, he joined COMSOF, a spin-off company of the Ghent University, were he is working on software modeling, design and optimization of optical networks. Bart Van Caenegem Ghent University - IMEC Department of Information Technology St-Pietersnieuwstraat 41, B-9000, Gent, Belgium [email protected] Bart Van Caenegem received his M.Sc. degree in electrical engineering in 1995 at the Ghent University, Belgium. From 1995 to 1999 he worked as a research associate with the Fund for Scientific Research of Flanders (FWO-V) at the Department of Information Technology (INTEC) at the Ghent University. He was involved in several European research projects dealing with optical networking and is finalizing his Ph.D. on modeling, design and performance evaluation of WDM network architectures. In September 1999, he joined the network design department of KPN-Orange, the third Belgian mobile operator. Philip Achten Comsof N.V. Ketelvest 16, B-9000 Gent, Belgium [email protected] Philip Achten received his M.Sc. degree in computer science in 1997 at the Ghent University, Belgium. Afterwards he joined the broadband communication networks group of the Department of Information Technology (INTEC) at the Ghent University, where he was working on several R&D projects with industry partners. Mid 1998 he was one of the pioneers who entered Comsof, a startup company that has strong relations with INTEC.Within Comsof he is mainly working on software architecture, design en development of various optical network planning tools.

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Piet Demeester Ghent University - IMEC Department of Information Technology St-Pietersnieuwstraat 41, B-9000, Gent, Belgium [email protected] Piet Demeester received his Ph.D. degree from the Ghent University, Belgium at the Department of Information Technology (INTEC) in 1988. He became professor at the Ghent University where he is responsible for the research on broadband communication networks (supervising about 25 engineers). He was involved in over 15 European ESPRIT, RACE and ACTS projects. His current interests are related to broadband communication networks (IP, ATM, SDH, WDM, access) and include network planning, network and service management, telecom software, internetworking, network protocols, QoS, etc. He is co-author of over 300 papers in the field of opto-electronics and communication networks. Paul Lagasse Ghent University - IMEC Department of Information Technology St-Pietersnieuwstraat 41, B-9000, Gent, Belgium [email protected] Paul Lagasse received his M.Sc. degree in electrical engineering in 1969 and his Ph.D. degree in 1972, both from the Ghent University, Belgium. In 1981 he became professor of electrical engineering at the Ghent University where he is now head of the department of Information Technology (INTEC). In 1985 he became also director of the INTEC division of the Inter-University Micro Electronics Centre (IMEC) in Leuven and since 1993 he is Secretary General of the International Union of Radio Science. After originally working in the area of surface acoustic waves, he is now mainly active in the fields of opto-electronics, high frequency technology and broadband telecommunications. He is member of the board of the Flemish Institute for Science and Technology, corresponding member of the Belgian Royal Academy of Science and since 1997 member of the board of governors of IEEE LEOS.

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