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Network element failure may occur due to natural disasters, malfunctioning components, or human errors. Ideally, network failure occurrence should not affect the network performance as it is experienced by the users. For this reason, robust protection mechanisms resort to standby network resources, which are readily available upon failure occurrence, e.g., dedicated path protection. In some network architectures and service models, a less ideal protection approach may still be applicable in exchange for a reduced network and service cost. Graceful degradation of performance upon failure occurrence may be acceptable in this case. The study in this paper explores preemption-based restoration mechanisms applied to optical connections (lightpaths) in Wavelength Division Multiplexing (WDM) networks. The rationale for preempting some of the still functioning lightpaths — while performing rerouting of the failing ones — is to avoid highly unfair scenarios in which some node pairs starve for lightpaths significantly more than other pairs do, as a consequence of a failing network element1.

In a recent study about Wavelength Division Multiplexing (WDM) networks, the authors showed that by decoupling the wavelength assigned to the outward circuit from the wavelength assigned to the inward circuit of the same bidirectional unprotected lightpath, it is possible to reduce the total required number of Wavelength Converters (WCs) when compared to the conventional Wavelength Assignment (WA) solution, in which both outward and inward circuits are assigned the same wavelength [1]. The goal of this paper is to generalize the study in [1] to account for dedicated path protection switching mechanisms applied to bidirectional lightpaths. In doing so, a second factor must be accounted for in the study, i.e., the wavelength assigned to the working circuit may be decoupled from the wavelength assigned to the protection circuit. As a result, four design options are possible by either enforcing or relaxing each of the following two constraints: 1) lightpaths are (are not) required to be assigned the same wavelength for the inward and outward circuit; 2) lightpaths are (are not) required to be assigned the same wavelength for the working and protection circuit. The four design options are compared in terms of their required average number of WCs in the network, revealing that the WC usage may differ significantly depending on the design used.

Multicast transmission offers a bandwidth efficient solution for delivering media content to multiple destinations over the Internet. However, in many existing networks, some (if not all) nodes do not support multicast, i.e., they cannot create multiple outgoing flows with one incoming data flow. In this paper, the authors propose an algorithm for multicast tree computation in networks with multicast incapable nodes. Paths that originate at the source and traversing all destinations are computed first; if such paths cannot be found, destinations are partitioned into subsets and traverse paths are computed over each subset, which is executed recursively until feasible trees can be built based on traverse paths found or no further partition is possible. Two procedures for traverse path computation are presented and their respective advantages are discussed, in terms of both complexity and solution optimality. The algorithm is also shown to be very effective in finding multicast trees even if only a few multicast capable nodes exist in the network.

Summary form only given. Media content delivery and more in general Cloud related data traffic can highly benefit from the availability of multicast connections in the optical transport network. One specific case is represented by optical multicast trees (light-trees), which are established in the wavelength division multiplexing (WDM) layer. With a light-tree, the optical signal from the root of the multicast session is propagated and split optically till the multicast leaves are all reached. To avoid blocking due to already reserved (unavailable) wavelengths, the creation of a light-tree may require a change of the signal wavelength which is assigned to some of the tree links, in which case wavelength converters (WCs) are required for the light-tree to be created. WCs are pricy and consume energy, thus a wisely designed network must minimize the required number of WCs.

A significant drawback when establishing multicast optical trees (light-tree) using lambda channels in wavelength division multiplexing (WDM) network is the potentially large amount of wavelength converters (WCs), which are required when the wavelength continuity constraint cannot be satisfied across each light-tree. Prior works address this challenge by solving the routing and wavelength assignment (RWA) problem for each individual light-tree sequentially, to locally minimize the number of WCs required by each light-tree. In this paper, the authors intend to solve the RWA problem for a group of light-trees with the objective of globally minimizing the number of WCs that are required to support the entire group of multicast requests. The rationale for using the proposed RWA solution is that a joint solution of the RWA problem across the whole set of requested multicast trees may significantly reduce the number of WCs overall.

The growing demand for online media content delivery and multi-player gaming is expected to increase the amount of multicast service requests in both public and private networks. Careful traffic engineering of multicast service requests is becoming increasingly essential, as establishing the lowest cost tree, e.g., shortest path tree, in the network for every individual multicast request does not always ensure a global optimization.

The introduction of end-to-end (multi hop) optical circuits (lightpaths) is often seen as the natural extension of a point-to-point (single hop) wavelength division multiplexing (WDM) transmission system, spanning across optical cross-connect nodes. Conventionally, a bidirectional lightpath (whether single or multi hop) comprises two lambda channels in fibers with opposite directions of signal propagation, which are assigned the same wavelength. This constraint is referred to as identical wavelength in bidirectional lightpath (IWBL). While IWBL is a natural and historical choice in point-to-point system, the authors are going to demonstrate that IWBL may unnecessarily increase the number of wavelength converters (WCs), which are required to establish lightpaths in the WDM network when the wavelength continuity constraint cannot be met. In the study, the IWBL constraint is relaxed, thus allowing the assignment of two distinct wavelengths to the same lightpath, one for each direction. A wavelength assignment (WA) algorithm is designed to both take advantage of the IWBL constraint relaxation and minimize the required number of WCs to establish a given set of lightpaths. The algorithm is then applied to a number of network topologies. The outcome is quite surprising, in that the amount of WC reduction obtained by relaxation of the IWBL constraint may be significant under certain conditions.

A careful wavelength assignment (WA) to lambda services must be performed to reduce the total number of wavelength converters (WCs) that are required when the wavelength continuity constraint cannot be met in wavelength division multiplexing (WDM) networks. With the successful introduction of reconfigurable optical add-drop multiplexers (ROADMs), WDM networks are now growing in size, both in the number of optical nodes and the number of wavelengths supported. Fast and memory efficient WA algorithms are required to design cost effective large WDM networks. This paper presents a scalable and efficient WA heuristic algorithm aimed at reducing the total number of WCs that are required in (large) WDM networks bearing static lambda services. The WA algorithm is applied to both unprotected and (dedicated) protected lambda services. In the latter case, the wavelength continuity constraint between the working and protection path of a lambda service is taken into consideration when non-tunable optical transceivers are employed.

Hierarchical (multi-core) Wavelength Division Multiplexing (WDM) networks present a challenging design problem to the network designer who wishes to establish all-optical circuits end-to-end and across multiple network cores. Due to the nature of the hierarchical structure and its traffi distribution, it is likely that the inner core requires more capacity when compared to the capacity required by the metro cores, which are individually connected to the inner core. This capacity mismatch cannot be addressed by assigning distinct transmission rates to each core, as this solution would result in using electronic time division add-drop multiplexer to interconnect the traffi across cores with distinct rates. An alternative solution to addressing the capacity mismatch between WDM metro and inner core is explored in this paper, which is based on a limited number of wavelengths (a subset of the full set) being used in the metro core, when compared to the full set of wavelengths being used in the inner core. Two available architectures are presented in the paper, discussing their respective advantages and disadvantages.

With the successful introduction of reconfigurable optical add-drop multiplexers (ROADMs) and related technologies, WDM networks are now growing in the number of optical nodes, wavelengths, and lambda services supported. In addition, shared path protection mechanisms — whereby lambda services are allowed to share protection wavelength channels — are possible at the optical (WDM) layer. Efficient strategies must be devised to both determine the set of services that must share a common protection wavelength channel and assign wavelengths to every service. One objective of these strategies is to minimize the total number of wavelength converters (WCs), which are required every time the wavelength continuity constraint cannot be met. This paper presents a scalable and efficient heuristic, whose goal is to minimize the number of WCs in resilient WDM networks supporting static sets of shared path protection lambda services. The heuristic comprises a set of polynomial algorithms that are executed sequentially to obtain a sub-optimal solution. In small size instances of the problem, the heuristic is compared against the optimal solution obtained from ILP formulation. For large size instances — tens of thousands of lambda services and hundreds of nodes — the heuristic yields an average number of WCs that is close to be linear in the number of services, despite the fact that the wavelength sharing factor increases.

This paper presents an algorithm to find a simple path in the given network with multiple must-include nodes in the path. The problem of finding a path with must-include node(s) can be easily found in some special cases. However, in general, including multiple nodes in the simple path has been shown to be NP-Complete. This problem may arise in network areas such as forcing the route to go through particular nodes, which have wavelength converter (optical), have monitoring provision (telecom), have gateway functions (in OSPF) or are base stations (in MANET). In this paper, a heuristic algorithm is described that follows divide and conquer approach, by dividing the problem in two subproblems. It is shown that the algorithm does not grow exponentially in this application and initial re-ordering of the given sequence of must-include nodes can improve the result. The experimental results demonstrate that the algorithm successfully computes near optimal path in reasonable time.

PlaNet is a multilayer network planning tool designed and developed at the University of Texas at Dallas. This paper illustrates some of the features of PlaNet-OTN, one of the modules available in the PlaNet tool. PlaNet-OTN can be used to design and plan an optical transport network (OTN), which is comprised of two layers: wavelength division multiplexing (WDM) layer, which deals with wavelength allocation and routing of WDM services, and optical transport network (OTN) layer, which deals with optical data unit (ODU) equipment provisioning and routing of ODU services. Features of the PlaNet-OTN module include: multiple protection schemes and routing constraints for both WDM and ODU services, network equipment cost minimization, load balancing of traffic, and user-controlled run time of the optimization process. As shown in this paper, the PlaNet-WDM and OTN layers and optimizing equipment usage, hence reducing the cost of the whole network.

PlaNet is a multilayer network planning tool developed at the University of Texas at Dallas. This paper illustrates some of the features of PlaNet-PTN, one of the modules available in the PlaNet tool. PlaNet-PTN can be used to design and plan a single layer packet transport network (PTN). Quality of protection, routing constraints, minimization of the network equipment cost, and user’s desired run time of the tool are just some examples of the features available in PlaNet. As shown in the paper, the PlaNet-PTN planning module is able to provide, among others, optimization of Label Switched Path (LSP) routes, link capacity placement, node and link equipment configuration.

Hierarchical (multi-core) Wavelength Division Multiplexing (WDM) networks present a challenging design problem to the network designer who wishes to establish all-optical circuits end-to-end and across multiple network cores. Due to the nature of the hierarchical structure and its traffi distribution, it is likely that the inner core requires more capacity when compared to the capacity required by the metro cores, which are individually connected to the inner core. This capacity mismatch cannot be addressed by assigning distinct transmission rates to each core, as this solution would result in using electronic time division add-drop multiplexer to interconnect the traffi across cores with distinct rates. An alternative solution to addressing the capacity mismatch between WDM metro and inner core is explored in this paper, which is based on a limited number of wavelengths (a subset of the full set) being used in the metro core, when compared to the full set of wavelengths being used in the inner core. Two available architectures are presented in the paper, discussing their respective advantages and disadvantages.