Distributed optical control plane for dynamic lightpath establishment in translucent optical networks based on reachability graph

In transparent optical networks, physical layer impairments (PLIs) incurred by non-ideal optical transmission media accumulate along an optical path, and the overall effect determines the optical feasibility of the lightpaths. In addition, transparent optical networks suffer from inefficient wavelength utilization, as a connection request may be rejected because of non-availability of a common wavelength on all the links along the chosen route. To increase optical reach, resource utilization, and average call acceptance ratio (and hence revenues), network operators are resort to translucent optical networks. In these networks a limited number of regenerators are placed at a selected set of nodes. In this scenario development of an optical control plane which is aware of PLIs, location and number of regenerators, is of paramount importance for on-demand lightpath provisioning. In this paper, we propose a novel approach of constructing a reachability graph of the physical network considering PLIs and regenerators. If there is no transparent path in the physical network, we route the connections with multiple transparent segments on the reachability graph. We propose efficient mechanisms and corresponding GMPLS protocol extensions for impairment and regenerator aware routing and wavelength assignment (IRA-RWA) in translucent optical networks. The simulation results suggest that our proposed approach together with LSP stitching signaling mechanism is feasible to implement and close to deployment.     

Dynamic routing in translucent WDM optical networks: the intradomain case

Translucent wavelength-division multiplexing optical networks use sparse placement of regenerators to overcome physical impairments and wavelength contention introduced by fully transparent networks, and achieve a performance close to fully opaque networks at a much less cost. In previous studies, we addressed the placement of regenerators based on static schemes, allowing for only a limited number of regenerators at fixed locations. This paper furthers those studies by proposing a dynamic resource allocation and dynamic routing scheme to operate translucent networks. This scheme is realized through dynamically sharing regeneration resources, including transmitters, receivers, and electronic interfaces, between regeneration and access functions under a multidomain hierarchical translucent network model. An intradomain routing algorithm, which takes into consideration optical-layer constraints as well as dynamic allocation of regeneration resources, is developed to address the problem of translucent dynamic routing in a single routing domain. Network performance in terms of blocking probability, resource utilization, and running times under different resource allocation and routing schemes is measured through simulation experiments.     

QoT-aware lightpath set-up in GMPLS-controlled WDM networks: A survey

GMPLS-controlled Wavelength Switched Optical Networks (WSON) often undergo upgrade in bit rate or in network element switching capability. In such a scenario many wavelength paths cannot be anymore considered pre-validated from the original network design phase. Thus, it is required to verify the quality of the optical path physical signal during path computation and signaling by considering the optical network impairments (e.g., attenuation, amplified spontaneous emission, etc.).In this paper, the general problem of impairment-aware path computation is presented and then solutions based on GMPLS extensions to be used during path set-up are considered. Specifically, an overview is presented where different approaches for implementing Quality of Transmission (QoT) estimation and QoT measurements in transparent networks are outlined. In addition, techniques for exploiting sparse regeneration in translucent networks are illustrated. Numerical results show that the proposed schemes are effective in finding or designating QoT-validated paths in only few set-up attempts when a fully transparent path is available. If the transparent path becomes unfeasible, the translucent approach is adopted showing how to optimally designate intermediate regenerators to satisfy the end-to-end QoT constraints.     

Multiple-Constraint-Aware RWA Algorithms Based on a Comprehensive Evaluation Model： Use in Wavelength-Switched Optical Networks

Because of explosive growth in Internet traffic and high complexity of heterogeneous networks,improving the routing and wavelength assignment (RWA) algorithm in underlying optical networks has become very important.Where there are multiple links between different the node pairs,a traditional wavelength-assignment algorithm may be invalid for a wavelength-switched optical networks (WSON) that has directional blocking constraints.Also,impairments in network nodes and subsequent degradation of optical signals may cause modulation failure in the optical network.In this paper,we propose an RWA algorithm based on a novel evaluation model for a WSON that has multiple constraints.The algorithm includes comprehensive evaluation model (CEM) and directional blocking constraint RWA based on CEM (DB-RWA).Diverse constraints are abstracted into various constraint conditions in order to better assign routing and wavelength.We propose using the novel CEM to optimize routing according to an assessed value of constraints on transmission performance.This eliminates the effects of physical transmission impairments in a WSON.DB-RWA based on CEM abstracts directional blocking conditions in multiple links between network nodes into directional blocking constraints.It also satisfies rigorous network specifications and provides flexibility,scalability,and first-fit rate for the backbone,especially in multiple links between WSON nodes.     

Generalized Sharing in Survivable Optical Networks

Shared path protection has been demonstrated to be a very efficient survivability scheme for optical networking. In this scheme, multiple backup paths can share a given optical channel if their corresponding primary routes are not expected to fail simultaneously. The focus in this area has been the optimization of the total channels (i.e., bandwidth) provisioned in the network through the intelligent routing of primary and backup routes. In this work, we extend the current path protection sharing scheme and introduce the Generalized Sharing Concept. In this concept, we allow for additional sharing of important node devices. These node devices (e.g., optical-electronic-optical regenerators (OEOs), pure all-optical converters, etc.) constitute the dominant cost factor in an optical backbone network and the reduction of their number is of paramount importance. For demonstration purposes, we extend the concept of 1:N shared path protection to allow for the sharing of electronic regenerators needed for coping with optical transmission impairments. Both design and control plane issues are discussed through numerical examples. Considerable cost reductions in electronic budget are demonstrated     

Provisioning Methods for Bit-Rate-Differentiated Services in Hybrid Optical Networks

This paper addresses the problem of routing and wavelength assignment of bit-rate-differentiated optical services in a hybrid network. Hybrid optical networks are composed of resources, such as fiber links and photonic/electronic switches, that vary in their capabilities and transmission qualities. These networks are also responsible for the realization of optical services with varying quality-of-service (QoS) guarantees. In such networks, it is required to have a cost-effective assignment of the optical and electronic resources to these services in order to maximize the revenue of the network operator. This paper deals with optical services that are defined according to their tolerance to transmission impairments. We first divide the provisioning problem into two phases: (1) routing and (2) wavelength assignment and regeneration reservation. In the routing phase, a set of k-routes are generated to select from in the second phase, where each route optimizes a specific aspect of the problem (e.g., number of hops, maximum accumulated noise, etc.). The second phase, using the information about the resources along each route, attempts at finding the best wavelength allocation on that route such that the signal quality meets the service-level agreement (SLA). The second phase also uses the minimum number of regenerator ports on intermediate nodes for the purpose of wavelength translation and signal clean-up. Comparisons of the above scheme with a probing-based method, reveal substantial enhancements to the blocking performance with a maximum running time increase of 60%. In addition, the use of multiple routes provides higher reduction in the blocking probability over single-routing schemes. Moreover, the proposed, non-pessimistic, provisioning approach has a major impact on reducing the regeneration budget of the network.     

Considering physical layer impairments in offline RWA

We consider the offline version of the routing and wavelength assignment problem in transparent all-optical networks. In such networks and in the absence of regenerators, the signal quality of a transmission degrades due to physical layer impairments. Certain physical effects cause choices for one lightpath to affect and be affected by the choices made for other lightpaths. This interference among lightpaths is particularly difficult to formulate in an offline algorithm, since in this version of the problem we start without any established connections, and the utilization of lightpaths are the variables of the problem. For this reason the majority of work performed in this field either neglects lightpath interactions or assumes a worst case interference scenario. In this article we present a way to formulate interlightpath interference as additional constraints on RWA and show how to incorporate these constraints in an IA-RWA algorithm that directly accounts for the most important physical layer impairments. The objective of the resulting cross-layer optimization problem is not only to serve the connection requests using the minimum number of wavelengths (network layer objective), but also to select lightpaths that have acceptable quality of transmission performance (physical layer objective).     

Optimal regenerator assignment and resource allocation strategies for translucent optical networks

Physical layer impairments in wavelength-routed networks limit the maximum distance, a signal can travel in the optical domain, without significant distortion. Therefore, signal regeneration is required at some intermediate nodes for long-haul lightpaths. In translucent WDM networks, sparsely located regenerators at certain nodes can be used to offset the impact of physical layer impairments. The routing and wavelength assignment (RWA) techniques in such translucent networks need to take into consideration the availability of regenerators and the maximum optical reach of the transparent lightpaths (without any regeneration). Although there has been significant research interest in RWA algorithms for translucent networks, much of the research has focused on dynamic RWA techniques. Only a handful of recent papers have considered the static (offline) case, and they typically propose heuristic algorithms to solve this complex design problem for practical networks. In this paper, we propose a generalized integer linear program (ILP) based formulation for static regenerator assignment and RWA in translucent WDM optical networks, with sparse regenerator placement. To the best of our knowledge, such a formulation that optimally allocates resources for a set of lightpaths for translucent networks, given the physical network, the locations of the regenerators, and the maximum optical reach has not been considered before. The proposed formulation is important for two reasons. First, it can serve as a benchmark for evaluating different heuristic approaches that may be developedin the future. Second, we show that using a novel node representation technique, it is possible to drastically reduce the number of integer variables. This means that unlike existing ILP formulations, our approach can actually be used to generate optimal solutions for practical networks, with hundreds of lightpath demands.     

Shareability in optical networks: beyond bandwidth optimization

We extend the famous path protection sharing scheme and introduce the generalized sharing concept. In this concept, we allow for additional sharing of important node devices in order to reduce network cost. These node devices (e.g., optical-electronic-optical regenerators and pure all-optical converters) constitute the dominant cost factor in an optical backbone network, and reduction of their number is of paramount importance. For demonstration purposes, we extend the concept of 1:N shared path protection to allow for the sharing of electronic regenerators needed to cope with optical transmission impairments. Considerable cost reductions in electronic budget are demonstrated through numerical examples.     

Benefits of the use of impairment constraint routing in optical networks

In transparent optical networks, the optical signal accumulates the effects of all physical impairments present along the path it traverses. The conventional selection of signal paths based on e.g. shortest path routing without considering the signal quality and its association with the physical impairments does not always provide the optimum solution in terms of network performance such as blocking and resource utilization. This paper proposes an impairment constraint based routing algorithm to achieve an optimal combination of physical and networking performance taking into account all physical linear impairments including noise, chromatic and polarization mode dispersion, crosstalk and filter concatenation effects in an integrated approach. The performance of a typical metropolitan area network is examined and the improvement achieved when using the proposed approach compared to the conventional shortest path routing is demonstrated.     

Link-State-Based Algorithms for Dynamic Routing in All-Optical Networks with Ring Topologies

The increased usage of large bandwidth in optical networks raises the problems of efficient routing to allow these networks to deliver fast data transmission with low blocking probabilities. Due to limited optical buffering in optical switches and constraints of high switching speeds, data transmitted over optical networks must be routed without waiting queues along a path from source to destination. Moreover, in optical networks deprived of wavelength converters, it is necessary for each established path to transfer data from source to destination by using only one wavelength. To solve this NP-hard problem, many algorithms have been proposed for dynamic optical routing like Fixed-Paths Least Congested (FPLC) routing or Least Loaded Path Routing (LLR). This paper proposes two heuristic algorithms based on former algorithms to improve network throughput and reduce blocking probabilities of data transmitted in all-optical networks with regard to connection costs. We also introduce new criteria to estimate network congestion and choose better routing paths. Experimental results in ring networks show that both new algorithms achieve promising performance.     

A distributed ant-based algorithm for routing and wavelength assignment in an optical burst switching flexible spectrum network with transmission impairments

This work describes a distributed algorithm inspired by ant colony optimisation for a solution to the problem of dynamic routing and wavelength assignment with wavelength continuity constraint in optical burst switched networks. The evaluation was conducted in a wavelength division multiplexed network environment with limited number of wavelength channels and in a flexible spectrum network environment undergoing transmission impairments. The simulations in the flexible spectrum network environment aim to replicate the effects of both linear and nonlinear physical layer impairments. Under these effects, an optical burst control packet could be lost during traversal of the network with no measure in place to notify the network of the loss or to free up optical resource reservations. The optical burst switching acknowledgement protocol has been modified in this work to account for a burst control packet failure by implementing a traversal acknowledgement to cater for its loss. The performance of the distributed ant-based algorithm has been extensively evaluated on several network topologies and compared with that obtained by shortest path routing and ant colony routing and wavelength assignment. The results show that the distributed ant-based algorithm significantly improves the burst transmission success probability in the wavelength division multiplexed network environment and provides a good solution in the flexible spectrum network environment undergoing transmission impairments.     

Challenges and requirements for introducing impairment-awareness into the management and control planes of ASON/GMPLS WDM networks

The absence of electrical regenerators in transparent WDM networks significantly contributes to reduce the overall network cost. In transparent WDM networks, a proper resource allocation requires that the presence of physical impairments in routing and wavelength assignment (RWA) and lightpath provisioning be taken into account. In this article a centralized, a hybrid centralized-distributed and two distributed approaches that integrate information about most relevant physical impairments in RWA and lightpath provisioning are presented and assessed. Both centralized and hybrid approaches perform a centralized path computation at the management-plane level, utilizing physical impairment information, while the lightpath provisioning is done by the management plane or the control plane, respectively. The distributed approaches fall entirely within the scope of the ASON/GMPLS control plane. For these two approaches, we provide functional requirements, architectural functional blocks, and protocol extensions for implementing either an impairment-aware real-time RWA, or a lightpath provisioning based on impairment-aware signaling     

Routing, wavelength and time-slot-assignment algorithms for wavelength-routed optical WDM/TDM networks

This paper studies the connection-assignment problem for a time-division-multiplexed (TDM) wavelength-routed (WR) optical wavelength-division-multiplexing (WDM) network. In a conventional WR network, an entire wavelength is assigned to a given connection (or session). This can lead to lower channel utilization when individual sessions do not need the entire channel bandwidth. This paper considers a TDM-based approach to reduce this inefficiency, where multiple connections are multiplexed onto each wavelength channel. The resultant network is a TDM-based WR network (TWRN), where the wavelength bandwidth is partitioned into fixed-length time slots organized as a fixed-length frame. Provisioning a connection in such a network involves determining a time-slot assignment, in addition to the route and wavelength. This problem is defined as the routing, wavelength, and time-slot-assignment (RWTA) problem. In this paper, we present a family of RWTA algorithms and study the resulting blocking performance. For routing, we use the existing shortest path routing algorithm with a new link cost function called least resistance weight (LRW) function, which incorporates wavelength-utilization information. For wavelength assignment, we employ the existing least loaded (LL) wavelength selection; and for time-slot allocation, we present the LL time-slot (LLT) algorithm with different variations. Simulation-based analyses are used to compare the proposed TDM architecture to traditional WR networks, both with and without wavelength conversion. The objective is to compare the benefits of TDM and wavelength conversion, relative to WR networks, towards improving performance. The results show that the use of TDM provides substantial gains, especially for multifiber networks.     

Benders decomposition algorithms for the fixed-charge relay network design in telecommunications

In translucent networks, signal regenerators that operate based on optical/electrical/optical (O/E/O) conversion are employed to handle the problems of physical impairments and resulting signal quality issues in long-distance transmissions. Regenerators are placed at the relay nodes in a translucent network in such a way that the optical signals are not transmitted farther than a threshold distance without a 3R regeneration (reamplification-reshaping-retiming). In this study, we consider a fixed-charge relay network design (FCRND) problem whose applications are found mainly in long distance translucent optical telecommunication networks. From a modeling perspective, FCRND combines prominent features of the uncapacitated single-assignment hub location (location of relays points) and the fixed-charge network design problems (choice of transmission links in the network and routing of signals) while additionally considering distance-based coverage constraints. We devise Benders decomposition based solution methodologies in which the algorithmic performance is further enhanced by devising strengthened and disaggregated Benders cuts, surrogate constraints for the master problem, and an upper bound heuristic to both obtain and tighten optimality Benders cuts. We obtain solutions within 2.0 % of optimality in very reasonable times as we illustrate in our computational study.     

Translucent optical networks: the way forward [Topics in Optical Communications]

The transmission reach of signals in optical transmission systems is limited. To go beyond these transparent reach limits, signal regeneration is necessary to re-amplify, reshape, and retime the optical signals. Translucent optical networks are a type of optical transport network specifically devised to address such a concern by allowing for sparse but strategic signal regeneration in the network. Translucent optical networks seek a graceful balance between network design cost and service provisioning performance, and can achieve performance comparable to that of an all-electronic switching network, but requiring far fewer signal regenerators. Despite massive progress, there are many outstanding issues regarding the implementation of translucent networks planning and operation. This article reviews a range of translucent optical networks and discusses various research issues, particularly involving network planning, lightpath routing and wavelength assignment, and network survivability. We also suggest other potential research topics such as traffic grooming, fault detection, and multicasting for translucent networks     

Cross-layer routing in clustered optical networks

Physical layer impairment aware routing algorithms have been proposed for optical transparent networks in order to calculate the feasibility of dynamically establishing an optical path when no regeneration is used. The benefit of node clustering in optical networks, regarding routing with physical layer impairment awareness, is investigated under the CANON network architecture where regenerators are conveniently placed and routing is confined among a small subset of nodes. The CANON architecture exhibits enhanced blocking performance, high resource utilisation and adequate physical performance; hence, it can serve Quality of Service.     

Impairment aware optimal diverse routing for survivable optical networks

In wavelength-routed optical networks, a lightpath connection can be provisioned only if a path, or a pair of paths in case of path protection is required, can be found which satisfies multiple constraints while simultaneously achieving optimal primary cost. The primary cost can be any metric set by network administrators, and the constraints concerned in optical networks include wavelength continuity constraint, accumulation effect of some transmission impairments in the optical domain, and SRLG-disjoint requirement in survivable networks. In this paper, the impact of these constraints on the optimal path calculation algorithms is studied. Three novel algorithms for solving this problem, which we call “Impairment Aware Optimal Path Pair” problem, are proposed, and their performance is evaluated through extensive simulations. This research has been supported by a National Natural Science Foundation of China (NSFC) grant (90604002, 60472008), and Program for New Century Excellent Talents in University grant (NCET-05-0807).     

Code-Empowered Lightwave Networks

In this paper, an architecture for code-empowered optical CDMA (OCDMA) lightwave networks is presented. The architecture is based on reconfigurable optically transparent paths among users of the network to provide high-bandwidth optical connections on demand over small areas such as local area networks or access networks. The network operates on the transmission of incoherent OCDMA codes, each network station being equipped with an OCDMA encoder and decoder. The routing at a network node is based on the OCDMA code itself. The destination address, as well as the next node on the path, is given by the code as in a code-empowered network. A node consists of an OCDMA router built from parallel code converter routers that perform switching, routing, and code conversion. The latter enables a virtual code path for increased scalability. Commonly available delay lines enable the tunability of the encoder, decoder, and router for a reconfigurable and flexible network. Flexibility and granularity are also accentuated by OCDMA encoding. An OCDMA lightwave network can therefore respond to changes in traffic load, traffic conditions, failure, and other network impairments. We describe the possible architectures and the routing constraints of such OCDMA lightwave networks. We present a power analysis and focus on the performance issues of dynamic routing. The effect of coding, topology, load condition, and traffic demand is analyzed using simulations. The obtained results show that the flexibility of OCDMA and the large offered cardinality can be a solution to the needs of local area and access networks.     

Multipath routing for video delivery over bandwidth-limited networks

The delivery of quality video service often requires high bandwidth with low delay or cost in network transmission. Current routing protocols such as those used in the Internet are mainly based on the single-path approach (e.g., the shortest-path routing). This approach cannot meet the end-to-end bandwidth requirement when the video is streamed over bandwidth-limited networks. In order to overcome this limitation, we propose multipath routing, where the video takes multiple paths to reach its destination(s), thereby increasing the aggregate throughput. We consider both unicast (point-to-point) and multicast scenarios. For unicast, we present an efficient multipath heuristic (of complexity O(|V|/sup 3/)), which achieves high bandwidth with low delay. Given a set of path lengths, we then present and prove a simple data scheduling algorithm as implemented at the server, which achieves the theoretical minimum end-to-end delay. For a network with unit-capacity links, the algorithm, when combined with disjoint-path routing, offers an exact and efficient solution to meet a bandwidth requirement with minimum delay. For multicast, we study the construction of multiple trees for layered video to satisfy the user bandwidth requirements. We propose two efficient heuristics on how such trees can be constructed so as to minimize the cost of their aggregation subject to a delay constraint.