Konstantinos Manousakis

Offline routing and wavelength assignment in transparent WDM networks

We consider the offline version of the routing and wavelength assignment (RWA) problem in transparent all-optical networks. In such networks and in the absence of regenerators, the signal quality of transmission degrades due to physical layer impairments. Because of certain physical effects, routing choices made for one lightpath affect and are affected by the choices made for the other lightpaths. This interference among the 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. We initially present an algorithm for solving the pure (without impairments) RWA problem based on a LP-relaxation formulation that tends to yield integer solutions. Then, we extend this algorithm and present two impairment-aware (IA) RWA algorithms that account for the interference among lightpaths in their formulation. The first algorithm takes the physical layer indirectly into account by limiting the impairment-generating sources. The second algorithm uses noise variance-related parameters to directly account for the most important physical impairments. The objective of the resulting cross-layer optimization problem is not only to serve the connections using a small number of wavelengths (network layer objective), but also to select lightpaths that have acceptable quality of transmission (physical layer objective). Simulations experiments using realistic network, physical layer, and traffic parameters indicate that the proposed algorithms can solve real problems within acceptable time.

Offline Impairment-Aware Routing and Wavelength Assignment Algorithms in Translucent WDM Optical Networks

Physical impairments in optical fiber transmission necessitate the use of regeneration at certain intermediate nodes, at least for certain lengthy lightpaths. We design and implement impairment-aware algorithms for routing and wavelength assignment (IA-RWA) in translucent optical networks. We focus on the offline version of the problem, where we are given a network topology, the number of available wavelengths and a traffic matrix. The proposed algorithm selects the 3R regeneration sites and the number of regenerators that need to be deployed on these sites, solving the regenerator placement problem for the given set of requested connections. The problem can be also posed in a slightly different setting, where a (sparse) placement of regenerators in the network is given as input and the algorithm selects which of the available regenerators to use, solving the regenerator assignment problem. We formulate the problem of regenerator placement and regenerator assignment, as a virtual topology design problem, and address it using various algorithms, ranging from a series of integer linear programming (ILP) formulations to simple greedy heuristic algorithms. Once the sequence of regenerators to be used by the non-transparent connections has been determined, we transform the initial traffic matrix by replacing non-transparent connections with a sequence of transparent connections that terminate and begin at the specified 3R intermediate nodes. Using the transformed matrix we then apply an IA-RWA algorithm designed for transparent (as opposed to translucent) networks to route the traffic. Blocked connections are re-routed using any remaining regenerator(s) in the last phase of the algorithm.

A dynamic impairment-aware networking solution for transparent mesh optical networks

Core networks of the future will have a translucent and eventually transparent optical structure. Ultra-high-speed end-to-end connectivity with high quality of service and high reliability will be realized through the exploitation of optimized protocols and lightpath routing algorithms. These algorithms will complement a flexible control and management plane integrated in the proposed solution. Physical layer impairments and optical performance are monitored and incorporated in impairment-aware lightpath routing algorithms. These algorithms will be integrated into a novel dynamic network planning tool that will consider dynamic traffic characteristics, a reconfigurable optical layer, and varying physical impairment and component characteristics. The network planning tool along with extended control planes will make it possible to realize the vision of optical transparency. This article presents a novel framework that addresses dynamic cross-layer network planning and optimization while considering the development of a future transport network infrastructure.

Joint Online Routing, Wavelength Assignment and Regenerator Allocation in Translucent Optical Networks

In translucent (or managed reach) WDM optical networks, regenerators are employed at specific nodes. Some of the connections in such networks are routed transparently, while others have to go through a sequence of 3R regenerators that serve as ?refueling stations? to restore their quality of transmission (QoT). We extend an online multicost algorithm for transparent networks presented in our previous study, to obtain an IA-RWA algorithm that works in translucent networks and makes use, when required, of the regenerators present at certain locations of the network. To characterize a path, the algorithm uses a multicost formulation with several cost parameters, including the set of available wavelengths, the length of the path, the number of regenerators used, and noise variance parameters that account for the physical layer impairments. Given a new connection request and the current utilization state of the network, the algorithm calculates a set of non dominated candidate paths, meaning that any path in this set is not inferior with respect to all cost parameters than any other path. This set consists of all the cost-effective (in terms of the domination relation) and feasible (in terms of QoT) lightpaths for the given source-destination pair, including all the possible combinations for the utilization of available regenerators of the network. An optimization function or policy is then applied to this set in order to select the optimal lightpath. Different optimization policies correspond to different IA-RWA algorithms. We propose and evaluate several optimization policies, such as the most used wavelength, the best quality of transmission, the least regeneration usage, or a combination of these rules. Our results indicate that in a translucent network the employed IA-RWA algorithm has to consider all problem parameters, namely, the QoT of the lightpaths, the utilization of wavelengths and the availability of regenerators, to efficiently serve the online traffic.

Reach Adapting Algorithms for Mixed Line Rate WDM Transport Networks

We consider the problem of planning a mixed line rate (MLR) wavelength division multiplexing (WDM) transport optical network. In such networks, different modulation formats are usually employed to support transmission at different line rates. Previously proposed planning algorithms have used a transmission reach bound for each modulation format/line rate, mainly driven by single line rate systems. However, transmission experiments in MLR networks have shown that physical layer interference phenomena are more severe among transmissions that utilize different modulation formats. Thus, the transmission reach of a connection with a specific modulation format/line rate depends also on the other connections that copropagate with it in the network. To plan an MLR WDM network, we present routing and wavelength assignment algorithms that adapt the transmission reach of each connection according to the use of the modulation formats/line rates in the network. The proposed algorithms are able to plan the network so as to alleviate cross-rate interference effects, enabling the establishment of connections of acceptable quality over paths that would otherwise be prohibited.

Comparison of Routing and Wavelength Assignment Algorithms in WDM Networks

We design and implement various algorithms for solving the static RWA problem with the objective of minimizing the maximum number of requested wavelengths based on LP relaxation formulations. We present a link formulation, a path formulation and a heuristic that breaks the problem in the two constituent subproblems and solves them individually and sequentially. The flow cost functions that are used in these formulations result in providing integer optimal solutions despite the absence of integrality constraints for a large subset of RWA input instances, while also minimizing the total number of used wavelengths. We present a random perturbation technique that is shown to increase the number of instances for which we find integer solutions, and we also present appropriate iterative fixing and rounding methods to be used when the algorithms do not yield integer solutions. We comment on the number of variables and constraints these formulations require and perform extensive simulations to compare their performance to that of a typical min-max congestion formulation.

Energy efficient RWA strategies for WDM optical networks

We consider the energy minimization problem in optical networks from an algorithmic perspective. Our objective is to plan optical WDM networks so as to minimize the energy expended, by reducing the number of energy-consuming components, such as amplifiers, regenerators, add/drop terminals, optical fibers, etc. We initially present an algorithm for solving the energy-aware routing and wavelength assignment problem based on an integer linear programming formulation that incorporates energy consumption and physical impairments (through a maximum transmission reach parameter) into routing and wavelength assignment. We then present a second algorithm that decomposes the problem and uses a linear programming relaxation to address the problem in large scale networks. Simulations are performed to evaluate and compare the performance of the proposed algorithms. In previously published works, energy minimization derives mainly from the reduction of the electronic processing of the traffic and the bypass in the optical domain, while the energy consumed by the optical devices is usually neglected. We focus on the optical layer and show that energy reductions can be obtained in that layer also.

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).

Impairment-Aware Offline RWA for Transparent Optical Networks

We consider the offline version of the routing and wavelength assignment (RWA) problem in transparent all-optical networks. In such networks and in the absence of regenerators, the signal quality of transmission degrades due to physical layer impairments. We initially present an algorithm for solving the static RWA problem based on an LP relaxation formulation that tends to yield integer solutions. To account for signal degradation due to physical impairments, we model the effects of the path length, the path hop count, and the interference among ligthpaths by imposing additional (soft) constraints on RWA. The objective of the resulting optimization problem is not only to serve the connection requests using the available wavelengths, but also to minimize the total accumulated signal degradation on the selected lightpaths. Our simulation studies indicate that the proposed RWA algorithms select the lightpaths for the requested connections so as to avoid impairment generating sources, thus dramatically reducing the overall physical-layer blocking when compared to RWA algorithms that do not account for impairments.

A Multicost Approach to Online Impairment-Aware RWA

We design and implement a multicost impairment-aware routing and wavelength assignment algorithm for online traffic. In transparent optical networks the quality of a transmission degrades due to physical layer impairments. To serve a connection, the proposed algorithm finds a path and a free wavelength (a lightpath) that has acceptable signal quality performance by estimating a quality of transmission measure, called the Q factor. We take into account channel utilization in the network, which changes as new connections are established or released, in order to calculate the noise variances that correspond to physical impairments on the links. These, along with the time invariant eye impairment penalties of all candidate network paths, form the inputs to the algorithm. The multicost algorithm finds a set of so called non-dominated Q paths from the given source to the given destination. Various objective functions are then evaluated in order to choose the optimal lightpath to serve the connection. The proposed algorithm combines the strength of multicost optimization with low execution time, making it appropriate for serving online connections.