Gokhan Sahin

Fast restoration in a mesh network of optical cross-connects

This paper proposes a distributed restoration method for mesh networks of optical cross-connects (OXCs) that is competitive with SONET ring restoration speeds. This approach is especially adapted to OXCs with optic fabrics where electronic signal performance or fault detection is available only at add/drop ports.

On monitoring transparent optical networks

Fault identification and localization problems in optical networks have become crucial. Due to network transparency and high data rates, optical networks are vulnerable to sophisticated attacks on the physical infrastructure, and hence require adequate fault monitoring in order to accurately identify and locate network failures. In transparent optical networks, faults may propagate to various parts of the network from the origin, and multiple alarms can be generated for a single failure. In order to reduce the number of redundant alarms, simplify fault localization, as well as lower financial investment in network monitoring equipment, fault monitor placement should be optimized for a given network. In this paper, we formulate a problem on the optimal placement of network monitoring devices and propose a solution approach. We provide a brief summary of available physical-layer monitoring devices, and present a scheme for optimal monitor placement.

Routing and Wavelength Assignment in All‐Optical Networks with Multicast Traffic

This paper considers multicast routing and wavelength assignment (MCRWA) in a wide area all-optical network. We present static and dynamic MCRWA algorithms, and evaluate their blocking performance under various fanout splitting policies. Through simulations and comparisons with a lower bound, we demonstrate that dynamic MCRWA can perform significantly better than its static counterparts. We then quantify the throughput gain due to link sharing in multicasting, and the performance penalty that would be suffered by considering each multicast request as multiple unicast connection requests. We also present an analytical model for estimating the blocking performance of the static MCRWA algorithms. The performance improvement with multifiber networks and with wavelength conversion are also investigated.

Active monitoring and alarm management for fault localization in transparent all-optical networks

Achieving accurate and efficient fault localization in large transparent all-optical networks (TONs) is an important and challenging problem due to unique fault-propagation, time constraints, and scalability requirements. In this paper, we introduce a novel technique for optimizing the speed of fault-localization through the selection of an active set of monitors for centralized and hierarchically-distributed management. The proposed technique is capable of providing multiple levels of fault-localization-granularity, from individual discrete optical components to the entire monitoring domains. We formulate and prove the NP-completeness of the optimal monitor activation problem and present its Integer Linear Program (ILP) formulation. Furthermore, we propose a novel heuristic whose solution quality is verified by comparing it with an ILP. Extensive simulation results provide supporting analysis and comparisons of achievable alarm-vector reduction, localization coverage, and time complexity, for flat and hierarchically distributed monitoring approaches. The impact of network connectivity on fault localization complexity in randomly generated topologies is also studied. Results demonstrate the effectiveness of the proposed technique in efficient and scalable monitoring of transparent optical networks.

Dynamic LSP provisioning in overlay, augmented, and peer architectures for IP-MPLS over WDM networks

We consider an IP/MPLS over WDM network, in which label switched routers (LSRs) in the IP/MPLS layer are interconnected through optical cross-connects (OXCs) in the optical core network (WDM layer) providing an end-to-end wavelength routing capability. In this paper, we study a dynamic label switched path (LSP) provisioning problem for the three different network models of the IP/MPLS over WDM network, namely, overlay, augmented, and peer models. For the overlay model, we propose an algorithm, called MLH/spl I.bar/OVLY, in which a network finds a path with the minimum number of logical hops for an LSP request. We also propose, for the augmented model, a simple and efficient dynamic LSP provisioning algorithm, called CAPA/spl I.bar/AUG, utilizing summarized capacity information from the WDM layer. We compare the proposed algorithms with the existing algorithms available for the overlay and peer models, and also provide an in-depth analysis of the algorithms. The algorithms are compared and evaluated using two key performance measures: LSP blocking probability and network (lightpath) utilization. Simulation results show that at low loads with a limited number of ports available in the network, CAPA/spl I.bar/AUG achieves an order of magnitude better blocking performance and outperforms all other algorithm. Considering the small amount of information that is exchanged between the layers in the augmented model, these results suggest that the augmented model can be a practically good compromise between the overlay and peer models.

Predictive scheduling in multi-carrier wireless networks with link adaptation

Channel-aware scheduling and link adaptation methods are widely considered to be crucial for realizing high data rates in wireless networks. However, predicting future channel states, and adjusting transmission schedules and parameters accordingly, may consume valuable system resources, such as bandwidth, time, and power. The paper considers the trade-offs between prediction quality and throughput in a wireless network that uses link adaptation and channel-aware scheduling. In particular, we study the effects on the throughput of the look-ahead window, i.e., the range of future time slots on which we have channel state estimates, and the reliability of the channel state estimates. We develop an online scheduling algorithm for a multichannel multiuser network that employs predictive link adaptation, and generalize it to incorporate imperfect channel state estimates. We apply this heuristic together with performance bounds to the offline version of the problem to evaluate the performance with varying prediction qualities. Our results suggest that it may be possible to reap most of the potential channel-aware scheduling benefits with a small look-ahead and imperfect channel state estimates. Thus, a modest consumption of resources for channel prediction and link adaptation may result in a significant throughput improvement, with only marginal gains through further enhancement of the prediction quality. Our results can provide meaningful guidelines in deciding what level of system resource consumption is justified for channel quality estimation and link adaptation.

Cost efficient LSP protection in IP/MPLS-over-WDM overlay networks

We consider an IP/MPLS-over-WDM overlay network, which an IP/MPLS service layer exists independently over a WDM transport network. In the overlay model, two adjacent layers communicate with each other only through a pre-defined user-network interface (UNI) and the WDM topology information is not available to the service layer. In this paper, we concentrate on network design with a fault tolerance mechanism provided only in the IP/MPLS service layer in which the label-switched path (LSP) protection takes into account both node failures in the IP/MPLS service layer and failures in the WDM transport layer. In this design problem, the service layer must decide how lightpaths or logical links should be provisioned and how primary and backup LSPs should be routed in a cost-effective manner, such that failures do not affect both LSPs simultaneously. We formulate this survivable IP/MPLS overlay network design problem as a mixed integer linear program (MILP). Since the MILP is computationally intractable for large networks, we solve it only for a small network and present the optimal solution found by CPLEX 7.5, a commercial MILP solver. In order to find efficient solutions in larger networks we propose a heuristic algorithm that performs near-optimally in the cases studied. we then compare total costs for LSP protection from WDM link failures and LSR failures may not be much more expensive than protection from LSR failures only.

Efficient alarm management in optical networks

As the capacity of optical transport networks increases, rapid fault identification and localization become increasingly important. These problems are more challenging than in traditional electronic networks because of optical transparency. In a transparent optical network which does not regenerate optical signals, a fault may propagate to various parts of the network from the origin, and multiple alarms can be generated for a single failure. Efficient alarm management and filtering requires a careful selection and placement of network monitoring equipment. In this paper we survey the capabilities of current optical monitoring equipment and formulate a problem of selecting monitors to be placed throughout the network. Simulation results suggest that the number of monitors and generated alarms can be significantly reduced if monitor locations are selected judiciously.

Analysis of multi-hop traffic grooming in WDM mesh networks

Traffic grooming is an essential functionality of WDM optical networks to provision multi-granularity subwavelength connections. Depending on the number of lightpaths allowed in a connection route, traffic grooming can be classified as single-hop traffic grooming (SH-TG) and multi-hop traffic grooming (MH-TG). MH-TG is more general and resource-efficient than SH-TG, because it allows connections from different source-destination pairs to share the bandwidth of a lightpath. In this paper, we propose a MH-TG algorithm, namely the fixed-order multi-hop (FOMH) grooming algorithm, based on the fixed-alternate routing approach. We introduce the grooming node selection (GNS) problem in MH-TG and propose three grooming policies, namely exhaustive sequential (ES), limited-hop sequential (LHS) and load sharing (LS) policies, to address the GNS problem. These policies represent different trade-offs among blocking probability, computational complexity and transceiver requirements. Given that the analysis of MH-TG is a relatively unexplored area, we propose an analytical model to evaluate the blocking performance of MH-TG using FOMH and the LS grooming policy. To address the multi-layered routing and multi-rate connection characteristics of traffic grooming, we introduce a novel multi-level decomposition approach in our analytical model which decomposes traffic at four different levels, namely alternate path, connection route, lightpath and link levels. The model also addresses various factors that affect connection blocking probability. These factors include wavelength continuity constraint, channel continuity constraint and route dependence. The Erlang fixed-point approximation method is used to solve the analytical model. Numerical results show that analytical results match well with simulation results. We also evaluate the effect of the grooming policies, the number of virtual hops (lightpaths) within a connection route and the number of alternate paths on the performance of the grooming algorithm.

Monitoring and alarm management in transparent optical networks

Rapid fault identification and localization in optical networks are crucial due to high data rates. These problems are more challenging than in traditional electronic networks because of optical transparency. In a transparent optical network which does not regenerate optical signals, a fault may propagate to various parts of the network from the origin, and multiple alarms can be generated for a single failure. In order to reduce the number of redundant alarms, simplify fault localization, and reduce the fault localization time, the number and location of fault monitors that are turned on should be optimized for a given network. In this paper, we formulate a problem on the optimal activation of network monitoring devices and propose a solution approach. First, we provide a brief summary of available physical-layer monitoring devices, and then present a scheme for optimal monitor activation. We show the NP-completeness of the problem and present a mixed-integer-linear-program (MILP) formulation of it. We also present a heuristic, whose performance is evaluated through comparisons with the solutions to the MILP, as well as a naive monitor activation approach.