Wavelength-routed Optical Networks Research Articles

Programmable select multiwavelength gigahertz Raman soliton pulse generation

The generation of programmable multiwavelength pulses based on the self-frequency shift of a Raman soliton is demonstrated. The approach produces tunable multiwavelength picosecond pulses. Only select multiwavelength signals with a tuning range of approximately 50 nm are generated with a repetition rate of 9.95 GHz at each wavelength channel. A bit error rate (BER) of better than 1 x 10(-9) was successfully obtained for all the measured multiwavelength Raman soliton pulses. Furthermore, it was found that the signal has an excellent relative intensity noise (RIN) of better than -135.5 dBc/Hz. The BER and RIN measurements show that the frequency-shifted Raman soliton pulses are promising for use in measurement systems, optical gating, signal processing, and wavelength routing optical packet networks with the ability to provide 1:1 communication and 1:N multicasting.

A computer aided tool for the performance evaluation of next generation WDM wavelength-routed optical networks

A computer aided tool for the performance evaluation of next generation WDM wavelength-routed optical networks

Signaling Schemes for Distributed Connection Management in Wavelength-Routed Optical Mesh Networks

The next-generation optical transport network will evolve from point-to-point connectivity to mesh networking, which can provide fast and automatic provisioning with enhanced flexibility and survivability. Signaling is used to support connection setup, maintenance, and teardown in such a network. In this paper, we study the performance of two hop-by-hop and one parallel signaling schemes in wavelength-routed optical mesh networks. Based on the sequence between optical crossconnect (OXC) switching and signaling message processing, we classify hop-by-hop signaling into two types that comply with the requirements of GMPLS signaling protocols. These two types are forward before switching configuration (FBSC) and forward after switching configuration (FASC). Also, we propose a parallel signaling scheme that is different from the existing hop-by-hop GMPLS signaling protocols. Considering OXC architectures and traffic patterns, we compare the FBSC, FASC, and parallel signaling schemes using simulation experiments, in terms of network blocking probability and reservation time. The simulation data reveal that the performance of a signaling scheme depends on the nature of the signaling as well as the network setting (e.g., the OXC architecture and traffic pattern). We analyze reasons for this result and discuss tradeoffs between these signaling schemes. This work offers some insight into designing an efficient signaling protocol for wavelength-routed optical mesh networks.

A computer aided tool for the performance evaluation of next generation WDM wavelength-routed optical networks

Wavelength-Routed Optical Network Simulator (WRONSIM) is a simulation tool intended for studying the dynamic behavior of distributed reservation protocols in wavelength-routed optical networks. Unlike telephony circuit-switched networks, wavelength-routed optical networks operating under wavelength continuity constraint requires that the same wavelength must be assigned to all links along the selected route. The user can specify the inputs to the simulator which includes a description of network topology, reservation protocols, traffic load and control parameters such as the number of wavelengths per link, the time to process a packet, the time to find an available route etc. The simulator provides statistics such as the average blocking probability of connection requests due to the load or reservation conflicts and the average setup time. A modular design is adopted so that new modules can be added to the simulator easily. As a case study, the simulator is used to compare the performance of two reservation protocols: the source initiated reservation protocol (SRP) and the destination initiated reservation protocol (DRP).

Preferred Link Based Distributed Adaptive Routing in Wavelength Routed Optical Network

As the WDM technology matures and the demand for bandwidth increases, dynamic provisioning of lightpaths at the WDM layer becomes an important and challenging problem. In this paper, we consider the problem of dynamic routing and wavelength assignment in wavelength-routed optical networks. The conventional approach to this problem is to select a route from a set of candidate routes, which has a common wavelength available on all the links of the route. In this paper, we propose a distributed algorithm which selects a route based on the state of the network (called preferred link approach). In this approach, a route is selected link by link based on a preference value given to each of the links. We propose three different heuristic functions for calculating the preference of the links, depending on the cost and congestion on the links. We evaluate our routing algorithm in terms of call acceptance ratio, cost of the path, hop length, and call setup time. Our experimental results suggest that our algorithm not only out performs the existing methods with respect to average call acceptance ratio, but, also improves the fairness among different hop connections, which is an important result in the case of WDM optical networks.

Dynamic routing and wavelength assignment in optical networks by means of genetic algorithms

We propose a novel genetic algorithm for solving the dynamic routing and wavelength assignment (DRWA) problem in wavelength-routed optical networks. The algorithm not only obtains low call blocking probability, but it also employs a very short computation time. Moreover, it is capable of providing fairness among connections, that is, to offer approximately the same quality of service (in terms of blocking probability) for all source-destination node pairs. Since requirements on optical network availability are highly severe, we also propose an extension of the algorithm to provide fault-tolerance capability at the optical layer. It is achieved by means of protection, where each optical connection request is provided with a pair of lightpaths (a primary and a backup lightpath). Again, the genetic algorithm proves to be highly efficient, in this case, at performing routing and wavelength assignment of pairs of lightpaths.

Investigation of the scalability of dynamic wavelength-routed optical networks [Invited

Feature Issue on Optical Interconnection Networks (OIN). We describe results of the scalability analysis for dynamic wavelength-routed optical networks with end-to-end lightpath assignment and central network control with electronic scheduling and processing of lightpath requests. We investigate the effect of the algorithm complexity in both the scheduling and the dynamic routing and wavelength assignment (DRWA) of lightpath requests. Scheduling theory and static performance-prediction techniques were applied to define the bounds on the electronic processing time of requests, and hence the maximum number of nodes supported by a centralized dynamic optical network for given blocking probability, latency, and network diameter. Scalability analysis results show that medium-sized centralized networks (~50 nodes) can be supported when these networks are reconfigured on a burst-by-burst basis. In addition, we found that real topologies showed a complex trade-off between the request processing time, blocking probability, and resource requirements. These findings can be used to determine the optimum combination of scheduling/DRWA algorithm, showing that the fastest DRWA algorithm does not necessarily lead to the minimum blocking probability and maximum scalability but that a careful consideration of both blocking and processing speed is required. The results are applicable both to dynamic network architectures with centralized request processing such as wavelength-routed optical networks and to the design of advanced optical switching matrices and routers.

Novel Devices for Wavelength Routing and Optical Access Networks

We report the recent research thrust at the Chinese University of Hong Kong that investigate novel devices to enhance the network flexibility, versatility, and robustness of wavelength routing networks and optical access networks. In particular, we will discuss wavelength and format converter, optical performance monitoring devices, low cost WDM transmitters for access networks, and the associated protection and restoration schemes.

Application of efficient transmission performance evaluation model to dynamic resource allocation in wavelength-division-multiplexing optical networks

In wavelength-routed optical networks, the transmission performance of the optical lightpaths available may not always satisfy the service requirements due to the degradation of signals induced by crosstalk, polarized mode dispersion (PMD), nonlinear effects, etc. This paper is to incorporate the role of transmission performance analysis in dynamic resource allocation by employing appropriate models of crosstalk accumulation and PMD so that the BER of a candidate lightpath can be estimated, in advance, to determine if this lightpath should be used for the call. An efficient algorithm that can improve the simulation speed significantly with no loss of accuracy is given in detail.

Analysis of Wavelength-Routed Optical Burst-Switched Network Performance

This paper studies a novel scalable network architecture combining optical burst switching (OBS) with dynamic wavelength allocation to guarantee quality of service (QoS), forming a wavelength-routed optical burst-switched network. All processing and buffering functions are concentrated at the network edge and bursts are assigned to fast tuneable lasers and routed over a bufferless optical transport core using dynamic wavelength assignment. Different burst aggregation mechanisms are evaluated for a range of traffic statistics in terms of delay and packet loss rate. New network performance parameters in an analytical model quantify the advantages of dynamic wavelength allocation. The results define the operational gain achievable with dynamic wavelength assignment compared to quasi-static wavelength routed optical networks.

A novel distributed control protocol in dynamic wavelength-routed optical networks

Path selection has long been one of the most important issues in the network design and management, which is also referred to as routing and wavelength assignment in the optical network domain. In this article we investigate the RWA problem for dynamic wavelength-routed mesh networks in a fully distributed controlled environment. We present a novel routing and signaling protocol called Asynchronous Criticality Avoidance (ACA), which is devised to reduce the mutual interference between lightpaths launched by different source-destination pairs to improve network performance in terms of blocking probability. With the fixed alternate routing architecture, the ACA protocol dynamically marks a set of wavelength channels as critical if the occupancy of the channels causes a bottleneck between an S-D pair with a width equal to or narrower than a predefined threshold. To support a distributed control environment, a suite of signaling processes is devised to realize the criticality avoidance mechanism, with which a two-stage routing and wavelength assignment (or criticality avoidance routing) is performed. The ACA protocol has been shown by simulations to be capable of better performance than existing schemes.

Optimization of splitting node placement in wavelength-routed optical networks

This paper addresses the placement of multicast nodes in wavelength-routed all-optical networks. This problem is motivated by the expected high cost of optical multicast cross-connects due to fabrication complexity and power considerations. An analytical model for the approximate blocking probability in multicast networks is developed. The blocking performance is used to guide an iterative algorithm, Multicast-ADD, for the placement of multicast nodes. We provide validation of the model and the techniques used. In addition, a simulation study is provided to evaluate the performance of the heuristic. Insights obtained from the simulation results reveal that: (1) only a subset of the nodes (found to be 50%) need to be multicast-capable for acceptable blocking performance; (2) the blocking performance of Multicast-ADD outperforms that of random placement; and (3) employing alternate routing trees is not helpful in the optimization procedure.

Investigation of dynamic resource allocation based on transmission performance analysis and service classification in wavelength-division-multiplexing optical networks

In wavelength-routed optical networks, the transmission performance of the optical lightpaths available may not always satisfy the service requirements due to the quality degradation of signals induced by crosstalk, polarized mode dispersion (PMD), nonlinear effects, etc. Based on the integrated consideration of signal transmission impairments and service classification, a novel model on dynamic resource allocation in optical networks is presented in this paper. The model classifies the service requirements with different priorities and provides differentiated QoS in the optical domain. By adopting a multi-path RWA algorithm, called DMC-OVWP, the lightpath candidates are found out for each connection request of the services. Afterwards, by analyzing the transmission qualities of the lightpath candidates, an appropriate lightpath matched the request priority are picked out and set up.

Fixed-alternate routing and wavelength conversion in wavelength-routed optical networks

Consider an optical network which employs wavelength-routing crossconnects that enable the establishment of wavelength-division-multiplexed (WDM) connections between node pairs. In such a network, when there is no wavelength conversion, a connection is constrained to be on the same wavelength channel along its route. Alternate routing can improve the blocking performance of such a network by providing multiple possible paths between node pairs. Wavelength conversion can also improve the blocking performance of such a network by allowing a connection to use different wavelengths along its route. This work proposes an approximate analytical model that incorporates alternate routing and sparse wavelength conversion. We perform simulation studies of the relationships between alternate routing and wavelength conversion on three representative network topologies. We demonstrate that alternate routing generally provides significant benefits, and that it is important to design alternate routes between node pairs in an optimized fashion to exploit the connectivity of the network topology. The empirical results also indicate that fixed-alternate routing with a small number of alternate routes asymptotically approaches adaptive routing in blocking performance.

Routing and wavelength assignment in optical networks from edge disjoint path algorithms

Routing and wavelength assignment (RWA) problems in wavelength-routed optical networks are typically solved using a combination of integer programming and graph coloring. Such techniques are complex and make extensive use of heuristics. We explore an alternative solution technique in the well-known maximum edge disjoint paths (EDP) problem which can be naturally adapted to the RWA problem. Maximum EDP is NP-hard, but now it is known that simple greedy algorithms for it are as good as any of the more complex heuristic solutions. In this paper we investigate the performance of a simple greedy maximum edge disjoint paths algorithm applied to the RWA problem and compare it with a previously known solution method.

Impact of polarization-mode dispersion on the design of wavelength-routed networks

In this letter, we address the impact of polarization-mode dispersion on the design of wavelength-routed optical networks. A mathematical formulation of the problem is provided which optimizes the cost of the network comprising the total number of regenerators and additional installed fiber for a given traffic forecast. The hill-climbing search algorithm is employed with random restarts. Numerical examples show the efficiency of the heuristic, especially in networks with nonhomogeneous fiber quality.

Influence of mode competition on the fast wavelength switching of an SG-DBR laser

Because the sampled grating distributed Bragg reflector (SG-DBR) laser is among the most attractive sources for wavelength division multiplexing (WDM) systems, it is important and necessary to investigate its wavelength switching characteristics. This behavior will set the capability limits for reallocation in wavelength-routed optical networks. In this paper, that mode competition plays an important role in the wavelength switching dynamics of DBR-type tunable lasers is confirmed experimentally. By using a time-resolved spectrum technique, the loss-dependent mode competition behavior has been directly observed, for the first time, from measurements of wavelength switching on an SG-DBR laser.

Analysis of a dynamically wavelength-routed optical burst switched network architecture

The concept of optical burst switching (OBS) aims to allow access to optical bandwidth in dense wavelength division multiplexed (DWDM) networks at fractions of the optical line rate to improve bandwidth utilization efficiency. This paper studies an alternative network architecture combining OBS with dynamic wavelength allocation under fast circuit switching to provide a scalable optical architecture with a guaranteed QoS in the presence of dynamic and bursty traffic loads. In the proposed architecture, all processing and buffering are concentrated at the network edge and bursts are routed over an optical transport core using dynamic wavelength assignment. It is assumed that there are no buffers or wavelength conversion in core nodes and that fast tuneable laser sources are used in the edge routers. This eliminates the forwarding bottleneck of electronic routers in DWDM networks for terabit-per-second throughput and guarantees forwarding with predefined delay at the edge and latency due only to propagation time in the core. The edge burst aggregation mechanisms are evaluated for a range of traffic statistics to identify their impact on the allowable burst lengths, required buffer size and achievable edge delays. Bandwidth utilization and wavelength reuse are introduced as new parameters characterizing the network performance in the case of dynamic wavelength allocation. Based on an analytical model, upper bounds for these parameters are derived to quantify the advantages of wavelength channel reuse, including the influence of the signaling round-trip time required for lightpath reservation. The results allow to quantify the operational gain achievable with fast wavelength switching compared to quasistatic wavelength-routed optical networks and can be applied to the design of future optical network architectures.

Performance of a dynamically wavelength-routed optical burst switched network

This letter describes a novel network architecture combining optical burst switching with dynamic wavelength allocation to achieve a guaranteed quality of service. All processing and buffering functions are concentrated at the network edge and bursts are assigned to fast tuneable lasers and routed over a bufferless optical transport core using dynamic wavelength assignment. Burst aggregation is evaluated for a range of traffic statistics in terms of delay and packet loss rate, and an analytical model is given to quantify the benefits of dynamic wavelength reuse. The results define the operational gain achievable with dynamic wavelength assignment compared to quasi-static wavelength-routed optical networks.

Logical topology design for linear and ring optical networks

The design of logical topologies in wavelength-routing multihop optical networks is a well-studied problem. We consider logical topology (LT) design over the popular ring and linear topologies. Our objective is the minimization of the electronic processing delay for the worst case traffic flow. For uniform traffic between nodes, this delay minimization corresponds to minimizing the number of hops on a shortest path between the farthest two nodes in the logical topology (the diameter of the logical topology). The simple structure of the physical topologies enables us to present a rigorous analysis of the problem. We present lower bounds for the achievable diameter wherever possible and propose practical logical topology design algorithms and corresponding upper bounds. We also present an application of the LT designs in the linear topology to the survivability of ring networks.