Showing papers in "Photonic Network Communications in 2000"

A Comparison of Allocation Policies in Wavelength Routing Networks

Abstract: We consider wavelength routing networks with and without wavelength converters, and several wavelength allocation policies Through numerical and simulation results we obtain upper and lower bounds on the blocking probabilities for two wavelength allocation policies that are most likely to be used in practice, namely, most-used and first-fit allocation These bounds are the blocking probabilities obtained by the random wavelength allocation policy with either no converters or with converters at all nodes of the network Furthermore, we demonstrate that using the most-used or first-fit policies gives an improvement on call blocking probabilities that is equivalent to employing converters at a number of nodes in a network with the random allocation policy These results have been obtained for a wide range of loads for both single-path and general mesh topology networks The main conclusion of our work is that the gains obtained by employing specialized and expensive hardware (namely, wavelength converters) can be realized cost-effectively by making more intelligent choices in software (namely, the wavelength allocation policy)

Study on the Implementation of Optical Transparent Transport Networks in the European Environment—Results of the Research Project COST 239

Abstract: This paper reports the main results obtained in the framework of the COST239 project on the implementation of transparent optical network schemes to connect main European cities. The investigation on transmission limitations has shown that transmission systems, for which 1662:5 Gbit/s WDM systems are used due to traf®c reasons, have a maximum range shorter than 1000 km. This condition does not allow a completely transparent network. Two solutions, the partitioned and the gridconnected network architecture, are proposed, described, and compared in the paper.

Allocation of Splitting Nodes in All-Optical Wavelength-Routed Networks

Abstract: In this paper, we introduce the splitter placement problem in wavelength-routed networks (SP-WRN) Given a network topology, a set of multicast sessions, and a fixed number of multicast-capable cross-connects, the SP-WRN problem entails the placement of the multicast-capable cross-connects so that the blocking probability is minimized The SP-WRN problem is NP-complete as it includes as a subproblem the routing and wavelength assignment problem which is NP-complete To gain a deeper insight into the computational complexity of the SP-WRN problem, we define a graph-theoretic version of the splitter placement problem (SPG), and show that even SPG is NP-complete We develop three heuristics for the SP-WRN problem with different degrees of trade-off between computation time and quality of solution The first heuristic uses the CPLEX general solver to solve an integer-linear program (ILP) of the problem The second heuristic is based on a greedy approach and is called most-saturated node first (MSNF) The third heuristic employs simulated annealing (SA) with route-coordination Through numerical examples on a wide variety of network topologies we demonstrate that: (1) no more than 50% of the cross-connects need to be multicast-capable, (2) the proposed SA heuristic provides fast near-optimal solutions, and (3) it is not practical to use general solvers such as CPLEX for solving the SP-WRN problem

Wavelength Conversion Technologies

Abstract: This paper reports a review of the different technologies used for wavelength conversion in wavelength-division multiplexed networks. The strengths and the weaknesses of the competing technologies, including optoelectronic wavelength conversion, four-wave mixing, difference frequency generation, cross-gain and cross-phase modulation in semiconductor optical amplifiers, are stressed with regard to conversion speed, transparency and regeneration of the converted signal.

Algorithms for Dynamic Routing in All-Optical Networks

Abstract: In this paper, we have studied a number of algorithms for routing in all-optical wavelength routed networks. We have considered situations where a lightpath is dynamically created in response to a request for communication and the WDM channels constituting the lightpath are reclaimed when the communication is over. We have looked at two schemes for dynamic wavelength allocation. In the first scheme we have assumed the existence of a central agent to maintain a database of existing lightpaths where the central agent attempts to generate a new lightpath in response to a request for communication. In the second scheme, we attempt to generate a new lightpath using a distributed algorithm. In the first scheme we have exploited the existence of multiple paths between any pair of nodes in a network in order to reduce the blocking probability. For the second scheme, we have proposed three distributed strategies to determine, if possible, a lightpath when there is a request for communication. Each of these strategies have their advantages and disadvantages in terms of the expected blocking probability and the set-up time. We have studied the performances of both the schemes using Monte Carlo simulation.

An Optical Layer Lightpath Management Protocol for WDM AONs

Abstract: In this work, we propose a control protocol for lightpath management in the optical layer of all-optical networks (AONs) AONs follow a layered structure, as used by various network standards, where each layer communicates with its peer through Protocol Data Units (PDUs) In the context of Open System Interconnection (OSI) Reference Model (RM), a new layer, called optical layer, has been introduced in the AON architecture to manage the lightpath related functions The optical layer lies in between the physical layer and the data link layer The objective of this paper is to define a specification for an optical layer protocol for managing lightpaths in AONs In order to study the dynamics of the optical layer protocol system, we first employ the communicating finite state machine model to represent the protocol Then a reachability analysis of the model is performed to verify the protocol This paper shows how the optical layer protocol can be specified formally and made error-free by the step-wise refinement of an initial specification, where validation is done after each refinement

Tutorial: Key Elements for WDM Transport Networks

Abstract: This paper reports a brief review of the key elements for WDM transport networks. In particular, the transmission aspects, the basic elements of WDM networking, and the issues relating to failure restoration and network management are taken into account.

Wavelength Conversion in WDM Optical Networks: Strategies and Algorithms for Limiting the Number of Converters in the Optical Cross-Connects

Abstract: This paper considers the problem of wavelength conversion in optical networks using wavelength division multiplexing technique. In the previous literature, two main wavelength routing and assignment strategies have been introduced: wavelength path (WP) and virtual wavelength path (VWP), depending on whether the signal stays on the same wavelength or is converted to another during its travel throughout the network. While the former method does not require any wavelength conversion, the latter needs wavelength conversion in each optical node and, in particular, a wavelength converter per each signal handled by the node itself. From the previous literature emerged that the VWP leads to optical cross-connect (OXC) with lower dimensions compared to the ones required by the WP scheme, and that the difference between the WP and VWP schemes increases as the number of wavelengths carried by each fiber increases. In this paper a new strategy is introduced, named partial virtual wavelength path (PVWP), with the related wavelength routing and assignment algorithm, which makes limited use of wavelength conversion compared to the VWP scheme, and allows the same advantages of VWP to be attained with lower OXC dimensions. The paper reports a comparative analysis among the different strategies, considering both the cases of a network without failures and a network with the possibility of failure restoration. The main result is that the proposed PVWP strategy allows the same advantages of the VWP scheme with a strongly reduced number of wavelength converters (around 5% of the number required by VWP scheme). This figure does not vary appreciably if failure restoration is considered. The new strategy can be adopted by using an opportune OXC architecture, as illustrated in the paper, which allow a limited number of converters to be shared among all the channels as a common pool.

Planning of WDM ring networks.

Abstract: The rapid introduction of wavelength division multiplexing (WDM) technology to cope with the increased bandwidth requirements of transmission networks has intensified the need for recovery mechanisms at the optical layer. A first step towards survivable optical networking will be seen through the introduction of optical rings. This paper presents different types of optical rings (dedicated and shared protection WDM rings) and the planning issues associated with these WDM rings. In particular, we give mathematical models as well as solution methods for the ring loading and wavelength assignment problem. We compare the wavelength requirement of dedicated and shared protection rings under scenarios with different demand patterns. We also discuss the influence of the WDM equipment cost, and present a mathematical model for the optimization of hybrid networks with both dedicated and shared protection rings.

Tunable and Agile Laser Transmitter Developments for Future DWDM Optical Networks: Towards Managed Wavelength Control and Switching Invited Paper

Abstract: The International Telecommunications Union (ITU) has defined the laser wavelengths to be used in the frequency plan for dense wavelength division multiplexed (DWDM) optical fiber networks. It is up to the equipment manufacturers to supply suitable transmitters which network planners must use. The options involve a rack or array of many lasers or tunable lasers. We review the latter which are complex, four-section devices and discus their operation. More importantly we consider how recent work has permitted characterization and then control of these so that a single laser can access the full ITU bandwidth. We also consider the new functionality only recently available through fast wavelength switching. This may permit improved management of the fiber's full capacity at all times despite peak-to-mean traffic fluctuations per wavelength channel. The possibilities for new functionality mean a closer interaction between device and network designers in future.

Efficient Wavelength Assignment Algorithms for Light Paths in WDM Optical Networks With/Without Wavelength Conversion

Abstract: In order to reduce the overall wavelength number required in a wavelength division multiplexing (WDM) network with static traffic loading, new heuristic algorithms for wavelength assignment are proposed in this paper. A new parameter called “one-wavelength-decrease cost” is defined and used to compare the efficiency of these algorithms. Comparative simulation studies have been carried out for various network topologies to investigate the effectiveness of the proposed algorithms.

Design and Analysis of a Synchronous Transmission WDMA Protocol

Abstract: In this paper, we propose a synchronous WDMA protocol for a passive star topology and suggest a new architecture for the network interface of each station. The multichannel nature of WDM networks involves receiver collision phenomena at the destination. We develop an analytical model based on a finite number of tunable receivers and a finite number of stations to investigate the performance of the proposed protocol. Numerical results are showing the performance behaviour for various number of channels, stations, and tunable receivers. Also simulation results are presented for comparison with the results obtained by the performance analysis.

Lightpath Versus Semi-Lightpath: Some Studies on Optimal Routing in WDM Optical Networks

Abstract: This paper deals with the problem of optimal dynamic routing in WDM optical networks with wavelength-changing facilities available at some of the nodes. The route may be either a lightpath (i.e., wavelength continuous channel) or a semi-lightpath (i.e., wavelength-converted channel). We attempt to estimate in this work the gain in blocking probability, when we move from lightpath routing to semi-lightpath routing, keeping the number of wavelengths fixed, in a given circuit switched network. We ensure optimal (minimum cost) routing in both the cases by using the algorithm of Banerjee et al. [7,8] (called Algorithm-I in this paper) for lightpaths and that of Chlamtac et al. [6] (called Algorithm-II) for semi-lightpaths. Our results indicate that, for both the algorithms, the blocking probability (PB), as expected, increases with network load. At light load, PB for Algorithm-I is always larger than that for Algorithm-II. But the rate of increase in PB is slightly higher in case of Algorithm-II, so that there is a crossover point where PB for Algorithm-II exceeds PB for Algorithm-I. This probably happens due to the irregularities in the semi-lightpaths at heavy loads when almost all routes are exhausted in the network. However, since this crossover phenomenon occurs at a very congested status of the network, it has little significance over the real life operation of a network. It only suffices to indicate that, under heavy load, both the algorithms are equally insufficient, and conversion does not improve the situation as might have been expected intuitively.

Optical Interconnection Networks for Terabit Packet Switches

Abstract: The challenge of building packet switches with terabit capacity is being met by wavelength division multiplexing (WDM) where the benefits of optical fiber are exploited. Two kinds of WDM-based bufferless optical interconnection networks are proposed in this paper to interconnect multiple electronic packet switch modules. One is based on 3-stage Clos principle and the other is based on broadcast-and-select principle. The proposed optical interconnection networks are implemented with small modular structures to provide capacities in the range of terabit per second. Their architectures, component and interconnection complexity, and power budget analyzes are presented. In addition, the crosstalk caused by the finite ON-OFF ratio of semiconductor optical amplifier is discussed. Bit error rates with respect to different ON-OFF ratios and extinction ratios are also evaluated. It is concluded that it is feasible to implement optical interconnection networks by using state-of-the-art WDM technology, and they are excellent candidates for future terabit packet switching systems.

Network Dimensioning in WDM-Based All-Optical Networks.

Abstract: The problem of network dimensioning, which involves optimal network designs using minimal resources for a given well-predicted traffic between individual nodes but without a pre-determined network topology, is analyzed in details in this paper. Such optimal designs are critical as the network resources are directly related to the cost of implementation. The problem complexity increases as the traffic between every node pair varies in a periodic manner. Furthermore, the presence of wavelength conflicts makes the network-dimensioning problem distinct and more complicated than that for traditional circuit-switching networks. Here, we adopt the approach based on a multi-commodity flow problem. A general cost function covering the resources of all system components is formulated, and two solution approaches are presented; one based on integer programming and one on heuristics.

Experimental and Theoretical Evaluation of Distortion Effects on SCM Optical Transmissions Due to the Joint Action of Static Chirping, Dynamic Chirping and Dispersive Channel

Abstract: In optical CATV systems based on subcarrier-multiplexing technique, the joint action of semiconductor laser chirp and fiber chromatic dispersion gives rise to intermodulation distortion phenomena affecting the transmission performance. This problem has been explained analytically, leading to a model valid for every kind of modulating signal and explored in the case of a Gaussian process. In this paper, the model is developed for the important specific case of an analogue SCM modulating signal. The results are assessed experimentally.

On the All-to-All Broadcast Problem in Optical Networks 1

Abstract: This paper considers the transmission of uniform deterministic traffic in an optical broadcast-star network using Wavelength Division Multiplexing. Lower bounds are established on the minimum time to exchange information between every node pair in such a network with tunable transmitters and fixed-tuned receivers. Three different scheduling algorithms are developed that are strictly optimal in three regimes of system parameters. The results are applicable to arbitrary tuning delays and arbitrary numbers of wavelength channels, and indicate the existence of a well-defined transition regime from tuning-limited operation to bandwidth-limited operation. Finally, the problem of computing the optimal number of wavelengths is addressed to achieve a schedule with minimum schedule length, and exact solutions are given.

Dynamic Bandwidth Allocation in WDM Passive Star Networks with Asymmetric Traffic

Abstract: When asymmetric traffic is offered to a WDM passive star network, the offered bandwidth must be allocated is such a way that each station takes a portion of the available bandwidth proportional to its needs. When the traffic characteristics are fixed and a priori known, then, the bandwidth allocation scheme can be based on these characteristics. Unfortunately, the traffic characteristics are often unknown and time-variable. In this paper, a dynamic bandwidth allocation scheme is presented, which is based on the network feedback information in order to be capable of adapting to the changing traffic characteristics. According to the proposed scheme, a set of learning automata processes the network feedback information and dynamically allocates the available bandwidth to the stations according to their needs.

Design of a Core Switch for an Optical Transparent Packet Network

Abstract: Design guidelines are given to dimension large switches in the optical packet network environment developed in the framework of the ACTS project KEOPS (KEys to Optical Packet Switching). Different possible choices in the architecture and operation of the switch are proposed according to the photonic technology availability. Simulation and analytical tools are used to evaluate switch performance and they have been applied to find out suitable switch configurations that meet packet loss requirements. Results and application examples are given to show the feasibility of a packet switch for high-speed photonic transport with the available technology.

Scalability of optical networks : crosstalk limitations

Abstract: Optical networks represent a promising solution for the future high capacity and flexible transport network. This paper presents a model for the performance evaluation of optical networks with respect to linear crosstalk and accumulated spontaneous emission noise. The proposed model is intended for the evaluation of the crosstalk requirements on the devices needed to support an optical network with a certain number of nodes and with a given level of bit-error rate.

Economic Optical Networking in the Metropolitan Area

Abstract: The rapid introduction of wavelength division multiplexing (WDM) technology in long distance fiber optic communication networks which has been observed in the recent years has been driven by line economy. Not merely the ability to overcome fiber scarcity, but also the savings associated with the replacement of many electronic regenerators by fewer optical amplifiers, have motivated the carriers to employ WDM. These arguments do not often apply to optical networking in metropolitan areas because transmission distances are short. For this reason, metro-WDM has been deployed in a few rare cases only, though it has been widely discussed by a broad scientific and industrial community. In this paper, an advanced operational concept for all optical transport networks in the metropolitan area is introduced. It focuses on node economy, i.e., the technical concepts to provide gigabit services access to the transport network at minimum cost, characterized by the following basic features: protocol independence, monitoring, protection and fast bandwidth provisioning.

All-Optical Wavelength-Bypassing Ring Communications Networks

Abstract: We introduce an all-optical WDM packet communication network that performs wavelength bypassing at the routers. Packets that arrive at a wavelength (optical cross-connect) router at designated wavelengths are switched by the router without having their headers examined. Thus, the processing element of the router is bypassed by such packets. For packet traffic that uses wavelengths that do not bypass a switch, the headers of such packets are examined to determine if this switch is the destination for the flow. If latter is the case, the packet is removed. Otherwise, the packet is switched to a pre-determined output without incurring (network internal) queueing delays. We study a ring network with routers that employ such a WDM bypassing scheme. We present methods to construct wavelength graphs that define the bypassing pattern employed by the routers to guide the traffic flows distributed at each given wavelength. Performance is measured in terms of the network throughput and the average processing path length (i.e., the average number of switches not being bypassed). For a fixed total processing capacity, we show that a WDM bypassing ring network provides a higher throughput level than that exhibited by a non-bypassing ring network, using the same value of total link capacity. By using WDM bypassing, the average processing path length (and thus the packet latency) is reduced. We study a multitude of network loading configurations, corresponding to distinct traffic matrices and client-server scenarios. Higher throughput levels are obtained for network configurations driven by non-uniform traffic matrices. The demonstrated advantages of WDM bypassing methods shown here for WDM ring networks are also applicable to more general network topological layouts.

A Distributed Fault Management Scheme for Wavelength Routed All-Optical Networks

Abstract: This paper presents a distributed algorithm to determine fault avoiding routes between source-destination end-node pairs in an all-optical network. Fault-tolerant routing for WDM routed all-optical networks has not been studied in details in the literature except for using protection switching or loopback recovery. Such schemes use predetermined protection mechanism and hence assume that the protection path is not faulty. This paper considers extensions to a dynamic routing scheme (presented earlier by these authors for fault-free networks) to handle rerouting in case of fault occurrence. The performance degradations of the network because of fault occurrence are then studied in terms of blocking probability and set-up time through simulations.

Dimensioning of the Wavelength Converters in a WDM Optical Packet Switch

Abstract: This paper analyzes the performances of a WDM optical packet switch making use of fiber delay lines to resolve output packet contentions; the optical packet switch is equipped with tunable wavelength converters which can shift optical packets to any wavelength of the output link which they are directed to. As not all packets need conversion, we propose a dimensioning technique allowing to reduce the number of converters and to improve the signal quality by reducing unnecessary conversions. The obtained results show that a remarkable reduction of the number of converters is obtained with respect to that needed by other switches described in literature. Such a saving is achieved by maintaining the packet loss probability below a prefixed threshold.

Modular and Re-Configurable Parametric Wavelength Interchanging Cross-Connect Architectures

Abstract: We report on a new technique that allows for modular expansion of strictly transparent wavelength interchangeable cross-connect utilizing parametric wavelength converters based on four-wave mixing or difference frequency generation. The proposed technique results in cost effective cross connect architectures with fewer components than other frequently proposed designs. It will be shown that employing FWM-based converters results in reconfigurable and simple to fabricate architectures.

Planning of Transport Networks based on Photonic and Electronic Cross-Connection

Abstract: This paper deals with the problem of dynamic allocation of bandwidth in a hierarchical transport platform comprised of both broadband (optical) and wideband (ATM) crossconnects. The crossconnect platforms will react to fluctuations of traffic on different time scales. The model developed in this paper will take advantage of both multi-service and multi-hour character of traffic profiles and includes off-line and on-line algorithms. The off-line model will generate the configuration of optical bearers and the routing of origin-destination paths in the network for each time interval. The on-line algorithms will adjust the capacity allocated to each path in accordance with the short-term fluctuations of demand to decrease blocking probability and maximize resource utilization. We formulate the problem of designing a minimum cost network for a multi-hour, multi-service traffic profile where the cost is a piecewise linear function of the maximum possible number of optical paths as well as a linear function of the number of ports and of the traffic flow through tandem nodes. The model is a large-scale mixed integer program. Further, we provide a second model to carry out the fine tuning necessary to cater for traffic fluctuations. The alternative versions of the second model are given, including a goal program that optimally matches the network capacity to the varying traffic requirements.