Carmen Mas

Failure location algorithm for transparent optical networks

Fault and attack management has become a very important issue for network operators that are interested to offer a secure and resilient network capable to prevent and localize, as accurately as possible, any failure (fault or attack) that may occur. Hence, an efficient failure location method is needed. To locate failures in opaque optical networks, existing methods which allow monitoring of the optical signal at every regeneration site can be used. However, to the best of our knowledge, no method exists today that performs failure location for transparent optical networks. Such networks are more vulnerable to failures than opaque networks since failures propagate without being isolated due to optoelectronic conversions. In this paper, we present a failure location algorithm that aims to locate single and multiple failures in transparent optical networks. The failure location algorithm developed in this paper can cope with ideal scenarios (i.e., no false and/or lost alarms), as well as with nonideal scenarios having false and/or lost alarms.

Performance engineering of metropolitan area optical networks through impairment constraint routing

We demonstrate the use of impairment constraint routing for performance engineering of transparent metropolitan area optical networks. Our results show the relationship between blocking probability and different network characteristics such as span length, amplifier noise figure, and bit rate, and provide information on the system specifications required to achieve acceptable network performance.

An efficient algorithm for locating soft and hard failures in WDM networks

Fault identification and location in optical networks is hampered by a multitude of factors: the redundancy and the lack of coordination (internetworking) of the managements at the different layers (WDM, SDH/SONET, ATM, IP); the large number of alarms a single failure can trigger; the difficulty in detecting some failures; and the resulting need to cope with missing or false alarms. Moreover, the problem of multiple fault location is NP-complete, so that the processing time may become an issue for large meshed optical networks. We propose an algorithm for locating multiple failures at the physical layer of a WDM network. They can be either hard failures, that is, unexpected events that suddenly interrupt the established channels; or soft failures, that is, events that progressively degrade the quality of transmission; or both, hard failures are detected at the WDM layer. Soft failures can sometimes be detected at the optical layer if proper testing equipment is deployed, but often require performance monitoring at a higher layer (SDH, ATM, or IP). Both types of failures, and both types of error monitoring, are incorporated in our algorithm, which is based on a classification and abstraction of the components of the optical layer and of the upper layer. Our algorithm does not rely on timestamps nor on failure probabilities, which are difficult to estimate and to use in practice. Moreover, our algorithm also handles missing and false alarms. The nonpolynomial computational complexity of the problem is pushed ahead into a precomputational phase, which is done off-line, when the optical channels are set up or cleared down. This results in fast on-line location of the failing components upon reception of the ringing alarms.

Optical networks security: a failure management framework

Network security is becoming a very sensitive and important topic for equipment manufacturers and network operators. In transparent optical networks, security is even more complex since the optical signals are not regenerated as in opaque networks and, therefore, the faults and attacks at the physical layer are more difficult to detect and isolate without significantly affecting the overall network performance. In this paper we define Failure Management as the prevention, detection, and reaction against failures. Failures are defined as the interruptions of the normal functioning of the network and comprise faults (accidental interruptions) as well as attacks (intentional interruptions which can be performed by service disruption or eavesdropping). Our work deals with a solution to detection of failures in transparent networks. For this purpose we have extended a Fault Location algorithm developed for opaque optical networks to be used in transparent networks and be able to also locate attacks. The proposed algorithm is called Transparent Failure Location Algorithm (TFLA). The first part of the extension is based on the study of other optical network elements such as Optical Add/Drop Multiplexers (OADMs), Optical Cross-Connects (OXCs), wavelength converters, Optical Line Terminators, etc. The vulnerability of these elements depends on their architecture and/or fabrication technology and, therefore, different attacks can be considered. A classification of these components based on the masking and alarming properties is proposed. The second part of the extension is based on the monitoring equipment that may be available in transparent networks. The TFLA was applied for the case of a transparent ring of the Pan-European network.

Cost comparison of IP/WDM vs. IP/OTN for European backbone networks

As the demand for Internet-based services grows rapidly, carrier IP networks are becoming more of a crucial social infrastructure. Network operators must target to offer higher speeds, larger capacities and higher reliability while trying to reduce cost of operation, flexible introduction of new services and easy adaptation of legacy low-speed traffic to high-speed networks. In this work we compare two proposed architectures from a cost point of view. These two emerging solutions are IP/WDM and IP/OTN. The former consists of core routers connected directly over point-to-point WDM links, whereas the latter connects the core routers through a reconfigurable optical backbone (OTN) consisting of electro-optical cross-connects (OXCs) interconnected in a mesh WDM network. The main result of our analysis is the quantitative cost difference between IP/OTN and IP/WDM for the coming years and the use of 2.5 Gbps vs. 10 Gbps interfaces. Indeed, IP/OTN leads to significant decrease in network cost through reduction of expensive transit IP router ports and by exploiting more scalable and cheap OXC ports.

Failure Location in WDM Networks

Fault identification and location in optical networks must cope with a multitude of factors: (i) the redundancy and the lack of coordination (internetworking) of the managements at the different layers (WDM, SDH/SONET, ATM, IP); (ii) the large number of alarms a single failure can trigger; (iii) the difficulty in detecting some failures and the resulting need to cope with missing or false alarms.

Impairment aware routing in metropolitan area optical networks

Impairment aware routing is considered to be a necessary precondition for the migration from opaque to all-optical networks. In opaque networks, the signal is regenerated at each node so that signal impairments are eliminated. However, in transparent optical networks, the optical signal reaches the receiver without having been regenerated, and, therefore, it accumulates all the impairments that have occurred along the path. The selection of paths that guarantee the required signal quality, taking into account physical impairments, does not always provide the best solution in terms of network parameters (e.g., blocking probability). We propose a new scheme that integrates routing and wavelength assignment (RWA) with a new impairment constraint based routing (ICBR) algorithm to achieve an optimal combination of physical and networking performance. This new scheme also takes into account chromatic dispersion and filter concatenation impairments (not used by previous ICBR schemes). The proposed RWA also helps in network savings by requiring less equipment in managed reach networks (e.g., transceivers, dispersion compensators, etc.). Performance engineering studies with the proposed scheme have been done on metro networks, both transparent and managed reach networks.

Fault Localization at the WDM Layer

A single failure in a communication network may trigger many alarms. When the communication network uses optical fibers as transmission medium and increases its capacity by using Wavelength Division Multiplexing (WDM), the number of alarms and the difficulty to locate the failure are considerably higher. In this case, a single failure may interrupt several channels which causes a large information loss. We propose an Alarm Filtering Algorithm (AFA) for the fault management of an optical network that supports multiple failures and works in the presence of non-alarming elements, that is, network components which may fail but never generate an alarm (e.g. optical fibers). The algorithm provides a list of components whose failure explain the observed alarms. It avoids the use of failure probabilities, which are difficult to estimate, and does not need a global knowledge of the network topology. Moreover it also tolerates alarm losses and false alarms. The algorithm is tailored to the specific behavior of the hardware components of an optical network when a failure occurs. The classification of the network components according to the alarm signals they generate enables a formalization of the alarm-filtering problem and results in an efficient algorithm for localizing the failure(s). This algorithm is applied to the WDM rings of the COBNET network [1] (COBNET is a European ACTS project) and to a meshed optical network with the ARPA2 topology.

An alarm filtering algorithm for optical communication networks

A single failure in a communication network can trigger many alarms. We propose an alarm filtering algorithm for the management of an optical network using Wavelength Division Multiplexing (WDM). The algorithm supports: (i) multiple failures and (ii) passive network elements that do not generate alarms but may fail. This algorithm will be applied to the network of the ACTS COBNET project.

Fault localization for optical networks

A single failure in a communication network may trigger many alarms. When the communication network uses optical fibers as a transmission medium and increases its capacity by using Wavelength Division Multiplexing and Space Division Multiplexing, the number of alarms and the difficulty to locate the failure are considerably higher. In this case, a single failure may interrupt several channels so that the quality of lost information is larger. We propose an alarm filtering algorithm for the fault management of an optical network that supports multiple failures and works in the presence of passive elements, that is, network elements which may fail but never generate an alarm (e.g. optical fibers). Our algorithm avoids the use of failure probabilities because they are difficult to estimate and it does not need a global knowledge of the network topology. Moreover it also tolerates alarm losses. The algorithm results in the presentation to the human manager of a list of faults which may have caused the observed alarms. The alarm filtering problem is defined and formalized at the level of abstraction for devising the Alarm Filtering Algorithm. The algorithm is then applied to the optical network of the ACTS COBNET project.