Carmen Mas, Patrick Thiran,
"A review on Fault Location methods and their application to optical networks",

to appear in Optical Networks Magazine, May 2001.

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.
This paper reviews some approaches used for fault location. We classify them in two main categories: the Model Based approaches and the Black box learning-based approaches. The first ones rely on an abstract model of the network, capturing the dependency relations between the elements and capable of pointing out the element(s) which is (are) most likely to be the cause of the received alarms. The second ones do not attempt to model the network in detail, but leave it as a black box. An abnormal situation is then diagnosed from a set of rules obtained by learning or thanks to expertise of the human manager.

Carmen Mas, Patrick Thiran,
"An efficient algorithm for locating soft and hard failures in WDM networks",

JSAC Special Issue “Protocols and architectures for next generation optical WDM networks”, Vol. 18, No. 10, October 2000, pp.1900-1911.

Fault identification and location in optical networks is hampered by 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. 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 non-polynomial computational complexity of the problem is pushed ahead into a pre-computational 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.

Download a postscript version: uncompressed, gziped.

Carmen Mas, Patrick Thiran, Jean-Yves Le Boudec
"Fault Localization at the WDM Layer",

"Photonic Network Communications" Volume 1, Number 3, November 1999, pages: 235-255

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 passive elements, that is, network elements which may fail but never generate an alarm (e.g. optical fibres). 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 behaviour 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 formalisation of the alarm-filtering problem and results in an efficient algorithm for localising the failure(s). This algorithm is applied to the WDM rings of the COBNET network (COBNET is a European ACTS project) and to a meshed optical network with the ARPA2 topology.

Download a postscript version: uncompressed, gziped.

Carmen Mas, Patrick Thiran,
"An efficient Fault Localization Algorithm for IP/WDM Networks",

IEEE/ACM/SPIE Workshop on Optical Networks
The University of Texas at Dallas
1st Febrary 2000

We propose an algorithm for localizing multiple failures in an IP/WDM network. They can be either hard failures (unexpected events that interrupt suddenly the established channels) or soft failures (events that progressively degrade the quality of transmission). Hard failures are detected at the WDM layer, whereas soft failures can be detected at the optical layer if proper testing equipment is deployed, and/or by performance monitoring at a higher layer, which is here IP. The algorithm also tolerates missing and false alarms.
Even without missing and false alarms, multiple fault localization is NP-hard. The diagnosis phase (i.e., the localization of the faulty components upon reception of the alarms) can however remain very fast, but at the expense of a very complex precomputation phase, carried out whenever the optical channels are set up or cleared down. We show how the algorithm performs on an example of an IP/WDM network.

Download a postscript version: uncompressed, gziped.

Carmen Mas, Olivier Crochat, Jean-Yves Le Boudec
"Fault Localisation in an Optical Network",

All-Optical Networking: Architecture, Control, and Management Issues Proceedings p.408-419
SPIE'98 Voice, Video, and Data Communications
November 98

A single failure in a communication network can trigger many alarms. We propose an alarm filtering algorithm for the management of an optical network that avoids the use of failure probabilities as well as timestamps. The algorithm locates failures in network elements that cannot generate alarms but may fail. This algorithm is defined and formalised at the level of abstraction and will be applied to the network of the ACTS COBNET project.

Download a postscript version: uncompressed, gziped.

Carmen Mas, Jean-Yves Le Boudec
"An Alarm Filtering Algorithm for Optical Networks",

MMNS97 Proceedings p. 205-218

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.

Download a postscript version: uncompressed, gziped.

"An Alarm Filtering Algorithm for Optical Communication Networks" SSC/1997/007 Carmen Mas, Jean-Yves Le Boudec

"Fault Localization at the WDM layer" SSC/1999/013 Carmen Mas, Patrick Thiran, Jean-Yves Le Boudec

WP1 "COBNET control and management evaluation report"
Deliverable 144 - December 98

This deliverable presents the final implementation of the control and management structure and the tests carried out. From the management part, the fault management is studied in detail by presenting the routines and the trap processing at the CPN level. From the control part, the communication of the node agent with the mezza's and with the CPN entity is introduced. The deliverable also shows the tests of the global management integration.

WP1 "Specification of Control and Management Functions for COBNET"
Deliverable 143 - September 97

This deliverable presents in deep the management and the control implementations and the relation between them. In particular it introduces the MIB definition and structure and the databases in COBNET as well as the interaction between the different management levels considering the protocol, signals and functions. Concerning the control implementation, the tools for the developing and testing have been presented together with the agent implementation and its requirements.

WP1 "Design of the Control and the Management Functions"
Deliverable 142 - March 97

This deliverable presents a refinement of the management structure of the COBNET network and introduces some points of its implementation as for example the chosen management protocol, the software tools, some implementation methods and the control structure with its implementation.