Passive Optical Networks (PONs) have been proposed and standardised to open up the bandwidth capacity of access networks. At present, network operators have begun to deploy Time-Division Multiplexing (TDM) -based PONs in highdensity urban areas, while Wavelength Division Multiplexing (WDM) PONs are still in the stage of research and standardisation. Concerning TDM-PONs, current standards such as the Gigabit PON (ITU-T G.984), the Ethernet PON (IEEE 802.802.3ah), and their recent enhancements XG-PON1 (ITU-T G.987) and 10G-EPON (IEEE 802.3av) use a 1xN passive splitter/combiner to divide the optical signal to all users in the downstream direction and aggregate the users’ data in the upstream direction. TDM access sharing is required in the upstream direction to avoid collisions between user’s data. On the other hand, for PONs based on WDM, the power splitter/combiner is replaced by a wavelength selective filter, usually an Array Waveguide Grating (AWG), thus allowing a dedicated wavelength with symmetric bandwidth between each user and the central office. Despite their differences, both types of PONs share a main drawback related with the high Operational Expenditures (OPEX) derived from their manually troubleshooting procedures, as follows: typically, most vendor equipment offer proactive alarms related with physical and link-layer aspects
such as link down, frame loss or power level events. These alarms are often followed by a set of manual measurements launched by the network manager to detect and locate the failure, most of the times comprising fibre breaks or dirty connectors. In this light, the network manager must devote some time in manually connecting an external measurement equipment with Optical Time-Domain Reflectometry (OTDR) capabilities to actually locate the failure and isolate it from the rest of the network. Such manual operational procedures comprise high OPEX, and it would be desirable to make them automatic. Indeed, the IEEE and the ITU-T have standardised a number of Operations, Administration and Maintenance (OAM) procedures for Ethernet networks under the IEEE 802.1ag [1] and ITU-T Y.1731 [2] recommendations. These mechanisms include the generation of loopback messages, measurements of packet delay or loss, etc. at the Ethernet layer which, in conjuction with the raw physical alarms provided by most vendor equipment and the OTDR measurements, can provide a means towards the automatic troubleshooting of WDM-PON networks. This article explores this idea of integrating troubleshooting information from multiple independent sources (equipment alarms, OTDR traces and Ethernet OAM features) and further proposes an Integrated Troubleshooting Box (ITB) for the effective and proactive (i.e. without user intervention) management of failures in WDM-PONs. Thanks to this box, the network manager will be provided with accurate real-time information about the PON status, including the detection, isolation and verification of failures upon their occurrence (Fig. 1). The remainder of this article is organised as follows: Section II describes the troubleshooting capabilities of OTDRs at the optical layer. Section III reviews the Ethernet OAM mechanisms described in the IEEE 802.1ag and ITU-T Y.1731 at the link layer. Section IV proposes the abovementioned ITB device which will integrate both physical and link-layer functionalities and automatise the process of detection, verification and isolationofthefailure.Finally,SectionVconcludesthisarticle withasummaryanddiscussionofitsmaincontributions,along with future work worth of investigation.