Elsevier

Computer Communications

Volume 34, Issue 8, 1 June 2011, Pages 1011-1021
Computer Communications

Time-, wavelength-, and code-domain optical reflection monitoring for next-generation access-metro networks

https://doi.org/10.1016/j.comcom.2010.11.005Get rights and content

Abstract

Distributed optical reflectors are proposed to implement essential fault management operations such as fault detection, localization, and notification, in next generation all-optical access-metro networks in the optical layer. Fixed time-slots, wavelengths, or optical codes are assigned to selected key network locations along the path of a monitoring signal where corresponding mirrors are placed. The reflection received at a transmitting node enables online all-optical monitoring of the selected locations. After detailing the network architecture, we explore fundamental system design issues such as real-time fault localization algorithms, fault notification delay upperbounds, and delay line calculation algorithms for synchronous operation. Our simulation results showcase standard and long-reach passive optical networks, and demonstrate that our algorithms achieve fault notification at light speed using off-the-shelf components.

Introduction

Recent research in optical access-metro network architectures points to the emergence of Wavelength Division Multiplexed (WDM) all-optical islands of transparency such as long-reach Passive Optical Networks (LR-PONs) [1], [2] and all-optical overlays in access-metro architectures [3]. The increased range and number of optical access-metro nodes compounds the need for Operation, Administration, and Management (OAM) technologies, particularly fault management. After a fault is detected, its location is determined (fault localization) and a network node is notified (fault notification) in order to initiate fault recovery actions. Upon receipt of the fault notification, fault recovery is initiated through widely deployed protection or restoration techniques [4].

It is well established that fault detection should be handled at the layer closest to the failure, thus reducing the cross-layer signaling required for fault notification [5], [6]. Furthermore, the fact that the optical-layer infrastructure is largely invisible to the Network Management System (NMS) complicates the fault notification process and compounds failure-related revenue losses [8]. Therefore, fault detection in mission-critical optical infrastructure is required at the optical layer. However, the vast majority of state-of-the-art fault management techniques (e.g., SONET/SDH Automatic Protection Switching) as well as photonic OAM proposals [7] require costly Optical-Electrical-Optical (OEO) conversions.

Optical-layer monitoring of PONs is attracting increasing attention owing to the accelerated deployment of PON technology [8]. Optical Time Domain Reflectometry (OTDR) is unpractical in point-to-multipoint (PMP) networks such as PONs [8], [9], [10]. This is due to the difficulty of distinguishing the back scattering signal emanating from each PON branch. Various modifications of OTDR have been proposed to overcome this problem. Among these is the combination of OTDR with so-called reference reflectors at the end of each PON branch [8], [9], or the embedding of OTDR functionality in Optical Network Units (ONUs) [12], [11].

Some recent proposals seek to avoid the expense and complexity of OTDR-based solutions. In [13], the PON distribution fibers are replaced by specialty fibers generating a unique signature Brillouin frequency shift. Optical Coding (OC) techniques have also been proposed to realize fault management schemes for wavelength-broadcasting PONs that overcome the shortcomings of OTDR [14], [15]. Subsequent work on OC-based monitoring extensively investigates the physical-layer feasibility and scalability of 1D and 2D OC-based monitoring solutions in a PON setting [16], [17].

We propose a fault management architecture for the optical layer that performs fault detection, localization, and notification all-optically and in real-time. In the proposed optical reflection monitoring (ORM) technique, fixed time-slots, wavelengths, or optical codes are pre-assigned to selected key network locations along the path of a monitoring signal, and corresponding reflectors are placed at those locations. The reflections, encoded with respect to time, wavelength, or optical code, are received at the transmitting node and enable it to all-optically monitor the status of the selected locations.

ORM provides a general framework for optical-layer reflection monitoring in the time, wavelength, and code multiplexing domains. Besides, our use of in-line reflectors generalizes reflection monitoring to more network topologies and introduces novel fault localization paradigms and algorithms. More importantly, unlike OTDR and OC-based PON monitoring, the emphasis in ORM is on the fast detection of the faults in the optical layer. Accordingly, our proposed real-time fault localization algorithms are designed to enable fast identification of a faulty segment during normal network operation. The major envisioned application is automatic protection and restoration measures in optical overlays, PONs, and LR-PONs where path redundancy is provided [18]. For such applications, a small number of reflectors that are strategically placed are sufficient to enable online resilience mechanisms. ORM and OTDR are thus complimentary, since the online identification of a faulty segment may subsequently facilitate accurate fault localization through OTDR offline.

ORM’s ability to detect faults within strategic fiber segments or photonic components in real-time leads to increased effectiveness and decreased restoration times compared to legacy fault management techniques. ORM thus introduces an additional all-optical monitoring layer that is capable of targeting a pre-defined subset of the fiber plant (e.g., mission-critical infrastructure), paving the way to a more deterministic Quality-of-Protection (QoP) differentiation that is based at the optical layer [19]. In addition, since ORM does not require OEO, it is expected to trade reduced power consumption for added optical layer processing.

In this work, we lay down the fundamental architectural and algorithmic principles of ORM. We describe the required architectures for ORM in bidirectional and unidirectional transmission media for both asynchronous and synchronous operation. To our knowledge, this is the first time optical-layer monitoring architectures are proposed for generalized access-metro networks, and include the use of in-line reflectors to enhance the visibility of the optical infrastructure. The foremost contribution of this work is the resulting fault localization algorithm that enables the real-time detection and localization of optical-layer faults in generic optical access-metro networks. The design principles of synchronous ORM are also detailed. Our simulations show the ability of ORM to deliver real-time fault notification capabilities in all-optical access-metro network architectures.

The remainder of this article is organized as follows. Section 2 explores architectures of time-domain (TD-), wavelength-domain (WD-), and code-domain (CD-) ORM in all-optical access-metro networks. In Section 3, we propose real-time fault localization algorithms and define fault notification delay as a performance metric. Section 4 introduces delay line (DL) calculation algorithms for synchronous operation and evaluates their performance in terms of DL length. The numerical simulations of Section 5 illustrate the notification delay, scalability, and power budget performance of ORM in selected access-metro scenarios. We conclude in Section 6.

Section snippets

Network architecture

We use the term optical overlay to designate the all-optical transparent portions of future access-metro networks. Prominent examples of optical overlays are not limited to LR-PONs [1] but extend to ring and tree topologies [3]. ORM aims at monitoring optical overlays through mapping any distinct location with a unique and distinguishable property of the reflected signal, such as its time-slot, wavelength, or code.

In ORM, the node hosting the NMS includes an optical monitor that transmits

Real-time fault localization

The method followed by the monitor for real-time detection and localization of a network fault operates in two stages. First, the loss of signal (LOS) emanating from any reflector is detected. Second, LOS events are translated into a unique localization of the failed segment. Fig. 5 shows the flow chart implemented at the RM and leading to an LOS alarm for any given reflector Ri in (a) CW and (b) pulsed operation.

In both cases, after the delay τi corresponding to the round time trip (RTT)

Synchronous ORM design

In pulsed ORM, pulse-synchronous operation is defined as the control of the time position of reflected pulses at the RM. The time-alignment, or synchronization, of pulses is accomplished through the implementation of DLs at the input of each reflector. The length of the individual DLs must be adjusted so as to control the time-domain overlap between the reflected pulses. In TD- and hybrid TD/WD-ORM, reflector DLs must be designed so that no overlap between reflected pulses occurs. In CD-ORM, we

Synchronous ORM

We start by studying a standard PON topology with a 10 km feeder segment and 16 distribution fibers having lengths uniformly distributed between 0 and 10 km. The monitor is located at the Optical Line Terminal (OLT), and N = 17 reflectors are placed at the termination of the feeder fiber and of each distribution fiber. Fig. 9 depicts the described architecture, where Ri and Di denote the reflector and delay line pair used at location i  [1, N].

Fig. 10 shows (a) the average (Lavg) and (b) the maximum (

Conclusions

This work presents a centralized optical-layer monitoring architecture capable of delivering fault notification to future access-metro networks at light speed. Based on the tapping and reflection of a source signal at remote network locations, ORM is introduced in its time, wavelength, and code domains of operation. Fault localization algorithms and notification delay upperbounds are investigated for continuous-wave and pulsed variants of the system. In the pulsed case, synchronous algorithms

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