Optical wdm networks principles and practice management

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Optical Network Management

Alan McGuire

Core Transport, Internet & Data Networks, OP7, B29, Adastral Park, Martlesham Haeth, Suffolk, United Kingdom IP5 3RE Email: alan.mcguire@bt.com

Abstract: Optical networks will play an essential role in meeting the demands for future

communications bandwidth With such large traffic volumes at risk network management is fundamental to the running of such networks To achieve this

on an industrial scale management solutions must be based on an unambiguous framework that describes the entities that need to be managed This chapter describes such a framework and provides a high level summary of how it can be applied to the management of both simple and complex structures Nevertheless management of this new technology is still at a very early stage and considerable effort is required within the industry before the vision of an optical transport network can be fully realized.

1 INTRODUCTION

The accelerating demand for bandwidth fuelled by growth in both theInternet and broadband services represents a major challenge to networkoperators Globally operators are facing up to a shortage of fiber capacity inparts of their networks Whereas this has been most acute in the US wheregrowth in traffic is greatest it is rapidly becoming an international problem.Conventionally an operator would have increased capacity by deployingmore fiber or introducing higher bit rate digital systems Approximatelythree years ago a new option became commercially available, wavelengthdivision multiplexing This technology has emerged from the research labs

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and in a relatively short time frame has become an essential weapon in thetransmission engineer’s arsenal Over 1000 of these systems have now beendeployed globally Depending on the number of channels utilized WDMincreases the capacity of a fiber from between 2.5 – 10 Gbps with existingsingle channel systems to between 40 and 200 Gbps With large trafficvolumes at risk network management is an essential component of suchsystems At the present time the management of these systems is relativelysimple In the next few years we can expect to see the introduction of morecomplex optical systems such as rings and cross-connect meshes There hasbeen a considerable amount of literature published regarding thetransmission characteristics of such networks but very little on the networkmanagement challenge Yet the lack of large scale industrial strengthmanagement systems represents a major barrier to the deployment of theoptical network This is particularly true in the existing competitiveenvironment where automation of many of the tasks involved in running andmaintaining the network will become a necessity to drive down operatingcosts and to provide faster provision of services

In this chapter we shall examine some of the features of existing opticalnetwork management solutions and describe what will be required in order

to manage the optical transport network of the future But first we need tounderstand what it is that we want to manage

2 OPTICAL TRANSPORT NETWORK ARCHITECTURE

In order to manage a communications network it is necessary to describethe entities that need to be managed in a rigorous and unambiguous manner.The integration of technology and function is now so great that it isimpossible to accurately describe what functions a network elementprovides by means of a semi-formal technique Furthermore, theachievement of successful scalable software systems is predicated on a cleardefinition of what is to be managed and its behaviour

Transport networks can be described in terms of layer networks inaccordance with the architectural principles cited in (ITU-TRecommendation G.805, 1995) Each layer network represents a set ofinputs and outputs (or access points) that can be interconnected and thelayer network is characterized by the information that is transported across

it This information, or characteristic information as it is termed, is a signal

of characteristic rate, coding and format Examples of layer networksinclude the VC and VP layers in ATM, the VC-12, VC-4, multiplex and the

regenerator sections in SDH A layer networks topology can be described interms of subnetworks and links between them, as illustrated in Figure 1.

Figure 1: Components of a layer network

A subnetwork can be decomposed into smaller subnetworksinterconnected by links, in other words it is recursive This decompositioncan, if required go from a global network right down to the smallestsubnetwork that is equivalent to a single network elements cross-connectfabric Connectivity in any layer network can be managed at the networklevel in the same manner independently of technology In other words, once

we know how to manage connectivity in one layer we should be able to do itfor all layers Managed objects that represent resources within a layerinclude connection, link, subnetwork, trail, network trail termination points(NW TTPs) and Network Connection Termination Points (NW CTPs).These managed objects represent an abstract view of the resource that can

be manipulated by a network manager

Layer networks have client/server relationships with each other, and inmany cases one server layer may support different types of clients (Figure2) An example is the VC-4 network layer which can support (is the serverof) VC-3, VC-2-nc, VC-12, ATM VP etc Whereas it can support these

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different clients the definition of its characteristic information is separateand distinct In turn the VC-4 layer network is the client of the multiplexsection To get from one layer to another requires some processing to alterthe characteristic information and this is provided by entities known asadaptation and termination functions Adaptation functions providefunctionality such as multiplexing/demultiplexing, frequency justification,timing recovery, alignment and soothing Termination functions providemeans of ensuring signal integrity supervision within a layer by means, forexample, of error detection codes, trail trace identifiers, remote indicatorsand performance monitoring The transfer of validated information istermed a trail

Figure 2: The client/server relationship

TCP – termination connection point, CP – connection point

In addition to providing a network view, the architecture can also beapplied to network elements where adaptation and termination functions arecombined to describe functionality as it can be observed from the inputs andoutputs of the network element The internal structure of the implementation(the equipment design) does not need to be identical to the structure of thefunctional model as long as the external observable behaviour is the same

The management view of a network element is based upon an informationmodel containing managed objects that can be manipulated by amanagement system The definition of a managed object is derived from aspecific part of the functional model For example, generic trail terminationpoint and connection termination point classes are defined generically such

as CTP and TTP, from which technology dependent subclasses such asrsTTP and rsCTP (in SDH) can be developed using the object orientedprinciple of inheritance These managed objects have attributes andbehaviour that is manipulated from a management system; i.e., they cangenerate alarms or change their connectivity to other objects The managedobject effectively hides the implementation of the resource that it representsfrom the management systems and only provides information about aspects

of that resource which are important from a network management view

In essence, element managers manipulate managed objects andrelationships between these managed objects within a network element,whereas network management is concerned with entities such as networkconnections, which may use resources from several network elements Thereader is warned however that this is a very simplified picture of the reality

ITU-T Recommendation G.872 (1995), which is a technology dependentversion of G.805, defines three optical layers, as shown in Figure 3, in theoptical transport network (OTN):

– an optical channel (OCh) layer network that provides end-to-endnetworking of optical channels for transparently conveying digital clientinformation of varying formats (e.g., SDH, PDH and ATM)

– an optical multiplex section (OMS) layer network that providesfunctionality for transport of a multi-wavelength optical signal

– an optical transmission section (OTS) layer network that providesfunctionality for transmission of optical signals on optical media

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Figure 3: The Optical transport network layers

The optical transport network architecture of G.872 is extremely flexibleand supports the following features (not all of which may appear in a singleinstance of a network):

– Unidirectional, bidirectional, and point-to-multipoint connections.– Individual optical channels within a multiplex may support differentclient types

– Cross-connection of optical channels can be accomplished by eitherwavelength assignment or wavelength interchange

– Optical transport network functionality can be integrated with clientfunctionality in the same equipment

– Interworking between equipment with existing single-channel opticalinterfaces (ITU-T Recommendation G.957, 1999) and equipmentcontaining optical transport network functionality

The last two points are significant since they provide network operatorswith a degree of flexibility in the design of their networks, both now and inthe future.

Each of these layer networks provides overhead for the operationsadministration and maintenance of its layer OAM functions includecontinuity and connectivity supervision, defect indications (upstream anddownstream), protection switching protocols and channels for transportingmanagement information Not all of these functions are found in each layer.Initially there was considerable debate with regard to the nature of theoverhead at the optical channel level as some people viewed the opticalchannel as a transparent entity within the network, but such an entity, almost

by definition cannot be managed Instead of optical transparency the opticalchannel provides service transparency, and this is perhaps more central tothe concept of optical networking as described by the ITU The opticalchannel overhead is created within a digital frame that takes the client layersignal in the form of a continuous data stream and adds both the overheadand a forward error correction mechanism for improving system margin.This frame is known as a digital wrapper

The concept of the digital wrapper can be viewed in one of two ways,firstly as an overhead that is carried end-to-end and so may be switched inoptical channel subnetworks However, this requires that the optical channelremain in the optical domain from beginning to end Alternatively atintermediate points part, but not all, of the overhead may be processed Atsuch points the optical channel is regenerated, the FEC is computed and therelevant parts of the overhead are processed There is however no need toobtain access to the payload In contrast to all optical networks, opticaltransport networks utilize the strengths of both electronics and optics toproduce much more scalable networks Nevertheless there is considerabledebate within the standards community as to the appropriate overheads andthe choice of FEC

The following figures (4, 5 and 6) show the relationships between theresources of the transport network and managed objects The actual objectsare subject to change in the standards’ body and the ones provided here arefor indication purposes With these in hand we have the basic resources ofthe optical transport network that need to be managed

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Figure 4: Relationship between equipment resources and managed objects The adaptation and termination functions and their connectivity are managed and controlled via managed objects

Figure 5: Relationship between optical transport resources and optical managed objects The example shown can be considered as an optical channel cross-connect with two ports

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Figure 6: Entity-Relationship between optical managed objects

3 OPTICAL NETWORK MANAGEMENT

ARCHITECTURE

Network management is an activity that allows a network operator toadministrate, plan, provision, install, maintain, monitor and operate atelecommunications network and its services Within the ITU-T, the generalarchitecture of the management network is described in terms of aTelecommunications Management Network (TMN) as described in(M.3010, 1996) The TMN concept can be applied to a variety of scenariosincluding public and private networks, exchanges, digital and analogtransmission systems, ISDN, circuit and cell-based networks, operationssystems, PBX’s and signalling systems It can also be applied to optical

networking Although there are some issues concerning the viability ofTMN, the principles described below are very general and can be applied toany management architecture

The basis for the management of the optical network and its networkelements is the following simple rule:

The management of the optical layers must be separable from its client

layers, or, put another way the management of the optical layers is not dependent on a particular client layer even if they are in the same box.

This rule is of fundamental importance; it ensures that the opticalnetwork can support both existing and future, unforeseen, clients It doesnot necessarily mean that a link in an optical network simultaneouslysupports a wide variety of clients, rather it suggests that regardless of whichprotocol is supported in an instance of an optical network, the architectureand management is the same as in all other instances of an optical network

It allows WDM in a stand-alone platform connected to existing SDHequipment to be managed in the same way as WDM and SDH integratedwithin the same physical platform This principle is also implicit in othertechnologies such as ATM.

An optical management network (OMN) is defined as a subset of a TMNthat is responsible for managing optical network elements An opticalnetwork element, ONE, is that part of a network element that containsentities from one or more OTN layer networks According to this definitionthe functions of an ONE may be contained in a stand-alone physical entity(an equipment) that may or may not support other layer networks Wherenon-OTN layer network entities are in the same equipment they should beconsidered as being administered separately from OTN entities Thisseparation is a logical separation and it indicates that the management ofdifferent technologies is not interdependent, and nor should it be It does notprevent them from both being available on a single element manager albeit

as separate applications