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The ASON Approach to the Control Plane for Optical NetworksAndrzej JajszczykDepartment o~Telecomm unicutions.AGH University of Science and Technolorn

AI. Mickiewicza 30, 30-OS9 Krak6w, Poland, e-mail: ajszcTk @kt. agh.e du.p IABSTRACTThe paper gives an overview of the autom atically switched optical network (ASON ), concentrating on its con trolplane. Drivers to automatically switched optical network are briefly presented. The ASON architecture, basedon transport, control, and management planes is discussed. Types of connections supported by ASON aredescribed, including permanent, switched, and soft permanent connections. Different kinds of internal andexternal interfaces are presented. Various issues related to the ASON control plane are enumerated.Keyw ords: optical networks, control plane, ASON, ASTN1 . INTRODUCTION

Optical backbone networks, based on SDWSONET and WDM technologies, and designed mainly for voiceapplications, do not match current needs triggered by rapid growth of data traffic. Available resources oftencannot be properly allocated due to inherent inflexibility of manually provisioned large-scale optical networks.This problem may be solved by using a control plane that performs the call and connection control functions inreal time. One of the most promising solutions is based on the concept of automatically switched opticalnetworks.Autom atically switched optical network (ASON) is an optical transport network that has dynamic connectioncapability. This capability is accomplished by using a control plane that pe rforms the call and connection controlfunctions [I]. A related, but more generic, term is automatic switched transport network (AS") [2]. ASTN istechnology inde pendent, i.e., it concerns not on ly optical networks.2. DRIVERS TO ASONas follows:The major features of an automatically switched optical network, expected by netwo rk operators, can be listedfast provisioning,easier network operation,higher network reliability,scalability,simpler planning and design.

Provisioning of optical channels in minutes or even seconds would open new opportunities related to betterresource utilization, creation of new services, such as bandwidth on demand, and a range of traffic engineeringmechan isms. Optical network resources can be auto matically linked to data traffic patterns in client networks.Creation of a separate control plane will significantly impact the network operation and management.Connections can be set up in a multi-vendor and multi-carrier environment without relying on interoperabilitybetween different management systems. Such system will be also relieved from route selection and the need tomanually update the network.topology. This, in turn, will increase scalability which is essential to supportswitched connections on a global scale.New protection and restoration schemes for mesh-type optical transport networks will improve the reliabilityperformance measures offered to customers. Such measures are especially important if we take into account v eryhigh bit data rates switched in optical networks. The control plane rapidly reacting to failures in the opticalnetwork will make it possible to reallocate traffic to reserve paths in real time.Large-scale transport networks are difficult to plan and design. Lack of reliable traffic data, uncertainty offuture service needs predictions, a large variety of available protocols and interfaces make the network designprocess a real challenge. The standardized control plane will enable the reuse of existing protocols and willreduce the need to d evelop operational support systems for configuration manage ment. Moreover, the possibilityto dyn amically allocate optical network resources to changing traffic patterns, will facilitate network planning incontrast to statically configured networks.

This work was suppo rted by the Polish Co mmittee for Scientific Research under grant KBN 4 T1 ID 012 2 50-7803-8343-5/04/$20.00 02 00 4 IEEE

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3. ASON ARCHITECTURE3.1 Functional planesF U I I C ~ ~ O M ~lanes of ASON are shown in Fig. 1[I]. The layered transport plane, referred also to as data plane,represents the functional resources of the network which convey user information between locations. Transfer ofinformation is either bi-directional or unidirectional. The transport plane can also provide transfer of somecontrol and network m anagement information.

Management PlaneUControl Plane

Transoort Plane

Figure 1.Transport, control, and managementplanes in ASONThe control plane p e r f o m the call control and connection control functions. The functions of the ASON controlplane are automated, based on networking intelligence that include automatic discovery, routing and signaling.The management plane performs management functions for the transport plane, the control plane and the systemas a whole, as well as coordinates operation of all the planes [ l ] . These management functions are related tonetwork eleme nts, networks an d services and, usually, they are less automated than those of the control plane.Although each plane is autonomous, some interactions occur because the planes operate on a commonunderlying resource. The following interactions can be distinguished management- ransport interaction, control-transport interaction, and management- ontrol interaction.The management plane operates on an appropriate information model of transport resources. Such a modelreflects an external management view of the equipment. Its managed objects interact with the functional model,represented by G.805 atomic functions, via the management information interfaces. Atomic functions representfunctionality of transport processing functions within network elements. Both the management objects andmanagement information interfaces are physically contained within the transport resource. Control planeoperation appears autonomous to the operation of the management plane, and vice versa, that is both planes areunaware of each others existence, and see only resource behavior. The information presented to the controlplane is similar to that presented to the management plane. In fact, the control plane information overlaps somebut n ot all management information [11.Every control plane component has a set of interfaces used for its monitoring as well as setting policies andaffecting internal behavior. These interfaces are employed by a management system. It should be noted tbat themanagement plane does not access resources via control plane components but only manages these componentsthemselves. The management plane interacts with control plane components by operating on a suitableinformation model.3.2 Optic al connectionsThe following three kinds of connections, differing in connection establishment type, can be distinguished inASON [2]: permanent, switched, and soft permanent.The permanent connection is set up either by a management system or by manual intervention and is alsoreferred to as a provisioned connection. Therefore, such a connection does not require any intervention of thecontrol plane and does not involve automatic routing or signaling. Usually, this is a static connection lasting for arelatively long time, such as months or years. The switched connection is established on demand by thecomm unicating end-points by using routing and sign aling capabilities of the control plane. In t h i s case we refer tos i p l e d connection set up. The switched connection requires a user-network signaling interface (UNI) and its setup may be the responsibility of the end user (the client network) [Z]. Th e soft permanent connection isestablished by specifying two permanent connections at the edge of the network and setting up a switched

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connection between the permanent connections withii the network. The relevant connection establishment isreferred to as a hybrid co nnection set up. In this case no UN1 is needed.The pennan ent con nection is set up by the network operator via the manag ement plane and is an equivalent toa traditional leased line. The switched connections, involving the co ntrol plane, are set up within seconds. Theyenable such a service as bandwidth on demand, The soft permanent connections, triggered by the managementplane, but set up wthn the network by the control plane, may support traffic engineering or dynamic re-establishing of failed connections.3.3 ASON reference model and interfacesAccording to G.8080 ITL-T Recommendation, the interconnection between domains, routing areas, and in somecases, also sets of control components is described in t e r n of reference points. A reference point represents acollection of services provided via interfaces on one or more pairs of components. The exchange of informationacross these reference points is described by the multiple abstract interfaces between control components. Thephysical interconnection is provided by one or more of these interfaces. A physical interface is provided bymapping an abstract interface to a protocol.A logical view of ASON architecture is shown in Fig. 2. Along with the transport, control, and managementplanes, a variety of ASON and non-ASO N interfaces is shown .

CC: Connection Controller \ e t w o r vCCI: Connection Control InterfaceE-"I: External Network-Network InterfaceI-"I: Internal Network-Network InterfaceNE: Network ElementNMI-A: Network Management Interface -ASON control planeNMI-T: Network Management Interface-Transport planeNMS: Network Management SystemPI : Physical InterfaceUNI: User-Network InterfaceX: Interface between management systems

Figure 2. Logical view of ASON architecture.

4. CONTROL PLANE REQUIREMENTSThe control plane is a set of communicating entities that are responsible for setup of end-to-end connections,their release, and maintenance. These capabilities are supported by signaling.The control plane in ASON is responsible for the call and con nection co ntrol. A call is an association betweenendpoints that support an instance of service, while a connection is a transport entity capable of transfeninginformation between its inputs and ou tputs. An important feature ofASON is separation of call and connectioncontrol functions. This separation allows to reduce call control information at intermediate connection controlnodes, since the call control is provided o nly at UN1 (an ingress port) and E-NN I (network boundaries), and notat I-NNI.

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The prin cipal functions of the control plane to supp ort the call and connection co ntrol are as follows:automatic network neighbor, resource and service discovery,address assignment and resolution,signaling,routing.A v ariety of requirements related to the control plane can be listed as follows [I]:.....fast and ieliab le call setup,ability to co ntrol admission of calls and co nnections,reliability, scalability, and efficiency of the contro l plane ,support for transport network survivability,support of various transport network technologies,support for sup plementary services,applicability regardless of the parhcular choice of control protocols,applicability regardless of the distribution of conn ection control functions,suppo rt for multi-homing,support of diverse connections,uossibilitvof division into d omains and routine areas.The call admission function at the originating node is respon sible for authentication of the user and ch eckingthe requested service parameters against a Service L evel Specification. These parameters may be renegotiated, ifnecessa ry. At the termina ting node, the call adm ission function has to check if the called user is entitled to ac cept

the call. The conn ection admission control checks if there are sufficient resources to admit a connection. In thecase of circuit switched networks, availability of physical resources (e.g., wavelengths, TD M channels, etc.) ischecked. For packet networks, the connection admission function has to ensure that a dmission of a new packetflow will not jeopardize quality of service contracts of the existing connections.The control plane has to be reliable, scalable and efficient. The reliability means that even in case of failuresof the control plane the existing transport connections are maintained. The separation of the call and connectioncontrol facilitates support for transport network survivability. T he impact.of failures affecting connections in thetransport plane can be m inimized by using ap propriate protection and restoration schemes. During the relevantprocedures the associated calls are maintained.Along with bearer services, such as SONETBDH, OTN, Ethemet, and others, the control plane shouldsupport supplemen tary services independent of the bearer service. An example of such a service is a closed usergroup [2]. ASON defines functionality of the control plane independen t of a particular choice of controlprotocols. Therefore, a variety of such protocols can he used in real networks, including those from the MPLSfamily, like RSVP-TE, or coming from the ATM world, like PNNI. Elementary control functions can bepackaged differently by different vendors. They can have also diverse approaches concerning eithercentralization or distribution of control functions.The control plane h a s to support multi-homing, allowing multiple links between a user and o ne or moretransport networks. Such an approach facilitates load balancing and resilience. A user can request diverselyrouted connections, i.e., connections using disjoint sets of network resources. The control plane can besubdivided into domains that match the administrative domains of the network.5. CONCLUSIONThe strength of the ASON concept is the fact that it employs well developed concepts of the IF world, such asautomatic discovery or routing, and allows reuse of some of its protocols, in the circuit switched environment ofoptical networks. Implementation of ASON enables fast provisioning, easier network operation, increasesnetwo rk reliability and sca lability as well as simp lifies plann ing and des ign. This, in turn, may be translate d intodirect benefits to operato rs and their clients.REFERENCES[ I][2]

ITU-T RecommendationG.8080N.304,Architecture f o r the Automatically Switched Optical Networks,Nov. 2001, andAmendment 1. March 2003.ITU-T Recommendation G.807N.1302, Requirements fo r Automatic Transport Networks (ASTN),July2001.