All-Optical Transport Layer and GPON Access forImmersive Communications

Pablo Beltran∗, Andrea Blanco†, Francisco J. Cortes∗ and Amador Pozo∗∗R&D Area, Telnet Redes Inteligentes (Zaragoza, Spain)

{pbeltran, fcortes, apozo}@telnet-ri.es†TECNALIA-Telecom (Zamudio, Spain)

ablanco@robotiker.es

Abstract—Immersive communications, such as high quality 3Dvideoconference, are the best example of what Next GenerationNetworks are capable in the short term. This work is beingcarried out within the VISION project framework, supportedby the Spanish Commitee for Industrial and Technology De-velopment and with the main objective of developing suchvideoconference system. The presented system provides opticaltransport, switching capability and enough flexibility to allocatea QoS stage between access and core network segments. Thearchitectural model relies on the optical circuit switching (OCS)paradigm in a first approach and it is ready to support burstswitching communications, optimizing even more the networkresources. We present the overall architecture of such network,the building blocks of a node, the control plane based on theDRAGON open-source suite and how interfacing with highercontrol layers is solved. Finally, the evolution from OCS toOBS and the enabling technologies which make it possible arediscussed.

Index Terms—Immersive Communications, GMPLS, OCS,OBS, GPON, PON, QoS.

I. INTRODUCTION

Nowadays, optical communications can be classified intothree different paradigms. Optical Circuit Switching (OCS) isthe dominant paradigm in the short term. Its best exponentare dynamic networks. In the near future, OCS will definitelyevolve to Optical Burst Switching (OBS), which is now atdevelopment stage, although the first practical applicationsare arising. With the arrival of real photonic buffering andswitching, OBS will become a purely packet switched net-work, namely namely Optical Packet Switching (OPS).

The VISION Project is the ideal framework for experimen-ting advanced communications. The general purpose of thismultidisciplinary project, financially supported by the SpanishCommittee for Industrial and Technology Development, is todevelop a 3D videoconference system with feeling of presence.There are several workpackages involved, from 3D displayprototyping to gesture recognition, passing by multimedia cod-ification, scene capture and reconstruction, ergonomic reportsand, what is more relevant for us, the design and developmentof a communication network able to support the data rate ofthe videoconference.

Since Generalized Multiprotocol Label Switching (GMPLS)was born, several testbeds have been implemented [1], [2],[3], [4]. Although our proposal could be used in any scenario,

it is clearly oriented to immersive communications, adding aQoS platform in order to give higher priority to specific dataflows, as some of them could be more sensible to electronicbottleneck between access and core segments. The presentedarchitecture also allows to extend GMPLS domain through aGPON access network [5], exploiting its dynamic bandwidthallocation to reserve data circuits.

This work is organized as follows. Section II describes theoverall architecture of the communication network, as wellas the building blocks. Section III is focused on the GMPLScontrol plane and section IV discusses the evolution to OBS.Finally, conclusion is given in section V.

II. OVERALL ARCHITECTURE

The present section shows the overall network architectureand how the network nodes are built.

The optical network designed within the VISION projectmust comply with low latency requirements to enable highquality 3D videoconference with feeling of presence. Lowlatency is achieved by means of all-optical dynamic circuits.Prior to sending videoconference data, such as video, audioor even metadata, an optical circuit must be created. Thevideoconference is started by an end-user, asking to the bordernode for a connection with another end-user, IP identified.

The set-up of a data circuit is as follows. The user connectedto the border node asks to the network controller for a connec-tion. The border node communicates with higher control layersand it receives the command for establishing a circuit betweentwo nodes. Then, control plane procedures begin, described inthe next section. Once the optical circuit is created, the bordernode receives another command to associate a data flow tothis optical circuit, with specific QoS characteristics. Noticethat the bandwidth of the optical circuit can be shared amongseveral data flows, each one with different QoS profiles.

The network core is all-optical, and thus latency is min-imized. Nevertheless, an opto-electronic conversion is stillneeded at the border nodes, in order to offer QoS management,IP routing, and, at the same time, acting as a bridge betweenaccess and core segments. End users are reached by means ofa GPON network, implemented as one of the node modules.Gigabit Passive Optical Network (GPON) foresees Dynamic

9781-4244-3941-6/09/$25.00 c©2009 IEEE

Bandwidth Allocation (DBA), thus enabling Resource ReSer-Vation Protocol Traffic Engineering (RSVP-TE) extensions toreserve bandwidth directly from the optical network unit, closeto the end user.

A. Building blocks

Each node is based on the same mechanical chassis, whichallows the insertion of several kinds of modules. Communica-tion from one module to another is accomplished by means ofa backplane, which routes both data traffic and managementsignals. The control module is required in every configurationas it executes control plane and chassis management pro-cesses. In a basic configuration, a border node includes opticaltransponding, access network and routing with QoS modules.Fig. 1 shows an overview of the nodes. The optical transpond-ing module is dedicated to core optical transport, which takesthe data traffic and adapts it to the fiber, either 10 Gbps or40 Gbps, depending on the link requirements. The currentimplementation of the access network module consist on aGPON Optical Line Termination and the required circuitry tomultiplexing and demultiplexing the traffic between Betweenoptical transponding and access modules there is an electronicstage called routing and QoS module which offers a platformto develop QoS management and the possibility to implementrouting algorithms.

Fig. 1: Overall network architecture and node modules.

This design takes into account GMPLS philosophy. GMPLSwas born in order to merge into one generalized protocol everyswitching technology (Ethernet, TDM, Wavelength Switching,Fiber Switching...), encapsulating each transport protocol intoa higher hierarchy, by means of labels. Label stacking is also akey concept in GMPLS, as several switching technologies canbe concatenated within one node The design modularity ena-bles gradual evolution to OBS. For example, migration couldstart by incorporating a fast optical cross connect, instead of amillisecond range OXC. Burst building can be accomplished

in the routing and QoS platform, using electronic bufferingtechniques.

III. CONTROL PLANE

GMPLS [6] [7] seems to be the dominant technology for thecontrol plane in OCS. However, the module functions shouldbe logically separated to allow gradual evolution to OBS [8].

Fig. 2: DRAGON components.

Our GMPLS [9], [10], [11] implementation is based on theopen source project DRAGON (Dynamic Resource Allocationvia GMPLS Optical Networks) [4]. The DRAGON suite isable to provisioning deterministic services or ”lightpaths”on an interdomain basis and across heterogeneous networktopologies. It includes traffic engineering features and AAAmechanisms. This GMPLS network consists of two differentcommunications planes: control plane and data plane. Thegenuine DRAGON control plane is composed of Unix PCs,where the DRAGON software is running on. GRE/IPIP/IPsectunnels are used by DRAGON in order to transmit OSPF-TEand RSVP-TE signalling across the control plane. On the otherhand, traditional network topology switches are connected viaoptical connections to perform data plane.

DRAGON processes (see Fig. 2) running on these PCsallocate optical circuits, by sending commands to switchesin order to create VLANs (Virtual Local Area Network).DRAGON defines serveral architectural components for thecontrol plane: Network-Aware Resource Broker (NARB) forrouting, path computation, and signaling on an interdomainbasis across topologies. Virtual Label Switch Router , withthe primary purpose of control Ethernet switches via theGMPLS control plane. However, the VLSR has also beenadapted (or can be adapted) to control TDM and every kindof optical switches. VLSR acts as a partner on control planefor a node on data plane, carrying GMPLS functionality out.The End-System Agent (ESA) is software that runs on, oron behalf of, the end-system that terminates the data plane(traffic engineering) link of the provisioned service. This is thesoftware that participates in the GMPLS protocols to enable

Fig. 3: Control plane addressing.

on-demand end-to-end provisioning from end system to endsystem.

The DRAGON implementation within the VISION projecteliminates the need of UNIX PCs for hosting control planeprocesses. Instead of this, an embedded architecture is pro-posed, especifically to host NARB and VLSR. MeetingDRAGON’s requirements, and adaptation from little endianarchitecture to big endian system architecture were the mosthard tasks we carried out.

We have developed an hybrid testbed, taking as start pointthe simulation environment provided in the DRAGON suite,which allows to simulate a virtual control plane network inone PC using several linux-based virtual machines, each onerunning DRAGON processes. The testbed consists of multiplereal embedded systems, virtual machines running DRAGONin virtual mode and the GMPLS emulator module of the N2Xfrom Agilent (see Fig. 3).

The main research focus is the experimental investigation ofthe GMPLS-based control plane and distributed managementplane, based on DRAGON software, and its implementationinto our equipment. Every module can be managed throughXML messages coming from control module.

The results shows that optical circuit reservation and releaseworked out fine, interoperability between DRAGON and N2Xwas successful and DRAGON implementation is a goodapproach to a GMPLS control plane [12] [13]. CurrentlyDRAGON software is being improved in order to implementactual lambda labels and transparent optical circuits allocationwith ROADMs and tunable transponders. A similar task hasbeen already developed [14], adapting DRAGON processes.

IV. EVOLUTION TOWARDS OBS AND ENABLINGTECHNOLOGIES

A flexibility enhancement is required to adapt transportnetworks to the necessities of todays traffic, mostly bursty andwith different characteristics and requirements. Although OPSis the paradigm networks must pursue, significant advancesin optical technology and control plane mechanisms must be

achieved before efficient OPS networks are made feasible. Amedium-term approach is OBS, whose principle is explainedbelow.

In terms of hardware, an OBS network can be seen as anOCS network where the switching speed of the components isfast enough to be considered small when compared to the burstsize. The proposed architecture allows gradual migration fromOCS to OBS. However, control plane migration requires moreresearch [8], as GMPLS is clearly dedicated to data circuitmanagement.

At the edge of OBS networks, optical packets sharinga particular parameter, such as destination or priority, aregrouped into bursts. Once a burst is ready to be transmitted,a control packet is sent to reserve bandwidth and set opticalswitch fabrics along a path for transporting the data containedin the burst. The term reservation is not exact in this case, sincethe resources are not uniquely dedicated to a certain burst.There are several protocols to carry out the setup process,the most significant are TAG (Tell-And-Go) [15] and JET(Just Enough Time) protocol [16]. All of them have commonfeatures: they use a one-way process, and more importantly,bursts can cut-through switches instead of being stored andthen forwarded. OBS provides an optimised network resourcesuse compared to OCS, owing to statistical multiplexing, andprovides high-bandwidth transport fitting the characteristics ofinternet traffic. However, this advantages come at the price ofa higher burst blocking probability and much higher controlcomplexity. Following, the issues appearing in the evolution toOBS, in terms of hardware and control plane, are addressed.

The one-way set-up process reduces vastly the set-up timerelative to OCS. However, since there is no acknowledgementand thus no guarantee that the desired resources will beavailable when the burst reaches, issues like how to dealwith conflict situations and reduce burst dropping must beaddressed. Optical buffering would be the ideal solution toavoid burst discards in case of congestion or conflict situations[17]. So far, the only practical solution to photonic bufferinghas been the use of fiber delay lines, which is not a very

desirable approach owing to its bulkiness, and its slow andfixed response time. An interesting alternative approach tooptical buffers is slow light, i.e. diminishing group velocityof light signals until nearly stopping it. Many approaches toslow light have been proposed: exploiting atomic resonances[18], [19] , using Brillouin [20] and Raman [21] scattering,coupled resonant structures [22], [23], [24], metamaterials [25]and plasmonics [26]. Among all of them, there is one type ofcoupled resonant structure constituting, to our understanding,the most promising solution: photonic crystals [27]. This is alow-power consuming, ultra fast-access approach which canbe fabricated in a very small foot-print and over silicon-on-insulator substrate. These characteristics make photoniccrystals an efficient approach, integrable even with electronics.However, before commercial photonic buffers, offering thecapacities required by current routers, can become a real-ity, much research in reducing losses and dispersion musbe carried out [28]. Considering the challenges of photonicbuffering, a medium-term alternative is to perform wavelengthconversion in conflict situations. Finally, one can simply allowfor burst discards and let the upper layers at the end nodes tosolve the problem, although this is not, obviously, a solvedtask.

Many studies have dealt with the operation, architectureand signaling of OBS nodes [29], [30]. Nevertheless, OBSis still in experimental state. The achievement of faster high-capacity switching matrices is the breakthrough which has toimpulse the transition from OCS to OBS. The duration of aburst can oscillate between 1µs and 1s, and thus switchingtimes in intermediate nodes must be under 1µs. Such lowswitching times are only achievable with a few technologies.Conventional MEMS (Micro Electro-Mechanical Systems), forinstance, present switching times in the range of hundredmicroseconds to a few milliseconds [31]. Very recently, high-speed MEMS operation with switching times of hundredsof nanoseconds have been demonstrated [32], although theyare still at an experimental stage. Other technologies capableto achieve sub-microsecond switching times are electro-opticswitches [33] and semiconductor optical amplifiers (SOAs)[34]. These two technologies, although incredibly fast, presentscalbility issues and, so far, are not suitable for buildinglarge switching fabrics as those required in current routers.Reseaarch must be done in order to achieve large-scale fastswitching fabrics, and thereby to enable the deployment ofefficient OBS networks.

Concerning the control plane, migration from OCS to OBSwill not be trivial. Control complexity is increased in burstswitching, since optical packets are not sent anymore to apredetermined and dedicated lightpath between the originand the destination. Aspects like routing, signalling, QoS,protection and scheduling, must be considered now for everyburst. GMPLS seems to be adecuate also for OBS for tworeasons: GMPLS and OBS share the same philosophy ofseparating control and data planes, and GMPLS tends toreduce network management complexity. However GMPLSis clearly dedicated to data circuit management. Research

efforts are taking place to adapt GMPLS to optical networkswith statistical multiplexing capapabilities [8], [35], but thedefinition of a control plane responding to the complexity ofOBS is not a solved task.

V. CONCLUSION

We have shown in this document the optical networkarchitecture developed within the VISION project, orientedto immersive communications and including a GPON accessnetwork segment. The inclusion of the GPON module enablesfurther development of GMPLS, making possible bandwidthreservation from the end-user connected through the PON.As the opto-electronic conversion between access and coresegments is a latency penalty, making the end-to-end networknot purely optical, our proposal take advantage of it byincorporating a QoS and routing stage, enabling a priorityhierarchy and accommodating data traffic conveniently. Thiselectric stage also enables future evolution to OBS, as burstbuilding can be made there. On the other side, the evolution toOBS is the natural step, considering the typology of networktraffic, to achieve an efficient and fast switching layer. In termsof hardware, the challenges of this evolution are the large-scalefast switching fabrics and the appearance of integrable andfast-access tunable buffers to allow for a good service avoidingburst discards. Relating to the control plane, the adaptation ofGMPLS to statistical multiplexing is still an open issue.

ACKNOWLEDGMENT

This work has been partially supported by the SpanishAdministration organism CDTI, under project CENIT-VISION2007-1007.

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