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A Full Featured and Lightweight Controlfor Optical Packet Metro Networks

Lida Sadeghioon, Annie Gravey, Bogdan Uscumlic, Philippe Gravey, Michel Morvan

Abstract—Metro networks support increasing traffic vol-umes and evolving traffic profiles. Revisiting metro networksarchitecture, optical packet rings with POADM features areproposed in order to support both optical transparencyand sub-wavelength granularity. A MAC structure for multi-ring POADM networks is proposed, that can support multi-protocol encapsulation and provides the support of differ-entiated QoS and differentiated protection on a per flowbasis. Unicast and multicast flows are efficiently transportedbetween stations, with a lightweight control within each sta-tion. An opportunistic insertion process associated with anappropriate scheduling process is shown to ensure transportnetwork QoS levels. Simple models are provided to assessthe transfer performance on the POADM ring. Per-flowprotection mechanisms are proposed and their efficiencyis assessed. Dimensioning costs are derived, that take intoaccount the number of used wavelengths and the number offixed receivers required to support a given traffic matrix.Finally, the paper shows that POADM rings can be usedto directly support Metro Ethernet services and thus allowcollapsing stack of transport network layers.

I. INTRODUCTION

S everal forecasts have emphasized that distribu-tion/aggregation networks, also called Metro Area Net-

works (MAN), are particularly impacted by traffic evolution.They have to support a sharp increase in utilization, togetherwith a strong demand on functions (such as video contentdistribution) that were not used by traditional telephonyservices.

Future MANs should fulfill several requirements. Flexi-bility, facilitating a quick adaptation to varying traffic de-mands, in terms of supported protocols and traffic profiles, ismandatory. An efficient support of both fine granularity andlarge volumes of traffic demands, for uplink and downlinktraffics, is necessary, as MANs have to interconnect bothlow activity nodes (e.g. small DSLAMs) and high activitynodes (e.g. data centers). MANs should provide methods forisolating different clients’ flows, together with an excellentQoS, including high reliability and fast protection. Lastly,energy efficiency is a must, in order to limit OperationalExpenditures (OPEX).

The above requirements seem difficult to be achieved si-multaneously. MAN networks currently rely on optical links;in opaque networks, nodes operate in the electronic layer(e.g. Ethernet), while transparent networks are fully opticalbetween source and destination. Currently deployed trans-parent networks present a coarse (wavelength) granularity,which precludes an efficient MAN usage. As contentionavoidance is easily implemented in opaque networks (thanksto electronic buffers), the current MANs are often Ethernet

Annie Gravey, Philippe Gravey and Michel Morvan are with Tele-com Bretagne, France. e-mail: name.lastname@telecom-bretagne.eu

Bogdan Uscumlic is with Alcatel Lucent Bell Labs, France,bogdan.uscumlic@alcatel-lucent.com

Lida Sadeghioon is currently with Orange Labs, France,lida.mollazadehsadeghioon@orange.com

rings implementing specific protection protocols [1], [2]. Themain issues with these networks are their high-energy con-sumption on the one hand and their fixed Ethernet packetgranularity that is adequate for Metro Access areas, but istoo fine for Metro Core areas.

Optical packet/burst switching (OPS/OBS) have been formany years considered as potential options to combine sub-wavelength granularity and optical transparency. However,technological immaturity and doubts on their ability toachieve a high multiplexing gain together with a QoS similarto the one provided by electronic switching have preventedtheir use in operational networks. The key issue is con-tention avoidance, as optical buffering is not a currentlyviable option. The present paper, which is an extendedversion of [3], shows that a specific OPS architecture, namelya Packet Optical Add and Drop Multiplexing (POADM) ringintroduced initially in [4], can meet most of these require-ments. It recaps and completes results presented in variousconferences, including an exhaustive description of the MACspecification and operation and also presents how this MACenables collapsing the protocol stack currently applied inmetro networks, while being compliant with a centralizednetwork control.

The paper is organized as follows: the next two Sectionspresent the POADM ring architecture and existing state ofthe art, while Section IV fully describes an original MediumAccess Control (MAC) for multi-ring POADM networks. Sec-tion V addresses protection issues and shows how a POADMarchitecture can provide differentiated, per flow protection.Next, Section VI presents network dimensioning as a manda-tory step for providing simple operation and transport net-work level QoS. This is illustrated by some quantitativeevaluation results regarding delivered QoS. Section VII re-lates network dimensioning to protection issues, highlightingvarious options leading to different network costs. Lastly, weconclude, showing how POADM rings can be used to directlysupport Metro Ethernet services and thus allow collapsingstack of transport network layers.

II. POADM RING ARCHITECTURE

We consider bi-directional rings. In each direction, aPOADM node is connected by a time slotted, WDM, unidirec-tional ring with a single out-of-band control channel using adedicated wavelength; the other wavelengths can be used asdata channels (up to 40 or 80 depending on the WDM grid).

Data channels are switched all optically whereas the con-trol channel packets experience O/E/O conversion at eachnode in order to be processed. The data and control channelsare synchronized and are time slotted; the bitrate of thecontrol channel (e.g. 2.5 or 10 Gbits) may be lower thanthe bitrate of the data channels (e.g. 10, 40 or 100 Gbits).Fig. 1 shows the node structure equipped with four keycomponents:

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• Tunable optical transmitter• Optical multiplexer and de-multiplexer• Semi-conductor Optical Amplifiers (SOAs) as optical fast

gates• One or multiple receivers

Fig. 1. Packet Optical Add/Drop Multiplexer structure [5]

The incoming WDM line is de-multiplexed passing throughthe optical DEMUX. The SOA gates ’ON’ or ’OFF’ statesdetermine if the packets pass through or are suppressed. TheSOA gate states are reconfigured according to the contentof control packet. Only the packets that are destined to thenode are received and processed in the electronic layer whilethe others are passed transparently. Each node can receiveon one, or several fixed wavelength receiver(s).

A single fast tunable transmitter can add a single packetper time slot; any empty wavelength can be used by this“opportunistic” insertion process, as long as the destinationof the selected PDU can receive traffic on the selected datachannel.

As stated in [4], a unidirectional POADM ring providesoptical transparency and sub-wavelength granularity. Thepresent paper contributes by showing how coupled unidirec-tional rings, in which each POADM node operates in bothdirections, can provide protection. Interconnected rings canbe used to cover large geographical areas, or to realize theclassical hierarchical MAN architecture that presents bothmetro-access and metro-core aggregation/distribution zones.This flexibility makes POADM an attractive solution formetropolitan area networks. However, some specific MACfeatures are necessary to support multi-protocol transport,QoS and protection support. This is addressed in Section IV.

III. EXISTING MACS FOR OPTICAL PACKET RINGS

Apart from POADM, many other optical packet/burstswitching rings have been proposed in previous years, as asolution for metropolitan area network.

A. Comparison of POADM with other optical packet/burstrings

DBORN [6] is a bidirectional optical packet switching ringwith slotted or un-slotted operation, where all the trafficpasses through a special node, called “hub”. In DBORN,the packet removal is done at the hub node, i.e. “desti-nation stripping” is not allowed. DBORN is characterizedwith an out-of-band control channel, and fixed receivers. InDAVID [7] and in a variant of RINGO [8], the packets travelthe entire ring before being removed (i.e. “source stripping”).DAVID exists in passive and active architectures, which

support, or not, the separation of upstream and downstreamwavelengths. The sources in DAVID can be equipped eitherwith fixed or a single tunable transmitter, while the receiversare fixed. RINGO sets one tunable laser and fixed arrayof receivers per node. It exists in both unidirectional andbidirectional versions. Another concurrent optical packetswitching solution to POADM is HORNET [9]: this tech-nology is defined for bidirectional slotted rings, and spatialreuse is leveraged. A HORNET node has tunable lasers, andthe number of transmitters and receivers per node is fixedto dW/Ne, where N is the ring size, and W is the numberof wavelengths in the network.

Regarding the optical burst switching solutions, the impor-tant examples are: TWIN [10], OBTN [11] and OPST [12].• TWIN is Time-Domain Wavelength Interleaved Network

proposed by Bell Labs that is characterized by a passivenetwork core, and active edge nodes which perform thescheduling of optical bursts. The network operation istime-slotted, as in POADM, but a single wavelength isallocated to each destination. TWIN and POADM havebeen compared in terms of dimensioning cost in [13].

• OBTN stands for “Optical Burst Transport Network”,and is a solution proposed by Huawei. In OBTN, eachsource is allocated a separate wavelength for the emis-sion (thus preventing wavelength sharing), and thenetwork is synchronized. Sources employ fixed-tunedlasers, while destinations use burst mode receivers thatcan receive on desired wavelengths.

• OPST is an optical packet switching network proposedby Intune. In this network, each destination has adedicated wavelength, and the network is asynchronous,which makes this network different to POADM. InOPST, the sources employ fast tunable lasers, and re-ceivers are fixed.

B. Various MAC optionsMACs for WDM OPS networks belong to several sub

categories depending on the technologies and topologies ofthe networks. For example pre-allocation and tell-and-gomethods with wavelength reservation mechanism per des-tination or per source are implemented in hub-and-Spokeand in tree based topologies [10].

In OPS rings, when and how the packet can be insertedto avoid collision is decided after examining the channel ortime slots status [14], but different methods are proposed forslotted and un-slotted access schemes.

A typical method for un-slotted OPS rings is token based.It operates by assigning to each channel a token that passesthrough the nodes on the control channel. When a token ar-rives, the node is allowed to transmit and can hold the tokentill it sends all its packets; as a result timing between tokenarrivals and releases should be controlled in order to avoidunfair access to bandwidth, which is rather complex [15]compared to methods used in slotted rings networks.

The Carrier Sense Multiple access with collision Avoidance(CSMA/CA) method is an early method proposed to detectan idle channel. For example, in HORNET, each node checksevery slot availability status by sensing the subcarrier multi-plexed (SCM) [9]. However MACs based on CSMA/CA causebandwidth fragmentation and unfair access to bandwidthunless specific controls are applied, as in [16] where adynamic intelligent algorithm is proposed to mitigate the

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bandwidth fragmentation problem by spacing local packettransmission epochs in order to release usable bandwidthfor downstream nodes. Such methods rely on complex pernode processes.

In time slotted OPS rings, the usual method to detect anidle channel relies on control channel information which ispart either of the time slot that is carried in-band [17] or ofa separate, dedicated, control channel as in [18].

The support of multicast service in [17] is either addressedin the simple case of a single ring with source strip method,or by constructing multicast trees with optical cross connects(OXCs). The first approach presents scalability issues relatedto the optical power budget, while the second is quite com-plex and potentially expensive.

In [18], the proposed MAC supports MPLS labels with adedicated control channel. The fairness issue is addressedby a credit based system, previously investigated in Multi-MetaRing [19], imposing quota per channel per node andrenewing them by control signal. MPLS labels are carriedin the MAC layer PDU and each node has to go throughan explicit label extraction/insertion process which adds tothe complexity of the data plane. Multicast is supported asin [17], with multicast trees.

IV. MAC STRUCTURE

A MAC protocol used to regulate the access to the sharedoptical fiber has to fulfill the next generation metro networkrequirements:• Multiple levels of granularity• Multi-protocol and multi-class• Unicast and multicast• Transport network level grade of service, e.g. low la-

tency, 99.999 availability, etc.Existing MACs for optical packet rings often rely on ded-

icating wavelengths either per destination or per source; inother cases they rely on specific hub nodes to control theaccess to the medium. A POADM ring as defined in [4] sharesthe wavelengths between all nodes and does not rely on hubs.

The MAC has to specify how client data is coded intooptical packets and how it is transported over the POADMnetwork. Taking into account the above list of requirements,and the analysis of already existing MACs (which oftenpresent complex mechanisms), we designed a MAC dividedinto two sub-layers: adaptation, transport.

A. MAC principles

Client protocols (IP, MPLS, Ethernet, SONET/SDH, etc.)present different encapsulation frame formats and may re-quest different transport modes. An adaptation sub-layershould encapsulate all client data in a holistic manner, whilesupporting the various requirements. The operation of theMAC adaptation layer is illustrated in Fig. 2.

Client Data Units are first encapsulated, adding POADMspecific information; this is described in Section IV-A1. Clientflows are then multiplexed, using possibly segmentation orpadding, in order to obtain fixed size bursts which shall behandled by the transport sub-layer; this is detailed in Sec-tion IV-A2, adding control information that shall be carriedin the control channel as explained in Section IV-A3. Lastly,Section IV-A4 explains how control information is used tocontrol switching in POADM nodes.

Fig. 2. POADM MAC layer structure and packet encapsulation;client data is encapsulated in Service Data Units (SDUs), which areaggregated into Protocol Data Units (PDUs).

1) Service Data Units (SDUs): A POADM node can receivevarious client data packets, carried over multiple protocols.Relevant control information carried in the client headerssuch as addressing, transport type, QoS class, etc. is ex-tracted and analyzed to identify the appropriate client flow,consisting in a Forwarding Equivalence Class (FEC). Thisflow shall be identified by a SDU level label carried in aheader associated with the cliend data. The SDU headeralso carries segmentation and reassembly (SAR) informationif necessary and SDU length in order to enable burst de-multiplexing at reception. The SDU level label can be usedto implicitly identify the client layer, or e.g. the VirtualPrivate Network (VPN), or the multicast flow to which theclient packet belongs; this is used for correctly and efficientlyaddressing de-multiplexed bursts at reception.

SDU levels labels are also used to identify how clientflows are to be multiplexed; priority mechanisms can be usede.g. to speed up the transport of client flows with stringentlatency requirements. At ring interconnection nodes, SDUflows can be groomed into new bursts without accessing theclient layers.

2) Protocol Data Units (PDUs): Fixed size PDUs are cre-ated by adding specific information regarding the physicallayer, framing, synchronizing, error correction and padding.PDUs can be considered as containers that group multipleSDUs with the same characteristics. Scheduling rules areused to select which PDU, if any, is inserted in each timeslot.

3) PDU control information: Since all the nodes can poten-tially receive traffic on all the wavelengths, we need a meanto recognise each flow per time-slot and per wavelength. Thisis done thanks to packet level information carried by thecontrol packet. Control packets are electronically processedat each node, in order to identify which optical packets needto be received and whether a new packet can be insertedon the ring. We adopt a label switching concept, similarto the one used in MPLS, as it is the most current andefficient technology for packet switched networks [20]. Anode identifies which PDUs to receive and extract using PDUlevel labels in the control packet characterizing each PDUcarried in a given time-slot. It is thus not necessary to carry

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an explicit destination address.4) Label-based operation: A classical label based operation

is implemented in each POADM node thanks to multiplelocal tables shown in Fig. 3:• the flow specifications (FEC) such as flow identifications,

type, QoS and path (i.e list of nodes that the flowpasses till it reaches the destination); there is such aspecification only for the flows inserted or extracted bythe node; the CTS and STP are used to bind labels toflows;

• the SIT maps PDU level label and protection levelinformation (which use shall be described in Section V)to the action (i.e. operation) that the node takes on thePDU;

• The Protection Information Table (PIT) is used to pro-vide per flow protection (see Section V)

Fig. 3. Node’s Local Information Database

At each time slot, the node processes the control packet inorder to identify which action(s) to take, based on the PDU-level labels characterizing each PDU. If the label is not listedin the SIT, the PDU is passed transparently. Otherwise, theSIT identifies which of the 4 following actions is to be taken:

1) Receive-Erase: The node receives the PDU and stripsit off.

2) Receive-Pass: The node receives the PDU but does noterase it from the ring.

3) Erase: The node erases the PDU without receiving it.4) Receive-Insert: The node extract the PDU and mod-

ifies the label stack (e.g pops the outer label), andretransmits the PDU into the network.

The first three actions occur when the node is either asource or a destination for the flow within a single ring. Thelast one occurs in a hub with POADM interfaces on two ormore nodes. Inter-ring traffic corresponds to optical PDUsthat a hub receives on one ring and inserts on the other.

The local databases and associated tables only containinformation regarding the labeled flows that are handled inthe electronic layer by the POAM node. The FEC, CTS, STPand FIT tables can be static (i.e. configured manually or bythe management plane) or dynamic (i.e. configured by controlmessages carried in control PDUs). They are updated if thereis a new entry or a change within the network by a centralor distributed control/management plane.

When a PDU label does not figure in the SIT, the flowpasses transparently through the optical layer. This is very

positive in terms of configuration and complexity, but mayraise issues, e.g. if a label is errored, as no node would thenever erase the PDU. This can be solved by a simple Time-To-Live field to be updated every time a PDU passes a POADMnode, even if it is passed transparently, as the control infor-mation for each data PDU is electronically handled in everyPOADM node.

Ring Interconnection is illustrated on a simple example inFig. 4. Two flows are sent from the backbone to respectivelyN3 and N4 located on a metro-access ring. Depending on thesize of the packets and on whether they have to pass the hubto reach their destinations, node N1 can groom them into onePDU with PDU Level label L20, identifying the hub as anintermediate destination. The final destination informationis coded in the SDU level labels L3 and L4. The hub derivesfrom its SIT that it has to send them in different PDUs to bereceived respectively by N3 and N4; we assume that this isdone by labelling this new PDUs with L3 and L4 (other labelvalues could be specified in the SIT). This method creates aflexible grooming tunnel in a multi-ring topology.

Fig. 4. Inter-Ring traffic grooming example

B. Control information

As explained in Section IV-A3, part of the control infor-mation is contained in SDU and PDU headers, and is usedwhen the data is managed in the electronic layer.

The other part of the control information is carried in theout-of-band control channel. It corresponds to the criticalinformation that each node requires in order to properlytreat each time slot. Thanks to this control channel, datathat just transits through a node is passed transparently inthe optical layer.

The structure and the content of the control packet de-termine the efficiency of control information processing andthe switch reaction time. A control packet carries both globalinformation and information relative to each data channel.Fig. 5 shows the structure of the control packet. In the follow-ing, we offer a general view of the control packet content thatmakes an integrated multi-service, multi-functional MAClayer for suitable for POADM WDM metro rings.

The global information used for configuration and OAMare carried in the global control fields:

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Fig. 5. Control packet structure

• Framing and synchronising information (used for theoperation of the physical layer);

• Length: control packet length (in order to fill the controlpacket, it may be necessary to add padding bits whichcan easily be discarded thanks to the Length field);

• Global information: includes global OAM notificationmessages (the use of these fields is described in Sec-tion V);

• Error correction information, which protects the com-plete control PDU contents.

Data channel specific information fields are:• Occupancy flag (OCC): indicates whether the data chan-

nel is free or occupied during this time slot;• Service type: shows to which class of service (Guaran-

teed or Best Effort) the PDU belongs;• Flow type: unicast or multicast;• Reservation flag : used for Best Effort support (see

Section IV-D);• Source and Destination interface IDs relative to the

reservation, if present (see Section IV-D);• Label stack: a single label is used to identify a flow,

a label stack can be used e.g. in a multi-ring POADMnetwork;

• Flow version: indicates whether the flow is original or abackup (see Section V);

• Protection method: In case of failure, it specifies themethod of extraction and behavior of the switch duringfailure recovery time (see Section V).

• TTL: Time-To-Live, is decremented by one every time acontrol packet is read by a POADM node. When TTL=0,the data PDU is erased, even if this is not configured inthe SIT (see Section IV-A4).

The maximum size for control channel packets consideringthree packet lengths and two bit rates has been investigatedin [20]. In the worst case (control channel operated at 2.5Gbps and slot time equal to 10 microseconds), the label stackdepth can be at least equal to 5 for a label size of 20 bits(as in MPLS). This allows covering all practical scenarioscurrently envisaged. This is also an upperbound on the labelstack depth in operational MPLS backbone networks.

C. Unicast and Multicast MAC Operation for Guaranteedtraffic

The focus here is on the insertion-extraction processesof the Guaranteed unicast and multicast traffic flows. It isassumed that the network is properly dimensioned, based

on a given service level agreement profile for a given traf-fic matrix. The insertion/extraction process for Best Efforttraffic (in excess of the agreed traffic matrix) is addressed inSection IV-D.

1) Guaranteed Unicast Traffic: We assume here thatthe unicast flows are original (i.e. not backup flows). Theinsertion-extraction process of the backup unicast flows de-pends on the protection method in use, which is explainedin Section V.• Insertion: As specified in [21], the insertion method

for Guaranteed unicast traffic is opportunistic, based onsimple “empty slot” policy. A PDU carrying Guaranteedtraffic is transmitted as soon as an empty time sloton a suitable wavelength is available. We assume inthe dimensioning process that the original flow choosesthe shortest distance direction on the bidirectional ringin normal operation mode (i.e. no failure). Since theamount of Guaranteed traffic is known, a given nodecannot starve or even degrade the QoS offered to theother nodes as long as the transmitted traffic conformsto the traffic matrix (i.e. negotiated SLA).

• Extraction: Extraction is performed when the SIT rec-ognizes the label of a flow that should be received anderased by the POADM node; this is the traditional “des-tination stripping” strategy that allows “spatial reuse”and thus increases network capacity. Indeed, spatialreuse characterizes the possibility for the network tosupport simultaneously concurrent transmissions onnon-overlapping portions of the same wavelength chan-nel. If the label is not listed in the SIT, the packet ispassed transparently.

2) Guaranteed Multicast Traffic: Multicast traffic is in-serted by one node and is usually received by more than onedestination nodes. Let us define a Multicast Service Point(MSP) as a node used by a multicast source and MulticastDrop off point (MUD) as a node where the multicast datais erased. For a given multicast flow, both MSP and MUDare part of a POAMD rings as illustrated in Fig. 6. In anintermediate node (located between the MSP and the MUD),the multicast extraction method relies on a “Receive andPass” action, where the node just copies the PDU, whichcontinues on its path.

In the following, we introduce unidirectional and bidi-rectional multicast services that are slightly different ininsertion and extraction methods:• Unidirectional Multicast Service: The MSP inserts

the multicast flow in one direction of the ring on a datachannel. The flow loops through the ring, comes back tothe MSP and the “source stripping” method is used todrop the flow off the ring. In this case, a single node (alsocalled source-stripped point) is both MSP and MUD.

• Bidirectional Multicast Service: The MSP inserts amulticast flow in both directions of the POADM ringwith the same labels in the control packets associated toeach direction. The drop-off point MUD in this approachis calculated to be approximately at equal distance inboth, directions from MSP. At MUD the flow is either“Received and Erased” if the MUD node receives themulticat flow, or just ’Erased’ if the MUD does notrequest the multicast flow.

The usefulness of implementing both unidirectional andbidirectional multicast can be seen on Fig. 6. The Multicast

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Fig. 6. Multicast service on a POADM multi-Ring deployingMulticast Drop off Point (MUD) method in the main ring and thesource-stripped point in the side rings

flows L1 and L2 are inserted using the bidirectional method,in the core ring, whereas the unidirectional method is usedto distributed L1 and L2 in their respective access rings.Since the core ring is the feeder of the multicast flows tothe access rings, and potentially spans a large area, it isworth specializing a node to play the role of MUD as themean latency is roughly half in the bidirectional approachcomparing to the unidirectional one. On the other hand, inthe smaller access rings a simpler configuration with thehub erasing unidirectional multicast traffic is has initiallyinserted is more efficient.

D. Support of Best Effort trafficAs mentioned earlier, unicast and multicast insertion-

extraction methods were defined for Guaranteed (G) trafficon a dimensioned network for an static traffic matrix. Theinsertion mode for G traffic is opportunistic: there is noexplicit reservation of slots, and the QoS delivered to Gtraffic is controlled as long as the sources are compliant withthe specified traffic matrix (see Section VI).

Paradoxically, we proposed in [22] to support Best Effort(BE) traffic using a reservation based method : in order totransmit a BE packet, a source would need to explicitlyreserve a slot. A simple analogy clarifies this apparentparadox: when an airplane is fully booked, there is a prioritylist and a waiting list. Members of the waiting list (BE)are served only if there are still some seats available whenall priority (G) customers have been served; there is noguarantee to serve the customers in the waiting list.

In the present context, a reservation sent by a node forBE traffic corresponds to entering the waiting list. If thereservation is not discarded, its return to the source nodeindicates that resources are available for a BE packet.

The reservation based method relies on the Res Flag inFig. 5, together with the Source and Destination node IDs

which indicate that the source node attempts to transmita BE packet to the destination node. All the nodes in thenetwork can add or drop reservations under the followingconditions:• Add reservation: A node can make one reservation

per time slot, the reservation indicates the source anddestination of the BE packet.

• Drop reservation:– At any intermediate node the reservations can be

dropped to protect the G traffic or to enforce a fairaccess to resources available for BE traffic;

– The destination node can drop the reservation if itconsiders that it can get congested.

When handling both types of packets, the POAM nodeshould go through the following steps:

1) Receive any G packets on one or more channels andrelease corresponding slots;

2) Insert a G packet if possible;3) If no G packet can be inserted, insert a BE packet if

possible (i.e. on a data channel that carries an appropri-ate reservation); any reservation that cannot be usedis erased by the node;

4) analyze the reservations on all data channels (if any)and drop them if necessary; insert a new reservation ifnecessary and possible.

We have shown in [22] that the reservation-based schemeallows to protect G traffic (the QoS of the G traffic is withinthe design limits) and to use part of, but not all, availableresources.

V. RESILIENT POADM BIDIRECTIONAL SINGLE RING

In the present Section, we propose protection mechanismsthat can alleviate performance degradation in case of a singlefailure (a ring network cannot be protected against morethan one failure).

Relying on a label based transport allows to consider dif-ferent protection mechanisms, and differentiated protectionclasses [23]:• Premium Traffic: traffic loss is (nearly or totally) not

tolerated, and the service must be constantly availableeven in case of failure.

• Regular Traffic: some traffic loss and limited serviceunavailability is tolerated in case of failure.

• Unprotected: no protection in case of failure.The network operator could support one or several protec-

tion classes. In the latter case, the protection class could beselected e.g. during SLA negotiation.

There are three main steps to provide protection:1) failure notification and localization: this triggers

the use of backup traffic on the POADM ring;2) failure recovery: backup traffic has replaced the orig-

inal traffic during the failure;3) restoration: at the end of the failure, the system

reverses to normal operation relying on original traffic.A node adjacent to a failure immediately detects it as it

stops receiving valid control PDUs. The first step is thenrealized thanks to a general failure localization method: incase of failure the two end points of the failure generatenotification failure messages and send them towards theoperational directions. The notification failure message car-ries node ID node, interface ID and other information such

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as time of the failure; this is carried in the field “GlobalInformation OAM” of the control packet (see Fig. 5).

Fig. 7. Protection Information Table content within the localInformation Database (node D4)

The other nodes rely on their local databases to identifywhich flows are affected by the failure. Fig. 7 shows the pro-tection information table (PIT) that is part of the nodes’ localinformation database. When receiving a global notificationfailure, each node uses the PIT to identify which flows areaffected by the failure thanks to the PIT. As the SIT, thePIT is generated when the switch is configured; it is updatedif there is any change in the services it offers as source ordestination.

The nodes adjacent to the failure also trigger the initiationof the restoration process. The restoration procedure beginsas soon as the 2 nodes at both ends of the failure startreceiving control PDUs again. They can thus send out aglobal “OAM RESTORE” notification message in the controlPDUs.

The second and third steps depend on the type of pro-tection which is applied; Regular and Premium protectionmethods are described in the following.

A. Regular (1:1) protectionIn the nominal operation mode (i.e. no failure), a single

copy of each PDU is sent on the shortest path to the desti-nation. It is received and erased at destination which enablesspatial reuse.

The reconfiguration process is as follows (see Fig. 8):• Reconfiguration of extraction process: the receiving node

has to receive (and erase) PDU on the interface to thelongest path.

• Reconfiguration of insertion process: The source node ofan affected regular flow stops sending the PDUs of theflow on the shortest path and sends them in the oppositedirection to the destination.

The restoration process for Regular flow that were affectedby the failure consists in simply reverting to nominal behav-ior when receiving the global “OAM RESTORE” notificationmessage in the control PDUs.

B. Premium (1+1) protectionIn the nominal operation mode (i.e. no detected failure),

two copies of each PDU of the flow are sent on both directionsof the ring; the Flow-Version field in the control packet(see Fig. 5) is set either to Original for the working pathand to Backup for the protection path, which allows thedestination to differentiate between received PDUs. Thereare thus an “original” flow (sent on the shortest path todestination) and a “backup” flow (sent on the longest path),

with the same label. The destination node receives anderases the Original PDUs and erases (but does not receive)the Backup PDUs.

The Failure recovery process for an affected flow is asfollows (see Fig. 9):• Reconfiguration of extraction process: The node starts

receiving all the PDUs marked with both Flow-Version=Backup and Flow-Version=Original.

• Reconfiguration of insertion process: The source nodestops sending the affected flow on the shortest path; itstarts marking the PDUs sent on the longest path asFlow-Version=Original.

When receiving the global “OAM RESTORE” notificationmessage in the control PDUs, the source nodes of affectedflows reverse to their nominal behaviors.

C. Performance of the Protection Schemes

Transient performance degradation may affect Regularand Premium flows in case of failure. By computing bothrecovery and restoration times, we identify below the poten-tial degradations (packet loss, packet duplication and packetdisordering) due to failures.

In order to illustrate typical situations, consider a bidirec-tional ring with n nodes sending unicast traffic, and let TRbe the total cycle time (identical in both directions). DenoteT si,j and T li,j be respectively the shortest and longest timedistances between 2 nodes i and j.

Fig. 8. Time Diagram of 1:1 Regular protection method. Failurehappens at tf , Tnotify = T s0,f .

1) Performance for Regular traffic: Assume that node N0

sends a Regular flow to node Ni (i < n − 1) on the shortestpath, while a failure occurs at tf between nodes Nf andNf+1, 0 ≤ f ≤ i − 1. The top part of the time diagramin Fig. 8 shows packets sent by N0 before the source nodeis aware of the failure. Those packets are all lost. LetTloss be the duration of the loss period. The worst case iswhen the failure occurs close to Nf+1. Let us first assumethat the failure detection and message processing time arenegligible comparing to point-to-point propagation time, N0

8

learns about the failure at T s0,f+1 and we obtain :

Tloss ≤ 2T s0,f+1 (1)

which shows that in the worst case (i = f + 1 and T s0,f+1 =T l0,f+1), Tloss is at most equal to TR.

The middle part of the time diagram in Fig. 8 shows theperiod of time during which the source, aware of the failure,sends the packets on the longest path. Ni may start receivingpackets on the longest path at time tf+T s0,f+1+T l0,i. Note thatNi is stopped from receiving packets from N0 due to thefailure during at most Tloss+T l0,i-T s0,i, which is also upperbounded by TR (the worst case being when i = f + 1).

During restoration, some packets sent by N0 on the longestpath during the fault recovery period may arrive after pack-ets sent on the shortest path by the source once it is awareof the restoration; this may yield packet disordering at Ni(but neither loss, nor duplication). This is shown on thebottom part of the time diagram in Fig. 8. The period of timeduring which Ni may receive disordered packets (Tdisorder)is equal to T l0,i − T s0,i. A solution within the POADM layerto the disordering problem would involve numbering thepackets, and buffering temporarily packets received duringthe restoration period. A simpler option would be to let theupper layer deal with the disordering; some client layerssuch as IP are well suited to this simple approach.

We will now look at the impact of non-zero failure detectionand message processing times (i.e. failure notifications andlocalization, recovery and restoration). When a cable failureoccurs between nodes Nf and Nf+1, 0 ≤ f ≤ i − 1, theneighbouring nodes will be notified of the failure at most afew time slots after the received signal is lost: more precisely,a corrupted packet shall first be received and then we mayconsider that (for instance to avoid false alarms in case of aburst of transient errors) the node will have to wait a few(e.g. 2 or 3) time slots before inserting a failure notificationmessage in its control PDUs.

By construction, a POADM node processes any controlPDU in a single time slot. Therefore, after one time slot, thecontrol PDU for the next time slot can be generated. PDUprocessing time is compensated by suitable delay lines usedby the transit data PDUs. This is true in particular for fail-ure notification messages (included in the OAM messages)and for the per flow protection dedicated fields, shown inFig. 5.

From the previous remarks, we may conclude that, as-suming a time slot duration of 10 microseconds, the re-ception of notification will be further delayed by roughly40 microseconds, in comparison to our previous estimationwhere only propagation delays where taken into account (40microseconds correspond to the propagation delay for 20km).This supplementary delay includes roughly 30 microsecondfor failure message emission and 10 microsecond for failuremessage reception. Thus the estimation of the worst caseloss period would be slightly larger, the accuracy dependingof course on the actual ring length. We may expect that thesame delay would be added to the transient phase occuringafter the failure repair. On the other hand, the processingdelay of OAM messages in the intermediate nodes can beincluded in the previous model just by adding a suitable fibrelength to each fibre section (2 km for 10 microseconds slots).

2) Performance for Premium traffic: We now assume thatnode N0 sends a Premium flow to node Ni in both directions,

Fig. 9. Time Diagram of 1+1 Premium protection method. N0 asSource node, Ni Destination node. tf failure time and Tnotify =T s0,f .

while a failure occurs at tf between nodes Nf and Nf+1,0 ≤ f ≤ i− 1.

The top part of Fig. 9 shows that the last instant of timewhen Ni, receives a packet from the shortest path is just be-fore Ni learns of the failure. The destination node then startsreceiving packets from the longest path, even when markedwith Flow-Version=Backup. Actually, the first packets hadprobably been received already on the shortest path! Thereis indeed a potential duplication period of duration T l0,i−T s0,i.Then, Ni starts receiving the packets sent on the shortestpath by N0 and lost due to the failure during the period of du-ration Tloss computed previously. However, in the Premiumcase, the packets are received on the longest path, althoughmarked with Flow-Version=Backup, and are thus not lost.After this second period, Ni continues receiving packets onthe longest path, but marked with Flow-Version=Original,as long as the failure is not repaired. Sending packets onboth shortest and longest paths ensure that no Premiumpackets are lost although Ni receives some duplicate packetsduring this interval of time.

If we take into account a non-zero notification delay, asdiscussed for Regular traffic, some marginal loss (typically3 to 4 packets) could be observed. Buffering a few backuppackets could enable a strictly lossless operation, even inthis case.

The impact of restoration on a Premium flow is somewhatsimilar the one for a Regular flow. Due to the differencebetween shortest and longest paths, some disorder can beobserved during a period of time of duration T l0,i−T s0,i, whenthe destination node receives Premium packets marked withFlow-Version=Original both on the shortest and longestpaths. This period ends when Ni receives the first packeton the longest path marked with Flow-Version=Backup.

As pointed out previously, duplication and disorder can behandled either by the POADM layer, by numbering PDUs orby the client layers.

Assuming a fiber link failure rate less than 3 per year

9

per 1000km, both Premium and Regular protection schemesyield an availability rate higher than 99.999%, with arestoration time significantly less than 50ms. Moreover,POADM protection schemes are flow based as in MPLS-TP [24], and not channel based as in RPR and EPR.

VI. THE COST OF QOS IN POADM NETWORKS

A transport network relies on physical resources that havebeen provisioned, based on conservative traffic predictions,as making new resources available involves lengthy pro-cesses such as deploying new optical fibers. In the past,transport networks relied on sets of permanent, fixed ratecircuits established between each (source, destination) pair.The circuit’s capacity could greatly exceed the required peaktraffic rate, depending on the available granularity of cir-cuits. A MAN build with POADM rings still relies on opticalfibers, but replaces fixed rate circuits by fixed rate virtualcircuits, which potentially allows optimizing bandwidth us-age.

However, the need to provision resources based on pre-dicted peak traffic rates should still be taken into account.We thus assume that the MAN operator monitors networkusage in order to regularly update a “Traffic Matrix” Tthat recaps peak traffic requirements. Assuming that thenetwork operator provisions enough resources to support T ,a POADM network relies on a simple “opportunistic” PDUinsertion scheme: a node can insert a PDU on the first freetimeslot present on a convenient data channel. As trafficarriving to a node is aggregated, the insertion process can bemodeled by a a simple discrete time Geo/Geo/1 queue [25],where PDU generation at a node is modeled by a Bernoulliprocess and the distribution of the number of slots betweentwo free timeslots is geometrically distributed [21].

As long as client layers conform with T (conformancecan be enforced at each node by a simple access controlprocedure), the performance delivered by POADM rings canbe shown to comply to QoS objectives set for Carrier Ethernetin a MAN [26]. Indeed, latency is mostly due to geographicaldistance, as POADM nodes are optically transparent fortransit data PDUs; one can safely assume that the distancebetween nodes in a MAN is less than 1000km, which yieldsa latency less than 5ms.

Fig. 10. 6-node POADM ring with symmetric traffic of amplitudes. Jitter in nodes X and Y [21].

Jitter, on the other hand, is due to the insertion process.Using a Geo/Geo/1 model, one can show that even in thepessimistic case where only one data channel is used, jitteris less than 0.5ms as shown in Fig. 10. Even if a flow has tocross 3 POADM rings, and therefore be inserted 3 times, the

end-to-end jitter is still less than the target value of 2ms.Lastly, POADM operation is loss free, except in case of linkor node failure.

It is worthwhile noting that “fairness” is not an issue for aproperly dimensioned POADM ring: although some labeledflows may receive a better level of QoS than others, allreceive Carrier Ethernet levels of QoS.

VII. THE COST OF TRAFFIC PROTECTION INBIDIRECTIONAL POADM RINGS

In this Section, we consider the problem of dimensioninga POADM ring, while minimizing its CAPEX cost. Theoptimization model is formalized as a multi-commodity flowproblem with a 0-1 Integer Linear Programming (0-1 ILP)formulation.

The CAPEX cost of the ring is composed of two main costs:wavelength leasing cost per link l, (Cl), and cost of fixedsingle-wavelength receiver (Cr). Since the wavelength costis defined per link, we are able to encompass the impactthat the physical topology has on the final network cost, asthe traffic routes over greater distances have higher price.Transmitter cost is neglected as each node is equipped witha single tunable laser or even several of them in the case ofhigh traffic demands [27].

The input data of the problem are the bidirectional ringtopology and the set of traffic demands. The result of thedesign is the network configuration in terms of number ofwavelengths needed in the network, the allocation of wave-lengths to the traffic flows and distribution of fixed single-wavelength receivers at ring nodes. The costs are normalizedand given in arbitrary units.

In the remainder of this Section, apart from the wave-length assignment problem that need to be resolved inthe bidirectional POADM ring, the two protection methodsexplained earlier are considered during the formulation thedimensioning problem.

A. Dimensioning bidirectional POADM ring with protectionThe input traffic matrix of ordinary working mode TDo can

contain flows of three types: for premium, regular and un-protected traffic, in one of two directions of communications(noted with D). It is supposed that between two stations (e.g.A and B) can exist only one single connection “A to B”, withone of the three possible protection schemes. This means thatconnection B to A is also possible and it can benefit fromanother protection mode. Also, it is assumed that “shortest-path” routing is applied for unprotected and regular flows(except when in the restoration phase) and for the “original”version of a Premium flow. The traffic matrices are given forboth directions simultaneously.

From TDo , we derive traffic matrices TDl (l ∈ L, where L ={L1, L2, ..., LN} is the set of network links), which containthe working and protected traffic for the case of failure ofthe link l. In the case of failure, the traffic matrices willchange because for regular traffic some new flows will beincluded. For premium traffic, the incident of link failuredoes not induce a traffic matrix change. Similarly, the flowsof unprotected traffic are given in all TDl and TDo matrices.

Given Parameters• G(V,L): a non-directed graph representing the bidirec-

tional ring, where V is the set of nodes, L is the set ofbidirectional links;

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• N = |V |: number of nodes in the ring;• D: direction of transmission: D = 1 for clockwise direc-

tion, D = 2 for counterclockwise direction;• TDo : traffic matrix in normal operation mode in directionD; in all traffic matrices, element, tk, is the traffic rate(in bits/s) requested by the k-th flow;

• TDl : traffic matrix in protection mode (if link l has fallen)in direction D;

• TDe : any of matrices TDl or TDo ;• TDa : general traffic matrix, containing all possible flows

in the ring, i.e. TDa = TDL1∪ ... ∪ TDLN

∪ TDo ;• P: matrix of protection with elements P(k), such that:P(k) = 1 for premium protection, P(k) = 2 for regularprotection, and P(k) = 0 for all other flows k;

• B: wavelength capacity (in bits/s);• Cr: cost of a receiver;• Cl: cost of link l ∈ L;• W : maximum number of wavelengths per fiber;• πDk : path of the k-th flow (i.e., set of links connecting

node s to node d) in direction D; flow k is the protectivetraffic of the same size sent in the opposite direction, i.e.from d to s.

Variables• binary pk,Dw indicates whether k-th flow is routed on

wavelength w in direction D;• binary riw indicates whether node i requires a receiver

on wavelength w in direction D;• binary variable xl,Dw defining a “link-route”, i.e. indicat-

ing whether wavelength w on link l is used to carry thetraffic in direction D.

0-1 ILP formulationThe objective function minimizes the overall ring cost, i.e.,

the cost of the receivers and the link-routes required in thering:

Min

(2∑

D=1

∑i∈V

W∑w=1

Cr · ri,Dw +

2∑D=1

W∑w=1

∑l∈L

xl,Dw · Cl

)(2)

Constraint ensuring that one (and only one) wavelength isassigned to each traffic demand, in each direction (valid forall ring connections, i.e. the elements of TDa ):

W∑w=1

pk,Dw = 1, ∀k : tk ∈ TDa , ∀D (3)

Constraint ensuring that, for each traffic matrix TDe , thetraffic rate routed on wavelength w does not exceed thewavelength capacity:∑

k:l∈πDk,tk∈TD

e

pk,Dw tk ≤ 1, ∀w, ∀l ∈ L, ∀e, ∀D (4)

Constraint defining “link-routes” xl,Dw and allowing multi-ple flows, belonging to different protection events, to besupported by reusing the same wavelengths, which in turnallows saving of network resources in the case or regularprotection: ∑

k:l∈πDk,tk∈TD

a

pk,Dw tk ≤ (N + 1)xl,Dw ,

∀w, ∀l ∈ L, ∀D (5)

Constraint limiting the overall traffic received on wavelengthw at destination d, in direction D, for each traffic matrix TDe :∑

k:d=dest(tk),tk∈TDe

pk,Dw tk ≤ 1 ∀d ∈ V, ∀w, ∀e, ∀D (6)

Constraint forcing the use of the receiver on wavelength w,at destination d, in direction D, for the flows sharing thisreceiver:∑k:d=dest(tk),tk∈TD

a

pk,Dw tk ≤ (N+1)·rd,Dw ∀d ∈ V, ∀w, ∀D (7)

B. Numerical ResultsWe consider a 5-nodes POADM ring, in diverse network

scenarios. The goal is to quantify the impact of protectionschemes on the network’s CAPEX.

In all the examples, the channel rate is set to 10 Gbps,while the wavelength cost per link per km can be calculatedwith the formula Cl = (10 ∗ Cr/(100km)) ∗ l, where Cr isthe receiver cost and l is the link distance in km. Thisformula is derived from [28], where the ratio between thewavelength cost per ring and the receiver cost is estimatedon a ring circumference of 100 km. Both wavelength costand receiver cost are given in arbitrary units (a.u.) and it isalways considered that Cr = 0.1.

The above 0-1 ILP formulation is solved by a commerciallyavailable optimization software. We have performed an ex-tensive set of simulations for “random centralized” trafficwhich is characterized by two parameters, namely the load, σ(overall amount of traffic supported by the network) and thetraffic distribution factor, θ (the proportion of traffic passingthrough a highly congested node S). For θ = 0%, all thetraffic passes through S, while for θ = 100%, the trafficmatrix is fully decentralized. The channel rate is set to 10Gbps, and the value of the load σ is in {100, 150, 200, 250, 300}Gbps.

The results are averages collected on 100 designs withrandomly generated traffic flow matrices. All the links aresupposed to be of the same size and the overall ring circum-ference is taken to be 100 km, resulting in Cl = 0.2.

Fig. 11. Design cost for different protection levels and θ = 40%

Fig.11 shows the results of the comparison of design costfor premium, regular and unprotected traffic for θ = 40%.Premium protection is clearly more expensive than Regularprotection (up to 20% here) and much more expensive thanthe absence of protection (up to 55%). At lower loads, thedifference in cost between Regular and Premium protectionsis smaller, so it is probably wise to use Premium protectionfor such loads.

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We have shown on this example that protecting a POADMring may double the cost of a network, and that Regular pro-tection is slightly less expensive than Premium protection,as supplementary resources planned for Regular traffic areshared between all Regular flows, making this mechanismsimilar to 1:N circuit-switched protection.

VIII. CONCLUSION

In the present paper we addressed several major issues forthe operation of a POADM-based metro network, includingan original label-based MAC with simple opportunistic ac-cess and fast and efficient per-flow protection mechanisms.We also investigated the cost, in terms of network resources,to achieve carrier-grade QoS and protection. Using labeledvirtual circuits enables the use of many traffic engineeringtools available in any connection oriented network, whilesupporting multiple multiplexing granularities. This is par-ticularly interesting for Metro networks that need to supportmultiple protocols and widely varying granularity (fromlarge flows between data centers to small flows backhaulingmobile nodes).

Our label-based MAC builds upon the fact that POADM isa very flexible and bandwidth efficient solution. Its flexibilityis due to the fact that wavelengths can be freely sharedby destinations, and that “destination stripping” is used inthe unicast mode. POADM’s bandwidth efficiency is due tothe fact that each node is equipped with fast-tunable laser,and that both opportunistic access and slot reservation arepossible. It was shown in [29] that the most efficient POADMconfiguration in terms of the capacity and latency is the “all-wavelength-shared” configuration, where the opportunisticaccess is achieved via FIFO queues, under assumption thateach node can receive packets on all of the available wave-length channels. In [30], it was shown that POADM ringswith opportunistic access can achieve the average wave-length occupancy of 80%, and the maximum wavelengthoccupancy of 95%. Thus, our MAC combines simplicity ofthe access technique for guaranteed traffic (opportunistic),high throughput, low latency and per-flow protection. Tothe best of our knowledge, a combination of these featureshas not been previously reported with previous OPS MACs.Moreover, we built a mechanism for inserting BE trafficwithout impacting these performances.

The proposed MAC is aligned with the current approachof plane separation, centralised control and intermediatenode function simplification. The fundamental data planeoperations are defined for any packet belonging to any typeof flow: Unicast, Multicast, protection, etc. via local table.These tables are derived by the control plane, potentially in acentralized manner in a high performance machine. We havealso defined the protocol and algorithms that generate theabstract table such as the protection mechanism in additionto Multicast and QoS services.

As shown in Fig. 12, Multiple client layers can be trans-ported thanks to a POADM network. Labelling each clientflow by encapsulating client information units within SDUsallows the sharing of a single PDU layer by multiple clientflows. This transport technology can support many protocols,thanks to SDU encapsulation.

The QoS delivered to PDUs is Carrier-Ethernet compati-ble, and the protection methods provided for POADM ringsare as flexible as those designed for MPLS-TP. A POADM

Fig. 12. POADM network as a multi-protocol transport network

network can offer a differentiated protection performance.Efficient protection mechanisms are based in particular onan easy distribution of global OAM information, as controlPDUs carried in the control channel can be used to dissem-inate OAM messages instantaneously.

In addition of having a dedicated control channel, usinglabel controlled mapping and switching provides POADMnetworks with a clean separation between data and thecontrol planes. The data plane is controlled by various localtables (FTS, STP, FIT and PIT) that can be static (as inthe current Metro networks), periodically distributed by acentralized control plane (as in a SDN framework), or locallycomputed thanks to some distributed procedures (as in a G-MPLS framework). The size of the control tables should berelatively as they don’t need to include label swap for transitpackets, as these packets are passed transparently.

POADM thus seems a good candidate to deploy for sup-porting a universal Metro architecture, which can be used tocarry any client protocol layer; it provides an optically trans-parent transport to traffic in transit and thus is more energyefficient that any technique relying on electronic operationin every node. An intermediate architecture such as MPLS-TP or PBB-TE is thus made redundant. A fully convergedarchitecture could thus present multiple client layers carriedover POADM, which is then carried over WDM. This doesnot preclude using wavelength channels with OTN for verylarge flows that do not require fine granularity.

ACKNOWLEDGMENT

The research leading to this paper has received fund-ing from the European Community’s 7th Framework Pro-gramme FP7/2013-2015 under grant agreement 317762(COMBO project) and from the French Ministry of Industryin the framework of the CELTIC+ SASER-Savenet project.

REFERENCES

[1] ITU-T, “ G.8032/Y.1344 Ethernet Ring Protection Switching.”[2] IEEE Computer Society, “IEEE 802.17 Resilient packet ring

(RPR).”[3] A. Gravey, P. Gravey, M. Morvan, L. Sadeghioon, and B. Us-

cumlic, “QoS of Optical Packet Metro networks,” in OFC 2014: the Optical Fiber Communication Conference and Exposition,2014, pp. paper W1C.5 –.

12

[4] D. Chiaroni, “Optical packet add/drop multiplexers for packetring networks,” in Optical Communication, 2008. ECOC 2008.34th European Conference on, Sept 2008, pp. 1–4.

[5] D. Chiaroni, G. Buforn, C. Simonneau, S. Etienne, and J.-C.Antona, “Optical packet add/drop systems,” in proc. OFC, 2010,paper OThN3.

[6] N. Bouabdallah, G. Pujolle, and H. Perros, “Cost-effectivesingle-hub WDM ring networks,” in Communications, 2006.ICC ’06. IEEE International Conference on, vol. 5, June 2006,pp. 2421–2426.

[7] A. Stavdas, S. Sygletos, M. O’Mahoney, H. L. Lee, C. Ma-trakidis, and A. Dupas, “IST-DAVID: concept presentation andphysical layer modeling of the metropolitan area network,”Lightwave Technology, Journal of, vol. 21, no. 2, pp. 372–383,Feb 2003.

[8] A. Carena, V. D. Feo, J. Finochietto, R. Gaudino, F. Neri,C. Piglione, and P. Poggiolini, “RINGO: an experimental WDMoptical packet network for metro applications,” Selected Areasin Communications, IEEE Journal on, vol. 22, no. 8, pp. 1561–1571, Oct 2004.

[9] I. White, M. Rogge, K. Shrikhande, and L. G. Kazovsky, “Asummary of the HORNET project: a next-generation metropoli-tan area network,” Selected Areas in Communications, IEEEJournal on, vol. 21, no. 9, pp. 1478–1494, Nov 2003.

[10] I. Widjaja, I. Saniee, R. Giles, and D. Mitra, “Light core andintelligent edge for a flexible, thin-layered, and cost-effectiveoptical transport network,” Communications Magazine, IEEE,vol. 41, no. 5, pp. S30–S36, May 2003.

[11] S. Cao, N. Deng, T. Ma, J. Qi, X. Shi, J. He, and J. Zhou,“An optical burst ring network featuring sub-wavelength- andwavelength-granularity grooming,” in Photonics Global Confer-ence (PGC), 2010, Dec 2010, pp. 1–3.

[12] J. Dunne, T. Farrell, and J. Shields, “Optical packet switch andtransport: A new metro platform to reduce costs and powerby 50% to 75% while simultaneously increasing deterministicperformance levels,” in Broadband Communications, Networks,and Systems, 2009. BROADNETS 2009. Sixth InternationalConference on, Sept 2009, pp. 1–5.

[13] I. Popescu, B. Uscumlic, Y. Pointurier, P. Gravey, M. Morvan,and A. Gravey, “A cost comparison of survivable subwavelengthswitching optical metro networks,” in ITC 26 : proceedings ofthe 2014 26th International Teletraffic Congress, 2014.

[14] M. Herzog, M. Maier, and M. Reisslein, “Metropolitan areapacket-switched WDM networks: A survey on ring systems,”Communications Surveys Tutorials, IEEE, vol. 6, no. 2, pp. 2–20, Second 2004.

[15] A. Fumagalli, J. Cai, and I. Chlamtac, “The multi-token inter-arrival time (MTIT) access protocol for supporting IP overWDM ring network,” in Communications, 1999. ICC ’99. 1999IEEE International Conference on, vol. 1, 1999, pp. 586–590vol.1.

[16] V. H. Nguyen and T. Atmaca, “Dynamic intelligent MAC proto-col for metropolitan optical packet switching ring networks,” inCommunications, 2006. ICC ’06. IEEE International Conferenceon, vol. 6, June 2006, pp. 2661–2668.

[17] D. Dey, A. Van Bochove, A. Koonen, D. Geuzebroek, andM. Salvador, “Flamingo: a packet-switched IP-over-WDM all-optical MAN,” in Optical Communication, 2001. ECOC ’01. 27thEuropean Conference on, vol. 3, 2001, pp. 480–481 vol.3.

[18] M. Salvador, S. Heemstra de Groot, and D. Dey, “MAC protocolof a next-generation MAN architecture based on WDMand all-optical packet switching,” Telecommunication Systems,vol. 19, no. 3-4, pp. 377–401, 2002. [Online]. Available:http://dx.doi.org/10.1023/A%3A1013894506809

[19] M. Ajmone Marsan, A. Bianco, E. Leonardi, A. Morabito, andF. Neri, “All-optical WDM multi-rings with differentiated qos,”Communications Magazine, IEEE, vol. 37, no. 2, pp. 58–66, Feb1999.

[20] L. Sadeghioon, A. Gravey, and P. Gravey, “A label based MACfor ops multi-rings,” in Optical Network Design and Modeling(ONDM), 2011 15th International Conference on, 2011, pp. 1–6.

[21] B. Uscumlic, A. Gravey, M. Morvan, and P. Gravey, “Impact ofpeer-to-peer traffic on the efficiency of optical packet rings,” inWOBS’2008 : international Workshop on Optical Burst/PacketSwitching, september 8-11, London, UK, 2008.

[22] A. Kabat, L. Sadeghioon, A. Gravey, and H. S. Darwish, “Amulti-class MAC for time-slotted WDM optical packet ring,”

Advances in Electronics and Telecommunications, vol. Vol. 2,no. 4, pp. 68–73, 2011.

[23] L. Sadeghioon, A. Gravey, and P. Gravey, “Rapid protectionschemes in an all-optical packet metro ring,” in Networks andOptical Communications (NOC), 2012 17th European Confer-ence on, june 2012, pp. 1 –6.

[24] H. van Helvoort and E. J. Ryoo, “MPLS-TP ring protec-tion switching (MRPS), draft-helvoort-mpls-tp-ring-protection-switching-06.txt,” in Work in progress, April 18, 2014.

[25] J. Cao, W. S. Cleveland, D. Lin, and D. X. Sun, “Inter-net Traffic Tends Toward Poisson and Independent as theLoad Increases,” in Nonlinear Estimation and Classification,C. Holmes, D. Denison, M. Hansen, B. Yu, and B. Mallick, Eds.New York: Springer, 2002, pp. 83–109.

[26] Metropolitan Ethernet Forum, “MEF 23.1, implementationagreement, carrier ethernet class of service.”

[27] B. Uscumlic, L. Sadeghioon, A. Gravey, and P. Gravey, “TheCost of Traffic Protection in Bidirectional Optical PacketSwitching Rings,” in ISCC 2013 : 18th IEEE Symposium onComputers and Communications, 2013.

[28] B. Uscumlic, I. Cerutti, A. Gravey, D. Barth, P. Castoldi,M. Morvan, and P. Gravey, “Optimal dimensioning of the WDMunidirectional Ecoframe optical packet ring,” Photonic networkcommunications, july 2011.

[29] B. Uscumlic, A. Gravey, P. Gravey, and I. Cerutti, “Trafficgrooming in WDM optical packet rings,” in ITC’21 : 21thInternational Teletraffic Congress, 2009.

[30] B. Uscumlic, A. Gravey, I. Cerutti, M. Morvan, and P. Gravey,“Stable Optimal Design of an Optical Packet Ring with Tun-able Transmitters and Fixed Receivers,” in ONDM 2013 : the17th International Conference on Optical Network Design andModeling, 2013.