Abstract Optical channels are currently able to carry10 Gbit/s and even 40 Gbit/s traffic flows. However, it isnot usual to have such amounts of traffic between anypair of client nodes. This article proposes using point-to-multipoint optical channels for the allocation of point-to-point connections in transparent wavelength-routedoptical networks. Specifically, when an optical connec-tion between a source-destination node pair has to beestablished, the optical signal is also sent to some adja-cent nodes by introducing passive optical splitters; inthis way a light-tree is built. Then, the already estab-lished point-to-multipoint optical channel can be usedto groom further point-to-point connections betweenthe same source node and each of the other nodes com-posing the light-tree. The benefits of this strategy are2-fold: first, the reduction of optical transmission equip-ments allowing cost savings with respect to the tradi-tional typical point-to-point approach and, second, theoptimization of the optical channels utilization meetingin such a way Traffic Engineering objectives. The meritsof proposed approach are evaluated by simulation.
S. Spadaro (B) · J. Comellas · G. D’Angelo · G. JunyentOptical Communications Group,Signal Theory and Communications Department,Universitat Politècnica de Catalunya (UPC),Jordi Girona 1-3,Barcelona 08034, Spaine-mail: [email protected]
The introduction of Dense Wavelength Division Multi-plexing (DWDM) technology in existing point-to-pointsystems (point-to-point fibre links) provides high-capacity links. The achieved advances in the opticalcomponents (e.g., Optical Cross Connects, OXCs)allow performing routing and switching directly in theoptical domain leading to the transparent optical net-works concept. OXCs are able to switch WDM chan-nels amongst different fibres, demultiplexing the opti-cal channels from any input fibre and switch them tothe required output ports [1].
The emerging requirement for Network Operators(NO) is to design cost-effective optical transport net-works in order to accommodate the increasing clientlayer bandwidth demands, trying at the same time tooptimize the available network resources (meeting Traf-fic Engineering objectives) as well as saving both oper-ational and capital expenditures (OPEX and CAPEX).
Optical channels (wavelengths) are currently able tocarry 10 Gbit/s and even 40 Gbit/s traffic flows. How-ever, it is not usual to have such amounts of trafficbetween any pair of client nodes [2]. Wavelength capac-ity is thus wasted due to the mismatch between thebandwidth requirements of the client data flows and theoptical channels capacity. Such mismatch implies lowwavelength bandwidth utilization and, from the net-work planning point of view, it increases the CAPEXof the transport network. In order to efficiently utilizethis capacity, a number of independent lower-rate traf-fic flows can be multiplexed into a single lightpath (so-called traffic grooming) in order to meet network designgoals such as hardware cost minimization.
72 Photon Netw Commun (2007) 14:71–81
With the emergence of MPLS and MPλS technolo-gies, service providers will eventually build a hierarchyof MPLS networks, with optical MPλS backbones occu-pying the top level of the hierarchy. Such network archi-tecture is feasible because GMPLS allows tunnelling aset of MPLS label switched paths (LSPs), with the sameoriginating and terminating LSRs, through a OXCs-based backbone by carrying these LSPs within the samelightpath. As a consequence, a new issue arises in whichthe goal is to groom a set of MPLS LSPs into light-paths for the transport of the traffic flows over WDMtransparent backbone networks of a mesh topology. Forsuch a kind of transparent networks (no O-E-O conver-sion in the intermediate nodes), the switching at OXCstakes place at the granularity of a whole wavelength,and therefore a point-to-point lightpath allows the wave-length capacity to be shared only among client connec-tions between the same OXCs pair.
The way we propose to increase the bandwidth uti-lization when allocating point-to-point connections intransparent optical networks consists in extending thelightpath concept to the light-tree concept [3]. Thisapproach makes possible to groom and tunnel a numberof lower-rate point-to-point flows/LSPs to several desti-nations, regardless of the egress OXC these destinationsare attach to. However, such a network will require opti-cal splitting capabilities and a greater power budget tocontrast the effect of power losses caused by the opticalsplitting operations. The reduction of the optical trans-mission equipments which allows network cost savingsis pursued.
The remainder of the article is organized as follows:in ‘Multicast-like scheme for unicast connections alloca-tion’, we propose the multicast-like approach. In ‘Opti-cal node architecture’, the architecture of an opticalnode capable to support the proposed method is shown.Section ‘Performance evaluation’ presents the simula-tion case studies carried out to assess the merits of themulticast-like approach. Finally, the last Section con-cludes the paper.
2 Multicast-like scheme for unicast connectionsallocation
To allocate client connections between two nodes (uni-cast connections), we propose to establish point-to-multipoint optical channels (e.g., light-tree). Since, asmentioned above, it is not usual to have amounts oftraffic between nodes of the order of the current wave-length capacities, we assume that the client networksgenerate data streams/flows with a bandwidth require-ment lower than the total wavelength capacity. Each
connection requires a bandwidth which corresponds to(1/NS)-th of the total wavelength capacity,1 where theparameter NS refers to the number of sublambdas perwavelength, namely the number of client connectionsthat can be theoretically groomed on to each opticalconnection. The already established point-to-multipointoptical channel can be then used to groom further uni-cast connections between the same source node andeach of the rest of nodes (leaves) composing the light-tree, using the free sublambdas over the already estab-lished light-tree. It has to be highlighted that by using ourapproach, the grooming functionality is applied only atthe nodes where the lightpaths are initiated. This is dueto the fact that it is intended to be used in transparentoptical networks in order to avoid O-E-O conversion inthe intermediate nodes of the lightpaths, reducing thusthe OPEX of the network.
The necessary devices to implement these nodes are1×N passive optical power splitters, being N the fanout(FO) of the optical splitters [5] (hereafter FO = N)and maybe some optical amplification to achieve thenecessary power values to cope with the optical receiverssensitivity [1].
Of course, the number of groomed connectionsdepends on the number of sublambdas per wavelengthand obviously, if the bandwidth requested by each clientconnection approaches the capacity of the wavelength,the unicast approach always performs better than themulticast-like one.
Different works are available in the literature aboutthe establishment of light-tree to accommodate mul-ticast client requests. In [3] the authors demonstratethe effectiveness of multicasting in all-optical networks;they show that, by introducing the light-tree concept,considerable savings can be obtained in terms of thetotal number of transceivers required in the network.For the efficient realization of multicasting, the interme-diate nodes have to be multicast-capable (MC-nodes).References [5] and [4] deal with the physical structureof MC-nodes. In [6] the authors introduce the power-efficient design for multicast networks which aims atreducing the number of power splitters with the objec-tive of maintaining an acceptable blocking performanceand in [7] the problem of the actual placement of the MCnodes to minimize the blocking probability is discussed.
On the other hand, the traffic grooming problemfor optical networks has been considered by several
1 For sake of simplicity we assume that all client connections havethe same bandwidth requirement. However, the approach is stillvalid in case of considering connection requests with differentbandwidth requirements in order to reproduce the heterogeneityof client networks.
Photon Netw Commun (2007) 14:71–81 73
researchers (e.g., see [8]–[11]). The objective consideredhas been either to maximize the network throughput orto minimize the connection-blocking probability. Specif-ically, in [11] the authors consider the problem of trafficgrooming, routing and wavelength assignment with theobjective of minimizing the number of transponders inthe network.
Recently, the multicast traffic grooming problem(grooming of multicast requests over light-trees) hasbegun to attract research attention [18]–[12] and [13].All these articles studied the multicast traffic groomingproblem based on the lightpath model andapproached the problem using integer linear program-ming (ILP) and heuristics. A more comprehensive studyon this topic can be found in [14].
With respect to these previous works, the noveltyof our approach relies on using light-trees to groomunicast connections (and not multicast ones) in trans-parent wavelength-routed networks, whilst the previousworks use light-trees to accommodate/groom multicastrequests.
When a request for connection between two nodesarrives, unlike point-to-point optical connection (theunicast approach in transparent optical networks), thelightpath established is routed not only to the destina-tion node but also to other (FO-1) nodes, building insuch a way a light-tree. It implies that the informationdestined to a given destination node is also shared by(FO-1) neighbour nodes, which, in principle, are consid-ered to not to use it. This may arise some problem of dataconfidentiality which can be effectively solved by usingdata encryption or similar strategies. On the other hand,some kind of optical power splitting has to be applied atthe first destination node allowing the optical signal toreach the other optical nodes. In the rest of the articlewe refer to it as multicast-like approach.
The multicast connection routing problem can be con-verted to the well-known Steiner-tree problem, whichis NP-complete [15]. Since in our approach, the light-tree is not built in order to accommodate a multicastrequest, we propose a very simple algorithm to buildthe light-tree. It is explained in details in ‘Light-treeset up’. Figure 1 shows the Pseudo-code of the multicast-like approach.
When a flow between the node-pair (a, b) arrives,(FO-1) nodes physically connected to node a are selectedon the basis of the number of free sublambdas/wavelengths or on the basis of the their nodal degree(See Fig. 2). This way the light-tree composed by thenode a and FO destination nodes is built. If (FO-1) nodesare not available (due to the nodal degree of node b orunavailability of wavelengths), a smaller-size light-treeis built. The SelectLightTreeLeaves function is describedin Fig. 2.
The goal of this article is, given the number of thenodes composing the network, the number of links andthe number of wavelengths per link (NL), to find outthe minimum number of sublambdas (SL) for whichthe multicast-like approach allows allocating more con-nections than the unicast approach for a given blockingprobability (Pb).
2.1 Light-tree set up
We have considered a very simple method for the light-tree setup. If a client connection has to be establishedbetween nodes a and b, at most (FO-1) of the ADJn
nodes, adjacent to the splitting node (b), will be addedas light-tree leaves. In order to select these nodes, aselection probability interval [0, Pi] is associated to nodei (i = 1, 2, . . ., ADJn) based on a Leaves Selection Mode(LSM) value: if the LSM parameter is set to 1, then Pi
Fig. 1 Multicast-likeapproach: pseudo-code
Given: NS, Pb
Input: Request for connection accommodation between (a,b)Output: Number of allocated connections
Algorithm:
While (actual_blocking_probability <= Pb)A connection request between node-pair (a,b) arrives If ((b is a leaf of a light-tree already established over λi) ← TRUE) If ((available sublambdas over (light-tree)λi) ← TRUE)
Allocate the connection request over (light-tree)λi
ElseRoute (a,b) // The RWA algorithm is run If (route (a,b) ← TRUE)
find up to (FO-1) neighbour nodes to b ← SelectLightTreeLeaves(a,b, FO)establish the light-tree (a, {b, b, b1, b2, …, b(FO-1)}λ j)
ElseConnection request blocked and lost Calculate actual_blocking_probability
EndwhileReturn (allocated connections)
74 Photon Netw Commun (2007) 14:71–81
Fig. 2 Light-tree set upalgorithm
SELECTLIGHTTREELEAVES(a, b, FO)
do LTS min(NDb , FO-1)n 0
For each node i adjacent to b
If (i a) and (i b)do n n + 1
ADJn i
If (n 0)For j 1 to n
If LSM = ‘nodal degree-based’
do Pjj
kADJn
kADJ
k
k
NDND 1
1
1
Else
do Pjj
kADJn
kADJ
k
k
NSNS 1
1
1
For j 1 to LTS
do valid_neighbor FALSEWhile valid_neighbor = FALSE
do r random number with uniform distribution over [0,1]m rPi
iminarg
If ADJm not already added to light-treedo valid_neighbor TRUE
bj+1 ADJm
Return(Bj, for j = 1, …, LTS-1)
Fig. 3 Unicast approach:Three different lightpathshave to be established
will be proportional to the nodal degree NDi of node i,otherwise (LSM = 2) Pi will be proportional to the totalamount of sublambdas available on its outgoing links.The first approach does not depend on the network state,so that it can be considered a static criterion, whilst thesecond one is adaptive since it relies on a time-varyingmetric. In the following Fig. 2, the pseudo-code of thelight-tree set up heuristic is depicted, where a and b arethe source and destination nodes respectively, b is alsothe splitting node, LTS is the light-tree size and bj arethe light-tree leaves whilst NDb is the nodal degree ofthe destination node b.
2.2 Multicast-like scheme: an illustrative example
This subsection is devoted to present an illustrativeexample in order to make clearer the proposedmulticast-like approach. Let us consider that the 9-nodetransparent optical network depicted in Fig. 3 has totransport traffic flows of bandwidth 1 on wavelengths ofcapacity 4 (then up to four connections can be allocatedon each optical channel, which means that the number ofNS is 4).
Let us suppose that the client connections/calls toallocate are (1,6), (1,4) and (1,5) (where the two numbers
Photon Netw Commun (2007) 14:71–81 75
Fig. 4 Multicast-likeapproach: Three differentconnections are allocatedusing just one lightpath
into the brackets are the source and the destinationnodes respectively). When using a typical unicast trans-parent optical network (Fig. 3) a new point-to-pointoptical channel will be established for each requestedconnection. On the contrary, when using the multicast-like approach (Fig. 4) only one optical channel (originat-ing at node 1 and terminating at nodes 6, 4 and 5) willbe able to accommodate the three connections. Specif-ically, when allocating the first connection, the opticalsplitter placed at node 10 (Passive optical splitting inFig. 4) enlarges the optical signal also to nodes 4 and 5.Then, if new connections from node 1 to nodes 6, 4, or 5have to be allocated, the already in-use wavelength canbe re-used and its capacity utilization is thus increased.Moreover, only one optical transceiver is used and onlyone of the available wavelengths on the links along thepath is used. Specifically, when examining Fig. 3, it can beseen how the link joining nodes 1 and 10 has three activewavelengths and each of them is only carrying a 25% ofits capacity. On the contrary, by using the multicast-likeapproach, the link joining node 1 and node 10 has justone active wavelength and its utilization is the 75% ofits capacity. On the other way, in the latter case, theinformation destined to a node is also shared by twoneighbour nodes, which, in principle, are considered tonot to use this information. This means that informationfrom node 1 to 6 will also reach nodes 4 and 5 becausethe signal is transported transparently to these nodes.
Of course, our proposed scheme is only useful undercertain network conditions, clearly determined by theratio between the capacity of the wavelength andthe bandwidth required by the client connections (i.e.,the number of sublambdas that can be theoreticallygroomed in each wavelength).
3 Optical node architecture
The structure of the node used in the optical multicast-like approach is a typical broadcast and select architec-
ture [16] and is similar to that presented in [17]. It isshown in Fig. 5 and as an example, 4 input/output fibres,and an add/drop capacity of 1 optical channel per fibreis considered.
Input signals are first amplified to recover the opticalpower level necessary. Then, a first splitter in each fibrewill allow directing the signals to the local tunablereceivers (TR). As the whole WDM multiplex will reachthe receiver, a filter selecting the appropriate opticalchannel is mandatory. The remaining part of the orig-inal signal is divided again by means of new 1 × NF(i.e., 1 × FO) power splitters and is sent to NF pools ofselective wavelength blockers (WB). Considering a spe-cific optical channel, the WB can let it pass or block it.The WB has an optical demultiplexer, NL variable opti-cal attenuators (VOAs), where NL corresponds to thenumber of wavelengths per fibre, and an optical mul-tiplexer to couple again the desired wavelengths. Theinternal structure of the WB is shown in Fig. 6.
A critical point on the considered node is theco-ordination between the WBs and the tunable trans-ceivers (TTs). A given wavelength could only be addedif it has been blocked at the corresponding WB.
The outputs of each WB are directed to the outputfibres where the locally added channels can be insertedthrough new couplers by means of tunable transceivers.Finally, the resulting signal is amplified with the aim ofhaving enough power to reach the next nodes.
4 Performance evaluation
To evaluate the performance of the multicast-likeapproach, an ad-hoc event-driven simulator reproduc-ing real scale network configurations has been set up. Wecarried out extensive simulations in order to comparethe number of the overall allocated client connectionsbetween node pairs (carried traffic by the wavelength-routed optical network) for a given blocking probabil-ity (Bp) using both the unicast and the multicast-like
76 Photon Netw Commun (2007) 14:71–81
Fig. 5 Node architecture
Fig. 6 Detailed structure ofthe WBs VOAVOA
VOAVOA
VOAVOA
1
DEMUX
MUX
2
NL
approaches. In fact we used the typical unicast approachas benchmark scheme to assess the merits of our pro-posal. It is worth to highlight that, even consideringthat the lightpath reaches more than one optical node,only one client connection is accounted as carriedtraffic.
As simulation method, we set a target Bp and thenconnections are allocated over the WDM-based trans-parent network both using the unicast and the multicast-like approaches. The simulation is terminated when thepredefined Bp is reached. To improve the accuracy ofthe simulation results, each point of the Figures in therest of the article is the average of the results obtainedover an enough number of simulations to assure a 95%confidence interval.
Establishing a lightpath in all-optical networksinvolves selecting a route and a wavelength. The prob-lem of routing connections is referred to as routing andwavelength assignment (RWA).
In our simulation scenarios, we route the lightpathsover the WDM network using an adaptive routing algo-rithm based on the shortest path (i.e., Dijkstra algo-rithm). Then, a free wavelength is selected on each linkscomposing the lightpath. In order to optimize the net-work resources utilization (maximizing the number ofallocated connections for the predefined Pb), the routingalgorithm takes into account not only the length of thepath (number of hops) but also the actual traffic load,namely the number of already in-use wavelengths oneach fibre link. This implies that the network state infor-mation databases, locally maintained at each node, areupdated when a new lightpath is established/tear down.The network nodes, to perform the routing and wave-length assignment algorithm, use such network stateinformation.
The simulation case studies have been carried out withthe following assumptions: (1) Each client connectionrequests a sublambda (1/NS of the overall wavelength
capacity); (2) Transparent networks have been assumed,which means that O/E/O conversion is not allowed atthe intermediate nodes; (3) The requests for connectionestablishment are generated using a exponential distri-bution and they are randomly generated and uniformlydistributed amongst the nodes of the network. Thesource-destination pairs are generated taking intoaccount the nodal degree parameter of each node. Inthis way, the probability to be source or destination nodeof the traffic flows is higher for those nodes with highernodal degree; (4) Connections that cannot be allocatedare blocked and lost; and finally, (5) We have consideredthat the optical nodes (OXCs) are full wavelength con-version capable (any input wavelength can be convertedto any output wavelength).
We simulated two meshed transparent networks,namely: the NSFNET network composed by 14 OXCs(connected by 21 optical fibre links) and the EuropeanOptical Network (EON), composed by 28 OXCs (con-nected by 36 links). Figure 7 shows both networks. Theaverage nodal degree of all the three networks is 3.Additionally we have also simulated two other networkscomposed by 20 and 24 nodes, respectively, with nodaldegree 3. In all the simulations we set the blocking prob-ability to 10−2, which is in line with the values used inthe literature.
Under the above assumptions, we carried out foursimulation case studies. The first one aimed to find outthe proper number of sublambdas for which, in specificnetwork conditions, the multicast-like approach allowsallocating more connections than the unicast one; thewavelength capacity is assumed to be constant and theeffect of the number of wavelength per link was alsoinvestigated. The second case study compared bothapproaches in terms of the wavelengths bandwidth uti-lization. The third one investigated the impact of the size
of the light-tree and finally the fourth case study aimedto show the reduction of the overall number of trans-ceivers required by using the multicast-like approach.
Regarding the first case study, as a sample of theobtained results, Fig. 8 shows the results for both theNSFNET and the EON networks. Specifically, for theNSFNET network the number of available wavelengthsper link (NL) is set to 12 whilst for the EON network itis set to NL = 36. In both cases, the parameter FO is setto 3.
It can be seen that, in this network conditions, as thenumber of sublambdas increases (higher than NSinv),the multicast-like approach allows allocating a highernumber of connections than the unicast one. Specifi-cally, Fig. 8a shows that the multicast-like approach per-forms better when the number of sublambdas is higherthan 9 (in this case NSinv = 9). When it is lower than 9,the multicast-like approach allows the allocations of anumber of connections lower that the unicast case. Forthe EON network, the obtained NSinv is 3. When fewresources are available over the transport network, theunicast approach is better than the multicast-like onesince it uses fewer resources. In fact, the latter approachimplies to enlarge the optical signal also to other two(since FO in this case is set to 3) nodes apart from thedestination one. Therefore, in these network conditionsit is clearly not efficient. The same behaviour has tobe expected when the number of wavelength per linksis higher. Therefore, we expect to find a range of NL([NLmin, NLmax]) for which there exists a NSinv startingfrom which the multicast-like approach outperforms theunicast one. From the results we obtained we can con-clude that the size of this range strongly depends on thesize and topology of the network. The higher the num-ber of the nodes composing the network, the higher theNL range is. In fact, for the NSFNET network we have
found that NLmin = NLmax is 12 whilst for the EONnetwork NLmin is 36 whilst NLmax is 70. This trend isconfirmed by further simulations results we have donewith network composed by 20 and 24 nodes, respectively.As additional result, we have found that the value of sub-lambdas starting from which the multicast-like approachoutperforms the unicast one (NSinv) does not stronglydepend on the number of NL ∈ [NLmin, NLmax].
We left for future investigations the study of the sizeof the range according to the network topology.
The second simulation case study consisted on thecomparison of the average wavelength capacity utiliza-tion using both the unicast and the multicast-likeapproaches. As a sample of the obtained results, Fig. 9depicts the average number of allocated client connec-tions per wavelength versus the theoretical maximum(i.e., the number of sublambdas). Specifically, Fig. 9arefers to the NSFNET network with NL = 12 and Fig. 9brefers to the EON network with NL = 36. Similar resultshave been obtained for the rest of the cases. When con-nection bandwidth approaches the wavelength capac-ity each connection occupies a full wavelength, so themulticast-like approach is not clearly efficient. Figure 9shows that by increasing the number of sublambdas theutilization of the bandwidth wavelength is always higherin the case of multicast-like approach. This not only pro-
vides better utilization of the available resources but alsoit allows allocating more connections.
We carried out a third simulation case study whichconsisted on evaluating the impact of the size of thelight-tree (e.g., parameter FO) on the performance ofthe multicast-like approach. Increasing FO means toreach more nodes with same point-to-multipoint light-path and then the possibility to reuse the already activewavelengths is higher; on the other hand, the higher isFO, the higher are the power losses due to the opticalsplitters, which increase the requirements of some opti-cal amplification; therefore, the proper light-tree sizehas to be investigated. The aim of this study was toevaluate if, by increasing the size of the light-tree, thereutilization of the already established lightpaths allowsto allocate more connections into the available wave-lengths. In particular, on the basis of the average nodaldegree of the networks under study, we compared thecase of setting FO equal to 3 to the case of setting FOto 4. We did not consider lower values since for thesecases both approaches tend to be the same. On the con-trary, for higher values of FO the optical power losseswould be excessively high and therefore we did not con-sider them. As a sample of the obtained results, Fig. 10,which is referred to the NSFNET (Fig. 10a) and EON(Fig. 10b) networks respectively, shows that for low
Fig. 10 Impact of the FO: (a)NSFNET, NL = 12, (b) EONnetwork, NL = 54
0
100
200
300
400
500
600
700
800
2 8 12 16 20Number of sublambdas
Allo
tacde
ocn
ntceio
ns
Multi FO= 4
Multi FO = 3
2 4 8 12 16 20 240
1000
2000
3000
4000
5000
6000
7000
lA
lotac
deoc
nn
tceio
ns
Number of sublambdas
Multi FO = 3
Multi FO = 4
(a) (b)
4
Table 1 Unicast versus Multicast-like approach, optical trans-ceivers cost saving
NSFNET → 38%24-nodes network → 30%EON → 20%
values of sublambdas it is better to built light-tree of sizeFO = 3 whilst for higher values of the number of sub-lambdas it is better to built light-tree of size FO = 4. Thismeans that as the available optical resources increase,the higher is the parameter FO the higher is the reuse ofthe already established lightpaths to groom new arrivingconnections. Therefore FO = 4 allows to allocate moreconnections than the case with FO = 3.
Finally, as the fourth case study, the cost savingsallowed by the multicast-like approach have been quan-tified. Even considering that the nodes will be more com-plex (optical devices capable of the splitting functionmust be added) there is an important reduction in thenumber of the required optical transceivers by using themulticast-like approach. We calculated the parameterR, which is the ratio between the numbers of the opticaltransceivers (Num_Trans) required for both approacheswhen the traffic carried is almost identical. R was cal-culated as R= (Num_Trans)multicast−like
(Num_Trans)unicast. Table 1 summarizes
the results in terms of (1-R), which represents thus thereduction on the number of transceivers required. It canbe seen as the reduction is about the 30%, dependingon the network topology.
Assuming that the optical active equipments are con-sidered to be the most expensive part amongst the opti-cal components, this is a very promising result.
5 Conclusions
This article proposes the multicast-like strategy throughwhich a transparent WDM transport network is ableto efficiently groom point-to-point client connectionsvia light-trees. It allows performing traffic grooming in
order to improve the wavelength capacity utilizationand, therefore, reducing the overall number of opticalactive equipments required in the network, althoughOXCs must be provided with some passive splittingfacility (MC-OXCs).
Simulation results indicate that our approach isclearly useful under certain network conditions, mainlydetermined by the number of wavelengths per fibrelink and the number of sublambdas. Specifically, oncea target blocking probability has been fixed, a rangeof values for the number of wavelengths exists overwhich our proposal achieves better connection provi-sioning performances. Such a range mainly depends onthe network size and physical topology. For a specificnumber of wavelengths (and a specific blocking prob-ability), there exists a minimum number of sublamb-das starting from which more client connections canbe allocated and the optical resource budget is usedin a more efficient way. Furthermore, when the num-ber of sublambdas is set to this minimum value (thenumber of connections setup in both cases is compara-ble) the Multicast-like approach requires a lower amountof optical transceivers with respect to the typical Uni-cast method, hence allowing a reduction of the capitalexpenditure for Network Operators. As regards hard-ware cost-saving issues, the decreasing of the overallnumber of optical active equipments has been quanti-fied for distinct network scenarios, and very promisingresults have been obtained.
Concerning the light-tree size, we have found outthat, the impact of the fan-out on the multicast-likeapproach performances mainly depend on the networksize and physical topology. For a generally meshed phys-ical topology, a reasonable choice is to set the opticalpower splitter fan-out to the value corresponding to theaverage network nodal degree.
Acknowledgements This work has been partially funded by theSpanish Science Ministry through the Project “Red inteligenteGMPLS/ASON con integración de nodos reconfigurables (RING-ING)”, (TEC2005–08051-C03–02).
80 Photon Netw Commun (2007) 14:71–81
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Salvatore Spadaro ([email protected]) received the M.Sc. andthe Ph.D. degrees in Telecom-munications Engineering fromUPC (Barcelona, Spain) in 2000and 2005, respectively. He alsoreceived the Dr. Ing. degreein Electrical Engineering fromPolitecnico di Torino, Italy, in2000. He is currently an assis-tant professor in the Optical Com-munications group of the SignalTheory and Communications De-
partment of UPC. Since 2000 he is a staff member of theAdvanced Broadband Communications Centre of UPC. He hasbeen involved in ACTS-SONATA, IST-LION as well as NOBELand in e-Photon/One (Network of Excellence) of the EuropeanVI Framework Program. Currently, he is participating in NOBEL-Phase 2 ad e-Photon/One phase 2 projects. He has co-authoredabout 50 papers in international journals and conferences. Hisresearch interests are in the fields of all-optical networks withemphasis on traffic engineering and resilience in GMPLS net-works.
Jaume Comellas ([email protected]) received M.S (1993) andPh.D. (1999) degrees in telecom-munications Engineering fromUPC. Since 1992 he has beena staff member of the OpticalCommunications Research Groupof UPC. His current researchinterests mainly concern opticaltransmission and IP over WDMnetworking topics. He has par-ticipated in different researchprojects funded by the Spanish
government and the European Commission. He has co-authoredmore than 100 research articles in national and internationaljournals and conferences. He is an associate professor and cur-rently head of laboratories at the Signal Theory and Communi-cations Department of UPC. Currently, he is also involved in theAdvanced Broadband Communications Center.
Giuseppe D’Angelo received theM.Sc. degree in Telecommuni-cations Engineering from UPC(Barcelona, Spain) in 2006. Healso received the Dr. Ing. degree inTelecommunications Engineeringfrom Politecnico di Torino, Italy,in 2006.
Photon Netw Commun (2007) 14:71–81 81
Gabriel Junyent ([email protected]) is a telecom-munications engineer (UniversidadPolitécnica de Madrid, UPM, 1973),and holds a Ph.D. degree in commu-nications (UPC, 1979). He has beena teaching assistant (UPC, 1973–1977), adjunct professor (UPC,1977–1983), associate professor(UPC, 1983–1985), and professor(UPC, 1985–1989), and has been afull professor since 1989. In the last
15 years he has participated in more than 30 national and inter-national R&D projects, and has published more than 30 journalpapers and book chapters and 100 conference papers.
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