Deployment case studies of an energy efficientprotected LR-PON architecture

Marco Ruffini, Linda Doyle, David B. PayneCTVR, the telecommunications research centreUniversity of Dublin, Trinity College, Ireland

Email: marco.ruffini@tcd.ie

Deepak Mehta, Barry O’Sullivan, Luis QuesadaCTVR, the telecommunications research centre

Cork Constraint Computation Centre,University College Cork, Ireland

Abstract—Current DSL products that provide broadband overcopper pair are reaching their capacity limits. In many cases,their peak rate struggle to deliver HDTV video streams, whichare widely acknowledged as one of the killer Internet applicationsin the near future. Fibre to the premises (FTTP) solutions arebeing deployed to address the problem of scarce capacity inthe network access. However, their ability to increase accesscapacity by over two orders of magnitude could create substantialscalability problems as power consumption of current IP-centricnetwork architectures in the metro and core nodes will scalelinearly (or worse) with capacity increase.

Long-reach passive optical networks were introduced as a so-lution to this scalability problem, since they considerably reducecost and power consumption for network operators, by allowingoptical bypass of a large number of metro nodes in the network.In this paper we report the results of a deployment study ofa dual-homed protected, nation-wide LR-PON installation forIreland and the UK, based on real dataset. We focus on furtherreducing the overall network power consumption by employinga protection mechanism that reduces the IP router capacityrequired for protection as well as the number of backup OLTs.We show that in a realistic LR-PON deployment up to 40%reduction in power consumption can be achieved, compared tosolutions adopting legacy 1+1 protection. This is accomplished byoptimising the network deployment for energy efficient protec-tion, and adopting a smart load balancing technique and a corenode architecture that allows N:M protection of backup links.

I. INTRODUCTION

There are two major factors that will influence the networksof the future. The first is the need to support extremely highbandwidths and the second is to do so using an architec-ture that is both cost and energy efficient. Internet traffic iscurrently growing at an annual compound aggregate rate of30 to 40%, with video services, dominating the scene [1]. Acompressed HDTV video stream, with bandwidth requirementbetween 8 and 15 Mbps, is sufficient to saturate most DSLsystems running on copper pairs. It is widely acknowledgedthat fibre to the premises (FTTP) is the only access technologycapable of supporting future service demands. Passive OpticalNetworks (PONs) are widely recognized as a viable solutionfor FTTP, by virtue of the ability to share costly equipmentand fibre among a number of customers. However the hugebandwidth capability enabled by FTTP will strain the totalnetwork viability as metro and core networks will need up-grading to support the bandwidth demand but with little returnon investment. This problem has stimulated next generation

PON investigations and among these the Long-Reach PON(LR-PON) [2] has recently gained increasing interest as aneconomic and potentially profitable solution. By extendingthe optical reach to about 100 Km, the number of networknodes can be reduced by as much as two orders of magnitude,bypassing most of the local exchanges and metro transmissionsystems, thereby reducing both cost and energy consumption.

Concerns about global warming has made power consump-tion become a significant design parameter in today’s net-works. Power consumed by world telecoms usage is 1% to 2%of total electricity generated but this could grow to 9% by 2030if business as usual network architectures are simply grown tomeet bandwidth demands [3]. Indeed global warming is notcaused by power consumption itself, but by the greenhousegases generated by electricity production methods based onfossil fuels combustion (e.g., coal, oil and natural gas). Whilein principle the use of renewable energy sources (e.g., solar,wind and tidal power) could halt global warming, in practicethese will only account for a relatively small percentage ofelectricity production in the near future. In addition the costof energy for telecoms operators is rising rapidly. It is thereforeimportant to design networks for minimal power consumption,a target that can be achieved by substituting where possibleactive and power hungry electronic components with passiveoptical elements. The target method of our study is bypassingall electronic packet processing in the metro network by meansof long optical links connecting the access directly to the core.

While the energy efficiency of a LR-PON architecturemight seem obvious, due to the fact there is a significantreduction in electronic equipment, it is important to look atreal deployments to understand exactly what is to be gained.The focus of this paper is on such deployments. In additionwe look at protection mechanisms for the network. Whileprotection mechanisms are optional features of the currentPON standards, we believe that their inclusion gives a realisticunderstanding of the energy usage. Hence in this paper weanalyse LR-PON deployment scenarios for two countries,Ireland and the UK. In our deployments PONs are dual-parented on two adjacent core nodes for protection, and usea load-balancing mechanism we have developed to drasticallyreduce the amount of backup equipment required. We haveused dataset obtained from the two countries major networkoperators, offering the exact position of the local exchanges

and the number of users per local exchange. Our contributionis to show the overall energy efficiency gains of this realisticdeployment. Specifically we show that energy consumptioncan be reduced by decreasing the protection capacity neededat the IP layer (by means of sharing available protectionresources) and by sharing backup OLT cards. Our resilientapproach can recover from the failure of an entire core area(serving an area of up to 14,000 Km2 or as much as halfa million customers), using considerably less IP and OLTprotection equipment compared to current solutions wheresuch equipment is simply doubled. We have built an optimi-sation model that from real local exchange information (e.g.,exchange position and population), locates the optimal positionof the LR-PON core nodes, ensuring that each exchange isprotected against the complete failure of its primary core node,and that the power consumption for the entire network isminimised.

The paper is organized as follows. Section II introduces theprotection issue, emphasizing why it is a requirement for LR-PON networks. Section III describes and explains the modelwe have developed to implement a deployment strategy thatminimizes power consumption. Finally, Section IV reports thepractical outcomes of the model, showing the optimal corenodes positioning for the two scenarios, and their respectivepower consumption.

II. A BRIEF OVERVIEW OF THE PROTECTION ISSUE

Protection mechanisms have been designed into PON stan-dards. However their implementation is an optional feature, asindicated for example in the ITU-T Gigabit Passive OpticalNetwork (GPON) standard: “protection shall be considered asan optional mechanism because its implementation dependson the realization of economical systems” [4].

Fig. 1. Example of LR-PON deployment with dual-parented feeder fibre

The situation, as illustrated in Figure 1, is different forLR-PON deployments. Since it replaces the metro network,which typically offers protection paths to the metro or outercore nodes, protection schemes need to be included in LR-PON deployments. Equipment failure or a fibre cut in the

long distance part (i.e. the feeder fibre) of a LR-PON or atthe optical line termination (OLT) can indeed affect over 500customers, while a cable cut or major equipment failure at acore node could affect tens to hundreds of thousands. In thispaper we propose the use of a dual-parented protection schemefor a LR-PON deployment where each PON is connected totwo afjacent core nodes. The part of the access network weprotect is the feeder fibre, i.e. the portion going from thecore node to the first split (where also the optical amplifieris located). We do not consider protection of the opticaldistribution network (i.e., the portion downstream of the firststage split), because its cost tends to be high as the links areshared among a progressively smaller number of users. We donot envisage that residential customers would require this levelof protection, while business users might opt for individuallyprotected solutions and thus bear its cost.

In order to reduce cost and energy consumption we proposea method that allows sharing the protection transmission pathsand terminal equipment over as many customers as possible.We originally introduced the concept of reducing backupequipment by using load-balancing in [5]. The idea consistsof sharing the IP traffic load of a failed portion of the networkamong a large number of other nodes. For example, looking atFigure 1, if we used 1+1 protection, the IP router in core node-2 would be required to provide enough protection capacityto bear the load of the illustrated LR-PON if core node-1failed. With our approach instead, if core node-1 fails, corenode-2 will offload part of its IP working capacity to othernetwork nodes (load-balancing) in order to free up enoughcapacity to support the traffic load of the illustrated LR-PON.This mechanism highly reduces the IP protection capacityrequirements for all core nodes, compared to 1+1 protection.In this paper we extend this concept by first including thepossibility to share backup OLTs in addition to IP capacity.Then we develop an optimization model that allows us to applyour protected LR-PON architecture to realistic case studies andcompute the energy savings it can achieve.

III. LR-PON DEPLOYMENT MODEL FOR MINIMALENERGY CONSUMPTION

We have modeled the energy-efficient protected LR-PONdeployment as a constraint optimization problem. The ob-jective is to find the optimal location of core nodes and anoptimal load transfer strategy that minimizes overall powerconsumption, by effective sharing of IP capacity and backupOLT cards. The system is designed to be fully resilient to thetotal failure of any individual core node.

Figure 2 shows the proposed interconnection of networkequipment within a core node. The fibre switch is used tocreate dynamic connections between the backup LR-PONlinks and the backup OLT cards. With this configurationthe number N of backup OLTs can be shared among theM backup links, in a N : M protection scheme, whereN is smaller than or equal to M . The value of the ratioN/M depends on the relative positioning between primary andbackup core nodes. While in more advanced architecture this

Fig. 2. Primary ad backup equipment used in the core node

fibre switch could also be used to create dynamic switchingof primary connections (for example to deliver wavelengthswitching between users), for this analysis the function of thefibre switch is limited to allow sharing of backup OLTs. Itsnumber of ports is thus equal to the number of protection links(which is equal to the number of LR-PONs that are protectedby the core node) plus the number of backup OLT cards.

The number of LR-PONs is proportional to the numberof customers served by a given core node. For this analysiswe assume each LR-PON can serve 512 users. Each LR-PON is directly terminated to one 10 Gbps OLT card forprimary service. Each OLT backplane has one managementcard for every 8 OLT cards, and one switch card (running at40 Gbps) for every IP router blade facing the OLT backplane.This configuration assumes that the OLTs cards and (Ethernet)switching cards can all be integrated within the same chassis,which can operate both inter-OLT switching, and aggregationof OLT traffic towards the IP router. Each OLT switching cardis connected to a 40 Gbps IP router blade, and additionalrouter blades are considered for connection to other corenodes, where we assume a ratio of inter-node to intra-nodetraffic of 0.5. Long-reach interfaces for transport in the corebackbone are not considered in this analysis, as they affect allthe scenarios considered equally, thus do not affect the resultsof our comparisons. The values we have considered for thedevice power consumption are reported in Table I and wereobtained by averaging a number of difference sources in theliterature [6], [7], [8], [9].

We have grouped the power consumption of a core nodeinto three contributions: the IP routing devices, the OLT cardsand layer-2 switching, and the optical fibre switch. Theirpower consumption is defined in terms of: the number ofusers connected to a metro node for primary coverage Lp

i ;the number of users the node needs to overprovision for Lo

i ;the number of users that can be simultaneously backed-up bymetro node Lb

i . We generated the traffic load by multiplyingthese quantities by the average sustained data rate per user ub.Let rib denote the data rate of an IP router blade (40 Gbps).Let pit denote percentage of inter-node traffic of metro node.In order to compute the power consumed on a metro-node i wefirst calculate the number of IP and OLT cards for metro-node

i:

IP_intrai = d(Lpi + Lo

i )× ub/ribe

IP_interi = d(Lpi + Lo

i )× ub/ribe × pit

OLT_Pi = dLpi /512e

OLT_Bi = dLbi/512e

IP intra are the number of IP blades connected to the switch-ing cards, IP inter those connecting to other core nodes.OLT P are the primary OLT cards, and are calculated bydividing the number of users connected to a metro node by thenumber of users per PON. OLT B are the backup OLT cards,and are calculated by dividing the number of users that canbe simultaneously backed-up by a metro node by the numberof users per PON.

TABLE IPOWER CONSUMPTION VALUES USED FOR THE CORE NODE EQUIPMENT

Let rcc be the router chassis consumption. Let rfc be therouter fabric consumption. Let icc be the consumption of anIP router blade. The power consumption of the IP-router fora metro-node i:

f1i =d(IP_intrai + IP_interi)/16e × rcc+

d(IP_intrai + IP_interi)/144e × rfc+

(IP_intrai + IP_interi)× icc

The first term is the power consumption of one IP chassis,serving 16 IP blade cards. The second is the power con-sumption of the fabric card shelves used where multi-shelvessystems are required. After the first chassis is full one fabriccard shelf is required for every 144 router blades. The thirdterm is the power consumption of the IP blades.

Let omcc be OLT management card consumption. Letoscc be OLT switch card consumption. Let occ be OLTcard consumption. The power consumption of the OLT and

layer-2 switching cards is:

f2i =d(OLT_Pi + OLT_Bi)/8e × omcc+

IP_intrai × oscc+

(OLT_Pi + OLT_Bi)× occ

The first term is the power consumption of the OLT manage-ment cards (one every 8 OLTs is required). The second term isthe consumption of the switching cards working at 40 Gbps.Since these aggregate traffic destined towards the IP router,their number is equal to the number of IP intra cards. Thethird term is the consumption of the OLT cards (running at10Gbps).

Let opc be optical port consumption. The power consump-tion of the optical switch is:

f3i = (dLs

i/512e+ OLT_Bi)× opc

where Lsi is the total number of users backed-up by metro

node i.After having defined the power consumption elements, we

now describe how we have modeled the core node positioningand the protection load-balancing, to minimise power con-sumption.

The input data considered are the set of all local exchangesin the country (they represent the location of the first-stagesplit of a LR-PON), together with their customer base. Thetraffic load is obtained by multiplying the number of customersby a sustained average data rate value

Constants:- E: set of all current local exchanges in the country (theyrepresent the location of the first-stage split of a LR-PON)- li: population of local exchange i (which will be served bya number of LR-PONs with first-stage split located at i)

Variables:- Mj : geographical position of a core node j (chosen withinthe set of local exchanges)- Pi, Si: respectively primary and secondary (i.e. backup) corenodes of LR-PON i- Lp

i , Lsi : sum of the users of the LR-PON sites for which i

is respectively the primary and secondary metro-node- Lb

i : number of users that can be simultaneously backed-upby metro node i; this is equal to the number of backup OLTsavailable at node i.- Lo

i : number of users for which IP capacity over-provision isrequired over a metro-node i, to protect against any individualcore node failure.- Uij : maximum number of users that can be transferred froma core node i to another core node j when core node i fails.- Tijk: actual total number of users that are passed from a corenode i to another core node j when core node k fails- Iik: sum of incoming users that core node i receives fromits neighbours when core node k fails- Oik: sum of outgoing users that core node i passes to itsneighbours when core node k fails.

Constraints: The number of core nodes to deploy is providedas a design parameter.- Pi 6= Si: The primary and secondary core nodes of anexchange site i are different- (Pi = j)→ ∀r((r 6= j)⇒ d(i,M(j)) ≤ d(i,M(r))),(Si = j) → ∀r((r 6= j ∧ r 6= Pi) ⇒ d(i,M(j)) ≤

d(i,M(r))): any LR-PON first-stage split must be closer toits primary node than to its secondary node- Lp

i =∑

Pe=i le: sum of primary users of a core node- Ls

i =∑

Se=i le: sum of secondary users of a core node- Uij =

∑Pe=i∧Se=j lk: the upper bound on the number of

users that can be transferred from a core node i to anothercore node j is equal to the sum of the users of all LR-PONsites whose primary metro node is i and secondary metro nodeis j. Here the condition Pe = i∧Se = j means that the usersof e are covered by both i and j, and i can off-load them toj, if required.- Lb

i = maxk(∑

j Tjik): the backup load of a metro-node i isthe maximum sum of incoming load that it can receive fromits neighbours j when a metro-node k fails.- Tijk ≤ Uij : for any node failure k the users transferred fromi to j is less than or equal to the corresponding upper bound.- Iik =

∑∀j Tjik, Oik =

∑∀j Tijk: respectively incoming and

outgoing number of users of a metro node i when a metro nodek fails- Ikk = 0: the incoming number of users of k is zero- Okk = Lp

k: when a core node k fails the outgoing numberof users of k is equal to the number of its primary users- Lo

i ≥ (Iik − Oik): when a core node k fails, the over-provision capacity (in terms of users) required on a metronode i such that i 6= k should be greater than the differencebetween the number of its incoming and outgoing usersObjective: The objective is to minimize the power consump-tion that arises from the use of optical fibre switch, OLT cardsand IP routers.

mini∈M

∑f1i + f2

i + f3i

Approach:We have decomposed our approach into two phases. In the

first phase we find a feasible placement of core nodes using amixed integer programing solver, then use a constraint-basedlocal search to improve the quality of the placement. Theprocess is repeated until the search terminates or the timespent reaches the given threshold. We avoid finding the samesolution by adding cuts to the MIP solver and randomizationto the local search. During the second phase we find a loadtransfer strategy that minimizes the over provision since thelatter is a critical factor in the power consumption.

IV. RESULTS

Figure 3, which we use for comparison, shows the broad-band coverage that could be achieved in Ireland using legacycopper pair connections, if all local exchanges were DSL-enabled. The circles in the Figure show the potential broad-band coverage areas of the local exchanges. The maximumcapacity of a DSL connection depends on the distance between

a user and the local exchange. We have considered a maximumgeographical distance of 4 Km, which, after applying a routingfactor of 1.5, gives a maximum link length of 6 Km. Forsuch length, the maximum downstream capacity of DSL overcopper pair is 1 Mbps.

Fig. 3. Areas that can potentially be covered by DSL in Ireland

In Figure 4 and 5 we show the results we obtained forthe LR-PON deployment case studies in Ireland and the UK.They show how core nodes could be deployed to ensure thateach PON is dual-parented to two overlapping core nodes(overlapping of core nodes coverage is required in orderto allow load redistribution in case of failure, see [5] foradditional information). We have found that deploying 20metro nodes was the best trade-off between having enoughnodes to obtain effective protection load balancing and keepingthe number of nodes as low as practicable. For the UK wehave deployed 85 core nodes. If we compare the Ireland LR-PON scenario, to the legacy DSL one reported in Figure 3,we notice that LR-PON provides two of obvious advantages.It increases coverage so that both urban and rural populationcan receive high-speed broadband services, and it reduces thenumber of active nodes by over 50 times (from about 1100local exchanges down to 20 nodes for Ireland).

Although we have not run deployment scenarios of GPONsystems, we can infer a number of advantages of the LR-PONsolution. The 20 Km reach of standard GPON means that themaximum reach is 5 times less than LR-PON, and the areacovered 25 times less. Thus we can expect that in order toachieve a dual-home GPON coverage we would require inthe order of 500 nodes. Although LR-PON does require theuse of optical amplifiers, their cost and power consumptionmore than compensates for the cost and consumption of thelarger number of nodes needed in GPON. In addition, thelonger distance between core nodes in the LR-PON solution,means that the backup node of a given LR-PON is locatedfurther away, compared to a GPON solution, which increasesits resiliency against geographical disasters.

In Figure 6 and 7 we show the power consumption of theLR-PON deployment, respectively, for the Ireland and UK

Fig. 4. High-speed broadband coverage of a dual-parented LR-PONdeployment case for Ireland, with 20 core nodes

Fig. 5. High-speed broadband coverage of a dual-parented LR-PONdeployment case for the UK, with 85 core nodes

scenarios. Each stacked bar in the plot shows the total energyconsumption broken down in: consumption of IP equipment,consumption of OLT termination and electronic switchingequipment, and consumption of the optical fiber switch. Thereader should note that this is only the power consumptionof the equipment shown in Figure 1 and is therefore anunderestimate of the total network energy requirements butis used here only for comparison purposes. The consumptionis shown as a function of the average sustained bandwidthper user, which we vary from 100 Kbps (typical of today’snetworks) to 20 Mbps (the maximum sustained rate of a10Gbps LR-PON shared among 500 users). For each value ofthe user bandwidth we compare the results obtained throughour proposed architecture (second bar) to a scenario where noprotection is provided (first bar - here LR-PONs have onlyone primary connection to the closest core node), and to ascenario (commonly used in today’s networks) where the latterconfiguration is protected with a 1+1 approach, i.e. by simplydoubling OLT cards and IP routing capacity (third bar). The

fiber switch is only used for sharing backup OLT, thus it is onlyincluded in our proposed model. Although we have includedit in the plot, its power consumption is so small compared tothat of OLTs and IP routers that it does not appear visible inthe Figure.

Fig. 6. Results of power consumption analysis for the Ireland networkscenario: a non-protected solution (first bar), our protected energy-optimizedapproach (second bar) and a protected approach obtained by doubling equip-ment (third bar)

Fig. 7. Results of power consumption analysis for the UK network scenario

For low volumes of traffic, most of the consumption is dueto the OLTs, as their number depends only on the number ofusers, and is assumed independent on the user rate. In thiscase the major power saving is due to the reduction of backupOLT cards, and for the Ireland case is about 21% (considering100 Kbps average rate). As user traffic increases the powerconsumption of the IP router becomes dominant, and ourload-sharing mechanism can bring substantial advantage. Bothplots show that our low-consumption protected architecturecan achieve considerable energy savings, up to 35% for Irelandand up to 40% for the UK scenario, compared to simpledoubling of equipment for protection (considering 20 Mbpsaverage rate). Although we have not reported any cost analysis,

we would like to draw attention to the fact that our proposedarchitecture will reduce core node equipment costs, becauseit reduces the amount of power hungry equipment (such asIP routers), which also tend to be among the most expensivenetwork elements.

We would like to observe that in principle 1+1 protectionmight have the advantage of additional protection comparedto our method of load and backup OLT sharing, as the formerdoubles the equipment at each metro node. However this isonly true under the assumption that we are protecting thenetwork against multiple concurrent geographical disasters thatmight occur in locations of the network that are far apart formeach other. Since, statistically, major geographical disasterstend to be localized, we believe our proposed protectionmethod can offer resiliency comparable to a 1+1 approachbut with much lower power consumption (and potentially atlower cost).

V. CONCLUSIONS

In this paper we have reported the results of two casestudies for nation-wide LR-PON deployment, where each PONis protected by dual-parenting to two adjacent core nodes.The study was conducted over two countries, Ireland and theUK, using real network information dataset. Our results showthat a nation-wide LR-PON installation can be designed toconsiderably reduce overall power consumption by adoptingsmart load-balancing and sharing backup OLT cards. We haveobtained overall savings of 35% and 40%, respectively, for theIreland an UK scenarios, compared to deployments that adoptlegacy 1+1 protection schemes. Since such power savings areobtained through optimal placement of the core nodes, it is ofparamount importance that considerations on smart networkprotection are taken into account at the outset of every LR-PON deployment.

ACKNOWLEDGMENT

This work is supported by Science Foundation Ireland GrantNo. 10/CE/I1853. We would like to thank Eircom and BT forproviding information about their internal network structure.

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