energy-saving in ip over wdm networks by putting ......ip over wdm networks achieved by switching...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 14, JULY 15, 2018 3003

Energy-Saving in IP Over WDM Networks byPutting Protection Router Cards to Sleep

Yongcheng Li, Lin Zhu, Sanjay K. Bose, Senior Member, IEEE, and Gangxiang Shen , Senior Member, IEEE

Abstract—Energy saving is of great interest in current informa-tion and communications technology applications. In this paper, weconsider energy saving for an IP over wavelength division multi-plexed (WDM) optical network by putting protection router cardsto sleep when the network is operating normally. Doing this re-quires that we must jointly consider several aspects. These aremulti-layer traffic grooming, network protection, and the assign-ment of router ports/cards for working and protection optical chan-nels. By employing different network protection techniques and themulti-hop traffic grooming strategy, we develop mixed integer lin-ear programming (MILP) models for the optimization problem byconsidering five different router port assignment schemes. Theseaim to minimize the total power consumption of the whole net-work. Subsequently, to tackle the computational difficulty of theMILP models, we propose an efficient heuristic algorithm to han-dle large networks. This heuristic algorithm consists of two steps,i.e., survivable traffic grooming and router port assignment. Re-sults show that the proposed energy-saving strategy is efficient andcan significantly reduce the total power consumption of an IP overWDM network, where the assignment strategy for router cards is ofprimary importance for energy-saving efficiency. We also observethat the proposed heuristic algorithm performs almost as well asthe MILP models in terms of the energy efficiency of the system.

Index Terms—Card and port protection, energy saving, routercard sleeping, shared backup path protection (SBPP).

I. INTRODUCTION

DRIVEN by the explosive growth of Internet traffic, morenetwork equipments with high transmission speeds and

switching capacity are being deployed [1]. This contributes toincreasing energy consumption in the ICT sector, which is esti-mated to reach about 4.6% of the total electricity consumption

Manuscript received November 26, 2017; revised April 4, 2018; acceptedApril 16, 2018. Date of publication April 26, 2018; date of current versionJune 8, 2018. Part of this paper was presented in OFC 2017 [34]. This workwas supported in part by the projects of “Famous City and Famous Univer-sity” of Suzhou City, National Natural Science Foundation of China (NSFC)(61671313), and in part by the Science and Technology Achievement Trans-formation Project of Jiangsu Province, China (BA2016123). (Correspondingauthor: Gangxiang Shen.)

Y. Li, L. Zhu, and G. Shen are with the School of Electronic and Informa-tion Engineering, Soochow University, Suzhou 215006, China (e-mail:, [email protected]; [email protected]; [email protected]).

S. K. Bose is with the Department of Electronics and Electrical Engineer-ing, Indian Institute of Technology Guwahati, Assam 781039, India (e-mail:,[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2018.2830380

in the world [2]. Moreover, this is expected to grow even furtherin the future as the total Internet traffic has been predicted toincrease by a factor of 50 in the next 10–15 years [3]. The fastgrowth of energy costs and their serious environmental impacton Green House Gases (GHG) emissions will adversely affectthe sustainability of this growth trend [4]. This has led to increas-ing research interest in methods to reduce energy consumptionin the ICT sector and in telecommunication networks [4].

IP over WDM is a dominant transmission and switching tech-nology widely deployed in today’s backbone and metro opticaltransport networks. These would therefore contribute a majorpercentage of the energy consumption in telecommunicationnetworks. To reduce the energy consumption of IP over WDMoptical networks, different energy-saving strategies and tech-niques have been proposed. These include lightpath bypass [5],multi-clock frequencies [6], solar energy [7], [8], adaptive linkdata rate [9]–[11], and network device sleeping [12]–[15]. Thisstudy focuses on energy saving by putting network devices orcomponents to sleep.

To maximize the energy saving, we consider different networkprotection techniques and scenarios, including Shared BackupPath Protection (SBPP), Path Restoration (PR), and port-basedand module-based protection. We also develop MILP modelssubject to various system constraints. In addition, for better nu-merical tractability in handling large networks, efficient heuris-tic algorithms are also proposed. The results show that theproposed protection module-based sleeping strategy is effi-cient and significantly reduces network power consumption.We also observe that the heuristic algorithms proposed are ef-ficient enough to perform close to the MILP models. Finally,by comparing different protection schemes, we find that theenergy-saving performance of SBPP is better than PR when thenetworks get larger.

The remainder of this paper is structured as follows. Section IIreviews the related work. Section III introduces the SBPP and PRprotection techniques and five different strategies for networkmodule assignment. Section IV presents the MILP model forthe energy-saving design based on the protection module-basedsleeping strategy. Section V describes our proposed heuristic al-gorithm for an energy-saving design that consists of the steps ofSBPP-based IP over WDM traffic grooming and module-basednetwork port assignment. In Section VI, we look at study casesunder different test conditions. The results for the different opti-mization approaches are presented and discussed in Section VII.Section VIII concludes the paper.

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3004 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 14, JULY 15, 2018

II. RELATED WORK

Extensive studies have been conducted to reduce power con-sumption by putting network devices or components to sleepwhen they are not needed. Depending on whether or not net-work protection is also being considered, these studies can bedivided into two categories. The first category does not considernetwork protection and saves power consumption by putting tosleep network equipment that provides working capacity whenthey are not needed. Cerutti et al. [12] proposed to enable link-based sleeping in a wavelength-switched optical network. Re-sults show that their proposed strategy can effectively reduce theenergy consumption of the whole network. Moreover, a tradeoffwas also observed between energy saving and network blockingperformance. Chiaraviglio et al. [13] evaluated the effectivenessof sleeping modes in a backbone network with limited config-uration capability and showed that the savings obtained with afew configurations (at most three) are efficient enough to be only10% less than the maximum saving that can be achieved. Pageset al. [14] evaluated how the duration of the wake-up time affectsthe network performance in terms of blocking probability. Theyalso proposed a high-performance routing algorithm to reducethe blocking probability. For a system where both sleep-enabledand non-sleep-enabled router cards are used, Zhao et al. [15]proposed a MILP model and an efficient heuristic algorithm toreduce the energy consumption of IP over WDM networks forgreater efficiency.

An undesirable side effect of putting working capacityto sleep is the impact that this has on the user applicationssupported by this working capacity. To avoid this, researchershave also considered putting only the equipment that providesprotection capacity to sleep, while leaving all the equipment’sworking capacity untouched. There have been several studiesin this direction. For example, Muhammad et al. [16] evaluatedthe sleeping option for optical devices (e.g., amplifiers, opticalswitches) installed for protection purposes. Results showed thatup to 25% overall power consumption can be saved compared tothe scenario where this is not done in the optical layer. Wiatr et al.[17] investigated the trade-off between green network operationsand the reliability performance of optical backbone devices. Thestudy also presented a number of models to estimate the reliabil-ity performance changes of a device as a function of its averageworking temperature, temperature variations, and average occu-pancy. Musumeci et al. [18] compared the power-saving perfor-mance of the sleeping mode for four protection strategies (i.e.,Shared-Link, Shared-Path, Dedicated-Link, and Dedicated-Pathprotection) and found that high power savings (up to about 60%)can be obtained for all these protection strategies. Moreover,Musumeci et al. [19] also provided an ILP model and threepower-reduction strategies for designing power-minimizedresilient optical core networks. Results showed that the overallenergy savings obtained over a whole day can reach as high as44%.

A side effect of protection capacity sleeping is the time itwould take to wake up the sleeping equipment when a networkfailure does occur and fast restoration is desired. The normal

standard of fast restoration in the SDH/SONET network is that itshould be done within 50 ms. For data communication networksin use today, this latency may be relaxed to 100∼200 ms. Ac-cording to the current state-of-the-art, it is possible to wake up asleeping network device (e.g., an ONU) within 1 ms. For exam-ple, Suzuki et al. [20] fabricated a self-sustained fast-lock clockand data recovery (CDR) circuit IC and achieved a fast wake-up time for an ONU within 497 ns. Similarly, Koizumi et al.[21] reduced the wake-up time to 16 ns, which makes efficientburst-by-burst power saving feasible. Thus, based on the sleep-ing technology of today’s network equipment, it is possibleto put network protection equipment to sleep and wake it upquickly without slowing down the network restoration speed.This is a key aspect motivating us to look into the protectioncapacity-based sleeping strategy for saving energy in an IP overWDM network.

In the existing literature available on protection capacitysleeping, we notice that most studies have been done basedon the assumption that each network port can be put to sleepindependently. However, today’s network equipment tends tobe modularized, where multiple modules are connected to acommon switching backplane and each module has multiplenetwork ports sharing a common circuit board and clock signal[22]. As a result, the strategy of putting each individual net-work port to sleep would be difficult and probably unrealistic.A more practical solution would be to put network equipmentto sleep in a modularized manner. Some studies have been car-ried out for energy saving in such a modularized configuration.Idzikowski et al. [23] estimated the potential energy savings inIP over WDM networks achieved by switching off router linecards during low-demand hours. In [24], they also presenteda detailed survey of approaches to reduce energy consumptionfor optical networks. Zhong et al. [25] presented a comprehen-sive study on applying stateful grooming to achieve an energy-efficient performance enhancement in IP-over-optical networkswhen tackling the tidal traffic problem. Lui et al. [26] evaluatedthe energy consumption of an IP over WDM network based onmodularized router cards and found that a mixed deploymentof different types of router cards consumes less energy thanthe case when a single type of router card is used. Coiro et al.[27] proposed an approach to switch the router card to a newstate with a low energy consumption, which can significantlyimprove the network’s energy efficiency.

In summary, even though energy saving in an IP over WDMnetwork has been extensively tried by putting network compo-nents to sleep and by module-based optimization design, thetwo efforts were carried out independently, not jointly. As akey contribution and novelty of this study, we consider energysaving for an IP over WDM network by putting to sleep modu-larized router cards being used for protection. Here we define acard as a protection router card if each of its ports is either un-used or used for protection; otherwise, we call it working routercard. This is different from prior studies which either focus onputting each network port to sleep or design an IP over WDMnetwork in a modularized manner without considering networkprotection.

LI et al.: ENERGY-SAVING IN IP OVER WDM NETWORKS BY PUTTING PROTECTION ROUTER CARDS TO SLEEP 3005

Fig. 1. Example of IP over WDM network. (a) Physical topology. (b) Multi-layer architecture with lightpath bypass.

III. RELATED CONCEPTS

A. Network Model

Our focus in this paper is on IP over WDM network, as shownin Fig. 1, where Fig. 1(a) shows the physical topology of a three-node network and Fig. 1(b) shows the multi-layer architecturefor this network. The nodes in the optical layer are Optical Cross-Connects (OXCs) and the nodes in the IP layer are routers. AnIP router connects to an OXC via a short-reach interface and theIP traffic flowing between routers are groomed or aggregatedvia optical channels in the optical layer. The efficiency of trafficgrooming can directly impact the number of optical channelsestablished and consequently the energy consumed. Lightpathbypass is one of the most effective ways to reduce the numberof lightpaths established and consequently the energy consumedby an IP over WDM network. As an example, a direct lightpath(A-C) is established between node pair (A, C) that bypasses anintermediate node B to avoid O-E-O conversion, which wouldsave on power consumption.

B. Network Protection

In this section, we introduce two path-based network protec-tion techniques, i.e., SBPP and PR, which are efficient in theircapacity utilization. To account for the level of equipment dis-jointedness between a pair of working and protection paths, wealso consider port and module-based protection scenarios.

1) SBPP and PR: SBPP and PR are two representative path-based protection techniques that enable protection capacitysharing between different protection paths [28]–[30]. The keydifference between them is the dependence on network failurewhen choosing a protection path for network recovery. SBPPis a failure-independent protection technique, which ignoresthe actual location of the network failure and always uses thesame protection path for failure recovery between a particularnode pair. In contrast, PR is more efficient as it can flexiblychoose different protection paths for different network failurelocations. As a result, PR would be more efficient than SBPP inspare capacity sharing, but its operation would be much morecomplicated than that of SBPP.

Fig. 2. Examples of SBPP and PR protection techniques. (a) SBPP. (b) PR.

Fig. 2 illustrates examples for SBPP and PR, respectively. InFig. 2(a), assume that there is a protected service establishedbetween node pair (A, I) via route A-D-F-I. Under SBPP, thesame protection path that is established along route A-C-E-H-Iis used to provide protection for any link failure on the work-ing path A-D-F-I. SBPP is efficient as it allows spare capacitysharing between different protection paths on their commonlinks as long as their corresponding working paths do not shareany common link. For example, the protection path A-C be-tween node pair (A, C) can share the protection capacity on link(A-C) with protection path A-C-E-H-I since their correspondingworking paths do not share any common link.

In contrast to the fixed protection path in SBPP, PR is moreflexible as it allows the use of different protection paths torecover from different link failures. In Fig. 2(b), for the failureof link (D-F), PR may employ A-D-C-E-F-I to recover fromthe failure, while for the failure of link (A-D), it may chooseA-C-D-E-F-I to recover from the failure. As a result, PR ismore efficient in spare capacity sharing than SBPP. However,its control system is more complex as it would now need tomaintain a more complicated network state database to allow itto store different protection paths for different network failurescenarios.

2) Network Card and Port Disjointedness: For different lev-els of network protection, we consider port disjointedness at thesource and destination nodes for a pair of working and protectionoptical channels as shown in Fig. 3. Different port protectionschemes are considered, which can be divided into two cate-gories, one at the network port level and the other at the networkcard level. For the port level, we require a pair of working andprotection optical channels to use different ports, which can befurther divided into three sub-cases, which are (1) 1+1/1:1 ran-dom port protection, (2) 1+1/1:1 interleaving port protection,and (3) port sharing by working and protection optical channels.


Fig. 3. Different port protection schemes.

Fig. 4. Different protection strategies at the port level. (a) W/P port sharing.(b) 1+1/1:1 random port protection. (c) 1+1/1:1 interleaving port protection.

Fig. 4(a) illustrates the scenario of port sharing by workingand protection optical channels, where a pair of working andprotection optical channels shares a common port. Fig. 4(b) il-lustrates the scenario of 1+1/1:1 random port protection, wherea pair of working and protection optical channels is assignedwith a pair of dedicated working and protection ports, but thereis no specific requirement on where these two ports should belocated on hardware modules. They can be on the same moduleor randomly on different modules. For example, W1 and P1 cor-responding to a pair of working and protection optical channelsare located on two different modules. In contrast, 1+1/1:1 inter-leaving port protection (as shown in Fig. 4(c)) sets a constrainton the locations of these two ports; they must be located on thesame module and assigned in an interleaving manner. That is,once the working optical channel is assigned with a port, itscorresponding protection optical channel should be assigned toa port neighboring to the working port on the same module.For example, W1 and P1 are neighboring, assigned on the samemodule.

In addition to the port level protection, we may further con-sider a higher level of module (or card) protection, under whicha pair of working and protection optical channels between eachpair of nodes should be established via ports on different networkmodules. As shown in Fig. 3, two different protection scenar-ios may be considered, i.e., (1) card protection with mixed porttypes and (2) card protection with dedicated port types.

Fig. 5(a) illustrates the scenario of card protection with mixedport types, where the working and protection optical channelsof each node pair are established via two ports on different cards

Fig. 5. Protection at the card level. (a) Card protection with mixed port types.(b) Card protection with dedicated port types.

and each card contains a mix of working and protection ports.For example, working and protection optical channels W1 andP1 are established via ports on cards C1 and C2, respectively,and each card contains a mix of working and protection opticalchannel ports.

As a comparison, Fig. 5(b) shows an example of card pro-tection with dedicated port type, under which each router cardcarries the same type of ports. For example, WC1 carries allthe working ports and PC1 carries all the protection ports. Fromthe energy-saving perspective, card protection with dedicatedport type has the advantage of putting protection router cardsto sleep when a network is in the normal state, through whichenergy consumption can be reduced. In contrast, no such a typeof sleeping option is allowed for the case of mixed port alloca-tion since on each card one can have both active working andprotection optical channel ports.

3) Protection Router Card Sleeping and Equipment PowerConsumption Model: In IP over WDM networks, the powerconsumption of the IP router is significantly higher than that ofthe devices in the optical layer [31]. Therefore, in this study, weonly consider the power consumption of the IP layer, but ignorethat for the optical layer. To estimate the power consumption ofan IP router, we develop a power consumption model as shownin Fig. 6. A router chassis consists of three parts, including(1) a power supply system, (2) router line cards, and (3) a fansystem. The power supply and fan systems are always on, andtherefore form an overhead θs of the power consumption ofthe whole chassis. Note that in this overhead, there can alsobe power consumption from systems like the chassis controller.Apart from the overhead power consumption, the remainingpower consumption is from the line cards, of which some couldbe fully loaded while the others could be sleeping. Consideran example where we assume that there are 8 line cards with4 functioning as working cards (fully loaded) and the other4 functioning as protection cards (sleeping). If the total powerconsumption of the chassis is assumed to be PF ull units when allof its line cards are fully loaded, then the power consumption ofeach active line card is calculated as Pa = (PF ull · (1 − θs)) /8units.1 Furthermore, if we assume that the power consumptionof a line card that is sleeping, i.e., Ps , is about 10% of the powerconsumption of its fully loaded status, then we can estimate the

1Note that this is a simplification of incorporating power consumed by switchfabric into power consumed by the router cards.


Fig. 6. Power consumption model of a router chassis.

total power consumption of this router chassis as

P = PF ull · θs + (‖W‖ + 0.1 · ‖S‖) · Pa

where ‖W‖ is the total number of working cards that are fullyloaded and ‖S‖ is the total number of protection cards that canbe put to sleep for energy saving. PF ull · θs is overhead powerconsumption of the chassis and (‖W‖ + 0.1 · ‖S‖) · Pa is thetotal power consumption of router cards.

IV. ENERGY-MINIMIZED IP OVER WDM NETWORK DESIGN

WITH PROTECTION ROUTER CARD SLEEPING

We adopt the lightpath bypass strategy for IP over WDM traf-fic grooming because of its verified energy efficiency [5]. Fornetwork protection, two types of path-based protection tech-niques, i.e., SBPP and PR, are considered jointly along withfive different schemes for router port protection (see Fig. 3). Asa representative design scenario, we focus in this section on theenergy-minimization optimization problem based on the SBPPprotection technique and card protection with dedicated porttype. For this problem, we have the following major inputs:

1) A physical topology Gp = (N, L), which consists of aset of nodes N and a set of links L.

2) A forecast demand matrix Λ, of which each element indi-cates the traffic demand between each node pair (s, d).

3) Each link carries W wavelengths with each wavelengthtransmitting a B-Gb/s capacity.

4) Other given inputs include the maximum total power con-sumption of a router chassis PF ull when all the routercards are fully loaded, the power consumption overheadof the chassis θs , the power consumption overhead of arouter card when it is put to sleep α, the total numberof router cards hosted by each router chassis β, and thenumber of ports on each router card Ω .

All these inputs are used as the parameters of the subsequentMILP model. Additional terms for the MILP model are givennext.

A. MILP Optimization Models

Indices:s and d Indices of the source and destination nodes of end-

to-end user traffic demand where this traffic demandis routed over a virtual topology created by opticalchannels established in the optical layer.

i and j Indices of nodes in the virtual topology. A virtual linkconnects the two end nodes, which correspond to apair of IP routers connected by end-to-end opticalchannels.

m and n Indices of nodes in the physical topology. A physicallink connects the two end nodes, which are neigh-boring nodes in the physical topology.

Sets:Nm Set of neighboring nodes of node m in the physical

topology.Si,j Set of physical links traversed by the working virtual

link between node pair (i, j). We assume that all theoptical channels on the working virtual link take thesame physical route.

S′i,j Set of physical links traversed by the protection vir-

tual link between node pair (i, j). We assume thatall the optical channels on the protection virtual linktake the same physical route.

K Set of optical channels contained on a virtual link.Because the exact number of optical channels to beestablished between each node pair is not pre-knownbefore a design, we set the size of K to be as large aspossible with its size just equal to the total numberof wavelengths in each fiber link W .

CIi Set of router cards at node i (similar to K, becausethe exact number of router cards at each node is notpre-known before a design, sufficient router cardsare assumed at each node).

P I Set of router ports on each router card (without losinggenerality, we assume that each router card containsfour router ports).

Parameters:Zi,j

l A binary value, which is equal to 1 if working virtuallink (i, j) traverses physical link l; otherwise, 0.

Z ′i,jl A binary value, which is equal to 1 if protec-

tion virtual link (i, j) traverses physical link l;otherwise, 0.

∇ A large value.

Variables:Vi,j Number of optical channels established on working

virtual link (i, j) (integer).V ′

i,j Number of optical channels established on protec-tion virtual link (i, j) (integer).

βs,di,j Amount of traffic demand between node pair (s, d)

that traverses working virtual link (i, j) (real).


β′s,di,j Amount of traffic demand between node pair (s, d)

that traverses protection virtual link (i, j) (real).δs,di,j Takes the value of 1 if the traffic demand between

node pair (s, d) traverses working virtual link (i, j);otherwise, 0 (binary).

δ′s,di,j Takes the value of 1 if the traffic demand between

node pair (s, d) traverses protection virtual link(i, j); otherwise, 0 (binary).

θs,dl Takes the value of 1 if the working virtual link of

node pair (s, d) traverses physical link l; otherwise,0 (binary).

ys,di,j,l Amount of traffic demand between node pair (s, d)

recovered via protection virtual link (i, j) upon linkfailure l (real).

Uai Takes the value of 1 if router card a is activated at

node i; otherwise, 0 (binary).Wa

i Takes the value of 1 if any port on router card a atnode i is used to establish working optical channels(which means that the card cannot be put to sleep);otherwise, 0 (binary).

Sai Takes the value of 1 if all occupied ports on router

card a at node i are used to establish protectionoptical channels (which means that the card can beput to sleep); otherwise, 0 (binary).

Ci Total number of activated router cards at node i(integer).

Csi Total number of activated router cards that can be

put to sleep at node i (integer).Si Total number of activated router chasses at node i

(integer). As long as there is an activated router cardin a chassis, then this chassis is considered activated.

Vi,j,k Takes the value of 1 if the kth optical channel onworking virtual link (i, j) is established; otherwise,0 (binary).

V ′i,j,k Takes the value of 1 if the kth optical channel on pro-

tection virtual link (i, j) is established; otherwise, 0(binary).

xa,bi,j,k Takes the value of 1 if port b of router card a at node

i is used for establishing the kth optical channel onworking virtual link (i, j); otherwise, 0 (binary).

x′a,bi,j,k Takes the value of 1 if port b of router card a at node

i is used for establishing the kth optical channel onprotection virtual link (i, j); otherwise, 0 (binary).

Objective: minimize

∑

i∈N

PF ull · θs · Si +∑

i∈N

Ci · PF ull · (1 − θs)β

+ α·

∑

i∈N

Csi ·

PF ull · (1 − θs)β

(1)

Objective (1) aims to minimize total power consumption ofthe whole network. Here, for the calculation of power consump-tion, we have only considered the power consumption of routers.We ignored the power consumption of the optical layer since itwas found that this is relatively much smaller than the powerconsumption of the routers [5]. The term

∑i∈N PF ull · θs · Si

is the total power consumption overhead of the chassis, the term∑i∈N Ci · PF u l l ·(1−θs )

β is the total power consumption of work-

ing router cards, and the term∑

i∈N Csi · PF u l l ·(1−θs )

β is the totalpower consumption of the protection router cards that can beput to sleep.

Subject to:− On virtual topology establishmentConstraints (2)–(15) are for the virtual topology establish-

ment, which include both working and protection virtual topolo-gies.

∑

j∈N :i �=j

βs,di,j −

∑

j∈N :i �=j

βs,dj,i =

⎧⎨

⎩

λsd i = s−λsd i = d0 otherwise

∀i, s, d ∈ N : s �= d (2)

∑

j∈N :i �=j

β′s,di,j −

∑

j∈N :i �=j

β′s,dj,i =

⎧⎨

⎩

λsd i = s−λsd i = d0 otherwise

∀i, s, d ∈ N : s �= d (3)

Constraints (2) and (3) ensure the flow conservation con-straints for both working and protection traffic flows in the IPlayer, respectively.

λs,d · δs,di,j = βs,d

i,j ∀i, j, s, d ∈ N : s �= d, i �= j (4)

λs,d · δ′s,di,j = β′s,d

i,j ∀i, j, s, d ∈ N : s �= d, i �= j (5)

Constraints (4) and (5) ensure that the IP traffic flow betweeneach pair of nodes is not split or transmitted via multiple pathsfor both working and protection traffic demand, respectively.

∑

s,d∈N : s �=d

βs,di,j ≤ B · Vi,j ∀i, j ∈ N : i �= j (6)

Constraint (6) finds the smallest number of optical channelsthat should be established on each working virtual link to ac-commodate all the working traffic demands traversing virtuallink (i, j).

βs,di,j = βd,s

j,i ∀i, j, s, d ∈ N : s �= d, i �= j (7)

β′s,di,j = β′d,s

j,i ∀i, j, s, d ∈ N : s �= d, i �= j (8)

Constraints (7) and (8) ensure that the IP flow between eachnode pair is symmetric and traverse the (lightpath) virtual topol-ogy bi-directionally for both the working and protection flows,respectively.

δ′s,di ′,j ′ · Z ′i′,j ′

l ≤ 1 − δs,di,j · Zi,j

l ∀l ∈L; i′, j′, i, j, s, d ∈ N : s �= d, i �= j, i′ �= j′ (9)

Constraint (9) ensures that the working and protection flowsbetween a pair of nodes (s, d) do not share any common physicallink. According to the definition of SBPP, there is only onefailure-independent protection path between a node pair for therestoration of any network failure.

θs,dl ≥ δs,d

i,j · Zi,jl ∀l ∈ L; i, j, s, d ∈ N : s �= d, i �= j (10)


Constraint (10) aims to ensure that the physical link l tra-versed by working virtual link (i, j) is also traversed by nodepair (s, d) when the traffic demand between node pair (s, d)traverses working virtual link (i, j).

∑

j∈N : i �=j

δ′s,di,j ≤ 1 ∀i, s, d ∈ N : s �= d (11)

Constraint (11) aims to ensure such a single protection pathin the IP layer.

ys,di,j,l ≤ λsd · θs,d

l ∀l ∈ L; i, j, s, d ∈ N : s �= d, i �= j (12)

ys,di,j,l ≤ β′s,d

i,j ∀l ∈ L; i, j, s, d ∈ N : s �= d, i �= j (13)

ys,di,j,l ≥ β′s,d

i,j − λsd ·(1 − θs,d

l

)∀l ∈ L; i, j, s, d ∈

N : s �= d, i �= j (14)

Constraints (12)–(14) jointly find the amount of traffic de-mand between node pair (s, d) to be recovered on protectionvirtual link (i, j) upon the failure of physical link l. When thefailure of link l does not affect the working flow between nodepair (s, d), i.e., θs,d

l = 0, then ys,di,j,l should be zero because of

constraint (12); otherwise, it should equal β′s,di,j .

∑

s,d∈N : s �=d

ys,di,j,l ≤ B · V ′

i,j ∀l ∈ L; i, j ∈ N : i �= j (15)

Constraint (15) means that a sufficient number of opticalchannels should be established on protection virtual link (i, j)so as to recover all the affected working paths upon the failureof physical link l. The term

∑s∈N,d∈N : s �=d ys,d

i,j,l finds the totalamount of traffic demand of all the affected node pairs that areto be recovered by protection virtual link (i, j).

– On router port assignmentConstraints (16)–(28) correspond to the constraints of router

port/card assignment.∑

k∈K

Vi,j,k = Vi,j ∀i, j ∈ N : i �= j (16)

∑

k∈K

V ′i,j,k = V ′

i,j ∀i, j ∈ N : i �= j (17)

Constraints (16), (17) find the total number of working andprotection optical channels to be established on their respectivevirtual links.

Uai ≥ xa,b

i,j,k ∀a ∈ CIi ; b ∈ PI; k ∈ K; i, j ∈ N : i �= j

(18)

Uai ≥ x′a,b

i,j,k ∀a ∈ CIi ; b ∈ PI; k ∈ K; i, j ∈ N : i �= j

(19)

Constraints (18), (19) mean that if a router card is used toestablish working or protection optical channels, then it must beactivated.

Wai ≥ xa,b

i,j,k ∀a ∈ CIi ; b ∈ PI; k ∈ K; i, j ∈ N : i �= j (20)

Constraint (20) means that a router card cannot be put to sleepif any of its ports is used to establish working optical channels.

∑

k∈K,j∈N :i �=j

xa,bi,j,k +

∑


x′a,bi,j,k ≤ 1 ∀i ∈

N ; a ∈ CIi ; b ∈ PI (21)

Constraint (21) ensures that each router card port can onlybe used to establish either a working or a protection opticalchannel.

Sai ≥ x′a,b

i,j,k ∀a ∈ CIi ; b ∈ PI; k ∈ K; i, j ∈ N : i �= j (22)

Constraint (22) means that under card protection with dedi-cated port type, as long as there is a router port used to establisha protection optical channel, then this router card is the one usedto establish optical channels and it can therefore be put to sleep.

Sai + Wa

i = Uai ∀i ∈ N ; a ∈ CIi (23)

Constraint (23) ensures that when a router card is activated,it can only be used as either a working or a protection card.

∑

a∈C Ii ,b∈P I

xa,bi,j,k = Vi,j,k ∀k ∈ K; i, j ∈ N : i �= j (24)

∑

a∈C Ii ,b∈P I

x′a,bi,j,k = V ′

i,j,k ∀k ∈ K; i, j ∈ N : i �= j (25)

Constraints (24), (25) ensure that there are sufficient numbersof ports provided by the router cards to establish the opticalchannels on the working or protection virtual link (i, j).

Ci =∑

a∈C Ii

Uai ∀i ∈ N (26)

Constraint (26) finds the total number of router cards activatedat node i.

Csi =

∑

a∈C Ii

Sai ∀i ∈ N (27)

Constraint (27) finds the total number of router cards that areactivated but can be put to sleep at node i.

β · Si ≥ Ci ∀i ∈ N (28)

Constraint (28) finds the total number of router chassis re-quired to accommodate all the router cards at node i.

B. Model Decomposition

The joint MILP model can be used to optimize the total powerconsumption of an IP over WDM network. However, the jointMILP model is only suitable for small size networks. Becauseof its high computational complexity, it is difficult to use this tofind an optimal solution for large-size networks. To alleviate thecomputational complexity, we divide the optimization probleminto two sub-problems and develop their corresponding MILPmodels.

The first sub-problem is to establish a virtual topology thatminimizes the total numbers of established working and protec-tion optical channels based on the SBPP protection technique.The second sub-problem is to compute the total network power


consumption under different card and port protection schemes.For these two sub-problems, we develop two separate opti-mization models, respectively. We present these two models asfollows.

The first MILP model is for the sub-problem of VirtualTopology Establishment (VTE), which is given as follows

Objective: minimize

∑

i,j∈N :i �=j

(Vi,j + V ′

i,j

)(29)

Subject to:This sub-problem is subject to constraints (2)–(15) in the

joint MILP model. Constraints (2)–(9) ensures the establishmentof the virtual topology and constraints (10)–(15) ensure thecondition of shared backup path protection in the IP layer.

Based on the virtual topology found by the first MILP model,the numbers of working and protection optical channels es-tablished on each virtual links (Vi,j & V ′

i,j ) are used as inputparameters in the second sub-problem, called the Router PortAssignment (RPA) problem. This aims to assign router portsto establish working and protection optical channels. There canbe five different port assignment schemes as shown in Fig. 3.Here as a representative one, we consider the scheme of cardprotection with dedicated port type. The objective of the secondMILP model is to minimize the power consumption by appropri-ately assigning ports to establish working and protection opticalchannels. This MILP model is given as follows.

Objective: minimize

∑

i∈N

PF ull · θs · Si +∑

i∈N

Ci · PF ull · (1 − θs)β

+ α·

∑

i∈N

Csi ·

PF ull · (1 − θs)β

(30)

Subject to:This sub-problem is subject to constraints (16)–(28).

C. Computational Complexities

The joint MILP model has a total of O (|N |4 · |L|) variables(due to variable ys,d

i,j,l) and O (|N |4 · |L|) constraints (due toconstraints (12)), where |N | is the total number of network nodesand |L| is the total number of network links. For the decomposedmodels, the first MILP model has the same total numbers ofvariables and constraints as those of the joint MILP model. Thesecond MILP model has a total of O(|N |2 · |CI| · |PI| · |K|)variables (due to variable xa,b

i,j,k ) and O(|N |2 · |CI| · |PI| · |K|)constraints (due to constraints (18)). Here, |CI| is the maximumnumber of router cards deployed at a network node, |PI| is thenumber of ports on each router card ( |PI| = 4 in this study),and |K| is the maximum number of optical channels establishedbetween each node pair. The computational complexities of theMILP models would increase significantly when the networksize becomes large.

D. Other Circumstances

By extending the MILP model for the basic scheme withSBPP and card protection with dedicated port type, we alsoconsider other circumstances with the PR protection techniqueand various port protection schemes. Their MILP models arepresented next.

1) Path Restoration (PR): We consider the PR technique forperformance comparison. The MILP model of PR is modifiedbased on the model of SBPP. For this, a new variable is definedas follows:ρs,d

i,j,l Takes the value of 1 if traffic demand between node pair(s, d) is recovered by protection virtual link (i, j) uponthe failure of physical link l; otherwise, 0. This variableis used to replace δ′s,d

i,j in the SBPP model.The MILP model of PR has the same objective as that of the

SBPP. For the constraints, we use the following new constraintsto replace constraints (3), (5), (8)–(14).

∑

j∈N :i �=j

ys,di,j,l −

∑

j∈N :i �=j

ys,dj,i,l =

⎧⎨

⎩

λsd · θs,dl i = s

−λsd · θs,dl i = d

0 otherwise

∀l ∈ L; i, s, d ∈ N : s �= d (31)

ys,di,j,l = ys,d

j,i,l ∀l ∈ L; i, j, s, d ∈ N : s �= d, i �= j (32)

ys,di,j,l = λsd · ρs,d

i,j,l ∀l ∈ L; i, j, s, d ∈ N : s �= d, i �= j

(33)

Constraints (31)–(33) ensure the flow conservation constraintin the IP layer, in which the restored IP traffic flow between apair of routers upon the failure of any link l is bidirectionallysymmetric and should not be split or transmitted via differentroutes.

We can also have the MILP models for the other port pro-tection schemes based on the basic MILP model. Though theseextensions are based on the SBPP protection technique, they arealso effective for the PR technique. We present these extensionsas follows.

2) Card Protection With Mixed Port Types: For this scheme,we need to ensure the card disjointedness between a pair ofworking and optical channels that have a protection relation-ship. However, it is not required for a card to carry the sametype of optical channels. We have the same sets, parameters,and variables as defined before. We define the following newvariables.ζai,j,k Takes the value of 1 if the router card a is used for es-

tablishing the kth optical channel on the working virtuallink (i, j); otherwise, 0 (binary);

ζ ′ai,j,k Takes the value of 1 if the router card a is used forestablishing the kth optical channel on the protectionvirtual link (i, j); otherwise, 0 (binary).

In addition to constraints (2)–(21) and (23)–(28), we also needto replace constraint (22) with the following new constraints.

ζai,j,k ≥ xa,b

i,j,k ∀k ∈ K; i, j, s, d ∈ N : s �= d, i �= j (34)

ζ ′ai,j,k ≥ x′a,bi,j,k ∀k ∈ K; i, j, s, d ∈ N : s �= d, i �= j (35)


Constraints (34)–(35) imply that if there is a port on a card thatis used to establish a working or protection optical channel, thenwe say that this card is the one that this working or protectionoptical channel is associated with.

ζai,j,k + ζ ′ai,j,k ≤ 1 ∀k ∈ K; i, j, s, d ∈ N : s �=

d, i �= j (36)

Constraint (36) ensures the constraint of card disjointednessbetween a pair of working and protection optical channels thathold a protection relationship; that is, the working and protectionoptical channels must be established on different router cards.

In addition, constraint (23) is used to judge whether a cardcan be put to sleep based on the current port assignment.

3) 1+1/1:1 Random Port Protection: This scheme only re-quires that a pair of working and protection optical channelshaving a protection relationship to be assigned to different routerports. These ports can be assigned on any router card. For itsMILP model, we have the same sets, parameters, and variablesas defined before. We also have the same objective as in (1). Forthe constraints, we only need to keep constraints (2)–(21) and(23)–(28), but constraint (22) can be removed since, under thecurrent port protection scheme, a pair of working and protectionoptical channels that have a protection relationship is not subjectto card disjointedness.

4) 1+1/1:1 Interleaving Port Protection: This scheme re-quires a pair of working and protection optical channels thathold the protection relationship to be assigned with ports in aninterleaving manner. That is, the two ports should be neighbor-ing, assigned on the same card. For its MILP model, we havethe same sets, parameters, and variables as defined before andneed to make the following extensions.

First, since a pair of working and protection optical channelsthat hold the protection relationship should be assigned in aninterleaving manner, we shall use a new variable to bind themtogether to indicate whether such a pair of working and pro-tection optical channels exists. This new variable is defined asfollows.Yi,j,k Takes the value of 1 if both the kth working and pro-

tection optical channels on the working and protectionvirtual links between node pair (i, j) are established;otherwise, 0 (binary). This essentially corresponds tothe equation of Yi,j,k = Vi,j,k & V ′

i,j,k .To decide the value of Yi,j,k , we have the following new

constraints.

Yi,j,k ≥ Vi,j,k + V ′i,j,k − 1 ∀k ∈ K; i, j ∈ N : i �= j (37)

Yi,j,k ≤ Vi,j,k ∀k ∈ K; i, j ∈ N : i �= j (38)

Yi,j,k ≤ V ′i,j,k ∀k ∈ K; i, j ∈ N : i �= j (39)

Constraints (37)–(39) are for Yi,j,k = Vi,j,k & V ′i,j,k .

∑

k∈K

Yi,j,k ≤ min{Vi,j , V ′

i,j

} ∀i, j ∈ N : i �= j (40)

Constraint (40) means that the total number of working andprotection optical channel pairs, each of which hold the protec-tion relationship, should never be greater than the total number

of working or protection optical channels on the correspondingvirtual links between node pair (i, j).

In order to assign ports to the working and protection opticalchannel pair that holds the protection relationship in an inter-leaving manner, in addition to constraints (34) and (35), we needto have one more constraint as follows.

∣∣ζai,j,k − ζ ′ai,j,k

∣∣ ≤ ∇ · (1 − Yi,j,k ) ∀k ∈ K; a ∈CIi ; i, j ∈ N : i �= j (41)

Constraint (41) ensures that a pair of working and protec-tion optical channels that hold the protection relationship areassigned with the ports on the same card. When Yi,j,k = 1,|ζa

i,j,k − ζ ′ai,j,k | ≤ 0, which can be derived to ζai,j,k = ζ ′ai,j,k .

In summary, for this port protection scheme, we need to haveconstraints (2)–(21), (23)–(28), (34)–(35), and (37)–(41).

5) W/P Port Sharing: This scheme allows a pair of workingand protection optical channels that hold the protection rela-tionship to share a common router port so as to reduce the totalnumber of router ports required. For this, we need a new variableZa,b

i,j,k as follows.

Za,bi,j,k Takes the value of 1 if port b of card a at node i is

used for establishing (working or protection) opticalchannels on virtual link (i, j); otherwise, 0 (binary).

In addition to constraints (2)–(20) and (23)–(28), we need toadd the following new constraints.

Za,bi,j,k ≥ xa,b

i,j,k ∀k ∈ K; i, j, s, d ∈ N : s �= d, i �= j (42)

Za,bi,j,k ≥ x′a,b

i,j,k ∀k ∈ K; i, j, s, d ∈ N : s �= d, i �= j (43)∑


Za,bi,j,k ≤ 1 ∀i, s, d ∈ N : s �= d (44)

Constraints (42)–(44) ensure that a pair of working and pro-tection optical channels that hold the protection relationshipshould be assigned with the same router port.

V. HEURISTIC ALGORITHM

It is easy to solve the MILP models for small networks. How-ever, for large networks, the computational complexities of themodels are high, and thus, it is necessary to develop an effi-cient heuristic algorithm to solve the optimization problem. Asa representative case, in this section we present the heuristicalgorithm for the SBPP-based protection in IP over WDM net-work. Note that due to the complexity of route difference foreach link failure recovery, we only consider heuristic algorithmsfor the SBPP-based protection technique.

Corresponding to the two sub-problems in Section III, the pro-posed algorithm consists of two steps, with each solving one ofthe sub-problems. Specifically, the first step is to groom surviv-able IP traffic flows onto the optical layer, which would establishvirtual topologies, i.e., a working and protection virtual topol-ogy. The working virtual topology is made up of working vir-tual links, each of which corresponds to a working lightpath. Theprotection virtual topology is made up of protection virtual links,each of which corresponds to a protection lightpath. When usingthe protection virtual topology to accommodate the protection


Fig. 7. Flowchart of grooming SBPP-protected services.

flows in the IP layer, the SBPP protection technique is employedfor spare capacity sharing among the protection flows. Once thevirtual topologies are established and the numbers of workingand protection optical channels that are to be established at eachnode are obtained, the second step is to assign router ports andcards to each of the optical channels based on different port pro-tection schemes. Next, we describe each of these steps in detail.

A. Virtual Topology Establishment

Fig. 7 shows a high-level flowchart for the step of groomingSBPP-protected flows to establish virtual topologies. Specifi-cally, we first create two empty virtual topologies GW and GP ,which are used to store the working and protection virtual linksestablished in the course of grooming protected IP traffic flows.The traffic grooming technique for IP over WDM networks canbe referred to [32] for more details. We are employing multi-hoptraffic grooming technique to improve the capacity utilizationof each established optical channel. Specifically, we can use theunused capacity of established lightpaths in a network at thefirst priority to serve new traffic demand. For example, assumethat the required traffic demand between a node pair (A-C) is20 Gb/s, and lightpaths (A-B) and (B-C) have unused capacitiesof 30 Gb/s and 40 Gb/s, respectively. Thus, we can use theseunused capacities to serve the traffic demand between the nodepair (A-C), not requiring to establish a new lightpath.

1) Establish Working Flow: For each SBPP-protected ser-vice, we first try to establish its working flow on the workingvirtual topology GW using its remaining capacity. All the virtuallinks that do not have sufficient remaining capacity would notbe considered when searching for the shortest route between theservice node pair based on GW . If a shortest route can be found,we use the remaining capacity on the virtual links along thisroute to establish the working flow; otherwise, we will establisha new optical channel (lightpath) directly between the sourceand destination nodes of the protected service in the opticallayer. In GW , this new lightpath corresponds to a new optical

channel on the working virtual link between the source and des-tination nodes of the service. We use this new optical channelto establish the working flow for the service. Here, since weassume that there are sufficient fiber spectrum resources in theoptical layer, a new lightpath can always be established uponrequest.

2) Establish Protection Flow: Once the working flow is es-tablished, the next step is to establish its corresponding protec-tion flow based on the SBPP protection technique. The detailedprocess is similar to the establishing the working flow. First,we check each virtual link in GP to see if there is sufficientcapacity for establishing the protection IP flow. Here, it shouldbe noted that under SBPP, the spare capacity on a virtual pro-tection link can be shared between different protection IP flowsas long as their corresponding working IP flows do not shareany common physical link. Thus, to tell if a protection virtuallink has sufficient protection capacity, we need to consider bothits remaining unused capacity as well as the sharable protectioncapacity. Only if the sum of these two capacities is enough toaccommodate the protection IP flow, can we say that this virtuallink is eligible to be used for protection.

To decide the maximum sharable capacity on each protectionvirtual link, we check each of the protection optical channels onthe virtual link and do the following examination. We get thelist of SBPP-protected services that are protected by the cur-rent optical channel L. Then we remove all the services whoseworking flows are sharing any physical link with the workingflow of the current service fromL since under SBPP they cannotshare spare capacity. Next, for the remaining protected servicesin L, we find the one with maximum amount of traffic demand.This amount would then be the maximum sharable protectioncapacity of the current optical channel eligible for the protec-tion flow of the current SBPP-protected service. We denote itas Pmax . In addition to the sharable protection capacity, thecurrent protection optical channel may also have some unusedremaining capacity, denoted as Cf . Using this, the total eligiblecapacity on the current optical channel eligible to providing ca-pacity for establishing the protection flow of the current serviceis Pmax + Cf . If this capacity is larger than the traffic demandof the current protected service, then we say that this opticalchannel is eligible. We will check all the optical channels onthe current protection virtual link. As long as there is at leastone eligible protection optical channel, we say that the currentprotection virtual link is eligible as well.

After checking all the protection virtual links and removingthose not eligible for protecting the current service, we have aremaining protection virtual topology. Based on this topology,we can employ the shortest path searching algorithm to find ashortest route. If such a route can be found, then we use thisroute to establish this protection flow; otherwise, we need torefer to the optical layer for a new protection lightpath (or opticalchannel). As for the procedure followed for the establishmentof a working optical channel, we will establish a new opticalchannel (lightpath) directly between the source and destinationnodes of the protected service in the optical layer and add it toGP . This new lightpath corresponds to a new optical channel onthe protection virtual link between the source and destination


nodes of the service. We use this new optical channel to establishthe protection flow for the service. As a key difference of thisfrom the working lightpath establishment process, it may benoted that in order for network protection, we need to ensurethat the established protection lightpath is link-disjoint from theestablished working flow in the previous step.

B. Router Port Assignment

Based on the above virtual topology establishment step, wecan obtain a working and protection virtual topology, in whicheach virtual link contains multiple optical channels. To establishthese optical channels, we need to assign router ports at boththe source and destination nodes of the virtual links. We havediscussed five possible port assignment schemes as shown inFig. 3. Here as a representative case, we consider the schemeof card protection with dedicated port. The key requirement forthis scheme is to use working router cards to establish workingoptical channels and protection router cards to establish pro-tection optical channels; a mixed situation with both workingand protection optical channels co-existing on the same cardis not allowed. This assignment is straightforward. We assignjust enough router cards to function as working cards to estab-lish working optical channels, and assign just enough protec-tion cards to establish protection optical channels. For example,Fig. 5(b) shows an assignment scenario, where no working andprotection optical channels co-exist on a common card. Basedon the total number of deployed working and protection routercards, we can then calculate the total power consumption foreach network node using (1), where the protection cards can beput to sleep in order to save power.

C. Computational Complexity

For the above heuristic algorithm, its computational complex-ity is analyzed as follows. In the first step of virtual topology es-tablishment, we have the two sub-steps of establishing workingand protection IP flows, where the latter is more complicated.As we need to first check the eligible capacity for the estab-lishment of a protection IP flow, we have the computationalcomplexity of O(N 4 · W ), where N is the total number of net-work nodes and W is the number of wavelengths in each fiberlink. Specifically, in each virtual topology, there can be up toO(N 2) virtual links if the network is fully connected. We alsoneed to check the capacity sharable status on each optical chan-nel of a virtual link; this may be up to O(W ) optical channelsand the sharing relationship may be up to O(N 2) different nodepairs. Once the capacity sharing status is decided, the next stepis to search for a shortest route based on the virtual topology,which has a computational complexity of O(N 2) according toDijkstra’s algorithm. Thus, the overall computational complex-ity of establishing the working and protection IP flows on thevirtual topology is at the level of O(N 4 · W + N 2). The sec-ond step of assigning router port is very simple and will have acomputational complexity at the level of O(N 2 · W ) since therecan be up to O(N 2 · W ) optical channels. In summary, the totalcomputational complexity of the whole algorithm is at the levelof O(N 4 · W + N 2 · W + N 2).

D. Other Circumstances of Port Protection

As shown in Fig. 3, there are different schemes for port pro-tection. Thus, we may extend Step B in the above heuristicalgorithm to support these port protection schemes as well.

1) 1+1/1:1 Random Port Protection: In this scheme, as longas a pair of working and protection optical channels are assignedwith different ports, they can be randomly connected to anyrouter card. Therefore, the number of router cards required isjust equal to the total number of optical channels divided by thenumber of ports supported by each router card (see Fig. 4(b)).

2) 1+1/1:1 Interleaving Port Protection: In this scheme, werequire a pair of working and protection optical channels be-tween a node pair to be assigned neighboring to each other on acommon router card (see Fig. 4(c)). Note that we can have a sit-uation where there is a working (or protection) optical channel,while its corresponding protection (or working) optical channelis not required because the remaining protection (working) ca-pacity provided by other protection virtual links can be sharedto set up the current protection flow. In this case, we just assignthe working (or protection) channel to a free port of a routercard.

3) Port Sharing by Working and Protection Optical Chan-nels: In this scheme, to save the number of required ports, weallow a pair of working and protection optical channels betweena node pair to share a common port (see Fig. 4(a)). This is atthe cost of lower network survivability as the failure of theport would lead to a situation where neither the working northe protection optical channels can be restored. Here, we canjust assign a pair of working and protection optical channelsto a router port. Again, there can be situation where there isonly a working or a protection optical channel. In this case, arouter port only supports the establishment of the working orprotection optical channel.

4) Card Protection With Mixed Port Types: This scheme isthe same as the case of dedicated port type except that the work-ing and protection optical channels that do not hold protectionrelationship can use the ports on the same router cards (seeFig. 5(a)). This is a type of card protection scheme. Comparedto the scheme of dedicated port type, it is expected to requirea smaller number of router cards since working and protectionoptical channels that do not hold protection relationship canshare common router cards.

VI. TEST CONDITIONS

To evaluate the performance of the proposed schemes, wedid simulation studies based on three test networks. Theseare (1) a six-node, eight-link (n6s8) network, (2) a 14-node,21-link (NSFNET) network, and (3) a 24-node, 43-link (US-NET) network, as shown in Fig. 8.

We assumed the following simulation parameters:1) The traffic demand between each node pair is as-

sumed to be uniformly distributed within a range of[10, 2X − 10] Gb/s, where X is the average demand in-tensity. Specifically, we set X∈ {20, 60, 100, 140, 180}Gb/s.


Fig. 8. Test networks. (a) n6s8. (b) NSFNET. (c) USNET.

2) The maximum number of router ports on each router cardΩ is set to be 4. The power consumption of each routercard is (PF ull · (1 − θs))/8 unit when it is activated (i.e.,not put to sleep), where PF ull is the total power consump-tion of a card chassis and θs is the power consumptionoverhead of the chassis. Here, PF ull and θs are assumedto be 1.0 and 0.3, respectively. The power consumptionoverhead of a router card when it is put to sleep α is setas 0.1. Each chassis is assumed to host 8 router cards, i.e.,β = 8.

For traffic grooming, the optical bypass strategy was em-ployed for lightpath establishment in the optical layer. Thisstrategy was seen to reduce greatly the number of optical chan-nels established. This helps to reduce the number of router portsrequired and, therefore, will reduce the power consumption fur-ther. As mentioned earlier, we find that, for an IP over WDMnetwork, it is the IP layer (i.e., its routers), which consumesmost of the power. (The power consumption of the optical layeris very small and can be ignored.) Therefore, we only estimatethe power consumption of the IP layer. We assume that the op-tical layer can always provide sufficient capacity to the IP layer,but will consume very little power.

We solved the MILP models using the commercially avail-able software AMPL/Gurobi [33] and implemented the heuris-tic algorithm using JAVA. For the n6s8 network, the longestcomputational time for solving the MILP models is about20,000 seconds with an MIPGAP of 5%. For the larger NSFNETand USNET networks, because of high computational complex-ity of the MILP models, we could not solve them within a rea-sonable short time; we therefore used the heuristic algorithmsto find the solutions for these networks. For the heuristic al-gorithms, all the results were obtained within several seconds,even for the larger test networks.

VII. RESULTS AND PERFORMANCE ANALYSES

For performance evaluation, we have considered the two pro-tection techniques - SBPP and PR. We also considered fiveschemes of router port assignment (see Fig. 3) for differentlevels of protection. Both the MILP models and heuristic al-gorithms were employed to find solutions. These form differ-ent study cases. In presenting the performance results, we usethe following legends to represent different study cases. We use“SBPP” and “PR” to represent the two different protection tech-niques studied. We use “MILP_J” to represent the results of thejoint MILP model and “MILP_D” to represent the results ob-tained based on the decomposed MILP model. In addition, forthe five port protection schemes, we use legends “P_Sharing,”“Random_P,” “Inter_P,” “Mixed_C,” and “Dedi_C” to denotethe schemes of (1) port sharing by working and protectionoptical channels, (2) 1+1/1:1 random port assignment, (3)1+1/1:1 interleaving port assignment, (4) card protection withmixed port types, and (5) card protection with dedicated porttype, respectively. We use legends “S” and “NS” to representthe cases of protection router card sleeping and non-sleeping,respectively.

A. Non-Sleeping Versus Sleeping

To evaluate the benefit of the proposed protection router cardsleeping strategy for energy saving, we compared the powerconsumption of the cases with and without protection routercard sleeping. Here as a representative study, we considered theschemes of SBPP-based protection under card protection withdedicated port type. For the sleeping case, three subcases areconsidered, including the results of the joint and the decomposedMILP models and the heuristic algorithm. For the non-sleepingcase, two subcases are considered, including the results of thejoint MILP model and the heuristic algorithm.

Fig. 9(a) shows the results of total power consumptionand power consumption distribution of the sleeping and non-sleeping cases in the N6S8 network. From the results, we firstcan see that the case of sleeping can significantly reduce thepower consumption by up to 35% compared to the case ofnon-sleeping. In addition, we see that the proposed heuristic al-gorithm can perform close to the MILP model for both sleepingand non-sleeping cases. This indicates the efficiency of the pro-posed algorithm. Moreover, comparing the results of the jointand decomposed MILP models under the sleeping case, we seethat they perform very close to each other. This in turn impliesthat the decomposed MILP model is good enough to performalmost optimally even though its computational complexity issignificantly less than that of the joint MILP model.

We also show the power consumption distribution of chassis,working router cards, and protection router cards. It is interest-ing to see that the power consumption of the chassis is dominantat a low traffic demand and the power consumption of workingrouter cards ranks below that. However, when the traffic demandgrows, the percentage of power consumption by the chassis re-duces and the power consumption by the router cards increases.This is reasonable since at a low traffic demand, the number ofactive router cards (including working and protection cards) is


Fig. 9. Performance comparison between sleeping and non-sleeping cases.(a) N6S8. (b) NSFNET. (c) USNET.

small, while the chassis should always be turned on even thoughthere are only few router cards in them. Therefore, the powerconsumption of the chassis is dominant. When there is moretraffic demand, more router cards should be turned on whicheventually fill up all the chassis and therefore the percentage ofpower consumption from the chassis becomes smaller. More-over, in the sleeping case, the working cards always consumemuch more power than the protection cards since they can beput to sleep during normal operation. This will not happen forthe non-sleeping case since there the protection cards cannot beput to sleep.

Fig. 10. Performance comparison between different port and card protectionschemes. (a) NSFNET. (b) USNET.

We made similar performance comparisons for the NSFNETand USNET networks as shown in Figs. 9(b)&(c). Here due tothe computational intractability of the ILP models, we only pro-vide the results of the heuristic algorithm. For the total powerconsumption, we see that the sleeping case can always reducethe total power consumption compared to the non-sleeping case.Specifically, for the NSFNET network, the reduction is up to41%, and for the USNET network, the reduction is up to 40%.Regarding the power consumption distribution, we can observea phenomenon similar to that in the N6S8 network. Specifically,under a low traffic demand, the power consumption of the chas-ses is dominant and when the traffic demand is higher, the powerconsumption of the router cards becomes more important. More-over, we see that the working cards always consume much morepower than the protection cards in the sleeping case for both thetest networks but this is not observed in the non-sleeping case.

B. Performance Comparison Between Different Port and CardProtection Schemes

In this section, we consider five different types of port and cardprotection schemes. Fig. 10 shows the results of NSFNET andUSNET. We first look at the power consumption of the differentschemes. Compared with the other port and card protectionschemes, the scheme of port sharing always has the lowest power


Fig. 11. Performance comparison between SBPP and PR protection schemes(N6S8).

consumption. This is because a pair of working and protectionoptical channels can share a common port, which requires asmaller number of ports and a correspondingly smaller numberof cards. It therefore consumes the least power.

Second, we see that the scheme of card protection with dedi-cated port type ranks second, while the other schemes consumemore power than the scheme of port sharing and the schemeof card protection with dedicated port type. This is because thecard protection with dedicated port type enables a card to carrydedicated optical channels, either working or protection. For allthe protection cards, we can put them to sleep in the normalstatus, thereby reducing power consumption. In contrast, for theother three schemes, including the schemes of 1+1/1:1 inter-leaving port protection, 1+1/1:1 random port protection, andcard protection with mixed port types, we do not guarantee toclassify the port types on each router card. Instead, the workingand protection ports on these cards would be arranged in a mixedway and therefore these cards cannot be put to sleep. Moreover,comparing the above the three schemes, though they performclose to each other, the scheme of 1+1/1:1 interleaving portprotection seems to consume the highest power since it alwaysforces a pair of working and protection ports to be assigned inan interleaving manner on a common card.

We also show the power consumption distribution of the dif-ferent components in Fig. 10. Similar observations can be madeto see that under a low traffic demand, the power consump-tion of the chasses is dominant and when the traffic demandis higher, the power consumption of the router cards becomesmore important. Moreover, the working cards always consumemuch more power than the protection cards and in some casesthere is actually no power consumption from protection routercards since there are actually no cards that can be put to sleep.

C. SBPP versus PR

We also evaluate how the protection techniques can impactthe benefit of power saving by putting protection cards to sleep.Fig. 11 shows the results of the two techniques for the N6S8network. The schemes of card protection with dedicated porttype and port sharing are considered, respectively. Only the re-

sults of the joint MILP model are presented. It is clear to seethat for both of the schemes, the two protection techniques, i.e.,SBPP and PR, always show very close performance. This im-plies that the PR technique cannot bring much benefit of powersaving although it is more efficient in spare capacity sharing. Inother words, it would be better to take SBPP as the protectiontechnique in the context of protection router card sleeping forthe best performance in terms of power consumption reductionand network operational simplicity. In addition, we have similarobservations of the power consumption distribution of differ-ent components under the two protection techniques. Moreover,similar observation can be made for the case of the decomposedMILP model, but we do not present the results here.

VIII. CONCLUSION

We proposed an efficient energy-saving strategy that puts pro-tection router cards to sleep when a network is in the normalstate. To evaluate the performance of the proposed strategy, weformulated the problem as an MILP model, which was furtherdecomposed into two sub-models for better tractability. Fur-ther, to solve the problem for large networks, we also developedan efficient heuristic algorithm that consists of two steps, i.e.,survivable traffic grooming and router port assignment to work-ing and protection optical channels. The results show that theproposed strategy of putting protection router cards to sleep isefficient and can significantly reduce (i.e., by more than 40%)the power consumption, compared to the case without this ef-fort. The proposed heuristic algorithm is efficient and performsvery close to the optimization model. By evaluating the impactof different protection schemes, we find that SBPP can per-form very close to PR even though the latter is more flexiblein choosing protection paths, depending on link failure scenar-ios. For the different port assignment schemes, we see that thescheme of card protection with dedicated port type is the mostpromising to ensure card-based protection while achieving thehighest power consumption saving by putting protection routercards to sleep. Finally, the simulation results show that it wouldalways be more efficient enough to employ the SBPP protectiontechnique in order to get the greatest benefit from power savingby putting protection router cards to sleep.

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Yongcheng Li received the B.Sc. and Ph.D. degrees from Soochow University,Suzhou, China, in 2011 and 2017, respectively. He is currently an Assistant Re-searcher with the School of Electronic and Information Engineering, SoochowUniversity. His research interests include optical networking, network design,and optimization.

Lin Zhu received the B.Sc. degree in 2015. She is currently working towardthe Master’s degree with the School of Electronic and Information Engineering,Soochow University, Suzhou, China. Her research interests include optical net-work design and optimization.

Sanjay K. Bose (SM’91) received the B.Tech. degree from IIT Kanpur, Kan-pur, India, in 1976, and the Master’s and Ph.D. degrees from S.U.N.Y. StonyBrook, Stony Brook, NY, USA, in 1977 and 1980, respectively. After workingwith the Corporate R&D Centre of the General Electric Co., Schenectady, NY,USA, until 1982, he joined IIT Kanpur as an Assistant Professor and becamea Professor there in 1991. He left IIT Kanpur in 2003 to join the faculty of theSchool of Electrical and Electronic Engineering, Nanyang Technological Uni-versity, Singapore. In December 2008, he left NTU to join IIT Guwahati, wherehe is currently a Professor with the Department of Electrical and ElectronicEngineering, and the Dean of Alumni Affairs and External Relations. He hasbeen working in various areas in the field of computer networks and queuingsystems, and he has published extensively in the area of optical networks andnetwork routing.

Prof. Bose is a Fellow of IETE (India), and a member of Sigma Xi and EtaKappa Nu.

Gangxiang Shen (S’98–M’99–SM’12) received the B.Eng. degree fromZhejiang University, Hangzhou, China, the M.Sc. degree from Nanyang Tech-nological University, Singapore, and the Ph.D. degree from the University ofAlberta, Edmonton, AB, Canada, in January 2006. He is a Distinguished Pro-fessor with the School of Electronic and Information Engineering, SoochowUniversity, Suzhou, China. Before he joined Soochow University, he was aLead Engineer with Ciena, Linthicum, MD, USA. He was also an AustralianARC Postdoctoral Fellow with University of Melbourne. His research interestsinclude integrated optical and wireless networks, spectrum efficient optical net-works, and green optical networks. He has authored and coauthored more than150 peer-reviewed technical papers, among which one of the papers receivedthe highest citations among all the papers published in IEEE/OSA JOCN. Hewas a Lead Guest Editor of IEEE JSAC Special Issue on “Next-GenerationSpectrum-Efficient and Elastic Optical Transport Networks,” and a Guest Edi-tor of IEEE JSAC Special Issue on “Energy-Efficiency in Optical Networks.”He is an Associate Editor of IEEE/OSA JOURNAL OF OPTICAL COMMUNICA-TIONS AND NETWORKING, and an editorial board member of Optical Switchingand Networking and Photonic Network Communications. He has served as TCPChairs for various international conferences in the area of optical networking,including general TPC Co-Chair of ACP 2018 and Symposium Lead Chairof GLOBECOM 2017. He received the Young Researcher New Star ScientistAward in the “2010 Scopus Young Researcher Award Scheme” in China. He wasa recipient of the Izaak Walton Killam Memorial Award from the University ofAlberta and the Canadian NSERC Industrial R&D Fellowship. He is a “HighlyCited Chinese Research Scholar” selected by Elsevier (from 2014 to 2017) andan “Excellent Young Research Scholar” sponsored by NSFC. He was a Sec-retary for the IEEE Fiber-Wireless Integration Subtechnical Committee. He isserving as a member of IEEE ComSoc Strategic Planning Standing Committeeand an IEEE ComSoc Distinguished Lecturer (2018–2019).

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