Energy-Efficient Communication Networks for Improved GlobalEnergy Productivity

Slavisa Aleksic

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Telecommunication Systems manuscript No.(will be inserted by the editor)

Energy-Efficient Communication Networks for ImprovedGlobal Energy Productivity

Slavisa Aleksic

Received: March 2011 / Accepted: September 2011

Abstract Recently, a lot of effort has been put into re-search and development of energy-efficient devices andsystems and into optimization of the network infras-tructure with regard to energy consumption. On theone hand, reducing total power consumption of globalcommunication networks has become an imperative andan important step towards the future highly energy-efficient Internet. On the other hand, capacity and per-formance of the Internet must be continuously improvedin order to meet the high requirements set by the con-tinuously increasing of both the number of broadbandsubscribers and the amount of user traffic. Thus, onlya highly energy-efficient and high-performance networkinfrastructure designed to efficiently support advancedapplications and services for improving energy produc-tivity in many other branches of business and societycan significantly contribute to global energy savings.This paper briefly discusses potentials and options forachieving such an energy-efficient network infrastruc-ture. Some examples of software applications and ser-vices for global energy savings are briefly reviewed. Theaim of this paper is to draw attention to the need fora holistic approach to evaluate energy efficiency andto improve the global energy productivity through theuse of high-performance and energy-efficient networks,services and applications.

Keywords Energy efficiency · power consumption ·network elements · core networks · access networks ·holistic approach

S. AleksicVienna University of Technology, Institute of Telecommuni-cations, Favoritenstr. 9/E389, Vienna, AustriaTel: +43-1-58801-38831Fax: +43-1-58801-938831E-mail: slavisa.aleksic@tuwien.ac.at

1 Introduction

The continuous growth of Internet traffic has led torapidly increasing capacity provided by network infras-tructure. Observations have shown that the main con-tributor to the traffic increase in the Internet is the traf-fic from residential customers [1]. This is mostly becauseof introduction of new bandwidth-hungry applicationsfor residential users but also due to the fast growth ofthe number of broadband subscribers [2] as it is evidentfrom Fig. 1 a).

Due to the concurrent growth of Internet traffic andthe number of subscribers, both the number of networkelements and their capacity are also expected to in-crease in the future. It has been predicted that aboutseven times more servers, thirteen times more routers,and sixteen times more PCs than we had in 2007 willbe a part of the global communication network in 2020[3]. For this reason, energy efficiency of both networkand end-user equipment has become a very importantissue. Network elements must be able to provide highperformance while featuring reduced power consump-tion. These very high requirements can only be sat-isfied through a combined effort of research commu-nity, network designers, and network operators. It canbe achieved by developing and applying power-efficientcomponents and structures as well as through design-ing new network concepts that are able to naturallysupport high capacities, high quality of service (QoS)standards, and low-power operation.

Historical development and some projections for fu-ture trends concerning the growth of Internet trafficand the increase in router capacity and line data rateare presented in Fig. 1b). In the past, user traffic wasincreasing by about 100% per year. Within the lastfew years, this growth has slowed somewhat, so that

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the traffic volume in global communication networksis currently increasing by approximately 50% per year[4]. In order to keep track with this increasing demandfor bandwidth, the capacity of underlying network com-ponents has to increase too. However, when consideringIP routers, the capacity per rack of equipment has beenincreasing with a lower rate than Internet traffic. As itcan be seen from Fig 1 b), the capacity per rack wasincreasing by about 75% per year for more than tenyears. The increase in capacity has slowed in the lastfew years, which is mostly due to increased energy den-sity and limited cooling rate. The increase of port (line)data rate has been also slowed down to some extent inthe last several years. While in the past, line data rateincreased by a factor of four each three years or by afactor of ten per approximately six years, the step from40 Gbit/s to 100 Gbit/s, which corresponds to an in-crease by a factor of 2.5, took more than three years. Inorder to keep the same increasing rate as during the lastdecade, the next generation optical transceivers operat-ing at the line data rate of 1 Tbit/s should be availablearound 2014.

According to [5], communication networks of to-day are responsible for about 2% of the global carbonemissions. Therefore, power consumption demands a lotof attention when considering scaling of network ele-ments to future required aggregate capacities. Figure 1c) shows the estimated total power consumption of elec-tronic routers for capacities up to 100 Tbit/s with someexamples of current high-performance routers [6]. Thus,the router’s power consumption increases linearly withbandwidth and each new generation of high-capacityrouters consumes more power than the previous one.Similar trends can be observed for large data centersand high-performance computers. Some examples of re-cently developed systems and projections for power con-sumption of high-performance computers (HPCs) areshown in Fig. 1 d). It is evident that already today,HPC systems consume several MW of electricity [7]. Fu-ture Exascale computer systems will probably consumemore than 20 MW, which will set very high require-ments on power supply and cooling systems. Therefore,a large effort has to be put into research and develop-ment of more energy efficient structures and technolo-

gies in order to make possible further scaling in bothcapacity and performance.

The structure of the paper is as follows. Section 2discusses some general issues related to energy efficiencyof communication networks. In Sections 3 and 4, someexamples of technologies and methods for improving en-ergy efficiency of network elements are given. Moreover,this Section presents and discusses power-efficient net-work concepts and some ideas about how to considerpower efficiency in designing network elements in theaccess and core areas. Section 5 briefly addresses appli-cations and services that can be used to increase theglobal energy productivity, while Section 6 proposes aholistic approach for evaluating the impact of the globalcommunication network on global energy savings. Fi-nally, Section 7 summarizes the paper and draws someconclusions. The aim of this paper is not to give a com-prehensive review or an exhaustive survey of methodsand technologies for reducing power consumption andfor increasing energy efficiency of communication net-works, but rather to mention some illustrative exam-ples of energy-efficient concepts and to draw attentionto complex interactions between different subsystems,layers and areas of communication networks as well asto interdependencies with other branches of businessand society through an broad use of advanced applica-tions and services.

2 Total Network Power Consumption

Let us now look at some general issues related to powerconsumption and energy efficiency of global networks.Sometimes, the terms energy efficiency and energy con-sumption are mixed up in the literature. Although re-ducing of energy consumption is often required to achievea high energy efficiency, minimizing energy consump-tion is not always the primary goal. Instead, energyefficient technologies and concepts are rather aimed atimproving performance and increasing capacity of theentire network while paying a particular attention toenergy consumption, i.e. to provide more functionalitywhile keeping the total energy consumption as low aspossible.

In order to obtain a complete picture it is crucialto concentrate on the total energy consumption fromthe system point of view. That means all contribut-ing elements should be taken into account and differentconcepts should be examined by dimensioning them allto provide the same performance. For example, a par-ticular device or technology may provide a low powerconsumption, but at the same time, it may deliver a lowperformance or its introduction into the system may

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Fig. 2 a) Power losses outside of network elements and b) to-tal power consumption of the global communication network(PSU: Power Supply Unit).

cause an increased complexity in other subsystems be-longing other network layers or areas. As a consequence,the introduction of such a device or technology in thenetwork could lead to a higher overall network powerconsumption [11,12]. Thus, it is beneficial to look atthe total power consumption of the entire network asit is represented in Fig. 2 b) and to consider differ-ent network layers and areas. Additional to power con-sumption of the network itself also power losses dueto inefficiencies of power supply and cooling equipmentas well as transmission losses in the power grid can beconsidered. [see Fig. 2 a)]

There are several reasons for concentrating on en-ergy consumption in the process of designing new net-work technologies. Maybe the most intuitive one is thatminimizing energy consumption of the global networkinfrastructure can have a positive environmental impactand, at the same time, it can lead to a reduced opera-tional expenditure of network providers due to a lowerenergy related costs.

Another important motivation for improving energyefficiency is to overcome scalability limitations due toan increased energy density in communication and in-

formation processing components and systems. For ex-ample, a traditional way to increase processing powerof data processing devices was to increase the clock fre-quency. However, a further increase in clock frequencybecame hard due to the fundamental issues related toincreased energy density and impossibility to rapidly re-move the huge amount of heat generated in the device.As the feature size, and thus the integration densityof semiconductor devices, is still increasing accordingto the Moore’s law, the limitation on clock frequencycan be overcome to some extent by implementing mul-tiple cores on a single die and by using dynamic clockand voltage scaling. Therefore, most of current high-performance processors are based on multicore archi-tectures and utilize sophisticated monitoring techniquesand dynamic power and thermal management.

Another example are intrasystem interconnects, whichare becoming increasingly complex and power consum-ing when increasing capacity. Since capacity of high-performance systems such as large routers and switches,large data centers and high-performance computers hasbeen increasing continuously in the last decade, therequirements on intrasystem interconnects have risenrapidly. The equipment of today’s large-scale systems ishoused by a large number of racks, so that complexity,required number of transmission links and transceiversas well as energy consumption of current intrasysteminterconnects is already very high. In order to enable afurther scaling in capacity, new technologies and con-cepts for large-scale, high-speed and energy-efficient in-terconnection systems are needed [8].

Finally, an energy-efficient global network infras-tructure that is optimized to efficiently support a rapiddevelopment and a broad use of applications and ser-vices for improved energy productivity can lead to verylarge indirect energy savings. Thus, complex interac-tions between the network and various advanced ser-vices and applications should be better understood inorder to optimize both network infrastructure and ap-plications for achieving an improvement in energy pro-ductivity in other branches of business and society. Thisis an important task because not only the savings inenergy by developing and using novel networking tech-nologies, i.e. the direct energy savings, can help to re-duce the total electricity consumption, but also a largenumber of applications that can be made possible by anubiquitous broadband network access could contributeindirectly even more to further reduce the global systemenergy demands. It has been estimated recently that thepotential reduction of greenhouse gas (GHG) emissionsthrough the use of advanced ICT applications and ser-vices is ten times higher than the ICT’s own emissions[9]. The estimated potentials for GHG emission reduc-

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tions [9] enabled by a broad use of various ICT servicesand applications in different other areas are graphicallypresented in Fig. 3.

To see the potential influence of direct and indi-rect energy savings on the environment let us assumetwo hypothetical scenarios for the year 2030. In the thefirst scenario (Case 1), the ICT related GHG emissions,which have been estimated to be 4.73 MtCO2 [10], arereduced by 80% in 2030, while the potentials for globalGHG emission reductions due to the use of ICT servicesand applications are not exploited at all. In this case,the relative reduction of the total GHG emissions fromthe estimated current value (236.5 MtCO2) is about1.6%. In the second case (Case 2), we assume that theICT related GHG emissions increase by 50% in 2030because of a strong increase of both network capacityand number of high-speed terminals. The global com-munication network is then able to support advancedservices and applications, so that the potentials of ICTservices and applications for improving global energyproductivity such as telecommuting, flexi-work, dema-

terialization and e-government can be fully exploited.In this scenario, the large indirect energy savings wouldlead to a reduction of other (non-ICT) GHG emissionsby 48.4 MtCO2 [10], which corresponds to a reductionof the greenhouse gas emissions by 19%. It is obvious,even from this simple example, that there is a need fora holistic approach to consider, within the same frame-work, technological, architectural and operational as-pects of communication networks. It is very importantalso to include interactions between the global commu-nication network and different other subsystems as wellas to estimate environmental implications of the globalsystem. In the next two subsections, we will briefly ad-dress two topics that may seem marginal at first sight,but in a holistic approach, they can gain in importancewhen considering the global system.

2.1 Greenhouse Gas Emissions

Results on energy efficiency are often presented as theenergy consumed in transmitting or processing a cer-tain amount of data in a component or a system underobservation. Thus, energy efficiency can be expressedin Watt/(bit/s) ≡ Joule/bit. Sometimes, it is expressedin bit/Joule, which represent the amount of data beingtransmitted or processed while consuming one Joule ofenergy. The total power (or energy) consumption is usu-ally presented in Watt (or Joule), but sometimes it isappropriate to present it in kilograms of greenhouse gas(GHG) emitted, as it can be seen in Fig. 3. The latterhas to be calculated carefully because the calculationof GHG emissions is not always straightforward. State-ments about savings in GHG emissions are meaninglessif not accompanied by all relevant parameters. The cal-culated savings strongly depend on assumptions madesuch as network/transport technology used, topology,nodes’ capacity, link and node utilizations, conversionfactors, energy sources mix, regional and time depen-dencies as it can be seen in the example depicted in Fig.4.

2.2 Alternative Energy Sources and Heat Reuse

An efficient removing of the heat dissipated by the net-working, data processing and storage equipment is nottrivial. A ”rule of thumb” for estimating the power con-sumption of the room cooling equipment says that, de-pending on climate factors, each Watt of power con-sumed by networking equipment would require 0.4 -0.6 Watt of power that needs to be supplied to theroom cooling equipment [15]. An optimal planning of

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the room structure and the placement of racks accord-ing to the basic thermodynamic principles is the firststep towards an efficient heat removing. The heat re-moved from the networking equipment can be used forother purposes such as for heating buildings, swimmingpools, domestic hot water preparation, etc. In this waythe overall system energy efficiency can be greatly im-proved.

Systems for power supply to IT infrastructure usu-ally have to be highly available and reliable. There-fore, developing and dimensioning of communicationspower supply systems have to be done very carefully in-cluding dimensioning the battery for backup, rectifiers,and cabling systems. Various alternatives for energy co-generation and storage that are able to deliver a highoutput capacity per unit volume should be consideredin order to achieve a further improvement in reliabilityand availability of the network infrastructure. Addition-ally, alternative energy sources can also improve thelevel of ”greening” of the whole system. In this spirit,one should consider exploiting ”green” energy sourcessuch as photovoltaic solar power [16], wind-based, fuel-cells, and pico-hydro co-generation systems [17].

3 Energy-Efficient Design of Core Networks

This section focuses on technologies and methods forimproving energy efficiency of core networks. The pre-dominant type of traffic in today’s communication net-works is the packet-based IP traffic. Thus, one couldconclude that packet switching is and will remain thekey technology in the Internet and that high-capacityIP routers will play the key role in forwarding data traf-fic in future core networks. However, their high com-plexity, large power dissipation and the need for largebuffers make further scaling to very high capacities ofhundreds of Tbit/s or even Pbit/s very difficult. Op-tical transmission and processing technologies providehigher speeds, larger transmission distances, and gener-ally consume less power than electronic ones, but com-plex optical signal processing is difficult to realize andoptical random access memories are not yet feasible [6,18].

One of the crucial preconditions for implementingcomplex optical switching and signal processing sys-tems is high-density photonic integration, which is stillin an early developing phase when compared with theintegration density of current electronic integrated cir-cuits. Moreover, scalability and cascadability of opti-cal switching and processing elements has to be im-proved in order to achieve very high capacities and toimplement complex data processing directly in the op-tical domain. Even if we would be able to implementdirectly in the optical domain all the functions thatare implemented in current electronic high-performancepacket routers, such an all-optical packet router wouldbe probably hardly able to provide any significant gainin energy efficiency in comparison to the all-electronicimplementation [11,18]. This fact is illustrated in Fig.5, which shows some examples of energy consumptionper bit of currently available switching elements [Fig.5 a)] and estimated values for large Clos-based switch-ing fabrics [Fig. 5 b)]. Although optical packet switchessuch as those based on semiconductor optical ampli-fiers (SOAs) provide a lower energy consumption per bitwhen comparing to electronic buffered packet switchesand considering a simple configuration and small size[see Fig. 5 a)], they are, however, not easily scalableto very large capacities and port counts without per-forming any signal regeneration. This is because in op-tically transparent packet switching, the noise gener-ated in each switching stage is accumulated through theentire switch, which can cause severe signal degrada-tion. Some other effects such as attenuation, dispersionand nonlinear effects also contribute to the degradationof the optical signal. Therefore, optical signals mustbe regenerated after traversing a number of switch-

ing elements in order to remove the accumulated noiseand other distortions. Additionally, in order to achieveproper function with an acceptable performance, opti-cal packet switches often comprise packet synchroniz-ers, wavelength converters and small optical buffers.The latter are often implemented as simple fiber de-lay lines (FDLs). On the other hand, electronic packetswitches are mostly buffered and provide high perfor-mance, but optical-to-electronic and electronic-to-opticalconverters have to be placed at their input and outputports because the signal transmitted over fiber-basedtransmission lines is an optical one and has to be con-verted in the electrical domain before switching andprocessing by electronic components.

As it can be observed form Fig. 5 b), which presentssome results on energy per bit for large optical and elec-tronic switches based on a number of smaller switch-ing elements in a Clos arrangement, SOA-based opticalpacket switches do not provide any significant bene-fit regarding energy efficiency against electronic packetswitches for a fixed data rate per port (i.e. per wave-length) of 10 Gbit/s. For lower aggregate switch ca-pacities below 10 Tbit/s, it provides slightly better en-ergy efficiency than its electronic counterpart, but asthe capacity increases the number of required SOA de-vices and 3R regenerators increases too, which leads toan increase in energy per bit. An order of magnitudelower energy per bit show electronic circuit switchesbased on cross-point devices. They are able to pro-vide high port counts and low energy consumption andare scalable to large sizes while consuming less energythan the both packet-switched options. Finally, opti-cal circuit switches such those based on micro-electro-mechanical systems (MEMS) provide the lowest en-ergy consumption and a very high port count of hun-dreds or even more than thousand ports. Thus, circuitswitches provide higher capacity and lower power perbit than packet switches, but they usually suffer frompoor bandwidth utilization because of their relativelylarge switching/ reconfiguration times and impossibilityto take advantage of ”statistical multiplexing”. There-fore a combined effort is needed to develop not only newenergy-efficient components, both optical and electronicones, but also to design energy-optimized network con-cepts, protocols and algorithms.

Methods for reduction of power consumption in-clude advanced optical transmission and processing tech-nologies as well as power-efficient electronic components,power-optimized structures, and transmission links thatscale both link bit rate and supply voltage to opti-mize power consumption with respect to network uti-lization. A significant reduction of power consumptionof the whole system can be achieved through developing

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and applying efficient and dynamic power monitoringschemes to optimize the usage of available resources.The aim of using optimized algorithms and protocolsfor a power-aware network management is to identifyinteractions between the power management and theresource management as well as to achieve benefits ofusing a dynamically adaptive network.

Additionally, new network concepts can be devel-oped, which are able to provide at the same time low-power consumption and high performance. Inactive orunderutilized interfaces can be dynamically switched offor put into a low-power mode. A number of recent stud-ies have concentrated on optimizing of both design andoperation phases in wide area networks through propos-ing more efficient resource provisioning and protectionschemes that utilize the low-power mode [27,28]. Theusage of power-on devices can be further increased andoptimized by applying power-aware routing and wave-length assignment (RWA) [25,26]. When looking at thecurrent Internet traffic, one can observe that many ap-plications require establishing a connection. Thus, de-spite the connectionless nature of IP, the use of thecore network is very connection oriented. Recently, it

has been shown that above 90% of traffic within back-bone networks is using the transmission control proto-col (TCP) [29]. Especially new applications such as IPtelevision (IPTV), voice over IP (VoIP), video confer-encing, and interactive gaming set very high require-ments on the quality of service that cannot be easilymet with pure packet switching.

Indeed, if we look more closely, we will find thatthere is plenty of circuit switching in core networks pro-vided through transport technologies such as SONET/SDH and WDM. However, the circuits in the Internetare considered by the IP as static point-to-point pathsprovided by layer 1 (e.g. SONET/SDH or OTN), whichare completely decoupled from the IP layer (i.e. networklayer - layer 3). Recently, different signaling mechanismsfor a more dynamic circuit provisioning in the corearea have been specified in different proposals includingmultiprotocol label switching (MPLS) [30], generalizedmultiprotocol label switching (GMPLS) [31], automaticswitched transport network (ASTN) [32], Optical Inter-networking Forum (OIF) [33], optical channel (OCh)and optical data unit (ODU) switching [34,35] as wellas TCP switching [36]. A network architecture that al-lows integration of circuit switching in the core of thepacket-switched Internet in an efficient way may be anappropriate solution [37]. Such an approach could be akind of dynamic circuit switching, optical flow switch-ing [38] or an integrated approach combining circuit,burst, and packet switching within a single network,which is referred to as hybrid optical switching (HOS)[39–43]. Through simplifying packet processing withinthe core network one can reduce the power consump-tion of core nodes at a large extent. Significant savingsin energy can be achieved if a part of Internet traffic isswitched in the circuit-switched manner rather than asindividual packets [44].

Hybrid optical switching (HOS) refers to a com-bined optical and electronic implementation of networknodes and coexistence of different switching paradigmswithin the same network. Recently, different realizationoptions for HOS network nodes have been studied. Therealization options include a combined optical and elec-tronic implementation of core network nodes able toswitch optical circuits, bursts, and packets in an effi-cient way [37,44,45]. A HOS node can be composedof a slow optical switch and a fast switch as shown inFig. 6 a). The fast switch can be implemented eitherin electronics, leading to a hybrid optical/electronic re-alization of the HOS node, or as an all-optical packetswitch, thereby achieving a hybrid all-optical realiza-tion. If the ratio of wavelength channels being switchedby the fast and by the slow switch can be dynamicallychanged according to the actual traffic situation, such a

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concept we refer to as the dynamically adaptive hybridswitching. In this concept, energy savings are achievedthrough switching off temporary unused ports of thefast switch.

In a recent study, a simulation model for evaluat-ing both blocking performance and total power con-sumption of dynamically adaptive HOS nodes has beenimplemented and applied to study energy efficiency ofhybrid optical/electronic and hybrid all-optical real-izations [45]. Capacities from a few Tbit/s to above100 Tbit/s were took into consideration. An integratedcontrol plane capable of supporting all three switch-ing paradigms was proposed and implemented. In thisproposal, control information is transmitted through adedicated control channel by means of exchanging con-trol packets. The control signal can be encoded together

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with the data signal on the same optical carrier using,for example, subcarrier multiplexing (SCM). This tech-nique has the advantages that control packets can betransmitted in parallel to data packets.

The structure of the control packet is shown in Fig.7. The control packet carries either an IP address ora stack of labels. That is, the HOS node can performforwarding functions on the base of either source anddestination IP addresses or GMPLS labels including la-bel swapping. Thus, all advantages of label switchingcan be fully utilized. Additionally, there is a 88-bit longfield containing information about traffic type (packet,burst or circuit), length of the incoming data unit, offsettime in case of the transmission of a burst as well as slotlength and number of free slots for time-domain multi-plexed (TDM) circuits. Finally, there are start and enddelimiters as well as a cyclic redundancy check (CRC)field for ensuring a reliable transmission of the controlpacket. The HOS core node receives control packets

that are generated by edge nodes, performs appropri-ate scheduling procedures, reserves required resourcesif possible and updates the corresponding fields in thecontrol packet. The updated control packet is then sentto the next node. At the present, the integrated con-trol plane includes only basic signaling and traffic man-agement functions, while it can be easily extended tosupport more sophisticated signaling, routing and linkmanagement protocols of a more standard frameworksuch as GMPLS. Also two different burst schedulingalgorithms were implemented, namely the first fit un-scheduled channel with void filling (FFUC-VF) and thebest fit with void filling (BF-VF). The study consideredtwo cases with and without short optical buffers (fiberdelay lines - FDLs) for contention resolution.

The main functional blocks of the simulator are shownin Fig. 6 b). Incoming control packets are received bythe node and processed by the integrated control plane.The desired output port is defined through an inspec-tion of the Address or Label Stack field and by per-forming the forwarding function. According to the typeof traffic field the control plane performs either packetor burst or circuit scheduling and controls the usage ofwavelength converters and optical buffers. Traffic gen-erators were used to simulate the load to the node andthe numbers of lost packets and bursts as well as re-jected requests for establishing circuits were countered.Additionally, a model for estimating the total node’spower consumption was developed, which takes intoaccount different realization options and actual utiliza-tion of low- and high-speed ports, wavelength convert-ers and optical delay lines. Thus, it was possible to ob-tain results on both achievable throughput and powerconsumption for different realization possibilities andunder different traffic conditions. For more informationabout the model, refer to [45].

Some exemplary results for different loads and per-centages of resources reserved for the circuit-orientedtraffic are shown in Fig. 8. In order to assess energyefficiency of the proposed concept (hyb), a new metricswas defined and used to estimate the achievable po-tential for improvement of energy efficiency (IE) withrespect to a conventional electronic IP router (el):

IE =Th

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where Th represents the achievable throughput andPcon is the power consumption of a network node pro-viding this throughput. From Fig. 8a) it can be seenthat an increase in energy efficiency above 100% is achiev-able if in average more than 50% of the active switch-ing ports are slow ones, i.e., if 50% of the availableresources is reserved for circuits. At a moderate load of

60% and higher percentages of circuit-oriented traffic,the parameter IE increases up to about 140%. However,for a very high load of 90%, the achievable improve-ment of energy efficiency is limited to approximately110% because both burst and packet losses become highat very high loads, especially when a large portion ofthe resources is reserved for circuits. As it can be seenform Fig. 8b), both packet and burst loss rates can bereduced when implementing the BF-VF algorithm in-stead of FFUC-VF and using fiber delay lines (FDLs)as optical buffers. Here, the total capacity of the sharedoptical buffers was set to N× 1 kByte, where N denotesthe number of switch ports. The improvement in node’sperformance increases rapidly with the increase in theamount of incoming traffic and for about 90% of the of-fered load, both relative reductions in packet and burstlosses reach the value of 103. Although both implemen-tation of an efficient scheduling algorithm and use ofshort FDLs for packet contention resolution may have astrong impact on switch performance, i.e. on burst andpacket loss rates, it has, however, a very low, almostnegligible influence on the achievable improvement inenergy efficiency as it is evident from Fig. 8 a). Thisis because the use of optical buffers and an implemen-tation of more effective scheduling algorithms usuallylead to a higher achievable throughput, but at the sametime it causes an increase in the total power consump-tion of HOS nodes, so that the ratio Th/Pcons|hyb doesnot change remarkably.

4 Energy-Efficient Design of Access Networks

Now, we will address methods for improving energy effi-ciency in access networks. Within the access area, thereis a variety of technologies and protocols currently inuse or proposed for use in the future to relax the bottle-neck of the first/last mile. The heterogeneity of proto-cols and a large number of network terminals lead to acomplex structure and a high total power consumptionof the access area. The equipment within the centraloffice (CO) of network providers is responsible for ag-gregating the data originating from a large number ofusers and forwarding it to regional or metropolitan areanetworks. In the downstream direction, the traffic com-ing from the metropolitan area is distributed to endusers through a distribution network that can be real-ized using different transmission media (copper, fiberor air) and different topologies (tree, ring or mesh).

Currently, there are many different standards andimplementation options for access networks. Broadbandsubscribers can choose between a wired access over tele-phone lines, in most cases a version of digital subscriber

line (DSL), and a kind of wireless access such as world-wide interoperability for microwave access (WiMAX),global system for mobile communications (GSM) oruniversal mobile telecommunications system (UMTS).Another widely deployed access technology is the hy-brid fiber coax (HFC) network, which reuse the widelydeployed Cable TV (CATV) network infrastructure.

Recently, there have been a large number of projectsconcentrating on fiber-based optical access networks forbroadband data transmission generally named fiber-to-the-x (FTTx). An attractive FTTx solution is a pas-sive optical network (PON) that utilizes passive opticalsplitters in the field in order to connect a number ofusers, usually up to 32 or 64, to a single port of an ag-gregation switch in the CO. Optical access technologiesthat have been investigated, standardized, and imple-mented in the last decade include point-to-point (P-t-P) optical Ethernet and point-to-multipoint (P-t-MP)passive optical networks (PONs) such as ATM-basedPON (APON/BPON), 1 Gbit/s Ethernet-based PON(EPON), 10 Gigabit Ethernet PON (10G-EPON), andwavelength-division multiplexed PON (WDM-PON). Hy-brid PONs that combine WDM and TDM in a singlenetwork and long-reach PONs (LR-PONs) are the lastproposed members of the PON family aiming to pro-vide a more cost-effective and extended-reach opera-tion. The reach of the network can be extended fromthe traditional 20 km range to 100 km and beyond byexploiting optical amplification and WDM technologies.Additionally, the maximum number of users connectedto a single LR-PON, i.e. to a single optical line termi-nal (OLT) port, increases to above 1,000. Thus, we canrecognize the trend of merging access and metropolitanareas and combining aggregation and access functionsinto a single integrated and high-performance network[46]

There are a number of methods proposed for reduc-tion of power consumption in access networks. Espe-cially for mobile devices, a very low consumption is ofgreat importance because the batteries have limited ca-pacity, size, and weight. Additionally, because wirelesstechnologies use shared transmission medium, there isa strong interference at receivers that needs to be miti-gated by using advanced modulation formats and cod-ing schemes as well as intelligent control of the trans-mitter power. Efficient channel selection algorithms areusually combined together with power-aware transmis-sion and routing protocols. Similar to wireless sensornetworks, also radio access networks are designed a pri-ori for low consumption. Although there has been a lotof progress towards energy efficient mobile devices andbase stations there is still some room for additional im-provement.

The requirements on low power consumption in wiredaccess networks are not as high as in wireless networks.Here, network terminals are usually supplied by thepower grid and capacity, size and weight of batteries donot play a significant role. However, due to a large num-ber of power-inefficient network terminals, the overallpower consumption is very high. Therefore, one shouldconcentrate on methods for improving the energy effi-ciency of access networks such as dynamic resource con-trol and power management, exploiting day/night uti-lization curves, and use of low-power consuming compo-nents [44]. Power-efficient architectures can be definedunder consideration of using optical technologies to re-duce the overall system energy consumption, and, atthe same time, to increase access rates.

Optical technologies have the potential to reducethe power consumption in the access area for an order ofmagnitude [47]. For a large number of users, high accessdata rates, and delivering of broadcast services such asstandard-definition (SD) and high-definition (HD) tele-vision, which are realistic assumptions for the accessarea of today and in the near future, passive opticalnetworks (PONs) seem to be able to provide a veryhigh energy efficiency. Due to the fact that in passiveoptical networks the downstream signal is simply splitin a passive splitter down to the optical network termi-nals located at the customer premises, these networkscan inherently provide broadcast and multicast opera-tion needed for the distribution of television programsto a large number of users without any reduction of theavailable bandwidth or introduction of additional de-lays. Indeed, various options for achieving broadbandaccess have been recently evaluated regarding achiev-able energy efficiency [48]. These options include fiber-to-the-curb (FTTC) topology with ADSL2+, VDSL2and HFC DOCSIS 3.0 as well as fiber-to-the-home (FTTH)options based either on point-to-multipoint (P-t-MP)topology such as Ethernet passive optical network (EPON),Gigabit PON (GPON) and 10G-EPON or point-to-point(P-t-P) optical Ethernet with 1 Gbit/s and 10 Gbit/sdata rates. In a recent study [48], the above mentionednetworks were compared in terms of energy efficiencyfor four network scenarios, namely when assuming ei-ther unlimited or limited uplink capacity of the aggre-gation switch and for two different percentages of thedownstream data rate dedicated to the broadcast traf-fic. For a detailed description of the model used forenergy efficiency evaluation, refer to [48].

Here, we use the following definition for energy effi-ciency:

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Fig. 9 Energy efficiency of various broadband access options including fiber-to-the-home (EPON, GPON, 10G-EPON, P-t-P1G and 10G Ethernet) and fiber-to-the-curb (ADSL2+, HFC and VDSL2). The results show the amount of data in Mbit thatcan be transmitted while consuming one Joule of energy. The considered access options are compared for different portionsof downstream data rate reserved for broadcast services and for various sizes of the network form 10 to 10,000 users per CO.Here, it is assumed that the uplink capacity of the aggregation switch in the CO is infinite.

where Eeff represents the amount of traffic trans-mitted in the network per unit of energy, which is givenin bit/s/W ≡ bit/J, Ptotal is the total network powerconsumption, while Nuser and Ruser represent the num-ber of active users and the calculated available data rateper user, respectively. Figs. 9 and 10 present the esti-mated energy efficiency of the considered access net-works for different network sizes and percentages ofthe downstream data rate reserved for delivering ofbroadcast services. As it can be seen form these figures,FTTC technologies generally show for an order of mag-nitude lower power efficiency than the FTTH options.In the theoretical case of unlimited capacity of the up-link in the aggregation switch (see Fig. 9), the highestefficiency can be achieved when using high-speed opti-cal access technologies with 10 Gbit/s data rates, whilewhen looking at a more realistic case with an limiteduplink (Fig. 10), EPON and GPON become the most

efficient options for large networks, i.e. for a large num-ber of users per central office. In the limited uplinkscenario, a remarkable decrease in energy efficiency canbe observed for larger network sizes, especially in thecase of the high-speed access options such as P-t-P 10GEthernet and 10G-EPON. The uplink limitation alsoaffects HFC and VDSL2 networks, while ADSL2+ re-mains unaffected. This is because ASDSL2+ provideslower data rates than the other two considered FTTCoptions, so that the limitation of uplink capacity, whichwe assumed to be 320 Gbit/s in this particular case, isnot reached even for a very large number of users.

The effect of decreasing energy efficiency in case oflimited uplink capacity and large network size can beexplained as follows. A growing number of users implyan increase in required number of network terminals,which consequently leads to a higher total power con-sumption. At the same time, the achievable data rate

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per user decreases due to the bandwidth limitation inthe CO, i.e., the users can not exploit their maximumdata rates anymore. The consequence is an increasein power consumption per bit/s, or equivalently, a de-crease in energy efficiency.

Delivering broadcast (and also multicast) servicesin a passive optical network leads to an improvementin energy efficiency because broadcast traffic is copiedin the optical domain and transmitted to all users inthe downstream direction without any additional pro-cessing and any further increase in energy consumption.The larger the amount of broadcast traffic the higherthe energy efficiency of PON networks. For example, anincrease of 60% in broadcast traffic leads to a four timeshigher energy efficiency. On the other hand, energy ef-ficiency of FTTC and P-t-P FTTH networks does notchange with changing the amount of broadcast traffic.This is because delivering broadcast services in P-t-P

networks requires copying of data in the electronic do-main and sending them separately to each user. There-fore, the amount of data transmitted per unit of en-ergy remains unchanged when increasing the amountof broadcast traffic in case of point-to-point FTTC andFTTH networks.

The results presented here suggest that the next-generation, high-speed optical access technologies suchas P-t-P 10G Ethernet and 10G-EPON have the po-tential to provide the highest energy efficiency for thecase of practically unlimited uplink of the aggregationswitch, i.e. if there would be a large enough capacityin the metropolitan area such that the very high band-width supported by these technologies could be con-currently provided to a large number of users. For thecase of a limited uplink capacity, which is a more real-istic assumption for current networks, the options pro-viding a lower data-rate per user such as EPON and

GPON show better efficiency. FTTC options provide ingeneral lower energy efficiency than optical access net-works, but in the case of a very large number of usersand strongly limited capacity of the uplink, the FTTCoptions become comparably efficient. In the case of ahigh percentage of downstream traffic used for deliver-ing broadcast services, passive optical networks are themost preferable option from energy efficiency point ofview.

Despite the efforts to develop new energy-efficienttechnologies, different other methods such as power-aware protocols and algorithms as well as efficient net-work and power management must be developed andapplied in order to make possible highly energy-efficientaccess networks that have the ability to dynamicallyadapt its performance, resource allocation, and powerconsumption to actual needs. Energy savings can bethen achieved by either using low-power modes or switch-ing off inactive devices. It has been recently shown [44]that if 60% of end users are not active then energysavings of up to 27% (or up to 58%) can be achievedwhen inactive equipment is operating in the low-powerstate (or CPE is switched off). This method has a hugepotential because network terminals are usually alwayson even if not used. The time period during which theequipment is actively used is relatively short in com-parison to its inactivity period. There are some timeperiods such as, for example, early morning hours, dur-ing which the most of the network terminals could beswitched off.

Finally, alternative and renewable energy sourcessuch as those based on solar and wind energy can beeffectively used either as backup sources or as the de-fault power source while mains are used as backup ora standby. Solar power has already been used to powerradio base stations and other network equipment. How-ever, it has mostly been used for disaster relief, in de-veloping countries and in vastly rural areas, where nodirect connection to the power grid is available. Sincerecently, renewable energy sources have been also con-sidered for powering wired network equipment for en-ergy savings purposes [16].

5 Advanced Applications Enabling EnergySavings

In this section, various applications and services thatcan be used to improve energy productivity in otherareas of business and society are briefly reviewed. Theintention of this section is to address the most impor-tant properties of different applications and to serve asa link between the previous two sections that focuseson network technologies and the next section that look

at the global system including not only networking as-pects but also interactions between the communicationnetwork, applications and other subsystems that makeuse of the global network infrastructure. The advancedapplications enabling energy savings can be groupedinto four categories, namely into applications for ad-vanced communication, those for remote automationand process control, for system optimization, and forsupport of transportation. Another promising applica-tion is the cloud computing, which tries to optimallyutilize scalable and virtualized resources of globally in-terconnected computing power. In Section 2, we al-ready discussed the potentials for environmental im-pacts of different applications. In the following, vari-ous advanced applications are briefly reviewed regard-ing their basic features and usability.

– Applications for Advanced CommunicationSome examples of advanced communications appli-cations are those for entertainment (video on de-mand, high-definition TV, interactive gaming), e-government, telemedicine, e-health, high-definitionand holographic videoconferencing, e-learning, tele-working, e-commerce, e-shopping and e-security. Dif-ferent applications set different requirements on thenetwork infrastructure. For example, high-definitionTV (broadcasting) requires a very high downstreambandwidth, while upstream bandwidth and roundtriptime have no relevance for such an application. Incontrast, for high-definition videoconferencing andinteractive gaming both high symmetric bandwidthand low end-to-end delays are of high importance.

– Applications for Remote Automation and Pro-cess ControlApplications for building energy management sys-tems can help a lot to reduce energy consumptionin hotels and office buildings. The manufacturingprocess controls that provide real-time data gener-ated by production machinery can be used to im-prove various processes and their communication inthe production sector. Such applications usually re-quire the utilization of highly interconnected sys-tems that consist of a large number of different sen-sor nodes. Since the most applications for remoteautomation and process control does not requirehigh bandwidth, capacity will not be a crucial is-sue for those systems. However, end-to-end networkdelay and data consistency are the most importantparameters for such an application, because the datagenerated and transmitted over the network are mostlytime critical and error sensitive.

– Applications for System Optimization

Smart grids are an example of applications for sys-tem optimization. They use advanced software ap-plications and communication network infrastruc-ture together with sophisticated sensing and moni-toring technologies to manage energy supplies, en-ergy demand, and energy transmission in a smartway. Among other benefits of smart grid systems istheir ability to integrate distributed and heteroge-neous energy sources including combined heat andpower systems, photovoltaics, wind turbines, andfuel cells. Also safety and availability of the commu-nication system is an important condition for stableand functional smart grids.

– Applications in Support of TransportationAn integration of high-tech supply chain logisticsand warehousing technologies are becoming increas-ingly important to governments and business. Somee-commerce and web shopping applications can alsobe integrated into the logistics and warehousing sys-tems. Traffic information services can be providedto logistics companies and transport operators car-rying people and goods, in order to better controlthe traffic situation and to avoid traffic jams. Anintelligent and networked traffic control system canrapidly react to an unbalanced traffic situation.

– Cloud Computing and VirtualizationCloud computing is a promising approach to sup-port ICT applications and to efficiently deliver ICTservices by improving the utilization of data cen-ter resources. It provides an inherently energy ef-ficient virtualization of networked data processingand storage resources. Savings in energy are achievedby allocating the workload to the minimum num-ber of physical servers and by switching the inactiveservers off in a dynamic manner. Apart from the en-ergy consumed by servers, i.e. by central processingunits (CPUs) and memory, a significant portion ofthe total energy consumed in data centers is dueto the internal interconnection network. Moreover,in order to be able to optimize the utilization ofprocessing and storage resources and to apply anenergy-efficient virtualization technique on a globalscale, an efficient and high-performance global net-work infrastructure is needed. Thus, an energy-efficientcloud computing requires, additionally to energy-efficient CPUs, memory systems and virtualizationtechniques, also energy-efficient and high-performancenetwork technologies, protocols and applications.

6 Improving Global Energy Productivity

As mentioned several times in this paper, advanced ICTservices and applications can potentially help a lot toreduce the global greenhouse gas (GHG) emissions. Onthe other hand, a further improvement in capacity andperformance of the global network infrastructure cancause a higher network related energy consumption.Thus, a combined effort is needed to develop new soft-ware applications and systems for optimizing processesin many different areas as well as to improve the globalnetwork infrastructure regarding high energy efficiencyand high performance. Consequently, there is a need fora holistic approach that is able to capture different sub-systems and interactions between them in an integratedand efficient way. The holistic approach should concen-trate on the global system comprising, additionally tothe global communication network itself, a number ofspecific software applications for use in many other ar-eas as shown in Fig. 11.

A broad use of such applications can lead to a reduc-tion of energy consumption in the respective subsystemand could also influence some other subsystems becauseof the interdependencies between them. For example,both applications for e-commerce and telecommutinginfluence directly the transportation sector, which canbe further optimized by introduction of applications forintelligent transport systems. Traffic information ser-vices can be provided to logistics companies and trans-port operators carrying people and goods, in order tobetter control the traffic situation and to avoid traf-fic jams. An intelligent and networked traffic controlsystem can rapidly react to an unbalanced traffic sit-uation. An integration of high-tech supply chain logis-tics and warehousing technologies are becoming increas-ingly important to governments and business. Somee-commerce and web shopping applications can alsobe integrated into the logistics and warehousing sys-tems. On the other hand, applications for building en-ergy management systems can help a lot to reduce en-ergy consumption in hotels and office buildings. Thus,all the complex interdependencies and interactions be-tween different subsystems, applications and the globalcommunication network should be efficiently consid-ered in the integrated and holistic approach, in orderto be able to identify applications and the main pa-rameters of their interaction with the global commu-nication network and other subsystems, to assess eachparticular application regarding its potential for pro-cess/system optimization and energy savings, to defineinteractions and interfaces between subsystems that in-clude the amount and characteristics of traffic and en-ergy transmitted through the boundaries of subsystems,

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Fig. 11 High-level illustration of the global system including the global communication network and other subsystems that canmake use of the global network infrastructure. The figure also shows information and energy flows as well as interdependencieswithin the global system.

and finally, to assess different concepts concerning theirpotential for energy savings.

The model of global communication networks, whichrepresents the central part of the global system as pre-sented in Fig. 11, should take into account not onlytechnological issues and different realization options butalso social, economical, and demographic peculiaritiesas well as possible future developments. Such a modelcould comprise a number of sub-models needed to studydifferent network areas and technologies in a detailedmanner, whilst the overall picture can be obtained byjoining the individual sub-models together into a globalhierarchical model with a manageable complexity. Forthis purpose, the global communication network canbe divided into different areas based on geographical,technological, and operational aspects. For each net-work area, a different simulation model can be devel-oped, which considers all peculiarities of the particulararea such as suitable network technologies, topologies,configuration parameters and realistic traffic scenarios.These sub-models can be then connected to each otherin a hierarchical manner, where the lowest hierarchyrepresents the network segment close to the traffic gen-erators (i.e. end user’s equipment) and the highest hier-archical level are the high-capacity long-haul transmis-sion links that interconnect continental-wide networks(i.e. intercontinental links). Presumably, the most log-ical way to divide the model of the global communi-cation network into sub-models is to follow the net-work area principle, namely to model separately local,metropolitan, regional, national-wide, and continent-wide networks. For each of the sub-models, we can con-sider different network concepts and transmission tech-nologies as depicted in Fig. 12 a). Statistical data onICT applications and services as well as on user behav-

ior can be then used to set parameters of traffic genera-tors. An illustrative example of how to define the trafficpatterns is shown in Fig. 12 b), c) and d), in which themain characteristics of various applications such as av-erage data rate, maximum data rate and appropriatedistribution functions as well as average usage of eachapplication over the day are taken into account. In thisway, we can study the influence of new applications andinclude user behavior in order to define different sce-narios for possible future developments. The interfacebetween two sub-models, i.e., between two hierarchicallevels, is defined by the amount and characteristics ofthe aggregated traffic sent/received by the gateway in-terconnecting these two network areas. The followingsteps can be made when developing a sub-model:

1. first, network nodes can be modeled by taking intoaccount technological and architectural aspects,

2. then, the network sub-models are developed underconsideration of different concepts, topologies, com-munication protocols, and traffic scenarios,

3. the total power consumption of a sub-model is calcu-lated using realistic values for each functional blockand according to the actual utilization of networkelements.

4. finally, the flow of information and energy throughthe entire global communication network can be iden-tified and evaluated by defining and implementingthe interfaces between the sub-models, which wouldlead to a holistic model of interconnected sub-systemsat a high hierarchical level.

In addition, various analogies between classical ther-modynamics and communication systems could be alsoincluded in such a holistic approach. Since all com-ponents of a communication system are physical ob-jects that need energy to properly function, any act

User Behaviour

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14Tel VoIP Radio SDTV HDTV VoD Inter File Sh. V-stream Gaming V-conf VPN Cloud RHM

00:00 50 20 700 30 10 10 70 10 5 100 5 10 2 4001:00 50 50 700 20 10 10 36 10 5 100 5 10 2 4002:00 50 100 700 20 7 5 26 10 5 100 5 10 2 4003:00 50 40 700 20 8 4 73 10 5 100 10 10 2 4004:00 50 100 700 20 10 5 45 30 5 100 10 10 2 4005:00 50 100 700 20 30 8 75 50 5 100 10 10 2 4006:00 50 100 700 20 60 6 2 60 5 100 10 10 2 40

Time of the Day

No. of users of the applications

Distribution of Packet/File Length

App. #2

Distribution of Packet/File Length

App. #2 App. #1

Distribution of Call Duration

App. #1

Distribution of Call Duration

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

#10#9#8#7#6#5#4#3#2#1

App.

Time of the Day [hours]

DS US Total#1 Telephone (local provider) 0.064 0.064 0.128#2 VoIP (over Internet) 0.064 0.064 0.128#3 Digital Radio 0.128 0 0.128#4 SDTV ( Broadcast) 2 0 2#5 HDTV (Broadcast) 20 0 20#6 VoD (Video on demand) 2 0 2#7 Classic Internet Services 0.04 0.04 0.08#8 Filesharing 20 10 30#9 Videostreaming 2 0 2

#10 Online Gaming 1 1 2#11 Videoconference 2 2 4#12 Home Office via VPN 1 1 2#13 Cloud Storage & Processing 1 1 2

#14 Remote Home Monitoring 0.005 0.8 0.805

Data rate [Mbit/s]App. No.

Description

a)

b) c) d)

User Behaviour

Day/Night Curves

Traffic Scenarios

Model of Access

Networks(e.g. HFC, xDSL,

FTTH, UMTS,WiMax, LTE)

InformationGeneration

InformationReception

InformationReception

InformationGeneration

Model of Metro/Regional

Networks(e.g. SONET/SDH,

Ethernet, OTN, Optical MANs,

WDM)

Model of National-wide

Networks(e.g. SONET/SDH,

OTN, WDM, OCS, OBS,

OPS)

Interface between Models (Gateway)

Model of Continent-wide

Networks(e.g. SONET/SDH,

OTN,WDM, Transatlantic, OCS, OBS,

OPS)

Inte

rcon

tinen

tal L

inks

Fig. 12 a) Hierarchical approach to model performance and energy efficiency of the global communication network; b), c) andd) hypothetical examples of applications characteristics and user behavior: b) applications’ characteristics, c) user behaviorrespective application use and d) an example presenting application-specific traffic over the day.

of communication is always a dissipative process. Onthe other hand, perceivable information itself is phys-ical in nature. We are not able to generate, transmit,receive or process information without its physical rep-resentation. Thus, the logical consequence is that anyphysical system for presenting and exchanging of per-ceivable information consumes and dissipates energy.Therefore, flows of information and energy within com-munication systems at both microscopic and macro-scopic levels can be identified and treated using well-established approaches and tools of thermodynamics.Some thermodynamic aspects of communication sys-tems have been presented and discussed by the authorin a recent paper [50]. Such an approach to model theglobal communication network by taking into accountinformation and energy (or entropy) flows and includ-ing thermodynamic aspects of communication systemscould be generally applicable and would have the po-tential to provide an efficient treatment of the globalsystem.

7 Conclusions

In conclusions, the paper briefly addresses technologiesand architectures for implementing high-performanceand energy-efficient network elements. Various methodsfor reducing power consumption and increasing powerefficiency of wired core and access networks are de-scribed. Through reducing the energy consumption while

concurrently increasing capacity and performance ofnetworks one can improve the energy efficiency of thenetwork infrastructure on the one hand and ensure anunhindered development of new broadband applicationson the other, which could consequently have direct andindirect positive influences on the global energy con-sumption and energy-related carbon dioxide emissions.A holistic approach integrating technological aspects,power-aware protocols and algorithms as well as inter-dependencies between the global communication net-work and different other subsystems can potentiallylead to an optimized network operation and large im-provements in global energy productivity.

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