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Review ArticleFiber-Wireless (FiWi) Broadband Access Networks inan Age of Convergence: Past, Present, and Future

Martin Maier

Optical Zeitgeist Laboratory, Institut National de la Recherche Scientique (INRS), Montreal, QC, Canada HA K

Correspondence should be addressed to Martin Maier; [email protected]

Received December ; Accepted May ; Published June

Academic Editor: Roberto Proietti

Copyright Martin Maier. Tis is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Afer describing the beginnings and state o the art o integrated ber-wireless (FiWi) broadband access networks in great detail,we briey review recent progress and point to various ongoing research activities, including the design o energy-efficient greenFiWi access networks, advanced survivability techniques, and integration o wireless and ber optic sensors, towards realizingadaptable, dependable, and ecoconscious uture-proo broadband access networks based on both wireless and shared passive bermedia. Furthermore, we discuss service, application, business, and operation related aspects, which motivate access technologyto move into a substantially different direction in the long run than continued capacity provisioning. Given that most G cellularmobile network researches so ar have been ocusing on the achievable perormance gains in the wireless ront-end only withoutlooking into the details o backhaul implementations and possible backhaul bottlenecks, we identiy open key research challenges

or FiWi broadband access networks. We explore ways o how they can be deployed across relevant economic sectors other thantelecommunications per se, taking major paradigm shifs such as the Tird Industrial Revolution, Energy Internet, smart grid, andexplosion o mobile data traffic in todays cellular networks into account.

1. Introduction

According to the Federal Communications Commission(FCC), broadband enables individuals and enterprises toaccess a wide range o resources, services, and productsrelated to education, culture, entertainment, telemedicine, e-commerce, public saety, and homeland security. In a detailedstudy carried out by the Organisation or Economic Coop-

eration and Development (OECD) [] it was shown that theimpact o providing residential and business subscribers withbroadband access is maniold. Among others, broadbandenables the emergence o business models, processes, andinventions as well as improved goods and services. Further-more, broadband increases competitiveness and exibility inthe economy by the increased diffusion o inormation atlower cost and by improving market access to increasinglylarger markets.

Figure (a) shows the xed wired and wireless accesstechnologies used by broadband subscribers in . Backthen, the majority o xed broadband subscribers deployeddigital subscriber line (DSL) or cable modem, while only %

o the million subscribers were connected via ber-to-the-premises solutions, that is, ber-to-the-home (FH) orber-to-the-building (FB) with local area network (LAN)deployment in apartments. (For completeness, we note thatthe remaining % o subscribers were provided broadbandaccess via satellite, WiMAX, or broadband over power line(BPL).) However, this situation is changing rapidly. Forillustration,Figure (b)shows the latest OECD data on xed

wired broadband subscriptions as o June . Clearly, thisgure illustrates that an increasing percentage o broadbandsubscribers rely on ber access technologies at the expense olegacy DSL solutions. Tis trend is expected to become evenmore pronounced over the next couple o decades. In theshort- to midterm, current state-o-the-art very high bit-rateDSL (VDSL) may be superseded by next-generation copperbased solutions supported by deep ber access networksgetting increasingly closer to subscribers.

We are currently witnessing a strong worldwide deploy-ment o deep ber access solutions to push optical ber closerto individual homes and businesses and to help realize Fxnetworks, whereby x denotes the discontinuity between

Hindawi Publishing CorporationAdvances in OpticsVolume 2014, Article ID 945364, 23 pageshttp://dx.doi.org/10.1155/2014/945364


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DSL

60%

Cable modem29%

Fibre + LAN9%

Other2%otal subscribers: 271 million

(a)

DSL54.8%

Cable modem30.4%

Fibre + LAN14.2%

Other0.6%

OECD xed (wired) broadband subscriptions, by technology, June 2012

otal subscribers: 321 millionSource: OECD

(b)

F : Broadband subscribers across OECD countries by technology: (a) xed wired and wireless access technologies used in and(b) xed wired access technologies used in [].

CO

CO

CO

CO

CO

CO

Passive splitter/combiner

Optical ber

Copper cable

FN (ADSL2+)

FH (PON)

DSLAM

DSLAM

End-user

End-user

End-user

End-user

End-user

End-user

Modem

Modem

Modem

Modem

Modem

Modem

Modem

ModemFB (PtP + VDSL2)

MDU

FCab (VDSL2)

Service provider Node/cabinet Building Home

FCab (HFC)

CAV

cabinet

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ONU

ONU

ONU

ONU

FH (PtP)

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F : Fx network architectures.

optical ber and some other transmission medium [].Figure shows a variety o possible Fx network archi-tectures, ranging rom ber-to-the-node (FN) to FHnetworks, depending on how deep optical ber reaches intothe rst/last mile. In FN networks, ber is used between

the central office (CO) and the DSL access multiplexer(DSLAM) located at the remote node. Te DSLAM connectsto each end-users modem through a separate legacy tele-phone twisted pair o copper wires. FN networks deployasymmetric DSL+ (ADSL+) on the twisted pairs offering


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data rates o up to Mb/s downstream and up to . Mb/supstream. In ber-to-the-cabinet (FCab) networks, alsoknown as ber-to-the-curb (FC) networks, the DSLAMis moved closer to the end-user in order to shorten thelength o the twisted-pair drop lines and thereby enable thedeployment o VDSL, which offers signicantly increased

data rates o up to Mb/s downstream and up to Mb/supstream. FCab is also used by cable network operatorsto build hybrid ber-coax (HFC) networks, where the droplines are realized through coax cables instead o twisted pairs.Bringing ber all the way to buildings gives rise to FBnetworks. ypically, FB networks use a multidwelling unit(MDU) optical network unit (ONU) in the basement o thebuilding to terminate the optical signal coming rom the COand distribute the converted signal across a separate networkinside the building. Finally, paving all the way to the homewith optical ber leads to FH networks, which come intwo avors: (i) point-to-point (PtP) star topology, where eachONU is connected to the CO via a separate ber or (ii) point-to-multipoint passive optical network (PON) tree topology,using a single shared ber link between the CO and a passiveoptical splitter/combiner at the remote node.

Deep ber access is a challenging mix o technologychoices, business models, and regulatory issues. But due toincreasingly personalized, interactive, on-demand, and high-denition broadband service demands and thanks to inno-

vations within low-cost ber-laying techniques, or example,microtrenching, there is no doubt that widespread FH/Bdeployment is just a matter o time []. According to recentmarket data by ABI Research, the number o xed broadbandsubscribers will rise to million by the end o , owhich million will subscribe to services delivered viaber. Among the three major xed broadband technologies,the number o ber subscribers is increasing astest at acompound annual growth rate o % rom to .Furthermore, FCC has recently unveiled the United Statesrst national broadband plan with the goal to provide everyAmerican with broadband access speeds o Mb/s by. oward this goal, ber (together with next-generationwireless) broadband technologies will play an increasingly

vital role in uture broadband access networks. Tis is alreadywitnessed by installed state-o-the-art VDSL equipment,which is almost exclusively based on optical ber backhaulsolutions. While copper will certainly continue to play animportant role in current and near-term broadband accessnetworks, it is expected that FH deployment volume will

keep increasing gradually and will eventually become thepredominant xed wireline broadband technology by [].

Optical ber provides an unprecedented bandwidthpotential that is ar in excess o any other known transmis-sion medium and offers signicantly longer ranges withoutrequiring any active devices. Optical ber has some urtheradvantageous properties such as longevity and low mainte-nance costs, which will eventually render ber the mediumo choice in wired rst/last mile access networks. In act, thistrend can already be observed in most o todays greenelddeployments where ber ratherthan copper cablesis installedor broadband access []. Optical access networks provide

transparency against data rate and signal ormat, which easedcarriers worldwide into deploying uture-proo PON outsideplants that can be exibly upgraded as new technologiesmature or new standards evolve []. Arguing that due to itsunique properties optical ber is likely to entirely replacecopper wires in the near-to midterm, we will elaborate on

the nal rontier o optical networks, namely, the convergencewith their wireless counterparts. Optical and wireless tech-nologies can be thought o as quite complementary and willexpectedly coexist over the next decades. Future broadbandaccess networks will be bimodal, capitalizing on the respec-tive strengths o both technologies andsmartly merging themin order to realize uture-proober-wireless (FiWi) networksthat strengthen our inormation society while avoiding itsdigital divide. By combining the capacity o optical bernetworks with the ubiquity andmobility o wireless networks,FiWi networks orm a powerul platorm or the support andcreation o emerging as well as uture unoreseen applicationsand services, or example, telepresence [].

In this paper, we provide a comprehensive overview othe beginnings, state o the art, and latest developments oFiWi broadband access networks. We discuss the differentthreads o FiWi access networking research and the rationalebehind their different design objectives. Afer describingrecent progress, we elaborate on the role o FiWi accessnetworks in thedawning ageo convergence andoutlinesomeexciting research directions or uture FiWi access networks.Te remainder o thispaper is structured as ollows. Section describes related research topics and denes FiWi accessnetworks as a new research area. InSection , we review thestate o the art o FiWi broadband access networks, whilerecent progress is described inSection .Section providesan outlook and outlines the road ahead or uture FiWi accessnetworks. Finally, we draw some conclusions inSection .

2. FiWi Broadband Access Networks

raditionally, wireless and optical ber networks have beendesigned separately rom each other. Wireless networksaimed at meeting specic service requirements while copingwith particular transmission impairments and optimizing theutilizationo the system resources to ensure cost-effectivenessand satisaction or the user. In optical networks, on theother hand, research efforts rather ocused on cost reduction,simplicity, and uture prooness against legacy and emergingservices and applications by means o optical transparency.

Wireless and optical access networks can be thought o ascomplementary. Optical ber does not go everywhere, butwhere it does go, it provides a huge amount o availablebandwidth. Wireless access networks, on the other hand,potentially go almost everywhere but provide a highly band-width-constrained transmission channel susceptible to a

variety o impairments.Future broadband access networks not only have to pro-

vide access to inormation when we need it, where we needit, and in whatever ormat we need it, but also, and arguablymore importantly, have to bridge the digital divide and offersimplicity and user-riendliness based on open standardsin order to stimulate the design o new applications and


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services. oward this end, uture broadband access networksmust leverage on both optical and wireless technologies andconverge them seamlessly, giving rise to FiWi access networks[]. FiWi access networks are instrumental in strengtheningour inormation society while avoiding its digital divide. Bycombining the capacity o optical ber networks with the

ubiquity and mobility o wireless networks, FiWi networksorm a powerul platorm or the support and creation oemerging as well as uture unoreseen applications and ser-

vices. FiWi networks hold great promise to change the way welive and work by replacing commuting with teleworking. Tisnot only provides more time or proessional and personalactivities or corporate and our own personal benet but alsohelps reduce uel consumption and protect the environment;issues that are becoming increasingly important in our lives.

Due to the difficulty and prohibitive costs o supplyingoptical ber to all end-user premises as well as the spectrumlimitations o wireless access networks, bimodal FiWi accessnetworks are more attractive than relying on either stand-alone access solution. FiWi access networks are realized byintegrating wireless access technologies, or example, cellular,WiMAX, and WiFi, with installed optical ber inrastructurethat has been pushed ever closer toward end-users over thelast ew years and has been the preerred medium o choicein most o todays greeneld deployments, where ber ratherthan copper cables are installed or broadband access. obetter understand the rationale behind the vision o FiWiaccess networks, we rst describe related research topics andthen dene FiWi access networks as a new research area inthe ollowing.

Fixed Mobile Convergence (FMC). According to the Euro-pean elecommunications Standardization Institute (ESI),FMC is concerned with developing network capabilities andsupporting standards that may be used to seamlessly offer aset o consistent services via xed or mobile access to xedor mobile, public or private networks, independently o theaccess technique [].

FMC can be done at different levels, or example, businessor service provisioning level. Note, however, that FMC doesnot necessarily imply the physical convergence o networks.In act, the convergence at the network acilities level, wherean operator uses the same physical network inrastructurewith common transmission and switching systems to provideboth mobile and xed services, is more accurately reerred toasxed mobile integration (FMI) []. While FMI considers

any type o access networks, or example, DSL, HFC, orwireless local area network (WLAN), the so-called opticalwireless integration (OWI) ocuses on emerging optical andwireless broadband access technologies, as explained next.

Optical Wireless Integration (OWI). Current copper basedaccess network technologies such as DSL and HFC ace seri-ous challenges to meet the requirements o uture broadbandaccess networks. While DSL suffers rom severe distance andnoise limitations, HFC alls short to efficiently carry datatraffic due to its upstream noise and crosstalk accumulation.Recent progress in optical ber technologies, especially thematurity o integration and new packaging technologies, has

rendered optical ber access networks a promising low-costbroadband solution. In particular, PONs are able to providelower network deployment and maintenance costs as wellas longer distances than current DSL and HFC networks.OWI aims at integrating PONs and other optical ber accesstechnologies with broadband wireless access technologies, or

example,WiMAX, in orderto increase the capacityo wirelessaccess networks and reduce access point complexity throughcentralized management [].

It is important to note that there is a difference betweenOWI and ree-space optical wireless (OW) communications.OW systems to test line-o-sight wireless communicationswere already designed in the s, well beore the develop-ment o optical ber communications []. Current radio re-quency (RF) wireless systems, or example, WiFi, WiMAX, orultrawideband (UWB), are limitedin bothdata rate andrangedue to their low carrier requencies. OW communicationslinks operate at much higher carrier requencies than theirRF counterparts. As the RF spectrum becomes increasinglycongested, OW represents an interesting alternative to RFwireless systems. OW may be deployed as a temporary back-bone or rapidly deployable mobile wireless communicationinrastructure, especially in densely populated urban areas.Note, however, that unlike OWI networks, OW links andnetworks do not involve any wired (ber) inrastructure.

Radio-over-Fiber (RoF) versus Radio-and-Fiber (R&F) Net-works. RoF networks have been studied or many years asan approach to integrate optical ber and wireless networks.In RoF networks, RFs are carried over optical ber linksbetween the CO and multiple low-cost remote antennaunits (RAUs) in support o a variety o wireless applica-tions, or example, microcellular radio systems []. It wasexperimentally demonstrated that RoF networks can havean optical ber range o up to km. However, inserting anoptical distribution system in wireless networks may have amajor impact on the perormance o medium access control(MAC) protocols []. Te limitations o RoF networkscan be avoided in so-called radio-and-ber (R&F) networks[]. While RoF networks use optical ber as an analogtransmission medium between the CO and one or moreRAUs with the CO being in charge o controlling access toboth optical and wireless media, in R&F networks access tothe optical and wireless media is controlled separately romeach other by using in general two different MAC protocolsin the optical and wireless media, with protocol translation

taking place at their interace. As a consequence, wirelessMAC rames do not have to travel along the optical ber tobe processed at the CO but simply traverse their associatedwireless access point and remain in the wireless ront-end,thus avoiding the negative impact o ber propagation delayon the network perormance.

FiWi Access Networks. FiWi access networks may deploy bothRoF and R&F technologies. By simultaneously providingwired and wireless servicesover the sameinrastructure,FiWiaccess networks are able to consolidate optical and wirelessaccess networks that are usually run independently o eachother, thus potentially leading to major cost savings.


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FiWi networking research deals with the OWI o opticaland wireless broadband access technologies, or example,wireless mesh network (WMN). FiWi research ocuses on thephysical (PHY), MAC, and network layers with the goal todevelop and investigate low-cost enabling FiWi technologiesas well as layer- and layer- protocols and algorithms.

Higher-layer network capabilities developed through FMCstandardization efforts can be exploited on top o the PHY,MAC, and network layers o FiWi networks. FiWi researchinquires new methods o optical RF generation exploit-ing ber nonlinearities and various modulation techniques.Specically, to avoid the electronic bottleneck, the generationo RF signals is best done optically. A variety o differentoptical RF generation techniques were experimentally stud-ied and demonstrated, including our-wave mixing (FWM)or cross-phase modulation (XPM) in a highly nonlineardispersion-shifed ber (HNL-DSF), cross-absorption mod-ulation (XAM) in an electroabsorption modulator (EAM),and external intensity modulation (IM) or phase modulation(PM) []. According to [], external intensity and phasemodulation schemes are the most practical solutions orall-optical RF generation due to their low cost, simplicity,and long-distance transmission perormance. Furthermore,FiWi research also includes the study o different remodu-lation schemes or the design o colorless (i.e., wavelength-independent) RAUs. Remodulation techniquesalso playa keyrole in ongoing research activities on ull-duplex wavelengthdivision multiplexing (WDM) PONs based on colorlessONUs. Remodulation capitalizes on a single light source atthe central office to generate a downlink wavelength that isreused at RAUs/ONUs or upstream transmission, therebyavoiding the need or an additional light source at eachRAU/ONU. A number o different remodulation schemeshave been proposed and investigated, or example, differen-tiated phase-shif keying (DPSK) or downstream and on-off-keying (OOK) or upstream, optical carrier suppression(OCS) or downstream and reused or upstream, or PM ordownstream and directly modulated semiconductor opticalamplier (SOA) or upstream. Te use o a colorless SOAas an amplier and modulator or upstream transmissionprovides a promising low-cost RoF solution that is easy tomaintain [].

While signicant progress has been made at the PHYlayer o FiWi and in particular RoF transmission systems,FiWi networking research on layer- and layer- relatedissues has begun only recently. Among others, FiWi layer-

/ research includes the joint optimization o perormance-enhancing MAC mechanisms separately used in the wirelessand optical network segments, or example, wireless rameaggregation and optical burst assembly, hybrid access controlprotocols, integrated path selection algorithms, and advancedresilience techniques. Layer-/ networking research is cru-cial to unleash the ull potential o FiWi broadband accessnetworks [,].

3. State of the Art

Beside cell-based RoF networks, a number o FiWi networkarchitectures were proposed, which can be classied based on

their wireless access technologies: WiMAX or WiFi. As wewill see shortly, different challenges were addressed such asrouting and wireless channel assignment, which can be per-ormed completely either in the wireless domain by the basestation (BS) or access point (AP), or by an optical networkelement, or example, CO or optical line terminal (OL). Te

level o provided quality-o-service (QoS) largely depends onthe perormance o the implemented routing and resourcemanagement algorithms, including bandwidth allocation andchannel assignment algorithms with absolute or relative QoSassurances. Reconguration is another previously addressedchallenging issue that involves resource management in thewireless and/or optical part.

.. Architectures

... Cellular Architectures. Cellular networks used or astmoving users, or example, train passengers, suffer rom re-quent handovers when hopping rom one BS to another one.Te requent handovers may cause numerous packet losses,resulting in a signicantly decreased network throughput. Aninteresting approach to solve thisproblem is the use o an RoFnetwork installed along the rail tracks in combination withthe so-calledmoving cellconcept [].

Recently, the moving extended cell concept was proposedto provide connectivity or any possible direction []. Teber optic network becomes a means or speedy hand-off between base stations that serve the mobile users. Ahybrid requency division multiplexing (FDM)/WDM net-work architecture wasused to support thedelivery o multipleRF channels in the GHz requency band over the samewavelength. Te extended cell involves the current userscell and the surrounding cells ensuring connectivity or anyrandom direction. Te extended cell is adaptively restruc-tured when the user enters a new cell. It was shown thatthe proposed concept can provide zero packet loss and calldropping probability in high-rate wireless services or a widerange o mobile speeds o up to m/sec, independently othe ber link distances.

RoF networks are attractive since they provide trans-parency against modulation techniques and are able to sup-port various digital ormats and wireless standards in a cost-effective manner. While single-mode bers (SMFs) are typi-cally ound in outdoor optical networks, many buildings havepreinstalled multimode ber (MMF) cables. Cost-effectiveMMF-based networks can be realized by deploying low-cost

vertical cavity surace emitting lasers (VCSELs). Apart romrealizing low-cost microcellular radio networks, optical berscan also be used to support a wide variety o other radiosignals. In [], a low-cost MMF network was experimentallytested to demonstrate the easibility o indoor radio-over-MMF networks or the in-building coverage o second-generation and third-generation cellular radio networks aswell as IEEE .b/g WLAN.

... WiMAX-Based Architectures. Te integration o Eth-ernet PON (EPON) and WiMAX access networks can bedone in several ways. According to [], the ollowing ourarchitectures can be used.


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Independent Architecture. In this approach, WiMAX base sta-tions serving mobile client nodes are attached to an ONUthe customer premises equipment (CPE) o an EPON

just like any other wired subscriber node. WiMAX andEPON networks are connected via a common standardizedinterace, or example, Ethernet, and operate independently

rom each other.

Hybrid Architecture. Tis approach introduces an ONU-base station (ONU-BS) that integrates the EPON ONU andWiMAX BS in both hardware and sofware. Te integratedONU-BS controls the dynamic bandwidth allocation o bothONU and BS.

Unied Connection-Oriented Architecture. Similar to thehybrid architecture, this approach deploys an integratedONU-BS. But instead o carrying Ethernet rames, WiMAXMAC protocol data units (PDUs) containing multiple encap-sulated Ethernet rames are used. By carrying WiMAX MAC

PDUs, the unied architecture can be run like a WiMAXnetwork with the ability to grant bandwidth nely usingWiMAXs connection-oriented rather than EPONs queue-oriented bandwidth allocation.

Microwave-over-Fiber Architecture. In this approach, theWiMAX signal is modulated on a wireless carrier requencyand is then multiplexed and modulated together with thebaseband EPON signal onto a common optical requency(wavelength) at the ONU-BS. Te central node consists o aconventional EPON OL and a central WiMAX BS, calledmacro-BS. Te OL processes the baseband EPON signal,while the macro-BS processes data packets originating rom

multiple WiMAX BS units.

... WiFi-Based Architectures. Besides the aorementionedintegration approaches o EPON and WiMAX networks,several other FiWi architectures based on WiFi technologyhave been studied, as described in the ollowing.

Unidirectional Ring. Tis type o FiWi network architectureinterconnects the CO with multiple WiFi-based wireless APsby means o an optical unidirectional ber ring []. Te COis responsible or managing the transmission o inormationbetween mobile client nodes (MCNs) and their associatedAPs as well as acting as a gateway to other networks. Each AP

provides wireless access to MCNs within its range. All MCNstake part in the topology discovery, whereby each MCNperiodically sends the inormation about the beacon powerreceived rom its neighbors to its associated AP. In doingso, APs are able to estimate the distances between MCNsand compute routes. Multihop relaying is used to extend therange. o enhance the reliability o the wireless link, the COsends inormation to two different APs (path diversity). Teproposed implementation can support advanced path diver-sity techniques that use a combination o transmission viaseveral APs and multihop relaying, or example, cooperativediversity or multihop diversity. Consequently, the CO mustbeable to assign channels quickly and efficiently by using one or

more wavelength channels on the ber ring to accommodatemultiple services such as WLAN and cellular radio network.

Bidirectional Ring. Tis WiFi-based architecture consists oa two-level bidirectional path-protected ring (BPR) archi-tecture or dense WDM (DWDM)/subcarrier multiplexing

(SCM) broadband FiWi networks []. In this architecture,the CO interconnects remote nodes (RNs) via a dual-berring. Each RN cascades APs through concentration nodes(CNs), where each AP offers services to MCNs. For protec-tion, the CO is equipped with two sets o devices (normaland standby). Each RN consists o a protection unit and abidirectional wavelength add-drop multiplexer based on amultilayer dielectric intererence lter. Each CN contains aprotection unit. Te AP comprises an optical transceiver, aprotectionunit, up/downRF converters, and a sleeve antenna.Each AP provides channel bandwidth o at least MHz andcovers up to MCNs by means o FDM. Under normaloperating conditions, the CO transmits downstream signals

in the counter-clockwise direction via RNs and CNs to theAPs. I a ber cut occurs between two RNs or betweentwo CNs, their associated controllers detect the ailure bymonitoring the received optical signal and then switch tothe clockwise protection ring. I a ailure happens at an AP,the retransmitted signals are protection switched throughother optical paths by throwing an optical switch insidethe affected AP. Tis architecture provides high reliability,exibility, capacity, and sel-healing properties.

Hybrid Star-Ring. Tis hybrid FiWi architecture combinesoptical star and ring networks []. Each ber ring accom-modates several WiFi-based APs and is connected to the CO

and two neighboring ber rings via optical switches. Teoptical switches have ull wavelength conversion capabilityand interconnect the APs and CO by means o shared point-to-point lightpaths. Te network is periodically monitoredduring prespecied intervals. At the end o each interval,the lightpaths may be dynamically recongured in responseto varying traffic demands. When traffic increases and theutilization o the established lightpaths is low, the load on theexisting lightpaths is increased by means o load balancing.Otherwise, i the established lightpaths are heavily loaded,new lightpaths need to be set up, provided enough capacityis available on the ber links. In the event o one or more linkailures, the affected lightpaths are dynamically recongured

using the redundant ber paths o the architecture.

Unidirectional Ring-PON. Tis FiWi network proposed in[] consists o an optical WDM backhaul ring with multiplesingle-channel or multichannel PONs attached to it. Moreprecisely, an optical add-drop multiplexer (OADM) is usedto connect the OL o each PON to the WDM ring. Wire-less gateways are used to bridge PONs and WMN. In thedownstream direction, data packets are routed rom the COto the wireless gateways through the optical backhaul andare then orwarded to the MCNs by wireless mesh routers.In the upstream direction, wireless mesh routers orwarddata packets to one o the wireless gateways, where they


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ONU

ONU MPP

ONU MPPSA

ONU

MP

MP

MP

InternetRN

Single subscriber

Multiple subscribers

Splitter/combineror

wavelength multiplexer/demultiplexer

DM or WDM

SA

MAP

OL

F : FiWi access network architecture based on single-stage DM or WDM PON and multihop WMN using legacy or VH WLANtechnologies.

are then transmitted to the CO on one o the wavelengthchannels o the optical backhaul WDM ring, as each PONoperates on a separate dynamically allocated wavelengthchannel. Since the optical backhaul and WMN use differenttechnologies, an interace is dened between each ONU andthe corresponding wireless gateway in order to monitor theWMN and perorm route computation taking the state owireless links and average traffic rates into account. When thetraffic demands surpass the available PON capacity, some othe single-channel time division multiplexing (DM) PONsmay be upgraded to WDM PONs. I some PONs are heavilyloadedand othershave less traffic,some heavily loaded ONUsmay be assigned to a lightly loaded PON by tuning theiroptical transceivers to the wavelength assigned to the lightlyloaded PON. Tis architecture provides cost-effectiveness,bandwidth efficiency, wide coverage, high exibility, andscalability. In addition, the recongurable DM/WDM opti-

cal backhaul helps reduce network congestion and averagepacket latency by means o load balancing. Moreover, thedynamic allocation o radio resources enables cost-effectiveand simple handovers.

Cascaded PON-WMN. Te majority o previous studies onFiWi access networks considered a cascaded architectureconsisting o a single-stage PON and a multihop WMN,as shown inFigure . ypically, the PON under considera-tion is a conventional IEEE .ah compliant wavelength-broadcasting DM EPON based on a wavelength split-ter/combiner at the RN, using one time-shared wavelengthchannel or upstream transmissions (rom ONUs to OL)

and another time-shared wavelength channel or down-stream transmissions (rom OL to ONUs), both operatingat a data rate o Gb/s. A subset o ONUs may be located atthe premises o residential or business subscribers, wherebyeach ONU provides FH/B services to a single or mul-tiple attached wired subscribers. Te remaining ONUs areassumed to be equipped with a mesh portal point (MPP)to interace with the WMN, which in turn consists o relaymesh points (MPs) andmesh accesspoints (MAPs) to providestations (SAs) with wireless access to the FiWi network.Most previous FiWi studies assumed a WMN based on IEEE.a/b/g WLAN technologies, offering a maximum rawdata rate o Mb/s at the physical layer.

Future FiWi access networks will leverage on next-generation PON and WLAN technologies to meet the ever-increasing bandwidth requirements o new and emerging

video-dominated applications and services. A variety o next-generation PON technologies are currently investigated toenable short-term evolutionary and long-term revolutionaryupgrades o coexistent Gigabit-class DM PONs []. How-ever, the most promising solutions or PON evolution towardhigher bandwidth per user are (i) data rate upgrades to Gb/s and higher and (ii) multiwavelength channel migra-tion toward wavelength-routing or wavelength-broadcastingWDM PONs with or without cascaded DM PONs, respec-tively [, ]. Similarly, to alleviate the bandwidth bottlenecko wireless mesh ront-end networks, uture FiWi accessnetworks are expected to be based on next-generation IEEE.n WLAN, which offers data rates o Mb/s or higherat the MAC service access point, as well as emerging Gigabit-

class IEEE .ac very high throughput (VH) WLANtechnologies that achieve raw data rates o up to Mb/s.

... Highlighted Architectures with a Physical Layer Focus

GROW-Net. Te so-calledgrid recongurable optical-wirelessnetwork (GROW-Net) is a scalable municipal FiWi networkarchitecture that adapts the street layout in a city and makesuse o available dark bers readily available in urban areas[,]. Te design objectives o GROW-Net are to providescalability in terms o bandwidth and allow or inrastructureextensibility. Te basic building block o GROW-Net is thegrid-cell, which provides the backbone ber connectivity to

wireless gateways. Te optical ber backbone inrastructureis extensible using the proposed H-tree partitioning process,whereby at each stage o the process the spacing betweentwo adjacent WMN routers is reduced by % and opticalterminals are inserted and interconnected by means ooptical WDM links that are laid out in an H-shape. In thedownstream direction, the central hub o GROW-Net deploysa tunable laser, which can reach different optical terminalsby tuning the laser to the respective wavelength channelsupported by the intended optical terminal. Each optical ter-minal is equipped with a thin lm lter (FF), which lets passonly the corresponding wavelength channel o a given opticalterminal. In the upstream direction, each optical terminal is


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made colorless by deploying a reective semiconductor opti-cal amplier (RSOA) or remote modulation o a continuouswavelength signal sent by the central hub. GROW-Net is ableto achieve increased ber protection than conventional tree-based PONs due to its mesh topology.

FUON. In the European research project FUON (FiberOptic Networks or Distributed, Extendible, HeterogeneousRadio Architectures and Service Provisioning), an integratedwireless/optical network architecture was developed thataims at enabling wireless services and meeting IUs IM-Advanced (IM-A) requirements in a cost-efficient manner[]. In the proposed architecture, low-complexity RAUsare connected via analog RoF links to a central processingunit (CU), which perorms joint signal processing andresource management. Signals rom the CU to differentantennas o a site and to different RAUs can be multiplexedonto the same optical ber by means o WDM and SCM.Te architectures ability was demonstrated by developing a

FUON prototype, which eatures downlink data transmis-sion rom two single-antenna RAUs to two single-antennamobile terminals. o separate the users spatially, an MMSEprecoding lter is applied at the CU. In the downlink, pilotsand data are transmitted to the mobile terminals, whereasthe uplink is used to eed channel state inormation rom themobile terminals back to the CU to adapt the precoding lter.Te obtained measurements demonstrated that the MMSEprecoding provides a large throughput gain compared toconventional noncooperative precoding. Tis is due to theact that the MMSE precoding is able to mitigate the mutualintererence o the RAUs such that both mobile terminalscan receive data at high data rates using the same time and

requency resources, at the expense o a signicant overheadin the uplink.

ACCORDANCE. Another interesting European researchproject is the so-called ACCORDANCE, an ultrahigh capac-ity extended reach optical access network architecturebased on orthogonal requency division multiple access(OFDMA) technology/protocols, implemented through theproper mix o state-o-the-art photonics and electronics[]. ACCORDANCE aims at realizing the concept ointroducing OFDMA-based layer- technology and layer-protocols to provide a variety o desirable characteristics suchas increased aggregate bandwidth and scalability, enhanced

resource allocation exibility, longer reach, lower equipmentcost/complexity, and lower power consumption, while alsosupportingmultiwavelength operation. In addition,it enablesthe convergence o the optical ber inrastructure withstandard wireless solutions, thus offering a way to integratedominant wired and wireless technologies in a hybrid accessnetwork supporting seamless ubiquitous broadband services.More specically, the OL manages the assignment o trafficto a large number o subcarriers, which travel all along theoptical distribution network to be demultiplexed only atthe user side and vice versa. Te subcarriers are groupedto orm OFDM channels (with several tens or hundreds osubcarriers contained within one channel), each o them

used or carrying traffic possibly by different providers andemploying different optical, wireless, and copper-based tech-nologies. ACCORDANCE was implemented using opticalI/Q (oIQ) modulation at the OL transmitter and simpledirect detection at the ONU receiver or downstream com-munications and seeded intensity modulation (IM) at the

ONU transmitter and coherent detection at the OL receiveror upstream communications. Sinceall ONUs share the same(seeded) optical source, upstream OFDMsignals are perectlyaligned and upstream OFDMA is made easible.

.. Network Planning and Reconguration. o maximize theperormance o FiWi networks and minimize their deploy-ment costs, network planning and reconguration play a keyrole in achieving these design objectives. In this section, wedescribe a number o algorithms that help solve importantFiWi network planning problems related to the optimalplacement o ONUs, mitigation o the detrimental impacto wireless intererences or peer-to-peer communications

between wireless end-users, and architectural modicationsor the support o direct inter-ONU communications. Fur-thermore, we discuss the previously proposed recongurableFiWi network architectures that are able to respond to varyingtraffic loads.

... ONU Placement. Te optimal placement o ONUs is animportant design objective o FiWi networks due to the actthat the cost o laying optical ber is signicantly higher thanthat o devices attached to either end o the optical ber, orexample, OL.

Several heuristics to solve the problem o optimally plac-ing ONUs in a FiWi access network consisting o a PON intandem with a WiFi- or WiMAX-based WMN were studiedin []. Te rst proposed heuristicis a greedy algorithm thataims at ndinga suitable placement o multiple ONUs to min-imize the average Euclidean distance between wireless end-users andtheir closest ONU; that is, this heuristic targets onlythe wireless ront-end and does not take the ber layout othe optical backhaul into account. Te greedyalgorithm startswith a given distribution o wireless end-users, which mightbe randomly or deterministically chosen, and consists o twophases. In the rst phase, the algorithm identies the primaryONU (i.e., closest ONU) or each wireless end-user. In thesecond phase o the algorithm, or each primary ONU a set owireless end-usersis obtained suchthatthe distancesbetween

the ONU and its wireless end-users are minimized. Teperormance o the proposed greedy algorithm was evaluatedin the Wildhorse neighborhood o North Davis, Caliornia,under the assumption o random and deterministic ONUplacement. Te presented results show that the greedy algo-rithm perorms better or deterministic rather than randomONU placement. Te second proposed heuristic optimizesthe placement o ONUs by means osimulated annealing,a widely used combinatorial optimization technique. For agiven initial ONU distribution, this heuristic perturbs oneo the high-cost ONUs to achieve a cost improvement interms o average Euclidean distance between ONUs andwireless end-users. For each successul perturbation, wireless


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OL

ONU 3ONU 2

client a

ONU 1 ONU 4

Router

RouterRouter

Router

Router

Router Router

Router

Router

RouterMesh

Mesh

client b

A

B

F : Peer-to-peer communications in FiWi access networks [].

end-users are reassigned to their nearest ONU. Te reportedresults show that simulated annealing is able to achieve anoverall cost improvement o typically close to % comparedto the aorementioned greedy algorithm. Unlike the two pre-

vious algorithms, the third proposed heuristic also considersthe cost o the ber layout rom ONUs to the OL as wellas intererence in the wireless ront-end. oward this end,the proposed heuristic deploys the wireless access points(or base stations) uniormly, determines their transmissionradius, and assigns channels to them such that the carrier-to-intererence ratio does not drop below a certain threshold.Afer deploying the access points, the heuristic determinesthe number o ONUs needed to support the access pointsand deploys the ONUs according to the above-describedgreedy algorithm. Finally, the heuristic nds a minimum-costspanning tree to lay ber rom the OL to all ONUs.

A powerul mixed integer programming (MIP) modelor the optimum placement o ONUs and WiMAX BSsin a PON/WiMAX-based FiWi network was developed in[,]. Te presented MIP model minimizes the numbero deployed ONUs and BSs by saving costs and reducingthe intererence among multiple BSs and nearby wirelessusers. Specically, the objective is to minimize the totalinstallation cost o all ONUs and BSs required, includingcost o connecting the BSs to an ONU by means o optical

ber. Furthermore, the MIP problem takes into account awide range o important constraints with regard to userassignment, ONU installation, BS installation, capacity, andchannel assignment, as well as signal quality and intererence.More precisely, each wireless user is associated with at mostone BS and the distance between a given wireless user and itsassociated BS must be within the transmission range o thatBS. Moreover, the number o channels assigned to each BS islarge enough to serve its wireless users, while not exceedingthe upper bound o channels assigned to any BS. Each BSmust be connected to only one ONU and the capacity oeach ONU is large enough to serve all traffic generated by itsattached BS. Finally, the quality o the wireless signal has to be

at least the threshold o an acceptable carrier-to-intererenceratio. For tractability, the authors usedLagrangian relaxationto relax some o the constraints and solve the MIP problemwith acceptable accuracy. Te presented simulation resultsindicate that the ONU/BS placement problem is quite sen-sitive to a number o parameters, or example, number oavailable wireless channels and wireless user coverage ratio.

... Inter-ONU Communications

Peer-to-Peer Communications. In FiWi networks, traffic maygo rom wireless end-users to the Internet or rom onewireless client to another wireless client. In the ormer case,the throughput o FiWi networks is limited by the bandwidthbottleneck and intererences o communications in the wire-less subnetwork. In the latter case, the bimodal nature o FiWinetworks provides an opportunity to mitigate the detrimentalimpact o wireless intererences on the network throughputor peer-to-peer communications between two wireless end-users. In FiWi networks, peer-to-peer traffic can be carriedin two ways: (i) either through the wireless ront-end onlyor (ii) along a wireless-optical-wireless path []. As shownin Figure , suppose that wireless mesh client wants tocommunicate with mesh client. Te resultant peer-to-peertraffic can be routed either through the multihop wireless

path within the wireless subnetwork or through the wireless-optical-wireless path, whereby traffic is rst sent rom meshclient via its associated mesh router to its closest ONU and is then orwarded upstream to the OL. Subsequently,the OL broadcasts the traffic downstream to all ONUs. Eachreceiving ONU determines to discard or orward the trafficaccording to the location o the destination mesh client. Inour example, ONU is closest to destination mesh client and thus ONU will orward the arriving traffic to

via its associated mesh router, while all remaining ONUsdiscard the traffic coming rom the OL. Note that trafficorwarded along the wireless-optical-wireless path alleviatesintererences in the wireless ront-end, thereby allowing more


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traffic to be carried in the wireless segment and resultingin an increased network throughput. Also note that thecapacity o the PON is much higher than that o the wirelessront-end such that peer-to-peer traffic can be easily carriedin the optical backhaul without suffering rom any seriousthroughput penalty.

Clearly, to exploit the benets o steering peer-to-peertraffic along wireless-optical-wireless paths, the placement oONUs has a great impact on the achievable throughput oFiWi networks. In [], the ONU placement was optimizedwith the objective to maximize the throughput o FiWinetworks or peer-to-peer communications, assuming thatsingle-radio single-channel wireless routers are deployed andONUs can communicate with each other by sending traffic tothe OL, which in turn broadcasts traffic back to all ONUs.Given the location o the wireless mesh routers, the optimalplacement oONUs was ound with the objective to min-imize the total number o required wireless hops. Once thelocation o the ONUs is xed, each wireless node calculatesthe shortest path (in terms o hops) within the wirelessmesh ront-end to each ONU and selects the ONU with theminimal hop count. o solve this optimization problem, theauthors applied atabu search based heuristic, a widely usedmetaheuristic algorithm or solving combinatorial problems.Te presented simulation results show that under differentnetwork sizes the proposed tabu search heuristic outperormsrandom and xed ONU placement schemes in terms oachievable network throughput, especially or an increasingnumber o ONUs.

FiWi versus WMN Networks. o measure the networkthroughput gain in FiWi networks, a linear programmingbased routing algorithm was proposed in []. Te objectiveo the routing algorithm is to route traffic in the FiWi networksuch that network throughput is maximized. Te proposedoptimal routing algorithm yields a bound on the throughputgain in FiWi networks. Extensive simulations wereconductedto studythe network throughput gain in FiWi networks underpeer-to-peer traffic among wireless mesh clients and comparethe achievable throughput gain to conventional WMNs with-out any optical backhaul. Te presented simulation resultsshow that the network throughput gain o FiWi networksis zero compared with traditional WMNs when all trafficis destined to the Internet, that is, no peer-to-peer traffic,since the intererence in the wireless ront-end representsthe major bandwidth bottleneck. In other words, when all

the traffic goes to the Internet, the throughput o FiWinetworks is the same as that o traditional WMNs. However,with increasing peer-to-peer traffic the intererences in thewireless mesh ront-end increase and the throughput oWMNs decreases dramatically. In contrast, the throughputo FiWi networks decreases to a much lesser extent orincreasing peer-to-peer traffic due to the above-describedwireless-optical-wireless communications mode o FiWi net-works. Tus, these ndings show that FiWi networks areparticularly benecial or supporting peer-to-peer commu-nications among wireless mesh clients and are able to achievea signicantly higher network throughput than conventionalWMNs.

Direct Inter-ONU Communications. Te throughput-delayperormance o FiWi networks or peer-to-peer communi-cations can be urther improved by means o direct inter-ONU communications. In conventional PONs, inter-ONUcommunications are done via the OL. In [], a newWDM/DM PON-based FiWi network architecture was

proposed and investigated to enable direct inter-ONU com-munications without going through the OL. Te proposedWDM/DM PON architecture requires major hardwareextensions at the OL, ONUs, and remote node. More speci-ically, the OL is equipped with an array o xed-tuned trans-mitters and an array o xed-tuned receivers, each operatingon a different dedicated wavelength channel, or transmittingandreceiving data to androm the ONUs. Each ONU deploysa pair o xed-tuned transmitter and xed-tuned receiver atdedicatedupstreamand downstream wavelength channels. Inaddition, each ONU uses a tunable transmitter and a xed-tuned receiver or inter-ONU communications. Te inter-ONU traffic rom all ONUs is combined into one ber bymeans o a passive combiner, which is attached to one otwo input ports o an arrayed-waveguide grating (AWG).Both the combiner and AWG are located at the remotenode o the WDM/DM PON. Te second input port o theAWG is used to interconnect the OL with the ONUs orcommunications between the OL and ONUs. Te additionaltransceiver enables ONUs to send data directly to each otheracross the AWG rather than going through the OL, resultingin an improved throughput-delay perormance under peer-to-peer traffic. At the downside, it must be mentionedthat the proposed architecture is quite complex and costly.Not only do the required tunable transmitters add to thecomplexity and cost o each ONU but also might opticalampliers become necessary to compensate or the couplingand insertion losses o the inserted combiner and AWG,thereby violating the unpowered nature o PONs and cost-sensitivity o access networks in general.

... Reconguration. Previously, we have introduced anoptical unidirectional WDM ring interconnecting multiplePONs integrated with a WiFi-based WMN. o adapt theconsidered FiWi network to traffic changes in differentdistricts, recongurability was implemented in the opticalbackhaul [,]. Instead o overprovisioning based on thepeak traffic demand, it is desirable to reallocate bandwidthamong multiple PONs by utilizing tunable transceivers atthe ONUs. For instance, let us assume that two PONs share

the same optical tree subnetwork and one PON is highlyloaded while the other PON is not, then by tuning thetransmitting and receiving wavelengths o a highly loadedONU associated with the ormer PON the ONU is enabledto join the latter lightly loaded PON in order to achieveload balancing between the two PONs. oward this end, inthe central hub o the FiWi network a network terminalcontinually monitors the buffer depth o each OL or thedownstream traffic. As a PON becomes heavily loaded, asindicated by the corresponding OL buffer depth, the heavilyloaded PON is instructed to deregister some ONUs andreregister them to the lightly loaded PON(s). Beore an ONUis deregistered, its packets queued at the OL will rst be


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emptied.Afer an ONUis deregistered, the incoming traffic tothat ONU is temporarily stored at the network terminal untilthe reregistration is achieved. o reduce the overhead o slowtransceiver tuning times during reconguration and increasenetwork throughput, the buffer size may be increased and ahigher threshold or the reconguration trigger may be set, at

the expense o longer queueing delays in the buffer.

.. Routing and QoS Continuity. Te wireless mesh ront-end o FiWi access networks provides multiple paths toroute traffic coming rom and going to the optical backhaul.In this section, we review a variety o proposed routingalgorithms that aim at optimizing the network perormancein terms o delay, throughput,packet loss, loadbalancing, andother important metrics such as path availability and powerconsumption. Te considered routing algorithms cover eitheronly the wireless ront-end or both the wireless and opticaldomains o FiWi access networks. Furthermore, we elaborateon various techniques to provide service differentiation

and end-to-end guaranteed QoS and enable QoS continuityacross the optical-wirelessinteraceo FiWi broadbandaccessnetworks.

... Wireless Routing Algorithms. In the ollowing, wedescribe various proposed routing algorithms or the wirelessront-end o FiWi access networks. All o the discussedwireless routing algorithms aim at nding the optimal paththrough a wireless mesh ront-end by meeting one or moreobjectives.

DARA. A proactive routing algorithm, reerred to as delay-aware routing algorithm (DARA), that minimizes the averagepacket delay between a router and any wireless mesh gatewaywas presented in [, ]. DARA is a link-state routingalgorithm, where each wireless mesh router and gatewayperiodically advertises its link conditions (i.e., traffic intensityandlink capacity with timestamp) in link state advertisement(LSA) messages. Upon reception o an LSA message, eachrouter/gateway predicts the traffic intensity, which is usedto predict the state o the corresponding link until the nextLSA message arrives. o make sure that the resultant linkstate prediction (LSP) is accurate, the authors proposed touseweighted moving average (WMA) to estimate the trafficintensity o a given link between the current LSA messageand the next LSA message. Based on the LSP inormation,each wireless link is assigned a weight according to its

predicted transmission, synchronization, and queuing delay.More precisely, links with higher predicted delays are givenhigher weights and vice versa. Subsequently, or each pair orouter and gateway a single (or alternatively > 1or loadbalancing) minimum-weight path(s) is (are) computed. Inaddition, DARA perorms admission control, where a packetis admitted into the wireless mesh only i the predicted delayalong the computed path is below a predetermined threshold.Otherwise, the packet is rejected.

Te presented simulation results indicate that DARAoutperorms other routing algorithms such as minimum hoprouting, shortest path routing, and predictive throughputrouting in terms o delay. Furthermore, DARA is able to

improve load balancing and alleviate congestion compared tominimum hop and shortest path routing algorithms. It alsoimproves the average hop count compared to the predictivethroughput routing algorithm.

DDRA. Te aorementioned DARA routing algorithm does

not allow or any sort o QoS differentiation. o do so, theso-called delay-differentiated routing algorithm (DDRA) wasintroduced in []. Te objective o DDRA is to decrease thedelay o upstream transmissions to a server located at theOLo a PON. oward this end, the authors dene the delay-to-server (DS) as the time rom a wireless mesh network clientto the server. raffic is divided into DS-sensitive and DS-insensitive traffic. Given that a signicant proportion o thepath delay generally occurs at the wireless bottleneck linksbetween gateways and their adjacent wireless mesh routers,the delay perormance can be improved by attaching externalbuffers to these routers. Te external buffers can store DS-insensitive data packets when the links are congested and

orward them to the server when the links to the gatewaysbecome less congested. In doing so, under high traffic loadsDDRA decreasesthe queuing delay o DS-sensitive traffic bytemporarily storing DS-insensitive data packets in externalbuffers. A given ingress wireless router computes a path to agateway, i the traffic is DS-sensitive. In the case o DS-insensitive traffic, a path rom the ingress router to one othe external buffers is computed. Specically, the path to agateway will be selected based on its computed delay, whereasthe path to an external buffer is chosen by taking both thedelay and size o the external buffer into account.

Te perormance o DDRA and DARA was compared bymeans o simulation. Te results reported in [] show thatunder light loads DDRA andDARAachieve roughly the samedelay perormance or DS-sensitive traffic, though DS-insensitive traffic suffers rom a slightly higher delay. Underincreasing traffic loads, DDRA outperorms DARA in termso delay and can support nearly . times the maximum loadthat DARA can handle, at the expense o additional hardwarerequirements at some o the wireless mesh routers.

CaDAR. Te capacity o wireless mesh networks is limited.Tis is particularly true or single-radio wireless mesh net-works, where each mesh node is equipped with only oneradio, whose capacity needs to be distributed among all thenodes outgoing links by means o time division multipleaccess (DMA). By assigning time slots o different duration

to a nodes outgoing links according to given traffic demands,each o these links has a different capacity. In [,], a newrouting algorithm, called capacity and delay aware routing(CaDAR), was proposed with the objective to distribute theradio capacity o a single-radio wireless mesh network nodeoptimally among its outgoing links such that the averagedelay is minimized. Specically, in CaDAR, each nodeinorms itsneighboringnodes about the current traffic load oall its outgoing links via periodic LSA messages. Based on theadvertised link loads, the CaDAR algorithm determines theraction o a nodes radio capacity that should be assigned toeach o its outgoing links. Subsequently, each link is assignedits delay as weight and CaDAR computes the shortest delay


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path or each pair o wireless mesh router and gateway.Similar to DARA, CaDAR may deploy admission controland WMA to estimate the traffic load o each link. It wasshown or a -node single-radio IEEE .g WLAN basedmesh network that CaDAR can support nearly three timesthe maximum load DARA can handle while maintaining low

delays.o increase the capacity o the wireless ront-end and bet-

ter match it to the optical backhaul, one promising approachis to deploymultiple radiosat nodes that carry higher traffic

volumes. In [], the problem o optimum placement o asecond radio at selected wireless routers/gateways was or-mulated as an integer linear program (ILP) with the objectiveto maximize the throughput-delay ratio o the wireless ront-end. By means o simulation it was shown that the maximumthroughput perormance o CaDAR can be almost doubledi only the gateways deploy an additional second radio whileleaving all mesh routers with a single radio. Equipping someo the wireless mesh routers with an additional radio does

not lead to an increased maximum throughput due to theact that the bottleneck links remain the same; however, ithelps reduce the average delay o the network to some extent.In act, the throughput-delay ratio o the wireless ront-end increases or a growing number o multiple-radio meshrouters.

In a wireless ront-end with multiple-radio mesh nodes,it is important to assign the wireless channels efficiently suchthatcontention is reduced. A channel assignment scheme thatbalances traffic loads among wireless channels to minimizecontention was investigated in []. Te proposed channelassignment scheme perorms load balancing o traffic owsacross different channels in order to maintain a similar level

o contention on each channel. Using CaDAR or routing, itwas shown by means o simulation that the proposed ow-aware channel assignment scheme effectively balances theload across different wireless channels, resulting in a reducedcontention throughout the wireless ront-end.

RADAR. In FiWi access networks, packet loss may occurdue to various ailures, or example, gateway or OL ailure.o cope with packet loss, the so-calledrisk-and-delay awarerouting (RADAR) algorithm was studied in []. RADARis an extension o the above-mentioned DARA routingalgorithm. Similar to DARA, each wireless router/gatewayadvertises the wireless link states in periodic LSA messages

and assigns weights to wireless links according to their delaysor minimum delay path computation. Unlike DARA, eachmesh router maintains arisk listto keep track o ailures. I aailure occurs, the risk list is updated and subsequent packetsare rerouted. Specically, a risk list contains the ollowing sixelds:

(i) path number,

(ii) primary gateway group (PGG),

(iii) secondary gateway group (SGG),

(iv) tertiary gateway group (GG),

(v) path status,

(vi) path delay,

wherebythe path status canbe eitherlive or stale.Te primarygateway or a given mesh router is the gateway with theminimum delay path. Te PGG contains paths to the primary

gateway and other gateways connected to the same ONU o aback-end PON. Te SGG contains paths to gateways that areconnected to different ONUs but the same OL as the PGG.Te GG contains paths to gateways that are connected to adifferent OL.

In the event o one or more FiWi network ailures, therisk list o affected mesh routers is updated accordingly. Moreprecisely, i theprimary gateway ails, then all the paths to thatgateway become stale and packets destined or the primarygateway are rerouted through live PGG paths. I the ONU othe PGG paths ails, all PGG paths become stale and packetsare rerouted along live SGG paths. I the common OL o theSGG paths ails, all SGG paths become stale and packets are

rerouted along live GG paths.RADAR provides protection against multiple wireless

ront-end and/or optical back-end network ailures. It min-imizes the average packet delay in the wireless ront-end andreduces the packet loss o FiWi access networks signicantly.It was shown by means o simulation that in terms o bothaverage delay and packet loss RADAR clearly outperormsminimum hop, shortest path, and predictive throughputrouting algorithms or a wide range o ailure scenarios suchas gateway, ONU, and OL ailures.

... Integrated Routing Algorithms. Integrated routing algo-rithms compute paths across the optical-wireless interace by

taking both the wireless and optical domains into account.

CaDAR with Optical Delay Awareness. Te CaDAR routingalgorithm, originally intended only or the wireless ront-end as explained above, can be extended to perorm inte-grated path computation across both the wireless and opticaldomains o FiWi access networks. In [], CaDAR wasused in conjunction with interleaved polling with adaptivecycle time (IPAC) with constant credit service as DBAalgorithm in a backhaul EPON. IPAC is used to arbitrate thecommunication between OL and ONUs. For downstreamdata transmissions to a wireless destination node, the OLselects a gateway such that data packets travel on the mini-

mum delay path across the EPON and wireless mesh ront-end. o select the correct gateway, all gateways periodicallysend LSA messages to the OL and the OL computesthe minimum delay path across the EPON and wirelessmesh ront-end. o calculate the minimum delay path orupstream data transmissions rom wireless mesh clients tothe OL, a gateway takes the current load o its collocatedONU into account and estimates the upstream delay on theEPON. Subsequently, the gateway disseminates the estimateddelay throughout the wireless mesh ront-end via periodicLSA messages. A given wireless mesh source node uses thisinormation or computation o the minimum delay path tothe OL.


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Te presented simulation results investigated the averagepacket delay or CaDAR under the assumption o equally andunequally loaded ONUs. It was shown that when the load oa particular ONU is increased, more traffic is diverted towardgateways collocated with other ONUs. When all ONUs haveequal load, traffic rom wireless mesh source nodes tends

to go toward the closest gateway using ewer wireless hops,which translates into shorter delays.

Availability-Aware Routing. Apart rom capacity, one o themajor differences between the optical and wireless parts oFiWi access networks is the act that optical wired links aremuch more reliable than their wireless counterparts, whosetransmission characteristics depend on a number o differentparameters. Tis major difference between optical ber andwireless links should also be taken into account in the designo integrated routing algorithms or FiWi access networks.

An availability-aware routing algorithm or PON/wirelessmesh based FiWi networks was examined in []. Availability

is the probability that a connection will be ound in theoperating state at a random time in the uture. It is given by

Availability= MF

MF + MR, ()

where MF denotes the mean time to ailure and MRdenotes the mean time to repair. Availability is in generalaffected by many actors and is an important parameter inthe service level agreement (SLA) between network operatorsand customers. Te availability o a path can be calculatedas the product o the availabilities o the individual links,whereby themost available path is denedas thepath with thehighest availability rom the source node to the destination

node o a given connection request. Te perormance othe availability-aware routing algorithm was investigated ordifferent link availabilities and compared to the shortest pathrouting under uniorm traffic. It was shown by means osimulation that the availability-aware routing algorithm isable to achieve a higher average availability or the selectedpaths than the shortest path routing, especially at low trafficloads. However, availability-aware routing is less bandwidth-efficient than the shortest path routing due to an increasedmean number o hops.

A link availability model or FiWi access networks wasdeveloped in [] and its perormance was analyzed or

various routing algorithms, including DARA, CaDAR,

availability-aware routing, and multipath routing. Teobtained results show that shortest path routing using linkunavailability as the link cost metric signicantly improvesthe availability o the selected paths. Moreover, it was shownthat multipath routing helps urther improve the availabilitycompared to single-path routing.

Multipath Routing. Multipath routing and its implicationshave been studied in greater detail in []. With multipathrouting, data packets can be sent upstream to the OLalong multiple paths in the wireless ront-end, resultingin improved load balancing and ault tolerance as well ashighernetwork throughput. However, these data packets may

arrive at the OL out o order due to the act that eachpath in general has a different delay. As a consequence,the out-o-order packets deteriorate the perormance o thetransmission control protocol (CP). In PON-based FiWiaccess networks, the OL serves as a convergence node oall upstream data ows and may as such resequence arriving

packets to some extent, thereby mitigating the detrimentalimpact o out-o-order packet delivery and increasing CPthroughput.

In [], the authors proposed an optimal ow assignmentand ast packet resequencing algorithm in order to reduce theout-o-order probability o upstream packets injected by theOL to the Internet. In this approach, packets belonging tothe same ow are sent to the OL along multiple paths. Tedelay along different paths may be different and may varyover time. Te OL maintains two rst-in-rst-out (FIFO)priority queues with preemptive priority scheduling. Out-o-order packets are stored in the high-priority queue, while theremaining packets are put in the low-priority queue. Flowsare optimally assigned among the different paths with theobjective to minimize the out-o-order probability beyondthe OL subject to given packet delay requirements.

... Energy-Aware Routing. FiWi networks and accessnetworks in general suffer rom a major shortcoming. Intodays Internet, the total energy consumption is dominatedby access networks and as access rates o tens o Mb/sbecome commonplace, it will be necessary to improve theirenergy efficiency in order to avoid a signicantly increasedgreenhouse ootprint o the Internet []. Green energy-efficient FiWi access networks can be realized by puttingselected nodes, or example, ONUs and wireless mesh routers,to a low-power (sleep) state in a coordinated manner duringlow traffic load hours in order to save energy [ ].

A couple o interesting techniques to design greenEPON/WLAN-mesh based FiWi access networks were stud-ied in [,]. Te authors proposed a centralized sleepingmechanism that allows the OL to put low-load ONUsasleep. Specically, the OL maintains two watermarks orthe traffic load ateachONU, one low watermark and one highwatermark. Te OL observes the traffic load at each ONUby monitoring the length o the corresponding queues. I agiven ONU operates under the low watermark, the OL putsit asleep and the wireless ront-end is used to reroute trafficto other active ONUs. Te OL wakes a sleeping ONU upwhen the traffic load o the active ONUs crosses the high

watermark in order to cope with the increased traffic load.Te authors ormulated this problem as a mixed integer linearprogram (MILP) to determine the minimum number oONUs that need to be kept active to support a given trafficload, while all remaining ONUs are put in the sleep mode,thereby minimizing the power consumption o the FiWiaccess network.

Te model can be solved or small FiWi networkinstances. However, or FiWi access networks with a largernumber o nodes and higher traffic loads, heuristics areneeded to solve the problem. oward this end, the authorsinvestigated an energy-aware routing algorithm. Te objec-tive o the proposed energy-aware routing algorithm is to


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use already existent paths and put the other FiWi networksegments asleep. o do so, the residual capacity o each link isassigned as its weight and LSA messages are periodically sentto inorm all other network nodes about the current residualcapacity o each link. Based on this inormation, data packetsare routed along the lowest residual capacity path between

each pair o source and destination nodes. Note, however,that this approach increases the average path length and pathdelay since the algorithm in general provides routes that arelonger than the corresponding shortest paths. o mitigate thisproblem, the authors proposed to use a so-called hop offset.Given a path o hops, or each hop a hop offset 1isadded to the path cost; that is, is added to the originalcost o the path. Te presented simulation results indicatethat a signicant number o ONUs o an EPON/WLAN-based mesh FiWi access network can be put asleep withoutcompromising its delay perormance.

... QoS Continuity. A plethora o enabling optical and

wireless network technologies and QoS provisioning tech-niques exists or FiWi access networks. In this section,we highlight some o the recently proposed techniques toprovide QoS continuity across the optical-wireless interaceo FiWi access networks. For a more detailed and compre-hensive description o QoS provisioning techniques or FiWiaccess networks the interested reader is reerred to [].

QoS-Aware Scheduling. A centralized QoS-aware schedulingalgorithm was proposed in [] to deliver services to bothwireline and wireless users o an integrated EPON-WiMAXnetwork. For QoS provisioning, the proposed schedulingalgorithm utilizes the integrated ONU/WiMAX outdoor

unit (ODU) to collect the resource inormation and servicerequests rom all wireless subscribers. Whenever the wire-less link conditions change, a WiMAX subscriber stationsends a bandwidth request message to its associated ODUto update its service requirements. Te collocated ONUaggregates the received bandwidth requests rom multiplesubscribers into a small number o multipoint control pro-tocol (MPCP) REPOR messages and sends them upstreamto the hybrid OL/WiMAX base station, which acts as thecentral controller o the integrated EPON/WiMAX network.Te aggregation o multiple bandwidth request messages intoa small number o REPOR messages helps reduce the sig-naling overhead and achieve an improved throughput-delay

perormance. For each polling cycle, the central controllerschedules the received bandwidth requests based on theirtraffic class and inorms the WiMAX subscriber stationsabout the resultant upstream transmission schedule.

Te presented simulation results show that the central-ized QoS-aware scheduling algorithm is able to improve bothnetwork delay and throughput by roughly % compared to adistributed multihop scheduling algorithm, where each hopmakes its own local transmission decision without any globalknowledge o the status o all wireless links.

Enhanced MPCP and Admission Control. A resource man-agement ramework with the objective o maximizing the

resource utilization and optimizing the QoS perormancein both the optical and wireless segments o an integratedEPON/WiMAX network was investigated in []. Te rame-work comprises two parts. Te rst part is the so-calledenhanced MPCP (E-MPCP), which aims at improving thesignaling between EPON and WiMAX. Te second part is

called theintegrated optical wireless admission control (IOW-AC)scheme, which is used to provide integrated bandwidthallocation in an EPON/WiMAX network.

Inthis approach, thecollocated EPON ONUand WiMAXbase station are integrated into a single device reerred to asaccess gateway. In E-MPCP, the GAE message is modiedsuch that it contains an additional -byte eld called the nextcycle time, which denotes the time interval between two con-secutive polling operations o a given accessgateway. In doingso, the access gateway knows the estimated waiting time orits next poll and can thus calculate the transmission delay inthe optical domain more accurately. Te IOW-AC works asollows. Upon reception o a new service request, the accessgateway calculates the transmission and propagation delaysby taking the wireless network conditions and traffic proleinto account. Subsequently, the access gateway calculatesthe queuing delay based on the current buffer occupancy.Finally, the access gateway also takes the aorementionedtransmission delay in the optical segment into account anddetermines the estimated overall delay across both WiMAXand EPON networks. I the overall delay is smaller thana prespecied delay bound the service request is accepted.Otherwise, the service request is rejected.

Simulation results were presented to compare the pro-posed integrated resource management ramework with aconventional resource management scheme that controls theEPON and WiMAX networks separately rom each other.Te reported results show that E-MPCP and IOW-AC aresuperior to the separated resource management scheme interms o channel utilization and dropping probability orboth best-effort and real-time traffic. However, it is importantto note that the perormance o the integrated resourcemanagement ramework largely depends on the network sizeand traffic prole.

QoS Mapping. One important aspect o QoS continuity acrossthe optical-wireless interace o FiWi access networks is theappropriate mapping o traffic classes o different optical andwireless broadband access technologies. In general, opticaland wireless broadband technologies differ in the number

and/or type o traffic classes. For instance, EPON deploysthe eight priority traffic classes dened in IEEE standard.D. On the other hand, xed and mobile WiMAX denesour and ve scheduling services to support different trafficclasses, respectively, while in IEEE .e QoS-enabledWLANs our different access categories are used, each asso-ciated with a different wireless channel access priority. Tesedifferent priority queues, scheduling services, and accesscategories are used to meet the specic QoS requirementso different services and applications such as voice, video,email, web browsing, and le transer. o provide end-to-endQoS support in FiWi access networks consisting o cascadedEPON, WiMAX, and/or WLAN networks or a variety o


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services and applications with different QoS requirements,the priority queues, scheduling services, and access categoriesmust be mapped properly, which can be easily done in astraightorward manner [].

Optical Burst Wireless Mesh Architecture. A metropolitan

area FiWi network architecture called optical burst wirelessmesh architecture (OBWMA), which interconnects multipleWMNs at the user access side through an optical burstswitching (OBS) core network, was proposed in []. Oneor more gateways o each WMN are connected to an OBSedge node. o provide end-to-end guaranteed QoS, OBWMAaccepts a new ow request only when the ow end-to-enddelay and bandwidth requirements can be met in the WMN.In the WMN, each mesh node has two radios with severalorthogonal channels per radio. Te wireless channels arestatically assigned to each radio such that intererence isminimized. Te OBS core network is a WDM ring networkconsisting o several OBS edge nodes. Each OBS edge node

aggregates received data packets into bursts based on thepackets destinations.OBWMA supports two traffic classes. Te high-priority

traffic class has QoS requirements with respect to delayand bandwidth, while the low-priority traffic class does nothave any QoS requirements (i.e., best-effort traffic). In theWMN, a certain percentage o the link capacity is allocatedto each traffic class at every mesh node and either trafficclass is allowed to exceed its allocated bandwidth only i theother traffic class has not reached its maximum bandwidth.Otherwise, excess traffic needs to be dropped. o enable loss-ree transport o high-priority traffic across the OBS corenetwork, each OBS network node is assigned a number olocal wavelength channels that are dedicated to the transmis-sion o high-priority traffic. Low-priority traffic is sent on anyo the remaining contention-based wavelength channels oron the aorementioned local wavelength channels, providedthat there is no high-priority traffic. o ensure end-to-endguaranteed QoS across the WMN and OBS networks, acontrol bridge is deployed at the OBS edge node, which hasglobal knowledge about the OBWMA network resources. Tecontrol bridge perorms QoS mapping between the WMNand OBS networks o OBWMA.

Te perormance o the OBWMA network was inves-tigated in terms o throughput, delay, and packet loss bymeans o simulation. Te obtained results show that at lowtraffic loads the end-to-end delay o the OBWMA network is

dominated by the OBS core network. Tis is due to the actthat the burst assembly takes more time at low traffic loads.Conversely, at high traffic loads the WMN gets increasinglycongested, resulting in an increased end-to-end delay. Fur-thermore, it was shown that the above-described bandwidthallocation schemes help decrease the packet loss and increasethe per-ow throughput o the network.

4. Recent Progress

Te design oenergy-efficient greenFiWi access networkshas been receiving considerable attention []. An overviewo recently proposed energy-efficient architectures as well as

energy-efficient MAC and routing protocols or FiWi accessnetworks was provided in []. Te authors also proposed anOBS based dynamic bandwidthallocation (DBA) protocol orthe energy-efficient integration o long-reach EPON andlongterm evolution-advanced (LE-A) technologies and investi-gated the tradeoffs between energy savings and QoS guar-

antees. Te simulation and experimental work in [] hasshed some light on the tradeoff between energy saving andQoS support in FiWi access networks. A unied analyticalramework or the throughput-delay perormance evaluationo a wide range o FiWi network routing algorithms wasdeveloped in [,], taking emerging IEEE .ac VHWLAN technologies and PON ber aults into account.

Beside the energy-efficient design o FiWi access net-works, their survivability has been another recent importantresearch area. A variety oadvanced survivability techniquesor PONs and FiWi access networks were investigated in[, ]. A centralized architecture incorporating RoF andcognitive radio technologies into a unied network calledcognitive WLAN over bers by replacing WLAN APs withRAUs was presented in [], including a testbed architec-ture built on a PON and sofware dened radio (SDR)platorm. Furthermore, next-generation PONs (NG-PONs)are expected to play an important role in the support ocoordinated multipoint (CoMP) transmission and receptionin G LE/WiMAX networks. It was shown in [] that byusing so-called XG-PONs, which offer symmetricaldata rateso Gb/s, instead o point-to-point bers, ber backhauldeployment costs in G CoMP architectures can be reducedby up to %. Network coding based energy managementsolutions or NG-PONs were presented in []. Te capac-ity and delay perormance o various types o NG-PONwere analyzed in []. A scheduling and DBA scheme thatallows the coexistence o high-speed Gb/s EPON andmultichannel WDM PON on the same optical inrastructurewas studied in []. Different low-latency polling schemesor long-reach PONs were analyzed in [, ]. A cost-effective and ully centralized RoF network architecturebasedon integrated long-reach WDM NG-PON, with an opticalber range o km, and LE/WiMAX networks, where theoptical and wireless transmission impairment compensationis done only at the CO, was proposed and experimentallyinvestigated or so-called quintuple-play services (high-speeddata, voice, television, management, and home security) [].In [], a novel MAC protocol or FiWi access networks toefficiently carry the ever-increasing amount o video traffic

was proposed and investigated.Recently, theintegration of wireless and ber optic sensors

into FiWi access networks has begun to receive increasingattention. In [], a hybrid approach combining genetic algo-rithm and tabu search techniques was proposed to determinethe optimal placement o ONUs in hybrid PON-wirelesssensor network (WSN) based networks, whereby the ONUshigh power and processing capabilities were used to serveas cluster heads with the objective to minimize the wirelesssensors energy consumption. In [], a DBA algorithm wasdeveloped or a hybrid PON-WSN network, where ONUsare connected to FH terminals or to WSN cluster headsor remote monitoring systems in a ubiquitous city, or


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example, medical sensors in a hospital monitoring system.Te integration o solar-powered wireless sensors and low-power ber optic sensors into a PON was experimentallydemonstrated in []. Finally, the research and development

vision o an ideal access system architecture was outlinedin [], identiying the ollowing three key design goals o

uture broadband access network architectures: (i) adapt-able, (ii) dependable, and (iii) ecoconscious. oward thisend, the authors concluded that passive (optical) networkinrastructures should be used as much as possible andpassive equipment may be shared in order to enable costreduction, promote open and air service competition, createnovel value-added services, and introduce original businessideas or the realization o uture-proo broadband accessnetworks based on both wireless and ber media.

5. The Road Ahead

In the PON community, researchers rom both industry and

academia have begun to contemplate on what the uture mayhold or evolutionary NG-PON and disruptive NG-PONtechnologies. Recently, at the OFC/NFOEC conerence,a workshop on Post NG-PON: Is it More About Capacity orSomething Else? was held to nd out whether it is reasonableto ever increase the system bandwidth or rather exploreservice and application as well as business and operationrelated aspects, which motivate access technology to moveinto a substantially different direction in the long run thancontinued capacity upgrades. Clearly, NG-PONs are expectedto play an important role in the support o CoMP coor-dination schemes among base stations in G LE/WiMAXnetworks. In act, in emerging LE-A heterogeneous networks(HetNets), where emtocells with small, inexpensive, and low-power BSs are introduced to supplement existing macrocellsor the sake o an improved (indoor) coverage, enhanced cell-edge user perormance, and boosted spectral efficiency perarea unit, a cellular paradigm shif is required that recognizesthe importance o high-speed backhaul connections, giventhat most G research so ar has been ocusing on theachievable perormance gains in the wireless ront-end onlywithout looking into the details o backhaul implementa-tions and possible backhaul bottlenecks []. Very recently,however, in their seminal work on quantiying the impact odifferent backhaul topologies (mesh versus tree) and back-haul technologies (e.g., WDM PON) on the perormance ocellular networks, Biermann et al. have shown that ultimately

the major actor limiting CoMP perormance in G mobilenetworks islatencyrather than capacity o the backhaul [].

o help identiy open key research challenges or NG-PONs and converged FiWi broadband access networks, it isimportant to consider emerging trends in related areas inorder to shif the post NG-PON research ocus rom merecapacity provisioning to more lucrative solution offerings bydeveloping holistic groundbreaking solutions across multipleeconomic sectors other than telecommunications per se.oward this end, in the remainder o this paper we willrst elaborate on emerging trends and open challenges orFiWi networking research. More interestingly, to better com-prehend the true potential o the aorementioned emerging

trends, it is helpul to put them into a wider nontechnicalcontext and see how they t into the bigger picture o present(and past) economic recessions. Te current global crisisis ar rom unexpected but rather represents a recurrenthistorical event that is typical or capitalist economies. Asrecently explained by Perez in her excellent work on the

implications o nancial collapses [], there have beenour previous situations equivalent to the current crisis inthe past two centuries since the rst industrial revolution.Figure illustrates the historical recurrence o recessions asturning points, where roughly every years the installationperiod o new technologies, the time when nancial capitalshapes the economy, has led each time to a major bubbleollowed by a major crash. Tis is the time or states to comeback actively to change the ocus rom the stock marketindices to the job-crea

fiwi - age of coverage (future)

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