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JOURNAL OF LIGHTWAVE TECHNOLOGY, 2016 1

Multi-Dimensional Convergencein Future 5G Networks

Marco Ruffini Senior Member, IEEE.

Abstract—Future 5G services are characterised by unprece-dented need for high rate, ubiquitous availability, ultra-lowlatency and high reliability. The fragmented network view thatis widespread in current networks will not stand the challengeposed by next generations of users. A new vision is required, andthis paper provides an insight on how network convergence andapplication-centric approaches will play a leading role towardsenabling the 5G vision. The paper, after expressing the viewon the need for an end-to-end approach to network design,brings the reader into a journey on the expected 5G networkrequirements and outlines some of the work currently carried outby main standardisation bodies. It then proposes the use of theconcept of network convergence for providing the overall archi-tectural framework to bring together all the different technologieswithin a unifying and coherent network ecosystem. The novelinterpretation of multi-dimensional convergence we introduceleads us to the exploration of aspects of node consolidation andconverged network architectures, delving into details of optical-wireless integration and future convergence of optical data centreand access-metro networks. We then discuss how ownershipmodels enabling network sharing will be instrumental in realisingthe 5G vision. The paper concludes with final remarks on therole SDN will play in 5G and on the need for new businessmodels that reflect the application-centric view of the network.Finally, we provide some insight on growing research areas in5G networking.

Index Terms—convergence, access-metro, next-generation 5G,multi-service, multi-tenancy, consolidation, sharing, end-to-end,datacentre.

I. INTRODUCTION

ONE of the prevailing dilemmas operators are facingtoday is how to persuade residential users to take up

faster broadband offers in areas where satisfactory speed (forexample of the order of fifty to a hundred megabit per second)is already available at competitive price. Blame is typicallygiven to the lack of marketable applications requiring speedof hundreds of megabit per second. We argue this perceptionderives from an incorrect approach to the problem, and it isthe cause of an un-converged vision of the network which isconsidered as a chain of separate sections only inter-connectedat the IP protocol level.

In this paper we envision a unified view of the networkthat is achieved by scaling up network convergence to a pointwhere the network can provide personalised connectivity to theapplication using it independently of the underlying technol-ogy, geographical location and infrastructure ownership. Whilea number of research projects have emphasised the economicbenefit of physical convergence of access and metro networks

Financial support from Science Foundation Ireland (SFI) grant 14/IA/2527(O’SHARE) and grant 13/RC/2077 (CONNECT) is gratefully acknowledged.

M. Ruffini is with the CONNECT research centre in The University ofDublin, Trinity College, e-mail: [email protected]

[1],[2],[3] we argue this new vision requires dealing withnetwork convergence at a multi-dimensional level and focus onthe following three dimensions: the spatial dimension dimen-sion, the networking dimension and the ownership dimension[4]. We also come to the conclusion that current businessmodels based on broadband offering of peak bit rates doesnot have a place in the future, as the value for the end userslies in the correct delivery of the application, which shouldbecome the starting point of the value-chain.

If we look at the past two decades we see that networkconvergence has driven the reduction in cost of ownership bymoving voice services from the synchronous TDM transmis-sion systems (e.g., Sonet and SDH) to the packet switchedarchitecture used to transport Internet data, through the adop-tion of Voice over IP (VoIP) technology. This technologicalconvergence also gave the operators the opportunity to offerbundled broadband services, such as triple play (e.g., voice,Internet and TV) and quadruple play (adding mobile phone tothe mix), as a means to reduce their cost and to benefit fromeconomies of scales associated to service consolidation. Thistrend has evolved over the years, moving today its focus onthe convergence of access and metro networks, which revolvesaround two key trends of infrastructure integration. The first isthe consolidation of the number of central offices (COs), whichis typically achieved by adopting fibre access architectureswith longer reach, to bypass some of the current network nodes[2]. The second is the convergence of wireless and wirelinenetworks, which typically focuses on the transport of data frommobile stations over shared optical access links.

This paper argues that while this access-metro infrastructureintegration is a step in the right direction, it only representspart of the contribution that network convergence can providein support of the application-centric network vision necessaryto deliver future 5G services.

The paper is structured as follows. The next section pro-vides an overview of the requirement of future 5G networks,based on the early work of industry fora. Section III givesa brief insight on the work carried out by some of the mainstandardisation bodies on 5G. While this short overview is farfrom being comprehensive, due to the large number of ongoingstandardisation activities, it provides an insight on the maintechnologies being considered on wireless, optical and highernetwork layer technology. After this, we investigate a numberof research activities on converged network architectures thataim at providing an integrated framework to bring togetherdifferent 5G technologies within a unifying and coherentnetwork ecosystem. We report these activities under threedistinct categories of convergence. The convergence in thespace dimension, in section V, explores the use of long-reach

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access technology to enable central office consolidation. Theconvergence in the networking dimension, in section VI, dis-cusses trends and options for end-to-end integration of differ-ent network types, focusing on fixed/mobile convergence andproposing a vision where agile data centres play a pivotal rolein the virtualisation of network functions. The convergence inthe ownership domain, in section VII, describes current workin the area of multi-tenancy for access network. Section VIIIprovides final remarks and discussions giving some insight onthe role that SDN will play towards converged 5G networksand proposing the use of application-driven business modelsas a foundation of the 5G vision. Finally, section IX concludesthe paper summarising the main key points and providing someinsight on future research areas.

II. FUTURE 5G NETWORK REQUIREMENTS

One of the main targets for operators in the design of nextgeneration network architectures is to plan for an infrastructurethat if 5G-ready, i.e., capable to support the next generationof applications and services. The Next Generation MobileNetworks (NGMN) Alliance was one of the first fora tocome up with a set of use cases, business models, technologyand architecture proposition for 5G [5]. The NGMN Allianceenvisages the existence of three large application groups:

1) Enhanced mobile broadband (eMBB), which aims atscaling up broadband capacity to deliver next generationof ultra high definition and fidelity video service withaugmented reality. From a networking perspective thetarget is to provide every active user with a least 50mbps everywhere, with enhanced targets of 300 Mbpsin dense areas and up to 1 Gbps indoor.

2) Massive machine type communications (mMTC), aim-ing at scaling up the network to support tens of billionsconnected devices, with densities of up to 200,000 unitsper square km. The target is also to simplify the devicescompared to 3G and 4G to enable ultra-low cost and lowpower consumption. mMTC is seen as a major enablerfor the Internet of Things (IoT), which is the end-to-endecosystem running on top of the machine-to-machinecommunication service.

3) Ultra reliable and low latency communication (uRLLC),aiming at decreasing end-to-end latency to below 1 msand reliability levels above five nines. It is envisagedthat such requirements will be crucial for applicationssuch as automotive (e.g., inter-vehicle communicationsystems for accident avoidance) and medical (e.g., re-mote surgery), but could be extended to other typesof tactile internet applications, which require ultra lowlatency user feedback mechanism.

Figure 1 (derived form [6]) shows a summary of the increasein key capabilities of the network as the technology movesfrom IMT-Advanced 1 (in red shade) towards IMT-2020 (ingreen shades). It should be noticed that although, collectively,IMT-2020 is expected to reach the most stringent requirementsshown in the figure, no one application is expected to require

1IMT-advances includes both LTE-Advanced and WiMAX release 2

them all simultaneously. Indeed the figure shows that suchrequirement can be further categorised according to the threeapplication categories above. For example it is envisagedthat eMBB will benefit from most of the enhancements,but it is not expected to require sub-ms latency (such asuRLLC applications) or density of devices above 105 perkm2 (unlike mMTC applications requiring a density of upto 106 per km2). eMBB and eRLLC applications are inparticular those that are expected to generate novel revenuestreams for operators, especially from vertical markets 2, asit is believed that 5G network infrastructures will enable thedigitalisation of society and economy, leading to the fourthindustrial revolution, impacting multiple sectors, especially theautomotive, transportation, healthcare, energy, manufacturingand media and entertainment sectors [7].

Finally, an aspect of 5G networks of increasing importanceis the ubiquity of broadband connection, as it is expected thatthe user experience continuity is not confined to urban districtsbut also extended to rural areas. The digital divide has becomea major social and political issue worldwide, as fast broadbandconnectivity is now a commodity, and like drinkable water aright of every citizen. The European Commission has clearlystated broadband speed targets for the year 2020 of 100%population coverage with at least 30Mbps, and 50% with atleast 100 Mbps [8]. These requirements put additional pressureon developing access broadband architectures that are capableof reducing connectivity costs in rural areas, and are compliantwith open-access models, which are necessary to operate inremote areas that require state intervention.

Latency (ms) Connection density (devices/km2)

Energy efficiency (x times)

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(Mbit/s/m2)

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Fig. 1: Key capabilities introduced by IMT-2020 networks,subdivided by application type, and compared to key capabil-ities enabled by IMT-advanced.

A. Design for 5GWhile much of the 5G architecture and technologies are still

undefined and under discussion, the diverse range of capabili-

2A vertical market is a market in which vendors offer goods and servicesspecific to an industry, trade, profession, or other group of customers withspecialised needs - Wikipedia definition


ties expected, often with conflicting goals, makes it clear that5G is not simply the next-generation of 4G, or confined to thedevelopment of new radio interfaces, but rather encompassesthe development of an end-to-end system including multiplenetwork domains, both fixed and mobile. Indeed we’ll seein the next section that there are different technologies andstandardisation bodies involved in the definition of the 5Gecosystem.

In order to provide a level of insight on how to designa 5G-ready network, while not providing specific guidelines,the NGMN Alliance has attempted a definition of two generaldesign principles [9]:

• Provide expanded network capabilities: with the idea ofpushing the network performance and capabilities overmultiple directions, as exemplified in figure 1.

• Design intelligent "poly-morphic" systems: provide amalleable system that can be tailored to the applicationrequired. It is envisaged that virtualisation of networkfunctions, control and data plane will play a pivotal rolein enabling network flexibility.

The envisaged flexible network design implies flexibilityin assigning network resources to applications, with diversequality of service, reliability and availability requirements andwill be reflected in updated business and charging models todrive the economics of the 5G ecosystem. A confirmation ofthis necessity is given by the difficulty current operators haveto convince end users to buy ultra-high speed fibre accessbroadband where there are already other lower cost optionsoffering satisfactory broadband speed. It is indeed increasinglydifficult for end user to understand the practical benefit offaster access speeds, and justify its higher cost, when thisdoes not assure the correct end-to-end delivery of services.In the current model the user pays for network connectivity,typically a flat rate, in addition to charges for the use of someapplications, which however do not have the ability to viewor influence the underlying end-to-end network performance.This model does not reflect the fact that the real value forthe user is in the applications, which have diverse capacityand latency requirements, and whose value per bit can varysubstantially (for example if we compare a video on demandto a voice call or a medical device remotely sending bodymonitoring values to a medical centre).

Clearly this model needs to shift towards one that insteadis linked to the application used, and makes the end usercompletely oblivious of the details of the underlying networkperformance. A practical, although basic, example of theimplementation of this model is a recent deal where Netflix hasagreed to pay Comcast for an improved delivery of its serviceover their network. Is the reliable service delivery throughend-to-end quality of service (QoS) assurance, not the isolatedincrease in access data rate, that will generate new forms ofrevenue. If the operators cannot deliver this type of service, itis likely that OTTs and industry verticals will continue to buildtheir own dedicated network, which will lead to a fragmentedsuboptimal network development.

B. Quality of service implementation

Although the basic mechanism for QoS have been standard-ised many years ago through Integrated Services (IntServ) [11]and Differentiated Services (DiffServ) [12], attempts at usingthem in practice for end-to-end quality assurance have failedin the past. This was due to the complexity of implementingit across different domains at the granularity of the individualapplication and over-provisioning was used instead as a meansto provide an average acceptable service.

We argue that today the situation has changed for the follow-ing reasons and that QoS assurance will play a fundamentalrole in future networks. Firstly, operators have now startedto look for higher efficiency in their network as their profithave constantly declined, and massive overprovisioning ceasesto be a valuable option. Secondly, the Internet architecturehas migrated over the years from a hierarchical model, whereend-to-end connectivity was provided by passing throughmultiple tiers, and crossing several network domains, to amodel where many peering connections provide direct linksbetween providers. This reduces substantially the numberof domains crossed by the data flows which representedone of the main obstacles to QoS delivery. The reductionin average number of IP hops was also achieved throughwidespread installation of new data centres and the use ofContent Delivery Networks (CDN) to reduce the distancebetween traffic source and destination. Indeed an analysis ofcurrent metro vs. long-haul traffic shows that the latter is inconstant decrease (see Fig. 2), suggesting that most end-to-endconnections are confined within the metro area. We argue thatthis change in circumstances will enable the success of large-scale end-to-end QoS delivery. This will be facilitated by thedevelopment of open and programmable networks, based onthe Software Defined Networking (SDN) paradigm, which willautomate most of the QoS configuration hiding its complexityto network administrators. Indeed many operators are alreadyinvestigating the use of SDN in their network, with somemaking it their main short-term goal [13], and discussions havealready started among industry fora for delivery of BroadbandAssured Services [14].

Fig. 2: Metro vs Long-haul traffic growth estimates (CiscoVNI 2015) [15]

III. STANDARDISATION ACTIVITIES TOWARDSTHE 5G VISION

While the requirements and use cases are still work inprogress and are progressively refined as discussions on 5Gcarry on, standardisation bodies have started building up aroadmap to evolve the current technology towards 5G. This


section reports on some of the most relevant standardisationactivities, considering both fixed and mobile networks, thatcontribute at different levels towards the realisation of theoverall 5G vision. Here we report some highlights of thestandardisation roadmap for major bodies like the ITU, IEEE,ETSI and ONF, which are summarised in figure 3.

2014 2015 2016 2017 2018 2019 2020

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Fig. 3: Roadmap of some of the major standardisation bodies

A. ITU

Having standardised most of the previous generations ofmobile communications, one of the most active bodies in thestandardisation of radio aspects of 5G is the ITU Radiocom-munication Sector (ITU-R), through their International MobileTelecommunication 3 IMT-2020 programme. Releases 13 and14 (recognised as part of the LTE-Advanced [16] framework)are setting up the basis towards the required 5G enhancements,considering technologies such as Full-Dimension MultipleInput Multiple Output systems (FD-MIMO) with 2D arrays ofup to 64 antennas, Licensed-Assisted Access (LAA) enablingthe joint usage of unlicensed and licensed spectrum, andenhanced carried aggregation to increase the number of carriesfrom 5 to 32 [17]. However it is expected that the full IMT-2020 specification will be provided with release 16 (with anexpected release date around the year 2020), where new airinterface for operations above 6GHz will be developed in addi-tion to evolutions that are backward compatible with LTE (i.e.on radio frequencies below 6GHz). IMT-2020 is also targetingthe Vehicle-to-X (V2X) type of use cases, focusing specificallyon Vehicle-to-Vehicle (V2V), Vehicle-to-Pedestrian (V2P) andVehicle-to-Infrastructure/Network (V2I/N) [18]. These effortsinclude studies on enhancements to resource allocation mech-anisms, to improve robustness, latency, overhead and capacityof the mobile communication system.

From an optical access network perspective, while notexplicitly considered a 5G evolution, ITU-T has recentlystandardised the XGS-PON [19], for cost-effective deliveryof symmetric 10 Gbps to the residential market in the shorterterm, and NG-PON2 [20], offering up to 80 Gbps symmetricrates over 8 10G wavelength channels. The ITU optical access

3This is the programme that has defined first, second, third and fourthgenerations of mobile networks

group is currently investigating what other technologies toconsider beyond NG-PON2, with proposals spanning fromamplified Optical Distribution Network (ODN) for longerreach to coherent transmission optical access. Consideringyearly traffic growth rates of about 30% (for most developedcountries) [15], it is expected that NG-PON2, although onlyexploiting a small portion of the available optical spectrum,will be able to deliver the capacity that residential users mightneed for the foreseeable future, and that future generationof standards should focus on aspects of cost reduction andnetwork flexibility, for example to support mobile transport,business and residential types of services in the same PONinfrastructure [21].

B. IEEE

Another body considering 5G adaptations to its standards isthe IEEE, with its 802.11 group working on two main activ-ities. The 802.11ax, labeled High Efficiency Wi-Fi, enhancesthe 802.11ac by aiming at throughputs of the order of the10Gb/s per access point, through a combination of the MIMOand Orthogonal Frequency Division Multiplexing (OFDM)technologies and working on frequency bands between 1 and6 GHz. The Next Generation 60GHz (NG60) aims insteadat evolving the 802.11ad air interface to support data ratesabove 30 Gb/s. The high frequency limits its operation to lineof sight transmission, targeting applications such as wirelesscable replacement, wireless backhaul and indoor short-distancecommunications.

From an optical access perspective, IEEE is also updatingthe 10Gb/s PON interface with the new 802.3ca standardi-sation effort, investigating possible physical layer data ratesof 25, 50 and 100 Gb/s, expected to be fully released bythe end of 2019. It is also worth mentioning the IEEE efforton Standard for Packet-based Fronthaul Transport Networks(P1914.1). This provides a practical solution to the fixed-mobile convergence problem, aiming to deliver an Ethernet-based transport, switching and aggregation system to deliverfronthaul services.

C. ETSI

In addition to physical layer standardisation, the 5G ecosys-tem requires standardisation also of higher layers. In particularNetwork Function Virtualisation (NFV) [22] is identified as atarget for 5G networks [23], to increase network programma-bility and reduce cost of ownership by moving network func-tions from proprietary hardware to software running on generalpurpose servers (a concept also known as "softwarisation").ETSI has been active on NFV from its emergence, publish-ing the first release of its specification in December 2014,where it provided an infrastructure overview, an architecturalframework, and descriptions of the compute, hypervisor andnetwork domains of the infrastructure. The architecture hasthree main constituent elements [24]: the Network FunctionVirtualisation Infrastructure (NFVI), which provides the com-modity hardware, additional accelerator components and thesoftware layer that abstracts and virtualises the underlyinghardware; the Virtualised Network Function (VNF), which is


the software implementation of the required network functionand runs on top of the NFVI; and the NFV Management andOrchestration entity (M&O) taking care of the lifecycle man-agement of the physical and software resources and providingan interface to external Operartion Support Systems (OSS)and Business Support Systems (BSS) for integration withexisting network management frameworks. The second NVFrelease, covering the working period from November 2014 tomid 2016, provided NFVI hypervisor requirements, functionalrequirement of management and orchestration, hardware andsoftware acceleration mechanism for the virtual interfaces andvirtual switch. It also expanded the architectural frameworkwith the further specification of the Management and Orches-tration entity (dubbed MANO) with the definition of [25]: theVirtualised Infrastructure Managers (VIM), which performsorchestration and management functions of NFVI resourceswithin a domain; the NFV Orchestrator (NFVO), performingorchestration of NFVI resources across multiple domains; andthe VNF Manager (VNFM) carrying out orchestration andmanagement functions of the VNFs. ETSI’s work towards thethird NFV release has only recently started and is expected totarget topics such as charging, billing and accounting, policymanagement, VNF lifecycle management and more.

D. ONF

The Open Networking Forum is the reference consortiumfor the standardisation of the Software Defined Networkingparadigm. After the release of the OpenFlow (OF) v1.0 inDecember 2009, their work has progressed to define a plethoraof updates, with new releases every few months (for this reasonthe ONF roadmap is not reported in figure 3). While the OFv1.0 specification has evolved to v1.3.5, other two releaseshave progressed in parallel to allow the development of moreunconventional versions of the protocol, that was not seenas essential for all vendors. For example v1.4 allowed fora more extensible protocol definition and introduced opticalport properties. Version 1.5 introduced the concept of Layer4 to Layer 7 processing through deep header parsing and theuse of egress tables to allow processing to be done in thecontext of the output port. While the OF specification targetsthe control plane functions, the management and configurationspecification where carried out through the ’Of-Config’ proto-col releases, currently at version v.1.2. Additional specificationwere also released to target transport networks, coveringaspects of multi-layer and multi-domain SDN control, togetherwith many technical recommendations on SDN architecturalaspects.

IV. ENABLING THE 5G VISION THROUGH NETWORKCONVERGENCE

The previous two sections have provided an insight onexpected 5G network requirements and given a brief descrip-tion of the roadmap pursued by some of the most relevantstandardisation bodies. A first set of activities, at the lowernetwork layers, focus on the enhancement of network per-formance, addressing higher cell density, higher peak rateand energy efficiency, as well as scalability, latency reduction

and higher reliability. This is reflected for example by thework carried out by ITU-R, ITU-T, and IEEE cited above.A second set of activities, at the higher network layers, aretargeting software-driven approaches to resource virtualisationand control, through network virtualisation, NFV and SDNcontrol layers, which is strongly driven by ONF and ETSI.This is believed to be a distinctive feature of 5G networks,to satisfy the ’Poly-morphic’ design principle and providethe flexibility and automation required to accommodate theenvisaged diversity of requirements. In addition, it is envisagedthat a third set of activities should focus on new business andnetwork ownership models that will have to emerge to makethe integration of all the various components profitable for themarket players. The NGMN Alliance for example recognisesthat the creation of a valid and all-encompassing businesscase for 5G is pivotal for the sustainability of the entire 5Gecosystem [9].

While standards are taking care of some of the potentialemerging technologies they only operate within specific tech-nologies and typically do not provide the end-to-end view thatis required for the 5G vision. In this tutorial we complementthe analysis on requirement and standardisation activities on5G with an overview of a number of recent research projectsand activities on network architectures aiming towards aunified end-to-end view of the network. We propose to cate-gorise such activities across three complementary dimensionsof network convergence, summarised in figure 4, which cancontribute the overall architectural framework to bring togetherdifferent 5G technologies within a unifying and coherentnetwork ecosystem. The convergence in the space domainsignifies the consolidation of many of the current networknodes: the the top left of figure 4 shows a node consolidationanalysis for the UK that could reduce the number of total nodesfrom over 5,000 to less than 100 [2]; the convergence in thenetworking dimension, at the bottom of the figure, designatesthe integration of heterogeneous infrastructure across differentnetwork segments to allow end-to-end control of networkingresources; the convergence in the ownership domain, in thetop right hand side of the figure, denotes the concept of multi-tenancy across network resources, allowing multiple operatorsto share physical network infrastructure [10]. Overall, whilethe network can benefit from all three types of convergence,some of them have contrasting requirements, for example con-sidering the trade-off between node consolidation, requiringlonger links with higher latency, and support for some of nextgeneration mobile services, which present today very tightlatency constraints. We anticipate that some of these issueshave not yet been resolved and should be addressed in futureresearch projects. The next three sections of this paper providean in-depth analysis of these network convergence topics.At the end of each section we summarise how the researchactivities described contribute towards the realisation of the5G vision, and provide an overall outline in figure 10.

V. CONVERGENCE IN THE SPACE DIMENSION:


HeterogeneousInfrastructure

Space dimension

Networking dimension

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Fig. 4: Introducing the concept of multi-dimensional networkconvergence

NODE CONSOLIDATION

The first category of the convergence we discuss is thatin the space dimension, which is achieved by integrating thephysical architecture of access and metro networks to enablenode consolidation, i.e. a significant reduction in the numberof central offices. The aim of this convergence is manifold,providing capital and operational cost saving through a mas-sive reduction in the number of electronic port terminations(i.e., optical-electrical-optical (OEO) interfaces), in addition tosimplification of network architecture and management. TheLong-Reach Passive optical network architecture, originallyintroduced in [26], and further developed by the EuropeanDISCUS consortium [2], targets exactly this idea. By extend-ing the maximum PON reach to distances above 100 kmthrough the use of in line optical amplifiers in the opticaldistribution network, LR-PON can transparently bypass thecurrent metro transmission network, linking directly the accessfibre to a small number of Metro-Core (MC) nodes, whichserve both network access and core.

This evolution is shown in figure 5, where starting from thetraditional architecture that separates access, metro and corein figure 5a, the access-metro convergence operated by LR-PON can remove at least two levels of OEO interfaces in thenetwork leading to the scenario in figure 5b. Studies carriedout for a number of European countries [27], [28], show thattypically the number of central offices can be reduced bytwo orders of magnitude. An additional benefit of LR-PONnetworks is that by bringing the total number of nodes (e.g.,within a national network) below a given threshold (typicallyin the order of a hundred), it enables a fully flat core network,moving to the architecture represented in figure 5c. The flatcore interconnects the metro-core nodes through a full meshof wavelength channels, using transparent optical switchingto bypass intermediate nodes, thus eliminating the inner coreOEO interface. With this architecture the only OEO interface isin the MC node, as packets are only processed electronicallyat the source and destination nodes. Studies carried out in

Metro Access

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(a) Traditional network architecture with separate access,metro and core

Access

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Fig. 5: Evolution of the network architecture from traditionalseparation of access, metro and core to Long-Reach PON withflat optical core

[29] have shown that for values of sustained user traffic abovea certain threshold (about 7Mb/s for their scenario) the flatcore becomes more economical than the current hierarchicalmodel based on outer and inner cores. The large reduction ofOEO ports can achieve large capital and operational saving,as shown in [30], and a 10 times reduction in overall powerconsumption [31] compared to "Business as Usual" scenarios.

It should be stressed that this architecture is capable ofachieving such advantages because it considers the networkfrom a converged, end-to-end perspective, rather than opti-mising separately access, metro and core. For example, one ofthe crucial parameters in long-reach access is the optimal valuefor the maximum optical reach, which cannot be determinedif the access-metro part is considered independently from the


core. The end-to-end perspective, which brings the core intothe picture, provides a convincing argument for identifyingsuch parameter, which is the value required to reduce the num-ber of MC nodes to a level that enables their interconnectionthrough a full mesh of wavelengths, (i.e., a flat core). Undersuch circumstances it was shown in [30] that the deployment ofthe Long-Reach architecture can be even more cost effectivethan fibre to the cabinet, as the cost reduction in the corenetwork can subsidise the cost of deploying fibre infrastructurein the access. In addition the Long-Reach access is beneficialfor lowering cost of fibre deployment in sparse rural areas, aprimary target for every government that wants to reduce theirextent of financial intervention.

The studies mentioned above on the benefits of imple-menting node consolidation reveal that it can contribute toimproving the business case for 5G, by reducing the overallnetwork cost and energy consumption, thus providing thefoundation for an architecture capable to deliver broadbandfibre connection to a larger number of users, creating aubiquitous optical access network ecosystem. The envisagedhigh-capacity and dynamic multi-wavelength PON architec-ture also allows flexibility in the type of service that canbe offered, as each user end point can avail, on-demand,of different types of services (e.g., from dedicated point-to-point to shared wavelength). However, while this architecturecan provide a basis towards the design of future networks,it lacks flexibility in the Optical Distribution Network side,as every user signal needs to be sent to the main centraloffice for further processing. Based on this idea, in [33]the architectural concepts were further developed to provideadditional flexibility with the inclusion of Branching Nodes(BN) positioned in place of some of the remote nodes inthe LR-PON architecture, providing wavelength routing andsignal monitoring, in addition to optical amplification. Thisfor example would allow the add/drop of optical signals closeto the end point for local processing, when and where required,thus significantly reducing transmission distance to satisfythe requirements of those applications with strict latencyconstraints. The addition of a flexible BN in the networkarchitecture further contributes to the 5G poly-morphic designprinciple.

VI. CONVERGENCE IN THE NETWORKING DIMENSION

The second category of convergence we discuss is inthe networking dimension, aiming towards the integration ofdifferent types of networks, mobile, fixed and data centres,to enable end-to-end resource management. Among the threeconvergence dimensions, this is the one that most impactsthe 5G vision, as it involves integrated development with thewireless access.

As mentioned in the introduction, an example of large-scalenetwork convergence was the migration of voice services fromcircuit-switched to packet switched IP networks, generatingsubstantial savings in cost of network ownership for opera-tors. Extending the concept to the access network, a strongintegration between mobile and fixed technologies within aubiquitous fibre deployment provides the resources to serve a

wide range of user and services for several years to come, asany point of access can in principle deliver several terabitsper second of capacity and bring such capacity from oneend of the network to the other. The idea is that to build aflexible network architecture where multiple technologies canconverge and provide a pool of diverse resources that can bevirtualised, sliced and managed to provide the required end-to-end connectivity to applications. An example of the convergedarchitecture vision is shown in figure 6.

In addition, configurability and openness to upcoming tech-nologies is a key factor when considering the unpredictabilityof technology adoption and financial return on investment. Thecurrent debate on GPON upgrade paths is an example. It waspreviously believed that most operators would skip the XG-PON standard (defining a single channel operating at 10Gbpsdownstream and 2.5Gbps upstream) [34] and transition fromGPON directly to NG-PON2. However, due to the high initialcost of NG-PON2 equipment and uncertainty on the additionalrevenue that could be generated, it is likely that residentialusers will be upgraded to XGS-PON (one single channeloperating with symmetric 10G rate), while, simultaneously,NG-PON2 will be adopted for offering enhanced servicesand higher flexibility for business users and mobile cellinterconnection (e.g., thought standard backhaul, fronthaul ormidhaul [35]).

Indeed it is envisaged that such higher flexibility will berequired in next generation 5G networks, where a ubiquitousand flexible fibre access network will provide connectivity toservices with different capacity, service type and reliabilityrequirements to different end points. For example, respec-tively, offering capacity of tens of Mb/s to hundreds of Gb/s,with quality ranging from best effort to assured capacity,with reliability spanning from no protection to dedicated 1+1end-to-end protection, to diverse type of customers such asresidential users, mobile stations, business and governmentalorganisation, data centres and local caches, etc.

Finally, among the available technologies in the access,we should consider that copper will still play an importantrole in the near future, as access fibre deployment is usuallycharacterised by a long return on investment, leading someoperators to use new high-speed copper access technology asintermediate steps towards FTTP. For example G.FAST [36]is currently being considered to deliver few hundreds Mbpsover distances up to about 300 m. In many cases G.FASTwill make use of a PON as backhaul, but use the existingcopper pair for the last drop. Even faster rates, up to 10Gbps,can be achieved over copper with XG.FAST technology [37],which can terminate the XGS-PON or NG-PON2 fibre anduse the existing twisted copper pair over few tens of metersto reach the household. This could be used for example incases where the fibre deployment over the very last tens ofmeters of the drop are particularly expensive or inconvenient.Other users might be connected through a cable system withthe new DOCSIS 3.1 standard capable of reaching 10Gbpsdownstream: these can already be backhauled through GPON[38], while new proposals are currently being developed toguarantee compatibility with NG-PON2 [39].


Large business

DOCSIS 3.1

XG.FAST

XGS-PON

NG-PON2

Micro DC

PtP fibre Premises protection

Premises protection

Feeder protection

Micro DC

RF backhaul/ midhaul

SME

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M2M, IoT, Sensor, etc...

ONU+

Core networ

k Processing

unit

Multiple 100/400G TRx

Stadium Local

fronthaul

Metro-Core Node

CE-Filter

Optical add/drop multiplexer

L2/L3 electronic switches (access

and core) IP peering points, IP service provider N

odes

Processing Unit Cloud for C-RAN

Radio-Core Equipment

PtP-WDM Primary PONs

Core fibers

Transponders for flex grid

Flex grid WSS Flex grid WSS

Flex grid connection

OLTs

Non-blocking optical space sw

itch

Connection to other M/C Nodes via Core Network (many fiber pairs)

Metro-Core Node

Dual-home protection

= OLT

Edge Processing ONU

400G channel Power

splitters

Wavelength routing and add/drop, signal monitoring, amplification

Fig. 6: A converged access-metro architecture

A. Fixed/mobile convergence

One interesting scenario of network convergence is theuse of PONs to provide connectivity to mobile base stations.Wireless network capacity has increased over the past 45 yearsby one million times, a trend also known as Cooper’s law ofspectral efficiency. If we look at how this was accomplished wesee that a factor 25 increase was enabled by higher efficiencyof transmission technology, an additional factor of 25 by theuse of more spectrum, but the vast majority, counting for afactor of 1600 was given by densification of cells, i.e. enablingspacial reuse of frequency channels. Looking at the next fiveyears, a major goal of some of the network vendors workingtowards 5G is to provide a further capacity increase of up to athousand times [40]. Although it is still uncertain whether thisis achievable, any such increase in capacity is likely to follow asimilar split between improvement of transmission technology,spectrum resources and densification. Beside the challengeswith delivering such a high density of capacity on the radiointerface (e.g., due to interference and frequency reuse issues)there is a comparable challenge for linking an ever increasingnumber of small cells to the rest of the network in a way thatis cost effective. A solution to this problem that is gainingtraction is to use PON networks: even though PONs wereinitially deployed as a means to bring ultra-fast broadbandto residential users, the high capacity the fibre provides andthe high degree of reconfigurability enabled by the simplepower split architecture makes it an ideal candidate also forconnecting base stations.

While with 3G systems, base stations were typically con-nected to the network through backhauling, i.e., at layer 2

or 3, massive densification of next generation mobile net-works brings new challenges that cannot be solved throughsimple backhauling. Deploying a large number of cells canbe prohibitively expensive when accounting costs for the basestations and the rental of the space where all its processingequipment is located. A solution that is becoming increasinglypopular is to use fronthaul. The idea stems from the extensionof a transmission protocol called Common Public RadioInterface (CPRI) that was used to link the antenna at the topof a base station mast with the BaseBand processing Unit(BBU) equipment at ground level though optical fibre. Thiseliminated the need for the RadioFrequency (RF) interfacebetween antenna and BBU, thus reducing complexity, spacerequirements, heat dissipation and ultimately costs. The con-cept was extended to BBU hoteling, where the link betweenthe antenna mast and the BBU is increased to a few kilometresto place BBUs from different masts into one building in orderto reduce deployment costs and enhance security, reliabilityand ease of management.

It should be stressed however that while adopting smallercells has the benefit of offering higher data rates per useras the total cell capacity is shared among a smaller numberof users, it also reduces cell utilisation as the lower numberof connected users reduces the statistical multiplexing gainscompared to larger cells. Thus the need for additional savingshas lead to the concept of BBU pooling, where multiple radioheads (RRH) are multiplexed into a smaller number of BBUs.Besides improving BBU utilisation, by operating statisticalmultiplexing also within the BBU units, this mechanism allowscentralised control of multiple RRHs, enabling the use ofadvanced LTE-A techniques such as Coordinated Multi-Point


(CoMP), coordinated beamforming, and Inter-Cell InterferenceCancellation (ICIC).

Finally, the third step of this convergence is to run the BBUas software on a virtual machine (software implementation ofthe LTE stack are already commercially available [41]), inpublic data centres, a concept known as Could Radio AccessNetworks (C-RAN). This is fully in line with the 5G visionof NFV.

From a technical perspective, the issue with fronthaulingis that it operates by transmitting I/Q samples, which in-creases the transmission rate over fibre by a factor of 30[35], compared to backhauling. In addition, since samplingoccurs whether or not an actual signal transmission is in placeover the radio interface, this rate is fixed, independently ofthe amount of data used in uplink or downlink by mobileusers. Thus a large cell providing an aggregate rate of about10Gbps, for example using an 8x8 MIMO array, 3 sectors and5 x 20MHz channels, will require an approximate fronthaultransmission rate of about 150Gbps, while a small cell withsimpler 2x2 MIMO, 1 sector on a single 20MHz channel,would still require 2.5Gb/s, while offering a radio data ratebelow 150Mbps.

In addition, fronthaul imposes an upper bound to the latencybetween the remote radio head and the BBU, due to amaximum Round Trip Time (RTT) of 3 ms between handsetand the Hybrid Automatic Repeat reQuest (HARQ) processingblock in the BBU. Considering the latency introduced by thedifferent mobile system processing blocks, typically a budgetof up to 400µs is allowed for the optical transmission system,imposing a maximum distance between RRH and BBU ofabout 40km.

Data compression techniques have been proposed in theliterature [42] in order to reduce the large capacity require-ments of fronthaul. Another concept, known as mid-haul,originally introduced in [43], that is becoming increasinglypopular is moving the physical split between RRH and BBU toa different point of the LTE stack. This can reduce the opticaltransmission rate by a factor of 5 [35] to 10 [44] comparedto fronthaul and restore the dependency from the mobile datatraffic volume, thus bringing back the possibility of operatingstatistical multiplexing of a number of mid-haul transmissionsystems. However there is a trade-off with some of the LTE-A functionalities as for example CoMP performs better forsolutions placing the split closer to the RRH [45].

For all cases above, latency remains an open issue: sincethe HARQ processing block is located in the MAC, a solutionthat would position the HARQ in the RRH would be almostequivalent to backhauling.

A number of research projects have worked to providearchitectural solutions to the fixed-mobile convergence issue.The FP7 COMBO project has proposed a number of solutionsbased on PONs to bridge the capacity gap between the basestation and the central office in cost effective manners. Theidea is to use different technologies in the ODN: sharedTWDM PON channels can be used to multiplex backhaul sig-nals from base stations with other users, while fronthaul chan-nels are transmitted through WDM point-to-point channels [3],in order to meet the strict latency requirements of fronthaul.

However using dedicated wavelengths for fronthaul is notalways a cost effective solution, especially when the remoteradio head is constituted by a small cell that does not require a10G channel capacity. The 5G-PPP project 5G-Crosshaul [46]is looking at architectures for transmission of both fronthauland backhaul traffic over a common packet switched network.The development targets a unified data plane protocol andswitch architecture that meet the strict latency and jitter de-mands. From a standardisation perspective, as previously men-tioned, the IEEE is currently working on similar topics withthe P1914.1 Standard for Packet-based Fronthaul TransportNetworks targeting architecture and technology to allow thetransport of next generation fronthaul systems over Ethernet.Another 5G-PPP project working on fixed/mobile convergenceis the 5G-Xhaul [47], which enhances the convergence byintroducing point-to-multipoint millimeter-Wave systems toprovide wireless fronthaul to remote radio head from a macro-cell location. The macro cell is then connected to the BBUpool through a CPRI interface across the metro network. Theoptical network transport is carried out through a Time-SharedOptical Network (TSON) system capable of providing sub-wavelength switching granularity to optimise resource usage.

These examples show how the integration of fixed andmobile networks has become a critical factor affecting the suc-cess of next generation mobile services. In addition, as NFVis progressively moving network functions from dedicatedtelecommunications vendor equipments to virtual machines,including data centres into the big picture becomes important.The next section provides additional insights on the integrationof data centres in the network convergence.

Before concluding this section, we would like to focus thereader’s attention to the trade-off between node consolidation,which is based on the use of longer access reach technologies,and services like fronthaul, which, due to their tight latencyconstraint, require a shorter optical reach. We believe that 5Gnetwork design should take both aspects into consideration,introducing in the ODN the flexibility to redirect latency-bound signals to local processing nodes, while allowing othersignals to benefit from the network equipment consolidationenabled by the use of longer-reach connections. However,this trade-off has not been yet properly investigated and willrequire further study, which should be addressed by futureresearch projects.

B. Data centre integration

Considering that much of the 5G framework revolves aroundvirtualisation of networks and functions, integration of access-metro and data centre networks becomes essential for deliv-ering the end-to-end vision. Taking as example a cloud RANsystem, where the processing chain goes from the radio inter-face to the server where the networking processing functionsare carried out, strict latency requirements for the service canonly be met if the network and processing resources can becontrolled across the entire chain [23]. As NFV moves networkprocessing towards general purpose servers, effective scale upof processing power will require a re-design of the centraloffice architecture, which will progressively migrate towards


Op#calspaceswitch

Intra-DC To/from DC

DC rack

DC cluster

Op#calspaceswitch

Micro-DC

Stadium

Massive MIMO BS

Home Small cell Small cell

PON fibre Large DC

P2P fibre

Core fibre

Coretransmission

Access-Metro network DataCentre

Core network

BBU

OLTBBU

Small cell

BBU

Drop

DropAdd

Drop

Op#calspaceswitch

Drop

Fig. 7: Integration of the data centre into the converged access-metro architecture through agile optical switching

a typical data centre network, a view shared by the CentralOffice Re-architected as a Datacentre (CORD) [48] collabora-tive project between AT&T and ON.Lab. Integration of virtualresources from the wireless and optical domains to the DCwas also one of the main research themes of the EuropeanCONTENT project, which introduced the idea of using theabove mentioned TSON in the metro node for convergence ofwireless, optical and DC resources. The CONTENT approachshows that it is possible to satisfy future content distributionwithout the use of cloudlets4, thus reducing overall energy con-sumption [49] and increasing resource usage efficiency [50],while only paying a small penalty in overall latency. It shouldbe noticed that these studies predate the 5G-Xhaul project andfocused on LTE backhaul, thus latency requirements were notan issues as for fronthaul. When considering X-haul of nextgeneration 5G systems, as anticipated in figure 6, it is expectedthat a 5G flexible architecture should be capable of offeringdata processing elements in different parts of the network, andassign them to the application depending on its latency andcapacity requirement. It should be expected that some networkfunctions will be processed near the access point, some in theODN near a branching or remote node, some at the centraloffice and others in larger data centres.

Due to the increasing role data centres are playing in theconverged network vision, the optical networking communityhas dedicated substantial effort in exploring novel technologiesand architectures for faster interconnection within the datacentre (intra-DC) and among data centres (inter-DC). Theformer has received particular attention on projects targetingnext generation Exascale computing systems, through networkarchitectures that make increasingly use of optical switchingtechnology. This includes study of hybrid electronic-opticalswitching architectures [51], [52], and circuit switching tech-nologies ranging from 3D Micro Electro Mechanical Systems

4A cloudlet is a small data centre located to the edge of the Internet, closeto the end user, to provide computation resources with lower reduce latency.

(MEMs) and fibre steering, capable of 10-20 ms switchingtimes, to faster 2D MEMs [53] capable of few microsecondsswitching times, to the combination of fast laser wavelengthtuning and Array Wave Guide (AWG) with nanosecondswitching times.

As the intra-DC network achieves faster optical switching,it becomes interesting to investigate the use of this technologyoutside the data centre, not only for connections among DCsbut between DCs and any other 5G infrastructure, which wedub DC-to-5G communication. Work in [54] has demon-strated the SDN-driven inter-DC switching of transparent end-to-end optical connections across multiple metro nodes, usingan architecture similar to that shown in figure 7, with commer-cial Reconfigurable Add Drop Multiplexers (ROADMs) forrouting wavelengths across the nodes. The authors showed anoverall path setup time of about 88ms, mostly dominated (72ms) by the reconfiguration time of the ROADMs and opticalswitch, showing the potential for fast inter-DC optical circuitswitching for applications such as virtual machine migration.

The ultimate vision is that of an agile access-metro opticalnetwork capable of transparently interconnecting end points,for example a RRH to the server or rack in the DC where theBBU is located, avoiding any intermediate packet processing,thus eliminating any further transport delay. A use case isshown in figure 7, operating over an Architecture on Demandtype of node [32], where an optical space switch is usedto enable dynamic reconfiguration of optical links, wherewavelengths can be routed to the desired destination, at anaccess PON, at a DC, or else towards the network core (afterundergoing any required aggregation into larger data streams,protocol conversion or other type of signal processing). Theuse case merges the concepts of C-RAN and NFV throughthe virtualisation of the PON Optical Line Terminal (OLT).It considers a number of small cells in a given area usingmid-haul as transport mechanism (traffic is only shown in theupstream direction for simplicity): the traffic they generateon the transport link is three to six times higher than the


traffic in the radio access side and is proportional to it 5.When the traffic from those cells is too low to justify theuse of dedicated wavelengths for each cell, their mid-haultraffic is aggregated over a TDM PON. The idea is to operatetransparent switching of the PON wavelength channel at theaccess-metro node, terminating it directly into a rack or serverin the DC (the violet link in figure 7), which implements bothOLT6 and BBU (purple line terminating on purple OLT andBBU boxes). The colocation of OLT and BBU will minimiseend-to-end latency, allowing for example synchronisation ofOLT and BBU scheduling mechanisms or advanced PHYfunctions such as CoMP and coordinated beamforming amongthe BBUs. A sudden increase in the small cells traffic howeverwill likely create congestion in the PON, as for examplea cell offering a peak rate of 1 Gbps could increase theload in the PON by 3 Gbps. The associated mid-haul trafficshould thus be quickly allocated more capacity, by moving thetransmission wavelength of the RRH to a dedicated channel(e.g., a 10G point-to-point channel), which gets transparentlyswitched in the access-metro node towards the same BBUserver (yellow line terminating on yellow BBU box), or ifneeded to a different server or even DC (red line), althoughthis would require ultra-fast Virtual Machine migration. Webelieve the switching time should be at least comparablewith that of the LTE scheduling time, i.e., of the order ofthe millisecond, in order to achieve seamless transition. Thisrequires the optical switches in the access-metro node andthe DC to operate in the tens to hundreds of microseconds,suggesting the use of 2D-MEMs type of devices, and theimplementation of an agile control plane capable of supportingthis fast switching (switching control times of 10 microsecondswere demonstrated for intra-DC environments in [55]).

Another issue worth mentioning is that mid-haul systems,similarly to fronthaul ones, require low transport latency(few hundreds microseconds), which cannot be achieved withDynamic Bandwidth Assignment (DBA) mechanism availableit today’s PONs (operating on the order of few milliseconds).However studies in [56] have shown the possibility to reducesuch values to few tens of microseconds by using DBAalgorithms synchronised with the BBU scheduler.

Finally, while this section has focused on a C-RAN casestudy, the concept of access-metro and agile data centre con-vergence can support other scenarios. Figure 8 provides furtherexamples, showing the potential mapping between servicesand transmission channels (i.e., shared TWDM PON channels,dedicated wavelength channels and dedicated fibre links). Inaddition, while it is recognised that sub-millisecond switchingmight be still a few years away in access-metro nodes, in theshorter term the ability to provide dynamic optical switching atlower speed will still prove useful to operate dynamic resourceallocation at the network management level.

5Considering that fronthaul can increase the data rate up to about 30 timesthat of the radio interface and that mid-haul can reduce this value by 5 to 10times according to [35],[44], we have that the midhaul rate will be just threeto six times that of the radio interface

6Notice that the MAC of the OLT might be operated in a merchant siliconhardware switch, as in the CORD architecture

100M 1G 10G 100G 1T 10T

Connection to home user

Small cell backhaul, mid-haul

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mini-DC

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and massive MIMO BS

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To other Data centre

To mobile BS

Aggregated PON capacity

Dedicated wavelength

Dedicated fibre/s

Connection type

Peak rate (bps)

Application types:

Fig. 8: Example of service mapping from data centre to accessnetwork users.

In conclusion, this section has shown how the seamlessintegration of wireless, optical and data centre networkingtechnologies will be instrumental to provide the necessarynetwork agility to satisfy dynamic end-to-end allocation ofconnections with assured data rate and latency requirements.The increasing use of transparent optical switching in themetro architecture will increase the ability to adopt new tech-nologies as they become available. In addition, the increasinguse of virtualisation to provide independency among networkresource slices and end-to-end control of the leased resourceswill pave the way to application-oriented, and multi-tenantsolutions, further described in the next section.

VII. CONVERGENCE IN THE OWNERSHIP DIMENSION:MULTI-TENANCY

The last aspect of the convergence we want to discuss inthis paper is in the ownership dimension, i.e. the ability ofthe network to support multi-tenancy. A simplified thoughcommon division of network ownership domains is in threelayers:

• the passive network infrastructure: ducts, sub-ducts, fibrewith any passive splitter element, and copper on somelegacy networks;

• the active network infrastructure: transmission andswitching equipment;

• the service layer: the provisioning of services to the enduser.

Multi-tenancy in access and metro networks has been exten-sively studied in the literature [57]-[60] and figure 9 [61]shows different possible ownership models, ranging from totalvertical integration, typical of incumbent operators that ownall three layers, to complete separation where each layer isowned by a different entity.

While it is out of the scope of this paper to furtherinvestigate the pros and cons of these models, we argue thatopen access, at least of the active network infrastructure, isrequired in order to share infrastructure costs and open upthe market to better competition. Access network sharing hasbeen implemented in the past, though at a basic level, as


telecommunications regulators have enforced Local Loop Un-bundling, (operating at the level of the passive infrastructure)and bitstream services [62] (at the active infrastructure level).Active network sharing has gained popularity as it allowsService Providers (SPs) to sell bandwidth services withoutowning physical network infrastructure, and the bitstreamservice has further evolved to Virtual Local Loop Unbundling(VULA) and next generation access (NGA) bitstream [63].However, although they give service providers more control,by adding some quality of service differentiation, they donot offer the ability to fully customise services to providehighly differentiated products to the users, and rely on theinfrastructure owner for functions such as performance andfault monitoring, which are essential for serving the businesssector.

Regulation

Competition

PhysicalInfrastructureProviders

NetworkProviders

ServiceProviders

Vertical Integration Passive Sharing Active Sharing Open Access

PhysicalInfrastructureProvider

PhysicalInfrastructureProvider

NetworkProvider

NetworkProvider

ServiceProviders

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IntegratedServiceProvidersVertically

IntegratedOperators

InfrastructureCompetition

ServicesCompetition

Access to ducts Regulated PassiveWholesale

Regulated ActiveWholesale

Regulated Passive andActive Wholesale

Fig. 9: FTTH business models [61]

Work is currently underway by standardisation bodies suchas the Broadband Forum under the Fixed Access NetworkSharing study group [64] to increase the control of SPs overthe access network through virtualisation. The vision is thatof an ownership model differentiating between [10]:

• an Infrastructure provider (IP) that owns and maintainsphysical networking infrastructure (for example the pas-sive and active network infrastructure), enables physicalresource virtualisation and carries out the network virtu-alisation, provides virtual resources controlling APIs forthe Virtual Network Operator (VNO) and gets its revenueby leasing resources to the VNO.

• a Virtual Network Operator that operates, controls andmanages its assigned virtual network instance, is able torun and re-designs customised protocols in its own vir-tual networks, provides specific and customised servicesthrough its own virtual networks, receives revenue fromthe end users and pays the usage of network resources tothe IP, while saving on network infrastructure deploymentcosts.

The proposed roadmap is through three sequential steps, assuggested in [65]:

1) the first step is to reuse existing network equipmentcontrolled by the infrastructure provider through theirmanagement system: virtual network operators could getaccess to the network through a standard interface whichcan provide raw access to the management layer withoptional monitoring and diagnostic functionalities.

2) the second step is to deploy new hardware in theaccess node capable of resource virtualisation, so thatthe virtual network operators could be assigned a virtualnetwork slice and get full control of the equipment. Forthis step however the interface remains, similarly to step1), to the network operator management system.

3) the third and final step is the full SDN integration, wherethe virtual operators access their network slice througha flexible SDN framework with standardised APIs.

In conclusion, multi-tenancy enables network infrastructuresharing, reducing the cost of ownerships and opening themarket to a plurality of new entities that can provide thediversity that is necessary to empower the 5G vision.

Although we do not suggest that the three dimensions ofconvergence described in this paper constitute a complete setfor satisfying all foreseeable requirements of 5G networks, weargue that they can contribute to its realisation. A summary ofthe mapping between the described convergence dimensionsand the 5G requirements, grouped into three activity areas asoutlined in section IV, is reported in figure 10.

Enhancednetworkperformance

Flexible,poly-morphicnetworkdesign

Businesscaseof5Gecosystem

Spa@al -  Lowerenergyconsump0on-  Ubiquitousfibreentrypoint

-  FlexibilityofbranchingnodesinODN

-  Lowercostofnetworkownership

Networking -  Transparentop0calswitchingallowslargercapacityalloca0onandfasterintegra0onofnewphysicallayertechnology

-  Integra0onallowsend-to-endcontrolofperformanceparameterslikecapacityandlatency

-  Flexibilityofvirtualisedend-to-endresourcealloca0onoverwireless,op0calandcomputa0on

-  Enableapplica0on-oriented(personalized)resourcealloca0on

-  BeKerbusinesscasefortransportofmobileX-hauldata

-  Virtualisa0onofresourcesfacilitatesmul0plexingofservicesandahigherandmoredifferen0atednumberofmarketplayers

Ownership -  Enablehigherdiversityofapplica0onswithinthesamenetworkinfrastructure

-  Reducecostofnetworkownershipthroughinfrastructuresharing

-  Openingthenetworkcreatesnewmarketopportuni0es

5G requirement area Network

convergence

Fig. 10: Summary of mapping between network convergencedomain and 5G requirements

VIII. FINAL REMARKS ON THE ROLE OF SDN AND NEWBUSINESS MODELS

After reporting on a number of research activities on net-work convergence and showing how they can contribute tomeet future 5G requirements, we finalise the paper by brieflyconsidering two further aspects. The first is a contemplationof the role that SDN could play in this future convergence andthe second is a vision on future end-to-end virtualisation andassociated business models.

We believe SDN will play a pivotal role in the multi-dimensional convergence discussed above, both as the mecha-nism to orchestrate the interaction among the different networkdomains and technologies, and the system to integrate andautomate many of the control and management operations thatare today still carried out through proprietary and closed inter-faces. While SDN has experienced a hype in the past few years(which has also caused disagreement over its capabilities)there is now firm interest from industry, with concepts mov-ing from research to production environment [13], and with


many related working groups being established in differentstandardisation bodies, including ETSI, ONF, BBF and IEEE.The number of enterprises, including many startups, workingprofitably in the SDN industry is constantly increasing, with anestimated overall value of $15 billion in 2015, and a projectedCompound Annual Growth Rate (CAGR) of 46% for the next5 years, bringing the market value to $105 billion by 2020[66].

Besides reducing cost of network ownership, by makingmore efficient use of the network capacity, reducing en-ergy consumption, and enabling the convergence of multipleservices (business and residential) into the same physicalinfrastructure, SDN will generate new form of revenues, byallowing operators to quickly deploy new services in theirexisting infrastructure.

One of these will come from enabling end-to-end virtu-alisation of the network, which will provide the opportunityfor new business models for network sharing. This process,started in the U.S. with the physical unbundling enforced bythe telecom deregulation act in 1996 [67], is now evolving tovirtualised bitstream concepts offering higher flexibility andproduct differentiation, and will continue to evolve towardsfull end-to-end network virtualisation [68]. The idea is tobreak down the network into a pool of virtual resources whoselease is negotiated to provide dedicated end-to-end connec-tions, across multiple network domains and technologies. Forexample a customer could submit a service request through auser portal that is forwarded to a network orchestrator actingas a resource broker. A digital auction is then carried out tosecure a chain of virtual resources to provide an ephemeralend-to-end connection [69] that only lasts for the time requiredby the service, releasing all resources immediately after theservice terminates. In principle this could be carried out withhigh granularity targeting network connection performances ofindividual flows, although more work is required to determineits scalability.

The practical implementation of such concept will likelyrequire a shift towards business models that better reflectthe value chain of the service offered. From an end userperspective, the value is in the service delivered and itsexpected performance. Acceptable performance values suchas data rate, latency and packet loss vary among services andthus need to be assured on a service base and not in averageacross all services delivered to an end point. For examplethe value per bit of a medical application monitoring andtransmitting vital body parameters to a medical centre willbe much higher than that of a video streaming service. It thusmakes sense for the end user to pay for the service ratherthan for the network connectivity. The network should be paidby the entity providing the service which would select themost appropriate quality of service that assures reliable servicedelivery (thus paying different price levels for personalisedephemeral connections as mentioned above). This will makenetwork operations completely transparent to the end users,which only interacts with the services and application ofinterest. Some basic forms of this idea have already beenimplemented by industry. For example, users of the Amazonkindle 3G pay for the purchase of their books but not for the

network delivering it; Facebook zero and Google free zone aresimilar examples where the user is able to access a small setof their services without paying for the underlying network.

Even if an agreement on potential business models is stillfar ahead, the idea of providing the option of better quality ofservice for targeted applications is gathering interest amongnetwork operators [14], as many believe SDN can now providethe level of programmability and network automation requiredto practically implement this idea on a large scale.

Finally, while it is still an open question whether SDNwill be capable of enabling the full network convergence andend-to-end virtualisation envisaged in future 5G networks,we would like to justify this optimism by pointing out thesuccesses it has already enabled. A pragmatic analysis revealsthat SDN has so far boosted research in several areas ofnetworking, by producing a framework where testing of newalgorithms, protocols and architectures [70], can be quicklymoved from simulation/emulation environments (e.g., usingthe mininet platform [71]) to real testbed scenarios [72],[73], enabling large-scale interoperability tests among differentresearch groups and physical laboratories [74], [75].

IX. CONCLUSIONS AND FUTURE WORK

The main contribution of this paper was to present anovel perspective on network convergence, proposing a multi-dimensional, end-to-end approach to network design to enablefuture 5G services. While providing a tutorial of the tech-nologies, architectures and concepts revolving around networkconvergence, we have presented a number of high levelconcepts that we believe are of paramount importance in the5G vision. One of the key messages is that an un-convergedview of the network based on the piece-wise developmentof the individual segments is suboptimal and can lead touneconomical decisions. A typical example is that viewingthe wireless domain only as a client of the optical domain haslead to issues in performance and deployment cost.

Network architectures should be designed with an end-to-end vision in mind, as the real value for the end usercomes only from a consistent support of the application fromits source to its destination. This is becoming increasinglyclear among the networking community, as for example theEuropean Commission is promoting an increasing numberof ICT funding calls targeting converged fixed and wirelessarchitectures towards 5G.

Finally, we have identified many open areas of research,which we believe will become increasingly relevant in thecoming years. We conclude the paper outlining a few:

• aggregation of mobile mid-haul links over TWDM PONs;• end-to-end optimisation of dynamic resource allocation,

including wireless spectrum and antenna resources, op-tical transport and processing resources in the data centre;

• architectural solutions capable of merging the benefits ofnode consolidations with latency-bound requirements ofsome of the 5G services and applications;

• joint development of protocols and scheduling algorithmsacross the fixed and wireless domains;

• convergence of access-metro and DC through faster op-tical switching enabling DC-to-5G communication;


• scalable, SDN-controlled, end-to-end QoS assurance;• new application-centric business models.

ACKNOWLEDGMENT

The author would like to thank Prof. David B. Payne forthe fruitful discussions on the Long-Reach Passive OpticalNetwork architecture and Prof. Linda Doyle for the insightfulconversations over 5G networks and services.

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Marco Ruffini received his M.Eng. in telecommu-nications in 2002 from Polytechnic University ofMarche, Italy. After working as a research scientistfor Philips in Germany, he joined Trinity CollegeDublin (TCD) in 2005, where he received his Ph.D.in 2007. Since 2010, he has been Assistant Professor(tenured 2014) at TCD. He is Principal Investigatorat the CTVR/CONNECT Telecommunications Re-search Centre at TCD, currently involved in sev-eral Science Foundation Ireland (SFI) and H2020projects, and leads the Optical Network Architecture

group at Trinity College Dublin. He is author of more than 80 journal andconference publications and more than 10 patents. His research focuses onflexible and shared high-capacity fibre broadband architectures and protocols,network convergence and Software Defined Networks control planes.



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