future service adaptive access aggregation network

696IEICE TRANS. COMMUN., VOL.E95–B, NO.3 MARCH 2012

INVITED PAPER Special Section on New/Next Generation Photonic Networking and Future Networks

Future Service Adaptive Access/Aggregation Network Architecture

Hiroki IKEDA†a), Member, Hidetoshi TAKESHITA††, Student Member, and Satoru OKAMOTO††, Fellow

SUMMARY The emergence of new services in the cloud computingera has made smooth service migration an important issue in access net-works. However, different types of equipment are typically used for thedifferent services due to differences in service requirements. This leads toan increase in not only capital expenditures but also operational expendi-tures. Here we propose using a service adaptive approach as a solution tothis problem. We analyze the requirements of a future access network interms of service, network, and node. We discuss available access networktechnologies including the passive optical network, single star network. Fi-nally, we present a future service adaptive access/aggregation network andits architecture along with a programmable optical line terminal and opti-cal network unit, discuss its benefit, and describe example services that itwould support.key words: optical access network, passive optical network, network ser-vice, network topology

1. Introduction

Fiber to the home (FTTH) service has grown rapidly overthe last five years in Japan, and there are now almost 20 mil-lion subscribers [1]. The installation of fiber infrastructuresfor access networks in Japan is almost completed. The mainFTTH providers offer Internet access service and video dis-tribution service. The Internet access service uses a passiveoptical network (PON) technology, such as Ethernet PON(E-PON) and Gigabit Ethernet PON (GE-PON), which aredescribed in the Institute of Electrical and Electronics En-gineers (IEEE) 802.3ah Specification, Ethernet in the FirstMile (EFM) [2], and broadband PON (BPON) and Gigabit-capable PON (GPON), which are described in the Interna-tional Telecommunications Union (ITU) G.984 Recommen-dation [3]. A PON, which is an attractive broadband tech-nology for the last mile, is a point-to-multipoint optical net-work that has no active elements between the optical lineterminal (OLT) and the optical network units (ONUs). Theadvantages of using a PON as an access network includelower fiber deployment cost due to subscribers sharing afiber line, a wider bandwidth, and lower maintenance costfor the fiber line. The video distribution service uses ra-dio frequency (RF) overlay technology as an alternative toproviding a high-definition television (HDTV) video service

Manuscript received July 25, 2011.Manuscript revised November 7, 2011.†The author is with Central Research Laboratory, Hitachi Ltd.,

Kokubunji-shi, 185-8601 Japan.††The authors are with the Department of Information and Com-

puter Science, Faculty of Science and Technology, Keio University,Yokohama-shi, 223-8522 Japan.

a) E-mail: [email protected]: 10.1587/transcom.E95.B.696

Fig. 1 Schematics of current network architectures including ATM,CATV, PON, and mobile backhaul.

over fiber lines [4]. This RF overlay technology enables aservice operator to deliver video contents in the same formatas a cable TV (CATV) or satellite provider. At the carrier’slocal office, a video optical line terminal (V-OLT) is alsorequired to transmit the video content using the 1550-nmwavelength. An optical amplifier boosts the optical videosignals, and a wavelength-division multiplexer (WDM) mul-tiplexes the signal over the same fiber line.

As shown in Fig. 1, a variety of access services areavailable to subscribers. The asynchronous transfer mode(ATM) is used to provide a unified transport function acrossservices. Since ATM originated in the early 1990’s, ATMequipment is coming to the end of its life due to economi-cal, technical, and/or political reasons. The unified transportfunction is being shifted to Ethernet and the multi-protocollabel switching (MPLS). Both of these technologies arewidely used in carrier networks, such as Ethernet-based E-PON/GE-PON for internet access services, in carrier-gradeEthernet for leased line services, in MPLS virtual privatenetworks (VPNs) for VPN services, and in MPLS traffic en-gineering for Internet protocol (IP) backbone services. TheMPLS transport profile (MPLS-TP) has recently emergedas a transport technology alternative to ATM [5]. MPLS-TPoffers a smooth migration path towards IP-based infrastruc-tures, where development is taking place. Moreover, carri-ers can implement MPLS-TP without having to take currentATM implementations off-line by using a pseudo wire tech-nology [6]–[8]. This is important to carriers from a prof-itability standpoint.

Another paradigm shift in transport technology is Eth-ernet, namely EFM. The idea of using Ethernet as the last-mile (or first-mile in the case of EFM) access technology

Copyright c© 2012 The Institute of Electronics, Information and Communication Engineers

IKEDA et al.: FUTURE SERVICE ADAPTIVE ACCESS/AGGREGATION NETWORK ARCHITECTURE697

for providing new services to businesses and residences isquickly gaining popularity. For enterprise customers, thechance to do away with conventional E1/T1 access linesand the expensive IP routers that convert IP-over-Ethernetpackets to and from IP over time-division multiplexing(TDM) — plus the order-of-magnitude increase in accessspeeds that Ethernet promises — combine for a very com-pelling solution. The corresponding opportunity for carriersextends beyond simplifying their networks to offering ad-ditional, Ethernet-based services such as toll bypass, voiceconferencing, video conferencing, bandwidth-on-demand,and storage.

In mobile networks, the evolution towards long termevolution (LTE) and 4th generation (4G) supporting higherbandwidths for mobile data and video applications is forcingmobile backhauls to adapt to new requirements for accessnetworks. To support these growing bandwidth demandscost efficiently, mobile backhauls are in a transition fromsynchronized TDM networks to asynchronous Ethernet net-works.

Timing has traditionally been delivered very accuratelyand reliably over TDM, but Ethernet-based networks do nothave an inherent mechanism for providing highly accuratesynchronization. There are several approaches to address-ing this challenge, with Precision Time Protocol (PTP) -IEEE 1588v2 [9] currently leading the way. The transmis-sion of clock information over an Ethernet network elimi-nates the need for alternative mechanisms, such as a globalpositioning system (GPS). The IEEE 1588v2 solution trans-mits dedicated timing packets, which flow along the samepaths as the data packets, reducing the cost of synchroniza-tion and simplifying implementation. Since the number ofcell sites will increase due to the need for a higher signal-to-noise ratio to support the increase in bandwidth deliveredto customers, this cost reduction will be welcome news foroperators.

A cloud computing service provider or an applicationservice provider (ASP) providing, for example, a computer-aided design service or a medical/health care service re-quires a high-speed response time for its applications. Toreduce the transmission delay time, ASPs attempt to placetheir data centers (DCs) as close as possible to users. Forexample, a DC might be placed on a floor in a building inan urban area. This type of DC is called a micro-DC to dis-tinguish it from the conventional giant DC. Micro-DCs areconnected to the access network.

A smart grid for automated metering infrastructure(AMI) application also needs to meet different requirement.For example, active demand response and emergency loadmanagement require higher reliability and lower latency asan integrated system than they do as stand-alone systems.

The services now emerging and growing include real-time enhanced reality, video-conferencing, on-line gaming,on-the-map positioning, and cloud storage. The number ofthese services is growing exponentially thanks to the latestgeneration of multimedia-ready devices like smart phones,tablet personal computers (PCs), personal digital assistants

(PDAs) and handheld game consoles. The increasing num-ber of services translates into an increase in the number ofaccess sites. Therefore, capital expenditures (CapEx) andoperational expenditures (OpEx) are increasing [10], [11].To reduce the number of access sites, it is necessary toimplement long reach access and to increase the numberof users per site. Long-reach PON (LR-PON) technology,which supports the migration of metro networks and accessnetworks into a long-reach access network, simplifies thenetwork and reduces CapEx and OpEx [10], [11]. The ac-cess line topology for the Internet service and video distri-bution service using FTTH is passive double star (PDS), asshown in Fig. 1, which reduces cost. The leased line and mo-bile backhaul use a single star (SS) topology because of theirdifferent requirements such as various qualities of services(QoS), different transmission/transport frame, and differentbandwidth guarantee.

The rest of the paper is organized as follows. In Sect. 2,detailed requirements for future access networks are dis-cussed. Section 3 provides current solution candidates forrequirements. Section 4 describes our proposed serviceadaptive network architecture. Finally, Sect. 5 summarizesthe key points.

2. Requirements for Future Access Networks

2.1 Service Requirements

Conventional access services including Internet access andvideo on demand have specific requirements and use a ded-icated network consisting of a core network, a metro net-work, and an access network that meets their requirements.Future access networks should support both conventionaland new services as shown in Fig. 2. New access services,such as micro-DC service, mobile backhaul service, leasedline and machine-to-machine (M2M) service, need addi-tional requirements.

Table 1 summarizes the technical requirements for fourtypes of service. A leased line service and a micro-DC ser-vice require high reliably and managed transmission. To en-hance the reliability, operation, administration, and mainte-nance (OAM) techniques and protection techniques shouldbe implemented. Since legacy synchronous digital hier-archy (SDH) and ATM leased line service equipment are

Fig. 2 Application reliability as function of data size.


Table 1 Technical requirements for four types of new services.

now being replaced with MPLS-TP equipment, future ac-cess networks require high transmission quality and highavailability, equivalent to those of MPLS-TP core transportnetworks.

A micro-DC network requires low latency and timesynchronization. Fiber channel (FC) networks are currentlyused to connect servers and back-end storage systems inmicro-DC environments. In addition, micro-DC networkrequires high reliability, meaning that it needs a protectionmechanism.

A mobile backhaul service requires time synchroniza-tion across devices, high availability transmission (i.e., pro-tection), and time distribution to client devices. As de-scribed in Sect. 1, mobile backhauls are in a transition fromsynchronized TDM networks to asynchronous Ethernet net-works. Therefore, future access networks should supporttree-like user and time distribution technologies, which isdescribed in IEEE 1588v2 solution and synchronous Ether-net [12], to accommodate the growing number of users.

M2M is a new development in telecommunication en-vironments, and M2M services require low latency and highQoS. In an M2M service, a communication device is con-nected directly to a network without a user interface or userinteraction. M2M services are expected to expand rapidlyand to generate significant revenues for operators becausethe traffic carried will be considered valuable. Therefore fu-ture access networks should support QoS technology.

2.2 Network Requirements

Because the number of ONUs is increasing, it is necessaryto implement long reach access and to increase the num-ber of users per site in order to reduce CapEx [11], powerusage [13], and operational and management costs. CapExand power usage reduction requires simplifying the networkstructure. An aggregation network has been introduced foraggregating the user traffic and/or service networks of eachservice into a single access/aggregation network [13]. Tech-

nologies supporting effective aggregation will be needed toaccommodate ONUs in a wide area. Therefore, it is neces-sary to extend multipoint-to-point control (MPCP).

The peak transmission rate per user in an access net-work is at least 1 Gb/s, and the committed data rate is above0.3 Gb/s [11]. However, a future access network requiresa much greater access reach (up to 100 km) and a largernumber of customers per fiber (e.g., 1024) [11] than cur-rent access network. The National Institute of Informa-tion and Communication Technologies (NICT) in Japan hasstarted the AKARI Project [14] to develop a new genera-tion network (NWGN). In the AKARI project, a new ac-cess network architecture has been proposed that will sup-port NWGN requirements in 2015 to 2020 [15]. For theNWGN, 10-Gb/s for each user and QoS support for eachservice are required. Meeting these requirements requiresseveral 10-Gb/s to Tb/s order transmission in one fiber andreal-time packet buffer processing. Technologies support-ing high speed and long reach transmission will be neededto accommodate ONUs in a wide area. Therefore, it is nec-essary to extend access reach by using new access networktechnologies.

The topologies for access networks vary depending onthe primary purpose of the network, so they are heteroge-neous. For instance, SS topologies, which can be considereda point-to-point link, are bandwidth inefficient and achievelow yield in terms of users served in comparison to activeand passive double star (ADS/PDS) networks. However,since SS topologies serve a business niche, e.g., leased lines,SS and ADS/PDS topologies must be supported.

2.3 Node Requirements

As the processing speed of terminals increases, higher band-width connections are needed. Specialized equipment andoptical fiber are thus been installed for each new service.This makes the deployment and provisioning of new ser-vices a lengthy and costly process. The service migrationof access networks can be made more cost effective by im-proving the availability of hardware and by avoiding dou-ble investment in the optical fiber infrastructure. To sim-plify operations, carriers are looking for ways to build ac-cess networks over a single infrastructure that will scale tomeet these diverse services while maintaining security, ser-vice levels, and reliability.

A QoS is needed for M2M and leased line services. Tomeet the QoS requirements for a variety of services providedin different physical topologies, a new media access control(MAC) frame and multi-point control protocol (MPCP) areneeded that can simultaneously handle Ethernet, MPLS, andFC frames while maintaining their QoS peculiarities. Mul-tiple service level agreements (SLAs) with different QoS re-quirements need to be considered. The QoS is also affectingthe dynamic bandwidth assignment mechanism in the accessnetworks as well as implementation of time synchronizationin the backhaul networks.

In DC environments, FC over Ethernet technology will


Table 2 Requirements for future access/aggregation networks.

be used to share network links. A network-based time syn-chronization technology will be used underground, in of-fices, in factories, and in locations where GPS radio wavesignals cannot reach or the cost of GPS receivers cannot besustained to support business applications that require timesynchronization. As M2M services require low latency aswell, future access networks should support low latency andtime synchronization technology.

2.4 Future Access Network Requirements

Table 2 summarizes the service, network, and node require-ments for future access/aggregation networks mentionedabove. Future networks need to meet these requirements.We will discuss solutions to these requirements in Sects. 3and 4. The sections describing solutions are shown in Ta-ble 2.

3. Technology Trend for Access Network

In this section, we describe available technology trend andcandidate solutions for some of the network requirementslisted in Table 2. They include current and future opticalaccess network technologies.

3.1 Current Optical Access Network Technologies

A PON is a point-to-multipoint network consisting of anOLT and multiple ONUs in a tree topology. The OLT islocated in the central office, connected to a metro area net-work or a backbone/core network. The ONUs are located

Fig. 3 IEEE and ITU-T TDMA-PON standardization road maps.

on subscribers’ premises.Commercial PONs use time division multiple access

(TDMA), which is cost effective for access networks. Forthe upstream TDMA, the ONUs share the upstream capac-ity and transmit data to the OLT. For the downstream TDM,the OLT broadcasts data to all ONUs, which share the down-stream capacity, and then the destination ONU extracts thedata on basis of the PON link identifier (ID), such as theport ID of the GPON or the logical link ID of the GE-PON.The logical connection between the OLT and each ONU ispoint-to-point.

TDMA-PON standards have been evolving under twodifferent umbrella organizations, the ITU and the IEEE. ThePON standardization road maps are both shown in Fig. 3.Each organization has been presenting over the last decadedifferent sets of recommendations and standards. However,they are now facing a technological challenge: TDMA-based systems have apparently reached a threshold in termsof cost vs. performance. Hence, new technologies are beingproposed to maintain control of the cost of the electronicswhile maximizing the available optical bandwidth.

MPCP is defined to provide bandwidth assignment andauto discovery in IEEE 802.3ah [2]. The bandwidth assign-ment mechanism relies on grant and request message. Theauto discovery is used to detect newly connected ONUs.

3.2 Future Optical Access/Metro Network Technologies

3.2.1 Network Topology Technologies

Here we discuss future network topology technologies thatuse current TDMA- and WDM-PONs.a) ADS/PON approach

Since WDM-PONs assign one wavelength to eachONU, they meet service requirements from a technical per-spective. However, from the economic point of view, aWDM-PON is not cost effective because a user who doesnot require a large bandwidth still consumes the entire band-width.

One way to solve this problem is to share one wave-length among ONUs to provide a TDMA-PON and to use


the other wavelengths as WDM-PON [15]. With this ap-proach, a 10 to 20 km range can be covered.b) SS approach

A WDM-PON basically assigns one wavelength toeach user. With an SS approach, one wavelength is assignedto each service. This results in a flexible transmission rateand bandwidth for each user. This is because an SS approachprovides a dedicated fiber for each user. If a user requestsa shared access service, a PON is emulated in the OLT. TheSS approach enables bandwidth to be shared in the opticaldomain by using optical couplers or in the electrical domainby using a layer two switch.

Miyazawa reported new architectures for optical accessnetworks [15], but their cost effectiveness in comparison toa WDM-PON was not mentioned. Cost reduction with theSS approach is the most important factor in achieving theNWGN access network.

3.2.2 Network Architecture Technologies

Network architecture technologies include TDMA-PON,WDM-PON, optical orthogonal frequency division multipleaccess (OFDMA)-PON, optical code division multiplexing(OCDM)-PON, and coherent WDM-PON.

TDMA-PONs have been widely deployed becausethey contribute to cost-effective high-speed access systems.However, a pure TDMA-PON is not sufficiently scalable interms of reach, user count, and per-user data rate mainlydue to the increased cost of the electronics beyond 10 Gb/s.Several candidate technologies for future access networksare thus now being investigated. They include WDM-PON, OFDMA-PON, OCDM-PON, coherent WDM-PON,and hybrid systems combining two or more of these tech-nologies. Their benefits and drawbacks are briefly discussedbelow.

WDM-PON [16] reduces the number of fibers throughthe use of wavelength multiplexes, provides a guaranteedbandwidth for each user, has a long reach due to opti-cal amplification, and ensures security and privacy withwavelength per user. However, it creates ONU inven-tory and OAM complexities and is less cost-effective whenONU with colorless sources is used (widely tunable lasers,wavelength-seeded reflective semiconductor optical ampli-fier (SOA), etc.).

OFDMA-PON provides higher capacity with spectralefficiency using many narrow-band orthogonal subcarriersthan TDMA-PONs and WDM-PONs, has a long reach withhigh linear dispersion tolerance, supports dynamic band-width allocation with subcarriers, and has a flexible modula-tion format (analog or digital) with orthogonal subcarriers.To date, it has been demonstrated to support 40-Gb/s trans-mission over 20 km with a 1:32 optical split [17]. However,for the achievement of 100-Gb/s, 100-km transmission re-quires the development of an ultra-high-speed digital analogconverter (DAC), an analog digital converter (ADC), and ahigh-performance digital signal processor (DSP).

OCDM-PON is based on the well-established spread

spectrum technique in microwave communications. It pro-vides data confidentiality and privacy by using a unique op-tical code, has low latency, provides a guaranteed bandwidthfor each unique user code, and operates asynchronously[16]. However, OCDM-PONs are still too complex for prac-tical use as they require unconventional optical devices andinline optical dispersion compensation [16]. An OCDM-PON has been demonstrated to support full-duplex, asyn-chronous, 10-Gb/s transmission over 50 km [18].

A coherent WDM-PON has been field tested in a 2.5-Gb/s coherent link over 68 km [16]. They are still cost inef-fective due to the use of digital coherent receivers based onadvanced DSP and the use of silicon photonics integrationof the single ONU and OLT optical components.

To enable the access reach to be extended in a cost-effective manner, optical amplifiers are being developed.Candidate devices are the erbium-doped fiber amplifier(EDFA), SOA, and distributed Raman amplifier (DRA) [16].

3.3 Time/Clock Synchronization Technologies

Time/clock synchronization is essential for reliable servicesat the different layers in networks, from the physical layer tothe application layer. As mentioned in Sect. 1, although GPStechnology can be used to distribute this information witha high level of accuracy, logistic and cost issues generallyrestrain network designers from using it widely. Instead,GPS signals are received at a node or server high in the net-work hierarchy, and the information extracted is distributedthroughout the network. Table 3 summarizes the proper-ties of four commonly used time and clock synchronizationtechnologies; the network time protocol (NTP), GPS, IEEE1588, and the synchronous Ethernet.

With IEEE 1588v2, the slave clock on each networkdevice is synchronized with the system grandmaster clock.Traffic time-stamping is used to achieve the high accuracysynchronization needed to ensure the stability of the basestation frequency and of the handovers in wireless networks.The timestamps between the master and slave devices utilizespecific PTP packets.

Synchronization is achieved using two procedures. Thefirst procedure is to run the slave clock at the same fre-quency as the master clock. The second procedure is to

Table 3 Properties of commonly used time and frequencysynchronization technologies.


run the slave clock at the same time of day as the mas-ter clock. IEEE1588-2008 [9] defines two clock types fornetwork nodes: boundary clock (BC) and transparent clock(TC). A BC behaves as a master and a slave. It synchro-nizes the slave clock with the adjacent master clock via oneport and synchronizes the other slave clocks via the otherports. A TC does not need to synchronize with an adjacentclock; instead, it measures the residence time of PTP eventmessages such as sync and delay requests.

One of the challenges when applying IEEE1588v2 to aPON is determining how to deliver IEEE1588v2 signaling.An ad hoc group within the IEEE P1904.1 working group[20] addressed this challenge but was unable to make a de-termination because it exceeded the scope of its mandatedue to the implementation dependency.

4. Future Network Architecture

In this section, we propose network solution for the require-ments shown in Table 2. We describe a future service adap-tive access/aggregation network and its architecture alongwith the OLT design. The protocol and services supportedare also discussed. Service adaptive means that the proposednetwork accommodates various kinds of services with uni-fied architecture.

4.1 Future Service Adaptive Access/Aggregation Network(SAAN)

An overview of a future network, from the common corenetwork to the proposed future access/aggregation net-works, is shown in Fig. 4. The core network is heteroge-neously composed of leased line networks, DC networks,mobile networks, and Internet backbone networks. Despitetheir differences, these networks have a feature in commonthat affects the design of the overall network: they are allhighly bandwidth intensive. MPLS-TP is currently the lead-ing candidate for use as the common core network transportprotocol.

The future access/aggregation network accommodates

Fig. 4 Future service adaptive access/aggregation network (SAAN).

all services by using adaptation technologies. Each ofthese service networks operates under different premises andwithin certain parameters, and the traffic they carry mayhave different data frames and types, protocols, and require-ments. The data frames include but are not limited to MPLS-TP, Ethernet, FC, IEEE 1588 (v1 and v2), and ATM. Oneapproach to unifying services with different requirementson the same platform is to have them share the physicalfiber network and hardware. Services can be unified notonly in the physical layer, which consists of the basic hard-ware transmission technologies, but also in the data linklayer, which provides the functional and procedural meansto transfer data between network nodes.

To accommodate all services in the access network, anadaptation function is required. This function will be ap-plied to deeply service optimized access network technol-ogy. We propose a service adaptive access/aggregation net-work (SAAN). The SAAN provides an adaptation functionfor service accommodation and a traffic aggregation func-tion for traffic grooming. SAAN consists of a programmable(P-OLT), programmable ONUs (P-ONUs), and optical fiberwith various topologies, including ADS, PDS, and SS, link-ing the core network with the end-users. The P-OLT is ableto support a variety of services, meaning that the SAANtransparently conveys the traffic while maintaining the spe-cific features of each traffic type.

Using a programmable OLT and programmable ONUshas several benefits:

• Resource utilization is improved because virtual re-sources are reserved to meet service requirements asnecessary.• Service migration is smoother because virtual re-

sources are configured for service deployment withoutinstalling additional equipment.• Installation costs are reduced because the logical OLTs

(L-OLTs) and logical ONUs (L-ONUs) can be config-ured remotely.

The P-OLT bridges the core network with the P-ONUs.The ONUs must be reconfigurable so that they can supportnot only existing services but also new services. Further-more, they must be reconfigurable regardless of the topol-ogy (e.g., ADS, PDS, and SS). The wide variety of traffictypes to be transmitted requires the development of a newMAC frame for the SAAN; this raises other technical is-sues regarding, for example, the discovery of new ONUs andthe bandwidth allocation technology used to achieve low la-tency. In addition, legacy equipment compatibility must beconsidered and supported, both at the physical layer and atthe management layer. The required technologies will bediscussed in the following subsections.

4.2 OLT and ONU Architecture

The basic logical architecture of a programmable OLT isshown in Fig. 5. An OLT must be able to provide differentservice modules and be able to operate the module speci-


Fig. 5 Architecture of SAAN with programmable OLT and ONUs.

fied by the system operator in a static or dynamic matter.The services include OAM, protection, time synchroniza-tion, bandwidth guarantee, low latency, and even power sav-ing. The physical layer may operate using Ethernet, MPLS-TP, ATM, or FC on a common physical layer device (PHY).Since current developments on PON systems in terms ofavailable bandwidth are blurring the boundary between op-tical transport and traffic distribution at the access point, theSAAN must natively support such technologies if servicesand applications at the end point are to operate correctly.

Figure 6 shows how the programmable OLT and ONUsare configured in accordance with the topology of the opticaldistribution network (ODN), that is, on the basis of whichtopology is used: ADS, PDS, or SS (an active network, apassive network, or a point-to-point link). A dynamic band-width allocation (DBA) method that is suitable for the topol-ogy is selected.

Figure 7 shows the basic layered architecture of theSAAN. The adaptation layer between the service layer andMAC layer enables a variety of services to be provided. Theservice adaptation layer selects the most suitable MAC pro-tocol for each service to operate over the ODN. This layerstack can be implemented only in reconfigurable systems.

This physical layer implementation plays a key role inthe SAAN. Because MAC is provided using TDM, WDM,OFDM, or OCDM, services can be allocated more effi-ciently by increasing the granularity at which the opticalbandwidth is accessed.

4.3 Programmable OLT and ONU

The use of a programmable OLT and programmable ONUsenables various services to be provided by a single device,without installing additional equipment, as shown in Fig. 5.The programmable OLT can configure multiple L-OLTs

Fig. 6 Configuration scenarios corresponding to (a) ADS, (b) PDS, and(c) SS topologies.

Fig. 7 Basic layered architecture between OLT and ONU.

within the one physical OLT. They are configured usingsoftware/firmware re-install or update. Each programmable


ONU can configure multiple L-ONUs within one physicalONU. Each L-ONU would be used for a different service.For example, one L-ONU could accommodate an FTTHterminal and another could accommodate a femto base sta-tion. In practice, services such as FTTH that use a low-endtype ONU would not need a programmable ONU function.Therefore, the programmable OLT must be compatible withnon-programmable ONUs.

The programmable OLT receives the required ODNtopology and service requirement parameters from the net-work management system via the service adaptive function.It then selects the most suitable type of DBA and the re-quired function modules. The L-OLT and L-ONU configurethe data link function, thereby enabling a variety of service-dependent MAC frames to be capsulated in an SAAN framefor transport.

The programmability and reconfigurability of the OLTand ONU depend not on dedicated access to a physicalmedium but on the service layers of the system. The ben-efit for carriers is reduced installation time, which reducesoperation expenses. The benefit for manufacturers is thatthey can produce a larger number of the same type ofONU even though they are to be used for different services,which reduces ONU cost. However the cost merit for pro-grammable technologies is important issues For example,the cost-effective development of programmable OLTs andONUs is thus a major challenge.

4.4 MAC Frame and MPCP for SAAN

Various possible structures for the proposed MAC frame forthe SAAN are shown in Fig. 8. The MAC frame can accom-modate various types of data frames (FC, Ethernet, MPLS-TP, etc.) and provide multi-level QoS service or even best-effort service. It can take various components, including async header, a logical link ID, a time stamp, and the payloadtype. The sync header is used to determine the timing of theclock synchronization. The logical link ID is used to con-nect between an L-OLT and an L-ONU. The payload type isused to specify the type of SAAN frame (data, OAM, etc.).

A new MPCP is required to coordinate virtualmultipoint-to-point upstream traffic in any topology becausea programmable ONU has multiple L-ONUs. In addition,in the ADS case shown in Fig. 6(a), an MPCP coordina-

Fig. 8 Examples structures for MAC frame: (a) FC frame, (b) Ethernetframe, (c) Ethernet frame with time stamp, (d) MPLS-TP packet.

tion mechanism with an optical switch (OSW) for upstreamand downstream traffic is required because of the requiredswitching scheduling of the OSW in the downstream andupstream directions.

An OAM function is required in the domain of an L-OLT/L-ONU although the discovery function of the PONsystem can be used as an alternative to the OAM function.In this scenario, GATE/REPORT messages, which are MACcontrol frames in MPCP, are sent periodically between theL-OLT and the L-ONUs. They are used as alarm indica-tion signals, remote defect indications, continuity checks,fault detections, and loop back tests. Performance monitor-ing functions will hence be needed to monitor parametersdirectly derived from physical impairments, such as the biterror rate (BER), or generated by the network, such as la-tency and jitter. Performance monitoring plays a key role indetecting probable causes of changes in the QoS and, even-tually, in detecting the inducing sources. It is also impor-tant for service allocation. Performance monitoring will beimplemented in various ways, including delay testing andinsertion loss measurement.

4.5 Multi-QoS Support

Current PON technologies can provide bandwidth controland traffic prioritization. However, a new mechanism op-erating on top of the PON system must be implementedto support requests for low jitter and low-latency features.These features are extremely important when implementingbackhaul mobile solutions, in which synchronization playsa key role. For the proposed SAAN, the DBA module needsto be reengineered to enable coordination mechanisms tohandle multiple QoS levels, such as the ones specified forMPLS-TP, ATM, and PON. It also needs to be able to co-ordinate with protocols operating at higher layers, such asIEEE 1588, to ensure proper operation of the network.

As can be seen Figs. 5 and 6, the DBA module has sev-eral functions:

• time slot assignment for each L-ONU (both upstreamand downstream) to meet requirements for each ser-vice;• coordination with function modules including time

synchronization and protection modules;• control of topology information (ADS, PDS, SS, etc.);

and• extraction of QoS information from incoming MAC

frames.

An example of multi-QoS assignment to L-ONUs isshown in Fig. 9. For a low-latency TDM service (L-ONU#1), a time slot can be assigned in a short cycle; but thisconsumes much bandwidth because of the high frame over-head. For a low-latency service (not TDM) (L-ONU #2),a time slot can be assigned to reduce latency. For a best-effort service (L-ONU #3), an unoccupied time slot can beassigned to consume as much unused bandwidth as possi-ble. Therefore, the development of a suitable DBA module


for SAAN is a major challenge.

4.6 SAAN Application Examples

The SAAN is an open architecture network characterized bythe reconfigurability of its OLTs and ONUs, enabling it toadapt to different network scenarios and technical require-ments.

The programmable OLTs comprise a hardware sidecontaining optical transmitters and receivers and a softwarereconfigurable set of functions and protocols. This indepen-dence between hardware and software not only has manytechnical benefits but also provides CapEx and OpEx reduc-tion. For example, the OLTs and ONUs can be reconfiguredremotely instead of locally.

An example mobile backhaul application of SAAN isshown in Fig. 10(a). Since the access line topology is PDS,DBA with PDS is selected. The OAM and time synchro-nization function modules are selected on the basis of therequirements of the mobile backhaul.

The L-OLT transfers user data to the L-ONU cooperat-ing with QoS messages in MPLS-TP. The L-OLT transfersa time-stamped PTP frame to the L-ONU, and the L-ONUsynchronizes its clock with that of the base station.

Fig. 9 Example multi-QoS assignment to L-ONUs.

Fig. 10 Example applications of SAAN: (a) mobile backhaul, (b) micro-DC.

A micro-DC application of SAAN is shown inFig. 10(b). Since the access line topology is ADS, DBA withADS is selected. A protection function module and a DBAmodule with low latency are selected on the basis of the re-quirements of the micro-DC. In micro-DC applications, ser-vice must be continued by using a protection function even ifa failure occurs. Low latencies for user service requests areachieved by using DBA for the low-latency function block.

5. Conclusion

The requirements for a future access network were dis-cussed, and the proposed future SAAN and its architec-ture were described. The programmable OLT and the pro-grammable ONUs it uses were also described, and the re-quired technologies and services supported were discussed.

The SAAN can accommodate various services withdifferent requirements and provided using different topolo-gies with sharing of network resources. Also discussed werethe need for a new media access control frame and a newmulti-point control protocol for the SAAN.

The SAAN provides several benefits. Virtual resourcesto meet service requirements are reserved as necessary,which improves resource utilization. Virtual resources areconfigured for service deployment without the need to in-stall additional equipment, resulting in smooth service mi-gration. The logical OLTs and logical ONUs can be config-ured remotely, which reduces installation costs.

Application examples were presented to demonstratethe potential use of the SAAN in practical situations. Thefunction modules can be chosen and updated by software asnecessary.

Acknowledgments

The authors thank Prof. Naoaki Yamanaka for his insightful


comments and stimulating suggestions.

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Hiroki Ikeda received B.S., M.S. and Ph.D.degrees in communication engineering fromOsaka University in 1993, 1995 and 2009. From1995 to 1999, he worked in Hitachi Ltd. CentralResearch Laboratory, studying optical transmis-sion systems. From 1999 to 2002, he workedin Hitachi America’s Network System ResearchLaboratory, studying optical cross-connect net-works. From 2003 to 2006, he worked at Hi-tachi (China) Research and Development Inc.in Beijing, studying future optical access net-

works. Since 2006, he has been working at Hitachi’s Central ResearchLaboratory, studying optical access network. He served as an editor on theIEICE’s technical committee on communication system (CS) from 2009 to2011.

Hidetoshi Takeshita graduated from Shizu-oka University, Japan, where he received a B.E.in electrical engineering. In 1974 he joined NECCorporation, Tokyo, Japan, where he workedon the development of electronic switching sys-tems, digital switching systems, and the systemengineering for fixed and mobile carriers. Since2009, he has been investigating high-energy effi-cient network architectures. Since 2010, he hasserved as a research assistant for Global COEProgram “High-Level Global Cooperation for

Leading-edge Platform on Access Spaces” of the Ministry of Education,Culture, Sports, Science, and Technology, Japan. He is an IEEE studentmember.

Satoru Okamoto received B.E., M.E. andPh.D. degrees in electronics engineering fromHokkaido University, Hokkaido, Japan, in 1986,1988 and 1994. In 1988, he joined Nippon Tele-graph and Telephone Corporation (NTT), Japan,where he conducted research on ATM cross-connect system architectures, photonic switch-ing systems, and optical path network archi-tectures and participated in the development ofGMPLS-controlled HIKARI router (“photonicMPLS router”) systems. He has led several

GMPLS-related interoperability trials in Japan, such as the Photonic In-ternet Lab (PIL) and the Optical Internetworking Forum (OIF) WorldwideInteroperability Demo. Since 2006, he has been an associate professorat Keio University. He is a vice co-chair of the Interoperability Work-ing Group of the Kei-han-na Info-communication Open Laboratory. Heis now promoting several research projects related to photonic networks.He was the chair of the Technical Committee on Photonic Network (PN).He received the Young Researchers’ Award in 1995 and the AchievementAward in 2000. He has also received the IEICE/IEEE HPSR2002 Outstand-ing Paper Award, the Certification of Appreciation ISOCORE and PIL in2008, and the IEICE Communications Society Best Paper Award and IEEEISAS2011 Best Paper Award in 2011. He is an associate editor of OSAOptics Express and was associate editor of IEICE Transactions. He is anIEEE Senior Member.

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