1. IntroductionSince the invention of viable optical communication fiber cable, it has been projectedthat fiber to the home (FTTH) would be an objective architecture for the access net-work. However, it has taken over 20 years to realize this goal. This paper reviews thedevelopment and deployment of passive-optical-network (PON)-based fiber access sys-tems in North America, tries to discern the key drivers for that deployment, reviewsthe lessons learned, and reviews the possible alternatives for next-generation PONsystems.
Optical fiber is very well suited for access networks in technical terms. Its ultimatebandwidth capacity is well beyond any foreseeable access demand, and so is “futureproof” from this perspective. Due to its low loss, fiber is intrinsically long reach, andtherefore holds the promise of an all-passive outside plant (OSP) (without repeaters orconcentrators). This eliminates electrical powering requirements in the outside distri-bution network (ODN). Fiber is also chemically inert, and is resistant to electricalinterference and other external disturbances. For this reason, a well-designed fiberplant is projected to last well over 25 years, and should have little trouble during thatlife. Thus, from a purely cable plant perspective, fiber is an obvious choice.
Of course, the raw fiber is only one half of the network, and it is useless withouttransmission equipment to light it and receive the transmitted signals. The cost ofthis equipment has been the limiting factor from the beginning of FTTH development,when prototype PON equipment cost was near U.S. $5000 per customer. The biggestsource of cost reduction has been technological advancement in the semiconductor andoptics industries. These advances have reduced the cost of key components such aslasers, detectors, and application-specific integrated circuits manyfold.
Beyond the raw cost of components, the system design plays an important role inthe overall cost of the network. The PON design is a key invention in that it reducescosts, not only in the equipment but also in the plant, through the principle of shar-ing. The PON shares one central office optical line termination (OLT) and feeder fiberover a number of subscribers [typically 32 optical network terminations (ONTs) usedin North America]. This reduces part count, and it also reduces the space and powerneeded at the central office (CO). PONs also fit nicely into the typical access networktopology of tree and branch, where the splitters replace the copper cross-connects anddigital loop carriers of the former copper network. PON also offers passive multiplex-ing deep in the network, which can be used to produce very efficient statistical gain,leading to higher peak bandwidths available to the subscriber.
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From a strictly topological point of view, PONs are similar to cable TV networks.They are both passively split and multiplex distribution networks. However, CATVsystems traditionally began from a broadcast-centric design where the RF spectrum isshared over quite a large number ��2000� of users. This then evolved toward nar-rower and narrower scope of transmissions through the use of hybrid fiber coax (HFC)designs. This evolutionary approach can reduce initial costs, but when carried to itsfinal conclusion in a FTTH deployment, HFC systems becomes increasingly inefficientdue to the carrying forward of many legacy modulation formats and network elementsover a coaxial cable distribution network with limits in bandwidth. It also has meantthat as the number and usage of data service users has increased on an HFC network,there has had to be a reduction in the actual spectrum set aside cables core set of ser-vices, namely, video.
Digital baseband PONs tend to be more revolutionary, since digital signaling isenabled by the use of a fiber-based network and legacy modulation formats are not arequirement to deliver video services. However, it is easy to show that baseband digi-tal signaling is the most efficient given the nearly limitless bandwidth of the fibermedium. This translates into lower ultimate costs, lower power consumption, and asimpler design of the network than an HFC network. Thus, deploying PON should beseen as a bold yet logical step forward for some network build situations.
2. PON Technology Trial Period (1996–2000)The invention of PON by scientists at British Telecom Laboratories [1] initiated a longsequence of technology development that has only recently come into the mainstream.The initial patents and publications occurred circa 1990 [2,3]. At that time, opticaltechnology was so immature that it clearly was not ready for rapid consideration. By1995, things had progressed to the point where serious system development couldbegin. This ushered in a period of standards development and field trials that lastedapproximately 5 years.
FSAN and A-PON standards. The first step in this sequence was the founding ofthe full-service access network (FSAN) consortium [4]. This informal group of networkoperators from around the world committed themselves to develop broadband accessnetwork solutions. This included copper-based digital subscriber line (DSL) and fiber-based solutions (PON), and every combination between. The FSAN group organizedinto separate working groups, with the optical access network (OAN) working groupbeing concerned with PON development. This has been, in fact, the longest livedgroup of FSAN, having survived the FS-VDSL, optical access management, and homenetworking groups. The OAN group has also been the most productive, having origi-nated well over a dozen standards in the ITU.
At the beginning of the work on optical access, the FSAN had consensus on thePON fiber arrangement and the use of TDMA, but there were many divergent areas ofdesign. Some of these were adapted into options of the standard, such as the choice oftwo- and single-fiber networks, the speed of transmission (there were two choices),and the optical budget classes (there were three). However, through 1996 the FSANtechnical membership agreed on an ATM-based transmission protocol. This was fullydeveloped into a recommendation that eventually become ITU G.983.1, commonlyreferred to as A-PON.
A-PON trials. Several operators endeavored to deploy A-PON systems on a trial oreven a limited service basis. In Japan, NTT set out to deploy A-PON systems as acountrywide solution for broadband data access services. The A-PON used by NTTwas a single fiber, symmetric 155 Mbits/s, class B system, and provided the user withan ATM-50 interface. The logical data service provided was a point-to-point privateline service, similar to what a bank would use to connect its branch offices together.Japan continues to lead the world in PON deployment ever since the initial deploy-ment of A-PON systems.
In the same time period, BellSouth ran an early North American trial with A-PONin the Dunwoody suburb of Atlanta, Georgia [5]. This trial was limited to a data-onlyoverlay on top of the standard copper telephony network. The PON used by BellSouthwas similar to that used in Japan, with two big exceptions. First, a two-level splitterarrangement was used in the distribution plant. Second, the optical network units(ONUs) for Dunwoody were built with Ethernet interfaces, rather than an ATM inter-face used in the NTT A-PON deployment.
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The main objectives of this trial were to assess the maturity of the network equip-ment, and to gain experience in deployment of PONs in the access network. The trialwas a success in this limited sense by confirming that PON equipment was indeedfield ready, and that PON systems were rugged enough to withstand the rigors ofdeployment. However, from the beginning it was clear that PON was considerablymore expensive than the other options available. As such, the trial did not progress toany wider deployments in BellSouth.
Verizon and SBC (now AT&T) had also analyzed or trialed A-PON systems, but forbusiness applications. They found A-PON did not have compelling cost advantagesagainst new access-oriented SONET-based platforms for smaller business locations,which leveraged mature support systems in the operators’ network.
Mixed results. At the close of the 1990s, then, we could see that PON was a mature,deployable technology. However, the business case for PON deployment was lackingfrom several perspectives in North America. First, the capital costs of the equipmentand the fiber plant were quite high. Second, the demand for bandwidth in amountsthat justified fiber was far from clear. Third, the competitive marketplace remained ina state of flux. Fourth, and last, the regulatory approach to new infrastructure wasuncertain.
As a result of all of these reasons, most operators decided to stick with a moreorganic broadband growth scenario that revolved around DSL, leveraging the existingcopper network. While such DSL networks were more limited in bandwidth (withmost offerings of that era being below 1 Mbits/s), they were economical to deploy tothe mass market, and were much closer to the telephone operators experience base.
There were a few exceptions to this basic DSL trend. BellSouth and their suppliersdeveloped a fiber to the curb (FTTC) system to serve new-build areas. This systemwas an active double star arrangement, with an active remote terminal serving aneighborhood of 4–8 home FTTC ONUs. The FTTC ONU fed a composite drop oftwisted pair and coax cable transporting rf-modulated video to the homes. This sys-tem was provably the least-cost solution for a deep-fiber network capable of providingvoice, broadband data, and broadcast video available for new builds. It did have thedownsides that it was a proprietary system and required a network powering system.Yet, it enjoyed some moderate success, and over 1�106 customers are served fromthis system in BellSouth footprint.
US West (now Qwest) also rolled out a fiber to the node (FTTN) system in Phoenix,Arizona, and Denver, Colorado, wherein the ONU was larger, and used VDSL to feeda residential gateway device in the home. This architecture minimized ONU costs anddeferred the construction costs associated with deeper FTTC, but required a switcheddigital video (SDV) solution so that the video could be carried over the VDSL drops.This early attempt to converge video, voice, and data services into a single copper pairwas a key stepping stone on the path to integrated networks with triple-play servicecapability. Deploying this SDV feature pushed the envelope of the current access tech-nology, but due to the lack of industry focus and standards at the time, limited indus-try expansion took place. This system serves about 60,000 homes currently in Qwest[6], but any significant expansion on this ATM-based platform is unlikely. High levelsof customer satisfaction were recorded for the video services, which served to validatethe ability to deliver high quality over last mile copper networks and spurring contin-ued interest in FTTN [7]. Improved home networking solutions over existing coaxialwiring, user interface improvements optimized for SDV networks, and features likeCaller ID on the TV screen emerging from early deployments of video by US West arenow table stakes for new access systems.
Many of the incumbent local exchange carriers (ILECs) experimented with HFCnetworks to add the delivery of video services to subscribers. Ameritech, SouthernNew England Telephone (SNET), and Pacific Bell all had trials or deployment of HFCnetworks. This was essentially a play to equalize the field with the burgeoning cableoperators. However, the HFC effort was cancelled as the providers were acquired bySBC and efforts were redirected to align with its DSL strategies.
In addition to the large network operators, there were many small telephone com-panies and municipalities that moved to deploy broadband networks. A shift wasbeginning from municipality-operated HFC networks focusing on video services to fullservice or triple-play capable networks. Many of these tended to use active Ethernetsystems, with fiber interconnecting many small Ethernet switches in the field. This is
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favorable for small operations, since the equipment is basically designed for the enter-prise customer, and the increased management effort is acceptable in a small networkcommon with municipal networks. But for these very same reasons, such networkshave a difficult time to scale into ILEC deployments.
3. Initial Deployment Period (2001–2005)At around the turn of the century, the telecom world was revolutionized by the “.com”and telecom business cycles (“the bubble”). Forecasts of ever-growing demand,whether real or imagined, drove massive investment, new competition, and opportu-nity for new ideas. Some of these business drivers moved PON development forward.
The existing A-PON system was functional, but the two companies that were con-sidering it (NTT and BellSouth) knew that it needed improvement [8]. In the residen-tial market, BellSouth wanted a way to add broadcast video services to the system ina cost-efficient way. The concept of the analog video overlay was developed, and wasmoved through FSAN and ITU, becoming the G.983.3 standard that specified thewavelength allocation for overlay signals. NTT needed a way to offer 100 Mbits/s ser-vice over broadband PON (B-PON), to compete with emerging competition. Given thatthat upstream PON speed was limited to 155 Mbits/s, dynamic bandwidth allocationwas required to allow customers to burst to 100 Mbits/s as needed. This work movedthrough FSAN and ITU, becoming G.983.4 standard. In a similar time period, opticstechnology advanced to where the prevalent PON speed was 622 Mbits/s down-stream, 155 Mbits/s upstream. With improvements like these, the A-PON system wasgiven a new name of B-PON. In parallel Verizon started to look at this technology forits broader residential market where operational savings could be realized over itspresent method of operation and did deploy a proprietary PON system as part of itsBrambleton, Virginia, deployment. However, it became clear that a standards-basedPON system would be more universally accepted and ensure the technology’s viability.The B-PON system was the focus of the 2003 Tri-Company (BellSouth, former SBC,and Verizon) PON technical specification, and is even now being deployed in volumein North America.
However, it was clear to FSAN members, the industry, and ITU PON-developinggroups in 2001 that there was a need to define the next generation of PON systemswith greater efficiencies and in line with the developments occurring in the rest of thenetwork. New efforts were started to build a PON that carried packets more natively.The system was dubbed G-PON for gigabit capable. FSAN considered three possiblealternatives. The first was a sort of ATM-like system. The second was an Ethernet-framing system. The third was a physical layer framing system, which borrowed basicconcepts from synchronous digital hierarchy (SDH) and generic framing procedure(GFP), as well as incorporated many different features to make sure that all servicescould be supported. It was this third approach that was selected by the FSAN opera-tors, and this became the basis for the G.984 series of G-PON standards.
By 2002, the major work of defining the protocol and physical layer was complete.However, the commercial production of the equipment would take several years. Thisconsiderable lag can be attributed to two major factors. First, the optical and elec-tronic devices needed to make G-PON work at the speeds desired (2.4 Gbit/s down-stream, 1.2 Gbits/s upstream) took a couple of years to develop. Second, the maturityof the full-service multiprotocol adaptation schemes was not sufficient in 2002. In anutshell, G-PON ultimately offered more functionality, and so it took longer to pro-duce.
Also in 2001, the IEEE began a parallel effort to define an Ethernet-based PONsystem. In contrast to the top-down requirements-driven approach of FSAN–ITU, theIEEE prefers a technology-driven development philosophy. The result was a systemthat took the existing Ethernet transmission protocols and devices, and applied themto the PON space with as few changes as humanly possible. The result was EthernetPON (EPON), which was simpler than the FSAN G-PON specifications. Of course,this EPON system was ideal for carrying best-effort Internet access services, and didso with a symmetric line rate of 1 Gbit/s.
Ethernet PON just happened to fit the needs of NTT, who needed a more advancedbroadband solution to fend off competition from aggressive and financially backedISPs [9]. This serendipitous alignment of a simple need with a simple technology pro-
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pelled EPON into mass deployment in Japan in 2004. In North America, however, thebusiness case for PON has always demanded full-service capabilities. This hasdirected the choice of PON technology to be either B-PON or G-PON.
Deployment of B-PON: AT&T. AT&T initiated deployment of B-PON systems in alarge development project in the San Francisco Bay Area in 2002 [10]. The $4 billion(U.S.) redevelopment project called Mission Bay consists of 6000 residential housingunits, 6�106 ft2 of commercial space, university, retail, hotel, schools, and fire/policestations, and is bordered by the San Francisco Giants baseball park. A B-PON systemoperating with 622 Mbits/s downstream and 155 Mbits/s upstream was deployedwith a 1�32 splitter and ONTs equipped with four voice ports, an Ethernet data port,and an rf-video port. The majority of Mission Bay residential buildings are multi-dwelling units where single-family-designed ONTs and power supplies with backupbattery are installed into each living unit. Voice and data service was initiated inApril 2003 to residential customers and continues to be installed in new residentialbuildings being constructed in the 6–10 year build-out of the Mission Bay Project. Atechnology trial delivering rf video was completed in 2004 in Mission Bay. Subse-quently, AT&T has pursued a strategy to deploy a mixture of FTTP (fiber to the pre-mises) and FTTN with VDSL in the residential last mile. This strategic move trig-gered development of an IP-based video architecture (IPTV) and ended the company’spursuit of rf-based video delivery. In 2006, AT&T launched their IPTV service in keymarkets over the FTTN architecture in existing residential areas called “brownfields.”IPTV service is planned over B-PON networks deployed in areas of new residentialconstruction, which are called “greenfields” deployments. Subsequent discussion ofAT&T B-PON deployments relate solely to the company’s plans for greenfield develop-ments.
Deployment of B-PON: Verizon. To date, Verizon has announced that it is deployingFTTP in parts of 16 states—more than half of the states Verizon serves, includingCalifornia, Connecticut, Delaware, Florida, Indiana, Maryland, Massachusetts, NewHampshire, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Texas,Washington, and Virginia. Verizon passed greater than 3�106 homes and businessesby the end of 2005—and it expects to add another 3�106 premises passed in 2006.This means that by the end of 2006, the technology and the products it provides willbe available to approximately a fifth of Verizon’s residential customer base. The aim isto pass 18�106 households by 2010 covering approximately 50% of Verizon’s customerbase.
Marketing of FiOS broadband products began in Keller, Texas, in August 2004.FiOS Internet service is now available to customers in approximately 775 communi-ties in the states where the FTTP network is being built. Verizon is currently sellingFiOS broadband products with downstream speeds of 5, 15, and 30 Mbits/s; with2 Mbits/s upstream on the first two products and 5 Mbits/s on the third product. Veri-zon also has recently introduced a 50 Mbits/s product set and will be increasing therates up to 100 Mbits/s in the foreseeable future.
On September 22, 2005, Verizon introduced its first video, digital broadcastproduct—competing directly with cable TV companies and providing another enter-tainment choice for consumers. Keller, Texas, was the first FiOS TV market. Verizonhas several super-head-ends in place, more than 270 COs supporting video and morethan 1.8�106 homes open for video services. Verizon has also opened new fiber solu-tions centers (FSCs). These are specialized work centers where employees are provid-ing support to customers of the new FiOS broadband and video products.
4. Architecture Considerations for Reducing Cost of DeploymentPrevious studies have looked at the architectural considerations at the choice of cas-caded splitters or locating them at one location [11] and have shown that there aremany considerations that need to be taken into account. AT&T and Verizon have cho-sen to deploy with a centralized location for splitters called a fiber distribution hub(FDH) to help simplify the operational turn-up and troubleshooting of the fiber plant.The splitters used in PON networks have matured with wide optical bandpass, reli-ability, and excellent optical performance [12] for PON networks. Verizon has chosento deploy a uniform deployment guideline for the OSP, which uses a 1�32 splitter outto 11 km or a 1�16 if the deployment is beyond 11 km total loop length. The Verizon
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optical network is illustrated in Fig. 1. The OSP utilizes fiber connectors at each sideof the fiber drop, the splitter hub, and the CO. AT&T currently deploys a similar fiberarrangement, but with greater use of fusion splicing rather than preconnectorizedfibers. Also, AT&T’s focus on IP-based video in the digital PON transmission does notrequire a central office WDM or optical amplifier for the rf-video overlay.
Because of the use of the overlay wavelength for analog-modulated rf-video signalsused by Verizon, the OSP contains only angled physical contact (APC) fiber-optic con-nectors to reduce the likelihood of multiple reflections, which can degrade rf-videoquality. AT&T has also chosen to use only angled faceted connectors in the outsideplant and nonangled, ultrapolished connectors (UPC) in the controlled environment ofthe CO for PON deployment. Since an APC connecter and a UPC connector cannot bemated together, AT&T maintained a single polish type in the CO to align with currentpractices for the fiber lineups used for all fiber transmission such as GbE, SONET,and PON. Use of angled connectors in the OSP ensures the fiber network deployed forB-PON and G-PON today can sustain over the life of the fiber network, a minimalreflection with long-term exposure of connectors to uncontrolled environments. Thelong-term exposure of nonangled fiber connectors and increases in reflectance at con-nector pairs could present a transmission degradation mechanism that would be diffi-cult to troubleshoot. As the next generation PON systems emerge, which are likely tobe higher rate including 10 Gbits/s line rates, an OSP network with controlled opticalreflections inherent in angled connectors allows some relaxation of the isolationrequirements of ONT lasers and associated optical components and thus reducesfuture cost.
The deployment of FTTP may be considered as split into two distinct deploymentscenarios. These are greenfields where no services are offered today and brownfields,which are areas served by copper distribution areas with existing Plain Old TelephoneService (POTS) and data service today. In greenfields, the operator could deploy FTTPto each house in the development and offer a triple-play service of voice, video, anddata. In those areas where the operator already provides POTS service one woulddeploy the fiber feeder plant and pass each house via a distribution cable. When asubscriber chooses to subscribe to data or video service the operator deploys the ONTto that subscriber and connects the drop, either aerial or buried to the ONT, on theoutside of the house. Both an overlay and greenfield PON deployment are illustratedin Fig. 2.
By adopting a splitter hub approach, it’s possible to more fully utilize the equip-ment back at the CO because the operator only connects the distribution fiber to thesplitter when we have a customer to support. The splitter hub approach to higher uti-lization applies equally well to brownfield as it does to greenfield, as newly con-structed homes turn up in a randomized fashion for service at different time periods.
Fig. 1. (a) Example fiber network used by Verizon showing the use of WDM for an rf-video overlay, together with (b) an example FDH.
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To serve the customers in multiple dwelling unit (MDU) buildings such as apart-ments or condominiums, there are several alternatives using either the single familyONT for each living unit or a shared ONT with the capability to serve multiple livingunits. Both scenarios are discussed starting with single family unit (SFUs).
One can deploy fiber to each living unit in an MDU deployment and then use theequivalent ONT that one would use for a SFU deployment as shown in Fig. 3. Thisoption deploys a SFU ONT within each living unit and is the preferred option as itprovides the highest service capability and quality, and simplifies operations andmaintenance requirements by using one ONT variant. By coupling the FTTP F1feeder fiber-optic facilities, with a fiber-optic F2 distribution network within the MDUbuilding the operator provides an all-optical network connection between the CO andthe end-user customer’s living unit. The FDH houses passive optical splitters andserves as the interface between the F1 feeder fibers and the F2 distribution fibers.Figure 3 shows FDTs on each floor. The fiber drop terminal (FDT) serves as the inter-face between the fiber drops to each living unit and the F2 distribution fiber.
In the SFU ONT design for an MDU deployment, both the ONT and associatedpowering equipment are located within the subscriber’s living unit within a struc-tured wiring cabinet, which simplifies installation, operation, and maintenance of thefacilities required to provide voice, data, and video service. This would be located closeto the living units wiring closet in brownfields, which would allow easy access to theroot of the living unit’s wiring. In new build MDUs, the ONT and power supply can befitted into a structured wiring cabinet shown in Fig. 3. The SFU ONT architecture isthe preferred method of supplying service to MDU customers in both greenfield andbrownfield applications. Interior single-mode fiber cabling to each living unit in anMDU should be done by the builder in all greenfield applications, and is applicable foroverlays in situations where new fiber distribution cables and fiber drops can eco-nomically be placed to each living unit using available ducting.
For the Verizon case of rf-video overlay, within each living unit the distribution ofthe data signals would be via multimedia over coax alliance (MoCA) with the broad-cast video signal being offered through the legacy coaxial distribution network of the
Fig. 2. Illustrative examples of overlay and greenfield PON deployment scenarios.
Fig. 3. Illustrative example of MDU deployment using a SFU unit in a wiring closet.
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living unit. For new builds, the operator works with the housing developer to placeinterior cabling optimal for the suite of services interfaces on the ONT. The use ofstructured cabling category 5 inside wiring is typically used by AT&T in new buildswhile home-phone-line networking alliance (HPNA) version 3 for home networking isthe direction for AT&T FTTP deployments in legacy coaxial wiring scenarios.
An alternate solution is to employ a common ONT deployed in a common area of anMDU as shown in Fig. 4. The MDU ONT architecture is applicable for deploymentswhere it is not feasible to install fiber in the building to each living unit (LU). TheMDU architecture places the MDU ONT in a common location, from which it servesmultiple LUs with voice, data, and video using the legacy copper distribution network(Cat 3/Cat 5) and a coax distribution network with coax drops. There are two basicdesigns for MDU ONT deployment; centralized and distributed. In the centralizeddesign the MDU ONTs are placed in a common location for simplified access andmaintenance, such as an apartment house basement. Legacy copper wiring is usedfrom that central basement location to all the LUs in the building. The other scenarioinvolves placing MDU ONTs in distributed locations, typically riser equipment clos-ets, to serve units on one or more floors. In large buildings the distributed design ispreferred because it minimizes the distance between the ONT and the LU, andreduces the length of cabling required to transport voice, data, video, and power.
The type of MDU network utilized in any building will depend on the size andarchitecture of the building itself as well as the availability of existing copper/coaxfacilities that can be reused for deployment. The design of each MDU building’s wiringsystem will be unique based on the availability of space, power, access to existingPOTS, and coax distribution networks and the cabling limits for POTS, data (Ether-net or VDSL), and video. In some cases the coaxial cable may not be available and itwill be necessary to use VDSL to offer SDV service to the units. In the apartment anoperator could deploy an integrated VDSL modem with the broadband home router(BHR) in a single small box.
5. Scaling to Volume (2006–2010)When the Tri-company PON group reformed in 2005, it published a request for infor-mation (RFI) that sought to learn the sensitivity of the market to various key param-eters of the G-PON standard. This was similar to the effort that was carried out suc-cessfully for the B-PON request for proposal, several years previously. These keyparameters were the cost impact of different speed options on the downlink anduplink, the effect of ODN fiber distance, and the different ODN classes (A, B, B+, andC). It became clear in the RFI process that given the volumes projected for G-PON,there was little cost penalty in designing the system to support the higher speeds, andthe participating companies decided to define a single downlink speed requirement of2.5 Gbits/s and a single upstream requirement of 1.25 Gbits/s, a single ODN class ofB+ or 28 dB, and a requirement for 20 km reach representing the need to serve longerloop lengths common in regions of North America. In parallel, operators from Asia,Europe, and North America in the FSAN group achieved a consensus set of commonphysical transport layer and the level of ODN support aligning with the Tri-companyrequirements for G-PON [13]. This helped bring clarity to the plethora of laser driverand receiver type modules within the industry and allowed the industry to focus on areduced set of options and exercise economies of scale in an aggressive time frame andat aggressive price points. While G-PON systems were available as early as 2006, the
Fig. 4. Illustrative example of use of the MDU ONT in a centralized location.
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Tri-company and FSAN operators consensus on key G-PON requirements in 2006,facilitated G-PON equipment being available from multiple vendors and ready fordeployment starting in early 2007.
AT&T will continue to rollout B-PON in greenfields into early 2007, at such time asG-PON is introduced and scaled. When this happens, existing B-PON areas will con-tinue to grow to reach full utilization, but new B-PON system deployments will becapped. Verizon has made an announcement of choosing three vendors for its G-PONdeployment. These G-PON vendors will allow Verizon to begin deploying G-PON earlyin 2007. Verizon has designed its ODN to support B-PON and G-PON, and will beshifting to G-PON soon to facilitate offering of higher speeds and increased service setto its customers.
An important feature of all these deployments is the lessons learned. These lessonsvalidate assumptions, or offer guidance as to what has to change to make the technol-ogy choice viable. Some of the key lessons learned to date are listed below.
Battery backup. Over the years since FTTP was first proposed one of the potentialstumbling blocks was the provision of battery support times in line with the coppernetwork. However over the years customers have gotten used to the support timesassociated with wireless phones and it is becoming clear that a design target of an 8 hbattery backup time commonly found in current copper networks is sufficient for resi-dential customers. In addition, Verizon utilizes a feature in the power backup thatreserves approximately an hour’s worth of capacity, which can be customer activatedby pressing a “blue button” on the backup power supply.
Trends in ONT design. The installation of the ONT is still a large cost contributorto the overall cost per subscriber, and in order to reduce the cost of FTTH one has tomake the installation simpler and cheaper. A large cost portion of the deployment hasbeen the time it takes to connect the ONT to the power supply and then to the AC out-let. Verizon sees the present placement of the ONT on the outside of the house evolv-ing to be on the interior of the home, but just inside. This is seen as reducing the timeto install the auxiliary equipment (power supply with battery backup and associatedAC–DC converter equipment) as well as increasing the reliability of the unit, since itwould then be protected from the elements. However, unlike the ONT deploymentmodels used in China and Japan, Verizon sees issues with moving the ONT too farinto the house, as the associated cost with the internal wiring to the ONT could becost prohibitive. In addition, the use of “new wiring” is contrary to Verizon’s overallstrategy.
Demand growth. PON is a robust technology option for meeting customers’demands for high bandwidth. The evidence shows that when a superior bandwidthoffering is presented to the mass market, an economically significant fraction of thecustomers want it and are willing to pay for it. The PON system’s flexibility allows thenetwork operator to dynamically modify its service offering to follow that demandgrowth, and do so without deploying additional equipment or OSP. So, it turns outafter all that, “If you build it, they will come.”
6. Follow-On Technologies and MigrationThe recent successes of B-PON and G-PON technologies have cemented PON as aleading North American architecture for broadband optical access in large-scaleoperator networks. Just as legacy copper has given the market drivers for the devel-opment of ever-increasing bit rate capability of DSL variants, the fiber in the groundnow will give PON a huge advantage over competing fiber architectures in the future.Several groups and companies have begun to consider what the next steps are in theevolution of PONs.
10 G TDMA PONs: IEEE and FSAN. The currently deployed Gigabit speed PONsystems operate with 1.2 or 2.5 Gbits/s downstream, and 1.2 Gbits/s upstream. Whilecertainly a very robust bandwidth for residential PON, some have considered scalingthe bit rate higher. The IEEE have begun a task force to consider 10 Gbits/s trans-mission, while the FSAN group is considering several higher bit rates including rateasymmetry (e.g., 10 Gbits/s downstream with 2.5 and 5 Gbits/s upstream).
Certainly, the move to 10 Gbits/s raises significant physical layer issues. The dis-persive effect of the transmission fiber makes directly modulated lasers operating inthe 1550 nm region difficult to use over 20 km reach used in North American build-
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outs for B-PON–G-PON. Therefore, external modulation is most likely going to berequired at the OLT for the downstream transmission. Because these externallymodulated lasers are significantly more expensive than directly modulated lasers, it isimpractical to use them at the ONT for the upstream direction. Therefore, the indus-try is seriously considering 10 G transmission in the 1310 nm band. This raises theissue of compatibility with existing 1 G PON systems, which also use 1310 nm. Thelikely resolution of this problem is the use of TDMA techniques to time share the 1310channel between the older 1 G system and the newer 10 G system. This in turn pre-sents the challenge of constructing a high-speed dual-rate OLT receiver. While not aninsurmountable problem, it is a new technology development.
Producing a loss budget of 28 dB or greater (important for backward compatibilitywith the fiber plant build-outs in North America and Japan) also looks challenging.The B+ budget that is used in B-PON and G-PON, and the similar 29 dB budgetavailable with EPON being deployed in Japan, is one of the unifying aspects of allPON technology. This common budget makes it possible at a minimum to keep thePON cable and splitter network constant through PON technology upgrades. Historyproduced the 28–29 dB value due to that fact that this budget was cost effectively fea-sible at �1 Gbit/s data rates. There is no guarantee that 10 G speed will also be costeffective. However, the optics industry is working diligently to find ways of reachingthis goal through the use of new technologies such as semiconductor optical amplifi-ers, special APDs, and so forth. A good cross section of presentations that present thestate of the art in these considerations is given at the referenced website [14].
Moreover, one of the justifications of the higher speed is to have the PON servemore customers, but this tends to drive the PON splitter losses even higher andreducing the deployment reach especially for FTTH applications with the current 1�32 ODN design requirements. New applications involving PON-based metro access,MDU-focused PON deployment, or business-focused PON may be a market opportu-nity for higher rate PON. Clearly, the scaled deployment of G-PONs by worldwideoperators in mostly residential areas indicate a stability point while faster PONs arestandardized, developed, and become cost competitive.
Advanced optical PONs and PON migration. On top of the data rate, there areother means to increase the capacity of a PON. The literature is rich with all mannerof wavelength, frequency, and code division concepts applied to PONs [15]. As animportant first step, the consideration of the wavelength spectrum plan has begun,looking to reserve wavelengths in the C and/or L bands for future PON standardsexpected to be developed in ITU-T. It is hoped that rapid standardization of this spec-trum will result in G-PON ONUs that are “blind” to wavelengths allocated for nextgeneration PON systems. Operators are actively developing PON migration scenariosfor the future to ensure PONs deployed today may be upgradeable when the timecomes for additional capacity in a smooth manner such as one ONT at a time ratherthan change out all ONTs at the same time. Such consideration and study is taken inFSAN to ensure coexistence of the legacy PON with a next generation PON systemallowing a smoother migration to the next-generation PON system on the same PON-based fiber network.
7. Closing CommentsIt is evident from the growth of PON networks in North America that fiber is an idealaccess medium to meet current service requirements and long-term future growth inmany deployment scenarios. In North America, fiber access in new-build areas isexpected to be the dominant access system, with PON architectures leading overactive Ethernet systems, in terms of homes served. This is due to the large volumedeployments from large operators that require higher scalability offered by PON sys-tems. However, cost reduction in installation procedures and methodologies need tocontinue to reduce the associated cost of connecting each customer to the passive opti-cal network. Fiber placement and overall construction costs in existing residentialareas with buried or large lot sizes do pose higher costs for fiber access network com-pared to dense, aerial neighborhoods. Therefore, we expect to see a continued deploy-ment of both copper and fiber networks in North America by the telephone companies.
A common requirement for North America is the need to provide full service accessnetworks delivering video, data, and phone services, with increasing reliability and
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quality, such that the customers’ expectations are met in the highly competitive mar-ket. Today, there is certainly much activity in the research community on next-generation PON alternatives, including systems for future PON migration. Operatorsdeploying PONs today can expect future systems that reuse the investment in fiberpathways all the way to the premises.
References and Links1. J. W. Ballance, “Optical communication network,” U.S. patent 4,977,593 (11 December
1990).2. J. R. Stern, J. W. Ballance, D. Faulkner, S. Hornung, and D. B. Payne, “Passive optical local
networks for telephony applications and beyond,” Electron. Lett. 23 1255–1257 (1987).3. K. Okada, F. Mano, and N. Miki, “Passive double star architecture and subscriber network
evolution,” Presented at the 2nd IEEE Workshop on Local Optical Networks for the LocalLoop, Dec. 1990.
4. D. Faulkner, K. Okada, W. Warzanskyj, A. Zylbersztejn, Y. Picault, R. Mistry, and T.Rowbotham, “The full service access network initiative,” IEEE Commun. Mag. 35, 58–68(1997).
5. G. Mahony, “Fiber to the home: A summary of Bellsouth’s first office application,” inNational Fiber Optic Engineers Conference (IEEE), pp. 275–283.
6. F. Wagoner, Motorola Wireline Access Systems group (personal communication, 2007).7. P. Whitehead, previously with USWEST, currently at AT&T Services (personal
communication, 2007).8. H. Ueda, K. Okada, B. Ford, G. Mahoney, S. Hornung, D. Faulkner, J. Abiven, S. Durel, R.
Ballart, and J. Erickson, “Deployment Status and Common Technical Specifications for aB-PON System,” IEEE Commun. Mag. 39, 134–141 (2001).
9. H. Shinohara and T. Manabe, “Broadband optical access technologies and FTTHdeployment in NTT,” in Broadband: Optical Access Networks and Fiber-to-the-Home, C. Lin,ed. (Wiley, 2006), pp. 1–16.
10. E. Edmon, K. McCammon, R. Estes, and J. Lorentzen, “Today’s broadband fiber accesstechnologies and deployment considerations at SBC,” in Broadband: Optical AccessNetworks and Fiber-to-the-Home, C. Lin, ed. (Wiley, 2006), 16–41.
11. F. Effenberger, K. McCammon, and D. Cleary, “Analog video and PON optical lossvariations,” in National Fiber Optics Engineers Conference (IEEE, 2003), pp. 589–595.
12. Recommendation ITU-T G.671 Amendment 1, “Transmission characteristics of opticalcomponents and subsystems” (ITU, 2006).
13. A. Cauvin, A. Tofanelli, J. Lorentzen, J. Brannan, A. Templin, T. Park, and K. Saito,“Common technical specifications of the G-PON system among major worldwide accesscarriers,” IEEE Commun. Mag. 44, 34–40 (2006).
14. http://www.ieee802.org/3/av/.15. R. Davey, J. Kani, F. Bourgart, and K. McCammon, “Options for future optical access
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