Special Feature

Trends in Optical Access Network Technologies
Toward “30 Million Optical Subscribers by 2010”
Osamu Yamauchi†
Abstract
With NTT group’s medium-term management strategy calling for
30 million optical subscribers by 2010, NTT Laboratories is pushing
forward to meet this goal. Before that date, an efficient optical accessnetwork will have to be constructed, and afterwards, when the era ofmass optical communications finally arrives, the facilities and equipment supporting the network will have to be operated and maintained
appropriately. This paper gives an overview of new technologies thatare being developed to address these issues and provides the overall
context for these technologies, which have been described separately
in detail in previous issues of NTT Technical Review.

1. BackgroundAs the development of optical fibers and opticalcables continues to progress and practical opticalfacilities are deployed in rapid succession, thedemand for B-FLET’S broadband services has been
increasing rapidly. Dealing effectively with the
installation work that this entails and reducing associated costs have become major issues.
The number of subscribers to B-FLET’S and otherfiber to the home (FTTH) services has been increasing dramatically and now stands at more than 3.4 million (as of June 2005), which means that most of the
FTTH subscribers in the world are in Japan. Looking
at FTTH use by region, the Tokyo, Nagoya, andOsaka areas stand out in terms of subscriber numbers.
Subscribers to the B-FLET’S “Mansion Type” service option for multi-dwelling apartment buildings
make up a large percentage of FTTH use in large metropolitan areas, while B-FLET’S for individual houses tend to be prevalent in suburban and rural areas. In
general, the percentage of Internet-connection services made up of broadband services has a high cor† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
E-mail: o.yamauchi@ansl.ntt.co.jp

Vol. 4 No. 4 Apr. 2006

relation with population size, as shown in Fig. 1 andbroadband has a higher penetration in municipalities
with higher populations. This trend is particularly
noticeable with FTTH, which is already being provided in more than 90% of municipalities with
50,000 people or more, but in only 3% of rural areas
with populations of less than 5000 people. From theviewpoint of eliminating the digital divide, these figures represent a serious problem.
Looking back at policies on the national level, the“e-Japan Strategy” initiated in 2001 focused on theconstruction of a broadband infrastructure, while the“e-Japan Strategy II” promoted effective and practical use of broadband services. The aim was first to
catch up and then provide a world-leading communications infrastructure by 2005. This was actuallyaccomplished faster than expected, so in 2004, thegovernment proposed the “u-Japan Strategy” advocating movement toward high-value services including ubiquitous networks with the goal of Japan
becoming a world leader in this field by 2010. This
new strategy represented a significant shift from acountry in catch-up mode to one intent on becoming
a frontrunner.
At present, however, there are still many regions inJapan without broadband access, and even in thoseareas where broadband services is claimed to be21

Special Feature• Provision of broadband services has a correlation with population size
• FTTH services show largest gap between low and high populations(as of March 2005)
Penetration of FTTH
services
Under 5000
people(676 groups)

3.0

97.0

5000-9999
people(783 groups)

11.6

88.4

10,000-49,999
people(1186 groups)

36.5

50,000 or
more people(478 groups)
0%

54.7

63.5

93.1

6.9

Not yet provided

45.3

92.3

7.7

99.7

0.3

13.3

86.7

18.0

82.0

30.4

20% 40% 60% 80% 100% 0%

69.6

80.8

100.0

20% 40% 60% 80% 100% 0%

Already provided

Penetration of cable Internet
services

Penetration of ADSL
services

19.2

20% 40% 60% 80% 100%

Source: White Paper, Information and Communications in Japan, 2005

Fig. 1. Provision of broadband services by population size.

already provided, a high percentage of users are able
to use only relatively low-speed broadband. In
response to this situation, Japan’s Ministry of Internal
Affairs and Communications (MIC) has proposed thefollowing goals in the form of the “Next-Generation
Broadband Concept 2010”.
• Eliminate the digital divide and broadband-deficient regions and make broadband services available to 100% of the nation’s people.
• Make ultrahigh-speed interactive broadband(UIBB) featuring upstream speeds of 30 Mbit/s orhigher available to at least 90% of all households.
Against this background, NTT group’s mediumterm management strategy calls for “30 million optical subscribers by 2010” in line with the u-Japan
Strategy. In short, NTT aims to provide optical services to 30 million users, which corresponds to about
half of the current number of fixed-line subscribers.
Thus, with its sights set on the coming of an era ofmass optical communications, NTT views R&D
efforts toward the construction of a highly efficient
optical access network to be a matter of great urgency.
2. Configuration of the optical access network
We considered the following three types of opticalaccess network topologies.
• Single star (SS)This is the traditional format featuring 1-to-1 connections between the central office and users (thesimplest structure of the three).
22• Passive single star (PSS)This topology features the placement of an optical coupler (splitter) on the office side so that a single optical line terminal (OLT: terminating equipment on the office side in an optical-fiber subscriber communications network) can be shared bymultiple users. In this system, fibers are still used in
a 1-to-1 manner and each user pays for what he/sheuses.
• Passive double star (PDS)
In this format, a splitter is also placed on the userside so that a fiber running from the splitter to theoffice can also be shared by multiple users in addition to the OLT.
Of these three topologies, PDS allows both an OLT
and a fiber to be shared by multiple users, so this system has the lowest initial costs. For this reason, PDS
has become the main topology in use.
From the viewpoint of constructing efficient network facilities, it is important to select a “wiring
method” for efficiently handling a wide variety ofusers at a central office. This wiring method should
include specifications for:• Unit of facility management (fixed distributionarea, dropping area)• Operation method of optical fiber (ribbon/singlefiber)• Subscriber cable-drop-off method• Number of fibers in the cable core
For example, if a fat cable is deployed all at once
even though demand is still low, each user will have
NTT Technical Review

Special Feature

to bear a high initial cost. On the other hand, deploying slender cables as new subscribers are added eventhough demand is expected to be high is extremely
inefficient. In short, the scale of facilities for optimizing cost depends on the number of users. Cost
also varies with the size of the dropping area. If
demand is small, a larger dropping area will lower thecost per user, but if demand is growing, a smaller
dropping area will lower that cost. An optimal dropping area must therefore be decided upon by weighing the number of users against the scale of facilities
and finding the minimum point for achieving that balance.
The main pieces of equipment composing an optical access network are shown in Fig. 2. To begin with,the equipment within the central office consists mainly of an integrated distribution module (IDM) capableof terminating 4000 optical fibers, a 4-branch optical
splitter, and a system called AURORA (automatic
optical fiber opeation support system) that includes
an optical testing module. The AURORA systemenables a maintenance center to test and monitor optical access paths remotely. It can be used to perform
optical pulse tests and optical-path-loss tests whenconstructing a network, fault-isolation tests when
performing maintenance, periodic testing to check forwater penetration and other problems, and test lightinsertion to check the condition of optical fiber.
In the underground segment (ducts and cable tunnels), plastic-sleeve-type closures are used at cable
jointing points, and water-blocking cables in which
optical fiber ribbons are stacked in slots in the cablecore are used to simplify maintenance.
In the aerial segment, an aerial optical closure is set
up at the feeder point. In this segment, slotless slender cables are the most commonly used cable typebecause they lower costs and make cable deployment
more efficient. At the end of this segment, an 8branch optical splitter is installed for branching toindividual user homes. Close to a user’s house, there
will be an aerial optical cable with a small number offibers as well as an aerial optical closure for cable
dropping, an optical drop cable, and an optical cabinet for running an optical fiber into a house [1].
3. Future direction of R&D
Future NTT R&D in optical access network technologies will be divided into two phases. Phase 1 willconsist of immediate support toward an era of mass
optical construction. Phase 2 will cover medium- andlong-term development work after the target of 30
million optical subscribers by 2010 has been reached.
In Phase 1, the focus will be on prompt installationand cost reduction as urgent issues that must be dealt
with prior to 2010. Specifically, there will be a needfor a do-it-yourself (DIY) approach and simplified,easy-to-use systems. Next, in Phase 2, which will
actually begin slightly before 2010, there will be aneed to maintain a large user base efficiently as partof a period requiring the operation and maintenanceof a large number of optical facilities. Targets in thisphase will include the reduction of field operations by
developing ways to perform installations work with-

ONU

OLT
8-branch optical splitter

4-branch optical splitter
Central office

Individual
house

Distribution
point

Feeder
point
IDM

OLT

ONU
AURORAHouse(premises)

AerialUndergroundFig. 2. Configuration of an optical access network.

Vol. 4 No. 4 Apr. 2006

23

Special Featureout dispatching workers to the costomer’s premises
and the elimination of manual database input toincrease the accuracy of facility databases, which are
the foundation of network maintenance and operation.

ty was high-performance fiber when the existing
optical trunk system was constructed. However, since
the introduction of optical access equipment in thesecond half of the 1990s, the need for low-cost systems has been growing. The coming era of mass optical communications will no doubt require simplicity
and even further cost reductions. One effective means
to this end is a DIY approach because it brings two
benefits: It simplifies installation work, so opticalcable installation time can be shortened and installation expenses can be reduced, and enables the work
involved in wiring on premises to be done by the user
himself, which decreases the cost to the user.

3.1 Technologies for mass optical construction(Phase 1)
In recent years, it has frequently been said that fiber
optic research has just about exhausted itself. Indeed,as shown in Fig. 3, various technologies have beensteadily improving. For example, under fiber class,single-mode fiber has reached the completion stage,and under fiber/ribbon structure, the trend is toward
more slender and thinner configurations. Likewise,for cable structure, cable waterproofing, and fiberconnection, technology has been advancing significantly. Thus, in future R&D, instead of efforts todevelop new elemental technologies, the emphasis
will be on enhancing the practical application ofexisting technologies. It will be necessary to investigate ways of using the optical network more efficiently, reducing field operations, and making network operations more efficient.
Although the three pillars of development are highperformance, low cost, and simplicity, the top prioriFiber class

Graded index

1989

Make fiber
common withtrunk system

3.2 Technologies for mass operation andmaintenance (Phase 2)
When the period of operating and maintaining
numerous optical facilities actually arrives, we can
expect to see an increase in tasks associated with service orders, optical-fiber cable removal work, andfault handling. Consequently, in addition to reducing
the cost of product development and facility deployment as in the past, there will also be a great need toreduce the lifetime cost (operating cost) of the optical
network.

Fiber/ribbon
structureCable
structure

0.9-mm single fiber

Bundling

Reduce coating
thickness andadopt masssplicing
0.4-mm ribbon

Single-mode(40 fibers)(100 fibers)(300 fibers)

0.3-mm ribbon

Eliminatedifficulty ofbundling

Fiberconnection
Mono-fusion splicing

Gas maintenancefor waterproofing
Make connection
work more
efficient

Slotted rod

Improve usage
rate of cables
with fewer fibers

1993

Cable
waterproofing

Mass-fusion spicing
Make maintenancemore efficient by
remote monitoring
No gas maintenancefor waterproofing(1000 fibers)

MT connector
Mechanical splicing

0.25-mm single fiber
1998
Application
development
based on existing
technologies• Technologies for mass optical communications• Technologies for mass operation and maintenance• Adopt DIY approach• Enable skill-free installation• Reduce lifetime cost

Fig. 3. Technology flow toward FTTH.

24

NTT Technical Review

Special Feature

4. Recent development achievements
4.1 Developments toward coping with demandfor optical accessTo achieve a true era of mass optical communications, we must develop products and components thatanyone can handle and must simplify construction
and installation techniques. Several aspects of thecurrent optical access network are inconvenient.
• Storing excess optical fiber in a cabinet and aerial
optical closure is time consuming and troublesome• Transferring fibers at a closure due to changes inservice/carrier is time consuming and troublesome• Troubleshooting at a cabinet is difficult• Wiring inside a house is complicated
If the storage of excess optical fiber could be madeunnecessary, problems associated with the housing offiber could be eliminated. Likewise, if fiber connections could be achieved through the use of connectors, transfer operations could be simplified. In addition, the development of cables with a smaller bending radius that are consequently easier to use shouldmake wiring easier to carry out. The development ofvarious optics-related components to meet theseneeds is moving forward at NTT (Fig. 4).
Of particular interest here is a cable sheath holding
connector, which can be directly attached to a cable.
So there is no need to deal with excess lengths of optical fiber [2].
To assemble this field-assembly (FA) connector, we

cut the optical fiber, insert the optical fiber into theconnector, and pull out the wedge fixing the fiber.
The cable sheath holding connector is located in a
newly developed optical cabinet. The use of this connector in conjunction with highly bendable opticalcable enables the cabinet volume to be reduced by asmuch as 65% compared with the existing optical cabinet. This is because the existing optical cabinet,though also including a mechanical splice, may haveto store up to 3 m of wound up excess optical fiber.
The new optical cabinet, in contrast, has no need tostore bare optical fiber making the previous storage
space unnecessary and shortening installation time
(Fig. 5). Similarly, for the aerial optical closure,application of cable sheath holding connectors andsplitter modules eliminates the need to store excess
optical fiber, which makes installation more efficient(Fig. 6).
Another new technology is hole-assisted fiber(HAF) [3]. This type of fiber has a number of holesarranged about the core, which produces a strong
light-confinement effect. In conventional opticalfiber, the optical loss factor increases rapidly once thefiber’s bending radius drops below 20 mm. In HAF,
however, almost no loss occurs even for small bending radii. The use of HAF-based free-bending opticalfiber cord for optical wiring inside a house can makewiring so simple that even ordinary users will be able
to do it by themselves. Free-bending optical fiber
cord can withstand not only being bent, bundled, andTo achieve mass penetration of NTT’s B-FLET’S optical broadband services, opticalcomponents and installation techniques that anyone can deal withmust be developed andtheir maintainability must be improved by preparing facilitytroubleshooting points.
Segment

OutsideofficeProblem

Countermeasure

Target component

Fiber storage in an AO
closure is troublesome

Eliminate need tostore fiber

New AO closure for
optical fiber distribution

Transferring fibers at a
closure to accommodate
service/carrier changes
is troublesome

Enable fiber connection
by connectors

AO fiber connector

Eliminate need tostore fiber

Technology
convergence
Cable sheath holding
connector

Storing fiber in acabinet/ONU istroublesome

Insidehouse

Troubleshooting at acabinet is troublesome

Enable fiber connection
by connectors

Wiring on premises iscomplicated

Reduce cablebending radius

Optical connector
cabinetFree-bending opticalfiber cord
AO: aerial opticalFig. 4. Coping with demand for optical access: problems and solutions.

Vol. 4 No. 4 Apr. 2006

25

Special Feature• Cable sheath holding connector enables direct connection to optical fiber cable(⇒eliminates need to store excess optical fiber)• Joint use of cable sheath holding connector and highly bendable optical cableenables downsizing of optical cabinet (⇒65% reduction in volume)
Features
Existing cabinet
MechanicalspliceOptical connector cabinet
Cable sheath
holding
connector(FA connector)

150 mm77 mm
Highly bendable
optical cables
183 mm

227 mmBare optical
fiber

Component forstoring bare
optical fiber

15 mm

No storageof excessoptical fiber

No component
for storing bare
optical fiber

Fig. 5. FA connector and optical connector cabinet.

• Installation is simplified through use of modular parts.
• Service-order processing time is shortened by making all fiber connections by connectors andeliminating storage of bare optical fiber.
• Carrier and service changes can be made without having to replace drop cable.
• Can be applied to existing cable routes in addition to new installations.

Existing AO closure

New AO closure for optical fiber
distribution

Fig. 6. New aerial optical closure for optical fiber distribution.

knotted, but also being folded, so it enables opticalwiring in the home to be laid in a more attractive
manner (Fig. 7).
4.2 Developments toward reducing lifetime cost
Various problems must be solved to reduce theoperating cost after optical facilities have been constructed (Fig. 8). To begin with, the application of
two-dimensional barcodes or other means of identification can be effective in managing facilities outside
the central office [4]. For example, the camera func26

tion of a cellular phone can be used by a field worker
to send on-site information back to the office forcomparison with a database and to receive the resultsof that query. Such queries could even be performed
while a worker is up a telephone pole. It is also conceivable that a cellular phone could be used to activate test equipment within a central office andreceive test results.
Progress is also being made in the development of
remote testing technology with the aim of reducing
field operations. The present method of conducting
NTT Technical Review

Special Feature

Allows bending, folding, and knotting of optical fiber as in metallic wiring
Application to opticalcabling in house
Right-angle bend Folded state

KnottedBundled state

PC
ONU
HAF
Core

Hole

Optical rosette

Cladding

Fig. 7. Free-bending optical fiber cord.

Operating costs after constructing optical facilities must be controlled in relation to SOs, cablemoving, and fault handling.
Segment

Serviceorder

Problem

Countermeasure

Target component• Wiring management
dependent on physicaltags creates cablecongestion in central office• There are no ID tags thatcan be safely and easily
attached to existing wiring
facilities• Inexpensive add-on
tags using lasermarking technology• Add-on tags andtools having no effecton existing circuits

Add-on tags, tools for
applying add-on tags

Cable
moving• Staff must be dispatched
to user’s house to perform
a medium test

Fault
handling• Quick troubleshooting andfault isolation is difficult
past an outside splitter• Technology for
conducting opticalpath-loss tests froman NTT office

Expand use of ID-code
management technology
in combination withcellular phones and PDAs
Technology for measuring
optical loss by comparingreflection from an opticalfilter against a referencevalueHigh-resolution OTDR

Fig. 8. Reducing lifetime cost: problems and solutions.

optical-path-loss tests after optical-fiber cables havebeen moved is to dispatch workers to the user’s
house. The key to enabling remote operations from acentral office is to reflect a signal from an optical filter placed just before the user’s optical network unit
(ONU). Furthermore, if the same type of optical filter
is installed inside the office, the difference in reflection levels in both directions can be read, enabling atrue optical-path-loss test to be performed without
Vol. 4 No. 4 Apr. 2006

requiring a visit to the user’s house.
Furthermore, it is difficult to identify a faulty
branched optical fiber using conventional OTDR
(optical time domain reflectometer), because reflections from optical filters are included in the OTDR
trace. We developed high-resolution OTDR that canidentify a faulty fiber because the reflections are separated. So we can isolate the faulty fiber without dispatching any workers.
27

Special Feature

5. Conclusion
As FTTH continues its rapid spread and becomes amore familiar means of connecting to the network,“ease of use” will become a matter of prime importance. And as the need to respond quickly to risingdemand for FTTH also grows, ways to reduce thetotal cost must be found. Recognizing the urgency ofthis situation, we are focusing our R&D efforts on
implementing DIY schemes as a key technique while
reducing lifetime cost.

Profile■ Career highlights
Project Manager, Promotion Project 2, NTT AccessNetwork Service Systems Laboratories.
He received the B.E. and M.E. degrees in telecommunication engineering from Tohoku University,
Miyagi in 1978 and 1980, respectively. He joined Nippon Telegraph and Telephone Public Corporation(now NTT) in 1980. Recently, he has been engaged in
the development of optical fiber cable systems.

References
[1]

[2]

[3]

[4]

28

S. Kiyosue, M. Hashiguchi, N. Ogawa, and M. Awamori, “Development of Aerial Optical Cables with a Small Number of Fibers andAerial Optical Closure for Connecting the Cables,” NTT Technical
Review, Vol. 3, No. 6, pp. 61-64, 2005.
H. Aoyama, H. Tanaka, Y. Hoshino, and Y. Oda, “Optical Wiring
Technology for Home Networks for a Service-ready and Low-cost
FTTH Service,” NTT Technical Review, Vol. 3, No. 4, pp. 33-37,
2005.
K. Tajima, K. Kurokawa, K. Nakajima, and I. Sankawa, “Photonic
Crystal Fiber,” NTT Technical Review, Vol. 3, No. 8, pp. 57-61,
2005.
S. Uruno, H. Izumita, and M. Arii, “Optical Wiring Facilities Identification Technology for Preventing Congestion in Intra-office Optical
Wiring,” NTT Technical Journal, Vol. 17, No. 8, pp. 55-57, 2005 (inJapanese).

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