Special Feature

Optical Access Network Architecture that Enables
Prompt Construction to Meet Growing DemandTamotsu Yonemoto†, Masahiro Sonobe, Hiroshi Kigami,
Yukio Hasegawa, and Shigehiro Miura
Abstract
This article introduces the optical access network design technologies we are using to achieve NTT’s
target of 30 million optical subscribers by FY 2010. It also describes our investigation of how to expand
optical service coverage afterwards.

1. Current optical access networks

2. Existing optical access network architecture

Optical access networks can be implemented withvarious topologies, including single star, passive double star, and active double star (Fig. 1). In the passive
double star, optical splitters are used to connect multiple subscribers via a single optical fiber, and passive
optical elements are used to configure the opticalaccess network. Hence, it is also referred to as a passive optical network (PON).
In the optical access network of the B-FLET’S service provided by NTT, we reduced the initial construction costs by using a PON configuration with
four-branch optical splitters in NTT’s central offices
and eight-branch splitters outside. Since its launch in
2001, the B-FLET’S service has steadily gained subscribers, and the number reached 3.42 million by theend of FY 2005. In FY 2006, we aim to add a further
2.7 million subscribers in the regions handled by
NTT East and West towards meeting our target of 30
million optical subscribers by FY 2010. To respondpromptly to this growing demand for optical services(which resembles the large-scale growth of telephoneservices in the last century), it is becoming essential
to establish optimum optical access network design
technologies to ensure that the network is developed
more efficiently based on economical planning.

Here, architecture means the design technologies
used to enable rising demand to be accommodated
efficiently and economically in NTT’s central offices.
When optical cables are installed, it is cost-effective
to establish individual connections between each customer and an NTT central office when there is low
demand. However, as demand increases, this could
lead to overcrowding of facilities, more difficult facility management, and the same engineering work
being performed for each customer. We haveachieved efficient construction, operation, and management of optical access network facilities by establishing optimum management units (Fig. 2).

† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
Email: yonemoto@ansl.ntt.co.jp

182.1 Distribution area
The distribution area is the area covered by an 8branch optical splitter outside an NTT central office.
When demand is low, larger distribution areas yield ahigher splitter utilization rate and allow the opticalcables and equipment in NTT’s central office to beused more efficiently. On the other hand, when
demand is high, smaller distribution areas are more
economical because they result in shorter lengths ofdrop fibers* between the optical splitters and customers’ premises (Fig. 3).

* Drop fiber: a single fiber, mainly used from an outside splitter to acustomer’s premises.

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Special Feature

Optical access networks
Central office
Splitter
OLT

CustomerpremisesAccess section
Optical cable

ONU
ONU
ONU
MCMC•••

Single star

Characteristics

MCMC

Splitter
Passive doublestar

OLT

Optical cable

ONUONU•••

Topology

Optical cable

ONURT
CT

Optical cable

NTNT•••

Active doublestar

Metallic cable

Initialconstruction cost

Multiple
services

Testing

Fair
Optical fiber
needed foreach subscriber

GoodGood
Multiple
End-to-end
services can testing is
be provided. possible.

ExcellentMore efficient
accesssections andequipment incentral office

Fair
Fair
Splitter must Problems of
be changed. testing andfacility
management
between splitterand customer’s
premisesGood
More efficient
access sections

Fair
Can provide
only lowspeed
servicesNT

Fair
Nodes (O/Econversion)requiremaintenance.

OLT: optical line terminater
ONU: optical network unit
MC: media converter
CT: central terminal
RT: remote terminal
NT: network termination
O/E: optical-to-electricalFig. 1. Topology of optical access networks.

Low penetration of FTTH service

High penetration of FTTH service

Basic points
1. Facility management units– Fixed distribution area
– Distribution area
2. Fiber structure– Four-fiber ribbon or
single-fiber
3. Distribution configuration– Drop fiber/cable

Efficient constructionof facilities
Distribution cable

4. Quantity of facilities– Number of optical fibers

Feeder cable
FTTH: fiber to the homeFig. 2. Optical access network architecture.

2.2 Fiber structureAerial optical cables include four-fiber ribbon
cables and single-fiber cables. When we use a fourfiber ribbon cable, we need to separate the ribbon’sfibers at the point where a fiber is needed. The
Vol. 5 No. 2 Feb. 2007

remaining fibers in the ribbon can be used only toserve homes in that immediate area. So when demandis low, it is inefficient to use four-fiber ribbon cables.
However, four-fiber ribbon cables have the advantageof simpler multi-fiber connections. When demand is19

Special Feature

Facility architectureCost trend(1) Area scale: large

Cost per subscriber

Size of distribution area

(2) Area scale: small

Large-scale

Small-scale

Low

Penetration of FTTH service

HighFig. 3. Distribution area.

Facility architecture

Cost trend(2) Single fiber

Cost per subscriber(1) Four-fiber ribbon
Fiber structure
RibbonSingleLow
High
Penetration of FTTH service

Fig. 4. Fiber structures: comparison of four-fiber ribbon and single-fiber.

high, four-fiber ribbon cables are economical because
they do not need as many connections as single-fiber
cables. Therefore, single-fiber cables are more economical in regions of low demand and four-fiber ribbon cables are more economical in regions of highdemand (Fig. 4).
2.3 Distribution area configuration
Optical cables tend to cost less per fiber as the number of fibers in the cable increases. Accordingly,
although it is more economical to distribute opticalfibers individually from the optical splitter to eachcustomer’s premises when demand is low, it is more
20

economical to distribute the optical fibers from thenearest telegraph pole via cables when demand ishigh (Fig. 5).
2.4 Number of fibers required in a fixed
distribution area
The number of optical fibers to be constructed
depends on the target penetration of FTTH (fiber tothe home) and the complexity of the distribution network. As the distribution network becomes more
complex, the number of fibers required becomes larger. When four-fiber ribbons are used, even more
fibers are required; therefore, the utilization rate ofNTT Technical Review

Special Feature

Facility architecture

Cost trend88(2) Optical cable + drop fiber

Cost per subscriber(1) Drop fiber only
Configuration indistribution area

Drop only
Cable + dropLowHigh
Penetration of FTTH service

8Fig. 5. Configuration in distribution area (drop/cable).

four-fiber ribbon is inefficient when demand is low.
3. New optical access network architecture
Since the demand for FTTH has reached a substantial level, we have developed a new optical networkarchitecture that enables prompt construction to meetthe growing demand by focusing on the following
two ideas.
(1) Optical splitter utilization rate
When demand is low, the number of subscribers
accommodated in an eight-branch optical splitter isalso low, resulting in a low splitter utilization rate. Ina PON configuration, the service’s systems and optical splitters are closely related to each other, so new
optical splitters have to be added separately when services are added or modified. Consequently, when
new services are provided in a short period of time,
the cost of the facilities becomes relatively high without any increase in the optical splitter utilization rate.
An effective method of increasing the optical splitter
utilization rate is to expand the area covered by the
optical splitter, i.e., the scale of the distribution area.
However, simply increasing the scale of the distribution area results in a longer length of drop fiber fromthe optical splitter to the customer’s premises. That is,there is more engineering work involved in meetingthe service demand.
(2) Service demand work
In the current distribution configuration, wheredirect drop fibers are distributed to customers’
premises from the optical splitters, the drop fibers
Vol. 5 No. 2 Feb. 2007

must span several telegraph poles, which means that
service demand work will be a considerable burden in
the future when demand grows. This service demand
work can be reduced if the length of drop fibers isreduced by using an architecture where cables are
pre-routed from the optical splitters so that dropfibers have to be installed only in the section between
the telegraph pole nearest to the customer and thecustomer’s premises.
To implement these two ideas, we conducted acomprehensive study of design parameters such as
the structure of distribution areas and the fiber structures mentioned in section 2, and we developed andintroduced a new distribution area configuration thatcombines cables and drop fibers, as shown in Fig. 6.
We have also developed facility management methods and products (aerial optical closures for distribution/dropping points and optical cables) that areneeded to introduce the new distribution configuration. For example, Fig. 7 shows how the number offibers required in aerial optical cables varies with themarket penetration of FTTH and the distribution configuration from the optical splitter to the customer’spremises. We developed optical cables with the optimum number of fibers based on this distribution of
the required number of fibers.
4. Optical access network architecture for
expanding optical service coverage
In the construction of optical access networks, it isalso important to consider the characteristics of each21

Special Feature

CurrentNew
Distribution area configuration (drop)
Distribution pointCable

Distribution area configuration (cable + drop)

Drop fiber

Distribution point Cable Dropping point Drop fiber

Optical cable

Optical cable

8888

Drop fiber

8

Drop fiber

Fig. 6. Distribution area configuration.

Market penetration of FTTH
20%

40%

60%

Cable extension (relative value)

Distribution of drop optical fiber
Distribution of optical fiber cableand drop optical fiber

4 8 24 40100 4 8 24 40100 4 8 24 40100

Required number of fibers

Fig. 7. Distribution of the number of fibers required in aerial optical cables.

area, such as the subscriber density and service coverage area. In practice, they differ widely amongareas, as shown in Fig. 8. The B-FLET’S service has
been preferentially provided in urban regions with ahigh density of subscribers, and by the end of FY
2005, approximately one quarter of all NTT’s central
offices were providing this service. To achieve aneven greater spread of FTTH, we will have to investigate an optical access network architecture that can22

be applied to areas with a low subscriber density andto wide areas outside urban regions. In particular, theprovision of FTTH services in rural areas where there
is an extremely low subscriber density has been taken
up as a target in NTT Group’s Mid-Term Management Strategy (announced in November 2004), in theplan of the Ministry of Internal Affairs and Communications to introduce next-generation broadband by2010 (announced in July 2005), and in the governNTT Technical Review

Special Feature(2) States of service provision(grouped by municipal population scale)(1) Subscriber density

Large

Area (number of telegraph poles)

SmallLow Subscriber density (subscribers per pole) High

23 wards in Tokyo
Government-designated cities
Cities of at least 100,000

Other cities

Area characteristics• Small area and high subscriber
density: 23 wards of Tokyo• Large area and low subscriber
density: other cities

Less than
6.9% 8.6%
10,000(34) (42)(491 groups)
10,000 to10.4%
50,000(82)(788 groups)

84.5% (415 groups)

52.7% (415 groups)

36.9% (291)

50,000 to100,000(282 groups)

22.7%(64)100,000 to300,000(200 groups)

30.0% (60)

69.5% (139 groups)

over 300,000(82 groups)

29.3% (24)

70.7% (58 groups)

All cities(1843 groups)

0%

14.3%
(264)

68.1% (192 groups)

39.2% (722 groups)

9.2%(26)
0.5%(1)

46.5% (857 groups)

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Customers can subscribein all regions
in some regions

in no regions

Reference:
The Ministry of Internal Affairs and Communications plan for the
introduction of next-generation broadband by 2010

Fig. 8. Area characteristics.

ment’s IT (information technology) strategy unit’s IT
New Reform Strategy (announced in January 2006).
We are therefore engaged in the following studies for
expanding optical service coverage.
(1) Further expansion of distribution areas
We are investigating methods of increasing the utilization rate of optical splitters by further expanding
the distribution area in our new distribution configuration.
(2) Application of 2-branch optical splitters
We are investigating methods of sharing a single
optical fiber between multiple customers by using
cascaded 2-branch optical splitters in areas wheredemand is thinly distributed over a wide region.
For these two issues, we must study ways of optimizing the total capital expenditure and the operating
expenditure, not only of the optical access networkarchitecture but also of the optical access network as
a whole, including the costs of products/construction,maintenance/operation, and facility management.

Vol. 5 No. 2 Feb. 2007

5. Future plans
In service demand work for B-FLET’S service,
there have been concerns that the construction of dropfibers across multiple telegraph pole spans might hinder efforts to improve efficiency. However, the new
optical access network architecture that we are developing to enable prompt construction to meet growing
demand will substantially reduce the amount of construction work involved in network deployment. It
will also enable us to increase the utilization rate of
optical splitters relative to that in times of low
demand. The ease of installation of related products
has also been improved to make them more modularand easier to connect. The optical access networkarchitecture forms the basis upon which opticalaccess networks are constructed and operated. In thefuture, these networks will continue to play an important role as we make cooperative efforts in researchand development to adapt to the needs of business andsociety.

23Special Feature

Tamotsu Yonemoto

Yukio Hasegawa

Research Engineer, Access Media DesignGroup, Access Media Project, NTT Access Network Service Systems Laboratories.
Since joining NTT Access Network Service
Systems Laboratories in 2004, he has been
actively engaged in developmental research of an
optical access network architecture.

Research Engineer, Access Media DesignGroup, Access Media Project, NTT Access Network Service Systems Laboratories.
Since joining NTT Access Network Service
Systems Laboratories in 2006, he has been
actively engaged in developmental research of an
optical access network architecture.

Masahiro Sonobe

Shigehiro Miura

Research Engineer, Access Media DesignGroup, Access Media Project, NTT Access Network Service Systems Laboratories.
Since joining NTT Access Network Service
Systems Laboratories in 2003, he has been
actively engaged in developmental research of an
optical access network architecture.

Senior Research Engineer, Supervisor, AccessMedia Design Group, Access Media Project,NTT Access Network Service Systems Laboratories.
Since joining NTT Access Network Service
Systems Laboratories in 2003, he has been
actively engaged in developmental research of an
optical access network architecture.

Hiroshi Kigami
Research Engineer, Access Media DesignGroup, Access Media Project, NTT Access Network Service Systems Laboratories.
Since joining NTT Access Network Service
Systems Laboratories in 2004, he has been
actively engaged in developmental research of an
optical access network architecture.

24

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