Special Feature: N T T R & D F o r u m 2 0 0 5 W o r k s h o p

NTT R&D Forum 2005
Workshop Lectures
The special feature in this issue presents five workshop lectures given at NTT R&D Forum2005 held at Yokosuka R&D Center on Feb. 24 and 25. They followed the keynote lectures“Actions to Achieve a New Broadband Society” by Norio Wada, President and CEO, NTT, and“R&D to Create the Resonant Communication Environment” by Yuji Inoue, Senior Vice
President and Executive Director of Department III, NTT. The workshop lectures review thetrend and state of the art of NTT’s most important technologies, describe NTT’s R&D efforts,
and predict their future development. They were originally given in Japanese.

Lectures were:• Access Network Technology for Diverse Services
by Takamasa Imai, NTT Access Network Service Systems Laboratories• Optical Core Networks
by Kazuo Hagimoto, NTT Network Innovation Laboratories• Secure Technology
by Tomoo Fukazawa, NTT Information Sharing Platform Laboratories• Home Network & Networked Appliance Technology
by Hisashi Ibaraki, NTT Cyber Space Laboratories• Digital Mediaby Hiroshi Fuju, NTT Cyber Space Laboratories

References• N. Wada, “Actions to Achieve a New Broadband Society,” NTT Technical Review, Vol. 3, No. 4, pp. 6-15,
2005.
• Y. Inoue, “R&D to Create the Resonant Communication Environment,” NTT Technical Review, Vol. 3, No. 4,
pp. 16-25, 2005.

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Lectures

Access Network Technology for Diverse Services
Takamasa Imai†
Abstract
This article introduces some access network technologies developed
for various different services. These are recently developed technologies for Gigabit Ethernet passive optical network (GE-PON) systems,
optical access system installation, and broadband wireless systems.
Technologies for future systems are also described.

1. Trends in broadband services
Since the 1990s, the evolution of the information
era has been described using a series of keywords
such as mobile, multimedia, Internet, and ubiquitous.
During this period, the demand for communications
has grown remarkably as services have changed fromcircuit-switched services for fixed telephones toInternet services using broadband technologies suchas xDSL (digital subscriber lines), FTTH (fiber to thehome) and CATV (cable TV).
As we move closer to a ubiquitous network society,the dominant form of communication will change toobject-to-object from the conventional human-toobject and human-to-human types. Therefore, theamount of Internet traffic will grow dramatically inthe near future. According to the experts, there areseveral key issues to be considered to achieve broadband and ubiquitous services: 1) security, 2) technicaldevelopment to achieve high-performance services,and 3) the provision of a suitable infrastructure toachieve reasonably priced services that are easy toinstall and use. Meanwhile, ordinary users also tend
to list security, reasonable price, large capacity (highperformance), and ease of use as important factors.
Furthermore, there are clearly very large regional dif† NTT Access Network Service Systems Laboratories
Chiba-shi, 261-0023 Japan
E-mail: imai.takamasa@ansl.ntt.co.jp

Vol. 3 No. 6 June 2005

ferences in the progress of FTTH deployment inJapan. FTTH access services are significantly used in
the suburbs of Tokyo and Osaka. However, they arestill not popular in other regions. For example, when
the percentage of homes with FTTH access was 3.5%
in Tokyo, that in Aomori prefecture was only 0.2%.
Accordingly, we need to consider these key issues indeveloping optical access technologies. It is alsoessential to conduct research and development toredress this regional imbalance.
2. Current state of access network technology
Now NTT plans to deploy a resonant network,
which consists of a service/network control platform,
core transmission networks, and access and user networks. For the access networks, a combination of
optical and wireless access techniques are promising.
For the successful introduction of broadband communication, we must reduce the cost of the accesssystems to a reasonable level and ensure that the systems have high performance and are easy to installand use. Here, I review some of the main technologies that have already been implemented for practicalaccess systems in response to user demands.
2.1 Optical access systems
Optical fiber transmission systems have a large
impact on telecommunication networks owing to theremarkable advantages of optical fiber cables such as
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Special Feature

low loss and broad bandwidth, which enable longdistance and large-capacity communication. However, the final connections between an optical network
and a home have still relied on metal wire transmission until recently. Recently developed optical accesssystems employing passive optical network (PON)
technology provide end users with optical fiber communication. In a PON, the time division multiple
access (TDMA) technique enables transmission
between equipment in the central office and a large
number of users’ devices. Moreover, most opticalfiber cables can be shared among users. The first
PON system was introduced in 1997. The most recent
system delivers a transmission rate of 1 Gbit/s via an
Ethernet interface, which is widely used for datatransmission in homes (Fig. 1). The main technology
employed in the system, Gigabit Ethernet PON (GEPON), includes the key basic technologies of encryption and burst mode optical reception and the highperformance technologies of bandwidth and delay
control.
(1) Encryption
PON downstream transmission is a broadcast, so aframe destined for a certain optical network unit
(ONU) in a user’s home also reaches other ONUs. To
prevent other ONUs from eavesdropping and analyzing the frames not destined for them, encryption is
performed and a different encryption key is used foreach ONU.

(2) Burst-mode optical reception
For upstream transmission, the system is designed
to avoid collisions among the signals from the large
number of ONUs. The central office equipment isdesigned to receive burst signals whose power levelvaries according to the distance between the opticalline terminal (OLT) and ONU (Fig. 2).
(3) Bandwidth control
Most users do not need a fixed large bandwidth all
the time. They sometimes want a large bandwidth but
normally need only a smaller bandwidth at othertimes. To match these needs, NTT has developed asystem that guarantees a minimum bandwidth foreach user, while additional bandwidth is delivered ona best-effort basis. This makes it possible to implement an economical system that can provide a large
bandwidth when other users do not need so much.
(4) Delay control
NTT has developed a system that can be used fordelay-sensitive services such as VoIP (voice over
Internet protocol) or video communication. The system can choose one of two delay classes, either lowdelay class for high transmission priority or normaldelay class for normal transmission priority, according to the delay requirement. Audio and video signalsof the low-delay class are reliably transmitted almost
immediately, while other (normal-delay class) signals
are transmitted during free intervals. This makes it
possible to provide a reliable yet economical service• The pursuit of efficiency and evolution of high speed• Combination with Ethernet technology (GE-PON)

STM-PON

Broadband-PON(B-PON)

GE-PON(1000BASE-PX)

Standards

NTT proprietary
specification

ITU-T
G.983.1-4

IEEE
802.3ahTransmission rate

16 Mbit/s

622 Mbit/s downstream
156 Mbit/s upstream

1 Gbit/s

Upstream/downstream
multiplexing method

Time compression
multiplexing (TCM)

Data transmission
frame

STM

ATM

Ethernet

Year of commercial
launch19971999 (leased line)
2002 (FTTH)

2004

Wavelength divisionmultiplexing (WDM)

STM: synchronous transfer mode
ATM: asynchronous transfer mode

Fig. 1. Fiber-to-the-home (FTTH) systems (PONs).

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NTT Technical Review

Special Feature(1) Encryption technology
1

User

Time division multiple access NTT

1 2 •• n

No. 1 • • • • No. n

ONU
No. 1

1 2 •• n
OLT

1 2 •• nn

Central officeequipment

ONU
No. n
Same data delivered to all users (ONU)
Can only extract data addressed to each ONU(2) Burst-mode optical receiving technology
1

User
ONU
No. 1
2
ONU
No. 2

Time division multiple accessNTT
No. n • • • •n •• 2 1

Central officeequipment
OLT

n
ONU
No. n

⇒Preventing collisions between upstream signals⇒Receiving signals with different levels

Fig. 2. Encryption/burst-mode optical receiving technology.

(Fig. 3).
In a GE-PON system, signals of different wavelengths are used in the upstream and downstream
directions, allowing them to be transmitted in thesame optical fiber by wavelength division multiplexing (WDM). Specifically, the upstream signals aretransmitted at around 1.3 µm, the downstream signals
are transmitted at around 1.5 µm, and a separate bandfor transmitting video signals is provided at 1.55 µm.
The characteristics of this GE-PON system can besummarized as follows:• It is capable of large-capacity transmission at a rateof 1 Gbit/s. It also incorporates techniques such as
bandwidth and delay control that achieve a balance
between high bandwidth efficiency and low delay.
• The use of standard Ethernet components reduces
the system cost because no conversion of the Ethernet frames is required for local user networks.
• The sharing of center office equipment and subscribers’ optical fibers also reduces the system cost.
2.2 Technology for optical access systemconstruction
We have also developed optical wiring technology,
Vol. 3 No. 6 June 2005

which reduces costs to a reasonable level and makes
FTTH services to be installed and used easily, to meetthe demands of both quick and inexpensive construction.
(1) Indoor optical fiber cableTo facilitate network construction, we have developed a bend-insensitive optical drop/indoor cable.
This cable can tolerate being bent to a radius of just
15 mm, which allows it to be installed much more
neatly and makes it much easier to work with thanconventional cable, which has a minimum bending
radius of 30 mm.
(2) Extendable optical cord (flexible opticalfiber curl cord)
We have designed optical fiber cords with a coiled
section similar to the coiled cords used for telephonehandsets. This optical fiber curl cord is as easy to use
as a metal cable and can, of course, be connected toexisting standard fibers.
(3) Cable sheath connector and optical connector cabinet/rosette
The conventional cabinets used to connect between
exterior cables and indoor cables employ a mechanical splicing technique whereby the two cables are15

Special Feature

Improved performance
Allows the transmission of realtime services (e.g., video and audio)
Includes a function for choosing one of two delay classes (low-delay or normal-delay) according to the delay requirements.

Data such as audio and video that is sensitive to delay istransmitted with greater priority more immediately.

Voice

Other

Other data is transmitted during free intervals.

Voice

Other

Voice

Other

Time

OLT
ISP
ONU

ISP: Internet service provider

Fig. 3. Delay control function.

stripped down to the bare fiber and clamped together,which requires the use of sophisticated assembly
techniques. On the other hand, our newly developed
cabinet uses optical connectors (cable sheath connectors) that can be attached directly to the cable. Thismethod does not require us to remove the optical fiber
cable jacket, so we have no need to store spare lengthsof fiber in the cabinet. Thus, it can improve construction efficiency. In combination with the abovementioned bend-insensitive optical cable, this method
enabled us to reduce the volume of the optical cabinet
by 65%.
(4) Non-conducting optical fiber drop-cable
The ends of optical fibers are supported by tension
members. Conventionally, these tension members
have been made out of metal and have thus had to beearthed to prevent damage from lightning strikes andthe like. By making these members out of fiber-reinforced plastic (FRP) instead, we have managed toeliminate the need for earthing, thereby making
cables much easier to install. As a result, we havebeen able to reduce the time to install optical fiber
cables by about 20%.
2.3 Wireless access
For areas where it is difficult to retro-fit opticalfiber networks into existing structures such as apartment blocks, we have developed a fixed wirelessaccess (FWA) system. We made an economical highspeed (46-Mbit/s) access system by providing wire16

less hubs on telegraph poles and installing an antennain each home to communicate with a hub. This system is also useful in areas where there is a demand forbroadband access service but the demand is not yet
sufficient for economical installation of an opticalfiber system, for example, in new towns in rural areas.
3. Prospects of access network technology
With a medium-term target of 30 million customers
for optical fiber access, we are setting our sights on
the arrival of an era when optical communication istaken for granted. In this respect, it is essential not
only to improve the cost performance but also todevelop technologies that enable greater ease of use
for both subscribers and installation engineers. Sometechnologies aimed at achieving this goal aredescribed below.
3.1 Optical access systems
In current optical access systems, a PON architecture based on the time division multiple access(TDMA) scheme improves the cost performance. But
the wide bandwidth of optical fibers should make itpossible to achieve more economical systems for services with even wider bandwidth. Wavelength division multiple access (WDMA) is one promising technology for systems with a wide effective bandwidth.
Furthermore, code division multiple access (CDMA)is a promising candidate as a future technology.
NTT Technical Review

Special Feature(1) Wavelength division multiple accessTo ensure a good return on investment (ROI) in network infrastructure deployment, it is very importantto suppress the initial investment and judge subsequent investment flexibly according to the actual
demand trend. Wavelength division multiplexing(WDM) technology has good potential as a costeffective investment. For example, by assigning awavelength to a service menu, we can simultaneously provide various services with different protocols
and bit-rates on only one transmission cable. To provide additional services, we do not need to install
additional optical transmission cables or reconstruct
the whole transmission system, but only need to prepare a low-cost optical component (WDM filter).
Moreover, by assigning a wavelength to each user ona system with shared transmission cables (i.e., a PON
system), we can independently provide and upgrade
the transmission bandwidth of each user (Fig. 4).
However, conventional WDM equipment (especially WDM transceivers) is so expensive that it is difficult to utilize WDM technology widely in access network systems. Conventional WDM equipment consists of the optical transceiver and electrical signal
processing parts, which are mounted in the same
package. As a result, whenever a service upgrade isrequired, we must replace the whole package, evenwhen we only need to change the WDM transceivermodule. To make matters worse, a spare package
must be kept for each type of wavelength, transmission distance, and user interface, which leads to the
manufacturing of a wide variety products in small
quantities and makes inventory control difficult.
These factors make the conventional WDM system
expensive.
To overcome these problems, we have recently
developed a “pluggable WDM system” in which theoptical transceiver part is separated from the electrical signal processing part. This makes service
upgrading and inventory control of a WDM systemmuch simpler. Instead of keeping various types ofspare packages, we only need to keep pluggable optical transceivers. By replacing just the pluggable optical transceiver instead of the whole package, we canupgrade various services flexibly and quickly according as user requirements. Moreover, this pluggable
design will strongly contribute to the standardizationof optical transceivers, which have been the mostexpensive component of a WDM system, and lead toa drastic reduction in the cost of optical transceivers.
In addition, we can easily join an installed transmission system to a pluggable WDM system by plugging a pluggable WDM transceiver directly intoalready-installed transmission equipment and assign-

Flexible access network configuration to cope with diverse services and user requirements
One wavelength shared bymultiple users and services

Usage of existing PON

Central office
VideoInternet

λ1

CDN

Services with greater
diversity and bandwidth

Users having different
service requirements

Wavelength allocation to services

Wavelength allocation to users

Services can be added without installing extra optical fibers.

Bandwidth can be possessed while sharing optical fibers.

A

Central office
λ1

Internet

λ3

CDN: WDM filter: Light source

λ2

Central office

Video
B

λ2
C

λ1

User A
User B

λ3

User C: WDM filter: Light source

Fig. 4. Application of WDM technology to access networks.

Vol. 3 No. 6 June 2005

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Special Feature

ing another wavelength to this equipment. In thisway, we can effectively utilize previously installed
equipment without additional investment in transmission equipment and cables.
Recently, we have proposed a fully pluggable
WDM system including multiplexing/demultiplexing
functions using a pluggable WDM filter that candirectly connect to a pluggable transceiver module.
By combining these pluggable optical modules as
easily as stacking toy blocks, we can flexibly meetusers’ requirements not only for various services, butalso for various network topologies (such as point-topoint, point-to-multipoint, and cascade configurations).
Thus, the pluggable WDM system, which is attractive for cost-effectiveness and flexibility, should leadto WDM technology being applied to the access network.
(2) Optical code division multiple access(CDMA)
Code division multiplexing (CDM) is a popular
multiple access technique used in cellular phone systems. It has many advantages compared with othermultiplexing techniques. We are investigating thepossibility of applying this technique to future optical

networks.
The characteristics of optical CDMA include a significant reduction in optical interference andimproved spectral efficiency. When a user connects
incorrectly, the signal is normally obscured by theinterference light and impossible to receive. But in a
CDMA system, most of the optical interference canbe eliminated by the despreading function and thesignal be received correctly (Fig. 5).
This technique can also be exploited to allow
upstream and downstream signals to be transmitted
simultaneously at the same wavelength. Furthermore,
CDMA makes it easy to reduce the signal crosstalk
between adjacent wavelengths and has the potential
to improve the spectral efficiency compared with
WDM schemes.
3.2 Optical access system construction
Various transmission characteristics can beachieved by providing holes around the fiber core. A“hole-assisted” bend-insensitive optical fiber is lesssensitive to bending, so no bending loss occurs evenwhen the bend radius is very small. This technique
can substantially reduce an optical fiber’s bending
loss while making it easier to install and easier to han-Resistant to optical interference

Transmitting side

Receiving side

Optical interference eliminated

Bad connection
Optical interference

LAN equipment, etc.

Optical CDM-OLT

Optical CDM-ONU••

Decoder
Optical signal•

ReceiverOptical CDM-ONU

Opticalintensity

Optical interference

Despreading

Opticalintensity

Desired signal
Wavelength

Desired signal

Optical interference
Wavelength
Optical interference eliminatedLAN: local area network

Fig. 5. Some advantages of optical CDMA technology.

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NTT Technical Review

Special Featuredle. Such an optical fiber cord can withstand beingtied in a knot, bent at right angles, or even folded backon itself, making it useful not only in situations where
the cable must be laid down neatly, but also in ones
where there is little space available for the installation
and accommodation of optical equipment. We areinvestigating hole-assisted bend-insensitive opticalfiber cord for future use in optical access system construction.
3.3 Wireless access
For wireless access technology, the main issues for
future study are to provide high-quality wideband
services on a par with optical transmission speeds(over 100 Mbit/s), expand service areas, simplify
indoor wiring, and reduce equipment and installation
costs more.

Profile■ Career highlights
Senior Manager, Optical Access Innovation Project,
NTT Access Network Service Systems Laboratories.
He received the B.E., M.E., and Dr. Eng. degrees inelectronics engineering from Osaka University,
Osaka, Japan, in 1980, 1982, and 1992, respectively.
In 1982, he joined the Electrical Communication Laboratories of Nippon Telegraph and Telephone Public
Corporation (now NTT), where he has been engaged
in R&D of optical fiber transmission systems including long-distance and access systems. He served as avice-chair of the Paper Committee of SubOptic 2001
in Kyoto. He is a member of IEEE, the Institute ofElectronics, Information and Communication Engineers (IEICE) of Japan, and the Japan Society ofApplied Physics. He received the Young Engineer
Award from IEICE in 1990 and an IEICE Communication Society Activity Testimonial in 2000.

4. Conclusion
The main research and development issues foraccess network technology are large capacity (better
performance), cost reduction to a reasonable level,
and ease of use. We increased network capacity by
developing GE-PON (optical fiber) links via ADSL
(asymmetric digital subscriber line). In the future, wewill exploit the wideband properties of optical fibers
in order to provide a more comfortable communication environment and quality suitable for the ubiquitous era. We have already achieved costs similar to
those of metal connections at the user end, and wewill go on to develop extremely cost-effective broadband services by exploiting WDM and CDM. We
have already developed a number of facilitating techniques for optical access network construction.
As future research, we plan to develop more userfriendly systems by simplifying the wiring process,developing pluggable WDM technology that can beconnected with ease, and adapting to whatever circumstances arrive by continuing our efforts in otherareas such as wireless networks.

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