Special Feature

Optical Core Networks
Kazuo Hagimoto†
Abstract
This article introduces our current work on optical network technology to drive core networks towards our target of 30 million FTTH
(fiber to the home) subscribers and describes future prospects of thistechnology. By responding to the need for IP (Internet protocol) integration of services ranging from telephones to broadband video
through the use of revolutionary optical amplification/relay/switching
technology and network control techniques, we are contributing to theconstruction of an advanced reliable network by establishing photonic
network technology that adapts promptly and flexibly to traffic concentration and diversification.

1. Previous trends in optical technology
Optical communication technology was introduced
as a network medium by NTT and other organizationsin the 1980s. Today it is being used to form terabitper-second-class networks that exploit the full potential of light as a communication medium. In the 21st
century, optical technology is entering a new stage
where more wavelengths are being used in broader
networks and where signals are switched optically.
In the case of high-capacity optical communication, the cost/performance ratio unfortunately does
not improve unless the volume of traffic increases.
When the traffic increases, the cost per bit decreases.
Recently, broadband traffic has continued to grow bya factor of 2–3 times per year. There is a strong trend
toward more traffic associated with music downloads
and the like, which is expected to continue to grow toa considerable volume. Therefore, there might be ashortage of optical fibers in the future.
A reduction in the point-to-point cost of opticalfiber communication can be achieved by making
breakthroughs in various optical technologies. A† NTT Network Innovation Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: hagimoto.kazuo@lab.ntt.co.jp

20trans-Japanese circuit was completed in 1985 withthe opening of a single-mode fiber network covering
the whole of Japan. In addition, various otherachievements have been made to create other opticalcommunication technologies. To facilitate the running of this fast high-capacity system, we made what
were then regarded as revolutionary expansions of
network operation based on SDH (synchronous digital hierarchy: an international standard for high-speed
digital communications using optical fibers, standardized by ITU-T (International Telecommunication Union Telecommunications Standardization
Sector)). This move enabled us to develop the network while maintaining high-capacity signals without incurring higher operating costs.
However, from the viewpoint of increasing capacity, the most important factor is optical amplification.
By doping trace quantities of the rare-earth element
erbium into a fiber, we can make the intrinsically
lossy fiber medium generate gains of 20 or 30 dB.
This is an important effect, not only because it allows
the span between relays to be increased, but also
because it allows the capacity to be substantially
increased through the use of wavelength divisionmultiplexing (WDM). At present, an 80-channel
× 10-Gbit/s link system with a capacity close to 1
Tbit/s is being operated in the national network of
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Special Feature

NTT Communications.
By continuing to research and develop other technologies, we hope to eventually arrive at the era offully optical networks where data is switched according to its wavelength.

such as SDH on a single chip, and (iii) optical devices
such as quartz-based planar lightwave circuits(PLCs).
1.2 Fiber amplifier technology
Optical amplifiers were first used to amplify transmissions in 1989, when it was confirmed in tests thata gain of 30 dB could be achieved. However, the characteristics of the first amplifiers were not necessarily
flat. The lowest losses in an optical fiber occur in the
1.55-µm wavelength band where the gain was foundto be no more than about 20 dB. However, by making
slight changes to the amplification parameters, thecharacteristics were gradually made smoother andtechnology capable of amplifying 100 or even 1000
channels began to look possible. Furthermore, byusing a various different materials, it became possible
to produce amplifiers capable of covering all the lowloss wavelength regions of optical fibers.
The optical capacity of WDM technology has thusincreased in leaps and bounds, but in practice thenumber of photons needed for WDM and TDM (timedivision multiplexing) has not changed much. For
both techniques a transmitter power of about 1 mW isneeded for each Gbit/s of bandwidth. In other words,
a 1-Tbit/s system with 100 channels of 10-Gbit/s signals would require a fiber output power of 1 W.
Although this is not a problem in terms of transmission per se, it is surprisingly dangerous in terms of the

1.1 Cost trends
From the changes in the costs of nodes and links, it
can be seen that the link cost has fallen dramaticallywhile the node cost has remained more or less constant despite various functional enhancements including the development of new LSIs (Fig. 1). In the telephone era there were 60 million subscribers with 64kbit/s lines, which corresponds to potential network
traffic of about 4 Tbit/s. To prepare for the ubiquitous
broadband era, NTT aims by 2010 to have increased
this capacity to 100 Mbit/s × 30 million subscribers,
or 30 petabits per second (30 Pbit/s)—an increase ofabout four orders of magnitude. With link costs
shrinking, the only way to reduce the total cost is tobring down the node costs. Consequently, there is aneed to introduce new technology including not only
ATM switches and IP switches but also opticalswitches and novel node technology.
It is thus essential to take steps to maximize thevalue of NTT Group’s facilities and to work towards
this target while tackling the issues from three viewpoints: (i) operations and protocols, (ii) SoC (systemon a chip) technology for implementing schemes
104

Telephone era
4-Tbit/s network(64 kbit/s × 60 million subscribers)

103
Node
102

Ubiquitous broadband era
30-Pbit/s network
100 Mbit/s × 30 million subscribers

Link/node costs

Link
10

ATM switch
Effect of introducing optical fiber

IP switch

1

Ways of maximizing the valueof NTT equipment

Based on “IEEE"IEEE Communication Magazine”
Magazine"
Optical switch

10–1

Target

10–2

10–3• Operation protocol• SoC approach• Photonic PLC and other optical devices

1975

1980

1985

1990

1995

2000

2005

2010

2015

Fig. 1. Cost trend and targets: reductions in both nodes and links.

Vol. 3 No. 6 June 200521

Special Feature

heat density at the fiber surface and might cause thefiber to melt if anything adhered to it. According to
the safety guidelines of the OITDA (Optoelectronic
Industry and Technology Development Association),a fiber should be shut down if reflections are seenfrom the connector surface, but considering productliability law and the like, it is probably about time thissituation was reviewed.
A fiber is a passive medium in which the power isgreatest at the transmitting end and weakest at thereceiving end. However, since it is the power itself
that determines the signal-to-noise ratio (SNR) of an
optical communication system, less power means asmaller SNR. Of course, ideally fibers would be lossless, but if we contrive to give the fiber itself a positive gain, then the loss conditions can be relaxed.
Even if the optical input level is reduced, the same
SNR can be achieved without reducing the relay
spacing. In other words, amplification can be performed inside the actual fiber transmission medium(Raman amplification technique).

ate, but the increased bit rate and narrower pulse
width also lead to severer conditions related to dispersion in the fiber (the phenomenon whereby pulses
traveling through the fiber gradually broaden out).
The reason why speeds have increased smoothly inspite of this is that the original 400-Mbit/s (FabryPerot laser diode) systems had a spectral width ofabout 5–8 nm, or about 600–700 GHz in terms of frequency spreading, which was very wide. In the drive
towards increased capacity, signals have been subjected to more and more spectral compression. As aresult, in a 1.6-Gbit/s (distributed feedback laser
diode) single longitudinal mode system, the data istransmitted with a spectral width of about 0.3 nm, i.e.,less than 50 GHz.
In the future, the frequency occupancy factor willcontinue to decrease, reaching about 1 for 10-Gbit/s
systems, but modulating a 10-Gbit/s signal will resultin a bandwidth of about 10 GHz. This is referred to as
direct laser modulation, although instead of the laser
being actually switched on and off directly, its output
is modulated on and off by a lithium niobate opticalcrystal.
As the bit rate increases, the overall spectral widthof the system conversely decreases. Thus, if a rate of
40 Gbit/s can be achieved, then it is likely to decrease

1.3 Bit rate and spectral width
Figure 2 shows the spectral widths of light sources
with higher-capacity bit rates. As the speed increases,
not only does the circuit itself become harder to operReduction ofspectral width

Automatic dispersion
equalization

R&D target

10
1000

F-400M
ML-LD

FP-LD
F-1.6G
1

100
F-10G

LD

FB-

-D
QW

M

0.1

tor

la

du

Mo

m
for

al

ctr

pe
it s

lim

th
wid

for

ary

bin

40G

10

ns

Tra
0.01

0.001

Modulation scheme
Digital processing

Frequency(GHz@1552 nm)

Spectral full width (nm)

-LD

DFB

1

SBS-limit
0.1

0.0001
0.1

1

10
Bit rate (Gbit/s)

100

ML-LD: mode-locked laser diodeSBS: stimulated brillouin scattering

Fig. 2. Modulation band vs. spectral width of light source.

22

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still further. However, this rate is close to the limit ofordinary modulation and demodulation, so the spectral width will increase if the bit rate is made any
higher. If this happens, the pulse widths will tend toincrease and become more susceptible to the effectsof fiber dispersion, and it will become necessary touse dispersion equalization techniques to alleviate
this problem. A variety of modulation and demodulation techniques are also being applied to wirelesscommunications and ADSL (asymmetric digital subscriber line), where it is thought that improvements
can be expected in the same way as with optical signals by applying similar measures to the modulationand demodulation techniques and by employing digital signal processing techniques.
2. From IP packets to wavelengths
The assertion that dealing with the transition to
WDM technology is essential in the 21st century isstrongly related to the fact that network are converging to IP technology (Fig. 3). When information issent in the form of IP packets, it is inevitably slowed
down by the need to read the packet headers at every
node in order to forward the packet to its destination,
just as a car must stop and give way to other traffic at
a series of crossroads. Since this inevitably leads tocongestion, one promising idea is to find some way ofswitching all the interchanges beforehand so that data

Wavelengthpath

2.1 GMPLS research and development
GMPLS (generalized multiprotocol label switching) is being studied as a network control technique

Wavelength
multiplexing link

λ3

λ1 λ3

λ 1 λ2

λ2

Optical burst switch network

IP router

Dynamic

IP router
Wavelength
routing(OXC)

Wavelength
control

can travel straight through without stopping. By cutting out all the electrical processing and controlling
traffic based on its wavelength, we can make considerable improvements in terms of latency and quality.
When attempting to achieve increased throughput
with IP traffic, or when high-end users need to
exchange large amounts of data, users are very sensitive to latency. They strongly claim that they want thelatency reduced and that it is achievable. Eliminating
unnecessary processing would also be beneficialfrom an environmental aspect because no energy
would be wasted.
An important aspect of this technology is that the
optical switching can be tweaked according to thetraffic so as to increase or decrease the number ofexits. Using techniques such as this, our researchers
aim to progress from point-to-point WDM transmission to a reconfigurable optical add/drop multiplexer(ROADM) equipped with optical switches, wavelength path networks based on optical crossconnects,
and WDM/GMPLS (a combination of optical and IP
techniques). The ultimate aim is to perform all theprocessing optically (optical burst switch networks),
although there are still many issues to be tackled
before this can be achieved.

Wavelength
multiplexing
link = lane

WDM

WDM
GMPLS
Optical and IP
cooperation

OXC

On-demandconnection

WDM
IP router

OXC

Wavelength path network
based on optical crossconnects

IP packet
ROADM with optical switch
StaticOptical ADM
WDM transmission

Fixed connection
Year

Fig. 3. Research and development of photonic networks.

Vol. 3 No. 6 June 2005

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Special Featurethat allows an optical physical network to be set up
quickly in response to IP bandwidth requests. Highcapacity IP routers have hitherto been used for thispurpose, resulting in high cost and high power consumption, but this load can be reduced at a stroke byallowing signals to pass straight through interchange
points according to their wavelengths.
One issue to be resolved in the background to thissort of research and development is the transition to
an IP integration network that connects services tothe physical layer. Other likely requirements are theneed for NTT to provide more reliable communication as a national platform in its role as a major network platform and the need for the network to protect
itself against interference by switching optical paths
in a suitable manner.

technologies such as ROADM might be used as abasis for development (mountain-climbing
approach). Alternatively, for large-scale technologythat uses large switches to handle large volumes ofdata, another approach would be to proceed withresearch in the form of a national project and pursue
development for private consumers only when its
usefulness has risen (parachute approach).
In practice, national testbeds are starting up not justin Japan but in other countries as well. Since these
testbeds provide a means of reliably evaluating thefeasibility of new technology and introducing it into
a community, they provide a chance to continue beinga player when the technology is subsequently commercialized. The same thing happened when theInternet was starting up. The Internet was initiallyused for military purposes, and when it was subsequently developed for commercial use, the equipment
and environments that had already been provided
were inherited directly. It is thus very important toparticipate in this sort of framework.

2.2 Demand for photonic networks
Optical network environments currently exhibit thefollowing characteristics:• Commercial traffic is growing vigorously, almost
doubling every year.
• Mass users are enjoying the availability of economical broadband environments.
• Academic computer users still have a large
appetite for more bandwidth.
User needs are varied, and more research and development from many different viewpoints is needed.
Specifically, for the commercial base, metro-system

2.3 LANs and WANs
Computers are still continuing to grow according to
Moore’s law. As computer technology develops, local
area network (LAN) technology also develops, and it
is also rapidly infiltrating carriers. Routers are thebest example of this. Figure 4 shows how Ethernet
has evolved. Ethernet started out with a speed of 10
Direct LAN with λExtra-LAN

Intra-LAN

100 Gbit/s

40G
100GbE ?

WAN: SDH/SONET
10GbE(10GBASE-X)

10 Gbit/s
1GbE(1000BASE-X)

1 Gbit/s
100 Mbit/s

LAN: Ethernet

100MbE(100BASE-FX)
1995

1996

1997

1998

1999

2000

2001

2002

LAN >> WAN• Economy and speed of LANs are now penetrating WANs.
• Issues when the simplicity of LANs extends to WANs: reliability and operability (determining failure, etc.).

Fig. 4. User-network interface (UNI) drives WAN interface speed.

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Mbit/s, then increased to 1 and 10 Gbit/s, and is now
aiming even higher. It may overtake the speed of wide
area networks (WANs). WANs have already reached
Tbit/s-class capacity. Using WDM technology,
WANs are gradually converging towards the sort ofuser-network interfaces (UNIs) needed by users. This
could be because the interfaces of LAN users are lessexpensive.
In terms of both sales value and the number of
installed networks, LANs are bigger than WANs, so
although WANs have their own technologies geared
towards transport, in other respects there is a trend
towards the use of LAN technology to achieve savings based on the economy of scale.
2.4 OTN frame standardization
Implementing 10-Gbit/s Ethernet (10GbE) involves
addressing the issue of how to interact with the transport layer. An ordinary Ethernet switch can only
works at short distances. On the other hand, by making frames that can be maintained and managed overlong distances by applying optical transport network(OTN) technology, e.g., forward error correction(FEC), it becomes possible to jump distances eitherwith Ethernet or STM (synchronous transfer mode)
networks. Work is therefore under way to standardize

the OTN frames, where the current hot topic is what
to do about the physical layer (PHY) of 10GbE LANs
(Fig. 5).
These problems occur because conventional networks are based on STM technology. When a 10GbE
network attempts to use LAN PHY, the 10GbE clock
rate is slightly too high, so compression has to be
used. However, users are sensitive to latency and losses and would probably rather be allowed to go connect directly. In that case, the bit rate would increase
and make it impossible to get the data through conventional digital cross-connect switches with ease, sothey might as well access the network optically.
Thus, there are an increasing number of applications where large user channels are operated in large
numbers, and a great deal of consensus has beenachieved with regard to their standardization.
2.5 Latency elimination technique of i-Visto
The introduction of i-Visto facilitates comfortablecommunication with improved latency. In a satellite +MPEG-2 system, the satellite alone has a latency of
200 ms, and the overall latency of 433 ms is verylarge. In the case of a fiber + MPEG-2 system, thelatency of the fiber part is just 50 ms, so the overalllatency is kept down to about 250 ms. On the other

Direct accommodation in optical channel (OCh) frames reduces GFP (generic framing procedure) mapping and SDH processing
to achieve lower costs, reduced delays, and full-rate transparent transmission.

10GbE LANPHY
10GbE LANPHY• PCS termination processing• IFG processing
GFP

Payloads directly
accommodated
1

1

16 17
OCh-OH

Payload4

Format(bytes)3824 3825 4080
Error
correction
encoding

SDH

OCh frame structure1
Scheme for direct accommodation in OCh frames(Scheme for 10GbE-LANPHY over OTN)

OH

1

OH

16 17
OCh-OH

4

Payload

PayloadAccommodated in Format(bytes)payload3824 3825 4080
Error
Payloadcorrection
encoding
OCh frame structure

Accommodation scheme over GFP(10GbE-LANPHY over GFP over SDH over OTN scheme)
Support for the scheme is now growing since its proposal tointernational standards bodies.
GFP: general framing procedure
IFG: interframe gap

Fig. 5. Proposals to international standards organizations: an accommodation scheme based on using OTN opticalchannel frames.

Vol. 3 No. 6 June 2005

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Special Feature

hand, if uncompressed data is used (fiber + uncompressed IP i-Visto), the latency can be reduced toabout 100 ms, allowing smooth communication totake place across the Pacific Ocean.
3. History and future prospects of transmission
schemes
Photonic networks are expected to be utilized atvarious levels from the metro system to the core system. For this purpose, it is important to make wellbalanced developments including communication
schemes such as OTN. Looking back on the long history of communication schemes up to the present, thetechnology has evolved through the analog era into
the digital era, from electrical to optical, and from
PDH (plesiochronous digital hierarchy) to SDH.
Important landmarks include the development of
optical amplifiers, which brought about opticalrelays. However, these relays are still only analog
devices, so as before they still need to be maintained
and supervised. The fact that they operate transparently means they have to be monitored.
The next big step is digital optical technology, butthere are difficult problems to overcome before thiscan be achieved. Optical switches are at a semi-digital phase, while OTNs are mostly digital in that they
employ FEC technology. So although they are notdigital circuits in the real sense, they are coming quite
close to digital technology.
Looking at the characteristics of FEC, such as theerror rate for example, the error rate does not decreaseno matter much light is received when there is deterioration in the SNR at the transmitting end. Error correction can be performed by applying FEC. Sincebinary 0 and 1 values are clearly different, half-way
error states do not occur.
4. Future prospects of photonic networks
We are focusing on GMPLS because it connects
well with photonic networks. GMPLS sets up thephysical layer according to the IP traffic, thereby
linking layers 1 and 3 together. By linking with therouters, it is possible to change the routing by operating optical switches to ease congestion or recoverfrom faults.

26

As traffic continues to increase, there is a limit towhat can be done by mass demonstration tactics
when faults or interference occur in large amounts oftraffic. It is essential to move ahead in combination
with operation technology.
Also, with regard to overseas relations, we are participating in the GLIF (Global Lambda Internet Facility) organization, and we are looking into connecting
WDM networks together for use in supercomputer
applications. In Japan, we are participating in a network called JGNII (Japan Gigabit Network II) madeby the National Institute of Information and Communication Technology (NiCT), and we also hope tocontribute to national policy.
Profile■ Career highlights
Executive Manager, Photonic Transport Network
Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in physicalelectronics engineering from the Tokyo Institute ofTechnology, Tokyo, Japan, in 1978 and 1980, respectively.
In 1980, he joined the NTT Electrical Communications Laboratories, Yokosuka, Japan, where he wasinvolved in R&D on high-speed optical communications systems including the F-1.6G system and the FA10G system. After serving as a senior manager ofOperation Support Systems group of network business
unit of NTT Communications, he is now an executive
manager of NTT Network Innovation Laboratories.
His current research interests include very large capacity optical network systems and media networking
technologies utilizing optical networks.
He is a member of the Optical Society of America
and the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan. He served asa program co-chair of OAA’93 in Yokohama, Japanand a general co-chair of OAA’94 in Colorado. He
served as a chair of Optical Communications Group of
IEICE in 2002–2003. He received the Sakurai Memorial Prize from the Optoelectronic Industry and Technology Development Association in 1989, the Oliver
Lodge premium from the IEE in 1991, the Kenjiro
Takayanagi memorial award in 1994, the achievement
award from IEICE in 1994, and the best paper awardof first Optoelectronics and Communications Conference (OECC’96) in 1996.

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