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i
Opportunities in Telecommunications
INTRODUCTION
Telecommunications is a big business. The market for network equipment
purchased by companies in the United States that offer public network
telecommunications services (e.g., local exchange carriers, long-distance
interexchange carriers) exceeds $20 billion per year. When one considers the
market for all telecommunications equipment in the United States, the number
is substantially larger. The total world market is several times greater than the
U.S. market. Thus, depending on what one defines as telecommunications
equipment, the total worldwide market exceeds $100 billion per year.
Up until the late 1970s, telecommunications systems depended on advances
in electronics to provide new capabilities, increased performance, and lower
costs. Until that time, the media used for transporting information from place
to place were copper cables and radio (including terrestrial microwave links and
satellites). As f~ber-optic technology emerged from the research lab and field
experiments and entered large-scale deployment, the impact of this photonic
technology on telecommunications was dramatic. For long-distance (>100
miles) and moderate-distance (5 to 100 miles) point-to-point applications, fiber
displaced copper cable and radio as the medium of choice. Today, the deployed
U.S. base of long-distance and moderate-distance point-to-point fiber systems
has an information carrying capacity (e.g., measured in equivalent voice circuits
or in bits per second) far in excess of the total deployed base that existed in the
late 1970s. Thus in less than 10 years, this technology has revolutionized many
9
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10
PHO TONICS
transmission aspects of telecommunication networks and has had a very big
impact on the competing technologies by displacing them.
In this chapter the panel examines a number of different telecommunica-
tions applications environments and the potential of emerging photonic
technologies to either facilitate those applications or displace existing tech-
nologies currently used in those applications.
TELECOMMUNICATIONS APPLICATIONS:
DESCRIPTIONS AND TECHNOLOGY STATUS
Long-Distance and Moderate-Distance
Point-to-Point Connections
In the late 1970s f~ber-optic technologies emerged from the research and
field-experiment phases and were deployed on a large scale in the transmission
systems that interconnect telecommunications switching systems and provide
the formation-carrying capabilities used by customers attached to networks
employing those transmission and switching systems. Fiber offered the ability
to transmit large amounts of information over long distances without requiring
as many repeaters to remove the effects of loss and distortion. This was in
contrast to existing metallic cable and radio facilities, which have limited
information-carrying capabilities for a variety of physical reasons.
The impact of fiber was twofold. The ability to transmit information for
long distances without repeaters or with fewer repeaters (and without the
interference and security problems of radio) made it possible to build transmis-
sion systems with lower initial equipment costs, lower right-of-way costs, and
lower maintenance costs. The large information-carrying capability of fibers is
a latent attribute that can be called on in the near future to economically
transport video signals that can make use of that capability. Since fiber was first
employed in long-distance and moderate-distance applications, the marketplace
(purchasers of transmission equipment) has demanded products with ever-
increasing information-carrying capability and ever-increasing distances that can
be spanned without repeaters.
The technology has passed through three generations since the late 1970s,
starting with multimode fiber and 800- to 900-nm wavelength light, then moving
briefly to multimode fiber and 1300-nm wavelength light, and now using single-
mode fiber and 1300-nm wavelength light. The promise of lower light losses
points toward the use of 1500-nm wavelength light in the future. In addition,
the promise of more sensitive receivers (and therefore longer achievable spans
without a repeater) points toward the potential future use of coherent
technologies.
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QPPOR7rUNITIES IN TELECOMMUNICATIONS
11
Long repeaterless spans are important because of the costs of locating,
powering, and maintaining repeaters, in addition to the cost of the repeaters
themselves. This is particularly true in long-distance terrestrial and undersea
cable systems. In undersea cable systems, reliability and power consumption
are, in addition, more critical than in terrestrial applications. A figure of merit
for point-to-point transmission is the product of repeaterless span and
information-carry~ng capability. The product of repeaterless span and data rate
in commercially available equipment has increased from 5 km x 45 Mbits/s in
1979 to over 40 km x 560 Mbits/s today. This represents an increase by a factor
of over 100 in 8 years (approximately doubling every year) for deployed
products. This progress is expected to continue. Commercial products
operating at nearly 2 Gbits/s have been announced. In the laboratory, systems
have been demonstrated with (simultaneously) data rates of several gigabits per
second and repeaterless spans beyond 100 km, for a data rate times distance
product more than 10 times that of the commercially available equipment cited
above.
Local Area Networks
In the context of this report, a local area network is defined as a com-
munications interconnection system deployed in an office, a factory, or a multi-
building campus (distances typically < 1 mile) to allow computing devices and
peripherals to exchange information in electronic form via a shared networking
facility.
Local area networks emerged about a decade ago in response to the
proliferation of moderate- and low-cost computing devices (and peripherals)
and the need to interconnect them. Early local area networks were based on
copper-cable media (e.g., coaxial cable). Because local area networking is a
relatively young concept, there is still much to be learned about requirements
that local area networks should meet in various applications. For example,
even the best choice for the physical layout of the local area network cabling
and electronics is still not completely understood, although much has been
learned as a result of early installations of various designs.
A number of alternative fiber-optic-based local area networks have been
proposed, designed, or deployed in recent years. These have various physical
topologies (star, ring, bus), various capabilities (peak data rate, delay, number
of accessing computing devices that can be accommodated), and various target
applications. The components used in these local area networks include many
of the components used in point-to-point telecommunications applications plus
some additional components unique to the local area networking application.
Examples of these additional components are access couplers, which allow light
to be partially added or removed from a fiber at an access point; star couplers,
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PHO TONICS
which allow light arriving at an input to the star coupler to be split amongst
several outputs of the coupler; and optical switches, which allow remotely
controlled reconfiguration of the network for maintenance or rearrangement of
connectivity patterns.
Low cost and high reliability are required in local area networks with a
large number of access ports on the network to which relatively modest cost-
computing devices (e.g., terminals) can be attached. Since the computing
devices to be attached might cost only a few hundred or a few thousand dollars
each, and since there are more places to attach computing devices than there
are attached devices themselves, each access port must be very inexpensive
compared to the cost of the accessing device. Local area networks with many
access ports require high reliability of individual components in order to provide
acceptable maintenance costs and acceptable system downtime. Local area
networks that have only a few accessing devices or accessing devices that are
relatively expensive (e.g., minicomputers and specialized peripherals) can
tolerate more expensive components, if that is the only alternative.
Metropolitan Area Networks
Metropolitan area networks, in the context of this report, are connectivity
systems for allowing communication between geographically dispersed local
area networks and isolated terminals (typical distances are 1 to 10 miles). They
have characteristics similar to those of a local area network. Typically,
information carried between points on a metropolitan area network can share
transmission paths with other types of telecommunications traffic. Costs of
optical components are not as critical in metropolitan area networking because
of the larger numbers of computing devices sharing the use of those components
(the local area network concentrates traffic onto the metropolitan area
network). However, given equally good technical alternatives, the least
expensive components will be selected. Because of the potentially large
amounts of information traffic carried by metropolitan area networks, higher-
speed components and wavelength/frequency multiplexing technologies are
increasingly desirable. Metropolitan area networks are early versions of the
broadband integrated service digital networks (for business applications)
described in the next section.
Broadband Integrated Service Digital Networks
The concept of a broadband integrated services digital network (BISDN)
is to provide a high-bit-rate communications transport capability to a community
of unrelated users for voice, data, image, and video communications. It is the
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OPPORTUNITIES IN TELECOMMUNICATIONS
SINGLE
MODE
FIBER
2.488 Gb/s
TRUNK
HIGH SPEED _
SIGNAL _
SWITCH
(155 Mb/s)
'M _ _ M'
. E/O u l r , E/O .
l
LOW SPEED
SIGNAL
SWITCH
(64 Kb/s) .
CENTRAL OFFICE OR
UNATTENDED REMOTE UNIT
FIGURE 2.1 Setup for a typical office BISDN.
SINGLE
MODE
FIBER
622 Mb/s
RADIO PORT
13
u
—TELEPHONE
~ DATA
—VIDEO
~ GRAPHICS
~ AUDIO
—TELEMETRY
~ OTHER
PORTABLE VOICE
OR DATA
CUSTOMER LOCATION
E/O = ELECTRICAL-TO~OPTICAL
CONVERTER
MUX = MULTIPLEXER/DEMULTIPLEXER
vision of the future. Fundamental to such a concept is bringing fiber to
businesses and residences since copper wire cannot carry the required data rate.
Figure 2.1 shows a typical realization of a BISDN. Various high- and low-speed
digital signals are combined by electronic multiplexing in a central office or
remote (unattended) switching unit and are converted to optical form for
transmission to the customer over an optical fiber. At the customer's premises
the optical signal is converted back to electronic form, and its various informa-
tion components are distributed to appropriate terminals. There are now over
100 million copper-w~re access lines in U.S. telephone networks. If 1 percent
per year of these were converted to fiber, this would amount to a demand for
millions per year of optical transmitters, receivers, and other optical components
as well as millions of kilometers per year of fiber. If the installed cost of a fiber
access line was initially about $3000 and declined toward $1000 as production
volume increased and technology improved, then converting 1 million copper-
wire access lines per year would represent a $1 to 3 billion market. If the
changeover rate grew to 5 percent a year, then the market would be $5 to 15
billion in the United States alone. The total U.S. market (100 million lines) for
conversion is more than $100 billion. Thus the BISDN marketplace represents
a very important application for photonic technologies. Key to this marketplace
will be low-cost fiber cables, low-cost transmitter and receiver modules, and
possibly novel approaches to the distribution network architecture to reduce the
need for active electronic components in the outside plant (in unattended
locations). Also key to the timely deployment of BISDNs in the United States
is an appropriate regulatory and long-term investment climate that will
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PHO TONICS
encourage the large capital investments required and will allow BISDNs to be
deployed in an economically efficient manner. There is a worldwide race to
develop the necessary technology.
Photonic Technologies Within Equipment
As telecommunications moves toward the higher data rates associated
with BISDNs, the ability to perform functions such as multiplexing, switching,
and internal component interconnects at these high data rates becomes a
limiting factor in cost and practicality. Higher-speed electronic circuits consume
more power, which creates the thermal management problem of keeping
components from overheating. Higher-speed electronic circuits also experience
bottlenecks in the flow of electronic signals between circuit elements because
of the limited bandwidth of metallic interconnects, even very short-distance
interconnects between circuit elements on a monolithically integrated circuit.
A question often raised is whether photonic (optical) devices can help to resolve
these problems in a way analogous to f~ber-optic technology's opening up almost
unlimited bandwidth in transmission systems.
Various scientifically interesting photonic components can perform a
switching or routing function in the laboratory, and there has been much
speculation as to how these components might revolutionize the capabilities of
switching and computing systems. However, the large technological gap
between these laboratory devices and practical systems needs to be filled by
scientific breakthroughs as well as by engineering. When these gaps will be
filled is an open question.
There are, however, some near-term possibilities. Very often it is the
interconnects between components that represent the most troublesome
engineering problem in equipment operating at high data rates. Here optical
interconnects in the form of optical waveguides (fibers) on plug-in boards and
backplanes, accessed directly by components with optical input-output
capabilities, show much promise. Optical interconnects between plug-in boards
or arrays of components based on free-space optics (arrangements of lenses and
mirrors) also appear attractive. Fiber interconnects between backplanes are
becoming increasingly popular.
Small numbers of incoming and outgoing optical fibers may be rearranged
by interconnecting them with optical switching devices that act as a remotely
controlled patch panel. Such remotely controlled patch panels have possible
applications for protection switching, which allows a network of interconnected
fibers to recover from a fiber or equipment failure, or for rearranging the
connectivity pattern of fiber networks in response to the physical movement of
users.
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OPPORTUNITIES IN TELECOMMUNICATIONS
Photonic Networks
15
Today's telecommunications networks have architectures that evolved to
conform to the capabilities of the technologies--copper cable and radio
transmission systems, and discrete electronic components--available when these
networks were being designed and developed. With the advent of fiber optics,
advanced optical components, and highly integrated high-speed electronic cir-
cuits, one can ask whether a telecommunications network should retain the
existing architecture and use modern technologies to substitute for older
technologies or should adopt a new architecture tailored to derive the maximum
benefit from the new technologies. New architectures that have been suggested
tend to capitalize on the ability of fibers to carry very large amounts of
information with very little marginal cost once a fiber is in place. They tend to
push switching toward the edges of the network: large amounts of information
are distributed throughout the network, and the desired information is selected,
as needed, by the terminals connected to the network or by gateways at the
edges of the network that stand between the network users and the network.
These approaches often build on optical wavelength-division multiplexing
(WDM) or optical frequency-division multiplexing (coherent techniques) to
deliver these large bundles of information throughout the network. It remains
to be determined whether these architectural concepts will be preferable to
architectures with more switching internal to the network.
Military Telecommunications
Military telecommunications applications for fiber-optic systems closely
parallel the commercial applications discussed previously. For example, there
are fixed land-based applications for high-data-rate, long-distance, point-to-
point links. There are land-based, shipboard, and airborne applications for
local area networks for computer interconnects and for BISDNs to support
combinations of voice, data, video, and other signals. Certain attributes of
optical fiber systems have greater importance in military applications than in
commercial applications. Among such attributes are immunity to electromag-
netic interference, relative security from eavesdropping, spanning of long
distances without electronic repeaters, and low cable weight. Military
applications also impose additional requirements on fiber-optic systems, e.g.,
wider operating and storage temperature ranges; ability to withstand severe
vibration, shock, and other mechanical stress; and robustness of system
performance in the presence of multiple simultaneous subsystem failures. Many
of these additional requirements are satisfied with appropriate packaging and
other physical design measures as well as with appropriate system design,
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PHO TONICS
without any direct implication for the photonic devices. However, certain re-
quirements place the fiber and photonic components in the critical path of
viable applicability and lead to photonic technology challenges. An example is
the design of optical fibers and cables that are resistant to radiation, can operate
over a wide temperature range without excessive attenuation increase due to a
phenomenon called microbending, are capable of sustaining high strains without
breaking (e.g., for fiber guided missiles), and can withstand severe chemical
environments (a requirement not necessarily more stringent than some for com-
mercial applications such as oil well logging cables). Another example is the
design of optical emitters and detectors that are radiation resistant and can
operate in high-temperature environments. On balance, however, the
requirements of the military applications for fiber-optic systems are being
addressed successfully. The only key enabling technology to tee emphasized, one
that is needed for commercial applications as well, is a high-reliability optical
transmitter and receiver module that can operate over standard military
specification ranges of temperatures with failure rates of less than 1 per million
device hours. For commercial applications low cost is also critical but should
follow from high-volume automated production.
ENABLING TECHNOLOGIES
All of the important technologies that are required for successful exploita-
tion of the markets identified in the section "Telecommunications Applications"
can be divided into two categories: (1) technologies requiring continued or
increased emphasis on development in the next 5 years so that U.S. corpora-
tions can maintain or strengthen their competitiveness in existing but evolving
markets, or can ensure that they will compete successfully in emerging markets,
and (2) technologies that are enabling but are underdeveloped so that increased
or continued research is appropriate at this point. In this report these two types
of technologies are referred to as enabling technologies ripe for development
and enabling technologies requiring continued research.
Long-Distance and Moderate-Distance
Point-to-Point Connections
Enabling Technologies Ripe for Development
· Fiber cables with low loss and low dispersion (variation of group velocity
with wavelength) at 1.3-micron wavelength, and with low microbending loss and
high strength. Note: to date the United States has demonstrated competitive-
ness in this technology, and there are several U.S. companies currently
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OPPORTUNITIES IN TELECOMMUNICATIONS
17
marketing high-quality, low-cost fiber cables. Continued development of
manufacturing technology is needed to reduce costs in this increasingly
competitive market.
~ Fiber cables with low loss and low dispersion at 1.5-micron wavelength,
and with low microbending loss and high strength. Note: U.S. fiber companies
appear to be competitive in this emerging technology.
· Transmitter modules with high performance in terms of output power
into an integral single-mode fiber, single-frequency (longitudinal mode)
operation, narrow line width, low chirp (frequency shift under modulation),
long lifetime in an unconditioned ambient temperature, high-modulation-speed
capability, tolerance to elevated ambient temperatures for short durations
without disruption of performance, and low power consumption. Note: although
U.S. companies have thus far been able to produce transmitters containing
lasers for systems deployed to date, the increasing performance demands of the
marketplace combined with the generally acknowledged lead of the Japanese
in laser technology suggest that escalated development effort will be necessary
to retain U.S. competitiveness. It is believed that this is a development
challenge rather than a research challenge. (In this context "development"
focuses on manufacturing technology as well as on design for low-cost
manufacturing.)
· Receiver modules with high performance in terms of sensitivity, dynamic
range, bandwidth, and linearity. Note: although U.S. companies have thus far
been able to produce receivers for their systems that result in reasonably
competitive system performance, non-U.S. companies (e.g., the Japanese) have
been aggressive recently in developing high-sensitivity, high-dynamic-range
receivers that appear to outperform U.S. counterparts. Since much of the
research in low-noise, long-wavelength avalanche photodiodes (APDs) and in
low-noise amplifiers originated in the United States, it is suggested here that
increased development effort is needed to retain U.S. competitiveness in this
key technology.
~ Passive components for wavelength multiplexing and demultiplexing and
for related technologies such as transmitters with stable and predictable
wavelengths. These are needed to deploy wavelength-multiplexed systems with
moderate numbers of concurrent wavelengths.
Enabling Technologies Requiring Continuing Research
· Transmitter and receiver subsystems incorporating very narrow line
width, single-frequency lasers for coherent communications systems (coherent
detection).
· New types of ultralow-noise APDs.
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OPPORTUNITIES IN TELECOMMUNICATIONS
Photonic Technologies Within Equipment
Enabling Technologies Ripe for Development
19
~ Low-cost, high-reliability transmitter and receiver modules for short-
distance point-to-point interconnections between boards and shelves to replace
metallic cable connections. Note: although a number of U.S. companies
successfully market transmitter and receiver modules for point-to-point
interconnections within equipment and between pieces of equipment, continued
diligence to increase reliability and reduce cost is required as this market grows.
Enabling Technologies Requiring Continued Research
· Optical traces ("conductors") on circuit boards and backplanes to replace
conventional metallic traces.
· Optoelectronic assemblies to interconnect to optical traces on circuit
boards and backplanes (transmitter and receiver components integrated with
electronic devices).
· Free-space optical interconnects.
· Optical transmitter and receiver arrays for parallel interconnects.
Photonic Networks
Enabling Technologies Requiring Continued Research
· Tunable optical transmitters and receivers.
· Optical amplifiers with gain over a wide wavelength range.
· Photonic switching devices and subsystems.
· Coherent communications technologies.
Military Applications
Enabling Technologies Ripe for Development
· Ruggedized versions of commercial technologies with increased toler-
ance to physical abuse (e.g., temperature range for storage and operation,
vibration, shock, corrosive environments, and radiation).
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20
Enabling Technologies Requiring Continued Research
PHO TONICS
~ Ruggedized versions of commercial technologies indicated previously
as candidates for continued research.
· Higher-performance versions of commercial devices (e.g., ultralow-loss
optical fibers, higher-power optical transmitters, ultrasensitive optical receivers)
that may not be practical for high-volume or widespread commercial deploy-
ment but that may be practical for specialized military applications with high
tolerance to cost.
THE IMPORTANCE AND IMPACT OF STANDARDS
AND MODULARIZATION
in commercial point-to-point telecommunications applications of f~ber-
optic technologies, network providers have procured various components of
these systems separately. By taking advantage of standard or open interfaces
(e.g., interfaces with publicly documented and stable specifications) between
major components of a system, network providers can purchase optical fiber
cables from one manufacturer, optical line and span-terminating repeaters from
another manufacturer, and multiplexing equipment from yet another manufac-
turer. This approach allows more competitors to enter the marketplace, since
potential competitors with specialized capabilities need not develop capabilities
in all aspects of the system in order to offer products. This approach will likely
be followed as BISDN (fiber to the home and business) procurements are made.
It is likely that large-volume purchasers of local networks will want to have the
option of procuring fiber cables separately from terminal equipment.
From a purchaser's viewpoint, this approach typically results in lower cost
due to increased competition. From a seller's viewpoint, it is a two-edged
sword. A seller offering a complete range of system technologies might prefer
that procurements be made on a system basis so that fewer competitors are
capable of making an offering. On the other hand, a seller who lacks one or
more key technologies (or who does not desire to invest in one or more key
technologies) would prefer the disaggregated procurement approach.
From a U.S. competitiveness viewpoint, disaggregation can prevent U.S.
companies from being locked out of a large market because of weakness in one
or more key technologies that individually represent only a small part of a much
bigger system.
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OPPORTUNITIES IN TELECOMMUNICATIONS
SUMMARY
21
Although fiber optics has become the dominant technology for point-to-
point long- and moderate-distance telecommunications applications and has
emerged as a multibillion-dollar-a-year business in the 1980s, the largest
applications of fiber and related photonic technologies in local area networking,
metropolitan area networking, and BISDNs are yet to come. As existing and
emerging markets evolve, low cost, high reliability (quality), and high perfor-
mance will become increasingly important. Whereas research and engineering
that precede transfer of technology to the factory will continue to be important,
increased emphasis on perfection of manufacturing processes (manufacturing
engineering) will likely be critical to U.S. competitiveness in these markets in
the future. Manufacturing success will require the close cooperation of people
concerned with materials, devices, and systems. Certain key technologies ripe
for development have been identified, all of which emphasize development of
lower-cost, higher-quality versions of existing technologies. Numerous areas
requiring continued research activity necessary to enable certain new markets
or to reduce costs in existing or emerging markets have also been identified.
The impact of standards and open interfaces on increasing competition and
preventing particular technologies from becoming bottlenecks to competition
in larger system procurements was also discussed. (Refer to Appendix D for
additional technical data.)
REFERENCES
1. Personick, S. D. 1981. OpticalFiber Transmission Systems. New York:
Plenum Press.
Personick, S. D. 1985. Fiber Optics Technology and Applications. New
York: Plenum Press.
Midwinter,J. E. 1979. OpticalFibers for Transmission. New York: Wiley
and Sons.
4. Kao, K. C. 1982. Optical Fiber Systems--Technology, Design, and
Applications. New York: McGraw Hill.
IEEE. 1983. Special Issue on Fiber Optic Systems. Journal on Selected
Areas in Communications SAC-13 (April).
IEEE. 1985. Special Issue on Fiber Optic Local Area Networks. Journal
of Lightwave Technology LT-3~3) (June).
7. IEEE. 1984. Special Issue on Undersea Cable Fiber Optic Systems.
Journal of Lightwave Technology LT-2~6) (December).
8. IEEE. 1986. Special Issue on Broadband Communication Systems.
Journal on Selected Areas in Communications SAC-4 (July).
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PHO TONICS
9. IEEE. 1988. Special Issue on Fiber Optic Local and Metropolitan Area
Networks. Journal on Selected Areas in Communications SAC-6 (July).
10. IEEE. 1988. Special Issue on Photonic Switching. Journal on Selected
Areas in Communications SAC-6 (August).
Representative terms from entire chapter:
continued research
