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Representative terms from entire chapter:
dark fiber
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32
The Fiber-Optic Challenge of Information Infrastructures
P.E. Green, Jr.
IBM T.J. Watson Research Center
This paper discusses the role of optical fiber as the one
physical medium upon which it will be possible to base national and
global infrastructures that will handle the growing demands of
bandwidth to the desktop in a post-2000 developed society.
A number of individual applications today demand large bit rate
per user, such as supercomputer interconnection, remote site backup
for large computer centers, and digital video production and
distribution, but these are isolated niches today. The best gauge
of the need for the infrastructure to supply large amounts of
bandwidth to individual users is probably to be found in the
phenomena associated with the use of the Internet for
graphics-based or multimedia applications.
Just as the use of noncommunicating computers was made much
easier by the emergence of the icon- and mouse-based graphical user
interface of the Macintosh, Windows and OS2, the same thing can be
observed for communicating computers with the World Wide Web. The
modality in which most users want to interact with distributed
processing capability (measured in millions of instructions per
second [MIPs]) is the same as it has always been with local MIPs:
they want to point, click, and have an instant response. They will
in fact want to have a response time from some remote source on any
future information infrastructure that is a negligible excess over
the basic propagation time between them and the remote resource.
They will want the infrastructure to be not only widebanded for
quick access to complex objects (which are evolving already from
just still graphics to include voice and video) but also to be
symmetric, so that any user can become the center of his or her own
communicating community. This need for an "any-to-any"
infrastructure, as contrasted to the one-way or highly asymmetrical
character of most of our wideband infrastructure today (cable and
broadcast), is thought by many political leaders to be the key to
optimizing the use of communication technology for the public
good.
Thus, a dim outline of many of the needs that the information
infrastructure of the future must satisfy can be discerned in the
emerging set of high-bandwidth usage modes of the Internet today 1R, particularly the Web. The picture
that emerges from examining what is happening in the Web is most
instructive. Figure 1 shows the recent and projected growth of Web
traffic per unit time per user assuming the present usage
patterns, which include almost no voice, video clips, or high
response speed applications such as point-and-shoot games or
interactive CAD simulations. As these evolve, they could exacerbate
the already considerable bit rate demand per user, which Figure 1
shows as a factor of 8 per year. If the extrapolations in Figure 1
are correct, this means that in the decade to 2005, the portion of
the available communication infrastructure devoted to descendants
of the Web must undergo a capacity growth of about 109 in order to keep up with demand.
There is only one physical transmission technology capable of
supporting such growth: optical fiber. Fortunately for the
prospects of an infrastructure that will provide society what it
needs, fiber has been going into the ground, on utility poles,
within buildings, and under the oceans at a rapid rate. The
installation rate has been over 4,000 miles per day for some years,
just in the continental United States alone, so that by now over 10
million miles of installed fiber exist here. Even more fortunately,
each fiber has a usable bandwidth of some 25,000 GHz,
roughly 1,000 times the usable radio spectrum on planet Earth, and
quite enough to handle all the phone calls in the U.S. telephone
system at its busiest. While this gigantic capacity is underused by
at least a
Page 249
Figure 1
Predicted World Wide Web bandwidth demand.
SOURCE: Data courtesy of the Internet
Society.
Figure 2
The ''last mile" bandwidth bottleneck.
factor of 10,000 in today's practice, which is based on time
division multiplexing, the technical means are rapidly evolving to
open up the full fiber bandwidth. This is the all-optical
networking technology, based on dense wavelength division, in which
different channels travel at different "colors of light."
So, why isn't it true that we already have the physical basis in
place over which to send the traffic of the future? Most of the
answer is summarized in Figure 2 2R.
All the communication resources we have been installing seem to be
improving in capacity by roughly 1.5 per year, totally out of scale
with the 8-times-per-year growth of demand shown in Figure 1. The
top curve of Figure 2 shows the capability of desktop computers to
absorb and emit data into and out of the buses that connect them to
the external world 3R. The next line
shows local area network capacity as it has evolved. The third one
shows the evolution of high-end access to the telco backbone that
allows users at one location connectivity to users elsewhere
outside the local LAN environment. The capacity of this third curve
has been available only to the most affluent corporations and
universities, those that can afford T1, T3, or SONET connections to
their premises.
Page 250
While all three of these capacities are evolving at the rate of
only a factor of 1.5 per year, they represent really substantial
bit-rate numbers. Current Web users who can afford 10 Mb/s LANs and
T-carrier connections into the backbone experience little response
time frustration. However, the situation of most of us is
represented more accurately by the bottom curve, which shows the
data rate available between our desktop and the backbone over the
infamous "last mile" using telco copper connections with either
modems or ISDN. There is a 104
performance deficiency between the connectivity available between
local and long-haul resources and the internal performance of both
these resources.
If one compares the rate of growth of Web traffic in Figure 1
with the data of Figure 2, it is clear that there is an acute need
to bridge the 104 gap of the last
mile with fiber and inevitably to increase the bandwidths of the
backbone also, probably at a greater rate than the traditional
factor of 1.5 per year.
As for bridging the gap between the desktop and the telco
backbone, the proposed solution for years now has been "fiber to
the home" 4R, expressing the notion
that it must pay for itself at the consumer level. The alternative
of coaxial cable to the premises, while having up to a gigahertz of
capacity, is proving an expensive and nonexpandable way to
future-proof the last mile against the kind of bandwidth demands
suggested by Figure 1, and the architectures used have assumed
either unidirectional service or highly asymmetrical service. What
is clearly needed is fiber, probably introduced first in
time-division mode, and then, as demand builds up, supporting a
migration to wavelength division (all-optical).
Figure 3 shows the rate at which fiber to the premises ("home")
has been happening 5R in the United
States. The limited but rapidly growing amount of fiber that is
reaching all the way to user premises today is mostly to serve
businesses. The overall process is seen to be quite slow;
essentially nothing very widespread will happen during the next 5
to 7 years to serve the average citizen. However, the bandwidth
demand grows daily. Meanwhile, all-optical networks are beginning
to migrate off the laboratory bench and into real service in small
niches.
What Figure 3 shows is the steady reduction of the number of
homes that, on the average, lie within the area surrounding the
nearest fiber end. In 1984, when fiber was used only between
central offices (COs), this figure was the average number of homes
or offices served by such a CO. As the carriers 6R, cable companies 7R, and competitive local access
providers 8R found great economies
in replacing copper with fiber outward from their COs and
head-ends, the number decreased. A linear extrapolation down to one
residence per fiber end predicts that 10 percent of U.S. homes will
be reached by fiber by about 2005, at best. In Japan, it is quite
possible that a strong national effort will be launched that will
leapfrog this lengthy process using large government subsidies 9R.
During the coming decade, several things will happen, in
addition to ever increasing end-user pressure for more bandwidth to
the desktop. Competition between telcos, cable companies, and
competitive access providers may or may not accelerate the
extrapolated trend shown in Figure 3. Advances in low-cost
optoelectronic technology, some of them based on mass production by
lithography, could also accelerate the trend, because analyses of
costs of fiber to the home consistently show a large fraction of
the cost to lie in the set-top box, splitters, powering 10R, and, in the case of wavelength
division multiplexing (WDM) approaches, ultiwavelength or
wavelength-tunable sources and receivers. It is widely felt that
the price of the set-top box itself will have to be below $500 for
success in the marketplace. This is probably true, whether the
"set-top box" is really a box sitting atop a TV set or a feature
card within a desktop computer. By 2005 it should become quite
clear whether the TV set will be growing keyboards, hard disks, and
CPUs to take over the PC, whether the PC will be growing video
windows to take over the TV set, or whether both will coexist
indefinitely and separately. In any case, the bottleneck to these
evolutions will increasingly be the availability by means of fiber
of high bit rates between the premises and the backbone, plus a
backbone bandwidth growth rate that is itself probably inadequate
today.
Meantime, looking ahead to the increasing availability of fiber
paths and the customers who need them to serve their high-bandwidth
needs, the all-optical networking community is hard at work trying
to open up the 25,000 GHz of fiber bandwidth to convenient and
economical access to end users. Already, the telcos are using
four-wavelength WDM in field tests of undersea links 11R. IBM has recently made a number of
commercial installations 12R of
20-wavelength WDM links for achieving fiber rental cost savings for
some of its large customers who have remote computer site backup
requirements. The rationale bhind both these commercialization
efforts involves not only getting more bandwidth out of existing
fiber, but also making the
Page 251
Figure 3
Predicted rate at which fiber reaches user premises.
Figure 4
Three wavelength division architecture.
installation "multiprotocol" or "future-proof" by taking
advantage of the fact that each wavelength can carry an arbitrary
bit rate and framing convention format, or even analog formats, up
to some maximum speed set by the losses on the link.
These successful realizations of simple multiwavelength links
represent the simplest case of the three different kinds of
all-optical systems, shown in Figure 4. In addition to the
two-station WDM link (with multiple ports per station), the figure
shows the two forms taken by full networks, structures in which
there are many stations (nodes), with perhaps only one or a few
ports per node.
The second type, the broadcast and select network, usually works
by assigning to the transmit side of each node in the network a
fixed optical frequency, merging all the transmitted signals at the
center of the network in an optical star coupler and then
broadcasting the merge to the receive sides of all nodes. The
entire inner structure, consisting of fiber strands and the star
coupler, is completely passive and unpowered. By means of a
suitable protocol, when a node wants to talk to another (either by
setting up a fixed lightpath "circuit" or by exchanging packets),
the latter's receiver tunes to the former's transmit wavelength and
vice versa. Broadcast and
Page 252
select networks have been prototyped and, while still considered
not quite in the commercialization cost range, have been used in
live application situations, for digital video distribution 13R and for gigabit supercomputer
interconnection at rates of 1 gigabit per second 14R.
Aside from high cost, which is currently a problem with all WDM
systems, there are two other things wrong with broadcast and select
networks. The power from each transmitter, being broadcast to all
receivers, is mostly wasted on receivers that do not use it.
Secondly, the number of nodes the network can have can be no larger
than the size of the wavelength pool, the number of resolvable
wavelengths. Today, even though there are 25,000 GHz of fiber
capacity waiting to be tapped, the wavelength resolving technology
is rather crude, allowing systems with only up to about 100
wavelengths to be built, so far 15R.
The problems of both cost and number of wavelengths are gradually
being solved, often by the imaginative use of the same tool that
brought cost reductions to electronics two decades ago:
lithography.
Clearly, a network architecture that allows only 100 nodes does
not constitute a networking revolution; some means must be provided
for achieving scalability by using each wavelength many places in
the network at the same time. Wavelength routing accomplishes this,
and also avoids wastage of transmitted power, by channeling the
energy transmitted by each node at each wavelength along a
restricted path to the receiver instead of letting it spread over
the entire network, as with the broadcast and select architecture.
As the name "wavelength routing" implies, at each intermediate node
between the end nodes, light coming in on one port at a given
wavelength gets routed out of one and only one port.
The components to build broadcast and select networks have been
available on the street for 4 years, but optical wavelength routers
are still a reality only in the laboratory. A large step toward
practical wavelength routing networks was recently demonstrated by
Bellcore 16R.
The ultimate capacity of optical networking is enormous, as
shown by Figure 5, and is especially great with wavelength routing
(Figure 6). Figure 5 shows how one might divide the 25,000 GHz into
many low-bit-rate connections or a smaller number of
higher-bit-rate connections. For example, in principle one could
carry 10,000 uncompressed 1 Gb/s HDTV channels on each fiber. The
figure also shows that erbium amplifiers, needed for long
distances, narrow down the 25,000 GHz figure to about 5,000 GHz,
and also that the commercially available tunable optical receiver
technology is capable of resolving no more than about 80
channels.
With broadcast and select networks the number of supportable
connections is equal to the number of available wavelengths in the
pool of wavelengths. However, with wavelength routing, the number
of supportable connections is the available number of wavelengths
multiplied by a wavelength reuse factor
17R that grows with the topological connectedness of the net
work, as shown in Figure 6. For example, for a 1,000-node network
of nodes with a number of ports (the degree) equal to four, the
reuse factor is around 50, meaning that with 100 wavelengths, there
could, in principle, be five connections supportable for each of
the 1,000 nodes.
As far as the end user is concerned, there is sometimes a
preference for circuit switching and sometimes for packet
switching. The former provides protocol transparency during the
data transfer interval, and the latter provides concurrency (many
apparently simultaneous data flows over the same physical port, by
the use of time-slicing). In both cases, very large bit rates are
possible without the electronics needing to handle traffic bits
from extraneous nodes other than the communicating partners.
The very real progress that has been made to date in all-optical
networking owes a great deal to the foresight of government
sponsors of research and development the world over. The three big
players have been the Ministry of Posts and Telecommunications
(MPT) in Japan, the European Economic Community (EEC), and the U.S.
Advanced Research Projects Agency (ARPA). The EEC's programs, under
RACE-1 and RACE-2 (Rationalization of Advanced Communication in
Europe), have now been superseded by ACTS (Advanced Communication
Technology Systems).
In 1992, ARPA initiated three consortia aimed at system-level
solutions, and all three have been successful. The Optical
Networking Technology Consortium, a group of some 10 organizations
led by Bellcore, has demonstrated an operating wavelength routing
network using acoustooptic filters as wavelength routers. The
All-Optical Networking Consortium, consisting of the Massachusetts
Institute of Technology, AT&T Bell Laboratories, and Digital
Equipment Corporation, has installed a network that combines
wavelength routing, wavelength shifting, broadcast-and-select, and
electronic packet switching between Littleton, Lexington, and
Cambridge, Massachusetts. With ARPA and DOE support, IBM (working
with Los Alamos National
Page 253
Figure 5
Capacity of broadcast and select networks.
Figure 6
Wavelength reuse.
Laboratory) has developed an extensive set of algorithms for
distributed control of very large wavelength-routing networks, and
has studied offloading of TCP/IP for supercomputer interconnection
in its Rainbow-2 network.
It is fair to say that the United States now holds the lead in
making all-optical networking a commercial reality, and that ARPA
support was one of the important factors in this progress. At the
end of 1995, ARPA kicked off a second round of 3-year consortia in
the all-optical networking area, with funding roughly five times
that of the earlier programs
18R.
Whether all-optical networking will be a commercially practical
part of the NII depends on three factors: (1) whether the
investment will be made to continue or accelerate the installation
of fiber to the premises and desktop (Figure 3), (2) whether it
proves feasible to reduce component costs by two to three orders of
magnitude below today's values, and (3) the extent to which
providers offer the fiber paths in the form of "dark
fiber"—that is, without any electronic conversions between
path ends.
Page 254
This last problem seems to be solving itself in metropolitan and
suburban areas of many countries, simply by competition between
providers, but the problems of long dark fiber paths that cross
jurisdictions and require amplification have yet to be faced. In
the United States, the Federal Communications Commission has viewed
dark fiber as being equivalent to copper, within the meaning of the
Communication Act of 1934 19R20R; that is, if the public interest
requires making dark fiber ends available, one of the monopoly
obligations implied by monopoly privileges is that the public
should be offered it at a fair price.
The optoelectronic component cost issue is under active attack.
Considering that there are significant efforts under way to use
lithography for cost reduction of tunable and multichannel WDM
transmitters and receivers, it seems possible to predict a
one-order-of-magnitude decrease in price by 2000 and two orders of
magnitude by 2005. This implies that the optoelectronics for each
end of WDM links of 32 wavelengths should cost $15K and $1.5K,
respectively, and that the optoelectronics in each node of a
broadcast and select network of 32 to 128 nodes should cost $1,000
and $100, respectively. If these last numbers are correct, this
means that broadcast and select MANs and LANs should be usable by
desktop machines some time between 2000 and 2005, since the costs
would be competitive with the several hundred dollars people
typically spend year after year on each modem or LAN card for
PCs.
The sources of investment in the "last fiber mile" are
problematical. In the United States the telcos and the cable
companies are encountering economic problems in completing the job.
In several other countries with strong traditions of centralized
telecommunication authority, for example Japan and Germany, a
shortcut may be taken using public money in the name of the public
interest. So far in the United States it is "pay as you go." This
has meant that only businesses can afford to rent dark fiber, and
even then this has often been economical only when WDM has been
available to reduce the number of strands required 12R.
Whether a completely laissez-faire approach to the last mile is
appropriate is one of the problems governments are facing in
connection with their information infrastructures. Fiber has ten
orders of magnitude more bandwidth (25,000 GHz vs. 3.5 kHz) and can
operate with ten orders of magnitude better raw bit error rates
that can voice grade copper (10-15
vs. 10-5), and yet on the modest
base of copper we have built the Internet, the World Wide Web,
ten-dollar telephones at the supermarket, communicating PCs and
laptops, prevalent fax and answering machine resources, and other
innovations. It is the vision of those working on all-optical
networking that a medium with ten orders of magnitude better
bandwidth and error rate than one that gave us today's
communication miracles is unlikely to give us a future any less
miraculous, once the fiber paths, the network technology, and the
user understanding are all in place.
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