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The Broadest Broadband
A Less Expensive Way to Bring Fiber to the Home
by Paul W. Shumate, Jr.
Optical fiber has successfully replaced metallic cables in the backbone networks of most local and long-distance telephone carriers,
cable television operators and utility companies. So why not extend lines of optical fiber all the way to customers' homes? Doing so
would eliminate the so-called last-mile bottleneck that restricts users' access to the Internet and other services. In current
fiber-to-the-home systems, customers can download data at up to 100 million bits per second (future fiber networks may perform
much more quickly), which is at least 10 times faster than transmissions using metallic cables today. The capacity of optical fiber is so
enormous that it can handle all types of communications signals simultaneously; for example, telephone, television, videoconferencing,
movies-on-demand, telecommuting and Internet traffic could all be carried together on one user's fiber.
Until recently, the major obstacle to fiber-to-the-home has been its high cost. Dedicating one or two lines of fiber to each customer,
along with the electronics needed at each end to transmit and receive the optical signals, is very expensive. For a typical suburban
home, the one-time installation cost--including the cost of all the equipment--is currently about $1,500. (Approximately half is
attributable to the electronics.) This figure has dropped from about $5,000 a decade ago and is continuing to fall as fiber technology
advances. But it is still higher, in most cases, than the cost of connecting a comparable home with metallic cable.
Because backbone networks are highly multiplexed--that is, a single fiber in the network
can carry many independent channels of signals--the cost per channel is relatively low.
Therefore, telephone and cable TV companies take advantage of fiber's astounding
capacity by installing it partway to homes. In these systems, often called
fiber-to-the-node, fiber-to-the-cabinet or fiber-to-the-curb, a few fibers connect a
service provider to an enclosure near a group of homes. This enclosure holds the
equipment for converting the optical signal in the fiber to an electrical signal that can
travel on metallic cables. The metallic connections to the homes vary in distance from
less than 30 meters (100 feet) to slightly more than one kilometer. Such systems are
economical because they share the cost of the fiber and electronics among all the subscribers in the group, who can number in the
hundreds. The transmission rates, however, are lower than those of fiber-to-the-home, because the final metallic connections act as
bottlenecks.
Networks of Light
Fiber-to-the-home has been technically successful in more than three dozen field trials and installations worldwide; the earliest was in
Higashi-Ikoma, Japan, in 1977. Now it is becoming commercially viable, thanks to recent advances in architectures and technologies.
The first important change was the development of the passive optical network, or PON, in which a single fiber extends from an
optical transceiver at the service provider's location to an optical splitter near a small group of homes. The splitter divides the light
signals equally among 16 or 32 output fibers, which then carry the signals to the customers' homes. In 1998 the International
Telecommunication Union (ITU) standardized the specifications for such networks. PONs are less expensive than earlier
fiber-to-the-home systems because they do not require the installation of an optical transceiver and a full-length fiber for each
customer. Other factors making fiber-to-the-home less costly include new lasers, optical components, fiber cables and digital
integrated circuits designed specifically for the application.
Distance isn't as important with fiber as it is with copper cables, because optical signals lose very little power as they move through
fiber. PONs work as well in rural areas, where homes are widely separated, as they do in suburban or urban settings. The network
does entail equipment and installation costs that are not fully shared by the customers, such as the expense of the final length of fiber
from the splitter to the home and of the terminal in the home where the optical/electrical conversions take place. A group of
manufacturers, though, has outlined a strategy for aggressively reducing those costs over the next few years.
In these optical networks, a primary challenge is to organize the digital traffic so that a single fiber can simultaneously accommodate
different types of signals. Voice and video, for example, require nearly constant data rates, whereas file transfers and e-mail can be
transmitted in intermittent streams. Several techniques are used to pack all these signals into the available bandwidth. The
ITU-specified network uses asynchronous transfer mode, which has been designed to handle such a mix of data efficiently. An older
protocol called Ethernet has the advantage that inexpensive Ethernet plug-in cards are widely available for personal computers.
Frequency-division multiplexing creates many channels in a single fiber by modulating the light at a different frequency for each
channel. The most advanced PONs will employ wavelength-division multiplexing, which assigns a unique wavelength, or color, of light
for each customer's signal. Most wavelength-multiplexing devices are now expensive, but costs will fall rapidly because the technique
is commonly used in backbone networks.
Although a few fiber-to-the-home systems dating from the trials of the 1980s and early 1990s remain in operation, the
telecommunications industry's focus is now on the newer technologies. Passive optical networks have been tested in England, France,
Belgium, Bermuda and Japan. BellSouth and NTT--Japan's largest telecommunications company--are planning to install products
compliant with the ITU standard later this year. Frequency-division multiplexed fiber systems are operating in several rural
communities in the Midwest, providing telephone, television and Internet services. In rural areas, fiber-to-the-home appears to be
more economical than conventional networks are, regardless of the type of multiplexing used.
With the introduction of inexpensive standardized products, many businesses are reconsidering the advantages of fiber-to-the-home.
The demand for high-speed digital services is sure to grow, which means that customers will be clamoring for systems that can
cheaply deliver extraordinary amounts of data. Optical fiber can transmit more data than any other medium on the market, and the
cost of connecting it to homes is dropping rapidly. So in the coming years more and more customers will decide that
fiber-to-the-home is the most attractive broadband option.
The Author
PAUL W. SHUMATE, JR., is executive director of the Lasers and Electro-Optics Society of the Institute of Electrical and
Electronics Engineers. Prior to August 1999 he was a chief scientist and assistant general manager at Telcordia Technologies
(formerly Bellcore), in Morristown, N.J. He can be reached at p.shumate@ieee.org. |
