Vendors Exploring Ways to Pump Up Fiber


Several potentially compelling but largely untried new
approaches to the use of fiber in regional backbone and long-haul networks promise to make
life more complicated for network planners in the year ahead.

While in some cases, companies pushing these new ideas are
offering their technologies as system solutions directly to carriers, more often than not,
they're working behind the scenes as OEMs (original-equipment manufacturers) for major
system vendors.

Either way, the new capabilities will require careful
scrutiny and comparison across a wide range of unfamiliar technical concepts.

A case in point is a new technique known as "DMT"
(Dynamic Synchronous Transfer Mode), which has been embraced for possible inclusion in
forthcoming optical systems by Ericsson Inc.

Developed by Sweden-based start-up Dynarc, the technology
involves the use of time-division multiplexing and a very simple set of protocols to make
it possible to transmit Internet-protocol packets at a level of efficiency that represents
a four-to-one improvement over the IP-carrying capacity of SONET (synchronous optical
network), according to Ken Feldman, CEO of Airpower Communications Inc., a new competitive
local-exchange carrier based in Los Angeles.

"DMT supports spatial reuse of bandwidth on a fiber
ring, which means that unlike SONET, the bandwidth on the portion of the ring not used in
a particular transmission is available for other transmissions," Feldman said.

"If you're using four line cards operating at 1
gigabit per second each over four fibers, your effective bandwidth translates to 16 gbps
before you even get to DWDM [dense-wave-division multiplexing]," he added.

Airpower, which is backed by private funding, has begun
building out infrastructure for its launch of integrated broadband services in the Los
Angeles area by the first quarter of next year. The CLEC is using the DMT technology over
fiber to link about 60 fixed-wireless-access base stations delivering 200 megahertz of
bandwidth in the unlicensed segment of the 5-gigahertz band.

The company -- which is already providing some services
over another, lower-frequency band in some parts of Los Angeles -- plans to build into
other cities quickly next year, giving it an opportunity to offer end-to-end connections
that fully exploit the DMT capability, Feldman said.

"We believe the use of DMT will separate us from the
competition," he added. "It's unbelievable technology that's priced shockingly
low for the type of thing it's capable of."

Dynarc -- which is supplying its technology directly to
Airpower as a complete system -- is also working with Ericsson to help integrate DTM into
that vendor's optical systems, president and cofounder Olov Schagerlund said.

"I'm not at liberty to say when Ericsson will release
product using our technology, but I can say that since we entered into an agreement with
them in the April/May time frame, it has become a major engineering project in our
lab," he said. "Our own product is ready for shipment, so you can assume that it
won't take too long for Ericsson to put it to use."

As explained by Schagerlund, DMT multiplexes all of the
native trafficking instructions from the IP-packet header into the
optical-signal-management time slots, adding only a small amount of information in
separate time slots to round out the instruction set required for directing and protecting
the signal over the optical link.

This approach allows direct coupling of IP routers into
fiber without requiring the intervening signaling reconfiguration of SONET, greatly
simplifying the end-to-end connectivity of customers in the IP domain, Schagerlund said.

Moreover, he added, in addition to the bandwidth efficiency
made possible by spatial reuse on the ring, DMT adds to efficiency by allowing signals to
be packed into the flow of time slots at a high level of granularity, thereby eliminating
the gaps that occur in SONET when the basic 51-megabit-per-second frame is unfilled by a
particular transmission.

This granularity also provides support for dynamic shifts
in how much bandwidth is allocated to any one signal at a given moment in time, he said.

"If you have a video transmission operating in MPEG
[Moving Picture Expert Group] at 4 megabits per second, and you want to raise the data
rate to accommodate a high-quality segment of the video stream, you can do that in
increments of 0.5 mbps," Schagerlund added.

While Dynarc is basing its strategy on the advantages of
tight coupling of the optical layer into IP routing -- thereby supporting electronic
switching speeds that obviate the need for optical switches -- a number of other new
companies are bringing gear to market that they said represents the first instance of
products that make switching and routing in the all-optical domain a practical option.

"We have achieved solid-state performance levels using
our liquid-crystal-polarization technology, which we'll be able to apply in a number of
products, including switches, that we'll be phasing in through the first half of next
year," Chorum Technologies Inc. vice president of marketing Doug Dickerson said.

Based in Richardson, Texas, Chorum is working with several
major systems manufacturers to provide them with the means to support a variety of
optical-signaling processes in next-generation networks, starting with a basic
DWDM-multiplexing technology that will ship in the fourth quarter, Dickerson said.

The various applications for the company's
"PolarWave" technology -- which uses the polarization-shifting capabilities of
liquid crystal to segment or combine wavelengths -- include DWDM routers, optical
processors, integrated optical systems and optical switches, he added.

"To get to multiterabit levels of transport using DWDM
over a single fiber, the industry requires extremely fine filtering to provide the
isolation and shaping of signals," Dickerson said. "We can apply the
liquid-crystal-polarization process to do this for basic DWDM multiplexing, and then use
it as the foundation for the other applications, as well."

Going all the way to solid state in the supply of new
components, Scotland-based Kymata Ltd. has just introduced a line of integrated optical
circuits -- including a 16-channel arrayed-wave-guide grating and a variable optic
attenuator -- which eliminate the need for thin filters or other discrete optical
components that typically support DWDM applications.

Kymata's goal is to address industry needs for extremely
compact high-performance components through production of 20-by-40-millimeter
silica-on-silicon chips on a planar-fabrication line that will churn out six-inch wafers
starting at the end of the year, marketing director Bob Press said.

"Of the handful of suppliers currently fabricating
optical ICs [integrated circuits], much of the production is geared toward satisfying the
needs of in-house systems departments," Press noted. "Kymata's foundry provides
a capacity injection that will stabilize the market."

Its next product release will involve use of the optical
ICs in thermo-optic switches, Press added.

Still another approach to maximizing the performance of
DWDM systems -- this one using the elusive phenomenon known as "soliton wave" --
is under way in Brittany, France, at start-up Algety Telecom.

Algety -- which is preparing to open an office in the
United States -- plans to bring its first soliton-based products to market by mid 2000,
said Jerome Faul, managing director of the privately held firm.

Solitons -- which are unusually long-lasting waves that
occur in nature when two or more separate waves combine under certain conditions -- have
long been pursued as a way to minimize the loss in coherency of optical waves as they
travel through fiber.

Researchers have found that the soliton can be broken back
into its original constituent waves by an interfering wave at the receive end, thereby
allowing the originating signals to be recaptured.

But achieving precise replication of the original signals
has proved to be far more difficult than originally anticipated, leading to an endless
stream of technical papers on the subject at optical-communications conferences, with no
development of products until now.

While there have been some early implementations of soliton
communications for undersea fiber links, Algety is the first company to announce a
soliton-based product designed for the terrestrial long-haul market.

A hint of what Algety can do with solitons was made public
recently in a lab demonstration, where the company's system delivered 51 wavelengths at
speeds of 20 gigabits per second each, achieving an aggregate throughput of 1 terabit per
second over a distance of 1,000 kilometers.

"I'm not going to say what our performance targets are
for our products, but I think that gives you an idea of what we are capable of," Faul

"If we reduce the distance, we can operate at higher
capacity," he added, noting that these performance levels can be achieved over any
type of single-mode fiber. He said the test system meets the typical carrier long-haul
standard of 10-15 in bit-error-rate performance.

Presently, the maximum carrying capacity of a commercially
available DWDM system is 1.6 tbps -- the rate touted by Nortel Networks for its new
"OPTera Long-Haul" system, which is now undergoing trials in various carrier
networks in preparation for commercial product shipments early next year.

Typically, this and other current-generation DWDM systems
use optical amplification to extend the distance between points of optoelectronic
regeneration of the signal, usually maxing out at 200 km to 400 km, depending on the
vendor and the performance specs set by the customer.

Along with achieving much greater distances before
regeneration by using solitons, the Algety system also delivers the signals at rates of 20
gbps per wavelength, which is twice the maximum speed per wavelength in other DWDM

This is done by combining two OC-192 (10-gbps) laser inputs
into a single soliton wave at the transmission point, with the output delivering the
signal as a dual OC-192 feed for conversion back to electronic signals, Faul said.

"We're in discussions with several potential
customers," he added. "We've had good feedback so far."