Optics Advances Boost Bandwidth

The cable industry's never-ending search for more bandwidth
is moving its suppliers into a new push for cutting-edge solutions, elements of which were
evident in new products introduced at the Western Show in Los Angeles last week.

Every portion of the network is under bandwidth pressure or
soon will be as the industry continues to add services, even in systems where upgrades
have just been completed.

Perhaps most significant, the industry's efforts to
maximize its broadband advantage against stepped-up competition from digital subscriber
line are likely to require support for high-bandwidth applications like video telephony
and home-grown content distribution in the return channel much sooner than previously

"I think we're entering a pretty intense phase where
manufacturers will be offering many different approaches to satisfying bandwidth
needs," Harmonic Inc. CEO Tony Ley said. "It will probably require a lot of
experimentation by MSOs before the industry decides which ones are best."

As one of the suppliers of optical systems in AT&T
Corp.'s trial of its "LightWire" architecture in Salt Lake City, Harmonic has
found that MSO engineers are paying close attention to the carrier's progress, whether or
not they believe the idea of pushing fiber to coaxial serving areas handling about 80
households is the best way to go.

"The mux node [multiplexing node used in the LightWire
design] is now a product, but people will wait to see what happens with AT&T before
they use it themselves," Ley noted.

One of the new solutions to conserving bandwidth that is
taking hold is the use of digital lasers to aggregate return signals at the secondary hub
from diverse node feeds for transport back to the headend in uncompressed baseband format.

Harmonic and General Instrument Corp. introduced their
first baseband-return systems at the Western Show, following in the tracks of
Scientific-Atlanta Inc. and Synchronous Group.

Synchronous is the first to offer an 8-bit digital-sampling
option, which increases the number of feeds that can be supported by a single laser.

"We're offering a four-channel return system with a
return rate multiplexed [from the four node feeds at 5- to 40-megahertz] to 3.2 gigabits
per second," Synchronous chairman Vince Borelli said. "We also offer a
two-channel system using 10-bit encoding and a single-channel system using 12-bit

The different coding rates translate into different levels
of quality in the transmission path. The noise-power ratio, which is used as a measure of
digital baseband-signal quality, is 37 decibels for the 8-bit system, 41 dB at 10-bit
sampling and 45 dB at 12 bits, Borelli noted.

"In most situations, 37-dB NPR is more than adequate,
so you can take advantage of the higher efficiency of a four-port system," Borelli

Synchronous introduced an option in its "Corvus"
digital return-path system last week that allows operators to choose different
bit-sampling rates for different Corvus links within a given cable system so that all
links can be tuned to maximum efficiency within acceptable performance margins.

Synchronous broke new ground on other fronts, as well, with
its introduction of a 32-wavelength dense-wavelength-division-multiplexing return-path
system and a new type of erbium-doped fiber amplifier that is designed to operate deep in
the network. The company also said its new 80-channel single-fiber supertrunk, operating
in the 1550-nanometer wavelength window, matches the performance level of two-fiber
supertrunking systems, with carrier-to-noise performance of 58 dB.

For its part, GI, along with introducing a digital
baseband-return system, also brought out a new type of gallium arsenide/silicon hybrid
amplifier for use in fiber nodes that is designed to maximize the distance signals can
travel over coax without requiring further amplification.

"This is an important cog in fiber-deep architecture,
where the operator wants to get as much reach as he can without using in-line
amplifiers," said Bickley Remmey, senior vice president and general manager for
transmission-network systems at GI.

Remmey, echoing Ley, said the paths operators take toward
expanding bandwidth are likely to be highly varied for a long time to come, no matter how
well AT&T does with its LightWire system. "There are operators that will follow
AT&T's lead, but many others have different ideas about how to expand their network
capacities," he added.

GI is now supplying DWDM systems to AT&T Broadband
& Internet Services in Chicago and Grand Rapids, Mich., for use in its current
architectural mode, which entails delivery of optical signals to coax serving areas of
about 600 homes.

"AT&T, by going to DWDM, is now setting up one
approach to expansion, while operators like Time Warner [Cable] and Cox [Communications
Inc.], which have already built out most systems to 500- or 800-home HFC [hybrid
fiber-coaxial] architectures, look at DWDM as a possible way of expanding later," he

One of the new developments that could have a major impact
on approaches to future system expansions involves tight integration of the electronic and
optical operating components at the chip level. S-A is working closely with U.K.-based
Bookham Technology Ltd. to quickly bring such integration capabilities to bear on
cable-network expansion.

Bookham has developed a means of forming complex optical
circuits on mass-produced silicon chips, which has the potential to miniaturize and reduce
the costs of products to be used in DWDM, return-path transmissions and fiber optic nodes.

One of its first products is an integrated
transmitter/receiver that's being used by Japan's Nippon Telegraph and Telephone Corp. and
other entities around the world to help lower the cost of fiber-to-the-home systems.

"We're seeing a tremendous surge in demand for our
technology from systems manufacturers that are looking for solutions that will overcome
some of the limitations of opto-mechanical technology," Bookham vice president for
business development Robert Green said. "We're still a long way from the ultimate in
solid-state optical networking, but we're moving a good way up the scale from where we've

Bookham's patented technique involves alignment of passive
optical components onto silicon chips while using the standard production techniques of
silicon chips to produce high volumes of devices at low costs.

The resulting miniaturization of the complex circuitry
involved in the interface between electronic devices and optical receivers, wave guides
and other components plays well with the needs of architectures such as LightWire, noted
Paul Connolly, vice president of marketing and network architectures at S-A.

"The technology has implications for increasing the
density of wavelengths for return-path transmissions, where the transmitter and receiver
operate on the same chip, and for many other applications," Connolly said. "It's
looking like we'll be able to make this transfer of their technology to our needs very

Longer term, S-A and Bookham are working on approaches to
integrating a wavelength demultiplexer and photoreceiver onto the Bookham chip, thereby
creating a miniaturized, low-cost means of handing off a wavelength from a multiwavelength
stream at a micronode on the coaxial cable plant. This way, a single fiber could serve
multiple micronodes with dedicated wavelengths, vastly lowering the cost of extending
fiber deep into the cable plant.

The technology could also be used to take in multiple
wavelengths and translate them to separate RF feeds, Connolly said.

Another approach to increasing bandwidth efficiency that
will surely get attention in the near future involves the use of signal-processing
technology to dynamically adjust the allocations of bit streams to specific applications.
Here, the idea is to reduce the bandwidth requirement for the application, rather than to
assign more bandwidth to it, Ley said.

"You're going to see the use of digital processing in
MPEG [Moving Picture Expert Group] and other formats pushed further into the network with
continuing gains in the amount of functions you can integrate onto chips," he added.

Ley declined to discuss the idea further, suggesting that
he didn't want to give away Harmonic's game plan. But with Harmonic's soon-to-be-concluded
acquisition of digital-encoding specialist DiviCom Inc., it's clear that the vendor is
looking at ways to lower the volume of bits required to transport MPEG and other signals
at a prescribed level of quality.

"You're also going to see processing applied to
eliminating the noise from the signal," Ley said. "Right now, with digital
baseband, you're returning most of the noise that's in the original analog signal back to
the headend."