Cable Puts The Squeeze On Broadband Content
Thanks to Stephen Temple on the VoIP Thread
January 12, 1999
Inter@ctive: Wavelength Division
Multiplexing may let operators deliver
services over existing plant
By Fred Dawson, Contributing Editor
New network technology now being
deployed by Tele-Communications Inc.
could help the cable industry avoid what
is shaping up to be a messy bandwidth
problem this year.
Many companies are preparing to test the
TCI approach in 1999, prompting a
growing list of vendors to introduce
products that represent both a departure
from and a low-cost migration supplement
to the 1,310-nanometer (nm) amplitude
modulation lightwave platform used
throughout the cable industry.
The question facing cable engineers is
how best to exploit a set of options based
on Wavelength Division Multiplexing
(WDM) technology at the 1,550-nm
wavelength.
"The combination of TCI and AT&T
backing this technology has everybody in
the industry looking at it," says Randy
Schmid, director of marketing for the
analog transport systems business unit at
ADC Telecommunications (www.adc.com).
There's urgency behind the interest,
because the bandwidth growth curve is
proving to be steeper than many
strategists predicted. That growth is
being driven in the short term by
accelerating preparations for entering the
data and voice businesses and in the
longer term by plans for rollouts of
video-on-demand (VOD) and
high-definition television (HDTV) services.
The immediate squeeze from data and
voice services is on the narrow 5- to
50-megahertz (MHz) upstream path. But
by year's end, even the 750 MHz of
downstream bandwidth at current node
divisions of 1,000 homes or more may
become a problem as operators look to
deliver 80 analog 6-MHz channels, a
sizable chunk of digital broadcast
channels, VOD and HDTV, which takes up
more than six times the amount of
bandwidth consumed by the average
Motion Picture Experts Group2 channel,
says Paul Connolly, vice president of
marketing and network architecture at
Scientific-Atlanta (www.sci-atl.com).
Harmonic Divergence
TCI (www.tci.com) is using gear from
Harmonic Lightwaves to distribute
dedicated digital signals from its headends
over separate wavelengths on the same
fiber to primary distribution hubs. At the
hubs, one of the dedicated wavelengths
carrying digital broadcast, data and, in
the future, VOD services for customers in
that part of the network is optically
amplified, passed through optical splitters
and then combined with a 1,310-nm
analog video feed onto a fiber to each
coaxial interface node.
This approach contrasts with today's
all-1,310-nm wavelength distribution
systems, in which high-capacity fiber
rings or dedicated fiber links deliver a
combined 750-MHz payload of analog and
digital services from the headend to each
hub, using time division or frequency
division rather than wavelength division to
apportion out dedicated signals among
the hubs.
The TCI approach takes advantage of the
fact that vendors have learned how to
combine multiple wavelengths of digital
radio frequency quadrature amplitude
modulated (QAM) signals onto a single
fiber. While makers of Dense WDM
(DWDM) gear are able to supply systems
that can carry dozens of baseband time
division Synchronous Optical Network
(SONET) streams over a single fiber, the
maximum number of wavelengths carrying
frequency-divided QAM signals achieved
so far is 16, a benchmark just reached in
a new distribution system from
Scientific-Atlanta.
"We're working on being able to offer a
DWDM system that would allow you to
carry the analog video signals on a
wavelength within the same fiber, but
we're not there yet," Connolly says.
TCI can combine a 1,310-nm analog video
wavelength with a 1,550-nm QAM
wavelength over fiber from the hub to
each node because of the difference in
power levels between the two signals,
says Mark Trail, director of product line
management for transmission systems at
Harmonic Lightwaves. "We have several
other [cable] customers making use of
this technology," he adds.
The new wavelength multiplexing strategy
lets operators expand incrementally into
the digital domain by adding 1,550-nm
wavelengths without reconfiguring or
replacing the 1,310-nm optoelectronics
already in place. It also lets operators
push fiber deeper into the network
without having to add more lasers at the
hubs to accommodate more nodes.
Deeper fiber means fewer customers per
coaxial serving area, which means there
are fewer customers contending for
upstream bandwidth, Trail says. That
means the new approach also has
ramifications for resolving the upstream
bandwidth crunch.
The VOD Curve
Synchronous Group, the first company to
offer optical amplifiers and DWDM
products for the cable industry, is working
on a new generation of technology that
fully exploits the advantages of new
optical technology with an eye toward
VOD deployment, says Al Johnson,
president and chief operating officer of
the company. "The more reach you have
with high-wavelength density, the more
centralized your VOD operation becomes,"
he says.
When signals are already being
regenerated at each hub, it makes sense
to put the VOD server there, Johnson
says. But with the centralized point of
distribution for dedicated signals offered
by DWDM, a single master server or small
server cluster can serve the entire cable
system, he adds.
Interest in DWDM also is driving vendors
to look at alternatives to SONET as a
means of assuring adequate protection
against system failures, says David
Berman, director of integrated video
solutions at Northern Telecom
(www.nortelnetworks.com), which
recently signed an agreement to acquire
DWDM system supplier Cambrian Systems
(www.cambriansys.com). "When people
are looking at connecting hubs to a single
master headend, they don't necessarily
want to use SONET, which requires that
signals be remodulated and regenerated
for distribution to nodes," Berman says.
Cambrian and Nortel Networks are
developing a ring-based DWDM product
based on Cambrian's OPTera technology,
which provides route protection over
DWDM for signals operating in native
protocols over different wavelengths.
"SONET is great if you're trying to get one
hell of a lot of bits through on a single
wavelength," Berman says. "But if you're
using QAM channels, that's the equivalent
of putting DS-3 [45 megabits per second]
on each wavelength. That's not an
efficient use of SONET."
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