The cable industry is preparing to accommodate an explosion
in bandwidth requirements by exploiting new options in fiber optics that are tightly
attuned to existing network designs.
With expansion requirements affecting every type of network
-- from the most up-to-date, 750-megahertz, 500-home-node facilities, to much older plant
-- vendors have developed an arsenal of tools incorporating technologies such as
dense-wave-division multiplexing and optical amplifiers. These products are meant to fit
whatever starting point an operator has to deal with.
Some of these new tools are now in the market, while others
are in development for release in the year ahead as the bandwidth crunch hits.
"Customers looking at bandwidth expansion aren't
just those with older plant," noted Paul Connolly, vice president of marketing and
network architecture at Scientific-Atlanta Inc.
"People recognize that with things like
video-on-demand, higher levels of data use, telephony and HDTV [high-definition
television], there could be pressure on bandwidth in the most sophisticated systems."
VOD is an especially significant ingredient in the new
thinking about bandwidth, as it will require operators to find efficient ways to transport
dedicated signals from centrally located or dispersed servers without resorting to
installing new fiber.
In some cases, this will mean having the ability to
transmit multiple QAM (quadrature amplitude modulation) channels of dedicated VOD from a
central point to hubs. In others, it might mean adding new transmitters operating at
different wavelengths in distribution hubs.
"We're seeing the need for more bandwidth,
particularly in the transport segment between the headend and hubs," Connolly said.
"The real question is: Where do you want to process and connect your video and data
signals?"
S-A has introduced a 16-wavelength transport system,
doubling the capacity of previous QAM-compatible DWDM systems. Much higher wavelength
counts are available for baseband digital applications, but providing multiwavelength
systems where signals are modulated on RF carriers is more difficult.
"We're also 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 said.
Vendors and operators reported that DWDM solutions have
gained a lot of credibility in the wake of Tele-Communications Inc.'s decision to
make this technology a cornerstone of its network evolution.
TCI, with gear supplied by Harmonic Lightwaves Inc., is
using DWDM to distribute dedicated signals to hubs, at which point signals are amplified
and split for distribution to each node, in combination with nondedicated broadcast-video
channels over the same fiber.
This combining of a single, dedicated service wavelength
with the 80 or so broadcast-video channels at another wavelength into a single fiber going
out to each node is not problematic with regard to the difficulties of combining QAM and
analog wavelengths, because only a single QAM wavelength is involved, noted Mark Trail,
director of product-line management for transmission systems at Harmonic.
The two signals operate at different power levels, with the
digital arriving at 8 decibels per milliwatt to 10 dBm at the receiver, thereby avoiding
interference between the two different wavelengths, Trail said.
This distribution topology avoids any need for regenerating
signals at the hubs, relying instead on erbium-doped fiber amplifiers and WDMs to complete
the hub-to-node links, he added.
"We have several other MSO customers that are making
use of this technology," Trail said. Most are applying the technique in single
systems before going to wider deployment, he added.
ADC Telecommunications Inc. is another vendor bringing this
type of solution to market, having introduced an eight-wavelength cable-DWDM-transport
system and other optical products at the recent Western Show.
"The combination of TCI and AT&T [Corp.] backing
this technology has everybody in the industry looking at it," said Randy Schmid,
director of marketing for ADC's analog-transport-systems business unit.
This doesn't mean that everybody will move to this
architecture, Schmid and others noted. "People are doing their homework and figuring
out where these new ideas fit with their infrastructures," he said.
Such thinking marks a radical departure from the
cable-industry tradition of using 1310-nanometer transmission equipment. In fact, many
operators are better positioned to build on the 1310 mode as they add dedicated services,
rather than jumping to the DWDM-compatible 1550-nm-wavelength option, noted Bill Moore,
vice president and general manager at Ortel Corp.
Ortel -- which produces 1310 and 1550 transmitters, as well
as EDFAs and other components -- expects shipments of 1310 lasers in the domestic cable
market to go up by 10 percent to 15 percent in 1999, Moore said. "We could also see a
significant increase in the use of 1550 transmitters," he added.
One approach to using 1550 transmitters among companies
that have already used 1310 technology to build fiber-rich HFC (hybrid fiber-coaxial)
systems involves the placement of the 1550 lasers at mini-headends or distribution hubs,
in conjunction with VOD servers and cable-modem-termination systems, Moore noted.
"We're seeing a lot of testing with this approach
in mind, but so far, no one is doing it commercially," Moore said.
The extent to which 1550 becomes a flexible option depends
in part on costs, he added. "For any given power output, a 1310 transmitter is about
one-fifth the cost of a 1550 laser, but that could change if the [shipment] volume goes
up," he said.
Operators are also wrestling with issues associated with
maximizing upstream bandwidth efficiency as traffic increases.
The industry has been looking at subdividing coax nodes
into separate upstream segments and block-upconverting those segments onto different tiers
on a single transmitter back to the hub. But the idea hasn't caught on, due to costs
and to the difficulties of balancing the noise levels from separate feeds to accommodate a
coordinated transmission over a single return feed.
"It's not clear what the solution [for
subdividing nodes] is," said Eric Schweitzer, director of product-line-receiver
systems for Harmonic. "The leading options are frequency stacking versus
wavelength-division multiplexing."
In the latter mode, each upstream feed from a coax node
segment is fed into a wavelength-specific laser, and the wavelengths are combined into one
fiber for the return. But this solution runs into the 1550-transmitter costs.
"We're also seeing some operators subdividing the
node on the return path by using a separate [1310] transmitter and fiber for each path,
but containing all of the electronics in a single housing," Schweitzer noted.
Keeping a low profile with regard to new solutions,
Synchronous Group -- the pioneer in the use of EDFAs and DWDM in cable -- is working on a
new generation of product that fully exploits the advantages of new optical technology,
said Al Johnson, president and chief operating officer of Synchronous.
"We have some proprietary thoughts on how DWDM can be
used to meet the needs of a lot of different types of situations, but we're not ready
to share those ideas," Johnson said.
One area of development at Synchronous concerns very
high-power QAM DWDM systems to accommodate the efficient distribution of VOD and other
services from central locations, Johnson said.
While some operators are looking at VOD systems that
support remote location of servers at the hubs, others believe that it would be cheaper to
centralize the VOD system, which requires distribution of dedicated signals to hubs over
relatively long distances.
The cost of receivers is another barrier that Synchronous
is addressing, Johnson noted. "If we're going to drive fiber deeper, receiver
costs are the dominant factor that has to be dealt with," he said.
As operators look at the use of QAM-based DWDM, they also
have to consider alternatives to SONET (synchronous optical network) as a means of
assuring adequate protection against system failures.
Nortel Networks, which is pursuing DWDM-transport solutions
for the cable industry, is focusing on this problem in conjunction with its recently
signed agreement to acquire DWDM-system supplier Cambrian Systems Corp., noted David
Berman, director of integrated-video solutions at Nortel.
"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 said.
"Cambrian and Nortel are deeply involved in telecommunications-industry efforts to
port some of the SONET [route-protection] functions to the optical layer."
The two companies are developing a ring-based DWDM solution
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 noted. "But if
you're using QAM channels, that's the equivalent of putting a single DS-3 [45
megabits per second] on each wavelength, which is not an efficient use of SONET."
