Reasoning that the sturdiness of digital technology could
vastly ruggedize the quirky cable upstream path, Scientific-Atlanta Inc. plans to detail a
new family of optical products this week that do just that.
Coincident with the Society of Cable Telecommunications
Engineers' Emerging Technologies conference in Dallas this week, S-A will unveil
plans for its "Prisma" family of optical products that will digitize the
upstream path, initially from the node to the headend.
Most upstream-communications problems come from electrical
noise or from signals leaking into plant located between homes and the node. Still, S-A
executives think that it's a good idea to digitize upstream traffic at the node, then
send signals upstream to hubs and, ultimately, to the headend.
"There are a lot of concerns in the industry about the
currently used analog upstream techniques -- about whether they provide good enough
performance over different temperature ranges and distances," said Paul Connolly,
vice president of marketing and network architecture for S-A. "As you load more and
more information, the question arises as to whether these analog reverse transmitters are
really good enough."
Connolly described the new technique as "a
breakthrough" enabled by advances in silicon technologies. In particular, DSP
(discrete-signal-processing) chips developed to handle analog-to-digital conversions for
the telecommunications industry are now rugged enough for outdoor use.
"We're jumping on the whole telecom-technology
bandwidth explosion," Connolly said.
He described the new digital upstream technique as a
"flexible way for cable operators to make the reverse path more efficient, in a way
that lets them save on expenses."
Tony Werner, executive vice president of engineering for
Tele-Communications Inc., said he's thrilled about the concept.
"If it does what it's supposed to do, it'll
be great," he said, noting that he's been active in the development process with
Connolly said the product sits at the node site and works
by sampling the incoming analog signal at a rate of 100 million samples per second. The
sampled signal is then converted at baseband to digital and transmitted to the hub via a
Off the bat, this means that cable operators need not
struggle with network decisions like whether to use less expensive Fabry-Perot lasers or
costlier but better-quality distributed-feedback lasers (DFBs).
"It allows the performance of uncooled DFBs at around
the cost of an F-P transmitter," or about $500, Connolly said. DFBs run from $900 to
At the hub, the signal runs through a digital-to-analog
converter, "so that operators can immediately get the high performance of digital at
a low cost," Connolly added.
Plus, most hubs are configured to handle about 20,000
homes, meaning that individual fibers from 40 different 500-home nodes are coming in. Each
of those fibers could be crammed with voice, video, data and control signals that either
need to be processed at the hub, which is expensive, or that must be sent straight back to
the headend with a technique like wave-division multiplexing.
As operators drive fiber deeper into the network,
subdividing nodes, the problem of how to best get information through the hub to the
headend only increases, Connolly said.
"Probably the most significant benefit to this digital
reverse-product line is that once the node signal is digitized at baseband, it
doesn't need to be converted to analog at the hub," he said. "By using WDM,
you can send it digitally all the way to the headend."
Connolly described the product as one in a scheduled family
of digital upstream techniques, which will eventually be augmented with time-division
multiplexing techniques to increase the upstream capacity even further, from the hub to
"By digitizing at the node and then bundling different
flavors of this, operators can decide how much bandwidth they want and where they want to
do multiplexing," he said.
The products will initially work only in S-A brand optical
nodes, but the manufacturer is considering licensing the technique to work with other
vendors' gear, Connolly said.
The project has been ongoing for just under one year. It is
in lab tests now, and it will go to field tests in the spring.
"This is more of a technology-framework
announcement," Connolly said, adding that the company will begin displaying products
at the National Show in Chicago in June.