AT&T Broadband & Internet Services last week
signaled a commitment to using baseband-digital-fiber technology that appears to mark the
beginning of a sea change in operators' approach to upstream architecture.
AT&T Broadband will use the new return-path technique
pioneered by Scientific-Atlanta Inc. in secondary-to-primary hub connections at its
Dallas, Denver and Pittsburgh systems, spokeswoman Tracy Hollingsworth said.
"The technology could be extended to the
[distribution] plant later, and it could be used with 'LightWire,' as
well," she said, referring to the new fiber-rich architecture the MSO is testing in
Salt Lake City.
AT&T Broadband's use of an optical-transmission
technique rooted in the time-division-multiplexing mode of telecommunications is part of a
wide-scale MSO shift in this direction as operators look for ways to maximize upstream
efficiency, Harmonic Inc. director of product-line management Eric Schweitzer said.
"This idea has taken over the industry in the past six
weeks," he said, adding that Harmonic was working on development of product to meet
the demand.
"S-A has pretty much shown that digital baseband has
some important advantages," Schweitzer added. "It's not necessarily the
right solution for every situation, but we believe it's an important option that
everyone is looking at."
S-A's "dbr" (digital-baseband-reverse)
system delivers upstream cable signals in uncompressed digital format using the
time-division-multiplexing technique common to digital telecommunications.
S-A's dbr systems are designed for use in hubs, rather
than nodes, but this will soon change, according to vice president of marketing and
network architectures Paul Connolly. "We'll have product out by the end of the
year that will extend baseband digital reverse to the node," he explained.
AT&T Broadband's initial application of the
technology will be to streamline transmission from secondary hubs back to the headend.
Amplitude-modulated return feeds in the 5- to 40-megahertz path from the nodes will be
digitally multiplexed together in pairs onto individual wavelengths at the secondary hubs
for transmission to headends or primary hubs over standard telecommunications lasers
operating at 2.4 gigabits per second.
When this technique becomes available at the node level,
Connolly said, operators will be able to combine two or, eventually, four 5- to 40-MHz
feeds from the segmented coaxial return paths onto a single baseband laser operating at a
prescribed wavelength in the 1550-nanometer window. The signal will then be passed through
the secondary hub via a wavelength combiner, onto the fiber back to the headend.
"This will meet the goal of eliminating the secondary
headend," Connolly said, in reference to the racks of electronics now needed to
convert signals at these intersection points.
By operating in pure digital mode without the encumbrances
of advanced modulation techniques, cable can exploit the descending cost curve of
off-the-shelf telecommunications technology while extending transmission distances beyond
the link lengths of amplitude-modulated systems.
With filtering and noise-suppressing techniques in the
multiplexing process, the QAM (quadrature amplitude modulation) or QPSK (quadrature phase
shift key) signal entering the multiplexer can be cleaned up to maximize the NPR
(noise-power ratio) at the baseband output. That delivers a better quality signal at the
far end of the digital link than the one that went into the link.
This ability to boost NPR and sustain it across the digital
link will facilitate the transition to four-port inputs in bdr systems, Connolly noted.
That relates to the number of bits used to form a byte in the signal code: fewer bits per
byte create lower NPR.
S-A uses either 10- or 12-bit encoding technology to
translate the amplitude-modulated signal to digital in order to maintain an NPR high
enough to meet industry specifications. That means only two 5- to 40-MHz AM feeds can be
digitally multiplexed onto a 2.4-gbps telecom laser, because the clock-sampling rate
required to perform this translation must be at least twice the rate of the signal
frequency, or twice 35 MHz.
"We're actually using 100-MHz clocks, which is
more than enough sampling speed," Connolly noted. "We could do four ports now,
but we want to get the bit rate down to where we can continue to use the 2.4-gbps lasers,
which means we have to be at OC-12 rates [622 megabits per second, per port]."
This means S-A will have to come up with the means to boost
NPR at the input so that lower-bit-rate encoding can be used. "I'd say
we're looking at getting to four ports within the next year, possibly by next
spring," Connolly said.
The first company to offer a four-port
digital-baseband-return solution is Synchronous Communications, which is shipping product
to customers starting in November, according to chairman Vince Borelli.
