AT&T Tries New Upstream Architecture

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AT&T Broadband & Internet Services last weeksignaled a commitment to using baseband-digital-fiber technology that appears to mark thebeginning of a sea change in operators' approach to upstream architecture.

AT&T Broadband will use the new return-path techniquepioneered by Scientific-Atlanta Inc. in secondary-to-primary hub connections at itsDallas, 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,' aswell," she said, referring to the new fiber-rich architecture the MSO is testing inSalt Lake City.

AT&T Broadband's use of an optical-transmissiontechnique rooted in the time-division-multiplexing mode of telecommunications is part of awide-scale MSO shift in this direction as operators look for ways to maximize upstreamefficiency, Harmonic Inc. director of product-line management Eric Schweitzer said.

"This idea has taken over the industry in the past sixweeks," he said, adding that Harmonic was working on development of product to meetthe demand.

"S-A has pretty much shown that digital baseband hassome important advantages," Schweitzer added. "It's not necessarily theright solution for every situation, but we believe it's an important option thateveryone is looking at."

S-A's "dbr" (digital-baseband-reverse)system delivers upstream cable signals in uncompressed digital format using thetime-division-multiplexing technique common to digital telecommunications.

S-A's dbr systems are designed for use in hubs, ratherthan nodes, but this will soon change, according to vice president of marketing andnetwork architectures Paul Connolly. "We'll have product out by the end of theyear that will extend baseband digital reverse to the node," he explained.

AT&T Broadband's initial application of thetechnology 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 bedigitally multiplexed together in pairs onto individual wavelengths at the secondary hubsfor transmission to headends or primary hubs over standard telecommunications lasersoperating 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-MHzfeeds from the segmented coaxial return paths onto a single baseband laser operating at aprescribed wavelength in the 1550-nanometer window. The signal will then be passed throughthe secondary hub via a wavelength combiner, onto the fiber back to the headend.

"This will meet the goal of eliminating the secondaryheadend," Connolly said, in reference to the racks of electronics now needed toconvert signals at these intersection points.

By operating in pure digital mode without the encumbrancesof advanced modulation techniques, cable can exploit the descending cost curve ofoff-the-shelf telecommunications technology while extending transmission distances beyondthe link lengths of amplitude-modulated systems.

With filtering and noise-suppressing techniques in themultiplexing process, the QAM (quadrature amplitude modulation) or QPSK (quadrature phaseshift 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 thefar end of the digital link than the one that went into the link.

This ability to boost NPR and sustain it across the digitallink 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 perbyte create lower NPR.

S-A uses either 10- or 12-bit encoding technology totranslate the amplitude-modulated signal to digital in order to maintain an NPR highenough to meet industry specifications. That means only two 5- to 40-MHz AM feeds can bedigitally multiplexed onto a 2.4-gbps telecom laser, because the clock-sampling raterequired to perform this translation must be at least twice the rate of the signalfrequency, or twice 35 MHz.

"We're actually using 100-MHz clocks, which ismore 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 boostNPR at the input so that lower-bit-rate encoding can be used. "I'd saywe're looking at getting to four ports within the next year, possibly by nextspring," Connolly said.

The first company to offer a four-portdigital-baseband-return solution is Synchronous Communications, which is shipping productto customers starting in November, according to chairman Vince Borelli.

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