"SFA leading the way with compressed digital-return techniques..."
Found this article: scroll to the bottom for mention of SFA.
multichannel.com
Broadband Week for June 12, 2000
Vendors Blaze Return-Path Trail as Ops Step Up Rollouts
By DAVID ILER June 12, 2000
Return-path technologies got plenty of attention at last week's Society of Cable Telecommunications Engineers
Cable-Tec Expo, as vendors previewed techniques to maximize upstream traffic flow.
With cable operators moving swiftly to roll out Internet services, the need for an optimized return path becomes more and more critical.
The latest technology to grab mind share in the return-path market is the concept of lacing cable-modem-termination-system functionality at the node level.
Harmonic Inc. last week introduced what it called the first optical node that incorporates physical- and
media-access-control-layer CMTS functionality. The node represents the "distributed CMTS" technology, which was designed to deliver wide upstream bandwidth for next-generation Internet protocol-based services.
The CMTS node operates as a layer-2 bridge, using baseband Ethernet to send upstream data back to the
headend.
Harmonic is betting that the new node will resolve upstream bottlenecks in systems that carry heavy loads of
IP-based traffic resulting from the rocketing popularity in bandwidth-consuming Internet applications, such as Napster Inc.'s music-download service.
Harmonic has developed several return-path techniques, including digital and analog over DWDM
(dense-wave-division multiplexing) and coarse WDM, for optimizing the return path.
Harmonic's node CMTS marks the latest evolution of return-path technology. Various system characteristics --
such as traffic, topology and subscriber penetration -- dictate which return-path technology is best, vice president
of marketing Patrick Harshman said, adding that the distributed CMTS model works best in systems where
subscriber penetration is very heavy and in systems that expect heavy IP-telephony traffic.
By placing CMTS functionality deeper into the network, the upstream signal carried by fiber from the node to the
headend will have a greater capacity than digitizing the return path. The technology represents a "quantum leap
forward in bandwidth capability to residential and business subscribers," Harshman said.
He added that the node CMTS could send two separate 50-megabit-per-second traffic streams from the node on a fiber link using Ethernet-combining technologies. A digitized return stream, assuming a fully loaded 5- to
45-megahertz spectrum, will typically result in a 1.16-gigabit-per-second payload due to the added information
resulting from digital sampling.
Theoretically, more 50-mbps streams consuming less bandwidth can be combined by utilizing distributed CMTS
technology, instead of by digitizing the return path.
Harshman was careful to not endorse one return-path technology over another because of varying system
requirements, and he said he hoped the distributed CMTS technology would "promote dialogue within the industry."
But digitizing the return path has several strong supporters. Debuting prototypes of such technology last week was start-up Aurora Networks, a Santa Clara, Calif.-based maker of optical-networking technologies. Aurora's team is
comprised of former Silicon Valley Communications Inc. executives.
The company previewed a module and platform that digitizes the return path. The design of the headend module is engineered to allow it to be hot-swapped without reconnecting optical lines and without having to access the back of the chassis.
Aurora hopes to land mini-fiber-node-architecture backer AT&T Broadband as a customer. But CEO Guy Sucharczuk said the headend and node modules were not designed with only one operator in mind.
The biggest benefit of digitizing the return path is cost, which is less than that of analog transmission, Sucharczuk said. Digitization extends the distribution to which a signal can be carried and allows for the segmentation of nodes to bring them closer to subscriber homes.
Sucharczuk added that Aurora's modules have "hooks" to support CMTS functions if an operator chooses to place CMTS units in nodes. Rather than an either-or scenario, digitization represents "one more tool in the arsenal" to optimize return-path performance, the vendor added.
A more forceful proponent of a digital return-path solution is Robert Howald, director of systems engineering, transmission-network systems for Motorola Broadband Communications Sector.
"The gap between CMTS and digital-return payload will get smaller," Howald said. As digital-return systems are optimized for data with techniques such as compression, digital-signal processing and variable sampling, the digital-return payload will decrease, he added. Meanwhile, as the Data Over Cable Service Interface Specification evolves and migrates to higher orders of modulation, its payload rate will increase.
Leading the way with compressed digital-return techniques is Scientific-Atlanta Inc., which, according to vice president of advanced technology Don Sipes, is running compressed return-path data schemes in its labs.
Sipes called the breakthrough the "coup de grƒce of digital-reverse technology." By eventually incorporating DSP technologies into the return path, many functionalities become possible, including filtering and subcarrier encoding.
With digital-reverse technologies, the "operational complexity of activating the return path goes away," he added.
But despite several quickly evolving return-path technologies, activating the analog return path in the first place still looms as a substantial challenge for many operators. |
