Wednesday, February 6, 2008

A closer look at 40 Mb/s DSL

What’s behind Rim Semiconductor’s recent claims of superfast DSL?

Rim Semiconductor turned some heads in January with a claim that its new chip can send traffic at 40 Mb/s over 5,500 feet of 26-gauge copper wire. The chip, which the company said will be commercially available later this year, promises big jumps in bandwidth for carriers such as AT&T that are trying to cram as much traffic as they can down existing copper lines.

But Rim’s approach is such a departure from currently dominant access technologies that it will have to work hard to establish new industry standards in order to get widespread deployment by major telcos.

Part of Rim’s technology involves changing how upstream and downstream bandwidth is allocated. The company is proposing an alternative to discrete multi-tone (DMT) line-coding, the encoding standard commonly employed in DSL networks including VDSL. Rather than reserve fixed allocations of upstream and downstream bandwidth, as DMT does, Rim’s chip uses time-division duplexing (instead of the frequency-division duplexing used in VDSL2) so that, when needed, downstream traffic can use bandwidth that would otherwise be reserved for upstream traffic, and vice versa. And it uses “rapid bi-directional switching” to transition in milliseconds from upstream and downstream transport.

“DMT’s fixed ratio of downstream and upstream [bandwidth] was absolutely appropriate at the time [it was created], but video has changed it all,” said Brad Ketch, Rim Semiconductor’s chief executive officer. “We feel and have felt that ultimately DMT would come to a point of diminishing returns.”

Redoing DMT is just part of Rim’s approach, said Ketch, a veteran of access vendor Advanced Fibre Communications (now Tellabs). Ketch is careful not to divulge too much about the company (which, despite not having a commercial product, has issued more than 30 press releases in the past two years). According to the firm’s Web site, Rim’s technology “defines” not just the encoding algorithms inside transport processors but also “the signal stream waveform.” Rim attacked rate and reach limitations in several ways, the company says, increasing payloads and decreasing noise and latency. Along the way, it made a few acquisitions to aid the effort, including that of 1020 Technologies and Broadband Distance Systems (a subsidiary of Utek).

The company has also hired Telcordia to, as Ketch put it, “study the impact of this technology in a binder group…to work over our shoulder make sure it makes sense to tier-one telcos.”

Ketch acknowledges that, in order to be useful, Rim’s technology would need to be deployed both in access networks and in customer premises gear, underscoring the need for standardizing the nascent technology.

“As far as the market goes, they’re sailing into the wind,” said Kermit Ross, a consultant with Millenium Marketing. “Given that the phone companies have a mix of [DSL] platforms in their networks as it is and a mix of different suppliers of DSL modems, there’s an awful lot of current flowing against making a thing like this work.”

To develop an industry standard for its technology, dubbed Internet Protocol Subscriber Line (IPSL), Rim has convened a group called the IPSL Special Interest Group, or IPSLSIG. Embarq is one of the few publicly named members of the group, which plans to meet a few times a year (its next meeting, slated for Europe, is being scheduled now, Ketch said).

“We invented IPSL, but we’re committed to making it available to other vendors because we’d like to see the ITU-T bring it in and standardize it,” Ketch said.

“Getting IPSL standardized will be one of the biggest hurdles for Rim Semi as it looks to bring its technology to market,” said Erik Keith, an analyst with Current Analysis.

Rim’s technology also requires IP DSLAMs rather than the legacy model based on ATM, though the majority of DSLAMs deployed today are IP-based..

Rim is currently testing a field-programmable version of its chip in the networks of small telcos such as Oregon’s Monroe Telephone. But with the help of partner eSilicon, Rim expects to introduce a less expensive ASIC version commercially this year.

Ketch imagines Rim’s gear making it possible for carriers to add more high-bandwidth service such as IPTV over existing copper, but he’s less certain about the technology’s potential to bring DSL to areas currently beyond reach. “That’s one application for the technology, but I think there are some question marks about the economics of it,” he said.

Meanwhile, vendors across the industry are working on separate efforts to increase the performance of DSL and copper. Established vendors are fine-tuning technologies such as Dynamic Spectrum Management and VDSL2 pair-bonding (Ketch says IPSL could potentially work in conjunction with both those technologies), while newcomers like Xtendwave and Phylogy promise their own new approaches.

“Since FTTP is prohibitively expensive for most telcos, getting more out of the existing copper access network is their most cost-effective option," Keith said.

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