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).

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