The Plus Side of ADSL

Asymmetrical DSL equipment vendors and chip makers continue to tweak ADSL technology to let service providers offer more bandwidth over greater distances than was previously thought possible.

The new version of ADSL, called ADSL+, allows providers to pack more bits into the downstream bandwidth. This makes two video streams, plus data and voice, possible for service providers going for the voice, video and data bundle. With conventional ADSL, only one video stream is feasible.

ADSL+ could become a more budget-conscious alternative to very-high data rate DSL for delivering video across local phone networks. But many VDSL backers are dismissing ADSL+ as a patchwork solution.

The new ADSL+ standards are still in the works, but chip makers and vendors have already been working on ADSL+ chip sets, so that when standards are set, equipment will only require software adjustments or new line cards, says Claudia Bacco, president of TeleChoice.

Next Level Communications has already launched a new platform based on ADSL+, and other vendors are expected to reveal similar video over ADSL products at Supercomm.

Although Next Level has been an advocate of VDSL, the company has developed a new system using ADSL+ chip sets made by GlobespanVirata to broaden its market, says Geoff Burke, director of marketing at Next Level.

“VDSL got a bad rap because it was too expensive to deploy,” says Burke. However, he stresses that the company isnt turning its back on VDSL and instead has lowered the cost per subscriber on new versions of its VDSL equipment.

As for ADSL+, Next Level is claiming speeds of 10 megabits per second at an operating distance of up to 10,000 feet. Bacco calls those performance estimates conservative.

Conventional ADSL usually tops out at a rate of 8 Mbps and a distance of 8,000 feet from the central office. The lower rate limits ADSL to a single stream of video.

ADSL+ cant match VDSLs capacity or its ability to carry three video streams simultaneously, but it offers a big edge over VDSL in terms of reach. VDSLs operating range typically tops out at 4,000 to 5,000 feet.

ADSL+ isnt the only innovation coming down the pike, though. Vendors are also looking at other alternatives such as changing the error correction setting on DSL modems to increase the ADSL payload, says Gary Bolton, VP of product marketing at Catena Networks.

“With high-quality modems, providers can increase the bandwidth to 10 Mbps,” says Bolton. “We are seeing a lot of good innovation in turbo-charging and fine-tuning ADSL technology.”

No Translation Overseas

But ADSL+ does have its drawbacks. Analog front ends have to be adjusted to go beyond the spectrum theyre currently designed for to be compatible with ADSL+, which is an added expense for service providers, says Bolton. “More importantly, ADSL+ uses a higher frequency, so it is limited to very clean loops,” he adds.

In addition, ADSL+s frequency level is not compatible with ISDN, which is a concern in Europe and Asia, says Richard Sekar, VP of marketing for Ikanos Communications, a chip maker that focuses on VDSL and Ethernet technology.

Putting the two technologies together results in ISDNs chopping off some of ADSL+s bandwidth. “ADSL+ comes across as a patchwork solution that was ideal for CLECs that would deploy whatever they could, but they dont exist anymore,” notes Sekar.

Many doubt that ADSL+ will supersede VDSL and some are expecting service providers to be slow to adopt the new technology. But Bacco thinks ADSL+ could appeal to local service providers that need ways to deliver enough bandwidth to enable high-bandwidth services.