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Exclusive: One of the UK's best-respected broadband providers has raised concerns about the reliability of the world's most popular ADSL chip.

Zen Internet has uncovered a potential problem with the Texas Instruments AR7. The chip is at the heart of about a third of routers in use worldwide today - including Linksys and Netgear kit.

Zen has told its customers not to buy models that contain the chip because they provide an unstable connection.

Phil Long, a technical support manager at the firm, said: "The evidence is saying there's something about these chips that causes intermittancy."

Long explained that Zen fingered the issue when BT engineers were called to test lines for subscribers suffering repeated disconnections. BT staff test for a wiring problem using hardware based on chips made by Speedtouch - rivals to the AR7.

In many cases, the BT man would report back that there was no problem with the line, and the customer would be stung with an £169 unnecessary call-out charge.

"We're not saying there's definitely a problem [with the AR7 itself], but it does seem to be a common factor," Long said. The "fault" manifests itself on lines that have a variable signal-to-noise ratio (SNR). Fluctuating SNR can be caused by submerged wires, interference from electrial equipment, and myriad other factors on Britain's aged copper phone network.

The AR7 is now produced by German chip giant Infineon, which bought Texas Instruments' DSL unit in June this year. When it was released in 2003, the AR7 was aimed at pushing ADSL to its bandwidth limits for applications like IPTV.

Michael Seidl moved to Infineon in the deal and is the firm's senior product manager for DSL chips. He was part of the team that designed AR7 at Texas Instruments as the first "router on a chip".

He insisted that the chip is mature and reliable, but conceded that ADSL drop-outs are a fact of life on decades-old infrastructure.

The connection problems AR7-based routers seem to be having is most likely caused poor optimisation for the UK's network last mile, he added. "It could very well be. You give up robustness if you don't compromise on data rates."

BT carries out lab testing on routers aimed at the UK market, but does not release the data to other internet providers. Phil Long of Zen said: "BT won't provide us with any information on hardware testing. That's why we've brought this issue to light like this." BT's testing is bound by confidentiality agreements with manufacturers.

BT told The Reg it is unaware of any problem with AR7-based routers.

Reg reader and Zen customer Steve fears he would be left out of pocket by a BT visit, despite his Linksys WAG354 constantly disconnecting. He said: "This is just ridiculous. At the end of the day anyone who uses an AR7-based modem and who has a less than perfect BT line can look forward to compatability problems."

Long said: "Our advice is to pursue the fault but not arrange for an engineer visit until alternative hardware - without the AR7 chipset - is tested."

There's a list of some of the dozens of routers that contain the chip here. ®

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