Buyer beware: ADSL filters

Know what you're buying

If you've just upgraded from ADSL to ADSL2+ broadband, you may want to check your line filters. Chances are that you're not getting the speed you should be.

Line filters, also known as microfilters, are the small devices used as intermediaries between the phone outlet on the wall and your phone or ADSL modem. The filter helps in separating analog or voice information from digital data, eliminating any possible static and ensuring that both telephony and broadband services operate at their peak.

The problem lies in microfilter standards. Broadband equipment manufacturer NetComm Australia claims that microfilter standards established during the initial rollout of ADSL1 didn't take into account evolving technology. As a result, microfilters built for standard ADSL weren't up to scratch for ADSL2+ requirements, often capping out at ADSL's theoretical maximum speed of 8Mbps.

With more and more people switching to ADSL2+ services in a search for more bandwidth, there's a significant risk that they aren't upgrading their microfilters to suit. You might still notice a small increase in speed with ADSL2+ over ADSL using the original filters, but you're probably not getting the speed you should be.

At a base technological level, there isn't too much different between ADSL and ADSL2+ microfilters — there remains the chance that your original ADSL might even reach 24Mbps speeds. However, ADSL2+ microfilters are built to stringent standards that ensure they reach the service's maximum possible line speed. If you're suffering from slow speeds or line interference, changing your filters might do the trick.

We tested a number of ADSL2+ microfilters. Pitting NetComm's EM1120 ADSL filter against its ADSL2+ EM1550 in-line microfilter shows the speed difference. The EM1120 had a maximum speed of 8Mbps, whereas the EM1550 managed line speeds of up to 24Mbps. We also managed to replicate these speeds with splitters from lesser known manufacturer C10.

Unfortunately, ADSL2+ filters can still fail. Many filters claim compliance with Telstra's RCIT.0004 standards, which don't acknowledge a number of factors vital to full ADSL2+ compatibility. Although filters complying with these standards were popular during the early days of ADSL2+, they are rarer these days. The best option is to buy filters compliant with the AS/ACIF S041:2005 standard, such as those from Netcomm, C10 and Telequip. It's worth spending around $25 for a filter as chances are a $10 filter simply won't do the job.

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James Hutchinson

PC World
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