Slow ADSL speeds: what you can do about it

Connectivity problems on ADSL services can be caused by many different factors, often making it difficult to pinpoint where the fault lies.

When your line is synchronising at lower speeds than it should, or if you’re experiencing intermittent drops in connectivity, one place to start is your DSL connection statistics.

Your ADSL router should report these numbers, using wording such as signal-to-noise ratio (SNR), and line attenuation (sometimes referred to as resistance).

SNR and attenuation values are typically given in decibels (dB), for both your upstream and downstream connection.

The higher the SNR the better, and the lower the attenuation the better.

Since ADSL download speeds are much faster than upload speeds, this article will focus on what your downstream SNR and line attenuation values should be.

In brief:

  • SNR should be at least 6dB, but preferably higher than 9 or 10dB.
  • Attenuation/resistance should be below 55dB, but preferably lower than 45dB.

Line attenuation

Telkom VDSL router unplugged - SATNAC 2012
Telkom VDSL router

Cybersmart CEO Laurie Fialkov explained that line attenuation, sometimes referred to as ATUC and ATUR SNR resistance, is typically an indication of how far away you are from the exchange.

If the value is too high it impacts the maximum speed at which your line can synchronise, and can even cause your router to not sync at all.

Though Telkom does provide limits for noise and resistance levels, Fialkov said the exact ratios you need for a stable connection vary from router to router.

For example, Telkom specifies that attenuation/resistance should not exceed 55dB.

However, Fialkov warned that while Telkom routers will sync at these levels, some router brands will not sync at all over 45dB.

He said at above 50dB attenuation, only 4Mbps connections are generally stable, with some routers able to sync at 6Mbps.

Signal-to-noise ratio

Asked what SNR is needed for ADSL to sync stably at 10Mbps, MWEB ISP boss Derek Hershaw said that the “textbook” Telkom answer is that it should be at least 6dB.

“Our experience is that 9dB is a safer number to work with, especially once you start moving to faster line speeds, such as 4Mbps and up,” said Hershaw.

Afrihost agreed with Hershaw, adding that while it does depend on the line, it generally likes a value of at least 10dB.

“Because a decibel is not an absolute value – it’s a ratio – we do sometimes see lines with values closer to Telkom’s minimum perform adequately,” said Afrihost.

“We generally find that higher attenuation on higher line speeds has a significant impact on performance.”

What you can do about bad DSL stats

DSL link statistics on TP-Link router
DSL link statistics on TP-Link router

Though there is little that can be done if your line attenuation is too high, there are a few things you can do to improve your SNR.

The first step is to eliminate any potentially faulty equipment between your ADSL router or modem and the telephone jack.

This includes POTS filters and telephone line extensions.

Such equipment is often first to get damaged when lightning hits your phone line, and to ensure they are not causing noise on your line they should be disconnected temporarily.

Other equipment connected to the same jack as your ADSL router or modem, such as telephones, should also be removed while testing.

If the source of the noise on your line has still not been found, Webafrica recommends a few other things to test:

  • Is your router or modem faulty? To test this you will need another (working) modem to connect to your line to see if you get similar SNR readings from it.
  • Electric fences. A faulty security fence can cause impulse noise on an ADSL link. This will cause the line to run errors and drop intermittently. In severe cases the ASDL line will not sync at all. Check for any arcing from the fence due to cracked or faulty isolators, as well as for plants or any foreign objects touching the fence.

If nothing you test improves your SNR, it’s time to call either your ISP or Telkom (depending on who manages your line) and have them look into the issue.

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Slow ADSL speeds: what you can do about it