How to test for ADSL exchange congestion

Many ADSL subscribers are complaining about poor ADSL speeds and high latency during certain periods. Telkom exchange congestion is often blamed for these problems.

Around 25% of Telkom’s DSLAMs are congested – a problem which Telkom is trying to address.

There can also be other problems which can affect an ADSL service. This includes poor copper quality, the distance from the exchange, and congestion on an ISP’s network.

This means that proper troubleshooting is needed to find the cause of an erratic ADSL connection.

Testing for ADSL exchange congestion

Troubleshooting ADSL exchange congestion can get technical, but the following symptoms may point to this problem:

  • Deterioration of line speed and performance, without normal line fault indicators (high attenuation, line noise);
  • High latency to destinations on the local internet. Typically most ADSL services would not exceed a 50ms ping to destinations inside the country unless line conditions are sub-standard;
  • Packet loss and intermittent spikes of very high latency on a continuous ping test; and
  • Quite often users will experience these symptoms coming and going at certain times of the day and they may also be more prevalent at certain times of the week as well.

Slowness in browsing, video buffering, and high latency (in gaming, for example) are all signs of exchange congestion.

Use traceroute to find the point of congestion:

  • Problems at the second hop would indicate exchange congestion – it should always be sub 20ms for it not to be service-affecting.
  • If the third or fourth hop is greater than 80ms it is more likely that the ISP is throttling the account in some way.

“You can learn nothing from a trace if the person is doing something else while doing the trace, like e-mail, Skype, or downloading”

“Anything else you are doing on the line may influence the traceroute results, so it should be done with everything unplugged – except the test machine – or if the router has a trace facility, from the router directly.”

Once an exchange congestion problem is expected, test the line using a SAIX account to prove or disprove the suspicion.

If the same problem exists using a SAIX (Telkom) account, the problem is most likely related to congestion on Telkom’s network.

How to do a Traceroute

The following is a basic guide to do a traceroute to Telkom.co.za. If you want to do a traceroute to another website, just replace ‘telkom.co.za’ with the other domain (like ‘google.com’)

Windows

Start >> Programs >> Accessories >> Command Prompt
Type the following in the window: tracert Telkom.co.za

Mac OS

Open Utilities >> Open up the Terminal (command line)
Type the following in the window: traceroute Telkom.co.za

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