In reality both the level of interleaving and the amount of FEC is adjustable it is simply that BT have decided on using maxium FEC when it applies interleaving. Probably on the basis that they appear to be aiming more for stability rather than speed to minimise support costs. LLU operators may well use different amounts of either if they use it at all.

In reply to:

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LLU operators may well use different amounts

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Yes - my LLU line has 6 FEC check bytes out of 254 on the downstream, and FEC disabled on the upstream, whilst still correcting 98% of my line errors. Rather less draconian that BT's 16 bytes each way or nothing at all. It's a shame BT haven't taken their DLM technology a little further so that it sets an appropriate level of error handling automatically for each line, instead of just full on or off.

I have got downstream interleaving on at level 64 and upstream at level 2. I have logged 100 receive CRC errors and 69 HEC errors in the last 8 hours. Would you advise me not to turn interleaving and error correction off? I am on a 9dB target noise margin with a 3-4dB variation every 5 seconds or so.

That really depends on how many errors are being corrected, and of course how much you're interested in the reduced latency and increased speed. The CRC error count is very low, but represents only the errors that are left uncorrected, and gives no reliable indication of corrected error rates. The 64 depth interleaving is giving you around 16msec of latency at 8064kbps. As you're on an LLU line I can't tell how much error correction overhead you have without seeing the check byte figures.

Many modems will display stats including the number of corrected errors - shown as FEC errors or sometimes RS uncorrected errors, although you may need to resort to a telnet commandline interface to get them. Failing that a very very rough estimate might be around 1 uncorrected error out of every 50 corrected, so that would be 5000 in 8 hours, or around 1 every 6 seconds. That's perfectly useable, although speeds and certainly gaming are probably beginning to suffer.

Thank you for that.