I was quite surprised the other day when I ran the Bit Error Rate (BER) test in my 8800NL and found some wildly different results, depending on the period of the test. Currently, I'm using the device in ADSL mode, so this test is found in the form of a button at the bottom of the 8800's Status/xDSL page.
The default period is 20 secs, but you can run it for longer if you wish. At 20 secs, the test returned an ideal value of 0.00e+00. However, at any period greater than this the result rapidly deteriorated, with me getting figures around 5.60e and so forth.
Now, I've known for years and years that the line to my exchange is long and is subject to large amounts of transient noise each day (the line actually traverses an electrified railway line at one point) and to deal with this I've deliberately increased my target SNR and also have ensured that a certain amount of interleaving is performed, but how soon (in terms of these timed tests) should the BER start to deteriorate? After 30 secs? 1 min? 2 mins? What's considered to be an acceptable BER and what's not acceptable? 20 secs on my slow (4M bps) connection doesn't represent all that many packets, I'd have thought.
What sorts of errors are these anyway? Are they CRC errors, or instead irrecoverable errors?