As Chrys says, its the reduction in SNRm that causes the loss of speed.
When a new line gets attached, the crosstalk generates more noise on the line which causes the SNRm to decrease and that in turn causes the reduction in sync speed.
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If you are using DSLstats, then you can spot the signs of a cross-talker coming online. You can sometimes even see when one of your crosstalkers has a resync or reboots their router.
For example heres my DSLstats SNRm graph and you can see that at about 1:20am one of my crosstalkers had a resync.
Note how my SNRm momentarily goes up to 7.8 dB when he's not using his connection.
Then look what effect it has on my connection speed and how the max attainable jumps up. This particular crosstalker 'costs me' 5Mbps of sync speed.
Hopefully it's ok to drag up an old thread but I have an update.
I have had the Zyxel since the middle of last year and it has been faultless but my line speed as been steadily dropping off as can be seen on the attached which i am guessing is due to cross talk as already mentioned. i am using version 8 firmware and my speed has dropped from around 38 Mbps to now around 32 Mbps. Is there anything i am doing wrong or that looks wrong from the attached please ?
i have DSLstats if there is anything i could post from that.
Apologies if i should have started a new thread.
Thanks