| Quote from: strider |
Do I need to turn my router off and on to let the system change my speed?
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Every time you turn the router on, it will go through a new synchronisation.
That process always decides a new speed value by measuring the relative signal level (which rarely changes) vs the current noise level (which is changing all the time) - the SNR - and subtracting an allowance for future additional noise (the "target noise margin").
Generally, noise levels are higher from dusk to dawn, and lower in the middle of the morning... so a synchronisation in the morning can be at a higher speed than one in the evening.
Once the modem has gone through synchronisation, it continues to measure the relative signal/noise levels, and reports this back to tell you what the
current margin is. When you view the modem's stats just after a synchronisation, the SNRM value will probably reflect the "target SNRM" value that applied during the synchronisation. However, the modem's reported value will then change over time to reflect current conditions. The reported margin will go up if the noise reduces, and will go down if the noise increases. If it reduces to zero (or close to zero), it is likely the modem will no longer be able to "hear" the exchange properly through the noise, and a new synchronisation will be necessary - which will obviously end up at a lower speed to account for the higher noise at the time.
However, the exchange's DSLAM also monitors the line conditions using a process known as DLM - which can determine that you either have an unstable line (too many disconnections) or have too many errors recently - and will then respond by increasing that "target margin" value, and triggering a new synchronisation - which will usually then give a slower speed because of the higher margin now necessary. The default value is 6dB, and it goes up in steps to 9, 12 or 15dB... and it can be a slow painful process to get the DLM system to drop the values back again (ie weeks).
If DLM *does* make a change to your settings, it will trigger an automatic synchronisation. There is no need for you to do anything to help this along, and there is nothing you can do to hinder it (unfortunately).
So, if you perform a new synchronisation (or power off/on), and get a slower speed, it can either be because there is
currently more noise on the line, or because the DLM process has decided you need a higher target margin for your line, or (worse), both together.
When you turn your modem off too often (either manually, or because you have set an idle-timeout), you run the risk of inadvertantly triggering this DLM process ... which is obviously a bad thing for your speeds. So... the advice is to keep the modem on 24/7 to ensure you don't trigger DLM, and consequently to keep speeds high. Because the bad DLM behaviour is triggered by a mixture of your personal behaviour *and* the line conditions, no-one knows whether that advice is absolutely correct for you personally, but it remains good advice.
The exact parameters governing BT's DLM algorithm are unknown to us, and can change. Normally, people reckon that disconnecting more than once within an hour is bad behaviour. Other people advide no more than 1 disconnection each day (mid-morning) can be a good thing - at least for getting maximum speed with the current settings.
You reported your line stats once, which showed a noise margin of 12dB. That will be a "current margin" figure, and we can't be sure it represents a "target margin" unless a synchronisation was done just beforehand.
It is a high margin, and a starting point for investigations (which JoJo has started, by asking for a reset).
However, with your line known to get interference from a radio station (even though it has an RF filter fitted), it seems possible that the noise-level on your line will vary more than average - and perhaps a 6db margin is just not suitable, and it needs to be higher.
Plusnet Customer
Using FTTC since 2011. Currently on 80/20 Unlimited Fibre Extra.
