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[Weaver]

(ADSL1, BTW 20CN)

Is there any reason why switching your modem off occasionally is good/bad for performance?

1) Suggestion: turning your ADSL1 modem off for the recommended half hour min gives it a chance to resync at a faster rate if it has earned enough brownie points with DLM. (A lack of SRA means it will never get the chance to improve when its record merits it.)

Is that a sensible argument?

[of course it is the way of utter madness, trying to second-guess DLM and hovering over the device all the time hoping you'll succeed in your bet.]

2) if you were to decide that as in (1) above the occasional turn-on is good for performance when would you do it, or avoid doing it?

[loonylion]

you do it when the SNRM is above 6, ideally when its as high as it gets.

[kitz]

If the DLM makes any changes to the line, then it should automatically force a resync so that any changes up or down are applied immediately. 
As LL mentioned performing a resync at any time is entirely dependant on your current SNR margin.  For those lines which vary over the course of the day in most cases the SNRm is lower in the evenings, so performing a resync at this time may give a lower sync speed.   



A (rarish) case where it can be beneficial to perform a resync is if a line has been through a noisy patch where there has been lots of bit-swap.  Bit swap can make the router mark certain tones as unusable.   A line is supposed to have a certain amount of surplus SNR built in to cover bit-swap but if its been happening over lots of tones, then the overhead allowance gets eaten away at.  If high bitswap carries on then at some point the router will perform a resync itself, but if the noise has cleared then performing a resync should bring those tones back into use.   
TBH this doesnt happen nor is it needed often, as the router will have either done it by itself, but sometimes it may just get its knickers in a twist and start doing something silly.    I suspect in some cases this may be linked in with the reason why occasionally if a line is seeing tons of errors,  then a router reboot will clear it as the bit allocation table is started afresh during channel analysis which is performed as part of the sync up process.

Another time it can be beneficial is if you are with an ISP (such as PN) who apply a throughput rate restriction in line with either the sync speed or IP profile.
Because the BTw reports lag by a few hours, if you've had a positive change then you may have to establish a new PPP session to pick up the higher throughput speed.  IIRC AAISP also restrict line rate, but based on sync speed rather than IPprofile so shouldn't impact quite as much.

[Weaver]

@kitz - A lot to ponder on in that very helpful post

[Kitz mentioned Andrews & Arnold: ds rate-limiting is indeed carried out by A&A who track BTW based on reports from BTW’s systems (somehow). This is mentioned on the A&A website, see
     http://aa.net.uk/kb-broadband-shaping.html
and reports relating to this mechanism are visible in the logs that are available for each line. A&A’s rate-limiting lags behind BTW, by quite a bit (I forget, something like 24 hours).]