OK; the two lines are about the same length (assuming the Sky one is using ADSL2+, which general gives a line attenuation a few dB higher than the same length line using ADSL1 or ADSL2).
The BT line is using ADSL2 (G.992.3 is ADSL2; G.992.5 is ADSL2+).
Quite often, with lines of this length (it's probably around 5km long) modems will connect at a slightly higher speed on ADSL2, rather than ADSL2+.
Reason 2 is the Sky line has a noise margin of 9dB, compared to 3dB (although the target is probably 6dB) for the BT line.
This will produce a speed difference of around 1Mbps (give or take maybe half a meg).
You haven't posted the connection speed for the BT line in the stats, but I assume it's the faster of the two.
I'd ring Sky and ask the to put you on a 6dB "target noise margin", which should go most of the way towards equalising the speeds.
Also remember the routers will connect at the fastest possible speed which produces a noise margin equalling the target, so with the same target and re-syncing at the same time should produce similar speeds.
Anything else (either different targets, or syncing one line at night and the other during daylight) will produce different connection speeds.
Generally sky will not work with 'target noise margins'
They prefer to apply a sync speed cap.
Obviously it works in the same way.
E.g. if sky see the line is performing badly and is currently synced at 5 meg...
With most ISPs the target noise margin is increased from 6db to 9db
With Sky, instead they place an artificial cap on the line at say 3.5 meg. The line cannot sync higher than this unless Sky increase the cap. Even if you 'force' the noise margin down with custom firmware it will not go higher.
Generally this means that if you sync at 7pm you get a 3.5 meg speed.
Sync at 10am, still 3.5 meg.
If you sync at 10am the noise margin might be 11db when you sync, with a 3.5 meg sync
at 7pm, you still sync at 3.5 meg, however the noise margin may only be 8db
The difference is the noise margin isn't a 'target' as such. The only thing Skys DLM is concerned with is sync speed. Same goes with customer services, they will increase the sync speed cap, but not the noise margin target generally.
EDIT: obviously lowering sync speed increases the noise margin naturally.
Edited by ukhardy07 (Sun 28-Aug-11 17:01:51)