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Standard User ukhardy07
(committed) Sun 28-Aug-11 17:53:51
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Re: 2 x ADSL with 2 different speeds

[re: adebov] [link to this post]
In reply to a post by adebov:
Strange cap (1333kbps) and just a coincidence the margin has turned out at 9dB I guess.
Surely such an unusual sync speed (1333kbps) can only be a result of syncing according to a target margin?
I'd understand if the sync was 1250 or 1500, but 1333kbps???

My lines set to 15.0 mbps however I sync at 15562 kbps, give or take a few kbps. it never syncs dead on a speed.

Sometimes, although rarely this occurs...

DLM sets the line to a sync speed e.g. 7mbps.

If the person syncs in the morning, they can easily get 7mbps with a 6 / 7 db noise margin

However in the late evening, their noise margin drops and is 4 / 5 db at 7mbps.

If they connect at this time they will sync below 7 mbps.

This is because sky does not allow a user to connect at a speed which results in an SNR below 7db (I'm pretty sure sky use 7db noise margin instead of the standard 6db)

The point is they could not sync above 7mbps even if they tried to force their snr down to 1db with custom firmware.


In the above case, if they are syncing below the cap that DLM has set. Manually lowering the SNR will increase the speed, but only upto the physical cap.

So yep, noise margin targets are in place to some degree. However the target is never usually set above 7db, if the lines unstable a sync speed cap is applied that increases the noise margin naturally.

So people often sync with an SNR of 10db, 11db etc.

So yes. I would say it's a coinsidence.

If you ring up sky they CAN apply a higher SNR target although they are VERY reluctant to do this.

If you login to your my sky account you can see what speeds your line has been set to. Often you can sync around 0.5meg higher than the speed in mysky as it's generally a rounded figure.

EDIT: The user can ring up and ask sky to UNCAP the line. This removes any artificial speed cap that has been placed on the line

In this case the line will sync at whatever speed a 7db noise margin gives.

So this CAN occur like you suggested, however DLM does not automatically apply it.

Edited by ukhardy07 (Sun 28-Aug-11 18:18:17)

Standard User actionorton
(newbie) Mon 29-Aug-11 17:41:54
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Re: 2 x ADSL with 2 different speeds

[re: ukhardy07] [link to this post]

Thank you for you great reponses, still trying to get my head around things.

One thing i picked up on was Batboy saying that the BT line may be causing noise on my Sky line? is this a possibility?

when i first moved in to my house the line was slow until i had the bell wire removed from my line. this increased the speed dramaticly.

The whole reason i had the 2nd line put in was so i could have twice the speed i had before, but at the moment i seem to have the same if not less between the 2.

First thing, i will call sky and ask them to uncap and see what happens.

Standard User BatBoy
(legend) Mon 29-Aug-11 17:47:42
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Re: 2 x ADSL with 2 different speeds

[re: actionorton] [link to this post]
Yes, crosstalk between the two pairs of wires. If they refuse/don't understand uncapping, ask for the Gaming profile to be applied.

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Administrator MrSaffron
(staff) Tue 30-Aug-11 14:07:57
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Re: 2 x ADSL with 2 different speeds

[re: BatBoy] [link to this post]
If crosstalk is suspected, then can be checked for

1. Unplug one modem, wait 30 minutes or so, and restart the remaining modem and see if its sync changes for the better. If around the same then no serious cross talk issues.

2. Do the same but the with the other line this time.

Cross talk is an issue, but remember that in the 100 pair cables going to an exchange you will have around 50% of the wires carrying DSL, and that is why BT use twisted pair cable.

Cross talk will generally only impact speeds significantly when using untwisted pairs. I would go on what the others in the thread said, not batboy in this case.

Andrew Ferguson, [email protected] - formerly known as
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
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