Modem tolerance for poor SNR Margins

[Hogrod]

Taking a stab in the dark, as I understand it poor SNR Margins result in a loss of line sync. A modem can only sync to a line if the SNR Margins are within its tolerances.

Therefore would it not be better to find and purchase a modem that is capable of syncing to a line with poor SNR Margins, than it would be to have Telkom decrease the line speed (which would increase the SNR margins and therefore the stability of the line)?

I for one would rather have a modem that can sync to a line with poor SNR margins and therefore keep my 4meg line speed rather than Telkom reduce the line speed to 1 meg?

What do you think, or is my overview too simplistic?
And what modems have better than average tolerances to poor SNR?

 

[linkinpark]

I had a similar problem, my billion router was only syncing at 740k on my 4mb line while my old telkom router synced at full speed...
Tried firmware upgrades and downgrades with no difference
The billion also had worse snr and att values
Contacted billion, they couldnt explain it, wanted me to courier the router back to them

 

[Yotch]

Some routers do work better on noisy lines than others such as the Thomson SpeedTouch 585 v.6 and Netgear DG834GT which are based on Broadcomm chipsets. In the UK some of the ISPs claim that many of the Texas Instruments AR7 based routers do not cope well with noisy lines. BT engineers are apparently regularly called out to investigate lines that keep disconnecting only to find that the problems disappear when using the BT test routers.

Before you consider a new router you must first carefully analyse your downstream noise margin using routerstats. If this fluctuates excessively a better router will probably not help much. If the noise margin is low (near 6dB) but stable then a better router might help. If, however, it fluctuates wildly then you may have to reduce your linespeed further. It may not be possible to get a decent linespeed on a long line if there is excessive noise. Crosstalk problems together with aging copper wiring/joints/earthing etc. can seriously degrade performance. If you post plots of your noise margin fluctuation somebody may be able to assist you further.

Routerstats
http://www.vwlowen.co.uk/internet/files.htm

TI AR7 problems
http://www.theregister.co.uk/2007/10/22/zen_ar7_infineon_bt_fault/
http://forum.zensupport.co.uk/thread/28529.aspx
http://www.channelregister.co.uk/2007/10/22/zen_ar7_infineon_bt_fault/comments/

Refurbished Thomson routers
http://www.dsldepot.co.uk/product_details.asp?idProduct=851

 

[Hogrod]

@Yotch - You're a wealth of knowledge thanks I'll take a look at those links. MY line was reduced to 2meg and its better than it's ever been 4meg or 6meg alike.

I've got a down SNR of 24dB and up of 12dB, which probably explains the good performance. Though I think I could probably get them to boost it to 3meg and I'll be alright.

 

[fusion01]

With an SNR of 24db, you should easily be able to run a 4mb line... easily. We were getting 22db on a 384 kbps at one stage - upgraded to 4mb - SNR dropped to 5-12db (often higher SNR in the day, decreasing in the evenings - odd that). Point is, even on a Negear DG834 @ 5DB, the router is hanging in there and all seems stable enough.

The upgrade is on a house I've just moved into (where I go, fast broadband has to follow) and I'm now trying to figure out how to get all cabling rewired / new earthing in place / new joints etc too. Should help improve the SNR somewhat I hope. If it was sitting around 12-16db I'd be most happy (as per last two addresses) but - as I said - the Netgear is coping - impressive.