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Standard User sir_lagalot
(newbie) Fri 24-Mar-17 03:57:49
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Low SNR on ADSL Modem, High SNR on VDSL?

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Hi guys!

i was wondering if i could get some help in figuring out an issue recently with my internet connection. I'm based in the middle east and was using an ADSL connection (4 Mbps). My ISP recently upped the quality of their lines in my area and as a loyalty upgrade, i got a bump to 20 Mbps (although i was told i have now been moved to VDSL) . The trouble was, the ISP ran out of VDSL modems which they were providing their customers, so while the port was configured as a VDSL they asked me and a few dozen other customers to continue using their ADSL modems till they received stock.

since i know a few of the other customers who also had to continue using ADSL modems till the VDSL ones arrived, we all faced something rather strange. The SNR was low and we were getting low throughputs the entire time we using the ADSL modems , and the SNRs literally doubled when we changed to VDSL modems.

how exactly does a change of modems increase your speed? the ISP's lines were rehab'd to support up to 20 Mbps on ADSL2+. The port had been upgraded at the ISP end to vDSL as per a confirmation message i got.

so my question is, with VDSL configured at ISP end, why would i get low SNR with my ADSL2+ modem and double the SNR when i switch to VDSL? what exactly is the logic behind this?

Standard User RobertoS
(elder) Fri 24-Mar-17 09:44:48
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Re: Low SNR on ADSL Modem, High SNR on VDSL?

[re: sir_lagalot] [link to this post]
It is possible that the system you are on is a bit like it is in the UK.

In broad terms, here the ADSL2+ comes from equipment in the telephone exchange along with the telephone service, with a cabinet (Primary Connection Point or PCP) nearer to your premises where a cable carrying many copper pairs is connected to the individual lines to premises. The attenuation measures the signal loss. The SNR margin is usually set to 6dB by that exchange equipment.

With VDSL a separate fibre cable is run to another cabinet near to your PCP. The two cabinets are linked and the phone signal is merged with the fibre signal, giving phone plus VDSL (VDSL2 in the UK) on copper to your premises. (Recently there are some cabinets containing the combined equipment so you may have those).

From your post it sounds as if the ADSL2+ continued to be sent to you as before but passing straight through the VDSL2 cabinet. When you got your VDSL modem that was able to pick up the new signal generated much closer to you, at the cabinets instead of the exchange.

With respect to the SNR margin it is possible that your VDSL could supply much more than 20Mbps at your premises but is capped at the cabinets. If you are fairly close to the cabinets then that capping would result in a higher SNR margin. If you were quite a distance from them so the signal attenuates/weakens more, the SNR margin would also fall.

Have a read of the UK-targeted SNR/SNRM page on my website for the general principle, and Cause 3 here. Again, although that is ADSL the same principle applies.

The FTTx page may also help.

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Edited by RobertoS (Fri 24-Mar-17 09:50:32)

Standard User sir_lagalot
(newbie) Fri 24-Mar-17 10:20:43
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Re: Low SNR on ADSL Modem, High SNR on VDSL?

[re: RobertoS] [link to this post]
makes sense, thank you!

your website is great, going over it now.


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