Low SNR on ADSL Modem, High SNR on VDSL?

[Sir_Lagalot]

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?

thanks!

[Philip]

A higher SNR means cleaner signal. The DSL modem will work with a minimum SNR of let's say 6, but may experience some drops as noise fluctuates. If the noise fluctuates a lot, it is usually prudent to bump up the SNR setting (that can be done by engineers, automatically by the modem firmware, or, in some cases even by the user, depending on the modem/firmware).

When the SNR is higher, fluctuations in noise will not cause drops and CRC errors, that's a good thing. The downside of this is that in order to increase the SNR, the modem syncs at a lower speed. If your new modem syncs at the same speed with a higher SNR, it is a better scenario for you than before.

SNR can be different on the same line between different standards. VDSL can achieve much faster speeds usually, more noticeable in the upstream.