Recently I noticed, my 8Mb ADSL subscription, can only download with 600KB to 920KB per second using IDM, while it should be downloading with 1MB speed steady and not fluctuating like that! I also tested the 16Mb subscription, and the download speed went from 950KB to 1.2MB at best while it must have been around 2MB per second!

The speed test, reported 6.4Mb connection speed for my 8Mb, and 9.1 Mb for my 16Mb subscription. I myself think they have set a "Burst" mode or something like that causes this but I'm, not sure.

By the way here is the stats for my modem:

ADSL Firmware Version:  FwVer: HwVer:T14.F7_11.2
Line State: Showtime
Modulation: ADSL2 PLUS
Annex Mode: ANNEX_M

8Mb subscription:

                   Downstream   Upstream     

SNR Margin:            15.5     26.8    db
Line Attenuation:      32.3     22.0    db
Data Rate:             8192     508     kbps
Max Rate:              17096    1836    kbps
CRC:                   10   0    

16MB subscription:

                   Downstream   Upstream     

SNR Margin:            6.1      32.6    db
Line Attenuation:      31.6     21.5    db
Data Rate:             16318    508     kbps
Max Rate:              17904    2423    kbps
CRC:                   24        0   

When I contacted the support, they said, your line noise is very high, and that is causing this, but I don't think so. Based on what I know, my Line Attenuation is not prefect but is fairly good, correct me if I'm wrong. And as far as I know, if the Line attenuation is high, I should be facing constant disconnection, and not that my speed decreases that much, am I right?

So what is the possible reason here? I don't know if SNR Margin affects the download speed or not. I know if too low, I'll face disconnection regularly, but I'm not sure if it affects the download speed! does it affect the download speed? for example a low SNR Margin causes a 16Mb to actually perform like a 9Mb? In that case, how should CRC look like?

  • Eliminate the obvious: you are testing using wired and not Wifi connection, correct? – LawrenceC Feb 4 at 15:52
  • Yes, Wired, my modem is wireless, however, it is connected to my PC through the LAN cord. – Breeze Feb 4 at 15:53
  • Isn't your ISP also responsible for the physical telecoms connection to the building? – Tetsujin Feb 4 at 16:38
  • @Tetsujin, no, they are different companies. – Breeze Feb 4 at 17:29
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    That's unfortunate - you will now have each of them claiming it's the other's fault - but in that case it's the telco not the ISP that is actually responsible. Get onto them & don't take no for an answer. – Tetsujin Feb 4 at 17:35

Based off a quick Google, and this article https://www.giantstride.gr/snr-margin-adsl/ your attenuation is fine. I don't know if I'd jump strait to "My ISP is cheating me", it could be a router setting, an old/bad router, or distance to the cable box that connects you to your ISP. Distance to the ADSL line exchange box "cable box" factors into your line attenuation, and ADSL is notoriously bad for a stable Internet connection (from my experience). When I lived way out in the country we had ADSL and lived about a mile from the cable box that connected to the ISP and we NEVER got the speeds advertised, not even close. I would maybe try a factory reset on your router first, eliminate all possible options on YOUR end before you start looking at the ISP. After that, call the ISP and complain again. If that doesn't work tell your ISP you'll switch Internet providers if they don't do something about it, then switch (if possible) if they don't. At the end of the day, if it is the ISP's fault, there's not much you can do about it besides complaining.

Best, Ryan

Edit: Also, your upstream/downstream SNR should not be below 10dB. Below 10 is a bad connection, and below 5 is basically no connection at all. When I lived at the farm we had 12dB SNR and it was a spotty connection sometimes. https://mybroadband.co.za/forum/threads/what-should-my-noise-margin-and-line-attenuation-be.46156/

  • Thanks a lot. I actually filled a complaint against the ISP and submitted to the Radio communication and regulatory organization in my country, without it they wouldn't budge at all. I checked my part and everything in my end is fine. I am a bit far from the center, however, I used to get 1MB per second when I had 8Mb subscription! and I could reach 1.8 1.9MB when I had 16MB subscription. but now, suddenly I see fluctuation in my download speed, which is against the law here, (if its not my problem, which most likely it is not) – Breeze Feb 4 at 17:44
  • by the way, here ( giantstride.gr/snr-margin-adsl ) says, SRN margin of 6db is fine (its the at the edge but its fine, and anything below that is problematic) I dont know if SNR Margin affects the download speed or not. I know if its too low, I'll face disconnection regularly, but I'm not sure if it affects the download speed! – Breeze Feb 4 at 17:47
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    So this is an interesting article <increasebroadbandspeed.co.uk/SNR-tweak>. This sentence especially "The exchange equipment attempts to set the downlink connection speed such that there is a sufficiently high SNR margin to guarantee a reliable connection (without regular drops and/or an unacceptable number of errors). This specific margin that the exchange tries to achieve is called the target SNR margin. The higher the target SNR margin, the lower your connection speed." – Ryan R Feb 4 at 19:23
  • So your ISP might be limiting your down-link speed because your SNR margin is so low. – Ryan R Feb 4 at 19:24
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    Also, check this out <superuser.com/questions/863298/…> – Ryan R Feb 4 at 19:35

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