VDSL2/FTTC - Increasing SNR Margin, to reduce packet loss and increase connection reliability

[g67575]

I found an old article that suggests packet loss should be less than 1% for reliable voice / audio calls. Is this still considered to be the case? This info was found here:
https://web.archive.org/web/20131010010244/http://sdu.ictp.it/pinger/pinger.html

I'm assung this will also apply for video streaming / game streaming over the Internet.

I have been running my SNR Margin at 2DB, by setting this option in my router's web interface, which boosts the download rate from 50mbps to approx. 60mbps. The problem with this, is that it seems to increase the amount of TCP transmission, which /i can measure by running the following online test:
https://speed.measurementlab.net/#/

At 3DB, I was sometimes getting 2/3% TCP retransmission (with an ethernet cable connected directly to my router). I tried setting it to 6DB instead, and it was still a little over 1% retransmission sometimes.

Then I set the SNR margin to 7DB, and I so far haven't had a TCP transmission result above 1%.

Any thoughts about this, and is running at a higher SNR margin a sensible thing to do?

 

[bremen1874]

On that test, my retransmission rates vary between 0.00% and 0.01%. That's with an old BT HG612 and leaving everything alone so DLM can do its thing. No idea what the SNR is currently, and I don't care.

 

[g67575]

Yup. Don't think I have the best FTTC line. I experimented with disabling the higher VDSL2 frequency band a bit ago, and this had a similar effect on TCP retransmission to increasing the SNR Margin.

 

[Caged]

Are you noticing any issues, or did you read about the packet loss and then look for a way of measuring it?

3% loss sounds weird, and interleaving should be taking care of that. What modem are you using?

 

[the-evaluator]

According to that site I've got > 5.5% packet loss on my FTTP connection. I don't believe that for one second.

 

[g67575]

@the-evaluator - It might be your ethernet adapter settings.

Maybe the retransmission only occurs when you are downloading at very high speeds? you can test that by lowering the duplex setting to 100mbps Full Duplex temporarily.

If it is related to the total throughput, some routers have an option called 'NAT acceleration', to speed up packet transfers over your network, so that might be worth trying.

On my own connection, I can reduce the TCP retransmission to 0%, by disabling the higher VDSL2 frequency bands (D2 and D3). But, that reduces the speed of the connection, down to about half.

I've found that enabling flow control allowed me to reduced the SNR Margin to 6DB, as it seems to resolve some intermittent issues.

 

[ChrisD.]

According to that site I've got > 5.5% packet loss on my FTTP connection. I don't believe that for one second.

Same and my connection is solid.

 

[the-evaluator]

@the-evaluator - It might be your ethernet adapter settings.

Maybe the retransmission only occurs when you are downloading at very high speeds? you can test that by lowering the duplex setting to 100mbps Full Duplex temporarily.

If it is related to the total throughput, some routers have an option called 'NAT acceleration', to speed up packet transfers over your network, so that might be worth trying.

On my own connection, I can reduce the TCP retransmission to 0%, by disabling the higher VDSL2 frequency bands (D2 and D3). But, that reduces the speed of the connection, down to about half.

I've found that enabling flow control allowed me to reduced the SNR Margin to 6DB, as it seems to resolve some intermittent issues.

This is the only test I've seen that reports packet loss. As I said, there may be packet loss to that particular test server but there's absolutely no way my connection in general has that amount of packet loss. No chance.

 

[g67575]

Maybe so. Anyone know any other websites that test packet loss /retransmission?

There's an older test here, that seems to work:
https://www.measurementlab.net/p/ndt-ws.html

This test reports congestion, packet loss and any other problems with the network, in the details tab. It's not reporting any issues in my case.

 

[bremen1874]

Do you actually have a problem with the connection (beyond anything self-inflicted from messing with the SNR)?

Generally, DLM will deal with any normal line fluctuations. If you believe that there's an actual line fault get it dealt with via the ISP.

 

[g67575]

It seems to be alright when tested on the NDT test. I have had issues with voice calls in the past, but this was when I was using a homeplug connection.

Changing the SNR margin doesn't appear to affect things too badly, but it may or may not cause problems with TCP retransmission. I think most people can work out that not messing with things is often the best thing to do

 

[bremen1874]

Are you noticing problems with services that use UDP?

TCP is self-correcting and will just slow things down to some extent with the retransmissions.

 

[g67575]

I'm not sure yet, we will see next time we have a voice / video call.

 

[g67575]

I don't know why, but when I restrict the upload speed to less than 1mbps, the packet loss / retransmission always seems to be around 0.07-0.08% on the mLab test. I've ran the test about 10 times and got this result. The download speed is still showing as approx. 44mbps.

So, the retransmission appears to be on the upload part of the connection.

I wonder if that could indicate a line problem?

 

[ChrisD.]

No, that's typical if upload is saturated. Try and set it at the router to 90% of what your max upload is, it generally fixes a load of problems.

 

[g67575]

I don't understand it, if I increase the upload bandwidth above 4% (in my router's QOS settings), the packet loss issues start occurring again.

I remember getting packet loss on previous routers too, but playing around with the QOS wasn't an option on those.

 

[Caged]

Are you seeing packet loss anywhere outside that in-browser test page?

 

[g67575]

I disabled the D3 part of the downstream VDSL2 spectrum. After running the mlab test 10 times, packet loss was very good (0.2-0.3%).

The D3 part of the spectrum contains higher frequency signals, which tend to have a weaker signal than the D1 and D2 bands.

Here is a diagram of what I'm talking about:
https://1.bp.blogspot.com/-48T2a0H_...6BHOvo9BACLcBGAsYHQ/s1600/VDSL-freq-bands.jpg

Another thing I tried was raising the SNR margin to 12DB. This also worked, but reduces the downstream rate more.

I'll see how it goes anyway... It's possible that it's an intermittent problem that can't be solved. One that I notice is the download rate seems pretty steady when downloading large files.

 

[g67575]

I found out that the packet loss was caused by network congestion, which I tested by downloading a file on another PC while running the mlab test. As far as I can tell, this is something that under normal conditions always occurs, as I noticed the same thing happening on another router, with 2-3% packet loss.

I did find that limiting the upload traffic on my Asus DSL-N16 seemed to resolve the problem though.

 

[Caged]

Asus DSL modems are complete trash, have you tried using a different one?