Intriguing ADSL2+ behaviour

[#1]

First of all this is not an urgent issue though I would be very interested in a PN technical appraisal of the line.

Basic stats long term:-

Rate c.22-23mbps

Sync c.20mbps

SNRM c6 to 7

Attenuation 15.5 - 16

Output power c.19

Router: Billion 7800DXL

The "problem":-

The connection has been stable since the DLM issue was resolved by PN in Q4 2024. There was a disconnect on January 9.
On the 27th at at 17:04 there was another disconnect followed by 4 more eventually culminating in a stable connection at 17:12.

Resulting stats (today):-

22496/8.1/18.5/19.8/19923

Example of error preceding every disconnect: "Jan 27 17:10:41 daemon warn kernel: HOST XTM tx ch 0 disabled"

Observations:-

The repeated disconnects are odd. Nothing untoward in the house but fairly windy. The jump in attenuation and SNRM would normally lead to a significant fall in sync rate: it hasn't. The increase in output power is also interesting.

My thoughts are that there was either a PN problem or more likely an exchange fault that the DLM tried to rectify or maybe it's a weather driven copper line issue with DLM intervention?

What do folks think? I am loathe to with a good connection but should I raise a ticket?

TIA

[#2]

@Mardler 

"Basic stats long term:-

Rate c.22-23mbps

Sync c.20mbps"

To clarify, are you saying that your router syncs at approx 20Mbps, but it is saying that the maximum data rate is nearer 22 to 23 Mbps?

AFAIK, that shouldn't happen / be possible. The max data rate should always be slightly less than the sync rate, to allow for overheads?

That might be your problem: the router trying to maintain a connection speed that the line cannot currently support?

.

[#3]

---

@Mardler wrote:

What do folks think? I am loathe to with a good connection but should I raise a ticket?

---

You can no longer raise a ticket for a line fault. Either the fault bot does it automatically or you ring in and an Agent does it for you.

Apart from odd reports on your third party router, from my reading, all that it appears that you have to report is that since the 4th January, during for some, the worst weather in living memory, you suffered 6 drops and temporarily reduced speeds that has now corrected itself.

Don't waste your time.

[#4]

Mr P: that is exactly what I am saying and every router I have used always had an "attainable" rate higher than sync. It's normal & not a problem. The attainable or available rate is the ideal rate, sync is what the exchange DLM allows given line conditions. In fact, many years ago there was a rule of thumb that the average sync is c.88.2% of attainable, it's often a pretty useless rule but it makes the point.

FWIW, during the day my router will show a different margin between the two as conditions change e.g. the rates are currently 22656/19923 which, oddly, is very close to 88.2%.

Thanks for your interest.

[#5]

@Mardler 

Yes, you are correct - I got that around the wrong way.

I'm going to have to stop posting whilst doing other things at the same time!

.

[#6]

Baldrick: that's my point, I don't want to waste PN's time.

The multiple disconnects within 8 minutes was a first and appears to have been exchange (possibly ISP) related. At the time the weather was well within normal bounds.

What intrigues me is the large increase in line attenuation and commensurate increase in SNRM which has had virtually no impact on sync whereas normally it would knock back sync quite a lot.

As I said, I am loathe to bother PN.

Thanks for your input.

[#7]

There’s several degrees of confusion here.

Max Data rate is the sync speed the line might support at a target SNRM of 6dB with no error management. If the line is running with a target of 3dB actual could be higher than the max.

Actual data throughput rate on ADSL should be circa 84% of the sync speed. The over head is different on VDSL. See https://blog.ipspace.net/2009/03/adsl-overhead/#:~:text=Assuming%20the%20average%20IP%20packet%20siz...

[#8]

Mr. P: been there! 😀

[#9]

@Mardler  The Attainable rate often referred to as the Max rate is calculated by the modem firmware with manufacturers using various unknown algorithms based on line conditions so it is often a totally unachievable speed but not always, it can also vary from below the sync speed to significantly above it. I have never heard of the pretty useless 88.2% figure, which is obviously not true as in my case at the moment on an FTTC line the max is 73384 Kbps with a sync speed of 79999 Kbps. 

In conclusion the Attainable rate is meaningless and best ignored.

[#10]

The overheads on ADSL and VDSL (FTTC) are quite different. IIRC the latter is just 6%.

[#11]

Hi TM,

No confusion on my part. Intrigue, yes.

I remain intrigued by the high attenuation and SNRM not affecting sync to any degree as it normally would.

Interesting link.

[#12]

@Townman  The OP's posts are discussing estimated and generally meaningless Max Attainable rates displayed by modems which are nothing to do with data transmission overheads.