topic Re: Intriguing ADSL2+ behaviour in Broadband

Intriguing ADSL2+ behaviour

Mardler — Wed, 29 Jan 2025 16:14:47 GMT

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

Re: Intriguing ADSL2+ behaviour

Anonymous — Thu, 30 Jan 2025 08:52:09 GMT

@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?

Re: Intriguing ADSL2+ behaviour

Baldrick1 — Thu, 30 Jan 2025 11:00:37 GMT

@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.

Re: Intriguing ADSL2+ behaviour

Mardler — Thu, 30 Jan 2025 11:01:36 GMT

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.

Re: Intriguing ADSL2+ behaviour

Anonymous — Thu, 30 Jan 2025 11:03:44 GMT

@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!

Re: Intriguing ADSL2+ behaviour

Mardler — Thu, 30 Jan 2025 11:10:46 GMT

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.

Re: Intriguing ADSL2+ behaviour

Townman — Thu, 30 Jan 2025 11:15:23 GMT

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%20size%20in,resulting%20in%20a%20total%20of%20over%2016%25

Re: Intriguing ADSL2+ behaviour

Mardler — Thu, 30 Jan 2025 11:20:01 GMT

Mr. P: been there! 😀

Re: Intriguing ADSL2+ behaviour

RealAleMadrid — Thu, 30 Jan 2025 11:30:08 GMT

@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.

Re: Intriguing ADSL2+ behaviour

Townman — Thu, 30 Jan 2025 14:12:41 GMT

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

Re: Intriguing ADSL2+ behaviour

Mardler — Thu, 30 Jan 2025 14:18:33 GMT

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.

Re: Intriguing ADSL2+ behaviour

RealAleMadrid — Thu, 30 Jan 2025 15:32:37 GMT

@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.

Re: Intriguing ADSL2+ behaviour

Townman — Thu, 30 Jan 2025 20:39:14 GMT

@RealAleMadrid

Max rates can be informative, but they need to be understood.

@Mardler (the OP) brought into the the discussion "there was a rule of thumb that the average sync is c.88.2% of attainable", which is not actually correct. That factor refers to usable data throughput.

max attainable rate - what might happen on a good day with a SNRM of 6dB and no errors
sync rate - what you've got
usable data throughput rate - what you measure, will never be the same as either of the above due to errors and the stated packet over heads

Re: Intriguing ADSL2+ behaviour

RealAleMadrid — Thu, 30 Jan 2025 22:12:17 GMT

@Townman I do understand why my max rate is well below the sync rate, my downstream SNRM is lower than normal because after a power cut, which are quite common in my semi rural location my modem syncs up at 79.99 Mbps and the SNRM reduces when other modems come online. My line runs at between 1 and 2 dB SNRM with no CRC errors for months until the next power cut.

I was going to suggest in my post that the 88.2% figure was actually more likely to be the sync to data throughput ratio as you say but didn't want to imply that the OP was confusing things. 🤔

Re: Intriguing ADSL2+ behaviour

Mardler — Mon, 24 Mar 2025 14:03:09 GMT

An update.

First an apology: I could have sworn that I mentioned the upload speed riddle before but apparently not.

Since I raised the matter the connection has been stable with these stats today (in brackets: nominal values before the bout of disconnects this year, some numbers* are taken from immediately after PN's DSLAM reset):-

Down:-

SNRM: 8.3 (6.3*)

Attenuation: 18.5 (15.5-16)

Output power: 19.8 (19)

Attainable rate: 22692 (23-24000)

Sync: 19923 (21000+)

Up:-

SNRM: 3.2 (7.5-8)

Attenuation: 11.7 (10*)

Output power: 12.3 (12*)

Attainable rate: 872 (1267*)

Sync: 971 (1267*)

As you can see the important stat is both acceptable at its rate of 19923 and for its stability of 55 days. No complaints!

As stated in my initial post, I am simply intrigued as to what happened at the exchange with that bout of disconnects/reconnects especially with the upload speed (a bit of a pain on the infrequent times I use it for largish files).

Re: Intriguing ADSL2+ behaviour

lahbrannan1 — Mon, 24 Mar 2025 16:18:23 GMT

Thank you for your recent message.

I can run an enhanced diagnostics test to see if there are any underlying issues with your service, please provide us with your account username via private message so we can arrange this.

Lee Plusnet Sheffield

Re: Intriguing ADSL2+ behaviour

198kHz — Tue, 25 Mar 2025 11:06:10 GMT

Just to clarify where this 88.2% came from.

On 21CN ADSL, the IP Profile is fixed at 88.2% of the sync speed.

Re: Intriguing ADSL2+ behaviour

Mardler — Tue, 25 Mar 2025 11:38:25 GMT

Thanks, Lee.
How do I send you a PM? I tapped on your profile and got the About page but there's no hyperlink on the Contact me "button".

Re: Intriguing ADSL2+ behaviour

jab1 — Tue, 25 Mar 2025 11:40:42 GMT

@Mardler Try the 'send private message' button (in grey).

Re: Intriguing ADSL2+ behaviour

Mardler — Tue, 25 Mar 2025 11:41:24 GMT

198khz, it's also described as:-

IP Profile Rate

edit

It is the maximum download rate using the current sync rate. This is also known as the bRAS profile. It is found initially by taking 88.2% of the sync rate. This fraction applies to all 21st Century Network (21CN) ADSL products."

here:-

https://en.m.wikibooks.org/wiki/Internet_ADSL2%2B_Speed