Having attempted to play devil's advocate, I think I've failed, as no-one has really taken me up on this. Although Bald_Eagle1 has stated some of the things we know supposedly happen (on ADSL at least).
| Quote from: Bald_Eagle1 |
| Quote from: Anotherone | I sometimes wonder what the point of the Profile is on Fibre other than indicating the maximum download speed possible to the EU as it's not used by ISPs for traffic management as with ADSL.
|
Plusnet set their own "Current Line Speed" based upon IP Profile to manage packet loss etc.
However, Plusnet's profile is only updated following a Delta report from BT.
These are supposedly sent a few times per day, but Plusnet's profile sometimes lags behind. |
We know from experience with ADSL that Delta reports don't seem to be sent when they should be, but in relation to FTTC I'm not sure where this "few times a day" comes from. SIN498 doesn't refer to a "Delta report" but para.1.2.1 says two things -
| Quote |
| The upstream and downstream VDSL2 data rate is reported to the CP BRAS/Radius upon PPP discovery |
It then talks about overheads before stating
| Quote |
As a result of these overheads, the actual achievable throughput in bits per second is
dependent on the reported VDSL2 rate and frame size of the data being transmitted.
Openreach advise CPs to consider carefully how they interpret the reported VDSL2 rate in relation to the services they sell, the specifics on an individual EU's use of the service, and any impacts of their own network. |
The implication of this is that the actual rates are reported not (just) the profile, and this is confirmed in para.2.1.6 that's already been quoted.
When you run the BT Diagnostic Speedtest, it's exactly the same test for ADSL as for FTTC and you have to enter your Phone number. It's pretty obvious that the tester has to look up the (type of) service from a database, possibly for a circuit ID to know where to get data.
It's also obvious some BT systems know when there has been a change of sync (DLM at least).
What we don't know is whether the tester gets the current sync data (& profile?) direct or get's it from an OAM database.
If I've understood Bald_Eagle1's comments correctly, he has had instances of the IP profile reported by the tester not being consistent with the current sync speed shown by the modem.
As the sync speed isn't reported by the Tester for FTTC we are none the wiser as to where it gets data, nor whether it gets data from somewhere different to ADSL. As the latter is probably unlikely, one could make some comparison to ADSL.
That is - I've never come across a case of the tester reporting an incorrect sync speed.
In the past for 20CN, I recall advise to run the BT Speedtester to ensure that the profile is upodated!
Yet there also seem to be 20CN/21CN cases where the profile reported at the bRAS is not the same as reported by the Tester - are these cases ones where PPP has not been lost?.
Also note para.2.1.6 of SIN498 says "The line re-train time for VDSL2 can be anywhere between 10 and 60 seconds"
I'll leave you to draw your own conclusions, but because actual rates are reported, it's likely that Plusnet's end isn't in accordance with SIN498 and some possibility that something at BT's end isn't either! I find it rather strange that sync data on BT systems could/need to/may rely on establishing a new PPP session with the CP/ISP.
