FTTC DLM

There have been several posts where it has been reported that following an engineer visit to cure very low speeds, the engineer has "phoned-home", (not that many OR engineers are from another planet but it seems some newer ones are ), to get the speed uncapped.

As the IP Profile reacts almost instantly to sync speed changes I was baffled by this.

I believe I have found the reason in the extract first below from a SIN. In the event of severe problems the DLM goes beyond setting the IP Profile and imposes an actual sync-speed cap.

Note that although some recent posters have referred to a 10-day training period as in the BT Wholesale DLMs, there is no mention of such here.

2.2.5 Dynamic Line Management
Dynamic Line Management (DLM) is employed in GEA-FTTC. DLM constantly
manages lines to maintain a target stability. It does this for as long as the product
exists.
At provision, the line is put on wide open profiles, allowing downstream line speeds
of up to 40Mbit/s, and upstream line speeds of up to 2Mbit/s or 10Mbit/s depending
on the upstream product option selected.
On the first day of operation, DLM will intervene if severe instability is detected.
Otherwise, DLM will wait until the day after provision before intervening, provided
that the line has been trained up for at least 15 minutes during the preceding day.
If DLM intervenes it will set a capped profile with a maximum rate and a minimum
rate, where the minimum rate is set at approximately half of the maximum rate. The
purpose of the minimum rate is to ensure that the line does not train at a rate which is
significantly below the level the line should be able to achieve. If this happened, then
the line is likely to remain at a very low rate till a re-train is forced by the user
powering off the Active NTE.
Note that the upstream throughput is also constrained on the DSLAM to the upstream
rate requested in the order, ie 2Mbit/s or 10Mbit/s, so even if the upstream line speed
is higher, the upstream throughput is constrained to the level ordered for the product.

The following extract shows that ISPs are expected to apply the IP Profile within their own systems, as some do on ADSLx, to avoid packet loss, and that this needs careful implementation. It also seems to refer to this aspect when applying any ISP-traffic shaping.

2.1.6 Downstream shaping
The CP is expected to shape the downstream traffic to match the actual VDSL2 line
rate in order to avoid excessive traffic loss.
CPs should be aware that the mechanism for reporting the downstream and upstream
line rates relies on a line re-train causing the CP, or the CPE, to initiate a new PPP
session or a new DHCP request. The success of this is down to the CP's choice of
timers used around PPP / DHCP handling.
If the PPP/DHCP survives a re-train, then the CP will be unaware of any change in
the line rate and will not be able to shape appropriately.
The line re-train time for VDSL2 can be anywhere between 10 and 60 seconds, with
typical values in the 20-30 second range.
As DHCP typically uses lease timeouts in the order of days rather than seconds, CPs
intending to use DHCP are advised to consider the impact of downstream line rate
changes on their service and any strategies they could adopt if they wish to shape
downstream traffic.

Edited by RobertoS (Thu 08-Sep-11 10:24:55)

Thanks for posting that information, I have often wondered how the profiles are set.

I have noticed an increase in latency from 12ms to 23/24ms on my line recently and have wondered how it would go back, if it's done automatically within the '10 day' period or its adjusted by someone at BTOR.


My Broadband Ping

Your latency is probably interleaving kicking in. Mine seems to be on fast path, but Billford's swaps frequently between the same figure as mine and your low one, and the high one you have.

Does your ISP have an option to maintain the Interleaving settings for FTTC? I don't think Aquiss have this option available to them last time I checked.

I don't see the relevance ? I'm on solid fast path and happy. Bill is also on IDNet.

Edited by RobertoS (Thu 08-Sep-11 13:44:12)

I was just wondering if its possible for either the ISP/BTOR to switch from Interleaved to Fastpath but I don't think this is possible with FTTC. I think with FTTC its solely determined by DLM.

Could you kindly expand on a point in 2.2.5:

the line is likely to remain at a very low rate till a re-train is forced by the user
powering off the Active NTE

How does the user power off the Active NTE? Switch the modem off perhaps? And what is an NTE in this context?

And a point in 2.1.6:

The line re-train time for VDSL2 can be anywhere between 10 and 60 seconds

Does this mean that if you are running slow because of DLM you can recover in a very short time by (somehow) powering off the Active NTE?

The Openreach modem is the "Active NTE". So yes, just disconnect the power.

The retrain time is what we usually call the time taken to re-sync.

Re running slow, there are two totally different cases.

The usual one would be if there is a sudden burst of noise which caused a low sync, just like on ADSLx. Yes you can recover straight away by power-cycling the modem, as you say. Assuming you get a normal sync, the IP Profile rises pretty well immediately and you are back to speed.

The other case is were there has been prolonged noise, usually due to a fault somewhere. The DLM can impose a sync speed cap as a result, (called a "capped profile" which is nothing directly to do with IP Profile), and the only way that cap gets removed is after the fault is fixed the engineer has to request a reset.

Hope that clarifies it a bit.

Thanks for that. I understand it to mean that if you are still stuck at an unusually low speed after power-cycling the modem, waiting in hope for the situation to improve is pointless - it never will. Contact the ISP straight away.

I must have had about 10 engineer visits to have the line reset due to the download I.P profile dropping down from 38717Kbps down to the 25000Kbps region. There was nothing I could do to raise the profile above 25000Kbps region, the only way to get back to 38717Kbps was for the engineer to call into BTOR to have the line reset. I am sure that I have a REIN issue in my area but BTOR are refusing to look into the issue any more. I think my line will be in an 'interleaved' state until the REIN issue is resolved, which will be never.