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