It looks as though the "max attainable" figure is actually "max attainable at 6dB SNRM" thus it is possible for this figure to be lower than the current sync if the SNRM drifts below 6dB. If you resync at that point your sync speed will probably drop. If you don't resync it will hold (unless the SNRM falls further) - I had a 39999Kbps sync at about 3dB SNRM for something like 10 days just before the switch to 17a but eventually the inevitable happened and the speed dropped to 37M.

"Probably" BTW because I have noticed that my modem does tend to report a gradual slight fall in max attainable and SNRM over a few days following a resync which tends to go back up if I force another resync. It never approaches 3dB though - usually about 0.5dB "improvement"

Good point, however, my SNRM is at 6dB...

Max: Upstream rate = 15133 Kbps, Downstream rate = 31988 Kbps
Path: 0, Upstream rate = 1997 Kbps, Downstream rate = 34699 Kbps

Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 8c
TPS-TC: PTM Mode
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 6.1 28.9
Attn(dB): 0.0 0.0
Pwr(dBm): 8.5 2.3

Also, yes, i've noticed that myself about the modem, my max attainable was around 36Meg when i first resynced (about 10 days ago), and over those 10 days the attainable has gradually crept downwards.

In reply to a post by Zarjaz:

My D-side line length is just over 500 meters.

Excellent numbers - I'm hoping (if I ever see an FTTC cab, sob!) that I might get similar, as my line to the PCP is about 500m on the various mapping tools. Yes I know the line route, I've seen the diagram on an OpenReach SFI's laptop!

In reply to a post by Zarjaz:

Day off, so I thought I'd plug the in a JDSU and see what it showed.
I do wonder if the stats from the unlocked modems being reported on here are correct with regard to the max attainable rate. Some posts are showing little improvement, mine looks like a pleasing hike is possible, I guess time will show whose will prove to be correct ? My D-side line length is just over 500 meters.

Current sync rate 40000/10000

Attenuation 17db Profile 17a d/s snr 16.7db

Max rate up and down 22057/77821

The instrument often shows a sync rate lower than that achieved by the Openreach modem, so maybe it will just miss the full 80000.

Wow if you 500 meteres away from your cab and you can get a potential of 78mbps... thats amazing. I'v worked out im 360 meters away from my cab (according to google maps) so hopefully I should be able to sync at the the full 80mbps or above

Edited by Bobby_Valentino (Mon 14-Nov-11 19:18:07)

I will, at some point, get an exact measurement of the D-side. I know it's 0.5 copper all the way, though much of it has been in the ground since 1967.

I suppose of more use to you guys might be to unlock the modem and see what it says, then a comparison might show something. But with the hacking process not reporting attenuation, it all gets a bit pointless.

Nice

i'm about 10 meters away

200mbps here we come

In reply to a post by Zarjaz:

I will, at some point, get an exact measurement of the D-side. I know it's 0.5 copper all the way, though much of it has been in the ground since 1967.

I suppose of more use to you guys might be to unlock the modem and see what it says, then a comparison might show something. But with the hacking process not reporting attenuation, it all gets a bit pointless.

Hi Zarjaz,

The hacking process DOES show attenuation, but spread over the was 2 8c profile frequency band plans, but now 3 VDSL2 17a profile frequency band plans.

It is a known issue that BT's modem (& other broadcom based VDSL modems) does not report a "collective or average value" for attenuation.

We don't know the algorithm that a JDSU uses to report a combined value either.

What WOULD be really, really informative would be to be able to regularly compare unlocked modem stats against JDSU stats for the same connection.

Incidentally, nobody has been able/willing to confirm my own FTTC connection's D-side length with any certainty.
Is there any way that you are aware of whereby the actual cable route can be confirmed (from BT records for example)?

Paul.

Edited by deleted (Mon 14-Nov-11 21:06:29)

The records, in themselves, will show you where it goes, but won't provide an accurate length. For instance, every time your pair runs through a box with a joint in it, there might be a meter or so of slack on the cable, sometimes more.

Anyone thought of contacting JDSU to ask what algorithm they used ? Their base is in Chineham, just outside Basingstoke.

In reply to a post by Zarjaz:

The records, in themselves, will show you where it goes, but won't provide an accurate length. For instance, every time your pair runs through a box with a joint in it, there might be a meter or so of slack on the cable, sometimes more.

Anyone thought of contacting JDSU to ask what algorithm they used ? Their base is in Chineham, just outside Basingstoke.

An engineer visited today & renewed approximately 100m of overhead cable, from 2 poles away, & also replaced the master socket.

He said my total D-side length was being reported from his JDSU as 1 km, with 0.9 km showing as the length from the cabinet to the pole from where he replaced the cabling.

He was really pushed for time, so was unfortunately unable to show me BT's Network Records for my D-side that may have confirmed the D-side route.
That's a great pity, as my D-side could take either of 2 routes, with probably 250m or so difference between them.

This re-cabling has produced a small speed improvement, & CRC errors have almost disappeared, but it is still a fair way below the original installation's actual download speeds of 32 - 33 Mb.
Downloads are currently running at 25.5 Mb or so, against a modem sync speed of 27.4 Mb.

He did say that the attenuation as reported by his JDSU was roughly comparable to the attenuation obtained from the modem ( 21 to 22 dB for DS band D1).
However, my attenuation for DS band D2 is 64.0 dB & is that high for for DS band D3 it is reported as N/A.

From your own experiences, what sort of actual download speeds would you say can be typically expected for various D-side lengths from say 0.6 km to 1.2 km, & what sort of JDSU DS attenuation value readings would be reported?


Paul.

Unfortunately the day I had FTTC installed, I was at work, and SWMBO was here instead. So I never got to ask him for JDSU stats.

I've tried to estimate my line length as best as possible. I'm 550m away from the cabinet going by the BT access covers along the footpaths, so this is the shortest possible length it could be. If it wraps around a couple of streets first, it could be considerably longer.

Add that to the fact that my line is aluminium, and the fact that plenty of others are on my cabinet (my cabinet is currently out of capacity), I was pleasantly surprised to sync at around 35Meg on 8c, I'm hoping that I might get a couple of extra meg when I switch over to 17a, possibly even hitting the 40Meg ceiling.

Here are mine. I'm about 100m from the cabinet. Still on 8c, but it looks like might be maxing it up easily with the switch to 17.

Text

1 23 45 67 89 1011 1213 1415 1617 1819

Retrain Reason: 0 Max:    Upstream rate = 17411 Kbps, Downstream rate = 48624 KbpsPath:   0, Upstream rate = 9995 Kbps, Downstream rate = 39999 Kbps  Discovery Phase (Initial) Band Plan US: (0,95) (696,1183) DS: (32,687) (1192,1627) Medley Phase (Final) Band PlanUS: (0,95) (696,1183) DS: (32,687) (1192,1627)        VDSL Port Details       Upstream        Downstream Attainable Net Data Rate:      17411 kbps         48624 kbpsActual Aggregate Tx Power:    -  22.6 dBm           7.0 dBm ============================================================================  VDSL Band Status        U0      U1      U2      U3      D1      D2      D3   Line Attenuation(dB):  2.4     9.4      N/A     N/A    5.1     11.6     N/A   Signal Attenuation(dB):  0.0     8.4      N/A     N/A    5.1     11.6     N/A           SNR Margin(dB):  21.9    21.9     N/A     N/A    12.1    11.9     N/A            TX Power(dBm): -33.9   -22.9     N/A     N/A    4.9     2.8      N/A