Very slow FTTC

Hi, I've just moved to vdsl in the last week, and while I *know* the cabinet is still a mile away I had been hoping to improve upon my reasonable 4Mb ADSL

With the ECI modem the speed is 4.5Mb/0.6Mb (adsl was 4Mb/0.9Mb)

Sadly the ECI gives no details so I've changed to the HG612 for testing (which is connected at a terrible 2.5Mb/0.5Mb)

Anyone able to help interpret this graph?

http://i.imgur.com/jbchU.png

What was your estimate?

Estimate was 10.9/1.0

So you are a distance from the cab, the only option for you is to contact BT.

According to the VDSL Band Status you only have 1 upstream and 1 downstream band.

At that distance I am surprised they accepted the install. As you are well down on estimate and sub 5Mb then you need to speak with your ISP and, if you want to, ask to be returned to ADSL at the lower cost.

That said, I have to ask where your modem was connected when you did the measurements? In the test socket? on a long extension? and how much internal wiring do you have from where it comes through the wall to the master? Can it be shortened?

I can tell you what I see is interesting.

Is that if BT sold adsl2 from cabinets that would help people in your position.

The attenuation on D1 is 30db. For adsl if you had 30db attenuation you would have a much improved signal over your old adsl. But from cabinets vdsl2 is the only service offered by BT. The problem with the VDSL2 service however is to protect existing adsl services the frequencies that conflict with adsl have the power cut back, and it seems your conneciton cant make use of the frequencies above those hence the poor sync speed.

This power cutback to protect adsl has probably lost you 2/3 to 4/5 of the signal you would otherwise have. Ultimately there is nothing you can do about this as openreach dont provide adsl from cabinets and the vdsl2 policy is there for good reason to protect existing adsl services. I think you would be around 10-12mbit sync if on a adsl from the cabinet service.

Edited by Chrysalis (Tue 08-Jan-13 03:43:40)

Surely, though, ADSL from a cabinet would have to have a power mask just as tough as the VDSL2 power mask applied in order to prevent crosstalk from trashing ADSL from the exchange and other VDSL2 connections. In that case, offering ADSL from the cabinet is of very little advantage. Further, ADSL from the cabinet would lessen the effectiveness of vectoring VDSL2 for reducing crosstalk.

The only way round this would be to require all DSL in areas with FTTx cabinet to be provided from the cabinet - but that would preclude DSL via LLU and SLU, which Ofcom are not going to allow.

yes it adds complexities in various ways hence its a no go and isnt future proof at all as vdsl2 replaces it.

I was just saying what could have worked well for long lines from cabinets.

Modem is connected to a 7m cat5e extension, which is connected to the punch down socket behind the front plate.

There is no extension wiring and it is no better/worse when plugged directly into the nte.

You mean you've taken the stats from the master socket and compared them line by line?

Yes, I ran the stats again plugged into the socket, it's almost identical, as it also is with the filter face plate removed.

To be honest there is not much that can be done - you might have a degraded external line but that is probably unlikely and getting it checked will be a struggle.

You have to flag your concerns with your ISP now and as I suggested before tell them whether or not you want to cancel the contract - given the abysmal performance you should be able to (if you wish).

I'm a bit unsure about this.

The ECI modem speeds I assume are throughput from speed tests. The HG612 obviously has a major issue - where did you get it?

The estimates I believe to be sync speeds, but that's not the most important bit at the moment. I'd like to see what you get from a BT Performance test, using the ECI modem.

If you do that, please use the Diagnostic option on the left, and paste the full contents of the downstream and upstream results text boxes .

Details from the ECI below!

Download speed achieved during the test was - 4.32 Mbps
For your connection, the acceptable range of speeds is 3.24 Mbps-0 Mbps.
IP Profile for your line is - 4.63 Mbps

Upload speed achieved during the test was - 0.58Mbps
Additional Information:
Upstream Rate IP profile on your line is - 2 Mbps

The HG612 is a 3B from ebay, but the cabinet went live about 3 weeks ago, so I'm assuming its ECI there too. I needed something that could display line status!

Ummm . I was hoping for something a bit more encouraging from that. Thanks for trying, anyway.

Trouble with the HG612 you have is that if it is getting under half the ECI speeds, the line status from it isn't really much use.

What might have been interesting with it would have been the CLI telnet output from:-

xdslctl info --pbParams to see the Discovery phase compared to the actual. But probably not worth trying that. It could be just as unreliable. Have you tried a factory reset on the HG612? In fact on the ECI as well?

What does this checker give for the speed multiplier for your postcode, and applying that to your previous real ADSLx speed what do you get?

I've factory reset both devices, I got as far as a working serial output from the ECI, but sadly I was unable to send any characters via the serial connection to login.

Checker gives 4.57, compared with stable 4Mb adsl2 I would get 18.3

From a saved file with the HG612 last night:

# xdslcmd info --pbParams
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason: 0
Max: Upstream rate = 614 Kbps, Downstream rate = 2372 Kbps
Path: 0, Upstream rate = 630 Kbps, Downstream rate = 2505 Kbps

Discovery Phase (Initial) Band Plan
US: (0,95) (880,1195) (1984,2771)
DS: (32,859) (1216,1959) (2792,4083)
Medley Phase (Final) Band Plan
US: (0,95)
DS: (32,859)
VDSL Port Details Upstream Downstream
Attainable Net Data Rate: 614 kbps 2372 kbps
Actual Aggregate Tx Power: 1.9 dBm 2.0 dBm
============================================================================
VDSL Band Status U0 U1 U2 U3 D1 D2 D3
Line Attenuation(dB): 10.4 70.0 66.3 N/A 30.3 0.1 65.7
Signal Attenuation(dB): 10.4 N/A N/A N/A 30.3 N/A N/A
SNR Margin(dB): 5.3 N/A N/A N/A 5.4 N/A N/A
TX Power(dBm): 1.9 N/A N/A N/A 2.0 N/A N/A

Anyone able to help interpret this graph?

I'm trying to understand why there are a bunch of channels in use at around 500 aka 2 MHz and a large gap down to the next one in use at 600 kHz - perhaps that's where the power/frequency mask cuts in for your line length.

on further reading it depends on the Cabinet Assigned Loss or CAL - if the cabinet is a long way from the exchange eg 50 dB attenuation at 300 kHz then the downstream power mask allows full power from about 600 kHz upwards. In this case the cutoff in action around 2M suggests a CAL of 30 so cabinet about 2-3km from exchange.

Edited by yarwell (Tue 08-Jan-13 18:21:21)

In reply to a post by MHC:

At that distance I am surprised they accepted the install.

Why do you say that?

I am more than a mile from the cabinet and the BT estimate was 10.7 down, 1.0 up.

Initially the sync was 39mb/sec, but in the nine months it has been live, the sync speed has dropped to 31mb/sec.

Because of the low estimate, BT refuse to let my ISP put me on a 40/10 profile, so I am stuck with a 2mb/sec uplink cap, though the attainable speeds are reported at 38 down, 6.8 up.

The various research and tests that I have seen suggests that for a line of over 1 mile then the data rate on a nice clean line without too much cross talk would be in the order of 10Mbps - the BT estimate is close. As for achieving 39Mbps - I would be suspicious of the distance.

I have several test reports filed away - including this one from Ericsson: scroll to the eighth page (43) and look at the Profile 17 Downstream - it appears to end at 700m but it may well follow the profile 12 line and at 1750m (1.1 miles) shows about 10Mbps.

There will be cases where the response is noticeably better but not by a factor of four.

do these graphs show sudden drop offs, when the frequencies above the power back off run out? thats what I would expect. I suspect 10mbit would be about right if there was no power backoff on the adsl frequencies, it would seem the estimate took no account of that.

BTO say nothing wrong with the line,

Back to adsl2 I guess

I'm confused, I thought VDSL2 was supposed to work just as well as ADSL2 at the same distances?

Is this perhaps a profile issue? Surely 8b would make more sense for such a long line than 17a.

In reply to a post by alexatkin:

Is this perhaps a profile issue? Surely 8b would make more sense for such a long line than 17a.

Presuming you were referring to the original 8c profile, I did seem to achieve higher speeds on my "up to" 1000m line.

Using 8c, all the 40Mb DS speed was crammed in below tone 1627 (around 7 MHz):-

Discovery Phase (Initial) Band Plan
US: (0,95) (696,1183)
DS: (32,687) (1192,1627)

With 17a it's spread out over higher frequency tones (around 8.5 MHz):-

Discovery Phase (Initial) Band Plan
US: (0,95) (868,1207) (1972,2783)
DS: (32,859) (1216,1963) (2792,3959)

My connection stops bitloading at around tone 1750 leaving some DS speed "missing".
I could use all the way up to tone 1627 on my 8c profile & achieved up to 35Mb sync speed.

These days using 17a, I'm lucky if I can achieve 27Mb to 30Mb sync speed.
So, yes. The 8c profile did seem better suited to longer lines that simply can't use the higher frequency tones.

The band plan tones mentioned above are from a Huawei DSLAM.
They are slightly different from an ECI DSLAM.


We can see the original poster is connected to an ECI DSLAM & due to high attenuation over the mile long connection can only use the lowest DS band plan tones.

I have however seen users achieve up to around 10Mb on that sort of distance on their FTTC connections, again only using the lowest frequency band plan tones:-

Max: Upstream rate = 558 Kbps, Downstream rate = 8568 Kbps
Path: 0, Upstream rate = 542 Kbps, Downstream rate = 9627 Kbps

Discovery Phase (Initial) Band Plan
US: (0,95) (880,1195) (1984,2771)
DS: (32,859) (1216,1959) (2792,4083)
Medley Phase (Final) Band Plan
US: (0,95)
DS: (32,859)

Edited by deleted (Sun 27-Jan-13 22:28:49)

It just seems puzzling to use the 17a profile on a long line as its common sense that those higher frequencies are going to be useless.

Just a thought though, is it actually possible to force ADSL2 from a VDSL cabinet by unticking VDSL in the options of the HG612? Or is the DSLAM locked to VDSL to prevent crosstalk with exchange connections?

I await correction from someone more knowledgeable than I, but I'm pretty sure BT Openreach run all their VDSL2 on profile 17a and prohibit negotiation of any form of ADSL to an FTTC cabinet. I don't want to unplug my HG612 or risk my FTTC DSLAM port disabling itself and needing to be reset. If you have an FTTC line and are feeling brave, you could try connecting an ADSL2+ device (or disabling all flavours of VDSL on a hacked HG612) and see what happens.


In theory, VDSL2 should perform roughly the same as ADSL2 on long lines. However, to protect the weaker ADSL signals that have come from the exchange from strong VDSL2 signals generated at the cabinet, the UK frequency plan limits the allowable power on the lower VDSL frequencies which are shared with ADSL2+. This means FTTC a long way from the cabinet can land up performing worse than ADSL, because there isn't much usable bandwidth above the ADSL2+ frequencies and the power is limited on the shared frequencies.

If you could negotiate ADSL2+ to an FTTC cabinet, it wouldn't help because you'd be subjected to the same power cut-offs as a VDSL2 device. You may find the discussion in this thread interesting, especially this post of mine.

Edited by deleted (Mon 28-Jan-13 03:35:46)

Very interesting, but I am on Digital Region which supposedly ARE removing their 70Mbit limit on the Max service soon.

Although considering they seemed to originally plan to offer a proper Max service from day one of Max being available,then pulled back. Iit makes you wonder what the specific problem was with the line cards preventing full speed?

It makes you wonder if it was an issue with interference with ADSL lines or something.

Getting back on-topic though, how do the ADSL2 bands compare to VDSL2?

Edited by alexatkin (Mon 28-Jan-13 03:00:06)

In reply to a post by alexatkin:

Getting back on-topic though, how do the ADSL2 bands compare to VDSL2?

VDSL2 profile 17a uses 4096 carriers at 4.3125kHz spacing. ADSL and ADSL2 uses the bottom 256 of these carriers. ADSL2+ uses the bottom 512 of these carriers. The higher the frequency, the greater the attenuation, so the lower the signal to noise ratio on a longer line.

All forms of ADSL put the upstream on the lowest frequencies. VDSL2 uses a more complicated structure, with up to three blocks of upstream and downstream frequencies spread through the available frequencies. This tries to ensure that available downstream and available upstream bandwidth decrease in proportion to each other.

In reply to a post by alexatkin:

Just a thought though, is it actually possible to force ADSL2 from a VDSL cabinet by unticking VDSL in the options of the HG612? Or is the DSLAM locked to VDSL to prevent crosstalk with exchange connections?

No. I tried that a long time ago & a connection couldn't be established.

I have also tried forcing it back to 8c via telnet commands & that didn't work either.