Theft ...

Someone has stolen 6 Mbps

On Monday, all was fine with a full 40 Mbps sync and speeds of 38Mbps - TBB, BT and speedtest.net all giving similar results.

Yesterday (Tuesday), I was doing a download and noticed the speed was a little slow, so followed up with a TBB test and it returned 31Mbps which was confirmed by speedtest.net. Did a few checks and rebooted the modem. Tested again with similar results.

Tried a BT speedtest and found my maximum throughput had dropped from 38717 down to 32xxx. Again went and checked everything, and still returning the slow speeds. BT ran a full set of diagnostics around midnight and reported no issues but could see that my sync and max throughput had dropped.

Nothing obvious here, external wires look OK, no roadworks from here to cabinet ...

Rebooted the modem again this morning and max has increased to 33978

But still scratching my head as to why the drop and why it has not recovered.

More users on your cabinet leading to increased cross-talk?

It's happened to me as users increased from 3 when I first got FTTC up to about 20 now... though I've only lost about 3Mbps.

Possible, but all in one step? I was the first in the cabinet and now up to around 30 without any noticeable changes.

My neighbour had his installation a few weeks back and we share a dropwire - no noticeable difference.

If it was another one or two installed, then it does not bode well for a full cabinet!

It's not easy to be sure because I also had another problem in the middle of it all, but it seemed to happen in one step for me.

Another big unknown of course is the DLM, it may have a long memory and be biding its time to catch your neighbour out

it does not bode well for a full cabinet!

No, it doesn't... but it's bringing peoples rates down towards those that the estimator gives, maybe they're based on a full(ish) cabinet?

It bodes even less well for the achievable speeds when BT uprate the cabs for 80Mbps, I'd guess that it's even more susceptible to crosstalk


eta: your comment that

In reply to a post by MHC:

Rebooted the modem again this morning and max has increased to 33978

could be indicative of cross-talk, I get a better sync if I reboot in the morning. I think a fair number of people turn the modem off when they're not using it, hence lower crosstalk.

Edited by billford (Wed 18-May-11 11:39:08)

You think people turn the OR modem off?

The HH or other router, possibly, but the modem I'm not so sure. I haven't done so since it was first connected, whereas I used to turn off my ADSL router every so often.

It's a pure guess... but it fits with getting better sync in the morning, and a lot of people are averse to leaving anything electrical running if not necessary.

Some want to "save the planet" and some think it's a fire risk (possibly with some justification going by recent reports!).

I'd start by getting the modem swapped. When the engineer comes to do this, make sure they test with their kit, and if seeing the 'original' full sync, ask them to get the sync rate reset via the helpdesk.

Asked about that last night - but there is "no fault" yet! Need to wait until it gets to 15000 !

The local Tech who seems to be doing most is quite good. He called in one day last week - wanted to park in my drive whilst he did an install next door (other side) rather than park on double yellows. No problem for me, as I know I'll get good service when he comes back to see me again.

I can also add to the original that I have even reduced the local RFI/noise close to the modem and across one drop wire by having my ISDN removed and replaced with PSTN. ISDN NTE went along with ISDN switch and small PBX.

How do you tell how many people are connected to your cab?

When the BT engineer comes round to fix a fault, you ask him

oh

I haven't had a fault with the sync speed yet, mind you my line is 350 meters so it should be ok.

Only problem I have had is low throughput in the evens, friend on a different cab cas same issue, but it has been better as of late.

Yes, it could well be due to increased crosstalk. Raised in a specific topic on FTTC crosstalk some week or more ago, I suggested that this could well increase to problematic levels as the takeup of FTTC increases, and indeed I would think it'd become highly troublesome at the even more elevated speeds that BT are now talking about, despite any clever means that BT might introduce to mitigate it.

It's a shame that, essentially, we have Hobson's Choice when it comes down to a VDSL modem and that, unlike with many ADSL2/2+ modem-routers, we've no means of interrogating the modem or of controlling the line conditions to some extent. It may, for all we know, get to a point where the OR VDSL modem will drop the line. Like I say, that may be unavoidable, since the OP modem cannot be tweaked by the user for Target SNR. At least with a user-tweakable modem, you could organise it so that the line would run stably if noise and crosstalk were a problem, albeit at a lower sync. But non-tweakability is the sort of sacrifice that has to be made to have FTTC, it seems.

Obviously, it's on the copper stretch that the crosstalk will be taking place; knowing myself the features of optical fibre communications, I think you can disregard crosstalk on the fibre stretch. And the longer that copper portion is, the worse will be the crosstalk. BT has assumed a typical copper length of some 400m but I reckon quite a lot of people will be on connections that are far longer than that. Mine would be around 750m.

With ADSL Max and ADSL2/2+, BT have a default Target SNR of 6dB. Does anyone know what the equivalent figure is for FTTC? The copper stretch on it, I mean. And does having FTTC mean that the copper stretch will always run in Fast Path mode, never Interleaved?

Edited by deleted (Wed 18-May-11 17:18:20)

In reply to a post by meditator:

Does anyone know what the equivalent figure is for FTTC? The copper stretch on it, I mean.

I'm pretty sure it's 6db, when the engineer was checking my line I got a look at his test kit- SNR was 6db.

And does having FTTC mean that the copper stretch will always run in Fast Path mode, never Interleaved?

Assuming I've interpreted my BQM graphs correctly- nope. It's switched between pings of ~9ms and about ~18ms, with a re-sync at each change, which I assume is a flip between fastpath and interleaved. Profile (thus sync) stayed pretty much the same each time.

In that case, I'm wondering if the engineer can be asked to set the Target SNR to 9dB instead, or even to 12dB. I would personally rather have a stable line connection than a fast one that was always disconnecting.

I'm on a line that's predicted to give a downstream sync rate of some 27M bps with FTTC. However, the copper portion would still be 750m long and so that particular stretch will be subject to noise and crosstalk. And, at present, on ADSL2, I have to operate at an SNR of 15dB, owing to a high amount of noise and crosstalk that gets coupled on to the 2.7km line. I accept a low-ish sync in preference to regular disconnecting.

In going FTTC, I wonder whether, in my case, the noise source - which seems to be spurious - might lie along that 750m stretch somewhere. If it does, then the engineer setting an SNR of 6dB would be useless. I don't doubt that the VDSL modem would sync OK initially but I suspect that within a day or so it'd drop the sync speed, maybe even disconnecting.

If it transpired that, with FTTC, I got 27M bps but only by having an SNR of 6dB, I'd gladly see that 27M bps lowered to 15 - 20M bps at a higher SNR. 15 - 20M bps would still be a huge improvement on my measly 4M bps with ADSL2.

There are those who will want to screw as much speed as possible out of their FTTC connection but I'm not one of them. With any connection medium, there has to be a limit somewhere to the bitrate.

The only means around this problem, in the end, is for BT to make the entire length to the exchange fibre. That, apparently, is their goal. The sooner they do it, the better, I think.

Edited by deleted (Wed 18-May-11 18:29:36)

In reply to a post by meditator:

In that case, I'm wondering if the engineer can be asked to set the Target SNR to 9dB instead, or even to 12dB. I would personally rather have a stable line connection than a fast one that was always disconnecting.

I don't think that can be done directly, but there are three(?) settings available which probably do the same thing.

I think they're called standard, stable and super-stable (or similar), but whether they can be requested on a line by line basis I have no idea, you'd need to ask the ISP.

Or just see what the DLM does in the first place. Second-guessing it before even being connected is just a recipe for ulcers.

Good point.

The speed around by me has not changed with more people taking up slots in the Cab. I know that as of yesterday there are 143 connections using Infinity. I mentioned this before in another thread and the only thing that may be different is that we had to have a new Phone Cab installed as the other 1 fell apart. Just had a check around and peeps are still getting the same osrt of speeds as before my is 38.08Mb and 9.02Mb.

iechyd da 

Whats your up speed like.. and which area are you in.. ?
I have the same problem.. I was until last week getting 37Mbs in the day consistently and nearly 8Mbps up.
Now.. its down to around 31 and only 2.5 up.

I am in Basingstoke.
There is a line fault apparently in the area (on the Exchange side of the Cab) so my phone line sounds like a shaver ! Theory says they shouldn't be linked.. but.. ??

I was the first in the Cab but not sure on take up in the area.

Up speed is fine.

Also lost service completely on Thursday from 00:10 to 01:40.