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Author Topic: 2nd Opinion on downstream speed  (Read 6500 times)

AlexS

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2nd Opinion on downstream speed
« on: March 16, 2009, 10:33:24 PM »

Hi, My ISP has capped my downstream speed to 2560k saying it's the maximum the line can support reliably.
However with my previous ISP I was getting 3400k downstream reliably no problem !
Could someone please inform me what downstream speed I should get with these stats ?

Connected to test socket on NTE5 master with ADSLNation XF-1e filter.
No noise on phone line.
ADSL mode : G.dmt

ADSL Link Downstream Upstream
Connection Speed 2560 kbps 768 kbps
Line Attenuation 49 db 29 db
Noise Margin 7 db 11 db

thanks
Alex
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HPsauce

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Re: 2nd Opinion on downstream speed
« Reply #1 on: March 16, 2009, 11:02:23 PM »

My ISP has capped my downstream speed to 2560k
How have they done that?
What EXACTLY have they told you?
(your stats don't indicate how and are inconsistent)
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kitz

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Re: 2nd Opinion on downstream speed
« Reply #2 on: March 16, 2009, 11:03:37 PM »

HI and welcome

A good 49dB attenuation line would be expected to get somewhere in the region of 4.5 - 5Mbps... with you being connected to the test socket and having a XF-1e filter  this should give your line the best possible chance.

Even though you say that the line is capped, there currently isnt sufficient SNR Margin to support higher speeds... which must mean its much noiser than most lines would expected to be :/

With you saying test socket I am assuming you have an NTE5 master socket....  but is there any wiring or boxes before the NTE5?
Are there any other sockets/phones/devices plugged in?

Have you tried grabbing a copy of routerstats and monitoring your SNR Margin to see if it is ever any better than the current reading?
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AlexS

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Re: 2nd Opinion on downstream speed
« Reply #3 on: March 17, 2009, 12:16:39 AM »

Hi and thanks for the Welcome.
Was with Zen Internet using a Speedtouch ST546v6 and getting 3400k downstream.
( can't remember exact stats sorry ) One day connection kept dropping. No joy with Zen &
Openreach so moved to Sky Max as cheaper deal even though speed/reliability still poor.

Now with Sky using Netgear DG934G ( DG834Gv3 ) with Sky's custom firmware.
Sky cap downstream to 2560k as line is classed as noisy. Finally find out suffering from
REIN interference. Locate source to neighbour when he goes on holiday and returns !
( connection improves then starts dying again ). Noise on AM radio, so contact OFCOM
and nice OFCOM Radio tech visits and chats to neighbour. Soon after connection is reliable
again but still capped by Sky to 2560k speed...
No noise on radio anymore. Nothing else connected to phone system whilst testing.
reconnecting phone makes no change to stats.
No wiring/boxes or extra kit fitted before the NTE5 master.
I've used routerstats all along to show Openreach engineers and OFCOM tech what was
happening.
Openreach guys had REIN training, but hadn't been issued with test gear !
OFCOM guy had fancy scanner, but told me he couldn't say which property/kit was the source
of the REIN interfence.  ;)
* Above events occured over approx 2 year timescale.

Since I'm on a LLU product with Sky, I wonder if I'm potentially connecting at a higher speed
( hence the standard 6.9db noise margin recommended by Sky ) but because Sky tech still
think I have a noisy line they've instructed my port on the DSLAM to only talk to me at 2560k ?
I understand a capped line on an IPstream connection would result in a high noise
margin thereby giving the game away.
Since my 12month Sky contract expired at the end of Feb 2009, I fear the only way to check this
for sure is to move back to an ISP running an IPStream product.

thanks
Alex

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kitz

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Re: 2nd Opinion on downstream speed
« Reply #4 on: March 17, 2009, 02:20:08 AM »

Both IPStream maxdsl and LLU both work in a similar way when it comes to SNR and setting the sync rate.

At the moment you dont have any 'spare' SNR Margin for your line to be able to sync at higher speeds - regardless of the fact that sky say they have capped it.

If they have capped it at 2560 kbps and if your line was capable of supporting a higher sync speed then it would show by an increase in your Noise Margin - which is probably what HPSauce meant when he said your line stats were inconsistent.
The other thing is sky tend to use 2000 when they cap, but I will admit that I am not familiar with all their profiles.

As it stands atm from your linestats, even if Sky removed any cap they have applied, your line doesn't have sufficient SNR to be able sync much higher than what it is currently.

The only other figures that would have been useful to double check this would be power output, but I dont think that info is available from the DG934G.  Theres something called power cut-back which the dslam/msan can do which does have the effect of reducing your SNR.... but this isnt done unless youre on a very short line and there is a risk of your ADSL signal drowning out neighbouring lines at the dslam causing NExT.
However in all honesty I dont even think that the dslam would have done this 1. because youre not on a short line and 2. power cut-back only reduces SNRM to around 12dBish, not any lower.

Is your SNRM always at or around 7dB?  Each additional 1dB of SNRM can be anything from approx 125 to 400 kbps of sync speed depending on your bit loading.
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AlexS

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Re: 2nd Opinion on downstream speed
« Reply #5 on: March 17, 2009, 09:11:17 AM »

Sometimes I can get 8 or 9db, but the noise margin on the Netgear bounces around constantly.
I believe Sky put the ADSL patch for the AR7 chip in their latest firmware, which is supposed to stabilse
the connection but Broadcomm based routers seem to be more stable on my line. ( tested with DG834v4 )
I think the reason I got slightly higher speed with the Speedtouch on Zen was simply because it was happier
with a low noise margin than the Netgear is.
I suppose I should be grateful I can get at least 2mbps with a 3.8km line. 
thanks.
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HPsauce

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Re: 2nd Opinion on downstream speed
« Reply #6 on: March 17, 2009, 09:32:11 AM »

probably what HPSauce meant when he said your line stats were inconsistent.
Indeed.  ;)
I was in a rush at the time so was just trying to move things on a bit by asking for some clarification.

Having read the follow-up it still doesn't make sense though, unless there's a constant background interference.
Maybe some DMT graphs might clarify?

Also, if you can get another router that gives more detailed stats it might be of use. It doesn't actually need to fully connect (log on) - just getting a sync can tell you a great deal in this situation.
« Last Edit: March 17, 2009, 09:35:52 AM by HPsauce »
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AlexS

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Re: 2nd Opinion on downstream speed
« Reply #7 on: March 17, 2009, 10:37:41 AM »

Update :
Sky appear to have "partially" agreed to remove the cap.
Just received an email stating I have been "uncapped to 3mbps for testing purposes"
I thought you were either capped or uncapped ?
Anyway stats are now :

ADSL Link Downstream Upstream
Connection Speed 3072 kbps 768 kbps
Line Attenuation 51 db 29 db
Noise Margin 9 db 11 db

Strange that all 3 Downstream values have gone up, or is that because of the "new" 3mbps cap ?
I'll let it run for 24 hours to see what happens overnight and then try my DG834v4.

Just put my AM radio next to the phone cable and can get perfect reception of a commercial
AM radio station at 612Khz that doesn't broadcast anywhere near 612Khz frequency!
( can also get it on the mains wiring below the light switch at the top of the stairs ! )
Nothing on 612Khz anywhere else, weird !

I think I'm living in the twilight zone ! 

Thanks.

Alex
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kitz

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Re: 2nd Opinion on downstream speed
« Reply #8 on: March 17, 2009, 11:01:28 AM »

ermm  theres something very weird with those stats that just doesnt make sense 

- a line should not see an increase in attenuation by 2dB*.
- The increase in Noise Margin could be that sky have now set you with a target SNRM of 9dB
and your SNR Margin varies quite a lot over the course of the day.
- You would not normally expect to see a line sync at a higher speed when it has a higher SNRM.

I'm still wondering just how much your SNRM fluctuates over the course of the day - hence me suggesting routerstats.  It is possible on some noisy lines to see a good sync in the morning..  but then it all go to pot in the evenings.

>> I'll let it run for 24 hours to see what happens overnight and then try my DG834v4.

It does have more logging capabilities.





*unless going from adsl1 to adsl2+ which uses higher frequencies which is doubtful for a 500 kbps increase
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HPsauce

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Re: 2nd Opinion on downstream speed
« Reply #9 on: March 17, 2009, 11:14:30 AM »

I'm wondering if Sky somehow can restrict the frequency range in use?
The attenuation could be different if it was using a different range of frequencies.

DMT may clarify as suggested earlier.

It's very odd as Kitz has said.
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kitz

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Re: 2nd Opinion on downstream speed
« Reply #10 on: March 17, 2009, 12:13:47 PM »

>> I'm wondering if Sky somehow can restrict the frequency range in use?

Ive never ever heard of anything like that before - it defies the way DMT modulation works.
Granted there are some frequencies that can be blocked but these are set on the dslam and would affect all users (eg like Be does on their MSANs).
AFAIK its not possible to restrict rdsl sync speed by blocking frequencies - it wouldnt work because different lines need different frequencies to be able to sync.
Power cut-back is the only way I could think of which could be manipulated - but Im pretty sure the dslam does that dynamically depending only if the SNRM is very high in order to prevent NExT.  Power cut back is distributed evenly across the adsl spectrum range.
Even with fixed speed lines you will see available frequencies still in use (providing the SNR at that frequency is good enough to load bits).  DMT compensates for this by loading fewer bits per bin across the usable range.  This is turn gives scope for bit-swapping to be able to work.

In days of old when fixed speeds of say 512k/1Mb, then this was set on the line card... then from about 2004ish it could be set & stored remotely on the RAS as a profile to be used during the initialisation process.
With rdsl the maximum speed is a set parameter at the first stage of the initialisation 'handshake'..  which DMT then adjusts for accordingly at the 'exchange' and 'channel analysis' state to ensure the maximum speed and loading the bits across all of the available frequencies it can.

Without seeing the power cut back and bit loading I guess we wont know for sure whats been going on

---
I'm still uncertain if this particular line could see large SNRM variations throughout the day - which could account for the usual symptom of good sync in the morning.. but low SNR at night....  and may well also account for the difference between last nights and this mornings figures.
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HPsauce

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Re: 2nd Opinion on downstream speed
« Reply #11 on: March 17, 2009, 12:44:13 PM »

So how do Sky restrict their LLU customers to 8mbps, and O2 to their various speed levels (8 and 16 or is it 20?). Or is that just standard ADSL variants?
Time for someone (more knowledgeable and authoritative than me) to ask Sky how they're doing what they're doing. ;)

BTW those (3072) last stats look more consistent, certainly compared to the earlier ones.
« Last Edit: March 17, 2009, 12:47:28 PM by HPsauce »
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kitz

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Re: 2nd Opinion on downstream speed
« Reply #12 on: March 17, 2009, 09:13:09 PM »

>> So how do Sky restrict their LLU customers to 8mbps, and O2 to their various speed levels (8 and 16 or is it 20?). Or is that just standard ADSL variants?

In the way you mean its a line parameter. 
Some ISPs will also restrict on their network such as AOL/tiscali may have users on a 2Mb account but if they looked at their router it would still sync at 8128 so Im ignoring ISP side capping as it doesnt apply for this discussion.


Maximum sync speed is one of the parameters set for the initialisation stage and before authentication.  Its one of the very first steps that occurs - even max dsl which is a "rate adaptive product" has several different parameters that can be assigned to your line.

With Maxdsl, BTw doesnt have that many profiles for the downstream its just:
576 fixed,
1152 fixed.
2272 fixed or
8128 fully rate adaptive.

Tiscali LLU downstream its something like:
574 fixed,   168 Adaptive
1148 fixed,   1148 Adaptive
2296 fixed,   2296 Adaptive
4096 fixed,   4096 Adaptive
6144 fixed,   8160 Adaptive

I dont know Skys :/


This isnt the only parameter set in the profile the other parameters included are
~ Interleaving / Depth of Interleaving,
~ Target SNR
~ adsl standard (eg G.992.1 / G.992.5 and Annex type A/M).
~ Upstream speed.

All of these line characteristics are set at the intialisation handshake... which then goes on to the training and analysis part.  If a line is fixed rate the dslam needs to know before doing the channel analysis and way before sync is actually achieved.

I may be wrong but I dont see anyway how capping by frequency could be stored in the profile.  It would make things way too complicated..  and now having thought this through a bit further - power cutback wouldnt be stored in the profile either. That would occur at a later stage of intialisation.  The dslam would/should recognise that the SNR is too high at this stage and cut back power to make sure neighbouring lines aren't being drowned out.  The reason Im not sure at which stage is because power cut back can also occur after sync and seems to be consistantly monitored by the dslam.  I think therefore there is 2 types of power cutback 1.) Spectral mask type (stored on the dslam) and 2.) Continually monitored by the dslam if theres later a large increase in SNR causing a surplus of too strong a signal which could cause NExT.

DMT is designed to work on the full 256/512 subchannels as long as the SNR is good enough.. and its the adsl standard in use (g.992.1/g.992.3) which defines the subchannels and therefore frequencies available for use.

The stages of initialisation are listed here. Any profiles set by the ISP would have to be passed during stage 1 and before the end of stage 2.

Theres also a difference between fixed rate lines and adaptive lines in that fixed rate will either sync at 'x'speed or it wont.  Theres no Target SNR stuff and "oh it wont sync so lets try again at a lower speed".


I dont know where the LLU providers store their line profiles - BTw do it at the RAS (see NCAS in the diagram).
A simplified version and the way I would imagine it to work would be something like..

Tel no 01234567890 attempts to connect to the dslam.
Dslam quickly looks up line profile for the phone no.
Dslam says hello... knowing at this stage what profiles to use.
Router then says hi I want to connect (and at this stage can pass any of its own parameters that it wishes to over-ride such as the target SNR).
All this would be the 'Handshake'




These are my ramblings on the subject - as you know I will sometimes theorise on the forums rather than the main site which provides the hard facts.  Therefore if Im wrong about anything then Im happy to be corrected.
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