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Author Topic: High SNR Margin, low speed  (Read 15084 times)

widdershins

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High SNR Margin, low speed
« on: November 16, 2010, 04:44:30 PM »

Hi guys, I've recently been on a mission to improve the abysmal speeds I've been getting in my (veeeery old) house. I read about the ring wire thing, and removed it. Here are the stats from before and after taking it out:

Before:
Line mode: G.992.5 (ADSL2+)
Maximum line rate: 1021 kbps (downstream) / 606 kbps (upstream)
Noise margin: 14 dB (downstream) / 15 dB (upstream)
Line attenuation: 53 dB (downstream) / 34 dB (upstream)
Output power: 20 dBm (downstream) / 12 dBm (upstream)

After:
Line mode: G.992.5 (ADSL2+)
Maximum line rate: 1021 kbps (downstream) / 606 kbps (upstream)
Noise margin: 23 dB (downstream) / 15 dB (upstream)
Line attenuation: 54 dB (downstream) / 34 dB (upstream)
Output power: 21 dBm (downstream) / 12 dBm (upstream)

As you can see, the only thing that's really changed much is the SNR margin. I've been told that a higher SNR margin is good, but it doesn't seem to be translating to higher connection speeds. I waited 24 hours, and still no change in the "Maximum line rate" readings.

Is it simply that I'm too far from an exchange? (about 3.9km as the crow flies). Or should I simply wait a bit longer? Does anyone know roughly what speeds should be achievable with stats like these?

Sorry for all the questions, but I'm a bit of a noob to this. Thanks in advance for any help!  ;D
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HPsauce

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Re: High SNR Margin, low speed
« Reply #1 on: November 16, 2010, 04:52:50 PM »

That looks like it might be capped in some way; who is your ISP?
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widdershins

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Re: High SNR Margin, low speed
« Reply #2 on: November 16, 2010, 05:05:20 PM »

Orange  :-[
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jeffbb

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Re: High SNR Margin, low speed
« Reply #3 on: November 16, 2010, 06:58:16 PM »

Hi
Quote  I've been told that a higher SNR margin is good

Not quite right  ;). High SNR is good , that is the Signal to Noise Ratio . When you connect to the exchange  the normal SNR Margin should be about 6db. This margin is the normal "elbow room " required so that a normal line remains stable . so if for example your SNR(total) is 30db average then you have 30-6 =24db available for data .

see further  SNR explanation  .

High SNR margin > 6db    basically has 3  possible causes .

1  your target has been increased  because of unstable line conditions (normally a maximum of 15db)

2  You have a Capped service so tat you are not synching at the optimum speed that  your line condition could support.

3 If you were very close to The  exchange then even connecting at the maximum  speed there is still more SNR to spare .

With your attenuation number 3 is not an issue

So it could be 1 or 2 ,or a bit of both .  It may be that you were capped  so that you  your SNR margin was 15db ,now even after improving your connection your SNR has increased ,but because of the capping you cannot connect to the full potential of the line .

Regards Jeff



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widdershins

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Re: High SNR Margin, low speed
« Reply #4 on: November 16, 2010, 08:48:11 PM »

Thanks Jeff, good to have narrowed it down. I think the line's fairly stable. We did used to have some problem with dropped connections some 2 years ago, but things seem to have been stable since then, so I'm inclined to believe it's option 2.

I contacted Orange and they've agreed to do a line test. Hopefully they will be able to see that the line can support a quicker connection and they should be able to lift the cap. When I was on the phone to them, the guy kept repeating "your line can only support a 1.5Mb connection sir." I told him that I know that's what BT say, but that my stats say otherwise. It seems they may be capping my rate based on BT's (conservative) estimation of what my line can support. Does anyone know if this is common practice?
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sevenlayermuddle

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Re: High SNR Margin, low speed
« Reply #5 on: November 17, 2010, 12:03:11 AM »

Just to add to what Jeff & HP have already said...

Since your download sync speed was exactly the same, before & after removing the ring wire, but your SNRM improved to a staggering 23dB, I'd say HP is right - your line has been capped.   The big SNRM is testament that your line is now a lot more stable, which confirms your line improvements have been effective and, in that respect, it is indeed a good thing.  But SNRM is a trade-off against speed, and you would obviously prefer a a faster speed rather than a pointlessly massive SNRM.

BT's 'estimation' is, in most cases, actually just a reflection of your line's recent performance, so it becomes a self-fulfilling prophecy.  IMHO they shouldn't call it an 'estimate'.

The evidence is that many ISPs help desks don't have the slightest understanding of line parameters, or DLM, or capping.   As for the speed estimator, I harbour a suspicion that even BT don't understand how how it works, despite the fact they implemented it.    But, unless you can persuade Orange to disregard the BT estimate,  you may struggle to have the cap removed.

One thing that may help is to ask for an 'SNRM reset', or to have the line 'retrained'.  But if Orange are as bad as other ISPs and that question is not on the script,  you may find the helpdesk have simply no idea what you are talking about, and they will fob you off with some BS about the line's capabilities.   

Judging from past threads concerning Orange,  I'm afraid you could have quite a fight on your hands.  I hope I'm wrong...
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widdershins

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Re: High SNR Margin, low speed
« Reply #6 on: November 17, 2010, 08:33:29 AM »

Just found this:

http://www.orangeproblems.co.uk/phpBB2/viewforum.php?f=40

Oh dear, as you say, this may be more difficult than I'd hoped...
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jeffbb

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Re: High SNR Margin, low speed
« Reply #7 on: November 17, 2010, 11:19:41 AM »

Hi

You should be able to synch at about 4128 Kbps with the normal 6ddb SNR Margin . This obviously possible as for each 3db above 6db margin can cost anything from 400 to 1200Kbps in synch rate ,dependant  on bit loading . Obviously on a fairly long line like yours the potential gain is nearer the lower end of that range . But even at the lower end ,you could expect to gain  23 -6= 17 db spare SNR  that equates to at the very least  more than 17/3 * 400Kbps =  2300Kbps (approx calculation) . That is the very minimum extra synch I would expect .

Regards Jeff
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widdershins

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Re: High SNR Margin, low speed
« Reply #8 on: November 22, 2010, 01:40:12 PM »

Finally managed to speak to someone from Orange in the UK who admitted that we're on a 1 meg profile and that the line can support more. He said he'd put in a request with the supplier (whatever that means) to raise the profile. Fingers are crossed  :-\
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HPsauce

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Re: High SNR Margin, low speed
« Reply #9 on: November 22, 2010, 02:01:25 PM »

That looks like it might be capped in some way; who is your ISP?
Orange in the UK who admitted that we're on a 1 meg profile ....
... request with the supplier
Got to the truth eventually; they don't make it easy though.  :-X
Supplier will probably be part of BT.
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Oranged

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Re: High SNR Margin, low speed
« Reply #10 on: November 22, 2010, 02:55:07 PM »

What you must remember is that the DLM system that Orange use on their ADSL2+ product works on a default Target SNR of 10dB rather than the 6dB of most other ISPs including BT who "manage" the Orange LLU products.

Only Orange know the reasoning behind that decision.
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sevenlayermuddle

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Re: High SNR Margin, low speed
« Reply #11 on: November 22, 2010, 03:13:15 PM »

Finally managed to speak to someone from Orange in the UK who admitted that we're on a 1 meg profile and that the line can support more. He said he'd put in a request with the supplier (whatever that means) to raise the profile. Fingers are crossed  :-\

Well done you.   :)

Let us know how you get on, and what speed you are able to connect at eventually.

- 7LM
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sevenlayermuddle

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Re: High SNR Margin, low speed
« Reply #12 on: November 22, 2010, 03:28:57 PM »

What you must remember is that the DLM system that Orange use on their ADSL2+ product works on a default Target SNR of 10dB rather than the 6dB of most other ISPs including BT who "manage" the Orange LLU products.

Only Orange know the reasoning behind that decision.

I wasn't aware of that.  But I've heard murmurs of Orange dabbling in IPTV.    IPTV lacks TCP's retransmission mechanisms for corrupted data, and hence the underlying error rate needs to be reduced,  most easily achieved by increasing the target margin.  That may have something to Orange choosing a more generous default?

Just a guess.
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Oranged

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Re: High SNR Margin, low speed
« Reply #13 on: November 22, 2010, 04:08:54 PM »

To my knowledge (while I was with Orange) they shelved the IPTV idea in the UK to concentrate on getting broadband correct.  :lol:

I believe they thought that the UK's more varied TV competition (cable, satellite, freeview) would never make it a successful product.

Perhaps the ADSL2+ DLM specification was agreed and implemented before the decision to shelve IPTV.

Of course, now with BT involvement in their broadband product, it's quite possible their strategy may have changed.
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