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SNR Margins. What and why?

SNR Margins. What and why?

SNR Margins. What and why?

Some of you may have seen the words "SNR Margin" bandied around a fair bit in the forums. Quite a few may wonder what it is people are actually talking about! The SNR (or Signal to Noise Ratio) is essentially the quality of your line versus the amount of "noise" that is present on it. If you have a high SNR on your line, this means that you have a good quality telephone line with very little noise present. Because of the way that ADSL Max works, BT will try to force the highest possible speeds against a "target" margin of 6dB. This means that you will get the highest possible speed before your SNR drops to below 6dB. If you still have a higher SNR than this and your connection speed is the maximum at 8128Kbps then you should consider yourself lucky! The target SNR is designed for those with poorer quality telephone lines. There are a number of cases where customers experience an intermittent connection when trying to connect against the target margin of 6dB. This is often resolved by increasing the target noise margin at the exchange, meaning that your line is being forced to attain an SNR of say, 9dB or 12dB. This will increase your stability, but you will see a drop in synchronisation speeds as the line isn't being pushed as close to its limits as it could be. Obviously some sacrifices have to be drawn and stability will always be taken over speed. That's BTs decision and not ours! It's a pretty sensible decision nonetheless - there's no point having a superfast connection that you can't use. Sometimes though, you will see cases where someone has had a fault on their line that has caused an increase in a customer's SNR margin. This is often done as part of BT Wholesale's diagnostic process or due to the actions of their Dynamic Line Management systems. The problem arises when the fault is fixed or the condition that caused the SNR increase is no longer present - Customers are sometimes left with an artificially high SNR margin and a lower than optimal sync rate. This is when getting the target SNR back to how it was proves to be rather tricky! I'm fairly certain that BT never envisaged that Service Providers would be asking them to lower a customer's target SNR, as it's this sort of thing that their automated systems are built to manage. However, we're now in the situation where we see a number of these requests and more often than not, the only way that we're able to get them actioned is by raising a fault and escalating it to a Team Leader within BT Wholesale's Faults team. This is obviously far from ideal. Because of the measures required to restore the SNR target to its original value, it will often take a considerable amount of time for such a request to be actioned - This isn't great for our customers or us and can be very frustrating at the best of times. We have to raise a fault which will then spend time bouncing between us and BT Wholesale because their systems and processes were not designed to deal with such a request. Eventually the fault will get escalated within BT however at this point the customer, us and BT Wholesale have invested a lot of time and resource for what is basically a very simple change, which BT Wholesale have the tools to make. We're going to be having an internal meeting here next week to discuss amongst ourselves how we can action these requests with the optimal level of efficiency, and to collate information which we can then present to BT. As far as we're concerned, the ideal solution would be for us to have our own ability to change target noise margins, as we're able to do for customers on our (Tiscali) LLU platform. However, BT Wholesale have a target audience in the region of >5,000,000 whereas Tiscali's subscriber base is considerably smaller and essentially less open to abuse. It would be extremely useful though, and BT are always extremely keen to implement changes and make improvements to the Max system. I for one, would hope that this would be something that they would seriously consider. Think of the number of man hours it would save both them and us, not to mention the improved level of response time we'd be able to offer our customers. I'll try to post an update when I get some further news. James

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Newbie
I should point out we have raised this as a Statement of Requirement in the recent past with BTW but failed to receive a positive response. I dont think we will ever get a tool a la Tiscali but am hopeful they will develop a process for us to request this outside of the normal faults flow.
Not applicable
My SNR bounced up to 15db recently, seriously dropping my D/S sync speed - it took me 15 days of posting tickets to PN support, initially they told me it couldnt be done, then they insisted I needed an engineer visit. A PN support guy 'Mike Holmes' managed to get this done for me with 1 simple phonecall. Now it's happened again for no apparent reason as I was having fantastic stability & 30% higher d/s sync speed @ 9db (30% is alot when you are cut from 3Mb to 1.5Mb service) already I've had more BS about line testing - doesnt seem like I'm gonna get this actioned any time soon. PN needs to review this - I spoke to Mike when he called to arrange a visit (actioned by a previous operator), I explained exactally what I wanted and he agreed with me that SNR reductions dont need visits or tests - He told me he would phone BTW and sort it - within 1 hour it was done! What is so difficult?