Thursday, April 7, 2011


Many users are currently asking how they can find out if they will be able to get higher speeds. One of the more accurate ways of finding out is asking your ISP to perform a WOOSH test which will advise your ISP of your line "Loop Loss" and SNR Margin. Most ISPs are reluctant to perform these tests due to the time element involved, unless it’s for fault diagnosis.
However you should be able to get line stats from your modem / router yourself. The figures you are looking for are Attenuation and the SNR (noise) Margin. Both of these figures are measured in decibels (dB).

Line Attenuation

Line attention is in relation to the "loop loss" on your line. The further you are away from the exchange the higher your attenuation figure will be as the signal loss increases. The lower your attenuation figure the more chance you have of getting the faster speeds.

SNR (Signal to Noise Ratio)

The SNR margin can only be measured from your own property and therefore to be measured correctly may need an engineer visit to check this figure from the ISP test socket behind your master phone socket. Your SNR Margin can fluctuate on a day to day basis and may vary depending upon local equipment, conditions and interference. An upgrade in speed often causes the SNR to decrease by up to 5 dB. The higher your SNR figure then the better your line quality and therefore a higher chance of getting faster speeds. It is very important to note that SNR plays much more importance in maxdsl than it did on the traditional ADSL product.

Line Length

Ideally your line length should be below 3.5km from the exchange to get a 2Mb connection and 6km for a 1Mb connection.*

*The new reach limits came into effect on the 6th of September 2004. Prior to this date the limits were 6km for a rate adaptive product (512k) and 3km for a fixed product (1 & 2Mb).

The Results

When determining what speed you can get the following figures may be used as a guide. Remember you must use the downstream figures.

Downstream Attenuation

0-42 dB - 2Mbps
43-60 dB - 1Mbps
> 60 dB - 512kbps
If you already have ADSL enabled you may be able to add an extra couple of dB on those figures e.g. 44 dB would be the limit for a 2Mb line.

Downstream SNR Margin

Should be at least 10 dB to get ADSL - ideally above 12dB to get traditional fixed ADSL. Anything less than this will see frequent disconnections and other problems. Maxdsl works slightly differently and your router will try to sync at the highest speed it can whilst maintaining a safe SNR Margin.
As previously stated these figures should be used as a guideline as it is possible to have a very good SNR Margin but poor attenuation figure and vice versa.

An explanation of SNR and SNR Margin *

The SNR Margin is not the same as the Signal to Noise Ratio (SNR), although it is related.
The SNR is the signal-to-noise ratio. It is the ratio between the strength of the signal and the background noise on the line and is universally expressed as a logarithmic ratio (decibels or dB). For a given line the SNR remains essentially fixed whatever the speed you are running.
ADSL routers and modems actually report the SNR Margin, and the term used to describe this varies but is generally “SNR Margin”, “Noise Margin”, “Margin”, “Receive Margin”.
The SNR Margin is the difference between the actual SNR and the SNR required to run at a given speed. For example, if you need 20dB of SNR to run at 512Kbps, and the actual line SNR is 45dB, then the SNR Margin is 25dB.  
The main reason why SNR margins fall as the line speed increases is that the required SNR increases. For example, if increasing the speed from 512Kbps to 2MBps raises the required SNR from 20db to 30dB then the margin left will fall from 25dB to 15dB (as 45dB-30dB = 15dB.) The one refinement is that if it is necessary for a faster speed to use some higher frequencies then these will have inherently lower SNR and that will also contribute to the reduced SNR margin somewhat.
The SNR Margin is there to absorb fluctuations in interference on the line, and therefore to keep the service reliable. The figure of 6dB is normally accepted as enough headroom to make the link reliable, but some lines need more - 10dB is often accepted as a safer target.
Note that the target figures in the list are a reasonable summary, but it is the "SNR Margin" that is being talked about, not the "SNR". Despite this, it also does not invalidate the point that the most important factor on the reliability of a line at a given speed is the SNR Margin.
Also, as neither the SNR nor SNR Margin can be established until a line is enabled (or validated with special test gear), then the estimated attenuation has to be used to come up with a viable speed. Once a line is enabled and working, it is possible - at least in theory - to gather information about the actual SNR and SNR Margin and use that to work out the actual speed the line will be able to sustain.

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