Guys,
In an attempt to sort out my confusion I have read up on several sites. I found a somewhat bewildering array of information but I think I am starting to get to grips with it.
I have tried to collate all the information I have found into one document. It's a work in progress but I'd like to put it up here to see if I am talking sence or pure gibberish (like I normaly do
)
Please feel free to comment, good and bad (I can take it
)
Any constructive critisism on anything I have wrong or could be explained better greatly appreciated....
ADSL Stats As I see them
Transmit Power The amount of signal sent from the exchange dBmw Tx Power
Line Attenuation The amount of power lost through line impedance, Joints etc dB Atten Generally the longer the line the higher the line attenuation will be. It can also be effected by joints in the line (especially if they are badly made) amongst other things. The data rate or frequency of transmission also effects line attenuation. The higher the frequency/data rate the higher the line attenuation.
Received Power What is left of the transmitted power received by the modem after the line attenuation dBmw Rx Power = Tx Power - Atten The receiver will need a minimum amount of power to be able to decode the signal. This is called receiver sensitivity.
Noise level The amount of noise power on a line. dBmw Noise Again the longer the line the more noise will be induced in it. Noise comes from many sources; Atmospheric noise (thunder storms etc.), electrical interference from white goods or vehicles in the area and crosstalk from other lines running parallel to your line. This figure will change throughout the day. Cable quality also effects noise levels.
Signal to Noise Ratio The difference between the wanted signal and the noise levels dB SNR =Rx Power - Noise Probably the most important factor in any transmission service. The receiving equipment must have enough received power in the signal and it must not be swamped by noise. i.e. the SNR must be at least a minimum value
Connection speed The data rate established between the modem and DSLAM at the exchange when synchronisation takes place. This is the maximum data rate that can be maintained point to point between the modem and DSAM Kbps or Mbps Note data throughput (speed test results will be lower due to overheads)
Signal to Noise Margin This is where I get a bit grey...but as I see it...
SNR Margin = Required SNR + a Margin for variations in noise levels. dB SNRM To maintain a given data rate the modem required sufficient Rx Power with a high enough SNR to enable it to decode the signal. Given that noise levels fluctuate the SNRM figure is used to ensure a stable connection.
Target Signal to Noise Margin Again a bit grey...
Given the historical noise fluctuations on your line this figure is set by the DLM (Dynamic Line Management (a misnomer if ever there was one)) to ensure a stable connection. dB Target SNRM
To summarise all this.
As your data rate increases the line attenuation for that signal also increases.
Therefore you will lose more of your signal and so your Signal to noise ratio will reduce.
If the SNR falls below a certain level you will start to get errors/packet loss etc. If this happens the DLM will reduce your profile and therefore your throughput speed until it sees a period of sustained improvement.
The only way to avoid/minimise the chances of this is to
a) Reduce your data rate. This is what DLM does through profiling.
b) Reduce noise levels by using quality cable, minimising joints, ensuring good separation from power lines and other electrical devices.
c) Make sure you run modem from master socket, remove bell wire etc.
d) Increase Transmit power level. This will raise your signal out of the noise. Unfortunately we can't control this.
e) Presumably some modems will be more sensitive than others and so better at digging low signal levels out of noise?
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