I'm currently using an adsl connection and last week an engineer from my ISP came and told me that my SNR margin and line attenuation is not good, and they need to replace the main telephone cable to provide cable tv. I tried researching on SNR margin and line attenuation but I'm not sure how accurate the information are. So, can anyone explain me what SNR margin and line attenuation are?
The SNR margin is the Signal to Noise Ratio margin.
Signal to Noise Ratio is the relation between the
data signal level and the
Even the most perfect cable absorbs some noise. This "noise" is electromagnetic interference produced by:
other cables running near the cable.
Motors or transformers near the cable.
If the noise is strong then the signal cannot be recognized.
Higher ratios means better cables. Below 10dB is very bad and more than 20dB is good.
At higher ratios, more speed can be achieved and lower ratios mean error-prone cable and lower speeds.
The SNR margin is the difference between the SNR of the cable and the SNR needed to get an specific speed.
Suppose that the SNR of the cable is 34dB and the SNR to sync at 6Mb is 30dB, then the SNR margin is 34-30 = 4dB
Every cable on earth suffers of attenuation. It is a measure of how the signal loses strength when running through the cable.
Suppose you put
5 volts in the end of a 1 Kilometer cable. If you measure the voltage in the other end you get, for example
4.5 volts. There was an attenuation of the original voltage. If the cable was longer, for example, 5 kilometers then the voltage on the far side could be very low.
For data signal this is key because if the original signal is strongly atternuated it could be gibberish for the equipment.
Signal-to-noise ratio (SNR)
It's a value measured in dB and it describes the relation between the signal strength to your house versus the noise, which affects your copper line. The noise is normally magnetic disturbance from high voltage cables etc. The higher the dB value is, the better your line will be, as the signal strength outperforms the noise.
Typical values are:
- 10dB and lower is bad
- 11db – 20dB is OK
- 20dB – 28dB is excellent
- 29dB and above is outstanding
Basically it's the distance between you and the ISP and the quality of the copper to your house. It's also measured in dBs and you can check the number on your ADSL statistics page. The closer you are to the ISP, typically the better speeds you can get. It also depends on the amount of users, who connect through the same copper. In short, the lower this number is, the better:
- 20dB and below is outstanding
- 20dB – 30dB is excellent
- 30dB – 40dB is very good
- 40dB – 50db is OK
- 50dB and above is bad
SNR Margin is a value to set in order to get the speed you desire. It's the difference between the actual SNR (dB noise) and the line attenuation required to sync at a specific speed. This also depends on the ADSL technology, ADSL (up to 8 Megabits), ADSL2 (up to 12 Megabits) or ADSL2+ (up to 24 Megabits).
- Desired downstream in Kilobits per second: 8096 (ADSL)
- SNR: 44 dB
- Attenuation: 35 dB
SNR Margin: 44-35 = 9 dB
In conclusion, in order to get speeds close to 8 Megabits for your normal ADSL, the SNR Margin should be set to 9 dB, when the noise measurements are the above values.