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[busby]

Hi All,

Is it normal to see minus figures for output power? I have a Billion 8800NL, and after some heavy rain a couple of weeks ago, the upstream has dropped significantly. BT of course aren't interested as the downstream is above their estimates and there's no guarantee for upstream. Maybe it's just the Billion router just getting the power output figures wrong. BTW, I'm am on an aluminium line according to OR.

If anyone has any any advise (especially how to get the aluminium replaced), I'd be most grateful.

Thanks,



            Downstream   Upstream
Line attenuation (dB):     23.5      0.0
Signal attenuation (dB):   Not monitored      
Connection speed (kbps):   32729      4157
SNR margin (dB):           6.3      6.2
Power (dBm):               -0.2      -0.1
Interleave depth:          641      1
INP:                       3.00      0
G.INP:                     Not enabled      Not enabled
Vectoring status:          5 (VECT_UNCONFIGURED)

[Dray]

The reason you see negative values is that they are representing small but positive numbers, on a logarithmic scale. In logarithms, the value indicated represents an exponent... for example, under a log 10 scale, a value of -2 represents 10 to the -2 power, which equals 0.01.  Likewise, a negative Decibel-milliwatt (dBm) means that you're applying a negative exponent in your power calculations; 0 dBm equals 1 milliwatt (mW) of power, so -10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth.  It's a lot easier, and more useful in some calculations, to describe a weak signal as -100 dBm as opposed to 0.0000000001 mW.

https://supportforums.cisco.com/document/12560446/why-almost-everything-negative-wireless

[busby]

Thanks for the link Dray. Cisco docs usually do my head in lol. It's just that I've seen stats posted by others with power outputs of 6 or 8 dBm for example. I just wondered if the negative figures indicated a problem. I have seen positive numbers before the current issue I'm experiencing.

[j0hn]

No problem. Negative dBm is not negative power.
A few of us users here have seen a sharp drop in upstream power recently, including myself.

[burakkucat]

To expand on what has already been typed . . .

The power level is stated using units of dBm. That is decibels relative to one milliwatt. A ratio of measured power relative to a standard power level.

A positive value is a power level greater than 1 mW, a negative value is a power level less than 1 mW and a value of zero is a power level of exactly 1 mW.

The Wikipedia page dBm should be quite readable.

[busby]

Thanks burakkucat, that explains it nicely. So that would indicate VERY low power levels between me and the cab? I've got no idea how the power levels are determined. Sometimes I've seen the downstream at 6.x dBm. I assume the power has an effect on the bit loading?

[ejs]

It's not necessarily the case that something has decided to turn down the transmitted power level, causing a reduction in the SNR and therefore reducing the bandwidth.

It could be that the number of usable tones has reduced, due to an increase in noise levels rendering more tones completely unusable. Then the unused tones would have very little or no signal transmitted on them. The single power figure is the total power added up over all the tones, so with fewer usable tones, the total power would be lower.