[AusNOG] Acceptable dBM loss over SM cross connect in a datacentre

[Paul Wilkins]

*Your switches are (probably) reporting the receive POWER at -12dBm (which
would sound about right). I doubt the switch any way to know what power the
other end is sending and therefore no way to know what the attenuation is.*
Not at the physical layer, no. Seems a missed opportunity this wasn't
incorporated in the BFD protocol. It'd be a natural mix of synergy, control
plane, link reliability, and TX power. I'd be tempted to post an RFC myself
but for other irons in the fire etc.

Kind regards

Paul Wilkins

On 28 August 2015 at 17:05, Ross Wheeler <ausnog at rossw.net> wrote:

>
> Hi Ross,
>>
>> Power loss, reported as -19dBm.
>>
>
> That doesn't make any sense to me. Perhaps "the industry" has adopted some
> perculiar term compared to RF, but "loss" doesn't have a reference.
>
> Expressing loss as a negative seems counter-intuitive too, so a "loss of
> minus 19dB" is a double-negative, suggesting a gain of 19dB (seems unlikely
> in a passive lump of glass!)
>
> Power will be given in dB from some known reference, eg:
> dB(m) (dB referenced to 1 milliwatt)
> dB(V) (dB referenced to 1 volt)
> dB(W) (dB referenced to 1 watt)
> etc...
>
> +30dBm = 1 watt.       (10^3 * 1mW = 1W)
> -20dBW = 10 milliwatts (10^-2 * 1W = 0.01W)
>
> To give a "loss" of -19dBm sounds like someone doesn't really understand
> what they're telling you, and they either mean "19dB loss", or "we're
> measuring -19dBm of power". While many people use the terms interchangably
> (wrongly) they then rely on you to interpret it correctly from the context.
>
> (Much like people getting kW and kWh mixed up and using the wrong ones)
>
>
> area where they've been plugging SC cables into SCA sockets), I don't have
>> photos, but the power loss is now measuring -12dBm.  Which my switches now
>> report as normal.
>>
>
> Your switches are (probably) reporting the receive POWER at -12dBm (which
> would sound about right). I doubt the switch any way to know what power the
> other end is sending and therefore no way to know what the attenuation is.
> (a 100km cable might have 10dB attenuation, transmit power of -3dBm would
> see a receive power of -13dBm. The same transmitter on the end of a short
> jumper cable would report close to -4dBm)
>
>
>
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