[MLB-WIRELESS] Re: [Syd-Wireless] D-Link POE Unit - DistanceAnswers

[Fulton, Darren J]

> -----Original Message-----
> From: vak at alphalink.com.au [mailto:vak at alphalink.com.au]
> 
> 
> Howdy,
> 
> You shouldn't refer to collision domains AT ALL when talking 
> about POE. If
> you've opened up one of these boxes (and any other POE box for that
> matter), what basically happens is that pairs 2 and 3 (pins 
> 1, 2, 3, 6) go
> straight through to the other side - no hub, no electronics, 
> etc in between.
> 
> The other two pairs are used to pump power to the other side. Some POE
> units (as is the case with the DLink) usually some sort of 
> power regulation
> at the input side - don't ask me why - seems pointless. All 
> of them should
> have a regulator at the output end (where the AP, etc resides).

The "power regulation" on the input side will usually boost the voltage on the cable to something higher (othen 48V). 
Combine this with a DC-DC converter on the AP end and you can reduce the current and hence power loss on the cable. Without this 100m distances for PoE would be impossible (at least with the original AP power supply)

Most cat5e cable has a DC resistance of about 8.5 ohms/100m. If two pairs are used then the total series resistance is 8.5 ohms. If the AP sucks 1000mA then 8.5volts will be dropped across the cable, so the original 9V AP supply will be reduced to 0.5V.
  
> 
> According to Ethernet standards, you are allowed a maximum of 
> 95 metres of
> solid cable (single core conductors) and 15 metres of patch 
> (flex). Total =
> 110 metres maximum allowable.
>
> Cheers,
> 
> Vaskos
> --------

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