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Bronze

Power over ethernet

Anyone have any power over ethernet horror stories?

We are looking at symbol access-points with separate power units. Management is worried about it getting hooked up backwards. Dont know if it is a detecting unit that shouldnt send power unless it is required. Just looking for feedback about such issues.

On the same subject. The wires used for power are 4,5,7,8. Anyone know if how those (unused for ethernet) wires get terminated in the switch. Could power on them hurt the switch?

3 REPLIES
New Member

Re: Power over ethernet

As far as I know cisco switches do not conform with the IEEE 802.3af power standard. I would not expect this to work correctly. Do you have cisco switches that supply POE? If so I would turn off the power on those ports and not worry about it

Gold

Re: Power over ethernet

Cisco switches that supply power inline do it over the same lines that carry the data. That is, pins 1, 2, 3, and 6. The power is applied to the line only if the device at the other end can draw power according to Cisco's specifications.

Cisco has a power patch panel that uses pins 5, 4, 7, and 8 to send power, but again the device must be compatible with Cisco's specifications.

Cisco inline power switches and power patch panels do not conform to IEEE 802.3af PoE standard, which uses pins 5, 4, 7, and 8.

I don't know if those pins are actually connected to any circuitry in the Cisco switch itself, but all eight metal contacts are there in the RJ-45 port. Maybe someone from Cisco can check on this detail and get back to us. (Hint, hint.)

So just be careful which way you point those access-point power units. Aren't they clearly labeled on the ports?

Be extra careful if plugging into data lines where 10/100/1000 or 1000BASE-T Gigabit Ethernet switch ports are present. Gigabit Ethernet uses all four pairs, so if you plug one of these things into the wrong patch cable the wrong way, you WILL be sending current to active pins in the switch port.

I don't know whether the 802.3af PoE spec has a negotiating protocol that must be completed before a device puts power on the line, or if it just always supplies the power regardless of what's at the other end. Will have to do a little research and get back to you on this.

Bronze

Re: Power over ethernet

Thanks for the response.

I wasn't aware that Cisco used used different wires for the power. I know they recently changed the voltage to accomodate other vendor's IP phones. I think they brought it down to 7 volts (?).

Would you know if they changed the wires they are using when they did that?

I've never had much luck finding free info on 802 specs. Is there an RFC that matches the 802.3af spec?

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