EMZETT.PoE: Power over the network cable
In short: Power over Ethernet supplies devices such as access points, cameras and phones with power and data over the same cable, so no second power outlet is needed.
In more detail: Many network devices hang in places without a power outlet: a Wi-Fi access point on the ceiling, a surveillance camera on the facade. PoE (IEEE 802.3af/at/bt) transmits direct current together with the data over an ordinary Ethernet cable (copper twisted pairs).
In Depth
Building blocks
- PSE (power sourcing equipment): supplies the power. A PoE switch, a PoE injector (adapter piece) or a router with PoE.
- PD (powered device): the powered device.
- A negotiation procedure ensures that power only flows to devices that support PoE, and only as much as needed. A normal device without PoE stays undamaged.
Standards and power
| Standard | Power at port | Available at device |
|---|---|---|
| 802.3af (PoE) | 15.4 W | 12.95 W |
| 802.3at (PoE+) | 30 W | 25.5 W |
| 802.3bt (PoE++) type 3 | 60 W | 51 W |
| 802.3bt (PoE++) type 4 | 90–100 W | 71 W |
| An access point often needs PoE or PoE+, cameras with heating and pan motor PoE+, pan-tilt-zoom cameras or thin clients more. |
Advantages
Fewer cables and power supplies; devices can be mounted wherever a network cable runs. The power can be backed up centrally by a UPS: if the power fails, cameras and phones keep running. Maintenance and restart of the device are possible through the switch by software (switching the port off and on).
Limits and notes
- As with Ethernet, cable length is limited to 100 metres.
- The switch’s power budget is limited: it cannot supply all ports fully at the same time. Check the total power in watts.
- Use real copper cables (not CCA, copper-clad aluminium), since high currents otherwise cause losses and heat. Cat5e or better.
- Non-standard “passive PoE” supplies power without negotiation and can destroy devices: follow the manufacturer’s information.
See also: Cat cable, Layer 2 switch, UPS, Ethernet