Now days there are loads of products that support POE, IP Phones, IP Cameras, Access Points, Routers, the list goes on and on.

The first thing to be aware of is how POE Works. 

 

POE Explained from a previous Blog:

When it comes to Power over Ethernet, there a couple of different terms that we need to be familiar
with, and what those different terms mean.

 

Power over Ethernet is a method of supplying power over network cables to power end devices such as desk phones or access points.

There are 2 standards when it comes to PoE:

802.3af:

  • This is referred to as PoE
  • The maximum amount of power it can provide is 15.4 watts
  • This will power most phones and access points
  • Compatible with gigabit Ethernet
  • Generally 48 volts, but is allowable to be anywhere from 44 to 57 volts.

802.3at:

  • This is referred to as PoE+
  • The maximum amount of power it can provide is 25.5 Watts.
  • Is backwards compatible with 802.3af
  • Generally 48 volts, but is allowable to be anywhere from 44 to 57 volts.

By default, ports enabled with 802.3af and 802.3at do not send power all the time, there is a negation that takes place with the end device before it sends power down the Ethernet cable, thus saving your non-PoE device from being powered and blowing up your laptops Ethernet port. The end device has to be 802.3af or at compliant for power to be sent down.

 

There is also a 3rd way of delivering Power over Ethernet and that is passively.

Passive PoE means that there is a constant feed over power over the the Ethernet wires, and it is not negotiate like 802.3at and af do. Because Passive PoE does not have any active components, this means that it is not defined by a IEEE standard.

This means if you plug in your device to a passive power supply, it will likely blow the Ethernet port up. 

  • Passive PoE can come in any voltage, usually anywhere from 12-57 Volts
  • Devices that are 802.3at & 802.3at can be powered by Passive PoE providing that the voltages & watts being delivered are correct

 

How to choose your POE Device:

 

Now that you have an understanding of the POE Standards, how do you decide which POE Switch to buy?

First things first, how many devices do you need to run off the Switch? what is the power requirements of each device?, keeping in mind that whilst the AF/AT standards can support 15.4Watts / 25.5Watt, in practice many devices only require 5-8 Watts, you will find this in the product specifications.

For my example, lets say we have 20 IP Phones, each using 5 Watts, 10 Access Points at 8 Watts each, this would give us a total power requirement of 180 Watts. (Keeping in Mind that the published specs for the products are the Max Consumption possible) this allows enough overhead.

The logical choice for this is the US-48-500W  - UniFiSwitch 48 Port Gigabit 500W Managed PoE Switch.

The reason? there are enough data ports and the power budget of this switch well exceeds the requirements, which allows for adding more at a later date. 

Typically the power budget of most POE Switches will well exceed your requirements, ie it would be unusual to require the US-48-750W version of this switch, but a case where this could be useful is powering devices such as the UAP-AC-IW Ubiquiti UniFi AP AC 300+867Mbps In-Wall Access Point, as this device has POE Passthrough, which would need to allow for an extra POE Device to be daisy chained off the original POE Device, ie One AP and One phone could be powered from each port, that is potentially 96 devices powered off one 48 Port Switch!