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Why do we need selectable Power over Ethernet classes?

With the recent launch of the new D24, M24 and Q24 IP cameras form Mobotix, we have noticed a new feature which initially didn’t seem to have a purpose:

Selectable Power over Ethernet classes

The new models will feature selectable Power over Ethernet classes which can be adjusted from within the web interface.

IEEE 802.3af Power over Ethernet (PoE) has four classes based on how much power a device requires in order to function. When connecting to a PoE device, the router or midspan will send a short burst of information asking if the device needs power and at what level. The device will then respond with the following information.

  • IEEE Class 0: 0.44 to 12.95W
  • IEEE Class 1: 0.44 to 3.84W
  • IEEE Class 2: 3.84 to 6.49W
  • IEEE Class 3: 6.49 to 12.95W

Mobotix cameras up until now have been using class 0. This means that they can draw as much or as little power as they need between 0.44 and 12.95W.

Mobotix cameras tend to run at around 3W so why not use Class 1 or 2? Well, you can also add USB drives and external sensors which are powered by the Mobotix cameras which require more power than a single class can supply.

However, the use of class 0 creates another problem. Most Power over Ethernet devices have a set limit to the Wattage that can be supplied to connected devices, for example a four port PoE switch might be able to supply 30W to all its devices. In theory you should be able to power around ten Mobotix cameras from a single unit.

However, in practice you could only power 2 devices. Even though Mobotix camera only ran at 3W the potential was there for them to run at almost 13W. As such PoE devices must allow for the maximum possible wattage that the class might require. In this case 3 x 13W could not run from the same 30W PoE switch.

With the selectable classes on the new Mobotix range this problem is solved as you can manually set the power class to 1 if you are using the camera on its own or higher should you connect USB drives or external sensors.

Published on February 10th, 2010 by James Drinkwater

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