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Wednesday, November 22, 2017

A 802.3af POE switch with a power budget of 80 watts can't power more than 5 or 6 IP cameras, rated to consume 6 watts max each, at a time. What is the most likely reason?

A 802.3af POE switch with a power budget of 80 watts can't power more than 5 or 6 IP cameras, rated to consume 6 watts max each, at a time. What is the most likely reason?

  • Cameras are 802.3at POE.
  • Cables are CAT5 instead of CAT6.
  • IP cameras and switch are on different IP subnets.
  • POE Class is not detected.
 
A 802.3af POE switch with a power budget of 80 watts can't power more than 5 or 6 IP cameras, rated to consume 6 watts max each, at a time. What is the most likely reason?

EXPLANATION

The most likely reason is the proper POE Class is not being detected. The Class determines how much power is reserved for the PD (powered device), in this case the IP cameras. If no Class is detected, then the switch defaults to Class 0 which reserves the max power per port of 15.4W (in 802.3af). 5 cameras reserving 15.4W per port is 77W, leaving nothing left over for the other cameras. If Class 2 was detected, then around 6.5W max is reserved for the port, so up to 12 cameras can be connected (assuming there is enough ports). 80W/6.5W = 12.3, or 12 devices rounded.

Even if the cameras were rated at 802.3at POE compliant, which is around 30W max, it is still a matter of Class detection since it should still work with a 802.3af switch since they are backwards compatible, and since it was stated the cameras used 6W max. POE is not dependent on CAT5 or CAT6 cable being used, and does not work at the Layer 3 level so does not use IP addressing.

With the citation, also see...
http://www.veracityglobal.com/resources/articles-and-white-papers/poe-explained-part-2.aspx

SOURCE

https://en.wikipedia.org/wiki/Power_over_Ethernet
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