Extreme Networks

I have some AP245X and would like to verify if using three separate omni-directional antenna will allow the use of MIMO or if I need to use one antenna with three connectors instead. The antenna we're using now are three of these, one on each of the ...

I've got a device template I'm trying to delete, but I get the error message "The Device Template cannot be removed because it is used by another object (item). Please disassociate references from other configuration items before removing the item." ...