Chapter 6 Deploying Wireless Networks for FortiOS 5.0 : Wireless Mesh : Overview of Wireless Mesh : Firmware requirements
  
Firmware requirements
All FortiAP units that will be part of the wireless mesh network must be upgraded to FAP firmware version 5.0 build 003. FortiAP-222B units must have their BIOS upgraded to version 400012. The FortiWiFi or FortiGate unit used as the WiFi controller must be running FortiOS 5.0.