BCN_TIMOUT,AP_PROBE_SEND_START

[RenJian]

Hi，
I have a mqtt pubsub library, but sometimes I reconnect to the mqtt server failed, serial device print log about wifi:
I (159982920) wifi: bcn_timout,ap_probe_send_start
I (159985420) wifi: ap_probe_send over, resett wifi status to disassoc
I (159985420) wifi: state: run -> init (1)
I (159985430) wifi: pm stop, total sleep time: 0/10091207

What does this serial message mean and how do I start to find what the cause of this?
Thank you very much.