MAC CCA fail Thread mbed-os-example-mesh-minimal

I’m trying to connect two x-nucleo-s2868a1 with s2lp but on router side I’m getting following error
[DBG ][mMCp]: MAC CCA fail
What does this mean? What is CCA?

Hi Stepan,

CCA means clear channel accessment. In practise MAC CCA means that Thread has try to send packet nine times to the channel but the radio channel is not free and the sended packet has returned to send queue. And soon it will be sent again (or try to send after channel check).
Before Thread (also 6lowpan and Wisun) can send it will listen the channel and if there is nothing it will send the packet.

I hope this helps.