Protocol used between a RADIUS client and a computer?

I can't find documentation about the protocol used when a computer wants to reach a network with 802.1x.

RADIUS client (switch, access point) use UDP/1812-1813 to communicate with RADIUS server. How does computer (at this point with no IP) and RADIUS client communicate? Is there some ARP traffic? Is local link used ?

I tried to look for this information on Google and used Wireshark, I really have no clue at the moment.

Thanks a lot.