A federation RADIUS server has a list of connected IdP and SP servers and the associated realms.Typically, a FLR is authoritative for all RADIUS realms ending in its own top-level domain (e.g. a FLR for Antartica would be authoritative for *.aq); it may also serve a number of domains in other top-level domains (e.g. .com, .net, .org, ...) but it is not authoritative for those entire top-level domains.
The FLR receives requests from the confederation servers and IdP/SP it is connected to and forwards them to the proper server. For its authoritative top-level domain, it rejects requests for non-existent realms inside the top-level domain.
The IdP RADIUS server is responsible for authenticating its own users (at its own premises, if it also an SP, or when they are visiting another SP) by checking the credentials against a local identity management system.
The SP RADIUS server is responsible for forwarding requests from visiting users to the respective federation RADIUS server. Upon proper authentication of a user the SP RADIUS server may assign a VLAN to the user.
Note that the IdP RADIUS server is the most complex of all. Whereas the other RADIUS servers merely proxy requests, the IdP server also needs to handle the requests, and therefore needs to be able to terminate EAP requests and perform identity management system lookups.
The Identity Management System contains the information of the end users; for instance usernames and passwords. They must be kept up-to-date by the responsible IdP.
A supplicant is a piece of software (often built into the Operating System but also available as a separate program) that uses the 802.1X protocol to send authentication request information using EAP. Supplicants are installed and operate on end-user computing devices (e.g. notebooks, PDAs, WiFi-enabled cell phones, and so on).
Access Points are Wireless LAN access devices conformant to IEEE 802.11 and need to be IEEE 802.1X capable. They must be able to forward access requests coming from a supplicant to the SP RADIUS server, to give network access upon proper authentication, and to possibly assign users to specific VLANs based on information received from the RADIUS server. Furthermore Access Points exchange keying material (initialisation vectors, public and session keys, etc.) with client systems to prevent session hijacking.