EAP-AKA is the protocol that is used in UMTS/LTE (3GPP) networks between the UE and the network.
The technology is known as eHRPD which acts as an evolution path between HRPD(CDMA2000) towards LTE. The UEs have dual mode radios,i.e LTE and eHRPD, so whenever the UEs are moving from LTE to eHRPD n/w they start the eHRPD procedures to acquire the eHRPD radio resources and viceversa switch to EUTRAN access procedures while moving into EUTRAN access.
The eHRPD access network connects to the EPC via a g/w called HSGW which acts as a 3GPP ePDG.
There are two levels of authentication that happen in HRPD n/w, which also apply to eHRPD UEs.
1. UE device authentication: with Access n/w via 3GPP2 A12 interface, which uses CHAP protocol(with the hardware addr either ESN/MEID as the key). Here the authentication is only between the UE device and the eHRPD access-AAA server(known as AN-AAA)
2. UE subscriber authentication with the core LTE n/w: Here the subscriber authentication and authorization occurs with the LTE core network. For this, the UE, during the establishment of the session(during PPP auth phase), performs EAP-AKA' authentication. The HSGW sends PPP-EAPIdentityRequest, UE responds with EAP response(with its NAI). The HSGW forwards the EAP response (encapsulated over Diameter based Pi interface) to the 3GPP2 AAA proxy server, which in turn relays the EAP response to the 3GPP AAA server in the EPC (over Diameter based STa interface), the 3GPP AAA server now in-turn retrieves the authentication vectors from the HSS(over Diameter based SWx). The 3GPP AAA server now sends a EAP-REQ(with AKA' Challenge) which transcends to the UE via 3gpp2AAA->HSGW->UE. UE responds with EAP-RESP(with AKA' Challenge response) which travels upto the 3gppAAA, which sends EAP-SUCCESS(after comparison of SRES and XRES).
Point to Note: In '1)UE device authentication' could also follow EAP-AKA procedures similar to as described for 2 (instead of CHAP) if it is a scenario of an LTE UE roaming into eHRPD network.
Hope this answers your doubt! Refer to 3GPP2 X.S0057-0 for more clarity.