The MME performs UE authentication on receiving NAS requests. Authentication procedures can be defined for Attach procedures, Service requests and Tracking Area Update (TAU) procedures. These authentication procedures increase signaling towards the RAN and HSS. Selective Authentication is adopted to reduce signaling traffic towards the RAN and HSS. Selective Authentication is achieved by implementing frequency and periodicity based authentication of UE.
In a frequency-based selective authentication scenario the UE is authenticated based on configured frequency of access attempts. The configured frequency specifies the access-attempts per-UE and not across UEs. For example if the configured frequency is "n", the UE is authenticated for every nth NAS request received. The decision to authenticate is based on every nth request and not based on 'n' requests since last authentication. Where the nth request is equal to a multiple of n. (for example if n = 2, it will be 2,4,6,8 and so on)
In a periodicity-based selective authentication scenario the UE is authenticated based on configured periodicity. For example if the configured periodicity is "t", the UE is authenticated at every "t" minutes.
The frequency-based authentication is independent of the configured periodicity. However, periodicity-based authentication attempts are relative to the last UE authentication time. The last UE authentication attempt time is updated whenever an UE authentication is attempted irrespective of the authentication trigger.