What is an AAA server?
AAA means Authentication, Authorization, and Accounting. The AAA server is very common in diameter protocol. A client can contact the AAA server for accessing a resource. Authentication means a user or client sends an authentication request with credentials. The credentials can be a simple user name and password. If the user is allowed, the AAA server sends an authentication response with success else failure.
Authorization means allows granting for a service. The service can be a call, SMS, or data.
Accounting means, AAA server keeps the record for all services specific units. E.g how many calls, how, much data a device has been consuming over time.
Authentication, The GGS/PGW sends an Initial CCR to the AAA server, the credentials are IMSI (user name ) and password (optional).
Authorization, In initial CCR and update CCR during a data session, PGW/GGSN requests for the bytes to a UE. AAA server grants/authorizes the same number, more or less.
Accounting, The AAA server keeps all data records against the IMSI. The records may use for billing, future resource planning, etc.