AKA: AKA stands for the authentication and key agreement security protocol. it is a mechanism which performs authentication and session key distribution in universal mobile telecommunications system (umts) networks

AKA( Authentication and Key Agreement) is a security protocol used in 3G networks. This is also used for one-time password generation mechanism in the process of Digest access authentication. symmetric cryptography is utilized in this system and it is a challenge and response system.
Any communication network must address the problems of Security, Authentication, Encryption and Access Control as the vital features. The radio waves are used to carry communications and as a result, the need for the security is implied. The integrated cryptographic mechanisms for authentication and confidentially was first implemented by the GSM. The weak authentication and encryption algorithms, secret key length of just 32 buts are the major disadvantages the security features of GSM suffered from. The UMTS (Universal Mobile Telecommunication System) was the first third generation mobile communication system and the security features incorporated were such that to cover the loopholes in GSM and other 2g networks. The features include:
-Mutual Authentication, Network to Network security, International Mobile Subscriber identity (IMSI) usage etc.

UMTS AKA Protocol
three components are present in the UMTS that participate in the authentication procedure which are:
-Mobile station (MS) and UMTS Subscriber Identity Module (USIM),
-A VLR(Visitor Location Register), and the Serving Network which the MS
-HN(home network) which also includes the Home Location Register as well as the AuC (authentication centre).

For more details see:
http://en.wikipediawiki/AKA_%28security%29#AKA_in_CDMA
http://gbspublisherijtacs/1009.pdf