Abstract: With the expansion of mobile communication network, incredible changes are taking place in the field of mobile technologies, as requirements are increasing day by day for mobile data services along with them security concerns are also expanding. This paper focuses on the architecture of GSM and UMTS along with Authentication and Key Agreement protocol description, which shows the encryption process used in the authentication of the network and the client over the air interface. In addition, this paper also gives an introduction to some concepts of UMTS security architecture.