Section: 4.3.2.1 Authentication request by the network
Content: The network initiates the authentication procedure by transferring an AUTHENTICATION REQUEST message across the radio interface and starts the timer T3260. The AUTHENTICATION REQUEST message contains the parameters necessary to calculate the response parameters (see 3GPP TS 43.020[ Security related network functions ]  [13] (in case of GSM authentication challenge) and 3GPP TS 33.102[ 3G security; Security architecture ]  [5a] (in case of an UMTS authentication challenge)). In a GSM authentication challenge, the AUTHENTICATION REQUEST message also contains the GSM ciphering key sequence number allocated to the key which may be computed from the given parameters. In a UMTS authentication challenge, the AUTHENTICATION REQUEST message also contains the ciphering key sequence number allocated to the key set of UMTS ciphering key, UMTS integrity key and GSM ciphering key which may be computed from the given parameters. Furthermore, the ciphering key sequence number is also linked to a GSM Kc128 if after the authentication procedure the network in A/Gb mode selects an A5 ciphering algorithm that requires a 128-bit ciphering key.