name: SQN
full_form: Sequence Number
context: In a UMTS authentication challenge, the authentication procedure is extended to allow the MS to check the authenticity of the core network. Thus allowing, for instance, detection of false base station. Following a UMTS authentication challenge, the MS may reject the core network, on the grounds of an incorrect AUTN parameter (see 3GPP TS 33.102 [5a]). This parameter contains two possible causes for authentication failure: a) MAC code failure: If the MS considers the MAC code (supplied by the core network in the AUTN parameter) to be invalid, it shall send an AUTHENTICATION FAILURE message to the network, with the reject cause 'MAC failure'. The MS shall then follow the procedure described in subclause 4.3.2.6 (c). b) SQN failure: If the MS considers the SQN (supplied by the core network in the AUTN parameter) to be out of range, it shall send a AUTHENTICATION FAILURE message to the network, with the reject cause 'Synch failure' and a re-synchronization token AUTS provided by the USIM (see 3GPP TS 33.102 [5a]). The MS shall then follow the procedure described in subclause 4.3.2.6 (d). In UMTS, an MS with a USIM inserted shall reject the authentication challenge if no Authentication Parameter AUTN IE was present in the AUTHENTICATION REQUEST message (i.e. a GSM authentication challenge has been received when the MS expects a UMTS authentication challenge). In such a case, the MS shall send the AUTHENTICATION FAILURE message to the network, with the reject cause "GSM authentication unacceptable". The MS shall then follow the procedure described in subclause 4.3.2.6 (c). If the MS returns an AUTHENTICATION_FAILURE message to the network, the MS shall delete any previously stored RAND and RES and shall stop timer T3218, if running.
description: The SQN is a part of the AUTN parameter used to verify the authenticity of the core network. If it's out of range, the MS sends an AUTHENTICATION FAILURE message.
explanation: Categorized as SQN (Sequence Number) to prevent unauthorized access.
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