Section: 6.4.8 Initialisation of synchronisation for ciphering and integrity protection
Content: The ciphering and integrity protection algorithms are driven by counters (COUNT-C and COUNT-I) that at connection establishment need to be initialised. For that purpose the ME and the USIM have the ability to store a START value. The ME and the USIM store a STARTCS value for the CS cipher/integrity keys and a STARTPS value for the PS cipher/integrity keys. The length of START is 20 bits. The ME only contains (valid) START values when it is powered-on and a USIM is inserted. When the ME is powered-off or the USIM is removed, the ME deletes its START values and the Kc128 if one was derived. After power-on or insertion of a USIM, the USIM sends its START values to the ME, and the ME stores them. At radio connection establishment for a particular serving network domain (CS or PS) the ME sends the STARTCS and the STARTPS value to the RNC in the RRC connection setup complete message. The ME and the RNC initialise the 20 most significant bits of the RRC HFN (for integrity protection), the RLC HFN (for ciphering) and the MAC-d HFN (for ciphering) to the START value of the corresponding service domain; the remaining bits are initialised to 0. Also the RRC SN (for integrity protection) and the RLC SN (for ciphering) are initialised to 0. During an ongoing radio connection, the STARTCS value in the ME and in the SRNC is defined as the 20 most significant bits of the maximum of all current COUNT-C and COUNT-I values for all signalling radio bearers and CS user data radio bearers protected using CKCS and/or IKCS, incremented by2, i.e.: STARTCS' = MSB20 ( MAX {COUNT-C, COUNT-I | all radio bearers (including signalling) protected with CKCS and IKCS}) +2. - If current STARTCS < STARTCS' then STARTCS = STARTCS', otherwise STARTCS is unchanged. Likewise, during an ongoing radio connection, the STARTPS value in the ME and in the SRNC is defined as the 20 most significant bits of the maximum of all current COUNT-C and COUNT-I values for all signalling radio bearers and PS user data radio bearers protected using CKPS and/or IKPS, incremented by2, i.e.: STARTPS' = MSB20 ( MAX {COUNT-C, COUNT-I | all radio bearers (including signalling) protected with CKPS and IKPS}) +2. - If current STARTPS < STARTPS' then STARTPS = STARTPS', otherwise STARTPS is unchanged. If any of the COUNT-C or COUNT-I assigned to the radio bearers of the same CN domain reaches its maximum value, the ME and SRNC shall set START of the corresponding CN domain to its maximum value. The handling of the START values for new keys obtained from an authentication and key agreement run is described in clause 4.1.1.8 of TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  [35] and clause 8.1.12.3.1 of TS 25.331[ None ]  [17].