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Idle mode RAT change from E-UTRAN to UTRAN/GERAN
Ref: [33.401:9.1.1, 10.2.1], [23.401:D.3.5, 4.3.5.6], [24.008:4.7.5.1.1], [23.003:2.8.2.1].
Below diagram depicts change from E-UTRAN to UTRAN. The same procedure is applicable to change to GERAN; GPRS security context is mapped after mapping EPS context to UMTS one.
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if
TIN=P-TMSI or RAT related P-TMSI
Existing UMTS
EPS NAS
eKSI=KSIASME/y
KKSME
UL NAS count
DL NAS count
UE security capability
KNASint/Enc algo id
KNASenc/Int algo id
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mapped UMTS
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NAS token, CK', and IK' would be generated from KASME and NAS UL Count. Truncated (two least octets of) NAS Token would be put in P-TMSI signature. NAS token help authenticating UE. Mapped UMTS context is stored in USIM. GPRS Kc too generated and updated in USIM.
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Routing Area Update Request
(KSI, P-TMSI signature)
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Even though security keys are generated, algorithms are still to be decided. This is handled in RRC Security Mode procedure.
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RRC Security Mode Command
(integrity/ciphering algoritm, MAC-I)
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Further procedure is outside the scope of this document. You may refer [33.102:6.4.5]. UE will now have complete (mapped) UMTS Security context.
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