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Handover from E-UTRAN to UTRAN/GERAN
Ref: [33.401:9.2.1, 10.3.1], [23.401:5.5.2.1], [36.300:19.2.2.5.6], [36.331]
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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Before HO, NAS security is considered to be activated. During HO procedure [23.401:5.5.2.1], network maps LTE context to UMTS one. NAS DL Count and KASME arr used to generate CK' and IK'.
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Network - while sending HO Command to UE - also includes 4 LSBs of NAS DL Count in nas-securityParamFromEUTRA [24.301:9.9.2.6].
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eRRC Mobility From EUTRA Command
(RRC HO to UTRAN command, NAS DL Count)
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UE generates mapped UMTS security context (Fig 1.5.2).
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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
KSI=y
CK'
IK'
selected Ciphering algo
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Further UTRAN procedure is outside the scope of this article.
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