Fig 1.5.2 "LTE to UMTS/GPRS"
Fig 1.5.3 "UMTS to LTE"*
* Change from GERAN to E-UTRAN is same as change from UTRAN to E-UTRAN as SGSN and UE shares/stores UMTS security context [33.401:10.2.2, 10.3.2].
Specifications considers two cases of RAT change: during Idle mode (MM connected, RRC idle) and during Handover (RRC connected).
