TM's Blog for 3GPP Spec

Before starting this topic, we should know what is IDLE and CONNECTED mode first. (#LTE RRC States)

1. Cell re-selection: IDLE mode -> IDLE mode

When a UE stays in RRC IDLE state and the signal of the region becomes very poor. The UE could perform Cell re-selection to switch to a better cell.

2. Handover: CONNECTED mode -> CONNECTED mode

If a UE is on the phone (CONNECTED mode) and the signal gets worse and worse. The UE could perform handover procedure. (e.g. LTE RRC CONNECTED to 3G RRC CONNECTED)

3. Redirection: CONNECTED mode -> IDLE mode

This procedure usually is performed during CS fallback procedure:

(1) UE sends the extended service request with service_type 0 or 1 (mobile originating/ terminating CS fallback)

(2) Network sends RRC Connection Release with info "redirectedCarrierInfo geran" (4G to 2G)

"redirectedCarrierInfo utra-FDD" for 3G

(3) CSFB procedures...

MGUofT