After the UE enters a good LTE serving cell (
Event A1), and then observes LTE signal degradation (
Event A2)…
It’s not just about poor coverage. It’s about evaluating better RATs!
That’s where Event B1 & Event B2 come into play:
Event B1
Triggered when a neighbour 3G/2G cell becomes better than a threshold
Used for adding Inter-RAT neighbours (monitoring purposes)
Event B2
Triggered when serving LTE becomes worse and target 3G/2G becomes better
This event is essential for Handover or CSFB decision
Outcome based on UE capabilities:
If VoLTE not supported / unavailable:
→ CSFB (Circuit Switched FallBack) initiated
→ UE moves to 3G/2G for voice call
If Data continuity needed:
→ IRAT Data Handover triggered (e.g., LTE → 3G)
These decisions are made by MME + eNB coordination, based on UE capability + network config
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