LTE to 3G UMTS circuit switched fallback

The flow below covers circuit-switched fallback from LTE to 3G UMTS, end to end. To analyze a CSFB capture of your own, VisualEther turns the Wireshark PCAP into a sequence diagram across the MME, MSC-VLR, and eNodeB — and Claude Code can explain where the fallback stalled.

LTE to 3G UMTS CSFB call flow

Describes the complete flow of circuit switched fallback from an LTE network to UMTS. You can click on most messages in the flow to reveal full details about individual messages.

Interactions between LTE and 3G

Summary call flow that focuses on the interactions between 4G and 3G networks for establishing a circuit switched call.

Medium blog post on LTE to 3G circuit switched fall back

Medium blog post on LTE to 3G circuit switched fall back

MME role

MME interactions in a CSFB call. MME interactions with MSC-VLR are covered as well.

MSC role

MSC role in LTE to 3G circuit switched fallback.

eNodeB role

eNodeB interactions in a CSFB call flow.