CSFB
Circuit-Switched Fallback: a mechanism where an LTE UE falls back to 2G/3G circuit-switched network to make or receive voice calls when VoLTE is not available.
Circuit-Switched Fallback was the bridge the industry used during the years when networks had LTE for data but hadn't yet rolled out VoLTE for voice. The logic is in the name: when a call comes in or you place one, the network can't deliver it over LTE, so it bumps your device down to the 2G/3G circuit-switched domain to handle the call, then (ideally) returns you to LTE afterward.
The cost shows up in two ways an engineer notices. First, call setup is slower, because moving the device between radio technologies adds delay before the phone even rings. Second — and this is the one users complained about — your data connection drops or crawls during the call, since you've left LTE. CSFB relies on the SGs interface between the MME and the legacy MSC to coordinate paging across domains. It was always a transitional measure, and as VoLTE coverage matured most operators retired it. It resurfaces in conversation now mainly because some markets are sunsetting 2G/3G entirely, which removes the fallback layer CSFB depended on.
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