What is CSFB (circuit-switched fallback)?

Circuit Switched Fallback (CSFB), conveys voice and SMS services to LTE devices with the assistance of a global system for mobile communication or another circuit-switched network. The requirement for a Circuit Switch FallBack for the packet-based all-IP network, LTE, which cannot uphold circuit-switched calls. The device “falls back” to a more accessible 3G or 2G network to complete the call or to transmit the SMS message when an LTE network is not available to make or get a call or SMS message.