Circuit Switched FallBack
Circuit Switched FallBack (CSFB) is a technology whereby voice and SMS services are delivered to LTE devices through the use of GSM or another circuit-switched network.
Circuit Switched FallBack is needed because LTE is a packet-based all-IP network that cannot support circuit-switched calls. When an LTE device is used to make or receive a voice call or SMS, the device "falls back" to the 3G or 2G network to complete the call or to deliver the SMS text message.
CSFB was specified in 3rd Generation Partnership Project (3GPP) Release 8. CSFB requires a software upgrade of the operators core and radio network.
CSFB is often seen as an interim solution for LTE operators. Voice over LTE (VoLTE) is considered to be the long-term goal for the delivery of voice services on LTE networks.