CSFB : LTE EPC co-existence with 2G/3G CS Core for voice service
In LTE, voice services can be provided either by fallback to legacy 2G/3G CS core network or by having IP Multimedia Subsystem (IMS).
In first case of circuit switch fallback (CSFB), few nodes of 4G EPC (Mobility Management Entity & Home Subscriber Subsystem) needs to connect with the 2G/3G core node (Mobile Switching Center) to provide voice services. Co-existence of these two core networks of LTE & legacy network is depicted here.
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