S3 and S4 interface support is a license-enabled feature that enables 2G and 3G networks to interface with the 4G
evolved packet core (EPC) network. The S3/S4 functionality ensures session continuity on handovers between 2G/3G
subscribers and 4G LTE subscribers. S3/S4 functionality simplifies core network operations the following ways:

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Replaces the GGSN in the network with the P-GW

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Replaces the need for an HLR by providing connectivity to the HSS

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Optimized idle mode signaling during 3G/2G to 4G handovers (when the ISR feature is enabled)

The S3 and S4 interfaces provide control and bearer separation, and offload the backward compatibility requirement
from the mobility management entity (MME) and serving gateway (S-GW) EPC elements to the UMTS core.

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S3 Interface: Provides a GTPv2-C signaling path connection between the MME and the SGSN (MPC). The S4-

SGSN to MME RAU/TAU context handovers are supported via the S3 interface.

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S4 Interface: Provides a data and signaling interface between the S-GW and the S4-SGSN (MPC) for bearer

plane transport (GTPv2-U). The S4-SGSN communicates with the P-GW via the S-GW.

With support for S3/S4 interface, soft-handoffs between 2G/3G and the EPC networks are possible for multi-mode UEs.
Without this functionality, the Gn/Gp SGSN can still inter-work with the EPC core using GTPv1, but soft-handoffs