S3: Interface between SGSN and MME
1. Enables user and bearer information exchange for inter 3GPP access.
2. Network mobility in idle and/or active state.
S4: Interface between SGSN and Serving SAE Gateway
1. It provides control and mobility support between GPRS Core & 3GPP.
2. Anchor function of Serving GW.
3. Alternate plane tunnel in case of no Direct Tunnel.
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