S3:Interface between SGSN and MME Functionality 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 Functionality 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.
Introduction to UMTS LTE Interfaces between various reference points.