S-GW (Serving Gateway)

The gateway that terminates the  interface toward Evolved UMTS Terrestrial Radio Access Network  (E-UTRAN). S-GW is responsible for handovers with neighboring  eNodeBs, as well as for data transfer of all packets across the user  plane from the eNodeB to the P-GW. It is also the mobility anchor  point for Inter-Radio Access Technology (RAT) handover between  different generations of mobile networks such as 2G/3G.

This critical function will be replaced with UPF in 5G.