Serving Gateway (S-GW) | Network Elements and Functions

The S-GW is the gateway which terminates the interface to the E-UTRAN. A particular LTE subscriber will always be connected by a single S-GW.

In the case of inter-eNB handover, the S-GW acts as the mobility anchor of the connection and remains the same while the path for the transport of signaling and user plane will be switched onto the S1 interface. Once a handover is executed successfully and the associated UE has left the S-GW, the old S-GW will send one or more "end marker" packets to the source eNB, source SGSN, or source RNC of the handover to assist the reordering of user plane packets in these network elements.

Mobility anchoring of the S-GW is also defined for inter-3GPP mobility. Here the S-GW acts as the terminating point of the S4 interface and routes the traffic between the 2G/3G SGSN and the PDN-GW of the EPC. In other words, when it comes to inter-RAT handover involving the S4 interface, the S-GW acts as the Gateway GPRS Support Node (GGSN) (of the 2G/3G PS core network) that enables usage of the EPC transport and functions for UTRAN/GERAN PS services.

If the UE is in ECM-IDLE mode the S-GW buffers user plane packets that will be sent to the UE after a successful paging response. The paging via the S1 and Uu interfaces is also triggered by the S-GW.

The S-GW is the network element that provides connectivity and software implementations for lawful interception.

On the IP transport layer the S-GW acts as a packet router. User plane packets are forwarded transparently in the UL and DL direction and their underlying transport units are marked by S-GW with parameters like DiffServ Code Point, based on the QoS Class Indicator (QCI) of the associated EPS bearer.

Also embedded in the S-GW software are various charging functions for UL and DL charging per UE, PDN, and QCI. These functions are used to charge the operator's own subscribers as well as roaming users (inter-operator charging).

The S-GW can be connected to SGSNs in non-roaming and roaming scenarios. However, connectivity to a GGSN is not supported.