What Is Socket S1?

The S1 interface is a communication interface between the LTE eNodeB (base station) and the EPC (Packet Core Network). The LTE system is divided into a radio access network and a core network.

The S1 interface is a communication interface between the LTE eNodeB (base station) and the EPC (Packet Core Network). The LTE system is divided into a radio access network and a core network.

S1 interface definition

The S1 interface is a communication interface between the LTE eNodeB (base station) and the EPC (Packet Core Network). The LTE system is divided into a radio access network and a core network.

Figure 1 Location of the S1 interface in the EPS network element

S1 interface classification

The S1 interface follows the idea of separate bearer and control, and is divided into two interfaces, one for the control plane (S1-MME) and one for the user plane (S1-U).

Table 1 S1 interface description

Interface name	Connected network element	Interface function description	Main agreement
S1-MME	eNodeB-MME	Used to transfer session management (SM) and mobility management (MM) information, that is, signaling or control plane information	S1-AP
S1-U	eNodeB-SGW	Establish a tunnel between the GW and the eNodeB device to transmit user data services, that is, user plane data	GTP-U

S1 interface function

The control plane interface S1-MME connects the base station and the mobility management entity (MME), and mainly performs functions such as radio access bearer control of the S1 interface and interface-specific operation and maintenance.

The user plane interface S1-U connects the base station and the serving gateway (S-GW) for transmitting user data and corresponding user plane control frames.