The S1 interface is the interface between the LTE RAN and evolved packet core. S1 interface protocol stack is described in Figure.
S1 performs the following functions:
- S1-UP (user plane)
- S1-CP (control plane)
LTE S1-UP (user plane)
The S1 user plane external interface (S1-U) is defined between the LTE eNodeB and the LTE S-GW. The S1-U interface provides non guaranteed data delivery of LTE user plane Protocol Data Units (PDUs) between the eNodeB and the S-GW. Transport network layer is built on IP transport and GTP-U. UDP/IP carries the user plane PDUs between the eNodeB and the S-GW. A GTP tunnel per radio bearer carries user traffic.
The S1-UP interface is responsible for delivering user data between the eNodeB and the S-GW. The IP Differentiated Service Code Point (DSCP) marking is supported for QoS per radio bearer.
LTE S1-MME (control plane)
The LTE S1-MME interface is responsible for delivering signaling protocols between the eNodeB and the MME. S1-MME interface consists of a Stream Control Transmission Protocol (SCTP) over IP and supports multiple UEs through a single SCTP association. It also provides guaranteed data delivery. The application signaling protocol is an S1-AP (Application Protocol). The LTE S1-MME is responsible for Evolved Packet System (EPS) bearer setup/release procedures, the handover signaling procedure, the paging procedure and the NAS transport procedure. LTE Transport network layer is built on IP transport, similar to the user plane but for the reliable transport of signaling messages SCTP is added on top of the Internet Protocol.