4 General aspects

36.4103GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17S1 general aspects and principlesTS

This clause captures the S1 interface principles and characteristics.

4.1 E-UTRAN architecture

The E-UTRAN is layered into a Radio Network Layer (RNL) and a Transport Network Layer (TNL). The E-UTRAN architecture, i.e. the E-UTRAN logical nodes and interfaces between them, are defined as part of the RNL.

The E-UTRAN architecture consists of a set of eNBs connected to the EPC through the S1 interface. The overall LTE architecture and E-UTRAN architecture are described in 3GPP TS 36.401 [2]. This subclause specifies only the architecture of the S1 interface, and shall not constrain the network architecture of either core or radio access networks.

The S1 interface is specified at the boundary between the EPC and the E-UTRAN. Figure 4.1 depicts the logical division of the S1 interface. From the S1 perspective, the E-UTRAN access point is an eNB, and the EPC access point is either the control plane MME logical node or the user plane S-GW logical node. Two types of S1 interfaces are thus defined at the boundary depending on the EPC access point: S1-MME towards an MME and S1-U towards an S- GW.

Figure 4.1: S1 interface architecture

The E-UTRAN may thus have several S1 access points towards the EPC. As a minimum, each S1 access point
(in E-UTRAN or EPC) shall independently fulfil the requirements of the relevant S1 specifications (3GPP 36.41x series – see clause 7).

S1 is a logical interface.

There may be multiple S1-MME logical interfaces towards the EPC from any one eNB. The selection of the S1-MME interface is then determined by the NAS Node Selection function as described in clause 5.

There may be multiple S1-U logical interfaces towards the EPC from any one eNB. The selection of the S1-U interface is done within the EPC and signalled to the eNB by the MME.

4.2 S1 interface general principles

The general principles for the specification of the S1 interface are as follows:

– the S1 interface should be open;

– the S1 interface shall support the exchange of signalling information between the eNB and EPC;

– from a logical standpoint, the S1 is a point-to-point interface between an eNB within the E-UTRAN and an MME in the EPC. A point-to-point logical interface should be feasible even in the absence of a physical direct connection between the eNB and MME.

4.3 S1 interface specification objectives

The S1 interface specification shall facilitate the following:

– inter-connection of eNBs with MMEs supplied by different manufacturers;

– separation of S1 interface Radio Network functionality and Transport Network functionality to facilitate introduction of future technology.

4.4 S1 interface capabilities

The S1 interface supports:

– procedures to establish, maintain and release E-UTRAN Radio Access Bearers;

– procedures to perform intra-LTE handover and inter-RAT handover;

– the separation of each UE on the protocol level for user specific signalling management;

– the transfer of NAS signalling messages between UE and EPC;

– location services by transferring requests from the EPC to E-UTRAN, and location information from E-UTRAN to EPC;

– mechanisms for resource reservation for packet data streams.

4.5 S1 interface characteristics

4.5.1 Use of the Stream Control Transmission Protocol (SCTP)

The SCTP protocol (IETF RFC 4960 [7]) is used to support the exchange of S1 Application Protocol (S1AP) signalling messages between an eNB and an MME.