For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905. Dual Connectivity: E-RAB:

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905. AS CMAS DC E-RAB eNB EPC EPS ETWS GTP-U IP LTE MeNB MME O&M PWS QoS RAT RIM RNL S1-U S1-MME S-GW S1AP SAP SCG SeNB SCTP TNL

For the purposes of the present document, the following notations apply: Procedure Message

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 TS 36.401. 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.

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.

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.

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.

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.

The following clauses describe the functions supported over S1-MME and S1-U to fulfil the S1 interface capabilities.

In order to support UEs in the LTE_ACTIVE state, UE contexts need to be managed, i.e. established and released in the eNB and in the EPC to support user individual signalling on S1. This includes security context management. The S1 UE context management function supports the establishment of the necessary overall initial UE context including E-RAB context, security context, roaming and access restrictions, UE S1 signalling connection ID(s), etc. in the eNB to enable fast idle-to-active transition. The establishment of the overall initial UE context is initiated by the MME. The S1 UE context management function also supports the release of the context previously established in the eNB to enable the active-to-idle transition. The release of the context is triggered by the MME either directly or following a request received from the eNB.

The E-RAB service management function is responsible for establishing, modifying and releasing E-UTRAN resources for user data transport once a UE context is available in the eNB. The establishment and modification of E-UTRAN resources is triggered by the MME and requires respective QoS information to be provided to the eNB. For Dual Connectivity when SCG bearer option is applied, the modification of the E-RAB is triggered by the MeNB towards the MME for the modification of the transport information. The release of E-UTRAN resources is triggered by the MME either directly or following a request received from the eNB (optional).

The paging function supports the sending of paging requests to the eNBs having one or more cells which correspond to one of the TAs in which the UE is registered.

The S1 interface supports the transfer of roaming and access restriction information from the EPC to the eNB for the UE in the network.

The error indication function is used by the eNB (respectively the MME) to indicate to the MME (respectively the eNB) that a logical error has occurred. The reset function is used to initialize the peer entity after node setup and after a failure event occurred. This procedure can be used by both the eNB and MME. The S1 setup (respectively the eNB and MME configuration update) function allows to exchange (respectively update) application level data needed for the eNB and MME to interoperate correctly on the S1 interface.

The RAN Information Management (RIM) function is a generic mechanism that allows the request and transfer of information (e.g. GERAN/UTRAN system information) between two RAN nodes via the Core Network (CN).

The Configuration Transfer function allows the request and transfer of RAN configuration information (e.g., SON information) between two RAN nodes via the Core Network (CN).

The radio network signalling over S1consists of the S1 Application Part (S1AP). The S1AP protocol consists of mechanisms to handle all procedures between the EPC and E-UTRAN. It is also capable of conveying messages transparently between the EPC and the UE without interpretation or processing by the E-UTRAN. Over the S1 interface the S1AP protocol is, e.g., used to:

• Facilitate a set of general E-UTRAN procedures from the EPC such as paging-notification as defined by the notification Service Access Point (SAP).
• Separate each User Equipment (UE) on the protocol level for mobile specific signalling management as defined by the dedicated SAP.
• Transfer of transparent non-access signalling as defined in the dedicated SAP.
• Request of various types of E-RABs through the dedicated SAP.
• Perform the mobility function.

The E-RABs are provided by the Access Stratum (AS).

Figure 6.2-1 shows the protocol structure for S1-MME, following the structure described in TS 36.401.

The Transport Network Layer (TNL) is based on IP transport, comprising the Stream Control Transmission Protocol (SCTP) on top of IP.

Figure 6.3-1 shows the protocol structure for S1-U, following the structure described in TS 36.401.

3GPP TS 36.411 specifies the physical layer technologies that may be used to support the S1 interface.

3GPP TS 36.412 specifies the signalling bearers for the S1AP for the S1-MME interface.

3GPP TS 36.413 specifies the S1AP protocol for radio network control plane signalling over the S1 interface.

3GPP TS 36.414 specifies the transport bearers for the user plane of the S1-U interface.