5 Functions of the S1 interface protocols and functional split

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1 General

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

5.2 S1 UE context management function

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.

5.3 E-RAB management functions

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).

5.4 S1 link management function

5.4.1 GTP-U tunnels management function

This function is used to establish and release GTP-U (3GPP TS 29.281 [8]) tunnels between the EPC and the
E-UTRAN upon an E-RAB service request. This involves assigning a tunnel identifier for each direction.

5.4.2 S1 signalling link management function

The S1 signalling link management function provides a reliable transfer of the radio network signalling between
E-UTRAN and EPC.

5.5 Mobility functions for UEs in LTE_Active

5.5.1 Intra-LTE handover

The Intra-LTE handover function supports mobility for UEs in LTE_ACTIVE and comprises the preparation, execution and completion of handover via the X2 and S1 interfaces.

5.5.2 Inter-3GPP RAT handover

The Inter-3GPP-RAT handover function supports mobility to and from other 3GPP-RATs for UEs in LTE_ACTIVE and comprises the preparation, execution and completion of handover via the S1 interface.

5.5.3 Mobility to CDMA2000 system

The mobility to non-3GPP radio system function supports mobility to and from other non-3GPP radio technologies for UEs in LTE_ACTIVE, namely to and from CDMA2000 systems. This comprises signalling for pre-registration of UE with CDMA2000 HRPD network and signalling for handover preparation for handover from E-UTRAN to CDMA2000 HRPD/1xRTT.

5.6 Paging function

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.

5.7 Void

5.7a Roaming and access restriction support functions

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

5.8 S1 interface management function

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.

5.9 Coordination functions

5.9.1 Network sharing function

The S1 interface supports the transfer of the UE’s serving PLMN and of equivalent PLMNs to the serving PLMN.

5.9.2 NAS node selection function

The interconnection of eNBs to multiple MME / S-GWs is supported by the LTE/EPS architecture (see 3GPP TS 23.401 [9] and 3GPP TS 36.300 [11]). Therefore a NAS node selection function is located in the eNB to determine the MME association of the UE.

This functionality is located in the eNB to determine and establish an association between a given UE and one of the MME nodes that comprise the pool area the eNB belongs to.

It then enables proper routeing via the S1-MME interface.

On S1, no specific procedure corresponds to the NAS node selection function.

The S1 interface supports the indication by the MME of its relative capacity to the eNB, in order to achieve load-balanced MMEs within the pool area.

When the eNB is configured to ensure that the selected MME serves the country where the UE is located, as described in TS 23.401 [9], the eNB takes into account UE location information, if available, when determining the MME.

5.9.3 MME overload control function

The S1 interface supports the indication by the MME of its overload situations to the eNB.

5.10 Security function

5.10.1 Data confidentiality

5.10.1.1 Radio interface ciphering function

The radio interface may be ciphered. Both, signalling and user data, may be subject to ciphering. The ciphering shall be done within the E-UTRAN.

5.10.1.2 Ciphering key management function

The MME provides the eNB with the key K_eNB, which is used for the derivation of the ciphering keys used for ciphering of the radio interface as defined in TS 36.300 [11] and TS 33.401 [12].

5.10.2 Data integrity

5.10.2.1 Integrity checking

The purpose of the integrity check is to ensure the integrity of the signalling messages over the air interface to make sure that the signalling continues between the same elements, i.e., the UE and the eNB that first established the secure connection. The integrity check shall be done within the E-UTRAN.

5.10.2.2 Integrity key management

The MME provides the eNB with the key K_eNB, which is used for the derivation of the integrity key used for the integrity protection of the radio interface control plane as defined in TS 36.300 [11] and TS 33.401 [12].

5.11 Service and network access function

5.11.1 Core network signalling data transfer function

The NAS EPC signalling data shall be transparently conveyed between the EPC and the UE. Over the S1 interface, the same S1 interface channel that is used for the E-UTRAN–EPC signalling shall be used.

5.11.2 UE tracing

This function allows tracing of various events related to the UE and its activities. This is an O&M functionality.

5.11.3 Location reporting function

The positioning function performs the determination of the geographical position of the UE.

This function allows the MME to request the eNB for a given UE either to report directly the current serving cell or to report upon change of serving cell.

If dual connectivity is configured for a specific UE, the location reported refers to the cell served by the MeNB.

5.11.4 LTE Positioning Protocol A (LPPa) signalling transfer function

The LPPa (3GPP TS 36.455 [10]) signalling data is transparently conveyed between the E-SMLC and the eNB.

5.11.5 Delivery of warning messages

This function provides the capability to start or stop the delivery of warning notification messages related to alerts (earthquake, tsunami, etc..). The messages belong to one of the independent warning services: ETWS, CMAS or the other services which use the same Access Stratum (AS) mechanisms as CMAS as described in 3GPP TS 36.300 [11].

In ETWS, the messages to be delivered can either be primary notifications which have stringent delivery delay or secondary notifications.

5.12 RAN Information Management (RIM) function

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).

5.13 Configuration Transfer function

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).