19.2.1 S1 Interface Functions

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

19.2.1.0 General

The S1 interface provides the following functions:

– E-RAB Service Management function:

– Setup, Modify, Release.

– Mobility Functions for UEs in ECM-CONNECTED:

– Intra-LTE Handover;

– Inter-3GPP-RAT Handover.

– S1 Paging function:

– NAS Signalling Transport function;

– LPPa Signalling Transport function;

– S1-interface management functions:

– Error indication;

– Reset.

– Network Sharing Function;

– Roaming and Access Restriction Support function;

– NAS Node Selection Function;

– Initial Context Setup Function;

– UE Context Modification Function;

– UE Context Resume Function;

– MME Load balancing Function;

– Location Reporting Function;

– PWS (which includes ETWS and CMAS) Message Transmission Function;

– Overload function;

– RAN Information Management Function;

– Configuration Transfer Function;

– S1 CDMA2000 Tunnelling function;

– Trace function;

– UE Radio Capability Match;

– Retrieve UE Information Function;

– UE Information Transfer Function;

– Report of Secondary RAT data volumes Function;

– UE Radio Capability ID Mapping Function.

19.2.1.1 S1 Paging function

The paging function supports the sending of paging requests to all cells of the TA(s) the UE is registered.

Paging requests are sent to the relevant eNBs according to the mobility information kept in the UE’s MM context in the serving MME.

19.2.1.2 S1 UE Context Management function

In order to support UEs in ECM-CONNECTED, UE contexts need to be managed, i.e. established and released in the eNodeB and in the EPC to support user individual signalling on S1.

19.2.1.3 Initial Context Setup Function

The Initial Context Setup function supports the establishment of the necessary overall initial UE Context including E-RAB context, Security context, roaming and access restrictions, UE capability information, Subscriber Profile ID for RAT/Frequency Priority, UE S1 signalling connection ID, etc. in the eNB to enable fast Idle-to-Active transition.

In addition to the setup of overall initial UE Contexts, Initial Context Setup function also supports the piggy-backing of the corresponding NAS messages. Initial Context Setup is initiated by the MME.

19.2.1.3a UE Context Modification Function

The UE Context Modification function supports the modification of UE Context in eNB for UEs in active state.

19.2.1.3b UE Context Resume Function

The UE Context Resume function supports the suspension and resumption of an established UE Context and allows keeping the UE Context in eNB for UEs that were sent to RRC_IDLE and may resume the RRC connection without the necessity to re-establish the UE Context in the eNB.

19.2.1.4 Mobility Functions for UEs in ECM-CONNECTED

19.2.1.4.1 Intra-LTE Handover

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

19.2.1.4.2 Inter-3GPP-RAT Handover

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

19.2.1.5 E-RAB Service Management function

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. The release of E-UTRAN resources is triggered by the MME either directly or following a request received from the eNB (optional).

For DC 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.

19.2.1.6 NAS Signalling Transport function

The NAS Signalling Transport function provides means to transport or reroute a NAS message (e.g. for NAS mobility management) for a specific UE on the S1 interface.

19.2.1.7 NAS Node Selection Function (NNSF)

The interconnection of eNBs or HeNB GW, if deployed, to multiple MME/Serving S-GWs is supported in the E-UTRAN/EPC architecture. Therefore a NAS node selection function is located in the eNB or the HeNB GW, if deployed, to determine the MME association of the UE, based on the UE’s temporary identifier, which was assigned to the UE by the CN node (e.g. MME or SGSN).

NOTE: In case the UE’s temporary identifier is assigned by the SGSN, respective mapping rules are defined in TS 23.003 [26].

Depending on the actual scenario the NNSF determines the UE’s MME association either based its S-TMSI (e.g. at service request) or based on its GUMMEI and selected PLMN (e.g. at attach or tracking area update in non-registered TA).

The NNSF in the eNB or HeNB GW, if deployed, may differentiate between a GUMMEI mapped from P‑TMSI/RAI and a native GUMMEI as described in TS 23.401 [17].

This functionality is located in the eNB or in the HeNB GW, if deployed, and enables proper routing via the S1 interface. On S1, no specific procedure corresponds to the NAS Node Selection Function.

eNB selects serving MME based on the DCN-ID provided by the UE and the configuration in eNB if a serving MME corresponding to the information provided by the UE (e.g. GUTI, etc.) cannot be found by eNB as described in TS 23.401[17].

19.2.1.8 S1-interface management functions

The S1-interface management functions provide

– means to ensure a defined start of S1-interface operation (reset);

– means to handle different versions of application part implementations and protocol errors (error indication).

19.2.1.9 MME Load balancing Function

MME Load balancing is the function which achieves load-balanced MMEs with respect to their processing capacity within a pool area during system operation. The means to load-balance MMEs is to distribute UEs newly entering the pool to different MMEs in the MME pool. In addition the MME load balancing function is able to achieve equally loaded MMEs within a pool area after the introduction of a new MME and after the removal of a MME from the network.

The support of the MME load balancing function is achieved by indicating the relative MME capacity in the S1 Setup procedure to all eNBs served by the MMEs of the pool area per MME. In order to support the introduction and/or removal of MMEs the MME initiated S1 setup update procedure may be used by the operator indicating relative MME capacity value changes. When there are more than one MME operational in the pool, the indicated relative MME capacity steers the UE assignment for UEs newly entering the MME pool. When there is only one MME operational in the pool, UEs may be assigned to this MME.

The MME Load balancing Function is described in detail in TS 23.401 [17].

When DCN are used, the MME Load Balancing function is only performed between MMEs that belong to the same DCN. The MME Load Balancing for DCN is described in TS 23.401 [17].

19.2.1.10 Location Reporting Function

The Location Reporting function supports the MME requests to the eNB to report the location information of the UE.

19.2.1.11 Warning Message Transmission function

The warning message transmission function provides means to transfer warning message via S1 interface.

19.2.1.12 Overload Function

The overload function comprises the signalling means:

– to indicate to a proportion of eNBs that the serving MME is overloaded;

– to indicate to the eNBs that the serving MME is back in the "normal operation mode".

19.2.1.13 RAN Information Management Function

The RAN Information Management (RIM) function is a generic mechanism that allows the request and transfer of information (e.g. GERAN System information) between two RAN nodes via the core network.

19.2.1.14 S1 CDMA2000 Tunnelling function

The S1 CDMA2000 Tunnelling function transports CDMA2000 signalling between UE and CDMA2000 RAT over the S1 Interface for mobility from E-UTRAN to CDMA2000 HRPD and CDMA2000 1xRTT and for circuit switched fallback to CDMA2000 1xRTT.

19.2.1.15 Configuration Transfer Function

The Configuration Transfer function is a generic mechanism that allows the request and transfer of RAN configuration information (e.g. SON information) between two RAN nodes via the core network.

19.2.1.16 LPPa Signalling Transport function

The LPPa Signalling Transport function provides means to transport an LPPa message on the S1 interface.

19.2.1.17 Trace Function

The Trace function provides means to control trace sessions in the eNB. The Trace function also provides means to control MDT sessions as described in TS 32.422 [30] and TS 37.320 [43] or Application Layer Measurement Collection, as described in clause 23.16.

19.2.1.18 UE Radio Capability Match

The UE Radio Capability Match function enables the eNB to provide an indication to the MME whether the UE radio capabilities are compatible with the network configuration for voice continuity.

19.2.1.19 Retrieve UE Information Function

The Retrieve UE Information Function enables the eNB to retrieve the UE information from MME.

19.2.1.20 UE Information Transfer Function

The UE Information Transfer Function enables the MME to send the UE information to the eNB.

19.2.1.21 Report of Secondary RAT data volumes Function

The Report of Secondary RAT data volumes Function enables the eNB to report Secondary RAT usage data information in case of EN-DC, either with a dedicated procedure or by including Secondary RAT usage data information in other messages.

19.2.1.22 UE Radio Capability ID Mapping Function

The UE Radio Capability ID Mapping function enables the eNB to request the MME to provide the UE Radio Capability information that maps to a specific UE Radio Capability ID.