7 Functions of S1AP

36.4133GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17S1 Application Protocol (S1AP)TS

The S1AP protocol has the following functions:

– E-RAB management function: This overall functionality is responsible for setting up, modifying and releasing E-RABs, which are triggered by the MME. The release and modification of E-RABs may be triggered by the eNB as well.

– Initial Context Transfer function: This functionality is used to establish an S1UE context in the eNB, to setup the default IP connectivity, to setup one or more E-RAB(s) if requested by the MME, and to transfer NAS signalling related information to the eNB if needed.

– UE Capability Info Indication function: This functionality is used to provide the UE Capability Info when received from the UE to the MME.

– Mobility Functions for UEs in LTE_ACTIVE in order to enable

– a change of eNBs within SAE/LTE (Inter MME/Serving SAE-GW Handovers) via the S1 interface (with EPC involvement).

– a change of RAN nodes between different RATs (Inter-3GPP-RAT Handovers) via the S1 interface (with EPC involvement).

– Paging: This functionality provides the EPC with the capability to page the UE.

– S1 interface management functions comprise the:

– Reset functionality to ensure a well defined initialisation on the S1 interface.

– Error Indication functionality to allow a proper error reporting/handling in cases where no failure messages are defined.

– Overload function to indicate the load situation in the control plane of the S1 interface.

– Load balancing function to ensure equally loaded MMEs within an MME pool area

– S1 Setup functionality for initial S1 interface setup for providing configuration information

– eNB and MME Configuration Update functions are to update application level configuration data needed for the eNB and MME to interoperate correctly on the S1 interface.

– NAS Signalling transport function between the UE and the MME is used:

– to transfer and reroute NAS signalling related information and to establish the S1 UE context in the eNB.

– to transfer NAS signalling related information when the S1 UE context in the eNB is already established.

– S1 UE context Release function: This functionality is responsible to manage the release of UE specific context in the eNB and the MME.

– UE Context Modification function: This functionality allows to modify the established UE Context partly.

– UE Context Resumption function: This functionality allows keeping the UE Context in the eNB for a UE in RRC_IDLE that has been enabled to use User Plane EPS Optimization (see TS 23.401 [11]) and to resume the RRC connection without the need to re-establish the UE Context.

– Status Transfer: This functionality transfers PDCP SN Status information from source eNB to target eNB in support of in-sequence delivery and duplication avoidance for intra LTE handover.

– Trace function: This functionality is to control a trace session recording for a UE in ECM_CONNECTED or to control an MDT session transferring MDT measurements collected by the UE.

– Location Reporting: This functionality allows MME to be aware of the UE’s current location.

– LPPa Signalling transport: This functionality transfers LPPa messages between eNB and E-SMLC over the S1 interface.

– S1 CDMA2000 Tunnelling function: This functionality is to carry CDMA2000 signalling between UE and CDMA2000 RAT over the S1 Interface.

– Warning message transmission function:
This functionality provides the means to start and overwrite the broadcasting of warning message.

– RAN Information Management (RIM) function: This functionality allows the request and transfer of RAN information (e.g., GERAN system information) between two RAN nodes via the core network.

– Configuration Transfer function: This functionality allows the request and transfer of RAN configuration information (e.g., SON information) between two RAN nodes via the core network.

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

– PWS Restart Indication function. The functionality enables the eNB to inform the MME that PWS information for some or all cells of the eNB are available for reloading from the CBC if needed.

– PWS Failure Indication function. The functionality enables the eNB to inform the MME that ongoing PWS operation for one or more cells of the eNB has failed.

– Connection Establishment Indication function. The functionality enables the MME to complete the establishment of the UE-associated logical S1-connection.

– Retrieve UE Information function. The functionality enables the eNB to request UE information from the MME.

– UE Information Transfer function. The functionality enables the MME to transfer UE information to the eNB.

– CP Relocation function. The functionality enables the initiation of the UE-associated logical S1-connection for a NB-IOT UE using Control Plane CIoT EPS Optimisation following a re-establishment request.

– Report of Secondary RAT data volumes function. The functionality enables the eNB to report Secondary RAT data usage information in case of EN-DC as specified in TS 23.401 [11].

– QMC function. The functionality enables the eNB to collect QoE measurements from the UE.