Section: 5.9.3 IMSI and APN information retrieval procedure
Content: This procedure is used by the RCAF to determine the UEs that are served by a congested eNodeB or E-UTRAN cell and the APNs of the active PDN connections of these UEs. This information is used to determine the PCRFs serving these UEs and subsequently report RAN user-plane congestion information (RUCI) to the PCRFs. The decision whether the RCAF requests MME to retrieve the list of UEs on eNodeB or E-UTRAN cell level is up to operator configuration. NOTE 1: The details of congestion reporting to the PCRF are specified in TS 23.203[ Policy and charging control architecture ]  [6]. The RCAF determines the MMEs that are serving the congested eNodeB or E-UTRAN cell based on the Tracking Area Identities served by the congested eNodeB or E-UTRAN cell. For further details on the related DNS procedure see TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61]. The following steps are applied to all MMEs serving the congested eNodeB or E-UTRAN cell. NOTE 2: In network sharing scenarios the RCAF belongs to the RAN operator. In this case it is up to inter-operator agreements and operator configuration which sharing partner's MMEs the RCAF queries IMSI and APN information from. Figure 5.9.3-1: IMSI and APN information retrieval procedure 1. The RCAF sends an IMSI/APN information request to the MME. The request shall identify whether the request refers to an eNodeB or an E-UTRAN cell and shall contain the related eNodeB ID or ECGI. NOTE 3: The eNodeB ID is defined in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]. 2. The MME sends the IMSI/APN information response to the RCAF. The response shall contain the list of UEs (identified by the IMSIs) served by the eNodeB or E-UTRAN cell and the list of APNs of the active PDN connections of each IMSI. If the RCAF requested the IMSI/APN information on E-UTRAN cell level, then the MME first determines the list of UEs served by that E-UTRAN cell. The MME may achieve this by querying the eNodeB that the E-UTRAN cell belongs to for the exact ECGI for all UEs served by this eNodeB using the Location Reporting procedure (clause 5.9.1). NOTE 4: Applying the Location Reporting feature due to an E-UTRAN cell level RCAF request can increase S1-MME interface signalling load. NOTE 5: In order for RCAF to maintain the list of impacted UEs (identified by the IMSIs) (and related APN information) for a congested cell, the RCAF needs to regularly receive IMSI/APN information updates from the MME. The details of whether the RCAF needs to query the MME regularly or whether the MME updates the RCAF regularly without further explicit requests from the RCAF is specified in Stage 3.