Section: 4.12 Supporting up to 15 EPS bearers per UE
Content: A UE attached to WB-E-UTRAN access, including for dual connectivity using E-UTRAN access as described in clause 4.3.2a, may support 8 or 15 EPS bearers. To enable support of establishing 15 EPS bearers, it requires the EPC connected to the E-UTRAN access to support 15 EPS bearers for such UEs. If the UE supports 15 EPS bearers, then the UE shall indicate this to the MME in NAS signalling as defined in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]. The network shall support E-UTRAN idle mode mobility and handover procedures in such PLMNs, where only part of the network nodes have been upgraded to support 15 EPS bearers. In the case of mobility procedures involving target nodes not supporting 15 EPS bearers, additional bearers not supported by the non-upgraded nodes should be thus released. The network shall homogeneously support 15 EPS bearers per UE, at least per MME pool area/SGW serving area, in order to avoid EPS bearer deactivations and attempts from services to re-activate the deactivated EPS bearers caused by UE mobility within that area, by defining the tracking areas accordingly (see clause 4.3.5.3). The MME shall provide the UE with a TAI List such that it triggers the UE to perform the TAU procedure, as defined in clause 5.3.3.0, when the UE enters and exits the area with support for 15 EPS bearers per UE. The TAU procedure execution enables the nodes in the supporting area to identify a supporting UE when it enters the area. When exiting the area, also enables the nodes to remove bearer resources for the UE towards nodes without support for 15 EPS bearers per UE. NOTE 1: In order for the above TAI List handling to be fully successful, the network requires the PGWs the UE is connected to from these MME pool area/SGW serving area to be upgraded to support 15 EPS bearers. For a UE supporting 15 EPS bearers, mobility to UTRAN or GERAN may also lead to selective release of EPS bearers, due to the lack of support of 15 PDP contexts in the GPRS core network and Radio Access networks. NOTE 2: EPS bearers need to be released either because the UE has more than 8 active EPS bearers, or because they are identified by EPS Bearer Identities not supported. In order to minimize the impact from release of bearers not supported by the target nodes during mobility, the MME should be able to allocate the EPS bearer IDs in such way that the bearers with higher operator preference will be preserved in the case of mobility involving legacy target nodes. This prioritised bearer allocation should be based on, at least, the Allocation Retention Priority of the EPS bearers and may take other bearer parameters (e.g. QCI, APN) into consideration. All PDN GWs in a PLMN shall support 15 EPS bearers. MME may be configured to take into account additional PDN GW information such as whether the HPLMN supports 15 EPS bearers when selecting PDN GW (e.g. in the case of roaming users Home Routed PDN GW selection) for UEs supporting 15 EPS bearers. For inter-PLMN handover, support of 15 EPS bearers is based on MME configuration according to operator policy (e.g. bilateral agreements between operators).