Section: 4.7.2 The EPS bearer 4.7.2.1 The EPS bearer in general
Content: For E-UTRAN access to the EPC the PDN connectivity service is provided by an EPS bearer for GTP-based S5/S8, and if IP is in use, by an EPS bearer concatenated with IP connectivity between Serving GW and PDN GW for PMIP-based S5/S8. In this release of the specifications, dedicated bearers are only supported for the IP and Ethernet PDN Connectivity Service. When User Plane (S1-U) is used for data traffic, then an EPS bearer uniquely identifies traffic flows that receive a common QoS treatment between a UE and a PDN GW for GTP-based S5/S8, and between UE and Serving GW for PMIP-based S5/S8. The packet filters signalled in the NAS procedures are associated with a unique packet filter identifier on per-PDN connection basis. NOTE 1: The EPS Bearer Identity together with the packet filter identifier is used to reference which packet filter the UE intends to modify or delete, i.e. it is used to implement the unique packet filter identifier. An EPS bearer is the level of granularity for bearer level QoS control in the EPC/E-UTRAN. That is, all traffic mapped to the same EPS bearer receive the same bearer level packet forwarding treatment (e.g. scheduling policy, queue management policy, rate shaping policy, RLC configuration, etc.). Providing different bearer level packet forwarding treatment requires separate EPS bearers. NOTE 2: In addition but independent to bearer level QoS control, the PCC framework allows an optional enforcement of service level QoS control on the granularity of SDFs independent of the mapping of SDFs to EPS bearers. One EPS bearer is established when the UE connects to a PDN, and that remains established throughout the lifetime of the PDN connection to provide the UE with always-on connectivity to that PDN. That bearer is referred to as the default bearer. Any additional EPS bearer that is established for the same PDN connection is referred to as a dedicated bearer. The EPS bearer traffic flow template (TFT) is the set of all packet filters associated with that EPS bearer. An UpLink Traffic Flow Template (UL TFT) is the set of uplink packet filters in a TFT. A DownLink Traffic Flow Template (DL TFT) is the set of downlink packet filters in a TFT. Every dedicated EPS bearer is associated with a TFT. A TFT may be also assigned to the default EPS bearer. The UE uses the UL TFT for mapping traffic to an EPS bearer in the uplink direction. The PCEF (for GTP-based S5/S8) or the BBERF (for PMIP-based S5/S8) uses the DL TFT for mapping traffic to an EPS bearer in the downlink direction. The UE may use the UL TFT and DL TFT to associate EPS Bearer Activation or Modification procedures to an application and to traffic flow aggregates of the application. Therefore the PDN GW shall, in the Create Dedicated Bearer Request and the Update Bearer Request messages, provide all available traffic flow description information (e.g. source and destination IP address and port numbers and the protocol information). For the UE, the evaluation precedence order of the packet filters making up the UL TFTs is signalled from the P-GW to the UE as part of any appropriate TFT operations. NOTE 3: The evaluation precedence index of the packet filters associated with the default bearer, in relation to those associated with the dedicated bearers, is up to operator configuration. It is possible to "force" certain traffic onto the default bearer by setting the evaluation precedence index of the corresponding filters to a value that is lower than the values used for filters associated with the dedicated bearers. Further details about the TFT and the TFT operations are described in clause 15.3 of TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [7]. The details about the TFT packet filter(s) are described in clause 15.3.2 of TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [7] for PDN connections of IP type and in clause 5.7.6.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [83] for PDN connections of Ethernet type. A TFT of an uplink unidirectional EPS bearer is only associated with UL packet filter(s) that matches the uplink unidirectional traffic flow(s) A TFT of a downlink unidirectional EPS bearer is associated with DL packet filter(s) that matches the unidirectional traffic flow(s) and a UL packet filter that effectively disallows any useful packet flows (see clause 15.3.3.4 in TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [7] for an example of such packet filter. The UE routes uplink packets to the different EPS bearers based on uplink packet filters in the TFTs assigned to these EPS bearers. The UE evaluates for a match, first the uplink packet filter amongst all TFTs that has the lowest evaluation precedence index and, if no match is found, proceeds with the evaluation of uplink packet filters in increasing order of their evaluation precedence index. This procedure shall be executed until a match is found or all uplink packet filters have been evaluated. If a match is found, the uplink data packet is transmitted on the EPS bearer that is associated with the TFT of the matching uplink packet filter. If no match is found, the uplink data packet shall be sent via the EPS bearer that has not been assigned any uplink packet filter. If all EPS bearers (including the default EPS bearer for that PDN) have been assigned one or more uplink packet filters, the UE shall discard the uplink data packet. NOTE 4: The above algorithm implies that there is at most one EPS bearer without any uplink packet filter. Therefore, some UEs may expect that during the lifetime of a PDN connection (where only network has provided TFT packet filters) at most one EPS bearer exists without any uplink packet filter. To ensure that at most one EPS bearer exists without any uplink packet filter, the PCEF (for GTP-based S5/S8) or the BBERF (for PMIP-based S5/S8) maintains a valid state for the TFT settings of the PDN connection as defined in clause 15.3.0 of TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [7] and if necessary, adds a packet filter which effectively disallows any useful packet flows in uplink direction (see clause 15.3.3.4 in TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [7] for an example of such a packet filter) to the TFT of a dedicated bearer. NOTE 5: The default bearer is the only bearer that may be without any uplink packet filter and thus, a packet filter which effectively disallows any useful packet flows in uplink direction will not be added by the PCEF/BBERF. The initial bearer level QoS parameter values of the default bearer are assigned by the network, based on subscription data (in E-UTRAN the MME sets those initial values based on subscription data retrieved from HSS). In a non-roaming scenario, the PCEF may change the QoS parameter value received from the MME based on interaction with the PCRF or based on local configuration. When the PCEF changes those values, the MME shall use the bearer level QoS parameter values received on the S11 reference point during establishment or modification of the default bearer. In a roaming scenario, based on local configuration, the MME may downgrade the ARP or APN-AMBR and/or remap QCI parameter values received from HSS to the value locally configured in MME (e.g. when the values received from HSS do not comply with services provided by the visited PLMN). The PCEF may change the QoS parameter values received from the MME based on interaction with the PCRF or based on local configuration. Alternatively, the PCEF may reject the bearer establishment. NOTE 6: For certain APNs (e.g. the IMS APN defined by the GSMA) the QCI value is strictly defined and therefore remapping of QCI is not permitted. NOTE 7: In roaming scenarios, the ARP/APN-AMBR/QCI values provided by the MME for a default bearer may deviate from the subscribed values depending on the roaming agreement. If the PCC/PCEF rejects the establishment of the default bearer, this implies that Attach via E-UTRAN will fail. Similarly, if the PCEF (based on interaction with the PCRF or based on local configuration) upgrades the ARP/APN-AMBR/QCI parameter values received from the MME, the default bearer establishment and attach may be rejected by the MME. NOTE 8: Subscription data related to bearer level QoS parameter values retrieved from the HSS are not applicable for dedicated bearers. For WB-E-UTRA, the decision to establish or modify a dedicated bearer can only be taken by the EPC, and the bearer level QoS parameter values are always assigned by the EPC. Dedicated bearers are not supported over NB-IoT. The PDN GW uses the RAT Type to ensure that no dedicated bearers are active when the UE is accessing over NB-IoT. In the case of inter-RAT mobility from WB-EUTRA to NB-IoT, the UE and MME indicate local deactivation of non-default EPS bearers at TAU as specified in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]. The MME shall not modify the bearer level QoS parameter values received on the S11 reference point during establishment or modification of a default or dedicated bearer (except when the conditions described in NOTE 9 or NOTE 10 apply). Consequently, "QoS negotiation" between the E-UTRAN and the EPC during default or dedicated bearer establishment / modification is not supported. Based on local configuration, the MME may reject the establishment or modification of a default or dedicated bearer if the bearer level QoS parameter values sent by the PCEF over a GTP based S8 roaming interface do not comply with a roaming agreement. NOTE 9: If the EPS QoS parameters are not compliant with the roaming agreement, the MME, based on local policies, can downgrade the ARP priority level, ARP pre-emption capability, ARP pre-emption vulnerability, APN-AMBR or MBR (for GBR bearers) parameters received over S8 and allow the bearer establishment or modification of a default or dedicated bearer. The HPLMN is expected to set EPS QoS parameters compliant with roaming agreements, therefore the HPLMN is not informed about any downgrade of EPS bearer QoS parameters. The consequences of such a downgrading APN-AMBR and MBR are that APN-AMBR and MBR enforcement at the HPLMN and at the UE will not be aligned. The consequence of downgrading the ARP is that the EPS bearer ARP at the HPLMN and at the eNodeB will not be aligned, and multiple EPS bearers created can possibly have the same EPS bearer ARP in the eNodeB. NOTE 10: In roaming scenarios, for IMS voice service (e.g. the IMS APN defined by the GSMA), the MME, based on local policy, can override the ARP (i.e. ARP priority level, ARP pre-emption capability, ARP pre-emption vulnerability) received over S8 if the ARP indicates lower priority than the local policy. The purpose of ARP override in the serving PLMN is to apply the same allocation and retention priority for IMS voice service for all users (i.e. roamers and non-roamers) and to apply the same allocation and retention priority for all MPS service users (clause 4.3.18) when roaming agreements are in place and where regulatory requirements apply. At inter-RAT mobility, based on local configuration, the MME may perform a mapping of QCI values for which there is no mapping defined in Table E.3 or which are not supported in the target RAT. NOTE 11: The PCRF ensures that the EPS bearer QCI values are aligned with the QCI values mapped by the MME for the current RAT as described in clause A.4.1.2 of TS 23.203[ Policy and charging control architecture ]  [6]. The distinction between default and dedicated bearers should be transparent to the access network (e.g. E-UTRAN). An EPS bearer is referred to as a GBR bearer if dedicated network resources related to a Guaranteed Bit Rate (GBR) value that is associated with the EPS bearer are permanently allocated (e.g. by an admission control function in the eNodeB) at bearer establishment/modification. Otherwise, an EPS bearer is referred to as a Non-GBR bearer. NOTE 12: Admission control can be performed at establishment / modification of a Non-GBR bearer even though a Non-GBR bearer is not associated with a GBR value. A dedicated bearer can either be a GBR or a Non-GBR bearer. A default bearer shall be a Non-GBR bearer. NOTE 13: A default bearer provides the UE with connectivity throughout the lifetime of the PDN connection. That motivates the restriction of a default bearer to bearer type Non-GBR.