EPS Bearer - PART1

Before knowing EPS Bearer we need to know about the PDN connectivity service. The PDN connectivity service is nothing but the Evolved Packet System provides IP conectivity between a UE and PLMN external packet data network. The PDN Connectivity Service supports the traffic flow aggregate(s), consisting of one ore more Service Dara Flows (SDFs).


Now coming to EPS Bearer,

 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 seperate 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 IP 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.


Every dedicated bearer is associated with a TFT. A unidirectional EPS bearer is either associated with an UL TFT or a DL TFT that matches the unidirectional traffic flows and a DL TFT or an UL TFT in the other direction that blocks all traffic flows.

 

For of E-UTRAN, 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. Therefore, the MME shall not modify the bearer level QoS parameter values received on the S11 reference point during establishment or modification of a dedicated bearer. Instead, the MME shall only transparently forwards those values to the E-UTRAN. Consequently, "QoS negotiation" between the E-UTRAN and the EPC during dedicated bearer establishment / modification is not supported. The MME may, however, reject the establishment or modification of a dedicated bearer (e.g. 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).

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.

A dedicated bearer can either be a GBR or a Non-GBR bearer. A default bearer shall be a Non-GBR bearer.