Bearers in LTE

  • e-RAB transports the packets of an EPS bearer between the UE and the EPC
  • Data radio bearer transports the packets of an EPS bearer between a UE and an eNB
  • S1 bearer transports the packets of an E-RAB between an eNodeB and a Serving GW
  • S5/S8 bearer transports the packets of an EPS bearer between a Serving GW and a PDN GW 
What is EPS Bearer?
For E-UTRAN access to the EPC the PDN connectivity service is provided by an EPS bearer for GTP-based S5/S8, and by an EPS bearer concatenated with IP connectivity between Serving GW and PDN GW for PMIP-based S5/S8.
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.
EPS Bearer is a virutal connection between UE and PGW which identifies a data send and received between these two end points with specific QoS attributes.
The procedure used to establish an EPS Bearer is called “EPS Bearer Activation” procedure. Either endpoint can trigger this procedure. For example, the PDN-GW may trigger this procedure when it determines that a new session is requested and a new EPS Bearer should be established to support this session.

What is Default Bearer and Dedicated Bearer?
Default Bearer: 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.
Dedicated Bearer: Any additional EPS bearer that is established for the same PDN connection is referred to as a dedicated bearer.

Are default bearers created on a per UE or a per PDN basis?
Default bearers are created on a per PDN basis. So if a UE is connecting to two PDNs it will need to establish two default bearers. The first default bearer will be established during the Attach process when the UE first powers up and the second will be done using Activate default EPS bearer context request procedure. When the second default bearer is established it will be dependent on the service and the UE.