• EPS is a connection-oriented transmission network and, as such, it requires the establishment of a “virtual” connection between two endpoints (e.g. a UE and a PDN-GW)
  • This virtual connection is called an “EPS Bearer”
  • It provides a “bearer service”, i.e. a transport service with specific QoS attributes.
  • The QoS parameters associated to the bearer are: QCI, ARP, GBR and AMBR.
A data radio bearer transports the packets of an EPS bearer between a UE and an eNB. 
When a data radio bearer exists, there is a one-to-one mapping between this data radio bearer and the EPS bearer/E-RAB. 
An S1 bearer transports the packets of an E-RAB between an eNodeB and a Serving GW. 
An S5/S8 bearer transports the packets of an EPS bearer between a Serving GW and a PDN GW. 
The QoS parameters associated to the bearer are: QCI, ARP, GBR and AMBR.
  • A low priority signaling (message) bearer (SRB1)
  • A high priority signaling (message) bearer (SRB2)
  • A data radio bearer (DRB), i.e. a bearer for IP packets

Part of the bearer establishment procedure are authentication and activation of encryption. The required data for this process is retrieved by the base station (the eNodeB or eNB in 3GPP talk) from the Access Gateway (aGW), or more precisely from the Mobility Management Entity (MME). The MME also delivers all necessary information that is required to configure the data radio bearer, like for example min/max bandwidth, quality of service, etc.