IP connection between a UE and a PDN is called PDN connection or EPS session. Each PDN connection (or EPS session) is represented by an IP address of the UE and a PDN ID (in other words, Access Point Name (APN)). It has more than one EPS bearer to deliver user traffic (IP packets), and applies the service quality (QoS) policy obtained from a PCRF to the EPS bearers. The minimum fundamental bearer that an EPS session has for a PDN is called a default EPS bearer.
Having an EPS session established means i) a PDN through which a user is to use services has been selected (by the user’s input or based on the subscription information provisioned by an HSS), ii) an IP address to be used in the PDN has been assigned to the user, iii) policy rules to be applied to the user IP packets (QoS and charging rules) have been selected, and iv) a default EPS bearer for delivering IP packets over the LTE network has been established. Through this EPS session established, IP packets can be exchanged between the user and the PDN according to the rules set by the operator.
Management and operation of sessions, including PCRF, will be explained in other document, and a PDN ID (APN) will be discussed as an ID relating to the EPS session in this document.
An EPS session is in charge of delivering and handling flows of the IP packets that are labeled with UE IP addresses and travel between a UE and a PDN (UE – P-GW – PDN). On the other hand, an EPS bearer is a pipe through which IP packets are delivered over the LTE network, i.e., between a UE and a P-GW (UE – eNB – S-GW - P-GW). A UE can have multiple EPS bearers concurrently. So, different EPS bearers are identified by their EPS bearer ID, which is allocated by an MME.
As seen in Figure 1, an EPS bearer actually is a concatenation of the following three bearers (DRB, S1 bearer and S5 bearer):
[UE] - [eNB]: Data Radio Bearer (DRB)
EPS bearer established over LTE-Uu interface. User traffic (IP packet) is delivered through a DRB. Different DRBs are identified by their DRB ID, which is allocated by an eNB.
[eNB] - [S-GW]: S1 bearer
EPS bearer established over S1-U interface. User traffic is delivered through a GTP tunnel. Different S1 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the endpoints (eNB and S-GW) of the GTP tunnel.
[S-GW] - [P-GW]: S5 bearer
EPS bearer established over S5 interface. User traffic is delivered through a GTP tunnel. Different S5 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the endpoints (S-GW and P-GW) of the GTP tunnel.
E-RAB is a bearer that has two endpoints of a UE and an S-GW, and consists of a DRB and an S1 bearer. Technically, E-RAB is a concatenation of a DRB and an S1 bearer, and connects from a UE to an S-GW (UE – eNB – S-GW). Different E-RABs are identified by their E-RAB ID, which is allocated by an MME. DRB IDs and E-RAB IDs are mapped with EPS bearer IDs on 1:1 basis.