Session & Bearer

From netmanias.

In my understanding,
- EPS session = PDN connection
- An EPS session can have single or multiple EPS bearers
- EPS bearer is transmission path between UE and P-GW to deliver user traffic (IP flows) with specficied QoS
- An EPS bearer can have single or multiple IP flows




1 EPS Session

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.

2.EPS Bearer
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.
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 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.

3. Dedicated Bearer

 Dedicated EPS bearers are linked to a default EPS bearer. The linked bearers are represented by a Linked EPS Bearer Identity (LBI), indicating they are all associated with the same default EPS bearer.  Since both of the bearers (the default and the dedicated EPS bearers) belong to the same PDN for the same user, an ID to indicate the two are intended for the same PDN is needed. For such purpose, an ID called LBI is used, and the default EPS bearer ID is used as the LBI.



Figure 1 shows the EPS sessions and EPS bearers of a user, with their IDs shown underneath.

Figure 1. Overview of Session/Bearer IDs

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