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Understanding LTE Bearers: EPS, E-RAB, S1, S5/S8, and Radio Bearers

lte bearer core network radio access quality service

In LTE, bearers are the essential pathways that manage data traffic between the user equipment (UE) and the core network. They ensure that specific Quality of Service (QoS) requirements are met. Various bearers, such as the EPS bearer, E-RAB, S1 bearer, S5/S8 bearer, and radio bearer, form an end-to-end communication chain, facilitating seamless data transfer across different network segments. Let’s explore the different types of LTE bearers and their roles.

EPS Bearer (Evolved Packet System Bearer)

  • Definition: The EPS bearer is the overall end-to-end service that enables data transfer in the LTE network, from the user equipment (UE) to the Packet Data Network (PDN), such as the internet.
  • Purpose: It ensures that a particular QoS is maintained across the entire LTE network, from the radio interface, through the core network, to the external network.
  • Components: The EPS bearer includes the E-RAB (E-UTRAN Radio Access Bearer), S1 bearer, and S5/S8 bearers.

E-RAB (E-UTRAN Radio Access Bearer)

  • Definition: E-RAB is the bearer between the UE and the Serving Gateway (SGW) that traverses the radio access network (E-UTRAN).
  • Purpose: It manages the user plane data between the eNodeB and the SGW while maintaining QoS characteristics.
  • Relation to EPS: E-RAB is the part of the EPS bearer that is specific to the LTE radio access network (E-UTRAN).

S1 Bearer

  • Definition: The S1 bearer connects the E-UTRAN (eNodeB) and the core network’s Serving Gateway (SGW).
  • Purpose: It carries user plane traffic between the eNodeB and the SGW in the core network.
  • Types:
    • S1-U Bearer: Carries user data (User Plane).
    • S1-MME Bearer: Responsible for control signaling (Control Plane).

S5/S8 Bearer

  • Definition: The S5/S8 bearer connects the Serving Gateway (SGW) to the Packet Data Network Gateway (PGW).
    • S5 Bearer: Used when both the SGW and PGW are in the same network (usually within the same mobile operator).
    • S8 Bearer: Used when the SGW and PGW are in different networks (for example, in international roaming scenarios).
  • Purpose: This bearer carries traffic between the SGW and PGW, allowing connectivity to external packet data networks (like the internet or corporate networks).

LTE Radio Bearer

  • Definition: The LTE radio bearer handles the data transfer over the air interface between the UE and the eNodeB (radio network).
  • Purpose: It ensures that data is transported between the UE and the eNodeB with the necessary QoS. There are different types of radio bearers, such as signaling radio bearers (for control messages) and data radio bearers (for user data).
  • Types:
    • SRB (Signaling Radio Bearer): Used for control plane signaling.
    • DRB (Data Radio Bearer): Used for the actual user data transmission.

External Bearer

  • Definition: The external bearer is the connection between the PDN Gateway (PGW) and the external packet data network (like the internet or private enterprise networks).
  • Purpose: It ensures that data traffic flows between the LTE core network and the external networks (like internet services).

LTE Bearers

LTE system provides end to end service using the hierarchy of LTE bearers shown in the figure.

Bearer Establishment Process

When a user attaches to the LTE network, the following steps occur:

  • Default EPS Bearer Setup: Upon initial attachment, a default EPS bearer is set up, which provides a “best-effort�? service. It usually connects to the internet.
  • Dedicated EPS Bearer Setup: If the user or application requires specific QoS (like for VoLTE), a dedicated EPS bearer is set up to provide this.

Summary of Bearer Relationships

  • EPS bearer: End-to-end between the UE and PDN (internet).
  • E-RAB: Part of the EPS bearer between the UE and SGW, traversing the radio access network.
  • S1 bearer: Between the eNodeB and SGW.
  • S5/S8 bearer: Between the SGW and PGW.
  • Radio bearer: Over the air interface between the UE and eNodeB.

Each bearer serves a specific purpose in enabling seamless and QoS-optimized data transfer across different parts of the LTE network.

Conclusion

Bearers in LTE play a crucial role in maintaining efficient and QoS-driven communication from the UE to external networks. By segmenting data paths across the radio, core, and external networks, LTE bearers ensure reliable and high-performance connectivity for diverse applications.

Reference: 3GPP reference TS 36.300