Default EPS Bearer

Description

The default bearer is an EPS bearer that gets established with every new PDN connection. Its context remains established throughout the lifetime of that PDN connection, and it is always a non-GBR (non-Guaranteed Bit Rate) bearer. It provides the UE with always-on IP connectivity to the PDN and serves as the anchor bearer for that PDN connection. Any traffic not matching the packet filters of a dedicated bearer is carried on the default bearer, making it the fundamental connectivity vehicle in the EPS architecture defined by 3GPP TS 23.401.

When a UE attaches to the LTE network or initiates a new PDN connection, the MME selects a P-GW and S-GW and triggers the creation of a default bearer. This involves establishing a radio bearer over the LTE-Uu interface, an S1-U bearer between the eNodeB and S-GW, and an S5/S8 bearer between the S-GW and P-GW. The default bearer is assigned a QCI (typically QCI 9 for best-effort or QCI 8/9 depending on operator policy), an ARP, and a set of QoS parameters. Because it is a non-GBR bearer, it does not reserve bandwidth; instead, it provides best-effort or differentiated best-effort treatment based on the QCI.

The default bearer is established using NAS signaling procedures defined in 3GPP TS 24.301. During the initial Attach procedure, the UE includes a PDN Connectivity Request, and the network responds with an Activate Default EPS Bearer Context Request that contains the EPS bearer identity, the QCI, the APN, the PDN address, and other parameters. The UE then completes the establishment by sending an Activate Default EPS Bearer Context Accept. For additional PDN connections, the UE initiates a separate PDN Connectivity Request procedure, each resulting in a new default bearer with its own IP address and APN.

Each PDN connection has exactly one default bearer, and the default bearer cannot be deleted without deleting the entire PDN connection. Dedicated bearers, by contrast, can be added and removed dynamically within the same PDN connection. The default bearer's EPS bearer identity is used as the reference for the PDN connection, and its TFT is typically empty or contains only a match-all filter, meaning it carries all traffic not explicitly matched by a dedicated bearer's TFT.

The default bearer plays a critical role in mobility and session management. During handovers, the default bearer is always maintained to preserve IP connectivity, while dedicated bearers may be subject to admission control and could be dropped if resources are insufficient in the target cell. The default bearer also carries the signaling traffic for the UE's NAS layer and is used for EPS session management procedures. In 5G, the equivalent concept is the default QoS flow within a PDU session, which similarly provides always-on connectivity and is established when the PDU session is created.

Purpose & Motivation

The default bearer was introduced in Release 8 as part of the EPS architecture to provide always-on IP connectivity to UEs. In earlier GPRS and UMTS networks, PDP contexts had to be explicitly activated for each service, and there was no concept of an always-on connection that persisted for the lifetime of a PDN session. The default bearer solves this by ensuring that whenever a UE connects to a PDN, a bearer is automatically established and maintained, giving the UE continuous IP connectivity without requiring repeated activation procedures.

This design supports the smartphone era's requirement for always-on applications such as push email, instant messaging, and background synchronization. It also simplifies session management by providing a stable anchor for the PDN connection, to which dedicated bearers can be added for specific QoS needs. The default bearer's non-GBR nature ensures that it can always be established without requiring bandwidth reservation, making it universally available even in congested conditions.

By separating the always-on default bearer from service-specific dedicated bearers, the EPS architecture achieves both ubiquitous connectivity and service differentiation. The default bearer guarantees that the UE can always reach the network and that basic IP services work, while dedicated bearers provide enhanced treatment for applications that need it. This model has been carried forward into 5G as the default QoS flow, demonstrating its enduring value.