IP Connectivity Access Network Bearer

Description

An IP-CAN bearer is a fundamental concept in 3GPP policy and charging control (PCC) architecture, defined in specifications such as TS 23.203, TS 29.212, TS 29.214, and TS 32.251. It represents an IP transmission path within an IP Connectivity Access Network (IP-CAN) that has defined capacity, delay, bit error rate, and other QoS parameters. The term 'bearer' itself is defined in 3GPP TR 21.905 as a transmission path with specific characteristics.

In the context of EPS (Evolved Packet System), an IP-CAN bearer is typically an EPS bearer, which can be a default bearer or a dedicated bearer. The default bearer is established when the UE attaches to the network and provides always-on IP connectivity. Dedicated bearers are established on demand to provide additional QoS treatment for specific services, such as VoIP or video streaming. Each bearer is associated with a set of QoS parameters, including QCI (QoS Class Identifier), GBR (Guaranteed Bit Rate), MBR (Maximum Bit Rate), and ARP (Allocation and Retention Priority).

The IP-CAN bearer is the unit of QoS enforcement in the network. The PCRF (Policy and Charging Rules Function) or PCF (Policy Control Function) makes policy decisions and installs PCC rules in the PCEF (Policy and Charging Enforcement Function) or SMF (Session Management Function). These rules are mapped to IP-CAN bearers, which are then established or modified by the access network (e.g., eNodeB, gNodeB) and the core network (e.g., SGW, PGW, UPF). The bearer's QoS parameters are enforced at various nodes along the path, ensuring that the service receives the appropriate treatment.

From a charging perspective, the IP-CAN bearer is used to correlate charging data. The charging system can generate records per bearer, allowing operators to charge differently based on the QoS and service associated with the bearer. For example, a dedicated bearer for IMS voice may be charged differently from a default bearer used for best-effort internet access.

The IP-CAN bearer concept is access-specific. In 3GPP accesses like LTE and 5G NR, the bearer is implemented as an EPS bearer or a QoS flow, respectively. In non-3GPP accesses (e.g., WLAN, fixed broadband), the bearer may be implemented differently, but the PCC architecture still uses the IP-CAN bearer abstraction to apply policies and charging. This abstraction allows the core network to manage QoS and charging uniformly across heterogeneous access networks.

Overall, the IP-CAN bearer is a crucial element in the 3GPP QoS and policy framework, enabling differentiated treatment of IP flows and providing the foundation for service-level agreements and charging.

Purpose & Motivation

The IP-CAN bearer concept was introduced to provide a standardized way to manage QoS and charging for IP-based services across different access networks. Before its introduction, QoS was often managed at the access level with limited coordination with the core network, making it difficult to offer end-to-end QoS guarantees and to apply consistent charging policies. The IP-CAN bearer abstraction allows the core network to request and enforce QoS on a per-bearer basis, independent of the underlying access technology.

The primary motivation was to enable the deployment of services that require specific QoS, such as VoIP, video conferencing, and interactive gaming, over packet-switched networks. By defining bearers with specific QoS parameters, operators can ensure that these services receive the necessary network resources and are not degraded by best-effort traffic. This is essential for meeting user expectations and for complying with regulatory requirements for emergency services.

Furthermore, the IP-CAN bearer supports flexible charging models. Operators can charge based on the QoS of the bearer, the volume of data, or the duration of the session. This granularity enables innovative pricing plans and helps operators monetize their investments in network infrastructure. The bearer concept also facilitates roaming, as QoS and charging policies can be applied consistently even when the user is visiting a foreign network.