Gateway Control Session

Description

The Gateway Control Session is a fundamental concept in the 3GPP Policy and Charging Control (PCC) architecture, specifically defined for scenarios where the Evolved Packet Core (EPC) does not employ GTP (GPRS Tunnelling Protocol) on the S5/S8 interfaces. In such cases, the Bearer Binding and Event Reporting Function (BBERF) resides in the access gateway (e.g., Serving Gateway or a non-3GPP access gateway) and requires a direct control relationship with the Policy and Charging Rules Function (PCRF). The Gateway Control Session establishes this relationship, enabling the PCRF to provision QoS rules and access-specific parameters to the BBERF, and allowing the BBERF to report events (e.g., bearer establishment, modification, or release) back to the PCRF.

The session is implemented using the Gxx reference point, which is a Diameter-based interface between the PCRF and the BBERF. The Gxx procedures include session establishment, modification, and termination. During session establishment, the BBERF provides the PCRF with information such as the UE identity, IP address, and access network type. The PCRF then installs QoS rules, which include packet filters, QoS parameters (e.g., QCI, ARP), and charging information. The BBERF enforces these rules in the user plane and reports events that may trigger policy decisions, such as bearer creation or modification.

The Gateway Control Session is distinct from the Gateway Control Session (GCS) used in the GPRS Tunnelling Protocol (GTP) scenario, where the PCRF communicates with the P-GW via the Gx interface. In the non-GTP scenario, the GCS ensures that the BBERF, which may be located in a non-3GPP access network (e.g., WLAN, WiMAX), can receive and enforce policy rules without relying on GTP signalling. This is particularly important for interworking with non-3GPP access technologies, where GTP may not be supported or desirable.

The session is maintained throughout the lifetime of the IP-CAN session. When the IP-CAN session is modified (e.g., due to mobility or changes in access network), the Gateway Control Session may be updated or re-established. The PCRF can also initiate session modifications to update QoS rules or to request specific event reports from the BBERF. The BBERF, upon detecting events, sends event reports to the PCRF, which may then make new policy decisions and update the rules.

In summary, the Gateway Control Session is a key enabler for policy control in non-GTP EPC deployments, providing a standardized mechanism for the PCRF to control the BBERF and for the BBERF to report events, ensuring consistent QoS and charging enforcement across heterogeneous access networks.

Purpose & Motivation

The Gateway Control Session was introduced to address the need for policy and charging control in EPC deployments where GTP is not used on the S5/S8 interfaces. In traditional GPRS and early EPC networks, the PCRF communicated with the P-GW via the Gx interface, and the P-GW enforced policy rules. However, with the advent of non-3GPP access technologies and the desire for more flexible architectures, the BBERF was introduced to handle bearer binding and event reporting in the access gateway. The Gateway Control Session provides a direct control link between the PCRF and BBERF, enabling the PCRF to provision QoS rules and receive event reports without relying on GTP signalling.

This separation of policy control from GTP signalling allows operators to deploy non-3GPP access networks (e.g., WLAN, WiMAX) that do not support GTP, while still maintaining consistent policy enforcement. It also enables more granular control over access-specific parameters, such as those related to the radio access technology, which may not be visible to the P-GW. The Gateway Control Session thus solves the problem of policy control in heterogeneous access environments, where different access technologies may have different capabilities and requirements.

Furthermore, the Gateway Control Session supports the evolution towards a more distributed and flexible core network, where policy enforcement points can be located closer to the access network. This aligns with the broader trend of network function virtualization (NFV) and software-defined networking (SDN), where control and user plane functions are separated. By providing a standardized interface (Gxx) for PCRF-BBERF communication, the Gateway Control Session ensures interoperability between multi-vendor equipment and facilitates the deployment of advanced services such as VoLTE and mission-critical communications.