Bearer Control Mode

Description

Bearer Control Mode (BCM) is a fundamental concept within the 3GPP packet core architecture, specifically defined in the context of the GPRS Tunnelling Protocol (GTP) for the S5/S8 and S4/S11 interfaces, and within the Evolved Packet Core (EPC) for the S5/S8 interfaces using GTPv2. It is a parameter negotiated during the establishment of a Packet Data Protocol (PDP) context in 2G/3G GPRS or a PDN connection in 4G EPS and 5G systems. The BCM value dictates which entity—the User Equipment (UE) or the network (specifically the Packet Data Network Gateway, PGW, in EPS, or the Gateway GPRS Support Node, GGSN, in GPRS)—is authorized to initiate procedures for modifying the characteristics of a bearer, such as its Quality of Service (QoS) parameters. This negotiation occurs in the Create Session Request/Response or Activate PDP Context Request/Response messages.

Architecturally, BCM is a control-plane attribute managed by the core network's policy and charging control (PCC) framework. The PGW/GGSN, acting as the Policy and Charging Enforcement Function (PCEF), receives the authorized QoS from the Policy and Charging Rules Function (PCRF). The BCM setting determines how these authorized QoS changes are enacted towards the UE and the radio access network. When the network is in control (a specific BCM value), the PGW/GGSN can proactively initiate a Bearer Resource Modification procedure to upgrade or downgrade a bearer's QoS based on PCRF decisions, application demands, or network conditions. This is signaled to the UE via the Mobility Management Entity (MME) or Serving GPRS Support Node (SGSN).

In operation, if BCM is set to 'UE-only' mode, the UE must request any QoS changes via a Bearer Resource Modification Request. The network then evaluates this request against its policies. In network-controlled modes, the PGW/GGSN can trigger modifications independently. This centralizes critical QoS and policy decisions within the network operator's domain, ensuring consistent application of business rules, traffic steering policies, and charging rules. It prevents the UE from arbitrarily requesting high-QoS resources, which is crucial for preventing fraud, managing congestion, and ensuring fair resource utilization among all subscribers.

The role of BCM is integral to dynamic policy enforcement and session management. It works in concert with the QoS Class Identifier (QCI), Allocation and Retention Priority (ARP), and other bearer-level parameters. By controlling the initiation point for bearer modification, BCM enables sophisticated network-driven services like sponsored data, on-demand QoS boosting for video streaming, and seamless QoS adaptation during handovers. It is a key enabler for the network's ability to provide differentiated services and implement advanced PCC scenarios defined in 3GPP specifications.

Purpose & Motivation

Bearer Control Mode was created to address the need for network operators to maintain authoritative control over the allocation and modification of bearer resources, which are directly tied to Quality of Service, network capacity, and revenue. In early mobile data systems, allowing the UE uncontrolled ability to request high-QoS bearers posed significant risks, including potential network resource exhaustion, QoS fraud (where a UE illegitimately requests premium QoS), and challenges in applying consistent policy and charging rules. BCM provides a standardized mechanism to dictate where the control point resides, solving these operational and business problems.

Historically, as services evolved from simple best-effort internet access to rich multimedia and latency-sensitive applications, the requirement for dynamic, session-aware QoS management became critical. The introduction of the PCC architecture in 3GPP Release 7 formalized the separation of policy control from bearer management. BCM is a logical component of this architecture, ensuring that the entity enforcing policy (the PCEF in the PGW/GGSN) also has the procedural authority to modify bearers to align with those policies. This was a significant advancement over static, pre-configured bearer QoS.

The creation of BCM was motivated by the limitation of earlier, more static bearer management approaches. It enables the network to be proactive and intelligent. For example, an IMS voice call requires a dedicated bearer with a guaranteed bit rate. With network-controlled BCM, the PCRF can instruct the PCEF to establish this bearer automatically when the call is set up, without relying on a UE request, ensuring faster and more reliable service establishment. It also allows the network to downgrade QoS during congestion based on subscriber tier or service type, optimizing the experience for all users.

Key Features

• Defines the authorized entity (UE or Network) for initiating bearer QoS modification
• Centralizes policy enforcement by enabling network-driven bearer updates
• Prevents unauthorized QoS requests from the User Equipment
• Enables dynamic service adaptation based on PCRF decisions and application function triggers
• Works in conjunction with the Policy and Charging Control (PCC) architecture
• Parameter negotiated during PDP Context Activation or PDN Connection Establishment

Evolution Across Releases

Defining Specifications