Priority Code Point

Description

The Priority Code Point (PCP) is a 3-bit field within the VLAN tag (IEEE 802.1Q) of an Ethernet frame header. It is used to indicate the frame priority level, enabling Quality of Service (QoS) differentiation at Layer 2. The PCP value maps to different priority levels, often corresponding to traffic classes such as Background, Best Effort, Excellent Effort, Critical Applications, Video, Voice, and Network Control. Ethernet switches use this PCP value to make forwarding decisions, including queue selection and packet drop precedence, which is crucial for managing congestion and ensuring low latency for priority traffic.

In the context of 3GPP 5G systems, the PCP gains specific importance on the N6 interface, which connects the User Plane Function (UPF) to a Data Network (DN). The 5G core network manages QoS through QoS Flows, each with a 5QI (5G QoS Identifier) that defines characteristics like priority, packet delay budget, and error rate. When user plane packets are sent over the N6 interface, which is often Ethernet-based, there needs to be a mapping between the 5G QoS Flow parameters and the Layer 2 QoS markings used on the transport network. The PCP field is one mechanism to carry this priority information.

The UPF is responsible for marking the PCP field in the outgoing Ethernet frames based on the QoS policy associated with the packet's QoS Flow. This ensures that the transport network between the UPF and the DN (e.g., an internet gateway or an enterprise network) respects the priority levels established by the 5G core. This alignment is vital for end-to-end QoS, preventing the transport segment from becoming a bottleneck that degrades the service quality guaranteed by the 5G radio and core network.

Purpose & Motivation

The PCP exists to provide a standardized, link-layer mechanism for traffic prioritization in Ethernet networks, as defined by IEEE 802.1Q. Its purpose is to enable switches to differentiate traffic and apply appropriate queuing and scheduling behaviors without needing deep packet inspection. This is fundamental for building converged networks that carry a mix of latency-sensitive (e.g., voice, gaming) and bulk traffic.

In 5G, the explicit motivation for referencing PCP in specifications like TS 29.244 (UPF protocol) is to ensure seamless QoS integration between the 5G core and external networks. 5G introduces sophisticated, flow-based QoS models. Simply relying on IP-layer DSCP markings might not be sufficient if the underlying transport is managed at Layer 2. The PCP provides a direct way to convey the 5G QoS Flow priority onto the Ethernet fabric, solving the problem of QoS transparency across administrative and technological domains. It addresses the limitation of having a QoS gap between the 5G system's granular control and the potentially simpler priority mechanisms of the attached data network transport.