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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (37 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the PCP function was enhanced to support priority services, introducing the capability for a UE to be configured for high priority access using access identities 1, 2, or 11-15. This allowed the network, specifically the AMF and SMF, to be aware of such UE configurations and apply exceptions in handling specific timers like T3396, T3584, and T3585. Furthermore, mechanisms were defined for the network to signal the validity of these access identities to the UE via the MPS or MCS indicator bits in system information.
- UE configuration for NAS signalling low priority via OMA-DM or USIM not applicable in 5GS TS 24.501CR0084
- Support of Priority Services TS 29.514CR0023
- AMF Region ID (8 bits), AMF Set ID (10 bits), and AMF Pointer (6 bits) TS 24.501CR0030
- SMF knowledge that a UE is configured for high priority access TS 24.501CR0032
- AMF pointer pointing one or more AMFs TS 24.501CR0305
- Exceptions for UEs configured for high priority access in handling T3396, T3584, and T3585 TS 24.501CR0677
+ 2 more changes
In Release 16, the PCP function was enhanced with a new SBI Message Priority mechanism specifically for emergency sessions, and clarifications were provided for the T3525 timer behavior when a UE is configured for high priority access. Furthermore, the release included corrections and refinements to the description and encoding of specific priority code points, such as for "128-5G-EA3," and removed a code-point from the Control Plane Service Type.
- Priority Sharing Indication TS 29.514CR0126
- Correction on the description of code point for 128-5G-EA3 TS 24.501CR1221
- Removal of a Code-Point in Control Plane Service Type TS 24.501CR1712
- T3525 clarification for UE configured with high priority access TS 24.501CR2561
- SBI Message Priority mechanism for emergency session TS 29.514CR0263
- Message Priority encoding TS 29.244CR0492
In Release 17, enhancements for the Priority Code Point (PCP) function clarified and aligned Network Slice Admission Control (NSAC) exceptions for emergency and priority services, ensuring these high-priority sessions are not blocked. The release also introduced procedures for high-priority access before Network Slice-Specific Authentication and Authorization (NSSAA) is completed and defined mechanisms for a UE configured for high priority access in a selected PLMN. Furthermore, it specified the handling of high-priority PLMN searches due to Steering of Roaming (SOR) and defined priority values associated with Network Slice Admission Control Groups (NSAGs).
- The exception in Network Slice Admission Control for Emergency and Priority Services TS 24.501CR3415
- High priority access before pass the NSSAA TS 24.501CR2513
- Clarification on NSAC for emergency and priority services TS 24.501CR3876
- High Priority PLMN search due to SOR TS 24.501CR3904
- Alignment for NSAC for emergency and priority services TS 24.501CR4144
- NSAG priority TS 24.501CR4484
+ 3 more changes
In Release 18, the PCP function was enhanced with new clarifications and corrections for handling high priority access in Standalone Non-Public Networks (SNPN). The release provided specific corrections and handling rules for the NSAG (Network Slice Admission Control) priority field and its processing at the AMF. It also included clarifications on the usage of the N30 and N5 reference points in the context of these priority mechanisms.
In Release 19, the PCP (Priority Code Point) function was enhanced to explicitly support Multimedia Priority Service (MPS) for messaging, including priority handling for Mobile-Terminated SMS over NAS and initial paging with priority in the 5G System. These changes introduced priority mechanisms for messaging services across interfaces like the N5 reference point and clarified procedures for UEs configured for high-priority access in selected PLMNs or SNPNs. Additionally, corrections were made to the mapping tables for 802.1Q tag PCP/DEI type values.
- MPS for Messaging Paging Priority TS 24.501CR6570
- Initial paging with priority in 5GS TS 24.501CR6948
- Support of MPS for messaging in N5 reference point TS 29.514CR0682
- Missing NOTE for T3540 for a UE with high priority access in selected PLMN or SNPN TS 24.501CR6546
- Missing NOTE for T3540 for a UE with high priority access in selected PLMN or SNPN TS 24.501CR6543
- MT SMS over NAS with priority for messaging and minor fixes in 24.501 TS 24.501CR6667
+ 3 more changes
Explore further
Broader topics and technologies where PCP plays a role.
Defining Specifications
3GPP specifications that define or reference PCP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.514 vj40 | 5G System; Policy Authorization Service; Stage 3 | Rel-19 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |