Quality of Service (QoS) in Industrial Networks — Prioritizing Control Traffic for Deterministic Performance

Introduction

Programmable Logic Controllers (PLCs), drives, and sensors must exchange control data within precise time intervals. Even small variations in latency can cause jitter, disrupt synchronization, or trigger system faults.

However, as modern plants integrate more devices—vision systems, data loggers, HMIs, and cloud gateways—the shared Ethernet network becomes increasingly congested. Not all traffic is equally time-critical, and without prioritization, essential control packets may compete with less urgent data.

That’s where Quality of Service (QoS) comes in. Managed and Lite-Managed industrial switches use QoS to classify and prioritize network traffic, ensuring that mission-critical control messages always get top priority.

Understanding QoS in Industrial Context

QoS works by assigning priority levels to Ethernet frames or IP packets based on certain criteria, such as:

• IEEE 802.1p Class of Service (CoS): Layer 2 tagging using VLAN headers.
• DiffServ (DSCP): Layer 3 tagging for IP-based prioritization.
• Port-based or protocol-based rules: Priorities assigned to specific switch ports or known industrial protocols.

The switch uses these tags to determine how packets are queued and transmitted when the network is under load. High-priority traffic is forwarded first, reducing latency and jitter for critical control loops.

Industrial Example: EtherNet/IP Control vs. HMI Data

Consider a packaging machine network with the following devices:

• A CompactLogix PLC controlling I/O modules and servo drives via EtherNet/IP.
• An HMI panel providing operator control and diagnostics.
• A vision system streaming images to a quality control station.
• A gateway uploading production data to a plant historian.

If all these devices share the same Ethernet backbone, their packets compete for bandwidth.

Without QoS, a burst of video traffic from the vision system could delay control packets between the PLC and drives, causing momentary lag or synchronization errors.

With QoS enabled:

• EtherNet/IP I/O messaging (real-time control data) is assigned highest priority.
• HMI and SCADA data are given medium priority.
• File transfers, camera streams, or remote logging are set to low priority.

The switch’s egress scheduler ensures the PLC’s control messages are always transmitted first, maintaining deterministic behavior even during congestion.

How Managed Switches Implement QoS

Most industrial switches allow engineers to configure QoS using:

1. Priority Queues: Each port typically has 4–8 hardware queues corresponding to different priority levels.
2. Mapping Rules: You can map CoS or DSCP values to specific queues.
3. Scheduling Algorithms:
   o Strict Priority (SP): Always sends high-priority traffic first.
   o Weighted Round Robin (WRR): Allocates bandwidth proportionally among queues to prevent starvation

Example configuration:
• CoS 6–7 → Queue 4 (real-time control)
• CoS 4–5 → Queue 3 (HMI)
• CoS 2–3 → Queue 2 (SCADA)
• CoS 0–1 → Queue 1 (background traffic)

This mapping ensures predictable latency for time-sensitive communication.

Best Practices for Industrial QoS

1. Identify critical traffic first. Determine which protocols (e.g., EtherNet/IP, PROFINET, Modbus TCP) require the lowest latency.
2. Standardize QoS markings. Configure PLCs, switches, and routers consistently across the network.
3. Monitor and verify. Use packet captures or switch statistics to confirm QoS tags are being applied and respected end-to-end.
4. Avoid over-prioritization. If every packet is marked “high priority,” QoS loses its purpose. Be selective.
5. Integrate with VLANs. Pair QoS with VLAN segmentation for greater isolation between control and non-control traffic.

Real-World Impact

On a bottling line or a robotic assembly cell, QoS can prevent microsecond-level delays that cause jitter or synchronization loss. It also helps ensure smooth operation when network bandwidth fluctuates—for example, during remote firmware updates or when multiple machines report to a MES simultaneously.

By implementing QoS, engineers can confidently converge IT and OT traffic on the same physical network without compromising deterministic control.

Why QoS Matters for Industry 4.0

As industrial networks evolve toward converged Ethernet architectures supporting real-time control and IIoT analytics, traffic prioritization becomes even more important. QoS ensures that:

• Control systems remain deterministic even when data and video traffic increase.
• Latency-sensitive applications like motion control and time synchronization perform reliably.
• Non-critical systems such as remote monitoring and cloud uploads coexist without interference.

QoS is not just a “nice-to-have” feature—it’s a foundational element of network design for Industry 4.0-ready automation systems.

Learn more with Maple Systems

Whether building complex industrial networks or setting up your first network switch, Maple Systems has an expansive library of technical and educational resources to support your success. Explore our support pages for tutorials, technical notes, and sample projects on a variety of topics. 

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