Glossary · Industrial communication
Quality of service (QoS)
Also known as: QoS, Traffic prioritization
German: Dienstgüte (QoS)
In networking, quality of service (QoS) is the set of mechanisms that prioritize, schedule or reserve network resources for certain traffic, so that time-critical data meet their latency, jitter and loss requirements. In Ethernet, priorities are commonly carried in the IEEE 802.1Q VLAN tag. In MQTT, QoS denotes the message delivery guarantee level.
- Industrial communication
In one sentence
QoS mechanisms prioritize or reserve network resources so time-critical traffic meets latency, jitter and loss requirements.
Example
Managed switches give PROFINET real-time frames priority 6, so large camera image transfers on the same network don't delay the I/O data.
How it applies
- Engineering: In converged networks where control traffic shares links with video, file transfers or IT traffic, QoS keeps the control traffic predictable. Real-time Ethernet systems define which priorities their frames use; switches must honor them.
- Configuration: QoS only works if all switches on the path are configured consistently. Time-Sensitive Networking (TSN) goes further with scheduled time slots.
- Messaging: In MQTT, QoS 0, 1 and 2 mean at most once, at least once and exactly once delivery between client and broker; this is a different concept from network prioritization.
- Documentation: The documentation team specifies which priorities a product uses and expects, and always states which meaning of QoS is intended, network or messaging.
Network QoS vs. MQTT QoS
Network QoS decides which frames go first. MQTT QoS decides how often a message is delivered and acknowledged. A German text saying "Dienstgüte" may mean either; check the context.