What Is Quality of Service? QoS Settings in Robotics Networks.

What Is Quality of Service?

Quality of Service, or QoS, refers to mechanisms and settings used to manage how network or software communications handle requirements such as reliability, delay, priority, and message delivery.

In robotics, different types of information have different communication needs.

For example, a robot's camera may generate a continuous stream of images, while a configuration application may occasionally send an important setting.

The appropriate communication behavior can differ between these applications.

Key Takeaways

• QoS mechanisms help manage communication characteristics such as reliability, timing, and priority.
• ROS 2 and DDS provide configurable QoS policies for different messaging requirements.
• QoS does not automatically guarantee network performance or qualify communication as safety-rated.

How Does Quality of Service Work?

Imagine a mobile robot sending camera images to its perception software.

The camera produces new frames continuously.

If one frame is lost, the application may prefer to process the next available image rather than wait for retransmission.

Now consider a software component receiving a new configuration parameter.

Losing that information could leave the component using the wrong setting.

The application may therefore require more dependable delivery.

QoS settings allow software developers to specify different communication behavior for these situations.

Reliable vs. Best-Effort Communication

Two common reliability options in ROS 2 and DDS are:

Reliable: The middleware uses mechanisms intended to deliver messages despite some communication losses.

Best effort: The middleware attempts delivery but may discard messages when communication conditions prevent successful transmission.

For example, a high-frequency sensor stream may use best effort when timely new information is more useful than delayed old measurements.

A configuration exchange may use reliable communication.

However, reliable delivery is not the same as a guarantee that a robot completed a commanded physical action.

What Other QoS Settings Matter?

Several policies can influence message handling.

History: Determines how previous message samples are retained.

Depth: Specifies how many samples are kept under certain history policies.

Durability: Influences whether information may remain available to subscribers that connect after publication.

Deadline: Defines timing expectations for receiving information.

Lifespan: Determines how long messages remain valid.

Different middleware implementations provide different supported policies and combinations.

QoS vs. Network Bandwidth

Bandwidth describes data-transfer capacity.

QoS describes mechanisms for managing communication behavior and requirements.

For example, a network may have enough bandwidth for several camera streams.

QoS settings can still influence how the communication system treats lost or delayed messages.

However, QoS cannot create unlimited bandwidth or eliminate physical network constraints.

Why Is QoS Important for ROS 2?

ROS 2 applications exchange information through topics, services, and actions.

Different topics can have different delivery requirements.

For example, LiDAR measurements may use settings appropriate for continuous sensor data.

A system configuration topic may require different reliability or durability behavior.

Publishers and subscribers must also use compatible QoS configurations to communicate successfully.

An incorrectly configured connection can prevent messages from being received even when both applications are otherwise functioning.

Does QoS Provide Functional Safety?

Not by itself.

Communication reliability features may support dependable operation, but safety-related functions require appropriate system architecture and validation.

A reliable message-delivery policy does not automatically make the communication suitable for a safety-rated stop function.

Frequently Asked Questions

Does QoS guarantee that no messages will be lost?

No. The guarantees and behavior depend on the protocol, configuration, and operating conditions.

Is QoS only used in robotics?

No. QoS mechanisms are also used in telecommunications, enterprise networking, cloud systems, and other applications.

The Bottom Line

Quality of Service allows communication systems to handle different types of information according to their needs. In robotics, suitable QoS settings help sensors, software, and controllers exchange information reliably and efficiently.