Quality of Service (QoS)

Introduction

A switch that supports QoS packet tagging can be configured to deliver higher priority messages before messages with a lower (or no) priority. This enhances system determinism and increases the timely delivery of prioritized messages.

In the absence of QoS tagging, the switch delivers various application messages on a first-in first-out basis. This can result in poor system performance caused by the long forwarding delay—and late delivery—of high priority application messages, which may be handled after lower priority messages.

Types of QoS

The tagging types are based on the switch configuration:

Tagging type	Priority mapping rule	Description
Explicit (QoS tag in Ethernet packet)	DSCP or TOS field in IP header	Each IP based Ethernet packet contains a value in the DSCP or TOS field in its IP header, indicating the QoS priority. The switch forwards packets based on this priority.
Explicit (QoS tag in Ethernet packet)	VLAN tag in Ethernet header	Each Ethernet packet contains a value in the priority field in the VLAN tag in its Ethernet header, indicating the QoS priority. The switch forwards packets based on this priority.
Implicit	Port based	Switch ports are mapped to different QoS priorities. For example, switch port 1 is mapped to QoS priority 1, switch port 2 is mapped to QoS priority 2, etc.

Recommendation

Schneider Electric recommends the use of devices—including switches—that support explicit QoS tagging.

NOTE: Some switches that support QoS tagging have this feature disabled by default. Confirm that QoS is enabled when deploying each switch.