Specifications

Optimizing Performance
EIO0000000051 6/2010 83
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 important application messages, which
may be handled after less important messages.
Types of QoS
The tagging types are based on the switch configuration:
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. Be sure to enable QoS when deploying each switch.
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.
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.