Rockwell Automation Ethernet Design Considerations Reference Manual - Manual (Page 45)

Rockwell Automation Publication ENET-RM002C-EN-P - May 2013 45
Ethernet Infrastructure Features Chapter 3
Quality of Service (QoS) Quality of service determines how packets are marked, classified, and treated
based on traffic type. Rockwell Automation EtherNet/IP devices prioritize traffic
internally. Implementing QoS at the switch level adds another level of
prioritization. QoS does not increase bandwidth—QoS gives preferential
treatment to some network traffic at the expense of others.
Not all network traffic can be treated equally. To minimize application latency
and jitter, control data must have priority within the cell or area zone. QoS gives
preferential treatment to some network traffic at the expense of others. Control
data is more sensitive to latency and jitter than information data.
To explain how QoS works, think about the last time you boarded a plane at the
airport. As boarding time gets close, everyone starts to crowd around the gate. It
is impossible for everyone to go down the jetway to the plane at once, so the
airline establishes a boarding procedure to avoid chaos. This can be compared to
the use of QoS on an Ethernet network. The network can have motion traffic,
voice traffic, and email traffic all being transmitted at the same time over the
network.
In the airline example, first class passengers board first, followed by families with
small children, followed by frequent flyers, and followed by the coach cabin
starting at the back of the plane. Similarly, QoS lets you set up priority queues in
the managed switches on the network. In the automation example, equate
motion traffic to the first class passengers and give it the highest priority for
network usage. Voice traffic can go second (it also has low tolerance for delay),
and email traffic has the lowest priority queue. This results in the least amount of
delay possible on the motion control.
Ingress Actions Egress Actions
Classification Policing/Metering Marking
Queue/Schedule
Congestion Control
Distinguish traffic by
examining Layer 2/3/4
labels and QoS fields.
QoS changed depending
on trust state at port. Make sure conformance
is to a specified rate. DSCP-CoS or
CoS DSCP Mapping 4 queues/or with
priority scheduling