STRIDE SE-MC2U-SC, Руководство пользователя

Страница: 113 / 230

Traffic Priority (Priority Queuing QoS, CoS, ToS/DS)
Without enabling special handling, a network provides a “best effort” service to all
applications. This means that there are no assurances regarding the Quality of Service (QoS)
for any particular application because all packets are treated equally at each switch or router.
However, certain applications require deterministic response from the network to assure
proper operation.
Consider a drilling machine in a plant that is controlled by a computer on a local network.
The depth of the machine’s drill is critical; such that if the hole is drilled is too deep, the
material will have to be thrown out. Under normal conditions, the drill process is running
smoothly (controller and computer are communicating efficiently over the network) but
when another user on the network accesses records from an online database, the large volume
of traffic can interfere with timely communication with the drill. A delay in communications
between the drill and controller causes the drill to go too far and the material has to be
thrown away. To prevent this from happening, we need to provide a certain QoS for all drill-
controller communications so delay is avoided.
Numerous mechanisms exist to help assure reliable and timely network communication. The
managed switch supports two common means of prioritizing messages: IP header and 802.1p
user priorities.
The IP header is present in all frames and contains a priority field, which defaults to 0 and
may be set as high as 255. This field is sometimes referred to as the Type of Service (ToS)
field, or the Differentiated Services (DS or DiffServ) field.
Applications may add IEEE 802.1p tags, which contain a priority field that may be set from
0 to 7. Each value has a traffic type associated with it. For example, a tag of 5 is prescribed for
video data.
The switch provides four priority queues for expediting outbound data. The 256 IP priorities
and the 7 IEEE priorities are mapped into these ports in a way that optimizes throughput of
high priority data.
Scheduling:
When choosing how to handle lower priority data, the switch can use strict or fair scheduling.
This choice affects all queues on all ports.
Send All Priority Frames before any others: With strict scheduling, all data in the highest
priority queue will be sent before any lower priority data, then all data from the second
highest priority, and so on. This assures that high-priority data always gets through as quickly
as possible.
Allow Lower Priority Frames through, a few at a time:
With fair scheduling, a round-robin
algorithm is used, weighted so that more high-priority than low priority data gets through.
Specifically, the switch will send eight frames from the urgent queue, then four from the
expedited queue, two from the normal queue, and one from the background queue, then start
over with the urgent queue. This assures that the lower priority queues will not be starved.
4-29
Chapter 4 - Managed Switch Software Setup
Stride Industrial Ethernet Switches User Manual 2nd Ed. Rev. A