wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

19-10 Industrial Communication Systems
Forwarded packets
Priority queuing system
High?
Tail-drop
Queue 1
Medium?
Normal?
Tail-drop
Tail-drop
Queue 2
Queue 3
Pre-emptive
scheduler
Hardware queuing
system
Hardware Q
Interface
Low?
Tail-drop
Queue 4
FIGURE 19.4
Priority queuing system.
PQ uses to be statically configured and implemented through the use of the expedite queue. One
major drawback is that low queues may never be sampled as long as higher priority traffic is being processed,
and this results in queue non-use.
Figure 19.5 shows the custom queuing system, which reserves a percentage of the available bandwidth
of an interface for each selected traffic type. If a particular type of traffic is not using the bandwidth
reserved for it, other traffic types may use the remaining reserved bandwidth.
Custom queuing must be configured statically and does not provide automatic adaptation for changing
network conditions.
The IP RTP Priority model provides a strict PQ scheme in which delay-sensitive data, such as voice,
can be dequeued and sent before packets in other queues. It is used typically on serial interfaces in
conjunction with CBWFQ on the same outgoing interface. Traffic matching the range of UDP ports
specified for the priority queue is guaranteed strict priority over other CBWFQ classes: packets in the
priority queues are always serviced first.
The weighted fair queuing (WFQ) approach classifies traffic into different flows based on such characteristics
as source and destination address, protocol and port and socket of the session, and is the default
Forwarded packets
Custom queuing system
Class 1
Tail-drop
Queue 1
Class 2
Tail-drop
Queue 2
Round
Robin
scheduler
Hardware queuing
system
Hardware Q
Interface
Class 16
Tail-drop
Queue 16
FIGURE 19.5
Custom queuing system.
© 2011 by Taylor and Francis Group, LLC