Queueing
The egress treatment is the same for customer-role and network-role ports.
Every port has four associated egress queues. The queues 1 and 2 are to be used for
delay-insensitive traffic (for instance file transfer); the queues 3 and 4 are to be used
for delay-sensitive traffic (for instance voice or video).
Please refer to “Traffic class to queue assignment” (p. 8-90) for the assignment of the
traffic classes to the egress queues.
Repeater mode
In the repeater mode, there is no queuing process as described above. All frames go
through the same queue.
Scheduler
The preceding functional blocks assure that all packets are mapped into one of the
egress queues, and that no further packets need to be dropped.
The scheduler determines the order, in which packets from the four queues are
forwarded. The scheduler on each of the four queues can be in one of two operational
modes, strict priority or weighted bandwidth. Any combination of queues in either of
the two modes is allowed. When exactly one queue is in weighted bandwidth mode, it
is interpreted as a strict priority queue with the lowest priority.
Provided that Quality of Service - Classification, Queueing, and Scheduling (QoS CQS,
cf. “Quality of Service configuration options” (p. 8-82)) is enabled, the queue
scheduling method can be configured as follows:
Queue scheduling method
Strict priority
The packets in strict priority queues are forwarded strictly according to the queue
ranking. The queue with the highest ranking will be served first. A queue with a
certain ranking will only be served when the queues with a higher ranking are
empty.
The strict priority queues are always served before the weighted bandwidth
queues.
Weighted
bandwidth
The weights of the weighted bandwidth queues will be summed up; each queue
gets a portion relative to its weight divided by this summed weight, the so-called
normalized weight. The packets in the weighted bandwidth queues are handled in
a Round-Robin order according to their normalized weight.
Each of the two modes has his well-known advantages and drawbacks. Strict priority
queues will always be served before weighted bandwidth queues. So with strict
priority, starvation of the lower priority queues cannot be excluded. Starvation should
be avoided by assuring that upstream policing is configured such that the queue is only
allowed to occupy some fraction of the output link’s capacity. This can be done by
setting the strict policing rate control mode for the flows that map into this queue, and
Traffic provisioning
Classification, queueing, and scheduling
....................................................................................................................................................................................................................................
365-312-807R7.2
Issue 4, May 2007
Alcatel-Lucent - Proprietary
See notice on first page
8-91
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