Nortel Networks NB5PLUS4/W - Manual (Page 46)

NB5Plus4/W User Guide YML754Rev1
4 www.netcomm.com.au
The Medium and Low priority queue lengths will be proportionally calculated via the
queue weights selected in 1.) Queue Priorities above.
Total queue length for all three queues will sum to the transmit queue length set in
the system.
Packets overflowing their queues will be tail-dropped, penalizing stochastically the
greediest flow within each queue.
Future implementations may introduce a “buffer stealing” policy. This policy will re-
move the fixed buffer limits and allow a particular queue buffer to decrease to some
predefined minimum limit.
De-queuing Policy
Expedited Forwarding Queue (High Priority) is always serviced first at each packet
scheduling cycle and serviced to extinction. Therefore, the EF queue is non-preemti-
ble by the Medium and Low priority queues.
WRR Queue Scheduler for Medium and Low priority queues
The L and M weights will be configured from the Web UI as stated above in 1.)
Queue Priorities.
A service scheduling array will be pre-computed for the Medium and Low priority
queues based on the user configurable weights assigned to these queues. Each ar-
ray slot corresponds to a scheduling cycle. The pre-computed algorithm will allo-
cate scheduling slots for each queue based on the Medium and Low priority queue
weights and uniformly interleave them through the scheduling array. This array will
provide an O(1) scheduler with a minimum possible average latency for each of the
two queues.
Configuration: