QoS and QoS Policies
116
Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
Strict priority scheduling in an exhaustive fashion takes place for the shaped VCs in
the order listed below:
1. CBR (always shaped)
2. rt-VBR (always shaped)
3. nrt-VBR (when shaped, user-configurable for shaped or unshaped)
UBR traffic is not shaped. To offer maximum flexibility to the user, nrt-VBR unshaped
(also known as scheduled) is implemented.
ATM traffic is serviced in priority order. CBR traffic has the highest priority and is
serviced ahead of all other traffic. After all of the CBR traffic has been serviced,
rt-VBR traffic is serviced. Then, nrt-VBR traffic is serviced.
After scheduling all the other traffic from the CBR and VBR service categories, UBR
is serviced. If there is no other traffic, UBR can burst up to the line rate. Scheduled
nrt-VBR is treated the same way as UBR. Both UBR and unshaped nrt-VBR are
scheduled using the weighted round-robin scheduler.
The scheduler weight assigned to queues hosting scheduled nrt-VBR and UBR
traffic is determined by the configured traffic rate. The weight used by the scheduler
for UBR+ VCs is dependent on the Minimum Information Rate (MIR) defined by the
user. UBR with no MIR traffic has an MIR of 0.
Similarly, the scheduler weight is dependent on the Sustained Information Rate (SIR)
for scheduled nrt-VBR. Weight used by the scheduler is programmed automatically
based on the user-configured MIR/SIR value and is not user-configurable.
For UBR+, Table 14 and Table 15 are used to determine the weight of a UBR+ VC.
These tables are also applicable to scheduled nrt-VBR weight determination. Instead
of the MIR, the SIR is used to determine the scheduler weight.
Table 14 Scheduler Weight Values (WRR) based on MIR for the T1/E1
ASAP Adapter Cards and 2-port OC3/STM1 Channelized Adapter
Card
Minimum Information Rate Scheduler Weight
<64 kb/s 1
<128 kb/s 2
<256 kb/s 3
<512 kb/s 4
<1024 kb/s 5
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