Quality of Service Guide ATM QoS Traffic Descriptor Profiles
Edition: 01 3HE 11014 AAAC TQZZA 383
8.1.1.5 ATM Queuing and Scheduling
The 7705 SAR provides a per-VC queuing architecture on the 16-port T1/E1 ASAP
Adapter card, 32-port T1/E1 ASAP Adapter card, and 2-port OC3/STM1
Channelized Adapter card with atm/ima encapsulation. The 7705 SAR provides a
per-VC queuing architecture on the 4-port OC3/STM1 Clear Channel Adapter card
and 4-port DS3/E3 Adapter card with atm encapsulation. In the egress direction
toward the ATM port, the scheduling priority at the ATM layer is as follows.
• CBR VCs are always shaped and are scheduled with strict priority over all other
service categories.
• rt-VBR VCs are always shaped and are scheduled next with strict priority over
nrt-VBR and UBR VCs.
• nrt-VBR shaped VCs are scheduled next with strict priority over nrt-VBR
unshaped VCs and UBR VCs.
• nrt-VBR unshaped VCs and UBR VCs are scheduled as a common class.
Scheduling among these VCs is done using a Weighted Round Robin (WRR)
scheduler, where the weight of each VC is determined by the configured SIR for
nrt-VBR and by the MIR for UBR VCs. The scheduling is work-conserving, so
each VC has access to excess bandwidth in proportion to its SIR/MIR. Under
congestion, the performance of each VC degrades proportionally to the weight
of the VC.
8.1.1.6 Congestion Avoidance
Congestion and potential discards are performed on a per-forwarding class basis in
the SAP queues in the CSM.
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