Existing Egress Port Based Virtual Scheduling - Alcatel-Lucent 7450 Quality Of Service Manual

Existing Egress Port Based Virtual Scheduling

The port based virtual scheduling mechanism takes the native packet based accounting results
from the queue and adds 20 bytes to each packet to derive the queue's frame based offered
load. The ratio between the frame based offered load and the packet based offered load is then
used to determine the effective frame based CIR and frame based PIR thresholds for the
queue. Once the port virtual scheduler computes the amount of bandwidth allowed to the
queue (in a frame based fashion), the bandwidth is converted back to a packet based value
and used as the queue's operational PIR. The queue's native packet based mechanisms
continue to function, but the maximum operational rate is governed by frame based decisions.
Queue Behavior Modifications for Frame Based Accounting
The frame based accounting feature extends this capability to allow the queue CIR and PIR
thresholds to be defined as frame based values as opposed to packet based values. The queue
continues to internally use its packet based mechanisms, but the provisioned frame based CIR
and PIR values are continuously revalued based on the ratio between the calculated frame
based offered load and actual packet based offered load. As a result, the queue's operational
packet based CIR and PIR are accurately modified during each iteration of the virtual
scheduler to represent the provisioned frame based CIR and PIR.
Virtual Scheduler Rate and Queue Rate Parameter Interpretation
Normally, a scheduler policy contains rates that indicate packet based accounting values.
When the children queues associated with the policy are operating in frame based accounting
mode, the parent schedulers must also be governed by frame based rates. Since either port
based or service based virtual scheduling is required for queue frame based operation,
enabling frame based operation is configured at either the scheduling policy or aggregate rate
limit command level. All queues associated with the policy or the aggregate rate limit
command will inherit the frame based accounting setting from the scheduling context.
When frame based accounting is enabled, the queues CIR and PIR settings are automatically
interpreted as frame based values. If a SAP ingress QoS policy is applied with a queue PIR
set to 100Mbps on two different SAPs, one associated with a policy with frame based
accounting enabled and the other without frame based accounting enabled, the 100Mbps rate
will be interpreted differently for each queue. The frame based accounting queue will add 20
bytes to each packet received by the queue and limit the rate based on the extra overhead. The
packet based accounting queue will not add the 20 bytes per packet and thus allow more
packets through per second.
Quality of Service Guide
QoS Scheduler Policies
525