User's Manual

DragonWave Inc.

Horizon Compact Plus Release 1.0.1 Wireless Ethernet Product User Manual – Volume 2

8.4.3 Committed Information Rate (CIR)

The Horizon Compact Plus system allows the user to assign a percentage of the maximum bandwidth

available to the data in each of the eight QoS queues (set cos queue cir [0-100 0-100 0-100 1-100 0-

100 0-100 0-100 1-100]). The default is 100% for each queue.

The behavior of the CIR is different for the two scheduling policies (Priority Queuing or Weighted Fair

Queuing - see Section 8.5). For the priority queuing policy the CIR for each queue can be set to any value

up to 100%. The “Expedite” option is not available in the Priority Queuing policy.

For Weighted Fair Queuing, with the expedite feature disabled, the sum of all CIR values in all queues

must not exceed 100%. With the expedite feature enabled, the sum of all CIR values in the non expedite

queues must not exceed 100%, but each of the expedite queues can be assigned CIR’s up to 100% of

system speed.

8.4.4 Committed Burst Size (CBS)

Since IP traffic is “bursty” by nature, the Horizon Compact Plus provides a feature, called QoS queue

Committed Burst Size (CBS), to handle Ethernet bursts. The Horizon Compact Plus system contains a

data buffer that is used to accommodate bursts of traffic in excess of the user allocated amount as

specified through the CIR setting for each queue.

The CBS defines the portion of the total amount of burst buffer available that is allocated to each queue.

At GigE rate, there is a total of 100 mS (16 MB) of buffer allocated to QoS queues. Each queue can be

allocated a percentage of this memory (set cos queue cbs [0-100 0-100 0-100 1-100 0-100 0-100 0-100

1-100]). The default CBS allocation for QoS queues Q1 through Q7 is 12%, with Q8 being allocated 16%.

The total percentage assigned to all queues must equal 100%.

The CBS function helps in smoothiong out the bursty input and transmit at the CIR of the queue by

queuing the traffic.

The CBS operates as a FIFO for each individual queue. Burst traffic will be queued as it arrives. This

may have the effect of creating a short delay for subsequent traffic arriving at that queue if the

subsequent traffic is at the CIR level of that queue. Conversely, if the burst is followed by traffic that is

less than the CIR level, then no delay should occur.

For example: Q3 has 25% CIR or 25 Mbps on a 100 Mbps link. The Q3 CBS is set to 25% or 25 mS at

GigE rate. If Q3 receives a burst of traffic in excess of 25 Mbps then Q3 would continue to transmit at 25

Mbps until the “burst buffer” is empty, regardless of whether or not the traffic source is sending packets. If

the traffic source continues to send packets, the packets will be queues whilr the que is emptied at the set

CIR. If there is no space in the queue, the incoming packets will be dropped.

8.5 QoS Scheduler Policies

A scheduler services all the queues, Q1 through Q8. Each of the eight queues, Q1 to Q8, is serviced by a

scheduler in a manner dependant on one of three selectable queuing policies (set qos policy

[strict_priority | wfq]).

These policies are:

• Strict Priority Queuing

• Weighted Fair Queuing

• Weighted Fair Queuing Expedite Mode

Regardless of the policy used, Q8 always gets serviced first. However, this is not strictly true if the Cut

Through option is enabled.