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Quality of Service (QoS) Algorithms

For the default weights, these calculations yield the following guaranteed minimum percentages:

•

p0 = 1.7%

•

p1 = 3.3%

•

p2 = 15%

•

p3 = 80%

Example Queue Cycle

You can compare the queue cycle to the operation of a gasoline pump with multiple dials. Some dials turn only

once during a cycle and are triggered by the smaller dials, which turn multiple times during a cycle. For example,

the dollars dial moves only slightly after many revolutions of the cents dials. Note that this comparison does not

exactly match because the slower dials on the gas pump are the more important dials. (For example, the dollars

dial has more effect on your pocket-book than the cents dials). Figure 9 shows an example of the queues

represented as dials.

Q u e u e 3

Pkt1

Pkt2

Pkt4

Pkt3

W eight = 4

(4 packets per revolu tion )

Figure 9

Queues represented as dials on a meter

Queue 3 receives the most servicing during a cycle. The routing switch interleaves service for the lower queues in

between servicing queue 3. QoS queue 0 receives the least servicing during a QoS cycle. Notice that queue 3

always makes a complete revolution before a lower queue advances. When a queue completes a revolution, the

system has serviced the number of packets equal to the weight of the queue. Assuming that queue 3 is using the

default weight 4, four packets are serviced in queue 3 before the system advances to the next queue to process a

packet.

Q u e u e 2

Pkt1

Pkt3

Pkt2

W eight = 3

(3 packets per revolu tion )

Q u eue 3: w eight=4, m inim u m percenta ge=80 %

Q u eue 2: w eight=3, m inim u m percenta ge=15 %

Q u eue 1: w eight=2, m inim u m percenta ge=3.3%

Q u eue 0: w eight=0, m inim u m percenta ge=1.7%

Q u e u e 1

Pkt1

Pkt2

W eight = 2

(2 packets per revolu tion )

Q u e u e 0

Pkt1

W eight = 0

(1 packet per revolution )