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Specifying Delay Buffer Sizes for Queues

You specify delay buffer sizes for queues using schedulers. The system calculates the buffer size of a queue based on the buffer allocation method you specify for it in the scheduler. See Table 145 for different buffer allocation methods and Table 146 for buffer size calculations.

Table 146: Delay Buffer Allocation Method and Queue Buffer

Buffer Size Allocation Method	Queue Buffer Calculation	Example
Percentage	available interface bandwidth x configured buffer size percentage x maximum delay buffer time = queue buffer	Suppose you configure a queue on a 1xDS0 interface to use 30 percent of the available delay buffer size. The system uses the maximum available delay buffer time (4 seconds) and allocates the queue 9600 bytes of delay buffer: 64 Kbps x 0.3 x 4 seconds=76800 bits=9600 bytes
Temporal	available interface bandwidth x configured transmit rate percentage x configured temporal buffer size = queue buffer	Suppose you configure a queue on a 1xDS0 interface to use 300,000 microseconds (3 seconds) of delay buffer, and you configure the transmission rate to be 20 percent. The queue receives 4800 bytes of delay buffer: 64 Kbps x 0.2 x 3 seconds=38400 bits=4800 bytes When you configure a temporal value that is greater than the maximum available delay buffer time, the system allocates this queue the remaining buffer after other queues are allocated buffer. Suppose you configure a temporal value of 6,000,000 microseconds on a 1xDS0 interface. Because this value is greater than the maximum allowed value of 4,000,000 microseconds, the queue is allocated the remaining delay buffer.

Buffer Size Allocation Method

Queue Buffer Calculation

Example

Percentage

available interface bandwidth x configured buffer size percentage x maximum delay buffer time = queue buffer

Suppose you configure a queue on a 1xDS0 interface to use 30 percent of the available delay buffer size. The system uses the maximum available delay buffer time (4 seconds) and allocates the queue 9600 bytes of delay buffer:

64 Kbps x 0.3 x 4 seconds=76800 bits=9600 bytes

Temporal

available interface bandwidth x configured transmit rate percentage x configured temporal buffer size = queue buffer

Suppose you configure a queue on a 1xDS0 interface to use 300,000 microseconds (3 seconds) of delay buffer, and you configure the transmission rate to be 20 percent. The queue receives 4800 bytes of delay buffer:

64 Kbps x 0.2 x 3 seconds=38400 bits=4800 bytes

When you configure a temporal value that is greater than the maximum available delay buffer time, the system allocates this queue the remaining buffer after other queues are allocated buffer. Suppose you configure a temporal value of 6,000,000 microseconds on a 1xDS0 interface. Because this value is greater than the maximum allowed value of 4,000,000 microseconds, the queue is allocated the remaining delay buffer.

When you specify the buffer size as a percentage, the system ignores the transmit rate and calculates the buffer size based only on the buffer size percentage.

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