Polycom 3725-76302-001LI manual + 5 *1 / 1 + 5 = 5/6, + 5 *5/6 / 1 + 5 = 31/36

MCU Pool Orders MCU Management

Table 6-15How failed calls reduce the apparent capacity of an MCU (continued)

Consecutive	Percentage of Ports
Failed Calls	Assumed To Be in Use
7	84%
8	88%
9	90%

Every 30 minutes, the reliability score of the MCU is increased so that it won’t be permanently removed from the pool due to failures in the distant past. To avoid trying the MCU every 30 minutes, monitor the DMA system and administratively take the MCU out of service.

By increasing the number of MCUs in the pool or increasing their capacity, you can decrease the usage of the working MCUs during a failover scenario. So, for example, if you want to avoid routing any more calls to an MCU after two consecutive failed calls, provide enough excess capacity that the remaining MCUs never all reach 43% port usage during a failure.

Note

After each call, the DMA system recalculates the reliability of an MCU as the weighted average of the result for the current call (1 for success, 0 for failure) and the reliability of all previous calls, using this formula:

reliability = (current_call + (weight * previous_reliability)) / (1 + weight)

For example, if weight is 5, previous reliability is 1 (no previous failed calls), and the call is sucessful, the reliability remains 1:

(1 + (5 *1)) / (1 + 5) = 1

If weight is 5, previous reliability is 1, and the call fails, the reliability becomes 5/6:

(0 + (5 *1)) / (1 + 5) = 5/6

If weight is 5, previous reliability is 5/6, and the call is successful, the reliability becomes 31/36:

(1 + (5 *5/6)) / (1 + 5) = 31/36

If the reliability is ever less than 1, it exponentially approaches 1 as more calls succeed, but it never quite gets there. It very quickly reaches the point where the weight of the past failed call counts less than a single call in progress. But it remains as the tie breaker between completely unused MCUs forever.

See also:

“MCU Pool Orders” on page 156 “MCU Selection Process” on page 159 “MCU Pool Order Procedures” on page 162

Polycom, Inc. 161