The time interval in seconds over which the channel utilization
index is calculated. All upstream/downstream channels use the
same docsIfCmtsChannelUtilizationInterval.
Setting a value of zero disables utilization reporting.
A channel utilization index is calculated over a fixed window
applying to the most recent docsIfCmtsChannelUtilizationInterval.
It would therefore be prudent to use a relatively short
docsIfCmtsChannelUtilizationInterval.