The average number of bits received per second on all
interfaces over the past 8.5 minutes.
The average is calculated by an 8.5 minute (512s) decaying average,
which is updated every 4 seconds using the following formula:
X' = 127/128X + 1/128i
where X' is the new average
X is the old average
i is the most recent poll result
The times and multiplier factors have been chosen to give the simplest
possible calculation using bit shifting operations.