NERO collects data from routers and switches at 5 minute intervals. Most variables are of the type COUNTER, which as the name implies, means that the router or switch is simply counting the number of packets, errors, etc. These counters are read once every five minutes, and by noting how much the packet counter (for example) has increased from the previous reading, we know the packet count for the most recent 5 minute interval. By dividing this count by 300 seconds we get a rate of packets/sec. Note that we can't say anything about the variation of the packet rate during this 5 minutes, we only know the average value. (A few data sources are read at 15 minute intervals instead of 5.)

The data collected above are stored using rrdtool. A little over one days worth of raw data is saved. In order to have a long term record without saving every 5 minute reading, data older than one day are combined into longer units of time. A weeks worth of data is kept by combining six 5 minute readings into one 30 minute reading which is an average of the original six readings. Since we may want to know something about the variability of the data over the 30 minute window, we also save the maximum 5 minute value out of the six we are combining. Sometimes we may choose to store the minimum instead of, or in addition to, the maximum.

Similarly, a months worth of data is kept by combining four 30 minute values from the weekly data into a single 2 hour average, and a years worth of data is kept by combining twelve 2 hour monthly values into a single one day average. In both the monthly and yearly cases, the maximum and/or minimum 5 minute value that occurred during the 2 hour or one day period is also preserved.

The data that we save can be viewed on these web pages in various ways. It is possible to view any combination of daily, weekly, monthly and yearly graphs for each variable we collect. By default these graphs are average values over the 5 minute, 30 minute, 2 hour, or 1 day intervals. If we are saving maximum and/or minimum 5 minute values for a variable, it is possible to view those also on the weekly, monthly, and yearly graphs. (Note that the maximum, average, and minimum values for the daily graph will always be the same, since 5 minutes is the basic collection interval).

Below a typical graph is a legend which indicates maximum, average and most recent (curent) values. These apply only to the data being represented on this particular graph, ie the maximum value listed on a monthly graph is the highest value seen on that monthly graph. The average value is the average for the month. Note that if you are graphing AVERAGE data, the maximum is the highest point seen on this graph. It is NOT the maximum 5 minute value for the month.

These graphs are produced using rrdtool.