With the implementation of Numerex’s excellent new rate plans, effective May 15, 2009, which give customers the choice of a number of attractive GPRS pooled data plans, the number of questions has intensified about how to estimate monthly data usage for modeling costs. Unfortunately, it’s very difficult to predict GPRS packet data based on reporting parameters.
People really have a hard time understanding that you just cannot accurately “model” GPRS packet data. They are used to simple methods – such as using SMS messages or Satellite messages – which is pretty easy – you just count and multiply.
With GPRX, there are way too many variables (which roaming carrier, how often you roam, rounding, etc.) The only way to really know is to test and sample in real life conditions. That’s one reason we encourage the use of the DEV servers to get started, so that there is a true history on a handful of devices in a configuration matching the planned application.
Attached is screen capture of a few lines from a 23 page spreadsheet that shows all the data reported over a month period on a SINGLE Xirgo XT2000 OBD device - used with a “Fleet Tracking” kind of configuration (meaning it’s reporting about 1x per minute when moving - and also reports on events such as direction change). There were about 1138 different “reports” total during that month - totaling about 667K in usage. The image shows just a few from one day, but if you closely examine the detail you’ll see that the individual packet size ranged from 122 bytes to 2 kilobytes. It’s likely that the smaller readings were due to just an hourly checkin, while the larger ones were when the vehicle was moving, but there is no firm way to estimate the usage.
In summary, the best approach is the power of statistical averages. We know for example, that most “Fleet Tracking” applications usually produce about 500K to 1000K per month - when averaged over a fleet of several vehicles. With pooling, you are protected on the few vehicles that are heavy users - for every vehicle like that, there is usually one or more that is out-of-service, the driver is away, it’s in the shop, etc. So we’d be confident telling a customer that a fleet tracking application would be safe with our 900K pooled plan.