Cellular Data Usage Challanges

It is common question to ask how much cellular data is your Digi device going to use.  This is most often done to determine billing costs.  

Here are some of the difficulties of counting these items up in trying to determine the total cellular data usage for a month:
--- the absolute best way to handle this is to test.  The ultimate measure is how much you get billed for (as opposed by some other metric).
--- many Digi products have a cellular statistics field.  You can use this as a rough guidline.   Warning: this may not reflect the true amount of data you are billed.  This value reflects the total data in and out of the digi’s USB interface, which is shared with the cell module.  This cell module plugs into the USB bus, which is how the digi gateway talks over the cell network.  In addition, the cell carrier may not count a particular byte of ‘data in/out’ as billable where it might on others.  To make matters even more confusing it can vary from carrier to carrier and possibly even between data plans as well.
--- 3G or 2G cell connection?  2G is cheaper for small amounts of data.  But it's largely been phased out.
--- There is also undesirable traffic as well.  There are a number of security threats by 3rd parties.  They are motivated by a variety of reasons.  They can do things like scanning for a vulnerability to exploit or a denial of service attack or something else.  This will likely be counted as billable traffic.  You can attempt to reduce this number by locking down unused features on your gateway.  An example might be to disable unused services/ports (reference: X4 web interface homepage -> configuration -> network -> network services settings)  
--- We believe there are 3rd party cell ‘agents’ that will shop for you to find the most competitive data plan.  This is like the concept of the insurance agent shopping around for you.
--- you can view the data in/out stats from the digi in it’s web interface at: homepage -> administration -> mobile
--- In addition to your data, which I’m going to call it, there is other metadata as well.  You can think of it as 'housekeeping' data and operations by the cellular carrier.  They can do things such as acknowledgements, retransmissions, keepalives, internal queries to remote nodes (gateway) for link status, routing, and other things.  
--- Another item of note is how frequently an app or the digi gateway transmits data.  For example, say the  app wants to send a query.  

Example 1:
Let’s assume it needs to send 40 bytes to each (80 bytes total).  Let’s also assume that in each of these cellular ‘packets’ there is 40 bytes of overhead (source and destination addresses, flags, type, rssi, etc.).  If the application sends all 40 bytes of data at a time it will need to send 2 packets (40 * 2 = 80 bytes).  There is also 40 bytes overhead for each packet as well.  So we have 40 + 40 = 80 bytes a packet with 2 packets.  (40+40)*2 = 160 bytes total.  

Example 2:
If the application sends only 10 bytes of data at a time it will need to send 8 packets (10 * 8 = 80 bytes).  There is also 40 bytes overhead for each packet as well.  So we have 10 + 40 = 50 bytes a packet with 8 packets.  (10+40)*8 = 400 bytes total.