There are ways to prevent against this negative scenario: Your application requires a very small airtime plan on a monthly basis to poll a 4-byte register 2 or 3 times a day. However when the bill comes from the cellular carrier, your M2M device has racked up 20MB of activity! How does this happen and how do you prevent against the egregious overage charges that result?
First remember that cellular data networks are IP-based. Therefore, the data is typically encapsulated in TCP or UDP, then in IP; then sometimes in IPsec or GRE , and finally into PPP. So the 4-byte register poll grows significantly beyond the size that you first calculated. Next, recall that TCP is connection based, so every packet sent must receive a handshake back. If there is a packet drop, the handshake may ask for a re-transmission, even if the missing data has already been sent. More data is pushed as a result Protocols like Modbus already have error-checking built-in; so why not use UDP instead if you can?
Other reasons why a handshake can go awry is that some M2M apps do not factor in latency. So if a handshake has not occurred in 100ms, another packet is transmitted. Although 4G speeds alleviate the latency issues, re-transmission bloat is still a concern. Signal quality is also important. Intermittent signal can lead to lost handshakes and the polling and transmission cycle can happen yet again. Using a better antenna is worth the monetary investment.
So how do you prevent overages? Many of the top-tier cellular device manufacturers have included the functionality on their device management systems where one can set a data threshold that will send out an alarm message prior to the data cap being exceeded or even DISABLE a connection before the data cap is reached. A USAT Corp. engineer can be engaged to show how this can be done in order to protect your organization from costly cellular bills. And, as in most cases, the best defense is a strong offense. Run a pilot with a single M2M cellular device/application software/and backend hardware all included in the test-bed. For the pilot, activate the M2M cellular device on a sizable rate plan like 2 GB. Analyze usage after one billable month, and then select the cellular data plan that is relevant. A well planned pilot project can save a lot of money in the long run.
In closing, also remember that it is the commonly the M2M application that initiates polling not the M2M 3g/4/g gateway or router. Make sure that you know how your own application operates so you can proactively enter the correct settings that limit polling. Blaming an M2M cellular network device for overages is like blaming a car for how fast it is driving on the highway…