The 20MB is fine for me since I am testing methods to keep data small.
My current tests have a sleep devices waking, then obtaining a cell-link. For example, I am working on a Digi gateway called an X3 (not on Digi's web site yet). The X3 can power itself down completely, using a real-time clock chip to wake it at a future time.
I ran a test for 5 days, waking the X3 device every 10 minutes and clocked how long it too for the GSM carrier to assign an IP to the X3:
- The fastest time was 23.7 seconds (half a minute)
- The slowest time was 97.0 seconds (1 and a half minutes)
- The average time was 28.6 seconds (so most near 30 seconds, with a few long outliers)
- Note that there will be several seconds of CPU start-up time in there - I can only know the RTC wake-up time (which I loaded into the RTC last cycle) compared to the RTC time when the IP address is assigned.
My next test will include actually doing work - probably sending a Modbus/TCP transaction to a DSL-based server.