Why does my LTE-M module keep losing association with the network?

Question

I have the development kit, I went through all the example rudimentary tests. Not one of the tests went, what I would call "smoothly". After completing the HTTP GET example it disconnected and has been like that for 15 minutes.

Is it also normal for a GET to take 15 seconds?

Not very impressed, so far. Thing seems barely functional. I updated XCTU and the module firmware to the newest prior to starting any of the functional tests.

Answer 1

You apparently haven't worked much with cellular.

Cell towers do NOT like idle nodes and remove bandwidth from them quickly. Unless your cell module sends data every few seconds, you will have a delay when you start to talk.

So when you start the GET, first your LTE-M needs to 'beg' the tower for radio bandwidth, which the tower likely needs to remove from other active peers. This takes time. Then you might do an DNS lookup. If you use the carrier's DNS server, they tend to be fast (aka: they want to track what names you look up). if you've set DNS to some alien host like OpenDNS, carriers (like Verizon) seem to punish you by delaying lookup response by dozens of seconds. They'll say they don't, but watch a network trace! I've seen OpenDNS replies show up over 45 seconds after I send them via VZ.

Only once all the above is complete can the LTE-M module attempt TCP/IP and HTTP. So 15 seconds from idle to completion is NOT BAD for cellular.

- CoraLyn

Comments

Oh, as for your 'association' question, I'm not sure what you mean by that. Cell module have varying levels of 'association'. There is 'can I talk to a tower?', and there is 'the tower accepted someone is paying my bill?'. Plus as I mention above, idle nodes eventually get removed by the tower. I doubt you can ever 'associate' and then sit idle forever. Cellular doesn't work that way.

EVENTUALLY, LTE-M promises to solve some of this, however this only works by scheduling future contact. So an LTE-M could say to the tower "I'll be quiet for 47 min and 30 sec", and the tower will be happy with this & retain association. However, your LTE-M *MUST* then talk in 47 min and 30 sec - not early nor late. So this feature only has use in a sleeping device on a strict schedule - like waking every 60 minutes to report.