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We had recently deployed a few Cellular IoT devices on the field, using SIMs from a certain provider (Airtel India). They had provided us with an APN, but we chose not to program our device with the APN, as it was working even without it. Further, we had earlier noticed that setting the APN didn't make any difference for all the cellular providers we tested here in India (in fact, APN of one provider would work for another also, without issues).

However, some of our devices deployed in a certain location stopped working recently. The issue was that we had not set the APN. However, devices deployed in other locations continued to work. The network connectivity was proper in all the locations and mobile phones were working.

Now, here are a few things that bewilder me about APN:

  • How does it happen that APN of one provider work for the other?
  • How does it happen that devices in one location stop working and in the other location continue to work, all of them NOT programmed with APN setting?
  • How exactly does changing the APN affect the connectivity?

I have read many articles on the Internet and questions on other forums to understand the mechanism of how APNs work, however the understanding on how it explains the behavior we are encountering, still eludes me.

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This confusing behaviour is typical. The underlying reason is that it is up to the carrier to decide what to do with the APN setting — unlike most configurations of this form, you're not actually configuring any connection setting. Instead, you're just passing to the carrier a text string.

The carrier may do a number of things with the APN you've specified:

  • Ignore it completely and connect you to the default network.
  • Ignore it if it isn't one that it recognises (e.g. your case of using a different provider's APN).
  • Use it to connect you to the default network, but with an IP from a restricted range.
  • Use it to connect you to a private network.

Additionally, the carrier may change the default/override settings at any time, which is why an APN setting might work for a while and then stop working.

There's some awfully detailed info here (LTE Quick Reference on ShareTechnote) on the confusing way APN settings behave.

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    Thanks!! This is one comprehensive answer :) you're just passing to the carrier a text string - this is key.
    – shivams
    Commented Jan 20, 2019 at 10:12
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    One question still remains unanswered though (the second question). How does it happen that devices in one location stop working and in the other location continue to work, while all of them were NOT programmed with APN setting? After programming the APN setting, all of them were working. This geographical disparity is confusing.
    – shivams
    Commented Jan 21, 2019 at 4:17
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    The behaviour of the carrier is even down to the cell tower that you connect to - some cell towers might prefer some routes over others, occasionally giving you a dud route.
    – Heath Raftery
    Commented Jan 21, 2019 at 20:17

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