The SIMs in your IoT devices can assist the radio module in optimizing how they scan and attach to networks.
You can achieve this by allowing the radio module to choose the best network in a given area, a function that involves PLMN lists on a SIM.
With automatic network selection, you can also shorten attach times, which will reduce the power consumption of your device.
What is a PLMN?
PLMN stands for Public Land Mobile Network and is a mobile operator’s cellular network in a specific country. Each PLMN has a unique PLMN code that combines an MCC (Mobile Country Code) and the operators’ MNC (Mobile Network Code).
When you receive a SIM from an operator, it will often have PLMN lists on it. These lists are a way to prioritise networks you would like to use on the SIM.
Typically, PLMN lists are based on commercial agreements. So, for example, an operator will have contracts in various countries so your SIM can connect to specific networks when you roam outside their network. You can read a guide on how to add user defined PLMN lists to your SIMs here: How to add a Network Preference List (PLMN) to a SIM.
A negative consequence of PLMN lists is that your device could prioritize networks with weak signals over networks with strong signals.
Here’s an example of a device prioritising a poor connection:
Example of a poor connection.
At Onomondo, we take a different approach and do not add PLMN lists to SIMs by default to avoid weak signal selection and reduce registration times.
Instead, we simply allow the radio module to choose a strong enough network according to 3GPP standards.
Example of a good connection with automatic network selection.
Automatic network selection
PLMN lists are records stored on the SIM that are used for saving priorities of PLMN’s and corresponding technologies. In case you'd like to learn more about these lists, ETSI TS 131 102 defines HPLMN, HPLMNwACT, EHPLMN, PLMNsel, PLMNwACT, OPLMNwACT, and others.
If there are no PLMN lists, as is the case for Onomondo SIMs, 3GPP states that the radio module should attach to a strong enough network. The signal strength that defines a “strong enough network” is equivalent to -85 dBm or better.
Leaving the PLMN list empty allows the radio module to choose the first available network with strong enough signal (not to be confused with best signal).
Here’s the full explanation of how the PLMN records are used during switch-on according to 3GPP TS 23.122:
4.4.3 PLMN selection The registration on the selected PLMN and the location registration are only necessary if the MS is capable of services which require registration. Otherwise, the PLMN selection procedures are performed without registration.
4.4.3.1 At switch on or recovery from lack of coverage
At switch on, the MS selects and attempts to perform a Location Registration on the registered PLMN, if it exists. On recovery from lack of coverage, the MS selects the registered PLMN (if it exists) and, if necessary (see subclause 4.6.2) attempts to perform a Location Registration.If successful registration is achieved, the MS indicates the selected PLMN.
If there is no registered PLMN, or if registration is not possible due to the PLMN being unavailable or registration failure, the MS follows one of the following two procedures depending on its operating mode.
A) Automatic Network Selection Mode Procedure
The MS selects and attempts registration on other PLMNs, if available and allowable, in all of its bands of operation in the following order:i) HPLMN (if not previously selected); ii) each PLMN in the "PLMN Selector" data field in the SIM (in priority order); iii) other PLMNs with received signal level above 85 dBm in random order; iv) all other PLMNs in order of decreasing signal strength.
Here’s what that process looks like:
NB, MS is short for Mobile Station and describes the device, e.g. cell phone, in 3GPP terminology. The MS includes the ME (Mobile Equipment) and SIM.
Network lists
As mentioned earlier, we keep PLMN lists blank by default at Onomondo. Otherwise, we operate with Network lists which are controlled in the Onomondo Mobile Core. These lists are typically created by users in the Onomondo App and tell the device which networks it can connect to without the order of priority that PLMN lists have.
This lack of a PLMN list ensures that we will always connect to the allowed network with the strongest signal in a given location.
FPLMN lists
Another essential SIM list to keep in mind is the FPLMN list, aka forbidden networks list.
The FPLMN list is a dynamic list that is stored on the SIM, but maintained by the device. In automatic selection mode, networks are added to the FPLMN list when a connection attempt is denied. This prevents the device from trying to connect to that network again. Keep in mind, this is with exceptions; according to 3GPP TS 23.122, a network won’t be added to the FPLMN if it’s on the EHPLMN or HPLMN lists (equivalent home network lists) and networks on the FPLMN can be selected when the device is in manual selection mode, for example.
It’s vital that you are aware of FPLMN as it can interfere with registration times. We have, for example, seen networks added to FPLMN lists by devices when a device has tried to connect to networks with SIM that was deactivated.
In such cases, you’ll want to clear the FPLMN list using AT commands. You can read how to do this here: How to clear the FPLMN (forbidden network) list on a SIM.