Improve coverage with automatic network selection

The SIMs in your IoT devices can assist the radio module in optimising 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 prioritise networks with weak signals over networks with strong signals.

Here’s an example of a device prioritising a poor connection:

We take a different approach and don’t 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.

Here’s an example of a device automatically selecting a good connection:

Automatic network selection

On the SIM, PLMN lists are records for storing priorities of PLMN’s and corresponding technologies. ETSI TS 131 102 defines HPLMN, HPLMNwACT, EHPLMN, PLMNsel, PLMNwACT, OPLMNwACT, and others.

If there are no PLMN lists, 3GPP states that the radio module should attach to a strong enough network (the signal strength that defines a “strong enough network” is called -85 dBm).

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.

The FPLMN list is a dynamic list that is maintained by devices. In automatic selection mode, networks are added to the FPLMN list when an attempt to connect 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 unactivated SIM.

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.

Conclusion

There are different PLMN strategies for optimising power consumption, quality of service, etc.

Suppose the PLMN lists contain one or multiple networks. In that case, it forces the radio module to scan and select those networks before any other – without taking the signal level and quality of service into account.

If the PLMN records are left blank, it forces the radio module to scan and select a network with a strong enough signal level and quality of service defined as good or better.

Experience with having PLMN records filled out has led to devices in Denmark using 10+ minutes to attach and getting a data connection that wasn’t stable enough to transmit data.

In our experience, allowing the radio module to choose the strongest available network in a given country is the preferred option. It has, on average, resulted in good signal strength and corresponding quality of service and dramatically reduced registration times.