Introduction
Many modern cellular modems developed for IoT are designed with LTE capabilities, including LTE-M and NB-IoT, offering higher data speeds, lower latency, and improved efficiency over older technologies.
In addition, the same modems often include 2G fallback to ensure connectivity in areas where LTE, LTE-M or NB-IoT coverage is weak or unavailable, providing a reliable backup for critical applications.
In practice, network selection for these modems is influenced by several factors, especially when using non-steered SIMs, such as those offered by Onomondo, which do not enforce a fixed priority on specific networks.
In this guide, we explore how network selection works for such setups and provide practical strategies to optimize modem performance. If you are deploying IoT devices that support the above cellular technologies, this guide aims to help you make smarter, more efficient network choices.
If you want to get familiar with automatic network selection fundamentals, see:
Automatic network selection in practice
Modern cellular devices typically use automatic network selection by default. In this mode, the device modem (UE) scans for and registers on an available Public Land Mobile Network (PLMN) without user intervention.
Learn more about network selection modes here:
According to 3GPP TS 23.122 and 3GPP TS 31.102, the UE follows a prioritized list when selecting a network:
HPLMN / EHPLMN: Home or Equivalent Home PLMN – The UE first tries to register on the Home PLMN (HPLMN), derived from the SIM’s IMSI, or if an Equivalent HPLMN (EHPLMN) list is present on the SIM, it will try the highest-priority EHPLMN available.
PLMNwAcT: User Preferred PLMNs – Next, the UE checks the “User Controlled PLMN Selector with Access Technology” (PLMNwAcT) list on the SIM. Any PLMN entries in this user-defined priority list are tried in order. This lets a user or device owner specify preferred networks (with specific radio access technologies like LTE, 3G, etc.) to use when away from home.
OPLMNwAcT: Operator Preferred PLMNs – The UE then tries PLMNs from the “Operator Controlled PLMN Selector with Access Technology” (OPLMNwAcT) list, derived from the SIM or device memory, in the priority order given. These are networks preferred by the service provider (e.g. preferred roaming partners).
High-Signal PLMNs – If no success yet, the UE will attempt any other available PLMN that has a "strong enough" signal, in random order. What counts as “strong" signal is defined by 3GPP as -85 dBm. Basically, in this step, strong signal networks are tried first to possibly get service quickly.
Other Available PLMNs – Finally, it will try all remaining found PLMNs in order of decreasing signal strength. This exhaustive search ensures the device attempts every possible network from strongest to weakest signal until it finds one that allows registration.
Onomondo SIMs do not come with a pre-determined HPLMN, EHPLMN or OPLMNwAcT.
Although this may prolong the initial registration process, it lets the UE choose the best available network and prevents prioritizing PLMNs with low quality signal. What this means, in essence, is that unless you implement a User Preferred PLMN (PLMNwAcT), the device will search for High-Signal PLMNs first, in line with the process outlined in this article.
If a suitable network is found and registration succeeds at any step, the selection process stops. The device will attach to that network and remain connected until the signal deteriorates beyond acceptable levels.
Based on 3GPP TS 22.011, if no network can be selected during this process (for example, no allowed networks are in range or registration fails on all), the UE indicates “No service” to the user. In that case, the device will periodically retry scanning for available networks until service is obtained (often using a back-off strategy to save battery).
Last Known Network (RPLMN) and Forbidden List (FPLMN)
Once your IoT modem registers with a local network, it will write this entry onto the Last Registered PLMN (RPLMN) file stored on the SIM. In the absence of an HPLMN, EHPLMN, User Preferred PLMNs (PLMNwAcT), and Operator Preferred PLMNs (OPLMNwAcT), the Last Registered PLMN (RPLMN) will be prioritized over other available networks, including high-signal PLMNs.
In practice, this could mean that upon power-up or when returning to coverage, your device will prioritize connecting with the last known network before scanning for other available options, even though those may offer stronger signal quality than the last known network. This speeds up re-connection if the last used network is available. By default, the RPLMN is given first priority at device boot-up (3GPP TS 22.011), unless it is the very first connection and the RPLMN on the SIM is empty. If that is the case, the process outlined in the previous section applies.
Aside from the above, the SIM holds a Forbidden PLMN list (FPLMN). Like the above, the list is stored on the SIM but managed by the modem. Networks on this list are avoided during automatic selection. The UE updates the forbidden list when registration to a network fails, either due to a deliberate rejection from the local operator, or in an instance where the network is absent on the Network List assigned to the SIM in the Onomondo app. This prevents the modem from repeatedly trying a network that rejected it, thereby saving time.
If you would like to learn more about FPLMN and how to manage it, see:
Scanning and re-selection behavior
When searching for a network (either at startup or after losing coverage), the cellular modem scans across frequency bands and radio technologies it supports to find available PLMNs. To improve efficiency, the scanning procedure may prioritize known frequencies or bands where networks were previously found. As described above, it may simply attach to the last known network.
If no networks are found initially, the UE doesn’t continuously scan at full throttle as this may drain its battery. Instead, it enters a cycle of periodic scans. It will wait for a short interval, then scan again, and repeat this process, possibly lengthening the intervals if no service is found.
Once attached to the network on LTE, it is not guaranteed that the signal quality will be sufficient to maintain a strong connection. If the LTE signal deteriorates, the device may first fall-back to 2G, instead of performing a full network scan and searching for alternatives. Although this may impact data transmission speeds, it preserves battery, too.
Implementing a thought-out back-off strategy and automatic FPLMN clearing could be highly beneficial in avoiding scenarios where a modem is continuously searching for networks in the area, although the coverage may be very poor, and / or FPLMN may have been unwillingly populated. It can also help avoiding scenarios where weak LTE signal leads to premature fallback.
This discontinuous search approach balances reconnect time with power saving – the device tries to regain service promptly but avoids fruitless constant searching.
Practical Optimization Strategies
Since Onomondo SIMs are roaming SIMs with no HPLMN or EHPLMN lists, the modem does not prioritize a single "home" network. Instead, selection is based on available PLMNs and last known networks.
Below are several optimization strategies to enhance performance and ensure reliable connectivity, adhering to 3GPP/ETSI standards:
Use the Onomondo app’s Network Lists for full control
The Network List feature is a solution designed to let you manage which networks your device is allowed to connect to. In the Network Marketplace, you can manually select and restrict networks based on supported technologies, data rates and known coverage limitations.
Networks excluded from the Network List will be added to the Forbidden PLMN (FPLMN) list on the SIM upon registration attempts, meaning the device will reject them and not attempt to reconnect, moving on to "better" options available in the area.However, you should keep in mind that:
If no networks from the list are available, the device will not connect to any network until you manually adjust the Network List.
The FPLMN list may need clearing if you later decide to allow previously rejected networks. For reference, see: How to clear the FPLMN List on a SIM.
Define a PLMNwAcT list for strongest coverage
Another way to optimize network selection is by configuring a User-preferred PLMN list (PLMNwAcT), mentioned earlier in this article. PLMN list allows the UE to prioritize networks with the strongest LTE coverage in your deployment area. By specifying preferred PLMNs on the SIM, the modem will attempt to register on those networks first before searching for others.
Configure LTE-only mode with conditional 2G fallback
Depending on your use case, you may choose to instruct your devices to prioritize LTE whenever available. Keeping devices in LTE-only mode ensures they do not prematurely drop to lower technologies like 2G. However, in cases where LTE coverage is weak or unavailable, a controlled fallback strategy can be applied:
If the LTE network scan fails for a predefined period, the device can be toggled to LTE+2G fallback mode, or even 2G only.
This allows the modem to prioritize high-signal 2G networks only when absolutely necessary, rather than dropping immediately.
Once LTE becomes available again, the device can switch back, ensuring optimal performance while maintaining backup connectivity.
Optimize re-selection and network scan behavior
Since Onomondo SIMs are not bound to a single preferred network, the modem will regularly scan for alternatives. To improve efficiency and avoid unnecessary switching:
Increase the periodic PLMN search interval to reduce frequent, battery-draining scans.
Limit unnecessary re-selection events by ensuring the modem does not prematurely leave an LTE network due to minor fluctuations in signal quality.
Use band-locking or frequency prioritization to focus searches on LTE bands where coverage is strongest, reducing time spent on unnecessary scans.
Adjusting modem behavior, including registration timeouts and search intervals requires a well thought-out approach, as it heavily depends on your use case, priorities and modem capabilities. If in doubt, consult your hardware manufacturer for advice.
Conclusion
For roaming SIMs without predefined network priorities, optimizing network selection requires proactive configuration. Whether by defining a User PLMN list, manually controlling allowed networks via Onomondo, using LTE-only mode with conditional fallback, or fine-tuning scanning behavior, these strategies can help ensure stable and efficient connectivity.
The best approach depends on your deployment scenario, balancing flexibility, reliability, and power consumption. By applying these strategies – aligning with the priorities defined in 3GPP/ETSI standards – one can achieve faster registrations, fewer network switching issues, and improved battery life, all while maintaining compliance with cellular network selection protocols.