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What You’ll Learn:
- Why automatic network selection is important.
- What one must do to pick the right network-steering method.
- How to prep firmware on when to clear the Forbidden Public Land Mobile Network (FPLMN) list and maximize data quality.
There many ways to extend the battery life of IoT devices on cellular networks, such as power-saving modes, communication protocol choices and additional binary translation to minimize data volume. A device’s automatic network selection is often overlooked when it comes to saving data quality and battery life, likely because getting the best network for a specific device is a surprisingly tricky issue.
SIM cards from mobile network operators (MNOs) and mobile virtual network operators (MVNOs) will force IoT devices to use networks based on commercial agreements instead of on the best available network signal at the current location. This affects quality of service and ignores the potential benefits of having several networks available.
Typically, operator-controlled network selection uses steering methods based on information stored on the SIM card. Standard SIM cards contain the information of the Public Land Mobile Network (PLMN, the geographical area covered by an MNO). A PLMN connects to other PLMNs to provide inter operators roaming and messaging.
However, these steering methods have nothing to do with delivering the best available network connectivity for the device. Even if a better network with a stronger signal is available, the device won’t switch over, which can be problematic for IoT device communications.
Network Steering Method Impacts Data Quality and Battery Longevity
Selecting the right network steering methods is more than just finding the best signal. It also affects data quality from packet losses due to a weak network signal and increases energy consumption when devices spend more time trying to send, receive, and verify data.
When evaluating network providers for cellular device communication, it’s useful to check how network steering is managed to leverage automatic network selection and get the best available network. This offers tremendous benefits, but first, the IoT device’s firmware must be ready for it.
Smarten Up Your Firmware for Best Network Selection
Firmware is traditionally built around saving energy. However, unlocking firmware to benefit from automatic network selection is an extra step that increases the device’s life, especially for battery-powered devices. A simple change in the firmware has shown to reduce energy use by around 4% over the life of a device, which could add valuable extra months of performance in the field.
The 3GPP specification defines that if there’s no PLMN list on the SIM, the device’s cellular module should attach to a sufficiently strong network. Signal strength determines which network is strong enough, with −85 dBm or better defined as the target.
It’s important to remember that the list of forbidden networks—the FPLMN list—is part of every SIM card. This list is handled by the device’s cellular module and defines the networks that a device should and should not connect to. The FPLMN list constantly grows as the device tries to connect to operator networks over and over during the device’s life. When there’s a failed connection, rejected networks go on a blacklist and the device won’t try reconnecting again, avoiding wasted time and energy.
But the limits on automatic network selection mean lots of networks will be rejected. Typically, PLMN is handled by the SIM and is fixed by the operator. Most device firmware doesn’t focus on the forbidden list, in part because even “allowed” networks may be added to the blacklist when there are other connectivity problems. This can be a failed connection attempt due to an unactivated SIM card or a SIM that’s run out of available data volume. Most firmware is programmed to clear the complete FPLMN list on every reboot.
But when trying to get the best performance and longevity out of a device, clearing the list on every reboot might not be the best choice.
Using the FPLMN List to Boost Data Quality
The device's firmware should be refactored to not clear the FPLMN on every reboot. Wipe the FPLMN list only when it’s necessary, maybe once a month or during firmware upgrades. This way, it remembers forbidden networks and more easily switches to the best network using fewer unnecessary connection attempts.
When the auto-selector properly connects to the true, best available networks—those with a strong signal—it leads to two main benefits:
- Energy efficiency of devices increases due to the use of stronger network signals.
- The device’s data quality improves with fewer connection issues, like lags or the loss of data packages.
Measurements show that the service performance and quality can be significantly enhanced by switching to networks with stronger signals. Just as with firmware, the numbers might look small at first, but adjustment automatic network selection uses 4% less energy and enjoys a 5% increase in connection success.
Switching more often between networks might seem like extra work. But even a few percentage points of saved energy use and better performance can make all the difference in making or breaking an IoT project.
This article appeared in Electronic Design.