For those working with automotive embedded systems and connected vehicles, maintaining a stable network connection is crucial. SIM cards play a vital role in this connectivity, but sometimes, devices can encounter issues connecting to cellular networks. One common culprit behind these problems is the Forbidden Public Land Mobile Network (FPLMN) list, also known as the forbidden network list. This list, stored directly on your SIM card, instructs your device to avoid attempting connections to specific networks.
While the FPLMN list is designed to optimize connection times by preventing futile attempts to access unavailable networks, it can occasionally become an obstacle, especially during debugging or when roaming. This article will delve into how the FPLMN list operates, why it might cause connectivity problems, and, most importantly, how to clear it, ensuring your automotive devices maintain optimal network access.
How the FPLMN List Works to Optimize Network Connections
The primary function of the FPLMN list is to streamline the network connection process. Imagine your device searching for a signal in an area with numerous networks. Without an FPLMN list, it would attempt to connect to each network sequentially, regardless of whether they are accessible or permitted. This process can be time-consuming and inefficient.
The FPLMN list acts as a smart filter. It’s a dynamic list, managed by the cellular module within your device. When your module tries to connect to a network and is denied access – perhaps due to roaming restrictions or temporary network unavailability – that network’s identifier is automatically added to the FPLMN list stored on the SIM. Subsequently, your device will bypass these forbidden networks during automatic network selection, leading to quicker connections with available networks.
Alt: Close-up of a SIM card illustrating cellular network technology for automotive devices.
When the FPLMN List Becomes a Problem for Connectivity
Despite its benefits, the FPLMN list can inadvertently cause connection issues in certain scenarios. A common situation arises when a SIM card is inserted into a device that is powered on but before the SIM has been properly activated. In such cases, the device, attempting to establish a connection, might encounter multiple local networks and, upon failing to connect (due to the inactive SIM), incorrectly add all of them to the FPLMN list.
Similarly, if a vehicle equipped with a SIM card travels to a region not included in the SIM’s “safe list” or “allow list” (regions where roaming is permitted), the device will likely attempt to connect to local networks and, upon denial, populate the FPLMN list with these networks.
If, after these scenarios, the SIM is activated or the vehicle returns to an allowed region, connectivity problems might persist. This is because, in automatic network selection mode, the device will still avoid the networks listed in the FPLMN, even if they are now available and authorized. Essentially, the FPLMN list, intended to be helpful, can become outdated and prevent legitimate network connections.
Understanding how devices utilize PLMN lists during automatic selection can provide further insight into these processes.
Methods to Clear the FPLMN List: Regaining Network Access
Fortunately, there are methods to clear the FPLMN list and resolve these connectivity issues. The following techniques generally require familiarity with AT commands, which are instructions used to communicate with cellular modules. If you’re not comfortable with AT commands, it’s advisable to consult your device manufacturer or supplier for guidance.
Clearing the FPLMN List Using AT+CRSM Command
The AT+CRSM command is a direct way to interact with the SIM card and clear the FPLMN list. First, you can verify if clearing is necessary by using the following AT command:
AT+CRSM=176,28539,0,0,12This command instructs the SIM to provide the data from the FPLMN file (identified by the file ID 28539) in binary format (indicated by 176). A typical response indicating a cleared or default FPLMN list would be:
+CRSM: 144,0,"FFFFFFFFFFFFFFFFFFFFFFFF"
OKThe “FFFFFFFFFFFFFFFFFFFFFFFF” response signifies that the FPLMN list is currently empty or contains only the default prefix. If you receive a different response, it suggests that networks are listed in the FPLMN and clearing is recommended.
To clear the FPLMN list, you can use the update binary command (code 214) with the same “FFFFFFFFFFFFFFFFFFFFFFFF” value:
AT+CRSM=214,28539,0,0,12,"FFFFFFFFFFFFFFFFFFFFFFFF"
OKThis command effectively resets the FPLMN list to its default state, containing only the prefix and no forbidden networks.
Important Warning: Some cellular modules might require specific operational modes before you can successfully clear the FPLMN list. Always consult your hardware’s technical manual for specific instructions and ensure you revert the module back to its normal operating mode after clearing the list. Incorrectly applying AT commands can lead to unintended module behavior.
Manual Network Selection with AT+COPS Command
Another effective method to remove networks from the FPLMN list involves using manual network selection. Most cellular modules offer the option to manually select a network, overriding the automatic selection process and the FPLMN list.
You can attempt to connect to a network that is currently forbidden by using the manual network selection AT command:
AT+COPS=1,,<MCC><MNC>Replace <MCC> with the Mobile Country Code and <MNC> with the Mobile Network Code of the network you wish to connect to manually. For example, to connect to a specific network manually, you would need to know its MCC and MNC.
During manual network selection, the module disregards the FPLMN list. If a data connection is successfully established with the manually selected network, the module will typically automatically remove that network from the SIM’s FPLMN list. This is because a successful connection indicates that the network is indeed available and should no longer be considered forbidden.
Important Warning: It’s worth noting that some cellular modules might be configured to prevent manual connection to networks listed in the FPLMN, even with the AT+COPS command. Again, referring to the manufacturer’s AT Command Manual for your specific module is crucial to understand its capabilities and limitations.
For a comprehensive understanding of network selection commands, refer to detailed resources on the AT+COPS command.
Alt: Automotive telematics system depicting connected car technology and network communication.
Alternative Methods Without AT Commands
If you lack direct access to AT commands or prefer simpler methods, there are a few GSMA standard-compliant approaches you can try to clear the FPLMN list. These methods rely on the device’s standard behavior when SIM cards are changed or relocated.
SIM Card Removal and Reinsertion
Some devices are designed to clear the FPLMN list when a new SIM card is inserted. This is a straightforward method if you are using standard plastic SIM cards and have physical access to the device.
- Power off the device.
- Remove the SIM card.
- Power on the device without the SIM card.
- Power off the device again.
- Reinsert the SIM card.
- Power on the device.
Upon restarting with the SIM reinserted, the device may attempt to reconnect to networks that were previously on the FPLMN list, assuming the device’s firmware is configured to clear the list on SIM change.
Unfortunately, this method is not applicable to devices utilizing embedded SIMs (eSIMs) as eSIMs are not physically removable.
Utilizing a Mobile Phone for SIM Card Reset
You can leverage a mobile phone to potentially clear the FPLMN list on your SIM card.
- Remove the SIM card from your automotive device.
- Insert the SIM card into a mobile phone.
- Check for Network Connection: Allow the phone to attempt automatic network connection. If it connects successfully, it might implicitly clear the FPLMN list.
- Manual Network Selection on Phone (if needed): If automatic connection fails, navigate to the phone’s network settings and attempt manual network selection. Force the phone to scan for available networks (this might take a few minutes). Then, manually select a network to connect to. Successful manual connection on the phone can also clear the FPLMN list.
If the SIM card fails to connect to any network, even with manual selection on a phone, it’s likely that the FPLMN list is not the primary issue. The problem might reside at the network operator level, such as service outages or account restrictions.
Location Change for FPLMN List Rollover
The FPLMN list typically has a limited storage capacity, often around four network entries. As the device encounters new networks and adds them to the FPLMN list, older entries are gradually removed in a rollover fashion.
While a more drastic measure, relocating the device to a different geographical area, especially to a different country with access to entirely new sets of networks, can indirectly lead to clearing the FPLMN list. As the device attempts to connect to these new networks and potentially gets denied from some, the FPLMN list will update, eventually overwriting older entries, which might include the problematic networks.
This method is less precise and relies on chance, but in situations where other methods are not feasible, changing location can be considered as a last resort to indirectly refresh the FPLMN list over time.
By understanding the FPLMN list and applying these troubleshooting techniques, you can effectively address SIM card connectivity issues in automotive devices and ensure reliable network access for critical vehicle functions and telematics services.
