1) How to ensure proper device interworking with a Multi-IMSI SIM solution
Device should have a 3 minute minimum active period:
o IMSI rotation may fail if this isn’t ensured. Sleep mode (or disabling modem) in between will disrupt the IMSI rotation sequence.
o This minimum threshold may be higher for devices which need to scan different NB-IoT/LTE-M bands. In this case, we recommend fine tuning your device by reducing the number of bands scanned.
Device (terminal and modem) should support SIM Toolkit Application protocol:
o Release 99 implementation is mandatory, in order to ensure successful applet logic.
o IMSI rotation will fail if support of this application is not guaranteed.
2) What is STK?
STK stands for SIM Toolkit Application. This protocol is specified by:
3GPP TS 11.14 (v8 / Rel99).
3GPP TS 31.111 (includes USIM Application Toolkit for 3/4G networks).
STK support in devices is mandatory to handle Multi-IMSI solutions.
3) Which commands within the STK are used for the Multi-IMSI solution?
Multi-IMSI applet is relying on the following standard STK Proactive commands:
Setup Menu
Provide Location Info
Timer Management
Refresh
Full details of the functioning of commands above can be found in both specifications 3GPP TS 11.14 and 3GPP TS 31.111.
4. Under which conditions will the Multi-IMSI applet will trigger an IMSI rotation?
Device will be forced to reboot and use a new IMSI identity after a 2 minute period when:
Network attach is rejected by all networks with the current IMSI (e.g. none of the available networks in this location are allowed in the Roaming Profile with the current IMSI).
Device lands in a new country where the priority IMSI isn’t currently active .
The SIM card being notified that the network has “Limited Service” (timeouts during the Authentication procedure or poor coverage for an extended period of time).
5. What is the forbidden PLMN list and how does it impact network searching?
The forbidden PLMN list is dynamically stored in the SIM and maintained by the device. When a device attempts to connect to a network and is denied, it will insert that network to the fPLMN list, thus preventing it from attempting to connect to the same network again.
If all available networks are added to the fPLMN list, the only way to ensure the modem will keep trying to connect will be to either make manual registration attempts (possible in phones but very unlikely in IoT devices) or clean the content of the fPLMN list.
The BICS Multi-IMSI applet will clear the fPLMN list content after every IMSI rotation.
6. Which network will my device choose to connect to?
The first time a SIM attempts to connect in a country, if no other preferences are explicitly set, a device will choose based on the signal strength coming from all the available networks. If the strength is better than -85 dBm, a network will be chosen randomly, if the signal strength is lower, the network with the highest signal strength will be selected.
Remark: Networks contained in the forbidden PLMN list will be ignored by the modem during this phase.
For subsequent connections, the SIM will normally instruct the device to register under the last used network (and with the same technology), minimizing the registration time.
Inside a country, BICS is not steering to any specific networks, apart from the screening based on the customer’s Roaming Profile or blacklisting.
What other best practices are recommended to optimize the device behaviour?
Recommendations we advise to firmware and app developers for an improved service:
The fPLMN list should be cleared by the device after every power cycle.
Perform manual network attach when device is out of service for an extended period of time.
TROUBLESHOOTING
My device is up and running but it’s not trying to connect to any network
This can be caused by one of following reasons:
Full fPLMN list.
To solve this situation, it’s highly recommended to have means on the device to clear the forbidden PLMN list periodically, like this new registration attempts will occur, and eventual IMSI rotation will be triggered.
Out of coverage.
Stuck on an IMSI without available networks.
My device is attached but it cannot exchange any data
This can be caused by one of following reasons:
APN isn’t configured, hence data session isn’t created.
APN in the device isn’t aligned with the APNs present in the APN group.
Network interface isn’t associated to the EPS bearer.
Under NB-IoT/LTE-M, can be due to VPMN policies.
My device is unable to switch IMSIs
This can be caused by one of following reasons:
Active period of the device is shorter than 3 minutes.
Unexpected STK call flow between device and applet.
Modem is not supporting STK or debugging mode is active.
For more information, please see full BICS Technical Specification