Meaning
Protocol failure sequences occur when a series of network attachment rejections triggers successive error handlers that can isolate a wireless device from the cellular network. In network rejection cascades, a single localized rejection, such as a full network or a subscription mismatch, causes the module to scan for alternative carriers. If these attempts are also rejected, the module might write multiple carriers to its forbidden list, eventually stranding the device.
Sequence Mechanism
The cascade begins when the primary network returns a rejection code during registration. The modem then scans for secondary networks, which may also return rejections due to the same subscription issue. This repetitive cycle can lead to a state where all available operators are added to the forbidden list, disabling the device.
Firmware Prevention
Telemetry firmware must be designed to break these failure sequences before they result in permanent lockouts. Implementing a monitoring system that counts consecutive rejections allows the controller to intervene. The controller can suspend connection attempts and wait for a long period before allowing the modem to clear its forbidden lists and try again.
Field Recovery
Automated recovery routines are required to restore connectivity without manual service calls. Forcing a SIM card refresh or cycling the power of the transceiver are common methods used to reset the registration state. This reset allows the device to restart the network selection process from a clean state.
