Managing Visited Network NAS Signaling Overrides during Cellular IoT Roaming
Managing visited cellular network NAS overrides requires firmware that accepts assigned timers, prevents aggressive attach retries, and protects battery life.
ESM Cause 26
Meaning
A session management error code generated by the core cellular network indicates that the requested packet data network connection cannot be established due to resource limitations in the network. When a cellular modem receives ESM cause 26 during a connection request, it must suspend further attempts to connect to that specific access point name. This action prevents the terminal from spamming the network during periods of high congestion.
Network Response
Congestion in the packet core or the radio access network triggers the generation of this specific error code by the mobility management entity. The transmission of ESM cause 26 usually includes a back-off timer to inform the module of the duration for which it must remain silent. This mechanism helps the operator manage network load.
Integration Guideline
Software drivers for the cellular module must intercept this error code and handle it according to the network operator requirements. The driver of the system parses the ESM cause 26 response and extracts the associated back-off value. If the timer is omitted, the terminal must implement a randomized exponential back-off to prevent simultaneous reconnect attempts by thousands of deployed devices, which would otherwise destabilize the cellular cell.
Diagnostic Validation
Testing the error handling routine involves using a network simulator to inject the rejection code during the attach procedure. The test engineer verifies that the device respects the timer. This verification is required for carrier acceptance.