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LAU Reject: PLMN Not Allowed Explained

mobile communication location area update network plmn wireless

There are many reasons why a Location Area Update (LAU) can fail. One such reason is when the network sends a message to the mobile station (MS) indicating LAU reject with the cause “PLMN not allowed.” This is a relatively straightforward reject cause that engineers sometimes need to analyze, whether in the lab or while testing the device under test (DUT) in the field.

The DUT can have one Radio Access Technology (RAT) or multiple RATs running on the chip. This DUT is often referred to as a multi-mode modem.

LAU reject-PLMN not allowed

This situation arises when a mobile station subscribed to PLMN ‘A’ tries to access PLMN ‘B’, for which it is not subscribed.

In this scenario, the network’s response to the location update request from the mobile will include the cause “PLMN not allowed.” This means the mobile station is not allowed to access any part of PLMN ‘B’. The MS needs to manually or automatically search for other PLMNs in its coverage area.

This is the simplest case: imagine an MS subscribed to Verizon Wireless attempting to register with T-Mobile, and there’s no roaming agreement between the two networks. The T-Mobile network will reject the Location Area Update (LAU) request with the cause “PLMN not allowed.”