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LAU Reject: Roaming Not Allowed Explained

mobile communication roaming gsm network location area

This scenario occurs when the network sends a message to the mobile station (MS) indicating an LAU (Location Area Update) reject with the cause “Roaming Not Allowed.”

LAU Reject-Roaming Not Allowed

A mobile subscriber (MS) usually receives an LAU reject with this cause from the network, particularly in a VPLMN (visited PLMN), when the MS can only access a portion of the VPLMN, while another part remains inaccessible. In such instances, the MS will attempt to register with another PLMN within the same area of the VPLMN. Essentially, this cause informs the MS that “this PLMN in this specific area is not suitable for you; try another PLMN.”

As mentioned, this cause is always sent by the VPLMN and never the HPLMN (home PLMN) because the HPLMN is inherently designed to serve its subscribed MS. To illustrate this, consider a PLMN operator that possesses both Verizon Wireless and T-Mobile GSM networks.

Let’s say the PLMN operator Verizon has a roaming agreement with T-Mobile for all Location Areas (LAs) except LA2. Therefore, Verizon subscribers can roam in any of T-Mobile’s location areas except LA2. If a Verizon subscriber attempts to register in the LA2 area of the T-Mobile network, T-Mobile will transmit an LAU reject message to the MS with the reject cause “Roaming Not Allowed in this Location Area.”