Accidental Border Roaming Prevention – Technical Overview

Border Roaming Prevention mechanism is an algorithm for rejecting registration of subscribers to neighbor networks accidentally. Signaling level solutions enable determining whether a customer is in an accidental roaming situation and acting accordingly. These solutions identify the cell in which the customer is located and apply decision logics that affect the customer’s registration process, allowing him to remain in his home network. 

Signaling-based techniques enable follow-ups on the customer’s next registration to improve the identification of cells that cause accidental roaming cases in real time. Similarly, they detect gaps in coverage from the home operator and allow the customer to register in the visited network in such cases. Accordingly, the home operator gains useful information for intelligently deploying the missing coverage. These solutions have the potential to significantly reduce cases of unintentional roaming.

As another method, Selective cell blocking for the nearby operator that causes inadvertent roaming can be applied for preventing accidental roaming. One of the deficits of the technique is the necessity of detecting the cells that originate the accidental roaming cases which obligates the coordinated workload of the mobile operators. Another disadvantage is that subscribers experiencing genuine roaming in the neighboring country are left without coverage. [1]

RaaS’s Border Roaming Prevention is a signaling-based service that probes international links in order to monitor Location Update (LU) and other related MAP operations. 

The system architecture is depicted below;

Internal database (DB) system is queried in order to decide if accidental roaming occurs or not. While looking at DB, RaaS uses Location Area Identifier (LAI) of new Location Update, counters that hold last periodic registration, etc. If RaaS decides that there is accidental roaming, then it rejects LU with an appropriate error. After MS gets rejected to LU request, it tries to register to another network that is found in the preferred PLMN list. This logic continues till MS tries to register to HPMN or RaaS figures out that subscribers could not get any service from HPMN. 

Basic call flow is described below;

  1. Subscriber powers on MS, MS sends Location Update message to VPLMN.
  2. Location Update is routed to HLR over international links, which are monitored by RaaS.
  3. RaaS detects Location Update and starts an evaluation process. It retrieves cell id from the HLR or VLR (depending on network features), and decides that this is an accidental roaming.
  4. In the meantime, since RaaS works as passive, registration of MS in VPLMN is completed.
  5. In order to invalidate the registration of MS in VPLMN, RaaS sends CL to VPLMN.
  6. After CL is acknowledged by VPLMN, RaaS sends PRN to VPLMN which forces the VLR to initiate data restore.
  7. RaaS intercepts the data restore process between HLR and VLR, and reports to VLR that roaming is not supported.
  8. Sending respectively CL and PRN forces MS to re-register at incoming/outgoing calls.
  9. MS searches for alternative networks and re-registers in HPLMN.

Border Roaming Prevention Signaling Flow
Border Roaming Prevention Signaling Flow

Triggering Scenarios

RaaS monitors international links and has an overlap database which contains mapping relationships between any two of the following: a NPMN VMSC, a HPMN VMSC, a NPMN cell-id and a HPMN cell-id. 

The prevention mechanism is triggered if the NPMN VMSC is border VMSC and NPMN cell-id is a border cell-id. Cell-id is learned by sending a MAP-ATI message to NPMN. After triggering, RaaS determines whether to start-up prevention mechanism or not. 

RaaS is triggered by the following scenarios;

In the scenarios; ‘HLR’ stands for ‘Home Location Register’ and ‘MSC’ represents ‘Mobile Switching Center’. ‘GW’ stands for ‘Gateway’, ‘HPMN’ represents ‘Home Public Mobile Network’ and ‘NPMN’ represents ‘Near Public Mobile Network’.

Why RaaS?

  • Better precision: Cell ID level registration is offered to create Border areas which would provide a better precision. LAC and Cell ID based registration for 2G; LAC, SAC and Cell ID for 3G; TAC and Cell ID for 4G registration is provided.
  • Customization: Subscriber group specific border rule management is offered. Attempt number and registration percentages can be customized per each network and subscriber group.
  • Efficiency: The system does not waste resources for genuine roaming cases into distant (non-neighboring) networks.
  • Easy management: All the processes can be managed through our user-friendly interface.
  • It allows complete control of location update attempts in border areas, through comprehensive rules and control parameters.



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