Prepaid and Postpaid Subscriber Roaming

International roaming allows a subscriber to access services virtually anywhere in the world. The visited network obviously needs to charge foreign subscribers for access time, transport and services. As the visited network is not in position to directly bill the roamers, it invoices their home network for the services usage. The home network then charge its own subscribers for the services used while roaming in a foreign network, using standard retail billing mechanism. This process works fine with postpaid subscribers as they are known to a service provider and bound by subscription agreement to pay for the services. The prepaid subscribers, on the other hand, pay for the services upfront. The service providers perform credit checks on each service initiation and decide if the service shall be provided or not. In case of roaming, the VPLMN, which processes and controls the calls/services initiated by a roamer has no information on available credit. To enable roaming for prepaid, the HPLMN must take control of all the services initiated by a prepaid subscriber in a foreign network. Therefore, roaming implementation for the prepaid subscribers is different from the implementation for their postpaid counterparts.

USSD callback.

Figure 1-1 USSD callback.

Today, most of the prepaid implementations are based on CAMEL or USSD callback. CAMEL is a network feature. CAMEL allows users to access services in a visited network transparently. Using CAMEL, the HPLMN has full control on the services used by a prepaid roamer in a visited network. Both the HPLMN and the VPLMN must be CAMEL-enabled to implement prepaid roaming services.

USSD callback allows a prepaid roamer to request to make a call in a foreign network by sending the called party number in a predefined USSD message. Figure 1-1 shows the USSD callback concept. The message is routed back to the prepaid system in the home network. The prepaid platform then performs the necessary credit checks and initiates two outgoing calls, i.e., one to the roamer and another call to the called party as requested by the roamer. The HPLMN monitors the call and may decide to disconnect after appropriate notification if the credit balance is running out.

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