**Pre-paid billing**

This mode is that the user pays first and the operator check user account information. The user uses the services according to the balance in his account. Pre-paid billing can get the cost of settlement at real-time . According to the balance of the user's account, the operator can control the user's usage of services to ensure the interest of itself.

The difference between pre-paid billing and post-paid billing is that pre-paid billing must pay first and then use the services. The user's accounting information is saved in the system previously. ASN-GW get the information of the balance and during the services ASN-GW diminish this balance.When the balance runs out, ASN-GW request the system to redistribute the account balance, and so on in turn . When all of the user's accounting balance runs out, the system will terminate user's services initiatively.

At present WiMAX can not support pre-paid billing . You can see the following exsiting reference structure of pre-paid billing:

Pre-paid billing entity includes:

AAA-server: Provide authentication, authorization and accounting services.

PPS: Pre-paid server. Control the services of pre-paid billing user.

PPC: Pre-paid client. Provide services according to the PPS pre-paid control strategy.

Fig. 10. Pre-paid Billing Reference Structure

#### **Roaming billing**

In the actual roaming environment, it may include many scenarios and network configuration . In order to manage this complexity, it defines an universal roaming model and it is shown in the following figure. There is only one accounting mode that the operator send the bill to users.

Fig. 11. Universal Roaming Model

In the above roaming model, WiMAX is divided into three parts : Visited Network, Roaming Agency and Home Network. By exchanging the information between these parts, it can provide authentication, authorization and accounting services to ensure the user to access to WiMAX.

In WiMAX, roaming agency is an optional part. It will use roaming agency or not according to the actual need. Roaming agency takes on an intermediary role in multiple roaming . In the actual roaming environment, an operator must communicate with several operators to realize the roaming of WiMAX. If they communicate with each other directly, it will be very difficult and its cost will be very large . However, if a operator access to the roaming agency, it can communicate with the other operators which are attached to the roaming agency. This is similar to the structure of Internet. Currently iPass and GRIC are most influential in roaming agency.

Roaming agency can be divided into two parts:

