3. THE SYSTEM DYNAMICS BASED RELIABLITY EVALUATION MODEL The system dynamics model is composed by the causal loop diagram and the stock and flow diagram.
3.1 THE CAUSAL LOOP DIAGRAM OF THE BILLING SYSTEM OF NGBOSS In the causal loop diagram, there are three loops: the recharging loop, the product using loop, and the unsubscribing loop. In the recharging loop, the balance would increase when the user recharge. In the product using loop, after the user subscribed and used the product, the balance would decrease. In the unsubscribing loop, after the user subscribe product, if the user unsubscribe the product, the balance would increase. The loops are presented in figure 4.
Figure 4. The causal loop diagram of the billing system of NGBOSS
3.2 THE STOCK AND FLOW DIAGRAM OF THE BILLING SYSTEM OF NGBOSS The stock and flow diagram is given in figure 5.
Figure 5. The stock and glow diagram of the billing system of NGBOSS
The balance is affected by the activities of recharging and billing. The user’s behavior of recharging would increase the value of his or her balance. The behavior of billing would decrease the value of balance. There are three ways of recharging: Internet bank recharging, telephone recharging and card/cash recharging. And the user’s actions of package subscribing, package unsubscribing and product using would take effect on the value of billing.
4. AN APPLICATION OF RELIABILITY EVALUATION TO BILLING SYSTEM OF NGBOSS The architecture of the billing system of NGBOSS is represented in figure 6.