Example: Limiting Subscriber Access Based on Account Balances

The sample data provides an example called Quota that limits a subscriber’s access rate based on the balances of accounts that record the subscriber’s use of network resources. Subscribers receive a quota of transfer (upload and download) volume in two ways:

• Periodic quota—Volume that is periodically added to a subscriber’s account. For example, a subscriber may receive a 25-MB periodic quota each month. The periodic quota is tracked in the periodic account.
• Bought quota—Additional volume that a subscriber can purchase and use at any time. For example, a subscriber may purchase 25 MB of bought quota in January, and use the bought quota between January and March. Bought quota is tracked in a bought account.

As a subscriber consumes volume, the SRC VTA debits the accounts, using first the periodic quota and, if no periodic quota is available, the bought quota.

Subscribers managed by the SRC VTA require a subscription to quota services—services for which the SRC VTA monitors and manages usage. You must configure these subscriptions to be activated when the subscriber logs in. When a subscriber logs in to the SAE, the SAE tries to activate the quota services. However, if neither the periodic account nor the bought account has a positive balance, the SRC VTA deactivates the quota services, and the SAE applies to the subscriber either the default policy or another policy that implements a service with a lower bandwidth.

The units of the accounts depend on the formula that you define to determine the use of network resources. With this formula, the SRC VTA calculates the change to the accounts and the interim accounting interval.

The Quota configuration example provides an SRC VTA that operates as follows:

1. When a service session for the quota service starts, the SAE sends a start event to the SRC VTA.
2. The SRC VTA starts a session that has the same identifier as the service session and a qualifier of zero.
3. When the SRC VTA receives the first interim update from the SAE in the SRC VTA session, it records a balance change that details the use of resources and the event time for the SRC VTA session. When the SRC VTA receives interim updates in the SRC VTA session, it updates the use of resources and the event time in the balance change recorded previously.
   • If the periodic account contains sufficient resources to cover the balance change, the SRC VTA changes only the balance of that account.
   • If the periodic account does not contain sufficient resources for the change, the SRC VTA records one balance change for the resources available in that account and records another balance change for the difference in the bought account. In this case, the SRC VTA records subsequent balance changes to the bought account.
   • If neither account has sufficient resources, the SRC VTA deactivates the quota service.
4. If the SAE session ends, the SRC VTA session ends. When a new service session starts, Steps 1 to 3 recur. However, a service session may last for several SRC VTA sessions. In this case, the SAE and SRC VTA continue the process described in Step 3.
5. When an administrator replenishes the periodic quota, the SRC VTA ends the session, finalizes all balance changes for the session, and records a credit to the periodic account.
6. When the subscriber buys additional volume, the SRC VTA ends the session, finalizes all balance changes for the session, and records a credit to the bought account.
7. When the SRC VTA next receives an interim update event from the SAE, it starts a new session. The SRC VTA obtains the start time for the session from the SAE event, and records debits to the accounts as described in Step 3.

   The SRC VTA always ends the session when an administrator replenishes periodic quota or the subscriber buys volume. However, a service session may last for several billing periods. In this case, when the SRC VTA starts a new session, it continues to assign the SAE session identifier to the session and increments the qualifier by one. Keeping the session within a billing period allows the SRC VTA to finalize balance changes.