Cisco Cisco IOS Software Release 12.4(22)XR

After the PDSN receives the access-accept message from the AAA server, it parses the RADIUS packet 
and retrieves the attributes inside it. The PDSN stores the information present in the packet regarding 
the quota allocated for the flow and the threshold corresponding to the allocated flow. It also stores the 
Quota-ID allocated in the user flow present in the message. Once the flow for the user comes up (IP 
address assigned for SIP, or MIP RRP received from the HA and sent to the MS), the PDSN starts 
metering the user’s traffic over the flow against the allocated quota.

User data (IP datagrams) that flow through each prepaid flow is accounted in both upstream and 
downstream directions. The bytes consumed are checked against the quota allocated for the flow by the 
billing server.

Once the volume threshold value reaches the allocated quota for the prepaid flow, the PDSN sends an 
access-request message to the AAA server to refresh quota for the user. This RADIUS packet contains 
a PPAQ VSA, which includes following parameters:

Update-Reason Sub-Type that is set to indicate “Threshold reached” (= 3)

Quota ID previously received 

Used volume in the VolumeQuota Sub-Type

HAAA authenticates the RADIUS packet and if authentication is successful, forwards the 
prepaid-related information present in the packet to the billing server.

The billing server updates its database with the amount of quota the user utilizes. Since the user indicates 
quota renewal, the billing server apportions a fraction of prepaid account balance of the user. It also 
assigns a new quota ID for the current allocated quota and a corresponding threshold value for the 
assigned quota. This information is passed on to HAAA.

The HAAA sends the information received from the billing server into a RADIUS access-accept 
message to be sent to PDSN. The attributes that are encapsulated into a PPAQ VSA include:

Quota ID

Allocated quota into VolumeQuota parameter

Threshold corresponding to the assigned quota into VolumeThreshold parameter.

After the PDSN receives the access-accept message from the AAA server, it parses the RADIUS packet 
and retrieves the attributes inside it. The PDSN stores the information present in the packet and updates 
the quota allocated for the flow and the current threshold value corresponding to the allocated flow. It 
also stores the new Quota-ID allocated for the current quota. 

User data (IP datagrams) continues to flow through the prepaid flow, and is accounted in both upstream 
and downstream directions. The bytes consumed are checked against the quota allocated for the flow.

The PDSN decides to close the prepaid flow based on the following criteria:

The access-request message was sent to renew the quota and the corresponding access-accept 
message was not received from the AAA server after a configurable time value. This time is the same 
as the RADIUS message timeout configured on the PDSN.

An access-accept was sent to retrieve quota and before access-accept can be received, the remaining 
VolumeQuota is consumed. This is when the VolumeQuota value and the VolumeThreshold values 
become same.

In this case, PDSN sends an access-request message containing the PPAQ VSA that includes:

Update-Reason Sub-Type to indicate 'Quota reached' (= 4)

Amount of quota used by the user in VolumeQuota attribute.

At this time, the PDSN marks the prepaid flow as being marked for deleted, such that it does not switch 
any packets through it for the prepaid flow. It does not delete the prepaid flow immediately and waits for 
the response of the access-request or timeout of the access-request message.