Manual
Cisco Packet Data Serving Node (PDSN) Release 2.0
Resource Management
35
12.3(11)T
How Prepaid Works in PDSN
When a prepaid mobile user makes a data service call, the MS establishes a Point-to-Point Protocol
(PPP) link with the Cisco PDSN. The Cisco PDSN authenticates the mobile station by communicating
with the AAA server. The AAA server verifies that the user is a valid prepaid subscriber, determines
what services are available for the user, and tracks usage for billing.
The methods used to assign an IP address and the nature of the connection are similar to those discussed in
the “How PDSN Works” section on page 4.
The following sections describe the IP addressing and communication levels in the prepaid environment for
each respective topic:
• Prepaid Simple IP Call Flow
• Prepaid Mobile IP Call Flow
Prepaid Simple IP Call Flow
In the following scenario, the prepaid user has sufficient credit and makes a Simple IP data call. The user
disconnects at the end of the call.
Step 1 The MS originates a call by sending an origination message. A traffic channel is assigned, and the MS
is authenticated using CHAP.
Step 2 The PDSN determines that a Simple IP flow is requested and sends an Access Request to the RADIUS
server.
Step 3 The RADIUS Server looks up the user’s profile and determines that user has prepaid service. It sends an
initial authentication request to the billing server.
Step 4 The billing server checks that the user has sufficient quota to make a call, and returns the result.
Step 5 The RADIUS Server sends an Access Accept message to PDSN indicating that this is a prepaid user.
Step 6 The PDSN completes the PPP connection, and an IP address is assigned to the MS.
Step 7 PDSN sends an Accounting Request (Start) as normal, and sends an Access Request to AR for initial
quota authorization. The request contains the Service Id VSA that indicates the call is Simple IP.
Step 8 The RADIUS Server, knowing that this is a prepaid user, sends an initial quota authorization request to
the billing server, which returns the quota information to the RADIUS Server. The RADIUS Server
includes the quota information in the Access Accept message and sends it to the PDSN.
Step 9 The PDSN saves the received quota information and monitors user data against this. When the quota is
used up, the PDSN sends an Access Request to AR indicating the usage and reason “Quota Depleted.”
Step 10 The RADIUS Server then sends a re-authorization request to PBS, which updates the user’s account,
allocates additional quota, and returns the new quota information to the RADIUS Server.
Step 11 The RADIUS Server includes the new quota information in the Access Accept message and sends it to
the PDSN. The PDSN updates the new quota information in its tables, and adjusts the usage to allow for
quota that was used since the Access Request was sent. The PDSN then continues to monitor the user
data. Steps 9 - 11 are repeated as long as the user has sufficient quota.