The PCA Application uses the DSR pplication Infrastructure (DAI), which provides a mechanism for Diameter messages routing and for status updates between the Diameter Routing Function and the DAI.
Table 1 describes two functions for communication between the Diameter Routing Function and the DAI.
| Function | Communication Direction | Description |
|---|---|---|
| Application Data | PCA <-> Diameter Routing Function | Either a Request or an Answer with supporting information |
| Application Status | PCA <->Diameter Routing Function | The PCA Operational Status of Available, Degraded, or Unavailable |
Request Routing
As shown in the following figure, the Diameter Request messages are routed from the Diameter Routing Function to the PCA based on the configured Application Routing Rule, and routed from the PCA to the Diameter Routing Function, all using the Application-Data function. The PCA will return the Request to the Diameter Routing Function for Peer Routing Rule processing and routing.
Request Processing at the Diameter Routing Function and PCA 
Answer Routing
When the PCA forwards a Request message to the Diameter Routing Function for routing, it must inform the Diameter Routing Function how to process the corresponding Answer. It can inform the Diameter Routing Function either to route the Answer to the PCA or to route the Answer to the downstream Peer without involving the PCA. Figure 2 shows the case where an Answer is transmitted back to the PCA. After the PCA completes processing of the Answer, it will send it to the Diameter Routing Function for transmission to the Diameter Transport Function so that it can be routed to the downstream Peer.
Answer Processing at the Diameter Routing Function and PCA
PCA Generated Answer
In some cases, the PCA needs to generate an Answer message in response to an incoming Request. For example, the Policy DRA function cannot find a PCRF to route the Request message to. Figure 3 shows the Diameter Routing Function routing for this scenario.
PCA Generated Answer Routing
PCA Generated Request
In some cases, the PCA needs to generate Diameter Requests. Figure 4 shows the Diameter Routing Function routing for this scenario.
PCA Generated Request Routing
Policy DRA Function Use Cases
The following typical Policy DRA signaling use cases demonstrate the Policy DRA and SBR capabilities to establish subscriber binding to some PCRF, and update and terminate the sessions when requested:
- Binding and Session Creation and Session Termination over the Gx Interface - A Policy Client requests to bind a subscriber for policy provisioning over a Gx interface. The Policy DRA creates the binding to a selected PCRF, generates the binding and session records in the Policy SBR database, updates the session as requested, and eventually terminate session as requested.
- Subscriber Session Creation and Termination over the Rx Interface - A Diameter Request is sent to the Policy DRA over the Rx interface for the same subscriber that has established a binding with the PCRF over the Gx interface. The Policy DRA coordinates the sessions over the Gx and Rx interfaces and routes the Diameter messages to the same PCRF.
- Policy DRA in Roaming Scenarios - In addition to communicating to the Policy Clients and Policy servers through Gx/Gxx and Rx interfaces in their own networks, the Policy DRAs can communicate to each other across the Visited Access and Home Access networks through the S9 interface, for session binding purposes. See Policy DRA in Roaming Scenarios.