In the most generic sense, a binding is a mapping between a subscriber and a PCRF assigned to handle policy decisions for that subscriber. In 3GPP networks, however, there is more than one way to identify a subscriber. So rather than having just one binding table mapping subscribers to PCRFs, there are four tables mapping subscriber identifiers to the PCRF that handles the subscriber's policy decisions.
P-DRA supports four subscriber identifiers: IMSI, MSISDN, IPv4 IP Address, and IPv6 IP Address. Of these, IMSI and MSISDN are relatively permanent in that they do not change from call to call. IP addresses, on the other hand, are assigned by PCEFs to a subscriber's device for temporary use in accessing the Internet or other IP services.
Regardless of the type of subscriber identifier, the relationship of a subscriber to a PCRF assigned by the P-DRA must be accessible from anywhere in the P-DRA network. This means that the tables in the binding database must be accessible from all P-DRA DSR sites. For example, a given IMSI, when bound, will appear in exactly one record in the binding database, but will be accessible from any P-DRA DSR in the P-DRA network
PCRF Pooling examines the APN along with the IMSI, in the mapping of the message to a Pool of PCRFs, but with the restriction that before a new binding is created, the logic must check for existence of another binding to the same PCRF Pool for the IMSI. If such a binding exists, the new APN is bound to the same PCRF as an existing APN mapped to the same PCRF Pool. After a binding exists, all sessions for that IMSI and APN are routed to the bound PCRF. Sessions for that IMSI and a different APN mapped to a different PCRF Pool can be routed to a different PCRF. With PCRF Pooling, an IMSI can have up to 10 binding-capable sessions, which can be bound to different PCRFs based on APN.
Binding-capable session initiation requests includes both IMSI and an APN. Policy DRA maps APNs, to a PCRF tool via Policy and Charging > Configuration > Access Point Names.
P-DRA then checks to determine whether a Sub-Pool exists by locating the PCRF Pool and the Origin-Host from the session initiation request via Policy and Charging > Configuration > Policy DRA > PCRF Sub-Pool Selection Rules.
If the PCRF Pool and Origin-Host are mapped to a Sub-Pool, the Sub-Pool is used; otherwise, the PCRF Pool that was mapped to the APN is used.
The PCRF Pool or Sub-Pool is mapped to a PRT table via Policy and Charging > Configuration > Policy DRA > PCRF Pool To PRT Mapping on the SOAM GUI. The P-DRA application instructs the Diameter Routing Layer to use the PRT table associated with the PCRF Pool or Sub-Pool to route the request.
The Diameter Routing Layer selects the actual PCRF based on the Route Lists and Route Groups selected from the PRT Rules in the PRT table.
If no binding exists, a new binding is created using the IMSI, APN, and PCRF Pool. For new bindings, the actual PCRF is not determined until a success answer is received from the PCRF that processed the session initiation request.
A split binding occurs when more than one PCRF has an active session for the same IMSI, APN combination. P-DRA avoids creation of split bindings by searching for and honoring applicable existing bindings before creating new bindings.