The Juniper Networks Session and Resource Control portfolio is a dynamic network resource allocation solution that enables service providers to deliver differentiated products and services.Juniper Networks' SRC portfolio delivers granular dynamic policy enforcement on a per-service level.This allows you to deliver revenue-generating services on top of existing sessions. The SRC Series modules readily interface with your existing subscriber management databases, so you can map available network resources to subscriber and service profiles.
SRC policy Engine Maintains customer, network, and service configuration information to enable automated, policy-based service and subscriber provisioning.The SRC PE also allocates network resources in response to subscriber and application requests to ensure quality of service and experience and to protect the quality of previously accepted sessions.
This Policy Engine facilitates or make its necessary in presence of SBR carrier .
If you still need more information on this, I recommend you to work with you local Juniper System ENgineer ( account Team).
Hope this clarifies your query.
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As mentioned above they could coexist (but it's not necessary as SRC could work with not-Juniper RADIUS servers, same applies to SBR itself: it could handle requests coming from non-juniper equipment).