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In the access control context, PDP cannot effectively function without PEP (Policy Enforcement Point) in a framework since their roles are complementary. PDP receives an access request from PEP; performs an evaluation against the predefined policies; makes a decision (usually permix X deny); sends the decision back to PEP which then enforces the decision.
Simply put, PDP is the ‘brains' and PEP is the 'muscle’ in the access control operation within a system.