AF
Application Function: an entity that interacts with the 5GC (via NEF or PCF) to influence traffic routing, policy decisions, and QoS for specific application services.
An AF is anything that wants to ask the network to do something on an application's behalf. A content provider's server, an enterprise platform, an IMS function, a MEC application — they reach into the 5G core to influence traffic routing, request a QoS treatment for a flow, or subscribe to events like a device coming online.
How it connects depends on trust. An AF the operator trusts and runs inside its own security domain can talk straight to the PCF (and other functions). An external, third-party AF goes through the NEF, which authenticates it, throttles it, and translates its requests into internal signalling. A common real flow: a low-latency gaming or video AF asks for better QoS on a specific flow; that request becomes a PCF policy, an SMF QoS flow, and finally enforcement on the UPF. The AF is the network's hook for letting applications shape how their traffic is treated.
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