XnAP

XnAP is the protocol gNBs use to talk to each other directly, over the Xn interface, without routing everything through the core. Its headline job is mobility: when a UE moves between two neighbouring gNBs, an Xn-based handover lets the source and target coordinate the move between themselves — faster and lighter than dragging the AMF into every handover. It also supports dual connectivity coordination and the exchange of load and interference information so neighbours can manage resources sensibly.

It's the 5G counterpart to X2AP in LTE, and the design intent is the same: keep local, frequent operations local. There's a fallback too — if there's no Xn relationship between two gNBs (or it's not usable), the handover goes the long way through the AMF as an N2-based handover instead. Like the other RAN control protocols, XnAP runs over SCTP. When you're investigating handover failures between adjacent 5G cells, whether Xn is configured and healthy between them is one of the first things to confirm.