How a 5G NSA phone attaches via LTE before adding 5G, traced in a lab capture

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In 5G Non-Standalone (EN-DC), the phone attaches to the 4G core through an LTE anchor, with 5G negotiated during attach but used later. A walkthrough of a 40-frame capture from an open5gs test network shows the UE setting the DCNR bit and the core marking NR as not restricted. A measurement event (B1-NR) triggers the addition of an NR secondary leg, after which the eNB hands off to a gNB via X2 and the bearer moves to NR. The capture is a lab setup with null encryption and no NAS authentication, and it covers only the LTE side, not the NR radio leg. The full frame-by-frame analysis is published on EventHelix.

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