  Untrusted node networks initially implemented by
measurement-device-independent quantum key distribution (MDI-QKD) protocol are
a crucial step on the roadmap of the quantum Internet. Considering extensive
QKD implementations of trusted node networks, a workable upgrading tactic of
existing networks toward MDI networks needs to be explicit. Here, referring to
the nonstandalone (NSA) network of 5G, we propose an NSA-MDI scheme as an
evolutionary selection for existing phase-encoding BB84 networks. Our solution
can upgrade the BB84 networks and terminals that employ various phase-encoding
schemes to immediately support MDI without hardware changes. This
cost-effective upgrade effectively promotes the deployment of MDI networks as a
step of untrusted node networks while taking full advantage of existing
networks. In addition, the diversified demands on security and bandwidth are
satisfied, and network survivability is improved.
