The Secondary Users (CR CPE) send UCS_Notification(Urgent Coexistence Situation) Packets to the Base Station when they detect the presence of Primary Users (incumbents) during Spectrum Sensing.

The UCS notifications enable CPEs to transmit measurements report to indicate the presence of incumbents. 

The exact time stamp when this packet was transmitted can be seen from the Packet Trace file in NetSim. 

Also refer: Network Simulator, NetSim, Emulator, 5G, Military Communication, Vehicular networks (