ZigBee Packet Sniffers and Security Models

Hello,

I am using Wireshark&TiWsPc and Wireshark&KillerBee to sniff ZigBee frames so in this scenario;

I need to understand

1-how the sniffers get Network Key (and-or also who),

2-how the sniffer decrypt the frames? Or 2-Who decrypts frames?

There are BBB as the gateway(Linux gw home design),cc2650 as sensor tag in my ZigBee network topology.

3-Which of these models(Distributed Security Model, Centralized Security Model) does have my network?