IEEE 802.15.4 topology

There are two fundamental device types in IEEE 802.15.4:

• Full function device (FFD): Supports any network topology, can be a network (PAN) coordinator and can communicate to any device PAN coordinator
• Reduced function device (RFD): Limited to only a star topology, cannot perform as a network coordinator, can only communicate with a network coordinator

The star topology is the simplest but requires all messages between peer nodes to travel through the PAN coordinator for routing. A peer-to-peer topology is a typical mesh and can communicate directly with neighbor nodes. To build more complicated networks and topologies is the duty of the higher level protocols, which we will discuss in the Zigbee section. 

The PAN coordinator has a unique role that is to set up and manage the PAN. It also has the duty of transmitting network beacons and storing node information. Unlike sensors that may use battery or energy harvesting power sources, the PAN coordinator is constantly receiving transmissions and is usually on a dedicated power line (wall power). The PAN coordinator is always an FFD.

The RFD or even low power FFDs can be battery based. Their role is to search for available networks and transfer data as necessary. These devices can be put into a sleep state for very long periods of time. Shown below is a diagram of a star versus peer-to-peer topology. 

Within the PAN, broadcast messages are allowed. To broadcast to the entire fabric, one only needs to specify the PAN ID of 0xFFFF.