The previous article in this series discussed the characteristics of the physical part of the radio including the frequencies, data rates, power and bandwidth of the radio. Now we are going to address the organization of the information on the “wire” or air in this case.
This part of 802.15.4, and really any other networking technology, is termed Media Access and the low level software that implements the Media Access is called the MAC or Media Access Controller. A MAC is responsible for both organizing the data received from the application into low level data frames, synchronizing access to the medium, checking for errors and a myriad of other things.
One of the tasks of an 802.15.4 MAC is to present the device to the network as one of the two types of devices defined by the 15.4 specification; a Full function Device (FFD) or a Reduced Function Device (RFD).
Reduced Function Nodes have very limited capabilities. RFDs are single purpose end device nodes like a temperature sensor or a light switch. These devices are often battery powered as they need only communicate intermittently. Reduced function Nodes can only communicate with a single Full function Nodes (FFD). FFD nodes can have device capabilities like RFDs but unlike RFDs they can extend the network by serving as network coordinators.
One way nodes can be organized is a Peer Network as shown in Figure 1. Any FFD can communicate with any other node in range. Each RFD associates with one and only one FFD. The PAN coordinator provides the interface and structure for the entire network.
A network containing only FFDs is a true peer network with the capability for any FFD to communicate with any other FFD in range. Figure 2 illustrates an FFD network composed of Full Function Devices.
There are two types of network coordinators; coordinators and PAN coordinators. Both are FFDs. Coordinators are FFDs that provide links or associations to other FFDs and RFDs. There can be multiple coordinator devices in a PAN but only one PAN coordinator. The PAN coordinator (Red Node in the diagrams) is the “owner” of the PAN and provides the PAN ID that uniquely identifies the network to the outside world.
The 15.4 standard does not define how the PAN ID is selected. Any FFD can decide to chose a PAN ID and become a PAN Coordinator. Other FFDs and RFDs can then associate with it and “grow” a wireless network. [If that sounds somewhat haphazard you need to remember that we are discussing the very lowest level software layer here. There are other higher layers that provide much more structure.]
A PAN coordinator and most FFDs are typically powered nodes while RFDs are often battery powered. The PAN coordinator will usually have access to a wired network and be …