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 the interface from wire to air. The FFDs and PAN coordinator may also have more computational capabilities then the RFDs which may do nothing more than pass their data to the next coordinator.
PAN coordinators address the devices associated to its network using a unique 64-bit address that is predefined by the device vendor. Alternatively, the PAN coordinator can assign a 16-bit short address to the device and address it with that address.
A second way of organizing a network is a Star network (Figure 3). In a Star network all nodes talk to a central coordinator and that coordinator synchronizes communications with other nodes. In a Star network there is only one coordinator and that is the PAN coordinator.
Star networks are limited geographically and numerically while a Peer network is theoretically unlimited. The PAN Coordinator in a Star network can only associate with a limited number of devices while coordinators of a Peer network could in theory extend the network continuously. Peer networks with higher level routing software form the basis of complex mesh networks with self organizing and self healing capabilities but those capabilities are not part of the 15.4 standard.
This is an area that generates a lot of confusion for the novice. There is no routing or organizing of nodes in a 15.4 network. Yes, it’s possible to have a very graphically diverse network of devices but there’s no mechanism in 15.4 to move information through the network.
An RFD can only transmit to its coordinator while an FFD can transmit to any node within “earshot”. To actually move data and route it around the network requires a higher level protocol, like Zigbee™. Zigbee adds the network and transport infrastructure required to support information transfer around a true Peer or Mesh network.
To associate with a network, nodes have to explicitly join a network by making themselves known to a Coordinator node that is already part of the network. If there are multiple networks available a node has the option to join any one of them. That’s where the PAN ID comes in. A node will typically be preconfigured to join a network with a specific PAN ID.
That does not mean that the node is associated with that network forever. Built into the 15.4 Media Access (MAC) is an association/disassociation strategy. This mechanism supports those applications where nodes are part of one network for a while and then leave to join some other network (think of a part moving through multiple production cells). Or an FFD can choose to start its own new network declaring itself the PAN Coordinator.
There’s a lot more to an 802.15.4 MAC and we’ll look at more of those important features in the next article.
John Rinaldi is the Technical Sales Manager for Real Time Automation in Brookfield, Wisconsin. RTA specializes in industrial and building automation software, hardware, systems and specialty controllers. He can be reached on 262-439-4999 or through the RTA website, http://www.rtaautomation.com/forms/contactus.html.
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