Wireless Sensor Networks : Technology, Protocols, and Applications

IEEE 802.15.4 LR-WPANs STANDARD CASE STUDY 179
TABLE 5.2
Device Types in ZigBee Networks
Logical Device Type
Physical Device Type Coordinator Router End Device
Full-function device Yes Yes Yes
Reduced-function device No No Yes
is equipped with the adequate resources and memory capacity to handle all the
functionalities and features specified by the standard. It can therefore assume multiple
network responsibilities. It can also communicate with any other network
device. An RFD is a simple device that carries a reduced set of functionalities,
for lower cost and complexity. It typically contains a physical interface to the wireless
modem and executes the specified IEEE 802.15.4 MAC layer protocol. Furthermore,
it can only associate and communicate with an FFD.
Based on these physical device types, ZigBee defines a variety of logical device
types. These logical devices are distinguished based on their physical capabilities
and the role they play in the network deployed. Table 5.2 illustrates the possible
combinations of device types that can be deployed in a ZigBee-enabled network.
There are three categories of logical devices:
1. Network coordinator: an FFD device responsible for network establishment
and control. The coordinator is responsible for choosing key parameters of
the network configuration and for starting the network. It also stores
information about the network and acts as the repository for security keys.
2. Router: an FFD device that supports the data routing functionality, including
acting as an intermediate device to link different components of the network
and forwarding message between remote devices across multihop paths. A
router can communicate with other routers and end devices.
3. End devices: an RFD device that contains just enough functionality to communicate
with its parent node: the network coordinator or a router. An end device
does not have the capability to relay data messages to other end devices.
Based on these logical devices types, a ZigBee wireless personal area network
(PAN) can be organized in one of three possible topologies: a star, a mesh (peerto-peer),
or a cluster tree. The three network configurations are illustrated in
Figure 5.15. The star network topology supports a single coordinator, with up to
65,536 devices. In this topology configuration, one of the FFD-type devices
assumes the role of network coordinator. All other devices act as end devices.
The coordinator selected is responsible for initiating and maintaining the end
devices on the network. Upon initiation, the end devices can only communicate
with the coordinator. The mesh configuration allows path formation from any
source device to any destination device, using tree and table-driven routing algorithms.
The table-driven routing algorithm employs a simplified version of the
on-demand distance vector routing (AODV) and Internet Engineering Task Force