wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

ZigBee 50-7
Coordinator
Router
End device
Star Cluster-tree Mesh
FIGURE 50.6
ZigBee network topologies and classes of device.
50.5.3 Network Layer
Network layer (NWK) enables routing and multi-hop communication between devices in star, mesh, or tree
topology. The NWK provides two types of basic services; data services and management services. The data
services include a network layer protocol data unit (NPDU), transmitting NPDU to the next hop, and applying
NWK security. The management services allow applications to interact with the ZigBee protocol stack.
The management services include configuring a new device, establishing a new network, joining and leaving
a network, assigning address to devices joining the network, neighborhood discovery, and route discovery.
The devices in a ZigBee network are either reduced function device (RFD) or full function device (FFD).
RFD is simple in terms of hardware and software complexity. It is only restricted to start topology, is incapable
of network routing, is low power, and is potentially low cost because of reduced functionality. On
the other hand, FFD is more resourceful, has routing capability, and is generally powered by mains power
supply. RFD only communicates with FFD while FFD can communicate with another FFD and/or RFD.
ZigBee devices are logically divided into three different classes; an end device, a router, and a coordinator
(Figure 50.6).
The responsibilities of a PAN coordinator comprise initiating new network, managing the network
nodes, and storing network information, for instance, security keys. There is exactly one PAN coordinator
per ZigBee network and is an FFD. Router is an optional FFD component in ZigBee network and is used to
extend network coverage, allocation and de-allocation of addresses, and routing between nodes. End devices
implement user applications and can either be FFD or RFD but do not participate in data forwarding.
In start topology, communication is done only through PAN coordinator and follows a master/slave
paradigm. In mesh topology, any device can talk to any other device. Such a topology is ad hoc, selforganizing,
and self-healing, and employs multi-hop communication. Mesh topology allows multiple
paths from a given source to destination and thus increases reliability. If one breaks, the data are sent
via another path transparently. Cluster-tree topology is a hybrid (star plus mesh) approach where PAN
coordinator designates itself as a cluster-head and forms a tree around itself. Although such a topology
increases coverage area, it introduces additional delays.
The routing algorithms that are used in ZigBee networks are hierarchical and have potential for table
optimizations. Commonly used algorithms include a routing algorithm similar to Ad-hoc on-demand
distance vector (AODV) and cluster-tree algorithm.
50.5.4 application Layer
The APL of ZigBee protocol stack is composed of application support (APS) sub-layer, application
framework, and ZDO. The APS provides an interface to the NWK. The services provided by APS comprise
data transmission, fragmentation and reassembly, reliable data transport, security, device binding,
and maintenance of APS information base (a database containing information about managed objects).
Application objects (manufacturer-defined components that implement the application) reside in application
framework. A total of 240 distinct application objects can be defined with IDs ranging from 1
© 2011 by Taylor and Francis Group, LLC