2.3 ZigBee Topologies
ZigBee uses the IEEE 802.15.4 2003 specification for its physical layer and MAC layer. IEEE 802.15.4 offers star, tree, cluster tree, and mesh topologies; however, ZigBee supports only star, tree, and mesh topologies.
It uses an association hierarchy; a device joining the network can either be a router or an end device, and routers can accept more devices.
Star topology: The star topology consists of a coordinator and several end devices (nodes), as shown in Figure 2.2. In this topology, the end device communicates only with the coordinator. Any packet exchange between end devices must go through the coordinator. The disadvantage of this topology is the operation of the network depends on the coordinator of the network, and because all packets between devices must go through coordinator, the coordinator may become bottlenecked. Also, there is no alternative path from the source to the destination. The advantage of star topology is that it is simple and packets go through at most two hops to reach their destination.
Figure 2.2 Star topology
Tree topology: In this topology, the network consists of a central node (root tree), which is a coordinator, several routers, and end devices, as shown in Figure 2.3. The function of the router is to extend the network coverage. The end nodes that are connected to the coordinator or the routers are called children. Only routers and the coordinator can have children. Each end device is only able to communicate with its parent (router or coordinator). The coordinator and routers can have children and, therefore, are the only devices that can be parents. An end device cannot have children and, therefore, may not be a parent. A special case of tree topology is called a cluster tree topology.
Figure 2.3 Tree topology
The disadvantages of tree topology are
- If one of the parents becomes disabled, the children of the disable parent cannot communicate with other devices in the network.
- Even if two nodes are geographically close to each other, they cannot communicate directly.
Cluster tree topology: A cluster tree topology is a special case of tree topology in which a parent with its children is called a cluster, as shown in Figure 2.4. Each cluster is identified by a cluster ID. ZigBee does not support cluster tree topology, but IEEE 802.15.4 does support it.
Figure 2.4 Cluster tree topology
Mesh topology: Mesh topology, also referred to as a peer-to-peer network, consists of one coordinator, several routers, and end devices, as shown in Figure 2.5. The following are the characteristics of a mesh topology:
A mesh topology is a multihop network; packets pass through multiple hops to reach their destination.
The range of a network can be increased by adding more devices to the network.
It can eliminate dead zones.
A mesh topology is self-healing, meaning during transmission, if a path fails, the node will find an alternate path to the destination.
Devices can be close to each other so that they use less power.
Adding or removing a device is easy.
Any source device can communicate with any destination device in the network.
Compared with star topology, mesh topology requires greater overhead.
Mesh routing uses a more complex routing protocol than a star topology.
Figure 2.5 Mesh topology