ZigBee Components:

• A zigbee system consists of several components. The most basic is the device.
• A device can be a full-functional device (FFD) or reduced-function device (RFD).
• An FFD can talk to an RFD or an FFD while an RFD can only talk to an FFD.
• The FFD can operate in three modes: a PAN coordinator, a coordinator, or a device.
• A network must include at least one FFD, operating as the ‘personal Area Network (PAN) coordinator'.

ZigBee Network Topologies:

Zigbee supports three types of topologies: star topology, peer-to-peer topology and cluster-tree.

1. Star Topology

• In the star topology, communication is established between devices and a single central controller, called the PAN coordinator.
• After an FFD is activated for first time, it may establish its own network and become PAN coordinator.
• Each star network chooses a PAN identifier, which is not currently used by any other network within the radio sphere of influence. This allows each star network to operate independently.
• The PAN coordinator may be powered by mains while the devices will most likely be battery powered.
• Applications that benefit from this topology are home automation, personal computer (PC) peripherals, toys and games.

2. Peer-to-peer topology:

• In peer-to-peer topology too, there is one PAN coordinator.
• Any device can communicate with any other device as long as they are in range of one another.
• Thus a peer-to-peer network can be ad hoc (unfixed), self-organizing, and self-healing.
• Peer-to-peer topology allows multiple hops to route messages from any device to any other device in the network. It can provide reliability by multipath routing.
• Applications such as industrial control and monitoring, wireless sensor networks and assets and inventory tracking would benefit from such a topology.

3. Cluster-tree topology:

• The cluster-tree topology is a special case of a peer-to-peer network in which most devices are FFDs.
• Any of the FFDs can act as a coordinator and provide synchronization services to other devices and coordinators. However, only one of these coordinators is PAN coordinator.
• The PAN coordinator forms the first cluster by establishing itself as cluster head (CLH) with a cluster identifier of zero (CID), choosing an unused PAN identifier.
• The CLH broadcasts beacon frames to neighboring devices.
• A candidate device receiving a beacon frame may request to join the network at cluster head.
• If the PAN coordinator permits the device to join, it will add this new device to its neighbor list.
• The newly joined device will add the cluster head as its parent in its neighbor list and begin transmitting periodic beacons such that other candidate devices may then join the network at that device.
• Once application or network requirements are met, the PAN coordinator may instruct a device to become the cluster head of a new cluster adjacent to the first one.
• In this way, an RFD may connect to a cluster tree network as a leaf node at the end of a branch.
• The advantage of the clustered structure is increased coverage at the cost of increased message latency.