ZigBee
ZigBee is the specification for a suite of protocols designed for wireless personal area networks (WPAN). It is based on the IEEE 802 standard. The term Personal Area Network is coined to provide differentiation from Local Area Network (LAN) and Wide Area Network (WAN). It should be understood to be of small size and not related to an ‘individual’.
The data rate is variable with a maximum achievable of 250 Kilobits per second.
The theoretical max distance between devices is 120m line of sight although 40-50m is considered a realistic maximum in an urban environment.
The radio band used is ISM (International Scientific and Medical). The band varies geographically, with Europe on 868MHz, USA and Australia on 915MHz and the RoW on 2.4GHz.
Channels
There are typically 12 different radio channels available to ZigBee. All devices on a PAN must be on the same channel. The channel selection is normally automated by the Coordinator (The Coordinator is a special task ZigBee device; every ZigBee PAN contains one and only one Coordinator).
Device Types
Three types of ZigBee device exist:
- Coordinator: Each ZigBee PAN has one and only one Coordinator. The Coordinator is responsible for formation of the network and address assignment. The Coordinator will be a continuously powered device.
- Router: The Router is a fully featured ZigBee device that can join PANs in exactly the same way as an ‘End Device’. It has the additional functionality of routing ZigBee traffic to the Coordinator (optionally via other Routers). The Router will be a continuously powered device.
- End Device: The End Device is typically a low power device without routing capability. This allows it to regularly drop into power saving mode. The End Device is most likely powered via battery or power harvesting.
Topology
The three ZigBee device types can form networks in four distinct modes:
- Paired or Point to Point:
This is the simplest network and consists of linking two nodes. One node must be a Coordinator.
- Star:
A Coordinator is located at the centre of the star, with all End Devices connected via radials.
- Mesh:
Multiple Routers provide a mesh to allow End Devices to access the Coordinator through multiple paths.
- Cluster:
The Cluster network sports multiple clusters of a single Router surrounded by End Devices. The Routers in each cluster then communicate with the Coordinator.
Addressing
A ZigBee device has four addresses:
- PAN Address. This is a 16-bit address that differentiates multiple PANs within a radio channel.
- Serial Number. This is 64-bit and permanently assigned address that is globally unique. It is very similar to a MAC address but the most significant four bytes form the organisation identifier.
- Short Address. This is a 16-bit address that the Coordinator assigns when building the network. These addresses are assigned so as to be unique within the PAN although not globally. The only purpose of this address is to reduce both memory and network overhead within the PAN.
- User Name: This is a human friendly text name. It is not guaranteed to be unique.
The drawing below shows a summary of the radio channel and addressing hierarchy.
