The Zigbee protocol stack includes a network layer (NWK) and an application layer (APS). Additional components include a security service provider, a ZDO management plane, and a Zigbee device object (ZDO). The structure of the stack illustrates genuine simplicity over the more complex but feature filled Bluetooth stack.
The NWK is used for all three principal Zigbee components (ZR, ZC, ZED). This layer performs device management and route discovery. Additionally, since it administers a true dynamic mesh, it is also responsible for route maintenance and healing. As the most basic function, the NWK is responsible for transferring network packets and routing of messages. During the process of joining nodes to a Zigbee mesh, the NWK provides the logical network address for the ZC and secures a connection.
The APS provides an interface between the network layer and application layer. It manages the binding table database, which is used to find the correct device depending on the service that is needed versus a service that is offered. Applications are modeled by what are termed application objects. Application objects communicate with each other through a map of object attributes called clusters. Communication between objects is created in a compressed XML file to allow for ubiquity. All devices must support a basic set of methods; however, a total of 240 endpoints can exist per Zigbee device.
The APS interfaces the Zigbee device to a user. The majority of components in the Zigbee protocol reside here, including the Zigbee device object (ZDO). Endpoint 0 is called the ZDO and is a critical component that is responsible for overall device management. This includes managing keys, policies, and roles of devices. It can also discover new (one-hop-away) devices on the network and the services those devices offer. The ZDO initiates and responds to all binding requests for the device. It also establishes a secure relationship between network devices by managing the security policy and keys for the device.
The bindings are connections between two endpoints with each binding supporting a particular application profile. Therefore, when we combine the source and a destination endpoint, the cluster ID, and profile ID, we can create a unique message between two endpoints and two devices. Bindings can be one-to-one, one-to-many, or many-to-one. An example of this binding in action would be multiple light switches connected to a set of light bulbs. The switch application endpoints will associate with light endpoints. The ZDO provides the binding management by associating the switch endpoints to light endpoints using application objects. Clusters can be created, allowing one switch to turn on all the lights while another switch is only able to control a single light.
An application profile delivered by the OEM will describe a collection of devices for specific functions (for example, light switches and smoke alarms). Devices within an application profile can communicate with each other through clusters. Each cluster will have a unique cluster ID to identify itself within the network.