Zigbee Wireless Relay Control and Power Monitoring System

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Introduction
We designed a system for wirelessly controlling relays and monitoring current. This is used for a home load simulation. By wirelessly turning relays on and off by sending commands from a PC to a microcontroller we can change the total load (current) to our simulated home. For wireless communication, we used XBee Series 2 Zigbee RF modules. One of these modules was connected to a microcontroller and the home load simulation, while another was connected to the PC, which was used for collecting and displaying data as well as for relay monitoring and control.



Background Theory
What we have built is a simple transmission system based on the Zigbee routing and networking protocol. This protocol and its details are discussed in greater detail in the Standards section (below); in this section, we focus on underlying network theory and the role this theory played in our project.
Data networks (and transmission systems) are typically divided into various layers based on functionality. This is sometimes called a “protocol stack” (in our case, we are using a “Zigbee stack”). Essentially, the lower the layer, the closer we are to worrying about actual physical electrons flying around. Conversely, the higher the layer, the less we are worrying about physical constraints and the more abstract the data structures are that we are dealing with and manipulating.



Physical Layer
The physical layer's job is to move individual digital bits from one place to another. The protocols in this layer depend on the actual physical medium. For example, in a wireless system, the actual physical medium is simply the atmosphere.

Link Layer
A network's page link layer routes a series of bits (sometimes called a datagram) from one node in a network to another. This can happen through a series of intermediate switches (or routers). Protocols at this layer provide more robust and full-featured services than protocols at the physical layer. WiFi is one example of a link-layer protocol.

Network and Transport Layers
Again, since these layers are higher in the model, protocols at this layer typically are more full-featured than protocols at the page link or physical layers. Protocols at these layers use the page link layer's routing capabilities to move the aforementioned datagrams between nodes in a network. The Internet Protocol (IP) is probably the most famous network layer protocol, while the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) are two examples of well-known and widely-used transport-layer protocols. Certain higher-level functionality is more prevalent in these two layers than in lower levels.