This article introduces the ZigBee” title=”ZigBee”>ZigBee protocol framework, the functions of each layer and the composition of the ZigBee wireless network, and describes the wireless sensor network based on Ember’s EM250″ title=”EM250″>EM250 SoC” title= “Wireless Sensor Network” > Wireless Sensor Network Solution, the hardware and software composition and design of the system are carefully explained.

ZigBee Protocol Framework

ZigBee protocol is a low-cost, low-power, low-speed networking solution for embedded devices to communicate with each other and with external networks. It is the ZigBee Alliance based on the IEEE 802.15. A technical standard for wireless local area network networking, security and application software developed by standardizing network layer protocols and APIs.

The ZigBee protocol stack structure is shown in Figure 1.

The ZigBee protocol physical layer and media access control layer adopts the IEEE 802.15.4 2003 protocol specification. The physical layer wireless frequency band is universal in the world. 2.4GHZ, 868MHz for Europe, 915MHz for the United States, using direct sequence spread spectrum technology, providing 27 channels, 20Kb/s, 40Kb/s, 250Kb/s three data rates for data transmission and reception The physical layer function includes activation and sleep radio frequency transceiver, channel energy detection, link quality indication of channel received data packets, idle channel estimation, sending and receiving data, etc.

The MAC layer of ZigBee is responsible for the establishment, maintenance and termination of wireless data links between devices, data transmission and reception in acknowledgement mode, and carrier sense multiple access/collision avoidance access mode. The maximum length of the data packet is 127 bytes. Each data packet consists of a header byte and a 16-bit CRC check value.

The network layer is the core part of the ZigBee protocol stack, which implements functions such as node access or departure from the network, route search and data transmission, etc. It supports three topology structures of star, tree and network. The network topology structure selects nodes according to specific ZigBee applications. According to the function, it is divided into full function device (FFD), reduced function device (RFD), coordinator (ZCRD) and router. Coordinator and router are usually full function devices.

The application layer of ZigBee includes the Application Support Sublayer (APS), ZigBee Device Object (ZDO) and the application object formulated by the manufacturer. The Application Support Sublayer is responsible for maintaining the binding table and passing information between the two binding entities according to services and requirements. ZDO is responsible for defining the role of device nodes in the network, discovering network devices, deciding what application services to provide, initializing or responding to binding requests, and establishing security relationships between network devices.

The ZigBee network adopts multi-point access, and there are two types of beacon-enabled networks, beacon-enabled network or non-beacon-enabled network, the coordinator periodically sends beacon frames in predefined time slots for node association, Join the network and transmit data synchronously In a network without beacons, the coordinator also periodically sends signals, but this is only used for terminal devices and to detect the presence of the coordinator. The device must be ready for point-to-point communication at any time, by sending data requests communicate with the response.

ZigBee supports two types of data formats: KVP key value pairs and MSG message frames. Generally, the KVP frame adopts the command response mechanism to transmit a simple attribute variable value; while the MSG frame does not have a specific format specification, it is usually used for the transmission of multiple information and complex information, it supports binary data transmission, and the data size is limited by Supported frame size limit.