## 6 Tracking and Tracing
### Technology Description
#### 6.2.2 ZigBee  
ZigBee is a communication option created and maintained by the ZigBee Alliance. It works around 900 MHz with one channel and around 2.4 GHz with up to sixteen channels. The technology has a low data rate and power consumption. An often-cited feature is the direct and specialized implementation of a mesh network structure (see Wi-Fi). These features allowed the technology to enter the market for smart home automation and expansion. Developed by the ZigBee Alliance based on IEEE 802.15.4, there are three basic module types in a ZigBee network. These modules generate the private, low-data, and short-range network (20 m to 100 m) [74]. The coordinator device establishes and handles the communication and paths between routers. They are arranging and managing the mesh structure. Therefore, these modules can only be connected to other ZigBee modules of the coordinator type. The ZigBee Router manages the actual data exchange between all three modules and forms vertices between the other two modules. The data rates are in the region of 20-250 kbps [75].  
Modules are capable of low-power sleep mode and can be used with over two hundred other units in one network. This has been shown via implementations of ZigBee networks for gas concentration monitoring [76; 77] as well as for tracking miners in dangerous areas for increased safety and accident alerts [78].  
Safety-critical environmental factors have a primarily three-part structure [79]. Due to the high mobility of the receiver/transmitter unit, which results from low weight, low power consumption and mesh coupling, it is possible to enable network-based stationary data acquisition underground, mobile workers/machines and data centres [79].  
Sensor units that collect data such as temperature, air quality and other environmental factors are located at fixed and critical points of operation in the underground mine. These obtain a temporary, on-the-spot evaluation and, if necessary, send an alarm event to workers and machines in the immediate vicinity of the network or of the sensor itself. Central monitoring units can record the data flow of sensors linked to central/active positions, monitor them over longer periods of time and link them to other measurement units.  
ZigBee was developed as a smart home communication protocol with dedicated hardware and a relatively short range of m to 100 m. However, this simple quadratic relationship for extensions in the dimension of length no longer applies in NLOS situations. This is the case, for example, in underground mines, where pillars or simply walls usually represent a total signal blockade. Due to the higher number of nodes in the same area, the necessary infrastructure is denser. However, this also provides the opportunity to implement an emergency tracking method for workers in dangerous sections of the mine, having a resolution instead of only a \"check-in\" tracking. One would know via the network the last closest node from which the signal of the mobile unit was received [76; 77; 80]. The execution of a rescue plan would be faster as \"in case of accidents or miners being trapped underground, the tracking unit [ZigBee] will help in locating the real time location of the miner\" [78].  
It is conceivable to add a ZigBee connection to existing measuring and monitoring sensors. This network would be able to be implemented without interaction with existing networks. The low hardware volume required for a ZigBee deployment brings with it the possibility of being deployed in adverse mine sections [81].  
ZigBee as a low-power mesh network, having a local range and option of twenty to one hundred meters, and therefore is highly suited for selective monitoring tasks in active mining environments. It is in use for different tracking and tracking use cases for locatingworkers and assets and can be utilized to create local low-power mesh networks but has not found wider adoption.