## 2 WSN for Monitoring of Column Shortening in High-Rise Buildings
### Wireless Sensor Network (WSN) System
#### 2.1.2 Sensor Node  
The existing technology for measurement of column shortenings generally involves installing a single data-logger on the floor where the sensors are installed and connecting all of the sensors to the logger in a given floor with cables. For the case of a high-rise, the cables connecting sensors to the logger can be several meters in length. To solve the problems associated with cables during construction, it is necessary to minimize the length of cable between the sensor and the data-logger, and automate the monitoring process, including data measurement, collection, and analysis.  
In this paper, to avoid lengthy cables between the logger and sensors, a VWSG-based wireless sensor node is used in measurement of column shortenings [19]. As can be seen in Figure 4a, the sensor node comprises a four channel VWS sensor module, a wireless communication module, and a processor and power-saving circuit. The sensor module excites the vibrating wire of the sensor and reads the resonance frequency from the sensor. To reduce the number of sensor nodes installed in high-rise buildings, this can simultaneously measure a maximum of four sensors using \"demuxing\" and \"muxing\" of signals. In buildings and civil infra-structures, a WSN using UHF or microwave frequency is generally employed [20, 21, 22]. The wireless communication module used in this study was the Industrial Scientific and Medical (ISM) frequency band at 424 MHz UHF, which has better diffraction, less electricity consumption, and less influence from signal disturbance than microwaves. This has valid communication distances of 300 m and within 70 m for line of sight (LOS) and non-line of sight (NLOS), respectively.  
The processor minimizes power consumption by controlling the operation of sensor nodes using the built-in memory program. In other words, the timer in the processor causes alternation between \"action mode\" and \"sleep mode\". In action mode, strains are measured and transmitted to master node or repeater node using a wireless communication module. Then, when the sensor node changes to sleep mode, the power is changed to an energy-saving mode.  
Figure 3: Vibrating wire strain gauge.  
The master node shown in Figure 4b comprises a short-distance wireless communication device (low-power wireless modem of ISM band) and a long-distance communication device (Code Division Multiple Access--CDMA). As shown in Figure 2, the master node receives measurement values through short-distance communication with sensor nodes installed in the building and transmits the collected data to the server outside of the building through long-distance communication. CDMA used in master node does not have distance limits, as a specific user's coded signal appears as noise to other users, and there is a low possibility of data noise from long-distance transmission. Therefore, there are fewer limitations to the position of the server receiving data, meaning it can be installed far from the site.  
The repeater node comprises only a short-distance wireless communication device. Thus, it is used in cases where communication between sensor node and master node is not smooth. A repeater node deciphers the communication between master node and sensor node by differentiating the frequency  
Figure 4: Wireless sensor node and master node. (**a**) Sensor node; (**b**) Master node.  
channel. According to the specific circumstances, there may be one or more repeater nodes between the sensor node and master node.