## 1 Introduction
In summary, the data-logger devices currently used for postharvest commodity monitoring are divided into: (i) passive communication devices with low power requirements but without real-time communication, i.e., they are not capable of transmitting the variables during the commercial life of the commodity; thus, it is possible only when the device is connected or approached to a reader in any of the phases of the product's commercial cycle (RFID, indicator strips, USB loggers, I-button, etc. ); (ii) IoT devices, which use some wireless communication bands to send the data to cloud servers on the Internet without operator handling (Bluetooth, Wi-Fi or ZigBee). IoT devices can be divided between those that use narrowband networks such as SigFox, (SigFox, Labge (France), NB-IoT, LTE-M, or LoRA with low power consumption that have data-sending limitations and, currently, limited coverage and other devices with wideband networks. These devices that use wide bands, such as GSM, Iridium and CDMA, require devices with high power consumption that depend on sampling, the data frame protocol and frequency of data sending but have worldwide coverage. The use of these devices for postharvest commodity monitoring makes it necessary to manage power consumption optimally to achieve the same autonomy that the theoretical commodity shelf life. The needs demanded by the land transportation sector in issues, such as integration, flexibility, autonomy and multi-point variable measurement, have motivated the design of the system presented in this article, which aims to overcome the limitations observed in the market equipment, highlighting the following:  
* Flexibility for any type of sensor measurement using analogic signals (voltage or current) or digital signals (1-wire, RS-232, SDI-12 [30]). * Compatibility with cloud servers. Standard J-Son type frames are used for real-time data allocation. * Use of non-proprietary networks and as standard as possible to allow for worldwide use. * Management of collaborative networks for master-slave multi-point measurement. * Energy management for a minimum autonomy of 1 month with 15 min of data sampling. * Reduced size for flexible placement. * Low-cost design for assuming its loss or breakage during all stages of the commodity handling life. The designed and described in this paper is a flexible multi-point measurement tool, able to communicate in real time the measured variables with a power consumption capable of providing a minimum autonomy of 1 month. The system is developed as a sensor nodes with two roles, the'slave' node and 'gateway' node. The'slave' node uses the Wi-Fi network for communication. The 'gateway' node uses GSM/GPRS for real-time cloud data allocating through the Internet but, in addition, it generates a Wi-Fi infrastructure for receiving multi-point measurement from the slave nodes. Slave nodes are designed for independent working. It means that they are able to connect and send data through any Wi-Fi network available during the postharvest chain and registered in the setup configuration of the device. The gateway node can be used as a mono-point measurement device and as a harvesting node in a WSN with the slave nodes. The objective of this article is to demonstrate that a wireless network system based on Wi-Fi communication is able to operate correctly under the specifications of flexibility, robustness to signal loss and power autonomy enough to be used in the cold supply chain of perishable commodities. This paper is divided into 8 sections. After the first section with the introduction, Section 2 reviews the commercial devices used for commodities controlling transportation. Section 3 describes the system architecture and the different working configurations. Section 4 describes the hardware design and the different components used for the nodes. Software design for communication, time synchronization and power management are described in Section 5.