Appendix S2 Technical description of EDAPHOLOG LOGGER - Communication unit of the EDAPHOLOG System The EDAPHOLOG Logger transfers data collected by the EDAPHOLOG probes to the EDAPHOWEB server. The device has two physical parts: (1) a GSM/GPRS communication device and (2) a radio communication device which is connected to the probes and is able to receive data from a maximum of 16 sensors simultaneously. The GSM user s software runs on the GSM A212 device, developed and manufactured by us, that is placed in an IP65 rated closed box supplied with a battery and solar panel. The device contains also a Telit GSM module which is able to send SMS and to communicate via GPRS connection, a radio transmitter module operating at 433 MHz which communicates with the probes, a CPU, flash memory and circuits responsible for power supply. After installation, the system connects to the GSM/GPRS networks, followed by the running of a service software. Then, it establishes and maps connections with the installed sensors. The communication with the probes is performed via a 433 MHz receiver. The device then goes into sleeping mode, receives only the signals coming from the probes, and saves them in the built-in memory. The received radio signals are processed by the communication processor which fixes the errors and forwards the data to the GSM/GPRS. Since radio signals may contain errors, in order to fix them, as well as to recognize data integrity, we further developed a convolutional encoding having high redundancy. After a pre-set time has passed or once a certain amount of data has arrived, the GSM A212 device reactivates and submits the collected data to the server of EDAPHOLOG. The GSM/GPRS communication device forwards the received information to the server without any modification, assigning only a time stamp to them. Finally it sends its own status data as the last data line: battery status, temperature, number of communication, etc. 1
The GPRS used by our device is a packet-switched, IP-based mobile data transfer technology used by GSM and IS-136 mobile phones. Its advantage is the higher security of data transfer. The data transfer is invoiced by the service provider on the basis of data amount and the useful information is more easily extracted from the message. Its disadvantage is the moderate speed of data transfer (171.2 kb/s) in GSM systems, and a computer permanently connected to the Internet is required to collect data. When a data packet arrives it is saved in a text document. The device is powered by approx. 6V and its current consumption is about 1mA. Figure 1. EDAPHOLOG Logger device Communication devices connected to the probes transfer the signals of sensors to the GSM/GPRS device. The device consists of a communication processor and a 433 MHz low-power radio transmitter. 2
When the sensor circuit wants to send the data, it starts the communication processor. After 100 ms the data transfer starts on the RS232 line. If a receiver is available in the system, it waits for the response of GSM/GPRS communication device; if not, it sends the data several more times with random timing. At the end of the communication, it sends back an acknowledgement signal on the RS232 line. In this case the sensor circuit resets its output wire, starting the communication to zero level resulting in a complete shutdown of the communication processor. The task of the sensor circuit is to reactivate the communication at given intervals and to send a message with a known speed and number of characters to the communication processor. The data transfer framework created by us consists of 8-bit head data and 24-bit (or 24+32) measured data. During transmission/reception, the current consumption of the communication processor is 1-2mA and is powered by approx. 3V. Structure of the GSM/GPRS communication device Via radio connection, the device is able to receive data from several sensors (EDAPHOLOG Probe) simultaneously by means of the communication device connected to the sensors. The EDAPHOLOG probes are installed in tens of meters around the GSM/GPRS communication device. The number of installable sensors depends only on the quantity of data sent by them. Content of the GSM A212 device: - a Telit GSM module which can send SMS to the previously defined phone number/numbers as well as establish connection with a software running on computer having static IP address. - a 433Mhz radio transmitter/receiver module which communicates with the remote counters/data LED S collecting modules. - CPU with flash memory. - circuits responsible for power supply with battery status meter and consumption optimizer. 3
Figure 2. Circuit diagram of the GMS module 4
Figure 3. Circuit diagram of the 433MHz radio receiver 5
Figure 4. CPU with flash memory 6
Figure 5. Block diagram of the radio communication module GSM/GPRS connections: The device is powered by approx. 6V and its average current consumption is about 1mA but in case of GPRS connection, peak current magnitude of some amperes can be expected. When the user turns on the GPRS device, it first scans the cell with automatic search to find a GPRS channel. If the suitable channel is found, in the first step it tries to connect to the network. The SGSN receives the request and searches for the subscriber's information from the HRL and then 7
an authentication procedure is performed. The encryption procedure is also performed in this step. Based on the information stored there, the SGSN specifies to which GGSN the traffic shall be routed. The selected GGSN reserves a dynamic IP address (PDP context activation) for the user before establishing the connection with the external network. When the mobile user is sending data, the SGSN routes the packets to the GGSN. The GGSN then forwards the data to the direction specified by the IP address included in the packet. In the opposite situation, if a packet arrives to the user, from the external network the packet gets into that GGSN which specified the IP address. The GGSN verifies the address of data packet, specifies the server SGSN and forwards the packet to the network accordingly. The SGSN forwards the packet to direction of the BSS in which the subscriber stays. When the device is turned off or it is in an uncovered area, the context is inactivated and the subscriber gets detached from the network. The device consists of a communication processor and a 433 MHz low-power radio transmitter. When the sensor circuit wants to send data it collected, it starts the communication processor by setting one of its output wires on a high logic level (approx. 3mA current demand). This level is kept until the response of the communication processor arrives. After 100ms the data transfer starts on the RS232 line. After the arrival of the last data byte the communication processor turns on the transmitter and sends the data. If a receiver is also available in the system, it waits for the response of the GSM/GPRS communication device, if not, it sends the data several more times with random timing. At the end of the communication it sends back an acknowledgement signal on the RS232 line - 05Ah, or what it got from GSM/GPRS communication device (if a 433 MHz receiver is also installed). In this case the sensor circuit resets its output wire, starting the communication to zero level, which results in a complete shutdown of the communication processor. 8
Figure 6. Communication processor with 433MHz radio transmitter circuit diagram 9
Figure 7. Communication processor with 433MHz radio transmitter Figure 8. 433 MHz radio board Coding: One letter represents a 8-bit number (from 0 to 255) (x)aattiiiszzzzzzzz x Optional, if several different message lengths are to be specified; if only one type (32- or 64-bit - greater value should not be specified) are to be created, it can be omitted AA 16-bit unique identifier 10
TT 16-bit type (gives information about the content of the message, it shall be specified by the designer of the sensor: e.g.: 1 if counter, 2 if thermometer, 3 if moisture content, 1+2+3 if all the three data are included in the telegram) seconds III 24-bit system time which (from the moment of turn-on) increments in every second or 10 S System status e.g.: battery status, internal temperature, alarm etc.) zzz.. - Data bits (24 or 24+32) During transmission/reception the current consumption of the communication processor is 1-2mA and is powered by approx. 3V. The supply voltage of the transmitter is 5-12V, its current consumption is about 10-15mA (it consumes only during transmission; its stand-by current: 0mA), its output power is 10mW. The data traffic between the remote sensors and the 433 MHz receiver module is structured according to the following table: Byte No. Content Note (hex) 00 55 Login, synchronization 01 AA 02 Unique identifier 03 04 Type identifier 05 06 Own time: Day 07 Hour 08 Minute 09 Second 10 Supply voltage 11 Collembola counter low byte 12 Collembola counter high byte 13 Collembola size 14 Temperature low byte 15 Temperature high byte 16 Soil moisture 1 17 Soil moisture 2 18 Soil moisture 3 19-30 empty 31 04 or 05 or Message type: EOT (ASCII) normal, ENQ (ASCII) installation test 11
Figure 9. The GSM A212 device under load testing Figure 10. Design of GSM antenna 12
Figure 11. EDAPHOLOG Logger 13