On a past IoT project (see here) I had used a Pi Zero to collect data via it's gpio interface from an Analogue to Digital Converter (ADC) connected to a temperature sensor chip. The actual collection being performed by a Zabbix Agent on the Pi Zero and passed back to Zabbix over a WiFi/Internet connection. While this works fine, it is limited the a location that has WiFi. To be able to use this in a more "Mobile" manner a Cellular Data connection method is required. To start with I have used Hologram for the Cellular Network and their Nova GSM 2G/3G Modem to connect with.

I'm just going to give an overview of how this all works and the issues, then go into detail on another occasion. I'm not going to cover the physical ADC and sensor setup as it's the same as in previous projects.

Hologram act as the Data Network Provider in that the SIM they provide can be used to connect to their Cellular connection APN (the gateway between their Cloud and the GSM Network). The Nova USB GSM Modem, gives the connected Pi Zero serial port access to the GSM chip (U-BLOX U201) in the Nova to process standard "AT" commands. Hologram provide an SDK, written in Python, that can be simply installed on the Pi Zero to take away all the complexity of managing "AT" command processing. The Hologram SDK provides a Command Line Interface (CLI) that allows simple text messages to be sent via a data connection to the Hologram Cloud, a full blown IP PPP connection can also be instantiated that allows Internet Access and inbound tunnelling access to the PI Zero. There are also commands to query and control the connections. Data usage and status can be monitored and controlled from the Hologram Cloud. The routing of messages can also be controlled via the Hologram Cloud. I have used this to route the simple text messages from the Pi SDK CLI to an end point that has a Node Red flow listening on it. This allows me to get the data from the text messages and push them into Zabbix via a Zabbix Sender.

I have tested the Cellular Network data usage of sending simple text messages (500 Bytes plus the message) vs. a Zabbix Agent connection over a full blown IP connection (14.5 KBytes). Giving the obvious conclusion that the text message approach should be used. Optimisation of traffic is a BIG thing with Cellular Network data plans. Hologram will provide a "Pilot" SIM for free with one MByte included. Subsequent data traffic is $0.40USD/MByte.

A separate niggle is the cost of getting a NOVA USB GSM Modem as they are only available from the States with pricy shipping and import duties. I will be looking at other devices available in the UK.