A defining decision in developing for the Internet of Things (IoT) is the choice of communication channel for the Things (the nodes with sensors etc., at the edge of the IoT). In choosing the protocol you have to bring into consideration the board footprint and power consumption – both critical for the small, often battery-powered Things – and the bandwidth needed for the application, whether limited, occasional transmission of short messages or significant up-stream and down-stream data-flows. If the communication is to be local, measured in metres or tens of metres, there is a range of protocol possibilities: Zigbee, WiFi, Bluetooth etc.
For long range communication the only option has, until recently, been cellular radio developed for mobile phones. This can provide transmission ranges measured in kilometres and even tens of kilometres. But it needs large and relatively power-hungry silicon. Each Thing requires its own SIM and an annual subscription to a phone service. And the messaging protocols, designed for voice communication or data streaming, produce a massive overhead for short messages, where payload can make up 1% or less of a transmission.
A new cellular approach, from French company Sigfox uses ultra-narrow band in the licence-free spectrum (868 MHz in Europe and 902 MHz in the US.) The ultra-narrow band is ideal for short messages. The specification provides 140 transmitted messages a day, each with a maximum payload of 12 bytes, transmitted at 100 bps. Downloads are limited to 4 messages (each of up to 8 bytes) a day. This leads to low power demands: the exact power is silicon implementation dependent. One modem (from Axsem) draws only 10mA @ 0dB when transmitting and receiving. Even with output power at 14 dB, the modem only draws 45mA at peak and idles at 950nA. This particular device cost only £1.50 in 1,000 piece orders and, like all Sigfox modems, includes a SIM within its firmware. A standard cellular module is around £9.00 in the same quantities, and requires an external, physical SIM.
Sigfox works in partnership with an infra-structure company in each of its markets. In the UK this is Arqiva, a company that, amongst other activities, provides the network and transmission towers for digital TV and radio. Sigfox is piggy-backing on this. In June 2015, Sigfox deployment had reached 10 cities with the aim of total population coverage by the end of 2017.
Airtime charges will be low, one example is £0.30 per month per device, but will vary according to the number of messages and number of Things connected to the network.
In summary, if you are connecting Things that are only going to require a small number of short messages, then Sigfox is going to be a good choice. If you are looking for larger amounts of data in each or either direction, conventional cellular wins.