LPWAN – What does low-power really mean for IoT devices?

Bruce Jackson


The key component of LPWAN (Low Power Wide Area Network) is low-power. For many remote devices, low-power is essential for preserving battery life and with the number of devices expected to rise exponentially, the environmental impact also needs to be considered. In this article, Bruce Jackson, Thingstream CTO puts a Thingstream IoT enabled device through its paces to find out what low-power really means and how Thingstream compares with GPRS.

Update – This test has been expanded to include Thingstream’s UDP-based data transmission as well as USSD:
Putting Thingstream data and power efficiency to the test

LPWAN low-power IoT - power lines

LPWAN – Why is low-power so important?

As the number of IoT devices increases, so too does the need to supply them with power. Many remote devices, such as those used in asset tracking applications, rely on batteries to provide power and in some cases may need to be able to work continuously for several days, weeks, or even years without being recharged or replaced. For this reason, it’s essential that devices use as little battery power as possible in order to operate for as long as required.

Along with operational concerns, there are also environmental concerns. The increasing number of devices means that the drain on the power grid and the resources that supply it will also increase. Increasing the overall efficiency of IoT deployments will go a long way toward easing the load on the power grid and the environment. On top of this, reduced energy costs to the business operating the IoT network will be a welcome bonus.

Achieving low-power with MQTT-SN over USSD

Building a low-power LPWAN device isn’t just about choosing the right hardware. Enormous power savings can also be made by choosing the right connectivity and message transport. Thingstream answers this by using MQTT-SN (Message Queuing Telemetry Transport for Sensor Networks) over USSD (Unstructured Supplementary Service Data) which compared to GPRS, draws significantly less current, as the test below shows.

MQTT-SN over USSD versus HTTP POST over GPRS

For this example, a simple “Hello World!” message was transmitted using our 2G/2.5G test button hardware. The orange line on the graph below shows current draw at the battery terminals (indicated in mA, peaking at 180mA) when sending the message “Hello World!” over Thingstream (MQTT-SN over USSD). The blue line shows the same message data being sent using HTTP POST over GPRS.


There are four really significant things to note:

  1. The peak current for Thingstream is significantly lower than GPRS (peak of 190mA versus 285mA)
  2. The total power usage (current draw) for the complete operation is 15% lower (the area under the current/time curve) for Thingstream
  3. The total operational time is shorter for Thingstream (the GPRS transfer is 32% longer)
  4. The current draw in real-world conditional at the battery is nothing like the 1500-2000mA theoretical maximum for 2G cellular.

Shorter operation cycles mean longer battery life

Along with the connectivity, engineers should also consider reducing the overall current drain by decreasing the amount of time the device needs to spend on air, sending and receiving data. The Thingstream SDK provides engineers with options to further improve power efficiency by shortening the overall operation cycle.

Thingstream is built on MQTT-SN, and fully supports the low power lifecycle optimisations defined in its specification (shown in the image below). In particular, Thingstream allows devices to:

  1. Publish data using a ‘fire and forget’ mode (defined as Quality of Service [QoS] -1). This means that a device does not need to go through the usual lifecycle stages of connect/publish/disconnect. It can use publish(QoS-1) without any additional setup. This obviously reduces the time that the device must have the radio turned on and the time taken to send data, both significantly reducing power consumption.
  2. Sleep. There are many cases where devices will both publish data and also subscribe to topics to receive data. Other methods require the device to be permanently connected to get the data sent from the server, and therefore the device must either keep the radio and all other processing components turned on, or implement a complex time windowing system synchronized with a server to allow inbound data at certain predefined times. Neither is ideal, and both are power-inefficient.

Thingstream allows devices to connect, subscribe and then disconnect with a timeout period. The Thingstream server will then hold all outbound messages for that device until it next connects, or until the timeout period expires. The device can then go into sleep mode, wake up before the timeout and reconnect to both publish data and receive any pending messages.

LPWAN cost savings can be achieved by a small code footprint

Thingstream has a very small SDK footprint (approximately 12k Flash/5k RAM for an ARM Cortex M0). This means engineers can build really useful connected applications with very power-efficient microcontrollers. There is no need to have external flash or RAM to support sizeable TCP/HTTP/MQTT stacks, saving on both component cost and power.

Overcoming power implications of roaming data

Thingstream is an overlay network designed for the Internet of Things. This means that its service ‘overlays’ the cellular network of our 600+ carrier partners in 190 countries. On average, Thingstream has three carrier partners in each country, and devices using our SIM cards will automatically roam onto the network which has the best signal strength at any point. This has enormous implications for power. The cellular radio front end can be driven at lower power because the chances of being in an area of good or excellent cellular coverage are greatly increased.

In Conclusion

For businesses deploying IoT projects, particularly those delivering IIoT (Industrial Internet of Things) at scale, it’s time to get serious about what low-power really means when referring to LPWAN. While there are often obvious savings that can be made by modifying hardware, you also need to literally think outside the box. Careful consideration of other factors such as connectivity, message size and code footprint can also create savings for the business whilst also lessening the load on the environment.

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