The gap they fill
Between short-range radios and full mobile data connections lies a gap. A soil sensor at the far edge of a field, a water meter in a roadside pit, or a tank level gauge on a hill does not need to stream anything. It needs to send a few bytes now and then, run for years on a battery, and reach a receiver that may be miles away.
Short-range options such as Bluetooth Low Energy cannot reach that far. Standard cellular data can, but modules, plans and power use may be more than the job justifies. Low-power wide-area networks, usually abbreviated LPWAN, were designed for that middle ground.
The core trade-off
LPWAN technologies buy range and battery life by giving up almost everything else:
- Low data rate. Payloads are typically small, from a few bytes to some tens or hundreds, depending on the technology and settings.
- Infrequent messages. Many networks limit how often a device may transmit, either by rules, by design or by the fair-use needs of sharing the radio channel.
- Limited or no downlink. Sending data to a device is often restricted. Many devices listen only briefly after they transmit, so commands may wait for the next opportunity.
- High latency. Messages are not suited to real-time control.
Long range is achieved by sending slowly and with robust coding, which lets the receiver pick out a faint signal. The slower a message is sent, the longer the radio is on, so there is a balance between distance and airtime.
Two broad families
Unlicensed-band networks. Some LPWANs use radio bands that anyone may use under local rules, usually in sub-gigahertz frequencies that are different in different regions. Because the spectrum is shared, local regulations commonly limit transmit power and how much of the time a device may be on air. Owners can often run their own gateways, so a farm, campus or city can build a private network, or join a community or commercial one. Coverage then depends on where gateways are placed.
Licensed-band networks. Others run on spectrum owned by mobile operators, using cellular towers. These are the low-power variants of cellular IoT. They benefit from the operator’s existing coverage, SIM-based identity and managed service, but require a subscription and depend on whether an operator offers the service in your area.
Neither family is categorically better. Private control and no subscription favor the first. Existing coverage and less infrastructure to run favor the second.
How a typical network is arranged
In an unlicensed LPWAN, many sensors transmit to any gateway in range, and the gateways forward what they hear to a central server over ordinary internet links. This is a star-of-stars layout. Sensors do not usually relay for one another, which keeps them simple but means coverage depends entirely on the gateways. The gateways take on much of the work described in the discussion of IoT gateways, but are usually simpler since they forward packets rather than interpreting them.
A single gateway can often serve a very large number of devices if each sends rarely, though capacity ultimately depends on message size, frequency and interference. It is worth remembering that these figures are limits, not promises.
Where LPWAN fits
Typical uses include:
- Water meters and gas meters read at long intervals, and some smart meters in places where other connectivity is poor.
- Soil moisture sensors and weather stations across farmland.
- Livestock monitoring tags, and other tracking where position updates are occasional.
- Waste collection bins reporting fill level.
- Parking sensors and other outdoor city infrastructure.
- Asset tracking of containers or equipment, when updates are infrequent.
These share traits: small payloads, readings that stay useful if delayed by minutes, and hard-to-reach locations. In these cases, a battery lasting several years is much more valuable than speed.
Where it does not fit
LPWAN is a poor choice for audio, images or video, for firmware images that are large, and for control loops needing quick responses. Updating software over a very slow link can be hard, so firmware updates for LPWAN devices need special care: they may need to be small, split into pieces, or avoided in favor of getting the design right the first time. Devices that need two-way interaction, such as a smart lock, would be frustrating.
Battery life, honestly
Marketing sometimes quotes battery lives of ten years or more. These figures usually assume a small battery load, infrequent messages, favorable temperatures and good signal. Poor signal causes retries, which cost energy. Cold weather reduces battery capacity. Treat vendor claims as best case, and test with your own message rate and environment. The device’s duty cycling matters as much as the radio.
Security and reliability
Short messages still need protection. Many LPWAN systems encrypt traffic and authenticate devices with keys stored in them, but how well this works depends on how keys are generated, loaded and protected. Unique keys per device are good practice, and shared or default keys are a weakness. Because messages are small and links are lossy, applications should tolerate lost or duplicate messages and avoid assuming that every reading arrives.
Choosing well
Start with the data: how many bytes, how often, and how late can it be? If the answers are “few”, “rarely” and “quite”, an LPWAN is a strong candidate. Then check coverage in your actual location, with a test rather than a map alone, and the long-term support for the technology in your region. Terrain, buildings and antenna placement can change results considerably.