Originally published on LinkedIn in December 2021; reviewed and updated for this site.
The UK's mobile operators now carry both NB-IoT and LTE-M (sometimes referred to as Cat-M1). Vodafone built out NB-IoT to near-nationwide coverage, with Telefónica O2 following on LTE-M, so the choice between the two is a real one for any UK deployment. It is worth being clear about the difference.
Both are 3GPP standardised technologies designed to support a massive IoT strategy: the deployment of a huge number of low-complexity devices that do not need to communicate frequently. High performance and low transmission latency are not requirements. The two technologies are complementary and address different use cases based on their respective strengths.
They do share some common ground. Both are inexpensive, both can run for a decade or more on battery power, both can connect even when there are hundreds of similar devices nearby, and both can be reached in remote rural locations, inside buildings, or underground.
Both provide low-bandwidth cellular communication, but there are important differences when choosing the right protocol for an industrial IoT or wearable application.
NB-IoT
NB-IoT supports very narrow bandwidth — 200 kHz — so the data rate peaks at around 250 kbps. It can be deployed in the guard band of an LTE carrier, using spectrum that would otherwise sit unused.
It costs less. NB-IoT uses half-duplex communication, meaning either the module is transmitting or the base station is — not both. Half duplex, combined with lower data speeds, lower RF bandwidth and a single antenna, reduces the complexity and therefore the cost of NB-IoT devices by as much as 50% compared with regular LTE-M modules.
It consumes less power. NB-IoT reduces the power consumed when transmitting by up to 75% compared with regular LTE-M modules, thanks to features like Power Saving Mode (PSM) and Extended Discontinuous Reception (eDRX), and its ability to optimise the energy used for small data transmissions. That is what allows devices to operate for a decade or more on battery power.
It provides more capacity. Narrowband transmission, signalling optimisation, adaptive modulation and hybrid automatic repeat request (HARQ) together allow as many as one million NB-IoT devices per square kilometre to connect to the network.
It delivers better coverage. NB-IoT employs large signal repetition. This lowers the data rate and increases power consumption during transmission, but improves coverage by five to ten times over other cellular technologies. That is why an NB-IoT device can still reach the network from deep inside a building, from a remote rural location, or from underground.
LTE-M
LTE-M is the second generation of LTE chips created for IoT applications. It provides low-cost LTE devices suitable for massive machine-type communications with enhanced coverage compared with normal LTE devices.
Three features matter most. Speed — up to 1 Mbps uplink and downlink, considerably more than NB-IoT. VoLTE — it supports voice. Mobility — LTE-M is designed for devices that move.
It operates at 1.4 MHz bandwidth, with higher device complexity and cost than NB-IoT. The extra bandwidth is what buys the data rate, the lower latency and the more accurate positioning.
Lower latency — the time taken to connect and send or receive a message — is what allows LTE-M to support voice alongside data, and applications like precision tracking or power grid control that need something closer to real time. It performs far better for mobile applications than NB-IoT, despite the mobility improvements in Cat-NB2. And as a natural extension of 4G LTE, LTE-M benefits from out-of-the-box roaming: a SIM from one operator works on another operator's network abroad.
What they have in common
Both NB-IoT and Cat-M1 devices can sleep for extended periods using eDRX and PSM, which greatly reduces power consumption, and both support enhanced signal coverage per base station.
So which is better?
LTE-M is more capable than NB-IoT, but that does not make it better. It depends on the use case. From an operator's perspective NB-IoT offers more deployment flexibility because of guard-band deployment, and it can also be deployed standalone where the operator's spectrum allows.
For applications needing higher data rates, low latency, full mobility and voice in typical coverage, LTE-M is the better choice.
NB-IoT has its own advantages. Although both provide better coverage than other technologies, most operators have deployed NB-IoT with the settings that deliver the best possible coverage improvement, and LTE-M with settings that deliver only partial improvement. In practice that means NB-IoT gives better coverage in warehouses, office buildings and underground locations, where signal loss and multiple layers of obstruction cause connectivity problems.