Cellular connectivity for IoT
Without connectivity, there is no Internet of Things. Only a few years back, getting an IoT device online was both complicated and costly. However, the drop in price and general availability of internet-enabled microcontrollers have spawned a vast number of IoT solutions in a wide range of business verticals. Applications range from self-driving cars with huge amounts of data being sent continuously over high-speed networks, to miniature sensor modules sending a few bits of data a few times a week and lasting a decade on a small battery.
One might think that a standard WiFi connection would be the best for an IoT application. After all, WiFi seems to be ubiquitous today, offering stable connections with data throughput in the gigabit range. However, WiFi networks suffer from a severe drawback: the reach of a WiFi network is very limited (under ideal conditions around 100 m). An alternative could be Bluetooth Low Energy (BLE). Unfortunately, BLE has pretty much the same limitations regarding reach as WiFi: about 100 m under ideal conditions. As we will see, there are also other options like Sigfox and LoRaWAN, but these offer very slow data transfer speeds and long latencies. So what is the alternative?
Why cellular IoT?
Cellular IoT uses existing cellular networks to connect physical devices to the Internet, i.e., the same type of networks our smartphones use to communicate. Cellular IoT relies on 2G, 3G, 4G, and more recently also 5G and Low Power Wide Area Networks (LPWAN) technologies such as LTE-M and NB-IoT to transmit and receive data. Cellular IoT is one of the most reliable and accessible ways to enable Internet connectivity for manufacturers building IoT devices. These cellular networks are available almost everywhere and there are some very nice advantages in using them:
- They provide excellent coverage
- They simplify global deployment
- They work right out of the box
- They support both low and high bandwidth
- Cellular networks built for IoT offer functionality to save power
Modems, SIM cards, and frequency bands
Before we dive into the plethora of available cellular networks there are a couple of concepts we need to sort out, namely cellular modems, SIM cards, and frequency bands. Typically, an IoT solution is built using some sort of microcontroller or Single Board Computer. To enable this hardware to communicate with a cellular network, a communication modem and a SIM card are needed. The traditional physical SIM cards have been around for decades, but more recently these have begun to be replaced by an electronic variant, called eSIM. Instead of inserting a physical SIM card into your communication device (i.e., your phone or IoT device), you install a SIM profile through software from your preferred network operator on your device.
While the SIM card you choose determines which cellular provider’s networks you can access, the modem you select impacts which network types and frequency bands your device can connect to. A frequency band is a range of frequencies within the radio frequency spectrum, which goes from 30 Hz to 300 GHz. Higher data rates typically require higher frequencies, but higher frequencies also imply poorer signal range and penetration, e.g., through solid objects like walls. Cellular networks use a fraction of the spectrum between 800 MHz and 5 GHz for 2G, 3G, and 4G connections whereas 5G can use bands up to 35 GHz. In practice, this means that a cellular modem is built for specific frequency bands and cannot communicate on other frequencies.
The cellular networks of today
2G/3G
The two oldest, digital cellular networks are the 2G/3G standards released in the 90s and 00s, respectively. These two technologies have worked well for many IoT applications, offering a data rate of a few kbit/s and thus allowing devices to transmit basic alerts, status updates, and location data while using relatively little power. However, 2G/3G networks will soon be a thing of the past and operators are beginning to shut down these networks.
4G LTE
Most modern cellular phones today connect to the 4G network, which offers significantly increased data rates compared to its predecessors. 4G theoretically offers 50/100 Mbit/s up/down, a figure that only a few years ago was reserved for WiFi. This data rate opened up a whole new universe of possibilities, including video live streaming and vehicle entertainment.
5G
The 5G cellular network is sometimes dubbed as the future of IoT connectivity. When it comes to data-intensive applications where speed is crucial, for example, self-driving cars, that is without a doubt true: 5G networks can offer nearly real-time data transmission from ~50 Mbit/s to over a Gbit/s which is on par with a cabled internet connection. In addition, 5G networks can maintain a stable connection with devices moving at very high speeds.
However, speed is not always the most important feature of an IoT cellular network. Power consumption is often just as important as data transfer speeds. The cellular networks described so far were never intended for the type of infrequent network connections and small data transmissions that an IoT device would do: phones connected to these networks are always checking for incoming calls, are always listening to radio signals, and send periodic information about location. This of course consumes lots of power, at least seen from an IoT device point of view. Enter LPWANs!
Cellular LPWAN
The Low Power Wide Area Networks for cellular connectivity include NB-IoT (Narrow Band-Internet-of-Things) and LTE-M (Long Term Evolution-Machine Type Communication) and these network types are tailor-made for IoT. Both NB-IoT and LTE-M actually cover a number of different categories with slightly different names but with the same basic underlying technology. Both LTE-M and NB-IoT are evolving technologies: the latest categories, LTE Cat M2 and LTE Cat NB2, are already deployed, fully functional, and offer global roaming. For a more comprehensive list of different network categories, this blog post might help.
From a technical perspective (outside the scope of this article), the two network types operate in different frequency bands. However, from the user’s perspective, the greatest differences are related to data rates and latency.
Both standards add one thing that is crucial for IoT devices: power saving. This is thanks to a combination of two important features, namely a Power Saving Mode (PSM) that allows the cellular modem to sleep for longer times without losing network connection, and a so-called Discontinuous Reception (DRX) functionality that allows the modem to listen for pending data without having to establish a full network connection. These additions have resulted in devices that may run up to 10 years on battery even though they go online at regular intervals.
It is also worth mentioning that even though LTE-M-based networks require more power than NB-IoT-based networks while transmitting, LTE-M-based networks can still be more power-efficient simply since the transmission goes faster and the latency is significantly lower than the NB-IoT-based networks. In addition, LTE-M can handle connected devices that move around. With NB-IoT, the device needs to reselect cells as it travels, but with LTE-M that isn’t the case. Hence, NB-IoT is less suited for mobile applications like vehicles.
The subscription cost has also evolved. Today, an IoT device can connect to a cellular network using a subscription that costs only a few dollars for the entire lifetime of the product.
To sum up, the choice between an NB-IoT-based network and an LTE-M-based network boils down to the use case at hand. If battery life and good signal penetration are crucial and low data rate acceptable, an NB-IoT-based network might be the preferred choice. If higher data rates are needed — possibly at the expense of battery life — and signal penetration is not crucial, an LTE-M-based network could be suitable.
So what are the alternatives to these standardized cellular networks? Well, there are only a couple of alternatives, and they differ in many ways compared to regular cellular networks.
Sigfox
Sigfox is the name of a French global network operator founded in 2010 that builds wireless networks to connect low-power objects such as electricity meters and smartwatches which need to be continuously on and emit small amounts of data. The Sigfox technology utilizes the license-free so-called ISM band (868 MHz in Europe) that typically is used for baby monitors, garage openers, microwave ovens, and other consumer electronics. It’s a wide-reaching signal that may pass freely through solid objects. The Sigfox device communicates with a base station that is operated by Sigfox so in that sense it resembles a classic cellular setup. Sigfox is focused on conserving energy, and the data rates available are hence quite low: a maximum of 140 messages per day may be sent and they may only contain 12 bytes uplink and 8 bytes downlink. This is of course only a minute fraction of the data amount that can be sent via a cellular connection; 140 messages at 12 bytes each corresponds to about 1.7 KB per day, compared to e.g., an NB-IoT connection that could theoretically transmit 20 KB (159 kbit/s) per second.
Sigfox as a company has gained traction. At the time of writing, they employ 400 people and their IoT network has covered a total of 72 countries reaching 1.3 billion of the world population.
LoRaWAN
LoRaWAN is not using cellular technology in any way. Instead, the technology uses the same license-free frequency bands below 1 GHz as Sigfox does. LoRaWAN-devices always communicate with a gateway in a star-topology and the gateway collects data and typically forwards it to a cloud service. The user can set up his own gateway and server for communication and with this setup, there are no data costs to this communication solution as in the case with Sigfox and cellular communication. The distance between gateway and a LoRaWAN device can be great; up to 10–15 km depending on conditions. However, the data rates are very low at a rate of 0.3–27 kbit/s depending on settings. As you probably noted, if LoRaWAN data rate is pushed to its maximum, it is on par with the slowest type of NB-IoT. This limits the use cases to very simple data transmissions. Nevertheless, LoRaWAN is suitable for remote locations where regular cellular connectivity is not available and where demands on data rates are low.
Summary
Cellular connectivity for IoT is a simple and efficient way of getting your IoT devices online. There are several advantages compared to a standard WiFi setup. Cellular connectivity offers both great coverage and building penetration and supports both high and low data rates depending on the user’s needs. The latency of a cellular network for IoT communication can be as low as 10–15 ms and data throughput up to 7 and 4 Mbit/s for uplink and downlink, respectively. In addition, tailor-made IoT network technologies like NB-IoT and LTE-M are very power efficient. There are several available cellular modems on the market and the prices are declining.
