Along with LoRa and Sigfox, mobile communication networks are also a good option for the transmission of IoT sensor data. The upgrade from EDGE to UMTS made this option even more attractive, since using a faster transmission system can in the end be better than using a lower-power but slower system. However, the immense bandwidth and power hunger of 4G/LTE make this rule of thumb a bit less relevant. The power consumption of the transmitters is significantly higher, and on top of that the modules are more expensive. Nevertheless, it can be worthwhile. In the framework of the specification 3GPP Release 13, designated by LTE as ‘informational’, the GSM Association defines two systems for the Internet of Things. The first is Narrowband Internet of Things (NB-IoT), and the second is LTE-M, also known as LTE Cat-M1 or eMTC. LTE-M is basically a ‘light’ version of LTE (4G) with a bandwidth of 1.4 MHz, while NB-IoT is a dedicated wireless communication standard for the Internet of Things. The key difference is that LTE-M additionally supports voice transmission with VoLTE, while an NB-IoT system exclusively transmits data messages.

The NB-IoT channels, each with a width of only 180 kHz, use a subset of the methods implemented in the full version of LTE. Uplink uses a simple version of the frequency division multiple access (FDMA) method, while downlink uses orthogonal FDMA (OFDMA). The quadrature phase shift keying (QPSK) modulation method does not require especially complex hardware in terms of processing power.

However, it should be noted that introducing NB-IoT usually incurs additional costs for the carrier for the new hardware. Due to the extremely narrow bandwidth, NB-IoT can easily fit into the guard band surrounding the LTE frequency packets. On the other hand, using NB-IoT in stand-alone mode is of course also possible.

NB-IoT Performance

Even the technically most attractive wireless communication standard is of no use if the transmission capability is insufficient for the intended task. In the case of NB-IoT, the version is an important consideration because there are differences between LTE Cat NB1 (Release 13) and LTE Cat NB2 (Release 14). The older version can only achieve 26 kbit/s in upstream, but Cat NB2 is significantly faster with 127 kbit/s upstream and 159 kbit/s downstream. For comparison, conventional (not HSDPA) 3G initially achieved 380 kbit/s. LTE Cat M1 currently runs at around 1 Mbit/s upstream and downstream, and Release 14 raises this to 4 Mbit/s upstream and 7 Mbit/s downstream.

The differences in latency times are enormous. LTE-M can usually achieve 15 ms, while with NB-IoT the recommended ‘working range’ is from 1.6 s to as much as 10 s. The module manufacturer Sierra Wireless, especially popular in the USA, describes the situation as follows:

“Another important fact to consider is that there are no NB-IoT use cases that LTE-M can’t also support. In other words, LTE-M supports any LPWA application, whereas NB-IoT is designed for simpler static sensor type applications.”

In addition, only version 2 of the NB-IoT standard supports position data provision by the network operator. If the module does not have GPS capability or you want to do without an external antenna, you can use this approach to obtain basic position data. Release 14 also accelerates searching for new cells, which is mainly beneficial for moving devices. Despite these new benefits of Cat NB2, LTE-M is still the best choice for automotive and other mobile applications because it provides smarter cell handover. The final improvement concerns transmit power: Super Low Power transmitters, which can operate with only 14 dBm, are only allowed in Release 14.

If at some point in time you got your hands on a 4G module for Verizon, the natural question here is which bands are used. Band 13, which is only important for North America, has caused problems for many Asian or European module providers. Table 1 is taken from the Deployment Guide of the GSM Association. You should make sure that the module you choose supports all the bands that are used by your preferred carrier.

Table 1: Frequency bands.
Region Bands
Europe 3, 8, 20
(Former) CIS countries 3, 8, 20
North America  2, 4, 5, 12, 66, 71, 26
Asia Pacific (APAC) 1, 3, 5, 8, 18, 20, 26, 28
Sub-Saharan Africa  3, 8
Middle East and parts of North America 8, 20
Latin America 2, 3, 5, 29

Availability and Agreements

It goes without saying that wireless communication standards are only worthwhile if they are also available in practice. In the case of the two IoT wireless communication standards, you should have a look at the interactive world map of the GSM Association in Figure 1 (status as of September 2021). As you can see, Mexico is the only country where only CAT-M is available (probably because of the larger range), while “NB-IoT only” is more widely available in the rural areas of Asian countries and, remarkably enough, in Eastern Europe. In the highly industrialised regions of Europe, North America, Asia, Australia and Oceania, both versions are available.

Figure 1: Comparison of geographic coverage of CAT-M only (red) and NB-IoT only (blue). (Source: GSMA)

CAT-M agreements are generally ordinary agreements in which the total usage volume and the number of SIM cards determine the overall cost. For the sake of completeness, it should be noted that with regard to cost, an IoT provider such as PodGroup is often a better choice than a prepaid SIM card purchased on the open market.

The claim that NB-IoT is not subject to duty cycle restrictions is not borne out by the author’s practical experience as a consultant. Talking with your mobile provider about IoT connectivity is and will remain a matter of negotiation, and all too often limits on the number of packets in a given time interval will be imposed. Operators rarely publish their exact conditions in this regard, which makes the following statement from T-Mobile USA all the more remarkable:

“Join the first nationwide NB-IoT network to power asset tracking, connected cities, and more. Limited time offer; subject to change. Taxes and fees may be additional. Plan includes 10 single-packet transactions per hour at up to 64 Kbps, up to 12 MB. Full service payment due at activation.”

Interestingly, this is only an individual opinion, and Hutchison Holding Ltd has confirmed that the total volume of data traffic (within the bounds of the agreed amount) can be used up in one day. Tom Tesch, the Austrian spokesperson of Hutchinson, says in this regard: 

“The data rate of NB-IoT — in accordance with the standard — is very low and primarily suitable for the transmission of individual measurements or status values. For this reason, more than 5 to 10 MB per month is very rarely needed for NB-IoT devices. For more bandwidth-intensive applications, such as the transmission of photos or videos, 3G/4G and of course 5G are more suitable technologies. There are currently no limits on when the volume can or may be used, which means that the entire volume can also be used up in one day.”

Getting Started with NB-IoT

After these basic considerations, it’s time to start thinking about how you can integrate NB-IoT into practical systems. Of course, development of customer-specific modems is not feasible for most companies, but in the past we have described the ‘design in’ process for wireless modules in detail, for example in Elektor 5-6/2021. 

If you don’t want to start developing your own board right away, one option is to use a ‘turnkey’ evaluation board — although the availability of Qualcomm ICs is proving to be a problem in this regard. 

Two possibilities are the NBIOT-BG96-SHIELD from Avnet, which integrates a Quectel BG96 module, and the 5G NB IoT click board from MikroElektronika, which hosts a Cinterion module. Arduino also offers a small development board in the form of the MKR NB 1500. However, both of these boards cost more than 50 dollars. 

In many cases, it is no longer permitted to supply evaluation board equipped with SIMs, so a massive rollout of applications based on NB-IoT is far from easy. The reason for this is that network operators have not yet packaged the technology for end users. This is also openly admitted by operators, as illustrated by the following statement from Hutchison: 

“NB-IoT is a very young and innovative network. As there are hardly any devices available on the market, the target group primarily consists of business customers in the hardware (and software) development environment. This means that our offering is currently exclusively oriented to business customers, for which we create a tailored offer in the course of a consultation process.” 

When working with ‘ordinary’ 2G/3G/4G systems, one way to get around this is to use a ‘virtual’ mobile communication provider such as PodGroup. When asked about this, they answered that NB-IoT is currently not really suitable, especially for ‘global’ solutions that need to work with a single SIM card. 

There are two reasons for this. Firstly, that the NB-IoT rollout is still relatively limited. And secondly, that roaming agreements between the different network operators have generally not yet been adapted to the new NB-IoT wireless communication standard. Liked tax treaties between countries, it takes a long time to achieve such adaptations. In short: international NB-IoT roaming is still in its infancy.

Is It Worthwhile?

Searching for a practical module that supports only NB-IoT is certainly a very tricky endeavour. Quectel, for example, offers two versions even with the smallest series (BC660): one with only NB-IoT, and the other with both eMTC and NB-IoT. Both wireless standards are also present in larger families, such as the very popular BG95 and BG96. Open market prices for these modules can only be found at SOS Electronic: the BC660K-GL costs €7.63 is small quantities, while the version with LTE-M and NB-IoT is not listed. The price there for the BG96 is €19. 

A search for u-blox yields more results. The SARA-N3 family includes a module exclusively intended for the NB-IoT set of protocols, but the Swiss company does not offer a pure CAT-M device (see Figure 2).

Figure 2: Searching for a pure CAT-M module doesn’t turn up many results. (Source: ublox)

At Gemalto, whose takeover by Thales has made the website even more confusing than it used to be, there is a pure CAT-M module in the form of the EMS31, along with a pure NB-IoT module (ENS22) with the same form factor. At the Czech distributor Sectron you can compare prices: the EMS31 costs €14, the ENS22 only €8. 

Information on current consumption (in roundabout form) can be found in the data sheets, which go by the name ‘Hardware Interface Description’. The highest current consumption of the EMS31 occurs when operating in Band 4 and is 239 mA with a supply voltage of 3.8 V. For the ENS22, the highest current listed is 404 mA in Band 28, but it should also be noted that wireless modules often require peak currents like this only for a very short time.

What’s In It for You?

From a technical point of view, NB-IoT works perfectly, and once you have arranged an agreement with a carrier, the effort for network operation is limited to a phone call to your lawyer, unlike the situation with a home LoRa WAN. The relatively low peak and quiescent current consumption of the modules also helps to keep your electricity bill within bounds. 

Whether or not it’s worthwhile in the end is primarily a matter of scale, just like tax havens such as Dubai or Monaco. If you buy five modems in a year, operate with a ‘full’ 4G module or, even better, a module with a beefier power adapter and costing a few euros more, the sad experience of the author is that in practice you will repeatedly need the ‘other’ wireless standard, if only because some base stations do not support every wireless communication standard. 

Naturally, the situation looks different if you are purchasing 50,000 modems that will all go to the same customer. If the mayor’s office of Großdorf am Klapotetz (Figure 3) needs NB-IoT, the local carrier will probably upgrade their network, and the cost savings from the large number of devices will also help.

Figure 3: A traditional Stierian windmill serving as a scarecrow. (Source: Martin Geisler, CC BY-SA 4.0)

Questions or Comments?

Do you have technical questions or comments about this article? If so, please contact the author at tamhan@tamoggemon.com or the Elektor editorial staff at editor@elektor.com.