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SIM Cards and Data Usage - [Dart3]


This article is intended to assist partners in selecting between LTE-M/NB-IoT, determining costings, and choosing SIM providers.

Advice is general and partners should conduct their own testing and make an informed decision. It should be noted that Digital Matter supply devices globally, but have offices based in the US, South Africa and Australia.

Our advice/the content of this article may be biased towards certain regions purely because we have more partner feedback from this region around what works. 

Cat-M1 and LTE-M are used interchangeably in this article, they mean the same thing. Networks like to change names to make things as confusing as possible! However more recently LTE-M is the common term (that wasn't what it was when the networks were first launched!).


This question comes up often, as partners need to choose a SIM data plan to assets costings. However this greatly depends on a few factors:

  1. Device Type: the type of device affects how much data is sent and how often
  2. Device Setup: what is the device setup to do?
  3. Network: what is the minimum connection charge?
  4. Network quality: how often is the device forced to reconnect?

Rule of thumb

Hard Wired Devices, 2G and 4G, will use about 5-10MB per month, dependent on settings and how often they are in trip. 

Keep in mind:

  • The minimum connection charge has a massive effect on the data usage. Consider Vodacom in South Africa. Each new data connection is charged a minimum of 10kB. The device may send less than 1KB on a heartbeat, but it is still charged for 10kB. With hourly heartbeats, this adds up fast. 10kB per heartbeat x 24 hourly heartbeats x 30 days = 7.2MB. That's just on heartbeats. MTN in South Africa charges a minimum of 25kB per connection. That's 18MB.
  • Dropped connections also have an effect: each reconnection could be charged the minimum connection charge.
  • Grouped/pooled data plans provide some 'breathing room' for devices that may upload more than planned, since there will be others in the pool which use less data that month.

LTE-M vs NB-IoT, what to choose?


  • Your hand may simply be forced as you may only have 1 technology available in your region. Or only one has widespread coverage. In that case go with the technology with better coverage
  • LTE-M is generally the choice for hard wired devices, but NB-IoT also has been shown to work with no issue. 
  • From the plans/costings we have seen, LTE-M overall is more flexible. You may initially be happy with 1 update per day, but if you change your mind later, you can move to a higher plan with ease. This is not generally the case on NB-IoT.
  • The NB-IoT pricing structures we have seen are low cost when you are using tiny amounts of data - as soon as you push the update rate up, it is equivalent or more expensive than LTE-M, and doesn't have the benefits of cell tower handover etc.
  • Firmware over the air is possible on NB-IoT, but since the data rate is much slower, it takes very long to complete the update.

NB-IoT data consumption

NB-IoT is an emerging technology geared around sending small amounts of data. Like infrequent location updates, temperature readings, or some other sensor values - powering the "Internet of Things", and many millions of interconnected devices. 

NB-IoT rollouts are in full swing globally. For a comparison between LTE-M and NB-IoT, along with examples of typical use cases, see 4G Connectivity - Cat-M1 vs NB-IoT, Coverage, Providers and Roaming. In general, NB-IoT is best suited to minimal data usage.

NB-IoT is attractive because the cost can be lower than LTE-M. Even a small saving each month can quickly add up across the lifetime of a device, particularly when rolled out across many thousands of devices. Getting monthly cost down is a key way in which many previously unviable applications are beginning to make cost sense. Now we really can track anything! However the cost is reflected in the data per plan. E.g. 5MB per year plans aren't uncommon. 

It is important to note however that many NB-IoT plans are structured to heavily discourage higher amounts of data usage, at which point they become more expensive than a Cat-M1 plan, so it is important to have clear how much data we expect to use, and test, before rolling out large-scale NB-IoT deployments. Typically it is not easy from a SIM or device level to simply switch to a higher plan, or from NB-IoT to Cat-M1, without retrieving the device and swapping SIMs. 

So how much data will I actually use?

The answer really is 'it depends'. We cannot simply look at the record size sent by our devices and multiply that by the number of uploads. Other things contribute to the overall data use including

  • TCP Overhead: Our devices communicate via TCP. TCP is a protocol which establishes and maintains a connection while two parties exchange data. This uses up extra data in sending hellos, acknowledgements, and other messages just as part of the actual connection - before we even send any data. This can often outweigh the actual message payload. 
  • Firmware updates
  • GPS aiding data. Our GPS devices periodically download information from the satellites, which helps to improve the GPS performance. Data usage in this case is normally around 50 kB.
  • Device System Parameters. The configuration of the device will impact the upload size and number of uploads.
  • Minimum connection charges typically do not apply on NB-IoT, you're only charged for the data you use, with no rounding up to 1kB, 2kB for example like on traditional networks. However this is not a given so be sure to check.

With all these various factors, we can make estimates, but the easiest and the most reliable way to work out how much data a device will use is to simply insert the SIM, let it run for a while with the desired settings, and then just look at the data usage at the end of the month.

What about Firmware Updates?

  • Firmware updates are not applied automatically, the partner must log in to OEM and apply them. So you should not be caught by surprise. 
  • Generally, they are around 300-500kB in size, which is of little consequence for a powered device.
  • If you are using a pooled plan, you could update a selection of your devices each month, in a staged process, to stay within your budget.
  • While extremely rare - critical firmware updates can happen, where all devices need to be immediately updated. In these cases it just makes sense to accept the cost/risk of overage charges, or factor this in initially.

Other Resources

For help in choosing a SIM to get online, see SIM Providers: LTE-M/Nb-IoT + Roaming, and Setup

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