SIGFOX – A new network technology for IoT comms?

It’s quite likely you've never heard of SIGFOX, or if you have it has been sketchy at best on details. So what is it, and why should you care?

Well, if you have a need for low cost, low power data collection which doesn't involve transferring megabytes of data and doesn't have to be particularly “real-time” e.g. you only need your data say once an hour, or even once a day, then SIGFOX may be just what you are looking for.

SIGFOX has been designed from the ground up as a low throughput data transmission technology, including a back-end managed data service, and is specifically intended to avoid many of the drawbacks of many current wireless technologies. 

SIGFOX Overview

The key advantages for a SIGFOX solution are:

SIGFOX uses a patented variant of what is called Ultra Narrow-Band (UNB) technology. The SIGFOX UNB radio uses the Industrial Scientific and Medical (ISM) frequency bands, which are free to use without the need for a licence, and allows data transmission over a very narrow spectrum to and from connected devices.

What are the advantages of Ultra-Narrowband?

For a given output power, the range of the RF link is determined by the data rate so that a lower rate provides longer range due to increased sensitivity for the receiver. As always there is a trade-off, since very low rates mean very long transmission/reception times, which affects battery life as well as increasing the likelihood of interference or collisions with other wireless systems.  In practice long-range systems typically use a reasonably low data rate to optimize the range and transmission time balance. 

Narrowband systems are typically defined as having less than 25 kHz bandwidth and have low in-band receiver noise as narrow band pass receive filters remove most of the noise. A typical narrowband system will use 12.5-kHz channel spacing with 10-kHz receive bandwidth. 

Ultra Narrow-Band systems reduce the bandwidth still further, in some cases down to a few hundred Hertz. By transmitting through an ultra-narrowband channel little power is required to transmit data over considerable distances as long as the data rate is also low. This typically allows a range of 5km in urban conditions with an in-building antenna, and up to 20-30km range in rural settings using an external antenna.

UNB systems are frequently asymmetric, for example with a sensor transmitting data to a basestation or concentrator. The use of this type of infrastructure allows the deployment of very efficient, low throughput communications network by limiting the number of base stations required. The use of UNB also allows excellent penetration, reaching inside buildings much more effectively than other low power radio technologies such as ZigBee.

Low cost

The impact on cost is significant and results in very affordable subscriptions when compared to the upfront investments and running cost of other network technologies. The costs will depend on the number and type of devices, but subscription costs of less than 1€ per device per year are being suggested.

SIGFOX communications costs are potentially more attractive and justifiable for almost any business case in comparison with existing connectivity solutions. SIGFOX provides connectivity, access to APIs, web-based administration and support as an all-inclusive annual subscription fee without any additional charges. The pricing model is based on actual daily or monthly usage defined by bandwidth and the quantity of devices deployed. 

Low energy consumption

To minimize energy consumption, the SIGFOX network is only used when the device needs to transmit a message. This means that the modem spends most of its time powered off and is the major reason why SIGFOX devices consume much less power than existing connectivity solutions when compared to competitive technologies such as GSM or broadband radio. This allows SIGFOX to keep their network costs down and to provide a cost effective low-throughput M2M and IoT communications solution.

The exact power consumption over time obviously depends on how many messages are sent and how often. SIGFOX claim that for a typical use case, such as a smart energy meter that transmits 3 messages a day using a 2.5 Ah battery, existing solutions would typically last for months whilst SIGFOX enabled smart meters run up to 20 years. Clearly the exact battery life will be influenced by the both the hardware and firmware design, but lifetimes of several years between battery replacements should be easily achievable with intelligent design.

Low bandwidth

As currently deployed each SIGFOX base station can potentially handle up to a million connected devices, but the network is scalable to handle more objects by adding additional base stations. The SIGFOX network is not targeted at any specific market or industry and can potentially be used in any scenario where there is a need for a low throughput connectivity solution. The definition of low throughput on the SIGFOX network can characterised as follows:

Collecting and Accessing Your Device Data

Once devices are deployed the messages they send are collected and aggregated by the SIGFOX network.  As a device owner the data transmitted is then available through standard Web Services APIs provided by SIGFOX servers.  Your application(s) can be built on top of these services and once this is set up then whenever a message is received from one of your connected devices it will be passed to your application for processing.

Use cases

There are a wide variety of possible use cases, a number centring around alarm or monitoring devices where the system needs do nothing other than continually check for a particular event and only send a notification or alarm if that event is detected. Examples here might be smoke detectors or air quality monitors. Because the network is powered down most of the time (other than perhaps daily keep-alive or heartbeat messages to confirm that the device is still on the network), the power consumption is very low and the device will last many times longer than similar devices using different wireless technologies.

Other use cases might be scenarios where periodic readings are required, such as daily totals from a smart gas meter, or regular temperature and/or humidity readings from a building or hot water supply. However the limited bandwidth available would make SIGFOX less suitable for advanced smart metering applications where half hourly reads and billing data is required as this could be too much data for the available message capacity.

So how real is it?

Well, as it originated in France, it will come as no surprise that the country is fairly well covered already (SIGFOX themselves funded a lot of the roll-out). Other European nations are following suit and the UK is scheduled to have good coverage in major cities by the end of 2015 (Arqiva are managing the roll-out in the UK). The US is coming along too, although on a slightly different frequency band, the equivalent ISM band in the US being 915MHz rather than 868MHz, so don’t just assume the same equipment will just work if you want to roll out in the US.

Silicon is also available from a number of vendors, ranging from the entire SIGFOX stack on a single chip from Atmel to complete modules from Telecom Design, Telit or Adeunis. For high volume, low cost solutions it would clearly be more cost effective to design a bespoke device from scratch, but for lower volumes where the requirements are reasonably straightforward then a solution based on one of these modules would be perfectly acceptable.

If you are developing your own design then check the datasheets carefully. For example the some devices do not currently support bi-directional data transfer, and some RF transceivers may not support the line coding scheme required for SIGFOX.

Are there downsides?

Well of course as with any new technology there are issues to consider when designing your system.

The SIGFOX technology is patented and the IP is proprietary, although it is readily available on a royalty free basis to members of the SIGFOX consortium. If you are putting together your own design from individual devices, be aware that there may be other patents that you need to take into account.

The bandwidth availability is very low and is only suitable for certain applications. Forget about over-the-air firmware updates as they could take months or even longer to complete!

There is obviously vendor lock-in on the subscriber side as you can’t switch from one provider to another in the way you can with GSM mobile networks so if SIGFOX were to decide to increase their subscription charges then your operating costs would go up.

Conclusion

SIGFOX is very much still at the early adopter stage, and as with any new technology there are bound to be teething troubles. However there are already many thousands of connected devices in France and the technology has the potential to provide a very cost effective solution in a variety of markets and industries.

There are of course other competing technologies such as LoRa or Weightless, but I’ll cover them in a future article.

Note: A shortened version of this article was previously published on the Abelon web site

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