Continuing with our series on IoT protocols, this breakdown of SigFox explains how it maintains low-power communication over long distances.
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SigFox is a Low Power Wide Area Networks (LPWAN). Devices connected using SigFox consume little power and operate over large distances compared to Wi-Fi and Bluetooth connection protocols, which consume more power and work best in short range. The chronology of a SigFox application follows these three basic steps:
- Numerous objects (devices) connected to the Internet send data through the SigFox network to a SigFox base station (gateway).
- The base station then detects, demodulates, and reports the messages to the SigFox cloud across 3 channels, at least every 10 minutes.
- The SigFox cloud then pushes these messages to many customer servers and IT platforms based on the client's application.
Technically, the SigFox network differs from other LPWAN networks in the methods it sends data and the electrical guidelines that govern the quantity, speed, and duration of the data being sent. SigFox is most used for low-power applications that only require sending small amounts of data, infrequently, over large distances. Perfect for Agro environments and asset management across vast distances.
How SigFox WorksThe SigFox network consists of these elements:
- Objects (devices)
- Base stations (gateways)
- Cloud (Internet)
The SigFox network consists of these three processes:
- Ultra narrowband (UNB) technology
- Differential Phase Shift Keying (DPSK)
- Frequency Shift Keying (FSK)
The diagram below illustrates the key elements and processes of data transmission via SigFox:
SigFox data transmission can be better understood as follows: Objects (devices) are connected to the Internet using the SigFox network. The object can be a temperature, humidly, and/or saturation (etc.) sensors located within 1,000 meters of a base station (gateway). Interacting with SigFox, the objects sends uplink messages to the base stations, where the hardware utilizes Differential Phase Shift Keying (DPSK) to demodulate the signal to send uplink data to the cloud for computation using 3rd party platforms like Ubidots.
What Is DPSK?
DPSK is a method used by base stations (gateways) to convert a signal, debug it, convert it back to be sent to the cloud. When a signal travels from a device to a base station, it inevitably encounters interference from the environment (think rain or dense forests). Interference is universal; any signal from any internet network will become impaired and look slightly different upon reaching its destination. SigFox alleviates this problem by utilizing DPSK. The role of DPSK is to make sure that the signal that leaves the base station is the same exact signal that left the device. The base station hardware accomplishes this by shifting the signal's phase to discover and eliminate impairments. SigFox hardware at the base stations accomplishes this by:
1. Object sends data to the base station in the form of digital bits. A "high" pulse occurs when there's a 1, and a "low" pulse occurs when there's a 0. Here is an input digital bit stream 11000110:
2. This bit stream is then converted into a different sequence of 1's and 0's as it passes through the demodulator circuit. The new sequence is not arbitrary, rather carefully calculated using sophisticated hardware. The purpose of this conversion is to prepare the signal for electrical analysis.
Whenever the state of the input signal goes from high to low (1 to 0), the hardware shifts the phase of the signal. To phase shift a signal simply means to impose a time gap between the signal's original and new paths. Once the phase is shifted, a signal will either lag/lead where the original path once was:
3. The more a signal at the base station becomes phase shifted, the more exposed its impairments become. Analogously, the more frequent an injured person visits the hospital, the more X-rays the doctors take to better understand and correct the injury. When a signal is "injured" from the environment, the injury is not noticeable until the signal is phase-shifted and run through the "X-ray" circuitry, which analyzes these phase shifts, discovers where the impairments exist, and subsequently "cleans" the data for transmission. In summary, the base station hardware shifts the phase in order to take an "X-ray" of the data to diagnose what interference/impairments exist and how it is to be corrected.
4. Following the phase, the hardware circuitry converts the original signal back to its basis sequence, but without the impairments, and sends the data uplink to the cloud.
When the cloud receives an uplink signal from the base station, it will respond with a downlink signal, either to a customer application or back to a base station. Downlink signals use Frequency Shift Keying.
What Is Frequency Shift Keying?
Frequency Shift Keying (FSK) is similar to Differential Phase Shift Keying (DPSK) in the sense that both processes convert the input signal, analyze/discover impairments, eliminate them, and convert the data back to the original signal. However, instead of shifting and analyzing the phase, FSK shifts and analyzes the frequency. Just like phase shifts in DPSK, the frequency shifts in FSK expose the signal's impairments, where sophisticated circuitry can debug them. The outstanding question now becomes, why does SigFox use DPSK for uplink transmission and FSK for downlink?
- DPSK is more bandwidth efficient than FSK, so it has fewer frequencies and channels available to transmit the signal.
- Less "space" to transmit the signal = lower data rate and throughput
- Lower data rate = more sensitive receiver (like a base station) of the signal
- Higher sensitivity = more achievable range. ie. data from sensor devices can be detected from farther away.
- Uplink signals typically encounter more interference than downlink signals, so having a narrow bandwidth in DPSK = power is more concentrated = more robustness to interference
- Since interference is not as big as a concern for downlink, downlink signals are more focused on reaching as many applications as efficiently as possible. In FSK, more bandwidth = more space to send a signal = more reachable applications
Ultra Narrowband (UNB) Technology
Thanks to DPSK and FSK, customers of SigFox can take advantage of sending data over long ranges. But how does SigFox ensure low power?
SigFox uses a technology called Ultra Narrowband where signals are sent very infrequently, therefore requiring less energy. SigFox structures its UNB technology to only allow up to 140 uplink messages a day, at 12 bytes per message, and 4 downlink messages a day, at 8 bytes per message, at low data rates. This is, in fact, a very infrequent data transmission — a message can be sent every 10 minutes at the minimum because of the size and time restraints that SigFox imposes on its data transmission protocol.
The technology used by SigFox contributes a long-range, low-power, low throughput communications network with excellent protection from environmental interference allowing data to reach many applications effectively. SigFox is still in the "early adopter stage" for connectivity solutions; however, there are already many thousands of connected devices around the world with SigFox technology, proving it has the potential to provide a cost effective solutions in a variety of markets and industries. For more of connectivity options in the IoT world, check out some thoughts on LoRaWAN or reference this great piece on popular wireless protocols around the world.
Published at DZone with permission of Thomas Michalski, DZone MVB. See the original article here.
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