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    Introduction

    As IoT nodes spread across broader geographical areas, these remote devices are expected to run on limited power (or batteries) for years. How can long distance communication be achieved under these requirements?  One answer may be SigFox, which is a low power wide area wireless network provider (LPWAN) who has successfully delivered a long-range solution using an intelligent trade off between bandwidth, power, and distance. In this post, you will learn some basics of the SigFox wireless network.

     

    Features

    Some main features of SigFox are:

     

    • Global Reach: SigFox has a global reach, and SigFox base stations are operated by different operators worldwide. As all the base stations are connected to the same Cloud, devices deployed anywhere can be accessed from a single interface. Devices can work anywhere irrespective of global boundaries provided they are in the operating frequency range based on the operating country.
    • Ultra-Narrow Band radio Modulation: SigFox uses the license-free ISM band (868 to 869 MHz and 902 to 928 MHz depending on the operation regions) for communication, and operates with a bandwidth of 200 KHz. The bit rate is also dependent on the region of operation and varies between 100-600 bps. Such a low bit rate not only makes it immune to noise but also helps to cover a significant distance. It uses DBPSK (Differential Binary PSK) and GFSK (Gaussian FSK) digital modulation techniques for uplinks and downlinks respectively. Its low bit rate provides higher spectral efficiency of 1 Bits/Hz (Information transferred over a bandwidth).
    • Small Payload: Uplink messages in SigFox are typically 12 Bytes, and the total bytes of data sent for such a payload is 26 Bytes. The typical time in the air for SigFox messages is 2s. The downlink message, however, is limited to 8 bytes only. SigFox Ready devices can even send empty messages with 0 bytes of data to indicate that the device is still online. Fewer data used for communication means less energy consumption, and hence longer battery life.
    • Lightweight Protocol: Designed for burst data communication, the SigFox protocol adds minimal overhead for uplinks and downlink data packets. SigFox also avoids signaling during communication.
    • Low Power Consumption: Small payload, lightweight protocol helps SigFox devices have low power consumption. Also, SigFox devices are not configured in always listening mode to save power.
    • Low Cost: SigFox uses low-cost hardware for modulation and demodulation. This makes SigFox Ready devices extremely inexpensive to manufacture.
    • Compatibility: SigFox is compatible with Bluetooth, GPS 2G/3G/4G, and Wi-Fi.
    • Network Architecture: SigFox employs a star topology. Base stations, which collect data from end nodes, are connected directly to the SigFox Cloud. Nodes get directly connected to the nearby base station when they try communication. However, nodes are not bound to any base station.
    • Implements Time/ Space/ Frequency Diversity To Avoid Interference: SigFox end nodes send each message three times in different frequencies and messages are picked up by all nearby base stations. This enables SigFox to have time (Three-time message send), space (Message picked up all nearby base stations.) and Frequency (the Same message gets transmitted in three different frequencies.)

     

    Architecture

    A SigFox network consists of 3 typical components:

     

    • SigFox Ready Devices
    • SigFox Base Stations
    • SigFox Cloud

     

    SigFox Ready Devices: The devices are equipped with the SigFox protocol stack responsible for SigFox communication, which is proprietary but royalty free for manufacturers. It helps generate radio frames and thus transmit messages over the SigFox global network.

     

    The protocol stacks consist of three layers instead of the seven layers as in the OSI model. They are:

     

    • Frame: Generates the radio frame from data received from the application and adds a systematic sequence number.
    • MAC: Adds unique identification for the device to the frame and also adds a CRC code to the frame. MAC layer does not involve any signaling for medium access control instead it uses patented Statistical Radio Medium Multiple Access schemes.
    • PHY: The PHY layer is responsible for modulation/ demodulation, Transmission Power, Bitrate, and Operating frequency.

     

    Each end node in a SigFox network has to be SigFox Ready certified. The first test is carried out for frequency hopping and frame repetition. The Cloud must receive copies of each message on different frequencies. Spreading is also checked in this test.

     

    The second test that the device goes through is its radiation performance. In an Anechoic chamber, the device is set in continuous wave mode, and the Effective Radiated Power (ERP) is measured and plotted in 2-D or 3-D. The ERP is calculated at every angle and done so by using a turntable.

     

    As per the ERP rating, SigFox devices are certified in 3 classes like Class 1 for 7-12 dBm, Class 2 for 0-7dBm and Class 3 if the ERP is below 0 dBm.

     

    The SigFox Ready program for end products doesn't substitute local regulations about, for example, electromagnetic compatibility, security, and safety. The certification scope covers only the performance of the end product and leads to the radiation class allocation to the connected device. The SigFoxReady certification is delivered per product model, and tests must be conducted on a pre-series or a final device. Prototypes are not eligible for certification.

     

    SigFox Base Stations: SigFox base stations are deployed and operated by SigFox operators or by SigFox itself throughout the world. They act as an intermediate between the Sigfox devices and the SigFox Cloud. The base stations are connected to the Cloud by using point-to-point links and are protected. They are responsible for demodulation and checking the messages, and then pass it to the Cloud.

     

    SigFox Cloud: The SigFox Cloud comes with three interfaces: web UI for the end user, API to integrate third-party services, and callbacks interface to push messages to third-party servers. Constant checking for data by the user application to the Cloud is not required due to the callback interface implementation. The SigFox Cloud also acts as one point to manage the complete network. It allows the user to register new devices, manage users, and manage APIs and Callbacks. The SigFox Cloud comes with a recurring cost for the user.

     

    How SigFox Communication Works?


    SigFox Ready devices act as end nodes and transmit data to the SigFox Cloud through base stations.

     

    The nodes transmit messages and are received by nearby base stations providing space diversity. The devices are not attached to specific base stations. The SigFox Cloud checks the authenticity of the message and action associated with the message. It also removes duplicate messages sent by all the base-stations who received the same message. Then the Cloud reports the collected data to consumers or partners, who can have their own IT infrastructure.

     

    Uplink/Downlink: Before communication handshaking, frequency and bitrate negotiation are not required in SigFox, which makes the communication non-synchronized. The node can choose any frequency within the operational band to transmit each message. SigFox Ready devices transmit each message three times in different frequencies employing frequency hopping, which provides frequency and time diversity. The base stations are capable of demodulating any SigFox signal in the operational frequency range.

     

    The uplink messages are limited to 140 messages per day, whereas the downlink messages are limited to 4 messages per day only. SigFox users are restricted to connect to deployed objects directly through downlink; the device can only request it. The end device always initiates the communication. When such request arrives with the message, the SigFox Cloud passes it to the customer service to which the customer can reply. If the customer replies it is forwarded to the end device (which is in listening mode) through a

    downlink.

     

    Initiation of communication from the end node limits the accessibility of it from the user end, but it helps in saving power. As the nodes are not always listening to incoming messages, it avoids reception of unauthorized packets sent by unauthorized entities, thus helping as a security measure.

     

    Security

    Security between the end devices and the SigFox Cloud is handled by using end-to-end authentication using a secret key. The key is stored in the device in a non-accessible memory location. A device specific ID is also stored in read-only memory. The key stored in the device is used to generate a signature for each message sent by the device which authenticates the origin device.  To avoid message replay, a sequence number is also embedded in each message.

     

    Each message is sent 3 times on different pseudo-random frequencies. This acts as a barrier against sniffing as a selection of a frequency for the messages are unpredictable and can be any frequency within the operational range.

     

    The messages from SigFox devices are picked up by all nearby base stations. Thus, with Ultra-Narrow Band technology, it provides high resistance to jamming. SigFox devices are not in always listening mode so a hacker can’t trick the devices using corrupted payloads.

     

    SigFox base stations are connected to SigFox Cloud by using point to point links in an encrypted Virtual Private Network. The Cloud is also virtualized on private servers, and hosted in secured data centers distributed in different physical locations. All the interfaces from the Cloud such as websites, APIs, and the callbacks are served over HTTPS (Secure HTTP).