Understanding SigFox Protocol Stack Layers

Introduction

The SigFox protocol stack is designed for efficient, low-power communication within its Low Throughput Network (LTN). It’s built with several layers, each handling specific tasks like physical signal transmission, data framing, and message processing. This guide breaks down the architecture of the SigFox protocol stack and explains how each layer contributes to smooth IoT communication.

Figure 1: A simplified protocol stack of the SigFox wireless system.

The stack consists of these layers:

• Radio Frequency (RF) Layer
• PHY (Physical) Layer
• MAC Layer
• Application Layer

RF Layer

The RF layer’s responsibilities include:

• SigFox systems use two radio layer implementations: Ultra Narrow Band (UNB) and Orthogonal Sequence Spread Spectrum (OSSS).
• Frequency spectrum allocations vary by region, such as 915 MHz in the USA, 868 MHz in Europe, 433 MHz in China, and Japan.
• For UNB uplink transmissions in the 868 MHz band, the maximum transmit power is specified as 25 mW.
• The SigFox receiver sensitivity should be better than -135 dBm.

PHY Layer

The PHY layer is responsible for:

• Handling MAC frames during transmission and reception.
• Using BPSK modulation for uplink and GFSK for downlink (in UNB implementations). OSSS implementations use DSSS with orthogonal signaling.
• Adding a preamble during uplink transmission from the end device to the Gateway and removing it during reception at the Gateway. The reverse happens in the downlink. This preamble is for synchronization.

MAC Layer

The MAC layer takes care of:

• Assembling MAC frames for transmission and disassembling them upon reception, following defined uplink and downlink structures.
• Implementing UNB for uplink message transmission. Downlink transmission can use piggybacking or broadcasting.
• Authenticating end users.
• Detecting errors using FCS (Frame Check Sequence).
• Piggybacking: LTN End Devices (LEPs) open a fixed window to receive downlink messages from LTN Access Points (LAPs). LAPs use this window to send messages to the LEP.

Application Layer

The Application layer’s function:

• Defining various applications based on user needs, such as web services or messaging.

References

ETSI GS LTN 001, 002, 003 (Low Throughput Networks) documents.

Conclusion

The layers of the SigFox protocol stack work together seamlessly to provide reliable and efficient IoT communication. Understanding these layers and their functions allows developers to leverage SigFox effectively for a wide range of IoT applications.