The protocol stack is similar to other stacks following the OSI model. There are three levels, detailed here:

• PHY Layer: Synthesizes and modules signals using DBPSK in the uplink direction and GFSK in the downlink direction as previously detailed.
• MAC Layer: Adds fields for device identification/authentication (HMAC) and error correcting code (CRC). The Sigfox MAC does not provide any signaling. This implies that devices are not synchronized with the network. 
• Frame Layer: Generates the radio frame from application data. Also, it systematically attaches a sequence number to the frame:

With regards to security, messages are not encrypted anywhere in the Sigfox protocol stack. It is up to the customer to provide an encryption scheme to the payload data. No keys are exchanged over a Sigfox network, however, each message is signed with a key unique to the device for identification.