Each device in a Sigfox network has a unique Sigfox ID. The ID is used for the routing and signing of messages. The ID is used to authenticate the Sigfox device. Another characteristic of Sigfox communication is that it uses fire and forget. Messages are not acknowledged by the receiver. Rather, a message is sent three times on three different frequencies at three different times by the node. This assists in ensuring the integrity of transmission of a message. The fire and forget model has no way of ensuring the message was actually received, so it is up to the transmitter to do as much as possible to ensure accurate transmission:

The frames contain a preamble of predefined symbols used for synchronization in transmission. The frame sync fields specify the types of frames being transmitted. The FCS is a Frame Check Sequence (FCS) used for error detection. 

No packet contains a destination address or other node. All data will be sent by the various gateways to the Sigfox cloud service. 

The data limit can be understood and modeled from the MAC layer packet format:

Given each packet is transmitted three times, and we know that European regulations (ETSI) limit transmission to a 1% duty cycle, we can calculate the number of messages per hour using the maximum payload size of 12 bytes:

Even though twelve bytes is the limit of a payload, that message may take over one second to transmit. Early versions of Sigfox were unidirectional but the protocol now supports bi-directional communication.