To have heterogeneous acoustic modems be inter-operable, they must, at the very least, share a common physical coding (which does not need to be their first or main choice). JANUS specifies a simple Frequency Shift Keying (FSK) scheme that is flexible in its centre frequency, incoherent and robust. JANUS encodes 64 bits of data that include identifying the sender, some of its capabilities, the intended interval before the message is repeated and the length of any data that is to follow.

JANUS provides a broadcast message with information that allows receiver(s) to react, based on that additional knowledge, including choosing to transmit their own JANUS message. Such communications function at least as beacons, and have been used for millennia. Examples include light houses, Instrument Landing Systems (ILS), wireless local area network access points (part of Wi-Fi 802.11), and Automatic Identification Systems (AIS). They are intentionally conspicuous and robust, usually sending information about the transmitter’s location and/or an “identity packet” of known structure.

JANUS may also be used to send arbitrary data, in a payload that follows the initial 64-bits and whose length is specified within the 64 bits. This empowers JANUS-compliant devices to not only broadcast their presence, but also to discover and establish two-way communications with similarly-compliant heterogeneous devices in their neighbourhood.