Wired extension: When an obstacle is too large (shielded wall, thick concrete wall, deep underground) the wired connection can considerably increase your voice communication range in hard to reach areas. In practice, the team member outside connects his audio device to the extender, and, in the isolated area, the extender is connected to the APPI-Com radio.
Relay box / Intercom relay: The intercom relay creates a bridge between two APPI-Com groups. In practice, an APPI-Com of a first group is connected to the first output of the relay, and an APPI-Com of a second group (different channel) is connected to the second output of the relay. Two independent APPI-Com networks are now communicating with each other.
Combination of wire extension + relay: The combination of these two technologies ensures communication between an independent network outdoors and another independent network in an inaccessible area.