Direct Mode Operation (DMO) provides the ability for TETRA radio terminals to communicate directly with each independent of the TETRA network infrastructure. DMO is not new and has been a facility mandated and used by many traditional PMR user organizations for several decades. The primary requirement for DMO has been brought about by the need to balance the RF Coverage, Grade of Service (GoS) and Reliability of a network with that of the network’s overall cost. The requirement for DMO makes the use of public cellular networks unsuitable.
As well as allowing direct communications in situations where network coverage is not available, DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. This functionality is called DMO gateway (from DMO to TMO) or DMO repeater (from DMO to DMO). In emergency situations this feature allows direct communications underground or in areas of bad coverage.