In FDMA communications between two or more primary radios (41, 42; 141,
142), use is made of a quiescent period in the primary communication. The
quiescent period is used by secondary radios (43, 44; 143, 144) to
communicate with each other. The quiescent period may be a transmit
interrupt (TI) frame, normally provided in a DIIS protocol in order to
permit return signaling from a called radio (43, 44). The secondary
radios (43, 44) may signal to each other during the TI frame.
Alternatively, the quiescent period may be a gap in channel activity, in
which case regulated access telegrams (96) are transmitted by either a
calling primary device (141), or a "last to call" primary device (141),
or a base station (141), the regulated access telegrams determining a
reservation window period during which secondary devices (143, 144) may
signal to one another.