A method of communication between a base unit (B1) and a plurality of portable units (P1, P2, etc) comprises; allocating a first set of predetermined timeslots (contact slots) in a frame for communication between respective portable units and the base unit and allocating a second set of predetermined timeslots (event slots) in the frame to be available for communication between any of the portable units and the base unit. The time slots are arranged in the frame such that the timeslots of the first set alternate with the timeslots of the second set. The invention allows radio data exchange between portable radio units and a base unit on a single RF channel. Periodical reliable telemetry data throughput is maintained by means of communication in the first set of timeslots, yet unforeseen events may be communicated successfully and quickly in timeslots of the second set.
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