A system which allows multiple senders to asynchronously transmit
identification codes via a common communication channel (e.g., RF) to
enable a central monitor to identify the presence (or absence) of each
sender within the monitor's detection zone. Each sender is configured to
repeatedly transmit a uniquely encoded identification frame. A frame, in
accordance with the invention, is comprised of pulses spaced to minimize
pulse collisions and configured to tolerate occasional collisions without
impairing the monitor's ability to separately identify each transmitting
sender. Each sender is configured to repeatedly transmit a unique
identification frame characterized by a pulse pattern comprised of active
intervals spaced by inactive (or "quiet") intervals. The inactive
intervals have variable length durations which are preferably
pseudorandomly selected so that each sender defines a unique sequence of
inactive interval durations.