A method is described for communicating among multiple devices over a shared communication medium. The method includes, in a schedule among multiple subsets of the devices that includes at least one respective time slot for each subset, transmitting from at least one of the devices in a given subset a presence signal associated with the given subset within a time slot for the given subset. A presence signal associated with a given subset is configured to indicate the presence of at least one device in the given subset. The method also includes communicating among devices in a given subset based on presence signals detected from one or more devices in at least one different subset.