Described is an animal containment system for containing an animal within a boundary, for maximizing the animal's usable area within the boundary, for not discouraging the animal from returning to within the boundary in the event the animal moves beyond the boundary, and for doing so without utilizing an independent polarity indicator. The animal containment system determines whether the animal is within the boundary or beyond the boundary by considering the polarity of an activation signal the animal containment system transmits from the boundary and receives at the location of the animal. The animal containment system determines the polarity of the activation signal by modulating the signal such that the modulation indicates the polarity of the activation signal. The animal containment system achieves the above-discussed features by delivering a stimulus to the animal only when it traverses the boundary from within the boundary.