Donot be fooled by the low aftermarket price. Perimeter Technologies is a brand you can trust with products that are loaded with exclusive features and benefits not found in the original. As a matter of fact, Perimeter Technologies' President is the same person that founded Invisible Fence brand back in 1976!Compare Perimeter Technologies with your current R21: PATENTED Comfort Contacts cushion your dog neck in soft rubber. PATENTED 3-Way, Omni-Directional antenna design eliminates the holesfound in lesser pet fencing systems. WATERPROOF and submersible collar receiver design. PROGRESSIVE STIMULATION allows your dog to choose the fence level that is right for their personality. ULTRA-SMALL and light weight collar design is a mere 1.1 oz! LONG LASTING replaceable lithium ion batteries. DESIGNED and MANUFACURED in the USA. Limited Lifetime warranty. 100% compatibility guaranteed!This replacement collar receiver for 700 Series Invisible Fence systems only and will not work with 800 series or later, including the Titanium Series. The Perimeter collar is not compatible with Shields Indoor Avoidance transmitters.FULLY COMPATIBLE WITH THE FOLLOWING INVISIBLE FENCE BRAND TRANSMITTERS: ICT-700, ICT-725, ICT-750, ICT-775, ICT-150, ICT-250, ICT-100, ICT-75, ICT-50, AT-950 Indoor transmitter, CT-4000Please indicate 7K or 10K frequency when ordering. 7K is the most common frequency, the 10K was used in installations experiencing frequency conflicts with neighboringcontainment systems.