Finally, the company needed to isolate each rack of phones from the others so their Wi-Fi networks didn't interfere with each other. This was necessary to create a consistent Wi-Fi environment for repeatable tests. Facebook built the chambers with insulation, copper tape, and a power filter. Each rack can hold 32 phones, plus computers that drive the phones to install, test, and uninstall applications. For iPhones, Facebook uses eight Mac Minis, and for Android phones, it uses four Open Compute Project Leopard servers.