How is my Wi-Fi signal affected by walls and when the signal stretches over several floors?

Under normal circumstances, one or two walls won't affect the router's Wi-Fi signal. This depends on the wall's thickness, direction, and type of material, as well as any interference source or other factors.

The router's vertical transmit power is about the same as its horizontal transmit power. The signal attenuation depends on the environmental layout. If your house has more than two levels, it is recommended that you have a router every one or two floors for better coverage.