A circulator is a passive electronic component with three or more ports in which the ports can be accessed in such an order that when a signal is fed into any port it is transferred to the next port, the first port being counted as following the last in numeric order. There are circulators for VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of propagating waves in a magnetized material, and 3-port "turnstile" or "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetized material.