A straight connector (ST connector) is a connector used in fiber-optic cables that utilizes a bayonet-style plug and socket. It has become the de facto standard for commercial wirings. The ST connector setup allows for unidirectional communication, so two ST connectors and two fiber cables are used for bidirectional communication.
he straight tip connector features a quick-release bayonet-style connector that is cylindrical with a twist-lock coupling, 2.5-mm keyed ferrule. It was developed by AT&T and became dominant during the 1980s and 1990s for long-line systems and short-distance applications. The most prominent feature of the ST connector is the straight ferrule, a rigid plastic tube, used to hold the fiber for proper alignment for interconnection or termination.
ST connectors are spring loaded, which means they are easily inserted and removed, but one also has to make sure that they are seated properly to ensure that there is no light loss. The typical insertion loss is 0.25 dB. The connector is rated for 500 mating cycles and is usable for both single- and multi-mode fibers.