ST stands for Straight Tip. A quick release bayonet style connector with a 2.5mm long ferrule. Common connector for multimode fibers. ST is the one of the most popular connectors for multimode networks, like most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. And because they are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference.