A crystal head is a plastic connector that can be inserted in a fixed direction and automatically prevent detachment, commonly known as a "crystal head". The professional term is RJ-45 connector (RJ-45 is a network interface specification, similar to the RJ-11 interface, which is commonly used as a "telephone interface" to connect telephone lines). The reason why it is called "crystal head" is because its appearance is crystal clear and translucent. The crystal head is suitable for on-site termination between equipment or horizontal subsystems, and the shell material is high-density polyethylene. Each twisted pair is connected to the network card and hub (or switch) by installing a crystal head at both ends.