RJ45 crystal head is a connector with 8 pins, which is mainly used in Ethernet. "RJ" represents the registered jack, "45" represents the serial number of the interface standard. RJ45 crystal head is usually terminated on Ethernet cable to connect various network devices, such as computers, routers, switches, etc.
RJ11 crystal head is very similar to the RJ45 crystal head, but it has only four pins. It is often used to connect the telephone and modem. It should be noted that RJ11 usually refers to a 6-position (6-pin) modular jack or plug, but only 4-pin is used. In addition, RJ11 is also used for a 4-pin version of the modular connector.
RJ12 is a 6-Piece connector with 6 positions and 6 conductors, which can be used as a telephone line. Although this technology is still in place, it seems to be a mobile technology that has been eliminated by many people. It is important to note that RJ11 and rj12 use the same switch, with the main difference between the two RJ11 using only 4 positions and using all positions rj12. The type that can be relied on can be a telephone or CAT5 cable.