1, RJ-45 interface this interface is the most common interface of network equipment at present, commonly known as "crystal head", and the technical term is RJ-45 connector, which belongs to twisted-pair Ethernet interface type. RJ-45 plug can only be inserted in a fixed direction, and there is a plastic shrapnel stuck in RJ-45 slot to prevent it from falling off. The interface can be used for 10Base-T Ethernet, 100Base-TX Ethernet and 1000Base-TX Ethernet. The transmission medium is twisted pair, but there are different requirements for the medium according to the bandwidth, especially when connecting with 1000Base-TX gigabit Ethernet, at least five types should be used. 2.SC optical fiber interface SC optical fiber interface has been applied in the era of 100Base-TX Ethernet, so it is called 100Base-FX(F is the abbreviation of optical fiber). But it was not popularized at that time, because its performance was not superior to that of twisted pair, but its cost was higher. Now the industry is vigorously promoting Gigabit network, and SC optical fiber interface is re-valued. There are many kinds of optical fiber interfaces. SC optical fiber interface is mainly used in local network switching environment. It is available on some high-performance Gigabit switches and routers. It looks similar to the RJ-45 interface, but the SC interface is flatter, and the obvious difference is the contacts inside. If it is eight thin copper contacts, it is RJ-45 interface, and if it is a copper column, it is SC optical fiber interface. 3.FDDI interface FDDI is one of the mature LAN technologies with the highest transmission rate. It has the characteristics of timed token protocol and supports various topologies. The transmission medium is optical fiber. Fiber Distributed Data Interface (FDDI) is a set of protocols formulated by American National Organization for Standardization (ANSI), which is used to send digital signals on optical cables. FDDI adopts double-ring token, and the transmission rate can reach 100Mbps. CCDI is a variant of FDDI, which uses twisted-pair copper cable as transmission medium, and the data transmission rate is usually 100Mbps. FDDI-2 is an extended protocol of FDDI, which supports voice, video and data transmission. It is another variant of FDDI, which is called FDDI full duplex technology (FFDT). It adopts the same network structure as FDDI, but the transmission rate can reach 200Mbps. Because using optical fiber as transmission medium has many advantages such as large capacity, long transmission distance and strong anti-interference ability, it is often used in metropolitan area network, campus backbone network and multi-building network distribution environment, so FDDI interface is more common in network backbone switches. Now with the popularity of Gigabit, some high-end Gigabit switches have begun to use this interface. 4. AUI interface AUI interface is specially used to connect thick coaxial cables. Early network cards had this interface, which was used to connect hubs and switches to form a network, but it is generally not used now. AUI interface is a "D" type 15 pin interface, which was previously used in token ring network or bus network. With the help of external transceiver (AUI to RJ-45), it can connect with 10Base-T Ethernet. 5.BNC interface BNC is a special interface for connecting thin coaxial cables, also known as "thin cables". Its most common application is separated display signal interface, that is, red, green, blue and horizontal and vertical scanning frequencies are input into the display interface respectively, and the interference between signals is small. Now BNC is basically not used in switches, and only a few BNC interfaces are provided in some early RJ-45 Ethernet switches and hubs.