"Hub" means "center". The main function of the hub is to regenerate, shape and amplify the received signal, so as to expand the transmission distance of the network, and at the same time concentrate all nodes on the node centered on it. It works in the first layer of the OSI (open system interconnect) reference model, that is, the "physical layer". Hub, like network card, network cable and other transmission media, belongs to the basic equipment in LAN and adopts CSMA/CD (a detection protocol) access mode. Hub belongs to the bottom equipment of pure hardware network, and basically does not have the ability of "intelligent memory" and "learning" similar to switches. It also has no MAC address table owned by the switch, so it sends data by broadcast, not as a target. That is to say, when it wants to send data to a node, it does not send data directly to the destination node, but sends data packets to all nodes connected to the HUB. Hub belongs to the basic equipment in data communication system. Like twisted pair and other transmission media, it is a hardware device that does not need any software support or only needs little management software. Widely used in various occasions. Hub works in the local area network (LAN) environment and is applied to the first layer of the OSI reference model like the network card, so it is also called the physical layer device. The hubs are electrically interconnected. When the environment for maintaining LAN is logical bus or ring structure, a physical star or tree network structure can be established with the hub. In this regard, the hub acts as a multi-port repeater. In fact, the hub is actually a kind of repeater. The only difference is that the hub can provide more port services, so the hub is also called a multi-port repeater. The signal forwarding principle of hub works in the physical layer of OSI/RM reference model and the MAC (Media Access Control) sublayer of data link layer. The physical layer defines electrical signals, symbols, line status and clock requirements, data coding and connectors for data transmission. Because the hub only shapes and amplifies the signal and then forwards it without coding, it is a physical layer device. 10M hub has four available standard interfaces in the physical layer, namely: 10BASE-5, 10BASE-2, 10BASE-T and10base-f. 10BASE-5(AUI) port of 10M hub is used to connect 1 layer and layer 2. Hub adopts CSMA/CD (Carrier Frame Multiple Access/Collision Detection) protocol. CSMA/CD is a MAC layer protocol, so hub also contains data link layer content.