Relay is the connecting device on the physical layer of the network. It is suitable for the interconnection of two types of networks that are identical. The main function is to extend the distance of network transmission by resending or forwarding data signals.

Relay is a physical layer device of the OSI model of network equipment that regenerates and restores signals.
Relay is the simplest and cheapest interconnected device used to extend the network distance in a local area network environment. It operates on the physical layer of OSI. Relay has the function of amplifying and regenerating the signal on the line. Used to extend the length of the LAN segment. It is only used to connect to the same LAN segment.
The role of relay:
Relay works at the physical layer of OSI and is the center of all nodes on the local area network. Its function is to amplify signals, compensate for signal attenuation, and support long-distance communication.
The working principle of relay:
Relay is a small invention. It is designed to boost your network signals so that they can be transmitted farther. Due to the influence of transmission line noise, the information-carrying digital signal or analog signal can only be transmitted for a limited distance. The function of the relay is to regenerate and send the received signal, thereby increasing the distance of signal transmission. It is the simplest network interconnection device, connecting two or more network segments of the same network. For example, Ethernet often uses relay to extend the cable length of the bus. The maximum length of each section of standard thin-cable Ethernet is 185 meters, and there can be up to 5 sections. Therefore, after adding the relay, the maximum network cable length can be increased to 925 meters.
Generally speaking, the network parts at both ends of the relay are network segments, not subnets. Relay can connect the cables of two LANs, retiming and regenerate the digital signals on the cables, and then send them out. These functions are the typical functions of the first layer in the OSI model-the physical layer. The role of relay is to increase the coverage area of ​​the local area network. For example, the Ethernet standard stipulates that the maximum length of a single-segment signal transmission cable is 500 meters, but after connecting 4 segments of cables with a repeater, the signal transmission cable in the Ethernet can be up to 2000 Meter. Some brands of relay can connect cable segments of different physical media, such as thin coaxial cables and optical cables.
Relay only sends data on any cable segment to another cable segment, regardless of whether there is error data in the data or data that is not suitable for the network segment.