Telephone switching refers to the process of temporarily connecting a call circuit between two (or more) telephones to achieve the continuity of calls between telephone users. There are manual telephone exchanges and automatic telephone exchanges. The wiring, dismantling, and other operations during the manual telephone exchange process are completely completed by the operator manually; Automatic telephone exchange involves the sender dialing the recipient's phone number, and the call is automatically connected. The wiring, disconnection, and other operations during the telephone exchange process are entirely carried out by the telephone exchange. The progress of telephone switching technology has gone through four main stages: manual switching, electromechanical switching, electronic switching, and program-controlled digital switching. Manual switching is a method of switching in which both parties of a call are manually connected through a telephone exchange operator. User phones are divided into two types: magnetic and shared phones. Magnetic phones require users to provide their own dry batteries, while shared phones are powered by the switch. Each user line is equipped with a plug hole (fixed) and a plug rope (extendable) on the manual exchange platform. When a certain user serves as the caller, the operator is informed of the called number or name through this phone. The operator pulls out the calling user's cord and plugs it into the other person's jack to be called, and rings the other person to complete the call; When a certain user is called, the calling party's plug rope will be inserted by the operator into the own plug hole, completing the connection of the communication circuit between both parties. The manual switch first appeared in 1878. Due to its use of humans as control devices for switching operations, it has low switching efficiency, slow speed, and high labor intensity, making it the most primitive switching method. Electromechanical exchange is an automatic exchange method that utilizes electromagnetic mechanical actions to control the connection of lines between two parties in a call. In 1891, American Almon B. Stundjo invented the "step by step automatic telephone connector". In 1892, the "step by step automatic telephone exchange" made of this type of connector was officially put into use. The step by step telephone exchange relies on the dialing pulse of the user's phone to directly control the opening and closing of the mechanical relay, causing the wiring terminals of the connector to climb and rotate, thereby completing the connection of the line contacts. This mechanical exchange method has high component wear, low lifespan, and high noise, but after all, it realizes the replacement of manual operation by machinery. In 1919, Swedish people Palmgren and Bertaland invented the "crossbar connector". The crossbar switch produced with crossbar connectors was first put into use in Sweden in 1926. The crossbar switch relies on user dialing to indirectly control the action of the wiring device through a common control device (recorder and marker). The function of this public control device is similar to that of a telephone operator in manual switching, responsible for receiving user dials, and performing storage, counting, forwarding, and other tasks. Compared to the step by step switching method, the mechanical action of the crossbar switch is much less, and the contact points are made of wear-resistant precious metals, resulting in low action noise, less wear and mechanical maintenance workload, and greatly extended working life