通信发展（Communication development）.docVIP

通信发展（Communication development） Switchboard The two telephones are connected by a pair of wires, enabling the communication between two users. If 3 users want to achieve the call between any two users, they need 3 pairs of wires; 5 users need 10 pairs of wires; 10 users need 45 pairs of wires; N users need N (N-1) / 2 pairs of wires. This connection is very uneconomical. The economical connection mode is that each users telephone is connected by a pair of wires to a switching device commonly used by each user. The switching device is located at the center of each user. This device is called a switch. The original switch was also called an artificial switch, and it was operated by the operator to complete the connection between the users. Later, there have been step-by-step switches and crossbar switches. They all belong to electromechanical automatic switches, but because they are transmitted by physical contact, the equipment is easy to wear and tear. At present, there are still some countries and regions using crossbar switches in the world. SPC exchange After the computer is generated, the functions of the switches are programmed and stored in the memory of the computer. This switch, which uses storage program to form a control system, is called stored program control switch, referred to as program controlled switch. SPC switches are essentially computer controlled switches. The worlds first program-controlled exchange was made by the American Bell Telephone company in 1965. The most outstanding advantage of SPC is to change the operation of the system, without changing the switching equipment, as long as the change of the instructions of the program can be, the exchange system has great flexibility, facilitate the development of new communication services, to provide a variety of services for users, such as telephone network data transmission. There are two types of signals that are transmitted or exchanged in a communication network: analog signals and digital