To access the mobile communication network, the mobile phone also needs an interface, which is the wireless transmission link between the base station and the mobile phone, also called the air interface. How many wireless transmission links a base station can provide means how many telephones can be connected at the same time. In order to ensure that wireless links do not interfere with each other, it is necessary to draw different wireless frequency bands for different transmission links.
Extended data:
Compared with ETACS mobile phones, GSM mobile phones have a very advanced technology, that is, the confidentiality is greatly improved. There are many encryption methods for GSM mobile phones. For example, when the mobile phone is turned on and registered to access the network, it is necessary to transmit the unique access number of each user, such as the IMSI number and the minimum number. These numbers are transmitted in the air by calculating the ki value stored on the SIM card, which corresponds to the switch, so it is basically impossible to intercept and crack these information in the air.
In addition, GSM system also has frequency hopping function. That is, the frequency of carrier frequency is constantly changing to protect information from being stolen.
Baidu Encyclopedia —— Concise Principles of Mobile Communication