Recently, I saw a slogan of China Telecom. It says new broadband 150 yuan monthly, with a bandwidth of 3M. What does 3M mean?

Bandwidth refers to the maximum data transmission speed allowed in a communication line, which can also be said to be the maximum data transmission frequency. Small bandwidth naturally limits the speed of surfing the Internet. However, the main factor affecting the frequency bandwidth is the transmission medium, and any medium can only accurately transmit signals below a certain frequency. When the fundamental frequency of the signal is higher than this frequency, the medium cannot transmit the signal accurately. Any medium has a maximum frequency of data transmission, that is, bandwidth. Due to the difference of material, diameter and length, the bandwidth of the medium is also different. For example, the bandwidth of twisted pair (that is, telephone line) is very low, and the maximum is only tens of Kbps, so there is no way to improve the speed of public network. Therefore, the network using optical fiber can theoretically reach the transmission speed of several hundred Mbps or even higher. But the speed of surfing the internet is not necessarily caused by the small bandwidth, but also closely related to the speed of modem, the capacity of ISP, the number of people visiting the website and so on.