Bandwidth of the access channel in the network

With the ubiquitous spread of computer networks, the term "channel capacity" became known to everyone. And if earlier interest was purely theoretical, now everything is completely different. Understanding what is hidden behind the words "network bandwidth" allows you to choose the best available provider (hereinafter referred to as LANs and the Internet), as well as optimally configure the work with the network.

Before delving into the theory, let us consider a practical situation, which, alas, is often encountered by Internet users living in post-Soviet countries. As you know, when you connect access services to the network, providers in their tariff plans indicate speeds with the prefix "before". For example, "up to 10 MB / s", "up to 50 MB / s", etc.

In fact, the bandwidth of the channel and this disclaimer are interrelated. Imagine a situation that at some point in time one subscriber is connected to the provider's network. As a rule, he is given a full tariff rate. Pursuing economic goals, the company-provider continues to recruit new subscribers. As a result, it becomes a natural situation when a lot of users initiate a connection to the network at a time. One has a tariff of "up to 50 Mb", the other has a third ...

The logical consequence is a drop in speed at all below declared (we remember the prefix "before"). Begin calls of disgruntled subscribers, common problems with communication, etc. In response, representatives of the support service mention that the bandwidth of the channel is limited. Surely, this is familiar to many users. What are we talking about and why does the speed drop?

In 1920, an American electronic researcher Ralph Hartley and physicist Harry Nyqvist, dealing with issues of information transmission in telegraphy, formulated the main features of the data transfer process. One of the most important is the relationship between the frequency of signal transmission and time. So, Hartley formulated the law, according to which the total amount of transmitted data is proportional to the used transmission frequency and operation time. In 1927, Nyqvist clarified that the transmitted volume is limited to twice the value of the used frequency (meaning transfer without loss of data per unit time). Only in 1940, Shannon summarized the results of their work, formulating the theory of data transmission and the concept of "communication channel capacity."

The frequency range used by the channel for transmitting information is called "bandwidth." From Shannon's theorem it follows that the maximum speed can be achieved by increasing the signal power, bandwidth, and reducing parasitic noise. The increase in speed by modulating the signal is difficult, since as the pulse increases, their total quantity decreases per unit time, and when multiplying by decreasing the duration of a single discharge, the amount of losses in the conductor increases. In general, the pulse width is calculated using a formula that takes into account the selected frequency.

It should be noted that the bandwidth of the channel includes not only a useful signal, but also noise. This can be electromagnetic interference, conductor properties, reflections, gauss process, etc. The receiver senses the full flow of signals and filters out the required component.

Returning to the example: when a large number of connected subscribers quickly reaches the limit of the total data flow for the transmission frequency used by the provider (optical line, radio channel, copper conductor). To solve the problem, it is necessary to increase the signal power, change the transmission medium and the frequency (it is expensive because it requires upgrading the equipment), or to limit the data flow from each subscriber, which is done.