Public Switched Telephone Network

If two computers in the same company or organization are close together, and now they need to communicate, the easiest thing to do is to connect a cable between them. That's how the LAN works. However, when it is far away, or if there are many computers, or if the cable has to go through a public road or other public place, the cost of laying a private cable is often impractical. Moreover, in almost all countries, it is illegal to erect (or lay underground) transmission lines on public property. Therefore, network designers must use existing telecommunications facilities.

These facilities. In particular, the Public Switched telephone network (switched telephone NETWORK<PSTN), usually designed many years ago, transmits human voice signals in a more or less recognizable form. They are used for communication between computers, which is only marginally qualified, but this has changed rapidly due to the inductive use of fiber optics and digital technology. In any case, the telephone system is always closely linked to the computer network, which is worth taking some time to study the telephone network carefully.

In order to see the importance of Cheongju Chu, we compare the characteristics of two kinds of connections roughly: one is to connect the computer to the computer through a cable, and the other is to connect by dialing telephone lines. The cable between the two computers can transmit data at a rate of 104bps (or even higher). The maximum data transfer rate for dial-up lines is 56kbps, which is nearly 20,000 times the difference. This distinction is like the difference between a duck slowly passing through a meadow and a rocket flying to the moon. If the dial-up line is replaced with an ADSL connection, there is still a 1000〜2000-fold gap.

Of course, the trouble is that users of computer systems are accustomed to dealing with computer systems, and when they suddenly face another system with 3 or 4 orders of magnitude worse, they spend a lot of time and effort trying to figure out how to use the system more effectively, which is no surprise.

structure of the telephone system

Shortly after Bell was granted a phone patent in 1876 (only a few hours earlier than his rival), his new invention had a lot of demand. The initial market was telemarketing, when the phone was paired, and then the customer pulled a line between a pair of phones. Electrons go through the earth and return. If a phone owner wants to talk to 0 other phone owners, he must pull 0 separate phone lines to their home. Within a year, the city was filled with telephone lines, which passed through houses and trees ^ the whole mess. Obviously, the mode of connecting each phone directly to every other phone is not feasible.

It is commendable that Bell saw this, he formed Bell Telephone Company, and in 1878 opened its first Exchange bureau (in Connecticut State only ~ The city of the company for each customer's home or office frame a telephone line. When the customer calls, first shake the handle, so that the telephone company's office to issue a ringtone, causing the operator's attention, and then the operator manually connect the caller and the callee through a view of the connected cable. The mode of the simple Exchange is shown in figure (b) 3

Soon, the bell system was spread all over the place, and people asked to be able to make long distance calls between different cities, so the bell System began to connect the Exchange Bureau. The initial question quickly emerged: connecting each Exchange station directly to each other, and then quickly becoming impossible to manage the entire structure, so the two-level Exchange Bureau was invented to solve the problem. After a while, multiple level two exchanges appeared, as shown in figure (c) ^ Finally, the entire hierarchy grew to level 5.

By the Year 1 0, three major parts of the telephone system had been available: The interchange, the customer and the Exchange line (now using a balanced type of insulated twisted pair, while the original used bare wire), a long distance connection between the Exchange Bureau. Although all of these three areas have improved since then, the basic Bell System model has remained unchanged for more than 100 years. A simple history of telephony system technology

Before the company was decomposed in 1984, the telephone system was organized into a highly redundant, multi-layered structure. The following description is highly simplified, but it illustrates the nature of the feature. Each telephone has two lines to reach out, directly connected to the telephone company's recent Jing, also known as the local Central Bureau. This distance is usually 1〜10km, the city is a little shorter than the rural 5 in the United States, there are about 22 000 end bureaus. The two-wire connection between the telephone and the end of each user is called the local loop in the telephony industry, and if all the local loops in the world are connected end to end, the length is equal to 1000 times the length of the moon.

There was a time when 80 of the company's assets were copper wire in the local circuit. At that time, the company was actually the world's largest copper mine. Fortunately, such facts are not widely known in the investment circle. If someone knows, some market troublemakers may buy the company, stop all telephone services in the United States, and take back all the phone lines and sell them to the copper smelters for a quick return.

If a user connected to one of the end offices calls another user who is also connected to that end, the exchange mechanism in the Board establishes a direct electrical connection between the two local loops. During a call, the connection remains the same.

The process of establishing a connection is different if the called phone is connected to another end station. Each end office has some external lines connected to one or more nearby Exchange centers, which are called long-distance bureaus, and if they are in the same local area, they are referred to as the receiving office. These lines are called long-distance connections

Trunk. If the caller and the callee's end-station happen to have a long-distance trunk connecting to the same long-distance office (which is very likely if they live close together) then the connection between the two sides may be established in this civil service. If the caller and the callee do not have a common long-distance office, the path between them will be set up somewhere on the higher level. The primary and regional bureaus form a network where long-distance offices connect to the network. Long-distance, primary, regional and district offices through the internal long-distance trunks of the large bandwidth, the number of different types of exchange centres and their topologies (there can be a direct connection between the two regional bureaux, or it must go through a regional bureau) as the country differs depending on the density of the telephones in each country. Figure 2.21 shows the possible Lu Zi of a middle distance connection,

-Typical path of a distance telephone

A variety of transmission media is used in the telephone system. Now, the local loop is made up of 3 twisted pairs, whereas in the early days of the telephone industry, two of the wires on the telephone were not insulated from the 25cm. In the Exchange Bureau, coaxial cable, microwave, especially the fiber KFC is widely used 5

In the past, the signals transmitted throughout the telephone system were analog ^ the actual voice signal was transmitted from the source to the monthly end of the voltage. With the advent of optical fiber, digital circuits and computers, all the trunk lines and switching devices are now digital, and only the local loop is still analog, and it is the last place in the system to retain the use of analog technology. Digital transmission is preferred because it does not need to be like analog transmission, and when a long-distance call signal passes through a number of amplifiers, it also requires a re-accurate generation of the analog wave signal. For digital transmission and 3, it is sufficient to be able to correctly differentiate between 0 and 1 undergarment. This feature makes digital transmissions more reliable than analog transmissions. and maintenance is easier, and maintenance costs are much cheaper. 6

In a word, the telephone system is made up of three main components;

(1) Local circuit (twisted pair family and business unit, analog signal)

(2) trunk (connecting the Exchange station via optical fiber, digital signal)

(3) Exchange Bureau (phone call from one main line pick up to another trunk line)