Facts | How does a modem era changes ?




Facts | How does a modem?

Modems, as a means of communication one computer to another, in the historical path of development has undergone many changes. First of all, the improvements did not have a material impact on the very essence of the transmission of digital information over distances. The speed of modern modem would hit the imagination of everyone who was close to the high-tech theme of the years, a couple of decades ago. But the basic principle of operation of these devices remains largely the same. Today we will learn how the modem, without which the people still had to be content with paper newspapers, radio and TV, and did not even dream of the Global Network and the tremendous opportunities it has brought to this world. And a little bit about the path that has done a data transfer rate: from three to forty billion bits per second . And it’s already in the wireless technology, which has traditionally lagged behind the cable.

“Silent” besprotsessornye terminals

Немой терминал

The word “modem” is an acronym formed from the English words “modulator” and “demodulator”. If you simplify the essence of his tasks, we can say that this device is able to send a signal and read a signal directed to him. Initially, the digital signal is sent from the modem to the modem using a conventional telephone line.

Before sending the data to the modem modulates the data signal compatible with the telephone line and the receiving modem demodulates the signal received by it, i.e. again converts it into digital data. Wireless modem transmitter converts digital data into radio signals and the modem receiver again returns them to the original shape. That is, the modems assigned the task of “translating” information “language” clear lines of communication. And, of course, the “translation” from this “language” is also beyond the scope of the modem.

Modems came into use as early as the 1960s. They were used to interconnect the terminals by conventional telephone lines. We describe the most typical of those early years, a situation in which the modem is used.

In the branch of a company or shop for a “dumb” terminal input-output without its own processor. These terminals were quite common in those years when the mainframe was considered quite expensive. Not every company can afford to own a computer. It was therefore common practice to hire a certain fraction of the time of a large machine. And some companies use this facility. To use the allotted time of the engine, its terminal was connected to a remote computer using a modem. The connection speed was only 300 bits per second.

“Dumb” terminals were only a screen and a keyboard to the remote computer. Today’s cloud technologies are largely a return to the sixties of the last century. The idea is the same: the implementation of programs rests with the resources of the remote computer. Another thing is that today, in the service of this idea put all the power and all the achievements of high-tech industry for the past with the relatively recent time the decade.

In those years were very common terminals, called DEC VT-100. They can even be considered a kind of “standard” of those early days. Until now, this type of such devices is the prototype for terminal emulators. The issue before us had a VT-100 display, which can accommodate 25 lines of text, with 80 characters per line. When a user types a character, a computer modem sent its ASCII-character. A remote computer again returned it to the terminal, making it possible to display a sign on the office located in the display. For the “dumb” terminals have been denied even their own editing tools, text and screen control.

In the late 1970s, personal computers have some distribution. And then came the first timid analog Internet: electronic bulletin boards; bulletin board systems (BBS). A person could make such a server boards a computer equipped with one or two modems. And other people had the opportunity to join this board. It is possible that this was the first more or less historically significant attempt to establish normal user digital communication. However, the “average user” of those years was, more often, a very competent engineer. And the modem cost is not cheap. For example, computer modems, IBM Portable PC 5110 cost 900 and even 2,000 dollars, depending on the capabilities of the modulator-demodulator.

In these ancient second modems, as noted above, can transmit only 300 bits per second. Modern user accustomed to giant graphics and sound streams transmitted over the network, it can seem very small. But in fact this is not true. 300 bits is about 30 characters of text. A lot more letters than able to read in a second person. Up to a point that speed just did not make sense to increase.

When bulletin boards are used to transfer programs and pictures, this rate has ceased to comply with the dictates of the time. And the industry, as is often the case, to respond to the real needs of users. From the sixties to the end of the nineties of the last century, several generations of modems. Since the 1980s, the next generation began to appear about every two years, although, of course, no strict rules do not exist:

300 bits per second. From 1960 to 1983
1 000 200 bits per second. Became popular in 1984 and 1985
2000 400 bits per second
9000 600 bits per second. First appeared in late 1990 and early 1991
19.2 kilobits per second
28.8 kilobits per second
33.6 Kbps
56 kilobits per second. Have become the de facto standard in 1998
But already in 1999, began to gain popularity ADSL-modem whose speed could theoretically reach 8 megabits per second

Modem with a speed of 300 bits per second

We begin our consideration of the principles of modems with device whose data rate is only 300 bits per second. In their example, the easiest way to explain the principle of the modem itself. These modems for transmitting information over a telephone line used by the frequency shift key; frequency shift keying (FSK). In this “language” of certain sounds to meet certain bits. These specific frequency sounds called modem tones.

Modem Terminal (referring modem) called a computer modem and asked him for permission to connect. If we talk about the modem terminal, the logical zero corresponded to 1070-hertz tone and a logical one – 1270 hertz. Computer modem (modem charge) pitched 2025-hertz tone that matched zero or 2225-hertz, corresponding to one.

As appealing and answering modems used different frequency tones, they can use the phone line at the same time. This mode of operation is called a duplex or two-sided at the same time. Modems that could not simultaneously transmit and receive information called half-duplex. But so imperfect device is quite rare even in those years when the digital information were making their first tentative steps.

Thus, the connection is established, and a user enters on the keypad letter «a». Its decimal code: 97; binary: 01100001. The terminal has a special device called UART or universal asynchronous receiver-transmitter. It was frustrating to individual bits bytes and send them through a serial RS-232 port. This port was connected to a modem, which is one bits transmitted to the remote computer through a phone line.
Modem with a speed of 56 kilobits per second

To control the speed modems in line with the needs of users, the designers did not have enough technology features FSK. At first, it was replaced by a PSK, and then QAM. We will not go into the details of these technologies. We note only that they are allowed to pass in those days unthinkable amounts of information, squeezing everything possible out of 3,000-hertz bandwidth of conventional telephone line, originally designed for voice conversations.

Modem, the theoretical maximum speed of which could be up to 56 kilobits per second, in fact connect to a remote computer at a speed of about 48 kilobits per second. Assuming ideal telephone line. The better the quality of the line, the higher the speed of the connection. To determine the achievable rate of such modems used the concept of “gradual degradation» («gradual degradation»). Tested the modem line and disconnected. Then joins again at a low speed. And so as long as the speed does not come into line with the real possibilities of the telephone line.

The next step in the evolution of ADSL-modems are devices in this category. ADSL is an acronym for the English words «asymmetric digital subscriber line» («Asymmetric Digital Subscriber Line”). What is expressed in its asymmetry? These modems can transmit data faster in one direction than in the second. Between home and office and telephone company proffers special copper cable. This cable is given the opportunity to share with the remote computer is much larger amounts of data than is possible using a conventional telephone line. Recall that the band reached its bandwidth of 3000 Hz.

If the home and in the office of the telephone company had ADSL-modems, cable stretched from one modem to another, playing the role of high-speed information highway. Between the house and the telephone company (ie in the direction opposite to the main traffic) could reach a top speed of about one million bits per second, that is one megabit per second. In the reverse direction (from the telephone company to the house) could theoretically speed up to 8 megabits per second. But this is under ideal conditions, virtually unattainable in the real world. This line may be used for telephone calls and not for transmitting digital data streams.

At its core the principle of ADSL-modem is very simple. The bandwidth of a new, more capacious telephone line, in the range of 24 thousand to 1 million cps 100 000 cps was divided into 4000 segments for every hertz. With each segment of the associated virtual modem. Just turned 249 virtual modems, each with its own section tested the bandwidth and tried to squeeze out the maximum speed. Thus the data rate ADSL-modem speeds calculated as the sum of virtual modems 249. If you simplify the description of this technology, we can say that almost two and a half hundreds of traditional modems together in one box and the same number of phone lines – a single cable.
From point to point. The PPP protocol

Our story would have looked unfinished and incomplete if we did not pay attention of PPP. These days, the user will not communicate directly with the computer of another user. Your computer modem accesses the ISP (Internet service provider; ISP). The Internet allows us to connect to any server in the network. The Global Network and providers, there is no need for a symbol unit of information transfer. Your modem is engaged in the routing of TCP / IP-packets between your computer and the ISP.

Standard technology for routing the packets to your modem protocol is PPP (Point-to-Point Protocol; protocol “point to point”). Its basic idea is quite simple: TCP / IP-stack your computer generates a TCP / IP-datagrams, i.e. data packets containing the address at which they are to be delivered. These packets are sent to the modem for onward transmission. Computer provider receives each datagram and sends it via the Internet at the address provided. The process of transferring data from the provider takes you to a similar scenario.

Yes, in just two decades since then, as computers were just starting to become friends of the family and by the end of the nineties of the last century has increased the speed of information transmission in the tens of thousands of times. As of today – is in the millions! Those fifteen years that lie between the two thousandth year and modernity, may be the subject of a separate interesting story. This is a story that is created and written before our eyes.

According to the materials computer.howstuffworks.
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