xdsl technologies in the far east. XDSL

We are all familiar with analog modems. These faithful guides to the world of the Internet for many years remained the only ones who could give us the opportunity to exchange information between computers. This situation continued for quite a long time. The need to change the foundations was caused by the rapid growth of the popularity of the Internet. New site-building technologies with the active use of multimedia components, packet voice transmission - all this required an increase in the transmission speed and expansion of accessibility. Unfortunately, analog modems can no longer cope with the flow of information that time requires. Currently, even the maximum available speed of 56k leads to painful waiting and nervous system disorders.

One of the technologies, as time has shown - the most successful, was xDSL (Digital Subscribe Line) technology. It allows you to reach megabit speeds data transmission over the good old telephone line (POTS). It can be assumed with a high degree of certainty that it was this circumstance, as well as the low price of terminal equipment, that played a decisive role in the development of xDSL.

For more than half a century, in almost every home, there is a telephone set connected by a twisted pair of copper wires to a telephone exchange. Under normal conditions, we use it for voice conversations with other subscribers of the telephone network. Those. at the same time, analog signals are transmitted over the line in a rather narrow frequency range, which is quite acceptable for communication. If you have a computer and a keen desire not to be alone, this line can be supplemented with a modem that uses digital-to-analog conversion on the subscriber's side to transmit a signal to the line and vice versa for reception. The same scheme is used on the receiving side.

But when transmitting an analog signal, only a small part of the bandwidth of a twisted pair of copper wires is used. The maximum available data rate can reach 56k. And this is a theoretical limit, that is, a further increase in speed using analog modems cannot be achieved.

As for DSL technology, it excludes the conversion of a digital signal into an analog one and vice versa. The data is sent to your computer in digital form, which allows you to significantly expand the usable frequency band. In addition, it is possible to separate the spectrum of signals used for telephone communication and DSL, which allows you to simultaneously enjoy high-speed Internet while talking on the phone, sending and receiving fax messages, etc.

What does the theory say?

The possibility of using a conventional pair of copper wires was due to the development of new methods of digital signal processing. Modems create multiple channels using the available bandwidth of the line using Frequency Division Multiplexing (FDM) or echo cancellers. FDM splits the range into two, one for delivery and the other for access.

The delivery channel is divided into multiple low and high speed channels by time multiplexing. The access path is multiplexed into low rate channels, superimposed on the delivery channels. Local echo cancellers are used to separate forward from reverse traffic, much like analog modems do.

With regard to modulation methods, the most widely used is "discrete multitone modulation" (Discrete Multitone, DMT). By the way, it is standard for ADSL.

ADSL uses frequencies ranging from 0 to 1.1 MHz. The range from 0 to 4 kHz is reserved for analog telephone lines. If traffic is transmitted only from the station to the subscriber, then DMT divides the range between 26 kHz and 1.1 MHz into 249 channels of 4 kHz, each of which can be considered as the equivalent of a modem. DMT also allocates 25 duplex channels for traffic in both directions. If the channel does not pass through interference, it may be excluded from operation. As the distance increases, there is more and more interference on the line, respectively, the data transfer rate drops.

Technology typesxDSL

DSL brings together several digital subscriber access technologies under its wing. It is important for the user to understand the difference between them when choosing equipment. Of greatest importance is the ratio of the distance to the base station to the data transfer rate, as well as the difference between the rates of "downstream" (from the network to the user) and "incoming" (from the user to the network) data flow.

So, DSL is a set of the following technologies:

ADSL (Asymmetric Digital Subscriber Line - asymmetric digital subscriber line)

It is most widely used due to simple installation, the possibility of simultaneous operation of the phone and high-speed data transfer, and the relatively low cost of connection. This technology is ideal for small offices and home users because of its asymmetry. As everyone knows, the data flow to the subscriber is significantly higher than the reverse, because. Basically, information from the network is obtained by the user (websites, files, etc.). ADSL provides data rates up to 8 Mbps to the user, and up to 768 Kbps from the user. Moreover, this speed can only be achieved at a distance of up to 2 km using wires with a diameter of 0.4 mm (the most common in our country). As the distance increases, the data rate decreases. The maximum range is approximately 4.5-5.5 km with a wire diameter of 0.4.

A simpler version of ADSL. Provides up to 1.5 Mbps downstream and up to 512 Kbps upstream

IDSL (ISDN Digital Subscriber Line - IDSN digital subscriber line)

Provides data transfer at speeds up to 144 Kbps in both directions (duplex). The difference from the usual ISDN is that IDSL is a non-switched technology, that is, the user does not need to call the provider. Actually, this is the highlight of the entire DSL line.

HDSL (High Bit-Rate Digital Subscriber Line - high-speed digital subscriber line)

HDSL technology provides for the organization of a symmetrical data transmission line, that is, the data transfer rates from the user to the network and from the network to the user are equal. Due to the transmission speed (1.544 Mbps over two pairs of wires and 2.048 Mbps over three pairs of wires), telecommunications companies use HDSL technology as an alternative to T1/E1 lines. (T1 lines are used in North America and provide a data rate of 1.544 Mbps, and E1 lines are used in Europe and provide a data rate of 2.048 Mbps.) Although the distance over which the HDSL system transmits data (which is about 3.5 - 4.5 km), less than with ADSL technology, telephone companies can install special repeaters to inexpensively, but effectively, increase the length of an HDSL line. The use of two or three twisted pairs of telephone wires to organize an HDSL line makes this system an ideal solution for connecting PBXs, Internet servers, local area networks, etc. HDSL2 technology is a logical result of the evolution of HDSL technology. This technology provides similar performance to HDSL technology, but uses only one pair of wires.

SDSL (Single Line Digital Subscriber Line - single line digital subscriber line)

Like HDSL technology, SDSL technology provides symmetrical data transmission at rates corresponding to T1/E1 line rates, but SDSL technology has two important differences. Firstly, only one twisted pair of wires is used, and secondly, the maximum transmission distance is limited to 3 km. Within this distance, SDSL technology provides, for example, the operation of a video conferencing system when it is required to maintain the same data transfer flows in both directions. In a sense, SDSL technology is the predecessor of HDSL2 technology.

VDSL (Very High Bit-Rate Digital Subscriber Line - ultra-high-speed digital subscriber line)

VDSL technology is the "fastest" xDSL technology. It provides downstream data transfer rates ranging from 13 to 52 Mbps, and upstream data transfer rates from 1.5 to 2.3 Mbps, and over one twisted pair of telephone wires. In symmetrical mode, speeds up to 26 Mbps are supported. VDSL technology can be considered as a cost-effective alternative to laying fiber optic cable to the end user. However, the maximum transmission distance for this technology is between 300 meters and 1300 meters. That is, either the length of the subscriber line should not exceed this value, or the fiber optic cable should be brought closer to the user (for example, brought into a building in which there are many potential users). VDSL technology can be used for the same purposes as ADSL.

So, we can sum up. We have seen many varieties of the DSL family of technologies. They differ in data transfer rates, distance, connection methods, but in any case, firstly, xDSL provides many times higher speed than analog modems. The second advantage is the convenience of work: no dialing, constant connection. You do not need to constantly dial the provider's number to enter the network and then worry that the connection may break at any time. And one of the tastiest features: your phone is always free. Home will finally gain the ability to painlessly communicate on the phone while you surf the world wide web and you will never miss such an important call for you.

All these features will allow you to truly forget about the problems associated with Internet access. The network will be at a distance from you by pressing the "Power" button on the computer's system unit.

ProfTelecom - An overview of xDSL technology. What does the theory say? Types of xDSL technologies

xDSL is a family of technologies that can significantly expand throughput subscriber line of the local telephone network by using efficient linear codes and adaptive methods for correcting line distortions based on modern achievements in microelectronics and digital signal processing methods. The xDSL abbreviation uses "x" to represent the first character in a particular technology name, and DSL stands for Digital Subscriber Line (DSL). xDSL technology allows you to transfer data at speeds that are significantly higher than those available even to the best analog and digital modems. These technologies support voice, high-speed data and video transmissions, creating significant benefits for both subscribers and providers. Moreover, many xDSL technologies allow high-speed data and voice transmission to be combined over the same copper pair. Existing types of xDSL technologies differ mainly in the form of modulation used and the data rate.

xDSL technologies are the most practical solution for maximizing the amount of data transmitted over existing telephone lines. The use of xDSL technologies for high-speed access to network services is especially noteworthy in that these technologies use the existing cable infrastructure of local telephone networks as a transmission medium. This allows service providers to save significant money and more quickly (and at a reasonable cost) to create a large number of new data services for their subscribers. Because xDSL technologies operate over standard lines, this system is critical for expanding capacity at the bottleneck, the last mile, of the existing telephone network.

To install DSL, you must have access to a cable telephone network. DSL modems are installed at both ends of the telephone line: one modem is installed at the subscriber, and the other at the telephone exchange.

Unlike earlier copper telephone line technologies, xDSL systems do not require manual setting when installing. The modem automatically analyzes the line and sets up the connection in seconds. This process continues during the connection, as the modem compensates for changes occurring in the line (for example, associated with temperature changes). The modems use advanced digital signal processing (DSP) algorithms that create mathematical models distortion introduced by the line, and carry out automatic correction. The data transfer rate is affected by the length of the line, which depends on the cross-section of the cable cores, the type of insulation and the level of interference present in the line.

The main types of xDSL are ADSL, HDSL, RADSL, SDSL, and VDSL. All these technologies provide high-speed digital access over a subscriber's telephone line. Existing xDSL technologies are designed to achieve specific goals and meet specific market needs. Some xDSL technologies are original developments, others are merely theoretical models, while still others have already become widely used standards. The main difference between these technologies is the modulation methods used to encode the data.

The following DSL technologies exist:

ADSL(Asymmetric Digital Subscriber Line - asymmetric digital subscriber line): A DSL variant that allows data to be transmitted to the user at up to 8.192 Mbps, and from the user at up to 768 Kbps.

DDSL(DDS Digital Subscriber Line): A broadband DSL variant that provides Frame Relay access at data rates from 9.6 Kbps to 768 Kbps.

ADSL G.lite: An ADSL variant having both an asymmetric transmission mode with a bandwidth of up to 1.536 Mbps from the network to the user, and a speed of up to 384 Kbps from the user to the network., and a symmetrical transmission mode with a speed of up to 384 Kbps in both transmission directions.

IDSL(ISDN Digital Subscriber Line): A low-cost and proven technology that uses BRI ISDN Basic Access Digital Subscriber Line chips to provide subscriber access at up to 128 Kbps.

HDSL(High Speed Digital Subscriber Line): Higher speed xDSL variant that allows transmission at over 1.5 Mbps (US T1 standard) or over 2 Mbps (European E1 standard) in both directions, usually on two copper pairs.

SDSL(Symmetrical Digital Subscriber Line - symmetrical high-speed digital subscriber line operating on one pair); Two modifications of this equipment are known: MSDSL (multi-rate SDSL) and HDSL2, which have a built-in mechanism for adapting the transmission rate to the parameters of the physical line.

VDSL(Very High Speed Digital Subscriber Line - ultra-high-speed digital subscriber line): xDSL technology that provides data transfer rates to the user up to 52 Mbps.

Permanent access
The main difference between constant access using xDSL technologies and dial-up access is that your computer is constantly connected to the Internet. Therefore, you don't need to dial into the ISP's dial-up pool to check email or visit a website. Turn on the computer, open the browser - and you are on the Internet!
free phone
If you surf the Internet using dial-up access, your phone will be busy. Conversely, if someone is talking on the phone, you will not be able to access the Internet. When using xDSL, the phone remains free. You can surf the Internet and talk on the phone at the same time.
High data rate
xDSL belongs to the class of broadband technologies. It provides data transfer speed in the direction to the subscriber - up to 7.5 Mbps for incoming and up to 768 Kbps for outgoing channels. High speed allows you to comfortably work with websites, quickly transfer large files and documents, work with multimedia, fully use interactive applications.
Ease of connection
Unlike dial-up access, the procedure for connecting an xDSL channel contains only one additional step related to preparing your line at the PBX (it must be switched to digital equipment). Further line configuration is performed by the subscriber independently (connection instructions) or with the help of our specialists (for a fee).

xDSL technologies have several significant advantages. Compared to satellite and wireless access systems, it provides a higher connection quality, close to the quality of fiber-optic lines. At the same time, the cost of services is much lower and comparable to the price for dialup access.

Unlike home networks, an xDSL subscriber receives a channel for individual use. In a home network, one dedicated channel is shared among housemates. This affects both speeds and connection reliability.

prospects

xDSL is practically the only technology capable of making broadband Internet access a truly mass service in Russia. Combined with its low price and ease of installation, it will soon make permanent Internet access as popular as dial-up access is today.

The modern world is ripe for the use of DSL technologies. Increasing the flow of information transmitted over the Internet by companies and private users, as well as the need to organize remote access to corporate networks, gave rise to the need to create inexpensive technologies for digital high-speed data transmission over the bottleneck of the digital network - the subscriber telephone line. DSL technologies can significantly increase the speed of data transmission over copper pairs of telephone wires without the need to upgrade subscriber telephone lines. It is the possibility of converting existing telephone lines into high-speed data transmission channels that is the main advantage of DSL technologies.

So what exactly is DSL technology?

The abbreviation DSL stands for Digital Subscriber Line (Digital Subscriber Line). DSL is enough new technology, which allows to significantly expand the bandwidth of old copper telephone lines connecting telephone exchanges with individual subscribers. Any subscriber currently using conventional telephone communication has the opportunity, using DSL technology, to significantly increase the speed of his connection, for example, to the Internet. It should be remembered that it is the existing telephone lines that are used to organize the DSL line; This technology is good because it does not require additional telephone cables. As a result, you get round-the-clock access to the Internet while maintaining normal operation conventional telephone connection. None of your friends will complain anymore that they cannot call you for hours. Thanks to the variety of DSL technologies, the user can choose the data transfer rate that suits him - from 32 Kbps to more than 50 Mbps. These technologies also allow the use of a conventional telephone line for broadband systems such as video on demand or distance learning. Modern technologies DSLs bring the possibility of organizing high-speed Internet access to every home or every medium-sized and small business, turning ordinary telephone cables into high-speed digital channels. Moreover, the data transfer rate depends only on the quality and length of the line connecting the user and the provider. At the same time, providers usually allow the user to choose the transfer rate that best suits his individual needs.

How DSL works

The telephone set installed in your home or office is connected to the telephone exchange equipment using a twisted pair of copper wires. Traditional telephony is for ordinary telephone conversations with other subscribers of the telephone network. In this case, analog signals are transmitted over the network. The telephone set perceives acoustic vibrations (which are a natural analog signal) and converts them into an electrical signal, the amplitude and frequency of which is constantly changing. Since the entire operation of the telephone network is based on the transmission of analog signals, the easiest way, of course, is to use this method to transfer information between subscribers or a subscriber and a provider. That is why you had to buy, in addition to your computer, a modem that allows you to demodulate an analog signal and turn it into a sequence of zeros and ones of digital information perceived by a computer.

When transmitting analog signals, only a small part of the bandwidth of the twisted-pair copper telephone wires is used; wherein maximum speed The transmission that can be achieved with a normal modem is about 56 Kbps. DSL is a technology that eliminates the need to convert a signal from analog to digital and vice versa. Digital data is transferred to your computer as digital data, which allows you to use a much wider telephone line bandwidth. At the same time, it is possible to simultaneously use both analog telephone communication and digital high-speed data transmission over the same line, separating the spectra of these signals.

Various types of DSL technologies and short description their work
DSL is a set of different technologies that allow you to organize a digital subscriber line. In order to understand these technologies and identify their areas practical application, you should understand how these technologies differ. First of all, you should always keep in mind the relationship between the distance over which the signal is transmitted and the data transfer rate, as well as the difference in the transfer rates of "downstream" (from the network to the user) and "upstream" (from the user to the network) data flow.
DSL brings together the following technologies under its roof.

ADSL(Asymmetric Digital Subscriber Line - asymmetric digital subscriber line)

This technology is asymmetric, that is, the data transfer rate from the network to the user is much higher than the data transfer rate from the user to the network. This asymmetry, combined with the "always connected" state (where there is no need to dial a phone number each time and wait for a connection to be established), makes ADSL technology ideal for organizing access to the Internet, access to local area networks (LANs), etc. When organizing such connections, users usually receive much more information than they transmit. ADSL technology provides downstream data rates ranging from 1.5 Mbps to 8 Mbps and upstream data rates from 640 Kbps to 1.5 Mbps. ADSL allows you to transfer data at a speed of 1.54 Mbps over a distance of up to 5.5 km over a single twisted pair of wires. A transfer rate of the order of 6 - 8 Mbit / s can be achieved when transmitting data over a distance of no more than 3.5 km over wires with a diameter of 0.5 mm.

R-ADSL(Rate-Adaptive Digital Subscriber Line - digital subscriber line with connection speed adaptation)

R-ADSL technology provides the same data transfer rate as ADSL technology, but at the same time allows you to adapt the transfer rate to the length and condition of the twisted pair wires used. When using technology R-ADSL connection on different telephone lines will have different data rates. The baud rate can be selected when the line is synchronized, during a call, or when signaled by a station.

G.Lite (ADSL.Lite) is a cheaper and easier-to-install version of ADSL technology that provides downstream data rates up to 1.5 Mbps and upstream data rates up to 512 Kbps or 256 Kbps in both directions.

IDSL(ISDN Digital Subscriber Line - IDSN digital subscriber line)
IDSL technology provides full duplex data transmission at speeds up to 144 Kbps. Unlike ADSL, IDSL is limited to data transmission only. Although IDSL, like ISDN, uses 2B1Q modulation, there are a number of differences between the two. Unlike ISDN, the IDSL line is a non-switched line that does not increase the load on the provider's switching equipment. Also, the IDSL line is "always on" (like any line organized using DSL technology), while ISDN requires a connection.

HDSL(High Bit-Rate Digital Subscriber Line - high-speed digital subscriber line)

HDSL technology provides for the organization of a symmetrical data transmission line, that is, the data transfer rates from the user to the network and from the network to the user are equal. With transmission speeds of 1.544 Mbps over two pairs of wires and 2.048 Mbps over three pairs of wires, telcos are using HDSL technology as an alternative to T1/E1 lines. (T1 lines are used in North America and provide a data rate of 1.544 Mbps, and E1 lines are used in Europe and provide a data rate of 2.048 Mbps.) Although the distance over which the HDSL system transmits data (which is about 3.5 - 4.5 km), less than with ADSL technology, for an inexpensive, but effective, increase in the length of the HDSL line, telephone companies can install special repeaters. The use of two or three twisted pairs of telephone wires to organize an HDSL line makes this system an ideal solution for connecting PBXs, Internet servers, local area networks, etc. HDSL2 technology is a logical result of the evolution of HDSL technology. This technology provides similar performance to HDSL technology, but uses only one pair of wires.

SDSL(Single Line Digital Subscriber Line - single-line digital subscriber line)

VDSL(Very High Bit-Rate Digital Subscriber Line)

VDSL technology is the "fastest" xDSL technology. It provides downstream data transfer rates ranging from 13 to 52 Mbps, and upstream data transfer rates from 1.5 to 2.3 Mbps, and over one twisted pair of telephone wires. In symmetrical mode, speeds up to 26 Mbps are supported. VDSL technology can be seen as a cost effective alternative to running fiber optic cable to the end user. However, the maximum transmission distance for this technology is between 300 meters and 1300 meters. That is, either the length of the subscriber line should not exceed this value, or the fiber-optic cable should be brought closer to the user (for example, brought into a building in which there are many potential users). VDSL technology can be used for the same purposes as ADSL; in addition, it can be used to transmit high-definition television (HDTV) signals, video-on-demand, and the like.

First, DSL technologies provide high data transfer rates. Different variants of DSL technologies provide different data transfer rates, but in any case, this speed is much higher than the speed of the fastest analog modem.
Secondly, DSL technologies leave you the opportunity to use ordinary telephone communication, despite the fact that they use a subscriber telephone line for their work. Using DSL technologies, you no longer have to worry about not receiving important news on time, or about the fact that for a normal phone call you will first need to log out of the Internet.

And finally, the DSL line is always working. The connection is always established, and you no longer need to dial a phone number and wait for a connection to be established every time you want to connect. You no longer have to worry about an accidental disconnection on the network, and you will lose connection at the very moment when you are downloading data that is simply vital for you from the network. Email you will receive at the time of receipt, and not when you decide to check it. In general, the line will always work, and you will always be on the line.

xDSL. Outside, everything is clear, but inside ...

Anyone who thinks about buying a computer inevitably thinks about connecting to the Internet. But the question is not whether to connect or not, but how to do it. Now it is difficult to find a person who has not heard about ADSL technology. But hearing is one thing, knowing how it functions is another. Many people think that this technology is intended for people who have "money pouring out of their pockets", but such a statement is not true. Connection using ADSL technology is not only not unprofitable - it is even profitable. But first things first...

I think everyone has heard about the new idea of RUE "Beltelecom" called ByFly. This is nothing more than an Internet connection using xDSL technology. The advantages of this service are obvious: the fee for 1 Mb of downloaded information is on average 72 Bel. rub. Now take a round number of traffic and calculate how much you will pay using Dial-Up and how much using ByFly. The benefit is clearly obvious. ByFly's system is somewhat similar to the system mobile communications- prepayment. Understanding this is quite simple: there is money in the account - you can go online, no - you can’t. After connecting, the user is provided with a personal web page - the so-called "user account", in which you can easily control the state of your account, downstream and upstream traffic.

xDSL technology

"Everything ingenious is simple, but you must first reach this simplicity," I saw this phrase in an article in the consumer magazine. It talked about how JVC engineers came up with a technology for manufacturing speakers with wooden cones (!). And the more I remember this phrase, the more I am convinced of its fidelity.

The abbreviation DSL stands for Digital Subscriber Line (Digital Subscriber Line). This technology allows to significantly expand the capacity of standard telephone lines connecting telephone exchanges with individual subscribers. When using DSL technology, data transfer rates from 32 Kbps to more than 50 Mbps are achieved. The transmission speed directly depends on the quality and length of the lines connecting the user and the provider. It all works quite simply. The fact is that the telephone set installed at home, in the office, etc., is connected to the equipment installed at the telephone exchange with a twisted pair of copper wires. These wires carry analog signals. The telephone set is tuned to receive acoustic vibrations, which are the natural analog signal. It processes them and converts them into an electrical signal with varying amplitude and frequency. That is why when working on the Internet through a Dial-Up connection, a modem is needed that modulates and demodulates analog signals, turning them into a sequence of zeros and ones that a computer understands. When transmitting analog signals, only a small part of the bandwidth of the telephone line is used. DSL technology does not involve converting information from analog to digital and vice versa. With this technology, data is transmitted over the lines in digital form initially, which allows the use of a wider bandwidth of the telephone line. By separating the spectra of these signals, it is possible to ensure that both analog and digital signals can be transmitted over one twisted pair cable, and without prejudice to both.

Types of xDSL technology

xDSL is nothing more than a set of different technologies that allow you to organize a digital subscriber line. These technologies differ in different data rates and range. xDSL includes the following technologies:

. ADSL (Asymmetric Digital Subscriber Line - asymmetric digital subscriber line). This technology is asymmetric, that is, the data transfer rate from the network to the user is much higher than the data transfer rate from the user to the network. This asymmetry, combined with the "always connected" state (which eliminates the need to dial a phone number each time and wait for a connection to be established), makes ADSL technology ideal for organizing Internet access, access to local area networks (LANs), etc. When organizing such connections, users usually receive much more information than they transmit. ADSL technology provides downstream data rates ranging from 1.5 Mbps to 8 Mbps and upstream data rates from 640 Kbps to 1.5 Mbps. ADSL allows you to transfer data at a speed of 1.54 Mbps over a distance of up to 5.5 km over a single twisted pair of wires. Transfer rates of the order of 6-8 Mbps can be achieved when transmitting data over a distance of no more than 3.5 km over wires with a diameter of 0.5 mm.

. R-ADSL (Rate-Adaptive Digital Subscriber Line - digital subscriber line with connection speed adaptation). R-ADSL technology provides the same data transfer rate as ADSL technology, but at the same time allows you to adapt the transfer rate to the length and condition of the twisted pair wires used. Using R-ADSL technologies connection on different phone lines will have different data rates. The baud rate can be selected when the line is synchronized, during a call, or when signaled by a station.

. ADSL Lite. ADSL Lite is a low-speed (relatively, of course) variant of ADSL technology, providing downstream data rates up to 1 Mbps and upstream data rates up to 512 Kbps. ADSL Lite technology allows data transmission over longer lines than ADSL, is easier to install and has a lower cost, which makes it attractive to the mass user.

. IDSL (ISDN Digital Subscriber Line - IDSN digital subscriber line). IDSL technology provides full duplex data transmission at speeds up to 144 Kbps. Unlike ADSL, IDSL is limited to data transmission only. Although IDSL, like ISDN, uses 2B1Q modulation, there are a number of differences between the two. Unlike ISDN, the IDSL line is a non-switched line that does not increase the load on the provider's switching equipment. Also, an IDSL line is "always on" (like any DSL line), while ISDN requires a connection to be established.

. HDSL (High Bit-Rate Digital Subscriber Line - high-speed digital subscriber line). HDSL technology provides for the organization of a symmetrical data transmission line, that is, the data transfer rates from the user to the network and from the network to the user are equal. Due to the transmission speed (1.544 Mbps on two pairs of wires and 2.048 Mbps on three pairs of wires), telecommunications companies use HDSL technology as an alternative to T1/E1 lines (T1 lines are used in North America and provide a data transfer rate of 1.544 Mbps , and E1 lines are used in Europe and provide a data transfer rate of 2.048 Mbps). Although the distance over which the HDSL system transmits data (which is about 3.5-4.5 km) is less than using ADSL technology, special repeaters can be installed by telephone companies to inexpensively but effectively increase the length of the HDSL line. The use of two or three twisted pairs of telephone wires to organize an HDSL line makes this system an ideal solution for connecting PBXs, Internet servers, local area networks, etc. HDSL II technology is a logical result of the development of HDSL technology. This technology provides similar performance to HDSL technology, but uses only one pair of wires.

. SDSL (Single Line Digital Subscriber Line - single-line digital subscriber line), like HDSL, provides symmetrical data transmission at rates corresponding to the rates of the T1 / E1 line, but it has two important differences. Firstly, only one twisted pair of wires is used, and secondly, the maximum transmission distance is limited to 3 km. Within this distance, SDSL technology provides, for example, the operation of a video conferencing system when it is required to maintain the same data transfer flows in both directions. In a sense, SDSL technology is the predecessor of HDSL II technology.

. VDSL (Very High Bit-Rate Digital Subscriber Line)- ultra-high-speed digital subscriber line). VDSL technology is the fastest xDSL technology. It provides downstream data rates ranging from 13 to 52 Mbps and upstream data rates from 1.5 to 2.3 Mbps, all over a single twisted pair of telephone wires. VDSL technology can be seen as a cost effective alternative to running fiber optic cable to the end user. However, the maximum data transmission distance for this technology is from 300 to 1300 m. That is, either the length of the subscriber line should not exceed this value, or fiber optic cable should be brought closer to the user (for example, brought into a building in which there are many potential users). VDSL technology can be used for the same purposes as ADSL, in addition to transmitting high-definition television (HDTV) signals, video on demand, and the like.

xDSL standards

The leading international organizations in the development and implementation of subscriber access standards (including xDSL digital subscriber line technologies) are the following organizations: International Telecommunication Union (ITU), American National Standards Institute ANSI, European Telecommunications Standards Institute ETSI, ADSL Forum (ADSLF) and Universal ADSL Working Group (UAWG). Below are the functions and results of the work of these standards organizations (see Fig. 1).

In the US, the ANSI T1 Committee is responsible for national telecommunications standards and participates in ITU-T's work to create international standards. Subcommittee T1E1 of the T1 subcommittee of ANSI deals with standards related to interfaces, bandwidth, and network security. Working group T1E1.4 of subcommittee T1E1 is responsible for DSL technologies. This group developed the first ADSL standard, known as T1.413, based on the use of the DMT line code and approved in 1995. This standard went through a series of revisions, called versions (issues). The latest version is known as T1.413 Issue 2, or simply T1.413i2. It was approved in 1998. The first edition of T1.413 of the ADSL standard contained the most important features and parameters of ADSL modems including echo cancellation, trellis code modulation, dual latency, supporting both delay-sensitive (for example, telephone and interactive video) and delay-insensitive traffic (such as data traffic). The second edition of this standard, T1.413i2, defines additional ADSL capabilities (transmission of a reference clock signal, the use of TDM and/or ATM methods for transporting signals, as well as a reduced service header mode). The first edition of T1.413 (Issue I) of the ADSL standard was limited to one user-premise interface (CPE). The second edition of this standard, T1.413i2, expanded the user's accessibility by introducing a special multiplexing interface, network configuration and control protocols, and a number of other improvements into the user equipment. Currently, the T1E1.4 working group is working on the third edition of the T1.413 standard, which takes into account the requirements of international standards (and primarily ITU-T recommendations G992.1 and G.992.2). Therefore, this latest edition of T1E1.4 13 has every reason to claim to be the international standard for ADSL technologies.

. ETSI website

ETSI is responsible for the development of European telecommunications standards and participates in the work of ITU-T to create international standards. ETSI has a Transmission and Multiplexing (TM) Technical Committee responsible for standardizing the functions and characteristics of transport networks and their elements. It includes the TM6 working group, whose area of activity is xDSL technologies. In particular, ETSI TM6 is in the process of adapting the ANSI T1.413i2 standard to European conditions. Regulations ETSI related to xDSL technologies are given in Table. 2. Attention should be paid to the technical specification ETSI TS 101 388 v1.1.1 (1998-11), which standardizes an ADSL line that allows ADSL signaling and ISDN Basic Access Signaling (ISDN-BA) to be carried together over a single pair. This specification is especially relevant for Western European countries (primarily Germany and Switzerland), where ISDN basic access is widely used. The rapid development of this standard perfectly illustrates the desire of carriers to protect the investments already made in ISDN.

. ADSL Forum (ADSLF), website

The purpose of the ADSL Forum is to assist telecom operators and access equipment manufacturers in the effective deployment of ADSL technology as the main way for the mass user to access broadband network services. The ADSL Forum is made up of 340 members representing service providers, equipment manufacturers and information providers from around the world.

The ATM Forum has defined ADSL as the physical layer of twisted unshielded pair transmission. The ADSL Forum was established in 1994 to promote the xDSL concept to the telecommunications market and facilitate the development of protocol and interface architectures for major xDSL applications. The work of the ADSL Forum is in seven main areas, each of which has its own working group:

1. ATM "over" ("over") ADSL (including signaling transport and architecture aspects between end terminals ("end to end")).
2. Packet transmission "over" ADSL (this work is not yet completed).
3. Interfaces and configurations of the customer premises equipment (CPE) and ADSL equipment of the local exchange or access point.
4. Normal operation and operational management.
5. Access network management.
6. Testing and ensuring the joint operation of equipment from different manufacturers.
7. Support for the working group involved in the study of VDSL.

Each working group initially creates the so-called "Working Texts", which are practically detailed plans for the work of the group. The output of these detailed plans are "Technical Reports" TR (Technical Report), which must then be reviewed and approved by the members of the ADSL Forum. After that, TRs become official documents that the administration of the ADSL Forum distributes to interested organizations. The ADSL Forum maintains formal links with key standards development organizations and working groups including ITU-T, the ATM Forum, ANSI T1.E1.4, ETSI TM6 and UAWG.

. Universal ADSL Working Group (UAWG) website

The Universal ADSL Working Group was formally established to address the following issues:
1. Development of a simple universal international standard with strong support from ADSL equipment manufacturers, currently known as ITU-T Recommendation G.992.2.
2. Simplify the installation of equipment in the user's premises of the CPE by eliminating the use of additional devices and / or keeping the existing cabling in the CPE.
3. Ensuring the throughput of the incoming direction of data transmission ("downstream"), at least 25 times greater than in the case of a standard telephone modem.
4. Ensuring that the ADSL modem operates in "always on" mode, which avoids time-consuming connection establishment procedures and allows the use of new classes of applications.

A key decision of the UAWG participants was to use the ANSI T1.413i2 standard and modify it to develop a new standard that optimizes the possibility of mass high-speed Internet access and implementation of other applications for private users. This lighter version of ADSL should provide, at an affordable price for the mass user, a sufficiently high throughput (up to 1.5 Mbps in the outgoing direction (downstream) and up to 512 Kbps in the upstream direction (upstream), which is sufficient, first of all, for efficient operation users on the Internet for almost all possible lengths of subscriber lines.An indicator of the high efficiency of the work of the UAWG is the submission of several reports to ITU-T SG-15, which allowed for the accelerated development of the previously mentioned recommendation G.992.2 (G.Lite). that the UAWG does not operate on a permanent basis.As a particular problem is resolved, it ceases to exist.The efforts made by the UAWG, especially towards ensuring interoperability of equipment from different manufacturers, will be transferred to the ADSL Forum.

ADSL is the most demanded technology from the xDSL family by ordinary users. Pretty decent speed characteristics and availability in all settlements where there is a communication center make it so popular. The scheme of operation of this technology is quite simple (see Fig. 2). The line connects two modems: one for the user and the other for the provider. At the same time, the user does not have to buy a modem for his own money - it is enough to rent it from a service provider.

In front of all devices telephone network a Spliter is connected, which, in fact, separates the digital signal and the analog one. The ADSL modem is connected via Ethernet or USB (depending on the modem interface). The bandwidth of a telephone line is divided into three channels: analog, downstream, and upstream (see Figure 3). The analog channel is used for conventional telephone communication. The downstream (data stream directed from the server to the subscriber) is usually twice as fast as the upstream (data stream directed from the subscriber to the server). So that an accident that occurred with an ADSL connection does not affect the operation of ordinary telephone communication, the latter is distinguished using filters.

For transmission in large volumes, information is compressed using digital signal processing, advanced analog filters and analog converters. The most complex is the bandwidth division system. The line is divided into several frequency bands, which are called carriers. With ADSL, different carriers simultaneously carry different parts of the transmitted data. This process is called link multiplexing (Frequency Division Multiplexing - FDM). With FDM, downstream and upstream data flows are allocated over a certain range, and the range, in turn, is divided into high-frequency and low-frequency channels (one or several) (see Fig. 4). Echo cancellation technology is also used.

And everything would be fine, but there is one "but". And this "but" lies in the fact that when providing a service to an end user, the provider cannot guarantee the correct operation of the ADSL connection. Yes, not everyone can connect, but only those who have a telephone line located at a distance of no more than 5 km from the communication center. And at the same time, it is not the direct line from the node to the subscriber that is taken into account, but the length of the wire. Moreover, the condition of the telephone cable should be acceptable. Probably, the vast majority of users encounter interference, interruptions, etc. when talking on the phone. Of course, for telephone communication this is not such a big nuisance, but for high-speed data transfer ... In this case, the line may start to malfunction both immediately (when connected) and after some time. The cause of the breakdown can even be insufficient insulation resistance of the twisted pair. To avoid such unpleasant incidents, the provider should test the user's line before starting to provide services. For this, rather expensive equipment is used, which makes testing of this kind economically unprofitable. Therefore, in each locality filtering is carried out, in which lines that require modernization and lines that do not require modernization are distinguished. Typically, the selection criterion is the length of the line (see Fig. 5).

Based on the foregoing, we can draw the appropriate conclusion. Yes, xDSL technologies are the future. In view of the cheaper services provided by these technologies, they will very soon "disperse" among ordinary users, as Dial-Up did in its time. In my opinion, using a digital subscriber line is much better than constantly dialing and waiting for a connection, and just astonishingly slow speeds of downstream and upstream traffic. However, I do not seek to impose my opinion on anyone. I told you what's what, and the choice is always yours.

In preparing the article, information from the site was used

DSL modem

DSL (Digital Subscriber Line) is an acronym for Digital Subscriber Line. DSL technologies make it possible to connect users to telephone exchanges, while expanding the usable frequency range of existing lines of the telephone cable network.

xDSL is a generic abbreviation for DSL technologies. xDSL technologies allow you to transfer data at speeds that are significantly higher than the speeds available to the best analog and digital modems. xDSLs support voice, high-speed data and video transmissions, creating significant benefits for both subscribers and providers. Moreover, many xDSL technologies allow high-speed data and voice transmission to be combined over the same copper pair. The existing types of xDSL technologies differ mainly in the form of modulation used and the data rate.

Types of xDSL technologies

So, DSL is a set of the following technologies:

· ADSL (Asymmetric Digital Subscriber Line -- asymmetric digital subscriber line)

A simpler version of ADSL. Provides up to 1.5 Mbps downstream and up to 512 Kbps upstream

· IDSL (ISDN Digital Subscriber Line -- IDSN digital subscriber line)

· HDSL (High Bit-Rate Digital Subscriber Line)

· SDSL (Single Line Digital Subscriber Line)

· VDSL (Very High Bit-Rate Digital Subscriber Line

cable modem

cable modem-- a modem with a built-in network bridge, which provides the possibility of two-way data transmission via coaxial (HFC, eng. hybrid fibre-coaxial) or optical cable(RFoG) Radio Frequency over glass). Cable modems are commonly used on cable television networks to provide broadband Internet access.

The first high speed asymmetric cable modem system was developed, shown and patented by Hybrid Networks in 1990. data computer cable modem

In the late 1990s, a group of US cable operators formed the MCNS consortium. multimedia cable network System) to develop an open and interoperable specification for cable modems. The group actually combined the two most popular proprietary protocols at the time, taking the physical layer from the Motorola CDLP system and the MAC layer from the LANcity system. After drafting the specification, the MCNS consortium handed over control of the specification to CableLabs.

The standard developed by CableLabs is called DOCSIS. Data Over cable Service Interface Specification). Almost all cable modems in use today are compatible with some version of DOCSIS. Due to the differences between European (PAL) and American (NTSC) television systems, there are two main versions of the standard - DOCSIS and EuroDOCSIS, which differ in the bandwidth of radio channels (6 MHz in the USA, 8 MHz in Europe). The third version of DOCSIS was developed in Japan.

Radio modems

A radio modem is a device designed to transmit digital data over a radio channel. There are narrowband and wideband radio modems. We manufacture narrowband radio modems with a transmission bandwidth of 25 kHz. The main difference of narrowband radio modems is a low transmission rate, but a significantly greater transmission range, with the same energy costs for transmission. Narrowband radios also require a small amount of bandwidth, making it much easier to obtain a frequency license. The main applications of narrowband radio modems in a communication system where it is not required high speed transmission, but requires a large area of coverage and high reliability of radio communications. Narrowband radio modems are used for remote control and receiving telemetry from stationary and especially moving objects, for various purposes. An alternative to narrowband radio modems are cellular communication systems, modern cell phones are inherently also radio modems. But unlike cellular communication systems, the use of narrowband radio modems, although it requires a license for a radio frequency, there is no subscription or other fee for subsequent operation. The communication channel is always available, the access time to the object is always minimal, the traffic in the network is predictable and manageable, unlike cellular systems, where the traffic and access time to the radio channel are not predictable.

The radio modem consists of two main blocks, the first block is an improved, in comparison with conventional voice radio stations, analog transceiver, the second digital block: interfaces, a microcontroller and a digital signal modulator.

A distinctive feature of the transceiver for the radio modem are: high stability of the reference frequency, short time to enter the mode! For the receiver generator, a low level of phase noise is important, and for the input circuits of the receiver, uniform: group time delays (GD) and frequency response (AFC) in the passband. And also high stability of the majority of parameters in temperature is necessary. For conventional voice radios, the requirements for many transceiver parameters and their stability in temperature are not so stringent. For them, changing the parameters in temperature or initially significantly worse parameters only leads to a deterioration in the audibility of speech and communication range, and in a radio modem this leads to the complete inoperability of the digital radio channel and the higher the speed of the radio modem, the more important the stability of the parameters in temperature.

The digital unit may consist of one or more microcontrollers and various interfaces: RS232, RS485, RS422, ethernet. To process the signal coming from the radio channel and modulate it during transmission, both high-performance microcontrollers such as digital signal processors (DSPs) and specialized microcircuits, which are essentially the same DSPs with a fixed operation algorithm, also called modems, can be used. In a scheme using a specialized chip modem, the microprocessor of the digital block only controls such a chip, collecting and buffering data.

Let's take a quick look at how a radio modem works. Digital data, from various interfaces, enters a digital microprocessor, which collects, buffers, encodes and sends digital data either to a specialized digital modulator chip (modem), or sometimes digitally modulates an analog RF signal itself. Further, the modulated signal is amplified and fed to a separate single-chip transmitter module and then goes through an external antenna to the radio channel. On the receiving side, a similar digital microcontroller continuously monitors and evaluates the received signal level called RSSI. As soon as this level exceeds a certain threshold, called the detection threshold, set in the region of 0.5-1 microvolts, the processor decides that a radio signal has appeared and turns on the receiver and modem in the adjustment mode and search for synchronization. After synchronization is detected, the microcontroller or modem begins to process and decode the digital data coming from the radio channel! Further, the received digital data is sent to the external interface of the radio modem.