One of the most popular and affordable ways to connect to the World Wide Web today is an ADSL connection. The abbreviation ADSL stands for "Asymmetric Digital Subscriber Line" - an asymmetric digital subscriber line. Despite its simplicity and almost 100% availability, the mobile connection is significantly inferior to the ADSL connection in terms of its capabilities: the data transfer rate is lower, the range of services is less, and the connection cost is much higher. Connection using ETTH technology (“Ethernet to every home”), GPON and FTTH (using fiber-optic cable) is currently available only for residents of the multi-apartment sector in large settlements, as it is economically justified with mass connections. Therefore, today ADSL connection is relevant for most users, especially in small towns.

ADSL connection problems

Despite its mass availability and fairly decent technical characteristics:

1. Practical access speed: up to 24 Mbps;
2. Subscriber line length for satisfactory operation: up to 7.5 km;
3. Service availability triple play- simultaneous transmission of voice, video and data.

This technology uses in its work a telephone subscriber line with all the ensuing problems.

Consider a typical subscriber connection scheme using ADSL technology:

The practice of operating this technology shows that the most common problems that lead to the fact that the user installs slow speed on adsl connection, or no Internet access at all, are:

1. Telephone line failure;
2. Access equipment port failure (DSLAM) on the provider side;
3. Incorrect connection on the user's side.

Telephone Line Trouble

This is the most common type of damage that occurs in the "Subscriber-Provider" chain. Unfortunately, the telephone line is far from perfect. While it “gets” from the Internet provider to the user, it can go through quite a lot of different sections: trunk, cable, distribution cables, cables between cabinets, and even the so-called air ducts - wires that go from the cabinet to the subscriber by air. Each of these sections, in addition to attenuation of the useful signal, can also introduce various interferences, leading both to a general decrease in speed and to the fact that the subscriber has frequent disconnections during adsl connection.

Of course, in order to measure the physical parameters of a telephone line in order to obtain its qualitative characteristics, it is necessary to have special devices and the ability to use them. But an ordinary user can also easily assess its condition in order to understand why certain access problems arise. To do this, you need to connect to an ADSL modem and view the ADSL connection statistics.

Not only problems with the communication line or with the provider's equipment lead to problems in working with the Internet. Asking the question - “How to increase the speed with an adsl connection?”, The user sometimes forgets that incorrectly working equipment or an incorrect connection on his side can also cause failures and low speed. Therefore, before calling the service technical support, you need to check if the telephone line, modem and telephone are connected correctly.

First of all, you should start with splitter- a special device that is designed to ensure that high-frequency noise from the modem does not interfere with telephone conversations. In fact, it is a special filter for separating the operating frequency bands of the modem and the telephone.

Consider the correct scheme for connecting user devices:

It should be remembered that telephone sets and any other telephone devices must not be connected before the splitter! All phones must be strictly connected to the PHONE jack! Otherwise, the connection will be unstable, and usually slow. Adsl connection breaks will be almost permanent in this case.

Connecting an adsl modem without a splitter will lead to noise during a telephone conversation and, as in the first case, to a poor connection quality. However, if you are not using a telephone set, then the modem can be connected to a telephone line without this device.

Excessively long telephone extension cords should be avoided. If you really can’t do without it, you need to choose those that use not four, but two conductors. This will reduce the level of interference and improve the quality of the connection.

Unfortunately, the adsl modem is also not immune from damage. Moreover, there are obvious damages, that is, when it simply does not work or does not work correctly, but there are hidden ones associated with damage to its linear part. Especially often, such malfunctions quite often occur after a thunderstorm. At the same time, the modem itself is working and can even establish a connection with the provider's equipment, but it is unstable, or the connection is at low speed. The first impression that arises is that the telephone line is malfunctioning, since the "symptoms" are very similar. In this case, you should take readings of the main characteristics of the connection from its menu in the "Statistics" section, and check it at the provider's stand, asking you to take the same data. If the readings are similar, most likely, the linear part of the modem is “burned out” and needs to be repaired.

1. If the speed of Internet access is periodically reduced, start the test by examining the stability of the established connection - the “link”. (The English version of the word is Link). Follow the indicator with the same name. On some models it is called ADSL. During operation, if the adsl connection is stable and established, it should just be on. If it blinks periodically, the connection with the provider is unstable, a check of the communication line is required.
2. Watch outgoing (upstream) speed in the line. Practice shows that the lower it is, the lower the quality of the connection. Ideally, it should be equal to or close to 1 Mbps (unless specifically limited by the tariff).
3. With constant disconnections, you can try to turn off the splitter and phone, turning on the modem for a while, directly into the line. This eliminates the possible influence of other devices on the connection. If in this case everything works stably, then you can, turning on the devices in turn, find out which one has an impact.
4. Always check the quality of the contact in the connectors. The modern RJ11 telephone jack is not a very high quality product, its contacts are often oxidized. Remove and reinsert it two or three times.

ADSL is an asymmetric Internet access technology. It is an asymmetric system in its structure and allows you to work with connections at speeds up to 8 Mbps. ADSL-technology, the transmission speed of which is calculated up to 1 Mbps, operates on average at a distance of more than 5 km. Today we will look at what this type of connection is and how it works.

History of appearance

Before answering the question: "ADSL - what is it?", we bring to your attention some historical data. For the first time, they started talking about the creation in the late 80s, when even the Internet in its modern form was only its main task in 1989 was to improve and modernize the technology of data transmission over copper telephone wires. Analog-to-digital conversion was created mainly for the rapid transfer of information between various interactive services, video games, video files, as well as for instant remote access to the LAN and other network systems.

Modern ADSL technology: how it works

The operation of the network is based on the subscriber's digital line, which provides access to the Internet through telephone communication channels. But telephone lines use an analog signal to carry voice messages. An ADSL connection is designed to convert an analog signal into a digital one and transfer it directly to a computer. At the same time, unlike already outdated Dial-up modems, ADSL-based devices do not block the telephone line and allow you to use digital and analog signals simultaneously.

The essence of the technology (asymmetry) lies in the fact that the subscriber receives a huge amount of data - incoming traffic, and transmits a minimum of information from himself - downstream traffic. As an input, various kinds of content are meant: video and media files, applications, objects. Downstream sends only important technical information - various commands and requests, emails and other secondary elements. The asymmetry is that the speed from the network to the subscriber is several times higher than the speed from the user.

The most important advantage that ADSL technology has is its budget and economy. The fact is that the same copper ones are used for the operation of the system. The number in them, of course, significantly exceeds the number of similar elements in cable modems. But at the same time, no modernization of switching equipment and complex reconstruction is necessary. ADSL connects quickly, and modern types of modems are intuitive to manage and configure.

What equipment is used for this connection?

In order for the technology to work, special types of modems are used, which differ in their structure, design, connection type:

• PCI modems (internal computer devices).
• External modems with USB connection type.
• Devices with Ethernet type interface.
• with Ethernet scheme.
• Profile types of modems (for security companies, private telephone lines).
• Router with internal Wi-Fi hotspots.

Additional equipment: splitters and microfilters

We must not forget that to connect such a gadget as an ADSL modem, you will need splitters and microfilters. Devices are selected in accordance with the design of the telephone cable. In a situation where a cable outlet is made (or can be done) to separate the modem and telephone channel, a splitter is used. In another case, the purchase of a microfilter is required, which is installed on each telephone present in the room.

The main task of the splitter is to separate frequencies - voice (0.3-3.4 kHz) and those used directly by the modem itself (25 kHz-1.5 MHz). It is in this way that the simultaneous operation of the modem and the telephone is ensured, which do not interfere with each other and do not interfere. Splitters are compact and will not cause unnecessary inconvenience. The miniature box is equipped with three connectors and is light in weight.

ADSL - what is it? Stages of connecting high-speed Internet

1. Provider choice. To date, each provider offers to use this technology. Different kinds and tariffs depend on the region, as well as on the technical capabilities of the company, the coverage area of ​​\u200b\u200bwhich may be limited.
2. Purchase of equipment. Currently, it is not necessary to buy a modem, splitters and microfilters. When drawing up a connection agreement, the provider offers to rent the necessary equipment, including an ADSL modem. In the future, upon termination of the document, the equipment is returned back. The client pays exclusively for the Internet connection. Modern Internet ADSL - what is it? This is a fast, cheap and high-quality connection method.
3. Account activation. For each client, the provider reserves an account, the activation of which can take up to 12 days. However, in most cases, with normal network coverage, the procedure does not require more than a few hours. First, the provider checks the phone number for the possibility of connecting ADSL. If the technology access zone is not enough, then high-speed Internet will not work.
4. Equipment setup. At this stage, devices are connected to the telephone line, splitters and microfilters are installed, modem drivers are installed on the computer, and the modem's network parameters are set in the Internet browser.

pros

What are the advantages of ADSL technology? Here are a few of them:

• High ADSL allows you to easily transfer files of any size without a long wait. The technology is constantly being improved, and the speeds are growing, significantly expanding the capabilities of the subscriber.
• Wireless connection. To use an ADSL system, you do not need to stretch the cable to the subscriber and install a large amount of equipment. The reliability, quality and functionality of the network is improved.
• No interference on the telephone line. The ADSL router operates in independent mode and does not create any problems for the phone to work. You can call and surf the virtual space completely freely.
• Permanent Internet access ADSL. What it is? This means that the network will not fail during operation. The technology does not require reconnection. The user gets access to the Internet constantly and can be online around the clock.
• Reliability and stability. Today, ADSL is the most reliable type of Internet connection.
• Profitability. The cost of connecting ADSL and installing a modem with a router is minimal and will not hit the family budget.

Flaws

1. Lack of crosstalk protection. If several dozen clients are connected to one channel, count on high speed won't have to. The more subscribers on one ADSL, the lower the quality of data transmission.
2. Although ADSL technology has disadvantages, they are not numerous. This also includes the minimum speed from the subscriber. The asymmetry of ADSL has an obvious minus - the transfer of files from the subscriber will be long and inconvenient. But the technology is intended, first of all, for quick access to the Internet, for surfing. In addition, the information transmitted from the subscriber takes up a minimum of space and does not require a large resource.

Speed ​​and factors affecting it

ADSL is a technology high speed internet, but there is no universal meaning and formula. For each individual subscriber, the speed is individual and is determined by a whole set of factors. Including some of them can affect the reliability and quality of equipment. Therefore, it is best for professionals to install modems and routers.

The main reason for the low speed of an ADSL connection is the quality of the subscriber line. We are talking about the presence of cable outlets, their condition, wire diameter and length. Signal attenuation is a direct consequence of an increase in the length of the subscriber line, and interference can be reduced by expanding the diameter of the wire. The standard length of an ADSL channel does not exceed 5 km - the optimal range for high-speed data transmission.

Speed ​​characteristics

Compared to other Internet connection technologies, ADSL has a significant speed advantage. An analog modem will give a maximum of up to 56 Kbps, while ADSL at the dawn of its appearance already allowed information to be transmitted at speeds up to 144 Kbps.

ADSL technology, the maximum speed of which is also determined by the characteristics of the modem and can reach 2048 Mbps, optimizes the process of information transfer. Digital lines significantly increase the user's capabilities, taking him beyond the limitations even with several connected computers, mobile phones, tablets and other gadgets.

Technology Perspectives

The possibilities and resources of ADSL technology are far from being exhausted. Even the ADSL2 and ADSL2+ standards, introduced back in the mid-2000s, still retain their relevance and capabilities. This is, in fact, the only technology that can provide wide Internet access without failures and software problems, therefore it is a competitor to many other methods of connecting to the Internet.

The minimum technical equipment is supplemented modern views modems. Manufacturers annually release new devices designed for continuous operation without the need for maintenance and service. In addition, ADSL speed is constantly growing and is not limited to megabits. The connection becomes relevant both for the home and for the whole office company with several dozen computer clients.

Conclusion

So, we found out what ADSL technology is, what is its essence and principle of operation. As you can see, this is one of those technologies that practically does not fail during operation (even if several dozen users are connected to the network). At the same time, it does not require constant reconnections and speed limits.

Almost everyone needs internet access these days. Whether it's work, entertainment, communication - the global network has entered our lives everywhere. To provide Internet access at home or in the office, you need a modem that will allow you to connect all the necessary devices to the network. In large cities, providers offer fiber-optic and fiber-coaxial systems that allow you to get a fast and stable connection. However, in order to run such cables, it is necessary that the number of users allows filling the entire bandwidth of the cable - otherwise it is simply not profitable. Therefore, the possibility of such a connection is not provided by business everywhere. This is especially true for small towns, towns and villages. But what if such services are not provided, but the Internet is still needed?

There are different options, and one of the best is the use of twisted-pair subscriber telephone wires. Many will recall with horror a non-working phone while using the Internet. However, technology has come a long way. Today, xDSL technologies are the most widespread and effective. DSL stands for Digital Subscriber Line. This technology allows you to achieve a fairly high data transfer rate over copper pairs of telephone wires, while not occupying the phone. The fact is that the frequency range from 0 to 4 kHz is used for voice transmission, while signals with a frequency of up to 2.2 MHz can be transmitted over a copper telephone cable, and it is the section from 20 kHz to 2.2 MHz that uses xDSL technology . The speed and stability of such a connection is affected by the length of the cable, that is, the farther from your modem is the telephone exchange (or another modem in the case of a network), the lower the data transfer rate will be. The stability of the network is due to the fact that the data flow goes from the user directly to the node, its speed is not affected by other users. Important factor: to provide xDSL connection, you do not need to change cables, which makes it theoretically possible to connect to the Internet wherever there is a phone (depending on the availability of such a service from the provider).

The xDSL modem will act as the link between the phone cable and your devices (or router), but there are a number of features to consider when choosing a particular model that will work for you.

What is the difference between xDSL modems

xDSL technologies

ADSL technology is translated as asymmetric digital subscriber line. This means that the transmission speed of incoming and outgoing data is different. In this case, the data reception rate is 8 Mbps, and the transmission rate is 1.5 Mbps. In this case, the maximum distance from the telephone node (or another modem in the case of a network) is 6 km. But the maximum speed is possible only at a minimum distance from the node: the farther, the lower it is.

ADSL2 technology makes much better use of throughput wires. Its main difference is the ability to distribute information across multiple channels. That is, it uses, for example, an empty outgoing channel when the incoming is overloaded, and vice versa. Due to this, its data reception speed is 12 Mbps. The transmission speed remained the same as in ADSL. At the same time, the maximum distance from the telephone exchange (or other modem) is already 7 km.

ADSL2+ technology doubles the downstream speed by increasing the usable bandwidth to 2.2 MHz. Thus, the data reception rate is already equal to 24 Mbps, and the transmission rate is 2 Mbps. But such a speed is possible only at a distance of less than 3 km from the node - further it becomes similar to ADSL2 technology. ADSL2+ equipment has the advantage of being compatible with previous ADSL standards.

SHDSL technology is a standard for high-speed symmetrical data transmission. This means that the download and upload speeds are the same - 2.3 Mbps. At the same time, this technology can work with two copper pairs - then the speed doubles. The maximum distance from the telephone exchange (or other modem) is 7.5 km.

VDSL technology has the maximum data transfer rate, but is significantly limited by the distance from the node. It works in both asymmetric and symmetrical modes. In the first variant, the data reception speed reaches 52 Mbps, and the transmission speed - 2.3 Mbps. In symmetrical mode, speeds up to 26 Mbps are supported. However, high speeds are available at a distance of no more than 1.3 km from the node.

When choosing an xDSL modem, you need to focus on the distance to the telephone exchange (or other modem). If it is small, you can safely focus on VDSL, but if the node is far away, you should choose ADSL2+. If there are two copper pairs of wires, you can pay attention to SHDSL.

Annex Standards

The Annex A standard uses frequencies from 25 kHz to 138 kHz for data transmission, and from 200 kHz to 1.1 MHz for receiving data. This is a common standard for ADSL technology.

The Annex L standard extends the maximum communication distance to 7 km by increasing the power at low frequencies. But not all providers use this standard due to interference.

The Annex M standard allows you to increase the speed of the outgoing stream up to 3.5 Mbps. But in practice, the connection speed ranges from 1.3 to 2.5 Mbps. For an uninterrupted connection, this standard requires a telephone line without damage.

DHCP server

USB ports

Setup and management

Telnet is a network protocol for remote access to a computer using a command interpreter. With it, you can configure the modem from devices that are not connected to it. This is useful for small chains of modems at home and in the office.

SNMP is a standard Internet protocol for managing devices on IP networks based on the TCP/IP architecture (means for exchanging information between networked devices). Using the SNMP protocol, network device management software can access information stored on managed devices. Due to this, it is most often used in the construction of office networks.

Criterias of choice

Recall that when choosing an xDSL modem, it is important to know the characteristics of the telephone network: the length of the cable to the telephone exchange, the number of copper pairs of the cable and its quality, the offers and capabilities of the provider. It is important that there is no interference on the line, which is caused by the intersection of cable pairs or its poor quality.

IN last years The development of the telecommunications services market has led to a shortage of bandwidth for access channels to existing provider networks. If at the corporate level this problem is removed by leasing high-speed data transmission channels, then what alternative can be offered to subscribers on existing lines, instead of a dial-up connection, in the residential sector and the small business sector?

Today, the main way for end users to interact with private and public networks is access using a telephone line and modems, devices that transmit digital information over subscriber analog telephone lines - the so-called Dialup connection. The speed of such a connection is low, the maximum speed can reach 56 Kbps. This is still enough for Internet access, however, the saturation of pages with graphics and video, large volumes of e-mail and documents, the ability to exchange multimedia information among users, has set the task of increasing the bandwidth of the existing subscriber line. The solution to this issue was the development of ADSL technology.

ADSL technology (Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) is the most promising at present, at this stage of development of subscriber lines. It is included in the general group of high-speed data transfer technologies, united by the general term DSL (Digital Subscriber Line - digital subscriber line).

The main advantage of this technology is that there is no need to lay a cable to the subscriber. Already laid telephone cables are used, on which splitters are installed to separate the signal into "telephone" and "modem". Different channels are used for receiving and transmitting data: the receiving one has a significantly higher bandwidth.

The common name for DSL technologies originated in 1989, when the idea first appeared to use analog-to-digital conversion at the subscriber's end of the line, which would improve the technology for transmitting data over twisted-pair copper telephone wires. ADSL technology was developed to provide high-speed (one might even say megabit) access to interactive video services (video on demand, video games, etc.) and equally fast data transfer (Internet access, dial-up LAN and other networks). To date, DSL technologies are represented by:

• 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 telephone 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 1.5Mbps to 8Mbps and upstream data rates from 640Kbps to 1.5Mbps. 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. The transfer rate 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)

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 R-ADSL technology, the connection on different telephone lines will have different data transfer rates. The baud rate can be selected at line synchronization, during connection, or by a signal coming from the station

• G. Lite (ADSL.Lite)

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

• HDSL (High Bit-Rate 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, telecommunications companies 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 inexpensive, but effective, extension of the HDSL line length, 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 remote PBX nodes, Internet servers, local networks and so on.

• SDSL (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 3km. 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.

• SHDSL (Symmetric High Speed ​​Digital Subscriber Line - symmetrical high-speed digital subscriber line

Most modern type DSL technology is aimed primarily at providing a guaranteed quality of service, that is, at a given speed and range of data transmission, to ensure an error level of at least 10 -7 even in the most adverse noise conditions.

This standard is an evolution of HDSL as it allows the transmission of a digital stream over a single pair. SHDSL technology has several important advantages over HDSL. First of all, these are better performance (in terms of line length limit and noise margin) due to the use of a more efficient code, a precoding mechanism, more advanced correction methods and improved interface parameters. This technology is also spectrally compatible with other DSL technologies. Because the new system uses a more efficient line code than HDSL, at any rate, the SHDSL signal occupies a narrower bandwidth than the corresponding HDSL signal at the same rate. Therefore, the interference from the SHDSL system to other DSL systems is less powerful than the interference from HDSL. The spectral density of the SHDSL signal is shaped so that it is spectrally compatible with ADSL signals. As a result, compared to single-pair HDSL, SHDSL allows a 35-45% increase in transmission speed at the same range, or a 15-20% increase in range at the same speed.

• 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.

• VDSL (Very High Bit-Rate Digital Subscriber Line)

VDSL technology is the "fastest" xDSL technology. It provides downstream data transfer rates in the range from 13 to 52 Mbps, and upstream data transfer rates in the range from 1.5 to 2.3 Mbps, with one twisted pair of telephone wires. In symmetrical mode, speeds up to 26Mbps 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. The technology is not standardized, different equipment manufacturers have different speeds.

So what exactly is ADSL? First of all, ADSL is a technology that allows you to turn a twisted pair of telephone wires into a high-speed data transmission path. The ADSL line connects the provider side DSLAM (DSL Access Multiplexor) access equipment and the client modem, which are connected to each end of the twisted pair telephone cable (see Figure 1). In this case, three information channels are organized - "downstream data transfer," upstream "data transfer and a regular telephone communication channel (POTS) (see Figure 2). This scheme allows you to talk on the phone simultaneously with the transfer of information and use telephone communications in the event of a malfunction of ADSL equipment.Constructively, the telephone splitter is a frequency filter that can be either integrated into an ADSL modem or be an independent device.

Rice. 1


Rice. 2

ADSL is an asymmetric technology - the rate of the "downstream" data stream (ie the data that is transmitted towards the end user) is higher than the rate of the "upstream" data stream (in turn transmitted from the user to the network side). It should immediately be said that one should not look for a cause for concern here. The data transfer rate from the user (the "slower" data transfer direction) is still significantly higher than when using an analog modem. Such asymmetry is introduced artificially, the modern range of network services implies a very low transmission speed from the subscriber. For example, MPEG-1 movies require 1.5 Mbps of bandwidth. For service information transmitted from the subscriber (command exchange, service traffic), 64-128 Kbps is quite enough. According to statistics, incoming traffic is several times, and sometimes an order of magnitude, higher than outgoing. This ratio of speeds ensures optimal performance.

ADSL technology uses digital signal processing and specially designed algorithms, advanced analog filters and analog-to-digital converters to compress the large amount of information transmitted over twisted-pair telephone wires. Long distance telephone lines can attenuate a transmitted high frequency signal (for example, at 1MHz, which is the normal transmission rate for ADSL) by up to 90dB. This forces the analog ADSL modem systems to work with a large enough load to allow high dynamic range and low noise. At first glance, the ADSL system is quite simple - high-speed data transmission channels are created over a regular telephone cable. But, if you understand in detail the work of ADSL, you can understand that this system belongs to the achievements modern technology.

ADSL technology uses a method of dividing the bandwidth of a copper telephone line into multiple frequency bands (also called carriers). This allows multiple signals to be transmitted simultaneously on a single line. Exactly the same principle underlies cable television, when each user has a special converter that decodes the signal and allows you to see a football match or an exciting movie on the TV screen. With ADSL, different carriers simultaneously carry different parts of the transmitted data. This process is known as frequency division multiplexing (FDM) (see Figure 3).

Rice. 3

With FDM, one band is allocated for the transmission of "upstream" data, and the other band for the "downstream" data stream. Information "downward" flow is divided into several information channels - DMT (Discrete Multi-Tone), each of which is transmitted on its own carrier frequency using QAM. QAM is a modulation method - Quadrature Amplitude Modulation, called Quadrature Amplitude Modulation (QAM). It is used to transmit digital signals and provides for a discrete change in the state of the carrier segment simultaneously in phase and amplitude. Typically, DMT splits the 4 kHz to 1.1 MHz band into 256 channels, each 4 kHz wide. This method, by definition, solves the problem of dividing the band between voice and data (it simply does not use the voice part), but is more difficult to implement than CAP (Carrierless Amplitude and Phase Modulation) - amplitude-phase modulation without carrier transmission. DMT is approved in the ANSI T1.413 standard and is also recommended as the basis for the Universal ADSL specification. In addition, Echo Cancellation technology can be used, in which the upstream and downstream ranges overlap (see Figure 3) and are separated by local echo cancellation.

This is how ADSL can provide, for example, simultaneous high-speed data transmission, video signal transmission and fax transmission. And all this without interrupting the usual telephone connection, for which the same telephone line is used. The technology provides for the reservation of a certain frequency band for ordinary telephone communication (or POTS-Plain Old Telephone Service). It is amazing how quickly telephone communication has turned not only into "simple" (Plain), but also into "old" (Old); it turned out something like "the good old telephone connection". However, one should pay tribute to the developers of new technologies, who still left telephone subscribers with a narrow band of frequencies for live communication. In this case, a telephone conversation can be carried out simultaneously with high-speed data transmission, and not choose one of the two. Moreover, even if your electricity is turned off, the usual "good old" telephone service will still work and you will not have any problems with calling an electrician. Making this possible was part of the original ADSL development plan.

One of the main advantages of ADSL over other high-speed data transmission technologies is the use of the most common twisted-pair copper wire telephone cables. It is quite obvious that there are much more such pairs of wires (and this is still an understatement) than, for example, cables laid specifically for cable modems. ADSL forms, so to speak, an "overlay network".

ADSL is a high speed data transfer technology, but how fast? Given that the letter "A" in the ADSL name means "asymmetric" (asymmetric), we can conclude that data transfer in one direction is faster than in the other. Therefore, there are two data rates to consider: "downstream" (transfer of data from the network to your computer) and "upstream" (transfer of data from your computer to the network).

The maximum reception speed - DS (down stream) and transmission - US (up stream) depends on many factors, the dependence on which we will try to consider later. IN classic version, ideally, the reception and transmission speed depends and is determined by DMT (Discrete Multi-Tone) splitting the bandwidth from 4 kHz to 1.1 MHz into 256 channels, each 4 kHz wide. These channels, in turn, represent 8 digital streams T1, E1. For downstream transmission, 4 T1,E1 streams are used, the total maximum throughput of which is 6.144Mbps - in the case of T1 or 8.192Mbps in the case of E1. For upstream transmission, one T1 stream is 1.536 Mbps. The maximum speed limits are indicated without taking into account overhead costs, in the case of classic ADSL. Each stream is provided with an error correction code (ECC) by introducing an extra bit.

Now let's look at how the real data transfer takes place in the following example. Informational IP-packets generated both in the local networks of clients and personal computers directly connected to the Internet will arrive at the input of the ADSL modem framed by the Ethernet 802.3 standard. The subscriber modem splits and "stacks" the content of Ethernet 802.3 frames into ATM cells, supplies the latter with a destination address and transmits them to the output of the ADSL modem. The one, in accordance with the T1.413 standard, "encapsulates" the ATM cells in the digital stream E1, T1, and then the traffic over the telephone line goes to the DSLAM. Station concentrator DSL multiplexor - DSLAM, carries out the procedure of "restoring" ATM cells from the T1.413 packet format and sends them via the ATM Forum PVC (Permanent Virtual Circuit) protocol to the backbone access subsystem (ATM network), which delivers the ATM cells at the address indicated in them, i.e. to one of the centers for the provision of services. When implementing the Internet access service, the cells arrive at the router of the Internet provider, which performs the function of a terminal device in a permanent virtual channel (PVC) between the subscriber terminal and the node of the Internet provider. The router performs the reverse (in relation to the subscriber terminal) transformation: it collects incoming ATM cells and restores the original Ethernet 802.3 frame. When traffic is transmitted from the service center to the subscriber, completely similar transformations are carried out, only in the reverse order. In other words, a "transparent" Ethernet 802.3 local network is created between the Ethernet port of the subscriber terminal and the virtual port of the router, and all computers connected to the subscriber terminal perceive the router of the Internet provider as one of the devices of the local network.

The common denominator in the provision of Internet access services is the IP network layer protocol. Therefore, the chain of protocol transformations carried out in a broadband access network can be represented as follows: client application - IP packet - Ethernet frame (IEEE 802.3) - ATM cells (RFC 1483) - ADSL modulated signal (T1.413) - ATM cells (RFC 1483 ) - Ethernet frame (IEEE 802.3) - IP packet - application on a resource on the Internet.

As mentioned above, the declared speeds are possible only in the ideal version and without taking into account overhead costs. So in the E1 stream, when transmitting data, one channel (depending on the protocol used) is used to synchronize the stream. And as a result, the maximum speed, taking into account overhead costs, will be Down stream - 7936Kbps. There are other factors that have a significant impact on the speed and stability of the connection. These factors include: the length of the line (the bandwidth of the DSL line is inversely proportional to the length of the subscriber line) and the cross section of the wire. The characteristics of the line deteriorate with an increase in its length and a decrease in the cross section of the wire. Also, the data transfer rate is affected by the general condition of the subscriber line, the presence of twists, cable outlets. The most "harmful" factors that directly affect the possibility of establishing an ADSL connection are the presence of Pupin coils on the subscriber line, as well as a large number of taps. None of the DSL technologies can be used on lines with Load Coils. When checking the line, it is ideal not only to determine the presence of load coils, but also to find the exact place of their installation (you still have to look for coils and remove them from the line). The load coil used in analog telephone systems is a 66 or 88 mH inductor. Historically, Pupin coils were used as a structural element of a long (more than 5.5 km) subscriber line, which made it possible to improve the quality of transmitted audio signals. A cable outlet is usually understood as a cable section that is connected to the subscriber line, but is not included in the direct connection of the subscriber with the telephone exchange. The cable outlet is usually connected to the main cable and forms a "Y" shaped branch. It often happens that the cable outlet goes to the subscriber, and the main cable goes further (in this case, this pair of cables must be open at the end). However, the suitability of a particular subscriber line for the use of DSL technology is affected not so much by the fact that there is a connection, but by the length of the cable outlet itself. Up to a certain length (about 400 meters), cable outlets do not significantly affect xDSL. In addition, cable taps affect different xDSL technologies differently. For example, HDSL technology allows cable outlets up to 1800 meters. As for ADSL, cable outlets do not prevent the very fact of organizing high-speed data transmission over a copper subscriber line, but they can narrow the line bandwidth and, accordingly, reduce the transmission speed.

The advantages of a high-frequency signal, which makes it possible to digitally transmit data, are its own disadvantages, namely, exposure to external factors (various pickups from third-party electromagnetic devices), as well as physical phenomena that occur in the line during transmission. Increasing the capacitive characteristics of the channel, the occurrence of standing waves and reflections, the isolation characteristics of the line. All these factors lead to the appearance of extraneous noise on the line, and faster signal attenuation and, as a result, to a decrease in the data transfer rate and a decrease in the length of the line suitable for data transmission. Some values ​​of the characteristics of the ADSL line, by which you can directly judge the quality of the telephone line, can be given by the ADSL modem itself. Almost all models of modern ADSL modems contain information about the quality of the connection. Most often, the Status-> Modem Status tab. The approximate content (may vary depending on the model and manufacturer of the modem) is as follows:

modem status

Connection Status Connected
Us Rate (Kbps) 511
Ds Rate (Kbps) 2042
US Margin 26
DS Margin 31
Trained Modulation ADSL_2plus
LOS Errors 0
DS Line Attenuation 30
US Line Attenuation 19
Peak Cell Rate 1205 cells per sec
CRC Rx Fast 0
CRC Tx Fast 0
CRC Rx Interleaved 0
CRC Tx Interleaved 0
Path Mode Interleaved
DSL Statistics

Near End F4 Loop Back Count 0
Near End F5 Loop Back Count 0

Let's explain some of them:

Connection Status Connected - connection status
Us Rate (Kbps) 511 - Up Stream speed
Ds Rate (Kbps) 2042 - Down Stream rate
US Margin 26 - Outgoing connection noise level in db
DS Margin 31 - Downlink noise level in db
LOS Errors 0 -
DS Line Attenuation 30 - Downstream signal attenuation in db
US Line Attenuation 19 - Signal attenuation in the outgoing connection in db
CRC Rx Fast 0 - number of uncorrected errors. There are also FEC (corrected) and HEC - errors
CRC Tx Fast 0 - number of uncorrected errors. There are also FEC (corrected) and HEC - errors
CRC Rx Interleaved 0 - number of uncorrected errors. There are also FEC (corrected) and HEC - errors
CRC Tx Interleaved 0 - number of uncorrected errors. There are also FEC (corrected) and HEC - errors
Path Mode Interleaved - Error correction mode enabled (Path mode Fast - disabled)

By these values, you can judge, as well as control yourself, the state of the line. Values:

Margin - SN Margin (Signal to Noise Margin or Signal to Noise Ratio). The level of interference noise depends on many different factors - wetting, the number and length of taps, the synchronism of the line, the "spread-out" of the cable, the presence of twists, the quality of the physical connections. In this case, the signal of the outgoing ADSL stream (Upstream) decreases until it is completely absent and, as a result, the ADSL modem loses synchronization

Line Attenuation - attenuation value (the greater the distance from the DSLAMa, the greater the attenuation value. The greater the signal frequency, and hence the connection speed, the greater the attenuation value).

What is ADSL

What lies behind this mysterious word:​

How does ADSL work?​

ADSL - broad channel to the future. ​

The modern world is unthinkable without the Internet and computer networks. High-speed channels have entangled the world in a web - satellites, optical fiber, cables - the nerves and blood vessels of the worldwide information network. Giant speeds, gigantic traffic, high technologies... But at the same time, for many years, high-speed channels with a data transfer rate of more than 1 megabit per second remained the lot of providers and large companies.
High technologies developed by leading Hi-Tech companies for high-speed data transmission turned out to be very expensive, having not only a huge implementation cost, but also a high cost of ownership. To gain access to the Internet, ordinary users had to be content with ordinary, very common and cheap Dial Up modems designed for use on analog telephone lines. Yes, and business, especially small ones, did not see the need to lay dedicated channels or install satellite Internet for themselves - expensive and inefficient. What to download at high speeds - news, prices, documents, kilobyte drivers? Over two decades of Dial Up access rules "last mile" - the very site through which information is delivered from the provider to the end user. Telephone lines, especially Russian ones, have become a wall in the way between users and providers owning high-speed data transmission channels. So an awkward picture turned out - between cities, countries and continents huge amounts of information were sent instantly, but on the last kilometer, on the last piece of telephone wire from the provider to the client, the speed dropped by orders of magnitude and the information came to the end user in uneven torn portions, moreover, with a constant disconnectome.
For a long time, the possibilities of Dial Up modems suited many people. This technology, developed at the dawn of the computer age for analog telephone lines, has evolved extremely slowly and slowly - over the past 15 years, the data transfer rate has increased from 14,400 Kbps to just 56,000 Kbps. Long years it seemed that this speed was enough for almost everything - download an HTML web page, a text document, beautiful picture, a patch for a game or program, or drivers for new devices, the size of which for a number of years did not exceed a few hundred kilobytes - all this did not take much time and did not require high-speed connections. But life has made its own adjustments.
Development of modern computer technology in addition to the increase in the frequency of central processors, the revolution in the field of 3D graphics accelerators and the explosive increase in the capacity of information storage devices, it also led to a dramatic increase in the volume of information sent. Computer evolution, which followed the principle of "bigger, higher, faster", has led to the fact that programs and files have grown to monstrous sizes. For example, a Word document that has now become a standard is dozens of times larger than a similar TXT file, the widespread introduction of 32-bit color has led to an increase in the size of pictures and video files at times, high quality sound, but Lately the bitrate of MP3 files from the standard 128 Kbps has risen to 192 Kbps, which also noticeably affects the size. Yes, compression algorithms that have been significantly improved lately help to some extent, but this is still not a panacea. Driver sizes have recently grown to gigantic sizes, for example, Detonator FX from nVidia takes about 10 megabytes (despite the fact that two years ago they occupied only 2 megabytes), and unified drivers for the nForce platform of the same company are already 25 megabytes, and this the trend captures an increasing number of manufacturers of computer hardware. But the main trouble that causes Dial Up modems to heat up, not giving them a minute of rest, is software patches or patches that correct errors in software. The widespread introduction of rapid development tools has led to the mass release of raw, unoptimized programs. And why optimize the program if the computer hardware is redundant anyway? Why engage in beta testing of the program, if there is an Internet network - just sell the raw program, then look at the list of the most frequently occurring problems and errors that users themselves will make when contacting support and then release a patch, after it another, third, and so on ad infinitum . Involuntarily, with nostalgia, we recall the times when the Internet was the lot of a handful of the elite, and unspoiled worldwide network programmers licked their programs to the last byte, knowing that after their product went to the end user, nothing could be fixed. Programs came out much less frequently, but they worked like a Swiss watch. And now, sadly looking at, for example, the fourth (!) Microsoft patch for Windows 2000 with a size of 175 megabytes, you understand that Dial Up access will not drain this lump even in a week, and how much will this patch cost if hourly pay! But there are also Microsoft Office and dozens of other programs that need fixing. And the gigantic deposits of music and videos on the Internet! I want to bite my elbow at the thought of all these treasures information technologies, which are practically not available to dialers.
All these gloomy thoughts lead to the idea that Dial Up Internet access has become obsolete and urgently needs to be replaced. What can replace moribund technologies? The classic ISDN (Integrated Services Digital Network) and the relatively new satellite Internet immediately come to mind. They come at once, but after much thought, both disappear. ISDN disappears due to the high cost of laying a dedicated channel, which is inappropriate in an apartment and the high cost of ownership ( subscription fee+ payment for traffic). In principle, this type of access is possible when laying a home network, when several users share a high-speed channel for themselves, and then distribute it over apartment building through the local network. But as the further material of the article will show, ISDN has a powerful competitor, nullifying all the advantages of this technology. Satellite Internet, of course, looks very attractive, but there are nuances, and not always pleasant ones. Yes, the satellite captures a large area of ​​the Earth's surface, but you need to see if the satellite of the provider providing this service in your area is visible and at what angle it is visible, it depends on what size satellite dish you have to install. In addition, the satellite channel is still not very fast - the best of them provide about 400 Kbps towards the user (for ordinary users, of course, there are higher-speed options, but they are several orders of magnitude more expensive). Data transfer from the user to the provider is carried out by phone, so the phone line is just as busy as when using a Dialup modem. Satellite systems of different providers have a number of common disadvantages, which are the high cost of the equipment used and the complexity of its installation and configuration. In addition, satellite providers are, to put it mildly, not reliable enough. There are reasons for this, both objective (satellites are not eternal, a telecommunications satellite will fall into the dense layers of the atmosphere when a replacement is put into the same orbit), and subjective - remember the fiasco satellite internet NTV +, which, it turns out, threw thousands of its users, leaving them with useless receivers.
It would be nice to have the same ISDN, but without any leased lines, but directly on a telephone copper cable. After all, a subscriber telephone line is nothing more than a cable for the network. Yes, the quality is terrible, but you can develop new data transfer technologies, convert everything to digital, modulate everything in a special way, correct errors that occur and get a broadband digital channel as a result. So it turns out that all hope for progress. And dreams and hopes turned out to be not at all fruitless - a holy place does not happen empty, and progress does not stand still - they received a technology that combines the best features of both Dial Up modems operating on analog telephone lines and high-speed IDSN modems. Meet - ADSL technology.

ADSL - what is it? ​

ADSL - how does it all work? ​

How fast is ASDL technology? ​

to be continued... ​

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For comparison, consider other technologies. ​

Glossary.

subscriber line- a pair of copper wires from the ATC to the user's phone. You can also meet its English designation - LL (Local Loop). Previously used exclusively for telephone conversations. With the advent of Dial Up modems, it has long served as the main channel for accessing the Internet, and is now used for the same purposes by ADSL technology.

analog signal- a continuous oscillatory signal, characterized by such concepts as frequency and amplitude. Analogue signals with specified frequencies are used to control telephone connections, such as a busy signal. A simple telephone conversation is a kind of analog signal with constantly changing frequency and amplitude parameters.

digital signal- digital signal, in contrast to analog intermittent (discrete), the signal value changes from minimum to maximum without transition states. The minimum value of the digital signal corresponds to the state "0", the maximum value "1". Thus, the digital transmission of information uses a binary code, the most common among computers. A digital signal, unlike an analog signal, cannot be distorted even in conditions of strong noise and interference on the line. In the worst case, the signal will not reach the end user, but the error correction system, which is present in the vast majority of digital communications equipment, will detect the missing bit and send a request to resend the corrupted piece of information.

Modulation- the process of converting data into a signal of a certain frequency, intended for transmission over a subscriber line, over a special cable, or, for wireless systems, over radio waves. The process of inverse transformation of the modulated signal is called demodulation.

carrier frequency- a special high-frequency signal of a certain frequency and amplitude separated from other frequencies by silence bands.

Cable modems- modems using cables from existing cable television networks. These networks are shared networks, that is, the data transfer rate is highly dependent on the number of users simultaneously on the network. Therefore, although the maximum speed of cable modems reaches 30 Mbps, in practice it is rarely possible to get more than 1 Mbps.
P.S. If any terms in the article are not clear to you, write, the glossary will be expanded.

ADSL (Asymmetric Digital Subscriber Line) technology is a type of xDSL technology that provides users with an affordable broadband transmission medium between relatively close network nodes.
ADSL research and development was spurred on by investments from telephone companies, which, unlike conventional broadcast television, wanted to provide on-demand video programming to users. Progress in the development of ADSL technology has made it suitable not only for digital television broadcasting, but also for many other high-speed interactive applications such as access to the Internet, delivery of corporate information to remote offices and branches, as well as audio and video information on demand. Under the best operating conditions and acceptable distances, ADSL technology can transmit data at speeds up to 6 Mbps in the forward direction (up to 9 Mbps in some versions) and 1 Mbps in the reverse direction.

ADSL equipment transfers data approximately 200 times faster than conventional analog modems, which have an average sustained transmission rate of about 30 Kbps, and in the same physical distribution medium.

Network Computing magazine employees tested ADSL modems manufactured by Amati Communications (ATU-C and ATU-R), Aware (Ethernet Access Modem) and Paradyne (5170/5171 ADSL Modem) in the MCI Developers Lab and evaluated the benefits of their work and disadvantages of ADSL technology.

As a result, when testing ADSL devices with a fairly large load, no significant flaws were identified, so from an engineering point of view, this technology is ready for implementation. Considering that the cost of equipment and services of any technology decreases as it is implemented, it makes sense to start negotiations with telephone companies now.

Additional wiring is not needed. ​

Signals are transmitted over a pair of wires between two ADSL modems installed at a remote network node and at a local PBX. An ADSL network modem converts digital data from a computer or other device into an analog signal suitable for transmission over twisted pair. For parity, redundant bits are inserted into the transmitted digital sequence. This guarantees the reliability of information delivery to the telephone exchange, where this sequence is demodulated and checked for errors.

However, it is not necessary to bring the signal to the telephone exchange. For example, if branch offices are located within a small town, pairs of wires are used between them. In this case, the "remote" ADSL modem operating in receive mode and the "central" transmitting ADSL modem can be connected by a copper wire without any additional intermediate elements between them. The connection of offices spaced over long distances from one another, provided that each of them is located relatively close to "its" automatic telephone exchange, is carried out using trunk lines provided by telephone companies.

The use of ADSL technology allows you to send several types of data at different frequencies at the same time. We were able to select the best transmission frequency for each specific application (for data, speech and video). Depending on the encoding method used in a particular implementation of ADSL, the signal quality is affected by the length of the connection and electromagnetic interference.

With the combined use of a data line and telephony, the latter will work without additional power supply, as is necessary in the case of ISDN. In the event of a power failure, conventional telephony will continue to operate using the current supplied to the line by the telephone company. However, ADSL modems must be connected to AC power to transmit data.

Most ADSL devices are designed to work with a frequency splitter used in Plain Old Telephone Service (POTS) called a frequency splitter. These functional features of ADSL give it a reputation as a reliable technology. It is also harmless, because in the event of an accident it does not have any effect on the operation of telephony. ADSL seems pretty elementary technology, in fact, that is what it is. Installing and running it is not difficult. Simply connect the device to the network and telephone line, and leave the rest to the telephone company.

However, this technology has some features that you need to consider when creating and operating your network. For example, ADSL devices can be affected by some of the physical factors inherent in signaling over a pair of wires. The most important of these is line attenuation. In addition, the reliability and throughput of the data transmission channel can be affected by significant electromagnetic interference on the cable, especially from the network of the telephone company itself.

Line coding types ​

DMT modulation is considered the best, as it provides more flexible bandwidth control and is easier to implement. For the same reason, the American National Standards Institute (ANSI) adopted it as the standard for ADSL line coding.

However, many do not agree that DMT modulation is better than CAP, so we decided to test both of them. And although the modems used in our tests were early implementations, they all worked perfectly. As a result, we were convinced of the following: DMT-based ADSL modems are indeed more stable during signal transmission and can operate over long distances (up to 5.5 km).

It should be noted that users only need to worry about the channel line coding method between modems (for example, from your office to the service provider's PBX). If these devices are used in packet-switched networks, such as the Internet, it is not your business to worry about possible conflicts between network nodes.

For testing, we used a copper pair with 24 gauge wire, which has a signal attenuation of 2-3 dB per 300 m. According to the specification, the length of the ADSL line should not exceed 3.7 km (attenuation is about 20 dB), but good ADSL- modems can function reliably over much longer distances. We also found that the actual range of most modems exceeds 4.6 km (26 dB). DMT-based ADSL modems operated at the maximum distance possible in our conditions - 5.5 km - at speeds of 791 Kbps in the forward direction and 582 Kbps in the reverse direction (measured signal attenuation in the line is 31 dB).

Both CAP-based ADSL modems operated at 4 Mbps forward and 422 Kbps in the reverse direction over a distance of 3.7 km. At a lower speed (2.2 Mbps), only one modem worked at a distance of 4.6 km.

In addition to the ones just described, we conducted tests in which we reproduced real conditions on the lines, for example, we checked the work with bridge taps, which are often used in telephony. A spur bridge is an open telephone line that branches away from the main line. As a rule, this additional line is not used and, therefore, does not create additional crosstalk in the main line, but significantly increases the attenuation in it. Therefore, it is surprising that some of the modems tested worked fine with a spur length of 1.5 km and a main line length of 3.7 km. With an increase in the length of the main line to 4.6 km, the reliability of signal transmission became below the permissible level only in the case of an increase in the length of the branch line to 300 m.

Electromagnetic interference ​

The electromagnetic field emitted by some wires interferes with other wires and causes data transmission errors. For the modems we tested, the impact of an adjacent busy T1 line on the ADSL data stream was minimal, and the signaling quality of the ADSL and T1 lines was not degraded. This impact on the PBX is likely to be exacerbated if multiple T1 lines and multiple ADSL lines are interleaved with each other. When laying ADSL lines, the telephone company must take into account this line interference.

Another interference that occurs during signal transmission over an ADSL line is amplitude modulation noise (Amplitude Modulation - AM). It is similar to the noise that occurs on a line passing near powerful electrical appliances, such as refrigerators and laser printers, or near powerful motors installed in the elevator shaft. The MCI engineers testing the modems applied up to 5 volts of pulse voltage to the twisted-pair cable running parallel to our ADSL line, but the bit error rate remained at an acceptable level. In fact, such an impact on modems in our tests could be neglected.

In our opinion, before widespread adoption ADSL technology in public networks is about a year away. It is currently under development and the possibility of its application is being evaluated. However, ADSL technology is already being used in the networks of corporations and small towns. Many firms have begun to produce products for ADSL. The high bandwidth and noise immunity of the first versions of ADSL modems that participated in our tests confirmed their high reliability. Now, when you modernize your network and increase the number of users, you can no longer neglect ADSL technology.

What is ADSL (another article) ​

ADSL (Asymmetric Digital Subscriber Line - Asymmetric Digital Subscriber Line) is one of the high-speed data transmission technologies known as DSL (Digital Subscriber Line) technologies and has general designation xDSL.
The name DSL technologies originated in 1989, when the idea first appeared to use analog-to-digital conversion at the subscriber's end of the line, which would improve the technology for transmitting data over twisted-pair copper telephone wires. ADSL technology was developed to provide high-speed access to interactive video services (video on demand, video games, etc.) and equally fast data transfer (Internet access, dial-up LAN and other networks).

Do you need an ADSL line? ​

It's up to you, but for you to accept correct solution Let's take a look at the benefits of ADSL.​

ADSL and SDSL ​

Asymmetric and balanced DSL lines​

DSL technology allows the user's premises to be connected to the central office (Central Office, CO) of the service provider via pre-existing copper telephone lines. If the lines meet the established requirements, then with the help of DSL modems the transfer rate can be increased from the mentioned 56.6 Kbps to 1.54 Mbps or more. However, the main disadvantage of DSL lines is that the ability to use them depends largely on the distance to the service provider's node.

DSL is not one technology for all occasions, it has many varieties, although some of them may not be available in a particular area. DSL variants usually follow one of two basic schemes, although they may differ in specific characteristics. Two main models - asymmetric (Asymmetric DSL, ADSL) and symmetrical (Symmetric DSL, SDSL) digital subscriber line - stood out in the early stages of technology development. In the asymmetric model, data flow is preferred in the forward direction (from the provider to the subscriber), while in the symmetric model, the flow rate in both directions is the same.

Private users prefer ADSL, while organizations prefer SDSL. Each system has its own advantages and limitations, rooted in different approach to symmetry.

ABOUT ASYMMETRY​

The asymmetric model fits well with the way the Internet works: large amounts of multimedia and text are transmitted in the forward direction, while the level of traffic in the reverse direction is negligible. US ADSL costs typically range from $40 to $200 per month, depending on expected data rates and service level guarantees. Cable modem-based services are often cheaper, around $40 a month, but the lines are shared by customers, as opposed to dedicated DSL.

The carrier line is able to support ADSL along with analog voice by allocating the digital signals to frequencies outside the frequency spectrum for the transmission of conventional telephone signals (see Figure 1), which requires the installation of a divider. The divider uses a low-pass filter to separate telephone frequencies at the lower end of the audio spectrum from the higher frequencies of ADSL signals. The available ADSL bandwidth remains intact regardless of whether analog frequencies are used. To support maximum ADSL speeds, splitters must be installed both at the user's premises and at the central site; they do not require power and therefore will not interfere with "life-saving" voice service in the event of a power loss.

Determining ADSL speeds is more of an art than a science, although speed reductions occur at fairly predictable intervals. Providers provide the best possible service, with results highly dependent on distance from the central hub. Usually "best possible" means that the ISPs guarantee 50% throughput. Attenuation and interference such as crosstalk become significant on links longer than 3 km, and at distances greater than 5.5 km they can make the lines unsuitable for data transmission.

At distances up to 3.5 km from the central node, ADSL speeds can reach 7.1 Mbps in the forward direction and 1.5 Mbps in the direction from the subscriber to the CO. However, DSL Reports editor Nick Braak believes that the upper limit is unattainable in practice. Braak states, "Actually, 7.1 Mbps is impossible to achieve, even under laboratory conditions." At distances over 3.5 km, the ADSL speed is reduced to 1.5 Mbps in the forward direction and to 384 Kbps - from the subscriber to the CO; as the length of the subscriber line approaches 5.5 km, the speed drops even more significantly - up to 384 Kbps in the forward direction of the flow and up to 128 Kbps - in the reverse direction.

Service contracts for ADSL services may contain a clause for the user to opt out of connecting to home networks or Web servers. However, DSL technology alone does not prevent home LANs from being connected. For example, even if an ISP provides a customer with a single IP address, using Network Address Translation (NAT) multiple users can share that single IP address.

One DSL connection is sufficient for a home with many computers. Some DSL modems have a built-in DSL hub as well as specialized devices called "resident gateways" that act as bridges between the Internet and home networks.

ADSL uses two ADSL modulation schemes: Discrete Multitone (DMT) and Carrierless Amplitude and Phase (CAP).

DMT provides for splitting the spectrum of available frequencies into 256 channels in the range from 26 to 1100 kHz, 4.3125 kHz each.

to be continued... ​

Attachments

Another important point regarding DSL, like any other communication channel, is security. Unlike cable modems, DSL users receive dedicated connections that are not affected by the activity of other users. Neighbors do not occupy the same lines as you, as is the case with cable modems, which is certainly a plus in terms of security. However, both technologies can be at risk of intrusion and denial-of-service attacks due to persistent connections and fixed IP addresses.

If data transmission systems could ever turn into living organisms, then the copper "twisted pair" would be the most tenacious of them. The Last Mile is a large and growing market, especially sensitive to affordable technologies with high supported bandwidth.

Free, unlimited, broadband access for everyone is impossible in our life, but if you are going to purchase DSL services, then you are going in the right direction.

speed and modulation.
ADSL connection speed.

First:
That the unit of information is a byte, there are 8 bits in one byte. Thus, when you download files, keep in mind that if your download speed is shown as, for example, 0.8 Mb / s (Megabytes per second), then the real speed is 0.8x8 = 6.4 Mbps (Megabits per second) !

Second:
The higher the set speed, the greater the likelihood of communication instability! The most stable speed is 6144 Kbps incoming and 640 Kbps outgoing with G.DMT modulation. For the Internet, high speed is not needed in principle - you simply will not feel the difference between 6144 Kbps and 24000 Kbps. However, when using the IP-TV service, you need to know that one channel occupies a bandwidth of 4-5 megabits per second. Therefore, if you want to watch IP-TV and have an Internet connection at the same time, then please note that for the Internet, the channel width will decrease by the amount indicated above. In addition, if for some reason you need to download information simultaneously in several streams, it also makes sense for you to ask to increase the speed.
Although you can ask to increase or decrease the speed by calling technical support at 062 (this is done right away!).

What are the characteristics of modulations.
Question: What are the characteristics of modulations?
Answer:
G.dmt is an asymmetric DSL modulation based on DMT technology, which provides data transfer rates up to 8 Mbps in the direction of the user, and up to 1.544 Mbps in the direction away from the user.

G.lite is a modulation based on DMT technology, which provides data transfer rates up to 1.5 Mbps towards the user, and up to 384 Kbps in the direction away from the user. "

ADSL - modulation provides a data transfer rate in the direction of the user up to 8 Mbps, and in the direction from the user up to 768 Kbps.

T1.413 is a discrete asymmetric multitone modulation based on the G.DMT standard. Accordingly, the speed limit is approximately the same as in the G.dmt modulation.

ADSL2+ ​

A rather funny situation has developed in our country. When there was a boom in ADSL providers around the world and almost no interest in home networks ETTH (Ethernet To The Home), in our country such networks began to be actively built. At the moment, the whole world is slowly beginning to realize that the development of multimedia and especially High-Definition (HD) content is severely limited by the speed capabilities of xDSL networks, and in Russia ETTH is already available in all major cities. Thus, we sort of stepped over one stage of network development (ADSL providers developed in parallel with ETTH, but there was no obvious dominance) and ended up among the leaders. You have to, at least in something! But that's not what we're going to discuss today. As you know, ADSL technology already exists in the second version and even in 2+. We will talk about their differences from a technical point of view and prospects in the Internet provider market.

General concepts

Let's briefly refresh our memory on the main distinguishing features of ADSL technology. It belongs to the xDSL family of standards designed to provide high speed data transfer over already existing telephone lines. Despite the fact that ADSL is far from being the “fastest” technology from the xDSL family, it is precisely this technology that has become most widespread in the world due to the optimal combination of speed and range.

The ADSL channel is asymmetric, that is, the upstream (from the user to the provider) and downstream (in the opposite direction) flows are not equivalent. Moreover, the equipment on both sides is different. On the user side, this is a modem, and on the provider side, it is a DSLAM (ADSL switch).

While only three versions of ADSL (ADSL, ADSL2, and ADSL2+) are widely known, there are actually many more specifications. I propose to take a look at the table, which presents all the major ADSL standards. By and large, the specifications differ in operating frequencies and are needed to enable the operation of ADSL technology on various types of telephone lines. For example, Annex A uses a frequency band ranging from 25 kHz to 1107 kHz, while Annex B's operating frequencies start at 149 kHz. The first was designed to transmit data over public telephone networks (PSTN or POTS, in English), and the second was designed to work together with ISDN networks. In our country, Annex B is most often used in apartments with burglar alarms, which also use frequencies above 20 kHz.

As is known, ADSL uses Quadrature Amplitude Modulation (QAM) with Orthogonal Frequency Multiplexing (OFDM). Without going into technical details, on the fingers, the situation is something like this: the available bandwidth (fits into the frequency range of 25-1107 kHz) is divided into channels (25 for transmission and 224 for reception); a portion of the signal is transmitted through each of the channels, which is modulated using QAM; further, the signals are multiplexed using the fast Fourier transform and transmitted to the channel. On the reverse side, the signal is received and processed in reverse order.

QAM, depending on the quality of the lines, encodes words of various depths and sends them to the channel at a time. For example, the QAM-64 algorithm used in ADSL2 uses 64 states to send an 8-bit word at a time. Moreover, ADSL uses the so-called equalizing mechanism - this is when the modem constantly evaluates the quality of the line and adjusts the QAM algorithm to a greater or lesser word depth to achieve greater speed or better communication reliability. Moreover, equalizing works for each channel separately.
ADSL2+ ​

In addition to this, in ADSL2+ implemented the possibility of combining ports (port bonding). Thus, by combining two lines into one logical channel, you will get a throughput of 48/7 Mbps. This, of course, is a rarity, but if there are two telephone numbers in the apartment, this is quite real. Or, alternatively, you can get a double increase in speed on one physical line in the case of using a cable with two copper pairs, crimped with an RJ-14 connector.

Instead of a conclusion

What would you like to say in the end? The advantages of the new standards are, in fact, more than obvious. From the point of view of an ordinary user, this is an increase in the speed threshold, which “pulled up” the ADSL speed to the level of cable networks. Purely nominally, both of them are capable of transmitting HD content. But as practice shows, where high-quality ETTH has reached, ADSL and cable companies are gradually starting to lose ground, feeling at ease only in the absence of serious competition. It would seem, why do we need such high speeds, because in many regions of our country a massive transition from dial-up access to broadband is just beginning? According to some forecasts, by 2010 traffic prices will drop by 3-4 times. And if the speed of the incoming channel (ADSL2+ - 24 Mbps) has a significant margin, then the low speed of the reverse channel (ADSL - 1 Mbps, ADSL2+ - 3.5 Mbps) severely limits ADSL users. For example, one of the main advantages of ETTH networks - internal resources - is technically possible to implement in ADSL, but the relatively low upload speed is a serious obstacle to fast internal file exchange between users. This also affects the efficiency of work in peer-to-peer networks, where users of large ETTH providers can often download files at speeds close to 100 Mbps.

Of course, ADSL has a future, and its "overclocked" versions will allow you to freely use fast internet just a couple more years. And what will happen next? Wait and see.

Glossary

Modulation– change in the parameters (phase and/or amplitude) of the modulated oscillation (high-frequency) under the influence of a control (low-frequency) signal.
Quadrature Amplitude Modulation (QAM) - with this type of modulation, information in the signal is encoded by changing both its phase and amplitude, which allows you to increase the number of bits in a symbol.

Symbol– signal state per unit of time.
Fourier multiplexing is the expansion of a carrier signal, which is a periodic function, into a series of sines and cosines (Fourier series) with subsequent analysis of their amplitudes.

Frame– a logical data block starting with a sequence indicating the beginning of a frame, containing service information and data, and ending with a sequence indicating the end of a frame.

Redundancy- the presence in the message of a sequence of characters that allows you to write it more concisely, using the same characters using encoding. Redundancy increases the reliability of information transmission.

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