General description of ADSL technology. ADSL - what is it? Operating principle, maximum speed, advantages and disadvantages of ADSL technology

ADSL refers to an asymmetric method of access to the global Internet information network. This is the so-called asymmetric system, which allows you to work with connections at speeds of up to eight Mbit per second. Thus, ADSL, the data transfer rate through which is calculated to be up to one Mbit per second, operates at a distance of more than five kilometers.

So let's take a look at what this type of connection is and how it actually works.
So, before we touch on the very concept of ADSL, let's dive a little into history. Today, high-speed connections do not cause surprise, but are perceived as something ordinary and a privileged property of modernity. But in order for the end consumer to be able to use this resource, the developers had to work hard and create the perfect option.

The idea of creating high-speed connections as such first appeared in the eighties, when no one even thought about the Internet. A high-speed connection was required to improve and speed data transmission over copper wires in telephony.

After some time, people became familiar with computer technology and the concept of the Internet. This is where it was necessary to develop a resource for the rapid transfer of electronic information units between various interaction services, video game products, as well as for access to other local network systems.

Modern ADSL technology is a network that is based on a subscriber’s digital line, through which a connection to the Internet resource is made through telephone channels. Since these telephone lines use an analog signal to transmit voice messages, ADSL transforms it into a digital format and transmits it directly to the computer.

If previously used Dial-up modems blocked the telephone line, then it is timely ADSL that allows you to simultaneously use both an analog signal and a digital signal at the same time.

So, the whole point of ADSL of the new generation is that a computer user has the opportunity to download a very large amount of information and save it on a hard drive, or simply view it, and transmit a minimum of information from himself in the form of requests. In other words, maximum traffic - minimum downstream traffic - this is the operating principle of modern ADSL technology.

Naturally, incoming traffic includes video files, media products, software applications, and graphic elements. Downstream traffic includes only technically important information at the level of commands and various requests, emails and some other minor components of working with the Internet.

So, the asymmetry in question implies the subscriber’s connection speed is significantly higher than the traffic speed from the user himself. The asymmetric high-speed connection system is the most budget and economical today. This system uses the same copper telephone wires. The only thing that has changed compared to the first samples is the number of twisted pairs in them; this fact did not require any action in the direction of modernizing the switches and measures for their reconstruction.

Modern ADLS connects very quickly and is accepted by all types of modern modems. But still, for optimal connection of this system, special types of modem devices are used. This list includes modems connected via USB ports, devices similar to an Ethernet interface, as well as routers and routers with the Ethernet circuit itself; profile modems and routers for Wi-Fi are also suitable.

Additional elements in the form of splitters and microfilters are also often used; they are selected for the type of telephone cable. Splitters are used when a cable outlet is made to separate the modem channel and the phone itself. In other cases, microfilters are suitable for installation; one such element is installed for each telephone in the room.

The use of splitters allows you to prevent interference in the operation of the phone and modem, which seem to work together, but one device receives voice calls, the other allows you to connect to the Internet.

Splitter devices are compact and do not interfere at all with their presence. This is a miniature box that has three light weight connectors.
In modern times, every second Internet provider recommends using ADLS technology. Naturally, the types and tariffs for connecting to the global information network are classified depending on the regional predisposition of PC users. And coverage area is important.

When setting up a network, it is inappropriate today to buy everything - a modem, a router, a router, and splitters. The network provider offers to rent all the necessary equipment today; this list also includes an ADSL modem. If the contract for the services provided is terminated, then all equipment is returned to the provider intact and intact.

This is the cheapest way to use the Internet network as such. The user pays only for the connection itself, without incurring the cost of purchasing all the necessary equipment for the connection.

So, we are convinced that ADLS is nothing more than the fastest, highest quality and cheapest method of connecting to the Internet. Each user using this type of connection must have his own account, which is assigned to him by the provider himself. It is activated within twelve days after registration. If there is normal uninterrupted coverage in the region, then this procedure does not exceed two hours.
Before using DDLS technology, the provider must check the phone for the presence of already used elements of the same ADLS. If the coverage is not effective enough, then you are unlikely to ever need to use a high-speed network connection.

To use this same ADLS connection, you must first connect and configure all the elements correctly. So, a modem, splitters, microfilters are connected to the phone, drivers are installed on the computer storage medium, the network parameters of the modem are set in the browser used to view sites located on the Internet.

Now let's touch on the advantages of modern high-speed technology for connecting to the global information network, which makes using the Internet much more effective and simply simple.

So, the most important advantages of ADLS include the high speed of transmission of electronic information data. In order to send or receive the necessary file, you do not need to wait long for the connection; it happens instantly.

This type of technology is constantly evolving and consumers are offered faster and faster connection speeds.
The second advantage of modern ADLS is that the phone works as a phone, and the modem as a modem, the work of these devices does not interfere with each other. Using ADLS does not require the installation of large equipment or cable laying to the subscriber. There is basically no interference on the telephone line.

ADLS is a reliable, stable system that does not fail and does not require reconnection; with such a connection, the user can surf the Internet around the clock. This is the most effective method of connecting to the Internet, to which there are no alternatives.
Minimum prices for connecting ADLS and installing a modem with a router spare the family budget. Despite these advantages, this technology still has its modern disadvantages.

None of the users of such a connection are protected from cross connections to the network and other Internet users. If tens or hundreds of subscribers are connected to such a network, there is no need to talk about high speed. Naturally, the more consumers we eat, the lower.
Disadvantages also include low file transfer speeds. It’s good to receive and quickly view information, but sending it is not very convenient. So keep in mind if you want to use a high-speed connection model that it is not aimed at sending data, but at constantly receiving them in large quantities.

The speed of such a perfect system as ADLS depends in most cases not on its perfection, but on many side factors. And this is the main prerequisite for the network to be laid by a specialist who will evaluate the effectiveness of the coverage, correctly connect all the elements and achieve a high-quality result.

The quality of communication is affected by the state of the subscriber line. That is, we are talking about the presence of cable outlets, their serviceability, the diameter of the wire and the length, which can reach several kilometers. If the signal is lost, this indicates that the subscriber line is too long; this defect can be eliminated by using a larger wire diameter.

A perfectly working ADLS is five kilometers long. This is the fastest system, as mentioned above. It allows you to transfer data at a speed of 2048 Mgb per second.

If the length of the wire is not excessive, then the user is practically not limited in anything - neither in speed, nor in the number of other connected subscribers, as well as mobile phones, tablets and other modern gadgets.

Development specialists say that ADLS has not yet fully exhausted its resource and there are promising plans for its development in the future.
So we figured out what modern technology for connecting to the Internet - ADLS - is, what its advantages and disadvantages are, why many today focus on this type of network creation.

If you decide to connect your computer device to the network, do not look for a better way, it does not exist today. Many personal computer users have become convinced of this. This method is used not only by individuals, but also by large companies that have to work with a large volume of information flow every day.

Trust the suggestions of specialists, try this method in practice, and you will see that today this is the limit of perfection in terms of achieving connection speed and connecting subscribers to the virtual space.

We hope that the information presented in this article was clear to you and you made the right conclusions for yourself. In modern times, it is necessary to use the most advanced high-quality communication systems, one of which, precisely, is the above-mentioned ADLS technology.

These days, almost everyone needs access to the Internet. Be it 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, to install 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 businesses everywhere. This is especially true for small cities, towns and villages. What to do if such services are not provided, but you still need the Internet?

There are different options, and one of the best is to use twisted pair telephone wires. Many will remember with horror a phone not working while using the Internet. However, technology has long gone far ahead. Today, xDSL technologies are the most common and effective. DSL stands for digital subscriber line. This technology allows you to achieve fairly high data transfer speeds over copper pairs of telephone wires, without occupying the phone. The fact is that voice transmission uses a frequency range from 0 to 4 kHz, while copper telephone cable can transmit signals with a frequency of up to 2.2 MHz, and it is the section from 20 kHz to 2.2 MHz that xDSL technology uses . The speed and stability of such a connection is affected by the length of the cable, that is, the farther the telephone node (or another modem in the case of creating a network) is located from your modem, the lower the data transfer speed 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. An important factor: to provide an xDSL connection, there is no need to replace cables, which makes it theoretically possible to connect to the Internet wherever there is a telephone (depending on the availability of such a service from the provider).

An xDSL modem will be the link between your phone cable and your devices (or router), but when choosing a specific model, you need to consider a number of characteristics that are right for you.

What are the differences between xDSL modems?

xDSL technologies

In the acronym xDSL, the “x” represents the first letter of DSL technology. xDSL technologies differ in signal transmission distance, data transmission speed, and also in the difference in transmission speeds of incoming and outgoing traffic.

ADSL technology translates to asymmetric digital subscriber line. This means that the transfer speed of incoming and outgoing data is different. In this case, the data reception speed is 8 Mbit/s, and the transmission speed is 1.5 Mbit/s. In this case, the maximum distance from the telephone exchange (or another modem in the case of creating a network) is 6 km. But the maximum speed is possible only at a minimum distance from the node: the further away, the lower it is.

ADSL2 technology makes much better use of wire bandwidth. Its main difference is the ability to distribute information over several channels. That is, it uses, for example, an empty outgoing channel when the incoming channel is overloaded, and vice versa. Thanks to this, its data reception speed is 12 Mbit/s. The transmission speed remains the same as in ADSL. In this case, the maximum distance from a telephone exchange (or other modem) is already 7 km.

ADSL2+ technology doubles the speed of the incoming data stream by increasing the usable frequency range to 2.2 MHz. Thus, the data reception speed is already 24 Mbit/s, and the transmission speed is 2 Mbit/s. But such a speed is only possible at a distance of less than 3 km from the node - then it becomes similar to ADSL2 technology. The advantage of ADSL2+ equipment is that it is compatible with previous ADSL standards.

SHDSL technology is a standard for high-speed symmetric data transmission. This means that the reception and upload speeds are the same - 2.3 Mbit/s. Moreover, 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 speed, but is significantly limited by the distance from the node. It works in both asymmetric and symmetric modes. In the first option, the data reception speed reaches 52 Mbit/s, and the transmission speed – 2.3 Mbit/s. In symmetric mode, speeds up to 26 Mbps are supported. However, high speeds are available up to 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 you have two copper pairs of wires, you can also pay attention to SHDSL.

Annex Standards

Annex is a type of ADSL standards for transmitting high-speed data in conjunction with analog telephony (regular telephone).

The Annex A standard uses frequencies from 25 kHz to 138 kHz to transmit data, and from 200 kHz to 1.1 MHz to receive data. This is the usual standard for ADSL technology.

The Annex L standard allows you to increase the maximum communication distance to 7 km thanks to increased 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 to 3.5 Mbit/s. But in practice, connection speeds range from 1.3 to 2.5 Mbit/s. For an uninterrupted connection, this standard requires an undamaged telephone line.

DHCP server

The abbreviation DHCP stands for Dynamic Host Configuration Protocol. A DHCP server is a program that allows you to automatically configure local computers to work on a network. It provides clients with IP addresses (unique identifiers of a device connected to a local network or the Internet), as well as additional parameters necessary for working on the network. This will allow you not to manually register an IP, which will make your work on the network easier. However, you need to take into account that for devices such as network printers and for constant remote access to a computer using special programs, a statistical rather than a dynamic IP will be desirable, since constantly changing the IP will cause difficulties.

USB ports

Today, there are two options for organizing an Internet connection using ADSL technology: via a USB port and via an Ethernet port.
An external USB ADSL modem is connected to a computer via a USB port. It receives power from the computer. The advantages of such modems: low cost and ease of use. The disadvantages include not being compatible with all computers, the need to regularly reinstall drivers, and working with only one device.
An ADSL modem connected to the device via an Ethernet port will work more stable. But to be used with multiple devices, it must have a router function or Wi-Fi technology.

Setup and management

Configuring and managing modems is most often carried out using three technologies: Web interface, Telnet and SNMP.
The web interface is a function that allows configuration and management via a computer browser. This option will be sufficient for home use of the modem.

Telnet is a network protocol for remotely accessing a computer using a command interpreter. With its help, you can configure the modem from devices not connected to it. This is useful for small modem circuits at home and in the office.

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

Criterias of choice

xDSL modems differ in a number of characteristics, the most important of which are the maximum distance from the telephone exchange, the speed of data reception and transmission, the presence of symmetric or asymmetric transmission. Understanding under what conditions and how exactly the modem will be used, you can choose the device that is right for you.

Let us remind you 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 recent years, the development of the telecommunications services market has led to a shortage of capacity for access channels to existing provider networks. If at the corporate level this problem is solved by providing high-speed data transmission channels for rent, then what alternative can be offered to subscribers on existing lines, instead of a dial-up connection, in the residential and small business sectors?

Today, the main way end users interact with private and public networks is access using a telephone line and modems, devices that provide digital information transmission over subscriber analog telephone lines - the so-called Dialup connection. The speed of such communication is low, the maximum speed can reach 56 Kbps. This is still enough for Internet access, but the saturation of pages with graphics and video, large volumes of email and documents, and the ability for users to exchange multimedia information have raised the challenge of increasing the throughput 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 part of a general group of high-speed data transmission technologies, united by the general term DSL (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 to receive and transmit data: the receiving channel has significantly greater throughput.

The general name for DSL technologies arose in 1989, when the idea first appeared to use analog-to-digital conversion at the subscriber end of the line, which would improve the technology of data transmission 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, remote access to LANs and other networks). Today DSL technologies are presented:

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 on” state (which eliminates the need to dial a phone number each time and wait for the connection to be established), makes ADSL technology ideal for organizing Internet access, local area network (LAN) access, etc. When organizing such connections, users usually receive much more information than they transmit. ADSL technology provides downstream data rates ranging from 1.5 Mbit/s to 8 Mbit/s and upstream data rates from 640 Kbit/s to 1.5 Mbit/s. ADSL allows you to transmit data at a speed of 1.54 Mbit/s over a distance of up to 5.5 km over one twisted pair of wires. Transmission speeds of the order of 6-8 Mbit/s can be achieved when transmitting data over a distance of no more than 3.5 km via wires with a diameter of 0.5 mm.

R-ADSL (Rate-Adaptive Digital Subscriber Line)

R-ADSL technology provides the same data transfer speed as ADSL technology, but at the same time allows you to adapt the transfer speed 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 data rate can be selected by line synchronization, during connection or by signal coming from the station

G. Lite (ADSL.Lite)

It is a cheaper and easier to install version of ADSL technology, providing downstream data speeds of up to 1.5 Mbit/s and upstream data speeds of up to 512 Kbit/s or 256 Kbit/s 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 transmission speeds 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 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), less than using 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 remote PBX nodes, Internet servers, local networks, etc.

SDSL (Single Line Digital Subscriber Line)

Just like HDSL technology, SDSL technology provides symmetrical data transmission at speeds corresponding to the speeds of the T1/E1 line, 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 necessary to maintain the same data flows in both directions.

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

The most modern type of DSL technology is aimed primarily at ensuring guaranteed quality of service, that is, at a given speed and data transmission range, ensuring an error level of no worse than 10 -7 even in the most unfavorable noise conditions.

This standard is a development of HDSL, since 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 characteristics (in terms of maximum line length and noise margin) due to the use of more efficient code, a pre-coding 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 speed the SHDSL signal occupies a narrower bandwidth than the corresponding HDSL signal at the same speed. Therefore, the interference generated by the SHDSL system to other DSL systems is less powerful than the interference from HDSL. The spectral density of the SHDSL signal is shaped in such a way that it is spectrally compatible with ADSL signals. As a result, compared to the single-pair version of HDSL, SHDSL allows you to increase the transmission speed by 35-45% at the same range or increase the range by 15-20% 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's capabilities are limited to data transmission only. Despite the fact that IDSL, like ISDN, uses 2B1Q modulation, there are a number of differences between them. 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 line organized using DSL technology), while ISDN requires a connection to be established.

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 Mbit/s, and upstream data transfer rates ranging from 1.5 to 2.3 Mbit/s, over one twisted pair of telephone wires. In symmetric mode, speeds up to 26Mbps are supported. VDSL technology can be seen as a cost-effective alternative to laying fiber optic cable to the end user. However, the maximum data transmission distance for this technology is from 300 meters to 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), video on demand, etc. signals. The technology is not standardized; different equipment manufacturers have different speed values.

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

Rice. 1

Rice. 2

ADSL is an asymmetric technology - the speed of the “downstream” data flow (that is, the data that is transmitted towards the end user) is higher than the speed of the “upstream” data flow (in turn, transmitted from the user to the network). It should be said right away that there is no cause for concern here. The data transfer rate from the user (the "slower" direction of data transfer) is still significantly higher than using an analog modem. This asymmetry is introduced artificially; the modern range of network services requires a very low transmission speed from the subscriber. For example, to receive videos in MPEG-1 format, a bandwidth of 1.5 Mbit/s is required. For service information transmitted from the subscriber (command exchange, service traffic), 64-128 Kbit/s is quite sufficient. According to statistics, incoming traffic is several times, and sometimes even an order of magnitude, higher than outgoing traffic. This speed ratio ensures optimal performance.

To compress large amounts of information transmitted over twisted pair telephone wires, ADSL technology uses digital signal processing and specially created algorithms, advanced analog filters and analog-to-digital converters. Long-distance telephone lines can attenuate the transmitted high-frequency signal (for example, at 1 MHz, which is the typical transmission rate for ADSL) by up to 90 dB. This forces analog ADSL modem systems to operate under a fairly heavy load to allow for high dynamic range and low noise levels. 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 how ADSL works, you can understand that this system belongs to the achievements of modern technology.

Rice. 3

In FDM, one band is allocated for the upstream data stream and another band for the downstream data stream. The downstream information stream 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 discrete changes in the state of a 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 bandwidth between voice and data (it simply does not use the voice part), but is more complex 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 of 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 transmission and fax transmission. And all this without interrupting regular telephone communication, for which the same telephone line is used. The technology involves reserving a certain frequency band for regular telephone communications (or POTS - Plain Old Telephone Service). It's amazing how quickly telephone communication turned not only into "simple" (Plain), but also into "old" (Old); it turned out something like “good old telephone communication”. However, we should pay tribute to the developers of new technologies, who still left telephone subscribers a narrow band of frequencies for live communication. In this case, a telephone conversation can be carried out simultaneously with high-speed data transfer, rather than choosing one of the two. Moreover, even if your electricity is cut off, the usual “good old” telephone connection will still work and you will not have any problems calling an electrician. Providing this capability 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 ordinary twisted pair copper telephone cables. It is quite obvious that there are much more such pairs of wires (and this is 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 technology, but how high-speed? Considering that the letter "A" in the name ADSL stands for "asymmetric", we can conclude that data transfer in one direction is faster than in the other. Therefore, there are two data transfer rates to consider: "downstream" (transferring data from the network to your computer) and "upstream" (transferring data from your computer to the network).

The maximum reception speed - DS (down stream) and transmission speed - US (up stream), depends on many factors, the dependence on which we will try to consider later. In the classic version, ideally, the reception and transmission speed depends on and is determined by DMT (Discrete Multi-Tone) dividing 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 down stream transmission, 4 T1,E1 streams are used, the total maximum throughput of which is 6.144 Mbit/s - in the case of T1 or 8.192 Mbit/s in the case of E1. For up stream transmission, one T1 stream is 1.536 Mbit/s. 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 additional bit.

Now let's look at how real data transfer occurs using the following example. IP information packets generated both in clients’ local networks and by personal computers directly connected to the Internet will be sent to the input of the ADSL modem framed by the Ethernet 802.3 standard. The subscriber modem splits and “packs” the contents 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. In accordance with the T1.413 standard, it “encapsulates” ATM cells into the digital stream E1, T1, and then the traffic over the telephone line goes to the DSLAM. The DSL multiplexor station concentrator - 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 service delivery centers. When implementing Internet access services, cells arrive at the Internet provider's router, which performs the function of a terminal device in a permanent virtual channel (PVC) between the subscriber terminal and the Internet provider's node. The router performs the opposite (in relation to the subscriber terminal) transformation: it collects incoming ATM cells and restores the original Ethernet 802.3 format frame. When transmitting traffic from the service delivery center to the subscriber, completely similar transformations are carried out, only in the reverse order. In other words, a “transparent” local network of the Ethernet 802.3 protocol 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 Internet provider’s router as one of the local network devices.

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) - modulated ADSL 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 stated speeds are only possible ideally 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 - 7936 Kbps. There are other factors that have a significant impact on the speed and stability of the connection. These factors include: line length (the throughput of a DSL line is inversely proportional to the length of the subscriber line) and wire cross-section. The characteristics of the line deteriorate as its length increases and the wire cross-section decreases. The data transfer speed is also affected by the general condition of the subscriber line, the presence of twists, and cable outlets. The most “harmful” factors that directly affect the ability to establish an ADSL connection are the presence of Pupinov coils on the subscriber line, as well as a large number of taps. None of the DSL technologies can be used on lines with Pupin coils. When checking a line, it is ideal not only to determine the presence of Pupin coils, but also to find the exact location of their installation (you will still have to look for the coils and remove them from the line). The Pupin 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 section of cable that is connected to the subscriber line, but is not included in the direct connection of the subscriber to 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 using DSL technology is influenced not so much by the fact of the connection itself, but by the length of the cable outlet itself. Up to a certain length (about 400 meters), cable outlets do not have a significant impact on xDSL. Additionally, cable outlets affect different xDSL technologies differently. For example, HDSL technology allows for a cable outlet of up to 1800 meters. As for ADSL, cable outlets do not interfere with 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 disadvantages, namely susceptibility to external factors (various interference from third-party electromagnetic devices), as well as physical phenomena that arise in the line during transmission. An increase in the capacitive characteristics of the channel, the occurrence of standing waves and reflections, and the insulation characteristics of the line. All these factors lead to the appearance of extraneous noise on the line, and faster attenuation of the signal and, as a consequence, to a decrease in the data transmission speed and a decrease in the length of the line suitable for data transmission. The ADSL modem itself can provide some values of the characteristics of the ADSL line, by which one can directly judge the quality of the telephone line. Almost all models of modern ADSL modems contain information about the quality of the connection. Most often, the Status->Modem Status tab. Approximate contents (may vary depending on the model and manufacturer of the modem) are 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 speed
US Margin 26 - Outgoing connection noise level in db
DS Margin 31 - Downlink noise level in db
LOS Errors 0 -
DS Line Attenuation 30 - Downlink 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 is enabled (Path mode Fast - disabled)

Based on these values, you can judge, and also control yourself, the state of the line. Values:

Margin - SN Margin (Signal to Noise Margin or Signal to Noise Ratio). The noise level of interference depends on many different factors - getting wet, the number and length of branches, line synchronicity, cable “breakage”, the presence of twists, the quality of physical connections. In this case, the signal of the outgoing ADSL stream (Upstream) decreases until it is completely absent and, as a consequence, the ADSL modem loses synchronization

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

General description of ADSL technology

ADSL (Asymmetric Digital Subscriber Line) is one of the high-speed data transmission technologies known as DSL (Digital Subscriber Line) technologies, collectively referred to as xDSL. Other DSL technologies include HDSL (High data rate Digital Subscriber Line), VDSL (Very high data rate Digital Subscriber Line) and others.

The general name for DSL technologies arose in 1989, when the idea of using analog-to-digital conversion at the subscriber end of the line first appeared, which would improve the technology of data transmission 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, remote LAN access and other networks).

So what is ADSL? First of all, ADSL is a technology that allows you to turn twisted pair telephone wires into a high-speed data transmission path. An ADSL line connects two ADSL modems that are connected to each end of a twisted pair telephone cable (see Figure 1). In this case, three information channels are organized - a “downstream” data stream, an “upstream” data stream and a regular telephone service (POTS) channel (see Figure 2). The telephone communication channel is allocated using filters, which ensures that your phone will work even if the ADSL connection fails.

Picture 1

Figure 2

To compress large amounts of information transmitted over twisted pair telephone wires, ADSL technology uses digital signal processing and specially created algorithms, advanced analog filters and analog-to-digital converters. Long distance telephone lines can attenuate the transmitted high frequency signal (for example, at 1 MHz, which is the typical transmission rate for ADSL) by up to 90 dB. This forces analog ADSL modem systems to operate under a fairly heavy load to allow for high dynamic range and low noise levels. 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 how ADSL works, you can understand that this system belongs to the achievements of modern technology.

ADSL technology uses a method of dividing the bandwidth of a copper telephone line into several frequency bands (also called carriers). This allows multiple signals to be transmitted simultaneously on one line. Exactly the same principle underlies cable television, when each user has a special converter that decodes the signal and allows them to see a football match or an exciting film on the TV screen. When using ADSL, different carriers simultaneously carry different parts of the transmitted data. This process is known as Frequency Division Multiplexing (FDM) (see Figure 3). In FDM, one band is allocated for the upstream data stream and another band for the downstream data stream. The downstream range is in turn divided into one or more high-speed channels and one or more low-speed data channels. The upstream range is also divided into one or more low-speed data links. 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.

Figure 3

This is how ADSL can provide, for example, simultaneous high-speed data transmission, video transmission and fax transmission. And all this without interrupting regular telephone communication, which uses the same telephone line. The technology provides for the reservation of a certain frequency band for regular telephone communications (or POTS - Plain Old Telephone Service). It's amazing how quickly telephone communication turned not only into "simple" (Plain), but also into "old" (Old); it turned out something like “good old telephone communication”. However, we should pay tribute to the developers of new technologies, who still left telephone subscribers a narrow band of frequencies for live communication. In this case, a telephone conversation can be carried out simultaneously with high-speed data transfer, rather than choosing one of the two. Moreover, even if your electricity is cut off, the usual “good old” telephone connection will still work and you will not have any problems calling an electrician. Providing this capability was part of the original ADSL development plan. This feature alone gives ADSL a significant advantage over ISDN.

Factors affecting the data transfer speed are the condition of the subscriber line (i.e., the diameter of the wires, the presence of cable outlets, etc.) and its length. Signal attenuation in a line increases with increasing line length and increasing signal frequency, and decreases with increasing wire diameter. In fact, the functional limit for ADSL is a subscriber line 3.5 - 5.5 km long with a wire thickness of 0.5 mm. Currently, ADSL provides downstream speeds ranging from 1.5 Mbit/s to 8 Mbit/s and upstream speeds from 640 Kbit/s to 1 Mbit/s. The general trend in the development of this technology promises an increase in data transfer speeds in the future, especially in the “downstream” direction.

In order to evaluate the data transfer speed provided by ADSL technology, it is necessary to compare it with the speed that may be available to users using other technologies. Analog modems allow you to transfer data at speeds from 14.4 to 56 Kbps. ISDN provides a data rate of 64 Kbps per channel (user typically has access to two channels for a total of 128 Kbps). Various DSL technologies give the user the ability to transmit data at speeds of 128 Kbps (IDSL), 768 Kbps (HDSL), downstream 1.5 - 8 Mbps and upstream 640 - 1000 Kbps ( ADSL), "downstream" stream 13 - 52 Mbit/s and "upstream" stream 1.5 - 2.3 Mbit/s (VDSL). Cable modems have data transfer rates from 500 Kbps to 10 Mbps. (It should be noted that the bandwidth of cable modems is divided among all users simultaneously accessing a given line. Therefore, the number of concurrent users has a significant impact on the actual data transfer rate of each of them.) Digital lines E1 and E3 have a data transfer rate , respectively, 2.048 Mbit/s and 34 Mbit/s.

When using ADSL technology, the bandwidth of the line through which the end user is connected to the backbone network always belongs to that user entirely. Do you need an ADSL line? It's up to you, but to help you make the right decision, let's look at some of the benefits of ADSL.

First of all, the data transfer speed. The numbers were stated two paragraphs above. Moreover, these numbers are not the limit. In subsequent years, we can expect the downstream speed to increase to 52 Mbit/s, and the upstream speed to 2 Mbit/s.
You no longer need to dial a phone number to connect to the Internet or LAN. ADSL creates a broadband data link using an existing telephone line. After installing ADSL modems, you get a permanent connection. A high-speed data link is always ready to go - whenever you need it.

The line bandwidth belongs entirely to the user. Unlike cable modems, which allow the bandwidth to be shared among all users (which greatly affects the data transfer speed), ADSL technology allows only one user to use the line.
ADSL technology allows full use of line resources. Typical telephone communications use about one hundredth of the telephone line's bandwidth. ADSL technology eliminates this "disadvantage" and uses the remaining 99% for high-speed data transmission. In this case, different frequency bands are used for different functions. For telephone (voice) communications, the lowest frequency region of the entire line bandwidth is used (up to approximately 4 kHz), and the entire remaining band is used for high-speed data transmission.

The versatility of this system is not the least argument in its favor. Since different frequency channels of the subscriber line bandwidth are allocated for the operation of different functions, ADSL allows you to simultaneously transfer data and talk on the phone. You can make and answer calls, send and receive faxes, while at the same time being on the Internet or receiving data from the corporate LAN. All this over the same telephone line.
ADSL opens up completely new possibilities in those areas where it is necessary to transmit high-quality video signals in real time. These include, for example, video conferencing, distance learning and video on demand. ADSL technology allows providers to provide their users with services that are more than 100 times faster than the current fastest analog modem (56 Kbps) and more than 70 times faster than ISDN (128 Kbps) ).

ADSL technology allows telecommunications companies to provide a private, secure channel to facilitate the exchange of information between the user and the provider.
We should not forget about costs. ADSL technology is effective from an economic point of view, if only because it does not require the installation of special cables, but uses existing two-wire copper telephone lines. That is, if you have a connected telephone at home or in the office, you do not need to lay additional wires to use ADSL. (Although there is a fly in the ointment. The company that provides you with regular telephone service must also provide ADSL service.)

Not much equipment is needed to make an ADSL line work. ADSL modems are installed at both ends of the line: one on the user side (at home or in the office), and the other on the network side (at the Internet provider or at the telephone exchange). Moreover, the user does not have to buy his own modem, but it is enough to rent it from the provider. In addition, in order for the ADSL modem to work, the user must have a computer and an interface card, for example, Ethernet 10baseT.

As telephone companies gradually enter the untapped field of delivering video and multimedia data to the end user, ADSL technology continues to play a large role. Of course, after some time, the broadband cable network will cover all potential users. But the success of these new systems will depend on how many users will be involved in the process of using new technologies now. By bringing movies and television, video catalogs and the Internet into homes and offices, ADSL makes the market viable and profitable for telephone companies and other service providers in a variety of industries.

04. 09.2017

Blog of Dmitry Vassiyarov.

What is ADSL - an old but current connection method

Hi all.

You can't live without the Internet now. Therefore, any modern person should know about the different options for connecting it in order to choose the right one for themselves. For these reasons, I will tell you about what ADSL is. What if you like this way of connecting to the World Wide Web? If not, you will simply be more knowledgeable about Internet technologies. In any case, after reading the article you will win ;).

Introduction to the xDSL family

In the mid-90s, a new family was born, and not just any simple one, but digital technologies that use a telephone line to connect to the Internet. It is called DSL, which means “digital subscriber line” (digital subscriber line). The abbreviation is usually preceded by an "x" to hide a specific member of that family.

There are quite a lot of them, but one of the most popular nowadays is Asymmetric. So our further conversation will be about ADSL. As the name implies, its feature is asymmetry. We are talking about the uneven distribution of downstream and upstream traffic.

The speed of the second one is lower. Practice shows that the first digit is more important for users. Because the volume of incoming traffic always exceeds the amount of outgoing traffic.

ADSL physical design

To understand the essence of our conversation, you must understand what ADSL actually is. Connecting to the network using this technology is performed through a telephone line and 2 modems (1 is located at the subscriber, the other at the provider).

There is usually an intermediary between the telephone cable socket and the user's modem - a splitter. It has 1 input for connecting a telephone line and 2 outputs - for the telephone itself and the modem. The splitter also eliminates communication interference and ensures the safety of devices from high-voltage pulses thanks to its inductors and electrical protection circuits on varistors.

By the way, there are modems that allow you to additionally connect one that distributes Wi-Fi.

A modem is no problem for your phone

Representatives of the “old school”, who remember how in the 1990-2000s they connected to the Internet via telephone using cards, do not rush to write off ADSL. For those who don’t remember this, let me explain: in those days you could either go online or talk on the phone - one of two things.

But in asymmetric technology this drawback is eliminated. The fact is that talking on the phone takes up a tiny percentage of the line's capabilities. Smart people figured out to use the rest of the channel to access the network so that one does not interfere with the other.

For undemanding voice communications, the lowest frequency band is used, for the Internet - everything else. In particular, the phone uses the range 400 - 3500 Hz, incoming traffic - 26000 - 138000 Hz, outgoing traffic - from the last digit to 1.1 MHz.

Which line is suitable for the Internet?

Connecting to the network via ADSL is cost-effective. Since you don’t have to buy a modem, but rent it from a provider, and you don’t have to lay new cables. But this is only if the telephone company provides Internet service. In addition, not just any line will do the job. It must meet the following requirements:

loop resistance is no more than 1200 Ohms, and insulation resistance is no less than 40 Ohms;
loop capacity - maximum 300 nanofarads;
capacitive asymmetry - maximum 10 nF;
signal attenuation: good - 5-20 decibels, in the range from the last digit to 30 dB there are glitches, and at 31-40 dB synchronization may be lost;
noise level: from -65 dB to -55 dB - excellent, up to -35 dB - good, up to -21 dB there may be malfunctions, and if lower, the equipment will not work.

The quality of the cable also matters. It is best to use shielded twisted pair cable. Often the telephone is connected through a single-pair distribution wire (SDC), especially in older houses. Which, of course, is not suitable for new technologies.

Data transfer rate

The primary question when choosing a method to connect to the Internet is what is its speed? Compared to other modern types of connections, ADSL “smokes on the sidelines,” although it is considered high-speed. Compare for yourself.

The latest generation of this technology is 2++. Its maximum input speed is 48 Mbit/s, output speed is 3 Mbit/s. While the currently popular family can offer customers an incoming traffic speed of 5 Gbit/s, although 1 Gbit/s remains more affordable, and, nevertheless, this is much more than the ADSL maximum.

For these reasons, a leased line is in greater demand than modem technologies. However, ADSL is still showing its ability to compete. For example, in government agencies and other enterprises where landline phones are indispensable, it is convenient and profitable to use their lines for the Internet, because high speed is not needed in such a situation.

You are always welcome on my blog page.