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. An important factor: to provide an xDSL connection, it is not necessary to change 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).

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

In the acronym xDSL, the "x" stands for the first letter of the DSL technology. xDSL technologies differ in signal transmission distance, data transfer rate, as well as the difference in the transfer rates of incoming and outgoing traffic.

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 wire bandwidth. 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

Annex - a kind of ADSL standards for high-speed data transmission in conjunction with analog telephony (ordinary telephone).

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

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 gives clients 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 enter the IP, which will make it easier to work on the network. However, it should be noted that for devices such as network printers, and for permanent remote access to a computer using special programs, a static rather than a dynamic IP will be desirable, since the constant change of 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 the computer via a USB port. It receives power from the computer. The advantages of such modems are low cost and ease of use. The disadvantages include compatibility not with all computers, the need for regular reinstallation of drivers, and work with only one device.
An ADSL modem connected to the device via an Ethernet port will work more stable. But for use with multiple devices, it must have a router function or Wi-Fi technology.

Setup and management

Modems are most often configured and managed using three technologies: Web interface, Telnet, and SNMP.
The web interface is a feature that allows configuration and control through a computer browser. This option will be enough for home use of the modem.

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

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

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.

As you know, humanity has always sought to contact each other more and more closely. The political and economic system , like a drop of water in weightlessness, seeks to pull the edges towards itself, to get as close as possible. But the methods of connection changed under the influence of the economic and social development of society. The adobe roads were replaced by railways, they were replaced by monorails. From the telegraph, mankind has now come to fiber-optic, and even wireless communication . So, we remember with admiration the Roman Appian Way, which has been invariably serving for two thousand years, but we prefer to walk along the Hermes Way of high-speed data transmission.

Emergence of a new communication standard

Twin brothers were born in the laboratory. The birthplace was the research center at the International Telecommunication Union (ITU). The brothers got one name for two, which was even very convenient for their creators, we know them under the name ADSL2 (Asymmetric Digital Subscriber Line - Asymmetric Digital Subscriber Line). However, at home - in the laboratory, they are called G.992.3 and G.992.4.

This standard is based on the already well-established ADSL system. It has a rich history and a good reputation among individual users and organizations, but it is not perfect, like all man-made. The purpose of creating a new standard based on ADSL was to correct the shortcomings and maximize the realization of the existing potential.

Work on ADSL2 was completed in July 2002. Well, a little later, these two standards had a younger brother - an even more nimble and modern ADSL2pus (ADSL2+).

Nature of ADSL

It should be said that DSL technology has several branches of development. For a long time, the connection through a "dedicated channel" - a separate cable - dominated. Good, reliable, but expensive ... Yes, and laying the cable is fraught with some difficulties. ADSL technology involves a technical trick - connecting to a provider using an existing telephone network. In this case, the information inside the twisted pair telephone wire is divided into three independent streams: the "incoming" signal, the "outgoing" signal and the telephone signal. The whole trick is that the analog connection with the PBX has a frequency different from the digital data stream: a band of up to 4 kHz is allocated to the telephone, from 4 kHz to 1 MHz - for data exchange with the provider. The provider filters the streams based on their frequency difference, and sends the telephone signal to the PBX. A positive feature of this integration is that the telephone line continues to function even if the ADSL connection is interrupted.

ADSL2 differs in maximum adaptability: it takes into account the difficulties that arose when working with ADSL.

ADSL2 Difference

The new standard has improved spectral and channel efficiency, modulation algorithms, channel initialization and adaptive data rate selection. Simply put, ADSL2 maximizes wire bandwidth. Imagine that at rush hour, a traffic controller with a striped baton appears on a busy street, directing the flow of cars to free routes, thereby unloading the busy ones. In the same way, ADSL2 has the ability to spread information across multiple channels, use an empty outbound when the inbound is particularly busy, and otherwise speed up the connection. Such an upgrade made it possible to increase the speed to 12 Mbps, and in addition to that, the connection distance has grown considerably. Also, the creators have improved the methods of automatic diagnostics on both ends of the connection. In addition, additional energy-saving modes have been introduced for a periodically inactive line.

The amount of service information has changed, automatically passing along with the "useful" amount of traffic. The amount of required technical information can vary from 4 to 32 kbps, this is important, because in some cases, when connecting an ADSL channel, 25% of all traffic was assigned to service information.

The crown of evolution - ADSL2+

Developed in January 2003 at UTI, the standard doubles the speed of the incoming data stream on lines up to 1500 meters. This is achieved due to the supported frequency - up to 2.2 MHz on the incoming channel. The speed of the outgoing channel is based on the quality of the line and the diameter of the copper wires.

ADSL2+ promises to be even more user friendly, and even provide a range of multimedia features: cable TV, for example.

What is very important - the ADSL2+ communication standard does not require specialized equipment, and can work on the same equipment on which an ADSL connection was previously made. Thus, connecting to ADSL2 today becomes even more attractive in the eyes of users. Who knows what new standards based on DSL technology will appear tomorrow?

If you have made your choice, now you can safely visit any sites, online games of any kind, be it hunting for a wild boar or tanks. And most importantly, remember that real life is no less interesting.

New ADSL standards: ADSL2 and ADSL2 plus

In July 2002, the International Telecommunications Union (ITU) completed the development of two new ADSL standards (G.992.3 and G.992.4), collectively referred to as "ADSL2".
In January 2003, just as the number of first generation ADSL chipset users passed 30 million, G.992.5 officially joined the ADSL2 family under the name ADSL2plus (or ADSL2+). Providers and users have played a key role in the development of the ADSL2 standard, as their feedback has enabled the ITU to incorporate many performance and functionality enhancements into the new standard.
As a result, ADSL2 will be more user-friendly and more profitable for providers, and promises to repeat the success of ADSL for the rest of the decade. ADSL2 (ITU G.992.3 and G.992.4) contains many innovations to improve network performance and interoperability and support new applications, services, and installation options.
Changes include performance improvements, speed adaptations, diagnostics, and more. ADSL2plus (ITU G.992.5) doubles the receive bandwidth, reaching speeds of 20 Mbps on 1500 meter telephone lines.
Solutions based on ADSL2plus will mainly be multi-modal, allowing interoperability with both ADSL2 chipsets and ADSL and ADSL2plus. ADSL2plus will allow providers to configure their networks to support advanced services such as "flexible" video with a single solution for both short and long distances. It includes all the features of ADSL2 while maintaining interoperability with existing equipment. Thus, providers can implement a gradual upgrade of equipment, and not immediately change the whole thing.

Speed ​​and Range Improvements

ADSL2 was specifically designed to improve ADSL speed and range, mainly to achieve better performance over long, noisy lines. ADSL2 can reach up to 12 Mbps and 1 Mbps upstream and downstream speeds, respectively, depending on range and other factors. This became possible due to the use of more efficient modulation methods, a reduction in the amount of overhead, an increase in coding efficiency, and the use of advanced signal processing algorithms.
Modulation efficiency in ADSL2 is enhanced by combining 4D, 16-phase trellis, and 1-bit quadrature modulation.
This allows you to get higher speeds on long lines with a low signal-to-noise ratio. ADSL2 systems use less overhead due to the programmable overhead frame.
Therefore, unlike the first generation ADSL, where the control bits in the frame were fixed and consumed 32 kbps of useful information, the number of control bits in the frame can vary from 4 to 32 kbps. In ADSL systems of the first generation on long lines, where the information transfer rate is already low (for example, 128 kbps), 32 kbps (or more than 25% of the total speed) is fixed for service information. On ADSL2 systems, this value can be reduced to 4 kbps, which will add an additional useful 28 kbps to the throughput. On long lines, where transmission rates are generally low, ADSL2 allows for greater Reed-Solomon coding efficiency.
This is possible due to improvements in frames that increase flexibility and programmability in creating codewords. In addition, the initialization mechanism contains many improvements that increase the transmission speed in ADSL2 systems:
- power reduction on both sides to reduce crosstalk;
- detection of the placement of the pilot signal by the receiver, eliminating interference from AM radio;
- carrier detection, used by the receiver for initialization messages to eliminate interference from AM radio and other annoyances;
- improvements in channel identification for receiver and transmitter tuning;
- Disabling the signal during initialization to enable RF interference suppression circuits.
Figure 1 shows the speed and range of ADSL2 compared to first generation ADSL. On long lines, ADSL2 will give a speed increase of 50 kbps for incoming and outgoing streams. This increase in speed is achieved on lines extended by 180 meters, which is equivalent to a 6% increase in coverage area.

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1. Diagnosis

The difficulty of identifying the source of problems has often been a barrier to using ADSL.
Advanced diagnostic capabilities have been added to ADSL2 transceivers to facilitate troubleshooting. They are designed to troubleshoot during and after installation, monitor performance during operation, and facilitate upgrades. To troubleshoot problems, ADSL2 transceivers can measure line noise, attenuation, and signal-to-noise ratio at both ends of the line.
The results of these measurements can be collected using a special diagnostic mode, even if the line quality is not sufficient to establish a normal ADSL connection. In addition, ADSL2 can perform real-time performance monitoring showing line quality and noise level at both ends of the line. This information is converted by the software and can then be used by the ISP to monitor the quality of the ADSL connection and prevent failures. It can also be used to identify opportunities to provide a user with a faster connection.

Power Consumption Improvements

First generation ADSL transceivers were active 24/7, whether they were in use or not. Considering that the number of installed ADSL modems can reach several million, a huge amount of electricity could be saved if the modems were able to enter sleep mode.

It would also save power for ADSL transceivers operating in small rooms where heating is a problem. To address these issues, ADSL2 power management provides two modes designed to reduce overall power consumption when serving an "always on" user connection. These modes include:

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2. L2 low consumption mode.
This mode performs statistical power saving on the Central Office (ATU-C) ADSL transceiver by quickly entering and exiting a low power mode based on the Internet traffic going through the ADSL connection. L3 low consumption mode.
This mode provides overall power saving for both the ATU-C and the remote ADSL transceiver (ATU-R) by going to sleep while the connection is idle for a long time. L2 mode is one of the most important innovations of the ADSL2 standard.
ADSL2 transceivers can enter and exit L2 mode based on the Internet traffic carried over the connection. When the user is downloading large files, the transceiver will operate at full power (also referred to as L0) to ensure maximum download speed. When Internet traffic decreases, such as when a user reads a long text, ADSL2 systems can enter L2 low power mode, which greatly reduces the transmission rate and thus reduces overall power consumption. While in L2 mode, the ADSL2 system can instantly return to L0 mode and increase the data transfer rate as soon as the user initiates file downloads.
The L2 entry/exit mechanisms and the resulting data rate adaptations operate without any service interruptions or even a single bit error, and thus are invisible to the user. The L3 power mode is a sleep mode and is used when the user is not using the network. When switching to it, no traffic is transmitted. When the user needs the network again, the ADSL transceivers only need about three seconds to re-initialize and establish a connection.

speed adaptation

Telephone wires are bundled together in multi-pair cables containing 25 or more twisted pairs.
As a result, electrical signals from one pair can cross over to adjacent pairs in the cable (Figure 3). This phenomenon is called "crosstalk" and can interfere with ADSL data transmission. Moreover, a change in the level of crosstalk in the cable can lead to a break in the ADSL connection.

Fig.3.

To solve these problems, ADSL2 adapts the data rate in real time.
This innovation, called Seamless Rate Adaption (SRA), allows ADSL2 systems to change the data rate on the link on the fly without service interruptions or bit errors. To do this, ADSL2 detects changes in the communication channel, for example, when the local AM radio station turns off its transmitter for the night - and transparently changes the transmission rate for the user. SRA is based on the separation of modulation level and frame level in ADSL2 systems.
Because of this, the modulation layer can change the data rate parameters without modifying the parameters at the frame layer, which would cause the modems to lose frame synchronization and hence uncorrectable bit errors or restart the system. SRA uses ADSL2's sophisticated online reconfiguration (OLR) procedures to seamlessly change the data rate on a connection. The protocol used for SRA works as follows:
1. The receiver monitors the signal-to-noise ratio for the channel and determines that it needs to adapt the data rate to the prevailing conditions.
2. The receiver sends a message to the transmitter to initiate a baud rate change.
This message contains all the necessary transmission parameters for the new rate. These parameters include the number of modulated bits and transmission power for each sub-channel of a multi-carrier ADSL system. 3. The transmitter sends a "Sync Flag" signal, which is used as a marker to determine the exact time for which the new transmission parameters will be used.
4. The "Sync Flag" signal is detected by the receiver, and now the receiver and transmitter switch to another speed mode without any system interrupts.

Consolidation to Achieve Higher Speeds

A common requirement for providers is the ability to provide different quality of service to different users. The data transfer rate can be significantly increased by using multiple telephone lines simultaneously. To support this capability, ADSL2 supports the af-phy-0086.001 "Inverse Multiplixing for ATM (IMA)" standard designed for traditional ATM architectures. Using IMA, ADSL2 chipsets can combine two or more copper pairs into a single ADSL connection. The result is much greater flexibility in the rate of the incoming data stream (Figure 4).

Fig.4.

IMA defines a new layer that sits between the physical layer and the ATM layer.
On the transmitter side, this sublayer, called the IMA sublayer, receives one ATM stream from the ATM layer and distributes it among multiple physical sublayers. On the receiver side, the IMA sublayer receives ATM parts from multiple physical sublayers, assembles them into a single ATM stream, and sends them to the ATM layer. The IMA sublayer defines the IMA framing, protocols, and control functions that are used to perform the above operations when the physical sublayers are bit errored, asynchronous, or have varying delays. In order to work under these conditions, the IMA standard also requires modifications to some of the standard ADSL physical layer functions, such as the receiver discarding empty or corrupted packets. ADSL2 supports a special IMA mode designed to be compatible with ADSL.

Channelization and Multichannel Voice over DSL (CVoDSL)

ADSL2 supports the ability to partition the bandwidth into multiple channels with different characteristics for different applications. For example, ADSL2 can simultaneously support voice applications that require low latency but tolerate high error rates and data applications that do not care as much about latency but want as low an error rate as possible. Channelization also provides support for CVoDSL, a method for transparently transporting derivatives of TDM voice traffic over DSL. CVoDSL reserves 64 kbit/s channels from the DSL bandwidth (Fig. 5) for delivery of PCM DS0 from the DSL modem to the remote terminal of the central office, similar to a conventional telephone system. Further, the access equipment via PCM transmits voice DS0s directly to the circuit switch.

Fig.5.

Fig.6.

Some Additional Benefits

ADSL2 also supports some other important features listed below.
Improved compatibility.
ICs from different manufacturers are compatible and can work together seamlessly. Quick start.
ADSL2 supports fast startup, which reduces initialization time from more than 10 seconds (required for ADSL) to less than 3 seconds. Full digital mode.
ADSL2 allows you to use the voice range for data transmission, adding another 256 kbps to the outgoing channel. This is quite an attractive opportunity for office use, since, as a rule, voice and data lines are separated in offices and a large bandwidth of the outgoing channel is required. Support for package-based services. ADSL2 includes a Packet Mode Transmission TransConvergence layer (PTM-TC) that allows packet-based services (such as Ethernet) to be carried over ADSL2.

ADSL2plus

ADSL2plus was developed by the ITU in January 2003 and is included in the ADSL standards as G.992.5.
The ADSL2plus recommendation doubles the downstream speed on lines shorter than 1500 meters. While the first two members of the ADSL2 family of standards set the downlink bandwidth to 1.1MHz and 552kHz respectively, ADSL2plus sets the downlink bandwidth to 2.2MHz. As a result, a significant increase in the speed of the incoming channel is achieved on shorter lines (see Fig. 8). The speed of the outgoing ADSL2plus channel depends on the quality of the connection and is in the region of 1 Mbps.

Fig.8.

ADSL2plus can also be used to reduce crosstalk. To do this, it can use tones between 1.1 MHz and 2.2 MHz, masking the incoming channel frequencies around 1.1 MHz. This can be useful when the ADSL terminals are connected to the central site through the same cable in the same order that they are connected to the customers' homes (Figure 9). Crosstalk from remote terminal lines to the link from the central site can significantly reduce data rates on the link from the central site.

Fig.9.

ADSL2plus can solve this problem by using frequencies below 1.1 MHz from the central site to the remote terminal and frequencies between 1.1 MHz and 2.2 MHz from the remote terminal to the user's home. This will eliminate most crosstalk between services and protect the data rate on the line from the central office.

Based on materials from the DSL forum, translated by Dmitry Geruss.