Digital subscriber lines. See what "VDSL" is in other dictionaries

Business travelers would like to receive email and other Internet services at the hotel. Until recently, they had to put up with dial-up modems and a wide variety of telephones. In addition to many difficulties, the telephone bill was quite impressive.

VDSL technology solves these problems. Although it uses the premises telephone line, voice and data are transmitted simultaneously. This way, the telephone in the guest's room will not be busy and, most importantly, the main lines on the hotel's PBX will not be blocked. In addition, VDSL is capable of operating at speeds typical for local networks.

Using VDSL, you can expand your hotel's local network by connecting rooms or even buildings that were previously inaccessible due to range limitations of conventional local network connections. Other potential applications (besides telephony and Internet services in guest rooms) are wireless hot spots in lobbies or conference rooms, and high-definition Video on Demand services for showing movies in individual rooms.

Group of technologies under general called Digital Subscriber Line (or Loop) - a “digital subscriber line” abbreviated as xDSL, for the most part has a very distant relationship with “digital” data transmission and leased lines. DSL technology is based on the fact that the telephone wire from the telephone exchange to the subscriber is capable of carrying signals not only in the 0-4 kHz frequency range used for telephone communications (POTS / PSTN - “regular telephone”), but also much higher - up to 1, 2, 4 and even 12 MHz. To transmit information, you can use a special analog modem, which differs from the usual “analog” V.34/V.90 only in the frequency range and modulation

(a way of presenting information in the form of a set of sinusoidal signals).

By placing the provider's equipment on the PBX, it is possible to provide individual high-speed connections to users without the need to create additional infrastructure. That is, to connect, the subscriber’s existing telephone line is sufficient—there is no need to rent an additional line. Although it is more profitable for telephone companies to sell expensive leased lines, only the most affordable technology can succeed in the home Internet market. Existing telephone channels are as simple as possible, unpretentious, and at the same time very reliable: the number of telephone breakdowns that a subscriber encounters throughout his life can be counted on one hand. Active equipment

Of course, there are also disadvantages. If a single pair of telephone wires is a good transmission medium, then several dozen such wires laid together in one telephone cable create strong crosstalk, which reduces the signal quality and, consequently, the transmission speed. The speed is also affected by the fact that high frequencies attenuate more strongly with distance, that is, the nominal speed can only be achieved in the immediate vicinity of the telephone exchange (up to 500 m) or within a certain distance, specific for each technology. The high-frequency signal can be blocked by various filters to increase the phone's operating range, suppress interference and echo. DSL operation may not be possible on a dual telephone line, on the same line as a burglar alarm or other specific devices.

Despite the use of high frequencies, complete independence is not achieved between regular telephone signals and DSL signals. This requires frequency dividers (“splitters”), which separate low frequencies from one common signal in the line and supply them only to telephone equipment, and high frequencies only to the DSL modem. On the PBX side, separators are always present as part of the access equipment. Since not every subscriber can install a separator independently, so as not to resort to the help of installers, on the subscriber’s side you can do without a separator, but this will limit the maximum speed (see).

Difficulty of Distribution DSL technologies stems from the same reason as the ease of deployment - from the need to rent telephone exchange space. Firstly, it is not so easy for a provider to agree on the very fact of installing their equipment on a PBX - and the equipment must be installed at all stations whose subscribers are planning to provide Internet access. Secondly, the cost of rent can be so high that it will require either a lot of wealthy clients or a lot of very wealthy clients to break even. Therefore, manufacturers of access points (DSLAMs - DSL multiplexers) are constantly increasing the concentration of ports per multiplexer and developing technologies for moving some of the equipment outside the telephone exchange.

The DSL connection is permanent, that is, it is established by the modem immediately after switching on and does not break. However, depending on the tariff policy of the provider, the connection may be forcibly reset, for example, every 24 hours.

ADSL

Asymmetric DSL is called asymmetric because the data speed from the Internet to the client and from the client to the Internet are not the same. Technically, this is due to the fact that the increase in speed is combined with an increase in signal strength, and powerful signal from the subscriber to the PBX leads to additional interference. In addition, simultaneous data transmission in both directions (duplex) is implemented in DSL, as a rule, using frequency division: in one direction the signal is transmitted, for example, to low frequencies, in the other - on high. Transfer speed depends on width frequency range, chosen for this direction. But since the width of the entire channel is limited to frequencies of 26 kHz at the bottom and 1.1 MHz at the top, you have to choose in which direction the speed has priority. Most users only consume data from the Internet by sending small requests there. Accordingly, high speed from the Internet to a typical client ( downdraft signals) is much more important than in reverse direction (updraft). This is where an asymmetric connection turns out to be the best option. Yes, it is possible to change the speed ratios in the opposite direction so that the subscriber can broadcast more information than to receive, but this leads to a deterioration in spectral compatibility with traditional connections in the same telephone cable and an increase in interference between lines, which means a decrease in speed and range.

In the first versions of ADSL, two frequency channels were used: 30-138 kHz for transmitting data from the client and 138-1104 kHz for receiving (Fig. 1a).

Rice. 1. Frequency spectrum of ADSL signals with various types of coding

To encode the signals, amplitude-phase modulation with suppressed carrier frequency(in English - Carrier-less Amplitude-Phase modulation), which is a type of quadrature modulation - Quadrature Amplitude Modulation (QAM). After 2001, this technology is not supported in new access equipment, but existing equipment may well have been preserved in some places. Don't forget to check with your provider which modem you will need.

The new ITU-T G.992.1 standard uses multi-channel coding to represent signals - Discrete Multi-Tone, DMT (Fig. 1b). The frequency range is divided into 256 channels with a width of 4312.5 Hz, the lower seven of which are not used, and two more are service. In each channel, the signal quality is constantly analyzed, on the basis of which the information capacity of this channel is selected (the number of bits transmitted per clock cycle) or the channel is ignored if it is very noisy or weakened. Due to the fact that the clock frequency in each channel is only 4 kHz, DMT technology is insensitive to signal reflections arising from inhomogeneity of wires.

Each channel uses the same QAM modulation as before to encode the signal. The difference is that if for single-channel modulation (Single-Channel Modulation, SCM) the flow rate and spectrum width were selected for the entire stream as a whole, then in the case of DMT the same thing happens for each channel separately, and instead of analog noise reduction, it is predominantly used digital. This ensures the highest possible speed in every situation. Please note: modem tries achieve high speeds, but no specific values ​​are guaranteed.

This distinguishes ADSL technology from slower, but more predictable types of DSL, for example, HDSL. IN ideal conditions By using all 256 channels, you can get speeds of up to 15.36 Mbit/s. Taking into account the fact that the line is not ideal (as a result of which the average number of bits per symbol is taken equal to 8), and that the channels are divided in a proportion of 25:224 in the direction from and to the subscriber, with one channel in each direction being used for for service purposes, the nominal limit is 768 kbit/s from the subscriber + 7136 kbit/s to the subscriber. In other words, in real conditions

you can expect speeds from 64 kbit/s to 7 Mbit/s towards the subscriber and 16-768 kbit/s from the subscriber. The speed depends on the length of the cable and its current condition. You can count on high speed at a distance of up to 1.5 km from the vehicle. The maximum distance is considered to be 5-6 km, where the deterioration in signal quality is so great that the download speed will be 128-512 kbit/s. But even at high speed, comparatively, a short distance The data delay time on just one section between the subscriber and the PBX can be 15-30 ms

Typically, DSL modems, also called CPE (Customer Premises Equipment), are made in the form external devices with a USB or Ethernet interface, but they also come in the form of expansion cards (Fig. 2).

Rice. 2. Subscriber devices ADSL external and internal versions

USB and PCI models are cheaper (from $30), but require connection directly to a computer, installation of drivers for the corresponding operating system on this computer, and configuration on this computer. Ethernet versions are more expensive (from $50), require a network card, but do not depend on the computers used: no specific drivers are needed, all settings are stored in the modem itself, you can easily provide Internet to several users by simply connecting the modem to a small switch, and Often such a switch is already built into the modem. It is believed that modems with an Ethernet interface provide slightly lower latency: the gain can be up to 5 ms compared to the USB version. To work on a line with an installed security alarm, you need special modems

marked “Annex B”, which are modified versions of the usual “Annex A” models with a price difference of 10-30 dollars. This modification consists of transferring the transmission frequency range to the 173-276 kHz section, which also ensured compatibility with ISDN.

ADSL Lite To simplify and reduce the cost of the connection procedure as much as possible, ADSL standard

provides for switching a user modem directly to the line, without a frequency divider. This eliminates the need for a qualified specialist to visit the customer’s home.

The ITU-T G.992.2 standard, called G.Lite for short, instead of 256 channels uses only the lower 128 (minus the reserved ones) and, due to the worse signal-to-noise ratio, instead of 15 bits per clock, only 8 bits are transmitted on each channel. Thus, the maximum speed to the subscriber is 4 times lower, and from him - 2 times lower. However, this is enough for many users: downstream 64-1536 kbit/s and upstream 16-384 kbit/s. Often, servers on the Internet are not capable of transmitting data even at this speed.

Full speed ADSL modems also support G.Lite. There are also “G.Lite only” modems, but, firstly, they are not always cheaper, and secondly, if you later decide to upgrade to a higher speed tariff, you may need to buy a new modem. So it makes sense to opt for a low-speed modem only if the provider provides it for free or on preferential terms. Or if such a modem is built into motherboard computer (laptop) - they say that in the near future G.Lite modems will be integrated instead of the usual V.34/V.90.

ADSL2 and ADSL2 Lite

The ITU-T G.992.3 standard significantly expands the capabilities of conventional ADSL. Due to the fact that more than 15 bits can be transmitted in one clock cycle in each channel, the speed of the downstream stream can be increased to 12 Mbit/s, upstream - to 1.5 Mbit/s.

If the telephone line is not used for conversations, the upstream is increased by 256 kbit/s. Service information has been optimized, the size of which now changes dynamically, reducing costs by 8 times. Improved mechanisms for adapting to the current state of the channel. On long lines, where the speed is low, the coding efficiency increases: with low noise, you can transmit 50 kbit/s more at the same distance or increase the distance by 180 m at the same speed. In the special RE-ADSL2 mode (Reach-Extended - “extended range”), also called “Annex L”, the range is 8.5 km at a speed of 384 kbit/s. In addition, ADSL2 provides power management: speed reduction during long periods of inactivity (return to

normal mode

takes no more than 0.5 ms), sleep mode when all client computers are turned off (waking up, as well as starting a session, requires less than 3 s instead of 10 s for ADSL). It is now possible to combine up to 4 telephone lines to increase speed up to 40 Mbit/s. Line testing mechanisms have been improved, both bilaterally and on the provider side, which simplifies fault diagnosis. ADSL2 Lite, again, is designed to work without a frequency divider. Compared to ADSL Lite, the new version does not increase speed, but puts all the other improvements of ADSL2 into the hands of the user. Many modem models on sale already support ADSL2, so bottom line

price range

By expanding the frequency spectrum to 2.2 MHz (512 channels), the ITU-T G.992.5 (ADSL2+) standard increases the transmission speed to 24 Mbit/s towards the client on short lines. But after 2.0-2.5 km the speed drops sharply, compared with ADSL and ADSL2. Accordingly, the maximum distance is the same - about 5.5 km.

As mentioned above, crosstalk in multi-core cable is a serious problem for ADSL. ADSL2+ provides the ability to operate only at frequencies of 1.1-2.2 MHz, without interfering with ADSL signals. Of course, this way the ultra-high speed is also lost. But in one house there can be twice as many clients accessing the Internet at full speed of regular ADSL.

This distinguishes ADSL technology from slower, but more predictable types of DSL, for example, HDSL. this moment The next version of the standard is still under development - ADSL2++, also known as ADSL4. The number “4” means that the frequency range and speed will increase by approximately 4 times compared to ADSL. That is, owners of very good telephone lines (I wonder how many of these we have?) who live near the telephone exchange will be able to receive information at a speed of about 50 Mbit/s, and send up to 3 Mbit/s.

ADSL2+ modems are still relatively new, and the choice of devices under $100 is small. IN retail network

We were able to find several router models with a built-in 4-port Ethernet switch starting at $80.

RADSL

Rate-Adaptive DSL is not an independent connection type, but a characteristic of client and provider DSL equipment, indicating the ability of modems to adaptively set the data exchange rate, instead of using a fixed value. The term RADSL is used in the context of asymmetric DSL connections. An analogue of RADSL for symmetric connections is MSDSL.

VDSL

The absence of the letter “A” in the name suggests that the technology is intended not only for asymmetric communication, but also for two-way transmission with the same downstream and upstream speeds. But since frequency division of streams is used for duplexing, the total channel capacity is simply divided equally between the streams, that is, the maximum speed in symmetric mode is 25 Mbit/s, at a distance of 1500 m - up to 6.5 Mbit/s.

Of all types of DSL, VDSL technology is closest in its characteristics to Ethernet, which provides 10-100 Mbit/s in both directions at a distance of up to 100 m. Therefore, based on VDSL, various “long-distance Ethernet” solutions have been developed: V-thernet and 10Base- S, also known as Long-Reach Ethernet. The IEEE 802.3ah “Ethernet in the first mile” (or last mile, depending on your preference) standard put all these home-made developments in their place, classifying them as short-range Ethernet and calling them 10Pass-TS. Here “10” means transmission of up to 10 Mbit/s in symmetric mode at a distance of 1200 m, “Pass” is abbreviated. from pass-band, that is, working on the same line along with other types of communication, “T” - transmission over a copper wire, “S” - abbr. from short-reach (“close range”). This standard eliminates some aspects of DSL that were included in the design of this family of protocols, but never received practical application . As a result, the complexity of the devices was reduced, and almost the same blocks of data ran along the wires as in a regular Ethernet network. It is now possible to alternate data transmission for which minimum time latency is more important than the complete absence of errors (voice, video), with ordinary data requiring high speed and reliability. The first type of data forms the so-called “ fast channel

As in its time with ADSL, the battle between supporters of single-channel (SCM) and multi-channel (DMT) coding was repeated - manufacturers really like to pull the blanket over themselves. As a result, it again turns out that the first VDSL products with single-channel QAM modulation are beginning to be supplanted by products supporting DMT (ITU-T G.993.1 standard), creating incompatibility.

But our joy would be incomplete if the differences were only this.

There are also several ways to split the frequency range between the downstream and upstream (Fig. 3).

Rice. 3. Frequency spectrum of VDSL signals of various standards

The partitioning used in 10Base-S technology (Fig. 3a) is suitable for both asymmetric connection and symmetric with priority downstream, because if the signal quality decreases at high frequencies, only the upstream will suffer. The G.993.1 standard "A" diagram, adapted from ETSI Plan 997, identifies 4 gaps (Fig. 3b). This partition is also universal and provides better symmetry guarantees. In scheme “B” (ETSI Plan 998) the picture is similar, only a larger range is allocated for the downstream flow, while maintaining, however, a fairly high upstream speed - exactly what most users need (Fig. 3c). It is also noteworthy in circuit “B” that the lowest range may also be used in. The “C” circuit, also known as the “Fx” circuit, has a floating cut-off frequency, which is selected according to the needs of the subscriber: the ratio of downstream and upstream speeds can be almost any (Fig. 3d). Multichannel coding (DMT) is even more flexible in this regard: there can be three, four, five, and six-band devices are just around the corner. In principle, it is not so important which technology the provider chooses. The problem is that if competing providers use different technologies, the mutual interference in adjacent cable cores will be much stronger than if uniformity were maintained.

Prices for VDSL modems start at $90 for a model with an Ethernet interface.

VDSL+ uses four-band encoding in the range up to 12 MHz, which ensures compatibility with existing products based on the same scheme, and adds a fifth band of 12-30 MHz for downstream. Over short distances, this allows you to achieve speeds of up to 200 Mbit/s towards the client.

The VDSL2 standard project is still in its infancy. It is only known that it will use multi-channel coding (DMT) and that it will be based on the principles modern version VDSL and .

Samsonov A.

Article "Types of Internet connections. Asymmetric DSL (2005-08-15)" You can discuss on

SDSL technology

Symmetrical or two-wire DSL (SDSL) is symmetrical and is based on the earlier HDSL technology, but has whole line improvements that allow more flexible organization of data transmission over one pair of wires. In addition, the maximum transmission distance is limited to 3 km. Within this distance, SDSL technology provides, for example, the operation of a video conferencing system when it is necessary to maintain the same data flows in both directions. SDSL technology can find applications in both the business and private sectors, giving it very high potential value.

It is worth noting that some modern manufacturers of narrowband switching equipment consider this technology as one of the ways to extend the life of this type of equipment. SDSL technology can be used in the form of embedded line cards capable of carrying 2 channels of switched traffic over switching network. Any other high-speed access capabilities are routed from the switched network to a non-switched IP or ATM high-speed data network. In addition, SDSL technology can be used as a complement to access technologies such as HDSL, ADSL and VDSL.

VDSL (Very High Speed ​​Digital Subscriber Line) technology is the natural evolution of ADSL technology towards faster data rates and an even wider bandwidth. This technology can be successfully implemented by reducing the effective length of the subscriber line by expanding the network of fiber-optic lines and their implementation in existing network access.

Ultra-high-speed digital subscriber line (VDSL) concept.

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 26 Mbps are supported. VDSL technology can be considered as a cost-effective alternative to laying fiber optic cable up to 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 given value, or the fiber optic cable must 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.

1.5.5.4.

VDSL technology

The signal transmission principles for ADSL and VDSL technologies are very similar to each other. Interesting - from the point of view of the issues raised in the monograph - are their main differences in the field of application of the corresponding equipment. They are most easily considered in terms of “length - transmission speed”.

It is obvious that the area of ​​potential application of ADSL technology can be almost the entire subscriber network. This is due to the fact that the Russian PSTN is characterized by fairly short AL. It is important to note that in practice difficulties may arise with ADSL equipment even on very short lines. The problem lies in the operational characteristics of the subscriber network, which may differ significantly from expectations. But in any case, the transmission speed provided by ADSL equipment will not exceed the values ​​​​given in Table 1.5.

Thus, for transmission speeds above 10 Mbit/s it is necessary to focus on VDSL technology. But such a solution limits the number of potential subscribers due to the fact that it is permissible to use only very short AL. In particular, a transmission speed of 52 Mbit/s is achieved for a line whose length is approximately 300 m. For this reason, VDSL technology is usually considered in combination with other methods of constructing a line. As a rule, solutions like FTTOpt+VDSL are of practical interest. One possible scenario based on such a trade-off is shown in Figure 1.16.

Example of using VDSL technology

There are five options for using VDSL technology for this scenario. In four cases it is assumed sharing OK and subscriber cable with copper conductors. For all five options, the lines of maximum length are designated as Li. These values ​​may vary, but the condition Li £ Lmax must always be met - the maximum length of the physical circuit that can be used to install VDSL equipment.

The first option illustrates the possibility of using VDSL equipment in AL located in the area direct supply. In this case, the signal propagation medium remains homogeneous. Obviously, such a solution will be very economical.

The second option involves sharing a subscriber cable with copper conductors and cables. IN in this case The OC is located between the cross-connect of the switching station and the multiplexer. Lines of different lengths are connected to the multiplexer, the maximum of which is designated L2. These lines can transmit digital information using VDSL technology.

A similar solution, but based on installing a concentrator, is represented by the third option. An essential feature of this solution can be considered the possibility of more efficient use of OC in the area between the hub and the cross-connection of the switching station.

The fourth option involves the use of PBX. This solution, from the point of view of VDSL technology, is equivalent to the third option. PABX, as a rule, supports a wider range of services compared to the PSTN switching equipment that serves residential subscribers. This circumstance allows us to predict the widespread use of VDSL technology by PBX owners.

The fifth option illustrates the use of VDSL technology to connect to a device called a Service Access Multiplexer (SAM). Similar devices can connect not only to switching stations, but also to any servers. A typical example of such a multiplexer is equipment that allows clients to connect to a server that supports the “Video on Demand” service.

Thus, there are several options for using VDSL technology. The corresponding scenarios, as a rule, are focused on the joint use of operated subscriber cables with copper conductors and cables that provide transmission broadband signals over a long distance.

When work on the final edition of the monograph was almost completed, I had the opportunity to take part in the seminar “Bell Labs - The Past. The present. Future", organized well-known company Lucent Technologies. The reports presented the results of new work carried out by an authoritative research center known to almost all Russian scientists - Bell Labs. In a talk given by John Amoss, a member of the Bell Labs Data Communications Division, I heard about a new technology called U-ADSL. Under the letter “U” is the word Universal, that is, universal or universal.

The main goals for which U-ADSL technology is being developed are to solve the following problems:

Simplify the installation of equipment, making maximum use of the “Plug and Play” concept, which can be translated as the motto “Plug and Play”;

Propose an implementation option that involves placing hardware in personal computer, which was practically agreed upon by such well-known suppliers of computer equipment as Compaq, Intel and a number of other large companies;

Enter in next Microsoft version Windows software for support U-ADSL equipment;

Ensure the ability of U-ADSL equipment to operate under almost any (within the limits acceptable for PSTN standards) AL parameters.

Naturally, such requirements are determined relatively low speeds information exchange: in the direction to the terminal this value is estimated at 1.5 Mbit/s, and in the direction to the network - 512 kbit/s. If such speeds are acceptable for large group potential users, then this new technology will be able to find a worthy niche in the xDSL equipment market.

VDSL technology

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

VDSL technology is the "fastest" xDSL technology. It provides downstream data transfer rates ranging from 13 to 52 Mbit/s, and upstream data transfer rates ranging from 1.5 to 2.3 Mbit/s, over one twisted pair of telephone wires. 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 ranges from 300 meters to 1300 meters (Table 2.1). 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.

Table 2.1 - VDSL distances and speeds

Advantages of xDSL technologies

Key advantages of xDSL technologies:

1. use of an existing subscriber line;

2. a significant increase in the speed of data transmission over a copper pair of telephone wires without the need to modernize them;

3. transmission of all various traffic over this single subscriber line (AL) mass user-- from traditional telephone conversation before Internet access;

4. transfer of all user data traffic (including Internet traffic) bypassing switched PSTN networks (telephone network common use) or ISDN directly into the data transport network;

5. A set of DSL technologies provides data transfer rates from 32 Kbps to 50 Mbps, so the user can make a choice depending on his own needs;

6. As a means of data transmission, xDSL equipment occupies an intermediate position between cheap analog modems and expensive leased lines. High speeds transmissions at relatively low costs make xDSL technologies an almost ideal means of data transmission for small and medium-sized businesses;

7. It is possible to simultaneously use analog telephone communication, and digital high speed transmission data on the same line, separating the spectra of these signals. Using DSL allows you to talk on the phone without disconnecting from the Internet.

Conclusion to the section

xDSL (eng. digital subscriber line, digital subscriber line) is a family of technologies that can significantly increase the capacity of the subscriber line of the public telephone network by using efficient linear codes and adaptive methods for correcting line distortion based on modern achievements in microelectronics and digital signal processing methods.

In the abbreviation xDSL, the character "x" is used to represent the first character in the name specific technology, and DSL stands for digital subscriber line DSL (English: Digital Subscriber Line - digital subscriber line; there is also another version of the name - Digital Subscriber Loop - digital subscriber loop). xDSL technologies allow you to transfer data at speeds that significantly exceed those that are available even to the best analog and digital modems. These technologies support voice, high-speed data and video, creating significant benefits for both subscribers and providers. Many xDSL technologies allow you to combine high-speed data transmission and voice transmission over the same copper pair. Existing types of xDSL technologies differ mainly in the form of modulation used and the data transfer rate.