What is adsl modem. What is the difference between xDSL modems. Transfer rate

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

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

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

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

The common name of DSL technologies originated in 1989, when the idea first appeared to use analog-to-digital conversion at the subscriber's end of the line, which would improve the technology 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, dial-up LAN and other networks). To date, DSL technologies are represented by:

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

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

• R-ADSL (Rate-Adaptive Digital Subscriber Line)

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

• G. Lite (ADSL.Lite)

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

• HDSL (High Bit-Rate Digital Subscriber Line)

HDSL technology provides for the organization of a symmetrical data transmission line, that is, the data transfer rates from the user to the network and from the network to the user are equal. With transmission speeds of 1.544 Mbps over two pairs of wires and 2.048 Mbps over three pairs of wires, telecommunications companies are using HDSL technology as an alternative to T1/E1 lines. (T1 lines are used in North America and provide a data transfer rate of 1.544 Mbps, and E1 lines are used in Europe and provide a data transfer rate of 2.048 Mbps.) Although the distance over which the HDSL system transmits data (which is about 3.5-4.5 km), is less than using ADSL technology, telephone companies can install special repeaters to inexpensively but effectively extend 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)

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

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

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

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

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

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

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

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

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

Rice. one


Rice. 2

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

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

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

Rice. 3

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

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

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

ADSL is a high speed data transfer technology, but how fast? Given that the letter "A" in the ADSL name means "asymmetric" (asymmetric), we can conclude that data transfer in one direction is faster than in the other. Therefore, there are two data rates to consider: "downstream" (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 - 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 and is determined by DMT (Discrete Multi-Tone) by 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 which is 6.144Mbps - in the case of T1 or 8.192Mbps in the case of E1. For upstream transmission, one T1 stream is 1.536 Mbps. The maximum speed limits are indicated without taking into account overhead costs, in the case of classic ADSL. Each stream is provided with an error correction code (ECC) by introducing an extra bit.

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

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

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

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

modem status

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

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

Let's explain some of them:

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

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

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

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

ADSL technology

In recent years, the growth in the volume of information transfer has led to the fact that there is a shortage of bandwidth for access channels to existing networks. If at corporate levels this problem is partially solved (by renting high-speed transmission channels), then in the residential sector and in the small business sector these problems exist.

Today, the main way for end users to interact with private and public networks is access using a telephone line and modems, devices that transmit digital information over subscriber analog telephone lines. The speed of such a connection 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 e-mail and documents will again raise the question of ways to further increase bandwidth in the near future.

The most promising at present is the ADSL (Asymmetric Digital Subscriber Line) technology. This is a new modem technology that turns standard analog subscriber telephone lines into high-speed access lines. ADSL technology allows you to transfer information to the subscriber at a speed of up to 6 Mbps. IN reverse direction speeds up to 640 Kbps are used. This is due to the fact that the entire modern range of network services involves a very low transmission speed from the subscriber. For example, MPEG-1 video requires 1.5 Mbps of bandwidth. For service information transmitted from the subscriber, 64-128 Kbps is quite enough (Fig. 1).

ADSL service organization principles

The ADSL service (Figure 1) is established using an ADSL modem and an ADSL modem rack called the DSL Access Module. Almost all DSLAMs are equipped with a 10Base-T Ethernet port. This allows the use of conventional hubs, switches and routers on access nodes.

A number of manufacturers have begun to provide DSLAMs with ATM interfaces, allowing them to be directly connected to WAN ATM switches. Also, a number of manufacturers create custom modems, which are an ADSL modem, but for software are ATM adapters.

There are three streams in the section between the ADSL modem and the DSLAM: a high-speed stream to the subscriber, a bidirectional service and voice channel in the standard frequency range of the PM channel (0.3-3.4 kHz). Frequency separators ( POTS Splitter) allocate a telephone stream, and direct it to a conventional telephone set. Such a scheme allows you to talk on the phone simultaneously with the transfer of information and use telephone communications in the event of a malfunction of the ADSL equipment. Structurally, the telephone splitter is a frequency filter that can be either integrated into an ADSL modem or be an independent device.

According to the theorem Shannon, it is impossible to achieve speeds above 33.6 Kbps using modems. In ADSL technology, digital information is transmitted outside the frequency range of a standard PM channel. This will cause the filters installed at the telephone exchange to cut off the frequency above 4 kHz, so it is necessary to install WAN access equipment (switch or router) at each telephone exchange.

Transmission to the subscriber is carried out at speeds from 1.5 to 6.1 Mbps, the speed of the service channel is from 15 to 640 Kbps. Each channel may be divided into multiple logical low rate channels.

Speeds provided by ADSL modems are multiples of speeds of T1, E1 digital channels. In the minimum configuration, transmission is carried out at a speed of 1.5 or 2.0 Mbps. In principle, today there are devices that transmit data at speeds up to 8 Mbps, but this speed is not defined in the standards.

Speed ​​of ADSL modems depending on the number of channels

base speed	Number of channels	Speed
1.536 Mbps	1	1.536 Mbps
1.536 Mbps	2	3.072 Mbps
1.536 Mbps	3	4.608 Mbps
1.536 Mbps	4	6.144 Mbps
2.048 Mbps	1	2.048 Mbps
2.048 Mbps	2	4.096 Mbps
2.048 Mbps	3	6.144 Mbps

The maximum possible line speed depends on a number of factors, including the length of the line and the thickness of the telephone cable. The characteristics of the line deteriorate with an increase in its length and a decrease in the cross section of the wire. The table shows several options for the dependence of speed on line parameters.

An ADSL modem is a device built on the basis of a digital signal processor (DSP or DSP), similar to that used in conventional modems (Fig. 2). IN general case, the entire bandwidth of the line is divided into two sections. The first section is intended for voice transmission, and is in the range of 0.3-3.4 kHz. The signal range for data transmission is between 4 kHz and 1 MHz. The physical parameters of most lines do not allow data to be transmitted at a frequency above 1 MHz. Unfortunately, not all existing telephone lines (especially long ones) even have such characteristics, so it is necessary to reduce the bandwidth, which entails a decrease in the transmission speed.

Two methods are used to create these streams: the frequency division method and the echo cancellation method.

Rice. 3 Stream separation schemes in the frequency band of a telephone line

The frequency division method consists in the fact that each of the streams is allocated its own frequency bandwidth. The high speed stream may be split into one or more low speed streams. These streams are transmitted using the " "(DMT).

The echo cancellation method is that the ranges of the high-speed and service streams are superimposed on each other. The streams are separated using a differential system built into the modem. This method is used in modern V.32 and V.34 modems. A high-speed stream can be divided into one or more low-speed streams. These streams are transmitted using the " discrete multitone modulation"(DMT).

When transmitting multiple streams, each of them is divided into blocks. Each block is provided with an error correction code (ECC).

Related technologies

There are a number of related technologies, some for end-users, others for high-speed backhaul. The principle of their operation is similar to ADSL. The common name for such technologies is xDSL.

High Data-Rate Digital Subscriber Line (HDSL)

HDSL is a technology that provides transmission at 1.536 or 2.048 Mbps in both directions. The length of the line can reach 3.7 km. Oriented as a cheaper alternative to dedicated channels E1, T1. Requires a 4-wire subscriber line.

Single-Line Digital Subscriber Line (SDSL)

It is similar to HDSL, differs in that a two-wire subscriber line is enough to organize a line. The length of the line can reach 3 km.

Very High Data-Rate Digital Subscriber Line (VDSL)

Similar to HDSL, speed up to 56 Mbps. Distance up to 1.5 km. The technology is very expensive and is not widely used.

Rate Adaptive Digital Subscriber Line (RADSL)

ADSL technology has one significant drawback. It does not allow you to change the speed depending on the quality of the line. In such modems, the choice of speed, a multiple of 1.5 or 2 Mbps, is made using software. Equipment built on the basis of RADSL technology allows you to automatically reduce the speed depending on the quality of the line.

Universal ADSL (UADSL)

ADSL technology has a number of minor drawbacks that prevent the wide implementation of the technology on subscriber access networks. This is the complexity of installing ADSL devices; they require serious tuning to a specific subscriber line (as a rule, with the participation of a technical employee of the company - network operator), have a relatively high cost.

Not so long ago there were reports of the creation new version ADSL technology, which is designed to eliminate these shortcomings. It is called Universal ADSL (UADSL), or DSL Lite. True, when using this technology, data is transmitted at lower speeds than in ADSL (with a subscriber line length of up to 3.5 km, the speed is 1.5 Mbps in the direction to the subscriber and 384 kbps in the opposite direction; with a length of subscriber line up to 5.5 km are provided with 640 kbps in the direction of the subscriber and 196 kbps in the opposite direction). However, these devices are easier to install; in addition, they include a frequency separator, so it does not have to be installed separately. Essentially, just plugging a UADSL modem into a telephone jack is as simple as plugging in a regular modem.

The cost of such devices does not exceed the cost of a conventional modem, so it is expected that this particular technology will be widely used in end-user access equipment.

Standards

The American National Standards Institute (ANSI) Working Group T1E1.4 recently approved a standard for ADSL up to 6.1 Mbps (ANSI Standard T1.413). ETSI has supplemented this standard with requirements for Europe. T1.413 defines a single terminal interface from the operator side. The second version of this standard, being developed by the T1E1.4 group, expanded the standard in which it defined: a multiplexed interface on the operator side; configuration protocols and network management.

Some numbers

Distances for short range modems depend on the diameter of the copper pair:

1. Telindus Crocus HDSL 2048Kb/s:

wire diameter (mm)	2-pair version (km)	3-pair version (km)
0.4	3.6	4.0
0.5	5.0	5.5
0.6	7.1	7.8
0.8	8.9	9.9
1.0	12.5	13.9

2. Telindus Crocus SDSL:

wire diameter	384 kbit/s	768 kbit/s	1152 kbit/s
0.4mm	5.0 km	4.3 km	3.6 km
0.5mm	6.9 km	6.0 km	5.0 km
0.6mm	9.8 km	8.4 km	7.1 km
0.8mm	12.4 km	10.6 km	8.9 km
1.0mm	17.3 km	14.9 km	12.5 km
1.2mm	19.3 km	16.6 km	13.9 km

3. Telindus Crocus HS (144Kb/s):

wire diameter (mm)	distance (km)
0.4	6.9
0.5	9.5
0.6	13.5
0.8	17.5
1.0	26.0

Addition1

The article is well written, everything is correct, however there are some comments regarding the implementation of ADSL in real life. Unfortunately, ADSL can only be used on ordinary Russian communication lines on an experimental basis, so far there is no talk of commercial operation. An ADSL line requires a TWISTED pair (not a noodle) and shielded, and if it is a multi-pair cable, then in compliance with the direction and pitch of the twist.

You can object (S.Zh.), noting that the noodles go only in the area from the cross in the house to the apartment, its replacement with a twisted pair does not present both technical and economic difficulties. In the cross-telephone station section, multi-pair cables are used, where each pair is twisted.

It seems to be convincing BUT have you tried to disassemble the telephone cable? Remove the insulation meter from the imported cable and from the domestic one. The imported one will dissolve into twisted pairs that will not fall apart even if they are pulled, and the domestic one almost immediately turns into a broom and a fair amount of skill is required to cut it without additional devices. Replacing noodles also doesn’t seem to be scary, but noodles won’t do here, you will need to replace the KRT (telephone distribution box), especially if it is plastic (remember how LANs are divorced) and it costs in each entrance and often more than one. The direction of the lay in domestic multi-pair cables is not respected (disassemble for example our 50-pair cable or 100-pair cable), because no one thought that such cables would be used to transmit wide-spectrum high-frequency signals, and, accordingly, no one also spoke about protection against crosstalk. thought. For the capitalists, perhaps, this benefit also arose by chance, because there is competition and in order for products to be bought, it must meet not even the obligatory, but the parameters recommended by all sorts of commissions (because these commissions do not eat their bread for nothing) and on the territory of one district (or even blocks) there may be two or more telephone service providers. Vooschem, as always, thanks to competition, quality goods and services are obtained.

For E1, a twisted pair is used with as many as two screens isolated from each other along the length of the cable and with a regulated number of cable spans, otherwise there is no need to talk about any mileage and stable communication.

This is true, but in my opinion (S.Zh.) DSL technology is more likely to find its application not in industry, but in the residential sector.

Yeah, that's what I can add (I.Sh.), a couple of years ago this technology was offered to ROSTELECOM for the reconstruction of short highways, and the trunk cable is not your home wiring for such a cable, you can skip 64 Mbps and this modernization was built according to the station-scheme cable station. Well, ROSTELECOM did not agree to use these technologies, because it is expensive. I doubt that now the equipment has fallen in price so much that it costs like an Ethernet hub? And if I'm wrong, then someone wants to warm their hands a lot on the modernization of cable lines and the introduction of new technology.

Well, now let's imagine that 2-6 Mbit is loaded into the telephone cable, but it (the cable) does not have the appropriate parameters (often the inter-wire insulation is underestimated - well, they wet the poor fellow, they probably heard cod and space conversations in the receiver), as a result, pickups will come out . I think that these pickups will be the result of combinatorial frequencies, and a very wide spectrum, which will interfere with television receivers so much that a real war can begin. So in practice, not everything goes smoothly, unfortunately.

That is why, personally, I think (S.Zh.) that the introduction of UADSL with low speeds (up to 640 Kbps). All these effects in this technology will be expressed to a much lesser extent.

I think (I.Sh.) that anyway, the price of such an implementation will be too high at this stage to seriously think about it. So here more problems than it seems at first glance and in any case a more serious approach is required.

And here is my information (S.Zh.): providers, in particular Rosnet, do not share your views on technical problems and can provide ADSL equipment. Installing a modem, setting up, connecting, costs about $2,500. At the same time, speeds up to 640 Kbps are provided. The monthly subscription fee is about $300.

ADSL modems now cost around $800-1500. UADSL modems should cost around $250-500, which is more acceptable.

As soon as each telephone node has installed access equipment for data transmission networks, this type of service will become much cheaper, and the introduction of such access equipment is directly related to the introduction of ATM.

Addition2

In the article, Stanislav Zhuravlev sets out the theoretical aspect well, but does not touch upon the specifics of the application of this technology in Russia. In the first addition, some gaps are eliminated, but there are several inaccuracies:

First, xDSL technologies were developed by Bell's research division specifically for use on the existing copper wire infrastructure, which, even in the USA, is old and built on a conventional copper telephone pair, rather than a shielded twisted one.

Secondly, "noodles" are really not suitable for xDSL lines, but "noodles" are used in the area from the telephone distribution box to the subscriber socket, which is usually about 5-15 meters. In fact, there are two limitations that, with a given line resistance (usually 1-1.5 kOhm), do not allow the use of xDSL devices, these are loading and assembly from wires of various sizes. Line loading is the introduction of an inductive component into the line in order to reduce signal attenuation, but in Russia such lines are almost never used. The second problem is quite common, but if the station part of the equipment is located at the PBX closest to you, then the likelihood of such a problem is small, in any case, this problem can be solved with a local telephone exchange. However, if you need a direct channel, for example, to connect two local networks, then this is not a problem either. In Moscow, there are a fairly large number of direct channels operating on copper at a distance of 5-7 km and a resistance of 1-1.5 kOhm.

The wide distribution of xDSL technologies in Russia is constrained, first of all, not by an insufficient number of telephone pairs with acceptable parameters (while the number of installed lines in Moscow is estimated at tens or hundreds), but by the price of equipment, $ 2000-3000 for a set of station and subscriber parts, the price of connection and the cost of a dedicated channel (for the sake of curiosity, look at any of the providers how much a synchronous channel costs 64K channel, the prices will unpleasantly amaze you). The speed of already installed lines usually ranges from 64-512K. xDSL lines operating at a speed of more than 2Mbit over copper, I have not seen at all and I think their appearance is unlikely in the near future. This is explained by the fact that the cost of a 2Mbit stream is so high that either very large commercial firms or telecommunications companies that are themselves engaged in providing services can afford it, and such a criterion as the probability of an error on the channel is very important for them. The smallest error probability is provided by optical fiber, the stability of which will in any case be several orders of magnitude higher than xDSL lines.

It seems to me that the most promising prospects are for equipment designed for speeds of 64-512K, especially those created in accordance with the UDSL standard, which should be adopted before the end of this year. Manufacturers promise the price for a subscriber UDSL modem not more than $300-400. If large telecommunications companies (ideally MGTS :--)) become interested in providing xDSL services, which will be able to place station equipment sets at a large number of telephone nodes at their own expense, we will soon see a sharp increase in the number of used xDSL lines.

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

History of appearance

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

Modern ADSL technology: how it works

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

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

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

What equipment is used for this connection?

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

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

Additional equipment: splitters and microfilters

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

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

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

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

pros

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

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

disadvantages

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

Speed ​​and factors affecting it

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

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

Speed ​​characteristics

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

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

Technology Perspectives

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

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

Conclusion

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

    Access via digital channels using xDSL technology - the ability to receive high speeds data transmission at a relatively low cost of equipment using a conventional telephone network. Ordinary telephone cables are becoming high-speed digital channels, and the data transfer rate depends only on the quality and length of the line connecting the user and the provider.

The xDSL abbreviation uses "x" to represent the first character in the name of a particular technology, and DSL stands for Digital Subscriber Line.

The existing types of xDSL technologies differ mainly in the form of modulation used and the data rate, which can reach 52 Mbit per second for VDSL technology (on a good link and a distance of up to 1.5 km). To date, the most common technology is ADSL (Asymmetric Digital Subscriber Line - Asymmetric Digital Subscriber Line). For data transmission, a regular telephone pair is used, but three information channels are organized - a "downstream" data transfer stream (downstream) with an exchange rate of up to 8 Mbps, an "upstream" data transfer stream (upstream) with an exchange rate of up to 1.5 Mbps sec. and the Plain Old Telephone Service (POTS) channel. Moreover, the telephone channel is allocated using filters, which guarantees the operation of your phone even if the ADSL connection fails. As a result, you get round-the-clock access to the Internet while maintaining the normal operation of a regular phone. The "asymmetry" of ADSL technology is simply expressed in the low cost of the equipment used and the significant difference in the speed of data transmission "to you" and "from you". Further development of ADSL technology has led to the emergence of its modifications with increased data transfer rates (ADSL2, ADSL2+)

    You connect to an ISP using an ADSL modem. According to the method of connecting to a computer, modems can be divided into USB and ETHERNET - modems. The same modem can have both USB and Ethernet ports (multiple ports). USB modems tend to be cheaper, but are only intended to be used in bridge mode. In this mode, the modem works similarly to dial-up modems. Instead of a standard dial-up connection, a PPPoE (Point-to-Point over Ethernet) connection is used. USB - modems in their pure form are now practically not produced.
    It is more common to connect to a computer via Ethernet, which assumes that it has a network card. With this connection, it is possible to use the modem both in bridge mode and in router mode. A modern ADSL modem is practically a specialized computer with its own software that performs not only routing and network address translation (Network Address Translation or NAT), but also support for device management via HTTP and (or) Telnet protocols, services domain name resolution (DNS), dynamic host configuration (DHCP), Firewall (firewall), TFTP server, etc. Naturally, all these internal functions are available if the modem is operating in router mode. Below is an example of a simple scheme for connecting a home LAN to the Internet using a Zyxel P660RU2 modem in router mode.

The Zyxel P660RU modem has only 1 Ethernet port, therefore, a switch is used to connect several computers. If the modem has several ports, the number of which is enough to connect your computers, a switch is not needed. The modem has an IP address equal to 192.168.1.1 on the LAN interface. Client computers have addresses 192.168.1.2, 192.168.1.3, and 192.168.1.33. The netmask is 255.255.255.0. The modem is used in router mode with NAT. DHCP is not used, the TCP/IP settings of client computers are done manually.

If you don't have internet access try:

1. Check if there is a physical connection to the provider's equipment. Almost all modems have an indication panel that displays the status of the ADSL line. The status indicator is usually labeled "ADSL", "DSL", "Link", "CD", etc.
In the case of normal functioning of your modem equipment, data line and provider equipment, the indicated indicator should light up on the modem indication panel. If it doesn't, try the following:

turn the modem off and on again. If the situation has not changed, try disconnecting all other devices from the telephone line (telephones, microfilters, splitter) and connect the modem to the telephone line directly. If the ADSL indication appears, deal with the disconnected equipment.

check if there is a dial tone on the telephone line. If not, try disconnecting all devices from the telephone line, and connect a known working phone. If the dial tone does not appear, contact the local telephone exchange.

Try, if possible, to replace the modem with a known good one. If this is not possible, and the fulfillment of the previous points did not lead to the indication of an ADSL line, contact the technical support service of your provider.

2. If there is an indication of the healthy state of the ADSL line, but there is no Internet access, a possible cause may be the lack of a PPPoE connection to the provider. In bridge mode (bridge or bridge), such a connection is made using operating system. In router mode, by modem software. Depending on the modem model, the indication panel may have a connection activity LED marked as "Internet", "PPP", "PPPoE", "WAN", etc. The reason for the lack of connection may be an incorrect username and / or password for connecting to the provider's network. If they are correct, the balance of your personal account may have been exhausted, or your account has been blocked by the provider for some other reason. Please contact technical support.
3. If the ADSL line and PPPoE connection are normal, but there is no Internet access, try the following:

make sure the cable that connects the modem to your computer is working. For most modems, there is an indication of the physical connection - LEDs "Ethernet", "LAN" or "USB". If the indicator is not lit, the modem port, connecting cable, or port to which the modem is connected may be faulty.

if all connections are normal - the problem is in the network settings.     All further checks and adjustments will be done in the Windows console. The case is considered when the modem is used in the router mode. Network settings can be obtained with the command:
ipconfig /all
As a result, we get:

NvidiaNforceNetAdapter - Ethernet adapter:
The DNS suffix for this connection. . :
Description. . . . . . . . . . . . : NVIDIA nForce Networking Controller
Physical adress. . . . . . . . . : 00-18-F3-EF-60-DC
DHCP is enabled. . . . . . . . . . . : No
IP address. . . . . . . . . . . . : 192.168.1.33
Subnet mask. . . . . . . . . . : 255.255.255.0
Main gate. . . . . . . . . . : 192.168.1.1
DNS servers. . . . . . . . . . . : 192.168.1.1

    The "IP address" line contains the IP address of the computer, "Subnet mask" - the mask that matches the router's mask (standard 255.255.255.0). The "Default Gateway" line should contain the IP address of your modem. In the line "DNS servers" - addresses of working DNS servers or the IP address of the modem, if its DNS server is used for name resolution. A combination of these addresses is possible. If you have any suspicions about the health of the provider's DNS servers, try manually changing their addresses in the TCP / IP settings to addresses

208.67.222.222 or 208.67.220.220 - OpenDNS servers
8.8.8.8 or 8.8.4.4 - Google servers
it is better to choose a combination of Google server and OpenDNS server.

    One of the main tools for diagnosing network problems is the host availability check commands ping.exe and the trace route command to the selected node tracert.exe. When using these commands, the ICMP protocol must be allowed in the firewall, or even better, turn off the firewall while diagnosing problems.

Brief help on using ping.exe can be obtained with the command:
ping /? Examples:

ping yandex.ru- check the availability of the site yandex.ru
ping 192.168.1.1- check the availability of our router.
When ping is performed without specifying keys, ICMP messages are sent 4 times (echo request) to the host specified on the command line, and a response is received with analysis of the response time (echo reply). The request and response data field contains a repeating string of Latin characters (from a to w). The default data length in Windows is 32 bytes. An example of the result of "ping yandex.ru":

Packet exchange with yandex.ru for 32 bytes:

Response from 77.88.21.11: number of bytes=32 time=5ms TTL=57

Response from 77.88.21.11: number of bytes=32 time=2ms TTL=57
Response from 77.88.21.11: number of bytes=32 time=1ms TTL=57

Ping stats for 77.88.21.11:
Packets: sent = 4, received = 4, lost = 0 (0% loss),
Approximate transmission and reception times:
smallest = 1ms, largest = 5ms, average = 2ms

    Ping statistics give a complete picture of the exchange between your computer and the pinged host. The TTL field in the echo response depends on the implementation of the IP protocol of the responding node (simplified, we can assume that it depends on the type and version of the operating system). Please note that some nodes are configured in such a way that they do not respond to ping (microsoft.com, for example)

More examples of using ping.exe:

ping -t yandex.ru- ping before pressing CTRL-C or CTRL-Break
ping -n 1000 -l 500 192.168.1.1- ping 1000 times using messages 500 bytes long.
ping -a -n 1 -r 9- perform ping 1 time (switch -n 1), determine the address by name (switch -a), issue a route for the first 9 hops (-r 9)

    Key Usage -r, to some extent, allows you to get a traceroute similar to that obtained using the tracert.exe command, but the maximum number of hops can be set to 9, which is usually not enough. Therefore, it is advisable to use tracert.exe.

tracert google.com- trace route to google.com node

Result:

Tracing a route to google.com with a maximum of 30 hops:
1 1 ms 2 498 ms 444 ms 302 ms ppp83-237-220-1.pppoe.mtu-net.ru
3 * * * .
4 282 ms * * a197-crs-1-be1-53.msk.stream-internet.net
5 518 ms 344 ms 382 ms ss-crs-1-be5.msk.stream-internet.net
6 462 ms 440 ms 335 ms m9-cr01-po3.msk.stream-internet.net
7 323 ms 389 ms 339 ms bor-cr01-po4.spb.stream-internet.net
8 475 ms 302 ms 420 ms anc-cr01-po3.ff.stream-internet.net
9 334 ms 408 ms 348 ms 74.125.50.57
10451ms 368ms 524ms 209.85.255.178
11 329 ms 542 ms 451 ms 209.85.250.140
12616ms 480ms 645ms 209.85.248.81
13 656 ms 549 ms 422 ms 216.239.43.192
14 378 ms 560 ms 534 ms 216.239.43.113
15511ms 566ms 546ms 209.85.251.9
16 543 ms 682 ms 523 ms 72.14.232.213
17 468 ms 557 ms 486 ms 209.85.253.141
18 593 ms 589 ms 575 ms yx-in-f100.google.com

The trace is complete.

    Let me remind you how it works. During the development of the IP protocol, in order to reach nodes whose addresses do not belong to the current network, routing was provided to transfer IP packets between different networks. When you issue the "tracert google.com" command, it first detects an IP address of google.com (74.125.45.100) that does not belong to the address range of your local network given by the network card's IP address value and subnet mask (192.168.1.0-192.168 .1.255). Such a packet will be sent to the router whose address is set as the default gateway. In the trace results, you see it first (192.168.1.1). Then (simplified, of course) the same algorithm works - if the google.com host is not reachable locally, it determines which router the packet should be sent through, and sends it.
    In the trace results above, it took 18 hops to reach google.com. Now imagine that on node number 10 (209.85.255.178) in order to reach the google.com node, the route is erroneously registered not to node number 11, but, for example, to node number 5. The result of such an error would be a loop and eternal circulation of the packet between nodes 5 and 10. In order to avoid such a situation, the developers of the IP protocol prudently introduced a TTL field ("Time to Live" - ​​Time To Live) in the header of IP packets with a length of 1 byte, taking values ​​from 0 to 255. In fact this field is not related to time, but is a count of the number of possible hops in the transmission of a routed packet. Each router, upon receiving a packet, subtracts 1 from this field and checks the value of the TTL counter. If the value becomes zero, such a packet is discarded and an ICMP time-to-live message is sent to the sender ("Time Exceeded" - value 11 in the ICMP header).
    When executing the tracert.exe command, it first sends an ICMP packet with a TTL field equal to 1 and the first router in the chain (your modem) resets the TTL and reports a time-to-live. This sequence is repeated three times, so the result line generated by tracert.exe shows three response times after the jump number:
1     1 - hop number (1 - first router, i.e. your modem)
1 ms 192.168.1.1 - his address (or name)
    Then the procedure is repeated, but the TTL is set to 2 - your router will decrement it to 1 and send it to the next one in the chain - i.e. provider router. That one, after subtracting 1, will reset the TTL and report that the lifetime has been exceeded. And so on, until the specified host (google.com) is reached, or until a failure is found that prevents the packet from being delivered to the recipient.
    Tracing results may contain entries about nodes in the form of asterisks (node ​​number 3 in the example) - this is not a sign of a malfunction and most likely indicates that the settings of this node prohibit the ICMP protocol for security reasons (combating DDoS - attacks)

    The ping command mentioned above has the key -i. It allows you to set the TTL value for the echo request, i.e. implement the same route tracing when executing ping -i with a successive increase in -i from one.
ping -i 1 yandex.ru
ping -i 2 yandex.ru
...

    In tracert.exe command line parameters, it is sometimes more convenient to specify not a host name, but any real non-private IP address. For example, commands:

Ping yandex.ru
tracert yandex.ru

May end with a message that the name yandex.ru (an unknown host yandex.ru) cannot be resolved. The reason for this may be the inoperability of the used DNS server (servers), its incorrect address, the stopped "DNS client" service, incorrect firewall settings, the intrigues of a caught virus, etc. And possibly lack of internet access. You can use the command:

Tracert 77.77.77.77
IP address 77.77.77.77 taken regardless of the actual node. The main thing is that it should be a correct, not reserved for local networks ("white") IP address. If the trace results for it show available nodes after the 2nd hop (after your modem and the provider's router), then with a high degree of probability it can be assumed that access to the provider's network exists, and you need to deal with name resolution.

A few more signs:

If after ping or tracert command:
arp -a
you will see that the ARP cache contains the IP and hardware (MAC) address of the router, then the path between the client machine and the router (modem) is working, the Ethernet and IP protocols are working.

If the route ends up to your modem, but there is the above entry in the ARP cache, then the ICMP protocol is most likely blocked by the firewall settings of your computer or router.

If the route terminates after your modem, then the problem is between you and the ISP.

If the trace shows the availability of nodes after the provider's network, then it is most likely that the problems are in the settings on the client computer.

That's all for now, but more is planned.

    Here I've put together some useful gizmos that can help Stream users, and not just Stream users.
    Standard unlimited tariffs, the user gets Internet access using ADSL technology (ADSL2+). The IP address is real, but dynamic. Traffic filtering is used - TCP ports 21, 23, 25, 69, 80, 135-139, 445, 8080, 254, 255, 161 UDP - 69, 135-139, 161. This is done to ensure the security of Widows systems and prevent spam -Activity and protection of subscriber equipment. In other words, on the client side, Windows network resources, standardly configured HTTP-FTP, TFTP, SMTP servers are not available from the outside. Sometimes the dynamic address and traffic filtering create some problems, the solution of which (free of charge) is offered below.

We solve the problem of a dynamic IP address.

    Go to the DynDNS.com website. To work with an existing or new account, use the button "Sign In"(at the top right of the page).
Create your account - click "Create Account". When registering, select free access (free account). The registration form changes periodically, but it is mandatory to enter the desired username, password and your E-mail. An email with a confirmation link will be sent to the email address you provided during registration. We confirm and enter the site. Click the button "My Services" on the left side of the screen and then select the menu item (on the left) Host Services

Click "Add new hostname" and fill out the form, where we indicate the desired computer name, the desired domain and IP address, no matter what, as long as it is valid, for example, offered by the form itself, your current address. Click the button "Create Host". If the chosen name is not occupied by anyone, "hostname" will be created.

    Now it remains to download and install a special client program DynDNS Updater
We go to the section "Support"- subsection "Update Clients"

    Download the client for our operating system. When installing on Windows (DynUpSetup.exe), you will be prompted to install the DynDNS client as a service. This will allow it to run before the user logs in. Otherwise, at the first start, after installation, the client will register in autoload, and will be executed after the user logs into the system. I do not like extra services on the computer and did not use the installation as a service.

    To log in, you will need to enter the username and password you received when you registered with DynDNS.com. You will then see a list of your HostNames created in DynDNS.com. We tick off the names for which the update will be performed by the client. The number of names is limited for a free account, which you will find information on the site. The program is very simple, the main settings are on the tab Advanced:

    Some modern ADSL modems have a built-in DynDNS client. The setup is usually very simple - fill in the username and password received during registration on DynDns.com and the name of your Host. Example for Zyxel P660RU2

    Using a modem DynDNS client can sometimes be very useful. The computer can be remotely turned off and on, as described in the "Wake On Lan Technology" example in the "Network" section, as well as deploy the servers you need on it, connect to its desktop and manage it from anywhere, there would be Internet access.

Solving the problem of switching to ADSL2+.

    A small digression. At the end of 2008 quite attractive tariffs appeared in Stream with connection using ADSL2+ technology. Besides, effective tariffs have changed since March 2009, except for ADSL2+ tariffs. The difference in payment for my unlimited tariff "Stream 6 Hit" (transmission speed 6144/768 kb / s) and "STREAM 10 HIT 2+" (speed 10240/896 kb / s) was 70 rubles. My Zyxel P660 RU2 modem supports ADSL2+, so there shouldn't be any problems with the transition to the new tariff - I switched. Like many other Stream subscribers. A lot has been written about the problems with the transition to ADSL2 on many thematic forums, people are spitting on Stream, many are changing providers, in short, everything turned out to be not at all simple. At first, I was also a little berserk, because in ADSL2 + mode the modem could try to establish a connection for half an hour, and in the case when it succeeded, the speed of the outgoing stream (upstream) could be 9 kb / s instead of 896 kb / s. Sometimes, however, it reached 500 kb / s, but this rarely happened, and be that as it may, such a speed is very far from the declared one for this tariff. True, the downstream speed was almost always appropriate tariff plan. As it turned out later, other subscribers could have the opposite - the upstream speed is normal, but the downstream speed is not. There were cases when it was impossible to make an ADSL connection for hours. A few days later everything suddenly worked, but not for long. Then I noticed that the speed dropped, became the same as on my previous tariff. I look at the state of the modem - it is - the speed is 6144/768 and DSL mode - G.DMT, i.e. I get my old "Stream 6 Hit", although in my account I see that my current tariff is "Stream 10 Hit2"
    Of course, all this is not encouraging. Even I (and I have been using Stream for about 5 years) had a desire to change the provider. But, in the end, he cooled down and decided to wait - it's a new thing for Stream, there are slips. I'm a techie, I get it. About a month later, I called Stream's technical support and asked to still turn on ADSL2+. After 20 minutes, the connection disappeared, then it was restored, and the old picture returned - there was no speed on the outgoing stream. From the messages on the forums, I concluded that "running into" technical support and Stream engineers is a thankless task and, I would say, harmful to your own nervous system. And then - the technology is new for them, the proven standard solutions do not work, and the transition to ADSL2 +, apparently, was carried out too hastily and without proper measures, such as testing settings, checking on different models of modems, personnel training, etc.

    It can be assumed that ADSL-modems supporting ADSL2 mode are mainly supplied with a configuration designed to work in normal ADSL. Also, we can assume that if you adjust the modem settings to the settings of the provider's equipment (DSLAM - Digital Subscriber Line Access Multiplexer - digital subscriber line access multiplexer), then everything will work stably and at the required speed. In my case, it turned out that way.

    To change the modem settings, connect to it using telnet.exe:
telnet 192.168.1.1
The modem will ask for a password.
Password:
Enter administrator password
To view the current modem settings, enter the command:
sysview autoexec.net
The modem will display the contents of the autoexec.net file
sys errctl 0
systrcl level 5
sys trcl type 1180
sys trcp cr 64 96
sys trcl sw off
sys trcp sw off
ip tcp mss 512
ip tcp limit 2
ip tcp irtt 65000
ip tcp window 2
ip tcp ceiling 6000
ip rip activate
ip rip merge on
ppp ipcp compress off
sys wdog sw on
ip icmp discovery enif0 off
bridge mode 1
sys quick enable
wan adsl rate off
ether driver qroute 2
wan dmt db tlb e

For ADSL settings, commands from the group are important wan. The "wan dmt..." command refers to normal ADSL settings, while for ADSL2 it should look like "wan dmt2..."
I did not find a clear description of the "wan dmt2 db ..." command, but I assumed that it would load the selected profile of optimal modem settings under the provider's DSLAM. Possible settings for a particular modem can be obtained with the command:
wan dmt2 db disp
For P660RU2 we have the result:
 
db_sel=ff db_final_sel=4
No    Compare    Reset    Pre       Mid       After         Help
0                                           
CTLM Database
GSPN Database
BCM Database
IFNEON Database
5                                                   TI Database

 
Total - 5 options, 0 - not used, 1-5 you can try.
(In short, equipment manufacturers - 1 = CTLM = Centillium, 2 = GSPN = Globespan, 3 = BCM = Broadcom, 4 = IFNEON = Infineon)
Command to load settings option:
wan dmt2 db tlb x - where x is from 1 to 5
We sequentially execute:
wan dmt2 db tlb 1
Resetting the line
wan adsl reset
After the connection is established, we look at the status of the channel:
wan adsl chandata
You will see connection details:
DSL standard: ADSL2+ Mode-ADSL2+ mode.
near-end bit rate: 10240 kbps - speed to you.
far-end bit rate: 1020 kbps- speed from you.

    When you decide which option you have the most stable work with, you need to write this command to the modem so as not to enter it every time after turning it off. To do this, enter the command:
sys edit autoexec.net
We see the message:
EDIT cmd: q(uit) x(save & exit) i(nsert after) d(elete) r(eplace) n(ext)
This is a hint for editing commands:
q - exit without saving the results;
x - exit with saving;
i - insert a line after the output line;
r - replace current line;
n - print the next line. You can just press ENTER;

        Now press ENTER until the line appears
wan dmt db tlb e
- click r without pressing ENTER after it
- we type the command for the selected option:
wan dmt2 db tlb 3- for option 3
- Press ENTER and press "x" to save and exit editing.
The result can be checked with the sys view autoexec.net command
sysview autoexec.net
sys errctl 0
systrcl level 5
sys trcl type 1180
sys trcp cr 64 96
sys trcl sw off
sys trcp sw off
ip tcp mss 512
ip tcp limit 2
ip tcp irtt 65000
ip tcp window 2
ip tcp ceiling 6000
ip rip activate
ip rip merge on
ppp ipcp compress off
sys wdog sw on
ip icmp discovery enif0 off
bridge mode 1
sys quick enable
wan adsl rate off
ether driver qroute 2
wan dmt2 db tlb 3

When working properly in ADSL2+ mode at the "Stream 10 Hit2" tariff, the modem state should be like this:

Just in case, some commands for P660RU2:

A list of commands for each level can be obtained by entering a question mark or an invalid command:
wan - will give a list of subcommands for wan, wan adsl - for the adsl level of the wan command. There is no hint for "wan dmt:" in the firmware of this modem. You can type commands incompletely - "wan adsl chandata" and "wa adsl c" are identical. However, there is information that in some modems the abbreviation does not work everywhere, and there is no diagnostics, therefore it is better not to use the abbreviated versions of the commands, or use it by controlling their operation.
Perhaps the inclusion of the OLR (Online Reconfiguration) mode, which allows you to change the configuration without breaking the connection, will help someone. It must be supported by the provider's equipment.
wan dmt2 set olr x
where
x=0 - OLR disabled
x=1 - OLR enabled
x=2 - SRA(Streamless Rate Adaptation) disabled.
X=3 - SRA enabled, adaptation to existing line possible.

    The "wan adsl linedata" command is used to evaluate line quality:
Line state for upstream (far end):
wan adsl linedata far
noise margin upstream: 11 db- at a value below 7 db, an unstable connection and low upstream speed were observed (although according to information from the Zyxel website, the noise immunity limit is 6 db)
output power downstream: 0 db
attenuation upstream: 2 db
tone 0-31:00 00 00 35 68 9a bb bc cc dd dd dc cc cb ba a9
tone 32-63: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 64-95: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 96-127: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 128-159: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 160-191: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 192-223: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 224-255: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 256-287: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 288-319: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 320-351: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 352-383: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 384-415: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 416-447: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 448-479: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 480-511: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Line status for flow to you (near end):
wan adsl linedata near
noise margin downstream: 21 db - for my line in the range of 20-24 db
output power upstream: 10 db
downstream attenuation: 0 db
tone 0-31: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 32-63: 00 00 00 00 00 01 00 11 42 45 53 55 55 47 65 57
tone 64-95: 55 57 75 57 58 55 76 68 34 53 55 48 77 65 77 75
tone 96-127: 55 65 45 57 65 77 74 44 76 76 57 77 55 88 55 97
tone 128-159: 79 94 76 75 78 48 49 75 76 59 56 88 44 57 85 45
tone 160-191: 55 55 73 58 76 53 45 53 55 67 45 54 57 76 55 55
tone 192-223: 56 64 56 66 55 54 46 35 56 25 14 31 53 02 54 35
tone 224-255: 15 54 33 02 31 04 15 00 11 01 30 55 41 33 14 46
tone 256-287: 64 34 31 56 63 65 67 56 55 47 67 67 55 46 78 79
tone 288-319: 69 58 89 99 79 76 97 98 79 76 98 79 89 87 79 74
tone 320-351: 76 88 89 99 99 9a 89 49 98 49 77 a9 4a 99 a9 98
tone 352-383: 6a 8a 86 86 a9 89 97 a9 97 98 9a a9 99 99 79 79
tone 384-415: 88 97 88 46 88 94 99 74 88 98 87 87 88 59 99 98
tone 416-447: 88 88 88 48 99 87 98 88 98 98 88 87 84 98 89 48
tone 448-479: 86 48 47 98 68 88 88 88 88 88 89 98 88 88 48 88
tone 480-511: 88 86 88 98 87 40 68 87 88 89 44 48 68 aa a8 80

    Noise margin downstream(Reception Noise Immunity Limit) - used as a criterion for judging the state of the line and defines the minimum limit at which the signal level is higher than the noise level. It is believed that if the ADSL modem is difficult to synchronize with the DSLAM, then the noise immunity limit on the switch should be reduced.
output power downstream(Output power when receiving data) - shows the output power when receiving data at the moment the modem is synchronized with the DSLAM.
Attenuation downstream(Data Receive Attenuation) - shows the Receive Attenuation when the modem is synchronizing with the DSLAM (this parameter must be

Cisco 827-4V and Stream.

   

Reset password for CISCO IOS

To reset the modem password, you will need a console cable and the Hyperterminal program (included with all Windows). The console cable connects to the console port (RJ-45 "Terminal" connector) on the modem and to the computer's serial port. The cable can be made independently using the diagram, which you will find on the pinouts.ru website.
Connect the console cable with the devices powered off. To exchange with the console port in HyperTerminal, use the parameters:

Speed ​​- 9600 bps
Data bits - 8
Parity - No
Stop bits - 1
Hardware flow control.

    After turning on the modem in the terminal window, you will see messages about the beginning of the download. After 30-60 seconds, press CTRL-Break, the download should stop and you will see the Rom Monitor prompt:

rommon 1>
Now we need to change the value of the modem configuration register so that when CISCO IOS boots, the configuration saved in the modem (startup-config) with an unknown password is not used:

rommon 1> confreg 0x2142

A value of 0x2142 means bypassing startup-config processing. We reboot Cisco with the reload (or reset) command. After the initial boot, a message will appear to select the configuration dialog:

Would you like to enter the initial configuration dialog? :

We answer - No
After that, a standard prompt for entering commands (Router>) will appear. To enter privileged mode, use the command

Router > enable

The Router> prompt will change to Router#
Now you can copy the saved configuration as the current one (running-config):

Router# copy startup-config running-config
We change the password to a new one, for which we set the configuration in the terminal:

Router# configure terminal
The prompt will change to reflect the level you are at - Router# will change to Router (config)# . To return to the previous level, use the exit command.
Enter a new password:
Router (config)# enable secret
This is the new password for privileged mode. To access via telnet, you also need to set a new password different from the previous one:
Router (config)# line vty 0 4
Router (config-line)# password
Route# (config-line)# exit
Now we have the previously saved configuration as the current configuration (running-config), with new passwords. But in this configuration, all network interfaces are stopped (administratively down). Therefore, for each of the interfaces used, you need to issue the no shutdown command. The list of interfaces can be obtained with the command:

Router# show ip interface brief

For each interface from the received list, we perform no shutdown. Example for Ethernet0:
Router (config)# interface Ethernet0
Router (config-if)# no shutdown
Router (config-if)# exit
Router (config)#
After starting all interfaces, you need to return the default value of the configuration register to load startup-config:

Router (config)# config-register 0x2102
Router (config)# exit
Now we have the current configuration (running-config) ready to go. It remains to copy it to Cisco NVRAM for processing at device startup (startup-config):

Router (config)# copy running-config startup-config

    To save and load the configuration, it is convenient to use the "Cisco TFTP Server", which can be taken from the CISCO website or Install it (by default, installation is performed in the "Program Files\Cisco Systems\Cisco TFTP Server" directory) and run.
To save the current configuration to the myconfig.txt file on the server with IP 192.168.0.10:
copy running-config tftp://192.168.0.10/myconfig.txt
To save the startup configuration to myconfig1.txt:
copy startup-config tftp://192.168.0.10/myconfig1.txt
To load the myconfig.txt file as the current configuration:
copy tftp://192.168.0.10/myconfig.txt running-config
To load the myconfig1.txt file as a start configuration:
copy tftp://192.168.0.10/myconfig1.txt
    Below is the contents of the configuration file for Stream:

!
version 12.1
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
! hostname stream-gw
! logging rate-limit console 10 except errors
enable secret TrxOXvO9bH8.
enable password password
! clock time zone MSK 3
ip subnet-zero
no ip finger
ip name-server 212.188.4.10
ip name-server 195.34.32.116
!
no ip dhcp-client network-discovery
vpdn enable
no vpdn logging
! vpdn-group pppoe
request-dialin
protocol pppoe
!
!
!
!
interface Ethernet0
ip address 192.168.1.1 255.255.255.0
ip nat inside
no ip mroute-cache
no cdp enable
!
interface ATM0
no ip address
no ip mroute-cache
no atm ilmi-keepalive
PVC 1/50
encapsulation aal5snap
protocol pppoe
pppoe-client dial-pool-number 1
!
bundle-enable
dsl operating-mode auto
!
interface Dialer1
mtu 1492
ip address negotiated
ip nat outside
encapsulation ppp
ip tcp adjust-mss 1452
dialer pool 1
dialer group 1
no cdp enable
ppp chap hostname [email protected]
ppp chap password XXXXXXX
!
classless ip
ip route 0.0.0.0 0.0.0.0 Dialer1
no ip http server
!
ip nat inside source list 101 interface Dialer1 overload
access-list 101 permit ip any any
dialer list 1 protocol ip list 101
no cdp run
!
line con 0
transport input none
stopbits 1
line vty 0 4
password password
login
!
scheduler max-task-time 5000
sntp server 192.43.244.18
end

    Create in directory Program Files\Cisco Systems\Cisco TFTP Server text file with this content, and copy it to the running config, change the access password in privileged mode (enable secret) and the password for telnet (enable password). If necessary, you can change the address for the Ethernet interface, VPI and VCI for your provider, if it is not Stream (pvc 1/50 - for Stream). And you need to enter your username and password to connect to the Stream network.
ppp chap hostname [email protected]- Username
ppp chap password XXXXXXX - password
These edits could have been made to the generated file before being copied to running-config. After making sure that this configuration works, save it to startup-config.

IntroductionAs the Internet developed, to ensure full-fledged work in it, more and more access speeds were required - if at first the Internet was predominantly text-based, then in the past few years, services related to the transmission of sound and video images in real time, and even volumes of typical pages, thanks to colorful graphics and flash animations, have grown from units and tens of kilobytes to hundreds of kilobytes, and sometimes even several megabytes.
However, if there were no problems with providing high-speed access to the Network of large organizations, then providing home access always rested on the same thing - the so-called "last mile". This term in telephony traditionally denotes a cable laid from a certain node (for example, a telephone exchange) to a subscriber, that is, an end user. The problem was that the cost of laying such a cable usually ranges from several hundred to several thousand dollars, and, obviously, in the case of connecting a home user, it falls entirely on his shoulders, making an individual high-speed connection to the Network prohibitively expensive.
For this reason, Internet access has traditionally relied on existing infrastructure, that is, the conventional telephone network. Indeed, in a modern city there is already a telephone in almost every apartment, in other words, if you use the telephone line also for Internet access, then the cost of laying the cable will be zero, and the client will have to pay only the cost of the final equipment, that is, the modem.
However, in a city telephone network, originally intended for voice transmission, the frequency band is forcibly limited at a level of about 4 kHz - this is more than enough for the usual tasks of a telephone, while a larger frequency range would only complicate the operation of the telephone network (audibility would only worsen due to the appearance high-frequency interference and increased mutual interference between adjacent lines). This limitation, of course, also applies to the signals transmitted by the modem, preventing high data transfer rates from being achieved - over the course of many years of development of modems, only 33.6 kbit / s was achieved.


The diagram above shows a somewhat primitive situation - in practice, everything is somehow major providers connect to the telephone network via digital channels; however, the 4-kHz filter on the user's side still does not disappear anywhere.
The situation slightly improved only with the advent of the V.90 standard, which made it possible to increase the transmission speed from the provider to the client up to 56 kbps, but even this speed was not always achieved - firstly, if more than one signal conversion from analog to digital (in modern telephone networks, the signal between exchanges is transmitted in digital form), then the V.90 protocol did not work at all; secondly, it turned out to be very sensitive to the quality of the line - far from all the lines where V.34 worked stably, it was possible to get quality work V.90. And, again, a further increase in speed in the existing telephone network was impossible (the theoretical limit is 64 kbps, but in practice the speed is deliberately limited to reduce mutual interference between adjacent lines).
As the usual modems ceased to satisfy the needs of users, all sorts of alternative options began to appear that did not use the telephone network, but somehow solved the problem of the high cost of laying " last mile". The most widespread are two technologies - radio access and satellite access.
The first technology consisted in installing a radio channel instead of a wired "last mile" - one transceiver was located directly at the client, the second - at a nearby station, which was already connected to the main channel, for example, fiber optic. Alas, such a solution, again, turned out to be quite expensive and by no means universal - the antennas must have been located in direct line of sight of each other, so each base station could serve only a relatively small number of customers, which negatively affected the cost of connection and further work.
The second technology is satellite Internet, which is also familiar to many. Since the transmitting satellite antenna is very, very expensive, a hybrid system was developed for connecting home users, in which the downstream data stream (from the provider to the user) was transmitted via satellite and received by a conventional inexpensive parabolic antenna, completely similar to those used in satellite television reception systems, and the upstream (from the user to the provider) was transmitted through the familiar telephone network using a conventional modem. Alas, such a system did not solve most of the problems either - the user still had to occupy a telephone line to work on the Internet, and the data transfer rate from him left much to be desired, which made it impossible, for example, to conduct two-way teleconferences. Yes, and with a one-way broadcast of a video signal, problems could arise - signal transmission via satellite generated quite noticeable delays.
Thus, none of the wireless (or partially wireless, as in the case of satellite Internet) technologies has been able to gain popularity, even remotely comparable to the popularity of the usual dial-up access through the city telephone network. Wired technologies continued to rest on the cost of laying the "last mile" ...
The way out of this impasse was quite obvious. After all, the bandwidth of the telephone network is limited by the equipment installed on the PBX itself, while the most common copper cable goes from the client to the PBX, capable of transmitting much higher frequencies than some three kilohertz ... Thus, the idea of ​​​​DSL (Digital Subscribers Line) - install one modem, as before, at the user's, connecting it to a regular telephone line, and another modem (more precisely, DSLAM - DSL Access Multiplexer) - not at the provider, but at the same PBX to which the telephone line is connected user, and enable it before PBX equipment. As a result, between the modems there was actually a simple piece of wire, without any restrictions inherent in the telephone network. Of course, due to the need to install equipment on each PBX network construction and maintenance costs were noticeably higher than in the case of classic dial-up access, when all provider modems were installed on one PBX, however, compared to the cost of other methods of providing high-speed Internet access, DSL technology turned out to be not just cheap, but very cheap.

Perhaps the only serious competitor for DSL was a technology that used another existing infrastructure - cable television networks. Technically, their use was more than justified - after all, they were originally designed to transmit high-frequency (tens and hundreds of megahertz) signals, but in practice, the prevalence of cable television is much lower than telephone networks, which led to the greater popularity of DSL.
ADSL (Asymmetric DSL) technology is a DSL variant in which the available channel bandwidth is distributed asymmetrically between downstream and upstream traffic - for the vast majority of users, downstream traffic is much more significant than upstream traffic, so providing it with most of the bandwidth is quite natural.
As I noted above, the conventional telephone network (in English literature, it is usually abbreviated as POTS, Plane Old Telephone System) uses a frequency band of 0 ... 4 kHz. In order not to interfere with the use of the telephone network for its intended purpose, in ADSL the lower limit of the frequency range is at the level of 26 kHz, that is, not only outside the frequency range of telephones, but even beyond the limits of human hearing. The upper limit, based on the requirements for data transfer rates and the capabilities of the telephone cable, is 1.1 MHz. This bandwidth is divided into two parts - frequencies from 26 kHz to 138 kHz are assigned to the upstream data stream, and frequencies from 138 kHz to 1.1 MHz are assigned to the downstream.
This frequency separation gives ADSL another advantage over dial-up access - if a regular modem occupies a telephone line, making it impossible to use the phone and access the Internet at the same time, then the ADSL modem does not interfere with the phone in any way - you can safely talk on it without disconnecting from Internet, and you will not feel any inconvenience. Of course, situations are possible when either the high-frequency signal of the ADSL modem negatively affects the electronics of a modern phone (obviously, it cannot affect old phones with rotary dialers - there is practically nothing to influence there), or the phone, due to some peculiarities its circuitry introduces extraneous high-frequency noise into the line or greatly changes its frequency response in the high-frequency region; to combat this, a low-pass filter is installed in the telephone network directly in the subscriber's apartment, which passes only the low-frequency component of the signal to ordinary telephones and eliminates the possible influence of telephones on the line. I note that a conventional analog modem connected through a filter continues to work as if nothing had happened, since it does not need any signals that go beyond the maximum 4 kHz allowed by the filter.
Generally speaking, filters are usually divided into microfilters and splitters. The former refers to filters that are switched on directly in front of telephones - between the telephone socket and the actual wire going to the telephone (I note that here telephones also mean ordinary analog modems), the latter are filters switched on at the input of the telephone network to the apartment and separating it into two parts - ADSL and regular telephone. As you can see, the difference is only in the place of installation, in terms of the device, both microfilters and splitters are exactly the same, so there is not much point in focusing on this.
Of course, the possibilities of the cable are not unlimited - as its length increases, the resistance increases, while ADSL equipment allows you to work with a cable resistance of no more than 1500 Ohms. Based on this, it is not difficult to determine the limits of ADSL operation - if a cable with a length of more than 5.2 km is laid from your apartment to the PBX, then the ADSL modem has every right not to work at all. If the cable length is exactly 5.2 km, then it should work, but the speeds are higher than 128 kbps. are not guaranteed. The ideal conditions are considered to be a cable length of no more than 1.8 km - while the ADSL modem can reach a maximum speed of 8 Mbps. from provider to user and 1.2 Mbps. from user to provider. Of course, these figures are indicative - in each case they depend on the cross-section of the cable used in the telephone line and its condition (the presence of connectors and "twists", all kinds of external interference, and so on), but practice shows that the speed is 1 Mbps. quite real for any city telephone line of any reasonable quality. Again, I note that only the quality of the wire from your apartment to the PBX matters for ADSL - everything that stands further has the most direct impact on ordinary dial-up access, but has nothing to do with ADSL. And even if there is a decade-step PBX built in the fifties of the last century in your area, you can only talk on the phone by shouting, and an ordinary modem refuses to connect to a provider at speeds above 9600 bps. - if your PBX can install ADSL equipment, then you have every chance to get Internet access at a speed of several megabits per second.
The most common, basic version of ADSL, also known as G.dmt and Full rate ADSL, has been described above. However, there is another "lite" option known as G.lite or Universal ADSL. Unlike G.dmt, it greatly reduces the bandwidth used and, accordingly, the maximum connection speed - it is only 1.5 Mbps. "down" and 512 kbps. "up". G.lite has two advantages - firstly, this standard allows you to slightly reduce the cost of equipment, and secondly, it is less demanding on the quality of lines and in most cases does not require the installation of a filter, allowing the user to simply connect the modem to the telephone socket, without any or interfering with the telephone wiring around the house (due to this, G.lite is sometimes also called "plug-n-play ADSL"). However, already now an ADSL modem that fully supports both G.lite and G.dmt can be bought for less than $50, and even G.lite can’t do without a filter in all conditions - it all depends solely on the phones you use and the quality of the telephone cable wiring in your apartment, so the benefit of using G.lite is not that high.

Other DSL Technologies

In addition to ADSL, there are several other DSL-based data transmission technologies with different characteristics and requirements. Firstly, the abbreviation DSL itself means not only the whole set of technologies, but also a very specific one, providing a speed of 160 kbps. (strictly speaking, the data rate is 144 kbps - two so-called B-channels at 64 kbps and one D-channel at 16 kbps; the remaining 16 kbps are protocol overhead) over a distance of up to 6 km on a single pair. "Classic" DSL uses a frequency band of 0 to 80 kHz (up to 120 kHz in some implementations) and is therefore incompatible with a conventional telephone. However, nothing prevents using one of the B-channels to transmit digitized voice (fortunately, digitizing the "telephone" range of 0 ... 4 kHz with a bit depth of 8 bits gives a data stream of just 64 kbps), moreover, DSL is often used for organizing two independent telephone lines (since there are two B-channels in total) on one pair of wires.
In the sixties, engineers at AT&T Bell Labs. created the first voice digitization system for telephone networks with subsequent multiplexing of twenty-four voice data streams (64 kbps each) into one data transmission channel operating at a speed of 1.544 Mbps. This system was called T1 (its European counterpart, which already combined thirty voice channels, was called E1 and operated at a speed of 2.048 Mbit / s.) And used a bandwidth of 1.5 MHz for data transmission with a maximum at a frequency of 750 kHz. The maximum data transmission range was about 1 km from the central station to the first repeater and about 2 km between subsequent repeaters, however, this technology was not suitable for connecting private users, not so much the need for repeaters, but too much interference, which did not allow organizing in one multi-core cable (which, in fact, goes from each residential building to the nearest exchange) more than one T1 / E1 channel. Moreover, mutual interference is so high that in general it is impossible to run another T1 / E1 channel even in an adjacent cable, so the networks of large telephone and telecommunications companies remained the lot of using T1 / E1 channels.
To eliminate this shortcoming, the HDSL (High data rate DSL) standard was developed, which is actually an improved technology for transmitting T1 / E1 over twisted pair. HDSL uses a bandwidth of only 80...240 kHz (depending on the specific implementation), allows you to easily place several lines in one cable, and also works at distances up to 4 km without any repeaters. The most serious disadvantage of HDSL is that in order to achieve a speed of 1.544 Mbps. (T1) it needs two pairs of wires at once, for a speed of 2048 Mbps. - already three pairs, which again complicated the installation of HDSL for private users, who usually have only one telephone line in the house. However, this HDSL was the first DSL standard to cross the 1Mbps threshold.
An improved version of HDSL, called SDSL (Single line DSL), used only one telephone pair to transmit all the same T1 / E1 streams, while providing speeds up to 1.544 / 2.048 Mbps at a distance of about 3 km from the PBX. In addition, the lower limit of the signal bandwidth in SDSL lies above 4 kHz, so nothing prevents you from using an SDSL modem and a regular phone on the same line.
I note that all these technologies are symmetrical, that is, they provide the same data transfer rates in both directions. This perfectly satisfies the needs of telephone companies, however, for home users, who, as a rule, have at least an order of magnitude more volume of received information than transmitted volumes, it is more profitable to use asymmetric channels, giving most of the bandwidth to the downstream data stream, which was done in the above described ADSL.
And finally, another standard created after ADSL is VDSL, Very high data rate DSL. The downstream data rate in VDSL can be up to 51.84 Mbps. - but you have to pay for this with a reduced stable communication distance, which at this speed is only about 300 m. In fact, VDSL is very good for use with a small - less than 2 km - distance from the exchange, but, since, according to statistics, the average distance from ATS to subscribers is about 5 km, then for widespread use, the more "long-range" ADSL is better suited.
In conclusion of this section, I will give a table with the main characteristics (speed and range) of modern data transmission technologies over a copper pair:

Introduction to ATM Technology

The transport protocol currently used for ADSL connections is ATM (Asynchronous Transfer Mode, asynchronous transfer mode), which has gained great popularity in recent years due to its flexibility, high efficiency and, at the same time, comparative ease of implementation.
Initially, ATM technology was developed as an efficient transport mechanism for the needs of the booming telecommunications market. In fact, two extreme options for organizing data transmission networks can be distinguished - a circuit switching network and a packet switching network. The first technology is perfectly illustrated by the familiar telephone network - for the entire duration of the conversation, you are provided with your own physical data transmission channel (that is, voice) with a certain bandwidth. On the one hand, this guarantees you that the channel will be enough for your needs under any conditions - after all, you and only you occupy it; but, on the other hand, when you pause in a conversation, the channel is actually idle, so on average its bandwidth is used relatively little. I note that such an explosive nature of traffic is typical for the vast majority of multimedia data transmission networks, and for many others too.
In the second option - in a packet-switched network - the same channel is provided to several clients. At the client end of this channel, there is multiplexing equipment that receives data packets from clients, arranges them in a queue, and sequentially transmits this queue over the existing channel. This approach ensures high efficiency of using the channel - it practically does not idle, but, on the other hand, it cannot provide you with a guaranteed delay time - if there is a large packet from another client in front of your packet in the queue, then sending your packet will be delayed for a time, necessary to transfer the previous one. And since the size of the queued packets can be very different, the delay is not only large, but also unpredictable, which leads to the virtual impossibility of transmitting real-time multimedia streams over packet-switched channels (for example, videoconferencing or even ordinary voice).
ATM technology is the middle ground between circuit and packet switching. First of all, ATM introduces the concept of a cell - a packet of a fixed length. In the modern standard, the length of a cell is 53 bytes, of which 5 bytes are for the address and 48 bytes for the actual information being transmitted. The packets received from the client are divided into cells at the so-called ATM adaptation level, each cell is supplied with address information and queued. It would seem that here we come to the same problem as with packet switching - to unpredictable delays due to the presence of a queue; however, the fixed cell size, and even so small, was not chosen by chance in ATM - cells containing 48-byte pieces of packets from different users are mixed in the queue, so the delays are so small that in the vast majority of cases they can be neglected. In addition, ATM introduced the concept of quality of service (QoS, Quality of Service) - cells can have different priorities: for example, cells in which a video stream is transmitted will have a higher priority than cells in which data that is not critical to the delay time is transmitted. This technology is completely analogous to the implementation of multitasking in modern computers - in fact, only one process is running at a time, but the switching time between processes is so short that, from a human point of view, they are all running simultaneously.
There are five ATM Adaptation Levels (AAL), depending on the type of service. In total, it is customary to distinguish three levels in ATM - physical (this is the data transmission medium itself, that is, in our case ADSL; in general, ATM technology is not tied to any particular transmission medium, therefore it makes it easy to combine heterogeneous networks into a single whole), the level ATM (it deals with the direct transmission and reception of cells) and the adaptation layer described above, which adapts the upper layer protocols to ATM cells.
In ATM technology, the concept of a virtual connection is also widely used. Unlike technologies that operate with physical communication channels, in ATM, binding to those (that is, specifying the address of the recipient of the packet) is carried out only at the connection setup stage. After that, a virtual channel is established between the two nodes participating in the data exchange, uniquely designated by two numbers - virtual path identifiers (Virtual Path Identifier, VPI) and virtual channel (Virtual Channel Identifier, VCI). This solution allows, firstly, to greatly reduce the size of the cell header and, accordingly, its processing time, without specifying the full address of the recipient in it, and, secondly, it is easy to build multi-connected networks (networks in which all nodes are connected in pairs with each other). another), thereby getting rid of transit nodes, which only introduce additional delays in data transmission. For each virtual path, you can create several virtual channels, which allows, for example, during a video conference, to transmit an image on one channel, sound on another, and other related information on a third.

Communication protocols

From the provider's point of view, using ATM over ADSL on the "last mile" allows him to create a homogeneous network - as I noted above, ATM is not tied to any particular physical transmission medium, nor to any particular speed, so the entire network of the provider , including external communication channels, can be built on the basis of ATM, which greatly facilitates its operation. But from the user's point of view, everything is not so simple - the vast majority of existing software is not designed to work directly with ATM, so using ATM "in its purest form" requires a serious update.
Protocol encapsulation in this case is extremely simple: applications work directly with ATM, nothing extra is involved (below, in all such tables, the "native" ATM protocols and the ADSL physical layer are marked in blue, and the "auxiliary" protocols that ensure compatibility with software are marked in yellow, those or other services and the like, and orange - the stages of encapsulation of these protocols in ATM):

The most common way to solve the software adaptation problem is to encapsulate legacy Ethernet frames in ATM cells (Ethernet over ATM, or EoA for short, is detailed in RFC 1483 and the newer RFC 2684). Encapsulation is performed at the fifth ATM adaptation layer (AAL-5) directly by an ADSL modem - accordingly, the client computer only needs a regular network card that supports it, which is the de facto standard for any modern system.
As you can see, the encapsulation scheme is noticeably more complicated - now applications work with their usual TCP / IP, then TCP / IP packets are transported via Ethernet, and in the modem Ethernet frames are converted to ATM cells (and vice versa) in accordance with RFC 2684:

To ensure user authorization, dynamic issuance of IP addresses and similar tasks, another protocol is often launched over the Ethernet network - PPPoE (PPP over Ethernet), which is well known to many home network users and is an analogue of the PPP (Point-to-Point) protocol familiar to any modem owner. protocol).

In the simplest case, the ADSL modem operates in the so-called bridge mode, converting ATM cells into Ethernet frames and vice versa and transmitting these frames to the user's computer, where, if necessary, the software for implementing PPPoE is already installed (in Microsoft Windows XP it, for example, is included in the standard delivery). However, there are also modems that can independently start a PPPoE session and log in with the provider.
Ethernet over ATM technology is good in terms of ease of connection and cost of user equipment (a modem that can work in bridged mode is enough - and this is the cheapest type of modem), but the efficiency of transporting large Ethernet packets by splitting them into 53-byte ATM cells is relatively low. To a large extent, this is compensated by the high (compared to conventional modems) speed of the ADSL connection, but it still somewhat complicates the organization of video conferences (and, in general, the transmission of multimedia traffic in real time).
However, since we traditionally use the PPP protocol for user authorization, what prevents PPP packets from being encapsulated in ATM cells, thereby getting rid of the intermediate layer in the form of the Ethernet described in the first version? This method is called PPP over ATM (PPPoA) and described in detail in RFC 2364. On the one hand, when using PPPoA, there is no need for double encapsulation (Ethernet over ATM, and then PPP over Ethernet), and on the other hand, all the advantages of the PPP protocol are preserved: a convenient user authorization mechanism, dynamic IP assignment algorithms - addresses, etc. Of course, this option means that either an ADSL modem that does not perform any conversions must be installed on the client computer and a PPPoA software client, or the modem must be able to independently maintain a PPPoA session, transmitting the received data to the client computer , for example, over an Ethernet network (I note that there is no question of data encapsulation ).

There is also another method - the transmission of IP packets over an ATM network (IP over ATM, or, for short, IPoA), described in RFC 2225 (formerly RFC 1577). Recently, this variant of encapsulation has become increasingly popular.

Plus, for each type of encapsulation, there are two possible modes - LLC (Logical Link Control) and VC-Mux (Virtual Channel based Multiplexing). I will not dwell on their differences in this article, I will only note that the choice of a specific mode, as well as the protocol itself among those presented above, depends on your ADSL provider.
Thus, it can be concluded that, from a theoretical point of view, the choice of specific protocols is a compromise between the complexity of configuration and work efficiency, on the one hand, and support for existing hardware and software, on the other.

User Equipment

From the user's point of view, all ADSL modems can be divided into four groups - internal PCI modems, external USB modems, external modems with an Ethernet interface, and external routers (routers) with an Ethernet interface.
Internal ADSL modems have the same advantages and disadvantages as classic modems compared to external ones. On the one hand, they do not take up space on the table, do not require a separate power supply and significantly reduce the number of wires, but, on the other hand, they require opening the system unit for installation (which is not always possible if the unit is under warranty and sealed), and also cannot work without drivers, and therefore, as a rule, they are suitable only for MS Windows users (as in the case of classic PCI modems, there are not always drivers for alternative systems, and their quality usually leaves much to be desired). The modem is configured using a special utility supplied with the drivers.

Micronet SP3300C PCI ADSL Modem

External USB modems provide exactly the same functionality as internal modems. They have only two connectors - USB and a connector for connecting a telephone line, and, as a rule, two indicators - one LED indicates that the modem is on, and the other indicates that an ADSL connection is established. Like PCI modems, they can only work in bridged mode - even if the modem is declared to support PPPoE, then in practice this will simply mean the presence of its own PPPoE client in its driver. Again, the modem requires drivers to work, and a special utility is required for configuration, so users of systems other than MS Windows should at least first find out the availability and quality of drivers for their OS, and even better, pay attention to modems with an interface ethernet.

USB ADSL modem Billion BIPAC-7000

ADSL modems with an Ethernet interface are more versatile - to work with them, the operating system only needs to support the TCP / IP protocol and any network card with a 10BaseT ("twisted pair") interface, to which the modem is connected. Setting up the modem also does not require any special drivers or utilities - it is done from any browser (the modem has its own HTTP server and web interface for configuration), and many modems also support telnet connection for command line supporters. There are also dual-standard modems with both USB and Ethernet interfaces (for example, the Efficient Networks SpeedStream 5100 has only a USB interface, while the SpeedStream 5200 has both USB and Ethernet).

Ethernet ADSL modem Zyxel Prestige 645M

Generally speaking, theoretically, such a modem can even be connected directly to a hub or switch on which a home local network is organized, but in practice, as a rule, this makes no sense - these modems do not support network address translation (NAT, Network Address Translation), nor any authorization methods (PPPoE or PPPoA), they can only act as a converter between ATM and Ethernet interfaces. Thus, their main advantage over USB modems is the presence of an interface supported by all modern operating systems and, accordingly, no need for any specific drivers.
As you know, the most common way to connect home (and not only home) networks to the Internet in conditions where the provider provides only one IP address is to use network address translation (NAT). In this case, so-called private IP addresses are distributed to computers within the network (often they are also called "gray") - these addresses can be used by anyone, but only within the local network, in the global network they do not have any meaning. Obviously, for this reason, computers with private IP addresses can only be accessed from the local network in which they are located - outside of it, such addressing loses all meaning; therefore, to provide access to the Internet, a server is installed that has two addresses at once - "gray", corresponding to the local network, and "white", accessible from the outside for everyone. If the server receives a packet from the local network that goes outside, the server replaces the "gray" address of the sender in it with its own "white" address and sends it further, while remembering which "gray" address this packet came from, so that when from the Internet will receive a response to it, forward this response to the sender of the original packet. This mechanism is called network address translation and provides the most transparent and least dependent on the applications and operating systems used to connect local networks to the Internet.
A variety of ADSL modems that have built-in NAT support are called ADSL routers. In addition to NAT itself, most ADSL routers also support PPPoE and PPPoA protocols (that is, if necessary, they can log in to the provider on their own without installing a PPPoE client on the user computer), they can work as a DHCP server, automatically distributing IP addresses and basic settings to connected to their computers, and also incorporate a DNS server and a firewall. In other words, an ADSL router can easily replace a separate server, fully ensuring the functioning and Internet access of a small local network. Of course, the capabilities of a modem are not enough for any serious network - it does not have traffic counting for each of the network computers, URL filtering, a caching proxy server, and much more, but for a small home network, usually consisting of a maximum of three or four computers (for example, one desktop computer and two laptops), such a modem is an almost ideal solution.

Ethernet/USB ADSL Router U.S. Robotics SureConnect 9003

Like the Ethernet ADSL modems discussed above, routers are connected via an Ethernet interface, and in this case, the ability to connect them directly to a switch or hub becomes much more tempting. Modems are also configured via a web interface using any browser, but many models also support protocols such as telnet and SNMP. Often, Ethernet ADSL modems turn out to be simplified versions of ADSL routers, the capabilities of which are limited by software - compare, for example, Zyxel Prestige 645M and 645R, or D-Link DSL-300G and DSL-500G.
ADSL routers are also very attractive for home users with only one computer. Firstly, such a router, through the use of NAT, allows you to isolate the computer from the network, completely protecting it from worms like MSBlast - the fact is that a computer with a "gray" IP address cannot be directly accessed from the Internet, because in as the recipient of the packet, the "white" address must be specified, that is, the address of the router. In general, there is no way to tell the router from the outside that this packet should be intended for any of the local computers connected to it - therefore, all attack attempts will fall on the router, which they cannot cause the slightest harm, if only because the OS on it has nothing to do with Windows. In addition, the ADSL router is a completely independent device, which is very convenient if you have several operating systems installed on your computer - for example, if you changed the password with the provider, then it is enough to change it once in the router settings, and not edit the PPPoE settings in each from systems. Yes, and actually setting up the OS comes down only to setting up the network interface to automatically receive an IP address and all related information from the router.
And, finally, the highest category of ADSL modems - ADSL routers with built-in switches, Wi-Fi access points, print servers... Such a router allows you to organize a small home network without using any additional equipment, which is not only very convenient, but also cheaper than buying two or three separate devices. The same part of the device that is responsible for ADSL and Internet access is no different from that in conventional ADSL routers.

D-Link DSL-604G+ ADSL router with Wi-Fi and 4-port switch

In addition to the modem, you will also need a splitter or microfilters, depending on how the telephone cable is laid in your apartment. If it is possible to make a separate branch for the modem between the cable entry into the apartment and the first telephone, then it will be more profitable to purchase one splitter, but if this is not possible, microfilters will be required, one for each of the telephones installed in the apartment.

ADSL splitter

Development prospects

A year and a half ago, at the beginning of 2003, the ITU (International Telecommunication Union - International Telecommunication Commission, FIE) completed the development of two new standards - ADSL2 (ITU G.992.3 and G.992.4 - these two options differ from each other in the same way, like G.dmt and G.lite - in the second, both the occupied frequency band and, accordingly, the speed are reduced) and ADSL2 + (G.992.5), which provides both an increase in the bandwidth of the ADSL connection and new functionality.
The ADSL2 standard is more aimed at increasing functionality, rather than speed - the latter has increased by only 50 kbps. compared to ADSL at the same line length (or, at the same speed, it became possible to extend the line by 200 meters). The noise immunity of communication has noticeably increased in the presence of narrow-band interference (for example, from radio stations of long and medium wave bands), it became possible to change the overhead of the protocol - if earlier they were 32 kbps. regardless of the connection speed, now at low speeds they can decrease to 4 kbps, which significantly increases the speed of user data transfer. In addition, ADSL2 allows real-time collection and processing of information about connection status and line quality (the latter - even if the connection failed), which can be extremely useful for providers and telephone companies when diagnosing problems.
The power consumption of ADSL2 transceivers has been greatly reduced - if in the current ADSL they always operate at full power, then ADSL2 has two additional levels of power saving, called L2 and L3. The ADSL2 transceiver operates at full power (L0 level) only when transmitting a continuous data stream (for example, if the user downloads a large file), if there is a small interruption in data transmission (for example, when the user is just browsing the Web, the data is downloaded in very small portions ), then the modem can automatically slow down and switch to L2 with more than half the power consumption compared to L0; transitions between L2 and L0 occur almost instantly and without any loss of information, so they are completely invisible to the user. If the interruption in data transfer is prolonged, then the modem can go into "hibernation" to the L3 level, turning off the transceivers altogether - however, it will take about three seconds to return from the L3 state to L0. By the way, 3 seconds is the connection setup time when the modem is first turned on, against more than ten seconds for current ADSL modems.
Those who use conventional analog modems for quite a long time will surely remember the appearance in the V.32bis protocol of the adaptive speed change (ASL) function, which allows the modem to change the speed depending on the quality of the line "on the fly", that is, without re-establishing the connection (retrain). A similar technology appeared in ADSL2 called Seamless Rate Adaptation (SRA) - now DSL modems can change the speed without breaking the connection or any errors, that is, imperceptibly to the user. For example, if a medium-wave radio station interfering with the operation of the modem stops broadcasting at midnight, then soon after turning off its transmitter, the modem will automatically increase the connection speed.
Undoubtedly, the old-timers remember the possibility of combining two analog modems into a pair that appeared in Windows 98 and Windows NT 4.0 SP5 - at that time it caused numerous disputes whether it can be considered that two modems of 56k each will give a total speed of 112k, or in reality an increase speed will not be as significant. However, due to the lack of support for this innovation by most providers, and, most importantly, the lack of a second telephone line for most users, the problem was more theoretical than practical ... Nevertheless, ADSL2 introduced a similar possibility of pairing modems (and even more), and this feature is implemented precisely at the modem level, and not the operating system, which allows manufacturers to produce multi-channel modems (that is, single-case devices that connect to several lines at once), allowing you to double or even triple the throughput. It is unlikely that they will be of interest to private users, but they may well be useful for organizations for which renting an extra telephone line is not a big problem.
The ability to create virtual channels has also appeared in ADSL2, which allows you to do something similar to traffic prioritization in ATM - for example, for voice or video transmission, you can select a channel with low latency, but a high percentage of errors, and for data transmission, a channel with a small percentage of errors, but also relatively long delay. Based on this technology, the so-called Channelized Voice over DSL (CVoDSL) function is also provided, which allows you to select one or more 64-kilobit channels for voice transmission from the general data stream, as in a conventional telephone system. Thus, since the bandwidth of an ADSL2 modem is much higher than 64 kbps, it is possible to organize several voice channels on one physical telephone line at once, and their support will be carried out by the modem at the DSL physical layer, in contrast to Voice over IP (VoIP) technologies. , this technology is implemented at the level of IP networks, and therefore requires special equipment - that is, roughly speaking, a computer) and even Voice over ATM (VoATM, this technology is implemented through the second AAL2 ATM adaptation layer).
After reading the previous paragraph, the thought naturally arises - is ADSL2 compatibility with conventional phones really needed now, because now we can easily organize several digital telephone channels at once? Indeed, ADSL2 modems provide the ability to disable the compatibility mode, after which the modem expands the frequency range it uses towards low frequencies, thereby increasing the speed updraft data at 256 kbps. Of course, it becomes impossible to use a regular phone at the same time as a modem.
From the point of view of the home user, the most significant changes have occurred in ADSL2+ - compared to ADSL2, the frequency band used for downstream data is doubled in it (in ADSL2 G.992.3 it extends from 140 kHz to 1.1 MHz, in ADSL2+ – from 140 kHz to 2.2 MHz), which increased the downstream speed to 24 Mbps. True, this works effectively only on lines about one and a half kilometers long - with a further increase in the length of the line, the difference between ADSL2 and ADSL2 + quickly decreases and already on a line 2.5 km long becomes equal to zero.
In addition, ADSL2+ allows you to reduce mutual interference in the cable between adjacent lines by using the range of 0.14 ... 1.1 MHz for one line and 1.1 ... 2.2 MHz for the other (while both lines receive such the same speed as in ADSL2) - however, here again it is understood that the second line should not be longer than one and a half kilometers, otherwise it will not be possible to make the modem work on it only in the high-frequency range.
Already existing hardware solutions allow both providers and users to gradually migrate to ADSL2 and ADSL2 + - for example, in June of this year, Texas Instruments introduced the Uni-DSL (UDSL) platform, which supports five standards at once - ADSL, ADSL2, ADSL2 +, VDSL and the VDSL2 standard not yet approved by the ITU (its approval is expected during 2005, and, unlike the current VDSL, it is not inferior to ADSL in speed over long distances, but is on a par with it). Thus, the transition from ADSL to ADSL2/2+ will occur gradually, without any restructuring of the existing infrastructure, as providers and users gradually upgrade equipment.

 

It might be useful to read:

• Conroy Avril Marie Ann (Avril Mari Ann Konroi);
• Discount coupons from Joki Joya park - maximum fun at a bargain price;
• Conducting quests for a children's holiday!;
• Exhibition of Japanese bonsai in the apothecary garden;
• Internet addiction: symptoms and how to get rid of it How to wean yourself from the Internet;
• Moscow Festival of Book Illustration Mors – How long does the festival last;
• How to find a job after graduation for a young specialist with no experience;
• Where to go to work without work experience?;