ADSL (English Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) is a modem technology in which the available channel bandwidth is distributed asymmetrically between outgoing and incoming traffic. There are two groups of factors that affect ADSL quality parameters:
Impact of terminals and DSLAMs on PBX
ADSL technology provides for the technological independence of the parameters of the ADSL modem and provider equipment (DSLAM). The devices are different, so any inconsistency affects the quality of ADSL access. The inconsistency factor can manifest itself in the fact that the modem and DSLAM may not establish the most efficient mode of operation. Violations in the process of negotiating coding schemes and failures in the SNR diagnostic algorithm can lead to deterioration in the quality of the ADSL connection.
Influence of Subscriber Line Parameters
The most significant operational factor affecting ADSL quality parameters are the parameters of the subscriber cable pair. Since the subscriber cable and its parameters in most cases are not upgraded, but are already available to the provider in the form and condition in which it has survived to this day, this contains the weakest element of the ADSL technological chain. It's no secret that the wear and tear of subscriber lines is extremely high, especially in rural areas.
Let's consider which of the parameters are the most critical for the quality of ADSL.
The basic parameters of subscriber cables are the parameters that are used to certify the operator's cable system:
Specialized options:
Common cable problems
Heterogeneity in the cable adversely affects data transmission. Soldering is a very common phenomenon in Russian wiring. The transmitted signal across the tap branches and then reflects off the mismatched end of the tap. As a result, 2 signals appear on the receiver side: direct and reflected. Reflected in this case can be considered as noise, so its effect on the transmission quality is very noticeable.
The mutual influence of subscriber cables on each other is characterized by crosstalk. The impact on the transmission quality is very complex and has a random factor. For example, the mutual influence of one pair on another may potentially exist, but not manifest itself in any way. But if another ADSL user connects, the quality of both connections can be affected.
The main problems that arise when using an ADSL modem
Connection breaks
This is the most common and very common problem. The nature of the breaks can be different: logical breaks, in which the ADSL modem breaks the connection to the server, while the physical connection to the PBX does not disappear. And physical breaks - at which the physical connection with the PBX is interrupted.
With logical breaks it is necessary to check the modem, update the software (firmware) of the modem to the latest version, in some cases, checking the connection with another modem will help to identify the source of the problem. If all these recommendations did not help solve the problem, it is possible that it is on the side of the provider.
On physical breaks communication, first of all, it is necessary to check the connection scheme, the quality of the connection and the condition of the telephone cables.
We can independently check the modem's connection parameters on the line through the modem's web interface. To do this, you need to go to the address http://192.168.1.1 (in some brands of the modem 192.168.0.1, 192.168.10.1) specifying the login admin, the password admin (login/password may be different if it was changed during the modem setup).
Usually, information about connection parameters is located in the system information sections. The information content of the parameters depends on the brand and model of the modem and the software (firmware) version, for example, in D-link modems of the 25xx series, it looks like this:
The main parameters to pay attention to:
Parameters for diagnostics
Signal to noise ratio (SNR)- used as a criterion for assessing the state of the line and defines the minimum limit at which the signal level is higher than the noise level:
6dB and below - a bad line, there are synchronization problems;
7dB-10dB - failures are possible;
11dB-20dB - a good line, no problems with synchronization;
20dB-28dB is a very good line;
29dB and above is a great line.
Signal attenuation (Line Attenuation)- shows the attenuation of the signal in the line at the time of synchronization of the modem with the DSL switch. This setting depends on the length of the cable between the modem and the DSL switch:
up to 20 dB - excellent line
from 20 dB to 40 dB - working line
from 40 dB to 50 dB - failures are possible
50 dB to 60 dB - intermittent loss of synchronization
from 60 dB and above - equipment operation is impossible
Technique for Diagnosing a Disconnection Problem
We check the scheme of connecting the ADSL modem to the telephone line. A certain percentage of problems arise precisely because of the incorrectly assembled scheme for connecting the modem to the telephone line.
Correct wiring diagram
We check telephone wires for poor-quality connections (twisting, "noodles", poor crimping of connectors).
To exclude the possibility of the influence of connecting cables, a splitter on the quality of the connection, it is necessary to check the quality of the connection directly, i.e. connect the ADSL modem directly to the telephone socket.
We are trying to check the connection using another ADSL modem. This is especially worth doing if the ADSL modem has been in operation for more than 3-4 years.
If the above actions do not correct the situation, then you need to contact your provider for a detailed check of the telephone line.
Low speed
ADSL technology is outdated and not the fastest compared to FTTB (optics to the home), but there are areas where, due to the lack of alternative connection schemes, this type of communication is the only possible one. In the private sector, to replace ADSL connections, it begins to be introduced new technology GPON. You can read more about it.
The low speed problem can manifest itself in different situations. Conditionally, problems can be divided into several types:
physical- incorrect connection scheme, a problem with the telephone line, the remoteness of servers, the distance from the PBX to the modem, etc.,
software- problems with software on the computer, incorrectly configured firewalls, antiviruses, peer-to-peer clients.
hardware- weak wi-fi transmitter, problems with network card, a problem with the router, etc.
In each case, the solution to the problem will be different, respectively, and troubleshooting methods will also differ.
When using an ADSL modem, a user without special technical knowledge can see for himself at what speed his ADSL modem connects. As mentioned earlier, to do this, just go to http://192.168.1.1. For example, on a 25xx series D-link modem, we can see the following:
Paying attention to parameter values Attainable rate (the maximum possible speed on the line). In our example, this is 26712 Kbps (26 Mbps), and Downstream rate (current connection speed) is 6141 Kbps (6 Mbps)
These figures tell us that the modem is connected at speeds up to 6 Mbps out of 25 Mbps possible. The speed equal to 6 Mbps is the speed value set on the DSLAM port and can be changed by the employee technical support.
If you change the tariff from 6 Mbps to a higher speed, for example 15 Mbps, then in fact the speed will remain the same 6 Mbps until the port settings on the station equipment (DSLAM) to which you are connected change .
Important when using ADSL technology is the distance to the PBX to which you are connected. The farther you are from the PBX, the lower the connection speed you can get.
For example, with a distance to the PBX of 4-4.5 km, given the condition of the wiring, it is unlikely that you will be able to get a stable Internet at a speed of more than 2-3 Mbps.
Usually, users use speedtest.net, 2ip.ru or the first available resource in the results to check the speed. search engines. And if the speed indicators do not correspond to the declared tariff, they begin to file complaints about low speed.
In this situation, many users do not take into account many factors: from the location of the selected server used for the test to the network activity on the computer from which the test is performed.
Test results will be objective if:
- disable all applications that can use the Internet channel
- make sure there is no update during testing operating system, antiviruses, other programs in which the auto-update mode is selected
- upload peer-to-peer clients (transmission, utorrent, skype, etc.)
- temporarily disable antivirus (especially if it is avast, kaspersky)
- check if the specified DNS servers are correct
- try to check the connection without using a proxy server
If the speed indicators on the test correspond to the selected tariff plan, but the pages load extremely slowly, you can try to restart the equipment: modem, router, switch, computer.
Slow upstream speed
Since ADSL technology is asymmetric, one more speed value - the outgoing speed (Upstream rate) will be much less than the incoming speed (Downstream rate). The asymmetry of ADSL implies the transfer of large amounts of information to the user and small amounts of information from the user. Usually, the contract with the provider states that the outgoing speed cannot exceed 800 Kbps. In real conditions - 600-700 Kbps.
Depending on the port settings on the DSLAM and ADSL modem, the state of the telephone line and the distance from the PBX, the outgoing speed can reach up to 1.5-2 Mbps.
So if we see Upstream rate 636 Kbps (0.6 Mbps) and Maintainable rate for upstream 1218 Kbps (1.2 Mbps), that is, the probability of increasing the outgoing speed upwards.
Pages do not load when using an ADSL modem
If you have problems opening pages, the indication on the ADSL modem will help you diagnose and identify the problem as quickly as possible. For example:
Some manufacturers of ADSL modems replace the inscriptions under the indicators with graphic symbols. In order to find out what the indication means, you need to refer to the user manual for the device.
Exist various ways Internet connections, but all these methods can be divided into wired and wireless. The disadvantage of wired is the wire (cable) itself - you cannot move further than the cable allows you to move away from the connection point. Wireless connections are preferable for a laptop because you can move freely within the range of a wireless network (or a network of a cellular operator or a wireless Wi-Fi network).
Let's start with a wired internet connection. Almost any laptop has a built-in modem that allows you to establish a regular modem connection. The advantages of a modem connection is that you can connect to the Internet in almost any apartment - everyone has a telephone. There are services for connecting to the Internet without concluding a contract - you either need to buy a prepaid access card or call a special phone number - then the Internet bill will be included in your phone bill. But that's where the benefits end. Disadvantages begin: low data transfer rate (in our realities - a maximum of 33.6 Kbps), constant disconnections, the high cost of the access itself. Don't believe that a dial-up connection is expensive? Let's count together. My local ISP charges 15 cents for an hour of access. Then 8 hours of access (an eight-hour workday) and 22 days a month will cost you $26.4. But an unlimited DSL connection with a speed of 5 Mbps in both directions (to and from the Network) costs only $15! Moreover, this connection can be used around the clock. If you use a modem connection in this mode, then it will cost you $108 per month, but the speed and quality of communication cannot be compared with a DSL connection, so we discard the modem connection right away. If there is a modem in your laptop, this does not mean that you need to use it. When there are no other ways to connect to the Internet, then you can use a dial-up connection.
Physical connection of the DSL modem
To connect a DSL modem to the telephone network, you need a special DSL splitter, which is usually included with the DSL modem. The splitter must be connected to a telephone line, and then a DSL modem and a regular telephone must be connected to it.
After that, you need to connect the modem to the laptop using an Ethernet cable. If you plan to use a wireless access point with a built-in DSL modem, then it must also be connected to the splitter. If the DSL modem and the wireless access point are different devices, then you need to connect the modem, as mentioned above, and then use an Ethernet cable to connect it not to a laptop, but to an access point. We will talk in detail about building your own network in the chapter Creating your own wireless network.
Setting up a DSL connection in Windows 7
Basic DSL connection setup
In most cases, setting up a DSL connection will not cause any problems. Let's start with the basic DSL connection setup and hope everything goes well. Make sure the DSL modem is turned on and connected to the laptop. Click the connection icon in the notification area and select Network and Sharing Center. If you do not have a connection icon, then through the control panel, run the Network and Sharing Center command.
Disabling and connecting the Internet connection
To terminate the connection, right-click on the connection icon and select Disconnect, Beeline Internet.
Change connection settings. IP address, DNS server
Usually, all network parameters (IP address, DNS server IP addresses) are set automatically when connecting, but sometimes the provider does not use a DHCP server for automatic configuration, and users must enter connection parameters manually. This is rare, but it does happen, and you should know how to configure the network interface manually.
Open the Network Sharing Center window and select the Change adapter settings command, right-click on the connection you need and select the Properties command. Note that, in addition to the Properties command, you will need the Disable and Status commands. The first allows you to terminate the connection, and the second - to view the status of the connection (including the number of received / sent bytes).
Internet connection diagnostics
To diagnose connection problems, we will use a number of Windows utility utilities. Let's start with the ipconfig utility, which displays information about your network settings. In particular, some providers are tied to the MAC address of clients. The MAC address is the scaled hardware address of the network adapter. When a client connects, the administrator enters its MAC address into the database. When a client connects to the network, the server checks if the user's login matches the user's MAC address. It turns out that even if someone steals your password to access the Internet, he will still not be able to connect, because his MAC address is different from yours. In other words, such control provides additional protection from "theft of the Internet" - you can access the Internet under your login only from your computer. But sometimes the MAC address needs to be changed, such as when you change your computer or network adapter. Then you need to inform the administrator of the new MAC address. To find out your MAC address, click the Start button, type cmd in the Search programs and files box, and press the key. Windows Command Prompt will open, enter the command:
Network adapter rate limit
When working with a DSL/PPPoE connection, I encountered the following problem: The DSL connection disappears by itself for no apparent reason, and several times a day. Re-connection then occurs without problems. It would seem that it's okay - you just need to reconnect, but when the connection breaks almost every 30-40 minutes, it's very annoying.
Network adapter speed limit helped me and other users. The network adapter is set to 100 Mbps by default. By limiting the speed to 10 Mbps, I got rid of the connection dropping problem. Not the fact that my advice will help you, but still worth a try. And don't worry too much about the slowdown - DSL connection rarely exceeds 10 Mbps anyway, so you won't feel a drop in the speed of opening web pages or downloading files.
Open Network and Sharing Center and select Change adapter settings. Then click on adapter local network right-click and select Properties. In the window that appears, click the Configure button.
Troubleshooting Wizard in Windows 7
The Network Sharing Center has a very "useful" troubleshooting wizard (called by the Troubleshoot command). The wizard helps you troubleshoot Internet connection problems, shared folder problems, homegroup problems, network adapter problems, incoming connections, DirectAccess workplace connection problems, and printing problems.
DSL technology
DSL technology. Any technology, first of all, provides for a specific physical model of the transport environment. One of the promising technologies that allows you to transfer digital information over copper wires (under “copper wires” is usually understood as the public telephone network - PSTN or POTS - Plain Old Telephone Service in the English abbreviation) are DSL technologies (Digital Subscriber Line - digital subscriber line) .
When using DSL technology (the abbreviation is often used xDSL, where the letter “x” means one of the possible subtechnologies, i.e. variant of the main technology) it is not required to build a new transport network, because the existing POTS network is used. This is precisely the main economic advantage of DSL technology.
The history of DSL can be traced back to the early 1980s, when Bellcore Corporation developed high-data-rate DSL (HDSL) DSL technology. Channel HDSL was designed to extend the capabilities of T1 technology by replacing interleaved coding based on the representation of two bits in one quaternary code (2 binary 1 quaternary - 2B1Q).
The development of Internet services that require high bandwidth (such as video) has created a demand for higher bandwidth connections. Observations show that most of the traffic received from the Internet is intended for the end user (downstream), and only a small percentage of the traffic is actually supplied by the user (upstream). As a result, a channel was developed ADSL(A - Asymmetric - asymmetric digital user line) used in traditional public telephone networks (PSTN - Public Switched Telephone Network).
ADSL technology uses a method that allows you to simultaneously use the same telephone line for both voice and data transmission, without increasing the requirements for switching equipment of the PSTN telephone network. To reserve a POTS channel with frequencies up to 4 kHz (a voice bandwidth of 4 kHz is set in telephony), frequency division multiplexing (FDM - Frequency - Division Multiplexing) is additionally used. In this case, digital streams (data) are transmitted at frequencies above 4 kHz (usually starting from 25 kHz).
Due to the continuous reduction of distance restrictions in DSL technology and the growth of available bandwidth, interest in DSL facilities in last years has grown. Before talking about DSL, let's look at the main types of DSL technology.
- ADSL is the most common DSL technology because it is asymmetric. This means that the speed of downloading data to the user's computer (modem) is higher than the speed of downloading data to a remote computer. To encode data in ADSL technology, CAP methods are used (Carrier less Amplitude and Phase modulation - amplitude and phase modulation without a carrier). The CAP method is not a standardized method for a DSL channel, but DMT has been standardized by ANSI (ANSI T1.413) and ITU (ITU G.992.1).
- EtherLoop - the patented technology of the Elastic Network company - an abbreviation for Ethernet local loop - a subscriber channel of the Ethernet network. EtherLoop technology uses an advanced signal modulation technique that combines with the half-duplex packetization of an Ethernet network. EtherLoop modems guarantee RF signals only for the duration of the transmission. The rest of the time they use low-frequency control signals. Due to the half-duplex nature of EtherLoop technology, constant throughput can be maintained in either downstream only or upstream only. The Nortel system was originally planned for speeds in the 1.5 to 10 Mbps range, depending on link quality and distance constraints.
- G.L.te – ADSL version with low data rate. It is an addition to the ANSI T 1.413 standard. It is known to the ITU standards committee as G .992.2. It, like ADSL, uses DMT modulation, but a POTS network splitter is not installed in the subscriber's building (usually signal splitting is performed by means of a local PBX exchange).
- G.SHDSL – this channel was defined in the ITU G.991.2 standard as a high-speed digital subscriber line on one twisted pair of wires. G.SHDSL technology is symmetrical, which allows you to transfer data at the same speed in forward and reverse streams, which is very important, because. it aims to replace older telecommunication technologies such as T1, E1, HDSL, HDSL2, DSL (SDSL), ISDN and ISDN-based DSL (IDSL).
- HDSL - this channel operates at a speed of 1.54 Mbps and has a range of about 2750 m on a wire with a cross section of 0.5 mm 2. HDSL technology uses 2B1Q line coding modulation.
- GDSL2 - this technology was developed in order to ensure the transmission of the T1 signal over the wires of one pair. The technology was designed to operate at a speed of 1.544 Mbps. It can provide all the services offered by HDSL technology.
- TDSL – This ISDN-based DSL service uses 2B1Q line coding and typically supports a data rate of 128 kbps. The IDSL service operates on a single pair of wires, and the channel itself can be up to 5800 m long.
- RADSL - used in all RADSL modems, but it is related in a special way to a patented modulation standard developed by Globespan Semiconductor. It uses DMT modems of the САР.Т1.413 standard. The uplink rate depends on the downlink rate, which in turn depends on the link condition and the S/N (signal to noise ratio) value.
- SDSL - the technology provides for a constant data transfer rate and does not have existing standards, which is why it is rarely used.
- VDSL – Very-high-data-rate DSL (Very-high-data-rate DSL) is a relatively new technology designed to increase the available data rate (up to 52 Mbps). VDSL technology takes advantage of fiber optic communications and the benefits of placing the end equipment closer to the subscriber. By placing the end equipment in offices and multi-apartment buildings, the length of the local communication line (ie the subscriber channel) can be reduced, which will increase the speed. VDSL technology assumes operation in both asymmetric and symmetrical modes.
Table 1 compares some of the DSL technologies and shows their most important comparable characteristics.
Encoding methods in DSL technology
In DSL technology, there are three main coding methods that are most widely used, which are briefly discussed below.
Table 1 Comparison of various DSL technologies| Technology | Max. upstream speed (Mbps) | Max. downstream data rate (Mbps) | Wire Diameter Standard | Max distance (meters) | Coding | Standards |
| ADSL | 0,8 | 8 | several | 5200 | ATS or DMT | ANSI T1.413 and ITU G.992.1 |
| EtherLoop | 6 | 6 | several | 6400 | QPSK, 16QAM, 64QAM |
Elastic Networks patented technology |
| G.Lite | 0,512 | 1,5 | several | 6700 | DMT | ITU G.992.2 |
| G.SHDSL | 2,304 | 2,304 | several | 6100 | TC PAM | ITU G.992.1 |
| HDSL | 1,544 T1 2 E1 | 1,544 T1 2.0 E1 | 26 AWG*) 24 AWG*) | 2750 3650 | 2B1Q | ITU G.992.1 |
| HDSL2 | 1,544 T1 2 E1 | 1,544 T1 2.0 E1 | 26 AWG*) 24 AWG*) | 2750 3650 | TS RAM | ITU G.992.1 |
| IDSL | 0,144 | 0,144 | several | 5800 | 2B1Q | ANSI T1.601 and TR-393 |
| RADSL | 1,088 | 7,168 | several | 5500 | ATS or DMT | ANSI T1.413 and ITU G.992.1 |
| SDSL | 0,768 | 0,768 | several | 3050 | 2B1Q | ITU G.992.1 |
| VDSL | 20 | 52 | several | 910 | CAP/DMT/ DWMT/SLC |
TBD |
1) Quadrature Amplitude Modulation (QAM) corresponds to a change (fixed offset) in the amplitude and phase of the signal to different bit values. Name quadrature amplitude modulation(i.e. QAM) arose because the signals are out of phase by 90 degrees, and 4 such phases (hence quadrature) together make up 360 o , or a full cycle. Figure 1 (QAM constellation) shows QAM encoding with three bits per baud (signal states are described by different amplitudes and phases). In each of the directions (0 o, 90 o, 180 o and 270 o) there are two points corresponding to two possible values of the amplitude, resulting in eight different states. If there are eight unique states, then 3 bits can be transmitted in each of them (2 3 = 8).
table 2
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Table 2 shows the possible values for 8 QAM encoding (8 possible bit combinations). The more different phase offsets and amplitude levels used, the more bits of information can be included in each dot or symbol. Problems arise when constellation points are placed so close together that noise on the line or in the receiving equipment makes it impossible to distinguish one point from another.
2) ATS coding - it's adaptive QAM code form. This method allows you to adjust the values of the symbols, taking into account the state of the line (for example, noise) at the beginning of the connection. When coding with this method the carrier frequency is removed from the output wave. In the CAP method, frequency division multiplexing (FDM) provides support for three subchannels - a telephone channel (POTS), a downstream data channel (downstream) and an upstream data stream (upstream) channel.
Voice signals occupy a standard frequency band of 0...4 kHz (see Fig. 2). The CAP method performs rate adaptation based on the channel condition by modifying the bit or frame number (ie constellation size + carrier baud rate). This is indicated by different pairs of carrier frequencies (eg 17 kHz and 136 kHz).
Figure 2 shows the frequency spectrum of CAP modulation. Access is supported in two frequency ranges: 25-160 kHz for upstream and 240-1100 kHz (up to 1.5 MHz) for downstream.
3) DMT (Discreate Multi - Tone modulation) coding is a signaling method in which the total bandwidth is divided between 255 subcarriers or subchannels with a bandwidth of 4 kHz each. The first subcarrier channel is used to carry the traditional voice signal and the POTS network. Upstream data is usually transmitted on channels 7-32 (26-128 kHz) and downstream data is usually transmitted on channels 33-250 (138-1100 kHz). In fact, the DMT method is a type of FDM compaction. The incoming data stream is divided into N channels having the same bandwidth but different average carrier frequency. Using multiple low bandwidth channels provides the following benefits:
- whatever the characteristics of the line, all channels remain independent, so they can be decoded separately;
- when using DMT, the transmission coefficient is selected so that each channel can operate independently in the presence of noise; this method changes the number of bits per subchannel or tone. As a result, the overall effect of noise with impulse noise at a constant frequency is reduced.
The main characteristics of the DMT method are:
Figure 3 shows the frequency spectrum for DMT modulation.
Typical inclusion of subscriber equipment for simultaneous viewing of TV programs and access to the Internet is shown in Figure 4.
The crossover filter (the crossover frequency is usually located in the range of 6...8 MHz) is sometimes unreasonably called a splitter. In essence, this is a frequency diplexer, which includes a low-pass filter (low-pass filter) and a high-pass filter (high-pass filter) in parallel. In particular, such a wiring scheme is carried out by the Stream-TV company.
Figures 5 and 6 illustrate the general possible schemes physical wiring at the customer's premises. In Figure 5, Customer Premises Equipment (CPE) has integrated POTS network splitters, and Figure 6 shows a line that branches out on a NID (Network Interface Device) device, usually the entry point to a customer's building. At this point, the local communication line goes into the building wiring). In the latter case, the signal (see Fig. 6) supplied to a regular telephone passes through a low-pass filter, 
and the data elements applied to the taps pass through the HPF. This approach ensures that the necessary signals are received in both cases. Both topologies are used depending on where the line should branch and where the wires will be physically placed.
DSL Immunity is estimated according to the criterion of the error rate (BER - Bit Error Rate) BER≤10 -7 . When the S/N (Signal - to - Noise) is lowered, an excessive number of errors appear in the data stream. The noise margin is understood as the difference in S/N (in dB) for a real line and for BER =10 -7 . When the S/N (Signal - to - Noise) is lowered, an excessive number of errors appear in the data stream. The noise margin is understood as the difference in S/N (in dB) for a real line and for BER =10 -7 .
At any time in the line, both the signal level and the noise level can change, as a result of which the implemented S / N value will also change. Note that the higher the DSL link rate, the lower the S/N, and the lower the DSL link rate, the higher the S/N. Therefore, the noise margin will be lower with longer cables (decreased signal level and increased noise) or with more high speed transmission in a DSL channel.
Rate adaptive DSL (RADSL) DSL technology is a technology in which the transmission rate is adjusted so that the necessary noise immunity value can be maintained, which allows the BER to be kept below 10 -7 . Tests show that the optimum noise margin for DMT services is 6 dB each for both downstream and upstream. You should not configure a DSL service with more than the optimum noise margin, because the system will prepare for a very low DSL connection to meet the specified margin. You should also not set too low a value for the noise immunity limit (for example, 1 dB), because a slight increase in noise will result in an excessive amount of errors and a re-provisioning process to establish a connection at a lower DSL link rate.
The noise immunity of a DSL link increases as the distance decreases (noise level decreases) and the wire diameter increases (losses decrease). Of course, increasing the link power level will also increase the S/N, but may result in interference with signals from other services on the same cable.
Forward Error Correction(FEC - Forward Error Correction) is performed mathematically at the receiving end of the transmission channel without a request for retransmission of erroneous data, which allows efficient use of bandwidth for user data. Nevertheless, we note that even in a situation where no error occurs during transmission, the use of the FEC method leads to some decrease in throughput, since this adds unnecessary service signals. The ratio of the number of corrected errors to the number of uncorrected ones shows the efficiency of the error correction algorithm or the relative intensity of errors. There are two main technologies associated with the FEC technique: FEC byte addition and interleaving.
FEC bytes also called control bytes or redundant bytes. The FEC bytes are added to the user data stream, thereby providing an opportunity to detect the presence of erroneous data. In many systems, you can choose the following number of FEC bytes: 0 (none), 2, 4, 8, 12, or 16. Obviously, the more FEC bytes, the more effective the error correction. However, keep in mind that the larger the number of FEC bytes, the more about Most of the bandwidth of the communication channel will be occupied only by service signals, which is very inefficient for low-noise channels. You can add that 16 bytes per frame (204 - 16 = 188 bytes useful information) at a transmission rate of 256 kbps occupy a percentage of b about more bandwidth than the same number of FEC bytes at 8 Mbps.
In most systems, the FEC signaling is separated and subtracted from the total stream before reporting the rate on the DSL link. Thus, the observed DSL link rate is actually the user's available bandwidth.
Interleaving is a process of permuting user data in a specific sequence, used to minimize the occurrence of consecutive errors in the Reed-Solomon FEC algorithm (Reed - Solomon - RS) at the receiving end of the channel. The efficiency of using the RS algorithm in the event of single or spaced errors (not sequential) is higher.
If a burst of noise occurs on a transmission line on a copper wire, it can affect several consecutive data bits, resulting in consecutive bit errors. Since the data is interleaved in the transmitter, deinterleaving the data at the receiver not only restores the original bit sequence, but also causes the error bits to be separated in time (error bits appear in different bytes). Therefore, the bit errors are no longer sequential and the FEC process with the RS algorithm works more efficiently.
Signal strength levels in DSL channels significantly higher than those used in the transmission of voice data. This is due to the fact that the per unit attenuation of a telephone line increases very rapidly with increasing frequency. So, for example, in order to normally receive a signal at the end of a line 5 ... 6 km long, a power of the order of 15 ... 20 dBm (dBmW) is required - the number of decibels (dB or dB), counted from a power equal to one milliwatt, calculated on a resistance of 600 Ohms .
Power levels of wideband signals are usually measured in dBm/Hz (dBmW/Hz). This value is called the power spectral density (PSD - Power Spectral Density):
| PSD = P - 60 | (1) |
Formula (1) is valid for a channel bandwidth of 1 MHz, i.e. applicable only to the ADSL channel.
Without going into technical features we state that the performance of DSL channels is played the following factors:
Cable losses increase with increasing frequency, primarily due to capacitive conduction distributed along the transmission line ( Y C \u003d j ω FROM).
Note also that the resistance of copper wire changes significantly with fluctuations in ambient temperature, especially when cables are laid on telegraph poles when they are in the sun. Therefore, under certain topological conditions, the characteristics of a DSL communication channel can vary greatly depending on the time of day. As the temperature rises, the resistance of the wire increases. The losses are growing. And with an increase in resistance (and associated losses), the S / N value decreases due to a decrease in the signal level.
Conclusion
DSL technology can be considered a full-fledged technology that can be used in the “last mile” segments for broadband networks. Different scenarios may use different flavors of DSL technology, depending primarily on distance and bandwidth requirements. There are many factors that affect the quality of a connection, and in order to improve DSL data rate and S/N margin, you need to adjust a lot of parameters. The solution lies in understanding the technology and which factors play which role in the connection.
The topologies of DSL networks can be very different for different service providers, so you should not think that if the subscriber equipment (CPE) for a DSL network works on one carrier, then it will work on another. Different topologies have their advantages and disadvantages, but all topologies are still widely used.
One of the most popular and available ways connection to the World Wide Web today is an ADSL connection. The abbreviation ADSL stands for "Asymmetric Digital Subscriber Line" - an asymmetric digital subscriber line. Despite its simplicity and almost 100% availability, the mobile connection is significantly inferior to the ADSL connection in terms of its capabilities: the data transfer rate is lower, the range of services is less, and the connection cost is much higher. Connection using ETTH technology (“Ethernet to every home”), GPON and FTTH (using fiber optic cable) is currently available only for residents of the multi-apartment sector in large settlements, as they are economically justified for mass connections. Therefore, today ADSL connection is relevant for most users, especially in small towns.
ADSL connection problems
Despite its mass availability and fairly decent technical characteristics:
- Practical access speed: up to 24 Mbps;
- Subscriber line length for satisfactory operation: up to 7.5 km;
- Service availability triple play- simultaneous transmission of voice, video and data.
This technology uses in its work a telephone subscriber line with all the ensuing problems.
Consider a typical subscriber connection scheme using ADSL technology:
The practice of operating this technology shows that the most common problems that lead to the fact that the user installs slow speed on adsl connection, or no Internet access at all, are:
- Telephone line failure;
- Access equipment port failure (DSLAM) on the provider side;
- Incorrect connection on the user's side.
Telephone Line Trouble
This is the most common type of damage that occurs in the "Subscriber-Provider" chain. Unfortunately, the telephone line is far from perfect. While it “gets” from the Internet provider to the user, it can go through quite a lot of different sections: trunk, cable, distribution cables, cables between cabinets, and even the so-called air ducts - wires that go from the cabinet to the subscriber by air. Each of these sections, in addition to attenuation of the useful signal, can also introduce various interferences, leading both to a general decrease in speed and to the fact that the subscriber has frequent disconnections during adsl connection.
Of course, in order to perform a measurement of the physical parameters of a telephone line to obtain it quality characteristics, requires the availability of special tools and the ability to use them. But an ordinary user can also easily assess its condition in order to understand why certain access problems arise. To do this, you need to connect to an ADSL modem and view the ADSL connection statistics.
Not only problems with the communication line or with the provider's equipment lead to problems in working with the Internet. Asking the question - “How to increase the speed with an adsl connection?”, The user sometimes forgets that incorrectly working equipment or an incorrect connection on his side can also cause failures and low speed. Therefore, before calling the technical support service, it is necessary to check whether the telephone line, modem and telephone are connected correctly.
First of all, you should start with splitter- a special device that is designed to ensure that high-frequency noise from the modem does not interfere with telephone conversations. In fact, it is a special filter for separating the operating frequency bands of the modem and the telephone.
Consider the correct scheme for connecting user devices:
It should be remembered that telephone sets and any other telephone devices must not be connected before the splitter! All phones must be strictly connected to the PHONE jack! Otherwise, the connection will be unstable, and usually slow. Adsl connection breaks will be almost permanent in this case.
Connecting an adsl modem without a splitter will lead to noise during a telephone conversation and, as in the first case, to a poor connection quality. However, if you are not using a telephone set, then the modem can be connected to a telephone line without this device.
Excessively long telephone extension cords should be avoided. If you really can’t do without it, you need to choose those that use not four, but two conductors. This will reduce the level of interference and improve the quality of the connection.
Unfortunately, the adsl modem is also not immune from damage. Moreover, there are obvious damages, that is, when it simply does not work or does not work correctly, but there are hidden ones associated with damage to its linear part. Especially often, such malfunctions quite often occur after a thunderstorm. At the same time, the modem itself is working and can even establish a connection with the provider's equipment, but it is unstable, or the connection is at low speed. The first impression that arises is that the telephone line is malfunctioning, since the “symptoms” are very similar. In this case, you should take readings of the main characteristics of the connection from its menu in the "Statistics" section, and check it at the provider's stand, asking you to take the same data. If the readings are similar, most likely, the linear part of the modem is “burned out” and needs to be repaired.
- If the speed of Internet access is periodically reduced, start the test by examining the stability of the established connection - the “link”. (The English version of the word is Link). Follow the indicator with the same name. On some models it is called ADSL. During operation, if the adsl connection is stable and established, it should just be on. If it blinks periodically, the connection with the provider is unstable, a check of the communication line is required.
- Watch outgoing (upstream) speed in the line. Practice shows that the lower it is, the lower the quality of the connection. Ideally, it should be equal to or close to 1 Mbps (unless specifically limited by the tariff).
- With constant disconnections, you can try to turn off the splitter and phone, turning on the modem for a while, directly into the line. This eliminates the possible influence of other devices on the connection. If in this case everything works stably, then you can, turning on the devices in turn, find out which one has an impact.
- Always check the quality of the contact in the connectors. The modern RJ11 telephone jack is not a very high quality product, its contacts are often oxidized. Remove and reinsert it two or three times.
Test Methodology for ADSL
The testing methodology is designed to evaluate and visualize the results of testing in case of problems when working on the Internet.
How to take a "screenshot" can be read
.
We draw your attention to some features of working on the Internet:
1) When the Subscriber is connected to its Data Transmission Network, the Provider is not responsible for the quality of communication outside the terminal subscriber device (if any) connected to the Provider's equipment.
The provider guarantees the speed of Internet access only if there is a direct connection, i.e. The Provider's cable connects directly to a laptop or personal computer. You can read more about the Procedure for the provision of services.
2) You can familiarize yourself with the division of areas of responsibility between the Provider and the Subscriber.
3) When using ADSL technology, the data transfer speed is always less than the connection speed by at least 13-15%. This is a technological limitation, which we will discuss in more detail below. It does not depend on the provider or the modem used.
Under ideal conditions, with a connection speed of 12 Mbps, you can count on the maximum real speed~ 10 Mbps.
Note!
You can read more about the factors affecting the data transfer rate when using ADSL technology.
Attention! If you are using wireless WiFi networks, It will be useful for you to read the information below.
1. Sources of interference that affect the operation of wireless Wi-Fi networks can be as follows:
- material of walls and partitions in your apartment or office;
- location WiFi hotspots your neighbors. For example, if the neighbor's point is located near the wall adjacent to your apartment, and your point, in turn, is located near this wall, then the signals of both points will interrupt each other;
- Wi-Fi module in your PC or other mobile device. A mobile device may not have the most modern module installed, which has a maximum speed limit;
- simultaneous downloading from different devices, both inside your apartment and at neighboring points outside your apartment;
- Bluetooth devices operating in the coverage area of your Wi-Fi device;
- various Appliances, which, when operating, uses the 2.4 GHz frequency band operating in the coverage area of your Wi-Fi device.
You can read more about possible sources of interference that affect the operation of Wi-Fi wireless networks.
2. To speed up the work on the Internet and make it more stable, you must:
- configure the router to work with mobile devices. How to do this on a TP-Link router, see ;
- choose a more free channel;
- choose the optimal location of the Wi-Fi point;
- purchase an external Wi-Fi adapter;
- use a dual-antenna wireless access point operating in the 2.4 GHz band;
- use a wireless access point operating in the 5 GHz band;
- work through an Ethernet cable.
You can learn more about ways to increase connection speed and Wi-Fi throughput.
- We measure the speed of the Internet.
a) We go by link and press the button Begin Test. We are waiting for the completion of the test.When the test is completed, you will be presented with a window similar to the following. Let's make it" screenshot” and attach to the results.
b) Download the file (about 75 MB in size) from here: http://www.apple.com/itunes/download/
Start downloading by clicking on the button "Download Now".
During the upload process, do "screenshot"
Attention! To display the download speed in the browser, go to the Downloads section by pressing the Ctrl + J key combination.With) Download a large file (about 2.3 GB) from here:
ftp://ftp.freebsd.org/pub/FreeBSD. During the upload process, do "screenshot" Your download manager or browser and attach to the test results.
Attention! You don't have to download the whole file! It is enough to wait a minute or two until a stable speed is established, then do 2-3 " screenshot» with an interval of 20-30 seconds and stop the download.d) Download the file using a torrent client. For correct speed testing it is necessary to exclude local retrackers. How to do this, you can see.
Attention! It is necessary to test the connection speed when downloading 3-4 files at the same time, in which the number of distributors is more than 100. During the download process, do " screenshot» of your torrent client and attach it to the test results. - We measure the speed from internal resources. For this Minsk subscribers go to the next link .
On the site click on "Change Server".
In the search bar write Atlant Telecom and select it as the server.
Then we press the button "GO".
We are waiting for the testing to be completed.As a result, a window with the results should appear.
We take a screenshot and attach it to the general results.
Regional subscribers go to the following links and download the file:
- link for Brest;
- link for Vitebsk;
- link for Grodno;
- link for Gomel;
- link for Mogilev.
During the download process, we take a “screenshot” of your download manager or browser (except for Internet Explorer) and attach it to the test results. - Download the program and install it (for modems of the D-link brand - the program).
Zyxmon- free Windows program to manage and monitor the status of Zyxel routers.
Unpack the zip folder using some archiver. For example, WinRAR or winzip. Run the executable " ZyxMon". The program window will open. Click on the button " Settings(circled in red).
The following window will appear. Fill in the fields RouterIP and router password. Press " OK».
After pressing " OK» we will return to the Main window of the program. We activate the connection with the modem. To do this, press the button " Telnet Router Connections” (circled in pink), while the indicators “ Telnet connection status" and " PPPoE session status» should change color from red to green . Description of bookmarks:
telnet: Modem connection status and PPPoE status.
Log: Modem text log;
SyslogD: Messages received from modem by Syslg Daemon;
SNMP: RealTime channel filling statistics;
DynDNS: Dynamic DNS state (not used);
line: Data required for line testing: noise margin , attenuation . To get the data, you need to press the button “ Get ”.
Doing " screenshot» of the result and attach it to the test results.
Test Methodology
Attention! If you are connected through additional equipment or use wireless Wi-Fi networks, you must first connect the Internet cable directly to your laptop or personal computer without additional devices, and then perform the speed test method.
To obtain adequate results during each of the points of the test, NO work on the Internet should be conducted!
For Windows OS
Downloading the archive. Unzip it to any folder on your computer. The file should appear in the same folder TEST.bat. We launch it and wait from 10 to 20 minutes (depending on the quality of the DSL connection).
Attention! For Windows 7 and Windows 8, you need to run the file as an administrator (right-click on TEST.bat and select "Run as Administrator"). When the BAT file performs all the actions, you will see the following window.
Press any key on the keyboard - the window will close. After that we go to Disk C and find text files there PING.txt, PATHPING.txt and CONFIG.txt . We attach these files to the results.
For Mac OS X
Downloading the archive. Unzip it to any folder on your computer. After unpacking, a file should appear in the same folder test.app. We launch it and wait from 10 to 20 minutes. After completing the test, press any key on the keyboard - the window will close.
Upon completion of testing, three text files will appear on the desktop - CONFIG, PING, TRACEROUTE. We attach these files to the results.
We check at what speed the modem receives / gives data.
a) telnet.
We go to command line: Start -> Run -> cmd -> Ok
. In the window that appears, write the command telnet
From the main menu of the modem, go to the menu 24.1 - System Maintenance - Status
. To do this, press on the keyboard 24 - "Enter", 1- "Enter".
Take screenshots of this window:
Explanations to the fields of interest to us in this menu:
Tx B/s
- transmission speed in Bytes per second;
Rx B/s
[Receiving speed, Byte/s] - receiving speed in Bytes in seconds;
Up Time
[Connection time] - the duration of the connection between the modem and the provider;
My WAN IP (from ISP)
[my ip address in global network(from provider)] - ip-address received by the modem from the provider;
line status
[Line state] - current state of the xDSL line: Up - up, Down - not up;
Upstream Speed
[Outgoing speed] - transmission speed of outgoing traffic in Kbps;
downstream speed
[Incoming speed] - transmission speed of incoming traffic in Kbps;
CPU Load
[CPU load] - percentage of modem CPU load.
b) For modems ZyXel 660R, ZyXel 660R-T1, ZyXel 660RU-T1, ZyXel 660HT1, ZyXel 660HW-T1 via WEB interface.
192.168.1.1
and press the key Enter.
1234
and press the button "login".
Ignore.
In the main menu of the modem, select System Status.
In the window that opens, find the button "Show Statistics"
and press it. Doing " screenshots» last window:
- first: during download from the Internet;
- second: during download from internal resources.
We name the files accordingly and attach them to the results.
c) For modems ZyXel 660R-T2, ZyXel 660RU-T2, ZyXel 660HT-2, ZyXel 660HW-T2.
Type in the address bar of your Internet browser (Chrome, Mozilla Firefox, etc.) the address 192.168.1.1
and press the key Enter.
Next, a window will appear asking for a password. Prescribe 1234
and press the button "login".
A window will appear in which you are advised to change the password for logging into the modem. Click the button Ignore.
In the main menu of the modem, press Status,
and in the window that opens, click the link Packet Statistics.
As a result, a statistics window will open, do it " screenshot»:
- first: during download from the Internet;
- second: during download from internal resources.
We name the files accordingly and attach them to the results.
d) For D-Link 2500/2540/2600/2640U v.2 modems
Type in the address bar of your Internet browser (Chrome, Mozilla Firefox, etc.) the address 192.168.1.1 and press the " Enter
". Next, a window will appear asking for a username and password. We register the user - admin
and password - admin
, press the button " Ok
».
Next, go to the menu Device Info -> Statistics -> WAN
As a result, a window will open, do it " screenshot»:
- first: during download from the Internet;
- second: during download from internal resources.
We diagnose the connection of the DSL channel.
For this we go: Start -> Run -> cmd -> Ok.
In the window that appears, write commands one by one (after each press the key "Enter"
):
netsh("Enter")
ras("Enter")
set tracing ppp enable("Enter")
exit("Enter")
Next, go to the Windows folder (usually c: Windows) and create a folder there tracing
. If you are told that such a folder already exists, do not be alarmed. We go into it (the path for the example: c: Windowstracing) and copy the ppp.txt file from there with the results of the commands we entered earlier. We attach this file to the results of the methodology.
We analyze the DSL channel on the modem.
a) For modems ZyXel 660R, ZyXel 660RT1, ZyXel 660RU1, ZyXel 660HT1, ZyXel 660HW-T1
We go to the modem configurator, as shown in paragraph 6-a, go to the menu
- modem command line. We write commands one by one (after each press the key "Enter"
):
wan adsl chandata ("Enter")
wan adsl opmode("Enter")
wan adsl linedata far("Enter")
wan adsl linedata near("Enter")
wan adsl perf("Enter")
wan hwsar disp ("Enter")
Doing " screenshots» obtained results. First of all, the state of the 1st (physical) level is analyzed. This information retrieved by the commands "xdsl state", "wan adsl linedata far", "wan adsl linedata near". Link for information: http://zyxel.ru/kb/1543.
The main parameters to control are "SNR margin value", "Loop attenuation" for 782 and 791, and "noise margin downstream", "attenuation downstream" for 642, 650, 650, 660. Both values are measured on the receive channel of the transceiver. The first universally characterizes the noise immunity margin of the line. Level 6 db roughly corresponds to an error rate of 10E-6 and is the threshold for reliable communication. This parameter obviously depends on the speed, i.e. the higher the speed, the lower the margin. It is also worth noting that the measured values at each end of the line may differ. This indicates that the source of interference is located closer to one of the ends of the line.
Attenuation downstream - signal attenuation in the line and obviously depends on the active resistance of the wire. The influence of noise on the quality of communication and the maximum speed is higher than that of attenuation. You need to do this several times at different times of the day. Attach the results to the results of the methodology.
b) For modems ZyXel 660RT2, ZyXel 660RU2, ZyXel 660HT2, ZyXel 660HW-T2, ZyXel 660RT3, ZyXel 660RU3, ZyXel 660HT3
When entering the modem settings through telnet
(as shown in paragraph 6-a), you will immediately be taken to the modem's command line, where you need to enter the commands indicated above.
c) For ZyXel 700 series modems (782 and 791)
In a similar way, go to the modem configurator (see paragraph 6-a) and go to the menu 24.8 - Command Interpreter Mode.
We write commands one by one (after each press the key "Enter"
):
xdsl cnt disp("Enter")
wan hwsar disp ("Enter")
xdsl state("Enter")
Doing " screenshots» obtained results and attach to the test results.
d) For D-Link 2500/2540/2600/2640U v.2 modems
We go into the modem configurator, as shown in paragraph 6-d, go to the menu Device Info -> Statistics -> ADSL
.
We take a screenshot and attach it to the results.
We save all the results of the testing methodology in one archive and send it to the technical support e-mail address [email protected] indicating client data (personal account number/organization name, contact phone number/address Email) for feedback.
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