What is snr in a modem. Dependence of ADSL operation on various parameters. Impact of Terminal Devices and DSLAMs on PBX
One of the most massive and affordable ways to connect 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 the simplicity and almost one hundred percent availability, mobile connection significantly loses in its capabilities to the ADSL connection: the data transfer rate is lower, the set of services is less, and the cost of connection is much higher. Connections using ETTH technology ("Ethernet to every home"), GPON and FTTH (using fiber-optic cable) are still available today only for residents of the multi-apartment sector in large settlements, since they are economically justified with mass connections. Therefore, today the ADSL connection is relevant for most users, especially in small settlements.
ADSL connection problems
Despite its massive 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 a telephone subscriber line in its work with all the ensuing problems.
Let's consider a typical scheme for connecting a subscriber using ADSL technology:
The practice of using this technology shows that the most frequent problems leading to the fact that the user is slow speed on adsl connection, or no Internet access at all, are:
- Telephone line malfunction;
- Failure of the access equipment port (DSLAM) on the provider side;
- Invalid user-side connection.
Telephone line failure
This is the most frequent 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 overhead lines - wires going from the cabinet to the subscriber by air. Each of these sections, in addition to attenuation of the useful signal, can also introduce various interference, leading both to a general decrease in speed and to the fact that the subscriber has frequent disconnections during the ADL connection.
Of course, in order to measure the physical parameters of a telephone line to obtain its quality characteristics, it is necessary to have special devices and the ability to use them. But an ordinary user can also easily assess its state in order to understand why certain access problems arise. To do this, you need to connect to an ADSL modem and view the statistics of the ADSL connection.
Not only problems with the communication line or with the equipment of the provider 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 incorrectly executed connection on his side can also cause failures and low speed. Therefore, before calling the service technical support, it is necessary to check if the telephone line, modem and telephone are connected correctly.
First of all, you should start with splitter - a special device designed to prevent high-frequency noise from the modem from interfering 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 connection scheme for user devices:
It should be remembered that telephones 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. In this case, the disconnections in the case of the adsl connection will be practically permanent.
Connecting an adsl modem without a splitter will lead to noise during a telephone conversation and, as in the first case, to poor connection quality. However, if you are not using a telephone, the modem can be connected to the telephone line without this device.
Excessively long extension cords should be avoided. If you really can't do without it, you need to choose those in which not four, but two conductors are used. This will reduce interference and improve connection quality.
Unfortunately, the adsl modem is also not immune to damage. Moreover, there is obvious damage, 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 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 a low speed. The first impression that arises is that the telephone line is faulty, 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 to take the same data. If the readings are similar - most likely, the linear part of the modem is "burnt" and requires repair.
- If the speed of Internet access periodically decreases, start the test by examining the stability of the established connection - "link". (English version of the word - Link). Follow the indicator of the same name. On some models it is called ADSL. During operation, if the adsl connection is stable and established, it should just light up. If it blinks periodically, the connection with the provider is unstable, and the communication line must be checked.
- Monitor the upstream line speed. 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 (only if not specifically limited by the tariff).
- In case of constant disconnections, you can try to turn off the splitter and the phone by 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 of them has an impact.
- Always check the quality of the contact in the connectors. The modern telephone jack RJ11 is not a very high-quality product, its contacts are often oxidized. Remove and reinsert it two or three times.
The intrinsic interference of DSL systems depends primarily on the implementation of the modems and is practically unalterable during the deployment phase. To minimize this type of interference, it is advisable to use equipment from reputable manufacturers.
The DSL system's immunity to internal interference can be improved through the use of better circuitry and advanced technologies such as TEQ adaptive time equalizers, adaptive echo cancellers, adaptive hybrid and programmable digital / analog filters.
The influence of external stationary interference, such as crosstalk, is also commonly considered when designing DSL modems. Spectral compatibility is ensured by appropriate shaping of the PSD masks of the transmitters of DSL modems. The operator can minimize crosstalk to other DSL lines by controlling the parameters of these masks. These techniques include the US and DS power back-off (UPBO / DPBO) techniques, the virtual noise technique introduced in the G.993.2 VDSL2 standard, and many others.
It should be noted that the interference cancellation mechanisms themselves can cause new interference to appear. So, for example, the well-known Viterbi algorithm is the cause of error propagation.
Figure: 19 Virtual noise method
The Impulse noise protection (INP) error correction strategy is a combination of Reed-Solomon coding and interleaving, the parameters of which are defined by the standard.
As the transmission rate decreases, the noise margin increases. As such, the receiver operates with a significant margin of noise to keep transmission errors within acceptable limits. In particular, these large margins can significantly reduce the impact on the transmission of impulse noise. Other disadvantages, such as increased interleaving memory, can be critical for real-time and low latency applications.
8.2 Methods for adapting DSL line capacity to interference fluctuations
8.2.1 Seamless adaptation methodSeamless Rate Adaptation (SRA).
For slow fluctuations in the noise level or its fluctuations at a moderate speed, a whole arsenal of adaptation mechanisms is used, including changing the bandwidth of subchannels (tone reordering), exchanging information bits between subchannels while maintaining the overall bandwidth of the connection (bitswap) and / or Seamless Rate Adaptation (SRA) ...
The bitswap and tone reordering methods were covered in the first chapter. Therefore, only the SRA method will be discussed here.
This method, based on decoupling the modulation and loop shaping processes, automatically changes the throughput DSL lines, keeping the noise margin unchanged. which guarantees its efficiency and excludes the possibility of interruption of communication.
it important property The SRA mechanism emphasizes the word seamless, which can literally be translated as seamless, or more correctly as infallible. Said decoupling mechanism using complex procedures for mutual reconfiguration of modems in real time - Online reconfiguration (OLR), allows changing the transmission speed of modems without affecting the parameters of the DMT signal cycles, which usually manifests itself in an increase in errors in the digital signal or even a complete loss of frame synchronization.
The SRA mechanism operates in 4 stages:
ADSL modem receivers, continuously monitoring the SNR of the connection, detecting a change in SNR (for example, due to water penetration into the cable), determine the need to change the transmission rate.
To initiate a change in baud rate, the modem receiver sends a message to the remote modem transmitter. The latter contains all the necessary transmission parameters at the new rate, including the number of bits and transmission power in each subchannel.
The transmitter of the remote modem starts transmitting the “Sync Flag” signal, which serves as a marker of the exact time of transition to the new baud rate.
The “Sync Flag” signal of the transmitter is detected by the receiver of the modem, which initiated the start of the process, after which the transition to the new baud rate begins.
8.2.2 Rapid adaptation mechanismRapid Rate Adaptation (RRA)
However, the SRA mechanism cannot cope with the rapid and profound changes in noise level caused, for example, by multiple DSL lines going active at the same time. A mechanism for suppressing such interference was proposed by Ikanos in 2006. It is called the Rapid Rate Adaptation (RRA) engine. It allows you to preserve the integrity of the connection and prevent the DSL modem retrain procedure in the event of an instant increase in external interference (Fig. 20) ..
Figure: 20 Effectiveness of Ikanos Rapid Speed \u200b\u200bAdaptation Mechanism
Table 3. Comparison of RRA and SRA properties
|
Functions / Parameters |
||
|
Maintain link integrity while strongly increasing crosstalk |
Supports |
Do not support |
|
Fast adaptation to 30a VDSL2 profile |
Several hundred milliseconds Any number of subchannels |
A couple of minutes |
|
SNR stock<0 |
The path is stable |
The path does not work |
|
Sharp and significant change in noise power |
The path is stable |
The path does not work |
|
SRA coexistence / addition |
RRA coexists with and complements SRA and bitmap. It is capable of suppressing all types of noise dynamics: 1. Dramatic and Significant Change (RRA) 2. Slow and Small Changes (SRA) 3. Long-term changes (bitmap) |
Not capable of handling harsh and significant Power changes |
8.2.3 Dynamic spectrum managementDynamic Spectrum Management (DSM)
It would seem that everything has been done - the most reliable modems have been developed that use all modern methods of not only signal transmission, but also damage diagnostics. But applications are multiplying, requirements are growing. Merciless competition requires more and more improvements.
And, lo and behold, they definitely appear!
An example of such solutions is the method.
The main obstacle limiting the bandwidth of the DSL line, as we recall, is transient interference.
The existing method for calculating DSL lines, the so-called Static Spectrum Management (SSM), guarantees the spectral compatibility of DSL lines of the same cable bundle for the worst case of transients. However, the actual transient influences very much depend on specific conditions, including the relative position of the influencing and influenced lines, as well as their state. Moreover, crosstalk varies significantly from one cable bundle to another and over time, as the modems themselves may be in operation or turned off. Therefore, the use of constant spectral masks of the transmission signals of DSL modems means a waste of bandwidth.
Dynamic Spectrum Management (DSM) overcomes this disadvantage by adapting the bandwidth of DSL lines to varying crosstalk. The meaning of the term DSM in its modern meaning also includes methods to mitigate crosstalk between the lines of a cable bundle by jointly processing the signals of these lines.
There are at least 6 known ways to increase the bandwidth of DSL lines:
1. Improving DSL technology (for example, creating based on ADSL).
2. Increasing transmission bandwidth (for example, creating ADSL2 plus based on ADSL2).
3. Increase the transmitted power and / or Power Spectral Density (PSD) (for example, creating a RE-ADSL2 based on ADSL2 or RE-VDSL2 based on VDSL2).
4. Shortening the subscriber line (AL) due to it using FTTx technologies.
5. Reducing the noise of DSL modems (for example, by replacing HDSL modems with narrower band SHDSL modems, which will reduce the transient effect of symmetric DSL on ADSL systems of the same cable bundle).
6. Increasing the throughput of a DSL connection through the use of several parallel twisted pairs, which is essentially a Spatial Multiplexing, also called Bonding.
These solutions are effective, but they cannot always be implemented.
In addition, all these methods are static in the sense that they do not take into account the real, current electromagnetic environment in the cable.
DSM is an alternative solution.
Its strong point is the property of adaptability, which ensures maximum use of the bandwidth of the DSL lines of the cable bundle by dynamically changing the transmission signal power of the DSL modems in accordance with the current values \u200b\u200bof the crosstalk power.
There are several DSM algorithms, the higher the ordinal number of which corresponds to the increasing complexity.
The first example of a DSM is DSM level 1, or simply DSM1. It relies on the Power Adaptive (PA) algorithm to maintain minimum transmit power while maintaining a fixed bit rate and noise margin within specified limits. This algorithm is also referred to as the Fixed Margin (FM) mode. The second example of DSM1 is the Iterative Water Filling (IWF) method, which is actually an extension of the PA algorithm. The only difference between these modes is that the IWF is not limited to a fixed transmit signal PSD spectrum mask, but allows its change, which is provided by reallocating the power of unused subchannels between used subchannels.
DSM level 2, or DSM2, solves the same problem as DSM 1. Only for its execution, information about the state of not only this line, but also all other lines of this beam is used. This requires coordination between all the beam lines. This optimizes the throughput of all lines of the cable bundle. An example of DSM level 2 is the rather complex Optimal Spectrum Balancing (OSB) algorithm.
Dynamic Spectrum Management (DSM) techniques rely on the ADSL and VDSL modem's ability to monitor multiple DSL connection parameters (including current modem configuration, noise, transmission errors, and other DSL line impairments) and the ability to access them for the network operator's OSS support systems.
The parameters of standard DSL modems can also be reconfigured by the network operator to improve the parameters of the DSL connection.
DSM methods take advantage of OSS capabilities to analyze the monitoring results of DSL modems. This allows, based on the received monitoring data, to automatically eliminate degradation of the DSL lines and predict their damage. This in-depth analysis can significantly reduce the number of Truck rolls required by the technician to repair damage.
The DSM method allows you to improve the parameters of the DSL line by:
1. Optimization of the bandwidth / overlapping distance parameter by choosing the best possible profile of the DSL line parameters. Moreover, the DSM1 method allows you to select this best profile, relying on the parameters of noise and crosstalk only for this DSL line. The more advanced DSM2 method allows you to improve the parameters of the DSL line based on the analysis and subsequent minimization of the transient effects between the DSL lines of the entire cable bundle. This improvement is achieved by reducing the output power of the transmitters of DSL modems, which means, which is especially important, and the power consumed by these modems. nutrition.
2. Detection in operating conditions, without disrupting the normal operation of modems, of such sources of possible deterioration of the AL condition, such as parallel BT taps or bad splices of bad splices (BS).
3. An accurate estimate of the achievable bandwidth of a DSL connection.
The key element of DSM is Spectrum Management Center (SMC), a simplified diagram of which is shown in Figure ... SMC is one of the subsystems of the OSS operational support system that performs the functions of DSM. The SMC collects information about the condition of the DSL lines and enables DSM algorithms that calculate the best parameter sets or diagnose degradation that has occurred. Based on this data, the SMC will automatically reconfigure the parameters of the DSL line under investigation or notify other operational support systems as needed.
Figure: 21 Simplified SMC Architecture
Example 1.
Influence of parameters of parallel VT taps on the AL insertion loss
A characteristic sign of the presence of VT is the violation of the monotonicity of the frequency response of AL attenuation, which acquires a wavy character. The parameters of this waveform (frequency and range of oscillations) depend on the number of BTs, as well as their length and proximity to the user's premises.
As an example, Fig. 22 shows the frequency characteristics of the attenuation of a 3 kft AL with a BT 50, 100, 200 and 400 feet long, 100 feet away from the user's premises.
For the curves shown in Fig. 22, there is a fairly simple algorithm for determining the location and length of the BT using known parameters.
Figure: 22 Effect of the influence of the length of parallel VT taps on the AL insertion loss
In the presence of several BTs, the regularity of violation of the monotonicity of attenuation is lost and the work to eliminate them has to be done manually. Fortunately, such cases are relatively rare.
Example 2
Determination of the presence of defective splices and their location
Defective splicesbad splices (bs) often act like a Cbs capacity (fig. 23) connected in series in the AL. Their effect is especially noticeable and unpleasant at low frequencies, since it is at these frequencies, where the DSL line transmits the bulk of the information, that poor-quality splices introduce increased attenuation.
The attenuation value depends on the equivalent capacity of the defective splice Cbs. The worse the splice, the greater the Cbs value and the greater the insertion loss at low frequencies (Fig. 23 (a) and (b)).
It is on this feature that the algorithm is based, which allows detecting bad splices, as well as predicting their appearance.
Defective splices are especially dangerous, since they not only increase the AL attenuation, but can also significantly worsen the line symmetry, as a result of which the transient influences between the AL of the cable bundle will increase.
One of the side effects of defective splices is to reduce the RMS deviation of cable parameters such as attenuation and crosstalk. There is a high probability that “bad” pairs of the same length of cable will be connected to “good” pairs of the next length, as a result of which the performance of the worst pairs will be improved at the expense of the performance of the best pairs. However, splices lead to impedance discontinuities and capacitive asymmetries and can therefore increase echoes and transient influences. The degree of this increase depends on the splice assembly technique; Usually, to assess the effect of splices, it is necessary to measure the parameters of typical cable samples.
Fig. 23 Influence of a defective splice on the insertion loss of a 3-kft AL at C bs \u003d 100 pF (a) and C bs \u003d 1.5 nF (b)
1 - AL without defective aggregates, 2 - AL with one defective aggregate.
Example 3
Figure: 24 Cable bundle structure with DSL lines terminated at the local exchange and at the RT
This example (Figure 24) illustrates the capabilities of the DSM method for one typical case, when one group of users receives DSL services from an access point located at a local exchange (CO), and another from an access point located at a remote terminal (RT). Both groups of users are served by one telephone cable, and the ALs of the first group of users are significantly longer than the ALs of the second group.
When transmitting DSL signals of the same power in the DS direction from the access nodes to the users, the ATU-C1 modem signal will arrive at a point coinciding with the position of the RT access node, which is greatly weakened. At this point and throughout the parallel run of both ALs from RT to the ATU-R1 modem, this attenuated ATU-C1 signal in the absence of a DSM mechanism will experience a strong transient effect from the transmitter of the ATU-C 2 modem of the RT remote terminal.
How does the DSM fix this situation?
Obviously, the situation can be corrected by reducing the transmission power of the ATU-C2 modem by the amount of signal attenuation between the access nodes CO and RT. Indeed, this reduction in the ATU-C2 transmit power, on the one hand, will weaken the transient effect of the second DSL line on the first, and on the other hand, will provide the necessary noise margin for the ATU-R2 modem signal, since AL2 is shorter than AL1. This ensures that both lines are equally protected against crosstalk. This is exactly what the DSM1 Level 1 algorithm solves.
Note that there is a so-called near-far problem with DSL access. Its essence lies in the fact that with the same transmission levels and spectral shapes of DSL modems, shorter lines worsen the parameters of longer lines due to the increased influence of the former on the latter.
Further improvement of the parameters of both lines can be achieved by taking into account the peculiarities of the DMT code. With the DMT code and the same transmission levels in the subchannels, most of the information flow is carried by the lower subchannels, since the AL attenuation increases with frequency. This more subtle mechanism is DSM2, which is the second stage of the DSM algorithm.
In our example, the DSM2 algorithm instructs the ATU-C2 to reduce the power transmitted on the lower subchannels, which will reduce crosstalk on the lower subchannels of the first DSL line.
At the same time, AL 2 of shorter length introduces less attenuation at high frequencies as compared to AL1. Therefore, the upper DMT subchannels can be used to carry the second DSL line signal.
Such a mutual redistribution of the DMT signal power in the subchannels of both lines will allow using a multi-step procedure of successive approximations to minimize the transient effect between them.
As a result, the transient effect between both lines will decrease and their throughput will increase, and, consequently, the stability of the operation of both lines will increase.
The above is naturally true not only for the two lines considered, but also for their arbitrary number with the structure of the two types indicated.
While DSM technology is still in its infancy, there are already hardware manufacturers who are integrating Spectrum Management Center (SMC) software into their management systems. As an example, we will indicate ASSIA Inc., whose DSL technology already uses DSM algorithms to optimize 15 million DSL lines.
The next most complex method, DSM3, builds on advances in two areas: the DMT modulation method and the MIMO (Multiple Input Multiple Output) digital signal processing method used in wireless networks (Figure 25a). Therefore this method is called DMT on MIMO
Fig. 25 (a, b) To an explanation of the MIMO algorithm
The MIMO method came to DSL from wireless radio.
The rapid development of wireless telecommunications systems such as cellular and satellite radio systems, local wireless networks and the Internet by technology Wi- Fi and Wi- MAX, found a serious problem. It turned out that almost the entire radio frequency range has been allocated and licensed by now. At the same time, studies by the US Federal Communications Commission (FCC) have shown that this spectrum, as a precious natural resource, is not used effectively enough. The dynamic spectrum management mechanism allows a significant increase in the efficiency of spectrum use, according to which secondary users (not assigned to a given frequency range) are given the opportunity to use the ranges of primary users (assigned to this range) for a time while this range is not used by the latter.
The mechanism for dynamic spectrum management is technically very complex and can only be used in so-called intelligent radio systems. A distinctive feature of such systems, distinguishing them into a separate group, is the ability to extract and analyze information from the surrounding radio space, predict changes in the communication channel and optimally adjust their internal state parameters, adapting to changes in the radio environment.
To describe such intelligent radio systems, D. Mitolla proposed the term "cognitive radio". The property of cognition, literally means the ability of the system to cognize and self-study.
It implies the ability of a radio system to estimate the so-called noise temperature of a radio environment and find unused spectral ranges (“spectral holes”) at a given time.
In a mathematical sense, the essence of the problem of monitoring the radiated power in a multi-user environment is to select the optimal level of radiated power among n users to maximize the aggregate bit rate without exceeding the maximum permissible noise temperature level and subject to a limited number of "spectral holes". The problem is that when the transmit power of one of the users is increased, the undesirable effect of increasing the level of interference at the input of the receivers of other users is observed.
To effectively control the radiated power in cognitive radio systems, the procedure can be applied Water Filling... Its essence is to cyclically increase the transmitted power by each user in order to increase the transmission rate (without exceeding the maximum allowable noise temperature level), and then adjust the power by each user to achieve the desired transmission rate.
The essence of the MIMO method as applied to DSL access is to treat DSL cable bundle systems as a single transceiver (Fig. 25b). The latter controls the signals of the DSL systems of this beam and creates a mathematical model of its transient influences. The DMT on MIMO system continuously monitors the DSL lines of the cable bundle and, accordingly, upgrades the DMT transmit spectra to maximize the DSL line capacity of the bundle.
The MIMO method can also be used to implement the bonding algorithm, which makes it possible to transmit a high-speed data signal using multiple parallel lines.
In principle, MIMO can be used with any DSL line code, but it is most efficient with DMT code.
Physically, MIMO can be thought of as a set of adaptive filters connecting each transmission channel in the system to all its other channels. In newer systems, such filters are implemented as discrete-time digital filters, and the filtered signal itself is represented in the frequency domain. Namely, such an operation is performed by DMT transceivers in the course of their normal operation.
There is another explanation as to why DMT is the most efficient code even without MIMO. DMT has the unique ability to suppress narrowband interference without disrupting the rest of the line spectrum. Other systems in the DSL family, such as HDSL and SHDSL, which use single-carrier transmission techniques, can be damaged by just one strong narrowband interference.
Let's immediately decide what we are discussing here gaps "Physics", but not "logic", i.e. gaps with loss of carrier. The greatest influence on the quality of communication is determined by the cable from the PBX to the RK (Junction Box - located on the staircase in the shield, usually above the switches and electricity meters). Moreover, it is not only the quality and length of this cable (distance from the PBX) that affects, but also the conditions in which they are laid. It makes no sense to list everything, there are a lot of them - from flooded wells to the cable from the intercom at the entrance. This is all that you cannot change, i.e. if the problem is on THIS site, then, most likely, after you zadolbat caliper, you will be written a technical failure. Now about what you can do yourself to diagnose the problem:
0.
a) Take a hammer.
b) Put your USB-modem on the stool.
c) make sure that people are not affected by the scale.
d) Hit the center of the box three times.
e) Collect the debris and throw it into the trash bin.
Dear, do not take usb modems. The use of half software devices with glitchy, ever-flying drivers and connecting via usb, which the system, "to save energy", cuts off for a second is at least not logical at the cost of NORMAL routers within 1k rubles.
1. Checking the connection diagram. Everything is simple here. I hope there is no need to explain the scheme. Are there breaks? Check the functionality of the splitter (read: try with another), and exclude ALL unfiltered devices using the telephone network (caller ID, fax, dialup modem, etc). Also check the wiring for twists, adhesions, breaks, insulation damage. Eliminate non-contact in the outlet or the presence of capacitors in it. We also try to disconnect all devices from the line, and connect the modem directly to the telephone socket (if there are several, to each one). Try to change wires from the modem to the splitter and from the splitter to the line. Line parameters (about them below) change over time, and, moreover, not for the better. In other words, if there are no complaints about the quality of telephone communication, this does not mean that the oxidation of the telephone line, for example (which by the way can be at any site up to the automatic telephone exchange), has not affected you. Perhaps it is still at an early stage. At a later one, crackling appears and noise occurs.
2. Diagnostics of linear indicators. The important indicators of the telephone line for adsl are the noise level (Noise margine or SNR ratio) and attenuation level (Attenuation Line)... They can be viewed in the router settings, usually located in the Status menu. Sample table of values:
Signal attenuation:
5dB to 20dB - excellent line.
from 20dB to 30dB - the line is good.
from 30dB to 40dB - the line is bad.
from 50dB and above the line is very bad.
Margins (signal to noise ratio):
6dB and below - sucks, ADSL may not work
7dB-10dB is average, with deterioration possible instability.
11dB-20dB good
20dB-28dB excellent
29dB or more - super
Accordingly, we look and draw conclusions. We connect the modem directly without a splitter and TA and draw conclusions again. If everything is also bad, then we connect from the switchboard. boxes, i.e. we physically remove the old cable going to the apartment and hook it with a short cable from the stamps in the pk to the modem. then again we measure the "margins".
Look carefully at the signal-to-noise ratio, it is very bad if it is not stable but "floating", that is, you have up / down 15/20, and after 10 minutes 9/14. If the SNR "sags", then the problem is probably bad contacts throughout the entire section from the modem to the RK. It is worth checking more closely. Also, all devices emitting electromagnetic waves significantly affect, as an example of dect-phones, the bases of which are placed next to the modem. IT IS WORTH REMEMBER - no unnecessary devices near the modem.
It is also worthwhile to closely monitor the cleaners who strive to rip out the wires, walk over them with a mop. It is also better to keep equipment for laundries and dry cleaners away from the modem - water will not lead to anything good, unless of course you have professional equipment from the company http://continent.com.ua/, which you should not doubt. For example, you can keep a high-speed washing machine at home.
3. Physical changes. PHYSLINK - the bandwidth of the channel between the modem and the automatic telephone exchange (dslam), i.e. the speed of the physical connection. We will not go into details. We will go into only one detail - the higher the channel speed, the more errors and, as a rule, disconnects, etc. etc. sorry for the "artisanal" explanation, if you wish, you can "google" and find out more. You need to realistically assess the capabilities of your line before asking to raise the physical link, especially since the majority does not need it. To set the optimal channel speed, you should contact the technical support service (8-125 for Rostelecom subscribers) and together try to choose a profile that would correspond to your capabilities / desire.
4. Checking the modem. It goes without saying that the reason for the breaks may be the modem. about the usb modem it was said in paragraph zero. Also, if you have been using a modem for 3-4 years, it makes sense to try with another modem. A sign of a “dying” modem is the noise in the phone and the margin slowly “sliding down”. Check.
5. Mysticism. Modem-powered cats and a military radio station at the veteran's grandfather have long become classics of the genre. In general, you have a 1/1000 chance of being in the anomaly zone and then ... in general, it is worth considering.
You can discuss this article on the forum
ADSL (Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) is a modem technology in which the available bandwidth is distributed asymmetrically between outgoing and incoming traffic. There are two groups of factors that affect the quality parameters of ADSL:
Impact of Terminal Devices and DSLAMs on PBX
ADSL technology provides technological independence of the parameters of the ADSL modem and the provider's equipment (DSLAM). The devices are different, so any inconsistencies will affect the quality of ADSL access. A mismatch factor can manifest itself in the fact that the modem and DSLAM may not establish the most efficient mode of operation. Violations in the negotiation of coding schemes and failures in the SNR diagnostic algorithm can lead to degradation of the quality of the ADSL connection.
Influence of subscriber line parameters
The most significant operational factor influencing the ADSL quality parameters is the parameters of the subscriber cable pair. Since the subscriber cable and its parameters in most cases are not upgraded, but the provider already has it in the form and condition in which it has survived to this day, it 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.
Basic parameters of subscriber cables are the parameters that are used for certification of the operator's cable system:
Specialized parameters:
Common cable problems
Inhomogeneity in the cable negatively affects data transmission. Unsoldering is a very common phenomenon in Russian wiring. The transmitted signal forks through the tap and then reflects off the unmatched end of the tap. As a result, there are 2 signals on the receiver side: direct and reflected. Reflected in this case can be considered as noise, so its impact 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 difficult and is a factor of randomness. For example, the mutual influence of one pair on another may exist potentially, but not manifest in any way. But connecting another ADSL user may affect the quality of both connections.
The main problems 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 cut off.
At 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 to solve the problem, it may be on the side of the provider.
With physical breaks communication, first of all, it is necessary to check the connection diagram, the quality of the connection and the condition of the telephone cables.
We can independently check the modem connection parameters on the line through the modem's web interface. To do this, 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 (the login / password may be different if it was changed when setting up the modem).
Usually, information about connection parameters is found in the system information sections. The informative value of the parameters depends on the brand and model of the modem and the version of the software (firmware), for example, in D-link 25xx series modems it looks like this:
The main parameters that you should pay attention to:
Parameters for diagnostics
Signal to noise ratio (SNR) - is used as a criterion for assessing the state of the line and determines the minimum limit at which the signal level is higher than the noise level:
6dB and below - bad line, there are synchronization problems;
7dB-10dB - malfunctions are possible;
11dB-20dB is a good line, no timing problems;
20dB-28dB is a very good line;
29dB and up is a great line.
Line Attenuation - shows the attenuation of the signal in the line at the time of synchronization of the modem with the DSL switch. This parameter 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 - malfunctions are possible
from 50 dB to 60 dB - synchronization periodically disappears
from 60 dB and more - equipment operation is impossible
Methodology for diagnosing a disconnected connection 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 of connecting the modem to the telephone line.
Correct connection diagram
We check the telephone wires for poor-quality connections (twists, "noodles", poor crimping of connectors).
To exclude the possibility of the influence of the connecting cables, the 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 test 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 did 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 in comparison with FTTB (optics to the house), but there are areas where, due to the lack of alternative connection schemes, this type of communication is the only possible one. New GPON technology is being introduced to the private sector to replace ADSL connection. You can read more about it.
The problem of low speed can manifest itself in various situations. Problems can be conditionally divided into several types:
physical - incorrect connection scheme, problem with the telephone line, the remoteness of the servers, the distance from the PBX to the modem, etc.
programmatic - problems with software on the computer, incorrectly configured firewalls, antiviruses, peer-to-peer clients.
hardware - weak wi-fi transmitter, problems with the network card, problem with the router, etc.
In each case, the solution to the problem will be different, respectively, and the 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 is connected. As it was announced earlier, for this it is enough to go to the address http://192.168.1.1. For example, on a D-link 25xx series modem, we can see the following:
Pay attention to the parameter values Attainable rate (maximum possible line speed)... 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 a speed of up to 6 Mbps out of 25 Mbps possible. The speed of 6 Mbps is the speed value set on the DSLAM port and can be changed by a technical support employee.
If you changed 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 are changed on the station equipment (DSLAM) to which you are connected ...
When using ADSL technology, the distance to the PBX to which you are connected is important. The further you are from the PBX, the slower the connection speed you can get.
For example, with a distance to the PBX of 4-4.5 km, given the state of the wiring, it is unlikely that it will be possible to get a stable Internet at a speed of more than 2-3 Mbit / s.
Usually, to check the speed, users use speedtest.net, 2ip.ru or the first resource they see in the search engine results. And if the speed indicators do not correspond to those stated in the tariff, they begin to complain about the 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 that during testing there is no update of the operating system, antiviruses, other programs in which the auto-update mode is selected
- unload peer-to-peer clients (transmission, utorrent, skype, etc.)
- temporarily disable antivirus (especially if it is avast, kaspersky)
- check the correctness of the specified DNS servers
- try to test 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.
Low outbound speed
Since ADSL technology is asymmetric, another value for the speed - the upstream rate will be much less than the downstream rate. The asymmetry of ADSL implies the transfer of large amounts of information to the user and small amounts 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 Mbit / s.
So if we see Upstream rate 636 Kbps (0.6 Mbps), a Attainable rate for upstream 1218 Kbps (1.2 Mbit / s), that is, the probability of an increase in the upstream speed.
Pages do not load when using ADSL modem
If there are problems with opening pages, the indication on the ADSL modem will help to diagnose and identify the problem as quickly as possible. For instance:
14:02, 07.02.2016Some manufacturers of ADSL modems are replacing the labels under the indicators with graphical symbols. In order to find out what the indication means, you need to refer to the user manual for the device.
There can be quite a few reasons for ADSL connection (link) breaks; in addition, there can be a combination of several factors leading to frequent link failures. Not all of them can always be eliminated, but you can try to reduce their impact. In this article, we will consider the settings for an ADSL modem using the Zyxel P660HTN EE as an example. We will do almost all the settings from the command line (CLI), via a telnet connection. You can read how to configure it in this article.
Possible causes of ADSL breaks
The first step is to look at the line parameters. Note that the downstream and upstream channels have their own parameters, but usually they are not very different.
1) The attenuation in the line (Attenuation) should be no more than 45 dB. If the value is more than 60 dB, then ADSL will not work.
2) The signal-to-noise ratio (Noise Margin) should be more than 6 dB.
You can check these parameters with wan adsl l n for downstream and wan adsl l f for upstream.
If these parameters are not met, then check:
- Is the splitter turned on correctly (if you have a telephone).
- Is there any damage to the wire.
- Telephone line connections available to you.
There should be no corrosion and loose contacts at the wire connections. The line must be made of solid wires and it should be noted that the fewer connection points, the better the line characteristics. If everything is connected correctly in the apartment and the line is in good condition, then you should contact the provider's support. So that they fix problems on the line.
The second reason could be the modem. Possible marriage of the modem, marriage of the modem's power supply, as well as the parameters of the household power supply. If there are power surges, then consider a modem transformer. Also, you can test the work on another modem, borrow for a while from a friend or ask the provider for a test. If little has changed from the replacement of the modem, then rather there is a third reason.
The third reason is the inconsistency / incompatibility of the settings of the provider's equipment and the modem. It is this reason that we will try to eliminate, using the Zyxel modem as an example. It has a fairly rich set of commands for configuring and diagnosing the connection, in addition, the commands are universal and suitable for most Zyxel modems, with rare exceptions.
Brief theory
First, it should be understood that the signal-to-noise ratio (noise margin) is a quantity that changes over time, for example, from the appearance / disappearance of interference from other subscribers in the trunk cable or radio interference. Also, you need to understand that the higher the noise margin value, the lower the speed. That is, higher speed means worse connection stability and vice versa.
Secondly, until the line has acceptable attenuation parameters, you will not get stable ADSL operation, here you need to change the line, not the settings.
Thirdly, you can change the connection parameters only for the downstream channel, the parameters of the outgoing channel are usually determined only by the DSLAM and the Annex version set on the modem.
Turn on the SRA mechanism
SRA (Seamless Rate Adaptation) allows you to change the connection speed on the fly, thus adapting to changes in the line without waiting for a break.
The modem is configured in bridge mode, empirically found out that the connection is kept stable with automatic synchronization, that is, when all ADSL standards and all types of Annex are allowed.
Now you need to find out which profile is configured on the DSLAM port. To do this, you need to connect to the modem via telnet and enter the command: wan dmt2 show cmsg1. In return, you will receive the profile settings on the DSLAM.
The important parameter here is RA-MODEds, it can have three options:
- RA-MODEds \u003d 1 (FIXED DATARATE) - fixed connection speed, usually cut by the provider, to a more or less stable connection.
- RA-MODEds \u003d 2 (RATE ADAPTIVE AT INIT) - the connection speed is consistent at start and does not change until the next break.
- RA-MODEds \u003d 3 (DYNAMIC RATE ADAPTATION) - the connection speed changes during operation.
The SRA engine will only work with a dynamic profile. Only the provider can change the profile to dynamic. It is best to apply in writing, because as soon as the first line of TP sees unfamiliar words, it will most likely send the question to specialists. For ease of communication, it is better to immediately write all the parameters in the profile. For example, such:
TARSNRMds \u003d 70 dB (desired S / N level \u003d 7 dB)
MINSNRMds \u003d 10 dB (minimum signal / noise level \u003d 1 dB)
MAXSNRMds \u003d 310 dB (Excess margin need not to be minimized) (maximum signal / noise level \u003d 31 dB)
RA-MODEds \u003d 3 (DYNAMIC RATE ADAPTATION) (DSLAM Profile Type, Dynamic Rate Adaptation)
PM-MODEds \u003d 1 0 (L2 is allowed) (L3 not allowed)
RA-USNRMds \u003d 85 dB (signal-to-noise level at which the speed will increase)
RA-UTIMEds \u003d 20 sec (time during which the signal-to-noise ratio must be greater than or equal to RA-USNRMds for the speed increase to occur)
RA-DSNRMds \u003d 60 dB (signal-to-noise level at which the speed will decrease)
RA-DTIMEds \u003d 20 sec (the time during which the signal-to-noise ratio must be less than or equal to RA-DSNRMds for a speed reduction to occur)
BIMAXds \u003d 15 bit
EXTGIds \u003d 0 dB
CA-MEDLEYus \u003d 6144 symbols (min)
Reserved1 \u003d 0 (Should be 0)
Then there are a few “ifs”.
If ping is not critical, then it is better to put the channel in interleave mode, while you need to set the delay in milliseconds, for example, 8ms. The latency must be the same for downstream and upstream. This will correct cell transfer errors and increase the stability of the connection.
It is worth asking to set the INP parameters, for bad lines it is worth setting the INP value min \u003d 1 or more. INP is set separately for downstream and upstream. 
If asked, the maximum speeds for downstream and upstream channels can be set at a maximum of 24000 kbps and 3500 kbps, respectively.
Next, for your part, you need to make sure that SRA is enabled in the modem. This is done by the wan dmt2 set olr 4 command. In response, you will receive the current state: OLR ON, SRA ON. From personal experience, I found that the line wan dmt2 db tlb 10 in the autoexec.net list interferes with the correct operation of the SRA and must be removed from there.
Below is an example of the contents of my autorun, in which the modem stably holds the line for 30 - 70 hours. Unfortunately, SRA is not a panacea; it will not save you from sharp and deep changes in line characteristics.
What to do if there is no SRA
If for some reason it is not possible to get a dynamic profile from the provider with the correct settings, then you can change some of the settings for the downstream channel.
You can try disabling the Annex M as it places higher demands on line quality.
Also, Zyxel modems have the ability to shift the signal-to-noise ratio up or down. The offset is set by the command wan dmt2 set snrmoffset X Y, where X or Y is the value obtained by the formula: 1280- (± Z * 512).
- X - sets the offset for the Fast mode;
- Y - sets the offset for the Interleave mode;
- Z is the offset value in dB.
For example, to increase the signal-to-noise ratio by +1 dB, we get the value 1280-512 \u003d 768, therefore, the command will look like wan dmt2 set snrmoffset 768 768.
Thus, you can select the optimal signal-to-noise ratio.
In order for the necessary commands to be saved after the modem is restarted, they must be registered in autorun. Enter the command sys edit autoexec.net, then use the N button to scroll through and press the i button in the right place, insert the desired command and press Enter. To save, press the X button. The location of the commands in autorun relative to each other matters. Presumably, the commands at the end have a lower priority than those at the beginning.
Commands are not for everyone
If the provider does not want to change the profile parameters, then you can try to change some parameters yourself. These commands don't work for all modems, but it's worth a try.
w dmt2 set INP V, where V is the value of INP_min
wan dmt2 set path 1 - set the Interleave channel mode.
wan dmt2 set txfltrgain 7FFFF - probably shifts the gain point of the frequency range, can take values \u200b\u200bfrom 00000 to FFFFF (7FFFF is the middle of the range).
To check the result, you need to reset the connection with the wan adsl reset command.
If you notice in the modem logs that the firewall often writes about a port scan attack, and the modem is configured in bridge mode, then it is better to disable the firewall in the modem. The fact is that as a protection against port scanning, the modem can block traffic or part of it, this can lead to breaks. This will not particularly affect security, since in bridge mode, the firewall of the host on which the PPPoE connection is established is used.