Been looking at a connection (stats to follow) its a long ADSL2 line, I noticed the HEC errors are high.. What would be the best way to reduce. Computer Network Accessories > Communication testers - ADSL/VDSL INTERFACE, OCD, FEC, CRC and HEC errors; Automatic detection and display of LOS, LOF. A CRC error indicates that part of the data packet is corrupt and requires retransmission. - see Cyclic Redundancy Check (CRC) for a more in depth explanation.
Hec errors adsl2+ - apologise, butedit source]
It is the ratio of the signal level to the noise level. Sometimes it is written as SNR or S/N. It is expressed in decibels (dB), because the logarithmic form allows convenient addition and subtraction of quantities instead of the multiplication and division of natural values. It is also useful in that it allows compression of what tend to be wide power and voltage ranges. Although of great use to the network provider, the SNR ratio itself is rarely known by the internet user, even from his router's connection statistics. Knowing it alone would not necessarily allow him any insight into the quality of transmission unless many other factors were also known. It is more useful to know how far from noise overload the current working system is. This quantity is called the noise margin.
Many line management functions make use of bit error rates (BER) rather than signal to noise ratio (SNR). However, when all of a link's parameters are known, there can exist a calculable relationship between the two. As such, once a profile is fixed, a knowledge of the one allows a sufficient estimate of the other. Bit error rate (BER) itself is found by taking a timed sample of the data stream, sufficient to give the necessary confidence level.
Profiles[edit edit source]
Clearly, having the computer equipment installed beside the main telephone box is not always suitable. If the router is to be placed beside the telephone main connection box, then a long Ethernet cable is needed to reach the computer. Alternatively, a long ADSL cable could be run from the phone box to a router placed near the computer. When the connection is always to be wifi, then it might be thought that a cable is unnecessary. However, the use of wifi will not always give as high a download rate as a cable run from the router, so having at least the option of a hardwired cable connection is always preferred, even for a laptop.
Whichever method is used, the user must consider the signal losses that might be incurred by the extension, so choose good quality items, and bear in mind that the losses and noise will increase with its length. For ADSL extension cables, if possible, choose the round ones marked as Cat5 rather than flat ones since the round cables contain twisted wire pairs to reduce noise. This cable is similar to that of the line itself, so in lengths under 10 meters or so will not cause any undue impairment of the data. ADSL cable extensions have connectors already fitted so it is just a question of choosing a length that is slightly longer than required.
If there is no filter in the main connection box, that is, if it is just an ordinary telephone connection, then a filter must be placed between that connection and the router. It is unimportant as to whether the filter is located before or after any ADSL extension cable, as long as the router is given a filtered data connection.
Profiles, Signals, and Noise[edit edit source]
Efforts at establishing a guaranteed line speed have varied for each country, and have included both voluntary and compulsory rules. The notes here draw heavily on the United Kingdom's (UK) recent experience.
- Voluntary Codes: The internet providers (ISPs) of some countries sign up to a voluntary and self-regulatory code of practice. It provides their clients some protection against what they feel are lower than advertised speeds.
- United Kingdom's Voluntary Rules. Prior to 23 May 2018 in the UK, the Minimum Guaranteed Access Line Speed was such a scheme. In this case the speed for a particular client was found statistically from a set of those with similar line profiles. The sync rates of the lowest 10% of such a set was taken and the highest of these gave the Minimum Guaranteed Access Line Speed for that client. Notice that it did not apply to the download rate, but to the sync or line connection rate. Those with speeds below this during the first 90 days of a contract had the right to nullify it, subject to giving reasonable opportunity to the provider to correct the matter. Most of the major providers in the UK were signed up to it. This voluntary code is being reconsidered after May 2018 however, when new compulsory advertising rules take effect. See the notes on the voluntary scheme here.
- Compulsory Rules: Some countries address the matter of guaranteed line speed through the regulation of advertising. Such rules are compulsory, are a legal requirement, and adverts can be banned if they do not comply.
- United Kingdom's Compulsory Rules. From 23 May 2018 (with 6 months to implement), new advertising standards apply to the sale of broadband. Speeds must be given for the average (median) download speed that is obtained, during busy times, 8pm to 10pm, for residential use. Note that this is no longer the line rate that is being described but the throughput, the download rate that is available to the client. The underlying intent of the new rules is for the purchaser to have a realistic idea of the download rate that he might expect. The new standards will likely require rewording of both the existing voluntary code, and most sales adverts. See the notes on the compulsory scheme at CAP and BCAP regulatory statement on speed claims in broadband advertising.
Line Management[edit edit source]
This is also known as the throughput or data rate. It changes slightly from hour to hour because of traffic management and traffic congestion. Traffic prioritization on the other hand, the giving of preference to some traffic types over others, is not applied to broadband products in the United Kingdom by the main providers, so the throughput tends to be quite close to the IP Profile rate for most of the time. In the United States, as of June 2018, providers are permitted to give priority to some traffic. Intuitively, the download rate can be thought of as the line rate, less the rate that is lost because of rework in the transmission system.
Guaranteed Line Speed[edit edit source]
This is sometimes referred to as signal to noise ratio margin or just margin. It is written as SNM and it too is expressed in decibels (dB). This quantity is the amount of noise that would need to be added to the prevailing signal to noise ratio to worsen it to the lowest expected signal to noise ratio. The lowest expected SNR for example, might be defined as the level that produces a bit error rate (BER) of say, 10-7, that is, one errored bit in ten millions. This would be the highest error rate that the internet user could reasonably tolerate without excessive download failures. Note that the margin is estimated with all of the advantages of error correction and other gains in place, so that when any of these change, the remaining available margin changes too. When the margin changes, parameters are adjusted until the margin returns to near its target value, at which point, the relationship is just:
SNR Margin = SNRprevailing - SNRlowest
To protect against the effect of noise and other changes, links are run, not at their best extremes but by allowing this operational margin for error. This fact is true of the initial synchronisation process and also for normal running. When a link is initialised, or synchronised, the margin that is used is called the target margin. It is the margin used in setting up the line. A high target margin might be chosen if widely fluctuating noise is expected; to do this, the initial signal to noise ratio would have to be high, and in turn to do that, the line rate might have to be lowered. Conversely, a low noise channel might allow a lower target margin to be used, even with a good line rate. The target margin is therefore selected to be no larger than necessary, to allow a high enough line speed with a given signal quality, and large enough to stay clear of historic noise levels. In most systems the value will be a preferred value of 3, 6, 9, 12, or 15dB, though in the system as designed, margins can be adjusted over a wide range in 0.5dB steps.
Ideally, links would be synchronised only when the prevailing SNR is equal to the sum of the lowest SNR and the target margin. This is to make sure that the link will work even when an additional noise, equal in value to that of the target margin, is applied. However, it is impractical to wait for such a SNR, so simulated conditions are constructed for the evaluation phase. That is to say, an existing SNR can be worsened with a noise generator to test the system under various conditions. This ideal SNR at which to set up the link is referred to as the Optimum SNR. That is:
SNRoptimum = SNRlowest + Target Noise Margin
The choice of sync rate and target margin affect both the stability and throughput of the line. If a link is unstable, it can be re-synchronized with a higher target margin (and a higher SNR), or interleaving can be applied. However, when using the same signal level, higher target margins require slower line rates. If a faster rate is needed, interleaving can be removed or a lower margin used. These two changes in turn contribute to a worsened stability. The process of synchronization attempts to find the best compromise between the two, bearing in mind the crosstalk effects associated with high signal levels. See figure 7 for a summary of these effects.
During normal running, the margin changes slightly to reflect both changes in the prevailing SNR and in coding gains from error correction. Continuous small changes to the rate and other parameters are made so that the margin is returned to a value near that of the target value. That is to say, when the prevailing SNR improves, the margin increases too, and the provider takes advantage of it to reduce his overheads or increase the rate; when it worsens, the margin worsens too, and he applies more resources, or decreases the rate. Clearly the closer that the prevailing SNR is to the optimum SNR, the one used for synchronisation, the fewer running changes will be needed to maintain the link. This situation can be approximated if in the selection of a target margin, the optimum SNR can be made to equal the SNR most likely found on the link for the busiest part of the working day. (8pm to 10pm for residential traffic).
The most commonly found value of noise margin is 6dB for lines shorter than about 2Km or so. Although lines might be synced with margins above 6dB for long or noisy lines, those synced with margins lower than 6dB are generally found only on good quality connections. The download rate of a connection can usually be increased if the provider syncs with a lower margin, but only at the cost to himself of more error correction and other overheads that compensate for impaired stability. When lines worsen too badly for effective control, or after the repair of a fault, new profiles are usually needed. See figure 6 for an example of how the longer term choices are made.
Digital Error Rates[edit
watch the thematic video Stop making data errors in HEC-ResSim edit source]
This is also known as the throughput or data rate. It changes slightly from hour to hour because of traffic management and traffic congestion. Traffic prioritization on the other hand, the giving of preference to some traffic types over others, is not applied to broadband products in the United Kingdom by the main providers, so the throughput tends to be quite close to the IP Profile rate for most of the time. In the United States, as of June 2018, providers are permitted to give priority to some perl ignore errors. Intuitively, the download rate can be thought of as the line rate, less the rate that is lost because of rework in the transmission system.
Guaranteed Line Speed[edit edit source]
Dynamic Line Management, (DLM) makes continuous changes to the operational profile. It makes use of error rate logs and other data that are continuously supplied. These allow an estimate of the ILQ or Indicative Line Quality. The process functions for synchronisation and hec errors adsl2+ day to day running. The parameters controlled by the DLM include the Sync rate, the Target noise margin, the depth of interleaving, the balance of frequencies used, and the bit loading of each, hec errors adsl2+. ADSL2+ can also make use of profile banding where hec errors adsl2+ link must sync within a restricted range.
Whereas the exact behavior of the line management system is known only to the network provider, it is possible to formulate some idea of its general functions from rhetorical accounts and from the patents themselves. The fact that a patent describes a particular process however does not guarantee that hec errors adsl2+ provider has built his system that way. The tables and flow charts in the section below should therefore be taken as a typical case rather than applying to any one in particular. For a detailed description of the initial synchronisation process itself, some will find the British Telecom PLC patent application entitled Optimised line synchronisation the most informative, hec errors adsl2+.
Table 2 shows the method of line categorisation, where MTBE and MTBR are used to decide on possible courses of corrective action. The actions that are possible are shown in Table 3. For a more detailed flow chart of how such a system might operate, refer to Figure 6 in the drop box below. Figure 7 shows the broad relationship between stability and speed, and its connection to interleaving. Figure runtime error 339 shows how a full retrain could be decided. Notice in Table 2 that there are three main levels of service; Hec errors adsl2+ can select a network service that prioritizes speed, hec errors adsl2+, error e3 mp240, or extreme stability, though the policy of an ISP is rarely known to the end user.
Table 3 lists the ADSL2+ fixed profile bands. It may have been noted that the flow chart of Figure 6 does not make clear whether or not reversion from fixed profile bands to fully rate adaptive service is automatic or not. Some clarification by network providers on this matter would go a long way to satisfying the curiosity of users.