Internet ADSL2+ Speed

Introduction

ADSL2+ is one of the faster ADSL internet variants. The term is short for Asymmetric Digital Subscriber Line. The subscriber circuits for this type of broadband use copper telephone wires all the way from the exchange to the house. Such broadband internet connections are commonly sold as having a maximum rate of 17 megabits per second. This page explains some of the ideas in a moderately technical way.

User-friendly Overview

The internet connection refers to the line connection between the user's equipment and the exchange. In viewing a web page there are many links in the connection chain, starting with the internet server at the far end, and with the user's link as the last. Each link corrects its own data before passing it on to the next, so that when one link in the supply chain has a poor throughput for any reason, it slows down the overall transmission while it is being sorted out. If the user's computer then runs out of data, a screen-freeze occurs, (buffering). Only if the links that follow the impaired link are fast enough to allow catchup, will the user get his data in time to make a seamless display. The description on this page deals only with the user's connection to the exchange.

On first connection to the line network, an automatic adjustment or synchronisation begins. Although the link can be used during this period, it takes about ten days or so for the system to put in place the necessary data for all of the line conditions that it encounters. At the end of this period it selects the best set of properties (profiles) for the prevailing conditions, and stores the rest for future use. Perhaps surprisingly, the fastest rate is not necessarily the best, since fast links have less stability. The network provider establishes the best compromise between speed and stability, bearing in mind factors such as the length of the line and the amount of electrical noise.

After the initial synchronisation period the network continuously monitors the line and makes profile changes to improve service. Most changes are made seamlesslly, without interrupting service, based on measured error rates. The network provider keeps some flexibility in his settings so that even the worst expected hourly variations cannot interrupt the link, but sometimes, in extreme cases, the line needs re-evaluated, at which time additional profiles are made.

The User's Equipment

The telephone provider's line terminates within the user's premises at a small connection box. From there, the user can plug in his phone and router, or run extension cables and wiring. This main connection box, like the line, is on rent from the phone company. Other items include frequency filters, and the computer or other equipment that is to connect to the network.

Figure 1: A typical ADSL filter or band splitter. It connects to a telephone socket, and provides both a phone output and another for the router, modem, or hub.

Filters

These serve to separate the telephone and data signals. Telephone and data travel on the same phone line. The low frequencies, (below 4Khz or so), belong to the telephone, and the higher frequencies to the data. The filters make sure that high data frequencies do not disrupt telephone use. Two types of filter are used. In one, the filters are housed inside a data connection box, (see figure 3), and the user plugs phone and data into its separate sockets; in the other, a filter accessory can be connected to an ordinary phone connection box to do the same thing, (figure 1). While the filter's phone sockets are common to all phones, those for the ADSL data are of a different type, called RJ11 data connectors.

Figure 2: A Typical Router/ Hub. The RJ11 line socket is shown on the left, and on its right are four RJ45 sockets (Ethernet) that connect to the local computers.

The Router or Hub

The equipment that manages the link's working rules is called a router. At times other terminology is used for the data handling equipment. Strictly speaking, units that convert between line frequencies and digital signals should be called modems. Sometimes the word hub is also heard; it is the process that time-multiplexes (distributes) the digital stream among the various computers. The hub also handles the wireless channels. However, because most manufacturers combine the modem, the router, and the hub into one unit, for example in the BT Home Hubs, the terminology has become somewhat blurred. In any case, the router is connected to line via the filter using an ADSL cable, with an RJ11 plug at both ends. A router yellow socket is connected to a computer's Ethernet socket by using a cable with an RJ45 plug at both ends. Figure 2 shows a typical router's socket panel. Note that not all computers connect to their modems with a cable. An increasing number of PCs make the connection with wifi in much the same way as would a laptop. In this latter wifi case the router would be located beside the incoming cable and the PC could be virtually anywhere in the room.

Figure 3: A main connection box with its own ADSL filters. It shows the line connection below, and the sockets for phone and data.

Extension Cables

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

Figure 4: The ADSL2 Frequency Spectra. Using combined modulation methods, ADSL2+ combines the equivalent of 512 x 4 KHz telephone bands to produce a theoretical download rate of 24 Mbps. The upload maximum is just 1 Mbps.

The throughput, the signal to noise ratio, and digital error rate are all related. As transmission rates and the interference from other cable users increase, the signal to noise ratio, (SNR), is reduced, and the correct detection of data levels becomes less certain. As a result, digital error rates increase, as does the rework (the overhead), and in the extreme a link can be interrupted or lose sync. The converse is also true; reducing transmission rates can improve all of these properties. In fact, when errors arise in data, error correction methods are first attempted, but when data packets are too badly damaged the data must be sent again, and at times the link rate must be reduced to obtain the necessary stability. The functions that can change line quality exist in sets, with variations in each component. These sets are called Profiles. To adjust a link, the system selects from a finite set of these profiles to suit the prevailing characteristics.

Signal to Noise Ratio

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

The making of changes to a link involves the application of profiles. These are database records of the properties needed to maintain line quality in particular circumstances. For example, signal to noise ratios differ throughout the day, the so-called diurnal changes, so different profiles will apply. At other times sporadic noise might be encountered, say from radio stations, and here too some particular choice of profile will prove best. When the service provider cannot deliver data that is sufficiently error free, and at a reasonable rework overhead, he will re-evaluate his choices of coding, interleaving and frequency allocations. Particular combinations of properties are referred to as profiles. There might be many profiles stored for a particular link and many trivial changes are possible within each. The first few profiles are stored for use during the link stabilisation process. This preliminary phase lasts about ten days, during which the link will usually be interrupted while profiles are made for the variety of conditions that it first encounters.

Not all parameter changes require a resync of the router. Retraining interruptions are needed mainly when a change to the framing is needed, whereas other changes, like frequency bit allocations and adjustments to the sync rate can be made seamlessly.

Figure 8 provides a crude flow chart of one such profile making process. It is repeated here as:

• The signal to noise ratios (SNR) of all of the frequencies used are monitored, triggered say, by a line quality issue (ILQ).
• If the signal to noise ratios found are within prescribed thresholds then there is no change to the selection of the currently applied profile. If on the other hand the SNR is poor, then there is a search for a suitable existing profile.
• If there is an existing profile for the job it is applied to the link. If there is no suitable profile available then one must be made, and a complete retraining of the link is initiated. This tends to interrupt service for a while.
• When the new profile is made it is added to the list of available profiles and then applied to the link.
• This procedure continues after the initial phase so that an ever-improving set of profiles becomes available for each new circumstance.

Noise Margin

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 = SNR_prevailing - SNR_lowest

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:

SNR_optimum = SNR_lowest + 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.

Table 1: Typical One Hour Error Rates for a Standard Option

GOOD	OK	POOR
ES <= 14	720 > ES > 14	ES >= 720
(MTBE > = 250)	(250 > MTBE > 5)	(MTBE < = 5)

Digital Error Rates

There are many error types used to control a link. Some have less significance than others. For example, successes in error correction (FEC) are likely to be of less concern than those that require the re-sending of packets, (CRC and HEC). Error counts are recorded by the link software and are used, in part, to decide the best sync rates. The following error counts are of particular interest in descriptions of the subject.

• FEC, Forward Error Correction. These are errors that have been successfully corrected using error correction methods. The larger the FEC statistic the harder that system is working to maintain stability. so very large values might signal an impending restriction in the selected profile.
• CRC, Cyclic Redundancy Check. CRC errors are those that persisted after other error correction. They initiate a resend process for the damaged packets. CRC checks detect such errors but cannot correct them; they just initiate the resending process.
• HEC, Header Error Correction. Packet headers contain a type of CRC code that can also correct one bit. This error count results only when there is more than one error, so that the data needs be sent again. Such errors contribute to rework.
• ES, Errored Seconds, or Error Seconds. This is the number of predefined seconds-slots in which there is any error at all. It is used in the calculation of MTBE, the mean time between errors, which in turn is used to categorize the quality of a line. It is expressed after all FEC error correction has been applied so it is an error that persists as visible to the user. See Tables 1 and 2.

Figure 5: Graph showing the improvement of Bit Error Rate (BER) with Forward Error Correction (FEC). The associated overhead is 7%.

• BER, Bit Error Rate. It is the proportion of bits sent that have any error at all. It is used to estimate SNR and margin. The target BER is the BER associated with the lowest SNR, the highest manageable error rate that the provider intends. It is commonly quoted as a value of 10^-7 or one in ten million bits at the output. It is common to refer to Pre-FEC BER or Transmission BER when describing the raw state of affairs before forward error correction (FEC) has been applied. The Post-FEC BER or Information BER value at the user's terminal is thus expected to be much improved. The graph of Figure 5 shows an improvement of BER from 10^-6 to 10^-14 using FEC with a 7% overhead. Overhead is the extra resending work needed to maintain the current BER figure.
• MTBE, Mean Time Between Errors. It is the ratio of the link Uptime in seconds divided by the Error Seconds. Notice that the other error quantities are not used directly in this ratio. It is used to categorize line quality. See Table 1.
• MTBR, Mean Time Between Retrains. Retrains essentially means profile realignments like speed changes, to avoid a loss of sync. It is the ratio of the link Uptime in seconds divided by the number of retrains. It too is used to categorize line quality. See Table 2.

Noise Types and Correction

The various noise types that affect a link include:

• Additive White Gaussian Noise (AWGN), thermal noise or just white noise for short. This noise exists at all levels, is random, continuous, and is not limited to any one frequency. Forward Error Correction (FEC) for the effects of AWGN is applied in three ways:
  • Convolutional coding, sometimes called trellis coding is used as the main method. Each unit of data output in trellis coding is a function of the data that preceded it and affects the data that follows it. At the price of added redundancy (payload length), an appropriate algorithm can then verify and correct errors. The method can be quickly adapted to work on data blocks of various length. The Viterbi algorithm is commonly used. Hard decision making (binary) algorithms are used for line transmission, whereas soft decision making (multi-level) algorithms can be used for other sub-systems. See A Basic Convolutional Coding Example for a description of the hard decision technique.
  • Reed Solomon (RS) block coding is used with trellis coding to handle any burst errors that may persist after convolutional decoding. It is a so-called linear block code. That is, it adds additional bits to the existing data unit, so that multiple errors can be detected and corrected.
  • Cyclic Redundancy Check (CRC) coding is applied at a late stage to detect the errors that escaped other methods. These methods do not correct errors, but merely detect them. The method applies a check value that is then recalculated at the receiver to detect errors. The CRC error count is therefore of data that increased overheads, needing to be retransmitted.
• Impulse Noise like the noise generated by lightning strikes, or electrical motor starts requires a different approach.
  • Interleaving. Convolutional coding cannot handle burst errors at all well and works best when these are not present. So, impulse noise that damages several adjacent bits at a time can greatly increase overheads. (The need to resend data). Interleaving removes most of the errors that would be found in close clusters, distributing them as single errors that can be more easily corrected. Interleaving functions by reading blocks of data into a matrix row by row, then reading it out again column by column. The setting of row and column lengths then determine the extent to which contiguous data bits become distributed in the channel stream; the so-called interleaving depth. A complementary process at the receiver reassembles the data to its pre-interleaved state ready for convolutional decoding. When convolution decoders do fail to make corrections they can themselves produce bursts of errors at their output. Reed Solomon decoding handles burst errors well, so is made to follow the Viterbi decoder. The combination of interleaving, convolutional coding and Reed Solomon coding can produce very low error rates.
• AM Radio Broadcast Noise. The frequency bands for AM radio and ADSL2+ overlap. There are two main methods used to reduce noise of this type
  • Adaptive RF Cancellation Filters. These are usually digital filters, and are self-adjusting. For example, they might shift an adjustable bandstop filter across the wider band until the detected interference is minimized.
  • Sub-carrier Adjustment. The link's sub-carrier channels near to the interference frequency can be switched off. Then because they are not in use, the interference that is band-passed with them is also removed from the process.
• Crosstalk. This is the unwanted signal from other lines. It is categorised as Near End Crosstalk (NEXT) or Far End Crosstalk (FEXT), and is further divided into self-crosstalk and foreign crosstalk. For long lines crosstalk is higher than for shorter lines because of the accumulative line capacitance; loading coils are then used to balance out the capacitive impedance. Because the near-end transmit power is high at source it has more influence than FEXT. NEXT is therefore the main offender.
  • Near End Crosstalk (NEXT). It is the signal induced into the receiver side by the transmitter side at this end. If the crosstalk is from the same ADSL installation it is called self-crosstalk, and if from another ADSL system nearby it is called foreign crosstalk. It is solved by having different frequencies for the send and receive paths, and by the restriction of levels for some low frequency bands.
• Bridged Taps. These are unterminated cables, or any other impedance mismatch in the path. Although some also act as antennas for AM radio interference their main problem is data reflection. An open connection gives rise to a reflection because it has a far end impedance mismatch. The reflection results in an additional leading edge in the data, one slightly delayed. This causes errors. Some methods to reduce the problem include the dropping of the leading bit, first made redundant in coding.

About rates

There is some confusion over internet rate terminology. To illustrate the point with a specific example, an ADSL2+ link might have an attainable rate of 24 Mbps, a maximum rate of 17 Mbps, a sync rate of 14.335 Mbps, an IP profile rate of 12.643 mbps, an actual download rate or throughput of 12.34 Mbps, and a Minimum Guaranteed Access Line Speed of 8 Mbps. All of these measures are quoted at times.

Attainable Rate

This is the rate that the technology is capable of provided that no service restrictions are applied (eg: 24Mbps). It is never available, but is of interest in systems design.

Maximum Rate

This is a term used to advertise broadband packages. It is perhaps the most vague. It is the attainable rate further modified to take account of the level of service. For example, ADSL2+ is commonly sold as having a maximum rate of 17 Mbps.

Sync Rate

This is also known as the downstream rate or the DSL connection rate. It is determined largely by the physical properties of the line and by any local noise that is characteristic of the location. When the nominal margin of 6dB is applied, the sync rate is always equal to or less than the advertised maximum rate. The sync rate is determined at the time of initial connection. At that time the sync process for ADSL2+ is fully rate adaptive (FRA), in that there is no imposed maximum rate. It then takes the system about 10 days or so to adjust the sync rate and other properties to produce the best line profile. During the initial sync process and at other times the line might be resynced, for example, after a more significant change is made, or a fault is repaired. At that time the IP profile rate is recalculated from the sync rate that is decided. Routers display the sync rate in their connection settings at times as just Downstream and Upstream. When the ILQ (Indicative Line Quality) is poor, (low values of MTBE or MTBR), and if the margin fluctuates greatly, then the sync rate might be made to operate in one of a set of fixed ranges, or so-called profile bands.

IP Profile Rate

It is the maximum download rate using the current sync rate. This is also known as the bRAS profile. It is found initially by taking 88.2% of the sync rate. This fraction applies to all 21st Century Network (21CN) ADSL products.

Download Rate

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

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

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 for 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. ADSL2+ can also make use of profile banding where a 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 the 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.

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 8 shows how a full retrain could be decided. Notice in Table 2 that there are three main levels of service; ISPs can select a network service that prioritizes speed, stability, 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.

MTBE and MTBR

The mean time between errors (MTBE) and the mean time between dropouts or retrains (MTBR) are both used to determine actions by the DLM. Despite the many available error types, only the Errored Seconds figure is used for the MTBE calculation. The two are calculated as:

Mean Time Between Errors (MTBE) = ADSL Uptime (secs) / Errored Second Count
Mean Time Between Retrains (MTBR) = ADSL Uptime (secs) / Retrain Count

BER Approximations

The BER or Bit Error Rate approximation depends on the size of the CRC block that is being error detected. Consequently there are two of them, one that assumes no interleaving and another that does. The two are calculated as:

For the Fast path (with maximum delay = S1):
Bit Error Rate (BER) = Number of bit errors / Number of transmitted bits
≡ 15 * CRC error count / data rate (bps) * ADSL Uptime (secs)

and For the Interleaved path (with maximum delay ≠ S1):
≡ 40 * CRC error count / data rate (bps) * ADSL Uptime (secs)

Determining Link Quality

Table 2 shows how, once the MTBE and MTBR have been calculated, the line status can be identified. The categories are arranged from good down to very poor. For example, a MTBE of at least 250 and a MTBR of at least 16800 are needed in a Standard network to qualify it as good. Notice that a so-called Stable link has a higher expectation, needing an MTBE of at least 6000 and a MTBR of at least 33600 to qualify for the good category. The Super-stable is more critical still. The categories have been color coded, and the same colors are used in Table 2 when identifying the relevant courses of action.

Table 2: Typical Line Categorization

Stability Option	Metric	Good	OK	Poor	Very Poor
Agressive (Standard)	Retrains	mtb ≥ 16800	mtb < 16800	mtb < 8640	mtb > 10 per hour
	Errors	mtb ≥ 250	mtb < 250	mtb < 5
Normal (Stable)	Retrains	mtb ≥ 33600	mtb < 33600	mtb < 16800	mtb > 10 per hour
	Errors	mtb ≥ 6000	mtb < 6000	mtb < 300
Stable Super Stable	Retrains	mtb ≥ 67200	mtb < 67200	mtb < 33600	mtb > 10 per hour
	Errors	mtb ≥ 60000	mtb < 60000	mtb < 3600

DLM Courses of Action

The line quality levels determined in Table 2 are used here in Table 3, to identify the associated courses of action.

Table 3: Typical Courses of Action

Line Classification	ILQ Status	Action
Good - Performing beyond expectations	Green	Possible removal or reduction of any barriers.
Within acceptable parameters	Amber	No changes will be made to the DLM profile.
Poor MTBR / MTBE	Red	Applies a further step to increase stability.
Poor MTBE upstream	Crimson	Applies a further step to increase stability.
Rapid Retrains (Dropouts)	Scarlet	Additional line monitoring for immediate changes.
No DLM data	Grey	No action.
Insufficient DLM data	Black	Days uptime was less than 15 mins. No action.

For a more detailed flow chart of DLM actions, refer to the drop box below. It shows the decision processes used in switching between the profile levels of a fully adaptive rate and those of fixed profile bands, as well as how a full retrain might be decided.

Sync rates are sometimes limited. Although ADSL2+ is fully rate adaptive at startup, when there are constant link failures or noise the link might be assigned to one of the fixed profile bands, as described in the flow chart. This band is a range of rates within which the link must sync. The associated IP profile rates will be 88.2% of these. A list of the fixed profile bands used for ADSL2+ is also given below.

Table3: Profile bands for ADSL2+

Lower Rate	Upper Rate
160 kbps	288 kbps
288 kbps	576 kbps
576 kbps	1152 kbps
1152 kbps	2272 kbps
1472 kbps	3072 kbps
2272 kbps	4544 kbps
3328 kbps	6656 kbps
4864 kbps	9728 kbps
7168 kbps	14336 kbps

Figures 6, 7, and 8

Figures 6, 7, and 8: Typical Profile Decision Making Figure 6: A typical decision tree for profile changes. Notice that both fixed profile bands and the fully rate adaptive (FRA) service are included. Figure 7: The SNR Margin-Stability Compromise: Dotted lines tend toward lower stability while solid lines tend toward better stability. Figure 8: Profile sets are built continuously. When the set of existing stored profiles cannot provide a solution, an additional one is made by completely retraining the link.

See Also

• A Basic Convolutional Coding Example : A description of convolutional coding, with links to a VBA simulator for assisted understanding.
• Viterbi Simulator in VBA:  : For those with an interest in VBA coding; an Excel simulator for convolutional coding that allows BER measurements for two configurations.

External Links

• The Effect of Signal-to-Noise margins on the performance of ADSL by J.H. van Wyk and L.P. Linde: In pdf format. An advanced treatment on copper line transmission limits.
• Optimised line synchronisation  : British Telecommunications PLC patent application. Good detail as to the relationship betwee margin and SNR.
• Data Communications : British Telecommunications PLC patent application. Flow charts show the adaptive rate decision process, including the use of the fixed profile bands.
• Basic Convolution Coding Example: Detailed working of a hard decision error correction system.
• UK Ofcom on Minimum Guaranteed Speeds.  : Notes from a UK authority on their proposals for improvements to the voluntary standard.
• Voluntary Code of Practice: Residential Broadband Speeds (Updated) 2015 (PDF, 104.1 KB)  : The details of the UK Ofcom voluntary code of practice for Minimum Guaranteed Speeds.
• Minimum Speed Change Rules 2018 : Changes approved in March 2018, for implementation March 2019, to the UK minimum guaranteed speed. This is the ISP voluntary standard in the UK.
• CAP and BCAP regulatory statement on speed claims in broadband advertising : This is the new UK compulsory advertising standard. This is likely to cause at least a partial realignment of the ISP voluntary standards