Internet ADSL2+ Speed
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 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.
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 sync itself correctly and to give the best line rate. 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 for errors and noise, and slight rate adjustments are made, both up and down, depending on how many people are using the internet at the time. The network provider keeps some flexibility in his settings so that hourly variations in noise cannot interrupt the connection, but sometimes, in extreme cases, the line is re-synced, for example, after a line fault is repaired.
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 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.
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. Two kinds of filter are are in use. In one, the filters are housed in the main telephone connection box, (see figure 3), and the user makes connections to separate sockets for phone and data; in the other, a filter accessory that can be connected to a telephone socket does the same thing. See 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.
The Router or Hub
The equipment that manages the link is called a router or sometimes a hub. At times other terminology is used for the data handling equipment. Units that convert between line frequencies and digital signals are called modems. Sometimes the word hub is heard; it is the process that distributes the data 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.
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
The throughput, the signal to noise ratio, and digital error rate are all related. As transmission rates and the interference from other cable users increases, the signal to noise ratio, (SNR), is reduced, and the 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 link 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, because 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.
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. 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 are 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.
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 is defined here as the level that produces a bit error rate (BER) of 10-7, that is, one errored bit in ten millions. This is the highest error rate that the provider intends to be manageable in the steady working state. 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 margin changes too. When the margin changes, parameters are adjusted until the margin returns to its target value, at whic 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. The target margin is initially selected to be no larger than necessary, to keep the line speed high, and large enough to maintain stability. 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 are synchronised only when the prevailing SNR is equal to the sum of the lowest SNR and the target margin. This makes sure that the link will work even when an additional noise, equal in value to that of the target margin, is applied. However, when this method is used the sync process could be delayed for several hours while the system waits for the right SNR to present itself, so other starting point compromises must usually be made. In any case, this near-to-lowest level of SNR 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, a higher margin can be used, or interleaving applied. Higher margins however, mean slower 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 synchronisation attempts to find the best compromise between the two. 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.
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, a resync is generally needed. See figure 6 for an example of how the longer term choices are made.
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. They did not need to be sent again, and did not slow throughput beyond their working overhead. 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 could not be corrected by FEC error correction. They require data packets to be resent. CRC checks detect errors but cannot correct them, so they represent a increase in rework.
- 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.
- 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 a value of 10-7 or one in ten million bits. 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 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 data and rework needed to achieve the result.
- 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:
- Convolution coding, sometimes called trellis coding is used as the main method. Each unit of data output in trellis coding is a function of the 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 operate on a continuous bit stream. The Viterbi algorithm is commonly used.
- Reed Solomon (RS) block coding is used with trellis coding to handle any burst errors that may be present. 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 lightening strikes, or electrical motor starts requires a different approach.
- Interleaving. Because error correction is often restricted to fixing only one bit at a time, impulse noise that damages several adjacent bits at a time can greatly increase overheads. (The need to resend data). So, instead of grouping data bits as units in the usual way, they are strung out, by adding redundant padding between each. The number of padding bits is adjusted so that the probability of damage to more than one bit of the original group is minimised. The process regroups the bits at the receiver, ignoring the padding (damaged or otherwise), before applying any one-bit error correction. The method is very effective.
- 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. Self explanatory.
- 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.
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.
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.
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.
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.
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 by the main providers, so the throughput tends to be quite close to the IP Profile rate for most of the time.
Guaranteed Line Speed
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. In the United Kingdom for example, the Minimum Guaranteed Access Line Speed is such a scheme. The speed for a particular client is found statistically from a set of those with similar line profiles. The sync rates of the lowest 10% of such a set is taken and the highest of these is the Minimum Guaranteed Access Line Speed for that client. Notice that it does 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 have the right to nullify it, subject to giving reasonable opportunity to the provider to correct the matter. Most of the major providers are signed up to it.
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
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.
|Stability Option||Metric||Good||OK||Poor||Very Poor|
|Retrains||mtb ≥ 16800||mtb < 16800||mtb < 8640||mtb > 10 per hour|
|Errors||mtb ≥ 250||mtb < 250||mtb < 5|
|Retrains||mtb ≥ 33600||mtb < 33600||mtb < 16800||mtb > 10 per hour|
|Errors||mtb ≥ 6000||mtb < 6000||mtb < 300|
|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.
|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.
|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
- 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