WO2007000547A2 - Method for correcting an impulse noise errors on an sdsl line - Google Patents

Method for correcting an impulse noise errors on an sdsl line Download PDF

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Publication number
WO2007000547A2
WO2007000547A2 PCT/FR2006/050470 FR2006050470W WO2007000547A2 WO 2007000547 A2 WO2007000547 A2 WO 2007000547A2 FR 2006050470 W FR2006050470 W FR 2006050470W WO 2007000547 A2 WO2007000547 A2 WO 2007000547A2
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WIPO (PCT)
Prior art keywords
segment
transmitter
receiver
request
sdsl
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PCT/FR2006/050470
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French (fr)
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WO2007000547A3 (en
Inventor
Amoikon Patrick Nda
Meryem Ouzzif
Ahmed Zeddam
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France Telecom
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Publication of WO2007000547A2 publication Critical patent/WO2007000547A2/en
Publication of WO2007000547A3 publication Critical patent/WO2007000547A3/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1867—Arrangements specially adapted for the transmitter end
    • H04L1/1874—Buffer management

Definitions

  • the present invention relates to a method for correcting transmission errors introduced by impulsive noises on an SDSL line between a transmitter and a data receiver. It also relates to a transmitter and a data receiver on an SDSL transmission line.
  • the invention aims to protect applications using SDSL as a transmission medium against the effects of impulsive noise.
  • the SDDS Symmetric Digital Subscriber Line
  • SHDSL Symmetric Digital Subscriber Line
  • the SDSL is capable of transmitting rates ranging from 192 kbps to 5696 kbps at frequencies up to 800 kHz and ranges up to 3 km.
  • the SDSL uses a 16-level trellis coded amplitude modulation (MAP 16, "Phase Coding Multilevel / Phase Signals", G. Ungerboeck, IEEE Transactions on Information Theory, Vol. No. 1, January 1982). Each level corresponds to a set of 4 bits called “symbol”. The symbols are grouped into sequences, or "segments", formed from the coded modulation.
  • MAP 16-level trellis coded amplitude modulation
  • Each level corresponds to a set of 4 bits called “symbol”.
  • the symbols are grouped into sequences, or "segments”, formed from the coded modulation.
  • SDSL signals are transmitted over twisted copper pairs without interleaving data during transmission, reducing processing times and making this technology particularly suitable for real-time applications.
  • severe electromagnetic disturbances, such as impulse noise combined with the lack of data interleaving, can cause errors to occur and thus degrade the transmission quality.
  • Impulsive noise is mainly characterized by a high amplitude and a very short duration. Its duration varies from a few hundred microseconds for impulsive sounds isolated at one millisecond for burst impulse noise, and its spectrum ranges from very low frequencies to a few MHz. It is therefore likely to disrupt systems operating in the same frequency band.
  • Impulsive noise has several origins: transient circuit of electrical circuits, radiation of antennas, electric motors, electrical appliances, etc.
  • the part of the access network consisting of overhead cables is very sensitive to electromagnetic waves.
  • the cables of the Customer's Terminal Installation (ITC) are subjected to the electromagnetic radiation of devices such as fluorescent tubes, motors, etc.
  • the channel coding techniques present in the SDSL only take into account stationary imperfections and, therefore, do not really combat the impact of impulse noise.
  • the technical problem to be solved by the object of the present invention is to propose a method for correcting transmission errors introduced by impulsive noise on an SDSL line between a transmitter and a data receiver, which would be really efficient and effective. would complement existing channel coding techniques.
  • the solution to the technical problem posed consists, according to the present invention, in that said method comprises the steps of: for the receiver, detecting on said line impulsive noises likely to affect at least one data segment,
  • At least one segment transmitted last by the transmitter is stored in a segment memory of said transmitter.
  • Said segment memory has a finite number of cells and the last transmitted segments each occupy one cell.
  • the filling of the cells is performed by successive offsets during transmission of the segments by the transmitter.
  • the transmitter can thus retrieve the segment or segments transmitted last if, at the request of the receiver, it must retransmit one of them after said receiver has detected an impulsive noise that affected the segment or segments considered.
  • the invention provides two provisions for ensuring the continuity of the transmission when the interruption due to impulse noise correction has stopped.
  • a first provision consists in that at least one segment received before response to said request is stored in a buffer memory of said receiver.
  • a second provision consists in that, when retransmitting said allocated segment, at least one segment to be transmitted by the transmitter is stored in a buffer memory of said transmitter.
  • each data segment comprises a header containing, at least, an identification index of said segment.
  • said segment identification index corresponds to a cell number of the segment memory of the transmitter.
  • each segment is identified in its header by the number of the cell of the segment memory in which the sender stored it.
  • the receiver indicates this cell number in the header of the segment carrying the request.
  • the transmitter can very easily find in its segment memory segment that it must retransmit.
  • said header further includes a field specifying the type of the segment.
  • the segment type is chosen from the following segment types: normal segment, request and response to a request.
  • the invention also relates to a data transmitter on an SDSL transmission line, remarkable in that said transmitter comprises means adapted to retransmit a data segment affected by an impulsive noise on request of a receiver of said segments.
  • An advantageous characteristic of the transmitter according to the invention consists in that it comprises a memory-segment capable of storing at least one segment transmitted last.
  • the transmitter according to the invention consists in that it comprises a buffer memory capable of storing at least one segment to be transmitted, during the retransmission of said affected segment.
  • the invention also relates to a data receiver on an SDSL transmission line, characterized in that said receiver comprises an impulse noise detector and means for identifying at least one segment affected by an impulsive noise and to issue a request for retransmission of said segment assigned to an issuer of said segments.
  • An advantageous characteristic of the receiver according to the invention consists in that it comprises a buffer memory capable of storing at least one segment received before responding to said request.
  • the receiver according to the invention comprises an impulsive noise detector comprising a comparator between an SDSL signal predicted by a prediction filter and the incoming SDSL signal.
  • the receiver according to the invention comprises an impulsive noise detector constituted by a branch metric amplitude variation detector.
  • Figure 1 is a communication diagram between two modems on an SDSL line.
  • Figure 2 shows a layered model of an SDSL transmission.
  • Figure 3 is a block diagram of the correction method according to the invention.
  • FIG. 4 represents the structure of a segment for implementing the correction method according to the invention.
  • FIG. 5 shows the structure of a segment memory used for the implementation of the correction method according to the invention.
  • Fig. 6 is a diagram describing the correction algorithm of the transmitter side (a) and the receiver side (b).
  • FIGS. 7a, 7b and 7c are diagrams of a first example of application of the correction method according to the invention.
  • Figures 8a and 8b are diagrams of a second example of application of the correction method according to the invention.
  • FIG. 9 is an implementation diagram of an impulse noise detector with a prediction filter.
  • Figure 10a is an SDSL chain schema in reception.
  • Figure 10b is an SDSL channel model diagram.
  • FIG. 1 schematically shows an SDSL communication between two modems, namely a modem 10 which will be considered as the transmitter modem of segments likely to be affected by impulsive noises during their transmission, and a modem2. intended to receive the segments transmitted by the transmitting modem 10 and to detect and correct any errors occurring in these segments due to impulsive noises.
  • a modem 10 which will be considered as the transmitter modem of segments likely to be affected by impulsive noises during their transmission
  • a modem2. intended to receive the segments transmitted by the transmitting modem 10 and to detect and correct any errors occurring in these segments due to impulsive noises.
  • each modem 10, 20 comprises transmission means 11, 21 and receiving means 12, 22, it being understood that the transmission on line 1 is symmetrical, in accordance with the nature of the SDSL transmission.
  • the transmitting modem 10 may be that of a central office and the receiving modem 20 that of a client. Then we will call downstream direction or DS ("DownStream”) the direction of communications from the central to the customer, and upward direction or UP (“UpStream”) the direction of communications from the customer to the central office.
  • DownStream downstream direction
  • UP upward direction
  • an SDSL line can be decomposed into layers.
  • the layered model at the central office is identical to the client-level model.
  • the modem 10 at the central office is the master modem. That is, it decides on the operating parameters each time the system is turned on.
  • a layered model of SDSL is shown in Figure 2.
  • the PMS-TC (Physical Media Specification-Transmission Convergence) layer can be considered as the top layer of SDSL. It depends on the transported application and is responsible for the functions of:
  • the PMD (Physical Medium Dependent) layer can be considered as the low layer of the SDSL. It depends on the physical medium used and is responsible for the functions of:
  • a noise detector 23 informs (1) the receiving modem 20, which identifies the affected data and requires (2) to the transmitting modem 10 the retransmission of these data.
  • the retransmission request from the receiving modem 20 is denoted ARQ ("Automatic Repeat reQuest").
  • ARQ Automatic Repeat reQuest
  • the data is transmitted in segments consisting of a set of symbols conforming, for example, to a 16-state PAM coded modulation.
  • Each symbol is therefore representative of 4 bits.
  • FIG 4 there is shown in (a) such a segment whose length L may be equal to a hundred symbols.
  • the implementation of the invention involves a modification of this basic structure. Indeed, for the affected segments can be identified by the modems 10, 20, it is appropriate to add identification data. As can be seen in (b) in FIG. 4, this is done by means of a header which, to respect the PAM modulation 16, extends also on 4 bit (bi, b 2 , b 3 , b 4 ), thus increasing by one unit the length L of the segment.
  • the two bits b 3 and b 4 constitute a field specifying the type of the segment. More specifically, it will be indicated, for example, by (00) that the segment is normally transmitted and (11) that it is a response to a retransmission request.
  • the values (01) and (10) are reserved for the formulation of a retransmission request from the receiving modem 20 and serve as a two-bit identification index (bi, b 2 ) appearing in the field " index "of the header.
  • the segments normally transmitted by the transmitting modem 10 will have a "request” field equal to (00) and an "index” field alternately equal to (01) or (10). If an error is detected on a segment, the receiving modem 20 will issue a segment having the field “request” the value (01) or (10) according to the index of the segment assigned and for field "index” a sequence index on two bits. Finally, the retransmission of segments by the transmitting modem 10 will be accompanied by a "request” field equal to (11).
  • the transmitter 10 is provided with a first memory, called segment memory, in which a number of transmitted segments are stored in order to be able to retransmit them following a request in this direction sent by the receiver 20.
  • segment memory a first memory
  • the segments transmitted are stored in the cells of the memory segment so that each segment is stored in a cell whose number corresponds to the index of the segment considered, as shown in Figure 5.
  • the indexes can take both values (01) or (10)
  • the memory-segment will have two numbered cells (01 and (10).
  • the transmitter 10 further comprises a second memory, called buffer memory, intended to store the segments received from the upper layers while waiting to be transmitted after an affected segment has been retransmitted.
  • This buffer memory is sized according to the maximum delay that can tolerate the system. Its structure is identical to that of Figure 5.
  • the receiver 20 also comprises a buffer memory of the same structure as the previous ones. When an erroneous segment retransmission request is made, the receiver stores in this buffer the segments consecutive to the affected segment received before receiving the response to the request. After receiving the retransmitted segment (s), the receiver re-places the segments in the correct order by placing the buffered segments after the response to the request.
  • the useful size of the buffer depends on the number of segments transmitted by the transmitter before processing the retransmission request. This number may vary depending on the flow. It will also depend on the length of the line. The maximum value will be determined by the maximum flow over a maximum range or the minimum flow over its maximum range.
  • the correction method according to the invention naturally entails additional delays compared to SDSL in its current version. These factors are:
  • the times of sending and response to the request are limited to the time of travel information on the medium, about 5 ⁇ s / km.
  • the processing time of the request will be considered equal to the decoding delay, ie 100 times the time of a symbol (1, 29 ⁇ s for a bit rate of 2304 kbit / s)).
  • FIG. 6 illustrates the correction algorithms used by the transmitter 10 (a) and the receiver 20 (b) during the application of the method according to the invention.
  • the transmission of the segments by the transmitting modem 10 and their reception by the receiving modem 20 proceed normally, according to the recommendations of the ETSI standard (Transmission and Multiplexing (TM); Access transmission System on metallic Digital Subscriber Line (SDSL) ", TS 101 524 V1.1.3 (03-2003)).
  • TM Transmission and Multiplexing
  • SDSL Access transmission System on metallic Digital Subscriber Line
  • the receiver 20 then sends an ARQ retransmission request for each of the segments affected by the impulsive noise. This request is made by registering each of the indexes associated with the erroneous segments in the "request" fields of the segments sent by the receiving modem 20.
  • the segments received by the receiving modem 20 before the response to the ARQ request are stored in the buffer memory of the receiver 20.
  • the receiving means 12 of the transmitting modem 10 interpret the request by decoding the header of the received segment, and sends the transmitting means 11 the index of the segment to retransmit, that is to say the number of the transmission cell. the segment memory containing the segment to retransmit.
  • the lower layers of the transmitting means 11 redirect the data segments to the buffer memory of the modem 10.
  • the transmitting means 11 of the modem 10 perform a reading in the segment-memory of the segment associated with the request and retransmits it to the receiver means 22 of the modem 20 by putting the bits of the "request" field to 1, (11) in the example supra. All queries are processed in the way just presented.
  • the receiving means 22 of the modem 20 After reception of the responses to the ARQ requests, the receiving means 22 of the modem 20 put in order the retransmitted segments and the segments already in the buffer memory.
  • the receiving means 22 of the modem 20 treat them as non-errored segments.
  • the duration of the impulsive noise is short, namely that the errors generated cause a fast retransmission without overload of the buffer memory of the transmitter 10 and without delay degrading the quality of service (QoS) of the applications of the upper layers.
  • QoS quality of service
  • the duration of the impulsive noise is average in the sense that the errors generated cause a retransmission to the limit of the delays accepted by the upper layers and cause the total use of the buffer memory of the transmitter 10.
  • the duration of the impulsive noise is long.
  • the retransmission without error is then impossible because of the absence of the required data in the buffer memory of the transmitter 10 or a delay exceeding the limit supported by the applications using the SDSL.
  • the bit rate is 2320 kbit / s, ie a symbol rate of 773.33 ksymbols / s ( 3 A yield).
  • the duration of a symbol is therefore 1, 29 ⁇ s.
  • a truncation length of 100 symbols we will have a duration of each segment of 129 ⁇ s.
  • the marking of the header will be 4 bits.
  • the total duration of the segment will therefore be 129 ⁇ s + 1, 29 ⁇ s or 130.29 ⁇ s.
  • the line has a length of 3 km.
  • the travel time on the support is taken equal to 5 ⁇ s / km.
  • the maximum delay that the system can tolerate in data processing is 500 ⁇ s, in accordance with the ITU recommendations.
  • FIGS. 7a, 7b and 7c This example is illustrated in FIGS. 7a, 7b and 7c.
  • the impulsive noise is detected by the detector 23 of the receiver 20, which does not perform any treatment on the affected segment, here the segment i as shown in FIG. 7a. This segment affected by the impulsive noise is ignored. Having received the previous i-1 segment, the receiver 20 knows the index associated with the erroneous segment i. It can therefore request a retransmission of this segment to the transmitter 10.
  • the receiver 20 then sends, via its associated transmitter means 21, an ARQ request with the index of the expected segment i and sets the segment being received, the segment i + 1, in buffer memory.
  • Figure 7b illustrates this operation.
  • the transmitter 10 receives the retransmission request, it sends the segment i again, as shown in FIG. 7c.
  • the receiver 20 will therefore process in sequence the segment i thus retransmitted and the segment i + 1 stored in buffer memory. The transmission normally continues thereafter.
  • the delays induced by the error correction are:
  • the receiver 20 after detecting each erroneous segment, successively sends 3 ARQ segment retransmission requests, always via its associated transmitter means 21.
  • the transmitter 10 for its part, buffers the segments to be transmitted and responds to requests from the receiver. To do this, it performs a decoding of the segments containing the ARQs, which induces a processing delay which is a function of the number of segments to be decoded.
  • Figure 8b illustrates the sending of requests and the buffering of the segments to be transmitted.
  • the receiver 20 When the impulsive noise is detected for the first time on the first transmission of the segment i, the receiver 20 generates a request ARQI i.
  • the transmission of the response to this ARQ1 i is also affected by the impulse noise, because it has a duration that can cover three segments.
  • the receiver 20 ignores this response and reformulates an ARQ2i request which, in turn, is affected by the impulsive noise and for which the receiver 20 reformulates an ARQ3i request for which it receives a correct response.
  • the noise will have finally affected the segment i, during its initial transmission, as well as the two ARQ responses formulated for this segment, ie in total three segments.
  • the delays induced by the error correction are:
  • the total delay is 435 ⁇ s. This delay always remains below the imposed delays and the segments to transmit use the entire buffer memory of the transmitter 10 without exceeding the capacity.
  • impulse noise lasts for more than 3 segments.
  • the transmitter can no longer store the segments received from the upper layers. Two choices are then possible:
  • FIG. 9 shows a first embodiment of an impulsive noise detector 23 capable of equipping the receiving modem 20. This embodiment is based on the use of a prediction filter arranged in parallel on the SDSL chain in reception.
  • impulsive noise can not be predicted.
  • the SDSL signal is stationary and can therefore be predicted.
  • the purpose of the prediction filter is to predict the SDSL signal received on the line. It will thus be possible to generate an error signal between the predicted signal and the initial SDSL signal. This error signal will usually be a stationary white noise. When the impulsive noise impacts the transmission line, the error signal is no longer stationary. This information can therefore be interpreted as impulsive noise coupled to the line.
  • This method has the advantage of being fast and giving precisely the moments when the impulsive noise is coupled on the line.
  • FIGS. 10a to 10c Another embodiment of an impulsive noise detector is illustrated in FIGS. 10a to 10c.
  • the SDSL does not provide an error detection means. It only performs a logic level CRC check to see if a frame contains an error or not.
  • Figure 10a shows a model of the SDSL chain.
  • the Viterbi decoder is a maximum likelihood type decoder. Decisions made by the decoder are based on the Euclidean distance between the received sequences and the possible sequences in the lattice.
  • r k is the kth received symbol.
  • n k is the kth sample of white noise.
  • This high amplitude is due to the loss of the correct path in the trellis during the decoding process and can be exploited for detecting interfering signals superimposed on the received SDSL signal, namely here the impulsive noise.

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  • Computer Networks & Wireless Communication (AREA)
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Abstract

The invention relates to a method for correcting errors introduced by impulse noises on an SDSL line between a data transmitter (10) and receiver (20). According to said invention, the inventive method consists, for the receiver (20), in detecting impulse noises affecting at least one data segment on said line, in identifying said affected segment, in requiring the transmitter (10) to retransmit said affected segment and, for the transmitter, in retransmitting the affected segment to the receiver (20). Said invention can be used for protecting SDSL applications against impulse noise effects.

Description

PROCEDE DE CORRECTION D'ERREURS DE BRUIT IMPULSIF SUR UNE METHOD OF CORRECTING IMPULSIVE NOISE ERRORS ON A
LIGNE SDSLSDSL LINE
La présente invention concerne un procédé de correction d'erreurs de transmission introduites par des bruits impulsifs sur une ligne SDSL entre un émetteur et un récepteur de données. Elle concerne également un émetteur et un récepteur de données sur une ligne de transmission SDSL. L'invention a pour but de protéger les applications utilisant le SDSL comme support de transmission contre les effets du bruit impulsif.The present invention relates to a method for correcting transmission errors introduced by impulsive noises on an SDSL line between a transmitter and a data receiver. It also relates to a transmitter and a data receiver on an SDSL transmission line. The invention aims to protect applications using SDSL as a transmission medium against the effects of impulsive noise.
Ces applications sont diverses. Elles peuvent être du type temps réel avec de fortes contraintes de délai, telles que les communications téléphoniques entre deux postes, les conférences d'appels, la visio- conférence, la télécommande, etc. Elles peuvent être aussi du type temps réel avec des contraintes plus faibles, comme le transfert de données, la mise à jour de bases de données, etc. Elles ne sont toutefois pas limitées au type temps réel, Internet par exemple.These applications are diverse. They can be of the real-time type with strong delay constraints, such as two-way telephone calls, call conferences, videoconferencing, remote control, and so on. They can also be of the real-time type with lower constraints, such as data transfer, updating of databases, etc. However, they are not limited to the real-time type, Internet for example.
Le procédé proposé pour la correction du bruit impulsif doit donc pouvoir répondre aux besoins de ces différents types d'applications.The proposed method for the correction of impulse noise must therefore be able to meet the needs of these different types of applications.
La technologie SDSL (« Symmetric Digital Subscriber Line »), dénommée SHDSL par l'UIT, est une technologie de transmission appartenant à la famille xDSL qui utilise le câblage téléphonique existant. Le SDSL permet de transmettre des débits variant de 192 kbits/s à 5696 kbits/s à des fréquences jusqu'à 800 kHz et sur des portées pouvant atteindre 3 km.The SDDS ("Symmetric Digital Subscriber Line") technology, called SHDSL by the ITU, is a transmission technology belonging to the xDSL family that uses existing telephone wiring. The SDSL is capable of transmitting rates ranging from 192 kbps to 5696 kbps at frequencies up to 800 kHz and ranges up to 3 km.
Le SDSL utilise une modulation d'amplitude codée en treillis sur 16 niveaux (PAM 16, « Phase Amplitude Modulation ») (« Channel Coding Multilevel/Phase Signais », G. Ungerboeck, IEEE Transactions on Information Theory, Vol. IT-28, No. 1 , January 1982). Chaque niveau correspond à un ensemble de 4 bits appelé « symbole ». Les symboles sont groupés en séquences, ou « segments », formées à partir de la modulation codée. Contrairement aux autres technologies xDSL, les signaux SDSL sont transmis sur des paires de cuivre torsadées sans entrelacement des données lors de la transmission, ce qui réduit les délais de traitement et rend cette technologie particulièrement bien adaptée aux applications temps réel. Toutefois, des perturbations électromagnétiques sévères, du type bruit impulsif, conjuguées à l'absence d'entrelacement des données, peuvent provoquer l'apparition d'erreurs et dégrader ainsi la qualité de transmission.The SDSL uses a 16-level trellis coded amplitude modulation (MAP 16, "Phase Coding Multilevel / Phase Signals", G. Ungerboeck, IEEE Transactions on Information Theory, Vol. No. 1, January 1982). Each level corresponds to a set of 4 bits called "symbol". The symbols are grouped into sequences, or "segments", formed from the coded modulation. Unlike other xDSL technologies, SDSL signals are transmitted over twisted copper pairs without interleaving data during transmission, reducing processing times and making this technology particularly suitable for real-time applications. However, severe electromagnetic disturbances, such as impulse noise, combined with the lack of data interleaving, can cause errors to occur and thus degrade the transmission quality.
Le bruit impulsif est caractérisé principalement par une forte amplitude et une durée très brève. Sa durée varie de quelques centaines de microsecondes pour des bruits impulsifs isolés à une milliseconde pour des bruits impulsifs en rafale, et son spectre s'étale des très basses fréquences à quelques MHz. Il est donc susceptible de perturber les systèmes fonctionnant dans la même bande de fréquence.Impulsive noise is mainly characterized by a high amplitude and a very short duration. Its duration varies from a few hundred microseconds for impulsive sounds isolated at one millisecond for burst impulse noise, and its spectrum ranges from very low frequencies to a few MHz. It is therefore likely to disrupt systems operating in the same frequency band.
Le bruit impulsif a plusieurs origines : régime transitoire de circuits électriques, rayonnement d'antennes, moteurs électriques, appareils électroménagers, etc.Impulsive noise has several origins: transient circuit of electrical circuits, radiation of antennas, electric motors, electrical appliances, etc.
La partie du réseau d'accès constituée de câbles aériens est très sensible aux ondes électromagnétiques. Par ailleurs, les câbles de l'Installation Terminale du Client (ITC) sont soumis au rayonnement électromagnétique d'appareils tels que tubes fluorescents, moteurs, etc.The part of the access network consisting of overhead cables is very sensitive to electromagnetic waves. In addition, the cables of the Customer's Terminal Installation (ITC) are subjected to the electromagnetic radiation of devices such as fluorescent tubes, motors, etc.
Cependant, les techniques de codage de canal présentes dans le SDSL ne tiennent compte que des imperfections stationnaires et, par conséquent, ne permettent pas de lutter véritablement contre l'impact du bruit impulsif. Aussi, le problème technique à résoudre par l'objet de la présente invention est de proposer un procédé de correction d'erreurs de transmission introduites par des bruits impulsifs sur une ligne SDSL entre un émetteur et un récepteur de données, qui serait réellement efficace et viendrait compléter les techniques de codage de canal déjà existantes. La solution au problème technique posé consiste, selon la présente invention, en ce que ledit procédé comprend les étapes consistant à : pour le récepteur, - détecter sur ladite ligne des bruits impulsifs susceptibles d'affecter au moins un segment de données,However, the channel coding techniques present in the SDSL only take into account stationary imperfections and, therefore, do not really combat the impact of impulse noise. Also, the technical problem to be solved by the object of the present invention is to propose a method for correcting transmission errors introduced by impulsive noise on an SDSL line between a transmitter and a data receiver, which would be really efficient and effective. would complement existing channel coding techniques. The solution to the technical problem posed consists, according to the present invention, in that said method comprises the steps of: for the receiver, detecting on said line impulsive noises likely to affect at least one data segment,
- identifier ledit segment affecté,- identify said affected segment,
- requérir auprès de l'émetteur la retransmission dudit segment affecté, pour l'émetteur,- to request from the transmitter retransmission of said affected segment, for the transmitter,
- retransmettre le segment affecté audit récepteur.- retransmitting the segment assigned to said receiver.
De manière à permettre une retransmission ultérieure d'un segment déjà transmis, il est prévu par l'invention qu'au moins un segment transmis en dernier lieu par l'émetteur est stocké dans une mémoire-segment dudit émetteur.In order to allow a subsequent retransmission of a segment already transmitted, it is provided by the invention that at least one segment transmitted last by the transmitter is stored in a segment memory of said transmitter.
Ladite mémoire-segment a un nombre fini de cellules et les segments transmis en dernier lieu occupent, chacun, une cellule. Le remplissage des cellules est effectué par décalages successifs au cours de la transmission des segments par l'émetteur. L'émetteur peut ainsi retrouver le ou les segments transmis en dernier lieu si, à la demande du récepteur, il doit retransmettre l'un d'entre eux après que ledit récepteur ait détecté un bruit impulsif ayant affecté le ou les segments considérés.Said segment memory has a finite number of cells and the last transmitted segments each occupy one cell. The filling of the cells is performed by successive offsets during transmission of the segments by the transmitter. The transmitter can thus retrieve the segment or segments transmitted last if, at the request of the receiver, it must retransmit one of them after said receiver has detected an impulsive noise that affected the segment or segments considered.
Du fait que la requête en retransmission émise par le récepteur et la réponse à cette requête fournie par l'émetteur induisent une interruption de la transmission normale des segments, l'invention prévoit deux dispositions permettant d'assurer la continuité de la transmission lorsque l'interruption due à la correction de bruit impulsif a cessé.Since the retransmission request sent by the receiver and the response to this request provided by the transmitter induce an interruption of the normal transmission of the segments, the invention provides two provisions for ensuring the continuity of the transmission when the interruption due to impulse noise correction has stopped.
Une première disposition consiste en ce qu'au moins un segment reçu avant réponse à ladite requête est stocké dans une mémoire-tampon dudit récepteur.A first provision consists in that at least one segment received before response to said request is stored in a buffer memory of said receiver.
Une seconde disposition consiste en ce que, lors de la retransmission dudit segment affecté, au moins un segment à transmettre par l'émetteur est stocké dans une mémoire-tampon dudit émetteur. Selon un mode de réalisation du procédé conforme à l'invention, chaque segment de données comprend un en-tête contenant, au moins, un index d'identification dudit segment. Ainsi, lors du procédé de correction d'erreur, l'émetteur et le récepteur peuvent communiquer entre eux en utilisant ledit index d'identification pour désigner le segment affecté qui doit être retransmis.A second provision consists in that, when retransmitting said allocated segment, at least one segment to be transmitted by the transmitter is stored in a buffer memory of said transmitter. According to an embodiment of the method according to the invention, each data segment comprises a header containing, at least, an identification index of said segment. Thus, during the error correction process, the transmitter and the receiver can communicate with each other by using said identification index to designate the affected segment to be retransmitted.
Plus spécialement, ledit index d'identification de segment correspond à un numéro de cellule de la mémoire-segment de l'émetteur.More specifically, said segment identification index corresponds to a cell number of the segment memory of the transmitter.
Dans ce cas, chaque segment est identifié dans son en-tête par le numéro de la cellule de la mémoire-segment dans laquelle l'émetteur l'a stocké. Lors d'une requête en retransmission, le récepteur indique ce numéro de cellule dans l'en-tête du segment portant la requête. A la réception de la requête, l'émetteur peut ainsi très facilement retrouver dans sa mémoire- segment le segment qu'il doit retransmettre.In this case, each segment is identified in its header by the number of the cell of the segment memory in which the sender stored it. During a retransmission request, the receiver indicates this cell number in the header of the segment carrying the request. Upon receipt of the request, the transmitter can very easily find in its segment memory segment that it must retransmit.
Comme on le verra en détail plus loin, ledit en-tête comprend en outre un champ spécifiant le type du segment. En particulier, le type de segment est choisi parmi les types de segment suivants : segment normal, requête et réponse à une requête.As will be discussed in detail below, said header further includes a field specifying the type of the segment. In particular, the segment type is chosen from the following segment types: normal segment, request and response to a request.
L'invention concerne également un émetteur de données sur une ligne de transmission SDSL, remarquable en ce que ledit émetteur comprend des moyens aptes à retransmettre un segment de données affecté par un bruit impulsif sur requête d'un récepteur desdits segments. Une caractéristique avantageuse de l'émetteur conforme à l'invention consiste en ce qu'il comprend une mémoire-segment apte à stocker au moins un segment transmis en dernier lieu.The invention also relates to a data transmitter on an SDSL transmission line, remarkable in that said transmitter comprises means adapted to retransmit a data segment affected by an impulsive noise on request of a receiver of said segments. An advantageous characteristic of the transmitter according to the invention consists in that it comprises a memory-segment capable of storing at least one segment transmitted last.
Une autre caractéristique avantageuse de l'émetteur conforme à l'invention consiste ce qu'il comprend une mémoire-tampon apte à stocker au moins un segment à transmettre, lors de la retransmission dudit segment affecté.Another advantageous characteristic of the transmitter according to the invention consists in that it comprises a buffer memory capable of storing at least one segment to be transmitted, during the retransmission of said affected segment.
L'invention concerne également un récepteur de données sur une ligne de transmission SDSL, remarquable en ce que ledit récepteur comprend un détecteur de bruits impulsifs et des moyens pour identifier au moins un segment affecté par un bruit impulsif et émettre une requête de retransmission dudit segment affecté auprès d'un émetteur desdits segments. Une caractéristique avantageuse du récepteur conforme à l'invention consiste en ce qu'il comprend une mémoire-tampon apte à stocker au moins un segment reçu avant réponse à ladite requête.The invention also relates to a data receiver on an SDSL transmission line, characterized in that said receiver comprises an impulse noise detector and means for identifying at least one segment affected by an impulsive noise and to issue a request for retransmission of said segment assigned to an issuer of said segments. An advantageous characteristic of the receiver according to the invention consists in that it comprises a buffer memory capable of storing at least one segment received before responding to said request.
Enfin, il est prévu que le récepteur conforme à l'invention comporte un détecteur de bruit impulsif comprenant un comparateur entre un signal SDSL prédit par un filtre de prédiction et le signal SDSL entrant.Finally, it is expected that the receiver according to the invention comprises an impulsive noise detector comprising a comparator between an SDSL signal predicted by a prediction filter and the incoming SDSL signal.
Un autre mode de réalisation propose que le récepteur conforme à l'invention comporte un détecteur de bruit impulsif constitué par un détecteur de variations d'amplitude de métriques de branche. La description qui va suivre en regard des dessins annexés, donnés à titre d'exemples non limitatifs, fera bien comprendre en quoi consiste l'invention et comment elle peut être réalisée.Another embodiment proposes that the receiver according to the invention comprises an impulsive noise detector constituted by a branch metric amplitude variation detector. The following description with reference to the accompanying drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be achieved.
La figure 1 est un schéma de communication entre deux modems sur une ligne SDSL. La figure 2 montre un modèle en couches d'une transmission SDSL.Figure 1 is a communication diagram between two modems on an SDSL line. Figure 2 shows a layered model of an SDSL transmission.
La figure 3 est un schéma de principe du procédé de correction conforme à l'invention.Figure 3 is a block diagram of the correction method according to the invention.
La figure 4 représente la structure d'un segment pour la mise en œuvre du procédé de correction conforme à l'invention. La figure 5 montre la structure d'une mémoire-segment utilisée pour la mise en œuvre du procédé de correction conforme à l'invention.FIG. 4 represents the structure of a segment for implementing the correction method according to the invention. FIG. 5 shows the structure of a segment memory used for the implementation of the correction method according to the invention.
La figure 6 est un schéma décrivant l'algorithme de correction du côté émetteur (a) et du côté récepteur (b).Fig. 6 is a diagram describing the correction algorithm of the transmitter side (a) and the receiver side (b).
Les figures 7a, 7b et 7c sont des schémas d'un premier exemple d'application du procédé de correction conforme à l'invention.FIGS. 7a, 7b and 7c are diagrams of a first example of application of the correction method according to the invention.
Les figures 8a et 8b sont des schémas d'un deuxième exemple d'application du procédé de correction conforme à l'invention.Figures 8a and 8b are diagrams of a second example of application of the correction method according to the invention.
La figure 9 est un schéma d'implantation d'un détecteur de bruit impulsif à filtre de prédiction. La figure 10a est un schéma de chaîne SDSL en réception.FIG. 9 is an implementation diagram of an impulse noise detector with a prediction filter. Figure 10a is an SDSL chain schema in reception.
La figure 10b est un schéma de modèle de canal SDSL.Figure 10b is an SDSL channel model diagram.
La figure 10c est un diagramme d'évolution des métriques de branche. Sur la figure 1 , est représentée de manière schématique une communication SDSL entre deux modems, à savoir un modemi 10 qui sera considéré comme le modem émetteur de segments susceptibles d'être affectés par des bruits impulsifs au cours de leur transmission, et un modem2 20 destiné à recevoir les segments transmis par le modem émetteur 10 et à détecter et corriger d'éventuelles erreurs survenues à ces segments du fait des bruits impulsifs.Figure 10c is an evolution diagram of branch metrics. FIG. 1 schematically shows an SDSL communication between two modems, namely a modem 10 which will be considered as the transmitter modem of segments likely to be affected by impulsive noises during their transmission, and a modem2. intended to receive the segments transmitted by the transmitting modem 10 and to detect and correct any errors occurring in these segments due to impulsive noises.
Comme le montre la figure 1 , chaque modem 10, 20 comprend des moyens d'émission 11 , 21 et de réception 12, 22, étant entendu que la transmission sur la ligne 1 est symétrique, conformément à la nature même de la transmission SDSL.As shown in FIG. 1, each modem 10, 20 comprises transmission means 11, 21 and receiving means 12, 22, it being understood that the transmission on line 1 is symmetrical, in accordance with the nature of the SDSL transmission.
De façon pratique, le modem émetteur 10 peut être celui d'un central et le modem récepteur 20 celui d'un client. On appellera alors sens descendant ou DS (« DownStream ») le sens des communications du central vers le client, et sens remontant ou UP (« UpStream ») le sens des communications du client vers le central.In practice, the transmitting modem 10 may be that of a central office and the receiving modem 20 that of a client. Then we will call downstream direction or DS ("DownStream") the direction of communications from the central to the customer, and upward direction or UP ("UpStream") the direction of communications from the customer to the central office.
Au niveau fonctionnel, une ligne SDSL peut être décomposée en couches. Le modèle en couches au niveau du central est identique au modèle au niveau du client. Toutefois, le modem 10 au niveau du central est le modem maître. C'est-à-dire qu'il décide des paramètres de fonctionnement à chaque mise en marche du système.At the functional level, an SDSL line can be decomposed into layers. The layered model at the central office is identical to the client-level model. However, the modem 10 at the central office is the master modem. That is, it decides on the operating parameters each time the system is turned on.
Un modèle en couches du SDSL est représenté sur la figure 2.A layered model of SDSL is shown in Figure 2.
La couche PMS-TC (« Physical Media Spécifie-Transmission Convergence ») peut être considérée comme la couche haute du SDSL. Elle dépend de l'application transportée et est chargée des fonctions de:The PMS-TC (Physical Media Specification-Transmission Convergence) layer can be considered as the top layer of SDSL. It depends on the transported application and is responsible for the functions of:
- multiplexage de canal,- channel multiplexing,
- mise en trame,- framing,
- synchronisation des trames,- frame synchronization,
- détection des erreurs par CRC (« Cyclic Redundancy Check), - justification,- error detection by CRC (Cyclic Redundancy Check), justification,
- maintenance. La couche PMD (Physical Médium Dépendant) peut être considérée comme la couche basse du SDSL. Elle dépend du support physique utilisé et est chargée des fonctions de :- maintenance. The PMD (Physical Medium Dependent) layer can be considered as the low layer of the SDSL. It depends on the physical medium used and is responsible for the functions of:
- génération et récupération d'horloge, - démarrage (start-up),- clock generation and recovery, - start-up,
- brassage et débrassage,- brewing and unloading,
- codage et décodage,- coding and decoding,
- modulation et démodulation,- modulation and demodulation,
- annulation d'écho, - égalisation de ligne.- echo cancellation, - line equalization.
Le procédé qui, conformément à l'invention, permet de corriger des erreurs de transmission introduites par des bruits impulsifs est implémenté entre les couches PMS-TC et PMD. Son principe est illustré sur la figure 3.The method according to the invention for correcting transmission errors introduced by impulsive noise is implemented between the PMS-TC and PMD layers. Its principle is illustrated in Figure 3.
Si, au cours d'une transmission de données dans le sens descendant (DS), un bruit impulsif survient affectant au moins un segment, un détecteur 23 de bruit en informe (1 ) le modem récepteur 20, lequel identifie les données affectées et requiert (2) auprès du modem émetteur 10 la retransmission de ces données. La requête en retransmission émanant du modem récepteur 20 est notée ARQ (« Automatic Repeat reQuest »). En réponse à cette requête, le modem émetteur 10 retransmet les données qui ont été affectées par le bruit impulsif.If, during a downstream data transmission (DS), an impulsive noise occurs affecting at least one segment, a noise detector 23 informs (1) the receiving modem 20, which identifies the affected data and requires (2) to the transmitting modem 10 the retransmission of these data. The retransmission request from the receiving modem 20 is denoted ARQ ("Automatic Repeat reQuest"). In response to this request, the transmitting modem 10 retransmits the data that has been affected by the impulsive noise.
Comme cela a déjà été mentionné plus haut, les données sont transmises dans des segments constitués d'un ensemble de symboles conformes, par exemple, à une modulation codée PAM à 16 états. Chaque symbole est donc représentatif de 4 bits. Sur la figure 4, on a représenté en (a) un tel segment dont la longueur L peut être égale à une centaine de symboles.As already mentioned above, the data is transmitted in segments consisting of a set of symbols conforming, for example, to a 16-state PAM coded modulation. Each symbol is therefore representative of 4 bits. In Figure 4, there is shown in (a) such a segment whose length L may be equal to a hundred symbols.
La mise en œuvre de l'invention implique une modification de cette structure de base. En effet, pour que les segments affectés puissent être repérés par les modems 10, 20, il convient de leur adjoindre des données d'identification. Ainsi qu'on peut le voir en (b) sur la figure 4, ceci est réalisé au moyen d'un en-tête qui, pour respecter la modulation PAM 16, s'étend également sur 4 bit (bi, b2, b3, b4), augmentant ainsi d'une unité la longueur L du segment.The implementation of the invention involves a modification of this basic structure. Indeed, for the affected segments can be identified by the modems 10, 20, it is appropriate to add identification data. As can be seen in (b) in FIG. 4, this is done by means of a header which, to respect the PAM modulation 16, extends also on 4 bit (bi, b 2 , b 3 , b 4 ), thus increasing by one unit the length L of the segment.
Les deux bits b3 et b4, désignés ensemble sous le terme de « requête », constituent un champ spécifiant le type du segment. Plus précisément, on indiquera, par exemple, par (00) que le segment est normalement transmis et par (11 ) qu'il s'agit d'une réponse à une requête de retransmission. Les valeurs (01 ) et (10) sont réservées à la formulation d'une requête de retransmission de la part du modem récepteur 20 et font office d'index d'identification à deux bits (bi, b2) figurant dans le champ « index » de l'en- tête.The two bits b 3 and b 4 , collectively referred to as "request", constitute a field specifying the type of the segment. More specifically, it will be indicated, for example, by (00) that the segment is normally transmitted and (11) that it is a response to a retransmission request. The values (01) and (10) are reserved for the formulation of a retransmission request from the receiving modem 20 and serve as a two-bit identification index (bi, b 2 ) appearing in the field " index "of the header.
Ainsi, les segments normalement transmis par le modem émetteur 10 auront un champ « requête » égal à (00) et un champ « index » alternativement égal à (01 ) ou (10). Si une erreur est détectée sur un segment, le modem récepteur 20 émettra un segment ayant pour champ « requête » la valeur (01 ) ou (10) selon l'index du segment affecté et pour champ « index » un index de séquence sur deux bits. Enfin, la retransmission de segments par le modem émetteur 10 sera accompagnée d'un champ « requête » égal à (11 ).Thus, the segments normally transmitted by the transmitting modem 10 will have a "request" field equal to (00) and an "index" field alternately equal to (01) or (10). If an error is detected on a segment, the receiving modem 20 will issue a segment having the field "request" the value (01) or (10) according to the index of the segment assigned and for field "index" a sequence index on two bits. Finally, the retransmission of segments by the transmitting modem 10 will be accompanied by a "request" field equal to (11).
L'émetteur 10 est muni d'une première mémoire, dite mémoire- segment, dans laquelle sont stockés un certain nombre de segments émis afin de pouvoir les retransmettre à la suite d'une requête en ce sens émise par le récepteur 20. Les segments transmis sont rangés dans les cellules de la mémoire-segment de sorte que chaque segment soit rangé dans une cellule dont le numéro correspond à l'index du segment considéré, comme représenté sur la figure 5. Ainsi, dans le cas mentionné plus haut où les index peuvent prendre les deux valeurs (01 ) ou (10), la mémoire-segment aura deux cellules numérotées (01 et (10).The transmitter 10 is provided with a first memory, called segment memory, in which a number of transmitted segments are stored in order to be able to retransmit them following a request in this direction sent by the receiver 20. The segments transmitted are stored in the cells of the memory segment so that each segment is stored in a cell whose number corresponds to the index of the segment considered, as shown in Figure 5. Thus, in the case mentioned above where the indexes can take both values (01) or (10), the memory-segment will have two numbered cells (01 and (10).
L'émetteur 10 comporte en outre une deuxième mémoire, dite mémoire- tampon, destinée à stocker les segments reçus des couches supérieures en attendant d'être transmis après qu'un segment affecté ait été retransmis. Cette mémoire-tampon est dimensionnée en fonction du retard maximum que peut tolérer le système. Sa structure est identique à celle de la figure 5. Le récepteur 20 comprend également une mémoire-tampon de même structure que les précédentes. Lorsqu'une requête en retransmission de segment erroné est effectuée, le récepteur stocke dans cette mémoire-tampon les segments consécutifs au segment affecté reçus avant réception de la réponse à la requête. Après réception du(es) segment(s) retransmis, le récepteur replace les segments dans le bon ordre en plaçant les segments tamponnés après la réponse à la requête.The transmitter 10 further comprises a second memory, called buffer memory, intended to store the segments received from the upper layers while waiting to be transmitted after an affected segment has been retransmitted. This buffer memory is sized according to the maximum delay that can tolerate the system. Its structure is identical to that of Figure 5. The receiver 20 also comprises a buffer memory of the same structure as the previous ones. When an erroneous segment retransmission request is made, the receiver stores in this buffer the segments consecutive to the affected segment received before receiving the response to the request. After receiving the retransmitted segment (s), the receiver re-places the segments in the correct order by placing the buffered segments after the response to the request.
La taille utile de la mémoire-tampon dépend du nombre de segments transmis par l'émetteur avant traitement de la requête en retransmission. Ce nombre peut varier en fonction du débit. Il dépendra aussi de la longueur de la ligne. La valeur maximum sera déterminée par le débit maximum sur une portée maximum ou le débit minimum sur sa portée maximum.The useful size of the buffer depends on the number of segments transmitted by the transmitter before processing the retransmission request. This number may vary depending on the flow. It will also depend on the length of the line. The maximum value will be determined by the maximum flow over a maximum range or the minimum flow over its maximum range.
Lors de sa mise en œuvre, le procédé de correction conforme à l'invention entraîne naturellement des délais supplémentaires par rapport au SDSL dans sa version actuelle. Ces facteurs sont :During its implementation, the correction method according to the invention naturally entails additional delays compared to SDSL in its current version. These factors are:
- le délai d'envoi de la requête (ARQ),- the deadline for sending the request (ARQ),
- le délai de traitement de la requête,- the processing time of the request,
- le délai de réponse à la requête.- the response time to the request.
Les délais d'envoi et de réponse à la requête se limitent au temps de parcours de l'information sur le support, soit environ 5 μs/km.The times of sending and response to the request are limited to the time of travel information on the medium, about 5 μs / km.
Le délai de traitement de la requête sera considéré égal au délai de décodage, soit 100 fois le temps d'un symbole (1 ,29 μs pour un débit de 2304 kbits/s)).The processing time of the request will be considered equal to the decoding delay, ie 100 times the time of a symbol (1, 29 μs for a bit rate of 2304 kbit / s)).
La figure 6 illustre les algorithmes de correction utilisés par l'émetteur 10 (a) et le récepteur 20 (b) lors de l'application du procédé conforme à l'invention.FIG. 6 illustrates the correction algorithms used by the transmitter 10 (a) and the receiver 20 (b) during the application of the method according to the invention.
En l'absence de bruit impulsif, la transmission des segments par le modem émetteur 10 et leur réception par le modem récepteur 20 se déroulent normalement, selon les recommandations de la norme ETSI (« Transmission and Multiplexing (TM); Access transmission System on metallic access cables ; Symmetric single pair high bit rate Digital Subscriber Line (SDSL) », TS 101 524 V1.1.3 (03-2003)). Si un bruit impulsif est détecté par le détecteur 23 associé au récepteur 20, une mise en mémoire des index des segments affectés par le bruit est effectuée. Le récepteur 20 émet alors une requête ARQ de retransmission de chacun des segments affectés par le bruit impulsif. Cette requête est réalisée en inscrivant chacun des index associés aux segments erronés dans les champs « requête » des segments émis par le modem récepteur 20. Les segments reçus par le modem récepteur 20 avant la réponse à la requête ARQ sont stockés dans la mémoire-tampon du récepteur 20.In the absence of impulsive noise, the transmission of the segments by the transmitting modem 10 and their reception by the receiving modem 20 proceed normally, according to the recommendations of the ETSI standard (Transmission and Multiplexing (TM); Access transmission System on metallic Digital Subscriber Line (SDSL) ", TS 101 524 V1.1.3 (03-2003)). If an impulsive noise is detected by the detector 23 associated with the receiver 20, the indexes of the segments affected by the noise are stored. The receiver 20 then sends an ARQ retransmission request for each of the segments affected by the impulsive noise. This request is made by registering each of the indexes associated with the erroneous segments in the "request" fields of the segments sent by the receiving modem 20. The segments received by the receiving modem 20 before the response to the ARQ request are stored in the buffer memory of the receiver 20.
Les moyens récepteurs 12 du modem émetteur 10 interprètent la requête en décodant l'en-tête du segment reçu, et envoie vers les moyens émetteurs 11 l'index du segment à retransmettre, c'est-à-dire le numéro de la cellule de la mémoire-segment contenant le segment à retransmettre.The receiving means 12 of the transmitting modem 10 interpret the request by decoding the header of the received segment, and sends the transmitting means 11 the index of the segment to retransmit, that is to say the number of the transmission cell. the segment memory containing the segment to retransmit.
A la réception de la requête ARQ, les couches basses des moyens émetteurs 11 redirigent les segments de données vers la mémoire-tampon du modem 10.Upon receipt of the ARQ request, the lower layers of the transmitting means 11 redirect the data segments to the buffer memory of the modem 10.
Les moyens émetteurs 11 du modem 10 effectuent une lecture dans la mémoire-segment du segment associé à la requête et le retransmet aux moyens récepteurs 22 du modem 20 en mettant les bits du champ « requête » à 1 , (11 ) dans l'exemple précité. Toutes les requêtes sont traitées de la façon qui vient d'être présentée.The transmitting means 11 of the modem 10 perform a reading in the segment-memory of the segment associated with the request and retransmits it to the receiver means 22 of the modem 20 by putting the bits of the "request" field to 1, (11) in the example supra. All queries are processed in the way just presented.
Après réception des réponses aux requêtes ARQ, les moyens récepteurs 22 du modem 20 remettent dans l'ordre les segments retransmis et les segments se trouvant déjà dans le mémoire-tampon.After reception of the responses to the ARQ requests, the receiving means 22 of the modem 20 put in order the retransmitted segments and the segments already in the buffer memory.
Notons que si le bruit impulsif affecte plus de segments que ne le permet la capacité de correction du système, les moyens récepteurs 22 du modem 20 les traitent comme des segments non erronés.Note that if the impulse noise affects more segments than the correction capacity of the system allows, the receiving means 22 of the modem 20 treat them as non-errored segments.
Pour illustrer le procédé de correction qui vient d'être décrit, trois exemples de situations vont maintenant être présentés en terme de retard engendré par le traitement des erreurs de bruit impulsif sur le système de transmission.To illustrate the correction method which has just been described, three examples of situations will now be presented in terms of the delay generated by the processing of impulse noise errors on the transmission system.
Dans le premier exemple, la durée du bruit impulsif est courte, à savoir que les erreurs engendrées provoquent une retransmission rapide, sans surcharge de la mémoire-tampon de l'émetteur 10 et sans retard dégradant la qualité de service (QoS) des applications des couches supérieures.In the first example, the duration of the impulsive noise is short, namely that the errors generated cause a fast retransmission without overload of the buffer memory of the transmitter 10 and without delay degrading the quality of service (QoS) of the applications of the upper layers.
Dans le deuxième exemple, la durée du bruit impulsif est moyenne au sens où les erreurs engendrées provoquent une retransmission à la limite des retards acceptés par les couches supérieures et provoquent l'utilisation totale de la mémoire-tampon de l'émetteur 10.In the second example, the duration of the impulsive noise is average in the sense that the errors generated cause a retransmission to the limit of the delays accepted by the upper layers and cause the total use of the buffer memory of the transmitter 10.
Dans le troisième exemple, la durée du bruit impulsif est longue. La retransmission sans erreur est alors impossible à cause de l'absence des données requises dans la mémoire-tampon de l'émetteur 10 ou à un retard excédant la limite supportée par les applications utilisant le SDSL.In the third example, the duration of the impulsive noise is long. The retransmission without error is then impossible because of the absence of the required data in the buffer memory of the transmitter 10 or a delay exceeding the limit supported by the applications using the SDSL.
Pour traiter ces exemples, les paramètres considérés sont les suivants :To deal with these examples, the parameters considered are as follows:
- le débit binaire est de 2320 kbit/s soit un débit symbole de 773,33 ksymboles/s (rendement de 3A). La durée d'un symbole est donc de 1 ,29 μs. En considérant une longueur de troncature de 100 symboles, on aura une durée de chaque segment de 129 μs.the bit rate is 2320 kbit / s, ie a symbol rate of 773.33 ksymbols / s ( 3 A yield). The duration of a symbol is therefore 1, 29 μs. Considering a truncation length of 100 symbols, we will have a duration of each segment of 129 μs.
- en choisissant une taille de mémoire-segment au niveau de l'émetteur 10 égale à 2 cellules mémoire (une cellule stocke un segment), le marquage de l'en-tête se fera sur 4 bits. La durée totale du segment sera donc de 129 μs + 1 ,29 μs soit 130,29 μs.- By choosing a memory-segment size at the transmitter 10 equal to 2 memory cells (a cell stores a segment), the marking of the header will be 4 bits. The total duration of the segment will therefore be 129 μs + 1, 29 μs or 130.29 μs.
- la ligne a une longueur de 3 km. Le temps de parcours sur le support est pris égal à 5 μs/km.- the line has a length of 3 km. The travel time on the support is taken equal to 5 μs / km.
- on supposera aussi que le délai maximum que le système peut tolérer dans le traitement des données est de 500 μs, ceci conformément aux recommandations de l'UlT.- it will also be assumed that the maximum delay that the system can tolerate in data processing is 500 μs, in accordance with the ITU recommendations.
Exemple 1Example 1
Cet exemple est illustré aux figures 7a, 7b et 7c.This example is illustrated in FIGS. 7a, 7b and 7c.
Lorsqu'il survient, le bruit impulsif est détecté par le détecteur 23 du récepteur 20, lequel n'effectue aucun traitement sur le segment affecté, ici le segment i ainsi que le montre la figure 7a. Ce segment touché par le bruit impulsif est donc ignoré. Ayant reçu le précédent segment i-1 , le récepteur 20 connaît l'index associé au segment i erroné. Il peut donc demander une retransmission de ce segment à l'émetteur 10.When it occurs, the impulsive noise is detected by the detector 23 of the receiver 20, which does not perform any treatment on the affected segment, here the segment i as shown in FIG. 7a. This segment affected by the impulsive noise is ignored. Having received the previous i-1 segment, the receiver 20 knows the index associated with the erroneous segment i. It can therefore request a retransmission of this segment to the transmitter 10.
Le récepteur 20 envoie alors, par l'intermédiaire de ses moyens émetteurs 21 associés, une requête ARQ avec l'index du segment i attendu et met le segment en cours de réception, le segment i+1 , en mémoire-tampon. La figure 7b illustre cette opération.The receiver 20 then sends, via its associated transmitter means 21, an ARQ request with the index of the expected segment i and sets the segment being received, the segment i + 1, in buffer memory. Figure 7b illustrates this operation.
Dès que l'émetteur 10 reçoit la demande de retransmission, il envoie à nouveau le segment i, comme indiqué sur la figure 7c. Le récepteur 20 traitera donc dans l'ordre le segment i ainsi retransmis puis le segment i+1 stocké en mémoire-tampon. La transmission continue normalement par la suite.As soon as the transmitter 10 receives the retransmission request, it sends the segment i again, as shown in FIG. 7c. The receiver 20 will therefore process in sequence the segment i thus retransmitted and the segment i + 1 stored in buffer memory. The transmission normally continues thereafter.
Dans ce cas, les retards induits par la correction d'erreur sont :In this case, the delays induced by the error correction are:
- réception et traitement (décodage et lecture) de la requête ARQ = (3 km)x(5 μs /km) + 130,29 μs = 145,29 μs- reception and processing (decoding and reading) of the request ARQ = (3 km) x (5 μs / km) + 130.29 μs = 145.29 μs
- envoi du segment demandé : (3 km)x(5 μs /km) = 15 μs Le retard total est donc de 160,29 μs. Ce retard est acceptable vu qu'il respecte le retard maximum imposé de 500μs. Par ailleurs, il convient de remarquer qu'une telle erreur n'aurait pas pu être corrigée par le codage de canal normalement associé à la transmission SDSL.- sending the requested segment: (3 km) x (5 μs / km) = 15 μs The total delay is therefore 160.29 μs. This delay is acceptable since it respects the maximum delay imposed of 500μs. Furthermore, it should be noted that such an error could not have been corrected by the channel coding normally associated with the SDSL transmission.
Exemple 2Example 2
Cet exemple est illustré aux figures 8a et 8b.This example is illustrated in Figures 8a and 8b.
Dans cet exemple, le bruit impulsif se superpose à trois segments (figure 8a).In this example, the impulsive noise is superimposed on three segments (Figure 8a).
Le récepteur 20, après détection de chaque segment erroné, envoie successivement 3 requêtes ARQ de retransmission de segment, toujours par l'intermédiaire de ses moyens émetteurs 21 associés. L'émetteur 10, de son côté, met en mémoire-tampon les segments à transmettre et répond aux requêtes du récepteur. Pour ce faire, il effectue un décodage des segments contenant les ARQ, ce qui induit un délai de traitement qui est fonction du nombre de segments à décoder. La figure 8b illustre l'envoi des requêtes et la mise en mémoire-tampon des segments à transmettre.The receiver 20, after detecting each erroneous segment, successively sends 3 ARQ segment retransmission requests, always via its associated transmitter means 21. The transmitter 10, for its part, buffers the segments to be transmitted and responds to requests from the receiver. To do this, it performs a decoding of the segments containing the ARQs, which induces a processing delay which is a function of the number of segments to be decoded. Figure 8b illustrates the sending of requests and the buffering of the segments to be transmitted.
Quand le bruit impulsif est détecté pour la première fois sur la première émission du segment i, le récepteur 20 génère une requête ARQI i. La transmission de la réponse à cet ARQ1 i se trouve également affectée par le bruit impulsif, car celui-ci a une durée pouvant couvrir trois segments. Le récepteur 20 ne tient pas compte de cette réponse et reformule une requête ARQ2i qui, à son tour, est affectée par le bruit impulsif et pour laquelle le récepteur 20 reformule une requête ARQ3i pour laquelle il reçoit une réponse correcte.When the impulsive noise is detected for the first time on the first transmission of the segment i, the receiver 20 generates a request ARQI i. The transmission of the response to this ARQ1 i is also affected by the impulse noise, because it has a duration that can cover three segments. The receiver 20 ignores this response and reformulates an ARQ2i request which, in turn, is affected by the impulsive noise and for which the receiver 20 reformulates an ARQ3i request for which it receives a correct response.
Le bruit aura finalement affecté le segment i, lors de sa transmission initiale, ainsi que les deux réponses ARQ formulées pour ce segment, soit au total trois segments. Dans cet exemple, les retards induits par la correction d'erreur sont :The noise will have finally affected the segment i, during its initial transmission, as well as the two ARQ responses formulated for this segment, ie in total three segments. In this example, the delays induced by the error correction are:
- traitement des requêtes ARQ : 3x(130,29 μs) = 390,87 μsARQ request processing: 3x (130.29 μs) = 390.87 μs
- acheminement : 3x(3 km)x(5 μs/km) = 45 μs- routing: 3x (3 km) x (5 μs / km) = 45 μs
Le retard total est de 435 μs. Ce retard reste toujours en dessous des retards imposés et les segments à émettre utilisent la totalité de la mémoire- tampon de l'émetteur 10 sans pour autant en excéder la capacité.The total delay is 435 μs. This delay always remains below the imposed delays and the segments to transmit use the entire buffer memory of the transmitter 10 without exceeding the capacity.
Exemple 3Example 3
Dans ce cas, le bruit impulsif dure sur plus de 3 segments. L'émetteur ne peut donc plus stocker les segments reçus des couches supérieures. Deux choix sont alors possibles :In this case, impulse noise lasts for more than 3 segments. The transmitter can no longer store the segments received from the upper layers. Two choices are then possible:
- effectuer un arrêt de la transmission jusqu'à disparition du bruit. Ceci évite une retransmission et une saturation de la mémoire-tampon de l'émetteur 10, donc une perte de données.- stop the transmission until the noise disappears. This avoids retransmission and saturation of the buffer memory of the transmitter 10, thus a loss of data.
- continuer la transmission et laisser le soin aux couches supérieures de gérer les opérations de requête de retransmission.- continue the transmission and let the upper layers handle the retransmission request operations.
La figure 9 montre une première réalisation d'un détecteur 23 de bruit impulsif apte à équiper le modem récepteur 20. Ce mode de réalisation est basé sur l'utilisation d'un filtre de prédiction disposé en parallèle sur la chaîne SDSL en réception.FIG. 9 shows a first embodiment of an impulsive noise detector 23 capable of equipping the receiving modem 20. This embodiment is based on the use of a prediction filter arranged in parallel on the SDSL chain in reception.
En effet, compte tenu de son caractère bref, le bruit impulsif ne peut être prédit. Par contre, le signal SDSL est stationnaire et peut donc être prédit. Le but du filtre de prédiction est donc de prédire le signal SDSL reçu sur la ligne. On pourra ainsi générer un signal d'erreur entre le signal prédit et le signal SDSL initial. Ce signal d'erreur sera en général un bruit blanc stationnaire. Lorsque le bruit impulsif impacte la ligne de transmission, le signal d'erreur n'est plus stationnaire. Cette information peut donc être interprétée comme étant du bruit impulsif couplé sur la ligne. Cette méthode présente l'avantage d'être rapide et de donner avec précision les instants où le bruit impulsif s'est couplé sur la ligne.Indeed, given its brief nature, impulsive noise can not be predicted. On the other hand, the SDSL signal is stationary and can therefore be predicted. The purpose of the prediction filter is to predict the SDSL signal received on the line. It will thus be possible to generate an error signal between the predicted signal and the initial SDSL signal. This error signal will usually be a stationary white noise. When the impulsive noise impacts the transmission line, the error signal is no longer stationary. This information can therefore be interpreted as impulsive noise coupled to the line. This method has the advantage of being fast and giving precisely the moments when the impulsive noise is coupled on the line.
Un autre mode de réalisation d'un détecteur de bruit impulsif est illustré sur les figures 10a à 10c.Another embodiment of an impulsive noise detector is illustrated in FIGS. 10a to 10c.
Au niveau physique, le SDSL ne fournit pas de moyen de détection d'erreur. Il effectue seulement un contrôle CRC au niveau logique pour savoir si une trame contient ou non une erreur. La figure 10a présente un modèle de la chaîne SDSL.At the physical level, the SDSL does not provide an error detection means. It only performs a logic level CRC check to see if a frame contains an error or not. Figure 10a shows a model of the SDSL chain.
Le décodeur de Viterbi est un décodeur de type maximum de vraisemblance. Les décisions effectuées par le décodeur sont basées sur la distance euclidienne entre les séquences reçues et les séquences possibles dans le treillis.The Viterbi decoder is a maximum likelihood type decoder. Decisions made by the decoder are based on the Euclidean distance between the received sequences and the possible sequences in the lattice.
En observant la métrique de branche (distance entre chaque symbole reçu et chaque symbole estimé) à chaque étape du décodage, on peut voir que, pour le chemin correct dans le treillis, les amplitudes des ces métriques se situent au niveau du bruit blanc.By observing the branch metric (distance between each received symbol and each estimated symbol) at each step of the decoding, it can be seen that, for the correct path in the trellis, the amplitudes of these metrics are at the level of the white noise.
Si on considère un canal sans interférence entre symboles (égaliseur parfait), il peut être modélisé comme sur la figure 10b.If we consider a channel without interference between symbols (perfect equalizer), it can be modeled as in Figure 10b.
Pour chaque symbole reçu, nous avons:For each symbol received, we have:
rk = xk + nk r k = x k + n k
où le kième symbole transmis xk appartient à la constellation de la modulation utilisée, rk est le kième symbole reçu. nk est le kième échantillon de bruit blanc. On a ainsi la métrique de branche évaluée pour le symbole rk :where the kth transmitted symbol x k belongs to the constellation of the modulation used, r k is the kth received symbol. n k is the kth sample of white noise. We thus have the branch metric evaluated for the symbol r k :
dk 2 = {rk -xkf ce qui représente bien (nk)2, le bruit additif.d k 2 = {r k -x k f which is good (n k ) 2 , additive noise.
Lorsque le canal est impacté par du bruit impulsif, on peut observer une forte variation de l'amplitude des métriques de branche pendant le processus de décodage, comme l'indique la figure 10c. La variation de métrique est due à l'addition du bruit impulsif. La forte amplitude observée lors de l'impact du bruit impulsif disparaît lorsque le bruit impulsif disparaît.When the channel is impacted by impulsive noise, a large variation in the amplitude of branch metrics can be observed during the decoding process, as shown in Figure 10c. The metric variation is due to the addition of the impulsive noise. The large amplitude observed during the impact of the impulsive noise disappears when the impulsive noise disappears.
Cette forte amplitude est due à la perte du chemin correct dans le treillis lors du processus de décodage et peut être exploitée pour la détection de signaux perturbateurs superposés au signal SDSL reçu, à savoir ici le bruit impulsif. This high amplitude is due to the loss of the correct path in the trellis during the decoding process and can be exploited for detecting interfering signals superimposed on the received SDSL signal, namely here the impulsive noise.

Claims

REVENDICATIONS
1. Procédé de correction d'erreurs de transmission introduites par des bruits impulsifs sur une ligne SDSL entre un émetteur (10) et un récepteur (20) de données, caractérisé en ce que ledit procédé comprend les étapes consistant à : pour le récepteur (20), - détecter sur ladite ligne des bruits impulsifs susceptibles d'affecter au moins un segment de données,A method for correcting transmission errors introduced by impulsive noises on an SDSL line between a transmitter (10) and a data receiver (20), characterized in that said method comprises the steps of: for the receiver ( 20), - detecting on said line impulsive noises likely to affect at least one data segment,
- identifier ledit segment affecté,- identify said affected segment,
- requérir auprès de l'émetteur (10) la retransmission dudit segment affecté, pour l'émetteur (10), - retransmettre le segment affecté audit récepteur (20).- Require the transmitter (10) to retransmit said assigned segment, for the transmitter (10), - retransmit the assigned segment to said receiver (20).
2. Procédé selon la revendication 1 , caractérisé en ce qu'au moins un segment transmis en dernier lieu par l'émetteur (10) est stocké dans une mémoire-segment dudit émetteur.2. Method according to claim 1, characterized in that at least one segment last transmitted by the transmitter (10) is stored in a segment-memory of said transmitter.
3. Procédé selon l'une des revendications 1 ou 2, caractérisé en ce que au moins un segment reçu avant réponse à ladite requête est stocké dans une mémoire-tampon dudit récepteur (20).3. Method according to one of claims 1 or 2, characterized in that at least one segment received before response to said request is stored in a buffer memory of said receiver (20).
4. Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que, lors de la retransmission dudit segment affecté, au moins un segment à transmettre par l'émetteur (10) est stocké dans une mémoire-tampon dudit émetteur.4. Method according to any one of claims 1 to 3, characterized in that, during the retransmission of said affected segment, at least one segment to be transmitted by the transmitter (10) is stored in a buffer memory of said transmitter.
5. Procédé selon l'une quelconque des revendications 1 à 4, caractérisé en ce que chaque segment de données comprend un en-tête contenant, au moins, un index d'identification dudit segment.5. Method according to any one of claims 1 to 4, characterized in that each data segment comprises a header containing, at least, an identification index of said segment.
6. Procédé selon la revendication 5, caractérisé en ce que ledit index d'identification de segment correspond à un numéro de cellule de la mémoire- segment de l'émetteur (10). The method of claim 5, characterized in that said segment identification index corresponds to a cell number of the segment memory of the transmitter (10).
7. Procédé selon l'une des revendications 5 ou 6, caractérisé en ce que ledit en-tête comprend en outre un champ spécifiant le type du segment.7. Method according to one of claims 5 or 6, characterized in that said header further comprises a field specifying the type of the segment.
8. Procédé selon la revendication 7, caractérisé en ce que le type de segment est choisi parmi les types de segment suivants : segment normal, requête et réponse à une requête.8. Method according to claim 7, characterized in that the segment type is selected from the following segment types: normal segment, request and response to a request.
9. Procédé selon l'une des revendications 4 à 8, caractérisé en ce que, en cas de saturation de la mémoire-tampon de l'émetteur (10), la transmission des segments de données est interrompue jusqu'à disparition du bruit impulsif.9. Method according to one of claims 4 to 8, characterized in that, in the event of saturation of the buffer memory of the transmitter (10), the transmission of the data segments is interrupted until the disappearance of the impulsive noise. .
10. Procédé selon l'une des revendications 4 à 8, caractérisé en ce que, en cas de saturation de la mémoire-tampon de l'émetteur (10), la transmission des segments de données est poursuivie sans retransmission de segments affectés.10. Method according to one of claims 4 to 8, characterized in that, in the event of saturation of the buffer memory of the transmitter (10), the transmission of the data segments is continued without retransmission of affected segments.
11. Emetteur de données sur une ligne de transmission SDSL, caractérisé en ce que ledit émetteur (10) comprend des moyens aptes à retransmettre un segment de données affecté par un bruit impulsif sur requête d'un récepteur (20) desdits segments.11. A data transmitter on an SDSL transmission line, characterized in that said transmitter (10) comprises means adapted to retransmit a data segment affected by an impulsive noise on request of a receiver (20) of said segments.
12. Emetteur selon la revendication 11 , caractérisé en ce qu'il comprend une mémoire-segment apte à stocker au moins un segment transmis en dernier lieu. 12. Transmitter according to claim 11, characterized in that it comprises a memory-segment capable of storing at least one last transmitted segment.
13. Emetteur selon l'une des revendications 11 ou 12, caractérisé en ce qu'il comprend une mémoire-tampon apte à stocker au moins un segment à transmettre, lors de la retransmission dudit segment affecté.13. Transmitter according to one of claims 11 or 12, characterized in that it comprises a buffer memory capable of storing at least one segment to be transmitted during the retransmission of said affected segment.
14. Récepteur de données sur une ligne de transmission SDSL, caractérisé en ce que ledit récepteur (20) comprend un détecteur (23) de bruits impulsifs et des moyens pour identifier au moins un segment affecté par un bruit impulsif et émettre une requête (ARQ) de retransmission dudit segment affecté auprès d'un émetteur (10) desdits segments.Data receiver on an SDSL transmission line, characterized in that said receiver (20) comprises an impulse noise detector (23) and means for identifying at least one segment affected by an impulsive noise and to issue a request (ARQ ) retransmitting said affected segment from an emitter (10) of said segments.
15. Récepteur selon la revendication 14, caractérisé en ce qu'il comprend une mémoire-tampon apte à stocker au moins un segment reçu avant réponse à ladite requête (ARQ).15. Receiver according to claim 14, characterized in that it comprises a buffer memory capable of storing at least one segment received before response to said request (ARQ).
16. Récepteur selon l'une des revendications 14 ou 15, caractérisé en ce qu'il comporte un détecteur (23) de bruit impulsif comprenant un comparateur entre un signal SDSL prédit par un filtre de prédiction et le signal SDSL entrant. 16. Receiver according to one of claims 14 or 15, characterized in that it comprises an impulsive noise detector (23) comprising a comparator between an SDSL signal predicted by a prediction filter and the incoming SDSL signal.
17. Récepteur selon l'une des revendications 14 ou 15, caractérisé en ce qu'il comprend un détecteur (23) de bruit impulsif constitué par un détecteur de variations d'amplitude de métriques de branche. 17. Receiver according to one of claims 14 or 15, characterized in that it comprises a detector (23) of impulsive noise consisting of a detector of amplitude variations of branch metrics.
PCT/FR2006/050470 2005-06-01 2006-05-22 Method for correcting an impulse noise errors on an sdsl line WO2007000547A2 (en)

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