FR2841711A1 - DEVICE FOR DETECTION AND LOCATION OF TRANSMISSION FAULTS IN HFC NETWORKS AND METHOD FOR IMPLEMENTING SAME - Google Patents

Abstract

The invention relates to a device for detecting and locating transmission faults in HFC-type networks, and a method for implementing this device. A transmission means (1) produces a pulse signal (P) which is injected into the downlink of the network. This pulse signal (P) causes intermodulation in network equipment, which generates the formation of an echo signal (E) conveyed on the upward path to a means (4) of reception. A control system (7) makes it possible to control the transmission means (1) and to process the received echo signal (E).

Description

The present invention relates to a device for detecting and

  localization of transmission faults in HFC-type networks, and a method for implementing

work of this device.

  The invention relates to the field of high-speed transmission systems suitable for HFC ("Hybrid Fiber Coax") type networks in English terminology. It typically finds its application in the systems of

  supervision of HFC type networks.

  The characteristics of HFC networks are defined in IUT-T recommendation J.83. These networks include optical fibers in a first part located near the interurban arteries, and cables

  coaxial in a second part serving users.

  HFC networks thus allow the broadcasting, to users' homes, of analog television signals

and / or digital.

  CATV video communication cable networks

  ("Community Antenna Television", in Anglo20 Saxon literature) generally have an HFC type architecture.

  They in fact comprise a first part made of fiber optic cables, located between a headend, which collects the information to be broadcast from analog and / or digital networks, and optical nodes, responsible for converting optical signals into electrical signals. and vice versa, and a second part in coaxial cables located between the optical nodes and the

users' homes.

  In order to allow an offer of interactive services, such as consultation of information on the Internet, telephony or video on demand for example, these HFC-type networks also have an upward path. To better understand the operation of HFC networks, the invention will now be presented with regard to an embodiment of the prior art, illustrated by the

  Figure 1, which shows a conventional HFC type network.

  The network head, referenced 100, is also called a "collection point" because it collects programs to be broadcast from networks of analog type, such as analog television 110 for example, and of digital type, such as Internet 120 or digital television 130 for example. However, the headend 100 can also be connected to the PSTN switched telephone network to allow transit of telephone communications. This headend 100 receives electrical signals which reach it via satellite or cable for example, then it multiplexes these signals. The multiplex thus formed is sent to an optical transmitter responsible for converting it into an optical signal which is sent to a

  optical fiber 151 intended for users.

  The headend 100 also collects signals from users on another fiber.

  optical 152 intended for the upward path.

  The flow of information coming from users C1 to Cn and going towards the head end of the network 100 is called "upward path" and the downward flow the information flow from the head end 100 to

users Cl to Cn.

  The optical nodes, on the other hand, constitute the points where the optical signals are converted into electrical signals and vice versa. They are in fact at the crossroads of coaxial cables 250 serving users Cl to Cn and optical fibers 151, 152 connected to the network head 100. For the sake of simplification, a single node

  optical, referenced 200, is shown in Figure 1.

  A connection point 280, also called TRP for "Passive Network Termination", located near the home of at least one user, marks the border between

  the HFC network and the installation of this user.

  Amplifiers referenced 260 are also arranged on the down and up channels of the coaxial part of HFC networks. These amplifiers 260, well known to those skilled in the art of cable networks, are in the form of boxes and include amplification modules respectively for the downlink and upstream, as well as diplexer filters arranged on either side of the amplification modules. In fact, a first diplexer filter is placed between on the one hand the upstream access to the HFC network and on the other hand the input of the amplification module of the downlink and the output of the amplification module of the uplink . A second diplexer filter is located between on the one hand the downstream access to the HFC network and on the other hand the output of the amplification module of the downlink and the input of the amplification module of the

upward path.

  The diplexer filters operate simultaneously in both directions of transmission. The frequencies which cross them are different according to whether they are on the downlink or on the upbound. They make it possible to avoid disturbances in the loop which they form with the amplification modules of the downlink and of the uplink. For this, they bring a

  sufficiently large attenuation in this loop.

  This attenuation is typically, by diplexer filter, between approximately 40 and 60 dB depending on the frequency and the

  model of amplifier used.

  In addition, when two sinusoidal signals whose frequencies are included in the frequency band of the downlink are injected into the downlink, there is an intermodulation phenomenon in the amplifiers 260. Intermodulation is well known in echometry and corresponds to a beat phenomenon between close frequencies which generates the production of one or more resulting frequency (s) which can penetrate the upward path via the diplexer filters. Intermodulation signal levels at the input of an amplification module

  of the uplink were measured up to 60 dBgV.

  HFC networks, and in particular the amplifiers of the coaxial part, are generally not supervised, so that an operator is not aware of a failure as soon as it occurs but only when a user reports it. However, transmission faults are more or less serious and require more or less rapid intervention. Thus, a transmission breakdown, for example, requires emergency intervention, while an abnormal variation in gain or attenuation can be considered as a minor defect to be treated in a non-urgent manner. A known method for supervising HFC type networks is based on the use of means for managing an interactive system installed in the HFC network which it is desired to supervise. In this case, the operator can know the modems, installed at the users, who no longer respond. Thus, it can detect and locate a fault on the network. However, a major drawback of this method lies in the fact that there is no systematic correlation between a modem which no longer responds, for example when it is disconnected, and a network failure. It is therefore very difficult for an operator to know precisely the reality of a failure, as well as

  the severity of this possible failure.

  Another known method consists in installing supervisable amplifiers which are currently commercially available. We can consider this solution for new networks, but this involves a relatively large surcharge. However, this solution is not possible for existing networks because it would require the replacement of all

  amplifiers in place, which does not seem feasible.

  Also, the technical problem to be solved by the object of the present invention is to propose a device for detecting and locating transmission faults in an HFC network, said network comprising a downlink intended for at least one user and a upward path from said user (s), said two channels being formed in a first part by optical fiber cables and in a second part by coaxial cables on which amplifiers are arranged, which would make it possible to detect and locate precisely, quickly and centrally, the transmission faults appearing in the network, without replacing existing amplifiers. The solution to the technical problem is obtained, according to the present invention, by the fact that said device comprises: - a transmission means intended to produce a pulse signal and to inject it into said downlink, said pulse signal being intended to cause intermodulation in said amplifiers, - means for receiving an echo signal generated by said intermodulation and conveyed on said upward path, - a control system intended to control the means

  transmit signal and process the received echo signal.

  Thus, each amplifier of the HFC network to be supervised sends back to the device an echo signal in response to each pulse sent. The control system then makes it possible to detect the existence of a fault by comparing the received echo signal with a reference echo signal, and to locate the defective amplifier having emitted the distorted echo signal, as a function the reception time of the corresponding echo signal. This device therefore makes it possible to supervise an HFC network so

  interactive and fully centralized.

  Advantageously, this device is arranged

  in the headend of the HFC network.

  The solution to the technical problem posed is also obtained, according to the present invention, thanks to a method of implementing the device for detecting and locating transmission faults. This method is remarkable in that it comprises the steps consisting in: - injecting into said downlink a pulse signal capable of producing, by intermodulation in said amplifiers of said HFC network, an echo signal which enters the upstream , - compare the received echo signal with a reference echo signal, - trigger an alarm associated with a fault when the received echo signal differs from the reference echo signal, - locate the fault as a function of time of reception of said echo signal on which said fault

has been detected.

  Other features and advantages of the invention

  will appear on reading the following description,

  made by way of illustrative but nonlimiting example, with reference to the appended figures which represent: - Figure 2, a functional diagram of a device for detecting and locating faults according to the invention, - Figure 3, a diagram d 'an embodiment of a transmission means of the device of Figure 2, - Figure 4a, a diagram of a first embodiment of a reception means of the device of Figure 2, - Figure 4b , a diagram of a second embodiment of a means for receiving the device of FIG. 2, - FIG. 5, a diagram of an embodiment of a means capable of producing an echo signal, - Figure 6, a diagram of a variant of

  realization of the device of figure 2.

  A functional diagram of a device for detecting and locating transmission faults in an HFC type network is shown in FIG. 2. This device, referenced 500, is preferably placed in the headend 100 of an HFC network in order to allow centralized supervision of this network, from the head of the network, without the need to replace all

existing amplifiers.

  An emission means, referenced 1, makes it possible to produce a pulse signal P. This signal is transmitted, via a coaxial cable 2, to the free input of a coupler 3 to allow its injection into the downward path. An optical transmitter, not shown in FIG. 2, also located in the headend, then converts this signal into a multiplexed optical signal which is then transmitted to the optical nodes of the HFC network via the fiber optic cable 151 of the channel. down. In known manner, intermodulation corresponds to a phenomenon of beat between close frequencies producing at least one resulting frequency (s). Thus, so that the intermodulation can take place, the pulse signal P is broken down into at least two signals centered respectively on a first frequency FI and on a second frequency F2 belonging to the passband of the downlink. These two frequencies are also fixed so that the intermodulation produces a signal whose resulting frequency F3, corresponding to the difference of the two frequencies such that F3 = (FI - F2) or F3 = + (2F1 - F2), is included in the bandwidth of

the upward path.

  The frequency bands used are very variable according to the generations of networks and according to the countries. For information, cable networks of the HFC type are currently designed, in Europe, to have on the one hand downlink channels whose bandwidths have a minimum between 47 and 87.5 MHz and a maximum, which depends on network implementation, generally between 300 and 862 MHz, and on the other hand upstream channels whose bandwidths have a minimum of 5 MHz and a maximum, which depends on the network implementation, generally between 25

and 65 MHz.

  Of course the choice of two frequencies to produce the intermodulation in the amplifiers is only an illustrative example, because it is quite possible to generate an echo signal from an intermodulation produced by signals carried by more than two frequencies. It should also be noted that a pulse signal conveyed by a noise signal, or a digital signal, may suffice to generate intermodulation in the amplifiers. In addition, so that intermodulation can take place in the amplifiers of the coaxial part of the HFC network, in order to generate an echo signal E in the uplink, the pulse signal P transmitted preferably has a level of high power. Typically, this power level must be between 15 and 30 dB higher than the power level of traditional traffic signals traveling on the downlink for the dissemination of information to users. More specifically, the power level

  is about twenty dB higher.

  Because a signal, of high power level, is injected on the downlink in the form of P pulses, each amplifier of the network will successively receive a transmitted pulse, after a propagation time linked to its geographical position in the network. , and in turn send an echo pulse. In this way, an amplifier can be identified according to the

  response time of the echo pulse it emits.

  It should also be noted that, in rare cases, the optical nodes can also be the seat of a

  intermodulation and return an echo signal.

  As in the known methods of echometry, the performance in terms of resolution, that is to say in the present case the ability to separate two amplifiers for which the round trip propagation times are

  neighbors, increases as the pulse duration decreases.

  On the other hand, a short pulse generates a low energy echo signal which has a wide frequency spectrum. Advantageously, a duration of the order of 50 to 100 ns proves to be a good compromise, and for such a duration and a pulse repetition frequency of less than about 50 Hz, no perceptible degradation has been observed. the visual quality of analog television images, as well as digital television images. These values are however given by way of illustration because acceptable results are also observed for a pulse duration of the order

20 ns or 150 ns.

  A reception means 4 also makes it possible to receive the echo signal E resulting from the intermodulation produced by the pulse signal P

  in the amplifiers of the coaxial part of the network.

  A distributor 6 makes it possible to take this echo signal E and to address it to the receiving means 4, via

coaxial cable 5.

  A data control and processing system 7 makes it possible to control the transmission means 1, by means of a control signal C, and to process the data transported in the echo signal E, which reach it from the reception means 4. This control and processing system 7 can for example be formed by

a specialized processor.

  The echo signal E from an upward path of a coaxial sector corresponds to a "signature" representative, at a given moment, of the state of this coaxial sector. It is therefore necessary, in order to be able to detect a modification of this state, to compare the echo signals received with a reference echo signal recorded at the time of the putting into service of the coaxial sector concerned, or, more generally, at a time when this sector can be considered to be in good working order. A fault is then detected when an echo signal received

  differs from the reference echo signal.

  Preferably, storage means contain all the pre-established parameters necessary for the various calculations and for the control program of the control system 7. Thus, the reference echo signal is stored in a storage means, such as a database,

  non-volatile memory (EEPROM) etc ...

  It is advantageous to define several categories of faults according to the nature and the amplitude of the received echo signal with respect to the reference echo signal. Templates can therefore be pre-recorded in one or more storage means. These templates each represent a threshold corresponding to a predetermined category of defects as a function of the nature and the amplitude of the signal. Thus, several templates can frame the reference echo signal and correspond

  each has a specific type of fault.

  The amplitude of an echo signal depends on the level of work of the downlink amplifier having generated the intermodulation phenomenon responsible for the transmission of this echo signal. The term "work level", still commonly called "operating level" in Anglo-Saxon literature, is a term well known to those skilled in the art of cable networks and is therefore not explained in more detail here. The decrease or increase in the amplitude of the echo signals emitted by an amplifier indicates a malfunction of this amplifier or a variation in gain in the upstream link. This type of dysfunction can be considered a minor defect that does not require immediate intervention. On the other hand, the total disappearance of the echo signals emitted by an amplifier reveals that this amplifier is faulty or that the upstream link is cut, which can be considered as a major fault, requiring

urgent repair.

  Another category of faults can reside in the abnormal appearance of echo pulses generated, for example, by a coaxial connector whose contact resistance has become non-linear due to corrosion. The control and processing system 7 therefore makes it possible to compare, with reference to the time of transmission of a pulse P injected into the downlink, the echo signal E, corresponding to said pulse, with one or more templates framing a reference echo signal

previously registered.

  When the recovered echo signal E is different from the reference echo signal and greater than one of the pre-recorded templates, an alarm is triggered. This alarm varies according to the type of template which has been exceeded, that is to say according to the category of

  fault to which the received echo signal E belongs.

  When the echo signal E has been recovered, processed and a fault has been detected, the control and processing system 7 then makes it possible to locate this fault with precision. For this, the location of the detected fault depends on the geographic position from which the echo signal was emitted. Indeed, each amplifier of the network receives the pulses P emitted after a propagation time linked to its geographical position in the network and transmits in return an echo signal. Depending on the instant of reception of this echo signal, on which the fault has been detected, it is therefore possible to identify the defective amplifier. The control and processing system 7 also manages a man-machine interface 8, of the computer type for example, or even a centralized supervision station 9, in order to allow an operator to supervise his HFC network in a centralized manner, from of the head

network.

  A distinction is generally made between line amplifiers which have a medium level of work, and terminal amplifiers which have a high level of work. Indeed, the terminal amplifiers having to supply a maximum of connection points 280, in order to avoid a massive and expensive implantation of these amplifiers in the network, they must have a high level of work. A compromise is therefore achieved on the overall intermodulation of the coaxial sector so that the line amplifiers work at low level and that the terminal amplifiers work at high level. The terminal amplifiers therefore send back easily measurable echo pulses whereas the line amplifiers send back hardly measurable echo pulses. However, not being able to measure echo returns from line amplifiers is not a problem because, if one of them breaks down, the downstream terminal amplifiers no longer send back echo pulse, which helps detect the failure and

  to locate the branch of the network concerned.

  FIG. 3 represents more precisely an embodiment of the transmission means 1 of the device of FIG. 2. This transmission means comprises a first generator 10 intended to produce a signal carried by a first frequency F1, and a second generator 11 intended to produce another signal carried by a second frequency F2. Each output of these two generators is connected to an input of a coupler 12 which supplies an electronic switch 13. A control signal C, coming from the control and data processing system 7, makes it possible to control this electronic switch in particular to transform the signal from the coupler 12 into a pulse signal P. Thus, one obtains, at the output of the switch, a signal consisting of periodic pulses P carried by frequencies F1 and F2. Preferably, these periodic pulses are sent with a period greater than the round-trip propagation time of the part of the network to be supervised in order to avoid a possible

  overlap of two echo returns.

  A first embodiment of the means 4 of

  reception of the echo signal is illustrated in Figure 4a.

  It includes a bandpass filter 14 whose bandwidth corresponds to the frequency spectrum of the echo signal E, an envelope detector 15 for supplying a baseband echo signal and a converter

analog-digital 16.

  A second embodiment of the means 4 for receiving the echo signal E is shown in FIG. 4b. It includes the same elements 14 and 16 as the embodiment of FIG. 4a, but the detector for supplying a signal

  baseband echo is here a synchronous detector 17.

  This detector receives a signal synchronized to the frequency F3 of the echo signal. The signal synchronized on the frequency F3 is produced by a mixer 18, supplied by the signal at the frequency F1 coming from the generator 10 and the signal at the frequency F2 coming from the generator 11, such

  so that F3 = + (Fl-F2) or F3 = (2F1-F2).

  The digitized echo signal from the receiver 4 is directed to the data processing and control system 7. The latter then calculates an average between several consecutive echo signals prior to comparison with the reference echo signal, or at least one template framing this reference echo signal. This averaging notably makes it possible to considerably increase the sensitivity, that is to say the ability to

  receive low amplitude echo signals.

  The control system 7 also performs a permanent test at the level of the received echo signal E and it suspends the averaging when this level exceeds a predetermined threshold corresponding to interference of the echo signal by another traffic signal present in the upward path. Indeed, although it is not desirable to use the same frequency band both for the echo signal and for a channel dedicated to traffic, this can be made necessary when the frequency resources of the upstream channel are limited. In this case, as the upstream traffic signals are generally transmitted according to a time multiplexing mode which provides moments of silence, the duration of the averaging is then increased in proportion to the inverse of the silence rate, where the silence rate is defined as the average total duration of rests per unit of time. The supervision of an HFC network can also be advantageously improved thanks to the installation at various points of the network, in particular on the unoccupied outputs of connection points, referenced 280 in FIG. 1, of retransmission devices 19 able to produce an echo signal. A simple and inexpensive embodiment of such a device 19 is shown in FIG. 5. It comprises a shielded box 20 to meet the EMC electromagnetic compatibility requirements (metal box for example), provided with a coaxial connector 21 , a nonlinear element 22, for example a diode, making it possible to produce intermodulation in the presence of a pulse signal, and an impedance adapter element 23 which can possibly provide attenuation in order to avoid, when the loss of passage between the different outputs of a connection point is not very high, to permanently produce perceptible intermodulation

in the HFC network.

  It is now recalled that at the head of an HFC network, a downlink start feeds several optical nodes 2001, 2002, ... 200m which each have their own uplink 1, 2, ... m. For this reason, the fault detection and localization device can be completed by a switching means 24 shown in FIG. 6. The control system 7 then controls this switching means 24, by means of a control signal C2, for successively connect the outputs 1 to m of the m upward channels, associated with the same downlink, to the input of the signal reception means 4

Echo to Em.

  The device and the method which have just been described with reference to the appended figures are only illustrations and are in no way limited to these examples. They typically find their application in the detection and localization of faults during the transmission of information in HFC networks. They thus make it possible to ensure supervision of the HFC network

  centralized from the network head.

Claims (12)

  1. Device for detecting and locating transmission faults in an HFC network, said HFC network comprising a downward path to at least one user (Cl, ..., Cn) and an upward path from said (said ) user (s), said two channels being formed in a first part by optical fiber cables (151, 152) and in a second part by coaxial cables (250) on which amplifiers (260) are arranged, characterized in what it comprises: - a transmission means (1) intended to produce a pulse signal (P) and to inject it into said downlink, said pulse signal being intended to cause intermodulation in said amplifiers (260)   - a means (4) for receiving an echo signal (E) generated by said intermodulation and conveyed on said upward path, - a control system (7) intended to control the means (1) of transmission and treat the echo signal (E) received.   2. Device according to claim 1, characterized in that the pulse signal (P) is carried by at least two frequencies (F1, F2).   3. Device according to one of claims   above, characterized in that the transmission means (1) comprises a first generator (10) producing a signal at a first frequency (F1), a second generator (11) producing another signal at a second frequency (F2), said first and second generators being connected to a coupler (12) supplying an electronic switch (13).   4. Device according to one of claims   above, characterized in that the receiving means (4) comprises a bandpass filter (14), a detector (15; 17) intended to produce an echo signal in baseband, and an analog-to-digital converter (16) . 5. Device according to claim 4, characterized in that the detector is a casing detector (15). 6. Device according to claim 4, characterized in that the detector is a synchronous detector (17) receiving a signal synchronized on the frequency (F3) of the echo signal (E) and coming from a mixer (18).   7. Device according to one of claims   previous, characterized in that one or more means (19) of retransmission, intended to produce an echo signal, is (are) also installed (s) on one (or more) output (s) unoccupied of one or more connection point (s) (280).   8. Device according to claim 7, characterized in that the re-emission means (19) comprises a shielded box (20) provided with a coaxial connector (21), a non-linear element (22) and an element (23) impedance adapter.   9. Device according to one of claims   above, characterized in that it further comprises a switching means (24) controlled by the control means (7) and intended to connect successively several channel outputs   going back to the entry of the receiving means (4).   10. Device according to one of claims   previous, characterized in that it is arranged   in the headend (100) of the HFC network.   ll. Process for implementing the device for detecting and locating faults in   transmission according to one of claims   above, characterized in that said method comprises the steps consisting in: - injecting into said downlink a pulse signal (P) capable of producing, by intermodulation in said amplifiers (260) of said HFC network, an echo signal ( E) which enters said upward path, - compare the echo signal (E) received with a reference echo signal, - trigger an alarm associated with a fault when said echo signal (E) received differs from the signal reference echo, - locate the fault as a function of the instant of reception of said echo signal (E) on which said fault has been detected.   12. Method according to claim 11, characterized in that the pulse signal (P) injected in the downlink has a power level higher than the power level of the signals of traffic carried in this lane.   13. Method according to one of claims 11 to 12,   characterized in that prior to the comparison with said reference echo signal, the control system (7) calculates an average between   several consecutive echo signals (E).   14. Method according to claim 13, characterized in that the averaging is interrupted during periods of time during which the echo signal (E) is interfered with by another signal   traffic present in the uplink.   15. Method according to one of claims 11 to 14,   characterized in that the received echo signal (E) is compared to at least one template (s) associated with a category (ies) of faults and framing the reference echo signal.