Abstract:
A system and method for maintaining an uninterrupted data flow in a communications network includes a transmitter, which transmits the data flow from a data source and a receiver, which receives the data flow. A channel interconnects the transmitter and the receiver, and communicates the data flow between the transmitter and the receiver. A switch is located in the channel between the transmitter and the receiver. The switch is openable for a period of time, which is sufficient to permit analysis of signal impairment in the channel without interrupting the data flow.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention concerns communications networks and more particularly to methods of analyzing signals used in the networks.  
       BACKGROUND OF THE INVENTION  
       [0002]     Community Antenna Television (CATV) networks carry television signals to subscribers in the form of analog and digital signals. Various methods are used to quantify the performance of this distribution network. In the analog world, the methods are well established, whereas in the digital world new methods are required to measure the performance under different conditions. CATV networks are used in the downstream to carry the digital signal in the form of Quadrature Amplitude Modulation (QAM), especially under the format of International Telecommunication Union (ITU) J-83, Annex A, B or C, according to the country of use. Downstream transmission is continuous, with so-called stuffing data, if no user traffic is present. In the upstream direction, burst transmission can follow the Data Over Cable Service Interface Specification (DOCSIS™) 1.0, 1.1 or 2.0 standard with modulation format. of Quadrature Phase Shift Keying (QPSK), QAM or Collision Detection Multiple Access (CDMA).  
         [0003]     Many types of impairments are known to degrade these signals. Test instrumentation has been developed to measure these impairments, in particular signal-to-noise, phase noise, static constellation distortion (I/Q gain and phase, compression) and Continuous Wave (CW) interference. The CW interference can be detected and measured within some limitations. The network distortion products known as Composite Second Order (CSO) and Composite Triple Beat (CTB) also cause interference; but they can be mistaken for CW even though they are different and cannot be measured the same way. It is important to clarify the difference between CW interference and CSO/CTB.  
         [0004]     CW is interference has a fixed frequency and amplitude. It generates a rotating shift of the constellation&#39;s soft decision, visible as the “doughnut” effect on the constellation.  
         [0005]     CSO and CTB on the other hand are the composite of many (hundreds, thousands) of very small distortion products of the second and third order between each one of the carriers on the system. Consequently, CSO/CTB have noise like characteristics. These characteristics include a spread in frequency related to the spread in frequency offset of the carriers to their nominal frequency (usually &lt;10 kHz); the phase of these carriers come in and out of alignment and cause a variation in level (the tighter the frequency spread, the less frequent the phase alignment occur but the longer it lasts). Variations of ±15 dB relative to the average have been reported, and may last up to a few hundred microseconds.  
         [0006]     The current impairment measurement methods rely on capturing an actual signal and applying processing algorithms to extract information on the impairment combined with the signal. For impairments external to the signal, there are four types of analysis instrumentation that are used to measure QAM Impairments, namely QAM Impairment Analysis, Rohde &amp; Schwarz™ analysis, as used in the EFA™ series of instruments, the Subtraction method, and spectrum analysis with narrow resolution bandwidth.  
         [0000]     QAM Impairment Analysis  
         [0007]     In this method, a large number of constellation&#39;s soft decisions are collected and the following algorithm applied: calculating distances from the actual soft decisions to the ideal point; generating a histogram of the distances; and comparing the histogram to a set of histogram templates with known amount of noise and CW interference. The best match gives the noise and CW levels. This method relies on a best match; but the difference between various templates is quite small when the CW interference is small. This method does not allow identification of the CSO/CTB impairment vs. CW because of its varying amplitude, so it is lumped into the noise measurement.  
         [0000]     Rohde &amp; Schwarz  
         [0008]     This method is described in the manual of the EFA™ series instrument (Rohde &amp; Schwarz™, EFA Operating Manual, Sections 2.5.9.1 and 2.5.9.2, document No.:2068.0950.12-11). This method relies on the calculation of the rms value of the distances for noise measurement and on the offset of the rms peak. When CW interference is small, the offset cannot be identified, which limits the accuracy of CW measurement. Again this method does not distinguish CSO/CTB from noise.  
         [0000]     Subtraction Method  
         [0009]     This method includes: capturing a sequence of constellation&#39;s soft decisions; generating a time reconstruction of the “original” signal by using the ideal symbols; subtracting the ideal signal from the actual received signal; and analyzing the difference as it represents the “impairments”. This method is computation intensive and it assumes that there have been no errors in transmission.  
         [0000]     Spectrum Analysis  
         [0010]     This method uses spectrum analysis with a narrow resolution bandwidth such that interference signal will emerge from the QAM spectrum. Assuming a 5 MHz signal bandwidth, if a 30 kHz resolution bandwidth (RBW) is used, the detected QAM signal is now 22 dB below the total average power; hence an interference that is 25/27 dB below the QAM total power could be identified with long enough averaging. To achieve a more sensitive detection, a narrower RBW may be used (36 dB with a 1 kHz RBW). This is usable with CW, but it is difficult to detect CSO/CTB that is spread over a 30 kHz bandwidth and whose amplitude varies in time.  
         [0011]     This there is a need for an improved method for analyzing signals in a network.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention reduces the aforesaid difficulties and disadvantages by providing a system and method for temporarily removing the actual QAM signal and measuring those impairments that are present in the channel in the absence of a signal. Advantageously, the present invention allows the removal of the signal without causing an interruption of service to the subscribers. Moreover, the method allows measurement of impairments that are external to, and independent of, the signal, that is, noise, CW interference, CSO, CTB and the like. The method does not directly measure impairments of the signal itself such as phase noise and static distortion. In addition, the method takes advantage of the characteristic that the data already has redundancy and voluntarily interrupts the signal to check line impairment.  
         [0013]     Accordingly, in a first embodiment of the present invention, there is provided a system for maintaining an uninterrupted data flow in a communications network, the system comprising: a) a transmitter for transmitting the data flow from a data source; b) a receiver for receiving the data flow; c) a channel interconnecting the transmitter and the receiver, the data flow being communicated between the transmitter and the receiver through the channel; and d) a switch located in the channel and disposed between the transmitter and the receiver, the switch being openable for a period of time sufficient to permit analysis of signal impairment in the channel without interrupting the data flow.  
         [0014]     Accordingly, in a second embodiment of the present invention, there is provided a method of maintaining uninterrupted data flow in a communications network during signal analysis, the network having a transmitter in communication with a receiver, the method comprising: a) temporarily removing a portion of the data flow from a communications channel, the channel communicating the data flow between the transmitter and the receiver; and b) measuring a source of signal impairment in the channel so as to maintain uninterrupted data flow to the receiver. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Further aspects and advantages of the present invention will become better understood with reference to the description in association with the following Figures in which:  
         [0016]      FIG. 1  is a simplified diagram showing a bi-directional CATV network with cable modem service;  
         [0017]      FIG. 2  is a simplified flow diagram of dual channel coding between a transmitter and a receiver;  
         [0018]      FIG. 2   a  is a simplified flow diagram of dual channel coding between FEC encoding and FEC decoding;  
         [0019]      FIG. 2   b  is a simplified flow diagram of an embodiment of a gated switch located downstream of a QAM modulator;  
         [0020]      FIG. 3  is a simplified flow diagram showing continuous QAM transmission;  
         [0021]      FIG. 4  is a simplified flow diagram of the gated switch interrupting a transmission;  
         [0022]      FIG. 4   a  is a representation of a transmission wave with an interrupted transmission;  
         [0023]      FIG. 5  illustrates interleave burst protection periods and latency for ITU J-83 annexes A, B;  
         [0024]      FIG. 6  is a diagrammatic representation of consecutive Burst transmissions in an upstream channel;  
         [0025]      FIG. 7  is a table of upstream Burst transmissions in the upstream channel; and  
         [0026]      FIG. 8  is a QAM signal detected in spectrum analysis showing different resolution bandwidths. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Referring now to  FIGS. 1 and 2 , a simplified a diagram of a bi-directional CATV network is illustrated, which includes a Cable Modem Termination System (CMTS) downstream transmitter  10  and a CMTS upstream receiver  12 . The network optionally includes a number of other signal sources  13 . The transmitter  10  and the receiver  12  are in communication with cable modems at the subscribers, the transmitter  10  receiving digital data, which is then communicated to the cable modems located downstream therefrom. A plurality of subscribers&#39; cable modems (subscriber receivers  15 ,  17 ) in the CATV network (only two are illustrated for clarity) receives the signals from the transmitter  10  and transmit upstream to the receiver  12  in assigned time slots.  
         [0028]     Typical CATV frequency bands are assigned as follows: 5 to 42 MHz for cable modem upstream; 55 to 550 MHz for analog television channels and 550 to 750 MHz for digital channels (television and cable modem downstream).  
         [0029]     Broadly speaking, the digital data includes a QAM digital signal, which travels between the downstream transmitter  10  to the cable modems and back to the upstream receiver  12  via a communications channel  14  to the subscribers. The channel  14  interconnects the transmitter  10 , the receiver  12 , and cable modems and may be a physical link, for example, fibres, coaxial cables and the like, or it may be non-physical, for example an air path. Although the following description will focus on downstream QAM signals, it is to be understood that the same principles apply to upstream transmission and to other signal formats and standards known to those skilled in the art.  
         [0000]     QAM Channel Coding  
         [0030]     Referring now to  FIGS. 2 and 2   a,  it is well known in the art that transmission of signals through transmission channels  14  is imperfect and typically leads to channel impairments. To combat the channel impairments, which include, for example, Gaussian noise, impulse noise, interference and the like, channel coding is added to the data stream to be transported.  
         [0031]     An ITU J-83 standard is used to illustrate the principle of channel coding and is set for digital video and data service in CATV network, and includes MPEG (Moving Pictures Expert Group) framing  16 . One skilled in the art will recognize, however, that similar schemes may be used in all digital channels with different parameters depending on the application.  
         [0032]     Channel coding is a dual process in which every step at the transmitter  10  side has a mirror process at the subscriber receiver  15  side such that the data stream is transparently transported. As best illustrated in  FIG. 2 , the ITU J-83 standard, on the transmitter  10  side, includes the MPEG framing  16 , an FEC encoder  18 , and a QAM modulator  20 . On the subscriber receiver  15  side, a QAM demodulator  22  is connected to the channel  14 , with an FEC decoder  24  and MPEG framing  26  being connected to the QAM demodulator  22 .  
         [0000]     Reed-Solomon Encoding, Forward Error Correction (FEC) Codes  
         [0033]     As best illustrated in  FIG. 2   a,  a Reed Solomon encoder  28  appends a data packet with Reed-Solomon codes such that at the reception, virtually all transmission errors are detected, and a number of errors can be corrected (one errored symbol can be corrected for two added FEC symbols per packet). Similarly, a Reed Solomon de-encoder  42  is located in an FEC decoding region  24  for detecting error and correcting those it can using the Reed Solomon codes from the signal.  
         [0034]     The content (data and FEC codes) of a set of data packets are interleaved by an interleaver  30 . For example, the transmitted symbols are a sequence of symbols from consecutive packets such as 1 st  symbol of 1 st  packet, then 1 st  symbol of 2 nd  packet up to the 1 st  symbol of the last packet, then 2 nd  symbol of 1 st  packet and 2 nd  symbol of 2 nd  packet, until the last symbol of the last packet. The feature of interleave is such that an impulse noise or a momentary interruption of transmission causes adjacent errored symbols in transmission, but when de-interleaved by a de-interleaver  40 , the adjacent errors are spread over a large number of packets. The FEC can then correct the few errors in each packet.  
         [0000]     Randomization  
         [0035]     Randomization by a randomizer  32  is a modulation characteristic in which long streams of 1 or 0 are prevented by multiplying transmitted data with a random sequence. At reception, a derandomizer  38  restores the original data sequence.  
         [0036]     Referring now to  FIGS. 2   a  and  2   b,  an embodiment of the present invention is illustrated in which, broadly speaking, the QAM signal is removed for a period of time from the data signal, which permits analysis of impairments in the channel  14 , but without disrupting QAM service to the subscribers. This is achieved by using a short interruption, typically in the order of 33 μsec, so that the Forward Error Correction (FEC) mechanism couples with the de-Interleaver  40  and the subscriber receiver  15  side can recover the missing piece of information, as will be described below.  
         [0037]      FIG. 5  is a table of the relationship between the interleaving characteristics and burst protection period. The burst protection period corresponds to the length of time during which the signal can be disrupted without loss of data due to the error correcting mechanism of the FEC. Mapping represents the process of grouping data bits into QAM constellation symbols (64 QAM or 256 QAM). In the J-83 Annex B format (for North American subscribers), the QAM mapping is combined with Trellis Coded Modulation by a Trellis encoder  34 , as that is a further defense against Gaussian noise. Consequently, transmission can be interrupted and yet the missing data for the subscriber can be regenerated. In the specific case of J-83 Annex B, at interleave 128×1 and QAM 256; the burst protection period is typically 66 μsec. Similarly, at interleave 128×4 and QAM 256; the burst protection period is typically 264 μsec.  
         [0038]     For QAM 64, at interleave 128×1; the burst protection is typically 95 μsec. Similarly, at interleave 128×4; the burst protection is typically 379 μsec.  
         [0000]     Gated Impairment Measurement  
         [0039]     If an interruption of QAM signal for 33 μsec (i.e. less than the burst protection period above) is generated at the transmitter  10 , the data should be recovered in the subscriber receiver  15 , but it reduces the margin for “normal” impairments. The test instrument can then sample the channel bandwidth during that period. Assuming a Nyquist sampling of the 6 MHz channel bandwidth at 12 Mega-samples/sec, 400 samples are produced that can be used in a FFT (Fast Fourier Transform) with a resolution of 6 MHz/200 (½ number of samples) or 30 kHz (in practice, a higher sampling rate is used for anti-aliasing purpose, but the effective resolution is the same). This 30 kHz is similar to the 30 kHz RBW used for CSO/CTB measurements. Consequently in-channel impairments can be measured, such as noise spectrum, CW interference, CSO/CTB interference. Further by repeating the interruption/sampling process, averaging can be implemented as well as time-graph or amplitude-histogram of interferers in particular CSO/CTB.  
         [0000]     Operational Trade-Offs  
         [0040]     Depending on the channel coding scheme in use, the burst protection period varies, but in practice 66 μsec is a lower limit in J-83 Annex B, for most schemes it is longer, which helps in terms of either larger number of samples (and resolution in analysis) or a relatively shorter interruption hence more headroom for normal channel impairments.  
         [0041]     In addition to the interruption period, the repetition rate of interruption is important. It should be slower than the Latency of the Interleaver so that two interruptions may not overlap the same interleave sequence. In addition, it is safer to space the interruptions so that, on a poor channel, if FEC is insufficient and data errors occur, they are widely spaced. For instance a repetition rate of one interruption per second, on Cable modem operation would cause one repeat packet per second, quite acceptable. On a video channel, the missing Moving Pictures Expert Group (MPEG) 16 packet per second, on a channel that carries 10/12 programs, means on the average a disturbance on the program every 10/12 seconds; notwithstanding that in video decoding a single MPEG packet in error can be concealed easily.  
         [0000]     Upstream or Bursty Transmission  
         [0042]     In addition to the downstream continuous transmission, the same principle may be applied to upstream bursty transmission.  
         [0043]     Referring now to  FIGS. 6 and 7 , most transmission media that use a common channel for multiple users rely on a traffic regulation mechanism to minimize collisions. This is the case of the DOCSIS™ standard where transmission slots are allocated by the network controller, called Cable Modem Termination System (CMTS), to the users to transmit upstream data without collision. Except for some specific slots allocated for users requesting transmission permission, the CMTS regulates precisely all traffic through MAP tables where cable modems are granted permission to transmit for a specific number of “mini-slots” within the period covered by the current Upstream Bandwidth Allocation Map (MAP). Consequently, the CMTS can insure that specific time periods can be free of user transmission. These periods are then used for upstream channel transmission Impairment measurements in a similar fashion to that of the downstream during signal interruption. A measurement instrument is conveniently located in the head-end (close to the CMTS) for network monitoring, but it could also be located in the network, provided it has a known delay relative to the CMTS so that “quiet” period is also known. Alternatively, a designated cable modem may not transmit when granted, consequently the channel is quiet and measurements on the channel can take place. The designated cable modem will transmit when required by the CMTS to replace the “missing” transmission and to satisfy protocol procedures.  
         [0000]     Alternate Measurement Methods  
         [0044]     Referring now to  FIG. 8 , instead of sampling the total signal bandwidth at once, the sampling can be accomplished with a sampling bandwidth narrower than that of the signal and multiple sampling processes are performed across the signal bandwidth to scan the entire band. A second alternate method relies on a receiver with a detector and Resolution bandwidth, such as a Spectrum Analyzer (in log mode or linear mode). This method gives information on the total power within the Resolution Bandwidth over time, but it does not differentiate the various components of this power. It has less discriminating capability than the bandwidth sampling method.  
       EXAMPLES  
       [0045]     Abbreviations used throughout include:  
         [0046]     I=number of taps;  
         [0047]     J=Increment;  
         [0048]     SID=Service Identifier;  
         [0049]     REQ=REQuest;  
         [0050]     MAC=Media Access Control;  
         [0051]     PMD=Physical Media Dependant; and  
         [0052]     IE=Information Element.  
         [0000]     1. Downstream Transmission  
         [0000]     1.1 Transmitter Implementation  
         [0053]     Referring now to  FIGS. 3 and 4 , the downstream QAM transmitter  10  includes a data source, the QAM modulator  20  and an up-converter  21 . The data source feeds the data to be transmitted to the QAM modulator  20  in digital wired format such as parallel data plus clock or an Asynchronous Serial Interface (ASI). The QAM modulator  20  receives the data, inserts the channel coding described above and modulates the carrier in the QAM format and filters the modulated signal at a fixed frequency band referred to as Intermediate Frequency (IF). The up-converter  21  translates the IF band to the assigned band for operational use. The gating, or transmission interruption of the present invention is implemented by inserting a high-speed switch  48  either at the output of the up-converter  21  or more conveniently at the IF link between the QAM modulator  20  and the up-converter  21 . The high speed switch  48  cuts-off momentarily the IF link causing an interruption of the transmitted signal. Typical switches used include, for example, solid-state switches, such as Gallium Arsenide Microwave Monolithic Integrated Circuit (GaAs MMIC) or the like, which can provide an isolation &gt;60 dB in tens of nano-seconds. A controlling circuit (not shown) for the high speed switch  48  is a rectangular signal whose repetition rate and pulse duration can be adjusted, conveniently using, for example, programmable digital dividers and a clock signal. The pulse duration corresponding to the QAM interruption is adjusted to the current QAM channel coding, not exceeding the Burst Protection Period. The repetition rate is adjusted to minimize subscribers&#39; inconvenience. The switching action needs not be synchronized with the data stream or the channel coding framing. In order to minimize possible degradation to service, the interruption mechanism may be turned ON-OFF either on a scheduled basis or on demand by manual operation or remote control.  
         [0000]     1.2 Measuring Instrument Implementation  
         [0054]     As best illustrated in  FIG. 4   a,  the measuring instrument is a receiver tuned at the QAM signal assigned frequency. This receiver can be a spectrum analyzer in the zero span mode or a fixed or variable tuned receiver. The IF bandwidth of the receiver is normally equal to that of the QAM signal, but a wider bandwidth can be filtered to correspond to the signal BW, and a narrower bandwidth is usable with the restriction that the analysis is limited to measuring the instrument receiver bandwidth.  
         [0055]     The receiver IF signal is continuously sampled at a rate above the Nyquist limit to insure a capture of the whole information content of the bandwidth. A continuous trigger mechanism tracks signal strength to detect a level below the lowest symbol level used in the specific format, such that, in the case of QAM256, the lowest level symbol is approximately 14 dB below the average level. Adding a 6 dB headroom, the trigger indicates transmission interruption when the received signal level is 20 dB below the averaged receiver level. This indicates the period of measurement with measurement valid samples. It is convenient to sample continuously and to use the trigger as “marker” on the total sample set. If the transmission interruption duration and repetition is known to the receiver, it is convenient to add a maximum valid samples duration, as well it is convenient for the receiver to rely on a known period without valid samples to simplify scheduling of sampling, samples retrieval and samples processing.  
         [0000]     1.3 Samples Processing  
         [0056]     The samples are processed with FFT and other known methods either in hardware (dedicated or programmable logic) or in software (Digital Signal Processors or more general purpose processor) or in a mix of both.  
         [0057]     In order to correlate the CSO and CTB impairments under the same conditions for analog and digital signals, that is an effective 30 kHz bandwidth, the sampling period should be 1/30 kHz=33 μsec (the signal blanking is slightly longer i.e. 35 μsec).  
         [0000]     1.4 Multiple Sampling Periods  
         [0058]     In the case of J-83 Annex A (the European Standard), as best illustrated in  FIG. 5 , the Burst protection period is quite short at 18 or 14 μsec hence an interruption of 35 μsec would cause un-correctable errors. It is then preferable to use a short interruption (such as 8 μsec) and make 4 sampling periods and then integrate these 4 sets of samples into one long set with an equivalent length of 32 μsec.  
         [0000]     2. Upstream Transmission  
         [0059]     Referring now to  FIGS. 2   a  and  2   b,  the overall process is similar in the upstream and downstream application but for one major difference, instead of interrupting a continuous transmission, a burst of the bursty transmission is omitted.  
         [0060]     At the receiver  12  end a similar threshold as for signal interruption is used, with the threshold adjusted for the modulation format, input signal level variation between users, and a “current signal level range” to memorize levels in the case of long periods without upstream traffic. Two implementations are possible, with or without CMTS active cooperation.  
         [0061]     In the active cooperation mode, more suited to upstream channel monitoring at the head-end, the CMTS grants mini-slots to a device that is either known to be inexistant or to a device designed not to answer, in both cases there is no transmission, reception for analysis can take place. The CMTS does need to know that no transmission will take place, hence no retransmission request are necessary. The receiver must also listen to the downstream channel to receive MAP tables and open its reception window accordingly.  
         [0062]     In the non-cooperative mode, more suited to field operation, the instrument emulates a cable modem, registers, ranges and then requests transmission permission. When granted, it does not transmit, allowing sampling of the channel for analysis purposes. The CMTS will then request a retransmission for the “missed” transmission, the instrument will then transmit a message to a given address to clear the retransmission request and clear the channel.  
         [0063]     Also the instrument that emulated the Cable Modem can be in a remote location and the sampling/measurement instrument in another such as the head-end.  
         [0064]     Applicant tested the system&#39;s performance using a QAM signal source modulated according to the ITU J-83 Annex B format (North American Standard). The source (Wavecom™ 4050+4040) fed a QAM signal receiver (AT2000™) following the same format.  
         [0065]     The receiver allowed a large number of measurements of the received signal, in particular the “Bit Error Rate” before and after the Reed-Solomon coding, the “Errored Second” after the Reed-Solomon coding. Between the source and the receiver, an electronic switch was located that completely interrupted the signal for a controllable lapse (period) with a controlled repetition rate. The “Bit Error Rate” was observed while modifying the interruption period from 66 μsec down to 10 μsec (66 μsec corresponding to the “Burst Protection Period” with QAM 256, Annex B, Interleave 128×1 configuration i.e. the utilised transmission mode). For an interruption of &gt;66 μsec, the Bit Error Rate (BER) before and after Reed-Solomon FEC was proportional to the interruption period. For an interruption &lt;66 μsec, the BER before FEC was still proportional to the period, but the BER after FEC decreased rapidly with shortening of the period. At about 45 μsec, the after FEC Bit Error Rate was substantially null i.e. the end-user (here the AT2000) would not “see” any signal reception error. Consequently, a preferred interruption period of 33 μsec (half of the Standard allowed time slot) was selected, which provides an even larger protection margin. Furthermore, a 33 μsec sampling allows, after Fourier Transforms (FT), a 30 kHz resolution when a channel analysis was performed in the absence of the QAM signal i.e. the final objective of the system.  
         [0066]     While a specific embodiment has been described, those skilled in the art will recognize many alterations that could be made within the spirit of the invention, which is defined solely according to the following claims.