Identifying an origin of a DOCSIS upstream burst

A method and an apparatus for identifying an origin of captured DOCSIS upstream bursts are disclosed. Upstream bursts are captured without knowing their allocated time slots in advance. Information from an upstream channel descriptor is used to generate RF waveforms of upstream burst preambles, which are then correlated to the captured upstream waveforms to determine the type of captured upstream bursts without having to decode the latter. Once the type of the captured upstream bursts is determined, information from the upstream channel descriptor is further used to demodulate and decode the upstream burst, so that CPE MAC addresses can be extracted. From the extracted CPE MAC addresses, the origin of the captured upstream bursts can be identified. The identification of origins of captured upstream bursts assists in locating faults in the cable network.

TECHNICAL FIELD

The present invention relates to testing of cable systems, and in particular to testing of upstream digital communications in cable systems.

BACKGROUND OF THE INVENTION

In a cable system, a network of interconnected electrical cables, referred to as a cable plant, is commonly used to deliver information to subscribers. A cable plant enables a broadband transmission of signals, such as television signals, from a head end facility to a multitude of home receivers. A broadband coaxial cable is advantageously used in this application because it supports a wide range of frequencies and provides signal shielding at a moderate cost in comparison to other media. The wide frequency bandwidth permits definition of a substantial number of information channels on the coaxial cable, thus allowing simultaneous broadcasting of many channels.

Cable systems have, in recent years, moved beyond merely broadcasting television signals over the cable to subscribers in their homes. Subscribers of a cable network nowadays have a transceiver, or a modem, which allows the transmission of digital signals upstream toward the head end of the network. Among many services afforded by cable modems are: an Internet service, a home shopping service using a television catalogue, and a voice-over-IP phone service.

In bidirectional cable networks, the upstream and the downstream signals occupy separate frequency bands called upstream and downstream spectral bands. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 860 MHz. Downstream information channel signals co-propagate in the downstream spectral band, and upstream signals co-propagate in the upstream spectral band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals propagating in a common cable in opposite directions.

To provide upstream communication capability to a multitude of subscribers, the upstream frequency channels are used in a so-called time-division multiplexing (TDM) mode. Each cable modem is assigned a time slot, within which it is allowed to transmit information. The lime slots are assigned dynamically by a cable modem termination system (CMTS) disposed at the head end. The time slot information is communicated to individual cable moderns via an allocated downstream channel. Subscribers access available network resources by using a data communication bridge established between CMTS and individual cable modems. Subscribers send data from their digital devices (such as personal computers, televisions, voice-over-IP telephones) into cable modems, which then relay the data to the CMTS. The CMTS, in turn, relays the information to the appropriate network elements. Information destined to the subscriber digital device is provided from the network elements to the CMTS, which in turn relays the information to individual cable modems. The cable modems then relay the information to the digital devices used by the subscribers.

One popular communication standard for bidirectional data transport over a cable network is the Data Over Cable Service Interface Specification (DOCSIS). DOCSIS establishes rules of communication between CMTS and cable modems in a cable network. Three revisions currently exist for a North American DOCSIS standard, DOCSIS 1.x, 2.0, and 3.0. In addition to the 6-MHz wide North American based DOCSIS standard, there exists a European (Euro-DOCSIS) standard formatted for 8-MHz wide bandwidth channels.

As cable communication systems grow and become more complex, the task of proper system maintenance and troubleshooting becomes more and more difficult. The difficulty results from a random nature of signal bursts from individual cable modems. Although the cable moderns are allocated time slots in which they are allowed to transmit, the actual transmission depends on network activity of individual subscribers. Furthermore, the upstream signal bursts from cable moderns have a very short duration and arrive intermittently from a multitude of locations in the cable network. Consequently, an upstream signal from a faulty location is interspersed with upstream signals from other locations. From the troubleshooting standpoint, it is important to identify faulty upstream bursts and the particular anomalous or faulty network location the faulty bursts came from. Therefore, the upstream burst troubleshooting equipment must possess a capability to determine geographical location of a cable modem or modems generating faulty upstream bursts.

Various systems have been devised to maintain and troubleshoot upstream communications in a cable network. Volpe et al. in US Patent application publication 2005/0047442, incorporated herein by reference, disclose a method and an apparatus for quantifying upstream communication signals transmitted by a remotely deployed cable modem. Referring toFIG. 1, a signal integrity analyzer100of Volpe et al. includes upstream/downstream diplex filter28, signal couplers30, a downstream tuner and demodulator32, an upstream tuner and demodulator34, a DOCSIS processor36, and a portable computer40having a storage medium44. The signal integrity analyzer demodulates a downstream signal carrying information about upstream signal time slots, which is then processed in the DOCSIS processor36to extract time slot information for various cable modems, not shown. The portable computer40is used to filter the cable modem information and program the US tuner and demodulator34to capture upstream bursts arriving in time slots corresponding to the cable modems of interest. The captured upstream bursts are then analyzed for signal distortions, decoding errors, and other faults.

Danzig et al. in U.S. Pat. No. 7,372,872, incorporated herein by reference, disclose a field-programmable gate array (FPGA) implemented network monitor for monitoring downstream and upstream traffic in a cable network. The network monitor of Danzig et al. has a functionality to fully analyze downstream signal information, obtain upstream signal time slots, and capture upstream bursts within the time slots corresponding to cable modems of interest.

Azenko et al. in US Patent Application Publication 2008/0089399, incorporated herein by reference, disclose a “sniffer” device having two cable modems, one to capture downstream data bursts and the other to capture downstream messages and to recover the downstream symbol clock and generate an upstream reference clock which is phase coherent with the recovered downstream symbol clock. The reference clock is used by a cable modern termination system to capture upstream bursts.

The prior-art approaches described above require complex and expensive equipment for receiving, demodulating, decoding, and analyzing both downstream and upstream signals, as well as complex processing circuitry for selecting and processing upstream bursts of interest based on the analyzed downstream time slot information. Accordingly, it is a goal of the present invention to provide a simple and inexpensive method and apparatus for identifying and processing upstream signal bursts in a DOCSIS cable network, which do not require an a priori knowledge of upstream signals timing for proper operation in the upstream domain.

SUMMARY OF THE INVENTION

The present invention uses upstream channel descriptor information associated with a particular DOCSIS channel to capture upstream bursts associated with that channel and to extract customer premise equipment (CPE) MAC address from the captured upstream burst. ROM the CPE MAC address, the location of the cable modem, which generated the captured burst, can be determined by referring to a database of subscribers.

The CPE MAC address extraction is performed without any prior knowledge of upstream burst timing or demodulation imperfections. The upstream bursts are preferably identified by correlating a captured radio-frequency (RF) upstream burst waveform with an RF waveform constructed using upstream channel descriptor information. Decoding the waveforms is not required, because it is the RF waveforms that are correlated. This allows one to considerably speed up and simplify the upstream burst identification process.

In accordance with the invention there is provided a method for identifying an origin of a captured DOCSIS upstream burst, comprising:

(a) acquiring an upstream channel descriptor associated with a DOCSIS upstream channel, the upstream channel descriptor including at least one burst type;

(b) capturing an upstream burst associated with the DOCSIS upstream channel;

(c) extracting a customer premise equipment MAC address from the captured upstream burst using the upstream channel descriptor; and

(d) determining the origin of the captured upstream burst from the customer premise equipment MAC address using a database of subscribers.

Preferably, step (b) is performed without any knowledge of a time slot reserved for the upstream burst.

In accordance with another aspect of the invention there is further provided an apparatus for identifying an origin of a captured DOCSIS upstream burst, comprising:

a burst capturing unit for capturing an upstream burst associated with the DOCSIS upstream channel;

an extractor for extracting a customer premise equipment MAC address from the captured upstream burst using an upstream channel descriptor; and

a determining unit for determining the origin of the captured upstream burst from the customer premise equipment MAC address using a database of subscribers.

Preferably, the apparatus includes a receiver for obtaining the upstream channel descriptor.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art.

Referring toFIG. 2, an upstream test instrument200of the invention for identifying an origin of a captured DOCSIS upstream burst includes a receiver202, a burst capturing unit204, an extractor206, and a determining unit208. The burst capturing unit204preferably includes a correlator216. The extractor206preferably includes a demodulator218. The correlator216and the demodulator218will be described in more detail further below.

In operation, the receiver202acquires an upstream channel descriptor (UCD)210associated with a DOCSIS upstream channel211. The UCD210is a table listing preambles and signal parameters for a cable modem to use for generation of an upstream burst associated with the upstream channel211. The upstream UCD210, an example of which will be given below, is extracted from a downstream DOCSIS channel212allocated for carrying communication information and data in a DOCSIS communication system. The burst capturing unit204captures an upstream burst associated with the upstream channel211, and the extractor206extracts a customer premise equipment (CPE) MAC address from the captured upstream burst using the UCD210.

The main function of the receiver202is to capture the UCD210. The receiver202is no longer needed when the UCD210has been stored in a UCD database, not shown, from which the correlator216and demodulator218can retrieve required information for burst type identification and CPE MAC address extraction.

The determining unit208determines the origin of the captured upstream burst from the CPE MAC address using a database214of subscribers. The origin of the captured upstream burst is outputted at209. The database214can be external to the determining unit208, or it can be internal.

Preferably, the database214is a subscriber database maintained by cable operators for billing purposes. Most of the subscriber databases include residential addresses of subscribers, MAC addresses of cable modems installed in subscriber residences, and MAC addresses of CPE connected to the installed cable modems. In the subscriber database214, the residential address of a subscriber can be successively mapped to the MAC address of CPE connected the cable modern installed in that residence. From a CPE MAC address extracted from the captured upstream burst, the corresponding residential address of a subscriber can be identified. Therefore, a geographical location of impaired upstream bursts can be identified by its CPE MAC addresses using the database214. Once the captured impaired upstream bursts are mapped to geographical locations of the cable network, one can determine the geographical location of problematic cable network area(s).

The process of upstream burst capturing by the upstream test instrument200will now be considered in more detail. Referring now toFIG. 3, a time trace of an upstream burst300is shown. When the upstream burst timing information is not available, beginning and end moments of the upstream burst300can be determined by detecting rising and falling edges302and304, respectively, of a burst power envelope at a predefined trigger threshold306. The upstream test instrument200is triggered to randomly capture the upstream burst300when the power envelope of the upstream burst300exceeds the trigger threshold306. The term “randomly” means that the upstream burst300is captured without any knowledge of a time slot reserved for the upstream burst300.

The estimated burst length of the captured burst300is the difference between the falling edge304and the rising edge302of its power envelope at the trigger threshold306. The trigger threshold306needs to be properly set for each type of upstream quadrature amplitude modulated (QAM) burst to prevent any false capture. A burst filter is implemented in the upstream test instrument200to filter out upstream bursts shorter than a minimum burst length L1. This minimum burst length L1is selected so that upstream bursts containing no CPE MAC information will be discarded because of their short estimated burst lengths. Any upstream burst longer than the minimum burst length L1will be captured and identified. CPE MAC information can be then extracted from the identified upstream burst.

Fixed-length upstream bursts from field test instruments can be identified by only capturing bursts longer than the minimum length L1and shorter than the maximum length L2>L1. For example, to detect a test burst from a field test instrument in QAM16 format with 21 bytes of preamble and 327 bytes of data bytes, forward error correction (FEC) bytes, and zero-padded bytes, the boundary burst lengths L1and L2can be calculated as follows. The total length of the test burst is (21+327)*2 symbols long=696 symbols. With +/−5 symbol window, L1can be set to 696−5, and L2can be set to 696+5 symbols. With this criterion bursts from this field test instrument will be captured and processed by the upstream test instrument200. The upstream test instrument200can also capture unrelated bursts that satisfy this filtering criterion. Bursts from other devices can be further filtered out by CPE MAC addresses as described below.

Once an upstream burst of length L satisfying condition L>L1or L1<L<L2is captured, the CPE MAC address of the burst source can be extracted to determine the origin of the burst as explained above. According to the invention, the CPE MAC address is extracted from the captured burst using information available from the upstream channel descriptor210. In a step (I), the burst type of the captured burst is determined. In a step (II), which is performed after the step (I), the captured burst is decoded to extract MAC information. For both steps (I) and (II), the information contained in the upstream channel descriptor210is used. The upstream channel descriptor must be acquired before the step (I). To explain how the information of the upstream channel descriptor210is used, the structure of the upstream channel descriptor210needs to be exemplified first.

Referring to Table 1 below, an example of the upstream channel descriptor210is presented. Parts of the example of the upstream channel descriptor210are omitted for brevity.

The UCD of Table 1 includes several sections. A first section labeled with “[UCD]:” defines general parameters of the upstream DOCSIS channel such as upstream channel frequency (15 MHz); upstream channel number (#13); downstream channel number (#38); symbol rate (16× base rate); preamble pattern length (128 bytes); and the actual 128-byte preamble super string(“03 f0 . . . ff ff”). Following sections contain so-called “burst descriptors” of types of bursts a cable modern can generate: “Request Burst”; “Initial Maintenance Burst”; “Station Maintenance Burst”; “A-TDMA Short Data Grant Burst”; “A-TDMA Long Data Grant Burst”; and “A-TDMA Unsolicited Grant Burst”. For each of these burst types, Table 1 defines such parameters as Interval Usage Code, or IUC; modulation type; preamble length; preamble value offset; FEC byte number; FEC codeword info byte number; interleaving depth; interleaving block size; and preamble type. Whenever a DOCSIS-compliant cable modem sends a data burst of a certain type, it has to use “preamble length” bits of the 128-byte preamble super string, offset by “preamble value offset” bits, as the preamble of the particular burst type. Therefore, the burst type can be determined from the burst preamble. “Request Burst”, “Initial Maintenance Burst”, and “Station Maintenance Burst” are burst types that do not contain CPE MAC addresses.

According to the invention, the step (I), that is, burst type identification, is performed by (a) generating preamble radio-frequency (RF) waveforms based on the information contained in the upstream channel descriptor210for all Data Grant Burst Types; and (b) correlating the preamble portion of the RF waveform of the captured upstream burst with all the preamble waveforms generated in step (a). The steps (a) and (b) are performed by the correlator216shown inFIGS. 2 and 4. Of course, no decoding of the upstream burst is required for simply correlating the two RF waveforms. Thus, no prior knowledge of decoding parameters is necessary to identify the burst type of the captured burst. Once the burst type is identified, the upstream channel descriptor210can be used to determine the decoding parameters, so the step (II) of decoding can be properly performed.

Referring toFIG. 4, the correlator216includes an analog-to-digital converter (ADC)402, a burst downconverter404, a burst upconverter406, a preamble symbol generator408, two root raised cosine (RRC) filters410, a waveform upconverter412, a correlation unit414, and an output unit416. All these units are preferably implemented in field-programmable gate arrays (FPGA), high-speed digital signal processing (DSP), or application-specific integrated circuit (ASIC), although other implementations are possible. For example, these units can be implemented in software.

The burst downconverter404includes an oscillator418, a 90-degree phase shifter420, two multiplication modules422, and two low-pass filters424. The oscillator418is tuned to the upstream carrier frequency, for example 15 MHz upstream carrier frequency of upstream channel #13 of Table 1 above, so as to produce a baseband in-phase (“P”) and quadrature (“Q”) signals at the output of the burst downconverter404. The burst upconverter406and the waveform upconverter412each include an intermediate-frequency oscillator424, the 90-degree phase shifter420, two multiplication modules422, and a summation module426. The baseband I and Q signals are transformed by the burst upconverter406into a RF waveform428at the intermediate frequency, which is applied to a first input430of the correlation unit414.

The preamble symbol generator408generates I and Q symbol components of preambles of various burst types. For CMTS configured for operation in advanced time-division multiple access (A-TDMA) mode, preambles of “A-TDMA Short Data Grant Burst”; “A-TDMA Long Data Grant Burst”; and “A-TDMA Unsolicited Grant Burst” burst types are generated. For CMTS configured in time-division multiple access (TDMA) mode, preambles of “TDMA Short Data Grant Burst” and “TDMA Long Data Grant Burst” burst types are generated. Finally, for CMTS configured for operation in “mixed” mode, preambles of all above-mentioned burst types are generated.

The preambles are taken from the acquired upstream channel descriptor210, or optionally from a database, not shown. The I and Q symbol components are filtered by the RRC filters410and are upconverted to the intermediate frequency by the waveform upconverter412. An upconverted preamble RF waveform432is applied to a second input434of the correlation unit414. The correlation unit414produces a correlation result for each burst type. The correlation results are preferably stored in the output unit416. The preamble symbols corresponding to the burst type of the captured upstream burst will show the highest degree of correlation. Thus, the correlator216can be used to determine the burst type of the captured upstream burst. Once the burst type of the captured upstream burst is known, the upstream channel descriptor210can be used to determine the FEC decoding and the interleaving parameters (such as “FEC Bytes”, “FEC Codeword Info Bytes”, “Interleaver Depth”, “Interleaver Block Size”—see Table 1 above) to decode the upstream burst and to derive the CPE MAC address.

To perform de-interleaving properly, it may be beneficial to know the exact burst length. The exact burst length can be used to derive the depth of the de-interleaver (if the interleaver is configured to operate in a dynamic mode) and the location of the last byte in the last codeword, so that a decoded byte can be de-interleaved to the proper codeword. Once the upstream burst type is identified, the exact burst length can be derived from the estimated burst length. The estimation is performed by rounding the estimated burst length of a randomly captured upstream burst. The length is rounded to the closest of: an integer number of mini-slots; or an integer number of FEC codewords plus preamble and guard-time symbols. These conditions can be represented as follows:
if |õ+m×CW−ŷ|>|n×S−ŷ|y=n×X(1)
if |õ+m×CW−ŷ|<|n×S−ŷ|y=m×CW+õ(2)

wherein Õ is the overhead=preamble+guard-time symbols; CW is the FEC codeword=K+2*T bytes, wherein T is FEC error correction bytes and K is FEC codeword information bytes; S is the mini-slot size provided in the upstream channel descriptor210; ŷ is the estimated burst length and y is actual burst length; and in and n are integers.

When at least two captured upstream bursts have substantially the same degree of correlation stored in the output unit416of the correlator216, the captured upstream burst300can be demodulated, and a parallel FEC decoding and deinterleaving can be performed to determine the burst type of the captured upstream burst300. Referring now toFIG. 5, the upstream burst demodulator218includes a downconverter unit502, two RRC filters410, an adaptive decision feedback equalizer (DFE)504, a symbol slicer506, a symbol demapper508, de-interleavers and FEC-decoders510-1. . .510-n, a decision unit512, and a CPE MAC address output unit514.

In operation, the modulated burst signal428at the intermediate frequency from the burst upconverter406is downconverted to I and Q baseband samples. The I and Q baseband samples are filtered by the RRC filters410and equalized by the DFE equalizer504to remove inter-symbol interference and demodulation imperfections. The symbol slicer506produces the equalized and demodulated symbols, which are converted into a bit stream by the symbol demapper508. The bit stream is then processed in parallel by deinterleavers and FEC decoders510-1,510-2, . . . ,510-nof n various burst types, for example the 2 Data Grant burst types of Table 1 above. During the parallel decoding process, multiple CPE MAC addresses are extracted. The decision unit512selects a CPE MAC address having a zero FEC error count. This CPE MAC address is outputted by the CPE MAC address output unit514. The modules of the upstream burst demodulator218can be realized in hardware, in software/firmware, and/or in FPGA. Sequential FEC decoding can also be used, although the parallel FEC decoding illustrated inFIG. 5is preferable.

Turning toFIG. 6, the DFE equalizer504has a feed-forward equalizer602, a feed-back equalizer604, and a decimator606connected in series. The feed-forward equalizer602has a series of fixed delay lines608coupled to taps610-1,610-2, . . . ,610-16having variable tap coefficients a1, a2, . . . a16, respectively. The feed-back equalizer604has a series of the fixed delay lines608coupled to taps612-1,612-2, . . . ,612-16having variable tap coefficients b1, b2, . . . b16, respectively. The initial values of feed-forward tap coefficients a1, a2, . . . , a16and the feed-back coefficients b1, b2, . . . , b16are initialized to zero, except for the coefficient a8, which is initialized to 1. The carrier frequency is estimated in advance to initialize the frequency oscillator424so that the coarse carrier frequency offset can be removed. Likewise, the symbol timing is estimated in advance to initialize the decimator606to skip appropriate number of samples (i.e., coarse symbol timing offset) before decimating samples at 4 times of symbol rate into symbols. Methods of rough estimation of the carrier frequency and the symbol timing is discussed further below.

The residual demodulation imperfections are removed by the two-stage adaptive DFE504running at 4 times of a symbol rate T. The adaptive DFE504first applies recursive least square (RLS) adaptation on preamble symbols to adjust the coefficients a1, a2, . . . a16of the feed-forward EQ taps610-1,610-2, . . . ,610-16and feed-back EQ coefficients b1, b2, . . . , b16of the feed-back EQ taps612-1,612-2, . . . ,612-16to remove the linear impairments and de-rotate the demodulated symbols to the correct carrier phase. The least mean square (LMS) adaptation is then applied on burst payload symbols to again adjust the coefficients a1, a2, . . . , a16of the feed-forward EQ taps610-1,610-2, . . . ,610-16and the coefficients b1, b2, . . . , b16of the feedback EQ taps612-1,612-2, . . . ,612-16to remove small carrier errors and small symbol timing errors. The output signal is then decimated by the factor of 4 by the decimator606, to bring the output signal back to the symbol rate T. The optimal number of feed-forward EQ taps and the number of feedback EQ taps is determined to be 16 based on a computer simulation. Other number of taps can of course be used. Other adaptation algorithms can be used. Furthermore, other types of adaptive equalizers known to one of skill in the are can be used instead of the DFE equalizer504.

Referring now toFIGS. 7A and 7B, the rough estimation of carrier frequency and symbol timing for proper setting of the intermediate-frequency of the oscillator424and the symbol phase of the decimator606is illustrated by means of block diagrams700A and700B, respectively. The block diagram700A includes two successive steps702of squaring the input QAM signal at the intermediate frequency, a fast Fourier transform (FFT) step704, and a step706of finding the frequency f4×IFof the spectral peak on the FFT spectrum obtained at the FFT step704. The frequency f4×IFis a “rough” estimate of 4 times of the intermediate frequency. The block diagram700B includes one squaring step702, one FFT step704, and a step708of finding the phase P of the spectral component at the symbol rate fs. The phase P is a “rough” estimate of the symbol timing.

The above disclosed embodiments of the apparatus and method for identifying an origin of a captured DOCSIS upstream burst are exemplary embodiments selected for the purpose of illustration and explanation. For this reason one is cautioned not to limit the invention to the disclosed embodiments, but rather encouraged to determine the scope of the invention only with reference to the following claims.