Abstract:
Disclosed are a method and bidirectional communication device, such as a cable modem, for iteratively performing the following steps for each of a plurality of communications channels until a downstream data signal is detected. Selecting a communications channel and checking for a downstream signal having a first bandwidth complying with a first system interface standard, and if the downstream signal having said first bandwidth is not detected, then checking the communications channel for a downstream signal having a second bandwidth complying with a second system interface standard.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application claims the benefit of U.S. Provisional Application serial number 60/305,219, filed Jul. 13, 2001, which is incorporated herein by reference in its entirety. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention relates generally to broadband data transmission. More particularly, the invention relates to bi-directional communication devices, such as cable modems, adapted for use in multiple standard systems such as the North American and European DOCSIS standard systems.  
         BACKGROUND OF INVENTION  
         [0003]    Bi-directional communication devices, such as cable modems, have been designed to specifically operate under a single standard, such as the North American Data Over Cable Service Interface Specifications (DOCSIS) or the European DOCSIS standards. The European version of the DOCSIS standard was not originally available when DOCSIS was first proposed to European customers. Many European cable operators started deploying the North American DOCSIS standard. These cable operators now express the need to change to a European DOCSIS-compliant system.  
           [0004]    There are three main differences between a European DOCSIS cable modem and a North American DOCSIS cable modem. First, a diplexer within a cable modem has different cross over points, depending on whether the modem is a European DOCSIS or North American DOCSIS device, since the forward (downstream) and the return (upstream) assigned data channel bandwidths are slightly different in the two standards. This difference in the diplexer crossover point is realized by the use of different high pass filter and low pass filter cutoff frequencies in the European and North American DOCSIS compliant devices. Second, the forward data channel bandwidth is 8 MHz for a European DOCSIS compliant device, while the forward data channel bandwidth for a North American DOCSIS compliant device is 6 MHz. This difference in channel bandwidth is accomplished through the use of a different surface acoustic wave (SAW) filter to maximize performance when additional channels are located next to the desired channel without any guard band. Third, the forward data channel for European DOCSIS stipulates an alternative forward error correction (FEC) scheme than that used for North American DOCSIS. Accordingly, as cable operators change over to use of European DOCSIS-compliant cable modems, the corresponding costs rise in economies of scale to manufacture a different cable modem for each DOCSIS standard.  
         SUMMARY OF INVENTION  
         [0005]    The disadvantages heretofore associated with the prior art are overcome by the present invention, a method and apparatus for processing multi-mode (multi-standard) communication signals through a bidirectional communication device, such as a cable modem. The method includes tuning to one of a plurality of channels, and searching for a downstream signal, which has a first bandwidth complying with a first service interface standard.  
           [0006]    In an instance where the downstream signal is not detected, the search is repeated for a downstream signal having a second bandwidth complying with a second service interface standard. In an instance where the downstream signal is not detected, the next channel is tuned and a search for a downstream signal is performed, which has a first bandwidth complying with the first service interface standard. In an instance where a downstream signal is not detected, a search for a downstream signal having the second bandwidth complying with the second service interface standard is performed. The method proceeds to iteratively tune and search through each channel until a downstream data signal is detected.  
           [0007]    The apparatus comprises a diplexer including a high-pass filter and a low-pass filter, downstream processing circuitry that is coupled to the high-pass filter, and upstream processing circuitry that is coupled to the low-pass filter. A detector searches a plurality of frequencies, where each frequency is checked for acquisition of at least one service interface standard (e.g. —European and North American DOCSIS) downstream data signal, prior to checking the next frequency.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 is a block diagram of an exemplary multi-mode bidirectional communications device for use in a data communications system in accordance with the principles of the present invention;  
         [0010]    [0010]FIG. 2 is an exemplary graphical representation of a response curve for the diplexer of FIG. 1, in accordance with the principles of the present invention;  
         [0011]    [0011]FIG. 3 is a flowchart of an exemplary method for detecting a downstream signal using the device of FIG. 1, in accordance with the principles of the present invention;  
         [0012]    [0012]FIG. 4 depicts an exemplary frequency response curve for data channels do operating under the North American DOCSIS standard in relationship to a SAW filter of the cable modem of FIG. 1, and in accordance with the principles of the present invention; and  
         [0013]    [0013]FIG. 5 depicts an exemplary frequency response curve for data channels having guard bands operating under the North American DOCSIS standard in relationship to the SAW filter of the cable modem of FIG. 1, and in accordance with the principles of the present invention.  
         [0014]    To facilitate understanding, identical reference numerals have been used, whenever possible, to designate identical elements that are common to the figures. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    While the invention will be primarily described within the context of a cable modem in a data communications system, it will be appreciated by those skilled in the art that other multi-mode/multi-standard, bi-directional communications devices, such as a satellite terminal, a digital subscriber line (DSL) modem, and the like may also incorporate and benefit from the present invention. Providing a single cable modem that could operate under multiple standard systems, such as the North American and European DOCSIS standard systems, would reduce the overall costs for the manufacturers, resellers, and renters, through the economy of scale.  
         [0016]    In accordance with one embodiment of the invention, a cable modem includes a single diplexer, which is used to facilitate the coupling of, for example, a computer device to a service provider via a cable transport network. In particular, the exemplary cable modem is utilized to provide downstream broadband data signals from the service provider to the computer device. Additionally, the exemplary cable modem is utilized to transfer upstream baseband data signals from the illustrative computer back to the service provider. More specifically, the exemplary cable modem is capable of selectively operating within the different downstream bandwidth constraints under both the North American DOCSIS and the European DOCSIS standards, which are incorporated by reference herein in their respective entireties. The cable modem of the instant embodiment is also capable of selectively passing through the upstream data signals in compliance with the European DOCSIS standard, as well as passing through a substantial portion of the upstream data signals in compliance with the North American DOCSIS standard.  
         [0017]    [0017]FIG. 1 depicts a block diagram of a data communications system  100  having a multi-mode bidirectional communications device  102 , the device being a cable modem in the instant illustrated embodiment. The data communications system  100  comprises a service provider  160  that provides electronically transmitted, digital data to an end user having an input/output (I/O) device  104 , such as a computer, handheld device, laptop, or any other device capable or transmitting and/or receiving data. The service provider  160  is coupled to the multi-mode bi-directional communications device (e.g., cable modem)  102  via a cable transport network  150 .  
         [0018]    The service provider  160  may be any entity capable of providing low, medium and/or high-speed data transmission, multiple voice channels, video channels, and the like. In particular, data is transmitted via radio frequency (RF) carrier signals by the service provider  160  in formats such as the various satellite broadcast formats (e.g., Digital Broadcast Satellite (DBS)), cable transmission systems (e.g., high definition television (HDTV)), digital video broadcasting ((DVB-C) (i.e., European digital cable standard)), and the like.  
         [0019]    The service provider  160  provides the data over the cable transport network  150 . In one embodiment, the cable transport network  150  is a conventional bidirectional hybrid fiber-coax cable network, such as specified under the North American or European DOCSIS standards.  
         [0020]    In operation, the service provider  160  modulates the downstream data signals with an RF carrier signal, and provides such signals via the cable transport network  150  to the cable modem  102 , where the RF signals are received, tuned, and filtered to a predetermined intermediate frequency (IF) signal. The IF signal is then demodulated into one or more respective baseband signals, and otherwise processed into, illustratively, data packets. The data packets are further transmitted, illustratively, through cabling  105  (e.g., universal serial bus (USB), coaxial cable, and the like) to the computer device  104 . Similarly, a user of the computer device  104  may send upstream data signals to the cable modem  102  via the cabling  105 . The cable modem  102  receives upstream baseband data signals from the computer device  104 , and then modulates and upconverts the data signals onto a RF carrier for transmission back to the service provider  160 , via the cable transport network  150 .  
         [0021]    The cable modem  102  comprises a diplexer  130 , upstream processing circuitry  106 , downstream processing circuitry  108 , a controller  126 , and a media access controller (MAC)  124 . The diplexer  130  communicates data between the cable transport network  150  and the cable modem  102 . The diplexer  130  comprises a high-pass filter  132  and a low-pass filter  134 . The high-pass filter  132  provides processed downstream RF signals to the computer device  104 . In particular, RF signals having a frequency greater than, e.g.,  106  MHz are passed through, while those frequencies below  106  MHz are attenuated.  
         [0022]    The diplexer  130  is coupled to the upstream  106  and downstream  108  processing circuitry. The high-pass filter HPF  132  passes the downstream data signals to the downstream processing circuitry  108  and the low-pass filter LPF  134  receives return signals (e.g., user requests) from the upstream processing circuitry  106 . As discussed in further detail below, both the HPF  132  and the LPF  134  may be utilized during operation under the European DOCSIS standard or during operation under the North American DOCSIS standard.  
         [0023]    Support circuitry  115 , such as voltage regulators, amplifiers, and the like, supports the operation of the downstream  108  and upstream  106  processing circuitry, as well as other circuitry in the cable modem  102 . Additionally, the controller  126  may be an embedded micro-controller that controls the modulator  110 , demodulator  118 , and the MAC  124 .  
         [0024]    The downstream processing circuitry  108  comprises a tuner  112 , a multi-rate demodulator  118 , which is selectively coupled to the tuner  112  through an 8 MHz bandwidth surface acoustic wave (SAW) filter  114 . The tuner  112  may illustratively be model type DIT9310, manufactured by Thomson Multimedia, Inc., of Indianapolis, Ind. When operating under the European DOCSIS mode, the SAW filter  114  provides an IF signal having an 8 MHz bandwidth to the multi-rate demodulator  118 , which operates within the requirements under the ITU J.83 Annex A standard. Alternatively, when operating in the North American DOCSIS mode, the SAW filter  114  provides an IF signal having a 8 MHz bandwidth to the demodulator  118 , which then operates within the requirements under the ITU J.83 Annex B standard. The multi-rate demodulator  118  may be model type BCM3350, manufactured by Broadcom Inc., of Irvine, Calif.  
         [0025]    The downstream processing circuitry  108  selectively tunes, demodulates, and otherwise “receives” at least one of a plurality of downstream data signals in response to a selection signal provided by the controller  126 . The diplexer  130  passes all downstream data signals above 106 MHz to the tuner  112  via the high-pass filter HPF  132 . The tuner  112  downconverts the received downstream RF signals from the HPF  132  to a predetermined IF frequency signal. The tuner  112  passes the IF frequency signal to the demodulator  118  via the 8 MHz SAW filter  114 . The filtered IF signals are demodulated by the demodulator circuitry  118  to provide one or more respective baseband signals. The baseband signals are sent to the MAC  124 , where the received signals are packetized into a bitstream, as discussed in further detail below.  
         [0026]    When operating under the North American DOCSIS standard, the SAW filter  114  provides a 36.125 MHz centered IF signal having a 8 MHz bandwidth to the demodulator  118 , where the demodulator  118  extracts the baseband signal(s) therein. Similarly, when operating under the European DOCSIS standard, the SAW filter  114  provides a 36.125 MHz centered IF signal having an 8 MHz bandwidth to the demodulator  118 , where the demodulator  118  extracts the baseband signal(s) therein. In any case, the baseband signals are sent to the media access controller (MAC)  124  for subsequent transport to the computer device, as managed by controller  126 .  
         [0027]    The baseband signals are illustratively formed into packets (e.g., MPEG elementary stream packets). The media access controller  124 , controller  126 , and other digital circuitry may further process the packetized data (e.g., attach or encapsulate in appropriate transport packets) and then distribute the processed, packetized data to the computer device  104  (or other information appliance). In particular, the MAC  124  sends the packetized bitstream to the controller  126 , where the data is processed (e.g., formatted) for interface with the computer device  104 . The controller  126  transfers the formatted packetized bit stream (via cabling  105 ) to the computer device  104  for further processing (e.g., extraction and upconversion of the data).  
         [0028]    The upstream processing circuitry  106  comprises a modulator  110  and other support circuits  115 , such as amplifiers, filters, voltage regulators, and the like (not shown). The modulator  110  modulates upstream signals from the computer device  104  for subsequent transmission to the service provider  160 . In particular, a user sends data, data requests, or some other user request to the service provider. The user request is up converted and modulated to an upstream RF signal. In one embodiment, the multi-rate demodulator  118 , modulator  110 , and MAC  124  may be physically integrated in one ASIC. In the alternative, separate components may also be utilized, as would be readily apparent to those skilled in the art.  
         [0029]    The controller  126  is coupled to memory  128 , which stores executable programs that control the cable modem  102 . The memory  128  includes non-volatile memory, such as an EEPROM, and may include volatile memory such as RAM and cache memory, as required. The memory  128  stores program code, which provides a method  300  for detecting the type of downstream signals received from the cable transport network  150 . In particular, the method  300  performs a search operation to determine if a tuned downstream signal falls under the European DOCSIS standard or North American DOCSIS standard. Once the type of downstream signal is detected, the demodulator  118  is set to further process the downstream signal (i.e., extract the baseband signals).  
         [0030]    Additionally, once the downstream signal is identified, the modulator  110  can also be set to the Cable Modem Termination System (CMTS) specified signal type to provide upstream signals. It is noted that the CMTS is an element of the DOCSIS standards that provides a set of specifications for high-speed data transfer over cable television systems. The CMTS specified signal type dictates, for example, the multiplexing technique (e.g., TDMA, CDMA, among others), compression technique (e.g., QPSK) symbol rate, and other parameters for setting the modulator  110 .  
         [0031]    [0031]FIG. 2 depicts a graphical representation of a response curve  200  for the diplexer  130  of FIG. 1. The response curve  200  comprises an ordinate  202  and an abscissa  204 . The ordinate  202  represents insertion loss (plotted in decibels (dB)), and the abscissa  204  represents frequency (plotted in megahertz (MHz)).  
         [0032]    Referring to FIGS. 1 and 2 together, it can be seen that the high-pass filter HPF  132  passes RF data signals having a frequency greater than 106 MHz, as shown by HPF response curve  206 . Under the North American DOCSIS standard, the downstream data signals are transmitted at a center frequency greater than 90 MHz (HPF response curve  208  (shown in phantom)). Specifically, the downstream signal is 6 MHz wide, such that the HPF  132  is capable of passing frequencies of at least 87 MHz, as shown as the corner frequency  214  of FIG. 2.  
         [0033]    Under the European DOCSIS standard, the downstream data signals are transmitted at a center frequency greater than 110 MHz. Specifically, the downstream signal is 8 MHz wide, such that the HPF  132  is capable of passing frequencies of at least 106 MHz, as shown as the corner frequency  216  of FIG. 2. In one embodiment of the invention, only a single high-pass filter HPF  132  is utilized in the diplexer  130 . Specifically, the HPF  132  passes RF data signals above a center frequency of 110 MHz. Under the North American DOCSIS standard, data signals having center frequencies below 110 MHz will not be passed to the demodulator  118 . However, most North American and European cable operators use frequencies higher than 106 MHz, so those signals having center frequencies less than 110 MHz are of little consequence to an end user.  
         [0034]    The North American cable operators usually put analog video signals at low frequencies (e.g., 54-300 MHz), since cable plants typically have better signal to noise ratios and less ingress at lower frequencies. It is noted that the number of analog signals varies from cable plant to cable plant. Analog video signals are more susceptible to these channel impairments than cable modem signals. As such, cable operators usually put the cable modem downstream data channel at higher frequencies (i.e., above 106 MHz). Specifically, digital signals are usually added at higher frequencies (e.g., above 300 MHz), since the higher frequencies are not used, and are available for channel formation. Further, since almost all digital downstream RF signals are above 106 Mhz under either the North American and European DOCSIS standards, the single HPF  132  is suitable for passing through such downstream RF data signals for further processing in the cable modem  102 . The HPF digital response curve  206  in FIG. 2 illustratively depicts a low-level insertion loss  302  for frequencies greater than 106 MHz.  
         [0035]    [0035]FIG. 4 depicts a frequency response curve  400  for data channels  402  operating in compliance with the North American DOCSIS standard in relationship to a SAW filter  114  of the cable modem  102  of FIG. 1. It is noted that the downstream to data channel is 8 MHz wide for the European DOCSIS standard, while the North American DOCSIS standard is 6 MHz wide. In prior art cable modems, a different SAW filter was used under each DOCSIS standard to maximize performance when additional channels are located next to the desired channel without any guard bands. In particular, a 6 MHz SAW filter was used under the North American DOCSIS standard, while an 8 MHz SAW filter was used under the European DOCSIS standard. In the embodiment of the cable modem  102  described herein, only a single 8 MHz SAW filter is utilized since the SAW filter has to be at least as wide as the largest bandwidth signal for the cable modem  102  to operate properly. Once the high-pass filter HPF  132  passes data signals above 106 MHz, as noted above with regard to FIG. 1, the SAW filter  114  provides a centered IF signal having an 8 MHz bandwidth to the demodulator  118 .  
         [0036]    Referring to FIG. 4, a desired data channel  404  having a bandwidth of 6 MHz under the North American DOCSIS standard is shown. Additionally, two adjacent data channels  406  and  408  also having 6 MHz bandwidths may also be present. For example, the desired channel selected by a user may have a center frequency of 120 MHz. The adjacent channels  406  and  408 , if present, will respectively have center frequencies of 114 MHz and 126 MHz (without guard bands disposed therebetween). As further illustrated by curve  410  in FIG. 4, the 8 MHz SAW filter passes the entire 6 MHz wide signal of the desired channel  404 , plus a 1 MHz signal portion of each adjacent channel  406  and  408 . The additional two MHz of data signals from the adjacent channels  406  and  408  may degrade the performance of the cable modem  102 . For example, if the total signal power is increased, additional adjacent channel power can distort active devices (e.g., demodulator  118 ) after the SAW filter  114  in the downstream processing circuitry  108 . Alternatively, if the total power is kept constant, then the desired signal will not use the full range of the analog to digital converter (not shown) in the demodulator  118 .  
         [0037]    [0037]FIG. 5 depicts a frequency response curve  500  for data channels having guard bands  502  operating under the North American DOCSIS standard in relationship to the SAW filter  114  of the cable modem of FIG. 1. A service provider  160  may provide 1 MHz guard bands  502  between the desired data channel  404  and adjacent channels  406  and  408 , thereby minimizing degradation of performance for the desired channel  404  caused by adjacent channels  406  and  408 .  
         [0038]    [0038]FIG. 3 depicts a flowchart of a method  300  for detecting a downstream signal using the cable modem  102  of FIG. 1. In particular, the method  300  tunes the downstream data channel to a specified frequency, and then both European and North American 64 QAM and 256 QAM signal acquisitions are attempted before tuning to the next frequency.  
         [0039]    The illustrative method  300  is optimized for the European DOCSIS mode of operation. In one embodiment, the method  300  searches for the downstream signal from a plurality of preset frequency channels first. If the downstream signal is not found in any of the preset channels, the method  300  searches for the downstream signal from a plurality of CCIR (Consultative Committee for International Radio) frequency channels. If the downstream signal is not found in any of the CCIR channels, the method  300  then searches for the downstream signal from a plurality of UK (United Kingdom) frequency channels.  
         [0040]    In particular, the method  300  starts at step  301 , and proceeds to step  302  where a counter is set equal to zero. At step  304 , the method  300  determines whether a scan list is present. Specifically, in one embodiment, the cable modem illustratively stores a scan list having a plurality of preset channels (e.g., ten preset channels). The preset channels may be, for example, unusual vendor channels that take a long time to find. The ten illustrative frequency settings are stored in the memory  128  (e.g., EEPROM). If the determination at step  304  is negatively answered, then the method  300  proceeds to step  318 , as discussed in detail below.  
         [0041]    If the determination at step  304  is affirmatively answered, then at step  306 , the tuner  112  is tuned to, illustratively, a first of a plurality of the scan list (i.e., predetermined) channel frequencies. That is, the cable modem  102  sequentially checks each of these ten preset channels prior to checking any other type channels (e.g., the European CCIR and UK channels or the North American IRC and HRC channels, and the like), since the preset channels may be frequently utilized. After tuning to a first channel frequency at step  306 , the method  300  at step  308 , searches for both 64 QAM and 256 QAM signal acquisition of an 8 MHz wide, European DOCSIS standard (Annex A) downstream signal. If at step  310 , the downstream signal is acquired, the method  300  proceeds to step  362 , where the upstream signal parameters are acquired.  
         [0042]    At step  362 , once the downstream signal is acquired, the upstream processing circuitry  106  must determine the appropriate power signal, modulation scheme, among other upstream parameters for transmitting information upstream. In particular, the modulator  110  sets the CMTS specified channel frequency in order to enable modulation of the baseband signals sent upstream from the computer device  104 . The method  300  then proceeds to step  364 , where the method  300  ends.  
         [0043]    If, at step  310 , an 8 MHz wide, Annex-A downstream signal has not been detected under the European standard for that tuned preset channel, then the method  300  proceeds to step  312 . At step  312 , the same channel frequency (e.g., preset channel) is checked under the North American DOCSIS standard (Annex-B). That is, the tuner  112  is kept at the same channel frequency and both 64 QAM and 256 QAM signal acquisition is attempted for a 6 MHz wide, Annex-B downstream signal. At step  314 , if a 6 MHz, North American QAM signal is detected, then the method  300  proceeds to step  362 , where the demodulator  118  and modulator  110  are set to enable further processing of the baseband signals, as discussed above.  
         [0044]    If, however, at step  314 , one of the North American standard QAM signals is not detected, at step  316 , the method  300  queries whether the tuned channel frequency is the last of its type to be searched. If the illustrative last preset channel frequency has not been searched, then the method  300  proceeds to step  306 , where the tuner  112  is sequentially tuned to the next preset channel frequency. Steps  306  through  316  are sequentially repeated for each preset channel in the scan list, until either a downstream signal is acquired from one of the preset channels (and the method proceeds to step  362 ), or at step  316 , the last preset channel is searched without acquiring the downstream signal. If at step  316 , the last preset channel is searched without acquiring the downstream signal, the method  300  proceeds to step  318 .  
         [0045]    It is noted that this technique of first tuning to a particular channel frequency, then checking for signal acquisition of either 64 or 256 QAM signals for both Annex-A and Annex-B on that same tuned channel, prior to tuning to a next channel, is performed throughout the method  300  for all the types of channels (e.g., CCIR, UK, among others). As such, the method  300  minimizes search time by only tuning the tuner  112  once for both downstream data channel modes.  
         [0046]    At step  318 , the tuner  112  tunes to a first CCIR channel. Presently, any one of 94 CCIR channels may carry a downstream signal. In particular, once the tuner  112  tunes a CCIR channel frequency (e.g. starting with the lowest frequency channel), at step  320 , a search is performed to acquire a 64 or 256 QAM, 8 MHz wide, downstream signal under the European (Annex A) DOCSIS standard. That is, both 64 QAM and 256 QAM signal acquisition is attempted at the currently tuned channel, before tuning to the next channel frequency.  
         [0047]    At step  322 , if a 8 MHz, European QAM signal is detected for that tuned CCIR channel, then the method  300  proceeds to step  362 , where the demodulator  118  and modulator  110  are set to enable further processing of the baseband signals, as discussed above. If at step  322 , an 8 MHz wide, Annex-A downstream signal has not been detected under the European standard for that tuned CCIR channel, then the method  300  proceeds to step  324 .  
         [0048]    At step  324 , the same channel frequency (e.g., CCIR channel) is checked under the North American DOCSIS standard (Annex-B). That is, the tuner  112  is kept at the same channel frequency and both 64 QAM and 256 QAM signal acquisition is attempted for a 6 MHz wide, Annex-B downstream signal. At step  326 , if a 6 MHz, North American QAM signal is detected, then the method  300  proceeds to step  362 , where the demodulator  118  and modulator  110  are set to enable further processing of the baseband signals, as discussed above.  
         [0049]    If, however, at step  326 , one of the North American standard QAM signals is not detected, at step  328 , the method  300  queries whether the tuned channel frequency is the last of its type to be searched. If the illustrative last CCIR channel frequency has not been searched, then the method  300  proceeds to step  318 , where the tuner  112  is sequentially tuned to the next CCIR channel frequency. Steps  318  through  328  are sequentially repeated for each CCIR channel, until either a downstream signal is acquired from one of the CCIR channels (and the method proceeds to step  362 ), or at step  328 , the last CCIR channel is searched without acquiring the downstream signal. If at step  328 , the last CCIR channel is searched without acquiring the downstream signal, the method  300  proceeds to step  330 .  
         [0050]    At step  330 , the tuner  112  tunes to a first UK (United Kingdom) channel. Presently, any one of  94  CCIR channels may carry a downstream signal. In particular, once the tuner  112  tunes a CCIR channel frequency, at step  332 , a search is performed to acquire a 64 or 256 QAM, 8 MHz wide, downstream signal under the European (Annex A) DOCSIS standard. That is, both 64 QAM and 256 QAM signal acquisition is attempted at each channel, before tuning to the next channel frequency.  
         [0051]    At step  334 , if a 8 MHz, European QAM signal is detected for that tuned UK channel, then the method  300  proceeds to step  362 , where the demodulator  118  and modulator  110  are set to enable further processing of the baseband signals, as discussed above. If at step  334 , an 8 MHz wide, Annex-A downstream signal has not been detected under the European standard for that tuned UK channel, then the method  300  proceeds to step  336 .  
         [0052]    At step  336 , the same channel frequency (e.g., UK channel) is checked under the North American DOCSIS standard (Annex-B). That is, the tuner  112  is kept at the same channel frequency and both 64 QAM and 256 QAM signal acquisition is attempted for a 6 MHz wide, Annex-B downstream signal. At step  338 , if a 6 MHz, North American QAM signal is detected, then the method  300  proceeds to step  362 , where the demodulator  118  and modulator  110  are set to enable further processing of the baseband signals, as discussed above.  
         [0053]    If, however, at step  338 , one of the North American standard QAM signals is not detected, at step  340 , the method  300  queries whether the tuned channel frequency is the last of its type to be searched. If the illustrative last UK channel frequency has not been searched, then method  300  proceeds to step  330 , where the tuner  112  is sequentially tuned to the next UK channel frequency. Steps  330  through  340  are sequentially repeated for each UK channel, until either a downstream signal is acquired from one of the UK channels (and the method proceeds to step  362 ), or at step  340 , the last UK channel is searched without acquiring the downstream signal. If at step  340 , the last UK channel is searched without acquiring the downstream signal, the method  300  proceeds to step  342 .  
         [0054]    At step  342 , the counter is incremented by one, which signifies a first pass through steps  304  to  340 . In one embodiment, steps  304  through  340  are repeated two times until either a downstream signal is detected or the counter is incremented to a value of two. Specifically, at step  342 , if a downstream data signal is not detected after a first pass though all the preset, CCIR, or UK channel frequencies, the method  300  performs a second pass through the preset, CCIR and UK channel frequencies as discussed above for steps  304  through  340 .  
         [0055]    Method  300  performs the second pass over each of the channels to provide the cable modem  102  a second opportunity to identify the downstream signal and to improve efficiency of the cable modem  102 . Such second opportunity may be necessary, for example, if a downstream, non cable modem related interruption occurs, (e.g., an intermittency caused at the head-end, a brown-out, or other downstream interruption), which may cause the cable modem  102  to lose the downstream signal.  
         [0056]    If, at any of the steps  310 ,  314 ,  322 ,  326 ,  334 , or  338 , one of the channel frequencies is detected during the second pass of steps  304  through  340 , the method  300  proceeds to step  362  as discussed above. If, however, none of the channel frequencies is detected during the second pass of steps  304  through  340 , and at step  344  the counter equals two (counter=2), method  300  proceeds to step  346 .  
         [0057]    At steps  346 , the tuner  112  is tuned to first of a plurality of frequencies (i.e., center frequencies) illustratively between 110 MHz and 862 Mhz, in incremental steps (e.g., 375 KHz steps). In particular, at step  346 , the tuner  112  is tuned to the first frequency (e.g., 110 MHz). At step  348 , both 64 QAM and 256 QAM signal acquisition is attempted first for a downstream European 8MHz, Annex-A signal. At step  350 , if the European 8 MHz, Annex-A signal is detected, then the method  300  proceeds to step  362  as discussed above.  
         [0058]    If at step  350 , the European 8 MHz, Annex-A signal is not detected, then at step  352 , signal acquisition for a North American 6 MHz Annex-B signal is attempted for the first tuned frequency. At step  354 , if the North American 6 MHz Annex-B signal is detected, then the method  300  proceeds to step  362  as discussed above. If at step  354 , the North American 6 MHz Annex-B signal is not detected, then the method  300  proceeds to step  356 .  
         [0059]    At step  356 , a query is made to determine if the tuned channel is the last channel to be searched. If, at step  356  the query is negatively answered, then method  300  proceeds to step  358 , where the tuner  112  is incremented to the next frequency. For example, the tuner is tuned to 110.375 MHz (i.e., 110 MHz+incremental 375 KHz). Steps  348  through  358  are repeated, until either a downstream signal is acquired, or at step  356 , the last channel has been searched. If at step  356 , the last channel (e.g., 862 MHz) has been searched, then the method  300  proceeds to step  301  where the entire method  300  is repeated until a downstream signal is acquired. Once a downstream signal has been acquired, and at step  362 , the upstream parameters are acquired, the method  300  ends at step  364 .  
         [0060]    Method  300  has been discussed in terms of optimizing the cable modem to search for known European channels, prior to methodically stepping through the frequency spectrum between 110 MHz and 862 MHz i.e., (steps  346 - 358 ), which is time consuming. It is noted that in another embodiment, the cable modem  102  may be optimized to search through North American DOCSIS by changing the order of acquisition to 6 MHz Annex-B channels first instead of the 8 MHz Annex-A channels as described in method  300 .  
         [0061]    In even another embodiment, the cable modem  102  may be further optimized to search through North American (e.g., harmonically related carrier (HRC) and incrementally related carrier (IRC)) channels instead of the CCIR or UK channels. That is, step  318  would include sequentially tuning through the 125 IRC channels, and step  330  would include sequentially tuning through the 125 HRC channels. Of course, detection for both 64 and 256 QAM downstream signals under the Annex-A and B standards would be performed at each tuned frequency, prior to tuning to the next IRC or HRC channel.  
         [0062]    In still another embodiment, method  300  could be designed to check European and North American frequencies evenly. Specifically, the method  300  would check for 64 and 256 QAM 8 MHz Annex-A, and then the 64 and 256 QAM 6 MHz Annex-B signal acquisitions for each of the preset channels (e.g., 10 preset channels). Next, the 94 CCIR channels would be checked in a similar manner, followed by the 125 IRC channels, the 94 UK channels, and the 125 HRC channels. If a downstream signal cannot be acquired, the method would search a second time through the known preset, CCIR, IRC, UK, and HRC channels, such as discussed at steps  342  and  344 . If a downstream signal cannot be acquired on the second pass through the known channels, then the method would proceed to check through all frequencies between 110 MHz and 862 MHz, as discussed at steps  346  through  358  or method  300 .  
         [0063]    As discussed above, the modem  110  settings are not set by the cable modem  102  until the downstream signal is detected and the DOCSIS standard (i.e., Annex A or B) is identified. Once the DOCSIS standard is known, the modulator  110  can be set to provide upstream signals back to the service provider  160 .  
         [0064]    In one embodiment of the invention, the cable modem  102  is provided with a single 65 MHz low-pass filter  134 , which may be used for upstream signals under both North American and European DOCSIS standards. In particular, a single 65 MHz low-pass filter  134  passes through the upstream data signals having a frequency between 5 Mhz and 65 MHz, as required under the European DOCSIS standard. The 65 MHz low-pass filter  134  may also be utilized under the North American DOCSIS standard, so that the additional bandwidth between 42 MHz and 65 MHz is available. Under the North American DOCSIS standard, the upstream data signals are typically transmitted in a frequency range between 5 Mhz and 42 MHz.  
         [0065]    Referring to FIG. 2, the LPF response curve  210  (drawn in phantom) illustratively depicts a low-level insertion loss  202  for frequencies less than 42 MHz when operating under the North American DOCSIS standard. Under the European DOCSIS standard, the upstream data signals are transmitted in a frequency range between 5 MHz and 65 MHz. The LPF response curve  212  of FIG. 2 illustratively depicts a low-level insertion loss  202  for frequencies less than 65 MHz when operating under the European, as well as the North American DOCSIS standards. As such, when the cable modem  102  is operating under the North American standard, the extra bandwidth availability (42-65 MHz) may be utilized to relieve a congested upstream path in instances where many users are actively accessing the upstream path.  
         [0066]    Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.