Abstract:
In a Hybrid Fiber Coaxial (HFC) network using a Set Top Box (STB) or a cable modem, the cable STB or the cable modem includes respective High-Pass Filters (HPFs) having different passband frequencies to transmit an upstream signal from the cable STB or the cable modem to the HFC network through paths having different passband frequencies depending on states of the HFC network so that the upstream frequency band of 5 to 42 MHz is available.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for CABLE MODEM AND FILTERING METHOD BASED ON FREQUENCY BAND IN THE SAME earlier filed in the Korean Intellectual Property Office on 30 Jan. 2004 and there duly assigned Serial No. 2004-6307. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cable modem including filtering based on a frequency band in the cable modem and, more particularly, to a cable modem including filtering based on a frequency band in the cable modem in which an upstream signal is transmitted via respective different Hybrid Fiber Coaxial (HFC) transmission paths embedded in the cable modem or a cable Set Top Box (STB). 
     2. Description of the Related Art 
     A cable modem transmits an upstream signal to a Cable Modem Termination System (CMTS) and receives a downstream signal from the CMTS over an HFC network. 
     A cable modem or a cable STB transmits and receives data and a video signal using the HFC network. The performance of the entire HFC network depends on communication between the CMTS/head end equipment and the cable modem/cable STB. An upstream signal transmitted by the cable modem or the cable STB plays the most important role. 
     The following patents each discloses features in common with the present invention but do not teach or suggest the inventive features specifically recited in the present application: U.S. patent application Ser. No. 2003/0012271 to McReynolds et al., entitled  MULTI - MODE BI - DIRECTIONAL COMMUNICATIONS DEVICE INCLUDING A DIPLEXER HAVING SWITCHABLE LOWPASS FILTERS , published on Jan. 16, 2003; U.S. patent application Ser. No. 2002/0178454 to Antoine et al., entitled  BROADCAST TELEVISION AND SATELLITE SIGNAL SWITCHING SYSTEM AND METHOD FOR TELEPHONY SIGNAL INSERTION , published on Nov. 28, 2002; U.S. patent application Ser. No. 2002/0023273 to Song, entitled  APPARATUS FOR PROVIDING A MULTIPLE INTERNET CONNECTION SERVICE USING A HYBRID FIBER COAXIAL CABLE NETWORK , published on Feb. 21, 2002; U.S. patent application Ser. No. 2003/0022631 to Rhodes et al., entitled  MULTI - MODE BIDIRECTIONAL COMMUNICATIONS DEVICE INCLUDING A DIPLEXER HAVING A SWITCHABLE NOTCH FILTER , published on Jan. 30, 2003; U.S. patent application Ser. No. 2002/0176524 to Popper et al., entitled  INGRESS NOISE REDUCTION IN A DIGITAL RECEIVER , published on Nov. 28, 2002; U.S. patent application Ser. No. 2002/0049038 to Sorrells et al., entitled  WIRELESS AND WIRED CABLE MODEM APPLICATIONS OF UNIVERSAL FREQUENCY TRANSLATION TECHNOLOGY , published on Apr. 25, 2002; U.S. patent application Ser. No. 2003/0033608 to Chang et al., entitled  BTI RF MODULE WITH FILTERING , published on Feb. 13, 2003; U.S. patent application Ser. No. 2003/0046706 to Rakib, entitled  ACTIVE CABLE MODEM OUTSIDE CUSTOMER PREMISES SERVICING MULTIPLE CUSTOMER PREMISES , published on Mar. 6, 2003; U.S. patent application Ser. No. 2003/0208775 to Roberts et al., entitled  SYSTEM, METHOD AND APPARATUS FOR COORDINATION OF CHANNEL QUALITY ASSESSMENT AND INGRESS FILTERING IN CABLE MODEM SYSTEMS , published on Nov. 6, 2003; U.S. patent application Ser. No. 2003/0066088 to Jung, entitled  BIDIRECTIONAL TRUNK AMPLIFIER AND CABLE MODEM FOR CABLE HYBRID FIBER AND COAX NETWORK WHICH UTILIZES AN UPSTREAM PILOT SIGNAL , published on Apr. 3, 2003. 
     SUMMARY OF THE INVENTION 
     An object of the present invention to provide a cable modem and filtering method based on a frequency band in the cable modem in which upstream signals are transmitted through respective different HPF transmission paths embedded in the cable modem or cable STB. 
     According to an aspect of the present invention for achieving the aforementioned object, a cable modem is provided comprising: a Central Processing Unit (CPU) adapted to output a control signal to perform different high-pass filtering depending on frequency bands of upstream signals transmitted to a Hybrid Fiber Coaxial (HFC) network; a multiplex High-Pass Filter (HPF) adapted to filter the inputted upstream signals through different paths based on their respective frequency bands, the multiplex HPF including a plurality of HPFs having different passband frequencies; and a High-Pass Filter (HPF) selection unit adapted to select one of the plurality of HPFs included in the multiplex HPF in accordance with the control signal to pass the upstream signals through the different paths. 
     The cable modem can further comprise an upstream signal control unit adapted to receive the upstream signals from the CPU and to adjust amplitudes of the received upstream signals and to output the resultant upstream signals. 
     The cable modem can further comprise a transformer adapted to receive an output signal of the upstream signal control unit and to isolate a next stage from the upstream signal control unit. 
     The cable modem can further comprise a Low-pass Filter (LPF) adapted to low-pass filter the upstream signals filtered by the HPF, and to send the resultant filtered signals to the HFC network, the HPF being selected by the HPF selection unit. 
     The cable modem can further comprise a tuner adapted to transmit the upstream signals to the HFC network, the high-frequency components of the upstream signals having been removed by the LPF. 
     The CPU is adapted to output different control signals according to respective frequency bands of the upstream signals. 
     The HPF selection unit includes a transistor adapted to be turned on or off in accordance with the control signal of the CPU, and a relay adapted to switch a connection to a corresponding HPF of the multiplex HPF in response to the on or off operation of the transistor. 
     The multiplex HPF includes a first HPF adapted to pass a frequency of at least 10 MHz to high-pass filter upstream signals having a frequency between 10 MHz and 20 MHz. 
     The multiplex HPF includes a second HPF adapted to pass a frequency of at least 20 MHz to high-pass filter upstream signals having a frequency of at least 20 MHz. 
     The HPF selection unit is adapted to bypass the upstream signals so that upstream signals having a frequency between 5 MHz and 10 MHz are not filtered by the multiplex HPF. 
     The cable modem can further comprise a control gate adapted to be turned on or off by the control signal outputted from the CPU when transmitting the upstream signals, the control gate being in an on state only upon transmitting the upstream signals to the HFC network. 
     The cable modem can further comprise a capacitor adapted to remove a DC current component flowing into the HPF. 
     The cable modem can further comprise a splitter connected to the multiplex HPF and adapted to isolate the delivered signals from each other. 
     According to an aspect of the present invention for achieving the aforementioned object, a filtering method is provided comprising: determining a transmission frequency band of upstream signals received from a Hybrid Fiber Coaxial (HFC) network, the upstream signals being transmitted to the HFC network; outputting a control signal to perform different high-pass filtering depending on the respective transmission frequency band of the upstream signals; selecting a high-pass filtering path corresponding to the frequency band from among a plurality of different high-pass filtering paths in response to the control signal; and filtering the upstream signal with the selected high-pass filtering path and transmitting the resultant signal to the HFC network. 
     The frequency bands for upstream transmission comprise: 5 MHz to 10 MHz, 10 MHz to 20 MHz, and 20 MHz to 42 MHz. 
     According to an aspect of the present invention for achieving the aforementioned object, a filtering method is provided comprising: outputting a control signal to perform different high-pass filtering depending on frequency bands of upstream signals transmitted to a Hybrid Fiber Coaxial (HFC) network; filtering the inputted upstream signals through different paths based on their respective frequency bands with a multiplex High-Pass Filter (HPF) including a plurality of HPFs having different passband frequencies; and passing the upstream signals through the different paths by selecting one of the plurality of HPFs included in the multiplex HPF in accordance with the control signal. 
     The method can further comprise receiving the upstream signals and adjusting amplitudes of the received upstream signals and outputting the resultant upstream signals. 
     The method can further comprise receiving an output signal of an upstream signal control unit and isolating a next stage from the upstream signal control unit with a transformer. 
     The method can further comprise low-pass filtering the upstream signals filtered by the HPF and sending the resultant filtered signals to the HFC network. 
     The method can further comprise removing the high-frequency components of the upstream signals and transmitting the resultant upstream signals to the HFC network. 
     The method can further comprise outputted different control signals according to respective frequency bands of the upstream signals. 
     The method can further comprise turning a transistor on or off in accordance with the control signal and switching a connection to a corresponding HPF of the multiplex HPF in response to the on or off operation of the transistor. 
     The method can further comprise passing a frequency of at least 10 MHz to high-pass filter upstream signals having a frequency between 10 MHz and 20 MHz with a first HPF of the multiplex HPF. 
     The method can further comprise passing a frequency of at least 20 MHz to high-pass filter upstream signals having a frequency of at least 20 MHz with a second HPF of the multiplex HPF. 
     The method can further comprise bypassing the upstream signals so that upstream signals having a frequency between 5 MHz and 10 MHz are not filtered by the multiplex HPF. 
     The method can further comprise turning a control gate on or off with the control signal when transmitting the upstream signal, the control gate being in an on state only upon transmitting the upstream signal to the HFC network. 
     The method can further comprise removing a DC current component flowing into the HPF. 
     The method can further comprise isolating the delivered signals outputted by the multiplex HPF from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments of the present invention with reference to the attached drawings in which: 
         FIG. 1  is a block diagram of a cable modem; 
         FIG. 2  is a block diagram of a cable modem according to an embodiment of the present invention; and 
         FIG. 3  is a block diagram of a cable modem according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of a cable modem. 
     Referring to  FIG. 1 , a cable modem  10  comprises a Central Processing Unit (CPU)  11 , an upstream signal control unit  12 , a transformer  13 , a Low-Pass Filter (LPF)  14 , a tuner  15 , and a 20 MHz High-Pass Filter (HPF)  16 . 
     The CPU  11  controls the entire operation of the cable modem  10 . It generates a first enable signal EN 1  and a second enable signal EN 2  and outputs data over a data bus. 
     The cable STB or cable modem will send an upstream signal to the HFC network to communicate with the CMTS or head end  20 . The CPU  11  of the cable STB or cable modem receives signal information that the CMTS or head end  20  transmits over the HFC network downstream, and determines the frequency of the upstream signal based on the received information. 
     That is, the frequency of the upstream signal is not determined by the cable STB or cable modem but is determined by the CMTS or head end  20  based on signal states on the HFC network. 
     In other words, if the CMTS or head end  20  sends a signal containing an upstream signal frequency and other information to the corresponding cable STB or cable modem  10 , the CPU  11  in the corresponding cable STB or cable modem  10  receives the signal to determine a frequency for upstream transmission. The CPU  11  uses the determined frequency as a frequency for transmission. 
     The upstream signal control unit  12  receives data as the upstream signal and adjusts gain of the signal under the control of the CPU  11 . 
     The transformer  13  receives an output signal from the upstream signal control unit  12 , and isolates the next stage from the upstream signal control unit  12 . 
     The LPF  14  receives an output signal from the transformer  13  to remove a high-frequency component from the output signal and to pass only a signal having a frequency of 42 MHz or less. That is, signals having a frequency of 42 MHz or more are removed by the LPF  14  since the upstream signal used in the modem employs only a 5 to 42 MHz frequency band. 
     The tuner  15  sends the upstream signal, the high-frequency component of which has been removed, to the HFC network. 
     The 20 MHz HPF  16  solves a noise problem in the upstream signal. The 20 MHz HPF  16  is adapted to block a 5 to 20 MHz frequency band and to improve the HFC network because the 5 to 20 MHz frequency band often includes noise in the HFC networks. In a typical case, therefore, a 5 to 20 MHz frequency band of the 5 to 42 MHz frequency band of the upstream signal is not conventionally used in the entire HFC network because that frequency band often includes noise. 
     A transmission unit of the cable modem shown in  FIG. 1  transmits the upstream signal to the CMTS over the HFC network. The upstream signal, data, has its amplitude adjusted by the upstream signal control unit  12  under the control of the CPU  11 . The signal transmitted from the cable modem  10  to the CMTS is not always constant in amplitude but is varied with commands from the CMTS. This is because the upstream signal can reach the CMTS only when the upstream signal is transmitted after being amplified in proportion to the distance between the cable modem and the CMTS. The transformer  13  serves to isolate the LPF  14  from the upstream signal control unit  12 . 
     The LPF  14  removes a high-frequency component of the upstream signal, which has been adjusted in amplitude by the upstream signal control unit  12 . The LPF  14  removes signals of a frequency equal to or greater than 42 MHz because the upstream signal employs only a 5 to 42 MHz frequency band. 
     However, since the HFC network using the cable STB or cable modem employs the 20 MHz HPF  16  positioned at a front stage of the cable STB or cable modem, it is impossible to use the 5 to 20 MHz frequency band in the entire HFC network regardless of whether or not that frequency band includes noise. 
     The upstream frequency band ranges from 5 to 42 MHz, which is not broad. Thus, use of the 20 MHz HPF results in blocking a 20 MHz band. This incurs a problem in that a bandwidth available for the frequency of the upstream signal becomes narrow. 
       FIG. 2  is a block diagram of a cable modem according to an embodiment of the present invention. 
     Referring to  FIG. 2 , the cable modem is composed of a CPU  31 , an upstream signal control unit  32 , a transformer  33 , a multiplex HPF  34 , an HPF selection unit  35 , an LPF  36  and a tuner  37 . 
     Although not shown, the multiplex HPF  34  consists of a first HPF and a second HPF. The first HPF passes a frequency band of 10 MHz or more, and the second HPF passes a frequency band of 20 MHz or more. 
     The HPF selection unit  35  selectively switches connections between the transformer  33  and the multiplex HPF  34 , and is controlled by the CPU  31 . That is, the HPF selection unit  35  performs a function of selecting a connection of either the first HPF or the second HPF included in the multiplex HPF  34  to the transformer  33 . The HPF selection unit  35  can make three connections between the transformer  33  and the multiplex HPF  34 ; a connection between the transformer  33  and the first HPF, a connection between the transformer  33  and the second HPF, and a connection in which the transformer  33  is neither connected to the first HPF nor to the second HPF so that the signal from the transformer is passed as is. 
     The LPF  36  receives an output signal from the multiplex HPF  34  and removes a high-frequency component from the output signal. It passes only signals having a frequency of 42 MHz or less. The LPF  36  remove signals having a frequency of 42 MHz or more since the upstream signal used in the modem employs only a 5 to 42 MHz frequency band. 
     The tuner  37  sends the upstream signal, the high-frequency component of which has been removed by the LPF  36 , to an HFC network. 
     When sending the upstream signal, the CPU  31  outputs upstream signal data and an enable signal to the upstream signal control unit  32 . The CPU  31  also outputs a control signal to control the HPF selection unit  35  depending on a frequency band for upstream transmission that the CMTS or head end  20  has transmitted. The frequency band for upstream transmission can be classified into three bands; less than 10 MHz (i.e., 5 MHz to 10 MHz), 10 MHz to 20 MHz, and 20 MHz or more (i.e., 20 MHz to 42 MHz). 
     When the CPU  31  transmits the upstream signal, the CPU  31  also outputs a corresponding control signal to the HPF selection unit  35  depending on the frequency band for upstream transmission that the CMTS or head end  20  has transmitted. 
     If the upstream transmission frequency that the CMTS or head end  20  has transmitted is less than 10 MHz, the CPU  31  outputs a first control signal to the HPF selection unit  35  so that the transformer  33  is neither connected to the first HPF nor to the second HPF of the multiplex HPF  34  and the signal from the transformer  33  is passed as is. By doing so, the upstream signal outputted from the transformer  33  is not filtered by the multiplex HPF  34  and is transmitted to the HFC network via the LPF  36  and the tuner  37 . 
     If the upstream transmission frequency that the CMTS or head end  20  has transmitted is 20 MHz or more, the CPU  31  outputs a second control signal to the HPF selection unit  35  so that the transformer  33  is connected to the second HPF of the multiplex HPF  34 . By doing so, the upstream signal outputted from the transformer  33  is filtered by the second HPF of the multiplex HPF  34  and is transmitted to the HFC network via the LPF  36  and the tuner  37 . 
     If the upstream transmission frequency that the CMTS or head end  20  has transmitted is between 10 MHz and 20 MHz, then the CPU  31  outputs a third control signal to the HPF selection unit  35  so that the transformer  33  is connected to the first HPF of the multiplex HPF  34 . By doing so, the upstream signal outputted from the transformer  33  is filtered by the first HPF of the multiplex HPF  34  and is transmitted to the HFC network via the LPF  36  and the tuner  37 . 
     When transmitting the upstream signal to the HFC network under the control of the CPU  31 , the upstream signal control unit  32  performs a function of adjusting the amplitude of the corresponding frequency of the upstream signal depending on the upstream transmission frequency transmitted by the CMTS or head end  20 . Thus, the upstream signal control unit  32  includes an Automatic Gain Control (AGC) module (not shown) to produce the upstream signal having a stabilized amplitude. 
     The transformer  33  receives the output signal of the upstream signal control unit  32  and isolates the next stage from the upstream signal control unit  32 . 
     If the upstream transmission frequency transmitted by the CMTS or head end is less than 10 MHz, then the CPU  31  outputs an upstream signal of the corresponding transmission frequency and an enable signal to the upstream control unit  32 . Accordingly, the upstream control unit  32  adjusts the amplitude of the upstream signal and outputs the resultant signal in response to the enable signal of the CPU  31 . The CPU  31  outputs a first control signal to the HPF selection unit  35 . Accordingly, the HPF selection unit  35  is operative to block the transformer  33  from being neither connected to the first HPF nor to the second HPF of the multiplex HPF  34  so that the upstream signal is passed without being filtering. Thus, the upstream signal outputted from the transformer  33  is not filtered by the multiplex HPF  34  and is transmitted via the LPF  36  and the tuner  37  to the HFC network. 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is 20 MHz and more, then the CPU  31  outputs an upstream signal of the corresponding transmission frequency and an enable signal to the upstream control unit  32 . The upstream control unit  32  adjusts the amplitude of the upstream signal in response to the enable signal of the CPU  31  and outputs the resultant signal. The CPU  31  also outputs a second control signal to the HPF selection unit  35 . In response thereto, the HPF selection unit  35  is operative to enable the transformer  33  to be connected to the second HPF of the multiplex HPF  34 . Thus, the upstream signal outputted from the transformer  33  is filtered by the second HPF of the multiplex HPF  34  and is thereafter transmitted to the HFC network via the LPF  36  and the tuner  37 . 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is between 10 MHz and 20 MHz, then the CPU  31  outputs an upstream signal of the corresponding transmission frequency and an enable signal to the upstream control unit  32 . The upstream control unit  32  adjusts the amplitude of the upstream signal in response to the enable signal of the CPU  31  and outputs the resultant signal. The CPU  31  also outputs a third control signal to the HPF selection unit  35 . In response thereto, the HPF selection unit  35  operates to allow the transformer  33  to be connected to the first HPF of the multiplex HPF  34 . Thus, the upstream signal outputted from the transformer  33  is filtered by the first HPF of the multiplex HPF  34  and then is transmitted to the HFC network via the LPF  36  and the tuner  37 . 
       FIG. 3  is a block diagram of a cable modem according to another embodiment of the present invention. 
     Referring to  FIG. 3 , the cable modem is composed of a Central Processing Unit (CPU)  41 , an upstream signal control unit  42 , a control gate  43 , a transformer  44 , first and second capacitors C 1  and C 2 , a Low-pass Filter (LPF)  45 , a tuner  46 , first and second switches  47  and  48 , a first High-pass Filter (HPF)  49 , a second HPF  50 , and a splitter  51 . 
     The first switch  47  and the second switch  48  respectively include a transistor Q 1  and Q 2 , a resistor (not shown), and a relay. The transistors Q 1  and Q 2  can be bipolar transistors or a Field Effect Transistors (FETs). 
     The CPU  41  has three ports P 1 , P 2  and P 3 . P 1  is a port that outputs a control signal to the control gate  43  to turn the control gate  43  on or off, and is used only to send an upstream signal in response to receiving information transmitted by the CMTS or head end  20 . 
     P 2  and P 3  are ports that output control signals to control the on/off states of the first switch  47  and the second switch  48  depending on the frequency band for upstream transmission transmitted by the CMTS or head end  20 . The frequency band for upstream transmission can be classified into three frequency bands; 10 MHz or less (i.e., 5 MHz to 10 MHz), 10 MHz to 20 MHz, and 20 MHz or more (i.e., 20 MHz to 42 MHz). 
     Accordingly, when having to send the upstream signal, the CPU  41  outputs the corresponding control signal at the respective ports P 1 , P 2  and P 3  depending on the upstream transmission frequency band transmitted by the CMTS or head end  20 . 
     That is, the CPU outputs a high signal at the port P 1  to turn the control gate  43  on only if the cable STB or cable modem transmits the upstream signal, but otherwise, always outputs a low signal at the port P 1  to turn the control gate  43  off, which prevents any upstream signals from the cable STB or cable modem from being transmitted to the coaxial cable (HFC). 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is less than 10 MHz, the CPU  41  outputs a high signal at the first port P 1  to turn the control gate  43  on and outputs low signals at the second port P 2  and the third port P 3  to respectively turn transistor Q 1  and transistor Q 2  off. Accordingly, the upstream signal is transmitted to the HFC network via the splitter  51 , the LPF  45  and the tuner  46 , and not via the first HPF  49  and the second HPF  5 . 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is 20 MHz and more, then the CPU  41  outputs high signals at the second port P 2  and the third port P 3  to enable transistors Q 1  and Q 2 , such that the upstream signal is transmitted to the HFC network via the second HPF  50 , the splitter  51 , the LPF  45 , and the tuner  46 . 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is between 10 MHz and 20 MHz, then the CPU  41  outputs a high signal at the second P 2  to enable only transistor Q 1 , such that that the upstream signal is transmitted to the HFC network via the HPF  49 , the splitter  51 , the low-pass filter  45 , and the tuner  46 . 
     The upstream signal control unit  42  performs a function of adjusting the amplitude of the corresponding frequency of the upstream signal depending on the upstream transmission frequency transmitted by the CMTS or head end  20  when sending the upstream signal to the HFC network under the control of the CPU  41 . Thus, the upstream signal control unit  42  includes an Automatic Gain Control (AGC) module (not shown) to produce an upstream signal having a stabilized amplitude. 
     The control gate  43  is used only when sending the upstream signal in response to receiving the information transmitted by the CMTS or head end  20 . The control gate  43  blocks noise components in the upstream signal control unit  42  from flowing into the HFC when the upstream signal is not transmitted. 
     For this purpose, the signal outputted from the first port P 1  of the CPU  41  determines the on/off of the control gate  43 . That is, if the cable STB or cable modem transmits the upstream signal, the CPU  41  outputs a high signal at the port P 1  so that the control gate  43  is on, but otherwise, the CPU  41  always outputs a low signal at the port P 1  to turn the control gate  43  off, blocking any upstream signal from the cable STB or cable modem from being transmitted to the coaxial cable HFC. 
     The transformer  44  receives an output signal from the upstream signal control unit  42  via the control gate  43  and isolates a next stage from the upstream signal control unit  42 . 
     The first capacitor C 1  and the second capacitor C 2  are used for AC coupling, and remove any DC component from the upstream signal. 
     The LPF  45  receives an output signal from the splitter  51  and removes a high-frequency component from the signal. It passes only signals having a frequency of 42 MHz or less. The low-pass filter  45  removes signals having a frequency of 42 MHz or more since the upstream signal in use in the modem only uses a 5 to 42 MHz frequency band. 
     The tuner  46  transmits the upstream signal, the high-frequency component of which has been removed by the LPF  45 , to the HFC network. 
     The first switch  47  is connected to allow the transformer  44  to be directly connected to the splitter  51  so that the upstream signal from the transformer  44  is outputted via the splitter  51  to the LPF  45  when the transistor Q 1  constituting the first switch  47  is off. On the other hand, the first switch is connected to allow the upstream signal from the transformer  44  to be outputted to the LPF  45  via the first HPF  49  or the second HPF  50  and the splitter  51  when the transistor Q 1  is on. 
     The second switch  48  performs a switching operation to connect the previous stage to the first HPF  49  when the transistor is off and to connect the previous stage to the second HPF  50  when the transistor is on. 
     The transformer  44  is directly connected to the splitter so that the upstream signal outputted from the transformer  44  is outputted via the splitter  51  to the LPF  45 , and the transformer  44  is connected at the on state so that the signal from the transformer is outputted to the LPF  45  via the first HPF  49  and the splitter  51 . 
     The first HPF  49  passes the 10 MHz or more frequency band and removes the 10 M or less frequency band. It is an HPF selected when the upstream transmission frequency is between 10 MHz to 20 MHz. 
     The second HPF  50  passes the 20 MHz or more frequency band and removes the 20 M or less frequency band. It is an HPF selected when the upstream transmission frequency is 20 MHz or more. 
     The splitter  51  is often called a “POTS splitter” in telephone communications. The splitter  51  is a device that splits a telephone signal into two or more signals each transferring selected frequency ranges. It can perform a function of reassembling signals incoming from several places into one signal. Some users connecting to the Internet through an Asymmetric Digital Subscriber Line (ADSL) service can locate the splitter at home and at an office. Others can use services that are not accompanied by a splitter, namely, services that do not require the splitter at home. In the ADSL, the splitter splits an incoming signal in order to send a low frequency to a voice device and a high frequency for data to a computer. A telephone station uses a Plain Old Telephone Service (POTS) splitter in order to send a low frequency voice signal to a telephone network and high-frequency data to a Digital Subscriber Line Access Multiplexer (DSLAM) for Internet transmission. 
     Thus, the splitter  51  splits the signal passing through the transformer  44  and the signal delivered through the selected one of the first HPF  49  and the second HPF  50  to block interference between the signals. 
     When the upstream transmission frequency transmitted by the CMTS or head end is less than 10 MHz, the CPU  41  in the cable STB or cable modem outputs a high signal at the first port P 1  to turn the control gate  42  on and outputs low signals at the second port P 2  and the third port P 3 . 
     When the CPU  41  outputs the low signals at the second port P 2  and the third port P 3 , transistor Q 1  and transistor Q 2  are each turned off and in turn the upstream signal outputted from the transformer  44  is transmitted to the HFC network via the splitter  51 , the LPF  45  and the tuner  46 , and not via the HPF  1  and the HPF  2 . 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is 20 MHz or more, then the CPU  41  outputs a high signal at the first port P 1  to turn the control gate  42  on and each outputs high signals at the second port P 2  and the third port P 3 . If the CPU  41  outputs the high signals at the second port P 2  and the third port P 3 , transistors Q 1  and Q 2  are enabled and the relay is activated to connect the transformer  44  to the second HPF  50 . Accordingly, the upstream signal outputted from the transformer  44  is delivered to the HFC network via the second HPF  50 , the splitter  51 , the LPF  45  and the tuner  46 . 
     If the upstream transmission frequency transmitted by the CMTS or head end  20  is between 10 MHz and 20 MHz, then the CPU  41  outputs a high signal at the first port P 1  to turn the control gate  42  on and outputs a high signal at the second port P 2  and a low signal at the third port P 3 . Accordingly, only transistor Q 1  is enabled and the relay is activated to connect the transformer  44  to the first HPF  49 . The upstream signal outputted from the transformer  44  is delivered to the HFC network via the first HPF  49 , the splitter  51 , the LPF  45 , and the tuner  46 . 
     The first port P 1  turns the control gate on/off and is activated to send the upstream signal in response to receiving the information transmitted by the CMTS or head end  20 . The CPU  41  outputs the high signal at the port P 1  so that the control gate  43  is on only if the cable STB or cable modem transmits the upstream signal, but otherwise, always outputs the low signal at the port P 1  to turn the control gate off, so that any upstream signal from the cable STB or cable modem is not transmitted to the coaxial cable HFC. In addition, the first capacitor C 1  and the second capacitor C 2  are used for AC coupling purposes. The capacitors reduce noise by blocking a DC current component from flowing into the HFC. 
     The HPF connection matrix depending on the states of the ports is as follows. Table 1 below corresponds to a case where the port  1  is at a high state. This is because a low state of the port  1  means that there is no upstream signal transmitted by the cable modem or cable STB. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Upstream 
               
               
                 Port2 
                 Port3 
                 Q1 
                 Q2 
                 Upstream Frequency Path 
                 Frequency 
               
               
                   
               
             
             
               
                 Low 
                 Low 
                 Off 
                 Off 
                 Via only LPF, not the first and 
                  5 to 10 MHz 
               
               
                   
                   
                   
                   
                 second HPFs 
               
               
                 Low 
                 High 
                 Off 
                 On 
                 Via only LPF, not the first and 
                  5 to 10 MHz 
               
               
                   
                   
                   
                   
                 second HPFs 
               
               
                 High 
                 Low 
                 On 
                 Off 
                 Via only first HPF and LPF 
                 10 to 20 MHz 
               
               
                 High 
                 High 
                 On 
                 On 
                 Via only second HPF and LPF 
                 20 MHz 
               
               
                   
                   
                   
                   
                   
                 or more 
               
               
                   
               
             
          
         
       
     
     In the cable modem  10  of  FIG. 1 , it is impossible to efficiently use the upstream bandwidth regardless of whether the coaxial cable HFC network includes noise because the 5 to 20 MHz frequency band is always not available when the 20 MHz HPF is used at all times. However, according to the present invention, it is possible to efficiently use the upstream bandwidth since different HPF paths are selected depending on the upstream frequency.