Abstract:
A synchronization system for establishing synchronization between a first communications device and a second communications device connected through a communications line transmitting an xDSL signal is disclosed that includes a clock extraction part configured to extract, from a synchronous network connected to the first communications device, the synchronization clock signal of the synchronous network; a synchronization signal generation and transmission part configured to generate a synchronization signal from the synchronization clock signal and to transmit the synchronization signal to the communications line, the synchronization signal having a frequency in a frequency band different from the frequency band of the xDSL signal; a synchronization signal extraction part configured to extract the synchronization signal from the communications line; and a clock reproduction and output part configured to reproduce the synchronization clock signal from the synchronization signal and to supply the synchronization clock signal to the second communications device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the technique of performing communications between communications devices that need clock synchronization using xDSL (Digital Subscriber Line) 
         [0003]    2. Description of the Related Art 
         [0004]    Popular are xDSL technologies that perform high-speed digital communications using metallic lines. Examples of xDSL include ADSL (Asymmetric Digital Subscriber Line), HDSL (High-bit-rate Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), and VDSL (Very high-bit-rate Digital Subscriber Line). 
         [0005]    Further, xDSL is becoming faster, and, for example, VDSL offers a transmission rate of 100 Mbps for each of upstream traffic and downstream traffic. Using xDSL technologies makes it possible to perform digital data communications with ease using existing metallic lines installed as telephone lines. 
         [0006]      FIG. 1  is a diagram showing a conventional communications system that performs xDSL communications using metallic lines. 
         [0007]    In the communications system shown in  FIG. 1 , an xDSL modem  1  is connected to an IP (Internet Protocol) network  5  through an Ethernet (registered trademark) line  3  at Point A. The xDSL modem  1  is a device having functions defined by ITU-T G.991, G.992, and G.993. A splitter  7  is connected to a telephone exchange  9  and the xDSL modem  1  through a metallic line  11  and a metallic line  13 , respectively. A voice signal and an xDSL signal are multiplexed by the splitter  7  to be transmitted to the Point B side through a metallic line  15 . Further, the splitter  7  splits a signal transmitted from the Point B side into a voice signal and an xDSL signal. 
         [0008]    On the Point B side, an xDSL modem  19  connected to a user terminal  17  such as a personal computer (PC) generates an Ethernet signal from the signal received through the metallic line  15 , and transmits the generated Ethernet signal to the user terminal  17 . Further, a splitter  21  extracts a voice signal from the signal received through the metallic line  15 , and transmits the extracted voice signal to a telephone set  23 . Each of the metallic lines shown in  FIG. 1  includes a pair of lines for differential transmission. Such an arrangement is also possible where the splitter  21  on the Point B side is provided as shown in  FIG. 2 . 
         [0009]    In general, a dedicated line formed of an optical cable or coaxial cable is necessary to connect communication devices (such as ATM communication devices or SDH transmitters) operating on a synchronous network. However, it may not be possible to newly install such a cable depending on the locations of installation of the communication devices. In such a case, the communications between the communication devices may be performed by xDSL using existing metallic lines. 
         [0010]    Reference may be made to the following documents for technologies related to the present invention. 
         [0011]    [Patent Document 1] Japanese Laid-Open Patent Application No. 10-233767 
         [0012]    [Patent Document 2] Japanese Laid-Open Patent Application No. 2004-129009 
         [0013]    [Patent Document 3] Japanese Laid-Open Patent Application No. 2004-533788 
         [0014]    [Patent Document 4] Japanese Laid-Open Patent Application No. 2005-303777 
         [0015]    Asynchronous communications are performed in xDSL. Therefore, in the case of communicating data through xDSL between the communication devices based on network synchronization as described above, there is a problem in that it is difficult to synchronize the communication devices. 
         [0016]      FIG. 3  is a diagram showing a configuration in the case of performing communications through xDSL between ATM communications devices  25  and  27  that are based on network synchronization. The case illustrated in  FIG. 3  is different from the configuration shown in  FIG. 1  in that the ATM communications device  25  is provided in place of the IP network  5  and an ATM-Ethernet conversion part  29  is further provided at Point A and that an ATM communication device  27  is provided in place of the user terminal  17  and an ATM-Ethernet conversion part  31  is further provided at Point B. Each of the ATM-Ethernet conversion parts  29  and  31  is one type of ATM communications device based on network synchronization. Further, as shown in  FIG. 3 , a part including the xDSL modem  1  and the ATM-Ethernet conversion part  29  is referred to as an xDSL-ATM interface unit  33 , and a part including the xDSL modem  19  and the ATM-Ethernet conversion part  31  is referred to as an xDSL-ATM interface unit  35 . 
         [0017]    ATM stands for “Asynchronous Transfer Mode.” However, since an SDH (Synchronous Digital Hierarchy) frame, which is a network synchronization type, is used in the physical layer of actual ATM communications devices, clock synchronization should be established between ATM communications devices for normal cell transmission and reception between the devices. 
         [0018]    The ATM-Ethernet conversion part  29  at Point A converts an ATM cell received from an ATM line  37 , which is a synchronous network, into an Ethernet frame, and inputs the Ethernet frame to the xDSL modem  1 . Further, the ATM-Ethernet conversion part  29  converts an Ethernet frame received from the xDSL modem  1  into an ATM cell, and outputs the ATM cell to the ATM line  37 . 
         [0019]    The ATM-Ethernet conversion part  31  at Point B converts an ATM cell received from an ATM line  39  into an Ethernet frame, and inputs the Ethernet frame to the xDSL modem  19 . Further, the ATM-Ethernet conversion part  31  converts an Ethernet frame received from the xDSL modem  19  into an ATM cell, and outputs the ATM cell to the ATM line  39 . 
         [0020]    A reference clock signal for establishing synchronization on the ATM network is supplied from a reference clock  200  of the ATM network to the ATM communications device  25  of Point A. The ATM-Ethernet conversion part  29  extracts a clock signal for synchronization (synchronization clock signal) synchronizing with the reference clock signal from a signal received through the ATM line  37 , and operates in synchronization with this synchronization clock signal. It is a common technique to extract a clock signal from a signal received from a line on a synchronous network. 
         [0021]    The ATM-Ethernet conversion part  31  at Point B has a built-in autonomous clock  202 . The autonomous clock  202  supplies a local clock signal for the ATM communications between the ATM-Ethernet conversion part  31  and the ATM communications device  27 , which clocks signal does not synchronize with the reference clock signal of the ATM network. 
         [0022]    According to the configuration of  FIG. 3 , the clock signal supplied by the autonomous clock  202  cannot be synchronized with the reference clock signal of the ATM network. Therefore, there is a difference between the clock frequencies of the clock signals, so that there is a difference in the phase of a clock signal used between the ATM communications devices  25  and  27 . This prevents normal ATM cell communications between the ATM communications devices  25  and  27 . For example, underflow/overflow occurs in the data buffers of the ATM communications devices  25  and  27  to cause loss of data. If control data requiring reliability are lost, this results in poor operation. 
         [0023]    Instead of by providing the autonomous clock  202  at Point B, synchronization may be established by inserting synchronization information into digital data (such as an Ethernet frame) at Point A, transmitting the digital data to Point B, and generating a synchronization clock signal from the synchronization information at Point B. 
         [0024]    In this case, the ATM-Ethernet conversion part  29  at Point A extracts the synchronization clock signal of the ATM network from the ATM line  37 , generates the synchronization information such as a synchronization frame and time stamp data, and transmits the generated synchronization information to the ATM-Ethernet conversion part  31  as digital data. 
         [0025]    Then, the ATM-Ethernet conversion part  31  reproduces the synchronization clock signal using the synchronization information transmitted from the ATM-Ethernet conversion part  29 , and operates in synchronization with the clock signal. 
         [0026]    However, xDSL transmission data greatly fluctuate, and a buffer for absorbing the fluctuation is required. Further, as the xDSL speed increases, the buffer is required to be larger in capacity. Accordingly, in the case of adopting the above-described method, there is the problem of an increase in the scale of a circuit for stabilizing synchronization if a control circuit is included. 
       SUMMARY OF THE INVENTION 
       [0027]    Embodiments of the present invention may solve or reduce one or more of the above-described problems. 
         [0028]    According to one aspect of the present invention, a technique is provided that facilitates establishing clock synchronization between communications devices based on network synchronization in the case of performing communications between the communications devices through an xDSL communications network. 
         [0029]    According to one embodiment of the present invention, a synchronization system for establishing synchronization between a first communications device and a second communications device connected through a communications line transmitting an xDSL signal is provided. The synchronization system includes a clock extraction part configured to extract, from a synchronous network connected to the first communications device, a synchronization clock signal of the synchronous network; a synchronization signal generation and transmission part configured to generate a synchronization signal from the synchronization clock signal and to transmit the synchronization signal to the communications line, the synchronization signal having a frequency in a frequency band different from a frequency band of the xDSL signal; a synchronization signal extraction part configured to extract the synchronization signal from the communications line; and a clock reproduction and output part configured to reproduce the synchronization clock signal from the synchronization signal and to supply the synchronization clock signal to the second communications device. 
         [0030]    According to one embodiment of the present invention, a synchronization signal transmitter transmitting a synchronization signal used to establish synchronization between a first communications device and a second communications device connected through a communications line transmitting an xDSL signal is provided. The synchronization signal transmitter includes a clock extraction part configured to extract, from a synchronous network connected to the first communications device, a synchronization clock signal of the synchronous network; and a synchronization signal generation and transmission part configured to generate the synchronization signal from the synchronization clock signal and to transmit the synchronization signal to the communications line, the synchronization signal having a frequency in a frequency band different from a frequency band of the xDSL signal. 
         [0031]    According to one embodiment of the present invention, a clock supplier supplying a synchronization clock signal for establishing synchronization between a first communications device and a second communications device to the second communications device is provided, wherein the first communications device and the second communications device are connected through a communications line transmitting an xDSL signal. The clock supplier includes a synchronization signal extraction part configured to extract a synchronization signal from the communications line, the synchronization signal being generated from the synchronization clock signal of a synchronous network connected to the first communications device and having a frequency in a frequency band different from a frequency band of the xDSL signal; and a clock reproduction and output part configured to reproduce the synchronization clock signal from the synchronization signal and to supply the synchronization clock signal to the second communications device. 
         [0032]    According to one embodiment of the present invention, a synchronization method for establishing synchronization between a first communications device and a second communications device connected through a communications line transmitting an xDSL signal is provided. The synchronization method includes the steps of (a) extracting, from a synchronous network connected to the first communications device, a synchronization clock signal of the synchronous network; (b) generating a synchronization signal from the synchronization clock signal and transmitting the synchronization signal to the communications line, the synchronization signal having a frequency in a frequency band different from a frequency band of the xDSL signal; (c) extracting the synchronization signal from the communications line; and (d) reproducing the synchronization clock signal from the synchronization signal and supplying the synchronization clock signal to the second communications device. 
         [0033]    According to one aspect of the present invention, a synchronization signal having a frequency in a frequency band different from the frequency band of an xDSL signal is transmitted onto a communications line, and a synchronization clock signal is reproduced from the synchronization signal and used at a receiving end. This makes it possible to establish synchronization between communications devices via an xDSL communications network with more ease than the system of transmitting synchronization information as digital data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
           [0035]      FIG. 1  is a diagram showing a conventional communications system that performs xDSL communications using metallic lines; 
           [0036]      FIG. 2  is a diagram showing another conventional communications system; 
           [0037]      FIG. 3  is a diagram showing a configuration for performing communications between ATM communications devices through xDSL; 
           [0038]      FIG. 4  is a diagram showing a configuration of a communications system according to a first embodiment of the present invention; 
           [0039]      FIG. 5  is a diagram showing a configuration of a communications system according to a second embodiment of the present invention; 
           [0040]      FIG. 6  is a graph showing the frequency spectrum of a synchronization signal transmitted onto a metallic line according to the second embodiment of the present invention; 
           [0041]      FIG. 7  is a diagram showing a first detailed configuration of xDSL-ATM interface units according to the second embodiment of the present invention; 
           [0042]      FIG. 8  is a diagram showing signal waveforms according to the second embodiment of the present invention; 
           [0043]      FIG. 9  is a diagram showing a circuit forming a synchronization signal transmission part according to the second embodiment of the present invention; 
           [0044]      FIG. 10  is a diagram showing a circuit forming a synchronization signal extraction part according to the second embodiment of the present invention; 
           [0045]      FIG. 11  is a diagram showing a second detailed configuration of the xDSL-ATM interface units according to the second embodiment of the present invention; 
           [0046]      FIG. 12  is a diagram showing another circuit forming the synchronization signal transmission part according to the second embodiment of the present invention; 
           [0047]      FIG. 13  is a diagram showing another circuit forming the synchronization signal extraction part according to the second embodiment of the present invention; 
           [0048]      FIG. 14  is a diagram showing a configuration of a communications system according to a third embodiment of the present invention; and 
           [0049]      FIG. 15  is a graph showing the frequency spectrum of a signal on a metallic line according to the third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]    A description is given below, with reference to the accompanying drawings, of embodiments of the present invention. 
       First Embodiment 
       [0051]      FIG. 4  is a diagram showing a configuration of a communications system according to a first embodiment of the present invention. 
         [0052]    According to the configuration shown in  FIG. 4 , Point A includes a communications device  40  that needs network synchronization, a synchronization signal transmitter  42 , and an xDSL-synchronous network interface unit  44 . Further, Point B includes an xDSL-synchronous network interface unit  46 , a clock supplier  48 , and a communications device  50  that needs network synchronization. 
         [0053]    The communications device  40  and the xDSL-synchronous network interface unit  44  are connected with a line  52  complying with the communication system of the communications device  40  (for example, a coaxial cable). The xDSL-synchronous network interface unit  44  and the xDSL-synchronous network interface unit  46  are connected with a metallic line  54  that is also usable as a telephone line. Further, the xDSL-synchronous network interface unit  46  and the communications device  50  are connected with a line  56  complying with the communication system of the communications device  50 . The line  52  connecting the communications device  40  and the xDSL-synchronous network interface unit  44  forms a synchronous network. Further, each of the xDSL-synchronous network interface units  44  and  46  is one type of communications device that requires network synchronization. 
         [0054]    The synchronization signal transmitter  42  is connected to the line  52  and the metallic line  54 . The clock supplier  48  is connected to the xDSL-synchronous network interface unit  46 . Further, the communications device  40  at Point A is supplied with a reference clock signal serving as a reference of synchronization in the synchronous network including the communications device  40  from a reference clock  204 . Alternatively, the communications device  40  may receive the reference clock signal from an external clock generator or have a built-in clock generator that generates the reference clock signal. 
         [0055]    Hereinafter, a clock signal that synchronizes with a reference clock signal and is transmitted between communication devices on a synchronous network is referred to as a synchronization clock signal (a clock signal for synchronization). The same applies to other embodiments. The synchronization clock signal may be the same as the reference clock signal or different in frequency from the reference clock signal. Although not graphically illustrated, each of the communications devices  40  and  50  is connectable to other communications devices on the synchronous network. 
         [0056]    The synchronization signal transmitter  42  at Point A includes a clock extraction part  58 , a frequency division part  60 , and a synchronization signal generation and transmission part  62 . Here, the clock extraction part  58  may be provided inside the xDSL-synchronous network interface unit  44 . The clock supplier  48  at Point B includes a synchronization signal extraction part  64  and a clock reproduction and output part  66 . 
         [0057]    According to the configuration shown in  FIG. 4 , the synchronization signal transmitter  42  generates a synchronization signal from the synchronization clock signal and multiplexes the synchronization signal as an analog signal of a frequency band different from that of an xDSL signal with the xDSL signal over the metallic line  54 . The clock supplier  48  at Point B extracts the synchronization signal from the metallic line  54 , reproduces the synchronization clock signal synchronizing with the reference clock signal based on the extracted synchronization signal, and supplies the reproduced synchronization clock signal to the xDSL-synchronous network interface unit  46 . Then, the synchronization clock signal is supplied to the communications device  50 . 
         [0058]    The communications device  50  can synchronize with the communications device  40  based on the supplied synchronization clock signal and can properly perform communications on the synchronous network. The synchronization-related operation of each part in the communications system shown in  FIG. 4  is as follows. 
         [0059]    The clock extraction part  58  extracts the synchronization clock signal from a signal transmitted from the communications device  40 . The frequency division part  60  performs frequency division on the synchronization clock signal extracted by the clock extraction part  58  so that the clock signal has an appropriate frequency, and inputs the frequency-divided clock signal to the synchronization signal generation and transmission part  62 . The synchronization signal generation and transmission part  62  generates a synchronization signal that is an analog signal of a frequency band different from that of the xDSL signal by shaping the waveform of the input clock signal, and outputs the generated synchronization signal to the metallic line  54 . 
         [0060]    The synchronization signal extraction part  64  at Point B extracts the synchronization signal from a signal transmitted over the metallic line  54 , and inputs the extracted synchronization signal to the clock reproduction and output part  66 . The clock reproduction and output part  66  reproduces the synchronization clock signal based on the synchronization signal, and supplies the reproduced synchronization clock signal to the xDSL-synchronous network interface unit  46 . 
       Second Embodiment 
       [0061]    Next, a description is given of a second embodiment of the present invention. 
         [0062]    (Basic Configuration) 
         [0063]      FIG. 5  is a diagram showing a configuration of a communications system according to the second embodiment of the present invention. In the second embodiment, the synchronous system between communications devices as described in the first embodiment is applied to the configuration shown in  FIG. 3 . In  FIG. 5 , the same elements as those described above are referred to by the same reference numerals. 
         [0064]    As shown in  FIG. 5 , the telephone exchange  9  is provided at Point A and connected to the splitter  7 . Further, the ATM communications device  25 , supplied with a reference clock signal from the reference clock  200 , and an xDSL-ATM interface unit  70  are provided at Point A. 
         [0065]    The xDSL-ATM interface unit  70  includes an ATM-Ethernet conversion part  72 , the xDSL modem  1 , a frequency division part  74 , and a synchronization signal transmission part  76 . The synchronization signal transmission part  76  is connected to the metallic line  13 . 
         [0066]    At Point B, the telephone set  23  and the splitter  21  are provided, and the splitter  21  is connected to the metallic line  15 . Further, an xDSL-ATM interface unit  80  and the ATM communications device  27  are provided at Point B. The xDSL-ATM interface unit  80  includes the xDSL modem  19 , an ATM-Ethernet conversion part  82 , a synchronization signal extraction part  84 , and a clock reproduction and output part  86 . 
         [0067]    In the communications system shown in  FIG. 5 , communications data based on network synchronization are relayed using asynchronous communications based on xDSL. An ATM cell output from the ATM communications device  25  is converted into an Ethernet frame to be output from the xDSL modem  1  as an xDSL signal, which reaches Point B by way of the metallic line  15 . 
         [0068]    Further, a synchronization clock signal synchronizing with the reference clock signal is transmitted onto the ATM line  37  from the ATM communications device  25 . The xDSL-ATM interface unit  70  extracts the synchronization clock signal from the ATM line  37 , generates a synchronization signal from the synchronization clock signal, and transfers the generated synchronization signal as an analog signal by way of the metallic line  15 . The xDSL-ATM interface unit  80  reproduces the synchronization clock signal from the received synchronization signal, and synchronizes the xDSL-ATM interface unit  80  and the ATM communications device  27  with the reference clock of the ATM network. A description is given below of the operation of each part in the xDSL-ATM interface units  70  and  80  in  FIG. 5 . 
         [0069]    The ATM-Ethernet conversion part  72  at Point A converts an ATM cell received from the ATM line  37  into an Ethernet frame, and inputs the Ethernet frame to the xDSL modem  1 . Further, the ATM-Ethernet frame  72  converts an Ethernet frame received from the xDSL modem  1  into an ATM cell. Further, the ATM-Ethernet conversion part  72  has the function of extracting the synchronization clock signal of the ATM network from a signal transmitted over the ATM line  37  and passing the extracted synchronization clock signal to the frequency division part  74 . 
         [0070]    The frequency division part  74 , which is, for example, a counter circuit, performs frequency division on the synchronization clock signal received from the ATM-Ethernet conversion part  72  so that the synchronization signal transmitted over the metallic lines  13  and  15  has an appropriate frequency. 
         [0071]    The clock signal extracted from the ATM line  37  generally has a high frequency. If the synchronization signal is generated and transmitted to the metallic lines  13  and  15  with this frequency unchanged, the frequency of the synchronization signal may overlap with the operating frequency band of xDSL. Further, since the frequency is high, the loss in the metallic lines  13  and  15  increases. Accordingly, the frequency of the synchronization clock signal received from the ATM-Ethernet conversion part  72  is reduced to a frequency of an order of magnitude of a few kHz in the frequency division part  74 . 
         [0072]    The synchronization signal transmission part  76  creates the synchronization signal by shaping the waveform of the clock signal received from the frequency division part  74 , and transmits the synchronization signal onto the metallic line  13 . The synchronization signal is of a single frequency, and the frequency is out of the operating frequency band in xDSL. This prevents the synchronization signal and a data signal transmitted in xDSL from interfering with each other. 
         [0073]      FIG. 6  is a graph showing the frequency spectrum of the synchronization signal transmitted onto a metallic line in this embodiment. As shown in  FIG. 6 , the synchronization signal uses a frequency in a frequency band that does not overlap the band of a voice signal transmitted over the metallic line or the band used in xDSL. 
         [0074]    The synchronization signal extraction part  84  in the xDSL-ATM interface unit  80  at Point B separates the synchronization signal from a multiplexed signal of the audio signal, the xDSL signal, and the synchronization signal on the metallic line  15 , and passes the separated synchronization signal to the clock reproduction and output part  86 . The clock reproduction and output part  86  generates the synchronization clock signal of the ATM network based on the synchronization signal received from the synchronization signal extraction part  84 , and supplies the generated synchronization clock signal to the ATM-Ethernet conversion part  82 . 
         [0075]    The ATM-Ethernet conversion part  82  converts an ATM cell received from the ATM communications device  27  into an Ethernet frame, and transmits the Ethernet frame to the xDSL modem  19 . Further, the ATM-Ethernet conversion part  82  converts an Ethernet frame received from the xDSL modem  19  into an ATM cell, and transmits the ATM cell to the ATM communications device  27 . At this point, the ATM-Ethernet conversion part  82  performs communications in synchronization with the ATM network using the synchronization clock signal of the ATM network received from the clock reproduction and output part  86 . 
         [0076]    (First Detailed Configuration) 
         [0077]      FIG. 7  is a diagram showing a first detailed configuration of the xDSL-ATM interface units  70  and  80 .  FIG. 7  shows in detail the synchronization signal transmission part  76  and a coupling circuit  90  in the xDSL-ATM interface unit  70 . The coupling circuit  90  supplies the synchronization signal to the metallic line  13 . Further,  FIG. 7  shows in detail a separation circuit  100  and the synchronization signal extraction part  84  in the xDSL-ATM interface unit  80 .. The separation circuit  100  separates the synchronization signal from a signal transmitted over the metallic line  15 . 
         [0078]    The synchronization signal transmission part  76  in the xDSL-ATM interface unit  70  includes a waveform shaping part  92  and a differential signal generation part  94 . Further, the coupling circuit  90  includes a high-pass filter (HPF)  96  for passing the xDSL signal and eliminating the synchronization signal and a low-pass filter (LPF)  98  for passing the synchronization signal and eliminating the xDSL signal. 
         [0079]    Further, the synchronization signal extraction part  84  in the xDSL-ATM interface unit  80  includes a differential amplification part  102  and a waveform shaping part  104 . Further, the separation circuit  100  includes a high-pass filter (HPF)  106  for passing the xDSL signal and eliminating the synchronization signal and a bandpass filter (BPF)  108  for eliminating the xDSL signal and the voice signal and passing only the synchronization signal. Next, a description is given of an operation in the configuration of  FIG. 7 . 
         [0080]    The ATM-Ethernet conversion part  72  at Point A extracts the synchronization clock signal of the ATM network and outputs the extracted synchronization clock signal to the frequency division part  74 . The frequency division part  74  reduces the frequency of the synchronization clock signal to a predetermined frequency, and outputs the clock signal of the reduced frequency to the synchronization signal transmission part  76 . Referring to  FIG. 8 , (a) shows an example of the synchronization clock signal extracted by the ATM-Ethernet conversion part  72 , and (b) shows an example of the output signal of the frequency division part  74 . 
         [0081]    Next, the output signal of the frequency division part  74  is input to the waveform shaping part  92  of the synchronization signal transmission part  76 . The waveform shaping part  92  converts a square or rectangular wave signal into a sinusoidal signal (of a single frequency). Referring to  FIG. 8 , (c) shows an example of the output signal of the waveform shaping part  92 . 
         [0082]    The output signal of the waveform shaping part  92  is input to the differential signal generation part  94 . The differential signal generation part  94  generates a differential signal (two sinusoidal waves different in phase) for a metallic line (having the Tip and Ring wires of a subscriber line) for differential transmission from the output signal of the waveform shaping part  92 . The level of the differential signal represents the potential difference of two signals. Referring to  FIG. 8 , (d) shows an example of the differential signal. The differential signal output from the differential signal generation part  94  is superposed on the metallic line from the coupling circuit  90 . 
         [0083]    The synchronization signal, which is a differential signal, is transmitted to the xDSL-ATM interface unit  80  through the metallic line, and is separated from other signals by the separation circuit  100  to be input to the differential amplification part  102  in the synchronization signal extraction part  84 . The differential amplification part  102  performs differential amplification to convert the synchronization signal of differential transmission into non-differential transmission. Then, the synchronization signal is converted into the square or rectangular wave signal shown in (b) of  FIG. 8  by the waveform shaping part  104  to be input to the clock reproduction and output part  86 . 
         [0084]    The clock reproduction and output part  86 , which includes, for example, a PLL circuit having a frequency conversion function, generates the synchronization clock signal ((a) in  FIG. 8 ) synchronizing with the reference clock signal of the ATM network based on the signal input from the waveform shaping part  104 , and outputs the synchronization clock signal to the ATM-Ethernet conversion part  82 . The ATM-Ethernet conversion part  82  establishes synchronization with the ATM network using the synchronization clock signal output from the clock reproduction and output part  86 . 
         [0085]      FIG. 9  is a diagram showing a specific circuit forming the synchronization signal transmission part  76 . As shown in  FIG. 9 , a low-pass filter (LPF) may be used as the waveform shaping part  92 . Further, a transistor  110 , an amplifier circuit having resistors R 1 , R 2 , and R 3 , and a transformer  112  may be used as the differential signal generation part  94 . A sinusoidal signal amplified by the amplifier circuit is input to the primary coil of the transformer  112 , and a differential signal is output from the secondary coil of the transformer  112 . 
         [0086]      FIG. 10  is a diagram showing a specific circuit forming the synchronization signal extraction part  84 . As shown in  FIG. 10 , a differential amplifier circuit  114  including an operational amplifier may be used as the differential amplification part  102 , and a comparator circuit  116  including an operational amplifier may be used as the waveform shaping part  104 . 
         [0087]    In the first detailed configuration shown in  FIG. 7 , the synchronization signal, the xDSL signal, and the voice signal are differential signals, and interfere with one another if their frequency bands overlap. Accordingly, their frequency bands should be separated. Since external noise enters a metallic line with an in-phase signal, the first configuration using a differential signal as the synchronization signal is less susceptible to external noise. 
         [0088]    (Second Detailed Configuration) 
         [0089]      FIG. 11  is a diagram showing a second detailed configuration of the xDSL-ATM interface units  70  and  80 . A description is given below of differences from the first detailed configuration. 
         [0090]    The second detailed configuration is different from the first detailed configuration in that an in-phase signal is generated as the synchronization signal to be superposed on the metallic line. Accordingly, the differential signal generation part  94  in the first detailed configuration is replaced with an in-phase signal generation part  120  in the second detailed configuration. Further, the differential amplification part  102  in the first detailed configuration is replaced with a summing amplification part  122  in the second detailed configuration. 
         [0091]    The in-phase signal generation part  120  generates an in-phase signal (two sinusoidal waves of the same phase) for a metallic line (having the Tip and Ring wires of a subscriber line) for differential transmission from the output signal of the waveform shaping part  92 . Referring to  FIG. 8 , (e) shows an example of the in-phase signal. The in-phase signal output from the in-phase signal generation part  120  is superposed on the metallic line from the coupling circuit  90 . 
         [0092]    The summing amplification part  122  receives the synchronization signal from the separation circuit  100 . The summing amplification part  122  performs summing amplification to covert the synchronization signal of differential transmission into non-differential transmission, and outputs the summing-amplified signal to the waveform shaping part  104 . 
         [0093]      FIG. 12  is a diagram showing a specific circuit forming the synchronization signal transmission part  76  in the second detailed configuration. As shown in  FIG. 12 , an amplifier circuit having a transistor  124  and resistors R 1 , R 2 , and R 3  and a transformer  126  may be used as the in-phase signal generation part  120 . A sinusoidal signal amplified by the amplifier circuit is input to the primary coil of the transformer  126 , and an in-phase signal is output from the two secondary coils of the transformer  126 . 
         [0094]      FIG. 13  is a diagram showing a specific circuit forming the synchronization signal extraction part  84  in the second detailed configuration. As shown in  FIG. 13 , a summing amplifier circuit  128  including an operational amplifier may be used as the summing amplification part  122 . 
         [0095]    In the second detailed configuration shown in  FIG. 11 , the xDSL signal and the voice signal are differential signals, while the synchronization signal is an in-phase signal. Therefore, there is the advantage that the in-phase signal is less likely to interfere with the xDSL signal and the voice signal. However, the synchronization signal, which is an in-phase signal, is susceptible to external noise, which is also an in-phase signal. Therefore, the bandpass filter  108  of the separation circuit  100  in the second detailed configuration has the characteristic of eliminating a noise signal and passing only the synchronization signal. 
       Third Embodiment 
       [0096]    Next, a description is given of a third embodiment of the present invention.  FIG. 14  is a diagram showing a configuration of a communications system according to the third embodiment. According to the third embodiment, a metallic line  130 , which is not used for voice communications, is used for the communications between the ATM communications devices  25  and  27 . As shown in  FIG. 14 , the configuration of the third embodiment is basically the same as the configuration of the second embodiment shown in  FIG. 5  except that no telephone exchange, splitters, or telephone set is provided. Further, the above-described first and second detailed configurations are applicable as its detailed configurations. 
         [0097]      FIG. 15  is a graph showing the frequency spectrum of a signal on the metallic line in the third embodiment. As shown in  FIG. 15 , since no voice communications are used in the third embodiment, a band not used by the xDSL signal may be employed as the frequency band of the synchronization signal without consideration of the voice band. This increases the latitude in selecting the frequency of the synchronization signal. Further, since no voice communications are used in the third embodiment, the bandpass filter  108  in the separation circuit  100  of the xDSL-ATM interface unit  80  in the first detailed configuration may be replaced with a low-pass filter that eliminates the xDSL signal. 
         [0098]    According to the systems described in the first through third embodiments, an xDSL signal and a synchronization signal are superposed on a metallic line, and a synchronization clock signal is generated from the synchronization signal at a receiving end of the synchronization signal. Using this synchronization clock signal makes it possible to synchronize communication devices based on network synchronization and connected through an xDSL line with each other. Accordingly, it is possible to install communications devices based on network synchronization using an existing metallic cable. Therefore, there is no need to prepare a dedicated line such as an expensive optical cable or coaxial cable, and it is possible to construct a system of network synchronization with ease. 
         [0099]    Further, the systems described in the first through third embodiments can establish synchronization between communication devices based on network synchronization and connected through an xDSL line with more ease than the system of transmitting synchronization information as digital data. 
         [0100]    The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
         [0101]    The present application is based on Japanese Priority Patent Application No. 2007-253673, filed on Sep. 28, 2007, the entire contents of which are hereby incorporated by reference.