Abstract:
Provided is an asymmetric digital subscriber line (ADSL) system that communicates via digital subscriber line (DSL) routing through a bundle of cables including a communication line for time compression multiplexing-integrated services digital network (TCM-ISDN) system that is synchronized with a TCM timing reference (TTR) signal. The ADSL system is synchronized with a data signal inputted after the TTR indication signal.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a telecommunication system, and more particularly to an asymmetric digital subscriber line (ADSL) system using a digital-subscriber line coexisting with a communication line for a time-compression multiplexing integrated services digital network (TCM-ISDN) line system.  
           [0003]    2. Discussion of Related Art  
           [0004]    In recent years, communication methods that are capable of facilitating high-speed data communications and minimal installation costs and user fees have become imperative to meet requirements for high-speed communications via Internet between users of personal computers. As a result, a Digital Subscriber Line and its variations (xDSL) system has been proposed. The xDSL system enables digital data communications by using existing general copper phone lines installed in homes and offices.  
           [0005]    The “xDSL” system, which generally refers to all kinds of communication methods using phone lines, includes a High data-rate DSL (HDSL) system taking the place of existing T1 lines, a Symmetric DSL (SDSL) system taking the place of T1 or E1 lines by using a single twisted-pair copper line, an Asymmetric DSL (ADSL) system capable of transmitting high-capacity data by a public switched telephone network (PSTN).  
           [0006]    In an ADSL system, the word ‘asymmetric’ is named because downstream data transmitted from a central office (CO) to remote terminals (RT) has wider bandwidth and more capable of data transmission than upstream data transmitted from the remote terminals to the central office. The ADSL system uses current phone lines or telephones as it is and enables high-speed data communications. Also, in the ADSL system, it is capable of data communications and a Plain Old Telephone Service (POTS), simultaneously. The ADSL system provides a transmitting speed of up to 8Mbps in a downstream direction and that of up to 640 Mbps in an upstream direction.  
           [0007]    The transmission of data in an xDSL system needs wider bandwidth and thus creates higher cross-talk interference between copper pairs in the same cable-binder group. The level of the crosstalk interference varies depending on different cable structures and materials. In particular, some countries such as Japan and Korea use telephone cables with a paper-based “pulp” insulator rather than plastic-insulated cables (PIC) used in the United States. These pulp cables produce much more cross-talk interference than the PIC cables. Thus, it is more difficult to provide wide-band DSL services in those countries which use the pulp cables because their existing telephone cables are prone to crosstalk interference.  
           [0008]    [0008]FIG. 1 is a diagram of conventional ISDN line telecommunication system which has interference.  
           [0009]    A central office  10  contains several Integrated Services Digital Network (ISDN) line cards  11  that connect a telephone network backbone to. local lines  20  that are strung to equipments of users. Remote ISDN terminal adapters or modems  30  are located at different remote customer sites within a few kilometers of the central office  10 .  
           [0010]    Local lines from ISDN line cards  11  to remote ISDN modems  12  are routed through one or more cable bundles  12 . These telephone-cable bundles  12  may contain dozens or more separate telephone lines or copper pairs. Standard voice services, ISDN services, and newer DSL services share the same cable bundle. Since lines run close to other lines in cable bundles  12 , mutual inductances can create cross-talk interference or noise between lines  20 . For voice services such as Plain Old Telephone Service (POTS), frequencies are so low that interference is negligible. However, interference cannot be neglected in ISDN digital services using a higher bandwidth of around 80 to 320 kHz. New xDSL services also use higher bandwidths. For example, bandwidths of an ADSL system are typically above 1 MHz and have significant cross-talk interference. To prevent this cross-talk interference in countries such as the U.S., where better-insulated PIC cables are used, a full-duplex data transmission system having echo cancellation has been employed.  
           [0011]    To eliminate the cross-talk interference in countries such as Japan, where the pulp cables are installed, a time-compression multiplexing (TCM) ISDN line system is used rather than the full-duplex data transmission system having echo-cancellation. The Operation of TCM-ISDN line system is based on a TCM timing reference (TTR) signal of 400 Hz. In such a system, the ISDN line cards at the central office transmit data when the TTR signal is at an active high. The ISDN line cards all receive data from ISDN modems during a different time period. Thus, near-end cross-talk (NEXT) interference is eliminated because none of the other ISDN cards at the central office are receiving data during the transmission time-period. Although far-end crosstalk (FEXT) interference still exists, it is usually much weaker than NEXT interference.  
           [0012]    [0012]FIG. 2 is a timing diagram for a TCM-ISDN line system. During window  22 , data is output from a central office to a remote ISDN modem installed in customer premises. This data arrives at the remote modem after a predetermined delay, during reception window  24 . An ISDN equipment of the customer premises includes a burst clock detector (not shown) to determine the timing of a receive downstream burst and to generate the timing for a transmit upstream burst thereof. During window  26 , upstream data is transmitted from the remote modem to the central office, which arrives at the central office after a predetermined delay, during window  28 . At any particular time, only one end of the TCM-ISDN line system is transmitting data, while the other end is receiving data. An echo cancellator is not needed because a transmitted signal does not have any echo that has to be removed.  
           [0013]    As an ADSL system as well as the existing public switched telephone network (PSTN) enable super-speed communications, data communications under other circumstances such as established ISDN and TCM-ISDN has been proposed so that an ADSL Annex B system and an ADSL Annex C system were developed. The ADSL Annex B system is a variation of the ADSL system in which frequency bandwidth of upstream data is not contained in that of ISDN to use the ADSL system under ISDN. The ADSL Annex C system is designed for using the ADSL system under TCM-ISDN of half-duplex data transmission system used in countries such as Japan.  
           [0014]    ADSL services use a full-duplex data transmission system. Therefore, a receiver at either side receives data all the time. If such an ADSL modem and the TCM ISDN are installed in the same cable bundle, the strong near-end cross-talk (NEXT) interference due to TCM ISDN modems will severely affect the reception of the ADSL signal during the transmission of data.  
           [0015]    [0015]FIG. 3 is a diagram of interference at a central office when several ISDN lines transmit data at the same time. During transmit window  22  of FIG. 2, a burst of data is sent from the central office to remote sites. Near-end crosstalk (NEXT) interference of the ADSL modem at the central office is particularly strong during transmit window  22 , when the ISDN devices at the central office are all transmitting data. During receive time window  28 , these ISDN devices at the central office are not transmitting data. Far-end-crosstalk (FEXT) interference is weaker than NEXT interference because it is attenuated by the length of the telephone line. However, in the customer premises ISDN modem, near-end crosstalk (Next) interference during the transmission of data is stronger than FEXT interference during the reception of data.  
           [0016]    Accordingly, the ADSL modem installed with TCM-ISDN in the same cable bundle has to be designed such that ADSL signals are transceived in consideration of FEXT interference and NEXT interference. Dual bit-map (DBM) and FEXT bit-map (BBM) minimize NEXT interference. DBM uses different bit-map according to FEXT/NEXT interference period, and BBM which uses singular bit map transmits data only at a period of FEXT interference which is weaker than NEXT interference. When ADSL services are provided in a situation of TCM-ISDN, Next interference is stronger than FEXT interference throughout the whole bandwidth. In this situation, precise network time synchronization can be performed between ADSL and TCM-ISDN services, and data transmission may be maximized when data is transmitted in a DBM type.  
           [0017]    As described above, in TCM-ISDN, data is transmitted from a central office to a remote terminal when a TTR signal is at a high level. Inversely, data is transmitted from the remote terminal to the central office when the TTR signal is at a low level. However, since ADSL modem cannot directly use the TTR signal of the TCM-ISDN modem, the TTR signal is transmitted as the downstream data with a certain frequency during the initialization. For example, the central office according to ADSL Annex C transmits pilot tone of 276 kHz and TTR indication tone of 207 kHz at the same time. The remote terminal performs clock synchronization using the pilot tone. According to a hyperframe synchronized with the TTR signal, phase information of +45° during FEXT interference period and phase information of −45° during NEXT interference period are included in the TTR indication tone by the central office. Accordingly, the remote terminal analyzes a received TTR indication signal of 207 kHz and determines the phase information on FEXT interference and NEXT interference periods to constitute a hyperframe of a received frame.  
           [0018]    [0018]FIG. 4 is a diagram illustrating hyperframe symbols of ADSL downstream signals, and 345 symbols constitute one hyperframe. One hyperframe includes 345 frames and 1 symbol represents a symbol of a frame. Here, a period of a hyperframe is 85 ms, which is a multiple (i.e. 34 times) of a period (2.5 ms) of a TTR signal. Although FIG. 4 illustrates an example of a hyperframe including a cyclic-prefix, the foregoing description may be also applied to a hyperframe including no cyclic-prefix. But, at this time, a period of one hyperframe (345 symbols) is 80 ms.  
           [0019]    [0019]FIG. 5 is a table illustrating one hyperframe of an ADSL signal during 34 periods of a TTR signal. A slant-lined region is a symbol representing FEXT interference period, and the other region is a symbol representing NEXT interference period. Here, a symbol is determined to be FEXT interference or NEXT interference on the basis of vertical dotted lines.  
           [0020]    [0020]FIG. 6 is a timing diagram exemplarily illustrating that a remote terminal recognizes a symbol included in a TTR indication signal transmitted from a central office. The ADSL central office according to ADSL Annex C is synchronized with the TTR signal of TCM-ISDN during the communication initialization and generates a hyperframe. The ADSL central office stores symbols of the hyperframe in a signal of 207 kHz and transmits the symbols as a TTR indication signal TTR_I. When the TTR signal is at a high level (i.e. during NEXT interference period), a phase of the TTR indication signal TTR_I leads 45° ahead of that of an original signal. On the other hand, when the TTR signal is at a low level (i.e. during FEXT interference period), a phase of the TTR indication signal TTR_I lags 45° behind that of the original signal.  
           [0021]    The remote terminal interprets the TTR indication signal TTR_I to determine symbols of the hyperframe. If the remote terminal starts to receive from A point the signal transmitted from the central office, the remote terminal may detect phases of frames  2000 - 2004  and precisely determine a symbol of each of the frames  2000 - 2004  whether it is FEXT interference or NEXT interference. As a result, data inputted after the TTR indication signal can be precisely decoded to data of FEXT interference or NEXT interference period.  
           [0022]    However, in the case that the remote terminal of customer premises (i.e. ADSL modem) starts to receive from B point the TTR indication signal (TX) transmitted from the central office, a phase of the frame  1003  is different from those of other frames  1000 - 1002 , and  1004 . Thus, a symbol of the frame  1003  cannot be determined as to whether it is FEXT interference or NEXT interference. It is. impossible to precisely determine whether data inputted after the TTR indication signal TTR_I is data of FEXT interference period or data of NEXT interference period. As a result, data cannot be normally decoded.  
           [0023]    A need therefore exists for a full-duplex telecommunication system capable of precisely detecting a start position of a data signal inputted after initialization, when the telecommunication system uses a communication line positioned in vicinity of a communication line of a telecommunication system adopting half-duplex data transmission system according to a reference signal.  
           [0024]    There is a further need for an ADSL telecommunication system capable of precisely detecting a start position of a data signal inputted after a TTR indication signal, when signals are transceived via a digital subscriber line (DSL) installed with a communication line of TCM-ISDN system in the same cable bundle.  
         SUMMARY OF THE INVENTION  
         [0025]    According to an embodiment of the present invention, a telecommunication system is provided which uses a full-duplex data transmission system that has a second communication line positioned in the vicinity of a first communication line of a half-duplex data transmission system which is synchronized with a reference signal. The telecommunication system comprises a analog-to-digital converter for converting an analog signal received via the second communication line into a digital signal. A first-in first-out (FIFO) buffer serially stores a digital signal outputted from the analog-to-digital converter and serially outputs the digital signal in stored order. A fast Fourier transformer (FFT) transforms the digital signal of a time region outputted from the FIFO buffer into signal of a frequency region, and outputs a phase of a transformed signal. A receive data processor receives a signal of the frequency region outputted from the FFT, and decodes a received signal to an original signal. Also, a synchronization circuit receives a series of the phase information from the FFT and controls the receive data processor to be synchronized with a data signal of output signals from the FFT.  
           [0026]    In another embodiment of the present invention, an ADSL system is provided for transceiving data with a central office via digital subscriber line (DSL) positioned in the vicinity of a data transmission line of a time compression multiplexing-integrated services digital network (TCM-ISDN) system. The ADSL system comprises a analog-to-digital converter for converting an analog signal received from the DSL into a digital signal. A FIFO buffer serially stores the digital signal outputted from the analog-to-digital converter and serially outputs the digital signal in stored order. A fast Fourier transformer (FFT) transforms the digital signal of a time region outputted from the FIFO buffer into a signal of a frequency region and outputs a phase of a transformed signal. A receive data processor receives the signal of the frequency region outputted from the FFT, and decodes a received signal to an original signal. A synchronization circuit receives a series of the phase information from the FFT and controls the receive data processor to receive from a start position a data signal of signals output from the FFT.  
           [0027]    Preferably, the analog signal received via the digital subscriber line is provided by a TTR indication signal for a predetermined time and then the data signal after that time is over.  
           [0028]    The TTR indication signal includes symbols that respectively represent a far end crosstalk (FEXT) interference and a near end crosstalk (NEXT) interference period according to a TTR signal, which is the reference signal of the TCM-ISDN system.  
           [0029]    A synchronization circuit comprises a symbol detector, a phase controller, a memory, and a correlator. The symbol detector receives a phase from a fast Fourier transformer (FFT) and detects which symbol corresponds to a currently inputted phase. Also, when a currently inputted phase corresponds to none of symbols, the symbol detector outputs a phase difference between a prior inputted phase and the currently inputted phase. The phase controller controls a FIFO buffer such that digital signals of a time corresponding to the phase difference are removed among the digital signals stored in the FIFO buffer. The memory stores the symbols detected by the symbol detector. The correlator correlates symbols stored in the memory and symbols of frames constituting a hyperframe to control the receive data processor to receive the data signal inputted after the TTR indication signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    A more complete appreciation of the invention will be readily apparent with reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:  
         [0031]    [0031]FIG. 1 is a diagram of conventional ISDN line telecommunication system which has interference;  
         [0032]    [0032]FIG. 2 is a timing diagram for communicating in conventional TCM-ISDN line telecommunication system;  
         [0033]    [0033]FIG. 3 is a diagram of interference at a central office when several ISDN lines transmit data at the same time;  
         [0034]    [0034]FIG. 4 is a diagram illustrating hyperframe symbols of an ADSL downstream signal according to an embodiment of the present invention;  
         [0035]    [0035]FIG. 5 is a table illustrating one hyperframe of an ADSL signal during 34 periods of a TTR signal;  
         [0036]    [0036]FIG. 6 is a timing diagram exemplarily illustrating that a remote terminal recognizes a symbol included in a TTR indication signal transmitted from a central office;  
         [0037]    [0037]FIG. 7 is a diagram illustrating an ADSL modem according to an embodiment of the present invention;  
         [0038]    [0038]FIGS. 8A and 8B are flowcharts illustrating a process that a symbol detector detects symbols included in a TTR indication signal;  
         [0039]    [0039]FIG. 9 shows a correlation between absolute values of phase differences and overlapped times; and  
         [0040]    [0040]FIG. 10 is a diagram exemplarily illustrating that a first frame stored in a file is a 21 st  frame of a hyperframe. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0041]    The present invention will now be described hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown.  
         [0042]    [0042]FIG. 7 is a diagram illustrating an ADSL modem according to an embodiment of the present invention. An ADSL modem  100  comprises an analog-to-digital converter (ADC)  110  for converting an analog signal received via a digital subscriber line (DSL) into a digital signal. A first-in first-out (FIFO) buffer  120  stores a digital signal outputted from the ADC  110  and outputs the digital signal in stored order. A fast Fourier transformer (FFT)  130  transforms a signal of a frame of a time region outputted from the FIFO buffer  120  into a signal of a frequency region and outputs a phase of a transformed signal of the frequency region. A synchronization circuit  140  receives a series of phase information outputted from the FFT  130  and controls the FIFO buffer  120  and a receive data processor  150 . Also, the receive data processor  150  decodes a signal outputted from the FFT  130  to an original signal. The receive data processor  150 , as is known to those in the art, includes a frequency equalizer, a constellation encoder, a gain scaler, a tone ordering, a rate-converter, a deinterleaver, a descrambler, a forward error corrector, a cyclic redundancy checker, a MUX/SYNC controller, and the like (not shown). Detailed description of the receive data processor  150  will be omitted here for brevity.  
         [0043]    When power of a central office (not shown) or an ADSL modem  100  of subscriber&#39;s premises is cut off and then supplied again, or when communications between the central office and the ADSL modems of the customer&#39;s premises are interrupted due to failure of telephone lines transceiving signals and restoration of the lines to be normal, initialization for communication between the central office and the ADSL modem  100  of the customer premises is performed. The central office according to Annex C outputs a TTR indication signal during the initialization. When the TTR indication signal is received, the ADSL modem  100  according to Annex C performs an operation of setting a start position of a data signal inputted after the TTR indication signal, which will be described in detail hereinafter.  
         [0044]    Referring to FIG. 7, an ADC  110  converts a TTR indication signal TTR_I received via a digital subscriber line (DSL) into a digital signal. Converted digital signals are serially stored in the FIFO buffer  120 . The FFT  130  transforms the digital signal of a time region during a frame outputted from the FIFO  120  into a signal of a frequency region and outputs a phase θ i  of a converted signal of the frequency region.  
         [0045]    The symbol detector  141  receives the phase θ i  outputted from the FFT  130  and detects a symbol of the frame. Since the TTR indication signal TTR_I has a phase difference of 90° between a FEXT interference and a NEXT interference period, the symbol detector  141  cannot determine the symbol by only one phase from the FFT  130 . Typically, the symbol detector  141  can determine a phase of each frame based on correlations between phases of a series of frames. FIGS. 8A and 8B illustrate a process that the symbol detector  141  detects symbols included in the TTR indication signal TTR_I.  
         [0046]    Referring to FIG. 8A, in step S 200 , the symbol detector  141  initializes a count value (i) to 0. In step S 201 , the symbol detector  141  receives a phase θ i  outputted from the FFT  130 . In step S 202 , the symbol detector  141  sets a reference phase θ ref  to a current phase θ i  and a start value j to a count value (i). In step S 202 , the count value (i) is increased by 1. In step S 204 , the symbol detector  141  determines whether or not an absolute value (i.e., |θ ref −θ i |) of difference between the reference phase θ ref  and ith phase θ i  is less than a critical value δ. A phase of the TTR indication signal TTR_I received via the DSL line is influenced by noise. The critical value δ is a permissible error value caused by the noise. When the absolute value of difference between the reference phase θ ref  and ith phase θ i  is less than the critical value δ, symbols of the frames corresponding to each of the reference phase θ ref  and ith phase θ i  are regarded as the same. When the absolute value of difference between the reference phase θ ref  and ith phase θ i  is equal to or larger than the critical value δ, the symbols of the frames corresponding to each of the reference phase θ ref  and ith phase θ i  are regarded as different. When the absolute value of difference between the reference phase θ ref  and ith phase θ i  is smaller than a critical value δ, the control proceeds to step S 206 . Otherwise, the control returns to step S 202 .  
         [0047]    In step S 206 , the symbol detector  141  determines whether or not a count value (i) is 2. As a result, if the count value (i) is 2, the control proceeds to step S 207 . Otherwise, the control returns to step S 203 . Accordingly, in steps S 202  to S 206 , when subsequent 3 phases out of phases inputted from the FFT  130  are the identical, the control proceeds to step S 207 . As can be known from the hyperframe illustrated in FIG. 5, the present embodiment is to use a characteristic that an unobvious frame, which cannot be determined as to whether it belongs to FEXT interference or NEXT interference, always follows at least 3 frames having an identical symbol of FEXT interference or NEXT interference. If a phase of a first frame outputted from the FFT  130  is different from that of a second frame, it cannot be discerned which of the first and second frames is the unobvious frame that cannot determine symbols. In this case, a frame having the different phase is determined as to whether it is an unobvious frame, when phases of next frames are received, at least 3 frames serially inputted are found to be identical, and a phase different from those of the prior frames is inputted.  
         [0048]    In step S 207 , the symbol detector  141  increases the count value (i) by 1. In step S 208 , the symbol detector  141  receives the phase θ i  from the FFT  130 . In step S 209 , the symbol detector  141  determines whether or not absolute value (i.e., |θ ref −θ i |) of difference between the reference phase θ ref  and ith phase θ i  is larger than a critical value δ. When an absolute value of difference between the reference phase θ ref  and ith phase θ i  is larger than a critical value δ, the control proceeds to step S 210 . Otherwise, the control returns to step S 207 . In the case that an absolute value of difference between the reference phase θ ref  and ith phase θ i  is larger than a critical value δ, symbols of frames corresponding to each of the reference phase θ ref  and ith phase θ i  are different from each other.  
         [0049]    In step S 210 , the symbol detector  141  determines whether or not an absolute value of difference between the reference phase θ ref  and ith phase θ i  is 90°. If the absolute value of difference between the reference phase θ ref  and ith phase θ i  is 90°, the control proceeds to step S 220  of FIG. 8B. Otherwise, the control proceeds to step S 211 .  
         [0050]    In step S 211 , the symbol detector  141  outputs an absolute value of difference between the reference phase θ ref  and ith phase θ i  to a slicer  142 . The slicer  142  calculates an overlapped time corresponding to a phase difference |θ ref −θ i | outputted from the symbol detector  141  by referring to a look-up table  143  and outputs a control signal CTRL 1  such that the FIFO buffer  120  deletes data corresponding to overlapped times.  
         [0051]    [0051]FIG. 9 is a diagram illustrating a correlation between absolute values |θ ref −θ i | of the phase differences stored in the look-up table  143  and the overlapped times. Referring to FIG. 9, for example, if an absolute value of difference between the reference phase θ ref  and θ i  is 40°, overlapped time is about 4.0 μs. At this time, the slicer  142  outputs a control signal CTRL 1  to delete data corresponding to the time of 4.0 μs among data stored in the FIFO  120 . Accordingly, when the TTR indication signal TTR_I outputted from the FIFO buffer  120  is fast Fourier transformed (FFT), a transformed signal of a frame includes one of FEXT interference and NEXT interference, and an unobvious signal, which cannot be determined to be FEXT interference or NEXT interference, is removed.  
         [0052]    Referring to FIG. 8B, in step S 220 , the phase detector  141  determines whether or not difference (θ ref −θ i ) between the reference phase θ ref  and ith phase θ i  is +90°. As a result, if difference (θ ref −θ i )between the reference phase θ ref  and ith phase θ i  is +90°, the control proceeds to step S 221 , thereby defining all symbols of jth to i-1th frames as FEXT interference. Inversely, if difference (θ ref −θ i ) between the reference phase θ ref  and ith phase θ i  is not +90° (i.e. −90°), the control proceeds to step S 221 , thereby defining all symbols of jth to i-1th frames as NEXT interference.  
         [0053]    In step S 223 , the symbol detector  141  sets θ 0  to θ i , θ ref  to θ i , and j to i. In step S 224 , the symbol detector  141  initializes a count value (i) to 0, and the control returns to step S 207 .  
         [0054]    Referring again to FIG. 6, the ADSL modem  100  of the subscriber&#39;s premises can precisely determine a symbol of each frame, when the TTR indication signal TTR_I is received from A point where TTR_I is synchronized with the TTR signal. However, in the case that an initial receive point is other than A point, there may be an unobvious frame, which cannot be determined to be FEXT interference or NEXT interference.  
         [0055]    For example, when the ADSL modem  100  of the subscriber&#39;s premises initially receives the TTR indication signal TTR_I from B point, operations of the symbol detector  141  will be described herein to explain existence of the unobvious frame. To begin with, the symbol detector  141  receives a phase θ 0  of 0th frame  1000  outputted from the FFT  130  (step S 201 ) and sets a reference phase θ ref  to the phase θ 0  of the 0th frame  1000  (step S 201 ). At this time, a start value j becomes 0. Next, the symbol detector  141  receives a phase θ 1  of 1st frame  1001  (step S 204 ) and determines whether or not an absolute value of difference between the reference phase θ ref  and the present phase θ 1  is smaller than a critical value δ (step S 205 ). As illustrated in FIG. 6, because phases of the 0th and 1st frames  1000  and  1001  are the identical and the count value (i) is 1, the control returns to step S 203 . Then, the symbol detector  141  receives a phase θ 2  of 2nd frame  1002  outputted from the FFT  130  (step S 204 ). The phase θ ref  of the 0th frame  1000  or a reference phase is equal to the phase θ 2  of the 2nd frame  1002  and the count value (i) is 2, so the control proceeds to step S 207 . The symbol detector  141  receives a phase θ 3  of 3rd frame  1003  outputted from the FFT  130  (step S 208 ). Because the phase θ 0  of the 0th frame  100  or the reference phase is different from the phase θ 3  of the 3rd frame  1003  (step S 209 ) and the difference therebetween is not 90° (step S 210 ), the symbol detector  141  outputs an absolute value of difference between the phase θ 0  of the 0th frame  1000  and the phase θ 3  of the 3rd frame  1003  (step S 211 ). The slicer  142  calculates an overlapped time corresponding to the difference between the reference phase θ 0  and the phase θ 3  of the 3rd frame  1003  and outputs a control signal CTRL 1 . The 3rd frame  1003  includes a signal  1003 F of the FEXT interference period and a signal  1003 N of the NEXT interference period. The difference between the phase θ 3  of the 3rd frame  1003  and the phase θ 0  of the 0th frame  100  is dependant on the signal  1003 N of the NEXT interference period included in the 3rd frame  1003 . The control signal CTRL 1  is for deleting signals among the TTR indication signal TTR_I stored in the FIFO buffer  120  based on a predetermined time, such that the signals of the FEXT interference and NEXT interference periods are not simultaneously included in a frame.  
         [0056]    After the foregoing processes, each frame outputted from the FIFO buffer  120  includes a signal of only one of the FEXT interference and NEXT interference periods. For instance, in FIG. 6, the ADSL modem  100  starts to receive from A point a TTR indication signal TTR_I. The symbol detector  141  continues to determine a symbol of the signal outputted from the FFT  130 . Since a frame includes a signal of only one of the FEXT interference and NEXT interference periods, the frames include the same symbol when phases of the subsequent two frames are the same. However, when the subsequent two frames have difference of +90° or −90°, the frames have different symbols.  
         [0057]    Referring again to FIGS. 6, 8A, and  8 B, when the FIFO  120  outputs a digital signal from A point the TTR indication signal TTR_I, operations of the symbol detector  141  will be described hereinafter. The symbol detector  141  receives a phase θ 0  of 0th frame  2000  outputted form the FFT  130  (step S 201 ) and sets a reference phase θ ref  to the present phase θ 0  (step S 202 ). At this time, a start value j is set to a count value (i). The symbol detector  141  serially receives a phase θ 1  of 1st frame  2001  outputted from the FFT  130  and a phase θ 2  of the 2nd frame  2001 , and the control proceeds to step S 207  (steps S 203 -S 206 ). The symbol detector  141  receives a phase θ 3  of 3rd frame  2003  outputted from the FFT  130  (steps S 207 -S 208 ). Because the reference phase θ 0  is equal to the phase θ 3  of the 3rd frame  2003  (step S 209 ), the symbol detector  141  receives a phase θ 4  of 4th frame  2004  outputted from the FFT  130 . Difference between the reference phase θ 0  and the phase θ 4  of the 4th frame  2004  is +90°. As stated above, the TTR indication signal TTR_I leads 45° ahead of an original signal ORIGIN at the FEXT interference period, but lags 45° behind the original signal ORIGIN at the NEXT interference period. Therefore, when a prior frame is a FEXT signal and a present frame is a NEXT signal, a phase difference therebetween is +90°. Also, when the prior frame is a NEXT signal and the present frame is a FEXT signal, a phase difference therebetween is −90°. Accordingly, the symbol detector  141  defines a symbol of each of the 0th to 3rd frames  2000 - 2003  as FEXT interference (steps S 210 , S 220 , and S 221 ). Defined symbols of the 0th to 3rd frames  2000 - 2003  are stored in a file  144  of FIG. 7.  
         [0058]    According to the foregoing method, symbols of the 345 frames are stored in the file  144 . The correlator  145  correlates a pre-defined hyperframe  146  to frames stored in the file  144  to discern which frame of the hyperframe  146  is the first frame stored in the file  144 . The hyperframe  146  is illustrated in FIG. 5. The first frame stored in the file  144  may be one of 0 th  frame to 344 th  frame of the hyperframe  146 .  
         [0059]    [0059]FIG. 10 exemplarily illustrates that the first frame stored in the file  144  is the 21 st  frame of the hyperframe  146 . Even if the first frame stored in the file  144  is the 21 st  frame of the hyperframe  146 , the central office typically starts to transmit data from the first frame of the hyperframe  146 .  
         [0060]    For example, if a central office transmits a TTR indication signal TTR_I during 3 hyperframes and then transmits a data signal, the ADSL modem  100  receives the TTR indication signal TTR_I during 3 hyperframes and then receives the data signal. However, as illustrated in FIG. 10, if the ADSL modem  100  starts to receive data from the 21 st  frame of the TTR indication signal TTR_I, the ADSL mistakenly detects a start position of the data signal. Thus, the correlator  145  outputs a control signal CTRL 2  such that FEQ  150  starts to receive data from a start position of the data signal outputted from the FFT  130  after the correlator  145  discerns which frame of the hyperframe is the first frame stored in the file  144 .  
         [0061]    The FEQ  150  receives the data signal outputted from the FFT  130  in response to the control signal CTRL 2 . Accordingly, a data received via the digital subscriber line DSL is decoded to an original data by the FEQ  150  and the receive data processor  150 .  
         [0062]    As set forth before, when signals are transceived via a digital subscriber line (DSL) installed with communication lines of a TCM-ISDN system in the same cable bundle, a telecommunication system according to the present invention can precisely detect a start position of a data signal inputted after a TTR indication signal.  
         [0063]    While the present invention has been described in connection with specific and preferred embodiments thereof, it is capable of various changes and modifications without departing from the spirit and scope of the invention. It should be appreciated that the scope of the invention is not limited to the detailed description of the invention hereinabove, which is intended merely to be illustrative, but rather comprehends the subject matter defined by the following claims.