Patent Publication Number: US-6658024-B1

Title: Data transmission device and data transmission method capable of minimizing delay of fast data both in normal conditions and noisy conditions

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a data transmission device and a data transmission method, and in particular, to a data transmission device and a data transmission method which are applied to XDSL (X Digital Subscriber Line (X: A, S, V, etc.)) which realizes high speed data transmission of the order of Mbit/s by use of metal cables such as telephone lines. 
     DESCRIPTION OF THE PRIOR ART 
     These days, techniques concerning XDSL (X Digital Subscriber Line (X: A (Asymmetric), S (Symmetric), V (Very high speed), etc.)) for realizing high speed data transmission of the order of Mbit/s via metal cables such as telephone lines are attracting attention. Most of all, ADSL (Asymmetric Digital Subscriber Line), whose bit rate asymmetry between upstream data transmission and downstream data transmission is suitable for the access to the Internet, is receiving much attention. 
     In an ADSL transceiver unit, digital signals are converted to analog signals according to a modulation/demodulation method called DMT (Discrete Multi-Tone), and the analog signals are transmitted by the ADSL transceiver unit. In the DMT method, modulation according to QAM (Quadrature Amplitude Modulation) is executed to 256 carriers, and the modulated carriers are multiplexed together by means of IDFT (Inverse Discrete Fourier Transform), and the multiplexed signal is transmitted. On the receiving side, each modulated carrier is extracted from the multiplexed signal by means of DFT (Discrete Fourier Transform), and each QAM-modulated carrier is demodulated, and thereby high speed data transmission is implemented. 
     However, in the case of the ADSL transceiver unit, if a cable used for ADSL (an ADSL cable) is included in a cable bundle that also includes an ISDN (Integrated Services Digital Network) cable, noise is caused in the ADSL cable due to a variety of effects of the ISDN cable, and thereby the transmission bit rate of the ADSL cable is necessitated to be deteriorated. Most of all, crosstalk noise from the ISDN cable has a large effect on the ADSL cable. 
     In the following, the crosstalk noise which occurs to the ADSL transceiver unit in the case where a TCM(Time Compression Multiplex)-ISDN line is placed adjacent to an ADSL line will be explained referring to FIG.  1 . FIG. 1 shows crosstalk noise which occurs to an ADSL terminal ATU-R (ADSL Transceiver Unit at the Remote end) due to data transmission on the TCM-ISDN line when downstream data transmission is being executed on the ADSL line. 
     In the TCM-ISDN, upstream data transmission (1.25 msec) and downstream data transmission (1.25 msec) are alternately executed. If upstream data transmission on the TCM-ISDN line is executed when downstream data transmission is being executed on the ADSL line, high power signals on the TCM-ISDN line before attenuation exert effects on attenuated signals on the ADSL line, and thereby NEXT (Near End X(cross)-Talk) occurs to the terminal ATU-R. If downstream data transmission on the TCM-ISDN line is executed when downstream data transmission is being executed on the ADSL line, signals on the TCM-ISDN line exert effects on attenuated signals on the ADSL line, and thereby FEXT (Far End X(cross)-Talk) occurs to the terminal ATU-R. Incidentally, the same effects also occur on the side of the central office ATU-C (ADSL Transceiver Unit at the Central office). 
     FIG. 2 is a schematic diagram showing noise levels of the NEXT and the FEXT. As shown in FIG. 2, the noise level of NEXT is higher than that of FEXT, since high power signals on the TCM-ISDN line before attenuation exert effects on attenuated signals on the ADSL line in the case of NEXT. There has been proposed a data transmission method which pays attention to the difference of the noise level, in which the data rate (data transmission bit rate) in FEXT periods (i.e. periods in which the lower noise due to the FEXT occurs) is set larger than the data rate in NEXT periods (i.e. periods in which the higher noise due to the NEXT occurs) as shown in the bottom of FIG.  2 . Such a method is called “dual bit-map method”. 
     Since the noise level changes periodically in the ADSL transceiver unit as mentioned above, generally, the SNR (Signal to Noise Ratio) of each carrier through the ADSL line is measured both in the upstream data transmission and in the downstream data transmission of the TCM-ISDN line (that is, both in the NEXT periods and in the FEXT periods), and thereby bit distribution and gain distribution onto the carriers are determined with respect to FEXT periods and with respect to NEXT periods, based on the SNR. FIG. 3 shows an example of such bit distribution. In FIG. 3, the horizontal axis denotes the frequency of each carrier to be used for the data transmission, and the frequency difference between adjacent carriers is 4.3125 kHz, and the number of carriers is 256. Each carrier is modulated when data is transmitted. The SNR of each carrier is evaluated in the NEXT periods and in the FEXT periods, as a result of transmission of a pseudo random signal which will be described below. Thereafter, actual data transmission is executed according to the bit distribution and the gain distribution which have been determined based on the measurement of the SNR. 
     In the following, a concrete example of the SNR evaluation and the bit/gain distribution calculation will be described referring to FIG.  3  through FIG.  5 . FIG. 4 is a block diagram showing an example of an ADSL data communication system. Referring to FIG. 4, an ATU-C  21  is provided to the system as the central office, and an ATU-R  22  is provided to the system as a terminal. The ATU-C  21  includes a transmission section  23 A and a reception section  24 A, and the ATU-R  22  includes a transmission section  23 B and a reception section  24 B. 
     FIG. 5 is a block diagram showing the composition of the ATU-C  21  and the ATU-R  22  of the ADSL data communication system of FIG.  4 . The ATU-C  21  on the left-hand side of FIG. 5 includes the transmission section  23 A, the reception section  24 A, a pseudo random signal generation section  25 A, an SNR measurement section  26 A, a bit/gain distribution calculation section  27 A and a bit/gain distribution table  28 A. The transmission section  23 A includes an IDFT (Inverse Discrete Fourier Transform) section  10 A, and the reception section  24 A includes a DFT (Discrete Fourier Transform) section  29 A. Similarly, the ATU-R  22  on the right-hand side of FIG. 5 includes the transmission section  23 B, the reception section  24 B, a pseudo random signal generation section  25 B, an SNR measurement section  26 B, a bit/gain distribution calculation section  27 B and a bit/gain distribution table  28 B. The transmission section  23 B includes an IDFT section  10 B, and the reception section  24 B includes a DFT section  29 B. 
     In the ATU-C  21 , the pseudo random signal generation section  25 A generates a pseudo random signal which includes all the carriers (256 carriers according to the standard of ANSI (American National Standards Institution), for example) and sends the pseudo random signal to the IDFT section  10 A of the transmission section  23 A so as to be transmitted to the ATU-R  22 . The SNR measurement section  26 A calculates the SNR of each carrier of a pseudo random signal supplied from the ATU-R  22 , with respect to the FEXT periods and the NEXT periods. The bit/gain distribution calculation section  27 A calculates and determines the bit distribution and the gain distribution of the carriers with respect to the FEXT periods and the NEXT periods, based on the SNR of each carrier in the FEXT periods and the SNR of each carrier in the NEXT periods which have been obtained by the SNR measurement section  26 A. The bit/gain distribution table  28 A receives bit distribution and gain distribution of the carriers which have been calculated and determined by the bit/gain distribution calculation section  27 B of the ATU-R  22 , and stores the bit distribution and the gain distribution. 
     In the ATU-R  22 , the pseudo random signal generation section  25 B generates a pseudo random signal which includes all the carriers and sends the pseudo random signal to the IDFT section  10 B of the transmission section  23 B so as to be transmitted to the ATU-C  21 . The SNR measurement section  26 B calculates the SNR of each carrier of a pseudo random signal supplied from the ATU-C  21 , with respect to the FEXT periods and the NEXT periods. The bit/gain distribution calculation section  27 B calculates and determines the bit distribution and the gain distribution of the carriers with respect to the FEXT periods and the NEXT periods, based on the SNR of each carrier in the FEXT periods and the SNR of each carrier in the NEXT periods which have been obtained by the SNR measurement section  26 B. The bit/gain distribution table  28 B receives bit distribution and gain distribution of the carriers which have been calculated and determined by the bit/gain distribution calculation section  27 A of the ATU-C  21 , and stores the bit distribution and the gain distribution. 
     In the following, an example of a process executed by the ADSL data communication system of FIG. 5 for determining the bit distribution and the gain distribution onto the carriers in downstream data transmission will be explained referring to FIG.  5 . First, the ATU-C  21  outputs a pseudo random signal from its pseudo random signal generation section  25 A for obtaining the bit distribution and the gain distribution of the carriers in the downstream data transmission. The pseudo random signal outputted by the pseudo random signal generation section  25 A is supplied to the IDFT section  10 A of the transmission section  23 A and IDFT (Inverse Discrete Fourier Transform) is executed to the pseudo random signal by the IDFT section  10 A. The pseudo random signal to which the IDFT has been executed is transmitted to the ATU-R  22  via the ADSL line. The pseudo random signal (to which the IDFT has been executed and which has been transferred to the ATU-R  22 ) is received by the reception section  24 B of the ATU-R  22 , and the signal is supplied to the DFT section  29 B. The DFT section  29 B executes DFT (Discrete Fourier Transform) to the pseudo random signal (to which the IDFT has been executed and which has been transferred to the ATU-R  22 ) and thereby obtains a received pseudo random signal which includes all the carriers (256 carriers, for example). The received pseudo random signal is supplied to the SNR measurement section  26 B. The SNR measurement section  26 B obtains the SNR of each carrier based on the received pseudo random signal including all the carriers, with respect to NEXT periods and FEXT periods. The SNR of each carrier with respect to NEXT periods and the SNR of each carrier with respect to FEXT periods which have been measured by the SNR measurement section  26 B are supplied to the bit/gain distribution calculation section  27 B, therefore, the bit/gain distribution calculation section  27 B receives 512 SNRs (256×2), for example. In the bit/gain distribution calculation section  27 B, the bit distribution and the gain distribution of the carriers are calculated with respect to NEXT periods and FEXT periods as shown in FIG.  3 . The bit/gain distribution information of the carriers with respect to NEXT periods and FEXT periods which has been obtained by the bit/gain distribution calculation section  27 B is transmitted by the transmission section  23 B to the ATU-C  21  via the ADSL line. The ATU-C  21  which received the bit/gain distribution information via the ADSL line stores the bit/gain distribution in the bit/gain distribution table  28 A. The above process for determining the bit/gain distribution is executed in initialization (when data communication is started, for example), in the fast retraining period (defined in ITU standard), etc. 
     In the following, an ATU (ADSL Transceiver Unit) transmitter for transmitting data according to the bit/gain distribution obtained by the above process will be described referring to FIG.  6 . FIG. 6 is a block diagram showing the composition of a conventional ATU transmitter. Incidentally, the ATU transmitter of FIG. 6 is a device which corresponds to the transmission section ( 23 A or  23 B) of the ATU (ADSL Transceiver Unit) ( 21  or  22 ) shown in FIG.  5 . 
     The ATU transmitter shown in FIG. 6 includes a multiplexing/synchronization control section  30 , a fast data CRC (Cyclic Redundancy Check) section  31 , scrambling/error correction sections  32  and  35 , an interleave data CRC section  34 , an interleaving section  36 , rate converters  33  and  37 , a tone ordering section  38 , a constellation encoding/gain scaling section  39 , and an IDFT (Inverse Discrete Fourier Transform) section  40 . 
     The ATU transmitter is provided with two types of channels: a fast channel which is suitable for communication of data such as video data, voice data, data for real-time application software, etc. to which delay is harmful, and an interleave channel which is suitable for data communication such as file transfer to which delay is not so harmful but burst error is very harmful. The difference between the two types of channels is the presence or absence of interleaving process, in which the order of transmitted data is changed and thereby the effects of the burst error is decreased. In the interleave channel, even if burst error occurred due to impulse noise etc., the receiving side of the interleave channel restores the order of the rearranged data into the original order and thereby the burst error is spread out, and thus the data can be correctly restored by error correction. However, data have to be temporarily stored in the interleaving process, and thus delay is caused in the interleave process. 
     In the following, the operation of the ATU transmitter of FIG. 6 will be explained briefly. Data (in which the ratio between the amount of fast data and the amount of interleave data is generally fixed according to initial settings) are supplied from an upper layer device (such as a computer) to the ATU transmitter at a prespecified data rate, and are separated by the multiplexing/synchronization control section  30  into fast data and interleave data. The fast data are supplied to a fast path (the upper path in FIG. 6) and the interleave data are supplied to an interleave path (the lower path in FIG.  6 ). In the fast path, the fast data CRC section  31  adds CRC code to the fast data. The scrambling/error correction section  32  scrambles the fast data supplied from the fast data CRC section  31 , adds Reed-Solomon error correction code to the scrambled fast data, and supplies the data to the rate converter  33 . Also in the interleave path, the interleave data CRC section  34  adds CRC code to the interleave data, and the scrambling/error correction section  35  executes the scrambling and the addition of error correction code. Thereafter, the interleaving section  36  in the interleave path changes the order of the interleave data supplied from the scrambling/error correction section  35  and thereby executes the interleaving, and sends the interleaved data to the rate converter  37 . 
     Each of the rate converters  33  and  37  recognizes change of the noise level due to the TCM-ISDN line by use of a TTR (TCM-ISDN Timing Reference) signal which is in sync with the change of data transmission direction in the TCM-ISDN line. Thereby, the rate converter ( 33 ,  37 ) increases the amount of data to be supplied to the tone ordering section  38  so as to raise the data rate in periods in which the noise due to the TCM-ISDN line is low (i.e. in the FEXT periods). In periods in which the noise due to the TCM-ISDN line is high (i.e. in the NEXT periods), the rate converter ( 33 ,  37 ) decreases the amount of data to be supplied to the tone ordering section  38  so as to lower the data rate. 
     The tone ordering section  38  obtains information concerning the number of bits to be assigned to each carrier, by use of the bit distribution information of the carriers which is supplied from the bit/gain distribution table, and determines fast data and interleave data to be assigned to each carrier. In the constellation encoding/gain scaling section  39 , the data assigned by the tone ordering section  38  to each carrier is converted into constellation position information using a constellation, and thereby corresponding Fourier transform coefficients are obtained. The Fourier transform coefficients with respect to each carrier are supplied to the IDFT section  40 . The IDFT section  40  which received the 512 Fourier transform coefficients with respect to the 256 carriers executes IDFT (Inverse Discrete Fourier Transform) to the coefficients and thereby obtains a voltage signal V(t) which varies in time. The IDFT section  40  outputs 512 sampled voltages of the voltage signal V(t) (i.e. V(t) corresponding to 512 times “t” in a symbol cycle) successively to the ADSL line. 
     FIG. 7 is a bar graph showing the ratio between the fast data and the interleave data which are assigned to each symbol (frame), in the case where data transmission is executed according to the dual bit-map method which has been described above. In FIG. 7, each bar corresponds to a symbol (frame), in which all the 256 carriers are included. The FEXT/NEXT cycle (2.5 msec) of the TCM-ISDN line corresponds to (10+α) symbols. As shown in FIG. 7, the amount of data to be assigned to a symbol is increased in the period in which the noise is low (in the FEXT period on the left-hand side of the graph), and is decreased in the period in which the noise is high (in the NEXT period on the right-hand side of the graph). However in FIG. 7, the ratio between the fast data and the interleave data assigned to one symbol is fixed, regardless of whether the noise is high or low. 
     In the data transmission according to the dual bit-map method, in the 2.5 msec FEXT/NEXT cycle (upstream/downstream data transmission cycle) of the TCM-ISDN line, the number of symbols in which the amount of transmitted data can be set large (hereafter, referred to as “FEXT symbols”) and the number of symbols in which the amount of transmitted data has to be set small (hereafter, referred to as “NEXT symbols”) can not be fixed, since the 2.5 msec FEXT/NEXT cycle of the TCM-ISDN line corresponds to (10+α (not an integer)) symbols on the ADSL line. 
     The phenomenon will be explained more concretely referring to FIG.  8 . FIG. 8 shows the timing when the data transmission direction is changed in the TCM-ISDN line, and symbols which are transmitted by the ADSL transceiver unit in each FEXT/NEXT cycle of the TCM-ISDN line. Symbols shown on the left-hand side of FIG. 8 are the FEXT symbols which can carry larger amount of data because of the low noise due to the FEXT from the TCM-ISDN line, and symbols shown on the right-hand side of FIG. 8 are the NEXT symbols which can carry smaller amount of data because of the high noise due to the NEXT from the TCM-ISDN line. As shown in FIG. 8, a little and various difference occurs between the timing when the data transmission direction is changed in the TCM-ISDN line and the timing when the amount of data transmitted by the ADSL line is changed based on the TTR signal in each FEXT/NEXT cycle. Therefore, the number of FEXT symbols which can be transmitted by the ATU transmitter in each FEXT/NEXT cycle of the TCM-ISDN line changes between 3 and 4 as shown in FIG.  8 . 
     In the ADSL transceiver unit, the bit rate of data which. are supplied from the upper layer device is fixed. Therefore, if the number of the FEXT symbols in a FEXT/NEXT cycle changes between 3 and 4, the amount of data which can be transmitted in a fixed cycle is necessitated to be changed, and thus data are required to be temporarily stored in the rate converter. The storage in the rate converter causes additional delay. 
     Further, in the conventional dual bit-map method, the ratio between fast data and interleave data which are assigned to one symbol is fixed as shown in FIG. 7, regardless of whether the noise is high or low (that is, regardless of whether the symbol is a FEXT symbol or a NEXT symbol). Therefore, the amount of fast data which are transmitted in a fixed time period changes in time, and thereby delay is necessitated to be caused in the transmission of fast data due to buffering (temporarily storing data), even though delay is harmful to the fast data such as video data, voice data, data for real-time application software, etc. 
     Furthermore, if the noise level due to the crosstalk from the TCM-ISDN line in the NEXT periods becomes very high, the amount of fast data which can be transmitted in each NEXT symbol is necessitated to be very small, and thereby a considerable amount of delay is caused in the transmission of the fast data. 
     SUMMARY OF THE INVENTION 
     It is therefore the primary object of the present invention to provide a data transmission device and a data transmission device, by which the delay of the fast data can be minimized. 
     In accordance with a first aspect of the present invention, there is provided a data transmission device which receives data including fast data of a first prespecified data rate and interleave data of a second prespecified data rate from an upper layer device and executes data transmission in environments where the noise level changes periodically by use of the dual bit-map method in which a larger amount of data are assigned to each frame that is transmitted in low noise periods and a smaller amount of data are assigned to each frame that is transmitted in periods including high noise periods. In the case (case “A”) where the amount of data which can be assigned to each frame that is transmitted in the high noise periods is larger than or equal to a specific amount which corresponds to the first prespecified data rate, fast data of the specific amount are assigned to each frame regardless of whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. In the case (case “B”) where the amount of data which can be assigned to each frame that is transmitted in the high noise periods is smaller than the specific amount, only fast data are assigned to each frame that is transmitted in periods including the high noise periods, and fast data of an amount larger than the specific amount are assigned to each frame that is transmitted in the low noise periods. 
     In accordance with a second aspect of the present invention, in the first aspect, the amount of interleave data which are assigned to each frame to be transmitted is controlled, based on the change of the amount of data which are assigned to each frame due to the periodic change of the noise level and based on the amount of fast data which are assigned to each frame. 
     In accordance with a third aspect of the present invention, in the first aspect, the data transmission device is a device which executes data transmission using multi-carrier, comprising a separation means, an interleaving means, a low noise frame number determination means, a fast data rate control means, an interleave data rate control means, a tone ordering means, a coefficient conversion means and a transform means. The separation means separates the data supplied from the upper layer device into the fast data and the interleave data. The interleaving means executes interleaving to the interleave data which have been separated by the separation means. The low noise frame number determination means figures out the number of frames which can be transmitted in each low noise period. The fast data rate control means controls the data rate of the fast data. The interleave data rate control means controls the data rate of the interleave data. The tone ordering means assigns the fast data and the interleave data to the carriers of the frame depending on whether it is in the case “A” or in the case “B”. In the case “A”, the fast data are assigned to the carriers of the frame starting from a carrier that can carry the smallest number of bits so that the amount of the fast data included in the frame will be equal to the specific amount, and the interleave data are assigned to the carriers of the frame starting from a carrier that can carry the largest number of bits. The data assignment is executed based on preobtained bit distribution of the multi-carrier with respect to the low noise periods or with respect to the high noise periods, depending on whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. In the case “B”, if the frame is a frame that is transmitted in the low noise periods, the fast data are assigned to the carriers of the frame starting from a carrier that can carry the smallest number of bits so that the amount of the fast data included in the frame will be larger than the specific amount by an amount that is determined by the number figured out by the low noise frame number determination means, and the interleave data are assigned to the carriers of the frame starting from a carrier that can carry the largest number of bits, based on preobtained bit distribution of the multi-carrier with respect to the low noise periods. In the case “B”, if the frame is a frame that is transmitted in periods including the high noise periods, only fast data are assigned to the carriers of the frame based on preobtained bit distribution of the multi-carrier with respect to the high noise periods. The coefficient conversion means converts data which have been assigned by the tone ordering means to each carrier of the frame into corresponding coefficients, based on the preobtained bit distribution and preobtained gain distribution of the multi-carrier with respect to the low noise periods or with respect to the high noise periods depending on whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. The transform means transforms the coefficients with respect to the carriers of the frame into a signal which varies in time. 
     In accordance with a fourth aspect of the present invention, in the third aspect, the transform means is an IDFT (Inverse Discrete Fourier Transform) means for executing inverse discrete Fourier transform to Fourier transform coefficients with respect to the carriers of the frame which have been obtained by the coefficient conversion means and thereby obtaining the signal which varies in time. 
     In accordance with a fifth aspect of the present invention, in the third aspect, the data transmission device further comprises a fast data CRC (Cyclic Redundancy Check) means, a first scrambling/error correction means, an interleave data CRC means and a second scrambling/error correction means. The fast data CRC means executes the cyclic redundancy check and adds CRC code to the fast data which have been separated by the separation means. The first scrambling/error correction means scrambles the fast data supplied from the fast data CRC means and adds error correction code to the scrambled fast data. The interleave data CRC means executes the cyclic redundancy check and adds CRC code to the interleave data which have been separated by the separation means. The second scrambling/error correction means scrambles the interleave data supplied from the interleave data CRC means and adds error correction code to the scrambled interleave data. 
     In accordance with a sixth aspect of the present invention, in the first aspect, the data transmission device is an ATU (ADSL (Asymmetric Digital Subscriber Line) Transceiver Unit) transmitter which is used as a data transmitter of an ATU (ADSL Transceiver Unit). 
     In accordance with a seventh aspect of the present invention, there is provided a data transmission method for receiving data including fast data of a first prespecified data rate and interleave data of a second prespecified data rate from an upper layer device and executing data transmission in environments where the noise level changes periodically by use of the dual bit-map method in which a larger amount of data are assigned to each frame that is transmitted in low noise periods and a smaller amount of data are assigned to each frame that is transmitted in periods including high noise periods. In the case (case “A”) where the amount of data which can be assigned to each frame that is transmitted in the high noise periods is larger than or equal to a specific amount which corresponds to the first prespecified data rate, fast data of the specific amount are assigned to each frame regardless of whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. In the case (case “B”) where the amount of data which can be assigned to each frame that is transmitted in the high noise periods is smaller than the specific amount, only fast data are assigned to each frame that is transmitted in periods including the high noise periods, and fast data of an amount larger than the specific amount are assigned to each frame that is transmitted in the low noise periods. 
     In accordance with an eighth aspect of the present invention, in the seventh aspect, the amount of interleave data which are assigned to each frame to be transmitted is controlled, based on the change of the amount of data which are assigned to each frame due to the periodic change of the noise level and based on the amount of fast data which are assigned to each frame. 
     In accordance with a ninth aspect of the present invention, in the seventh aspect, the data transmission is executed using multi-carrier. The data transmission method comprises a separation step, an interleaving step, a low noise frame number determination step, a fast data rate control step, an interleave data rate control step, a tone ordering step, a coefficient conversion step and a transform step. In the separation step, the data supplied from the upper layer device are separated into the fast data and the interleave data. In the interleaving step, interleaving is executed to the interleave data which have been separated in the separation step. In the low noise frame number determination step, the number of frames which can be transmitted in each low noise period is figured out. In the fast data rate control step, the data rate of the fast data is controlled. In the interleave data rate control step, the data rate of the interleave data is controlled. In the tone ordering step, the fast data and the interleave data are assigned to the carriers of the frame depending on whether it is in the case “A” or in the case “B”. In the case “A”, the fast data are assigned to the carriers of the frame starting from a carrier that can carry the smallest number of bits so that the amount of the fast data included in the frame will be equal to the specific amount, and the interleave data are assigned to the carriers of the frame starting from a carrier that can carry the largest number of bits. The data assignment is executed based on preobtained bit distribution of the multi-carrier with respect to the low noise periods or with respect to the high noise periods, depending on whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. In the case “B”, if the frame is a frame that is transmitted in the low noise periods, the fast data are assigned to the carriers of the frame starting from a carrier that can carry the smallest number of bits so that the amount of the fast data included in the frame will be larger than the specific amount by an amount that is determined by the number figured out in the low noise frame number determination step, and the interleave data are assigned to the carriers of the frame starting from a carrier that can carry the largest number of bits, based on preobtained bit distribution of the multi-carrier with respect to the low noise periods. In the case “B”, if the frame is a frame that is transmitted in periods including the high noise periods, only fast data are assigned to the carriers of the frame based on preobtained bit distribution of the multi-carrier with respect to the high noise periods. In the coefficient conversion step, data which have been assigned to each carrier of the frame in the tone ordering step are converted into corresponding coefficients, based on the preobtained bit distribution and preobtained gain distribution of the multi-carrier with respect to the low noise periods or with respect to the high noise periods depending on whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. In the transform step, the coefficients with respect to the carriers of the frame are transformed into a signal which varies in time. 
     In accordance with a tenth aspect of the present invention, in the ninth aspect, the transform step is an IDFT (Inverse Discrete Fourier Transform) step in which inverse discrete Fourier transform is executed to Fourier transform coefficients with respect to the carriers of the frame which have been obtained in the coefficient conversion step and thereby the signal which varies in time is obtained. 
     In accordance with an eleventh aspect of the present invention, in the ninth aspect, the data transmission method further comprises a fast data CRC (Cyclic Redundancy Check) step, a first scrambling/error correction step, an interleave data CRC step and a second scrambling/error correction step. In the fast data CRC step, the cyclic redundancy check and addition of CRC code are executed to the fast data which have been separated in the separation step. In the first scrambling/error correction step, the fast data as the result of the fast data CRC step are scrambled and error correction code is added to the scrambled fast data. In the interleave data CRC step, the cyclic redundancy check and addition of CRC code are executed to the interleave data which have been separated in the separation step. In the second scrambling/error correction step, the interleave data as the result of the interleave data CRC step are scrambled and error correction code is added to the scrambled interleave data. 
     In accordance with a twelfth aspect of the present invention, in the seventh aspect, the data transmission method is applied to an ATU (ADSL (Asymmetric Digital Subscriber Line) Transceiver Unit) transmitter which is used as a data transmitter of an ATU (ADSL Transceiver Unit). 
     In accordance with thirteenth through eighteenth aspects of the present invention, there are provided computer-readable record mediums storing programs for instructing a computer or a DSP (Digital Signal Processor) of a data transmission device to execute the data transmission methods of the seventh through twelfth aspects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram for explaining crosstalk noise which occurs to an ATU-R (ADSL Transceiver Unit at the Remote end) due to data transmission on a TCM-ISDN line when downstream data transmission is being executed on an ADSL line; 
     FIG. 2 is a schematic diagram showing noise levels of NEXT (Near End X(cross)-Talk) and FEXT (Far End X(cross)-Talk) and explaining the dual bit-map method; 
     FIG. 3 is a graph showing an example of bit distribution; 
     FIG. 4 is a block diagram showing an example of an ADSL data communication system; 
     FIG. 5 is a block diagram showing the composition of an ATU-C and an ATU-R of the ADSL data communication system of FIG. 4; 
     FIG. 6 is a block diagram showing the composition of a conventional ATU transmitter; 
     FIG. 7 is a bar graph showing the ratio between fast data and interleave data which are assigned to each symbol, in the case where a conventional dual bit-map method is employed; 
     FIG. 8 is a timing chart showing the timing when the data transmission direction is changed in the TCM-ISDN line, and symbols which are transmitted by the ADSL transceiver unit in each FEXT/NEXT cycle of the TCM-ISDN line; 
     FIG. 9 is a block diagram showing the composition of an ATU transmitter as a data transmission device according to an embodiment of the present invention; 
     FIG. 10 is a bar graph showing assignment of fast data and interleave data to each carrier which is executed by a tone ordering section of the ATU transmitter of FIG. 9; 
     FIG. 11 is a schematic diagram showing two examples of constellations: a 4-bit constellation and a 5-bit constellation; 
     FIG. 12 is a graph showing the relationship between each position (A, B) in the 4-bit constellation of FIG. 11 and a trigonometric function (A cos θ+B sin θ); 
     FIG. 13 is a graph for explaining the relationship between gain and noise; 
     FIG. 14 is a bar graph showing the assignment of fast data and interleave data to each symbol which is executed by the ATU transmitter according to the embodiment of the present invention, in the case where the noise level in NEXT periods is very high (case “B”); and 
     FIG. 15 is a bar graph showing the assignment of fast data and interleave data to each symbol which is executed by the ATU transmitter according to the embodiment of the present invention, in the case where the noise level in the NEXT periods is not so high (case “A”). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, a description will be given in detail of preferred embodiments in accordance with the present invention. 
     FIG. 9 is a block diagram showing the composition of an ATU transmitter as a data transmission device according to an embodiment of the present invention. Referring to FIG. 9, the ATU transmitter of the embodiment includes a multiplexing/synchronization control section  1 , a fast data CRC (Cyclic Redundancy Check) section  2 , scrambling/error correction sections  3  and  6 , a fast data rate converter  4 , an interleave data CRC section  5 , an interleaving section  7 , an interleave data rate converter  8 , a symbol selection section  9 , a tone ordering section  10 , a constellation encoding/gain scaling section  11 , and an IDFT (Inverse Discrete Fourier Transform) section  12 . The components of the ATU transmitter shown in FIG. 9 are implemented by, for example, one or more DSPs (Digital Signal Processors) and appropriate software. 
     The multiplexing/synchronization control section  1  receives data (in which the ratio between the amount of fast data and the amount of interleave data is generally fixed according to initial settings) which are supplied from an upper layer device at a prespecified data rate, separates the data into fast data and interleave data, and supplies the fast data and the interleave data to a fast path (the upper path in FIG. 9) and an interleave path (the lower path in FIG. 9) respectively. 
     The fast path is provided with the fast data CRC section  2 , the scrambling/error correction section  3  and the fast data rate converter  4 . In the fast path, the fast data CRC section  2  adds CRC code to the fast data supplied from the multiplexing/synchronization control section  1 . The scrambling/error correction section  3  scrambles the fast data supplied from the fast data CRC section  2 , adds Reed-Solomon error correction code to the scrambled fast data, and supplies the data to the fast data rate converter  4 . 
     The interleave path is provided with the interleave data CRC section  5 , the scrambling/error correction section  6 , the interleaving section  7  and the interleave data rate converter  8 . Also in the interleave path, the interleave data CRC section  5  adds CRC code to the interleave data, and the scrambling/error correction section  6  executes the scrambling and the addition of error correction code. Thereafter, the interleaving section  7  in the interleave path changes the order of the interleave data supplied from the scrambling/error correction section  6  and thereby executes the interleaving, and sends the interleaved data to the interleave data rate converter  8 . Incidentally, delay is caused by the interleaving process. 
     The symbol selection section  9  figures out the number of FEXT symbols which can be transmitted in each FEXT period (in which the noise level due to the adjacent TCM-ISDN line is low), by use of the TTR (TCM-ISDN Timing Reference) signal which is in sync with the change of the data transmission direction in the TCM-ISDN line. Concretely, the symbol selection section  9  figures out whether the number of FEXT symbols which can be transmitted in each FEXT period is  4  or  3  in FIG.  8 . The FEXT symbol number information obtained by the symbol selection section  9  is supplied to the fast data rate converter  4 , the interleave data rate converter  8  and the tone ordering section  10 . The aforementioned TTR signal is also supplied to the fast data rate converter  4 , the interleave data rate converter  8 , the tone ordering section  10  and the constellation encoding/gain scaling section  11 . 
     The fast data rate converter  4  is provided with memory (data buffer) for temporarily storing fast data which are supplied from the upper layer device at a prespecified data rate, since the transmission data rate of the fast data has to be controlled depending on the level of the noise due to the TCM-ISDN line. The fast data rate converter  4  controls the amount of the fast data to be supplied to the tone ordering section  10  using the FEXT symbol number information supplied from the symbol selection section  9 , as will be described later. 
     The interleave data rate converter  8  is also provided with memory (data buffer) for temporarily storing interleave data which are supplied from the upper layer device at a prespecified data rate, since the transmission data rate of the interleave data has to be controlled depending on the level of the noise due to the TCM-ISDN line. The interleave data rate converter  8  controls the amount of the interleave data to be supplied to the tone ordering section  10  using the FEXT symbol number information supplied from the symbol selection section  9 , as will be described later. 
     The tone ordering section  10  obtains the bit distribution information of each carrier (with respect to FEXT periods and with respect to NEXT periods) from the bit/gain distribution table  28 , and thereby rearranges the carriers from the viewpoint of the number of bits which can be carried by the carrier (that is, obtains a new order of the carriers based on the number of assignable bits) as shown in FIG.  10 . Referring to FIG. 10, a carrier that can carry the largest number of bits is placed at the left-hand end of FIG. 10, and a carrier that can carry the smallest number of bits is placed at the right-hand end of FIG.  10 . When the symbol to which data are assigned is a FEXT symbol (i.e. a symbol that is transmitted in FEXT periods), the tone ordering section  10  assigns (distributes) the interleave data to the carriers starting from the carrier that can carry the largest number of bits and assigns the fast data starting from the carrier that can carry the smallest number of bits, as shown in “A” of FIG.  10 . When the symbol to which data are assigned is a NEXT symbol (i.e. a symbol that is transmitted in periods including NEXT periods), the tone ordering section  10  assigns fast data only to the carriers, as shown in “B” of FIG.  10 . Incidentally, the data assignment shown in “B” of FIG. 10 is executed only in the case (case “B” which will be explained in detail later) where the noise level due to the adjacent TCM-ISDN line in the NEXT periods is very high (and thereby the amount of data which are transmitted in each NEXT symbol is necessitated to be very low). In other cases (case “A” which will be explained in detail later), the tone ordering section  10  assigns the interleave data to the carriers of the NEXT symbol starting from the carrier that can carry the largest number of bits and assigns the fast data starting from the carrier that can carry the smallest number of bits. The details of the data assignment will be described in detail later. 
     The constellation encoding/gain scaling section  11  obtains the bit distribution of the carriers and the gain distribution of the carriers from the bit/gain distribution table  28 . Thereafter, the constellation encoding/gain scaling section  11  refers to a constellation that corresponds to the number of bits which are assigned to each carrier according to the bit distribution. For example, in the case where 4 bits of data has been assigned by the tone ordering section  10  to a particular carrier according to the bit distribution, a constellation for 4-bit data (a 4-bit constellation) is prepared for the particular carrier and referred to. FIG. 11 is a schematic diagram showing two examples of constellations, in which a 4-bit constellation and a 5-bit constellation are shown. Subsequently, a gain scaling coefficient “g” is determined for each constellation corresponding to a carrier, according to the gain distribution information with regard to the carrier. Subsequently, with respect to each carrier, a position (i.e. coordinates (A, B)) in the constellation that corresponds to the bits (data) which have been assigned by the tone ordering section  10  to the carrier is found out, and the coordinates (A, B) is multiplied by the gain scaling coefficient “g”. For example, in the case where 4-bit data “0101” has been assigned by the tone ordering section  10  to a particular carrier according to the bit distribution and the gain scaling coefficient “g” of the particular carrier should be set at 1.5 according to the gain distribution, the 4-bit constellation shown in FIG. 11 is referred to and thereby a position (coordinates (A, B)) corresponding to the 4-bit data “0101” (i.e. a number “5” in decimal digits) is found out (that is, (1, −1) in FIG.  11 ), and thereafter the coordinates (1, −1) are multiplied by the gain scaling coefficient 1.5, thereby coordinates (gA, gB)=(1.5, −1.5) corresponding to the 4-bit data “0101” are obtained as Fourier transform coefficients. Thereafter, the Fourier transform coefficients (gA, gB) with respect to each carrier are supplied to the IDFT section  12 . Incidentally, FIG. 12 is a graph showing the relationship between each position (A, B) in the 4-bit constellation of FIG. 11 and a trigonometric function (A cos θ+B sin θ) as a Fourier component (before gain scaling) of a voltage signal V(t) which will be described below. 
     The IDFT section  12  which received the 512 Fourier transform coefficients with respect to the 256 carriers executes IDFT (Inverse Discrete Fourier Transform) to the coefficients and thereby obtains the aforementioned voltage signal V(t) which varies in time. The IDFT section  12  outputs 512 sampled voltages of the voltage signal V(t) (i.e. V(t) corresponding to 512 times “t” in a symbol cycle) successively to the ADSL line. 
     Here, an explanation on the relationship between gain and noise will be given. FIG. 13 is a graph for explaining the relationship between gain and noise. As shown in FIG. 13, the distance between the origin and each point becomes large when gain is increased. When the number of transmitted bits becomes large as indicated by the downward arrows in FIG. 13, the probability of error due to noise is necessitated to be high if the bits are transmitted without raising the gain since the distance between points in the constellation is small. Therefore, even when the noise level is high, the probability of error due to noise can be reduced by raising the gain and increasing the distance between points in the constellation. Incidentally, information concerning the gain (i.e. the gain scaling coefficient “g”) with respect to each carrier is transmitted to an ADSL transceiver unit on the receiving side. 
     The bit distribution and the gain distribution which are stored in and outputted by the bit/gain distribution table  28  can be obtained by the ADSL data communication system shown in FIG. 5, as described before in the “Description of the Prior Art”. For example, the process executed by the ADSL data communication system of FIG. 5 for determining the bit/gain distribution in downstream data transmission is as follows. First, the ATU-C  21  shown in FIG. 5 outputs a pseudo random signal from its pseudo random signal generation section  25 A. The pseudo random signal outputted by the pseudo random signal generation section  25 A is supplied to the IDFT section  10 A of the transmission section  23 A and IDFT (Inverse Discrete Fourier Transform) is executed to the pseudo random signal. The pseudo random signal to which the IDFT has been executed is transmitted to the ATU-R  22  via the ADSL line. The pseudo random signal (to which the IDFT has been executed and which has been transferred to the ATU-R  22 ) is received by the reception section  24 B of the ATU-R  22 , and the signal is supplied to the DFT section  29 B. The DFT section  29 B executes DFT (Discrete Fourier Transform) to the pseudo random signal (to which the IDFT has been executed and which has been transferred to the ATU-R  22 ) and thereby obtains a received pseudo random signal which includes all the carriers (256 carriers, for example). The SNR measurement section  26 B obtains the SNR of each carrier based on the received pseudo random signal including all the carriers. The measurement of SNR is executed both with respect to NEXT periods and with respect to FEXT periods. The SNR of each carrier with respect to NEXT periods and the SNR of each carrier with respect to FEXT periods which have been obtained by the SNR measurement section  26 B are supplied to the bit/gain distribution calculation section  27 B, and the bit/gain distribution calculation section  27 B calculates the bit distribution and the gain distribution of the carriers with respect to NEXT periods and FEXT periods as shown in FIG.  3 . The bit/gain distribution information of the carriers with respect to NEXT periods and FEXT periods which has been obtained by the bit/gain distribution calculation section  27 B is transmitted by the transmission section  23 B to the ATU-C  21  via the ADSL line. The ATU-C  21  which received the bit/gain distribution information via the ADSL line stores the bit/gain distribution in the bit/gain distribution table  28 A. The above process for determining the bit/gain distribution is executed in initialization (when data communication is started, for example), in the fast retraining period (defined in ITU standard), etc. 
     The conventional ATU transmitter of FIG. 6 employing the conventional dual bit-map method sets the ratio between fast data and interleave data in one symbol (frame) at a fixed ratio, regardless of whether the symbol is a FEXT symbol (i.e. a symbol that is transmitted in the FEXT periods in which noise due to the adjacent line is low) or a NEXT symbol (i.e. a symbol that is transmitted in periods including the NEXT periods in which noise due to the adjacent line is high), as has been explained referring to FIG.  7 . Therefore, when the number of FEXT symbols which can be transmitted in a FEXT period decreases (4 to 3, for example), the amount of transmitted fast data decreases, and thereby delay is caused in the transmission of fast data. 
     For example, in the TCM-ISDN, upstream data transmission (1.25 msec) and downstream data transmission (1.25 msec) are alternately executed, and thus the noise level due to crosstalk from the TCM-ISDN line to the ADSL line changes periodically with a noise cycle (FEXT/NEXT cycle) of 2.5 msec. Therefore, the ATU transmitter has to execute data transmission in sync with the change of the noise level. As explained before referring to FIG. 8, a little and various difference occurs between the timing when the data transmission direction is changed in the TCM-ISDN line and the timing when the amount of data transmitted by the ADSL line is changed based on the TTR signal in each FEXT/NEXT cycle. Therefore, the number of FEXT symbols which can be transmitted by the ATU transmitter in each FEXT/NEXT cycle changes between 3 and 4, thereby the amount of fast data transmitted in a fixed time period changes, and thereby delay due to buffering (temporarily storing data) is caused in the transmission of fast data. Further, if the noise level due to the crosstalk from the TCM-ISDN line in the NEXT periods becomes very high, the amount of fast data which can be transmitted in each NEXT symbol is necessitated to be very small, and thereby a considerable amount of delay is caused in the transmission of the fast data. The delay of fast data has to be minimized since the delay is harmful to the fast data such as video data, voice data, data for realtime application software, etc. 
     FIG.  14  and FIG. 15 are bar graphs showing the assignment of fast data and interleave data to each symbol (frame) which is executed by the ATU transmitter according to the embodiment of the present invention. FIG. 14 shows the data assignment executed by the ATU transmitter of this embodiment in the case where the noise level due to the adjacent TCM-ISDN line in the NEXT periods is very high and thus a desired amount of data can not be assigned to each NEXT symbol according to the bit distribution information with respect to the NEXT periods (i.e. the aforementioned case “B”). The “desired amount” means a specific amount that corresponds to (matches) the fixed data rate of the fast data which are supplied from the upper layer device. FIG. 15 shows the data assignment executed by the ATU transmitter of this embodiment in the other cases where the noise level in the NEXT periods is not so high and thus the “specific amount” of data can be assigned to each NEXT symbol according to the bit distribution information with respect to the NEXT periods (i.e. the aforementioned case “A”). 
     Referring to FIG. 15 depicting the case “A”, the amount of fast data which are assigned to each symbol is fixed (at the aforementioned “specific amount” which matches the fixed data rate of the fast data which are supplied from the upper layer device), regardless of whether the symbol is a NEXT symbol or a FEXT symbol. By such assignment, the amount of fast data which are transmitted in a fixed time period can be made constant, thereby the delay of fast data due to buffering (temporarily storing data) can be eliminated, and thereby the delay of the fast data (such as video data, voice data, data for real-time application software, etc. to which delay is harmful) can be minimized. Incidentally, by the constant assignment of the fast data, even if the number of FEXT symbols in each FEXT/NEXT cycle changed, the amount of fast data which are transmitted in each FEXT/NEXT cycle can be made constant, differently from the case of the conventional data transmission device shown in FIG.  7 . 
     Referring to FIG. 14 depicting the case “B”, the amount of data which can be assigned to each NEXT symbol (according to the bit distribution information with respect to the NEXT periods) is necessitated to be smaller than the “specific amount” due to the very high noise in the NEXT periods, and thus it is impossible to assign the “specific amount” of fast data to each NEXT symbol as in FIG.  15 . In such cases (i.e. the case “B”), only fast data are assigned to the NEXT symbols as shown in FIG.  14  and “B” of FIG.  10 . To each FEXT symbol, fast data of an amount larger than the “specific amount” is assigned so as to compensate the insufficient assignment of fast data to the NEXT symbols. By such assignment, the delay of fast data due to the very high noise in the NEXT periods can be minimized. The amount of the compensation varies depending on whether the FEXT symbol number (i.e. the number of FEXT symbols which can be transmitted in one FEXT/NEXT cycle) is 3 or 4. Therefore, in the case “B”, the fast data rate converter  4  changes the amount of fast data which are supplied to the tone ordering section  10  to be assigned to each FEXT symbol, depending on whether the FEXT symbol number (designated by the FEXT symbol number information supplied from the symbol selection section  9 ) is 3 or 4. The amount of interleave data to be assigned to each FEXT symbol also varies depending on whether the FEXT symbol number is 3 or 4, therefore, the interleave data rate converter  8  also changes the amount of interleave data which are supplied to the tone ordering section  10  to be assigned to each FEXT symbol depending on whether the FEXT symbol number is 3 or 4. The amount of interleave data supplied by the interleave data rate converter  8  to the tone ordering section  10  to be assigned to each FEXT symbol is determined as: ((the amount of data which can be assigned to each FEXT symbol according to the bit distribution information with respect to FEXT periods)—(the amount of fast data which are supplied by the fast data rate converter  4  to the tone ordering section  10  to be assigned to each FEXT symbol). 
     In the following, the operation of the ATU transmitter of FIG. 9 for implementing the data assignment explained above will be described in detail. 
     Data (in which the ratio between the amount of fast data and the amount of interleave data is generally fixed according to initial settings) are supplied from an upper layer device (such as a computer) to the ATU transmitter of FIG. 9 at a prespecified data rate. The data are separated by the multiplexing/synchronization control section  1  into fast data and interleave data, and the fast data and the interleave data are supplied to the fast path and the interleave path respectively. 
     In the fast path, CRC code is added by the fast data CRC section  2  to the fast data, and the fast data outputted by the fast data CRC section  2  is scrambled by the scrambling/error correction section  3 . The scrambling/error correction section  3  adds Reed-Solomon error correction code to the scrambled fast data, and supplies the data to the fast data rate converter  4 . Also in the interleave path, CRC code is added by the interleave data CRC section  5  to the interleave data, and the scrambling and the addition of error correction code are executed by the scrambling/error correction section  6 . Thereafter, the interleaving section  7  in the interleave path changes the order of the interleave data supplied from the scrambling/error correction section  6  and thereby executes the interleaving, and sends the interleaved data to the interleave data rate converter  8 . 
     The FEXT symbol number (the number of FEXT symbols which can be transmitted in each FEXT period) is figured out by the symbol selection section  9 , by use of the TTR (TCM-ISDN Timing Reference) signal which is in sync with the change of the data transmission direction in the TCM-ISDN line, and the FEXT symbol number information is supplied to the fast data rate converter  4 , the interleave data rate converter  8  and the tone ordering section  10 . 
     The fast data rate converter  4  operates in two ways depending on whether it is in the case “A” or in the case “B”. In the case “A” shown in FIG. 15, the fast data rate converter  4  supplies the “specific amount” of fast data to the tone ordering section  10  so as to be assigned to each symbol, regardless of the symbol is a FEXT symbol or a NEXT symbol. In the case “B” shown in FIG. 14, the fast data rate converter  4  changes the amount of fast data which are supplied to the tone ordering section  10  to be assigned to each symbol, depending on whether the symbol is a FEXT symbol or a NEXT symbol. In the case where the symbol is a NEXT symbol, the fast data rate converter  4  sets the amount of fast data (which are supplied to the tone ordering section  10  to be assigned to each NEXT symbol) so that the NEXT symbol is filled with the fast data only, that is, sets the amount so as to be equal to the amount of data which can be assigned to each NEXT symbol according to the bit distribution with respect to NEXT periods. In the case where the symbol is a FEXT symbol, the fast data rate converter  4  changes the amount of fast data which are supplied to the tone ordering section  10  to be assigned to each FEXT symbol, depending on whether the FEXT symbol number (designated by the FEXT symbol number information supplied from the symbol selection section  9 ) is 3 or 4, as described before. 
     The interleave data rate converter  8  sets the amount of interleave data which are supplied to the tone ordering section  10  so as to be assigned to each symbol as: ((the amount of data which can be assigned to each symbol according to the bit distribution information)—(the amount of fast data which are supplied by the fast data rate converter  4  to the tone ordering section  10  to be assigned to each symbol). Therefore, in the case “A” shown in FIG. 15, the interleave data rate converter  8  sets the amount of interleave data which are supplied to the tone ordering section  10  so as to be assigned to each symbol as: ((the amount of data which can be assigned to each symbol according to the bit distribution information)—(the “specific amount”). In the case “B” shown in FIG. 14, the interleave data rate converter  8  sets the amount of interleave data (which are supplied to the tone ordering section  10  so as to be assigned to each symbol) according to the first expression in this paragraph, therefore, the interleave data rate converter  8  supplies no fast data to the tone ordering section  10  so as to be assigned to NEXT symbols. For FEXT symbols, the interleave data rate converter  8  changes the amount of interleave data which are supplied to the tone ordering section  10  to be assigned to each FEXT symbol, depending on whether the FEXT symbol number is 3 or 4. 
     Incidentally, the fast data rate converter  4  and the interleave data rate converter  8  can judge whether it is timing for a FEXT symbol or timing for a NEXT symbol and execute the control on the amount of fast/interleave data supplied to the tone ordering section  10 , depending on whether it is in the FEXT periods or in the NEXT periods. Incidentally, in such control, a little delay due to processing by the tone ordering section  10 , the constellation encoding/gain scaling section  11  and the IDFT section  12  is taken into consideration by the fast data rate converter  4  and the interleave data rate converter  8 . 
     Or, it is also possible to let the fast data rate converter  4  and the interleave data rate converter  8  recognize the noise cycles (FEXT/NEXT cycles) (0˜33) shown in FIG. 8 by use of the TTR signal supplied thereto, and grasp the changing timing between the FEXT symbols and NEXT symbols in each noise cycle, and thereby change the amount of fast/interleave data to be supplied to the tone ordering section  10  in sync with the changing timing. Also in this example, a little delay due to processing by the tone ordering section  10 , the constellation encoding/gain scaling section  11  and the IDFT section  12  is taken into consideration by the fast data rate converter  4  and the interleave data rate converter  8 . 
     The tone ordering section  10  obtains the bit distribution information of each carrier (with respect to the FEXT periods and with respect to the NEXT periods) from the bit/gain distribution table  28 , and rearranges the carriers from the viewpoint of the number of bits which can be carried by the carrier (that is, obtains a new order of the carriers based on the number of assignable bits) as shown in FIG. 10 (in which a carrier that can carry the largest number of bits is placed at the left-hand end, and a carrier that can carry the smallest number of bits is placed at the right-hand end) according to the bit distribution information. The above process is executed by the tone ordering section  10  with respect to the FEXT periods and the NEXT periods, in initialization (when data communication is started, for example), in the fast retraining period (defined in ITU standard), etc. 
     In the case “A” shown in FIG. 15, the tone ordering section  10  assigns the interleave data to the carriers starting from the carrier that can carry the largest number of bits and assigns the fast data starting from the carrier that can carry the smallest number of bits as shown in “A” of FIG. 10, according to the above rearranged bit distribution with respect to the FEXT periods (for the FEXT symbols) or according to the above rearranged bit distribution with respect to the NEXT periods (for the NEXT symbols). In the assignment of the fast data and the interleave data to the carriers of a (FEXT or NEXT) symbol, the tone ordering section  10  keeps the amount of fast data included in the symbol constant (so as to match the amount of the fast data supplied from the upper layer device to the ATU transmitter in each symbol cycle) regardless of whether the symbol is a FEXT symbol or a NEXT symbol, as shown in FIG.  15 . 
     In the case “B” shown in FIG. 14, for FEXT symbols (which are transmitted in the FEXT periods in which the noise level due to the adjacent TCM-ISDN line is low), the tone ordering section  10  assigns the interleave data to the carriers starting from the carrier that can carry the largest number of bits and assigns the fast data starting from the carrier that can carry the smallest number of bits as shown in “A” of FIG. 10, according to the above rearranged bit distribution with respect to the FEXT periods. In the assignment of the fast data and the interleave data to the carriers of the FEXT symbol, the amount of fast data (and the amount of interleave data) included in the FEXT symbol is controlled depending on whether the FEXT symbol number (designated by the FEXT symbol number information supplied from the symbol selection section  9 ) is 3 or 4. For NEXT symbols (which are transmitted in periods including the NEXT periods in which the noise level due to the adjacent TCM-ISDN line is high), the tone ordering section  10  assigns fast data only to the carriers of the NEXT symbol as shown in “B” of FIG. 10, according to the above rearranged bit distribution with respect to the NEXT periods. 
     The constellation encoding/gain scaling section  11 , which is supplied with the bit distribution information and the gain distribution information (with respect to the FEXT periods and the NEXT periods) from the bit/gain distribution table  28 , figures out the Fourier transform coefficients (gA, gB) with respect to each carrier of the symbol (to each carrier of which data have been assigned by the tone ordering section  10 ), according to the method which has been described before. The process for obtaining the Fourier transform coefficients (gA, gB) with respect to each carrier of a symbol is executed based on different bit/gain distribution (the bit/gain distribution with respect to FEXT periods or the bit/gain distribution with respect to NEXT periods) depending on whether the symbol is a FEXT symbol or a NEXT symbol. The Fourier transform coefficients (gA, gB) with respect to each carrier of the symbol are supplied to the IDFT section  12 . 
     The IDFT section  12  which received the 512 Fourier transform coefficients with respect to the 256 carriers of the symbol executes IDFT (Inverse Discrete Fourier Transform) to the coefficients, thereby obtains the voltage signal V(t) corresponding to the symbol, and outputs 512 sampled voltages of the voltage signal V(t) (i.e. V(t) corresponding to 512 times “t” in a symbol cycle) successively to the ADSL line. 
     As described above, in the ATU transmitter according to the embodiment of the present invention, the amount of fast data which are assigned to each symbol to be transmitted is set constant as long as possible, differently from the conventional dual bit-map method in which the ratio between the amount of fast data and the amount of interleave data in each symbol is set at a constant ratio. 
     In the case “A” (where the noise level in the NEXT periods is not so high and thus the “specific amount” of data can be assigned to each NEXT symbol according to the bit distribution information with respect to the NEXT periods), the amount of fast data assigned to each symbol is set at the “specific amount” which corresponds to (matches) the fixed data rate of the fast data supplied from the upper layer device. By such assignment, the amount of fast data which are transmitted in a fixed time period can be made constant, thereby the delay of fast data due to buffering (temporarily storing data) can be eliminated, and thereby the delay of the fast data such as video data, voice data, data for real-time application software, etc. to which delay is harmful can be minimized. By the constant assignment of the fast data, the effect of the change of the number of FEXT symbols in each FEXT/NEXT cycle (that is, the change of the amount of fast data which are transmitted in each FEXT/NEXT cycle) is also eliminated, differently from the case of the conventional data transmission device shown in FIG.  7 . 
     In the case “B” (where the noise level in the NEXT periods is very high and thus the “specific amount” of data can not be assigned to each NEXT symbol according to the bit distribution information with respect to the NEXT periods), only fast data are assigned to the NEXT symbols, and fast data of an amount larger than the “specific amount” is assigned to each FEXT symbol, so as to compensate the insufficient assignment of fast data to the NEXT symbols. By such assignment, the delay of fast data due to the very high noise in the NEXT periods can be minimized. 
     Therefore, by the ATU transmitter according to the embodiment of the present invention, delay of the fast data such as video data, voice data, data for real-time application software, etc. can be minimized both in normal conditions (where noise is not so high) and in noisy conditions (where the amount of data which can be transmitted in each NEXT symbol is limited). 
     As set forth hereinabove, in the data transmission device and the data transmission method according to the present invention, a new dual bit-map method is employed. In the case (case “A”) where the amount of data which can be assigned to each frame (symbol) that is transmitted in high noise periods is larger than or equal to a specific amount which corresponds to the fixed data rate of the fast data supplied from an upper layer device, fast data of the “specific amount” are assigned to each frame regardless of whether the frame is a frame that is transmitted in the low noise periods or a frame that is transmitted in periods including the high noise periods. In the case (case “B”) where the amount of data which can be assigned to each frame that is transmitted in the high noise periods is smaller than the “specific amount”, only fast data are assigned to each frame that is transmitted in periods including the high noise periods, and fast data of an amount larger than the “specific amount” are assigned to each frame that is transmitted in the low noise periods. Therefore, the amount of fast data which is transmitted in a fixed time period is made constant (in the case “A”) or almost constant (in the case “B”), and thereby delay of the fast data (such as video data, voice data, data for real-time application software, etc. to which delay is harmful) can be minimized both in normal conditions (where noise is not so high) and in noisy conditions (where the amount of data which can be transmitted in high noise periods is limited). 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention. For example, the data transmission device according to the present invention is not limited to ATU (ADSL Transceiver Unit) transmitters, but can also be applied to various types of transmitters of XTUs (XDSL Transceiver Units) which are used for data communication according to XDSL (X Digital Subscriber Line (X: A (Asymmetric), S (Symmetric), V (Very high speed), etc.)). The present invention can be applied to various types of data transmission devices which are employed for data communication in environments where the noise level (by any causes including causes other than TCM-ISDN lines) changes periodically. The present invention can also be applied to data transmission devices which employs modulation/demodulation methods other than DMT (Discrete Multi-Tone). Further, the present invention is not limited to wire data transmission devices, but can also be applied to data transmission devices which execute data transmission by radio waves. Moreover, while the above explanation has been given on the assumption that “fast data” means video data, voice data, data for real-time application software, etc., the classification of data into the “fast data” and the “interleave data” can be done in various ways depending on design requirements etc.