(1) Field of the Invention
The present invention relates to a resynchronization control apparatus and a resynchronizing method suitable for use in a subscriber communication machine that can communicate in synchronization with a transmission cycle in, for example, ISDN (Integrated Services Digital Network) ping-pong transmission [TCM (Time Compression Multiplex) transmission] over a predetermined communication line such as a subscriber line.
(2) Description of Related Art
Recent widespread of multimedia-type services such as Internet and the like to the whole society including general home strongly demands early provision of an economical, highly reliable digital subscriber line transmission technique by which the user can enjoy such services. Enormous cost and time are required to newly lay communication lines, so that there have been proposed various methods for high-speed data communications using existing communication lines.
For instance, xDSL (Digital Subscriber line) attracts attention as a digital subscriber line transmission technique that uses existing telephone lines as high-speed data communication lines. The xDSL is a transmission system using existing subscriber lines, being also one of modulation-demodulation techniques. The xDSL is roughly classified into two according to whether the upstream transmission rate from the subscriber's premise (hereinafter referred to as subscriber's side) to the accommodating office (hereinafter referred to as office's side) and the downstream transmission rate from the office's side to the subscriber's side are symmetric or asymmetric.
As the symmetric type, there are HDSL (High-bit-rate DSL) whose upstream and downstream transmission rates are approximately 1.5 to 2.0 Mbps (megabit per second), SDSL (Single-line DSL) whose upstream and downstream transmission rates are approximately 160 k to 2.0 Mbps and the like, for example. As the asymmetric type, there is ADSL (Asymmetric DSL) that is tentatively operated in these years, extensively. The ADSL is further classified into “G.dmt” whose downstream transmission rate is approximately 6 Mbps, and “G.lite” (also called as light ADSL) whose downstream transmission rate is approximately 1.5 Mbps, both of which employ a distinctive modulation system called DMT (Discrete Multiple Tone) modulation.
In brief, the DMT modulation system divides the transmission frequency bandwidth into subcarriers each of about 4 kHz (in the case of “G.lite”, approximately a maximum of 128 carriers in the downstream although it depends on conditions), and modulates each of the subcarriers. The DMT modulation system has a feature to be resistible to noise at a specific frequency since, even when a certain subcarrier is unusable due to an effect of the noise at the specific frequency, it allows the communication using another subcarriers.
Hereinafter, an ADSL transmission system employing such the DMT modulation system will be described in detail.
(1) Description of ADSL Transmission System
FIG. 6 is a block diagram showing an example of the ADSL transmission system. The ADSL transmission system shown in FIG. 6 is configured with an ADSL machine 650 installed on the office's side 610 and an ADSL machine 660 installed on the subscriber's side 620, which are connected to each other over a metallic line (telephone line) 70. Incidentally, the ADSL machine 650 on the office's side 610 will be occasionally referred to as an office ADSL machine 650, whereas the ADSL machine 660 on the subscriber's side 620 a subscriber ADSL machine 660, hereinafter. When not particularly discriminated between the office's side 610 and the subscriber's side 620, they will be referred to merely as ADSL machines 650 and 660.
As shown in FIG. 6, the office ADSL machine 650 comprises, as a transmitter (transmitting block) 910, a serial to parallel buffer 10, an encoder 20, an IFFT (Inverse Fast Fourier Transformer) 30, a parallel to serial buffer 40, a D/A (digital/analog) converter 50, and a transmit bit map memory 60.
On the other hand, the subscriber ADSL machine 660 comprises a receiver (receiving block) 960 including an A/D converter 80, a TEQ (Time-domain Equalizer) 90, a serial to parallel buffer 100, an FFT (Fast Fourier Transformer) 110, an FEQ (Frequency-domain Equalizer) 120, a decoder 130, a parallel to serial buffer 140, a receive bit map memory 150, an AGC (Automatic Gain Controller) 160, and multipliers 170 and 180.
FIG. 6 shows only a downstream structure from office's side to the subscriber's side. However, the office ADSL machine 650 is practically provided with a receiver having a function equivalent to that of the receiver 960 in the subscriber ADSL machine 660, whereas the subscriber ADSL machine 660 is provided with a transmitter having a function equivalent to that of the transmitter 910 in the office ADSL machine 650, whereby communication in the upstream is performed similarly to communication in the downstream, in principle. Here, the description will be made by way of the light ADSL (G.lite).
In the office ADSL machine 650, the transmit bit map memory 60 holds data (bit map) defining assignment of transmission bits to each (sub) carrier of a DMT signal to be generated. The serial to parallel buffer 10 stores transmit data that is serial data of one symbol duration (¼ kHz) converts the stored data into parallel data, and outputs the data. At this time, the assignment of a transmission bit number to each carrier (division of the frequency bandwidth) is performed according to the transmit bit map stored in the above transmit bit map memory 60. For example, when the number of subcarriers is i+1 where the subcarriers are C0 to Ci, a bit group bi of the above parallel data is assigned as a bit group to be transmitted on a subcarrier Ci in FIG. 6.
The above subcarriers sometimes include a carrier for synchronizing a timing called a pilot tone [a carrier positioning in the center of the 128 carriers is a pilot tone in the case of the downstream in the light ADSL], as shown in FIG. 7, for example. This pilot tone is used to transmit only a signal for synchronizing a timing; no data for the pilot tone exists in the transmit bit map memory 60.
The encoder 20 performs a predetermined modulation process such as quadrature amplitude modulation (QAM) on parallel data outputted from the serial to parallel buffer 10 for each of the above subcarriers according to the above transmit bit map. The IFFT 30 performs inverse fast Fourier transform on data (frequency-domain data) outputted from the encoder 20 to convert the data into time-domain data, thereby obtaining a DMT signal. Namely, the encoder 20 and the IFFT 30 function as a DMT modulating unit that DMT-modulates transmit data.
The parallel to serial buffer 40 converts the data (DMT signal) that has been subjected to the inverse fast Fourier transform in the above IFFT 30 into serial data, and adds a cyclic prefix to be described later thereto. The D/A converter 50 converts an output (serial data) of the parallel to serial buffer 40 to an analog signal using a predetermined sampling frequency (for example, 1.104 MHz). The obtained analog signal is outputted to the metallic line 70.
In the subscriber ADSL machine 660, the multiplier (for analog signal) 170 multiplies an analog signal received from the office ADSL machine 650 over the metallic line 70 by an arbitrary coefficient. The A/D converter 80 samples an output (analog signal) of the multiplier 170 at a predetermined sampling frequency (for example, 1.104 MHz) to convert the signal into a digital signal. The multiplier (for digital signal) 180 multiplies the digital signal from the A/D converter 80 by an arbitrary coefficient. The AGC 160 controls the coefficients to be multiplied in the above multipliers 170 and 180.
Multiplying a signal by an arbitrary coefficient is equivalent to amplifying the signal. Namely, the AGC 160 controls the coefficients to be multiplied in the multipliers 170 and 180, thereby controlling an amplification gain of a signal received over the metallic line 70. Incidentally, the amplification gain (coefficient) of an analog signal in the multiplier 170 will be called an analog AGC value, whereas the amplification gain (coefficient) of a digital signal in the multiplier 180 will be called a digital AGC value.
The TEQ 90 is an equalizer in time domain configured with, for example, an FIR (Finite Impulse Response) filter, which performs a predetermined process so that intersymbol interference (ISI) with an inputted signal is placed in the cyclic prefix added in the parallel to serial buffer 40 (detailed of which will be described later). The serial to parallel buffer 100 removes the cyclic prefix from data outputted from the TEQ 90, converts the data to parallel data, and outputs the same.
The FFT 110 converts output data from the above serial to parallel buffer 100 to data in frequency domain in the fast Fourier transform. The FEQ 120 is an equalizer in frequency domain, which equalizes data converted into the data in frequency domain by the FFT 110 as above according to transmission characteristics (frequency characteristics) of the metallic line 70, thereby compensating effects on the amplitude and phase exerted when the data passes through the metallic line 70 for each carrier at a different frequency.
The decoder 130 performs a predetermined demodulating process (QAM demodulation or the like) on output data of the above FEQ 120. The parallel to serial buffer 140 converts parallel data outputted from the decoder 130 into serial data, and outputs it. The receive bit map memory 150 holds information (receive bit map) on the transmission bit number of each carrier assigned to each carrier on the transmitting side according to the transmit bit map in the transmit bit map memory 60. On the basis of this information, the demodulation process by the decoder 130 and the parallel to serial buffer 140 mentioned above is carried out.
Next, description will be made of an operation of the above-structured ADSL transmission system.
When transmit data is inputted to the office transmitter 910, the transmit data of one symbol duration (¼ kHz) is held in the serial to parallel buffer 10. The held data is divided into groups each of the transmit bit number per carrier determined beforehand according to the transmit bit map 60, and outputted to the encoder 20.
The encoder 20 converts the inputted bit sequences into signal points to be quadrature-amplitude-modulated, and output them to IFFT 30. The IFFT 30 performs the inverse fast Fourier transform on outputs of the encoder 20 to quadrature-amplitude-modulate each of the signal points, and outputs them to the parallel to serial buffer 40. Meanwhile, the encoder 20 and the IFFT 30 performs the DMT modulation.
The parallel to serial buffer 40 adds 16 samples (240 to 255 samples) out of outputs of the above IFFT 30 as the cyclic prefix to the head of the DMT symbol (details of which will be described later). The data to which the cyclic prefix has been added is sent from the parallel to serial buffer 40 to the D/A converter 50, converted to an analog signal at a sampling frequency of 1.104 MHz therein, and transmitted to the subscriber receiver 960 over the metallic line 70.
In the subscriber receiver 960, the analog signal received over the metallic line 70 is amplified by the multiplier 170, converted into a digital signal at 1.104 MHz by the A/D converter 80, and inputted to the multiplier 180. The multiplier 180 again amplifies the inputted digital signal, and outputs it to the TEQ 90. At this time, the AGC 160 measures a magnitude of each signal at the multipliers 170 and 180, and sets the AGC values to each signal and changes the same as needed.
The TEQ 90 equalizes an output of the multiplier 180 in time domain such that ISI is placed within the cyclic prefix of 16 samples, and makes the serial to parallel buffer 100 hold data of one DMT symbol. The serial to parallel buffer 100 removes the above cyclic prefix from the data of one DMT symbol inputted from the TEQ 90, then converts the remaining data into parallel signals, and outputs them to the FFT 110.
The FFT 110 performs fast Fourier transform on outputs of the above serial to parallel buffer 100 to convert signals in time domain into signal point data in frequency domain. The FEQ 120 compensates effects on the amplitude and phase of the converted signal point data on each carrier having a different frequency exerted when the signal passes through the metallic line 70. The decoder 130 then demodulates the data according to the receive bit map in the receive bit map memory 150 holding the same values as the transmit bit map 60. The data demodulated by the decoder 130 is held for awhile in the parallel to serial buffer 140, converted into a serial bit string, and outputted as receive data.
(2) Description of Equalizer
(2-1) Detailed Description of TEQ 90
Next, description will be made of a role of the above TEQ 90.
When the DMT symbol inputted to the above parallel to serial buffer 40 in FIG. 6 is in a state shown in FIG. 8(A), the parallel to serial buffer 40 copies 16 samples in the tail of the DMT symbol, and attaches them to the head of the DMT symbol, as shown in FIG. 8(B). The copied portion is the above cyclic prefix.
The DMT symbol added thereto the cyclic prefix as shown in FIG. 8(C) is sent to the D/A converter 50, converted to an analog signal at the sampling frequency of 1.104 MHz in the D/A converter 50, and transmitted to the subscriber ADSL machine 660 over the metallic line 70. The receive signal received over the metallic line 70 whose amplitude characteristics and phase characteristics to the frequency are not constant is distorted due to an effect of ISI, as shown in FIG. 8(D), for example.
The TEQ 90 described with reference to FIG. 6 performs such a process (ISI compressing process) as to place the ISI within only the cyclic prefix of 16 samples, and the serial to parallel buffer 100 removes the cyclic prefix, whereby a DMT symbol in which the effect of the ISI has been eliminated is obtained as shown in FIG. 8(F).
The TEQ 90 functions to eliminate the effect of ISI from the received signal using the cyclic prefix. In more detail, since the metallic line 70 has non-linear low pass filter (LPF) characteristics that deteriorate the transmission characteristics in the high frequency bandwidth, an impulse response having a constant length is generated at a discontinuous portion existing between symbols. This impulse response is overlaid on the data signal to cause deterioration of the signal.
Accordingly, 16 bits in the tail are added to the head of the symbol to form the cyclic prefix. A portion at which the cyclic prefix is combined with the symbol becomes continuous, so that no impulse response is generated at this portion. On the other hand, a portion at which the head of the cyclic prefix is combined with the preceding symbol becomes discontinuous, so that the impulse response is generated at this portion.
By inputting the received data to the TEQ 90 having characteristics of a high pass filter (HPF) reverse to those of the metallic line 70, the impulse response can be placed within the cyclic prefix. The cyclic prefix so processed is removed, whereby data not affected by the impulse response can be obtained.
(2-2) Adaptive Operation Algorithm of TEQ 90
In order that the TEQ 90 performs the above-mentioned process to place ISI within only the cyclic prefix of 16 samples as shown in FIG. 80(E), an adaptive operation to make the TEQ 90 have such the characteristics is required. For this purpose, the TEQ 90 comprises, as a block for the adaptive operation only, a reference signal generation block 920, a delay unit 930, a target channel block 940, and an adder 950, as shown in FIG. 9, for example. Incidentally, identical numerals in FIG. 9 identify identical or like parts in FIG. 6 described above. The structural elements of the receiving side block 960 shown in FIG. 6 other than the TEQ 90 are not shown in FIG. 9 in order to simplify the description.
The reference signal generation block 920 generates the same signal x(t) as a transmit signal (known received signal in the receiving side block 960) transmitted in the past from the transmitting side block 910, and outputs it. The delay unit 930 delays the reference signal x(t) generated by the reference signal generating block 920 by a predetermined time, and outputs it.
The target channel block 940 outputs a result b(t)*x(t), which is to be a target when the characteristics of the TEQ 90 are adjusted, obtained by convolution-integrating a characteristic b(t) (provided a delay quantity caused by the metallic line 70 is excluded) synthesized from the characteristics of the metallic line 70 and the characteristics of the TEQ 90 for the reference signal x(t) delay by the delay unit 930. The adder 950 computes a difference between an output z(t) of the TEQ 90 and an output b(t)*x(t) of the target channel block 940, and supplies the obtained result e(t) to the TEQ 90 and the target channel block 940. The TEQ 90 and the target channel block 940 thereby perform the adaptive operation such that the output e(t) of the adder 950 becomes “0.”
Next, description will be made of the adaptive operation of the above TEQ 90.
When a transmit signal x(t) is transmitted from the transmitting side block 910, the transmit signal x(t) is received by the receiving side block 960 over the metallic line 70. In the receiving side block 960, a result z(t) obtained by adding the characteristics of the TEQ 90 to the received signal by the TEQ 90 is supplied to the adder 950.
At this time, the reference signal generation block 920 generates a reference signal x(t) that is assumed to be the same as the transmit signal, and outputs it. The delay unit 930 delays the reference signal x(t) by a predetermined quantity such that the phase of an output z(t) of the TEQ 90 coincides with the phase of an output b(t)*x(t) of the target channel block 940. The target channel block 940 convolution-integrates the characteristic b(t) for the reference signal x(t), and supplies the obtained result b(t)*x(t) to the adder 950.
The adder 950 computes a difference e(t) between the output z(t) of the TEQ 90 and the output b(t)*x(t) of the target channel block 940, and supplies the obtained result to the TEQ 90 and the target channel block 940. The TEQ 90 and the target channel block 940 perform the adaptive operation on the basis of the difference e(t) supplied from the adder 950. Namely, the TEQ 90 and the target channel block 940 carry out the adaptive operation such that the output e(t) from the adder 950 becomes “0.”
As a result, the TEQ 90 has such a processing characteristic as to place ISI within only the cyclic prefix of 16 samples, as shown in FIG. 8(E).
(2-3) Adaptive Operation Algorithm of FEQ
Next, description will be made of an example of an adaptive operation algorithm of the above FEQ 120.
As described above, the FEQ 120 is an equalizer in frequency domain, which equalizes effects (changes in amplitude characteristics and phase characteristics) on a plurality of carriers having different frequencies exerted when the carriers pass through the metallic line 70 such as to equalize the characteristics of all the carriers. For this purpose, the FEQ 120 is provided with circuits shown in FIG. 10 in number corresponding to the number of the above carriers. Incidentally, identical numerals in FIG. 10 designate corresponding parts in FIG. 6.
In FIG. 10, a coefficient unit 1010 multiplies an input signal (received signal) Yi by a coefficient Wi. A value of the above coefficient Wi can be changed according to an output of adder 1020. In FIG. 10, a decision unit (Decision) 1030 detects a decision value Xi that is assumed to correspond to the input signal Yi, and outputs it. The adder 1020 computes a difference Ei between an output Xi of the decision unit 1030 and an output Zi of the coefficient unit 1020, and sets a coefficient Wi of the coefficient unit 1010 according to a result of the computation.
In the circuit structured as above, a decision value Xi is determined by inputting an output Zi of the FEQ 120 to the decoder 130, a difference Ei between the decision value Xi and an output Zi of the FEQ 120 is determined by the adder 1020, and a coefficient Wi of the coefficient unit 1010 is such adjusted that the difference Ei becomes “0.” As a result, the above equalizing process in frequency domain is adaptively performed. Incidentally, the above decoder 130 converts a decision value Xi into a bit sequence bi, and supplies the bit sequence bi to the parallel to serial buffer 140 shown in FIG. 6.
(3) Crosstalk from ISDN Ping-Pong Transmission Line
Since ADSL is a technique using communication lines, there are some instances where an ISDN ping-pong transmission line [TMC (Time Compression Multiplex) line] 70′ exists in the vicinity of the metallic line 70 (hereinafter referred to as an ADSL line 70) (in concrete, the ADSL line 70 and the TCM line are laid in parallel in the same cable). In such case, the ADSL line 70 is affected by crosstalk (TCM crosstalk) from the TCM line 70′.
In TCM transmission, in synchronization with a signal [TTR (TCM-ISDN Timing Reference)] 310 at 400 Hz as shown in, for example, FIG. 12(A), an office ISDN machine 630 shown FIG. 11 transmits downstream data in the first half cycle of the TTR 310, and a subscriber ISDN machine 640 transmits upstream data in the latter half cycle. For this, the office ADSL machine 605 is affected by near-end crosstalk (NEXT) 320 from the downstream data of the office ISDN machine in the first half cycle of the TTR 310, and affected by far-end crosstalk (FEXT) 330 from the upstream data of the subscriber ISDN machine 640 in the latter half cycle, as shown in FIG. 12(B).
In contrast to the office ADSL apparatus 650, the subscriber ADSL machine 660 is affected by FEXT 340 from downstream data of the office ISDN machine 603 in the first half cycle of the TTR 310, and affected by NEXT 350 from upstream data of the subscriber ISDN machine 640 in the latter half cycle, as shown in FIG. 12(C) Hereinafter, a noise section affected by NEXT will be referred to as an NEXT section, and a noise section affected by FEXT will be referred to as an FEXT section. Incidentally, FIG. 12(D) shows the NEXT section and the FEXT section in the subscriber ADSL machine 660. The effect in the NEXT section is generally larger than the effect in the FEXT section.
(4) Description of Sliding Window
In order to transmit well ADSL signals in the environment where TCM crosstalk (NEXT section, FEXT section) mentioned above exists, “sliding window” is introduced. The “sliding window” is used to specify the FEXT section in which the effect of TCM crosstalk is smaller than that in the NEXT section. Effective use of the specified FEXT section can minimize the effect of TCM crosstalk, thereby certainly transmitting data.
In the downstream direction where an ADSL signal is transmitted from the office ADSL machine 650 to the subscriber ADSL machine 660, states of the ADSL signal are defined as follows using the “sliding window”:
As shown in FIG. 12(E), when a transmit DMT symbol 360 is completely involved in the FEXT section in the subscriber ADSL machine 660, the office ADSL machine 650 transmits the symbol as an FEXT symbol using a sliding window 370. When the transmit symbol is at least partly involved in the NEXT section of the subscriber ADSL machine 660, the office ADSL machine 650 transmits the symbol as an NEXT symbol. Such transmitting method is called a dual bit map method.
In the upstream, the subscriber ADSL machine 660 transmits the DMT symbol in the similar manner. In the downstream, the office ADSL machine 650 may transmit a pilot tone instead of the NEXT symbol in the NEXT symbol section. Such transmitting system is called an FEXT bit map system. In the FEXT bit map system, the subscriber ADSL machine 660 transmits nothing in the downstream in a section of the NEXT symbol.
(5) Description of Frame Structure in ADSL Communication
The above “sliding window” is asynchronous with the TTR in TCM transmission. Here, a hyperframe used in ADSL communication in synchronization with the TTR will be explained.
In ADSL communication, one frame corresponds to one symbol. In steady communication, 69 frames, which are 68 ADSL frames 410 for user data and one frame for a synchronization symbol S, form one superframe 420, as shown in FIG. 13(C), for example. Further, as shown in FIG. 13(B), five superframes 420 form one hyperframe 430.
Into the hyperframe 430, an inverse synchronization symbol I may be inserted instead of the synchronization symbol S. The inverse synchronization symbol I corresponds to a signal whose phase is rotated 180 degrees from a phase of the synchronization symbol S in the case of carriers other than the pilot tone, as shown in FIG. 14(B). In the case of the pilot tone, the inverse synchronization symbol I corresponds to a signal having the same phase as the synchronization symbol S, as shown in FIG. 14(A).
In the case of a hyperframe in the downstream from the office ADSL machine 650 to the subscriber ADSL machine 660, the inverse synchronization symbol I is decided to be placed (inserted) in the fourth superframe 420 in one hyperframe 430, as shown in FIG. 13(B). In the upstream, the inverse synchronization symbol I is involved in the first superframe 420 in one hyperframe 430. As shown in FIG. 13(A), one hyper frame 430 synchronizes with 34 cycles of the TTR 310 in the above-mentioned TCM transmission.
(6) Report on TTR 310 Phase Information to the Subscriber ADSL Machine 660.
In order that the above mentioned ADSL machine 650 or 660 transmits and receives data using the sliding window 370 and the hyperframe 430, it is necessary for the ADSL machine 650 or 660 to recognize in which frame in the hyperframe 430 data now being transmitted/received in synchronization with the TTR 310 is, or whether the data is transmitted/received as an FEXT symbol or an NEXT symbol.
As shown in FIG. 11, the ADSL machine 650 on the office's side 610 can easily obtain phase information 670 on the TTR 310 from the ISDN machine 630 when installed on the same office's side 610 as the office ISDN machine 630. However, the subscriber ADSL machine 660 is separately installed in the subscriber's premise (on the subscriber's side 620), in general, so that the subscriber ADSL machine 660 cannot obtain the phase information 670 on the TTR 310 from the ISDN machine 640 installed in other subscriber's premise. For this, the subscriber ADSL machine 660 is required to receive the phase information 670 on the TTR from the office ADSL machine 650.
The office ADSL machine 650 therefore transmits the phase information 670 on the TTR 310 received from the office ISDN machine 630 using a carrier for reporting to the subscriber ADSL machine 660, at the time of initialization before the ADSL communication starts. Namely, the office ADSL machine 650 transmits the phase information 670 on the TTR 310 as the FEXT symbol [refer to FIG. 15(A)] or the NEXT symbol [refer to FIG. 15(B)] as decided by the sliding window 370 described above with reference to FIG. 12(E). At this time, the FEXT symbol and the NEXT symbol differ from each other in only phase.
Which frame in the hyper frame 430 should be transmitted as the FEXT symbol, or whether the frame should be transmitted as the NEXT symbol is decided in relation to the sliding window 370. Accordingly, the subscriber ADSL machine 660 can recognize which frame in the hyperframe is now received by receiving the above signal (FEXT symbol or NEXT symbol) transmitted from the office ADSL machine 650.
Since one hyperframe 430 synchronizes with 34 cycles of the TTR 310, as stated above, the subscriber ADSL machine 660 can obtain the phase information 670 on the TTR 310 from the above frame position information in the received hyperframe 430, thereby synchronizing with the TTR 310. After synchronization with the TTR 310 is established, the synchronization is kept using the above pilot tone.
As above, when the TCM line 70′ exists in the vicinity of the ADSL line 70, the ADSL machines 650 and 660 transmit/receive data using the sliding window 370 and the hyper frame 430 to minimize the effect of TCM crosstalk, thereby realizing reliable data transmission.
However, the above ADSL machine of “G.lite”, for example, is not provided with a splitter, that is, a low-pass filter (LPF). Accordingly, when the subscriber ADSL machine 660 cannot accurately receive a pilot tone for maintaining synchronization transmitted from the office ADSL machine 650 for a long time (1.8 ms or longer, for example) in steady communication due to effects of impulse noise caused by off-hook or higher harmonic noise caused by ringer generated in a telephone connected to the same line as the subscriber ADSL machine 660, or crosstalk noise caused by off-hook or ringer generated in a telephone on the adjacent line, the subscriber ADSL machine 660 cannot synchronize with the TTR 310. In such case, the ADSL communication thereafter cannot be continued. In order to re-communicate, it is necessary to initialize once more in the present condition. As this, the ADSL communication is interrupted for a long time once the synchronization with the TTR 310 becomes off.