Abstract:
A band control system for a digital subscriber line network in which a first apparatus and a second apparatus situated at a subscriber station and a center, respectively, are interconnected by a cable for interchanging at least a digital data signal with each other. The system may cause one of the first apparatus or the second apparatus to monitor receipt of signals from the other of the first apparatus or the second apparatus; send, based on a result of monitoring, a band variation command to the other apparatus for causing the other apparatus to vary a band by using a frequency band not used for signal transfer; cause the other apparatus to receive the band variation command; and vary the band in accordance with the band variation command.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a band control system for a digital subscriber network and a band control method therefor. More particularly, the present invention relates to a band control system implementing flexible control and efficient use of a band between IADs (Integrated Access Devices) and a DSLAM (Digital Subscriber Line Access Multiplexer) situated at subscriber stations and a center, respectively, and a control method therefor.  
         [0003]     2. Description of the Background Art  
         [0004]     A VoDSL (Voice over Digital subscriber Line) network using an ATM (Asynchronous Transfer Mode) communication system and a DSL technology provides a transfer path for multimedia communication using voice, data and image. DSL technologies implement high-speed digital transfer over metallic cables, i.e., existing telephone subscriber lines.  
         [0005]     A problem with the conventional DSL technologies is that the transfer rate is dependent on the quality of metallic cables and transfer distance and therefore indefinite despite a preselected transfer rate. Consequently, a communication band statistically set beforehand brings about the congestion of ATM cells and thereby causes some users to be blocked. It is therefore necessary to dynamically control band assignment in order to obviate the congestion of ATM cells.  
         [0006]     Japanese Patent Laid-open Publication No. 2000-184061, for example, discloses a technology for dynamically controlling band assignment in a DSL communication system. It has been customary with conventional technologies, including the above technology, to send band control information indicative of the variation of a communication band by using special ATM cells, e.g., RM (Resource Management) cells. This, however, presses the communication band and thereby makes the use of the frequency band uneconomical when such special ATM cells are used to dynamically guarantee the band during communication.  
         [0007]     Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No-11-331192.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide a band control system for a digital subscriber line network capable of solving the problem stated above and a band control system therefor.  
         [0009]     A band control system of the present invention is applicable to a digital subscriber line network in which a first apparatus and a second apparatus situated at a subscriber station and a center, respectively, are interconnected by a metallic cable for interchanging at least a digital data signal with each other. The band control System includes a commanding device included in one of the first and said second apparatuses for monitoring the receipt of ATM (Asynchronous Transfer Mode) cells from the other apparatus and sending, based on the result of monitoring, a band variation command to the other apparatus to thereby cause it to vary a band by using a frequency band not used for signal transfer. A band varying device is included in the other apparatus for receiving the band variation command and varying the band in accordance with the command.  
         [0010]     A band control method for the band control system is also disclosed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:  
         [0012]      FIG. 1  is a schematic block diagram showing a specific band control system in accordance with the present invention;  
         [0013]      FIG. 2  is a flowchart demonstrating the operation of the hand control system shown in  FIG. 1 ;  
         [0014]      FIG. 3  is a schematic block diagram showing a specific configuration of a DSLAM included in a first embodiment of the present invention;  
         [0015]      FIG. 4  shows a specific frequency characteristic of signals to be interchanged between IADs shown in  FIG. 1  and the DSLAM;  
         [0016]      FIG. 5  is a schematic block diagram showing a specific configuration of each IAD;  
         [0017]      FIG. 6  is a schematic block diagram showing a specific configuration of a DSLAM included in a second embodiment of the present invention;  
         [0018]      FIG. 7  is a schematic block diagram showing a specific configuration of an IAD included in the second embodiment;  
         [0019]      FIG. 8  is a flowchart demonstrating the operation of the second embodiment;  
         [0020]      FIG. 9  is a schematic block diagram showing a third embodiment of the present invention; and  
         [0021]      FIG. 10  is a schematic block diagram showing a specific configuration of an ATU-R (Asymmetric digital subscriber line Termination Units-Remote) included in the third embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Briefly, in a VoDSL network that allows a plurality of digital voice signals and a plurality of digital data signals to be interchanged via metallic wires, or existing telephone subscriber lines, by use of a DS; technology, the present invention provides a band control system realizing flexible control and efficient use of communication band between IADs and a DSLAM situated at subscriber stations and a center, respectively, and a band control method therefor.  
         [0023]     Further, the system and method of the present invention measure the amount of received ATM cells or the ratio of discarded ATM cells VC (Virtual Channel) by VC and then send a band variation command to any one of the IADs by using a frequency band not used for signal transfer in the VoDSL network, e.g., a frequency band lower than 4 kHz inclusive. This dynamically optimizes the frequency band for thereby promoting efficient operation of the VoDSL network.  
         [0024]     Referring to  FIG. 1  of the drawings, a specific configuration of a band control system in accordance with the present invention. As shown, the band control system includes a public switched telephone network  101 , an IP (Internet Protocol) network  102 , a voice GW (GateWay)  103 , a BAS (Broadband Access Server)  104 , an ATM network  105 , a DSLAM  106 , IADs  107 , telephones  108 , and personal computers or similar data terminals  109 . A band control method in accordance with the present invention is applied to the DSLAM  106  and IADs  107 . The telephones  108  and data terminals  109  are assigned to voice telephone services and Internet access and other data communication services, respectively.  
         [0025]     More specifically, the telephones  108 - 1  through  108 - m  (m being a positive integer) and data terminals  109 - 1  through  1 - 9 - n  (n being a positive integer) each are accommodated in one of the MDs  107 - 1  through  107 - x  (x being a positive integer). The IADs  107  each are connected to the DSLAM  106  by one of metallic cables  111 - 1  through  111 - x.  The voice GW  103  is existing network equipment that repeats, when any one of the telephones  108  effects a voice telephone service, all protocols necessary for connecting the ATM network  105  and switched telephone network  101 . Likewise, the BAS  104  is existing network equipment that repeats, when any one of the data terminals  109  effects an Internet access or similar data communication service, all protocols necessary for connecting the ATM network  105  and IP network  102 .  
         [0026]      FIG. 2  demonstrates the operation of the band control system in accordance with the present invention. As shown, the DSLAM  106  measures the amount of ATM cells received from each IAD  107  or the ratio of discarded ATM cells VC by VC (step S 1 ). The DSLAM  106  then commands, based on the result of measurement, the IAD  107  to vary the frequency band by using a frequency band not newly occupied by signal transfer in a VoDSL network, e.g., a frequency band lower than 4 kHz inclusive (step S 2 ). In response, the IAD  107  varies the frequency band of a VC newly designated by the DSLAM  106  (step S 3 ).  
         [0027]     As stated above, the band control system in accordance with the present invention sends band control information to the IAD  107  not by using conventional special ATM cells, but by using a frequency band not occupied by signal transfer. The system therefore solves the previously stated problem particular to the conventional DSL communication system. It is therefore possible to dynamically maintain an optimal communication band between each IAD  107  and the DSLAM  106  for thereby promoting efficient use of the VoDSL network.  
         [0028]     A first embodiment of the present invention also practicable with the configuration shown in  FIG. 1  will be described hereinafter. The description on the individual blocks shown in  FIG. 1  will not be repeatedly made in order to avoid redundancy. The voice GW  103  has the previously stated function as existing network equipment. More specifically, the voice GW  103  communicates with the IADs  107  with a signaling procedure using, e.g., an LES (Lop emulation Service). Also, the voice GW  103  communicates with the switched telephone network  101  with a signaling procedure using an FR-303 or similar time-division communication system.  
         [0029]     The BAS  104  also has the function stated earlier as existing network equipment. More specifically, the BAS  104  communicates with the ATM network  105  with a signaling system using, e.g., a PPPoA (Point to Point Protocol over ATM) system. Also, the BAS  104  interchanges IP packets with the IP network  101  by using an IP signaling system.  
         [0030]     A specific configuration of the DSLAM  106  will be described with reference to  FIG. 3 . As shown, the DSLAM  106  includes an ATM network interface  201 , an ATM cell multiplexer/demultiplexer  202 , x (x being a positive integer) ATM cell queues  203  ( 203 - 1  through  203 - x ) and x center DSL modems  204  ( 204 - 1  through  204 - x ) as conventional. In the illustrative embodiment, the DSLAM  106  additionally includes x band control information transmitters  205  ( 205 - 1  through  205 - x ) and x signal couplers  206  ( 206 - 1  through  206 - x ).  
         [0031]     Each band control information transmitter  205  measures, VC by VC, the amount of ATM cells received from the associated LAD  107  or the ratio of discarded ATM cells. Assume that either the amount or received ATM cells or the ratio of discarded ATM cells exceeds an allowable range implementing preselected communication quality. Then, to command the IAD  107  to vary the communication band assigned to the corresponding VC, the transmitter  205  modulates a band control information signal to a conventional modem signal or similar signal that can be sent in a frequency band lower than 4 kHz inclusive. The modulated signal is fed to the associated signal coupler  206 .  
         [0032]     The signal coupler  206  couples a DSL signal received from the center DSL modem  204  and the band control information signal received from the band control information transmitter  205  to thereby produce a signal, which will be described with reference to  FIG. 4  later. This signal is sent from the signal coupler  206  to the IAD  107  connected to the DSLAM  106  by the associated metallic cable  111 . In addition, when the signal coupler  206  receives a DSL signal from the IAD  107 , the signal coupler  206  simply transfers the DSL signal to the center DSL modem  203  without any processing.  
         [0033]      FIG. 4  shows specific frequency bands assigned to the signals to be interchanged between each TAD  107  and DSLAM  106 . As shown, the signals consist of a DSL signal or main information signal  121  and a band control information signal  122 . A frequency band higher than 4 kHz and a frequency band lower than 4 kHz inclusive are assigned to the DSL signal  121  and band control information signal  122 , respectively, by way of example.  
         [0034]      FIG. 5  shows a specific configuration of each IAD  107 . As shown, the IAD  107  includes a terminal DSL modem  302 , an ATM cellularizer/decellularizer  304 , a telephone interface  305 , and a data terminal interface  306  as conventional. In the illustrative embodiment, the IAD  107  additionally includes a signal uncoupler  301 , and a band control information receiver  303 .  
         [0035]     The conventional telephone interface  305  allows various kinds of telephone terminals  108  available for voice telephone services to be accommodated in the IAD  107 . For this purpose, the telephone interface  305  functions to terminate POTS (Plain Old Telephone Service) interfaces assigned to traditional analog telephones and SIT point interfaces assigned to ISDN (Integrated Services Digital Network) terminal adapters.  
         [0036]     Likewise, the conventional data terminal interface  306  allows various kinds of data terminals  109  available for internet access and other data communication services to be accommodated in the IAD  107 . For this purpose, the data terminal interface  306  functions to terminate a USB (Universal Serial Bus), 10/100 Base-T or similar interface.  
         [0037]     The signal uncoupler  301  separates the signals shown in  FIG. 4  and received from the DSLAM  106  via the metallic cable  111  into the DSL signal  121  and frequency control information signal  122  that lies in the frequency band lower than 4 kHz inclusive. The DSL signal  121  and band control information signal  122  separated from each other are input to the terminal DSL modem  302  and band control information receiver  303 , respectively. On the other hand, when a DSL signal is input from the terminal DSL modem  302  to the signal uncoupler  301 , the signal uncoupler  301  simply transfers the DSL signal to the metallic cable  111  without any processing.  
         [0038]     The band control information receiver  303  separates the band control information sent from the DSLAM  106  from the band control information signal  122  and analyzes the information. The receiver  303  then causes the ATM cellularizer/decellularizer  304  to vary the communication band assigned to the corresponding VC directed toward the DSLAM  106 .  
         [0039]     Specific operations of the illustrative embodiment will be described hereinafter. First, how signals flow when the telephone  108  accommodated in any one of the IADs  107  communicates with the public switched telephone network  101  by using a voice telephone service will be described.  
         [0040]     Referring again to  FIG. 1 , as for the flow of signals from the telephone network  101  toward the telephone  108 , a digital voice signal based on the time-division communication system is sent from the telephone network  101  to the voice GW  103  and transformed to ATM cells thereby. The cellularized digital voice signal is sent from the voice GW  103  to the DSLAM  106  via the ATM network  105 . In the DSLAM  106 , the ATM cell multiplexer/demultiplexer  202  delivers the cellularized digital voice signal to the center DSL modem  204  via one of the ATM cell queues  203  corresponding to the MAD  107 . The office DSL modem  204  modulates the cellularized digital voice signal to a DSL signal and sends the DSL signal to the IAD  107  connected thereto by the metallic cable  111 .  
         [0041]     In the IAD  107  shown in  FIG. 5 , the terminal DSL modem  302  demodulates the DSL signal to thereby restore the original ATM cellularized voice signal. Subsequently, the ATM cellularizer/decellularizer  304  decellularizes the cellularized digital signal input from the terminal DSL modem  302 . As a result, the decellularized digital voice signal is input to the telephone interface  305 . The telephone interface  305  transforms the digital voice signal to a voice signal format matching with the telephone  108  and then sends the transformed voice signal to the telephone  108 .  
         [0042]     The flow of signals from the telephone  109  toward the telephone network  101  is identical with the flow described above except that the procedure is reversed in direction and will not be described specifically.  
         [0043]     As stated above, the illustrative embodiment implements a bidirectional voice telephone service between the telephone  108  accommodated in the IAD  107  and the public switched telephone network  101 .  
         [0044]     Next, how signals flow when the data terminal  109  accommodated in any one of the IADs  107  effects a data communication service with the IP network  102  will be described hereinafter. Referring to  FIG. 1 , as for the flow of signals from the IP network  102  toward the data terminal  109 , the IP network  102  sends an IP packet or an IP-packeted digital data signal to the BAS  104 . The BAS  104  transforms the received IP packet or the IP-packeted digital data to an ATM cell. The ATM-cellularized IP packet or the IP-packeted digital data signal is sent to the DSLAM  106  via the ATM network  105 . In the DSLAM  107 , the multiplexer/demultiplexer  202 ,  FIG. 3 , delivers the ATM-cellularized IP packet or the IP-packeted digital signal data to the center DSL modem  204  via the ATM cell queue  203  corresponding to the IAD  107 , which accommodates the data terminal  109 . The center DSL modem  204  modulates the ATM-cellularized IP packet or the IP-packeted digital data signal to a DSL signal and sends the DSL signal to the IAD  107  via the metallic cable  111 .  
         [0045]     In the IAD  107 , the terminal DSL modem  302 ,  FIG. 5 , demodulates the DSL signal to thereby restore the ATM-cellularized IP packet or the IP-packeted digital data signal and feeds it to the ATM cellularizer/decellularizer  304 . The ATM cellularizer/decellularizer  304  decellularizes the ATM-cellularized IP packet or the IP-packeted digital data signal and inputs the resulting IP packet or the IP-packeted digital data signal to the data terminal interface  306 . The data terminal interface  306  transforms the IP packet or the IP-packeted digital data signal to a format matching with the data terminal  109  and then sends the transformed IS packet or the transformed data signal to the data terminal  109 .  
         [0046]     The flow of signals from the data terminal  109  toward the IP network  102  is identical with the flow described above except that the procedure is reversed in direction and will not be described specifically.  
         [0047]     As stated above, the illustrative embodiment implements a bidirectional data communication service between the data terminal  109  accommodated in the IAD  107  and the IP network  102 .  
         [0048]     Hereinafter will be described a band control procedure to be executed between each IAD  107  and the DSLAM  106 . When a plurality of voice telephone services and a plurality of data communication services, both of which are bidirectional; are effected at the same time, importance should be attached to the communication quality of voice telephone services. This is because voice telephone services allow information to be interchanged between persons and therefore need realtime communication more than data communication services. It is therefore necessary to reduce propagation delays as far as possible. In addition, voice quality falls with an increase in the number of ATM cells discarded due to the failure of retransmission. On the other hand, data communication services should also be effected at high speed as possible for users&#39; convenience.  
         [0049]     In light of the above, in the illustrative embodiment, each band control information transmitter  205 ,  FIG. 3 , measures the amount of ATM cells received from the associated IAD  107  and present on the associated ATM cell queue  203  or the ratio of discarded ATM cells VC by VC (step S 1 ,  FIG. 2 ). Assume that the amount of ATM cells or the ratio of discarded ATM cells exceeds an allowable range assigned to the communication quality of a voice telephone service, which is determined by the provider of the VoDSL network beforehand. Then, the band control information transmitter  205  modulates the band control information signal  122 ,  FIG. 4 , to a conventional modem signal or similar signal that can be sent in the frequency band lower than 4 kHz inclusive. The band control information signal  122  is sent to the IAD  107  via the signal coupler  206  in order to command the IAD  107  to vary the frequency band of the VC on which the corresponding data communication service is held (step S 2 ,  FIG. 2 ).  
         [0050]     In the IAD  107  shown in  FIG. 5 , the band control information signal  122  is routed through the signal uncoupler  301  to the band control information receiver  303 . The receiver  303  separates the band control information signal  122  and analyzes the communication band designated by the DSLAM  106 . The receiver  303  then controls the ATM cellularizer/decellularizer  304  in order to narrow the communication band assigned to the VC of the corresponding data communication service and directed toward the DSLAM  106  (step S 3 ,  FIG. 2 ).  
         [0051]     On the other hand, assume that the amount of ATM cells or the ratio of discarded ATM cells decreases below the allowable range assigned to the communication quality of the voice telephone-service. Then, the DSLAM  106  sends the band control information signal  122  to the IAD  107  in the previously stated manner. Again, the band control information receiver  303  separates the band control information signal  122  and analyzes the communication band designated by the DSLAM  106 . The receiver  303  then controls the ATM cellularizer/decellularizer  304  in order to broaden the communication band assigned to the VC of the corresponding data communication service and directed toward the DSLAM  106 .  
         [0052]     A second embodiment of the present invention will be described hereinafter. The second embodiment is essentially similar to the first embodiment except that the DSLAM  106  is also configured to vary the frequency band of the VC designated by the IAD  107  for thereby further promoting efficient operation of the VoDSL network.  
         [0053]     As shown in  FIG. 6  specifically, the DSLAM  106  includes signal coupler/uncouplers  501  ( 501 - 1  through  501 - x ). Each signal coupler/uncoupler  501  has, in addition to the function of the signal coupler  206 ,  FIG. 3 , a function of separating the signal received from the IAD  107  into the DSL signal  121  and the signal  122  lying in the frequency band lower than  4  kHz inclusive and feeding the signal  122  to a band control signal transmitter/receiver  502 .  
         [0054]     The band control signal transmitter/receiver  502  has the following function in addition to the function of the band control information transmitter  2051   FIG. 3 . The additional function is to separate the band control information signal sent from the IAD  107  from the signal  122  input from the signal coupler/uncoupler  501 , analyze the signal  122 , control the ATM cell queue  203  in accordance with the result of analysis, and vary the communication band assigned to the corresponding VC and directed toward the IAD  107 .  
         [0055]     As shown in  FIG. 7  specifically, the IAD  107  includes a signal coupler/uncoupler  601  and a band control signal transmitter/receiver  602 . The signal coupler/uncoupler  601  has, in addition to the function of the signal uncoupler  301 ,  FIG. 5 , a function of coupling the DSL signal  121  received from the terminal DSL modem  302  and the band control information signal  122  received from the band control information transmitter/receiver  602  and sending the resulting signal to the DSLAM  106 .  
         [0056]     The band control information signal transmitter/receiver  602  has the following function in addition to the function of the band control information receiver  303 ,  FIG. 5 . The additional function is to measure the amount of ATM cells received from the DSLAM  106  and input to the ATM cellularizer/decellularizer  304  or the ratio of discarded ATM cells VC by VC When the amount of ATM cells or the ratio of discarded ATM cells increases above an allowable range assigned to communication quality, the band control information transmitter/receiver  602  modulates the band control information signal to a conventional modem signal or similar signal that can be sent in the frequency band lower than 4 kHz inclusive. The modulated signal is sent to the signal coupler/uncoupler  601  in order to command the signal coupler/uncoupler  601  to vary the communication band of the corresponding VC.  
         [0057]     More specifically, as shown in  FIG. 8 , the IAD  107  measures the amount of ATM cells received from the DSLAM  106  or the ratio of discarded ATM cells VC by VC (step S 11 ). The IAD  107  then commands the DSLAM  106  to vary the frequency band by using the band lower than 4 kHz inclusive (step S 12 ). In response, the DSLAM  106  varies the frequency band of the VC designated by the IAD  107  (step S 13 ).  
         [0058]     As stated above, in the illustrative embodiment, the IAD  107  can command, based on the amount of ATM cells received from the DSLAM  106  or the ratio of discarded ATM cells, the DSLAM  106  to vary the communication band VC by VC. It follows that the DSLAM  106  can narrow or broaden the band of the VC designated by the IAD  107  accordingly.  
         [0059]     A third embodiment of the present invention will be described hereinafter. This embodiment is applicable to a DSL network configured to promote high-speed Internet access and other data communication services by using metallic cables. A DSL network transforms only digital data signals to ATM cells and transfer the ATM cells via metallic cables.  FIG. 9  shows a band control system representative of the third embodiment. As shown, the third embodiment includes x ATU-Rs  701  ( 701 - 1  through  701 - x ) in place of the IADs  107 - 1  through  107 - x,    FIG. 1 . The x ATU-Rs  701  are connected to the ISLAM  106  by the metallic cables  111 .  
         [0060]      FIG. 10  shows a specific configuration of one of the ATU-RS  701 . As shown, the ATU-R  701  includes the band control information transmitter/receiver  602  and signal coupler/uncoupler  601  in addition to the conventional terminal DSL modem  302 , ATM cellularizer/decellularizer  304 , and data terminal interface  306 . How the illustrative embodiment executes bidirectional control over the communication band between the DSLAM  106  and each ATU-R  701  will be described hereinafter.  
         [0061]     When the data terminal  109  accommodated in any one of the ATU-Rs effects a data communication service with the IP network  102 , signals flow in exactly the same manner as when the data terminal  109  accommodated in the IAD  107  effects a data communication service with the IP network  102 .  
         [0062]     First, a specific procedure for controlling the communication band directed from the ATU-R  701  toward the DSLAM  106  when a plurality of data communication services are held will be described. In the DSLAM  106  shown in  FIG. 6 , each band control information transmitter/receiver  502  measures the amount of ATM cells received from the associated ATU-R  701  and present on the associated ATM cell queue  203  or the ratio of discarded ATM cells VC by VC (step S 1 ,  FIG. 2 ). Assume that the amount of ATM cells or the ratio of discarded ATM cells increases above an allowable range assigned to the communication quality of a voice telephone service, which is determined by the provider of the VoDSL network beforehand. Then, the band control information transmitter/receiver  502  modulates the band control information signal to a conventional modem signal or similar signal that can be sent in the frequency band lower than 4 kHz inclusive. The band control information signal is sent to the ATU-R  701  via the signal coupler/uncoupler  501  in order to command the ATU-R  701  to vary the frequency band of the VC on which the data corresponding data communication service is held (step S 2 ,  FIG. 2 ).  
         [0063]     In the ATU-R  701  shown in  FIG. 10 , the band control information signal is routed through the signal coupler/uncoupler  601  to the band control information transmitter/receiver  602 . The transmitter/receiver  602  analyzes the communication band designated by the DSLAM  106 . The transmitter/receiver  602  then controls the ATM cellularizer/decellularizer  304  in order to narrow the communication band assigned to the VC of the corresponding data communication service and for transmission to the DSLAM  106  (step S 3 ,  FIG. 2 ).  
         [0064]     On the other hand, assume that the band control information transmitter/receiver  502  included in the DSLAM  106 ,  FIG. 6 , determines that the amount of ATM cells or the ratio of discarded ATM cells has decreased below the allowable range assigned to the communication quality of the voice telephone service. Then, the DSLAM  106  sends the band control information signal to the ATU-R  701  in the previously stated manner. Again, the band control information transmitter/receiver  602  separates the band control information signal and analyzes the communication band designated by the DSLAM  106 . The transmitter/receiver  602  then controls the ATM cellularizer/decellularizer  304  in order to broaden the communication band assigned to the VC of the corresponding data communication service and adapted for transmission to the DSLAM  106 .  
         [0065]     Next, a specific operation for controlling the communication band directed from the DSLAM  106  toward any one of the ATU-Rs  701  will be described. In the ATU-R  701  shown in  FIG. 10 , each band control information transmitter/receiver  602  measures the amount of ATM cells received from the DSLAM  106  and present in the ATM cellularizer/decellularizer  304  or the ratio of discarded ATM cells VC by VC (step S 11 ,  FIG. 8 ). Assume that the amount of ATM cells or the ratio of discarded ATM cells increases above an allowable range assigned to the communication quality of a voice telephone service, which is determined by the provider of the VoDSL network beforehand. Then, the band control information transmitter/receiver  602  modulates the band control information signal to a conventional modem signal or similar signal that can be sent in the frequency band lower than 4 kHz inclusive. The band control information signal is sent to the DSLAM  106  via the signal coupler/uncoupler  601  in order to command the DSLAM  106  to vary the frequency band of the VC on which the data corresponding data communication service is held (step S 12 ,  FIG. 8 ).  
         [0066]     In the DSLAM  106 , the band control information signal is routed through the signal coupler/uncoupler  501  to the band control information transmitter/receiver  502 . The transmitter/receiver  502  separates the communication band control signal and analyzes the communication band designated by the ATU-R  701 . The transmitter/receiver  502  then controls the ATM cell queue  203  in order to narrow the communication band assigned to the VC of the corresponding data communication service and adapted for transmission to the ATU-R  701  (step S 13 ,  FIG. 8 ).  
         [0067]     On the other hand, assume that the band control information transmitter/receiver  602  included in the ATU-R  701  determines that the amount of ATM cells or the ratio of discarded ATM cells has decreased below the allowable range assigned to the communication quality of the voice telephone service. Then, the transmitter/receiver  602  sends the band control information signal to the DSLAM  106  in the previously stated manner. Again, the band control information transmitter/receiver  502  in the DSLAM  106  separates the band control information signal and analyzes the communication band designated by the ATU-R  701 . The transmitter/receiver  502  then controls the ATM cell queue  203  in order to broaden the communication band assigned to the VC of the corresponding data communication service and adapted for transmission to the ATU-R  701 .  
         [0068]     As stated above, the illustrative embodiment provides high-quality data communication services by dynamically optimizing the frequency bands between the DSLAM  106  and the ATU-Rs  701  in opposite directions, thereby promoting efficient operation of the DSL subscriber network.  
         [0069]     In summary, it will be seen that the present invention obviates uneconomical use of a communication band by preventing it from being pressed. In addition, the present invention provides high-quality data communication services by dynamically optimizing frequency bands between a DSLAM and ZADs for thereby promoting efficient operation of a VoDSL network.  
         [0070]     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.