Abstract:
The present invention is intended to prevent from billing an unnecessary charge to an user by omitting AIS signals transmitted/received between a transmission side ATM unit and a receiving side ATM unit when a loss of service occurs in a communication network in which existing communication lines are connected by an ATM communication unit. In order to achieve the aforementioned goal, the transmission side ATM unit is adapted to monitor a signal received from a communication line on the transmission side and to stop the transmission of ATM cells to the receiving side ATM unit for a certain period of time when it detects an AIS signal indicating a loss of service on the transmission side and the receiving side ATM unit is adapted to monitor a state for receiving the ATM cells transmitted from the transmission side ATM unit and when it recognizes that the receiving of the ATM cells is stopped for a certain period of time, to generate an AIS signal indicating the loss of service on the transmission side and to send it on the communication line on the receiving side.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of application Ser. No. 08/682,958, filed Jul. 18, 1996 now abandoned, which is a division of application Ser. No. 08/347,950 filed Dec. 1, 1994 and now U.S. Pat. No. 5,553,057. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an AIS (Alarm Indication Signal) signal transmission method and a unit for informing a loss of service in a broad band ISDN and particularly in an ATM communication system in which existing communication lines are interconnected through an intermediary of ATM, and to a transmission side ATM unit and an ATM communication system. 
   2. Description of the Related Art 
   With the late diversification of information modes including voice, data and image and the increase of information amount, a broad band ISDN (Integrated Service Digital Network) is being actively developed. For the broad band ISDN, a system for connecting existing communication networks such as LAN and LAN and PBX PBX by an ATM (Asynchronous Transfer Mode: Asynchronous Network) is being considered. 
     FIG. 24  shows an arrangement of a communication network in which existing communication networks are connected by the ATM. In this communication network, existing units of routers of LAN, relay stations or PBX are connected by the ATM. 
   ATM units  200 A and  200 B and an ATM switching unit lie between an existing unit  100 A on the transmission side and an existing unit  100 B on the receiving side. The transmission side ATM unit  200 A is connected to the transmission side existing unit  100 A via an interface (DS1/DS3) in DS1 (Digital Signal Level 1) or DS3 (Digital Signal Level 3). On the other hand, the receiving side ATM unit  200 B is connected to the receiving side existing unit  100 B via an interface (DS1/DS3). 
   The ATM switching unit  300  for switching ATM cells lies between the transmission side ATM unit  200 A and the receiving side ATM unit  200 B. Each of the ATM units  200 A and  200 B and the ATM switching unit  300  are connected via user interfaces (UNI). 
   The interface (DS1/DS3) on the transmission side transmits a DS1 or DS3 signal transmitted from the transmission side existing unit  100 A to the transmission side ATM unit  200 A. The transmission side ATM unit  200 A converts the DS1 or DS3 signal transmitted from the interface (DS1/DS3) into ATM cells and transmits them to the receiving side via the ATM switching unit  300 . 
   The receiving side ATM unit  200 B restores the ATM cells received via the ATM switching unit  300  into the DS1 or DS3 signal. The interface (DS1/DS3) on the receiving side transmits the DS1 or DS3 signal sent from the receiving side ATM unit  200 B to the receiving side existing unit  100 B. 
     FIG. 25  is a functional block diagram showing an internal structure of the ATM units  200 A and  200 B (hereinafter generally referred to as the ATM unit  200 ). The conventional ATM unit  200  has transmission and receiving systems. When the transmission system receives the DS1 or DS3 signal from the interface (DS1/DS3) on the transmission side, a DS1/DS3 terminating section  201  converts the DS1 or DS3 signal into a signal mode which can be processed by the ATM unit  200  and sends it to a selector  202 . 
   The selector  202  switches an input terminal A 1  and input terminal A 2  and outputs data input from the DS1/DS3 terminating section  201  or data input from an AIS generating section  204  to an ATM cell generating section  203 . Normally, the input path of the selector  202  is connected to the input terminal A 1  to output data input from the DS1/DS3 terminating section  201  to the ATM cell generating section  203 . At this time, ATM cell generating section  203  outputs the DS1 or DS3 signal by converting into ATM cells. The ATM cells are then input to the receiving side ATM unit  200 B via the ATM switching unit  300  and are restored to the DS1 or DS3 signal by the ATM unit  200  to be transmitted to the receiving side existing unit  100 B (See ( 1 ) in  FIG. 26 ). 
   On the other hand, a LOS detecting section  205  monitors the DS1 or DS3 signal input to the DS1/DS3 terminating section  201  and detects a loss of service (LOS) when the DS1 or DS3 frame is a signal indicating “0s” successively for a certain number of bits. When the loss of service is detected, this LOS detecting section  205  outputs an instruction to the selector  202  to switch the input terminal. 
   Receiving the instruction, the selector  202  switches the input terminal from A 1  to A 2 . At this time, an AIS generating section  204  outputs an AIS (Alarm Indication Signal) indicating the loss of service. The AIS signal output from the AIS generating section  204  is input to the ATM cell generating section  203  via the selector  202 . 
   The ATM cell generating section  203  outputs the AIS signal by converting into ATM cells. This AIS signal is then transmitted to the receiving side ATM unit  200 B via the ATM switching unit  300  and to the receiving side existing unit  100 B (see ( 3 ) in  FIG. 26 ). Thereby the receiving side existing unit  100 B can recognize the loss of service on the transmission side. 
   When the transmission side existing unit  100 A transmits the AIS signal indicating the loss of service, the transmission system of the ATM unit  200  outputs the AIS signal by converting into ATM cells. The AIS signal converted into the ATM cells is then input to the receiving side ATM unit  200 B via the ATM switching unit  300 . Then the receiving side ATM unit  200 B restores the AIS signal converted into the ATM cells to the DS1 or DS3 signal to transmit to the receiving side existing unit  100 B (see ( 2 ) in  FIG. 26 ). 
   Next, in the receiving system of the ATM unit  200 , an ATM cell receiving section  209  receives the ATM cells transmitted from the ATM switching unit  300 . The ATM cell receiving section  209  transfers the received ATM cells to a receiving buffer  208 . The receiving buffer  208  temporarily stores the ATM cells input from the ATM cell receiving section  209  and appropriately outputs them. 
   A DS1/DS3 terminating section  207  decomposes the ATM cells output from the receiving buffer  208  to restore to the DS1 or DS3 signal. An input path of a selector  206  is normally connected with an input terminal B 1  to transmit the DS1 or DS3 signal input from the DS1/DS3 terminating section  207  to the existing unit  100 . 
   A starvation detecting section  210  monitors a number of ATM cells stored in the receiving buffer  208  and outputs to the selector  206  an instruction to switch the input paths when it detects that the number of cells is “zero” (starvation) for a specific period of time. 
   At this time, the AIS generating section  211  outputs an AIS signal indicating a loss of service. Then the selector  206  inputs the AIS signal output from the AIS generating section  211  via an input terminal B 2  and transmits it to the existing unit  100  (see ( 4 ) in  FIG. 26 ). 
   By the way, in the conventional method and system described above, because the transmission system of the ATM unit  200  outputs an AIS signal by converting into ATM cells when a loss of service occurs, the ATM cells are transmitted on the user interfaces (UNI) even if no data to be communicated exists. That is, because communication charge is made based on the number of cells transmitted on the user interfaces (UNI) in the ATM, a user is charged even for the cells of AIS signal. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to prevent from making an unnecessary charge to an user by omitting AIS signals transmitted/received between a transmission side ATM unit and a receiving side ATM unit when a loss of service occurs in a communication network in which existing communication lines are connected by an ATM communication system. In order to achieve the aforementioned goal, the present invention adopts the following arrangement. 
   [AIS Transmission Method in ATM Communication System] 
   An AIS transmission method of the present invention is applied to an ATM communication system in which a transmission side ATM unit is disposed between a communication line on the transmission side and an ATM switching unit and a receiving side ATM unit is disposed between a communication line on the receiving side and the ATM switching unit. 
   The AIS transmission method includes the following steps: 
   (1) ATM cell generating step 
   (2) Signal analyzing step 
   (3) ATM cell transmission stopping step 
   (4) ATM cell receiving step 
   (5) ATM cell storing step 
   (6) ATM cell decomposing step 
   (7) Starvation detecting step 
   (8) AIS generating step 
   (9) Signal sending step 
   Here the steps (1) through (3) are processing procedures to be executed by the transmission side ATM unit and the steps (4) through (7) are those to be executed by the receiving side ATM unit. 
   The ATM cell generating step in (1) includes a process of converting the signal transmitted from the communication line on the transmission side into fixed length ATM cells and for transmitting them to the receiving side ATM unit via the ATM switching unit. 
   The signal analyzing step in (2) includes a process of analyzing the signal transmitted from the communication line on the transmission side to discriminate whether it is an AIS signal indicating a loss of service on the transmission side. 
   The ATM cell transmission stopping step in (3) includes a process of stopping the ATM cell generating and transmitting processes for a certain period of time when the AIS signal is detected in the signal analyzing step in (2). 
   The ATM cell receiving step in (4) includes a process of receiving the ATM cells transmitted from the transmission side ATM unit. 
   The ATM cell storing step in (5) includes a process of temporarily storing the ATM cells received by the ATM cell receiving step in a receiving buffer and for appropriately outputting them. 
   The ATM cell decomposing step in (6) includes a process of decomposing the ATM cells output from the receiving buffer to restore to a signal mode transmittable on the communication line on the receiving side. 
   The starvation detecting step in (7) includes a process of monitoring a storing state of the receiving buffer to issue an AIS transmission command when it detects a state in which no ATM cell is stored. 
   The AIS generating step in (8) includes a process of generating the AIS signal indicating the loss of service on the transmission side in accordance to the AIS transmission command issued by the starvation detecting step. 
   The signal sending step in (9) includes a process of sending the AIS signal generated in the AIS generating step onto the communication line on the receiving side. 
   Here the signal analyzing step in (2) includes a process of discriminating whether the signal received from the communication line on the transmission side contains normal data. At this time, when a signal in which normal data is not stored is detected in the signal analyzing step, the steps (3) through (8) are executed in the similar manner as described above. 
   Further, the signal analyzing step in (2) may comprise steps of discriminating an attribute of the signal received from the communication line on the transmission side to discriminate whether it is an AIS signal indicating a loss of service on the transmission side and issuing a cell stop command for stopping the transmission of ATM cells when the AIS signal is detected. At this time, the ATM cell transmission stopping step in (3) includes a step of stopping the generation and transmission of the ATM cells for a certain period of time when the cell stop command is issued. 
   Further, the signal analyzing step in (2) may comprise steps of analyzing the signal received from the communication line on the transmission side to discriminate whether normal data is stored in it and issuing a cell stop command when a signal indicating “0s” successively for a certain number of bits is detected: 
   Furthermore, the signal analyzing step in (2) may comprise an AIS detecting step of analyzing the signal transmitted from the communication line on the transmission side to discriminate whether it is an AIS signal indicating a loss of service on the transmission side and a LOS detecting step of analyzing the signal transmitted from the communication line on the transmission side to discriminate whether it is a signal indicating “0s” successively for a certain number of bits. 
   The signal sending step in (9) includes a step of sending the signal restored in the cell decomposing step onto the communication line on the receiving side when at least one ATM cell is stored in the receiving buffer. 
   It also includes a step of stopping the transmission of the signal output from the cell decomposing step and in the same time, transmitting the AIS signal generated in the AIS generating step onto the communication line on the receiving side when no ATM cell is stored in the receiving buffer and the starvation detecting step issues the AIS sending command. 
   Next, an AIS transmission unit of the present invention will be explained. 
   [AIS Transmission Unit in ATM Communication System] 
   The AIS transmission unit of the present invention is configured by disposing a transmission side ATM unit between a communication line on the transmission side and a switching unit and by disposing a receiving side ATM unit between the switching unit and a communication line on the receiving side. The transmission side ATM unit comprises ATM cell generating means, signal analyzing means and ATM cell transmission stopping means. 
   The ATM cell generating means converts the signal received from the communication line on the transmission side into ATM cells and transmits them to the receiving side via the ATM switching unit. 
   The signal analyzing means analyzes the signal transmitted from the communication line on the transmission side to discriminate whether it is the AIS signal indicating a loss of service on the transmission side and issues a ATM cell stop command when it detects the AIS signal. 
   The signal analyzing means also discriminates whether normal data is stored in the signal received from the communication line on the transmission side and issues the ATM cell stop command when it detects a signal indicating “0s” successively for a certain number of bits. 
   The ATM cell transmission stopping means stops the generation and transmission of the ATM cells for a certain period of time when it receives the ATM cell stop command. 
   The ATM cell transmission stopping means also stops the operation of the ATM cell generating means for a certain period of time when the ATM cell stop command is issued from the signal analyzing means. 
   On the other hand, the receiving side ATM unit comprises ATM cell receiving means, receiving buffer, ATM cell decomposing means, starvation detecting step, AIS generating means and signal sending means. 
   The ATM cell receiving means receives the ATM cells transmitted from the transmission side ATM unit 
   The receiving buffer temporarily stores the ATM cells received by the ATM cell receiving means and appropriately outputs them. 
   The ATM cell decomposing means decomposes the ATM cells output by the receiving buffer to restore to the signal mode transmittable on the communication line on the receiving side. 
   The starvation detecting means monitors the storage state of the receiving buffer and issues an AIS transmission command when it detects that no ATM cells is stored. 
   The AIS generating means generates the AIS signal indicating a loss of service on the transmission side in accordance to the AIS transmission command issued by the starvation detecting means. 
   The signal sending means sends the AIS signal generated by the AIS generating means onto the communication line on the receiving side. 
   Further, the signal analyzing means may comprise an AIS detecting section for monitoring the attribute of the signal transmitted from the communication line on the transmission side to detect the AIS signal indicating a loss of service on the transmission side and a LOS detecting section for monitoring the signal transmitted from the communication line on the transmission side to detect a signal indicating “0s” successively for a certain number of bits. 
   The signal sending means of the receiving side ATM unit sends the signal restored in the cell decomposing means onto the communication line on the receiving side when at least one ATM cell is stored in the receiving buffer. It also stops the transmission of the signal output from the cell decomposing means and in the same time, transmits the AIS signal generated in the AIS generating means onto the communication line on the receiving side when no ATM cell is stored in the receiving buffer and the starvation detecting means issues the AIS sending command. 
   According to the present invention, the transmission side ATM unit stops the transmission of ATM cells to the receiving side for a certain period of time when it receives the AIS signal indicating a loss of service on the transmission side from the communication line on the transmission side. 
   At this time, the ATM cells are deprived to zero in the receiving buffer of the receiving side ATM unit (starvation state), thereby allowing to recognize that the transmission side ATM unit has stopped to transmit ATM cells, i.e. that a loss of service has occurred. Then the receiving side ATM unit can generate the AIS signal and transmit this AIS signal onto the communication line on the receiving side, thereby allowing the receiving side to recognize the loss of service in the same manner with the conventional method. 
   Further, when a loss of service occurs on the communication line on the transmission side and data region of the signal transmitted from the communication line is destroyed, the transmission side ATM unit detects an abnormality of the received signal and stops the transmission of the ATM cells. Thereby the receiving side ATM unit can recognize the loss of service, generate the AIS signal and transmit on the communication line on the receiving side. 
   Further, when a loss of service occurs on the line connecting the transmission side ATM unit and the receiving side ATM unit and ATM cells cannot be transmitted to the receiving side ATM unit, ATM cells are deprived to zero in the receiving buffer of the receiving side ATM unit. Thereby the receiving side ATM unit can recognize the occurrence of the loss of service and transmit the AIS signal onto the communication line on the receiving side. 
   As described above, the AIS signal needs not be transmitted from the transmission side ATM unit to the receiving side ATM unit when a loss of service occurs on the transmission side, so that the present invention allows to prevent from making unnecessary charge to communication users. Furthermore, the AIS signal needs not be transmitted/received on the ATM network, so that the transmission band width on the ATM network may be effectively utilized. 
   Still more, because the present invention can execute the same sequence with the conventional one between the ATM network and existing communication networks, existing interfaces may be used. 
   The above and other advantages of the present invention will become more apparent in the following description and the accompanying drawings in which like numerals refer to like parts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a structure of an ATM communication system according to a first embodiment of the present invention; 
       FIG. 2  is a schematic block diagram showing a structure of a communication network according to a second embodiment of the present invention; 
       FIG. 3  is a block diagram showing a functional structure of an ATM unit according to the second embodiment; 
       FIG. 4  is a diagram showing a frame structure of a DS1 signal; 
       FIG. 5  is a diagram showing a structure of an ATM cell; 
       FIG. 6  is a diagram showing a hardware structure of the transmission system; 
       FIG. 7  is a concrete example of a B8ZS signal; 
       FIG. 8  is a timing chart showing a processing of a terminating section of the receiving side DS1; 
       FIG. 9  is a timing chart showing processing of an AIS detecting section; 
       FIG. 10  is a timing chart showing processing of a LOS detecting section; 
       FIG. 11  is a timing chart showing operations of an OR circuit; 
       FIG. 12  is a timing chart showing processing of an ATM cell generating section; 
       FIG. 13  is a timing chart showing processing of an ATM cell transmission stopping section; 
       FIG. 14  is a diagram showing a hardware structure of the receiving system; 
       FIG. 15  is a timing chart showing processing of an ATM cell receiving section; 
       FIG. 16  is a timing chart showing processing of a receiving buffer; 
       FIG. 17  is a timing chart showing processing of a starvation detection section; 
       FIG. 18  is a timing chart showing processing of a terminating section of the transmission side DS1; 
       FIG. 19  is a timing chart showing processing of a selector; 
       FIG. 20  is a block diagram showing a communication network when a loss of service occurs; 
       FIG. 21  is a block diagram showing the communication network when the loss of service occurs; 
       FIG. 22  is a block diagram showing the communication network when the loss of service occurs; 
       FIG. 23  is a diagram showing transmission sequences of signals in the second embodiment; 
       FIG. 24  is a schematic block diagram showing a structure of a conventional communication network; 
       FIG. 25  is a block diagram showing a structure of a conventional ATM unit per function; and 
       FIG. 26  is a diagram showing a transmission sequences of an AIS signal in the prior art communication network. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring now to the drawings, preferred embodiments of the present invention will be described in detail. 
   First Embodiment 
     FIG. 1  shows a structure of an ATM communication system according to the first embodiment of the present invention. The ATM communication system comprises a transmission side ATM unit  3 , an ATM switching unit  4  and a receiving side ATM unit  5 . 
   The transmission side ATM unit  3  converts a signal transmitted from a communication line  1  on the transmission side into ATM cells and transmits them to the receiving side via the ATM switching unit  4 . When an AIS signal indicating a loss of service on the transmission side is transmitted from the transmission side communication line  1 , the transmission side ATM unit  3  stops the transmission of the ATM cells to the receiving side for a certain period of time. 
   On the other hand, the receiving side ATM unit  5  decomposes the ATM cells transmitted from the transmission side ATM unit  3  to restore to a signal mode transmittable on a communication line  2  on the receiving side and transmits the restored signal to the receiving side communication line  2 . The receiving side ATM unit  5  of the present invention also has a function to monitor a receiving state of the ATM cells and to recognize that the transmission side ATM unit  3  has stopped to transmit cells. By recognizing that the transmission side ATM unit  3  has stopped the transmission, the receiving side ATM unit  5  generates an AIS signal and sends out it onto the receiving side communication line  2 . 
   The transmission side ATM unit  3  also has a function to monitor whether correct data is stored in the signal received from the transmission side communication line  1 . When correct data is not stored in the received signal, the transmission side ATM unit  3  stops the transmission of ATM cells to the receiving side ATM unit  5  for a certain period of time. 
   A structure of the transmission side ATM unit  3  and the receiving side ATM unit  5  will now be explained. 
   The transmission side ATM unit  3  may comprise a ATM cell generating section  3   a , ATM cell transmission stopping section  3   b  and signal analyzing section  3   c,    
   The ATM cell generating section  3   a  converts the signal transmitted from the transmission side communication line  1  to ATM cells and transmits them to the ATM switching unit  4 . 
   The ATM cell transmission stopping section  3   b  stops the operation of the ATM cell generating section  3   a  for a certain period of time. That is, it has a function to stop the ATM cells generating process and ATM cells transmitting process of the ATM cell generating section  3   a  for a certain period of time. 
   The signal analyzing section  3   c  discriminates an attribute and normality of the signal received from the transmission side communication line  1  and issues a command to stop the transmission of cells to the ATM cell transmission stopping section  3   b  when it detects an AIS signal indicating a loss of service on the transmission side or a signal indicating “0s” successively for a certain number of bits. 
   Here the signal analyzing section  3   c  may comprise an AIS detecting section  30  for detecting the AIS signal and a LOS detecting section  31  for detecting the signal indicating “0s” successively for a certain number of bits. 
   On the other hand, the receiving side ATM unit  5  comprises an ATM cell receiving section  5   a , receiving buffer  5   b , ATM cell decomposing section  5   c , signal transmitting section  5   d , starvation detecting section  5   e  and AIS generating section  5   f.    
   The ATM cell receiving section  5   a  receives the ATM cells transmitted from the transmission side ATM unit  3 . 
   The receiving buffer  5   b  temporarily store the ATM cells received by the ATM cell receiving section  5   a  and outputs the stored ATM cells with an adequate timing. 
   The ATM cell decomposing section  5   c  decomposes the ATM cells output by the receiving buffer  5   b  and restores to a signal mode transmittable onto the receiving side communication line  2 . 
   The starvation detecting section  5   e  monitors the storage state of the receiving buffer  5   b  and issues a command to transmit an AIS signal when it detects no ATM cells is stored. 
   The AIS generating section  5   f  generates an AIS signal indicating a loss of service when the command to transmit the AIS signal is issued from the starvation detecting section  5   e.    
   The signal sending section  5   d  transmits the signal restored by the ATM cell decomposing section  5   c  as well as the AIS signal generated by the AIS generating section  5   f  onto the receiving side communication line  2 . The signal sending section  5   d  transmits the signal restored by the ATM cell decomposing section  5   c  onto the receiving side communication line  2  when at least one ATM cell is stored in the receiving buffer  5   b . The signal sending section  5   d  also stops the transmission of the signal restored by the ATM cell decomposing section  5   c  and transmits the AIS signal generated by the AIS generating section  5   f  onto the receiving side communication line  2  when no ATM cells is stored in the receiving buffer  5   b  and the starvation detecting section  5   e  issues the command to transmit AIS. 
   Now the operation of the first embodiment will be explained. 
   The transmission side ATM unit  3  stops the transmission of the ATM cells to the receiving side for a certain period of time when it receives the AIS signal indicating the loss of service in the transmission side from the transmission side communication line  1 . At this time, no ATM cell is left in the receiving buffer  5   b  of the receiving side ATM unit  5  (starvation), allowing to recognize that the transmission side ATM unit  3  has stopped to transmit ATM cells, i.e. that the loss of service has occurred. Then the receiving side ATM unit  5  generates the AIS signal and transmits it onto the receiving side communication line  2 , thereby allowing to recognize on the receiving side in the same manner with the conventional method that the loss of service has occurred. 
   Further, the signal analyzing section  3   c  of the transmission side ATM unit  3  detects an abnormality is the received signal and stops the transmission of the ATM cells when the loss of service has occurred on the transmission side communication line  1  and data region of the signal transmitted from the transmission side communication line  1  has been destroyed. Thereby the receiving side ATM unit  5  can recognize the occurrence of the loss of service and generate and transmit the AIS signal onto the receiving side communication line  2 . 
   Further, when a loss of service occurs on the line connecting the transmission side ATM unit  3  and the receiving side ATM unit  5  and no ATM cell is transmitted to the receiving side ATM unit  5 , no ATM cell is left within the receiving buffer  5   b  of the receiving side ATM unit  5 . It allows the receiving side ATM unit  5  to recognize that the loss of service has occurred and to transmit the AIS signal onto the receiving side communication line  2 . 
   Second Embodiment 
   A second embodiment of the present invention will be explained with reference to the drawings. 
     FIG. 2  is a diagram showing one example of a communication network of the second embodiment. In the network, A PBX (X)  6  and PBX (B)  7  are connected by the ATM communication system. 
   The PBX (A)  6  and the ATM communication system are connected via an interface DS1 (A) and the ATM communication system and the PBX (B)  7  are connected via an interface DS1 (B). 
   By doing so, a DS1 signal (A) transmitted from the PBX (A)  6  (or the PBX (B)  7 ) is input to the ATM unit (A)  3  (or the ATM switching unit  4 ) is input to the ATM unit (A)  3  (or the ATM switching unit  4 ) via the interface DS1 (A) (or the interface DS1 (B)). In turn, the DS1 signal output from the ATM unit (A)  3  (or the ATM unit (B)  5 ) is transmitted to the PBX (B)  7  (or the PBX (A)  6 ) via the interface DS1 (B) (or the interface DS1 (A)). 
   The ATM communication system comprises the ATM unit (A)  3 , the ATM switching unit  4 , the ATM unit (B)  5 , wherein the ATM unit (A)  3  and the ATM switching unit  4  are connected via an user interface UNI (A) and the ATM switching unit  4  and the ATM unit (B)  5  are connected via an user interface UNI (B). A charge for using the ATM communication network is billed based on a number of cells passing through the user interface UNI on the receiving side. By the way, the present embodiment will be explained exemplifying an ATM communication system which receives AAL type 1 ATM cells. 
     FIG. 3  is a functional block diagram of the internal structure of the ATM unit  3  and the ATM unit (B)  5  (hereinafter generally referred to as the ATM unit) of the second embodiment. 
   The ATM unit comprises transmission and receiving systems. The transmission system comprises a DSJ terminating section  3   d , ATM cell generating section  3   e , ATM cell transmission stopping section  3   b , AIS detecting section  30 , LOS detecting section  31  and OR circuit. 
   The DS1 terminating section  3   d  converts the DS1 signal transmitted from the PBX (A)  6  (or the PBX (B)  7 ) into a signal mode processible by the ATM unit and outputs to ATM cell generating section  3   a.    
   The ATM cell generating section  3   a  divides the signal output from the DS1 terminating section  3   d  to generate ATM cells. 
   In concrete, the DS1 signal has a multi-frame structure in which 24 frames each comprising one bit F bit region and 192 bits data region, i.e. 193 bits length, are combined (in total 4,632 bits) (see  FIG. 4 ) and the AAL type 1 ATM cell has 53 bytes comprising 5 bytes ATM header region, 1 byte SAR-PDU header region and 47 bytes SAR-PDU payload region (see  FIG. 5 ). Due to that, ATM cell generating section  3   a  generates the ATM cell by dividing the data region of the DS1 signal per each 47 bytes to store in the SAR-PDU payload region and by adding the SAR-PDU header and ATM header with it. 
   The AIS detecting section  30  monitors the DS1 signal input to the DS1 terminating section  3   d  and detects the AIS signal indicating an occurrence of loss of service in the PBX (A)  6  (or the PBX (B)  7 ). In concrete, the AIS signal conforming to the DS1 is a signal which has no frame and in which the value of bits composing the signal indicates “1” in 99.9%. Accordingly, the AIS detecting section  30  monitors the signal input to the DS1 terminating section  3   d  at least by 1,000 bits and recognizes that the signal is the AIS signal when it detects a signal in which more than 999 bits out of those 1,000 bits indicate “1”. Detecting the AIS signal, the AIS detecting section  30  outputs a request to stop the transmission of cells to the ATM cell transmission stopping section  3   b.    
   The LOS detecting section  31  also monitors the normality of the DS1 signal input to the DS1 terminating section  3   d  and discriminates whether an abnormality has occurred on the line connecting the PBX (A)  6  (or the PBX (B)  7 ) and the ATM unit. In concrete, when an abnormality occurs on the line connecting the PBX (A)  6  (or the PBX (B)  7 ) and the ATM unit, the DS1 signal becomes LOS state in which “0” is indicated successively for 100 to 250 bits. Due to that, the LOS detecting section  31  monitors the DS1 signal input to the DS1 terminating section  3   d  successively at least by 100 to 250 bits and recognizes the LOS state when all the values of those 100 to 250 bits are “0”. Recognizing the LOS state, the LOS detecting section  31  outputs a request to stop the transmission of cells to the ATM cell transmission stopping section  3   b.    
   While the ATM cell transmission stopping section  3   b  normally sends out the ATM cells generated in ATM cell generating section  3   a  to the ATM switching unit  4  as they are, it stop the cell generating process and cell transmitting process of ATM cell generating section  3   a  for a certain period of time. That is, when it receives the request to stop the transmission of cells from either the AIS detecting section  30  or the LOS detecting section  31 , it stops the cell generating process and cell transmitting process of ATM cell generating section  3   a  for the certain period of time. 
   Next, functions of the receiving system of the ATM unit will be explained. 
   The receiving system comprises an ATM cell receiving section  5   a , receiving buffer  5   b , DS1 terminating section  5   c , selector  5   d , starvation detecting section  5   e  and AIS generating section  5   f.    
   The ATM cell receiving section  5   a  receives the ATM cells transmitted from the ATM unit of the sender and transfers them to the receiving buffer  5   b.    
   The receiving buffer  5   b  temporarily stores the ATM cells transferred from the ATM cell receiving section  5   a  and outputs them in accordance to the request from the DS1 terminating section  5   c.    
   The DS1 terminating section  5   c  is a concrete example of the ATM cell decomposing section of the present invention which decomposes the ATM cells output from the receiving buffer  5   b  to restore the frame structure of the DS1 signal. 
   The starvation detecting section  5   e  monitors a number of ATM cells stored in the receiving buffer  5   b  and when the number of ATM cells becomes zero (starvation state), outputs a request to transmit the AIS signal to the selector  5   d  as well as a request to generate the AIS signal to the AIS generating section  5   f.    
   The AIS generating section  5   f  generates the AIS signal when it receives the request from the starvation detecting section  5   e  and outputs the AIS signal to the selector  5   d.    
   The selector  5   d  comprises two input terminals B 1  and B 2  and one output terminal and has a function to switch among the connection of the input terminal B 1  and the output terminal and that of the input terminal B 2  and the output terminal. By the way, the DS1 terminating section  5   c  is connected to the input terminal B 1  and the AIS generating section  5   f  is connected to the input terminal B 2 . Normally, the selector  5   d  connects the input terminal B 1  and the output terminal and transmits the DS1 signal output from the DS1 terminating section  5   c  to the PBX (A)  6  (or the PBX (B)  7 ). On the other hand, receiving the AIS transmission request from the starvation detecting section  5   e , the selector  5   d  cut off the connection between the input terminal B 1  and the output terminal and connects the input terminal B 2  and the output terminal. Then the selector  5   d  transmits the AIS signal output from the AIS generating section  5   f  to the PBX (A)  6  (or the PBX (B)  7 ). 
   Next, a structure of each part section described above will be explained in detail. 
     FIG. 6  is a diagram showing a hardware structure of the transmission system according to the second embodiment. 
   The terminating section  3   d  of the receiving side DS1 comprises a B/U converting circuit (bipolar/unipolar converting section)  301 , B8ZS decoder  302  and clock sampling section  303 . 
   According to the present embodiment, when successive eight data values are “0”, the PBX transmits this data as a B8ZS signal. This B8ZS signal is a bipolar pulse signal which is what when the eight successive data “0, 0, 0, 0, 0, 0, 0” are codes inverted into “0, 0, 0, 1, 1, 0, 1, 1” and violations are added (see  FIG. 7 ). 
   Here, the clock sampling section  303  has a function to detect clocks RC from the bipolar signal LINE IN input from the PBX (see  FIG. 8   d ). Then, the clock sampling section  303  inputs the clocks RC to the B/U converting section  301 , B8ZS decoder  302 , AIS detecting section  30  and LOS detecting section  31 . 
   The B/U converting section  301  has functions to covert the bipolar signal LINE IN (see  FIG. 8   a ) received from the PBX into an unipolar signal RDT (see  FIG. 8   b ) and to detect violations RV (see  FIG. 8   c ) from the bipolar signal. 
   The B/U converting section  301  inputs the code signal RDT after the conversion and the violations RV to the B8ZS decoder  302  and in the same time, inputs the code signal (RDT) to the LOS detecting section  31 . 
   The B8ZS decoder  302  restores the code signal RDT input from the B/U converting section  301  into the original signal “0, 0, 0, 0, 0, 0, 0, 0” (see  FIG. 8   e ) and inputs the restored signal SDATA to the ATM cell generating section  3   a.    
   The AIS detecting section  30  comprises an OR circuit  304  and counter  305 . The counter  305  updates a counter value when a data value of the unipolar signal RDT input from the terminating section  3   d  of the receiving side DS1 is “1” and resets the counter value if the data value of the unipolar signal is “0”. Then, when the counter value becomes “999”, i.e., successive 999 clocks RC are input while the data value is “1”, the counter  305  outputs the AIS signal to the OR circuit  32  (see  FIG. 9 ). 
   The LOS detecting section  31  comprises an OR circuit  306 , counter  307  and inverter  308 . 
   The counter  307  inputs the unipolar signal RDT input from the B/U converting section  301  via the inverter  308 . Then, the counter  307  updates the counter value if the data value is “1” and resets the counter value when the data value is “0”. When the data value of the unipolar signal RDT is “1”, the inverter  308  converts the data value “1” to “0” and inputs it to the counter  307 . At this time, the counter  307  resets the counter value. 
   On the other hand, when the data value is “0”, the inverter  308  converts the data value “0” to “1” and inputs it to the counter  307 . At this time, the counter  307  updates the counter value. Then, when successive 175±75 clocks RC are input while the data value is “1”, i.e., successive 175±75 data values “0” of the unipolar signal are input, the counter  307  outputs the LOS signal to the OR circuit  32  (see  FIG. 10 ). 
   When the AIS signal is input from the AIS detecting section  30  and when the LOS signal is input from the LOS detecting section  31 , the OR circuit  32  transmits the cell transmission stop signal to the ATM cell transmission stopping section  3   b  (see  FIG. 11 ). 
   The ATM cell generating section  3   a  comprises a buffer  309 , ATM header generating section  310  and header adding section  311 . 
   The buffer  309  stores data SDATA input from the terminating section  3   d  of the receiving side DS1. Then, when fixed length data is stored in the buffer  309 , the header adding section  311  adds an ATM header generated by the ATM header generating section  310  to this fixed length data (see  FIG. 12 ). 
   The ATM cell transmission stopping section  3   b  comprises an AND circuit  312 , inverter  313 , scrambler  314  and E/O (Electrical/Optical) transducing section  315 . 
   The AND circuit  312  operates a logical product of a signal SATM from the ATM cell generating section  3   a  and a signal from the inverter  313 . 
   When the inverter  313  receives the cell transmission stop signal from the OR circuit  32 , it converts this signal to “0” and inputs it to the AND circuit  312 . At this time, the output of the AND circuit  312  becomes “0” (see  FIG. 13 ). Thereby, the transmission of the ATM cell is stopped. The output of the AND circuit  312  is further output to the ATM network via the scrambler  314  and E/O circuit  315 . 
     FIG. 14  shows a hardware structure of the receiving system according to the second embodiment. 
   The ATM cell receiving section  5   a  comprises an O/E (Optical/Electrical) transducing section  501 , clock generating section  502 , de-scrambler  503  and ATM header checking section  504 . 
   An optical signal RUNI transmitted from the transmission system is input to the ATM header checking section  504  via the O/E transducing section  501  and de-scrambler  503 . 
   The ATM header checking section  504  has functions to remove the ATM header from the ATM cell and to output data of the payload section as successive data RATM (see  FIG. 15 ). 
   The clock generating section  502  generates clocks RCLK of the receiving system based on the optional signal RUNI. This clock RCLK is input to the receiving buffer  5   b.    
   The receiving buffer  5   b  comprises a FIFO  505 , fluctuation absorbing buffer amount counter  506 , OR circuit  507  and AND circuit  508 . 
   The FIFO  505  stores the data RATM from the ATM cell receiving section  5   a  up to a predetermined amount. The clock RCLK is input to the FIFO  505  as a write pulse. Thereby, the FIFO  505  writes the data RATM sequentially in accordance to the write pulse. 
   The clock RCLK is also input to the fluctuation absorbing buffer amount counter  506  via the OR circuit  507 . The fluctuation absorbing buffer amount counter  506  then counts the clocks RCLK and when the counter value reaches to the predetermined value, it recognizes that the predetermined amount of data is being stored in the FIFO  505 . When the predetermined amount of data (N 47 byte data) is stored in the FIFO  505 , an output CO of the fluctuation absorbing buffer amount counter  506  is fixed to “H”. This output CO “H” is input to the AND circuit  508  and OR circuit  507 . 
   The AND circuit  508  executes a logical product of the read enable signal CO and clock SC for transmitting data and inputs the result to the FIFO  505 . Then, when the output of the fluctuation absorbing buffer amount counter  506  is fixed to “H”, an output of the AND circuit  508  becomes the same value with the clock SC. Thereby, the FIFO  505  reads data in accordance to the clock SC for transmitting data and inputs this data RDATA to the terminating section  5   c  of the transmission side DS1 (see  FIG. 16 ). 
   The starvation detecting section  5   e  comprises a counter  509  and ALL “0” decoder  510 . The counter  509  is connected with the write clock and read clock of the FIFO  505 . The counter  509  increments the counter value when the write clock is input and decrements the counter value when the read clock is input. 
   The ALL “0” decoder  510  outputs the starvation signal STV to the selector  5   d  when the counter value of the counter  509  becomes “0” (see  FIG. 17 ). 
   That is, when a starvation is caused, the data amount in the FIFO  505  becomes “0” and the counter value of the counter  509  becomes “0”. 
   The AIS generating section  5   f  includes a 5-volt power source  511 . That is, this 5-volt power source  511  outputs a 5-volt electrical signal always. This electrical signal is a signal logically representing “1”, and indicating AIS signal in the case of DS1. 
   The terminating section  5   c  of the transmission side DS1 includes a B8ZS encoder  512 . The output data RDATA from the receiving buffer  5   b  and the clock SC for transmitting data are input to the B8ZS encoder  512 . 
   When values of the data RDATA are “0” successively by eight, the B8ZS encoder  512  converts this data into “0, 0, 0, 1, 1, 0, 1, 1” and produces violation signals SV. Then, the B8ZS encoder  512  outputs data SDT after the conversion and the violation signals SV to the selector  5   d  in accordance to the transmission side clocks (see  FIG. 18 ). The violation signal SV is a signal indicating a position to which the violation is to be added. 
   The selector  5   d  comprises an AND circuit  513 , inverter  514 , selector  515 , U/B converting section  516  and line clock generating section  517 . 
   The line clock generating section  517  generates a clock SC for transmission. This clock SC is input to the U/B converting section  516 , receiving buffer  5   b  and terminating section  5   c  of the transmission side DS1. 
   The output data SDT of the B8ZS encoder  512 , the AIS signal from the AIS generating section  5   f  and the starvation signal STV from the starvation detecting section  5   e  are input to the selector  515 . 
   When the signal STV from the starvation detecting section  5   e  is “0”, the selector  515  outputs the output data SDT from the terminating section  5   c  of the transmission side DS1 to the U/B converting section  516 . When the signal STV from the starvation detecting section  5   e  is “1”, the selector  515  outputs the signal AIS from the AIS generating section  5   f  to the U/B converting section  516 . 
   When the signal STV is “0”, it is converted by the inverter  514  to “1” and is input to the AND circuit  513 . When the AND circuit  513  inputs the signal “1” from the inverter  514 , it inputs the violation signal SV to the U/B converting section  516  as it is. 
   Then, the U/B converting section  516  converts the data SDT output from the selector  515  from the unipolar signal to the bipolar signal and adds the violation following to the violation signal SV. 
   On the other hand, when the starvation signal STV becomes “1”, it is converted by the inverter  514  to “0” and is input to the AND circuit  513 . When the signal “0” is input from the inverter  514 , the AND circuit  513  stops the output of the violation signal SV (see  FIG. 19 ). 
   Now the operation steps of the ATM unit of the second embodiment will be explained. Here the explanation will be made exemplifying a case in which the signal from the PBX (A)  6  is transmitted to the PBX (B)  7 . 
   The signal transmitted from the PBX (A)  6  is transmitted to the ATM unit (A)  3  via the interface DS1 (A). In the ATM unit (A)  3 , the DS1 terminating section  3   d  receives the signal transmitted from the interface DS1 (A). At this time, the AIS detecting section  30  discriminates whether the signal received by the DS1 terminating section  3   d  is an AIS signal or not and in the same time, the LOS detecting section  31  monitors the signal to discriminates whether it is normal or not. 
   If the signal received by the DS1 terminating section  3   d  is not an AIS signal and data region of the signal is normal, ATM cell generating-section  3   a  divides the signal output from the DS1 terminating section  36  into units of 47 bytes to generate ATM cells. Then ATM cell generating section  3   a  outputs the generated ATM cells to the ATM cell transmission stopping section  3   b . The ATM cell transmission stopping section  3   b  transmits the ATM cells output from ATM cell generating section  3   a  to the ATM switching unit  4  as they are. 
   The ATM switching unit  4  refers to an ATM header of the ATM cells transmitted from the ATM unit (A)  3  to discriminate an address. When the address of the ATM cells is the ATM unit (B)  5 , the ATM switching unit  4  transmits the ATM cells to the ATM unit (B)  5 . 
   In the ATM unit (B)  5 , the ATM cell receiving section  5   a  receives the ATM cells transmitted from the ATM switching unit  4 . The  5   a  then transfers the received ATM cells to the receiving buffer  5   b.    
   The receiving buffer  5   b  temporarily stores the ATM cells transferred from the ATM cell receiving section  5   a . It then outputs the ATM cells in accordance to a request from the DS1 terminating section  5   c.    
   The DS1 terminating section  5   c  decomposes the ATM cells output from the receiving buffer  5   b  to restore the frame structure of the DS1 and outputs to the selector. 
   Here the starvation detecting section  5   e  discriminates whether a number of ATM cells stored in the receiving buffer  5   b  is “zero” or hot. If the number of ATM cells stored in the receiving buffer  5   b  is not zero, the selector  5   d  and the AIS generating section  5   f  do not operate. Then the selector  5   d  transmits the DS1 signal output from the DS1 terminating section  5   c  to the interface DS1 (B). 
   The interface DS1 (B) transmits the DS1 signal received from the ATM unit (B)  5  to the PBX (B)  7  (see ( 1 ) in  FIG. 23 ). The same applies to a case when the signal is transmitted from the PBX (B)  7  to the PBX (A)  6 . Thereby the signal may be transmitted/received quickly between the PBX (A)  6  and the PBX (B)  7 . 
   Next, operation steps of the ATM unit when a loss of service occurs will be explained. 
   (1) When Fault Occurs on the PBX (A)  6  Side (see  FIG. 20 ): 
   When a loss of service occurs on the PBX (A)  6  side, the PBX (A)  6  transmits an AIS signal. The AIS signal transmitted from the PBX (A)  6  is then transmitted to the ATM unit (A)  3  via the interface DS1 (A). 
   In the ATM unit (A)  3 , the DS1 terminating section  3   d  receives the signal transmitted from the interface DS1 (A). At this time, the AIS detecting section  30  discriminates that the signal received by the DS1 terminating section  3   d  is an AIS signal and outputs a request to stop the transmission of the ATM cells to the ATM cell transmission stopping section  3   b . The transmission stop request output from the AIS detecting section  30  is input to the ATM cell transmission stopping section  3   b  via the OR circuit  32 . 
   Receiving the transmission stop request from the AIS detecting section  30 , the ATM cell transmission stopping section  3   b  stops the transmission of the ATM cells output from ATM cell generating section  3   a  and at the same time, stops the ATM cell generating process of ATM cell generating section  3   a  for a certain period of time. Thereby the ATM unit (B)  5  cannot receive the ATM cells and the number of ATM cells stored in the receiving buffer  5   b  becomes zero. 
   Detecting that the number of ATM cells stored in the receiving buffer  5   b  has become zero, the starvation detecting section  5   e  outputs a request to transmit the AIS signal to the selector  5   d  and at the same time, outputs a request to generate the AIS signal to the AIS generating section  5   f.    
   Receiving the AIS signal transmission request from the starvation detecting section  5   e , the selector  5   d  switches the connection from that between the input terminal B 1  and the output terminal to that between the input terminal B 2  and the output terminal. At the same time, receiving the AIS signal generation request from the starvation detecting section  5   e , the AIS generating section  5   f  generates the AIS signal and output it to the selector  5   d . The AIS signal output from the AIS generating section  5   f  is transmitted from the selector  5   d  to the PBX (B)  7  via the interface DS1 (B) (see ( 2 ) in  FIG. 23 ). 
   Thereby the AIS signal is informed to the addressee without being transmitted on the user interfaces UNI (A) and UNI (B) of the ATM communication system. 
   (2) When a Fault Occurs on Line Connecting the PBX (A)  6  and the ATM Unit (A)  3  (see  FIG. 21 ): 
   When a loss of service occurs on the line connecting the PBX (A)  6  and the ATM unit (A)  3 , the signal transmitted from the PBX (A)  6  becomes all “0” and is input to the ATM unit (A)  3 . 
   The LOS detecting section  31  in the ATM unit (A)  3  detects that the signal input to the DS1 terminating section  3   d  is “0” successively for 100 to 250 bits. At this time, the LOS detecting section  31  outputs a request to stop the transmission of ATM cells to the ATM cell transmission stopping section  3   b . This transmission stop request is input to the ATM cell transmission stopping section  3   b  via the OR circuit  32 . 
   Receiving the transmission stop request from the LOS detecting section  31 , the ATM cell transmission stopping section  3   b  stops the transmission of the ATM cells output from ATM cell generating section  3   a  and at the same time, stops the ATM generating process of ATM cell generating section  3   a  for a certain period of time. Thereby the ATM unit (B)  5  cannot receive the ATM cells and the number of ATM cells stored in the receiving buffer  5   b  becomes zero. 
   Detecting that the number of ATM cells stored in the receiving buffer  5   b  has become zero, the starvation detecting section  5   e  outputs a request to transmit the AIS signal to the selector  5   d  and at the same time, outputs a request to generate the AIS signal to the AIS generating section  5   f.    
   Receiving the AIS signal transmission request from the starvation detecting section  5   e , the selector  5   d  switches the connection from that between the input terminal B 1  and the output terminal to that between the input terminal B 2  and the output terminal. In the same time, receiving the AIS signal generation request from the starvation detecting section  5   e , the AIS generating section  5   f  generates the AIS signal and output it to the selector  5   d . The AIS signal output from the AIS generating section  5   f  is transmitted from the selector  5   d  to the PBX (B)  7  via the interface DS1 (B) (see ( 3 ) in  FIG. 23 ). 
   Thereby the AIS signal is informed to the addressee without being transmitted on the user interfaces UNI (A) and UNI (B) of the ATM communication system. 
   (3) When a Fault Occurs on Line Connecting the ATM Switching Unit (B)  4  and the ATM Unit (B)  5  (see  FIG. 22 ): 
   When a loss of service occurs on the line connecting the ATM switching unit  4  and the ATM unit (B)  5 , the ATM cells sent out from the ATM switching unit  4  cannot reach the ATM unit (B)  5 . Thereby the ATM unit (B)  5  cannot receive the ATM cells and the number of ATM cells stored in the receiving buffer  5   b  becomes zero. 
   Detecting that the number of ATM cells stored in the receiving buffer  5   b  has become zero (starvation state), the starvation detecting section  5   e  outputs a request to transmit the AIS signal to the selector  5   d  and at the same time, outputs a request to generate the AIS signal to the AIS generating section  5   f.    
   Receiving the AIS signal transmission request from the starvation detecting section  5   e , the selector  5   d  switches the connection from that between the input terminal B 1  and the output terminal to that between the input terminal B 2  and the output terminal. In the same time, receiving the AIS signal generation request from the starvation detecting section  5   e , the AIS generating section  5   f  generates the AIS signal and output it to the selector  5   d . The AIS signal output from the AIS generating section  5   f  is transmitted from the selector  5   d  to the PBX (B)  7  via the interface DS1 (B) (see ( 4 ) in  FIG. 23 ). 
   Thereby the AIS signal is informed to the addressee without being transmitted on the user interfaces UNI (A) and UNI (B) of the ATM communication system. 
   While preferred embodiments have been described, variations thereto will occur to those skilled in the art within the scope of the present inventive concepts which are delineated by the following claims.