Patent Publication Number: US-6212164-B1

Title: ATM switch congestion control method of connection setup requests and priority control method for receiving connection requests

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to an ATM (Asynchronous Transfer Mode) switch, and particularly to a congestion control method and a priority control method during reception of the connection setup requests in an SVC (Switched Virtual Channel) service to establish the connection among the ATM switches on demand. 
     2. Description of Related Art 
     The ATM switch is equipped with an ATM interface through which a plurality of ATM devices (ATM apparatus) such as ATM terminals, ATM switches, ATM concentrators, ATM routers etc. are respectively connected. An ATM network comprises a plurality of these ATM devices. 
     When ATM devices are required to be connected in the SVC service of an ATM switch, the ATM device sends a signaling message requesting a SETUP of the connection to the ATM switch to negotiate a destination, a frequency band and so on, so as to establish the connection between the ATM device and the ATM switch. 
     When a plurality of the ATM switches exist between a calling ATM device (originator) and the called ATM device (destination), the above negotiation process is executed for every two adjacent ATM devices. To process the signaling messages transmitted from a plurality of the ATM devices, the ATM switch is equipped with a buffer for receiving the signaling messages. The ATM switch stores the signaling messages from the ATM devices in the buffer temporarily and then extracts the signaling message from the buffer to execute signaling processes such as establishment of the connection. 
     Congestion may occur when a large number of signaling messages in excess of the signaling process capability of the ATM switch are received at the ATM switch because of, for example, booting of all ATM devices, requesting of a short period connection setup by a high performance ATM device, malfunction/failure of ATM devices, and so on. Once the buffer which receives and stores the signaling messages sent from ATM devices has overflowed, all the signaling messages received after the overflow are discarded at cell level or packet level until a condition of the buffer is restored in a conventional ATM switch. 
     The possibility of overflow may be reduced in the conventional device if a capacity of the buffer for receiving the signaling message is made larger. However, the signaling message may be stored in the buffer for a longer period of time if a larger buffer capacity is implemented. The ATM device which sent the signaling message to request the connection setup usually monitors a time from transmission of the signaling message to receipt of the response message. Thus, if the signaling message is stored for a longer period of time, it is possible for the originator of the signaling message to re-send another signaling message because of overtime delay. Particularly, once congestion has occurred, discarding of the connection setup requests from all ATM devices connected to the ATM switch may cause re-sending of the connection setup requests from these ATM devices. The signaling process capability may also be increased. However, the signaling process capability still has its own limit and may not resolve the congestion problem. 
     Furthermore, in the ATM switch, each of the signaling messages from ATM devices is processed according to its receiving sequence. The signaling message is provided with information elements defining a type of the connection, such as a type of service to be used, cell speed, etc. Since some types of signaling messages require the use of a fixed frequency band, the signaling message coming after such a signaling message may be rejected because of a lack of available frequency band. 
     Among ATM devices, there are important devices such as ATM ARP (Address Resolution Protocol) servers, LAN emulation servers, which make interconnections with a conventional LAN on an ATM-LAN system, and devices such as file serves, and printer servers, which are frequently accessed by users. In some cases, it is desirable to process the signaling messages from these ATM devices giving them preference over the signaling messages from ATM devices of general users. 
     When the connection is established via a plurality of ATM switches, it means that a frequency band to be used has been secured in every one of the ATM switches on the route. If the latter ATM device rejects the connection setup request because of a lack of frequency band or the like, these secured frequency bands in the rest of the ATM switches on the route will be wasted. It also possible that the signaling messages may not be received by the ATM devices on the route. 
     SUMMARY OF THE INVENTION 
     The first object of the present invention is to provide an ATM switch and a congestion control method of the connection setup request, which enable handling of the connection setup requests from the ATM devices without being influenced by congestion that has occurred due to a large number of connection setup requests from other ATM devices. 
     The second object of the present invention is to provide an ATM switch and a priority control method for receiving the connection setup requests, which enable efficient establishment of connections. 
     The first object of the present invention is accomplished by an ATM switch for exchanging inputted connections, comprising; reception means receiving a connection setup request, storage means storing the connection setup request received by the reception means, establishment means extracting the connection setup request stored in the storage means and establishing the connection according to the extracted connection setup request, detection means detecting a class having a predetermined number of connection setup requests assigned to the class being equal to the number of connection setup requests, which are stored in the storage means and classified into the class, the stored connection setup requests being classified into a plurality of classes, each respectively assigned to the predetermined value, and discard means discarding the connection setup request received by the reception means if the detection means detects the class and the received connection setup request belongs to the class. 
     In a preferred aspect of the present invention, there is provided a congestion control method of an ATM switch receiving a connection setup request, storing the received connection setup request, extracting the stored connection setup request and establishing the connection according to the extracted connection setup request, comprising the steps of; detecting a class having a predetermined number of connection setup requests pre-assigned to the class being equal to a number of the stored connection setup requests which are classified into the class, the stored connection setup requests being classified into a plurality of the classes, and discarding the received connection setup request if such a class is detected and the received connection setup request belongs to the detected class. Alternatively, in another preferred aspect of the present invention, the congestion control method comprises the steps of; detecting a class having a predetermined number of connection setup requests pre-assigned to the class being equal to a number of the stored connection setup requests which are classified into the class, the stored connection setup requests being classified into a plurality of the classes, and discarding the stored connection setup request if the class is detected and the stored connection setup request belongs to the detected class. Here, the connection setup request to be discarded is the one stored. Therefore the number of the connection setup request to be discarded may be determined beforehand. 
     According to the present embodiment, it is possible to classify the connection setup request based on the port or the content of the information elements of the setup request message and thus control the congestion in each of the classes. For example, even when a large number of connection setup requests are received in a short period of time due to booting of all ATM devices, or malfunction/failure of ATM devices or the like, the connection setup requests in the other classes may still be received without being influenced by the congestion. 
     The second object of the present invention is accomplished by an ATM switch for exchanging connections entered, comprising; reception means receiving a connection setup request, distinction means distinguishing a priority corresponding to content of the connection setup request which is received by the reception means by using a condition in which the priority is defined for preferentially receiving the connection setup request in correspondence with the content of the connection setup request, storage means storing the connection setup request for each of the priorities distinguished by said distinction means, extraction means extracting the connection setup request corresponding to the priority to be extracted according to an algorithm which is defined based on the priority for extracting the connection setup request, and establishment means establishing the connection according to the extracted connection setup request. 
     According to the present invention, it is possible to establish the connection efficiently according to the priority since the connection setup request is extracted according to the predetermined priority assigned in correspondence with the content of the connection setup request before the establishment of the connection. 
     The priority may be defined, for example, by the ports from which the connection setup request is received, or defined in correspondence with the predetermined information element parameter added to the connection setup request. Alternatively, the priority may be defined in correspondence with a number of external devices through which the connection setup request passed, the number corresponding to a originator of the connection setup request. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration showing a configuration and basic processes of the ATM switch; 
     FIG. 2 is a schematic diagram of the connection number setup table; 
     FIG. 3 is a schematic diagram of the receive counter (initial state); 
     FIG. 4 is a schematic illustration showing a principle of the congestion control method of the connection setup request; 
     FIG. 5 is a flowchart showing a congestion detecting process; 
     FIG. 6 is a flowchart showing a counting down process; 
     FIG. 7A shows an graphical image of the signaling message receiving buffer while the messages are being stored; 
     FIG. 7B is a flowchart showing a message discarding process; 
     FIG. 8 a schematic illustration showing a RELEASE COMP message format; 
     FIG. 9 is a flowchart showing a message discard notification process; 
     FIG. 10 is a flowchart showing a message discard notification process; 
     FIG. 11 is a flowchart showing a resetting process of the congestion detection level; 
     FIG. 12 is a schematic illustration showing a SETUP message format; 
     FIG. 13 is a schematic illustration of a connection setup table; 
     FIG. 14 is a schematic illustration showing a configuration and basic process of the ATM switch; 
     FIG. 15 a schematic illustration of a priority table; 
     FIG. 16 is a schematic illustration showing a principle of the priority control method for receiving connection setup requests; 
     FIG. 17 is a flowchart showing a branching process; 
     FIG. 18 is a flowchart showing an extracting process; 
     FIG. 19 is a schematic illustration showing a SETUP message format; 
     FIG. 20 is a schematic illustration of a priority table; 
     FIG. 21 is a flowchart showing a branching process; 
     FIG. 22A is a schematic illustration of a discarding priority table classified by port; 
     FIG. 22B is a schematic illustration of a discarding priority table classified by AAL type; 
     FIG. 23A is a flowchart showing an initialization step of the discarding process; 
     FIG. 23B is a flowchart showing a discarding process; 
     FIG. 24 is a schematic illustration of an ATMLAN system; and 
     FIG. 25 is a schematic illustration of a HOP number table. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments corresponding to the first object of the present invention will now be explained with reference to FIGS. 1 to  13 . First described will be the congestion control method in the ATM switch where the congestion is classified according to the port. 
     FIG. 1 shows a configuration of an ATM switch  100  to which the present invention is applied. The ATM switch  100  comprises three major sections, i.e. a switch control section  110  for controlling the switch, a switch section  120  and a line interface section  130 . The line interface section  130  is equipped with a plurality of ports of various physical specifications for connecting ATM terminals  140 A,  140 B,  140 C or ATM switch  100 B. When the ATM terminal  140  is connected the port, the connection specification in between is based on UNI (User-Network Interface). When the ATM switch  100  is connected to the port, the connection specification in between is based on NNI (Network-Network Interface). The ATM Forum and the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) are working on the standardization of these connection specifications. Further, each port also functions as reception means for receiving the connection setup request from each ATM terminal. 
     The switch section  120  is provided with a VPI (Virtual Path Identifier)/VCI (Virtual Channel Identifier) conversion table  182  for converting routing information according to the established destination, a cell disassemble/assemble section  183  for disassembling and assembling a cell, and a switch  181  for exchanging the connections. 
     The switch control section  110  includes a message receiving buffer  184  functioning as storage means for storing the signaling message, a connection congestion control section  157  for controlling the connection congestion in each class, a UNI signaling process  153  for executing a UNI signaling process, a NNI signaling process  154  for executing an NNI signaling process, a connection control section  151  for controlling connections, a routing table  152  for storing ATM addresses and corresponding connection ports, a PNNI (Private Network-to-Network Interface) process  160  for setting the routing table  152 , a ILMI (Interim Local Management Interface) process  170  for setting ATM addresses to ATM terminals during the initialization and a connection management table  156  for being referenced by the connection control section  151 . The UNI signaling process  153 , the NNI signaling process  154  and the connection control section  151  together function as establishment means for establishing the connection. 
     The connection congestion control section  157  includes a message transmitting/receiving process  185  which transmit/receives the message and is provided with detection means for detecting the congestion, a signaling message receiving buffer  186  for temporarily storing the received messages, a signaling message transmitting/receiving process  155  for transmitting and receiving the signaling message with the UNI signaling process  153  and the NNI signaling process  154 , a message transmitting buffer  187  for temporarily storing the messages to be transmitted, a message discarding process  460  which is discard means for discarding the messages during the congestion, a connection number setup table  440  for defining a number of connections which the signaling message receiving buffer  186  is capable of memorizing for each class and for counting a number of connections stored in the signaling message receiving buffer  186  for each class. The signaling message receiving buffer  186  is provided for each type of protocol process. As shown in FIG. 1, one of the signaling message receiving buffers is provided for the UNI signaling process  153  and the NNI signaling process  154 , and one of the signaling message receiving buffers is provided for each of the PNNI process  160  and the ILMI process  170 . The message transmitting/receiving process  185  detects a message type included in the message, determines the protocol corresponding to the message type, and stores the message in the signaling message receiving buffer corresponding to the protocol process. Further, the message transmitting/receiving process  185  counts a number of the messages stored in the signaling message receiving buffer for each class (in this example, for each port to which the messages are inputted) using a counter, compares the counted number and a predetermined number of messages for each class, declares detection of congestion if both numbers are the same, and sends messages to the message discarding process  460  to discard the received messages when congestion has been detected. 
     An overview of procedure is described with reference to FIG. 1 using an example in which the ATM terminal  140 A tries to communicate with the ATM terminal  140 B using the SVC service. The ATM addresses are assigned to the ATM terminals  140 A and  140 B by the ILMI process  170  during the initialization, and ports which correspond to these ATM addresses are registered in the routing table  152  by the PNNI process  160 . 
     The ATM terminal  140 A transmits the connection setup request message (hereafter called SETUP message) for the ATM terminal  140 B via a signaling channel which is provided for each port. VPI/VCI=0/5 is used as a default value of the standard specification for the signaling channel. The signaling channel is switched to a control port  188  by the VPI/VCI conversion table  182 . In the cell disassemble/assemble section  183 , the cell designated to the control port  188  is stored in the message receiving buffer  184 , and a plurality of the cells designated for each destination are assembled into single frame formation to form a service data unit. Further, the cell disassemble/assemble section  183  adds an input port number of the port to which the message was inputted to a field of the input port number  194  in the message  190 . The input port number of the port to which the message is inputted is given by the switch  181 . Since the message receiving buffer  184  is used for disassembling and assembling of the cell, it is provided with ample capacity for storing a plurality of the service data units. In the message receiving buffer  184 , there are messages which have completed being assembled, and uncompleted ones. The message  190  in the message receiving buffer  184  is stored with such information indicated by numerals  191  to  196  as shown in FIG.  1 . In the cell disassemble/assemble section  182 , an assembly complete flag  191  is set for messages for which assembly has been completed. The message transmitting/receiving process  185  checks if assembling of the message has been completed or not by detecting the assembly complete flag  191  of the message stored in the message receiving buffer  184 , then detects the message type  192  of the message for which assembly has been completed, and transfers the message to the signaling message receiving buffer  186  of the signaling process corresponding to the detected message type. The signaling message transmitting/receiving process  155  extracts the message from the signaling message receiving buffer  186 , and sends the message to the UNI signaling process  153  or the NNI signaling process  154 . As a result of the process described above, the SETUP message from the ATM terminal  140 A is transferred to the UNI signaling process  153 . In the connection control section  151 , the UNI signaling process  153  of the ATM terminal  140 B, the calling party, is identified with reference to the routing table  152 . Further, checking operations are executed to see, for example, whether or not the necessary resources are available for establishing a new connection using the connection management table  156 . If the necessary resources are available, the SETUP message is transmitted from the ATM switch  100 A to the ATM terminal  140 B. According to a series of the processes  158 , the connection  159  may be established for communications between the ATM terminal  140 A and the ATM terminal  140 B. 
     The congestion control operation of the present invention will now be described. In the following, the congestion control operation is illustrated by an example of the ATM switch  100  provided with four ports which are classified into four classes respectively. The class is defined in advance by the connection number setup table  440  in correspondence with each port. 
     FIG. 2 shows a connection number setup table  200 , an example of the connection number setup table  440  shown in FIG.  1 . The connection number setup table  200  defines a number of connections the signaling message receiving buffer  186  can store for each class. The connection number setup table  200  has a plurality of entry sets provided in correspondence with a respective one of the four classes, each set comprising the class  210  and the connection number  220  of the corresponding class. The connection number  220  is a number of messages the signaling message receiving buffer can memorize, and may be set by a system supervisor. In this example, the setup values are defined so as that the ports 0, 1, 2, 3 respectively correspond to the ATM terminals  140 A,  140 B,  140 C, and the ATM switch  100 B. 
     FIG. 3 shows a receiving counter  300  which is an embodiment of the connection number setup table  440  shown in FIG.  1 . The receiving counter  300  counts the messages stored in the signaling message receiving buffer in each of the classes. The receiving counter  300  has a plurality of entry sets, each set comprising the class  310  and the counter  320  for the corresponding class. In the counter  320 , initial values are set to 0. The connection number setup table  440  shown in FIG. 4 may also be defined by adding an entry of the counter  320  to the connection number setup table  200  shown in FIG.  2 . 
     FIG. 4 shows a configuration of the connection congestion control section  157  of the present invention. The connection congestion control in the present invention is executed at the connection congestion control section  157 . As shown in FIG. 4, the connection setup table  200  and the receiving counter  300  shown in FIGS. 2,  3  are now defined as the connection number setup table  440  including entry sets, each comprising the class x  422 , the connection number T(x)  444  and the counter C(x)  446 . The message transmitting/receiving process  185  includes a congestion detecting process  410  for detecting the congestion and a counting-up process  430  to increment the counter C(x)  446 . The message discarding process  460  for discarding the messages when the congestion is detected is connected to the congestion detecting process  410 . The signaling message transmitting/receiving process  155  includes a counting-down process  450  for decrementing the counter C(x)  446 . 
     FIG. 5 shows a flowchart of the congestion detecting process  410  and the counting up process  430  in the message transmitting/receiving process  185 . 
     In the example of FIG. 5, the message transmitting/receiving process  185  extracts the assembled signaling message from the message receiving buffer  184 , and checks a value of the input port number of the SETUP message  190  to identify the class x which corresponds to the input port number. In this example, the value of the input port number is set the same as the identification value of the corresponding class. Thus the port number is substituted for x (Step  510 ), and values of the counter C(x)  446  of the connection number setup table  440  and the connection number T(x)  444  are compared (Step  520 ). If the value of C(x)  446  is less than the value of T(x)  444 , it is judged that congestion is not detected. The message is then transferred to the signaling message receiving buffer  186  (Step  530 ). When the transfer operation is completed (Step  540 ), the counting up process is executed by incrementing the value of the counter C(x) by one (Step  430 ). If the value of C(x)  446  is not less than the value of T(x)  444 , it is judged that congestion is detected (Step  520 ). The message discarding process is then executed to discard the SETUP message (Step  460 A). 
     FIG. 6 shows the counting down process in the signaling message transmitting/receiving process  155 . The signaling message transmitting/receiving process  155  extracts the signaling message from the signaling message receiving buffer  186 , and at the same time checks the SETUP message for identifying its input port number, and substitutes the identified port number for x (Step  610 ), and executes the counting down process by decrementing a value of the counter C(x) by one (Step  450 ). 
     Because of these processes described above, it is possible to control a number of connections stored in the signaling message receiving buffer  186  at each port, and to thus control the congestion at each port. Even when the congestion occurs due to a large number of connection setup requests from any particular ATM devices, the present invention makes it possible to eliminate the influence of such congestion upon the connection setup requests sent from other ATM devices. 
     The second embodiment of the present invention will be described with reference to the figures. In the second embodiment, the message discarding process  460 B is described using an example in which the messages stored in the signaling message receiving buffer  186  are to be discarded. 
     FIGS. 7A and 7B show an illustration explaining the signaling message receiving buffer  186  and a flowchart showing the message discarding process  460 B. FIG. 7A shows an graphical image of the signaling message receiving buffer  186  with the messages being stored. M(x,y) is a message array, with x, y indicating the class and the storage sequence of the message. The maximum number of messages that may be stored is T(x). The signaling message subjected to the discarding process here is the SETUP message. 
     An operation of the second embodiment will now be described. In the following section, the operation is described by an example where a rate of discarding messages from the signaling message receiving buffer  186  during congestion is defined as a discarding rate p (0≦p ≦1), and an integer value, which is obtained by multiplying the connection number T(x) by the discarding rate p and rounding up the result, is defined as a discarded connection number D. 
     FIG. 7B shows the message discarding process  460 B. As shown in FIG. 7B, the number of older SETUP messages among the messages stored in the signaling message receiving buffer  186 , i.e. the SETUP messages stored earlier, that are discarded is as many as a number indicated by the discarding connection number D, and the storing sequence y is reset in the remaining SETUP messages, whose number is the connection number (T(x)−D) indicated. 
     When the congestion is detected in FIG. 5 (Step  520 ), as shown in FIG. 7B, the discarding rate p is multiplied by the connection number T(x) of the class x in which the congestion is detected, and the result of multiplication is rounded to an integer value for substituting the discarding connection number D (Step  720 ). The parameter y indicating the sequence of the message storing is initialized to a value of one (Step  730 ). The parameter y is incremented by one (Step  748 ) until it reaches T(x) while following processes (Steps  744 ,  746 ,  750 ) are looped. When a value of the parameter y is not more than the value obtained by subtracting the discarding connection number D from the connection number T(x) (Step  744 ), the message M(x,y) is reset for M(x,y+D) (Step  746 ). When a value of the parameter y is more than the value obtained by subtracting the discarding connection number D from the connection number T(x) (Step  744 ), the message M(x,y) is set to an unused state (Step  750 ). According to these processes, a plurality of SETUP messages that is as many as a number indicated by the discarding connection number D, starting from the one which arrived the earliest, are discarded from the SETUP messages stored in the signaling message receiving buffer  186 . 
     After discarding the messages, the counter C(x) is decremented by an amount of the discarding connection number D (Step  760 ) and the process in step  520  shown in FIG. 5 is executed (Step  550 ). Here, it is now possible for the SETUP message received when the congestion is detected to transfer to the signaling message receiving buffer  186 , since the congestion is resolved by the message discarding process  460 B. 
     According to the present embodiment, the SETUP messages stored in the signaling message receiving buffer  186  may be discarded during the congestion. Instead of discarding the SETUP messages that arrived earlier, the other SETUP messages stored in the signaling message receiving buffer  186  may be alternatively discarded. 
     The third embodiment of the present invention will be explained with reference to the figures. The third embodiment is an example in which a message discard notification process is executed after discarding of the messages in the message discarding process  460 . 
     FIG. 8 shows a format of a RELEASE COMP message  800 , which is an example of the signaling message for executing a message discard notification process. The RELEASE COMP message  800  comprises headers ( 810 - 840 ) common for signaling messages and an information element  850  indicating the cause of discard. The cause of discard may be one of the causes defined in the ATM Forum standard specification, such as cause number 21 “call rejected”, cause number 41 “temporary failure”, cause number 47 “resource unavailable, unspecified”, or cause numbers which are not defined in the standard specification may also be specified. 
     An operation of the third embodiment will now be described. 
     FIG. 9 shows the process in which the message discard notification process is added to the message discarding process  460 A. In the process shown in FIG. 9, the RELEASE COMP message  800  is generated (Step  910 ) after execution of the message discarding process  460 A when congestion has been detected in the process in Step  520  shown in FIG.  5 . Then the RELEASE COMP message is transferred to the message transmitting buffer  187  shown in FIG. 1 (Step  920 ), and the RELEASE COMP message  800  is transmitted to the originator of the SETUP message. 
     FIG. 10 shows the process in which the message discard notification process is added to the message discarding process  460 B of the second embodiment. The RELEASE COMP message  800  is generated in the same way as that of the process shown in FIG. 9 (Step  910 ) after the process  744  of FIG.  7  and the message discarding process  750 , and then the RELEASE COMP message is transferred to the message transmitting buffer  187  of FIG. 1 (Step  920 ). 
     According to the present embodiment, it is possible to send notifications of discard of the messages after discarding of the messages by the message discarding process  460 , so that the ATM terminals of the originators are able to recognize their discarded messages. 
     The fourth embodiment of the present invention will be explained with reference to the figures. In the fourth embodiment, a procedure to change the congestion detecting level dynamically during the congestion detecting process is described. 
     FIG. 11 shows a resetting process of the congestion detecting level. L(x) is a level adjusting parameter which can be preset to a value different for each class within a range of 0≦L(x) ≦1. In the procedure of step  520  shown in FIG. 5, the connection number T(x)  444  is multiplied by the level adjusting parameter L(x), and the result of multiplication is compared with a value of the counter C(x)  446  (Step  520 A). 
     According to the present embodiment, it is possible to change the congestion detection level. For example, a CPU utilization rate in the signaling process task may be reflected in the level adjusting parameter L(x), or a buffer utilization rate of the shared buffer may be reflected in the level adjusting parameter L(x). 
     The fifth embodiment of the present invention will be described with reference to the figures. In the fifth embodiment, the connection congestion control method is described for a case where the congestion is classified based on information elements included in the SETUP message. 
     FIG. 12 shows a format of the SETUP message  1200 . The SETUP message is provided with common headers for the signaling message ( 1210 - 1240 ) and several information elements such as AAL (ATM Adaptation Layer) parameter  1250 . 
     FIG. 13 shows a connection setup table  1300  where the congestion is classified based on the AAL parameter  1250  of the SETUP message  1200 . The AAL parameter  1250  specifies an AAL type and one of service types, each suited for communicating different types of information such as voice, data or video. Type 1, Type 2, Type 3/4 and Type 5 are specified as the AAL type by the ATM Forum and IT-U, and these types may be set as the class  1310  as shown in FIG.  13 . 
     An operation of this embodiment is the same as the operation of the message transmitting/receiving process  185  in the first embodiment described above except that it further includes procedures to read out the AAL type from the SETUP message and to determine the class based on the AAL type. 
     Alternatively, the classification may be carried out based on other information elements included in the SETUP message besides the AAL type. 
     According to the present embodiment, it is possible to control the congestion in each class by classifying the number of connection setup requests stored in the signaling message receiving buffer  186  based on the contents of the information elements and limiting the number of connection setup requests in each class. According to the present invention, even when the congestion has occurred due to a large number of connection setup requests of the particular class, the connection setup requests of the other classes may remain unaffected by the congestion. 
     According to the first to fifth embodiments of the present invention, it is possible to control the congestion in each class by classifying the connection setup request based on the port or contents of the information elements of the setup request message. For example, even when a large number of connection setup requests are received in a short period of time due to booting of all ATM devices, or malfunction/failure of ATM devices, or the like, the connection setup requests in the other classes may still be received without being influenced by the congestion. 
     Embodiments corresponding to the second object of the present invention will now be explained with reference to FIGS. 14 to  25 . First, the priority control method for receiving the connection setup requests in the ATM switch is described as the sixth embodiment where the priority of the connection setup request is classified according to port. 
     FIG. 14 shows a configuration of the ATM switch  100 A in the sixth embodiment. The ATM switch  100 A comprises three major sections, i.e. a switch control section  110  for controlling the switch, a switch section  120  for exchanging cells, and a line interface section  130  which is provided with an ATM interface function. The switch control section  110  has a hardware configuration consisting of elements such as a CPU, a memory, and an interface to the switch section  120 . The switch control section  110  of FIG. 1 shows functional blocks of software executed by the CPU and data structures. The line interface section  130  is equipped with a plurality of ports with various physical specifications to connect the ATM terminals  140 A,  140 B,  140 C or the ATM switch  100 B respectively. When the ATM terminal  140  is connected the port, the connection specification in between is based on the UNI (User Network Interface). When the ATM switch  100  is connected the port, the connection specification in between is based on the NNI (Network Network Interface). The ATM Forum and ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) are working on standardization of these connection specifications. Each port is also provided with functions of reception means for receiving the connection setup request from the ATM terminal or the ATM switch. 
     The switch section  120  is provided with a VPI (Virtual Path Identifier)/VCI (Virtual Channel Identifier) conversion table  2122  for converting routing information according to the established destination, a cell disassemble/assemble section  2124  for disassembling and assembling the cell, a switch  2121  for exchanging the connections and a control port  2123  for connecting the switch  2121  and the cell disassemble/assemble section  2124 . 
     The switch control section  110  includes a message receiving buffer  2111  functioning as storage means for storing the signaling message, a message transmitting/receiving process  2112  for transmitting and receiving the messages, a connection receiving priority control section  2150  for carrying out the priority control for receiving the connection setup requests in each class, a signaling message transmitting/receiving process  2115  for transmitting/receiving the signaling message, a UNI signaling process  2161  for executing an signaling process of the UNI, a NNI signaling process  2162  for executing a signaling process of the NNI, a connection control section  2163  for controlling the connections, a routing table  2164  for storing ATM addresses and corresponding connection ports, a PNNI (Private Network Network Interface) process  2170  for setting the routing table  2164 , a ILMI (Interim Local Management Interface) process  2180  for setting the ATM addresses to the ATM terminals during the initialization, a connection management table  2165  for being referenced by the connection control section  2163  and a message transmitting buffer  2114  for temporarily storing the message to be transmitted. The UNI signaling process  2161 , the NNI signaling process  2162  and the connection control section  2163  are establishment means for establishing the connection. 
     The connection receiving priority control section  2150  includes a branching process  2151  for checking the priority class of the received messages and branching the message according to its priority class, a priority table  2152  for storing values of the priority classes in correspondence with each of the ports, queues  2153  provided for each priority to store the received messages of corresponding priority class temporarily, an extracting process  2154  for extracting the message according to a algorithm predetermined for extracting the connection setup request, and a discarding process  2155  for discarding the messages during the congestion. The message transmitting/receiving process  2112  detects the message type included in the message, determines the protocol corresponding to the message type, and stores the message in the signaling message receiving buffer corresponding to a process of the determined protocol. 
     An overview of the procedure is described with reference to FIG. 14 using an example in which the ATM terminal  140 A tries to communicate with the ATM terminal  140 B using the SVC service. The ATM addresses are assigned to the ATM terminals  140 A and  140 B by the ILMI process  2180  during the initialization. The routing table  2164 , which registers the connecting ports and the corresponding ATM addresses, is generated by the PNNI process  2170 . 
     The ATM terminal  140 A transmits the connection setup request message (SETUP message) to the ATM terminal  140 B via a signaling channel which is provided for each port. VPI/VCI=0/5 is used as a default value of the standard specification for the signaling channel. The signaling channel is switched to the control port  2123  according to the VPI/VCI conversion table  2122 . In the cell disassemble/assemble section  2124 , the cell designated to the control port  2123  is stored in the message receiving buffer  2111 , and a plurality of the cells designated for each destination are assembled into single frame formation to construct a service data unit. Further, the cell disassemble/assemble section  2124  adds the input port number of the port to which the message was inputted to a field of the input port number  194  in a message  190 . The input port number of the port to which the message is inputted is given by the switch  2121 . Since the message receiving buffer  2111  is used for disassembling and assembling of the cell, it is provided with ample capacity to store a plurality of the service data units. In the message receiving buffer  2111 , there are messages that have completed being assembled and uncompleted messages. The message  190  in the message receiving buffer  2111  is stored with such information indicated by numerals  191  to  196  as shown in FIG.  14 . In the cell disassemble/assemble section  2124 , an assembly complete flag  191  is set for the message which has completed being assembled. The message transmitting/receiving process  2112  checks if assembling of the message has been completed or not by detecting the assembly complete flag  191  of the message stored in the message receiving buffer  2111 , then detects the message type  192  of the message which has completed being assembled, and transfers the message to the signaling message receiving buffer  2113  of the protocol process corresponding to the detected message type. The signaling message transmitting/receiving process  2115  extracts the message from the signaling message receiving buffer  2113 , and sends the message to the UNI signaling process  2161  or the NNI signaling process  2162 . These processes are provided for every one of the ports. As a result of the process described above, the SETUP message from the ATM terminal  140 A is forwarded to the UNI signaling process  2161 . The connection control section  2163  identifies the UNI signaling process  2161  of the ATM terminal  140 B, the calling party, with reference to the routing table  2164 . Further, checking operations are executed to see, for example, whether or not the necessary resources are available to establish a new connection using the connection management table  2165 . If the necessary resources are available, the SETUP message is transmitted from the ATM switch  100  to the ATM terminal  140 B. According to the series of processes  2167 , the connection  2168  may be established for communications between the ATM terminal  140 A and the ATM terminal  140 B. 
     An operation of the connection receiving priority control section in the sixth embodiment will be described. The operation is illustrated by an example of the ATM switch  100  with eight ports which being divided into four classes. A value of the class corresponding to the every port is defined in a priority table  2200  ( 2152 ) in advance. 
     FIG. 15 shows the priority table  2200  ( 2152 ). The priority table  2200  has a plurality of entry sets, which are provided in correspondence with a respective one of the priority classes, each entry set comprising the priority class  2210  and the port  2220 . The port  2220  may be specified in advance by a system supervisor. In the following, the priority is assumed to be higher for a smaller value of priority. In this example, a device type  2230  is provided as information and it is not required for an actual table. In the example shown in FIG. 15, the port 0 is connected to an ATM switch, the ports 1 and 2 are connected to routers or LAN servers such as ATM ARP servers, or LAN emulation servers, the ports 3 and 4 are connected to server terminals such as file servers or printer servers, and the ports 5 to 7 are connected to general terminals. 
     FIG. 16 shows a configuration of the connection receiving priority control process in the sixth embodiment. The priority control process for receiving connection in the present embodiment is executed in the connection receiving priority control section  2150  of FIG.  14 . Once the message  190 , which is constructed by adding a header specifying information  191  to  196  to the signaling message, is received from the message transmitting/receiving buffer  2112 , the branching process  2151  extracts the input port number  194  of the signaling message. Then the branching process  2152  stores the signaling message in the queue of the priority class which corresponds to the extracted input port number  194  while referencing the priority table  2200 . The priority table  2200  shown in FIG. 15 may be alternatively constructed like the table shown in FIG. 16, which comprises a plurality of entry sets, each comprising the port x and the priority class P(x). The extracting process  2154  extracts the signaling message from each of the priority queues comprising the priority 0 queue  2300  to the priority 3 queue  2303  by using a predetermined algorithm (hereafter, it is called a priority algorithm) for extracting the connection setup request based on the priority class, and transfers the signaling message to the signaling message transmitting/receiving process  2115 . 
     FIG. 17 is a flowchart showing the branching process executed in the connection receiving priority control section  2150 . 
     As shown in FIG. 17, when the assembled signaling message is received from the message transmitting/receiving process  2112 , the branching process  2151  identifies an input port number by checking the input port number  194  of the message  190  and substitutes the input port number for x (Step  2410 ). If the priority class P(x) of the priority table  2200  is  0  (Step  2420 ), the signaling message is transferred to the priority 0 queue  2300  (Step  2450 ). If the P(x) is 1 (Step  2430 ), the signaling message is transferred to the priority 1 queue  2301  (Step  2460 ). If the P(x) is 2 (Step  2440 ), the signaling message is transferred to the priority 2 queue  2302  (Step  2470 ). If the P(x) is not equal to 0, 1 or 2 (Step  2440 ), the signaling message is transferred to the priority 3 queue  2303  (Step  2480 ). These processes are carried out when the signaling message is received. 
     FIG. 18 shows a flowchart showing the extracting process executed in the connection receiving priority control section  2150 . 
     A suitable priority algorithm shall be implemented in extracting the signaling message from each of the priority queues  2300  to  2303 . If the algorithm is strictly based on the priority, the processing of signaling messages in the priority queue having a lower priority may rarely be executed, and “sinking” of the process may occur. Therefore the priority algorithm shown by the flowchart in FIG. 18 is defined a priority algorithm in which a value is set for each of the priority queues to define the minimum repetition number of a process, which is repeated with a constant frequency within the process to check the priority queue to see if there are any signaling messages stored, so as to prevent such sinking of signaling message processing in the lower priority queue. The repetition number of the checking process (the repetition number of the extracting process if the signaling messages are still stored) may be set for each of the priority classes. For example, the repetition number may be set as four times for the priority 0, three times for the priority 1, twice for the priority 2, and once for the priority 3. 
     As shown in FIG. 18, initial values of the parameters are set (Step  2510 ) while the priority class is defined as the parameter x and the minimum repetition number of the checking process, which checks the priority queues  2300  to  2303  to see if there are any signaling messages left, is defined as the parameter SC(x). In this example, the initial value of the parameter x is set to 0 so as to execute extraction from the priority queue with higher priority (Step  2520 ). The priority 0 queue  2300  is checked and if the parameter SC( 0 ) is not less than 1 (Step  2530 ), the parameter SC(x) is decremented by 1 (Step  2532 ). The priority 0 queue is then checked to see if there are any signaling messages left (Step  2534 ). If the signaling messages exist, the first signaling message in the priority 0 queue, which was stored at the earliest time, is extracted (Step  2536 ) and transferred to the signaling message transmitting/receiving process  2115  (Step  2538 ). If there is no signaling message left (Step  2534 ), the processes following step  2530  are repeated. If the SC( 0 ) is less than 1 (Step  2530 ), the extraction process of the priority 0 queue ends for this period of time. If the parameter x is less than 3 (Step  2540 ), the parameter x is added by 1 (Step  2550 ). By incrementing the value of the parameter x by one, the same processes mentioned above (Steps  2530  to  2538 ) are repeated for each of the priority 0 queue  2300  to the priority 3 queue  2303 . When the checking of the priority 0 queue  2300  to the priority 3 queue  2303  is completed and the parameter x becomes not less than 3 (Step  2540 ), it returns to the initialization process of the parameter SC(x) and the parameter x (Steps  2510 ,  2520 ) so as to repeat these processes. 
     According to the present embodiment, the connection setup request may be received in the ATM switch based on the priority which is assigned in correspondence with a level of importance of the ATM device connected to the port of the ATM switch. Here, the signaling message described above may be any one of the signaling messages, such as the SETUP message. Alternatively, it is possible to store only the SETUP messages in the priority queues while providing new queues besides the priority queues to store the other types of signaling messages. To prevent congestion due to the overflow of each queue, the discarding process  2155  shown in FIG. 14 may be executed when a number of signaling messages stored in each queue reaches a predetermined number of the signaling messages so as to discard the signaling messages after a certain number of signaling messages are extracted by the extracting process  2154 . When the signaling messages are discarded, the discarding process  2155  may also send a message for notifying discard of the connection setup request to an originator of the discarded connection setup request. 
     The seventh embodiment of the present invention will be described with reference to the figures. In the seventh embodiment, the priority control method for receiving the connection setup request will be described in such a case that the connection setup request is classified based on information parameters included in the SETUP message. The information element is an element constructing the signaling message or the like, and includes a plurality of parameters. The seventh embodiment has the same structure as that of the sixth embodiment except for the content of the priority table and the branching process. These points of difference are described in the following part. 
     FIG. 19 shows a format of the SETUP message  2600  according to the UNI version 3.1 of ATM Forum. The SETUP message  2600  is provided with common headers among the signaling messages ( 2610 - 2640 ) and several information elements such as an AAL (ATM Adaptation Layer) parameter  2650 . In the following, an operation is described when the priority is divided into four priority classes based on the AAL parameter. 
     The AAL parameter  2650  specifies an AAL type and one of the service types, each suited for communicating different types of information such as voice, data or video. Type 1, Type 3/4 and Type 5 are specified as the AAL type by the ATM Forum. This information element is defined as optional. In the present embodiment, the priority class is set for each of the AAL type. 
     FIG. 20 shows the priority table  2700 . The priority table  2700  has a plurality of entry sets provided in correspondence with a respective one of the priority classes, each entry set comprising the class  2710  and the AAL type  2720 . The AAL type may be classified in advance by the system supervisor. In this example, a service content  2730  of the AAL type is provided as information and it is not required for an actual table. In the example shown in FIG. 20, the service content of the AAL type 1 is for voice and video, the service content of the AAL type 3/4 is for the connection-type data, the service content of the AAL type 5 is for the connectionless-type data. 
     FIG. 21 shows a flowchart illustrating the branching process of the connection receiving priority control section  2150  of the seventh embodiment. 
     As shown in FIG. 21, when the signaling message that has completed being assembled is received, the branching process  2151  checks if it is the SETUP message or not (Step  2810 ). If it is the SETUP message, the branching process  2151  further checks if the AAL parameter is available (Step  2820 ), and identifies the AAL type if the AAL parameter is available. If the AAL type is 1 (Step  2830 ), the SETUP message is transferred to the priority 0 queue  2300  (Step  2860 ). If the AAL type is 3/4 (Step  2840 ), the SETUP message is transferred to the priority 1 queue  2301  (Step  2870 ). If the AAL type is 5 (Step  2850 ), the SETUP message is transferred to the priority 2 queue  2302  (Step  2880 ). If the AAL type is different from any of the above, or is not available (Step  2820 ) and if the message received is not the SETUP message, the message is transferred to the priority 3 queue (Step  2890 ). 
     The extracting process of the connection receiving priority control section  2150  may be executed in the same way as the process shown in the flowchart in FIG. 18, or based on a different priority algorithm. 
     According to the present process, the connection setup request may be received according to the priority which is assigned in correspondence with a level of importance of the information element parameter included in the connection setup request. 
     The eighth embodiment of the present invention will be described with reference to the figures. In the eighth embodiment, a method is described in which a discarding priority is additionally defined so as to specify the sequence of discarding the connection setup requests for each of the priority queues, which store the classified connection setup requests and are described in the sixth and seventh embodiments, and the connection setup requests are preferentially discarded from the priority queue of the higher discarding priority if a total number of messages stored in the priority queues exceeds a predetermined value. 
     FIG. 22A shows the discarding priority table  2900 , and FIG. 22B shows the discarding priority table  2920 . 
     The discarding priority table  2900  of FIG. 22A is configured so as to include an additional discarding priority item in the entry of the discarding priority table  2200  shown in FIG.  15 . Values of the discarding priority in this table are set to NONE at the priority queues with the priority class value of 0 to 2 since no message is discarded in these priority queues, and set to 0 at the priority queue with the priority class value of 3 to assign the highest discarding priority. The discarding priority table  2920  of FIG. 22B is configured so as to include an additional discarding priority item in the entry of the discarding priority table  2700  shown in FIG.  20 . Values of the discarding priority in this table are set to NONE at the priority queue with the priority class value of 0 since no message is discarded in this priority queue, and respectively set to 2, 1, and 0 at the priority queues with the priority class values of 1, 2, 3 to assign a higher discarding priority to the priority queue with a lower priority. 
     FIGS. 23A,  23 B are flowcharts showing the discarding process. 
     In the initialization process shown in FIG. 23A, a predetermined value for a total number of messages to be stored in all of the priority queues is substituted for MS, and a predetermined value of the discarding priority is substituted for D(x) which is defined as the discarding priority of the priority class x (Step  21010 ). D(3)=0, D(0), D(1), D(2)=NONE if the discarding priority table  2900  is referenced, or D(3)=0, D(2)=1, D(1)=2, D(0)=NONE if the discarding priority table  2920  is referenced. 
     The discarding process of FIG. 23B, which is shown as the discarding process  2155  in FIG. 14, is assumed to be executed following step  2540  of the extracting process shown in FIG. 18 if the judgment at step  2540  becomes NO. D(x) is set to 0 so that the discarding process may be executed starting from the priority queue with higher discarding priority D(x), i.e. D(x)=0 (Step  21010 ). In this example, a value of the discarding priority is assumed to be 0 to 3 or NONE, and the discarding process is not executed for the discarding priority of NONE. Therefore, a value of the discarding priority is checked to see if the value exists within a range of the discarding priority value, i.e. from 0 to 3 (Step  21012 ). Then, a number of messages stored in each of the priority queues is detected, and the detected numbers are substituted for MC(0)-MC(3) respectively. A result of MC(0)+MC(1)+MC(2)+MC(3) is substituted for MC which is defined as a total number of messages stored in all of the priority queues. A number of messages to be discarded is defined as MD, and MD is calculated by subtracting MS from MC (Step  21020 ). 
     If MD is larger than 0, i.e. if there is at least one message to be discarded (Step  21030 ), the queue of the priority class x, which corresponds to the discarding priority D(x), is detected (Step  21040 ). If MD is 0 or less, i.e. if there is no message to be discarded, the process is ended. 
     If the message number MC(x) of the priority x queue is equal to MD or larger (Step  21050 ), MD messages are discarded from the queue of the priority class x corresponding to the discarding priority D(x) (Step  21052 ), and the process is then ended. 
     If the message number MC(x) of the priority x queue is less than the discarding message number MD (Step  21050 ), all of the messages stored in the priority x queue, i.e. MC(x) messages, are discarded (Step  21054 ). The value of D(x) is then incremented to check the higher discarding priority (Step  21014 ). A series of processes from the step  21020  to the step  21054  is repeated within a range of the discarding priority (Step  21012 ). 
     For example, the discarding process is described in such a case that ten messages are stored in each priority queue, and MC(0)=10, MC(1)=10, MC(2)=10, MC(3)=10, and the predetermined value MS, the total number of messages for all of the priority queues, is set to 25. When the discarding priority table  2900  is referenced, ten messages are discarded from the priority 3 queue which corresponds to D(x)=0 by the single cycle of execution of this message discarding process. When the discarding priority table  2920  is referenced, ten messages are discarded from the priority 3 queue corresponding to D(x)=0 and five messages are discarded from the priority 2 queue corresponding to D(x)=1 by the single cycle of execution of this message discarding process. 
     According to the present embodiment, the overflow of the priority queue may be prevented even when unprocessed connection setup requests are accumulated in a particular queue and the number of connection setup requests stored in this queue becomes equal to a predetermined number or larger. This is accomplished by assigning the discarding priority in advance to define the sequence of discarding the connection setup requests for each of the queues, then extracting the connection setup requests from the queue in which the connection setup requests are stored, and discarding the extracted connection setup requests according to a predetermined algorithm for discarding the connection setup requests based on the discarding priority. 
     The ninth embodiment of the present invention is described with reference to the figures. In the ninth embodiment, a message discard notification process is added after discard of the messages in the discarding process described in the eighth embodiment. 
     In the message discard notification process executed by the discarding process  2155  shown in FIG. 14, a message discard notification operation using a RELEASE COMP message, which is one of the signaling messages, is executed after discard of the messages in the discarding process shown in FIGS. 23A,  23 B, i.e. after step  21052  or  21054 . The RELEASE COMP message may additionally include, following the header section, a message type specifying the message discard notification, identity information of the discarded SETUP message, and information indicating one of predetermined discarding causes. 
     According to the present embodiment, it is possible to execute the message discard notification operation after discarding of the messages in the discarding process  2155  shown in FIG. 14, and let the ATM terminal of the originator know of unsuccessful reception of the connection. 
     The tenth embodiment of the present invention will be described with reference to the figures. In the tenth embodiment, the connection receiving priority control method is described for such a case that the priority is given to the connection setup request which is sent from the other ATM switch and relayed through by more ATM switches during the reception of the connection setup requests. 
     FIG. 24 shows an example of an ATM-LAN system configuration where ATM switches  100 A- 100 C and ATM terminals  140 A- 140 C are connected. The present embodiment is described using an example where the connection receiving priority control is executed at the ATM switch  100 A. 
     The ATM switch receives the SETUP message from the ATM switch  100 B when either calling party at the ATM terminal  140 B or at the ATM terminal  140 C tries to establish communications with the ATM terminal  140 A. According to the sixth embodiment, the SETUP messages sent from the ATM terminals  140 B and  140 C will be stored in the same priority queue since the priority is defined by the port to which the ATM switch  100 B is connected in the sixth embodiment. According to the tenth embodiment, the SETUP message which is relayed by more ATM switches is preferentially received in preference to the others when the same priority is given to all the SETUP messages. The number of ATM switches which relayed the SETUP message is defined as a hop number hereafter. In the example, the SETUP message from the ATM terminal  140 C is preferentially received since the hop number of this message is larger than that of the SETUP message from the ATM terminal  140 B. 
     FIG. 25 shows a hop number table  21200 . The hop number table  21200  includes a plurality of entry sets, each comprising the ATM address  21210  and the hop number  21220 . The hop number table  21200  included in the ATM switch  100 A specifies the ATM addresses of a, b, c and the hop numbers  21220  of 0, 1, 2 when the ATM terminals  140 A,  140 B,  140 C are assumed to be the originator respectively. An example of a method for calculating the hop number is described. In this example, the SETUP message is provided with information specifying the hop number, and the hop number is incremented at each of the relaying ATM switches every time the SETUP message is relayed. The ATM switch  100 A is structured so as to be able to generate the hop number table  21200  by extracting and reading the hop number from the received SETUP message. 
     In the present embodiment, a plurality of queues are additionally provided at the priority queue described in the sixth to ninth embodiments in correspondence with values of the hop number (called priority sub-queue hereafter), and the SETUP message is branched and stored in the priority sub-queue which corresponds to the hop number of the SETUP message. In this example, the ATM address of the originator is stored as the calling party number  2660  of the SETUP message. Therefore, by reading the calling party number  2660  and consulting the HOP number table  21200 , the SETUP message is branched and stored in the priority sub-queue corresponding to its hop number in the branching process shown in FIG.  14 . In the extracting process shown in FIG. 14, the SETUP messages stored in the priority sub-queue is extracted according to a predetermined algorithm. Here, the SETUP message with a higher hop number may be preferentially received in the receiving operation by assigning a higher priority to the priority sub-queue corresponding to higher hop number. 
     According to the present embodiment, the connection receiving priority control may be accomplished while a number of relayed ATM switches is taken into account. In the example described above, the ATM switch  100 A is able to receive the SETUP message from the ATM terminal  140 C preferentially over the SETUP message from the ATM terminal  140 B. Furthermore, it is able to reduce occurrences of such a case where resources once appointed at the relayed ATM switches are to be wasted. 
     Although the priority sub-queue corresponding to the hop number is provided in the priority queue described in the sixth-ninth embodiments in the present embodiment, it is also possible to have only priority queues provided in correspondence with values of the hop number in advance. 
     According to the sixth-tenth embodiments, it is possible to establish the connection efficiently at the ATM switch by classifying ATM devices connected to the ATM switch and processing the classified SETUP message according to the sequence of priority so as to preferentially process an important connection setup request or to give preference to the connection setup request which has been relayed by many ATM switches. Furthermore, it is possible to reduce occurrences of such a case where resources once appointed at the relayed ATM switches are to be wasted by taking the number of relayed ATM switches into account.