ATM exchange system and priority control method

Abstract: A call reception control circuit 1 sets each guaranteed band width required for assuring communication quality of the respective band width demand classes in the corresponding N counter Nc, Ntv and Nv at a certain time interval Ts. The call reception control circuit 1 also sets values defined by the ratio of the W counter Wabr and Wubr of the respective best effort class to use the remained band width. The value 1 is subtracted from each value of the N counter Nc, Ntv and Nv and W counter Wabr and Wubr at every output of the cell from the corresponding class. A priority control process circuit 2 outputs cells to an output channel 3 according to priority levels giving a priority to the band width demand class having the value of the N counter not set to 0 over the band width demand class and the best effort classes having the value of the N counter set to 0.

Description: This invention relates to an Asynchronous Transfer Mode (referred to as ATM, hereinafter) exchange system used in a multimedia communication system operating basically through a buffer memory. More specifically this invention relates to a priority control method preventing an adverse influence caused by an abnormal traffic generated in a certain communication quality class among those defined by the required communication quality from being exerted to the rest of the classes. The multimedia communication system operating basically through the buffer memory provides services requiring communication quality under various aspects such as real time processability, low destruction rate, "Best effort" transfer and the like. The following services have been provided, for example: Circuit Emulation; Constant Bit Rate Video service (referred to as CBR class, hereinafter) at a constant transmission bit rate requiring strict standards on delay or cell loss; Audio and visual service at a variable bit rate requiring real time processability (referred to as rt-VBR class, hereinafter); Data service at a variable bit rate requiring no real time processability (referred to as nrt-VBR class, hereinafter); service at a variable bit rate requiring the best effort transfer and strict standards on cell loss (referred to as ABR class, hereinafter); service requiring destruction and fairness at every cell (referred to as UBR class, hereinafter) and the like. The priority control method in the ATM exchange system is effective for meeting the above-described different types of required communication quality simultaneously. In the conventional priority control method, an initial priority level is set to each communication quality class defined by the required communication quality. If a waiting queue length of each communication quality class is smaller than a threshold value specified with respect to the respective communication quality classes, priority control is executed according to the initial priority level set to the respective communication quality classes. If a certain communication quality class has a waiting queue length equal to or larger than a threshold value, the priority level of the class is raised by one level and the priority control will be executed according to the changed priority level. In case of communication quality classes provided with the same priority, a rotational priority control is executed. The rotational priority control is done by outputting the cell from buffers of those classes one by one alternately. Fig. 16 shows a more detailed explanation of the above-described prior art. In Fig. 16, it is assumed that three classes are priority controlled. The initial priority level is set to communication quality classes C1, C2 and C3, respectively according to the communication quality required by those communication quality classes C1, C2 and C3. In Fig. 16, priorities 2, 3 and 4 are given to the communication quality classes C1, C2 and C3, respectively. Threshold values Th1, Th2 and Th3 are also set to the communication quality classes C1, C2 and C3 according to the communication quality required by C1, C2 and C3. When each of waiting-queue lengths Q1, Q2 and Q3 of the communication quality classes C1, C2 and C3 is less than Th1, Th2 and Th3, respectively, the cell stored in a buffer B1 of the communication quality class C1 is output with the highest priority according to the initial priority level. A cell stored in a buffer B2 of the communication quality class C2 is output only when the buffer B1 stores no cells. A cell stored in a buffer B3 of the communication quality class C3 is output only when both buffers B1 and B2 store no cells. When the waiting queue length Q2 exceeds the threshold value Th2 owing to increased traffic amount in a certain communication quality class, i.e., C2, the priority level of the communication quality class is changed from 3 to 2, thus changing the priority levels of C1, C2 and C3 to 2, 2 and 4, respectively. As a result, the rotational priority control is executed between the classes C1 and C2 at the same priority level. When the waiting queue length Q2 becomes equal to or smaller than the threshold value Th2, the priority level of the communication quality class C2 is restored to 3 as the original priority level. With the conventional priority control method, when the waiting queue length exceeds the threshold value owing to increased traffic amount reaching a certain communication quality class, the priority level of the communication quality class is raised by one level. When the traffic amount is increased for some reason to have the waiting queue length exceed the threshold value in a certain communication quality class, communication quality of the other classes may be adversely influenced. For example, when the waiting queue length Q2 exceeds the threshold value Th2 due to the increased traffic amount in the communication quality class C2, the number of cells in the communication quality class C1 output in a unit of time is decreased, resulting in degraded communication quality. The waiting queue length Q3 of the communication quality class C3 further exceeds the threshold value Th3. Even though the priority level of C3 is raised from 4 to 3, the priority of the class C2 has been set to 2 and no cells are output from the communication quality class C3 even if the buffer for the communication quality class C1 stores no cells. EP 0 398 037 discloses a method of communication resource allocation in a packet network. The method comprises the steps of: subdividing a communication resource of the packet network into a plurality of sub-resources in correspondence with communications classified into a plurality of communication classes; allocating one of the sub-resources to each one of the communication classes; and controlling the amount of communications within each sub-resource independently in order to obtain a required quality of service within each sub-resource. It is an object of the present invention to provide an improved Asynchronous Transfer Mode exchange system overcoming the aforementioned problems. It is another object of the present invention to provide a priority control method for preventing an abnormal traffic generated in a certain communication quality class from exerting an adverse effect on the communication quality of any other communication quality classes. The above objects of the present invention are achieved with the features of the claims. An ATM exchange system is provided for receiving a plurality of calls each having different required communication quality comprising: a buffer for providing each of a plurality of bandwidth demand classes that have been defined according to the required communication quality for storing call cells belonging to the respective bandwidth demand classes; a counter for providing each of said respective bandwidth demand classes; call reception control means for setting a guaranteed bandwidth represented by the number of cells in a certain time required to be output to an output channel for assuring each communication quality of said respective bandwidth demand classes at said certain time interval; and priority control process means for outputting cells stored in a buffer for a bandwidth demand class having a counter value not set to 0 to said output channel, subtracting 1 from a value set in a counter of a bandwidth demand class that has output cells at every output of cells to said output channel, and outputting cells stored in a buffer for a bandwidth demand class having a counter value 0. In a counter for every bandwidth demand class, a call reception control means sets a guaranteed bandwidth represented by the number of cells in a a certain time required to be output for assuring communication quality of the respective bandwidth demand classes at said certain time interval. A value of the counter of each bandwidth demand class is subjected to subtraction through a priority control process means at every output of the cell from the corresponding bandwidth demand class buffer. The priority control process means outputs the cell from the buffer for the bandwidth demand class in which the N counter value has not reached 0 and outputs the cell stored in the bandwidth demand class outputting the cell with the highest priority. That is, the number of cells required to assure the communication quality of each bandwidth demand class is output. BRIEF DESCRIPTION OF THE DRAWINGS This and other objects, features and advantages of the present invention will become more apparent upon reading the following detailed description and drawings, in which: Fig. 1 is a block diagram of an embodiment of the present invention; Fig. 2 is a flowchart showing a process example of a call reception control circuit 1 upon generation of the call; Fig. 3 is a flowchart showing a procedure executed by the call reception control circuit 1 for setting the N counter at a certain time interval Ts; Fig. 4 is a flowchart showing a procedure executed by a priority control process circuit 2; Fig. 5 is an example of priority levels used by the priority control process circuit 2; Fig. 6 is an another example of priority levels used by the priority control process circuit 2; Fig. 7 is a block diagram of another embodiment of the present invention; Fig. 8 is an example of priority levels used by a priority control process circuit 12; Fig. 9 is another example of priority levels used by the priority control process circuit 12; Fig. 10 is a block diagram of a further embodiment of the present invention; Fig. 11 is an example of priority levels used by a priority control process circuit 22; Fig. 12 is another example of priority levels used by the priority control process circuit 22; Fig. 13 is a block diagram of a still further embodiment of the present invention; Fig. 14 is an example of priority levels used by a priority control process circuit 32; Fig. 15 is another example of priority levels used by the priority control process circuit 32; and Fig. 16 is an explanatory view of a prior art. EMBODIMENTS Embodiments of the present invention are described in detail referring to the drawings. Fig. 1 is a block diagram of an embodiment of the present invention comprising a call reception control circuit 1, a priority control process circuit 2, an output channel 3, bandwidth demand class buffers CBR, rt-VBR, nrt-VBR, best effort transfer demand class buffers ABR and UBR, N counters (Nc, Ntv and Nv) and W counters (Wabr and Wubr). The bandwidth demand class buffers CBR, rt-VBR and nrt-VBR store the cell related to the call belonging to the corresponding CBR class, rt-VBR class and nrt-VBR class resulting from classifying the call for bandwidth demand according to the required communication quality. The best effort transfer demand class buffers ABR and UBR store the cell related to the call belonging to the corresponding best effort class ABR class and UBR class resulting from classifying the call for the best effort transfer demand to use the remained bandwidth according to the required communication quality. The N counters Nc, Ntv and Nv are provided for the CBR class, rt-VBR class and nrt-VBR class, respectively. The W counters Wabr and Wubr are provided for the ABR class and the UBR class, respectively. The call reception control circuit 1 has various functions for setting a guaranteed bandwidth represented by the number of cells in a certain time Ts which is required to be output to the output channel 3 for assuring each communication quality of the CBR class, rt-NBR class and nrt-VBR class in the N counter Nc, Ntv and Nv of the CBR class, rt-VBR class and nrt-VBR class, respectively and for setting values defined by a ratio of the ABR class and UBR class to use the remaining bandwidth represented by the number of cells output to the output channel 3 in the certain time Ts resulting from taking the guaranteed bandwidths for the CBR class, rt-VBR class and nrt-VBR class from the whole bandwidth in the W counters Wabr and Wubr when at least one of Wabr or Wubr has a value set to 0 and both the ABR class and the UBR class are suspended the value of the W counter either Wabr or Wubr is 0 or the best effort transfer demand class buffers ABR and UBR are empty). The priority control process circuit 2 has functions for outputting cells stored in the buffers CBR, rt-VBR, nrt-VBR, ABR and UBR to the output channel 3 according to the priority level defined by each value of N counter Nc, Ntv, Nv and the W counter Wabr, Wubr and the respective class, and for subtracting 1 from the value of the N counter Nc, Ntv and Nv or the W counters Wabr and Wubr not set to 0 in the class outputting the cell. Figs. 2 and 3 are flowcharts representing an example of processing in the call reception control circuit 1. Fig. 4 is a flowchart representing an example of processing in the priority control process circuit 2. The operation of the embodiment is hereinafter described referring to these drawings. As shown in the flowchart of Fig. 2, upon generation of a call, the call reception control circuit 1 determines the class to which the generated call belongs (S1). When the call is determined to belong to any of the CBR class, rt-VBR class or nrt-VBR class for bandwidth demand, it is further determined whether or not the call reception results in consuming the remaining bandwidth (S2 and S3). When it is determined that the remaining bandwidth is consumed, the call is rejected (if YES at S3, the process proceeds to S5). However, when it is determined that the remained band width is not consumed, the call is received (if NO at S3, the process proceeds to S4). The call reception control circuit 1 increases the guaranteed bandwidth for the class to which the received call belongs among guaranteed bandwidths for the respective bandwidth demand classes (CBR class, rt-VBR class and nrt-VBR class) by the value corresponding to the communication quality required by the received call as well as decreasing the value equal to the increment from the remaining bandwidth under control. Even if the cell transmission source transmits cells at a constant bit rate, those cells do not always reach an exchange office side at the above-described bit rate owing to the influence of switchings and the like. So the guaranteed bandwidth is set to be wider than that logically obtained. At the end of the call, the call reception control circuit 1 decreases the guaranteed bandwidth for the terminated call by a value corresponding to the communication demand quality of the call as well as increases the remained bandwidth by the value equal to the above-described decrement. When it is determined that the call belongs to the ABR class or UBR class (if NO at S2), the call reception control circuit 1 executes the process at S4. The call reception control circuit 1 sets each guaranteed bandwidth of the CBR class, rt-VBR class and nrt-VBR class under control to the corresponding N counters Nc, Ntv and Nv, respectively at a certain time interval Ts as shown in the flow chart of Fig. 3 (S11). The call reception control circuit 1 sets values defined by a ratio of the W counters Wabr and Wubr to use the remained bandwidth in case at least one of either Wabr or Wubr has a count value 0 and both the ABR class and UBR class are suspended. As shown in Fig. 4, the priority control process circuit 2 outputs one cell according to the priority level shown in Fig. 5. When the counter value of the class outputting the cell is not 0, the processing for subtracting 1 from the counter value is executed repeatedly (S21 and S22). The following process is executed at a certain time interval Ts by outputting the cell according to the priority levels shown in Fig. 5. That is, the cell is output from the bandwidth demand class buffer CBR until the value of the N counter Nc of the CBR class becomes 0 or the bandwidth demand class buffer CBR becomes empty (in case of number of cells stored in the buffer CBR less than the N counter Nc value) (priority 1). Then the cell is output from the bandwidth demand class buffer rt-VBR until the value of the N counter Ntv of the rt-VBR class becomes 0 or the bandwidth demand class buffer rt-VBR becomes empty (in case of number of cells stored in the buffer rt-VBR less than the N counter Ntv value) (priority 2). The cell is output from the bandwidth demand class buffer nrt-VBR until the value of the N counter Nv of the nrt-VBR class becomes 0 or the bandwidth demand class buffer nrt-VBR becomes empty (in case of number of cells stored in the buffer nrt-VBR less than the N counter Nv value) (priority 3). The number of cells required for assuring each communication quality of the CBR class, rt-VBR class and nrt-VBR class are preferentially output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR, respectively. The cells stored in the best effort transfer demand class buffers ABR and UBR for the ABR class and UBR class are output to the output channel 3 through a weighted rotational priority control (Priority 4). The weighted rotational priority stands for outputting cells from the respective buffers according to a predetermined ratio. For example, supposing that the weighted rotational priority control is executed between the ABR class and UBR class in the ratio of 2:1, two cells are output from the best effort transfer demand class buffer ABR for the ABR class. On the other hand one cell is output from the best effort transfer demand class buffer UBR for the UBR class. In the above case, the ABR class has better delay characteristics than those of the UBR class. Then cells stored in the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class, having the value of the N counter Nc, Nt and Nv set to 0 are output to the output channel 3 (Priority level 5 to 7). In case the class having a higher priority than those of classes outputting the cell according to the given priority level is generated, the higher priority class buffer preferentially outputs the cell. The number of cells required for assuring each communication quality of the CBR class, rt-VBR class and nrt-VBR class is output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR, respectively at a certain time interval Ts. Therefore even if an abnormal traffic is generated in a certain bandwidth demand class among the CBR class, rt-VBR class and nrt-VBR class, communication quality of the rest of the bandwidth demand classes is guaranteed. When using the priority levels shown in Fig. 5, the delay characteristic of the higher priority class influences the lower priority class (especially in view of delay fluctuation). Priority levels shown in Fig. 6 can be used for reducing the aforementioned adverse influence. Fig. 7 is a block diagram of an other embodiment of the present invention. This embodiment is constructed by adding addition means 4, and S counters Sc, Stv, Sv, Sabr and Subr for the CBR class, rt-VBR class, nrt-VBR class, ABR class and UBR class, respectively to the construction of the embodiment shown in Fig. 1. Additionally a call reception control circuit 11 and a priority control process circuit 12 are provided in place of the call reception control circuit 1 and the priority control process circuit 2 in the embodiment shown in Fig. 1. The call reception control circuit 11 has a similar construction to that of the call reception control circuit 1 of Fig. 1. The call reception control circuit 1 of Fig. 1 sets a value larger than the logically obtained guaranteed bandwidth in the N counters Nc, Ntv and Nv. On the contrary, the call reception control circuit 11 sets the logically obtained guaranteed bandwidth in each of the N counters Nc, Ntv and Nv. The priority control process circuit 12 has the following functions: outputting cells to the output channel 3 according to the priority level defined by each value of the N counters Nc, Ntv, Nv, W counters Wabr, Wubr, S counters Sc, Stv, Sv, Sabr, Subr and the CBR class, rt-VBR class, nrt-VBR class, ABR class and UBR class; subtracting the value 1 from the value of the N counter Nc, Ntv and Nv not set to 0 in the class outputting the cell when the cell is output from the CBR class, rt-VBR class and nrt-VBR class having an N counter value Nc, Ntv and Nv larger than 0,; subtracting the value 1 from the value of the W counter Wabr and Wubr not set to 0 in the class outputting the cell when the cell is output from the ABR class and UBR class having a W counter value Wabr and Wubr larger than 0; subtracting the value 1 from the value of the S counter Sc, Stv and Sv not set to 0 in the class outputting the cell when the cell is output from the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv set to 0; and subtracting the value 1 from the value of the S counters Sabr and Subr not set to 0 in the class outputting the cell when the cell is output from the ABR class and UBR class having a value of the W counter Wabr and Wubr set to 0. The addition means 4 has the following functions: setting values. (Sc+Nc), (Stv+Ntv) and (Sv+Nv) obtained by adding each value set in the S counters Sc, Stv and Sv to each value set in the N counter Nc, Ntv and Nv, respectively before setting the guaranteed bandwidth in the N counters, Nc, Ntv and Nv through the call reception control circuit 11; and setting values (Sabr+Wabr) and (Subr+Wubr) obtained by adding each value set in the S counters Sabr and Subr to the value set in the W counters Wabr and Wubr in the S counter Sabr and Subr, respectively before setting the value in the W counter Wabr and Wubr through the call reception control circuit 11. Operations of the present embodiment of this invention shown in Fig. 7 are described. Responding to a call, the call reception control circuit 11 executes the same processes shown in the flowchart of Fig. 2. The call reception control circuit 11 sets the guaranteed bandwidth in each of the N counters Nc, Ntv and Nv and in the W counter Wabr and Wubr, respectively in a similar way. The addition means 4 sets the values (Sc+Nc), (Stv+Ntv) and (Sv+Nv) obtained by adding each value set in the S counters Sc, Stv and Sv to the value set in the N counters Nc, Ntv and Nv in the S counter Sc, Stv and Sv, respectively before setting the guaranteed bandwidth in the N counter Nc, Ntv and Nv through the call reception control circuit 11 and sets the values (Sabr+Wabr) and (Subr+Wubr) obtained by adding each value set in the S counters Sabr and Subr to the value set. in the W counters Wabr and Wubr, respectively before setting the value in the W counter Wabr and Wubr through the call reception control circuit 11. The priority control process circuit 12 outputs one cell according to the priority levels shown in Fig. 8. When the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv larger than 0 outputs the cell, the value 1 is subtracted from the value of the N counter Nc, Ntv and Nv in the class outputting the cell. When the ABR class and UBR class having a value of the W counter Wabr and Wubr larger than 0 outputs the cell, the value 1 is subtracted from the value of the W counter Wabr and Wubr in the class outputting the cell. When the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv set to 0 outputs the cell, the value 1 is subtracted from the value of the S counter Sc, Stv and Sv in the class outputting the cell. When the ABR class and UBR class having a value of the W counter Wabr and Wubr set to 0 outputs the cell, the value 1 is subtracted from the value of the S counter Sabr and Subr in the class outputting the cell. By outputting the cell according to the priority levels shown in Fig. 8, the following process is executed at a certain time interval Ts. First the bandwidth demand class buffer CBR outputs cells until the value of the N counter Nc of the CBR class is set to 0 or the bandwidth demand class buffer CBR becomes empty (in case of number of cells stored in the buffer CBR less than the N counter Nc value) (Priority 1). Then the bandwidth demand class buffer rt-VBR outputs cells until the value of the N counter Ntv of the rt-VBR class is set to 0 or the bandwidth demand class buffer rt-VBR becomes empty (in case of number of cells stored in the buffer rt-VBR less than the N counter Ntv value) (Priority 2). The bandwidth demand class buffer nrt-VBR outputs cells until the value of the N counter Nv of the nrt-VBR class is set to 0 or the bandwidth demand class buffer nrt-VBR becomes empty (in case of number of cells stored in the buffer nrt-VBR less than the N counter Nv value) (Priority 3). The bandwidth demand class buffer CBR outputs cells until the value of the S counter Sc is set to 0 or the bandwidth demand class buffer CBR becomes empty (in case of number of cells stored in the buffer CBR less than the S counter Sc value) (Priority 4). The bandwidth demand class buffer rt-VBR outputs cells until the value of the S counter Stv is set to 0 or the bandwidth demand class buffer rt-VBR becomes empty (in case of number of cells stored in the buffer rt-VBR less than the S counter Stv value) (Priority 5). The bandwidth demand class buffer nrt-VBR outputs cells until the value of the S counter Sv is set to 0 or the bandwidth demand class buffer nrt-VBR becomes empty (in case of number of cells stored in buffer nrt-VBR less than the S counter Sv value) (Priority 6). The cells stored in the best effort transfer demand class buffers ABR and UBR for the ABR class and the UBR class are output to the output channel 3 through the weighted rotational priority control (Priority 7). The cells stored in the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having both values of the N counter Nc, Ntv and Nv and the S counter Sc, Stv and Sv set to 0 are output to the output channel 3 (Priority 8 to 10). In case the class having a higher priority than those of classes outputting cells according to the given priority level is generated, the higher priority class buffer preferentially outputs the cell. The present invention sets a higher priority to the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv larger than 0 over the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0. The following is the reason why the above-described priority setting has been defined. Even if the cell transmission source transmits the cell at a constant bit rate, the cells do not always reach the exchange office at the above-specified bit rate. For example, supposing that the cell transmission source outputs 1000 cells in a certain time Ts, it is often the case that only 900 cells arrive within the certain time Ts and then in the next time Ts, 1100 cells arrive. It is assumed that a logically guaranteed bandwidth for assuring the communication quality is set to 1000 cell/Ts and only 900 cells are output in the first certain time interval Ts, then 1100 cells are output in the next certain time Ts to the bandwidth demand class buffer CBR for the CBR class. In the above case, the N counter Nc has the guaranteed bandwidth set to 1000 cells at a certain time interval Ts. When only 900 cells have arrived at the certain time interval Ts, the N counter Nc is not set to 0 but to 100 even though all the cells stored in the bandwidth demand class buffer CBR are output. The Nc value 100 is added to the value currently set in the S counter Sc. For example, the Sc value is changed from 0 to 100. When 1100 cells have arrived at the subsequent time interval Ts, 1000 cells are output from the bandwidth demand class buffer CBR until the N counter Nc value is set to 0. Cells of the rt-VBR class and nrt-VBR class having the value of the N counter Ntv and Nv larger than 0 are output from the bandwidth demand class buffers rt-VBR and nrt-VBR. As the value of the S counter Sc has been set to 100, 100 cells left in the bandwidth demand class buffer CBR are output. As a result, 1100 cells are output in this time interval Ts. Therefore 2000 cells can be output within two consecutive time intervals Ts, meeting the requirement of the guaranteed bandwidth, i.e., 1000 cells/Ts. In the embodiment shown in Fig. 1, supposing that 1000 cells/Ts is set as the guaranteed bandwidth to the N counter Nc, if only 900 cells have arrived at a certain time interval Ts, 900 cells are output. When 1100 cells have arrived at the next time interval Ts, 1000 cells are output. Therefore only 1900 cells in total are output within two consecutive time intervals Ts, failing to meet the requirement of the guaranteed bandwidth, i.e., 1000 cells/Ts. In the embodiment of Fig. 1, the guaranteed bandwidth value has to be larger than 1000 cells/Ts (for example, 1100 cells/Ts). In this embodiment, the guaranteed bandwidth for assuring the same communication quality can be reduced compared with the embodiment of Fig. 1. As a result, the number of received calls can be increased compared with the embodiment of Fig. 1. That is in case a total of each guaranteed band width of the CBR class, rt-VBR class and nrt-VBR class becomes larger than the whole bandwidth of the output channel 3, the call has to be rejected. When the guaranteed bandwidth is reduced, the number of calls for reception can be increased. In the aforementioned embodiment, the priority levels shown in Fig. 9 can be used in place of the priority levels shown in Fig. 8 for preventing the higher priority class from giving an adverse effect on the lower priority class. Fig. 10 is a block diagram of another embodiment of the present invention. In this embodiment, a priority control process circuit 22 is used in place of the priority control process circuit 12 shown in Fig. 7. The priority control process circuit 22 outputs cells to the output channel 3 according to the priority defined by each value of the N counter Nc, Ntv and Nv, the W counter Wabr and Wubr, the S counter Sc, Stv, Sv, Sabr and Subr, the CBR class, rt-VBR class, nrt-VBR class, ABR class and UBR class and each of first threshold values Thc1, Thtv1 and Thv1 for the respective CBR class, rt-VBR class and nrt-VBR class. Other functions of the priority control process circuit 22 are the same as those of the priority control process circuit 12 shown in Fig. 7. The threshold values Thc1, Thtv1 and Thv1 correspond to the number of cells (waiting queue length Qc, Qtv and Qv) stored in the respective bandwidth demand class buffers CBR, rt-VBR and nrt-VBR. Those threshold values are set conforming to the communication quality required by each of the CBR class, rt-VBR class and nrt-VBR class. Operations of this embodiment shown in Fig. 10 are explained. The call reception control circuit 11 and addition means 4 execute the same processing as aforementioned. The priority control process circuit 22 outputs one cell according to the priority levels shown in Fig. 11. When the CBR class, rt-VBR class and nrt-VBR class having the value of the N counter Nc, Ntv and Nv larger than 0 outputs the cell, the value 1 is subtracted from the value of the N counter Nc, Ntv and Nv not set to 0 in the class outputting the cell. When the ABR class and UBR class having the value of the W counter Wabr and Wubr larger than 0 outputs the cell, the value 1 is subtracted from the value of the W counter Wabr and Wubr not set to 0 in the class outputting the cell. When the CBR class, rt-VBR class and nrt-VBR class having the value of the N counter Nc, Ntv and Nv set to 0 outputs the cell, the value 1 is subtracted from the value of the S counter Sc, Stv and Sv not set to 0 in the class outputting the cell. When the ABR class and UBR class having the value of the W counter Wabr and Wubr set to 0 outputs the cell, the value 1 is subtracted from the value of the S counter Sabr and Subr not set to 0 in the class outputting the cell. By outputting cells according to the priority levels shown in Fig. 11, the following process is executed at a certain time interval Ts. First cells are output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv larger than 0 (Priority levels 1 to 3). Cells are output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0 and a waiting queue length Qc, Qtv and Qv larger than the threshold value Thc1, Thtv1 and Thv1, respectively (Priority levels 4 to 6). Cells are output from the band width demand class buffers CBR, rt-VBR, nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0 and a waiting queue length Qc, Qtv and Qv smaller than the threshold value Thc1, Thtv1 and Thv1, respectively (Priority level 7 to 9). The cells stored in the best effort transfer demand class buffers ABR and UBR for the ABR class and UBR class are output to the output channel 3 through the weighted rotational priority control (Priority level 10). The cells stored in the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having both values of the N counter Nc, Ntv and Nv and the S counter Sc, Stv and Sv set to 0 are output to the output channel 3 (Priority levels 11 to 13). In case the class having higher priority than those of classes outputting the cell according to the given priority level is generated, the higher priority class buffer preferentially outputs the cell. The highest priority is given to the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv larger than 0. The second highest priority is given to the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0 and each having a waiting queue length Qc, Qtv and Qv larger than the first threshold value Thc1, Thtv1 and Thv1, respectively. Then the third priority is given to the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0 and a waiting queue length Qc, Qtv and Qv smaller than the first threshold value Thc1, Thtv1 and Thv1, respectively. Therefore the communication quality of the CBR class, rt-VBR class and nrt-VBR class having a prolonged waiting queue length owing to abnormality generated in the traffic can be guaranteed to some extent without degrading the communication quality of other bandwidth demand classes. The priority levels shown in Fig. 12 can be used in place of those shown in Fig. 11 for preventing the higher priority class from giving an adverse effect on the lower priority class. Fig. 13 is a block diagram of a still further embodiment of the present invention. This embodiment uses a priority control process circuit 32 in place of the priority control process circuit 22 shown in Fig. 10. The priority control process circuit 32 outputs cells to the output channel 3 according to the priority level defined by each value of the N counter Nc, Ntv and Nv; W counter Wabr and Wubr; S counter Sc, Stv, Sv, Sabr and Subr; the CBR class, rt-VBR class, ABR class, UBR class; a first threshold value Thc1, Thtv1 and Thv1 for the CBR class, rt-VBR class and nrt-VBR class; and a second threshold value Thc2, Thtv2 and Thv2 for the CBR class, rt-VBR class and nrt-VBR class, respectively. Other functions are the same as those of the priority control process circuit 22 shown in Fig. 10. The threshold values Thc2, Thtv2 and Thv2 correspond to the waiting queue length Qc, Qtv and Qv for the CBR class, rt-VBR class and nrt-VBR class,respectively which are set in consideration of a traffic generated unexpectedly. Operations of this embodiment shown in Fig. 13 are explained. The call reception control circuit 11 and the addition means 4 execute the same processing as aforementioned. The priority control process circuit 32 outputs one cell according to the priority levels shown in Fig. 14. When the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv larger than 0 outputs the cells, the value 1 is subtracted from the value of the N counter Nc, Ntv and Nv not set to 0 in the class outputting the cell. When the ABR class and UBR class having a value of the W counter Wabr and Wubr larger than 0 outputs the cells, the value 1 is subtracted from the value of the W counter Wabr and Wubr not set to 0 in the class outputting the cell. When the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv set to 0 outputs the cells, the value 1 is subtracted from the value of the S counter Sc, Stv and Sv not set to 0 in the class outputting the cell. When the ABR class and UBR class having a value of the W counter Wabr and Wubr set to 0 outputs the cell, the value 1 is subtracted from the value of the S counter Sabr and Subr not set to 0 in the class outputting the cell. By outputting cells according to the priority levels shown in Fig. 14, the following process is executed at a certain time interval Ts. First cells are output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the N counter Nc, Ntv and Nv larger than 0 (Priority levels 1 to 3). Cells are output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0 and a waiting queue length Qc, Qtv and Qv larger than the threshold value Thc1, Thtv1 and Thv1, respectively (Priority levels 4 to 6). Cells are output from the bandwidth demand class buffers CBR, rt-VBR, nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv larger than 0 and a waiting queue length Qc, Qtv and Qv smaller than the threshold value Thc1, Thtv1 and Thv1, respectively (Priority levels 7 to 9). Cells are output from the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv set to 0 and a waiting queue length Qc, Qtv and Qv larger than the second threshold value Thc2, Thtv2 and Thv2, respectively (Priority levels 10 to 12). The cells stored in the best effort transfer demand class buffers ABR and UBR for the ABR class and the UBR class are output to the output channel 3 through the weighted rotational priority control (Priority level 13). The cells stored in the bandwidth demand class buffers CBR, rt-VBR and nrt-VBR for the CBR class, rt-VBR class and nrt-VBR class having both values of the N counter Nc, Ntv and Nv and the S counter Sc, Stv and Sv set to 0 are output to the output channel 3 (Priority levels 14 to 16). In case the class having a higher priority than those of classes outputting the cell according to the given priority level, the higher priority class buffer preferentially outputs the cell. The higher priority is given to the CBR class, rt-VBR class and nrt-VBR class having a value of the S counter Sc, Stv and Sv set to 0 and a waiting queue length Qc, Qtv and Qv larger than the second threshold value Thc2, Thtv2 and Thv2, respectively over the ABR class and the UBR class using the remained bandwidth. As a result, in case an unexpected traffic is generated in a certain bandwidth demand class to prolong the waiting queue length abnormally, the remaining bandwidth is available for the bandwidth demand class. Therefore communication quality of the bandwidth demand class having the unexpected traffic can be guaranteed to a certain extent without degrading the communication quality of other bandwidth demand classes. The priority levels shown in Fig. 15 can be used in place of those shown in Fig. 14 for preventing the higher priority class from giving an adverse effect on the lower priority class. The present invention is provided with a call reception control circuit for setting a guaranteed bandwidth represented by the number of cells in a certain time required to be output for assuring the communication quality of the bandwidth demand class in the N counter for the respective bandwidth demand classes at a certain time interval and a priority control process circuit for outputting cells stored in the bandwidth demand class buffers for the respective bandwidth demand classes according to the priority levels giving a priority to the bandwidth demand class having a value of the N counter not set to 0 over the bandwidth demand class having a value of the N counter set to 0 to the output channel. Even if an abnormal traffic is generated in a certain bandwidth demand class, other bandwidth demand classes are allowed to output the number of cells set in the N counter required for assuring the communication quality. In spite of the abnormal traffic generated in the certain bandwidth demand class, the present invention allows the rest of the bandwidth demand classes to keep the communication quality. The present invention is provided with a call reception control circuit for setting the guaranteed bandwidth in the N counter of the respective bandwidth demand classes as well as setting the value defined by the ratio of each W counter in the best effort classes to use the remained bandwidth and a priority control process circuit for outputting cells stored in the respective bandwidth demand class buffers and best effort transfer demand class buffers according to the priority levels giving a priority to the bandwidth demand class having a value of the N counter not set to 0 over the bandwidth demand classes and the best effort classes having a value of the N counter set to 0. In the ATM exchange system accommodating calls of the bandwidth demand classes and best effort classes, even if an abnormal traffic is generated in a certain bandwidth demand class, communication quality of the rest of the bandwidth demand classes can be protected from being deteriorated. The present invention is provided with a priority control process circuit for outputting cells stored in the bandwidth demand class buffers and best effort transfer demand class buffers according to the priority levels giving a higher priority to the bandwidth demand class having a value of the S counter containing the value of unused guaranteed bandwidth not set to 0 over the bandwidth demand class and best effort class having a value of the S counter set to 0. In case of a guaranteed bandwidth set in the N counter/which has been logically obtained, the communication quality of the bandwidth demand class can be kept. Therefore the number of calls for reception and the ratio of using the output channel for the bandwidth demand class can be increased. In the present invention, bandwidth demand classes at the same priority level are subjected to a weighted rotational priority control by using the guaranteed bandwidth of the respective bandwidth demand class as the weight, thus further improving the delay characteristic. The present invention outputs cells to the output channel according to the priority levels giving a priority to the bandwidth demand class having a value of the S counter not set to 0 and a number of cells stored in its buffer equal to or larger than the first threshold value over the band width demand classes and best effort classes having a value of the S counter not set to 0 and a number of cells stored in the bandwidth demand class buffer smaller than the first threshold value. Therefore an abnormal traffic generated in a certain band width demand class never gives an adverse effect on the other bandwidth demand classes, and the communication quality of the bandwidth demand class having the abnormality in the traffic is guaranteed. The present invention outputs cells stored in the respective bandwidth demand class buffers and the best effort transfer demand class buffers according to the priority levels giving a priority to the bandwidth demand class having a value of the S counter set to 0 and a number of cells stored in the bandwidth demand class buffer equal to or larger than the second threshold value over the best effort class. In case the waiting queue length is prolonged owing to unexpected traffic generated in a certain bandwidth demand class, the bandwidth demand class is allowed to use the remained bandwidth. As a result, the bandwidth demand class having abnormality in the traffic is able to keep its communication quality to a certain extent without giving an adverse effect on the communication quality of any other bandwidth demand classes.

Claims: An Asynchronous Transfer Mode ATM exchange system for receiving a plurality of calls each having a different required communication quality comprising: a buffer for providing each of a plurality of bandwidth demand classes that have been defined according to the required communication quality for storing call cells belonging to said respective bandwidth demand classes; a counter for providing each of said respective bandwidth demand classes; call reception control means (1) for setting a guaranteed bandwidth represented by a number of cells in a certain time required to be output to an output channel (3) for assuring each communication quality of said respective bandwidth demand classes at said certain time interval; and characterized by priority control means (2) for outputting cells stored in a buffer for a bandwidth demand class having a counter value not set to 0 to said output channel, subtracting 1 from a value set in a counter of a bandwidth demand class that has output cells at every output of cells to said output channel, and outputting cells stored in a buffer for a band width demand class having a counter value 0. The ATM exchange system of claim 1, wherein said priority control means (2) outputs cells stored in buffers to said output channel (3) according to a priority defined by said bandwidth demand classes. The ATM exchange system of claim 1 or 2, wherein said call reception control means (1) comprises means for determining a bandwidth demand class to which a generated call belongs. The ATM exchange system of claim 1, wherein said counter is a first counter and said buffer is a first buffer; said system further comprising: a second buffer for providing each of plurality of best effort classes that have been defined according to the required communication quality for best effort transfer for storing call cells belonging to respective best effort classes; a second counter for providing said respective best effort classes; and said call reception control means (1) is configured to set a guaranteed band width represented by a number of cells in a certain time required to be output to an output channel for assuring each communication quality of said band width demand classes in a first counter of said respective band width demand classes at said certain time interval and to set each value defined by a ratio of each of said respective best effort classes to use a remaining bandwidth resulting from taking a guaranteed bandwidth from a whole bandwidth of said output channel that is represented by said number of cells that can be output to said output channel within said certain time in said respective second counters when at least one of said second counters has a count value 0 and all best effort classes are in suspended status; and said priority control means (2) is configured to output cells stored either in said first buffer or said second buffer for a bandwidth demand class or a best effort class, respectively having a counter value not set to 0, to subtract 1 from a value of either a first counter or a second counter of a bandwidth demand class or a best effort class, respectively that has output cells at every output of cells to said output channel (3) and to output cells stored in said first buffer or said second buffer of a bandwidth demand class or a best effort class, respectively having a counter value 0 to said output channel. The ATM exchange system of claim 4, wherein said priority control means (2) outputs cells stored in a buffer for said bandwidth demand class with priority over cells stored in a buffer for said best effort class to said output channel (3) and, when outputting cells of said bandwidth demand classes, outputs cells stored in a buffer to said output channel (3) according to a priority defined by band width demand classes. The ATM exchange system of claim 4 or 5, wherein said suspended status represents that said second counter value is 0 or said second buffer has an empty area. The ATM exchange system of any of claims 1 to 6, wherein said call reception control means (1) comprises means for setting a guaranteed bandwidth wider than a logically obtained guaranteed bandwidth in said counter. The ATM exchange system of claim 1, wherein said buffer is a first buffer and said counter is a first counter; said system further comprising: a second buffer for providing each of plurality of best effort classes that have been defined according to the required communication quality for best effort transfer for storing call cells belonging to said respective best effort classes; a second counter for providing each of said respective best effort classes; a third counter for providing each of said bandwidth demand classes and said best effort classes; and addition means (4) for setting a value obtained by adding a count value of said first counter to a count value of a third counter corresponding to a bandwidth demand class of said first counter in said third counter before setting a guaranteed bandwidth in a first counter of a bandwidth demand class, and for setting a value obtained by adding a count value of said second counter to a count value of a third counter corresponding to a best effort class of said second counter in said third counter before setting a value in a second counter for each of said respective best effort classes; said call reception control means (11) is configured to set a guaranteed band width represented by a number of cells in a certain time required to be output to an output channel for assuring each communication quality of said respective band width demand classes in a first counter of each of said respective band width demand classes at said certain time interval and to set each value defined by a ratio of each of said respective best effort classes to use a remaining bandwidth resulting from taking a guaranteed bandwidth from a whole bandwidth of said output channel that is represented by said number of cells that can be output to said output channel within said certain time in said respective second counters when at least one of said second counters has a count value 0 and all best effort classes are in suspended status; and said priority control means (12) is configured to output cells stored in said first buffer of a bandwidth demand class having a value of said first counter not set to 0 to said output channel (3), to subtract 1 from a value of a first counter of a bandwidth demand class that has output cells at every output of cells to said output channel, to output cells stored in said first buffer of a bandwidth demand class having a value of said first counter set to 0 and a value of said third counter not set to 0 to said output channel (3), to subtract 1 from a value of a third counter of a bandwidth demand class that has output cells at every output of cells to said output channel (3), to output cells stored in said second buffer for a best effort class having a value obtained by adding a value of said second counter to a value of said third counter not set to 0 to said output channel, to subtract 1 from a value of either a second counter or a third counter of a best effort class that has output cells at every output of cells to said output channel, and finally to output cells stored in said first buffer for a bandwidth demand class having both values of said first counter and third counter set to 0 to said output channel. An ATM exchange system of claim 1, wherein said buffer is a first buffer and said counter is a first counter; said system further comprising a second buffer for providing each of plurality of best effort classes that have been defined according to the required communication quality for best effort transfer for storing call cells belonging to said respective best effort classes; a second counter for providing each of said respective best effort classes; a third counter for providing each of said band width demand classes and said best effort classes; and addition means (4) for setting a value obtained by adding a count value of said first counter to a count value of a third counter corresponding to a bandwidth demand class of said first counter in said third counter before setting a guaranteed bandwidth in a first counter of a bandwidth demand class and for setting a value obtained by adding a count value of said second counter to a count value of a third counter corresponding to a best effort class of said second counter in said third counter before setting a value in a second counter for each of said respective best effort classes; said call reception control means (21) is configured to set a guaranteed bandwidth represented by a number of cells in a certain time required to be output to an output channel for assuring each communication quality of said respective bandwidth demand classes in a first counter of each of said respective bandwidth demand classes at said certain time interval and to set each value defined by a ratio of each of said respective best effort classes to use a remaining bandwidth resulting from taking a guaranteed band width from a whole band width of said output channel that is represented by said number of cells that can be output to said output channel within said certain time in said respective second counters when at least one of said second counters has a count value 0 and all best effort classes are in suspended status; and said priority control means (22) is configured to output cells stored in said first buffer for a band width demand class having a value of said first counter not set to 0 to said output channel (3), to subtract 1 from a value of a first counter of a bandwidth demand class that has output cells at every output of cells to said output channel (3), to output cells stored in said first buffer for a bandwidth demand class having a value of said first counter set to 0, a value of said third counter not set to 0 and a number of cells stored in said first buffer equal to or larger than a first threshold value set with respect to said bandwidth demand class, to subtract 1 from a value of a third counter of a bandwidth demand class that has output cells at every output of cells to said output channel (3), to output cells stored in said first buffer of a bandwidth demand class having a value of said first counter set to 0, a value of said third counter not set to 0, and a number of cells stored in a first buffer smaller than a first threshold value set with respect to said bandwidth demand class, to subtract 1 from a value of a third counter of a bandwidth demand class that has output cells at every output of cells to said output channel (3), then to output cells stored in said second buffer for a best effort class having a value obtained by adding a value of said second counter to a value of said third counter not set to 0 to said output channel (3) and to subtract 1 from a value either of a second counter or a third counter of a best effort class that has output cells at every output of cells to said output channel (3), and finally to output cells stored in said first buffer for a bandwidth demand class having both values of said first counter and said third counter set to 0. The ATM exchange system of claim 9, wherein cells of said bandwidth demand classes stored in buffers are output to said output channel (3) according to a priority defined by bandwidth demand classes. The ATM exchange system of claim 9 or 10, wherein said suspended status represents that said second counter value is 0 or said second buffer has an empty area. The ATM exchange system of any of claims 4 to 11, wherein said call reception control means (21) comprises means for determining a class to which a generated call belongs. The ATM exchange system of any of claims 1 to 12, wherein said call reception control means (21) comprises means for rejecting a call when receiving said call is expected to exhaust a remaining bandwidth. The ATM exchange system of any of claims 8 to 13, wherein said call reception control means (21) comprises means for setting a logically obtained guaranteed bandwidth in said counter. The ATM exchange system of any of claims 4 to 14, wherein said priority control means (22) comprises means for weighted rotational priority controlling bandwidth demand classes having the same priority using a guaranteed bandwidth for each of said respective bandwidth demand classes as a weight. The ATM exchange system of any of claims 9 to 15, wherein said priority control means (22) comprises means for outputting cells stored in a first buffer for a bandwidth demand class having a third counter value set to 0 and a number of cells larger than a second threshold value larger than said first threshold value that has been set with respect to a bandwidth demand class next to a bandwidth demand class having said third counter value not set to 0. A priority control method for an ATM exchange system for receiving a plurality of calls each having a different required communication quality comprising steps of: setting a plurality of bandwidth demand classes according to the required communication quality; a first guaranteed bandwidth setting step for setting a guaranteed bandwidth represented by a number of cells in a certain time required to be output to an output channel for assuring communication quality of said respective bandwidth demand classes in a counter of each of said respective bandwidth demand classes at said certain time interval; determining a bandwidth demand class to which a call belongs and storing call cells in a buffer of a corresponding bandwidth demand class; characterized by an output step for outputting cells stored in a buffer for a band width demand class having a counter value not set to 0 with priority over cells stored in a buffer for a bandwidth demand class having a counter value set to 0 to said output channel; and a step of subtracting 1 from a value of a counter of a bandwidth demand class that has output cells at every output of said cells to said output channel. The priority control method of claim 17, wherein said output step outputs cells stored in buffers to said output channel according to a priority defined by bandwidth demand classes in the presence of a bandwidth demand class having values of two or more counters not set to 0. The priority control method of claim 17 or 18, wherein said step for setting a guaranteed bandwidth sets a guaranteed bandwidth wider than a logically obtained guaranteed bandwidth in said counter. The priority control method of any of claims 17 to 19 further comprising steps of: setting a plurality of best effort classes according to the required communication quality for best effort transfer; dividing a remaining bandwidth resulting from subtraction of a guaranteed bandwidth which has been set at said first guaranteed bandwidth setting step from a whole bandwidth of said output channel represented by a number of cells that can be output to said output channel in a certain time according to a ratio of each of said best effort classes to use said remaining bandwidth, and setting resultant values in a counter for each of said best effort classes when at least one of counters for said best effort classes has a count value 0 and all best effort classes are in suspended status; outputting cells stored in a buffer for either a bandwidth demand class or a best effort class having a counter value not set to 0 with priority over cells stored in a buffer for either a bandwidth demand class or a best effort class having a counter value 0; and subtracting 1 from a counter value of either a bandwidth demand class or a best effort class that has output cells at every output of cells to said output channel. The priority control method of claim 20 further comprising steps of: providing a second counter for each of said bandwidth demand classes and said best effort classes, respectively; setting a value obtained by adding a count value of said counter to a count value of a second counter corresponding to a bandwidth demand class of said counter in said second counter before setting a guaranteed bandwidth in a counter of each of bandwidth demand classes; setting a value obtained by adding a count value of said counter to a count value of a second counter corresponding to a best effort class of said counter in said second counter before setting a value in a counter of each of best effort classes; outputting cells stored in a buffer for a bandwidth demand class having a counter value not set to 0 to said output channel and subtracting 1 from a counter value of a bandwidth demand class that has output cells at every output of cells to said output channel; outputting cells stored in a buffer for a bandwidth demand class having a counter value 0 and a second counter value not set to 0 to said output channel and subtracting 1 from a value of a second counter of a bandwidth demand class that has output cells at every output of cells to said output channel; outputting cells stored in a buffer for a best effort class having a value obtained by adding a counter value to a second counter value not set to 0 to said output channel and subtracting 1 from a counter value of a best effort class that has output cells at every output of cells to said output channel; and outputting cells stored in a buffer for a bandwidth demand class having both counter values 0 to said output channel. The priority control method of claim 21 further comprising steps of: setting a first threshold value in a buffer for a bandwidth demand class; and giving a priority to a band width demand class having a number of cells stored in a buffer for said bandwidth demand class larger than said first threshold value over a bandwidth demand class having a number of cells stored in a buffer for said bandwidth demand class smaller than said first threshold value. The priority control method of claim 22 further comprising steps of: setting a second threshold value larger than said first threshold value in a buffer for a bandwidth demand class; and giving a priority to a bandwidth demand class having a number of cells stored in a buffer for said bandwidth demand class larger than said second threshold value over a bandwidth demand class having a number of cells stored in a buffer for said bandwidth demand class smaller than said second threshold value.

Inventors: FAN RUIXUE, SHINOHARA MASAYUKI, FAN, RUIXUE, SHINOHARA, MASAYUKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: H04L12
IPC codes: H04Q3, H04L12