Abstract:
A packet switching system arbitrates between Virtual Output Queues (VoQ) in plural input buffers, so as to grant the right of transmitting data to a crossbar switch to some of the VoQs by taking both an output data interval of a VoQ and the queue length of a VoQ as parameters. The system suppresses the delay time of the segment of a VoQ having a high load, thereby preventing buffers from overflowing; and, also, the system permits a VoQ having a low load to transmit segments under no influence of the VoQ that has a high load and is just reading out the segment.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a packet switching system having an input buffer and an output buffer (referred to as an input/output buffer packet switching system, hereinafter), particularly to a packet switching system adopting an arbiter system. 
   A conventional input/output buffer packet switching system, which has a First-In First-Out (FIFO) memory for each input line, has a disadvantage in that, if plural packets inputted from plural input lines converge at a predetermined output path, “Head Of Line (HOL) blocking” is caused, in which there is only 58.6% throughput of data transfer. To avoid HOL blocking, there is provided a well known method in which a Virtual output Queue (VoQ) is provided for each output path at an input buffer. 
   The input/output buffer packet switching system associated with a crossbar switch, because a crossbar switch has no buffer, adopts a way to arbitrate between the VoQs of input buffers, so as to prevent data on the crossbar switch from being converged. The arbitration is performed for the purpose of selecting a combination of an input port and an output port to which the right of transmitting data (grant) to some of the VoQs is given. Accordingly, higher throughput of data transfer of the switching system depends on efficient arbitration. 
   There are two ways to effect arbitration for selecting a combination of an input port and an output port: one, where arbitration is performed using the unit of a fixed-length internal packet into which variable length packets inputted to the switch have been divided; and another, where, as indicated in USP Ser. No. 09/362,134, arbitration is performed using the unit of a fixed-length container into which plural variable length packets are packed. In this way, two units are used in the switching processing: one unit in the form of an internal packet having a small fixed length and another unit in the form of a container having a large fixed length. 
   As regards a unit used in the processing carried out in the switching system of the invention, the volume of data per arbitration is defined as one segment. Note that hereafter the term “segment” is also used as a generic term to indicate data to be treated inside the switch, data such as an internal packet, a packet, and a cell. 
   As conventional arbitration, the following three methods have been proposed:
         First, there is a method of selecting a sending queue on a Round Robin basis by taking into consideration whether a segment of the VoQ exists as a parameter, as disclosed in “A Study of structuring a Large Capacity Packet Switching Systems,” Koji WAKAYAMA, et al., SHINGAKUGIHOU SSE98-160, and also disclosed in JP-A-2000-78148;   Second, there is a method of selecting sending queues by taking the waiting time of the segment in the VoQ as a parameter, as disclosed in “A Study of Scheduling an Input Buffer Switch and Trial manufacture thereof,” Toshiyuki SUDO, et al., SHINGAKUGIHOU SSE99-118; and   Third, there is a method of selecting an output data queue by taking the length of the VoQ as a parameter, as disclosed in “A Proposal of Balanced Packet Scheduling Algorithm and Performance Evaluation,” SHINGAKUGIHOU SSE96-56.       

   Each of the three methods has a problem that is caused when an unbalanced load is applied to the switch. Referring to  FIG. 16 , which is a conceptual view of an input port  30  of a 4×4 switch, an example of the problems will be explained. In the figure, reference numbers  31 - 1 ,  31 - 2 ,  31 - 3 , and  31 - 4  denote VoQs, each being directed along its output path; a quadrangle in each VoQ represents a segment. The VoQ  31 - 1  has traffic which represents a higher load than the other queues; and the VoQ  31 - 4  has traffic H which represents a lower load than other queues. 
   In the first method of selecting a sending queue on a Round Robin basis by taking into consideration whether segment of a VoQ exists as a parameter, if unbalanced loads are applied to the switch, the Round Robin approach that equitably reads out segments from all VoQs permits the VoQ having a low load to transmit segments without regard to the VoQ that has a high load and is just reading out a segment. However, the queue of a VoQ having a high load is liable to be long, causing its delay time in segment transmission inside the switch to be longer than those of the other VoQs. As shown in  FIG. 16 , this brings about an overflow of segments at VoQ  31 - 1  having a high load, and might result in a segment  32 A being abandoned. 
   In the second method of selecting sending queues by taking the waiting time of a segment in the VoQ as a parameter, in the same way as the first method, the queue of a VoQ having a high load is liable to be long This also causes the waiting time of the segment in a VoQ having a low load to be long, since the method that takes the waiting time of the segment in VoQ as a parameter transmits a segment having a long waiting time in a priority manner. In addition, management of the waiting time of all segments needs a lot of counters, so the method is not practicable. 
   Therefore, in most cases, the second method counts the waiting time from when the segment has arrived at the top of the queue. Specifically, it adds 1 to the counter of the VoQ when the segment in the VoQ is not transmitted during one arbitration, while it resets the counter when the segment is transmitted during one arbitration. This treats both a VoQ having a high load and a VoQ having a low load in the same way, because the counter resets once the segments of the VoQ which even has a high load are transmitted. Thus, the delay time of the VoQ to which a high load is given under the unbalanced load condition finally becomes large, and thus this might eliminate the overflow segments of the buffer. 
   In the third method of selecting sending queues by taking the length of the queue as a parameter, the segment can be effectively read out from a VoQ having a high load under above-said condition where an unbalanced load is applied to the switch. Thus, the delay time of the segment of a VoQ having a high load becomes small, also preventing a buffer from overflowing The method, however, suffers from a phenomenon called starvation, in which the grant of a transmission right is not given to a segment  32 B of VoQ  31 - 4  that has a low load. 
   Accordingly, the method might result in a deterioration of the quality of data in the packets, especially in both packets of voice data, which are required never to be late in data transfer, and packets of important data, which are required never to be abandoned in data transfer, both for keeping good quality in data. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a packet switching system that arbitrates between VoQs to select a combination of an input port and an output port, and thereby grant the right to transmit data to some of the VoQs by taking both an interval in sending a segment from a VoQ and the queue length of the VoQ as parameters. 
   According to one aspect of the invention we provide the packet switching system having: a queue length manager for managing the volume of segments queued in each VoQ per input line; an output data interval manager for managing an output data interval of the segment of each VoQ; and an arbiter-request (ARB-REQ) generator for allocating a level of transmission to the VoQs according to information received from the queue length manager and the output data interval manager, wherein arbitration is performed on the level assigned each VoQ so as to determine which VoQs will be allowed to send. 
   According to another aspect of the invention, we provide a packet switching system having: means for putting segment transfer interval prior to queue length in arbitration so as to determine a VoQ level; and means for putting queue length prior to segment transfer interval in arbitration so as to determine a VoQ level; 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing one embodiment of a packet switching system of the present invention. 
       FIG. 2  is a block diagram of the structure of an ARB-REQ generator  13  and a VoQ controller  12  as used in the system of  FIG. 1 . 
       FIG. 3  is a diagram showing one example of a level assignment matrix for assigning a level to a VoQ by the ARB-REQ generator of  FIG. 13 . 
       FIG. 4  is a diagram showing one example (prioritizing output data interval) of the level assignment matrix for assigning level to a VoQ by the ARB-REQ generator of  FIG. 1 . 
       FIG. 5  is a diagram showing one example (prioritizing queue length) the level assigning matrix for assigning the level to by the ARB-REQ generator of  FIG. 1 . 
       FIG. 6  is a block diagram of an example of the structure of an arbiter  14  as used in the system of  FIG. 1 . 
       FIG. 7  is a diagram illustrating a concept of a VoQ level matrix of the present invention. 
       FIG. 8  is a diagram illustrating concept of a tournament of the present invention. 
       FIG. 9  is a table showing a combination of win and defeat results of the tournament for every input and every output in accordance with the present invention. 
       FIG. 10  is a level reassignment table according to the present invention. 
       FIG. 11  is a flowchart illustrating an algorithm which the arbiter  14  of  FIG. 1 . performs. 
       FIG. 12  is a diagram which illustrates processing of the arbiter  14  of  FIG. 1 . 
       FIG. 13  is a block diagram of the overall structure of another embodiment of a packet switching system of the present invention. 
       FIG. 14  is a graph indicating 99% delay of the conventional arbiter compared to an arbiter of the present invention. 
       FIG. 15  is a graph of indicating queue length distribution of the conventional arbiter and the arbiter of the present invention when an unbalanced load is given. 
       FIG. 16  is a diagram illustrating a concept of overflow of a VoQ to which high load traffic is applied and starvation of a VoQ to which low load traffic is applied. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates one embodiment of a packet switching system of the present invention. In the system, ARB-REQ information is transmitted to an arbiter through a separate line  18 , which is different from a data line for connecting a VoQ  11  and the Crossbar Switch  19 . 
   An input line processor  16 - i  (i=1 to n) extracts address information of a packet by analyzing a header of a packet which is input from an input line  103 - i.  An input buffer  10 - i  includes n VoQ 11 - i  in correspondence to an output port. An input processor  16 - i  gives address information which has been extracted to a VoQ controller  12 - i . The VoQ controller  12 - i  gives an indication to an input buffer  10 - i  to write the packet in a VoQ corresponding to the output according to the address information. In this way, the packet is written in VoQ  11 - i  which has been designated. 
   The VoQ controller  12 - i  manages, per VoQ  11 - i , information between the volume of queuing segments and a waiting time (that is to say, an output data interval of each segment) of the segment at the top of a queue buffer An ARB-REQ generator  13 - i  assigns each level to each VoQ according to such information. 
   Each VoQ level is collected to an arbiter  14  by a signal line  18  during one arbitration period. It is determined by the arbiter  14  to which VoQ to give the grant according to the received information. 
   Grant information is transmitted to each of the VoQ controllers  12 - i  (i=1, . . . , n) as ARB-ACK by a signal line  15 , and, at the same time, the arbiter reflects the results of its arbitration to the structure of a path inside the crossbar switch. The VoQ controller  12 - i  informs the input buffer  10  of which VoQ should send a segment according to its ARB-ACK information. 
   The segment transmitted from the input buffer  10 - i  is switched by crossbar switch  19  and then transmitted to an output line processor  17 - i.  The output line processor  17 - i  restructures the packet from the segment which has been received from the crossbar switch  19  and then sends it to an output line  104 - i.    
   With reference to  FIG. 2 , the structures of the VoQ controller  12  and the ARB-REQ generator  13  will be described in detail. In this regard,  FIG. 2  shows one example of a 4×4 switch for simple explanation. 
   The header of the packet, which has been processed in the header analyzer  401  inside the input processor  16 , is transmitted to a write address (WA) generator  403  in the VoQ controller  12 , and the WA generator  403  determines to which VoQ 11  to write an input packet. The WA generator  403  sends memory address information to the input buffer  10  by way of a WA control signal line  412 , and gives an indication of writing the packet to the VoQ  11  corresponding to a destination. At the same time, the WA generator  403  transmits information of the packet which has been written in the input buffer  10  to a queue length manager  405  and an output data interval manager  406 . The queue length manager  405  has a queue length counter  410  corresponding to each of the VoQs inside the input buffer  10 . 
     FIG. 2  illustrates the case of a 4×4 switch as an example. Since four VoQs exist in the input buffer  10 , the queue length manager has four queue length counters  410 . The queue length manager  405  increases the length of the segment of the input packet to the numeric value of the queue length counter  410  for the current length of the queue. The output data interval manager  406  has an output data interval counter  411  corresponding to each of one or more VoQs inside the input buffer  10 . The output data interval manager  405  does nothing to the VoQ in which the packet has been input in the case where the segment has already existed. If the segment has not existed, the output data interval manager  406  gives an indication to the output data interval counter  411  corresponding to the VoQ so as to add 1 to the numeric value per arbitration period, and manages the output data interval time. In other words, the numerical value which the output data interval counter  411  shows indicates how long the segment has not been transmitted from corresponding VoQs. 
   A read address (RA) generator  404 , according to ARB-ACK information transmitted from the signal line  15 , transmits to the input buffer, through a signal line  413 , information indicating from which VoQ the segment is to be sent. At the same time, the read address (BA) generator  404  transmits information of the segment which is read out from the input buffer  10  to the queue length manager  405  and the output data interval manager  406  as well. The queue length manager  405  decreases the queue length counter  410  corresponding to the VoQ which has transmitted out a segment to the crossbar switch  19 . Further, the output data interval manager  406  resets the value of the output data interval counter  411  corresponding to that VoQ. 
   Information of the queue length manager  405  and of the output data interval manager  406  is transmitted to the ARB-REQ generator  13  by way of a signal line  414 . The ARB-REQ generator  13  has an ARB-REQ generating part  409  corresponding to each queue inside the input buffer  10 . Respective ARB-REQ generating parts  409 - 1  to  409 - 4  assign some level to the corresponding queue according to information received from the ARB-REQ generator  13 . When the level is assigned to the queue, a VoQ level assignment matrix  416  is referred to. For the VoQ level assignment matrix  416 , it is possible for a user to tune the arbiter in accordance with the characteristics of the traffics which are input to the node thereof. The level of each VoQ which has been created in the ARB-REQ generator is transmitted to the arbiter  14  by way of the signal line  18 . 
     FIG. 3  shows one embodiment of the VoQ level assignment matrix  416 . The level assignment matrix has a segment transfer interval  71  along the horizontal axis and the number of segments queued in VoQ along the vertical axis  72 . The longer an output data interval time is and the more the number of segments queued in the VoQ is, the bigger the level assigned to the VoQ is. The level assignment matrix is calculated from a queue length (the number of segment in VoQ) and the segment transfer interval. 
   By assigning the level to the queue in this way, it is possible to send, within an arbitrarily set delay time, the packet which has entered into the switch. 
   The time for sending the packet from the queue of a VoQ having a high load is longer than that of a VoQ having a low load Thus, a high level is assigned to a VoQ in which the number of segments stored in the queue is big, whereby transmission grant is given thereto in a priority manner. In other words, this level indicates information of the degree of priority in obtaining grant. 
   Since the packet is transmitted within a delay time which is set arbitrarily by way of a VoQ having a low load, it functions to increase the VoQ level if the transmitting interval becomes long. 
   The VoQ level L is obtained from the following expression. 
           L   =       1     ln   ⁡     (         M   -     a   ·   t         b   ·   s       ×   ⅇ     )         ×   15           
where, M: time out, t: output segment interval, s: the number of segments at the present time, a: output data coefficient, b: queue length coefficient.
 
   When (M−at)/b·s≦1 is attained, the VoQ level attains the maximum value. When the VoQ level has reached the maximum value, it is indicated that the VoQ thereof has reached a condition having a high possibility of obtaining a grant. 
   M is a value which is defined from both a delay time that can arbitrarily be set by the switch and one arbitration time. M is also a value for deciding both the maximum value of segment transfer interval  71  in the level assignment matrix and the maximum value of the number of segments  72  queued in a VoQ. Where T is the delay time which is required by the switch and is determined arbitrarily, ta is a time for one arbitration time, n is the number of input ports of the switch. M can be obtained by the following expression. 
   
     
       
         
           M 
           = 
           
             
               T 
               
                 t 
                 a 
               
             
             - 
             n 
           
         
       
     
   
   T/ta can define the number of times arbitration is performed during the delay time T, which can be defined arbitrarily. On the assumption that the levels of all VoQs attain the maximum values at the same time, until the grant is given, a VoQ, to which the time for maximum n arbitration will be waited, appears. Therefore, in order to transmit the segment within the delay time which is arbitrarily defined, even if a VoQ has only one segment, a VOQ level takes the maximum value, when the output data interval becomes T/ta−n. 
   As apparent from the level assignment matrix shown in  FIG. 3 , since the VoQ level becomes high as the output data interval becomes large in a VoQ having only one segment, it is not necessarily concluded that the segments are not transmitted until the maximum delay time is requested. 
   The level assignment matrix in  FIG. 3  is limited to time out M=20, a=1, and b=1. By changing an output data interval coefficient a and a queue length coefficient b, it is possible for them to be changed to arbitration which regards the output data interval as important and arbitration which regards the queue length as important. 
   When the packet switching system of the present invention is employed in a place where, for example, a lot of voice data is processed which is required never to be late in data transfer, the setting is changed so as to suppress the delay time as much as possible where such data are queued in a VoQ having low load traffic. 
   More specifically, setting the value of the output data interval coefficient a to 1 or more permits the VoQ in which the segments are not yet filled to get a large level within a short output data interval.  FIG. 4  shows the condition of the VoQ level assignment matrix at the time of defining the value of the output data interval coefficient a as 2. When the level of the matrix in  FIG. 4  is compared with the level assignment matrixof  FIG. 3 , the VoQ level already becomes large when the output data interval of the segment is small. Thus, it is also possible for a VoQ having a low load to transmit the segment in a short delay time. 
   On the contrary, when the packet switching system of the present invention is employed in a place where a lot of data is processed which is required never to be abandoned in data transfer, though the delay time of the VoQ having a low load traffic lengthens slightly, it is preferable to suppress an overflow of a buffer by outputting the segments from a VoQ having high load traffic in a priority manner. In such a case, by defining the value of the queue length coefficient b as a value greater than 1, it is permitted to assign the VoQ level which acts in response sensitively to a change of the length of the queue. When a high load is applied, the length of the queue becomes long.  FIG. 5  exemplifies the condition of the level assignment matrix at the time of assigning the queue length coefficient as 2. When the level assignment matrix is compared with the level assignment matrix of  FIG. 3 , large levels are found in places in which the number of the segments of VoQ is small. Therefore, for VoQ in which the length of the queue becomes longer, it is possible to prevent a VoQ, in which the length of the queue becomes longer, from buffer overflowing by making it easy to give a grant by assigning a larger level as soon as possible. 
   Further, where the output data interval is not considered at all and it is desired that arbitration is performed using only the length of the queue, it becomes possible by defining the value of the output data interval coefficient a as 0. 
   All of the VoQ levels calculated by a numeric expression 1 are rounded off and they are expressed in the level assignment matrix as integers. Further, when M−a·t&lt;b·s or M≦a·t is attained, they become values other than 0&lt;L≦15. However, when such a situation occurs, since it is expressed that the VoQ level already exceeds the maximum level 15, the level 15 is given to a V0Q to which the values other than 0&lt;L≦15 are given by this expression. 
   Information of the level per VoQ is collected to arbiter  14  from each ARB-REQ generator. 
     FIG. 6  is a block diagram of an embodiment of the arbiter  14 . In all VoQ level collectors  121 , information of the level of all VoQs is collected. 
     FIG. 7  illustrates a condition for all VoQs to which requests have been made as of arbitrary points of time. The columns of the matrix express an output line number  131 , and the rows thereof express an input line number  132 . For example, in the case where an input line number is 1 and an output line number is 1, the level, which is assigned to VoQ  11 - 1 - 1  of an input buffer  10 - 1  of  FIG. 1 , is stored. Further, a “O”  133  of the matrix has the same VoQ level and a level smaller than “O” is assigned to an empty portion. 
   In this example, giving grant to a VoQ having an output line number  4  in an input line number  2  and a VoQ having an output line number  2  in an input line number  4  obtain the best combination of inputs and outputs. In order to give grant to a VoQ efficiently all the time, a tournament for each of the inputs and a tournament for each of the outputs are performed. 
     FIG. 8  shows a concept of tournament processing. In  FIG. 8 , numerals represent the level of a VoQ which is a member of the same input line number or output line number. In  FIG. 8 , where there are two VoQs having the same levels, it is made not preferable for either one to win, but for all VoQs having the same levels to win. This is to give grant efficiently as shown in  FIG. 7 . 
   This tournament processing is performed for each of the input line directions and for each of the output line directions in every input line tournament processor  122 - 1  and every output line tournament processor  122 - 2 , and then a VoQ is selected, which has the highest VoQ level (the request for transmitting is the highest among their line numbers) among them. 
   As a result of the tournament, a win/defeat combination  141  in  FIG. 9  is capable of being considered for an input line direction and an output line direction. The level reassignment part  124  evaluates information of each VoQ level by reducing to four levels, 0 to 3, according to a level reassignment table  61 , an example of which is shown in  FIG. 10 . 
   VoQs which have been reevaluated into four levels, 0 to 3, are picked up sequentially from VoQs of the level 3 in a selector  125  of a VoQ having the same level. Grant is given to a VoQ which has been picked up herein by a Round Robin selection in a grant assignment part  126 . 
   Since it is not possible to give grant in the same arbitration period from both a VoQ to which grant is given and a VoQ which is a member of the same input line number or the same output line number, grant is taken away in a grant deprival part  127 . 
   Information of a VoQ, the grant of which has been deprived, is communicated to all VoQ level collector  121  by way of a signal line  123 . From this information, the tournament is performed once more among VoQs, the grant of which has not been deprived at all, and then the levels thereof are reevaluated. Then, in the same way as the aforementioned processing, a VoQ having level 3 is picked up by way of a same level VoQ selector  125 , and then grant is given to the V0Q by a grant assignment part through the Round Robin selection. By repeating such a repetitive operation, it is possible to create the best combination of input line and output line. 
   Since it is not possible to give grant at the same time in the same arbitration period from a VoQs that are members of the same input line number or the same output line number as VoQs having grant in the aforementioned process, the grant deprival part  127  deprives grant from the VoQs. Information of a VoQ, the grant of which has been deprived, is communicated to the same level VoQ selector  125  by way of a signal line  129 . The same level V0Q selector  125  picks up a VoQ having level 2 still having a grant and then gives a grant by the grant assignment part through the Round Robin selection. 
   Then, by way of the same process as giving a grant to a VoQ having a level 2, grant is given to a VoQ having a level 1. Grant is also given to a VoQ with the level 0 having a grant to be transmitted next. 
   Grant information is changed to ARB-ACK information by an ARB-ACK generator  128 , and then it is transmitted to the BA generator  404  of the VoQ controller  12  by way of the signal line  15 . The BA generator  404  transmits the segment-transmitting signal  413  to the VoQ, according to the ARB-ACK information. At the same time, VoQ information for sending the segment is communicated to the queue length manager  405  and the output data interval manager  406 . The queue length manager  405  decreases the number of transmitted segments from the value of the queue length counter  410 , which manages the number of segments of the VoQ to which the grant has been given. In the output data interval manager  406 , the value of the sending out interval counter  411 , which manages the output data interval of the VoQ to which the grant has been given, is reset. 
     FIG. 11  is a flowchart of a sequential process performed by the aforementioned arbiter  14 . A tournament is performed for the VoQ levels which have been collected from each of the input buffers with respect to the input and the output (S 81 ). The VoQ levels thereof are reevaluated using the levels of 3 to 0 in order of highly requested output data for each VoQ (S 82 - 1  to - 4 ). First of all, VoQs of the level 3 are picked up by way of the same level VoQ selector  125  (S 83 ). Grant is given to them through Round Robin selection (S 83 - 1 ) (it may be considered to adopt 2DRR (Namoru TAKAHASHI, et al., “Improvement of Packet-Priority-considered-Packet-Switch having Input Queue corresponding to each Output Port,” SHINGAKUGIHOU SSE97-13) and the like, which has a pointer in order to maintain the state of being equal as this Round Robin selection). Since it is not possible to transmit the segment at the same time in the same arbitration period from the VoQ for the same input and the same output as the VoQ to which a grant has been given, a grant of the VoQ is deprived (S 83 - 2 ). 
   In the present embodiment, in order to improve the efficiency of the arbiter, not to give a grant to VoQ having level 2, but the level of VoQ, to which grant is not given, is back to the level of 0 to 15 before reevaluating, the tournament is performed once more, and then reevaluation of the level (S 84 ) is performed. The more this process is repeated, the more the combination of the queues selected by arbitration are closed to the most suitable one. 
   As a result of reevaluating, a grant is given to a VoQ having level 3 (S 83 - 1 ). Grant of a V0Q, which has the same combination of input lines and output lines as the V0Q obtaining a grant, is cancelled (S 83 - 2 ). 
   Subsequently, a VoQ having the level 2 is picked up (S 85 ), grant is given (S 85 - 1 ), and then the grant of a VoQ which has the same combination of input lines and output lines as the V0Q to which grant has been given is canceled (S 85 - 2 ). 
   Subsequently, a VoQ having a level 1 is picked up (S 86 ), grant is given (S 86 - 1 ), and then grant of VoQ which has the same combination of input lines and output lines as the VoQ to which grant has been given is canceled (S 86 - 2 ). 
   At last, if there is a VoQ having a grant at the level 0, grant is given by way of the Round Robin selection (S 87 ). In this way, the process is ended to give a grant to the combination of all of the inputs and the outputs, and then the process of arbitration is finished. 
     FIG. 12  shows the results of processing in accordance with the flowchart of  FIG. 11 . This figure indicates arbitration of the 4×4 switch. It is possible to express the VoQ level, which have been collected in all VoQ levels collectors  121 , visually in way of a matrix  21 . The rows of the matrix indicate the input line number, and the columns indicate the output line number. This matrix shows that, for example, in the matrix  21 , the VoQ level for the input line number  3  and the output line number  1  is 10. 
   The tournament of  FIG. 8  is performed for each of the input lines and the output lines, and, thereafter, the aforementioned reevaluation is performed in  FIG. 10 . The matrix  22  indicates the results thereof. 
   The matrix  23  shows that a VoQ having level 3 has been selected from the matrix  22  and grant has been given thereto by way of the Round Robin selection. Since grant cannot be given to a VoQ which has the same input line number or output line number as a VoQ to which grant has been given at the same arbitration period, grant is deprived therefrom. The symbol “x” of the matrix  24  indicates that grant has been deprived. 
   Next, The level of a VoQ to which grant has not yet been given is back to the level of a VoQ which has been created in the ARB-REQ generator. A matrix  25  indicates the matrix which has already been converted. Once more, the tournament and reevaluation of the levels are performed. 
   A matrix  26  indicates the result thereof. In the matrix  26 , grant is given to VoQ having level 3. At this point, grant is given to VoQ having the input line number  2  and the output input number  4 . 
   A matrix  27  indicates that grant has been given thereto. Grant is deprived from a VoQ which has the same input line number and the output line number as VoQ to which a grant has been given. 
   A matrix  28  indicates that a grant has been deprived therefrom. Next, grant is given to a VoQ having level 2. 
   A matrix  29  indicates that grant has been given thereto. 
   In the case explained with reference to  FIG. 6 , since grant has been given to all of the combinations of inputs and outputs in accordance with the aforementioned processes, the series of arbitrations has terminated. For the case other than that, there may be a case where a process of giving a grant to a VoQ having a level 1 and a level 0 is required. In such a case, grant is given in accordance with the flowchart of  FIG. 11 . 
     FIG. 13  shows another embodiment of the packet switching system of the present invention. In this embodiment, by giving ARB-REQ information to a header portion of the segment without using another line, it is transmitted to an arbiter via an in-channel. 
   The differences between this embodiment and the packet switching system of  FIG. 1  are that ARB-REQ information is first transmitted to an ARB-REQ assignment part  111  by way of a signal line  118  and then to the arbiter  14  by giving ARB-REQ information  114  to the header portion of a segment  113 , and that the arbiter  14  is included inside the crossbar switch  19 . 
   In the same way as the embodiment of  FIG. 1 , ARB-REQ information is collected in the arbiter  14 , and then it is decided to which VoQ to give the grant from ARB-REQ information. Then, grant information is assigned to a header  116  of a switched segment  115  as ARB-ACK from the arbiter by way of the signal line  129 . Grant information is collected in an ARB-ACK collection part  112 , and then it is transmitted to the VoQ controller  12  byway of a signal line  119 . The VoQ controller  12  instructs an input buffer from which VoQ to transmit the segment. 
   The advantageous point of this method is that it is possible to simplify the structure of the hardware, since the number of signal lines can be decreased, because it is not required to prepare the signal line for the arbiter. 
     FIGS. 14 and 15  show results of the simulation of a queue information management arbiter of the present invention and an arbitration method (referring to Koji WAKAYAMA, et al., “A Study of structuring a Large Capacity Packet Switching Systems,” SHINGAKUGIHOU 1N98-160) for performing the Round Robin selection by judging the presence of the segment of a VoQ under the same condition. It is supposed that the condition of the simulation is a 4×4 input output crossbar switch having four input lines and four output lines. 
     FIG. 14  shows a distribution graph of an average delay time of the conventional arbiter and the proposed arbiter of the present invention at the time of having uniform traffic. The vertical axis  92  denotes Delay (Segment), and the horizontal axis  91  denotes Load Rate (%). The higher the load of a line is, the bigger the volume of the delay of the conventional arbiter is. However, it is possible for the proposed arbiter to suppress the delay time from increasing. 
   Even for uniform traffic, if the load of the line becomes higher, the traffic condition tends to be unbalanced. Therefore, the proposed arbiter, which takes both the output data interval and the queue length of a VoQ as parameters, can suppress the delay time better than the conventional arbiter, which considers only whether the segment exists as a parameter. 
     FIG. 15  shows the results of the simulation of the delay distribution of a VoQ having a low load traffic and a VoQ having a high load traffic when traffic having a load higher than others is given to one input line among four input lines. The vertical axis  102  denotes Probability (Delay time&gt;d), and the horizontal axis  101  denotes Delay (segment). 
   Quadrilateral plots denote the delay distribution of the input port having traffic of the higher load, and triangular plots denotes the delay distribution of the input port having traffic of the lower load. 
   As shown from the delay distribution of a VoQ having traffic of the higher load, the proposed arbiter suppresses the delay time better than the conventional type arbiter. The length of the queue of a VoQ having the higher traffic is longer than that of the other VoQs. The proposed way of performing arbitration by taking the length of the queue as a parameter tends to give much consideration to a VoQ having a long queue length. This can suppress the delay time of a VoQ having traffic of a higher load. 
   On the other hand, though an impact is given to a VoQ having traffic of a low load by said effect, the switching system of the present invention can suppress the effect on a VoQ having a low load traffic because it takes the output data interval as a parameter. 
   Since the length of the queue is managed, it is possible to perform highly effective switching even when unbalanced loads are applied to the switch. It is possible to suppress the delay time of a VoQ effectively, to which traffic of a high load is applied. Further, it is possible to transmit a segment without giving an effect to a VoQ to which traffic of a low load is applied. 
   Even for a VoQ to which traffic of a low load is applied, it is possible for a VoQ level to take the maximum level when the time predetermined by a user arrives. Thus, it becomes easy to remain undefeated in the input direction and the output direction for the tournament of next processing, and thus it becomes possible to attain the maximum level when the level is reevaluated. Therefore, it becomes easy for its VoQ to obtain grant of the segment. Therefore, it becomes possible to prevent starvation of a VoQ to which traffic of a low load is applied. 
   By employing the present invention, it is possible to provide an arbiter that is capable of managing any of balanced loads or unbalanced loads.