Cell switching apparatus

Abstract: An exchanging apparatus for cells each comprising a data portion and a header portion including destination information. The cells input through incoming lines are stored in respective buffer memoires selected by an incoming line spatial switch. Addresses of the stored cells in the buffer memories are managed for eachof the destinations of the cells. In accordance with the addresses under manage­ ment for each destination, the cells stored in the buffer memories are read out and output to desired outgoing lines connected to the buffer memories through an outgoing line spatial switch.

Description: This invention relates to a cell switching (or: exchanging) apparatus according to the pre-characterizing part of claim 1. Fig. 1 is a block diagram showing a conventional cell exchanging apparatus disclosed in Japanese Patent Disclosure No. 117241/90, for example. In Fig. 1, the reference numerals 1 1 - 1 n designate n (n ≧ 2) incoming lines through which packets are input. Each packet has a fixed length and has a header portion including encoded destination information. 2 1 - 2 m designate m (m ≧ 2) outgoing lines through which the packets are output dependent on the destinations specified by the header portions. 3 1 - 3 ℓ designate ℓ (ℓ ≧ n) buffer memories in which the input packets are temporarily stored. 4 designates an empty buffer selection switch for connecting each of the incoming lines 1 1 -1 n , through which a packet is input, to an empty one of the buffer memories 3 1 - 3 ℓ . Denoted by 5 1 - 5 ℓ are header memory circuits provided in correspondence to the buffer memories 3 1 - 3 ℓ for extracting and storing only the header portions of those packets which are stored in the corresponding buffer memories. 6 1 - 6 ℓ denote outgoing line selection circuits provided in correspondence to the header memory circuits 5 1 - 5 ℓ for making the outputs thereof significant, i.e., "1", from which the stored contents of the corresponding header memory circuits are sent to output lines. Denoted by 7 1 - 7 m are encoders provided in correspondence to the outgoing lines 2 1 - 2 m for receiving outputs from the outgoing selection circuits 6 1 - 6 ℓ to encode the outputs as buffer numbers of the buffer memories 3 1 - 3 ℓ . 8 1 - 8 m are first-in, first-out type (hereinafter referred to FIFO) memories provided in correspondence to the encoders 7 1 - 7 m such that the buffer numbers encoded by the encoders 7 1 - 7 m are written into the FIFO memories and then read out therefrom in the same order that the buffer numbers are input. 9 1 - 9 m are buffer connection switches controlled in accordance with the buffer numbers delivered from the corresponding FIFO memories for outputting the packets stored in the buffer memories to the outgoing lines specified by the header portions of the packets. Although the packet is used here instead of a cell to describe a transmitted unit of information, both the terms "cell" and "packet" mean the same thing in that multimedia information is divided into blocks and a header including destination information is added to each block. Generally, however, the two terms are different in that the packet is handled as having blocks of variable length, while the cell is handled as having a fixed length prescribed by the International Standard. An operation of the conventional cell exchanging apparatus will now be described. Fig. 2 is a time chart showing a timing relation between signals at the various points. This figure illustrates a flow of control in the case of receiving, at the same time, packets sent from the incoming lines 1 1 and 1 n to the outgoing line 2 1 when the buffer memories 3 1 and 3 ℓ are empty. It is also assumed that the packets handled here have a fixed length as mentioned above and the header portions include, as destination information, encoded outgoing line numbers. When a packet arrives through any one of the incoming lines 1 1 - 1 n , the empty buffer selection switch 4 selects an empty one of the buffer memories 3 1 - 3 ℓ and connects it to the incoming line along which the packet has arrived. Here, if two packets having the same outgoing line number "1" designating the outgoing line 2 1 arrive through the incoming lines 1 1 and 1 n at the same time, as shown in Figs. 2(a) and 2(b), the empty buffer selection switch 4 selects the incoming lines 1 1 - 1 n and the buffer memories 3 1 - 3 ℓ one by one in the ascending order, for example, and interconnects a selected pair of the incoming line and the buffer memory. In this case, therefore, the empty buffer selection switch 4 connects the incoming line 1 1 to the buffer memory 3 1 and the incoming line 1 n to the buffer memory 3 ℓ , whereby a packet A having arrived along the incoming line 1 1 is stored in the buffer memory 3 1 and a packet B having arrived along the incoming line 1 n is stored in the buffer memory 3 ℓ , respectively. With the switching operation of the empty buffer selection switch 4, the packet A is also supplied to the header memory circuit 5 1 corresponding to the buffer memory 3 1 and the packet B is also supplied to the header memory circuit 5 ℓ corresponding to the buffer memory 3 ℓ . The header memory circuits 5 1 - 5 ℓ serve to extract only the header portions of the received packets and to store the contents of the header portions, i.e., the outgoing line numbers. Accordingly, the outgoing line number "1" designating the outgoing line 2 1 is stored in the header memory circuits 5 1 and 5 ℓ . The contents of the header memory circuits 5 1 and 5 ℓ are sent to the corresponding outgoing line selection circuits 6 1 and 6 ℓ , respectively. Among the outputs of the outgoing line selection circuit 6 1 - 6 ℓ , those outputs which identify the output lines corresponding to the outgoing line numbers specified by the contents of the header memory circuits turn to a significant level, that is, "1", and outputs which identify other output lines remain insignificant, i.e., "0". Accordingly, the outgoing line selection circuit 6 1 sets the output to the encoder 7 1 to "1" as shown in Fig. 2(c), and the outgoing line selection circuit 6 ℓ also sets the output to the encoder 7 1 to "1" as shown in Fig. 2(d). When any one of the outputs of the outgoing line selection circuits 6 1 - 6 ℓ is turned to "1", a corresponding one of the encoders 7 1 - 7 m encodes the buffer number of the buffer memory which corresponds to one of the outgoing line selection circuits 6 1 - 6 ℓ , and causes the encoded buffer number to be stored in corresponding one of the FIFO memories 8 1 - 8 m . In the case the output of the two outgoing line selection circuits 6 1 and 6 ℓ turn to "1" at the same time as shown in Figs. 2(c) and 2(d), the encoder 7 1 causes the buffer numbers to be stored in the FIFO memory 8 1 in the ascending order, for example. Therefore, the FIFO memory 8 1 first stores the buffer number 1 ○ of the buffer memory 3 1 and then stores the buffer number ℓ ○ of the buffer memory 3 ℓ . Each of the buffer connection switches 9 1 - 9 m reads out the buffer numbers, stored in the FIFO memories 8 1 - 8 m , in the order of the buffer numbers stored, and connects the buffer memories designated by the read-out buffer numbers to the outgoing line connected to the buffer connection switch. Specifically, as shown in Fig. 2(e), the buffer connection switch 9 1 first reads the buffer number 1 ○ out of the FIFO memory 8 1 and, after the completion of the necessary connection process, reads the next buffer number ℓ ○ therefrom. When the buffer number 1 ○ is read out, the buffer memory 3 1 is connected to the outgoing line 2 1 and the packet A stored in the buffer memory 3 1 is output to the outgoing line 2 1 as shown in Fig. 2(f). After the completion of outputting the packet A to the outgoing line 2 1 , the buffer number ℓ ○ is read out as mentioned above, whereupon the buffer memory 3 ℓ is connected to the outgoing line 2 1 and the packet B stored in the buffer memory 3 ℓ is output to the outgoing line 2 1 , as shown in Fig. 2(g). As a result, the packets A and B are output to the outgoing line 2 1 successively as shown in Fig. 2(h). Whenever a packet is delivered to one of the outgoing lines 2 1 - 2 m , the buffer connection switches 9 1 - 9 m release a corresponding one of the buffer memories 3 1 - 3 ℓ and inform the empty buffer selection switch 4 of the fact so as to make it ready for receiving further packets. In the conventional cell exchanging apparatus, only one cell can be stored in each of the buffer memories 3 1 - 3 ℓ thus avoiding any collision between cells (packets) when the cells are read out of the buffer memories. If the number of cells to be written exceeds the number of buffer memories 3 1 - 3 ℓ , the excessive cells are discarded. To reduce the number of discarded cells, it is required to prepare a large quantity of buffer memories. This necessarily increases the size of the empty buffer selection switch 4 for connecting the incoming lines to the buffer memories and the buffer connection switches for connecting the buffer memories to the outgoing lines 2 1 - 2 m . EP 0 300 876 Al discloses a system for switching data blocks between a plurality of incoming multiplex ways and a plurality of outgoing multiplex ways. Each of the blocks in the incoming ways is intended for at least one addressee of the outgoing ways. The system comprises input means for detecting data blocks in the incoming multiplex ways to multiplex detected blocks into multiplexed blocks, in combination with buffer means comprising block cells for memorizing the multiplexed blocks, write addressing means that derive write addresses of block cells for writing the multiplexed blocks in block cells in accordance with the derived write addresses, read addressing means for arranging the addresses of the write addressed block cells dependent on addressee outgoing multiplex ways for which the written blocks are intended to cyclically read the written blocks into read blocks in accordance with the arranged block cell addresses, and means for demultiplexing the read blocks and transmitting them onto the addressee outgoing multiplex ways. This invention has been made to solve the problems as explained above, and has for its object to provide a cell switching (or: exchanging) apparatus which can reduce the number of cells discarded due to collisions using a small number of buffer memories, and which can also reduce the size of switching devices for connecting the buffer memories to incoming lines and outgoing lines. To achieve the above object, a cell switching (or: exchanging ) apparatus as defined in the pre-characterizing part of claim 1 according to the present invention is characterized in that the number of buffer memories means is at least the number of incoming lines plus the number of outgoing lines minus one (p ≥ n + m - 1), and the incoming line spatial switch means respectively selects a single of the plurality of buffer memory means to write a cell into in such a way that it is avoided to store two or more cells in the same buffer memory means which are to be read simultaneously. Preferred embodiments of the invention are defined in the subclaims. The cells input through the incoming lines are stored in the buffer memories selected by the incoming line spatial switch after detection of the cell destinations. The addresses of the stored cells in the buffer memories are managed for each of the destinations of the cells. In accordance with the addresses under management for each destination, the buffer memories are accessed to cause the cells stored therein to be read out and output to the outgoing lines connected to the buffer memories through the outgoing line spatial switch. Consequently, when reading the cells out of the buffer memories, a plurality of cells can be led to the outgoing lines through the spatial switch while avoiding collision between cells. Also, a processing speed (rate) is the same throughout the circuits, whereby the cells can be switched without any necessity to increase such a speed. Furthermore, the number of cells is lowered, which are discarded when the number of written cells exceeds the capacity of the buffer memories, without any necessity to increase the size of the spatial switches for connecting the buffer memories to the incoming lines and the outgoing lines. Preferably, the cells input through the incoming lines are held in the header processing circuits for a period of one time slot. The buffer control circuit controls the incoming line spatial switch such that the cells held in the header processing circuits are written in the buffer memories which are selected so as to avoid the overlapping of the cells, i.e. to prevent a plurality of cells from being written in one buffer memory at the same time within one time slot. The outgoing line spatial switch is controlled such that, when the cells have already been written in the buffer memories and are managed using the addresses in the buffer memories for each of the destinations and for each of the reading time slots so as to avoid the overlapping of the destinations within the same time slot, the cells are output to the outgoing lines specified by the cell header portions in a predetermined order. Thus, by setting one time slot as the operation cycle unit, the cells held in the header processing circuits are written in the buffer memories selected so as to avoid the overlapping of the cells within one time slot. The cells which are managed using the addresses in the buffer memories for each of the destinations and reading time slots so as to avoid the overlapping of the destinations within the same time slot are read out to the outgoing lines specified by the header portions. Consequently, it is possible to reduce the number of cells discarded due to collision therebetween or delay of the cells during the writing and reading operations. Advantageously, outgoing line speed adjusting buffers connected to the buffer memories through the outgoing line spatial switch for storing the cells to adjust an outgoing line speed are additionally provided in correspondence to the outgoing lines. The buffer control circuit functions to control the incoming line spatial switch to select the buffer memories in which the cells are to be written, to manage the addresses of the cells written in the buffer memories for each destination, to read the cells out of the buffer memories in accordance with those addresses under management in a predetermined order at a speed r (2 ≦ r < number of outgoing lines) times the outgoing line speed, to control the outgoing line spatial switch to write the read-out cells in the specified outgoing line speed adjusting buffers in a predetermined order, and to read out those written cells in accordance with the outgoing line speed to output the cells to the outgoing lines. Thus, the cells the destinations of which have been detected are stored in the selected buffer memories. The addresses of the cells stored in the buffer memories are managed for each destination, and the buffer memories are accessed in accordance with those addresses for reading out the stored cells at a speed r (2 ≦ r < number of outgoing lines) times the outgoing line speed. Consequently, when reading the cells out of the buffer memories, a plurality of cells can be led to the outgoing lines through the spatial switch so as to avoid any collision between cells, while allowing the cells up to a number of r to be read out of the same buffer memory in overlapped relation. The reading speed is raised as high as r times the outgoing line speed at its maximum and, therefore, the cells can be switched without any necessity to increase the speed. Because the opportunities of reading the cells out of the same buffer memory is increased, it is possible to further reduce the number of cells discarded by collision during the reading operation. In a further preferred embodiment, incoming line speed adjusting buffers for storing the cells to adjust an incoming speed are additionally provided in correspondence to the incoming lines. The buffer control circuit functions to control the outgoing line spatial switch to control the connection between the buffer memories and the outgoing lines, to control the incoming line spatial switch to select the buffer memories in which the cells are to be written, to write the cells read out of the incoming line speed adjusting buffers to the selected buffer memories at a speed w (2 ≦ w < number of incoming lines) times the incoming line speed, to manage the addresses of the cells written in the buffer memories for each destination, and to control the outgoing line spatial switch in accordance with those addresses under management to output the cells to the specified outgoing lines in a predetermined order. Thus, the cells for which destinations have been detected are written in the buffer memories selected by the incoming line spatial switch as a speed w (2 ≦ w < number of incoming lines) times the incoming line speed. The addresses of the cells stored in the buffer memories are managed for each destination, and the buffer memories are accessed in accordance with those addresses for reading out the stored cells. Consequently, even in a condition that the buffer memories become almost full with cells and a plurality of cells must be written in one buffer memory in one time slot, it is possible to write up to w cells in one buffer memory. Also, when reading the cells out of the buffer memories, a plurality of cells can be led to the outgoing lines through the spatial switch, while avoiding collisions between cells. The writing speed is raised as high as w times the incoming line speed at its maximum and, therefore, the cells can be switched without any necessity to increase the speed. As a result, it is possible to further reduce the number of cells discarded due to collision during the writing operation. The above and other advantages of the present invention will be become clearer from reading the following description with reference to the accompanying drawings which illustrate several preferred embodiments by way of example. Fig. 1 is a block diagram showing a conventional cell switching (or: exchanging) apparatus; Fig. 2 is a time chart showing a timing relation between signals at various points in the cell switching apparatus of Fig. 1; Fig. 3 is a block diagram showing a cell switching (or: exchanging) apparatus according to a first embodiment of the present invention; Fig. 4 is a time chart showing a timing relation between signals at various points in the cell switching apparatus of Fig. 3; Fig. 5 is a block diagram showing a cell switching (or: exchanging) apparatus according to a second embodiment of the present invention; Fig. 6 is a time chart showing a timing relation between signals at various points in the cell switching apparatus of Fig. 5; Fig. 7 is a block diagram showing a cell switching (or: exchanging) apparatus according to a third embodiment of the present invention; Fig. 8 is a time chart showing a timing relation between signals at various points in the cell switching apparatus of Fig. 7; Fig. 9 is an enlarged time chart showing an main part of the time chart of Fig. 8; Fig. 10 is a block diagram showing a cell switching (or: exchanging) apparatus according to a fourth embodiment of the present invention; and Fig. 11 is a time chart showing a timing relation between signals at various points in the cell switching apparatus of Fig. 10. Hereinafter, several preferred embodiments of the present invention will be described by referring to the drawings. Fig. 3 is a block diagram showing the structure of a cell switching (or: exchanging) apparatus according to a first embodiment of the present invention. In Fig. 3, denoted by 1 1 - 1 n are n (n ≧ 2) incoming lines through which cells are input, each cell comprising a data portion and a header portion including an outgoing line number as destination information. 2 1 - 2 m designates m (m ≧ 2) outgoing lines through which the cells are output dependent on the designations specified by their header portions. The above arrangement is identical to that shown in Fig. 1. Denoted by 10a 1 - 10a n are header processing circuits provided in correspondence to the incoming lines 1 1 - 1 n for detecting the outgoing lines designated by the header portions of the cells received through the incoming lines. 11 1 - 11 p designate p (p ≧ 2) buffer memories, each buffer memory being capable of storing cells at specified addresses and reading out the stored cells by specifying the addresses regardless of the order in which the cells are stored or written. These buffer memories 11 1 - 11 p are different from the conventional buffer memories 3 1 - 3 ℓ shown in Fig. 1 in that a plurality of cells can be stored in each buffer memory. 12 1 - 12 p designate storage control circuits provided in correspondence to the buffer memories 11 1 - 11 p , each storage control circuit supervising empty addresses using, for example, an FIFO-type memory to give an associated one of the buffer memories 11 1 - 11 p READ and WRITE addresses. Denoted by 13 is an incoming line spatial switch for selectively connecting the header processing circuits 10a 1 - 10a n to predetermined buffer memories 11 1 - 11 p , and 14 designates an outgoing line spatial switch for selectively connecting the buffer memories 11 1 - 11 p to predetermined outgoing lines 2 1 - 2 m . Denoted by 15a is a buffer control circuit which functions to (i) control a switching operation of the incoming line spatial switch 13 to select the buffer memories in which cells are to be stored, (ii) manage the addresses of the stored cells in the buffer memories on the basis of the respective destinations of the cells, and (iii) control a switching operation of the outgoing line spatial switch 14 in accordance with the addresses under management for the respective destinations, thereby outputting the cells, in a predetermined order, to the outgoing lines specified by the cell headers. The buffer control circuit 15a comprises a first buffer selection circuit 16 which, when a cell arrives along one of the incoming lines 1 1 - 1 n , receives the outgoing line number of the cell detected by the header processing circuits 10a 1 - 10a n associated with that incoming line, selects one of the buffer memories 11 1 - 11 p in which the cell is to be stored, and controls a switching operation of the incoming line spatial switch 13 so that the selected buffer memory is connected to the cell-detecting header processing circuit. 17 designates an address exchanging circuit which functions to sort out the arriving cells on the basis of the destinations, i.e., the outgoing lines, by referring to the outgoing line numbers detected by the buffer selection circuit 16, and obtain, from the storage control circuits associated with the buffer memories, the WRITE addresses in the buffer memories in which the cells are written to write the obtained WRITE addresses in address queues (described later). Denoted by 18 1 - 18 m are address queues each constituted by an FIFO-type memory and provided in corresponding to the outgoing lines 2 1 - 2 m . The address exchanging circuit 17 writes, in the address queues, the WRITE addresses in the buffer memories in which the cells destined for the outgoing lines are stored, in the order in which the addresses arrive, for every outgoing lines 2 1 - 2 m corresponding to the address queues. 19 designates a second buffer selection circuit which functions to determine the cells to be read out of the buffer memories by referring to the address queues 18 1 - 18 m , send the address read out of the address queues, as READ addresses, to the storage control circuits associated with the corresponding buffer memories, and control a switching operation of the outgoing line spatial switch 14 to cause the buffer memories to the corresponding outgoing lines. An operation of this cell exchanging apparatus will be described below. Fig. 4 is a time chart showing a timing relation between signals at various points in the cell exchanging apparatus. The time chart illustrates a control flow in which the number n of the incoming lines 1 1 - 1 n and the number m of the outgoing lines 2 1 - 2 m are respectively four (4) and the number p of the buffer memories 11 1 - 11 p is ten (10). It is also assumed that the cells handled here are input at random and have a fixed length, and that the phases of the respective cells are adjusted prior to being input to the incoming lines 1 1 - 1 n in such a manner that the cells are supplied at the same phase through all the incoming lines. Figs. 4(a) - 4(d) show examples of the cells input to the incoming lines 1 1 - 1 4 , Figs. 4(e) - 4(n) show examples of the cells stored in the buffer memories 11 1 - 11 10 in that case, the Figs. 4(o) - 4(r) show examples of the cells output to the outgoing lines 2 1 - 2 4 . It is assumed that all the circuits are synchronized with each other, and one cell per one time slot can be input and output. When the cells are input to the incoming lines, the header processing circuits 10a 1 - 10a 4 provided in corre spondence to the incoming lines 1 1 - 1 4 detect the outgoing line numbers from the header portions of the input cells. While referring to the header processing circuits 10a 1 - 10a 4 , the first buffer selection circuit 16 of the buffer control circuit 15a instructs the incoming line spatial switch 13 to respectively connect the incoming lines along which the cells have arrived to the buffer memories which are selected for storing the cells. The manner in which the incoming line spatial switch 13 connects between the incoming lines and the buffer memories can be described in various ways. It is undesirable that two or more cells to be read exist in the same buffer memory at the time the cells are stored and then read out of the buffer memories. To avoid this, the cells are preferably distributed to a number of buffer memories. To accomplish this the same number of buffer memories as that of the incoming lines is not sufficient. Control is facilitated by using as many buffer memories as possible. As a simpler example of control, it can also be envisaged to select a buffer memory which holds the least number of cells therein, and to write cells in the buffer memory. In other words, when x cells arrive simultaneously, x buffer memories which hold the least number of cells therein are selected, and the incoming lines receiving the x cells are spatially connected to the selected buffer memories. Fig. 4 shows, as a further simpler example of control, a manner in which the buffer memories 11 1 - 11 10 are sequentially selected and arriving cells are written in the buffer memories. In other words, the buffer memories 11 1 , 11 2 , 11 3 , ..., 11 10 are written therein successively. In a time slot 1, the first cell of a signal f (referred to as an "F1 cell" herein; the remaining cells are similarly referred to) is input from the incoming line 1 1 , a G1 cell of a signal g from the incoming line 1 2 , and an I1 cell of a signal i from the incoming line 1 4 . The header portion of the respective cells include outgoing line numbers; that is, the F1 cell has the number 0 1 specifying the outgoing line 2 1 , the G1 cell has 0 4 specifying the outgoing line 2 4 , and the I1 cell has 0 3 specifying the outgoing line 2 3 , respectively. In a time slot 2, the incoming line spatial switch 13 connects the incoming line 1 1 to the buffer memory 11 1 , the incoming line 1 2 to the buffer memory 11 2 , and the incoming line 1 4 to the buffer memory 11 3 , respectively. Therefore, these cells are stored, in the time slot 2, in the buffer memories 11 1 - 11 4 at the addresses specified by the storage control circuits 12 1 - 12 3 . At this time, the storage control circuits 12 1 - 12 3 send the WRITE addresses of the buffer memories to the address exchanging circuit 17. The WRITE addresses are selected from among those addresses which are managed by the storage control circuits 12 1 - 12 3 as empty addresses. The address exchanging circuit 17 sorts out the input cells for each destination outgoing line in reference to the first buffer selection circuit 16, and writes the WRITE address of the buffer memory 11 1 in the tail end of the address queue 18 1 , the WRITE address of the buffer memory 11 2 in the tail end of the address queue 18 4 , and the WRITE address of the buffer memory 11 3 in the tail end of the address queue 18 3 , respectively. Then, in a time slot 3, the second buffer selection circuit 19 takes out the addresses stored in the address queues 18 1 - 18 3 , sends the addresses to the storage control circuits 12 1 - 12 3 associated with the buffer memories 11 1 - 11 3 , and instructs the outgoing line spatial switch 14 to connect the buffer memories 11 1 - 11 3 to the outgoing lines 2 1 , 2 3 and 2 4 , respectively. The outgoing line spatial switch 14 connects the buffer memory 11 1 to the outgoing line 2 1 , the buffer memory 11 2 to the outgoing line 2 4 and the buffer memory 11 3 to the outgoing line 2 3 , respectively, in the time slot 3. The storage control circuits 12 1 - 12 3 send the received addresses to the associated buffer memories 11 1 - 11 3 as READ addresses and, thereafter, manage those addresses as empty ones. The cells read out of the respective buffer memories 11 1 - 11 3 are output to the destination outgoing lines 2 1 , 2 4 and 2 3 specified by their headers, respectively. While the outgoing lines specified by the input cells are different from one another in the above example, the cells input in the time slot 2 have the header portions specifying the same destination outgoing lines. An F2 cell, a G2 cell and an H1 cell input in the time slot 2 are respectively written in the buffer memories 11 4 , 11 5 and 11 6 in a like manner, but the header portions of these three cells include the outgoing line number 0 4 specifying the same outgoing line 2 4 . In the example of Fig. 4, higher priority is given for an incoming line of a smaller number, thereby to causing cells to be delayed. The F2, G2 and H1 cells are read out of the buffer memories 11 4 , 11 5 and 11 6 in this order in time slots 4, 5 and 6, respectively, and output to the outgoing line 2 4 . Subsequently, such cell exchange will be executed in the manner described above. In a time slot 8, an I2 cell and an H6 cell are stored in the buffer memory 11 3 . Although these I2 and H6 cells have different destinations, i.e., the outgoing line 2 2 and the outgoing line 2 3 , respectively, they are stored in the same buffer memory 11 3 and hence cannot be taken out simultaneously. In such a case, a collision of the two cells can be avoided by giving the outgoing lines 2 1 - 2 4 fixed preferential orders or variable preferential orders to be changed on the basis of a random number and by taking out only one cell, e.g., the I2 cell while delaying the other H6 cell. A similar event also occurs in other time slots 9, 10 and 15, but not cell is lost due to collision. In the cell exchanging apparatus according to the first embodiment, as described above, the cells input through the incoming lines are stored in the respective buffer memories selected by the incoming line spatial switch, after the detection of the cell destinations. The addresses of the stored cells in the buffer memories are managed for each of the destinations of the cells. In accordance with the addresses under management for each destination, the buffer memories are accessed for reading out the cells stored therein and outputting those cells to desired outgoing lines connected to the buffer memories through the outgoing line spatial switch. As a result, it is possible to achieve a cell exchanging apparatus which can lower the number of discarded cells due to collision by using a relatively small number of buffer memories, and can reduce the size of switch devices for connecting the buffer memories to incoming and outgoing lines. Fig. 5 is a block diagram showing the construction of a cell switching (or: exchanging) apparatus according to a second embodiment of the present invention. In Fig. 5, the same or corresponding elements are denoted by the same reference numerals as in the cell exchanging apparatus according to the first embodiment and not explained here. Referring to Fig. 5, denoted by 10b 1 - 10b n are header processing circuits provided in correspondence to the incoming lines 1 1 - 1 n for detecting the outgoing lines from the header portions of the cells input through the incoming lines and for holding the cells therein for a period of one time slot. The buffer memories 11 1 - 11 p are provided in a number corresponding to p = (n + m - 1). 15b designates a buffer control circuit which (i), for a period of one time slot, controls a switching operation of the incoming line spatial switch 13 to select the buffer memories in which the cells are to be written (i.e., the writing of cells), (ii) manages, for each of the destination of the cells and for each time slot for reading out the cells, the addresses of the stored cells in the buffer memories and (iii) outputs the cells in each reading time slot to the outgoing lines specified by their headers in a predetermined order. The buffer control circuit 15b comprises an address table 21 provided in correspondence to the outgoing lines 2 1 - 2 m . When the cells arrive through the incoming lines, the address table 21 receives the outgoing line numbers of the arriving cells which are detected by the header processing circuits associated with the incoming lines, and selects the buffer memories in which the cells are to be stored, and then stores the buffer memory numbers and the corresponding WRITE addresses in the table for each of the destination outgoing lines. 20 designates an incoming line connection instructing circuit which receives the number of the buffer memories storing the cells determined by the address table 21 and the WRITE addresses and controls a switching operation of the incoming line spatial swtich 13 to connect between the header processing circuits 10b 1 - 10b n and the buffer memories 11 1 - 11 p . 22 designates an outgoing line connection instructing circuit which determines the cells to be read out of the buffer memories in reference to the address table 21, sends the addresses read out of the address table 21, as READ addresses, to the storage control circuits associated with the buffer memories, and controls a switching operation of the outgoing line spatial switch 14 to connect the buffer memories to the corresponding outgoing lines respectively. An operation of this cell exchanging apparatus will be described below. Fig. 6 is a time chart showing a timing relation between signals at various points in the cell exchanging apparatus. The time chart illustrates a control flow in which the number n of the incoming lines 1 1 - 1 n and the number m of the outgoing lines 2 1 - 2 m are each four (4) and the number p of the buffer memories 11 1 - 11 p is equal to (n + m - 1), i.e., seven (7). It is also assumed that the cells handled here have a fixed length, and that the input phases of the respective cells are adjusted prior to being input to the incoming lines 1 1 - 1 n in such a manner that input cells are supplied at the same phase through all the incoming lines. For convenience of explanation, those incoming lines 1 1 - 1 4 are indicated by I 0 , I 1 , I 2 and I 3 , and those outgoing lines 2 1 - 2 4 are indicated by 0 0 , 0 1 , 0 2 and 0 3 , respectively. Further, the buffer memories 11 1 - 11 7 are indicated by buffer #0, buffer #1, ..., buffer #6, respectively. Fig. 6(a) shows time slot numbers, Fig. 6(b) shows examples of the cells input to the incoming lines I 0 - I 3 , Fig. 6(c) shows the internal state of the address table 21, Figs. 6(d) and (e) show the internal states of the incoming line connection instructing circuit 20 and the outgoing line connection instruction circuit 22, respectively, Fig. 6(f) shows the state of the buffer memories 11 1 - 11 7 storing cells, and Fig. 6(g) shows the cells output through the outgoing lines 0 1 - 0 3 . It is assumed that all the circuits are synchronized with each other, and only one cell can be input and output per one time slot. When the cells are input to the incoming lines, the header processing circuits 10b 1 - 10b 4 provided in correspondence to the incoming lines I 0 - I 3 detect the outgoing line numbers from the header portions of the input cells, and hold the cells for a period of one time slot. In reference to the header processing circuit 10b 1 - 10b 4 , the address table 21 in the buffer control circuit 15b determines in which buffer memory the cells input through the incoming lines are to be written. When the determined result is instructed to the incoming line connection instructing circuit 20, the incoming line spatial switch 13 connects the incoming lines at which the cells have arrived to the buffer memories which have been selected for storing the cells. Because only one cell can be written in and read out of one buffer memory within the same one time slot, the buffer memories 11 1 - 11 7 must be so selected that the writing and reading of a plurality of cells do not occur in the same time slot. The address table 21 includes a table rows of which represent destination outgoing lines and columns of which represent reading time slots. Each of the boxes in the table contains the buffer memory number and the address in the buffer memory in which the cell to be read out in that time slot is written. Null signals are put in those boxes corresponding to the outgoing lines through which no cells are to be output in any time slots. Upon arrival of the cells, the incoming line spatial switch 13 can be considered to connect the cells to the buffer memories 11 1 - 11 7 in various ways. It is, however, undesirable for two or more cells to exist in the same one buffer memory in the case where the cells are stored and then read out of the buffer memories. To prevent this situation, the cells are preferably distributed to a number of buffer memories. To do this, employing the same number of buffer memories as that of the incoming lines is not sufficient. Also, when the cells are written, different buffer memories must be selected. Therefore, the number p of the buffer memories should be at least (number n of incoming lines) + (number m of outgoing lines) - 1. (In this second embodiment, the number p of the buffer memories is 7.) In Fig. 6, the matrix-like table in the address table 21 is partitioned for each of the reading time slots. Because the capacity is finite, the table is cyclically reused in various time slots. To this end, a READ pointer rp is employed to point a READ position in the next time slot, and is cyclically updated. The table is arranged to prepare queues for the arriving cells in the order of arrival for each of the destination outgoing lines. Fig. 6 shows an example of WRITE pointers wp 0 - wp 3 employed for the respective destination outgoing lines to point the next writing positions next to the tail ends of the respective queues. In order to determine the buffer memories in which the cells are to be written, when the cells arrive at the incoming lines, the buffer memories 11 1 - 11 7 are checked sequentially from the incoming lines I 1 - I 3 to confirm that the writing of a plurality of cells in the same buffer memory will not be effected in the same time slot and that the reading of a plurality of cells out of the same buffer memory will not be effected in the same time slot, and then buffer memories in which the cells to be written are determined. In this case, those buffer memories which are not filled with cells are selected in a cyclic manner. In Fig. 6, it is assumed that no cell arrives before a time slot 1, and that three cells arrive through the incoming lines I 0 , I 1 and I 3 in the time slot 1. Each cell is identified using the incoming line number, the outgoing line number and the time slot in which the cell arrived. For instance, a cell 312 indicates that this cell arrives through the incoming line I 3 in the time slot 2 and is destined for the outgoing line 0 1 . The READ pointer rp is positioned at the column No. 2 in the table in the time slot 1, and, it is assumed that, though not shown in Fig. 6, the WRITE pointers wp 0 - wp 3 are all positioned at the column No. 3 in the table at the beginning of the time slot 1. A cell 011 arriving through the incoming line I 0 is stored in, for example, the buffer #0 because no other cell exists the table at this time slot. This buffer memory 11 1 is associated with the storage control circuit 12 1 for managing the WRITE addresses. The WRITE address of the cell 011 is assumed to be 0. As shown in Fig. 6(c), in each box the upper side represents a buffer memory number and the lower side represents an address in the buffer memory. A cell 121 arriving through the incoming line I 1 is destined for the outgoing line 0 2 and, therefore, a buffer memory other than the buffer #0, for example, the buffer #1, used for storing the cell 011 is selected as a candidate. In reading the cell 121, it is required to check the column No. 3 in the table. Because only the buffer #0 is used in addition to the buffer #1, the buffer #1 meets the requirements. Accordingly, the cell 121 is stored at an address 0 in the buffer #1. A cell 311 arriving through incoming line I 3 is destined for the outgoing line 0 1 and, therefore, stored at an address 0 in a buffer other than the buffers #0 and #1 used for storing the cells 011 and 121, that is, in the buffer #2, for example. When the writing and reading of the cells in the next time slot are determined, as explained above, the address table 21 sends constructions to the incoming line connection instructing circuit 20 and the outgoing line connection instruction circuit 22 to cause the incoming line spatial switch 13 and the outgoing line spatial switch 14 to perform necessary exchanging operations. Thus, the incoming line spatial switch 13 connects the incoming line I 0 to the buffer #0, the incoming line I 1 to the buffer #1 and the incoming line I 3 to the buffer #2, respectively and the cells are written in these buffers in the time slot 2. The column No. 2 of the table pointed by the READ pointer rp is fully occupied by null signals and, therefore, the outgoing line connection instructing circuit 22 is instructed to cause the outgoing line spatial switch 14 to perform no exchanging operation. In the time slot 2, cells arrive through all the incoming lines I 0 - I 3 . Similar to the operation in the time slot 1, some of the buffer memories 11 1 - 11 7 are selected so that a plurality of cells will not be written in or read out of the same buffer memory in the same time slot. In the time slot 2, the READ pointer rp is updated by one as compared with the time slot 1 and positioned in the column No. 3 of the table. The cell information in the column No. 3 of the table is sent to the outgoing line connection instructing circuit 22. The outgoing line spatial switch 14 then connects the buffer #0 to the outgoing line 0 1 and the buffer #1 to the outgoing line 0 2 in a time slot 3, respectively, and the reading of the cells is effected in the time slot 3. As can be understood from the above description, when a cell arrives through the incoming lines, the buffer memories 11 1 - 11 7 are checked in turn to confirm that a plurality of cells will not be written in the same buffer memory in the same single time slot, and a plurality of cells will not be read out of the same buffer memory in the same single time slot. Then, a buffer memory in which the cell is to be written is determined as a candidate. At this time, those buffer memories which are not filled with the cells are selected as candidates in a cyclic manner. In a time slot 4, a cell 134 being destined for an outgoing line 0 3 , the buffer #5 was first selected as a candidate. However, because the cell in the buffer #5 is read out in the same time slot, the buffer #6 is determined as a candidate. Likewise, the buffer #3 is finally selected for a cell 304. These two examples are indicated by o in Fig. 6. As described above, in the cell exchanging apparatus according to the second embodiment, one time slot is set as the unit of an operation cycle, and cells are written in buffer memories selected so as to avoid overlapping of the cells within each time slot. Cells are written in buffer memories, and are managed, in addresses in the buffer memories, for each destination and for each reading time slot in such a manner as to avoid overlapping of destinations within the same time slot. Those cells are then output to predetermined outgoing lines. Consequently, as compared with the case of merely allocating the buffer memories 11 1 - 11 7 in a cyclic manner, it is possible to avoid any collision between cells, and hence discarding of cells due to collision and delayed reading of the cells during the writing and reading operations can be avoided. The third embodiment of the present invention will be described below with reference to Figs. 7 - 9. Fig. 7 is a block diagram showing the construction of a cell switching (or: exchanging) apparatus according to the third embodiment of the present invention. In Fig. 7, the same or corresponding elements are denoted by the same reference numerals as those in the cell exchanging apparatus according to the first embodiment and are not explained here. Referring to Fig. 7, outgoing line speed adjusting buffers 23 1 - 23 m are provided in correspondence to the outgoing lines 2 1 - 2 m to be connected to predetermined buffer memories through the outgoing line spatial switch 14. The speed adjusting buffers store the cells read out of the buffer memories at a speed r (2 ≦ r < number of outgoing lines) times the outgoing line speed, and then output the cells of the corresponding outgoing lines in accordance with the outgoing line speed. A buffer control circuit 15c comprises a first buffer selection circuit 16, an address exchanging circuit 17, address queues 18 1 - 18 m and a second buffer selection circuit 19. The buffer control circuit 15c functions to (i) control a switching operation of the incoming line spatial switch 13 to select the buffer memories in which cells are to be written, (ii) manage the addresses of the written cells in the buffer memories for each of the destinations of the cells, (iii) cause the cells to be read out of the buffer memories in accordance with the managed addresses in a predetermined order at a speed r (2 ≦ r < number of outgoing lines) times the outgoing line speed, (iv) control the outgoing line spatial switch 14 to cause the read-out cells to be output to the outgoing lines specified by the header portions, (v) write the cells in the corresponding outgoing line speed adjusting buffers, and (vi) read out the stored cells in accordance with the outgoing line speed for outputting them to the corresponding outgoing lines. An operation of this cell exchanging apparatus will be described below. Fig. 8 is a time chart showing a timing relation between signals at various points in the cell exchanging apparatus. Similar to the first embodiment, the time chart illustrates a control flow in which the number n of the incoming lines 1 1 - 1 n and the number m of the outgoing lines 2 1 - 2 m are each four (4) and that the number p of the buffer memories 11 1 - 11 p is ten (10). Figs. 8(a) - 8(r) show the same states as those in Figs. 4(a) - 4(r), respectively. It is also assumed that cells handled here are input at random and have a fixed length, and that the phases of respective cells are adjusted prior to being input to the incoming lines 1 1 - 1 n in such a manner that the cells are supplied at the same phase through all the incoming lines. The basic sequence of exchanging cells proceeds in a manner similar to the first embodiment. It is noted that an I2 cell and an H6 cell are both stored in the buffer memory 11 3 in a time slot 8 as illustrated. Although the I2 and H6 cells have destinations different from each other; that is, the I2 cell is destined for the outgoing line 2 2 and the H6 cell is destined for the outgoing cell 2 3 , because these cells are stored in the same buffer memory 11 3 , they cannot be taken out simultaneously at a speed equal to the outgoing line speed in the outgoing lines 2 1 - 2 4 . Fig. 9 is a time chart partially showing the time slots 6 - 13 in Fig. 8 on a larger scale. Fig. 9 shows the case where the cells are read out of the buffer memories 11 1 - 11 10 at a speed 3 times the outgoing line speed in the outgoing lines 2 1 - 2 m . In Figs. 9(e) - 9(n) show examples of the cells stored in the buffer memories 11 1 - 11 10 , Figs. 9(s) - 9(v) show states of the cells written in the outgoing line speed adjusting buffers 23 1 - 23 4 , and Figs. 9(o) - 9(r) show examples of the cells output to the outgoing lines 2 1 - 2 4 , respectively. In Fig. 9, the I2 cell and the H6 cell in the buffer memory 11 3 are destined for different destinations in the same time slot 8. However, both the I2 and H6 cells would be simultaneously output to the corresponding outgoing lines 2 2 and 2 3 by reading these cells out of the buffer memory 11 3 at a speed 3 times the outgoing line speed. Generally, by reading cells out of the buffer memory 11 1 - 11 10 at a speed 3 times the outgoing line speed, it becomes possible to write in each of the buffer memories 11 1 - 11 10 up to 3 cells which are to be read out in the same time slot. Similar events occur in other time slots 9, 10 and 15 as well, but the cells are not in any case required to delay the reading of a cell in order to avoid any collision. The speed of reading cells out of the buffer memories 11 1 - 11 10 is not limited to 3 times the outgoing line speed, and may be generally set at r (2 ≦ r < number of outgoing lines) times the outgoing line speed. While it is assumed that the buffer memories 11 1 - 11 10 are of the type of a dual-port memory, this embodiment is also made feasible by using single-port memories which can operate at a speed double or more the outgoing line speed. Even in the case that the number of cells read out of the same buffer memory in one time slot exceeds r, a collision between the cells can be avoided by setting the outgoing lines 2 1 - 2 m in a fixed preferential order or variable preferential order changeable at random and by taking out r cells while delaying the reading of the remaining cells. In the cell exchanging apparatus according to the third embodiment, as described above, since the cells stored in the buffer memories are read out at a speed r (2 ≦ r < number of outgoing lines) times the outgoing line speed, opportunities of reading the cells out of the same buffer memory increase, which enables a reduction in the rate of cells discarded by collision during the reading operation. Fig. 10 is a block diagram showing the construction of a cell switching (or: exchanging) apparatus according to a fourth embodiment of the present invention. In Fig. 10, the same or corresponding elements are denoted by the same reference numerals as those in the cell exchanging apparatus according to the third embodiment and are therefore not explained here. Referring to Fig. 10, incoming line speed adjusting buffers 24 1 - 24 n are provided in correspondence to the incoming lines 1 1 - 1 n for storing the cells output from the corresponding header processing circuits 10a 1 - 10a n and for then reading the stored cells at a speed w (2 ≦ w < number of incoming lines) times the incoming line speed to deliver the cells to predetermined buffer memories 11 1 - 11 p through the incoming line spatial switch 13. A buffer control circuit 15d comprises a first buffer selection circuit 16, an address exchanging circuit 17, address queues 18 1 - 18 m and a second buffer selection circuit 19. The buffer control circuit 15d functions to (i) cause the cells stored in the incoming line speed adjusting buffers to be read out at a speed w (2 ≦ w < number of incoming lines) times the incoming line speed, (ii) control the incoming line spatial switch 13 to select the buffer memories in which the cells are to be written, (iii) cause the read-out cells to be written the selected buffer memories at a speed w times the incoming line speed, (iv) manage the addresses of the cells written in the buffer memories for each of the destinations of the cells, and (v) control the outgoing line spatial switch 14 in accordance with those addresses under management for outputting the cells in a predetermined order to the outgoing lines specified by the header portions. An operation of this cell exchanging apparatus will be described below. Fig. 11 is a time chart showing a timing relation between signals at various points in the cell exchanging apparatus of Fig. 10. Similar to the first embodiment shown in Fig. 4, the time chart illustrates a control flow in which the number n of the incoming lines 1 1 - 1 n and the number m of the outgoing lines 2 1 - 2 m are each four (4) and that the number p of the buffer memories 11 1 - 11 p is ten (10). Figs. 11(a) - 11(r) show the same states as those in Figs. 4(a) - 4(r), respectively. It is assumed that the buffer memories 11 1 - 11 p each have capacities equal to two cells, that cells handled in this embodiment are input at random and have a fixed length, and that the phases of respective cells input are adjusted prior to being input to the incoming lines 1 1 - 1 n in such a manner that the input cells are supplied at the same phase through all the incoming lines. When cells are input through the incoming lines, the header processing circuits associated with the incoming lines detect outgoing line numbers from the cell headers and cause the cells to write in the corresponding incoming line speed adjusting buffers. In reference to the header processing circuits, the first buffer selection circuit 16 in the buffer control circuit 15d instructs the incoming line spatial switch 13 to interconnect the incoming line speed adjusting buffers in which the cells have been written and the buffer memories selected for storing the cells, respectively. A speed of reading cells out of the incoming line speed adjusting buffers 24 1 - 24 4 , i.e., a speed of writing cells in the buffer memories 11 1 - 11 10 is assumed to be set 2 times the incoming line speed in the incoming lines 1 1 - 1 4 , thereby allowing two cells to be written in the same buffer memory in one time slot. Assuming that the buffer memories 11 1 , 11 2 , 11 3 , ..., 11 10 are selected in this order to cause the arriving cells to be written in the buffer memories in the order of arrival, if some of the buffer memories in which cells are to be written in any time slot are already filled with the cells, the filled buffer memories are skipped over, and a cell is written in the next buffer memory. In order to make sure that the cells are written in different buffer memories, the cells are preferably distributed by being written in the buffer memories. In this embodiment, because the speed of writing cells in the buffer memories 11 1 - 11 10 is set at 2 times the incoming line speed, it is possible to write a plurality (two) of cells in which of the buffer memories 11 1 - 11 10 in one time slot if any cells are unavoidably discarded, thereby reducing the number of cells discarded. In Fig. 11, because the buffer memory 11 6 is empty to be able to receive two cells and the buffer memory 11 7 is empty to be able to receive one cell in a time slot 11, three cells, i.e., F10, H10 and I8 cells input in the time slot 11 cannot be written in different buffer memories. By utilizing the fact that the speed of writing cells in the buffer memories 11 1 - 11 10 is twice the incoming line speed, therefore, the both F10 and H10 cells are written in the buffer memory 116 in order to prevent the cells from being discarded. The state after those three cells have been written in the buffer memories is indicated in Figs. 11(i) and 11(j) by putting the symbols F10, H10 and I8 in circles. The basic sequence of exchanging cells proceeds in a manner similar to the first embodiment. Although the speed of writing cells in the buffer memories 11 1 - 11 10 is set at 2 times the incoming line speed in the above case, that speed may generally be set to w (2 ≦ w < number of incoming lines) times the incoming line speed. While it is assumed that the buffer memories 11 1 - 11 10 are each constituted by dual-port memories, this embodiment is also made feasible by using single-port memories which can operate at a double speed or more. In the cell exchanging apparatus according to the fourth embodiment, as described above, destinations of cells are detected and the cells are then written, at a speed w (2 ≦ w < number of incoming lines) times the incoming line speed, in the buffer memories selected by the incoming line spatial switch. It is therefore possible to write no more than w cells in the same buffer memory and hence to reduce the number of cells discarded due to collision during the writing operation. The present invention has been described in detail with particular reference to several preferred embodiments thereof, but variations and modifications can be effected without departing from the scope of the invention as defined in the claims. For example: (i) Although the first to fourth embodiments show a single unit cell exchanging apparatus, a plurality of cell exchanging apparatuses may be linked to form a multi-stage configuration; (ii) Instead of directly designating, as destination (address) information in the header portions, the outgoing line numbers corresponding to the outgoing lines of the cell exchanging apparatus, some conversion process may be allowed; for example, encoded numbers may be provided as destination information in the header portions; (iii) In Figs. 4, 6, 8 and 11, for simplicity of explanation, when the cells arrive through the incoming lines, the cell exchanging apparatus selects buffer memories are candidates from one end thereof to the other (in the order of the buffers #0, #1, ... in Fig. 6) for writing the cells in the selected memories. But, it may be possible when there is a difference in the arrival time of the cells to further reduce the number of discarded cells by selecting the buffer memories which hold the least number of cells, as memory candidates, to write the cells in the selected memories, so that the individual buffer memories operate as if as a whole they substantially have capacity equivalent to that of one large-sized buffer memory shared by all the incoming lines; (iv) Although one cell is output to one outgoing line in the above first to fourth embodiments, it may be possible that the cell selection circuits in the output stage can be arranged to cause one single cell to be output to a plurality of outgoing lines by means of a proper determination of addresses. This permits the addition of a broadcasting function to the cell exchanging apparatus; (v) It may be possible that, as an alternative structure of the cell, a data portion and a header portion of a cell are separated and transmitted through separate circuits having different speeds, the data and the header portions being respectively allocated to a plurality of signal lines arranged in parallel; (vi) Although a link speed of the incoming lines is constant in any of the above first to fourth embodiments, traffic convergence can be made possible by setting the reading speed at the buffer memories higher than the link speed of the incoming lines. Conversely, it is also possible to make the link speed of the incoming lines higher than the outgoing line speed. When a plurality of cell exchanging apparatuses are linked with each other as mentioned above, the number of discarded cells between the adjacent stages of the cell exchanging apparatuses can be further lowered by setting a transfer speed between the stages higher than the incoming line speed; (vii) In any of the above first to fourth embodiments, the address queues are provided in correspondence to the outgoing lines of the cell exchanging apparatus. But, it is also possible to allocate to each outgoing line a plurality of address queues having different priorities and read the cells out of the buffer memories in the order of priority based on symbols placed in addition to the outgoing line numbers in the cell headers for indicating the priority; and (viii) If it is required to restrict the operation speed, a serial/parallel converter and a parallel/serial converter may be added before and after the cell exchanging apparatus, respectively, thereby carrying out a parallel signal processing.

Claims: A cell switching apparatus comprising: a plurality of incoming lines (1 1 ,...,1 n ) to which cells, each including a data portion and a header portion including destination information, are input; a plurality of outqoing lines (2 1 ,...,2 m ), to which the cells are output in accordance with the destinations specified by the header portions; header processing circuits means (10 a1 ,...,10 an ) provided in correspondence to said incoming lines (1 1 ,...,1 n ) for detecting the destination outgoing lines (2 1 ,...,2 m ) from the header portions of the cells input through said incoming lines (1 1 ,...,1 n ) ; a plurality of buffer memory means (11 1 ...,11 p ) in which the cells are written at empty addresses provided by storage control circuit means (12 1 ,...,12 p ) and from which the cells are read out at the addresses thus specified independently of the order of the cells written; an incoming line spatial switch means (13) for selectively connecting said incoming lines (1 1 ,...,1 n ) to said buffer memory means (11 1 ,...,11 n ) selected by a buffer control circuit means (15a, 15b, 15c, 15d); an outgoing line spatial switch means (14) for selectively connecting said buffer memory means (11 1 ,...,11 p ) to said outgoing lines (2 1 ,...,2 m ) determined by said buffer control circuit means (15a, 15b, 15c, 15d) according to the destination information; said buffer control circuit means (15a, 15b, 15c, 15d) controls said incoming line spatial switch means (13) to select the buffer memory means (11 1 ,...,11 p ) in which the cells are to be written, and manages the addresses of the cells written in the buffer memory means (11 1 ,...,11 p ) for each of the outgoing lines (2 1 ,···,2 m ), to control said outgoing line spatial switch means (14) in accordance with the addresses under management so that the are output in the order in which the addresses arrive to the outgoing lines (2 1 ,...,2 m ) specified by the header portions of the cells, characterized in that the number of buffer memories means (11 1 ,...11 p ) is at least the number of incoming lines (1 1 ,...,1 n ) plus the number of outgoing lines minus one (p ≥ n + m - 1), and that the incoming line spatial switch means (13) respectively selects a single of the plurality of buffer memory means (11 1 ,...,11 p ) to write a cell into in such a way that it is avoided to store two or more cells in the same buffer memory means (11 1 ...11 p ) which are to be read simultaneously. A cell switching apparatus according to claim 1, further characterized in that said header processing circuit means (10 a1 ...,10 an, ) hold the cells input through said incoming lines (1 1 ,...,1 n ) for a period of one time slot, and that said buffer control circuit means (15b) causes the cells held in said header processing circuit means (10 a1 ,...,10 an ) to be written, in one time slot, in the buffer memory means (11 a ,...,11 p ) which are selected so as to avoid writing of a plurality of cells in the same buffer memory means (11 a ,...,11 p ) in the same time slot and reading of a plurality of cells out of the same buffer memory means (11 a ,...,11 p ) in the same time slot. A cell switching apparatus according to claim 1, further characterized in that the cells are read out of the buffer memory means (11 a ,...,11 p ) at a speed r times the outgoing line speed, where 2 ≤ r < number of outgoing lines, and that outgoing line speed adjusting buffer means (23 1 ,...,23 m ) are provided in correspondence to said outgoing lines (2 1 ,...,2 m ) for storing the cells. A cell switching apparatus according to claim 1, further characterized in that incoming line speed adjusting buffer means (24 1 ,...,24 n ) are provided in correspondence to said incoming lines (1 1 ,...,1 n ) for storing the cells and that the cells are written into the buffer memory means (11 1 ,...,11 p ) with a speed w times the incoming line speed, where 2 ≤ w < number of incoming lines.

Inventors: MIURA SETSUKO, OSHIMA KAZUYOSHI, YAMANAKA HIDEAKI, MIURA, SETSUKO, OSHIMA, KAZUYOSHI, YAMANAKA, HIDEAKI, MIURA, SETSUKO, C/O TSUSHIN SYSTEM KENKYUSHO, OSHIMA, KAZUYOSHI, C/O TSUSHIN SYSTEM KENKYUSHO, YAMANAKA, HIDEAKI, C/O TSUSHIN SYSTEM KENKYUSHO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: G06F5
IPC codes: H04L12, G06F5