A supervision control system for an ATM switching system

Abstract: A supervision control system (23) for an ATM cell switching system counts (21) the number of cells transmitted from a subscriber in a predetermined duration unit, attaches a sign to the cells when the counted value exceeds a predetermined value, and discards the cells to which the sign is attached when a buffer does not have enough capacity during a cell multiplexation.

Description: This invention pertains to a supervision control method and apparatus for supervising the inflow of cells, e. g. to an ATM switching system. The paper "An ATM Switching System based on a distributed Control Architecture" by T. Koinuma (International Switching Symposium, vol. 5, June 1990, New York) describes a policing function for use in ATM networks in which a marking method allows excess cells to enter the network with violation tags in their headers and discards them only at congested nodes. There is no suggestion of marking excess cells in any other way, or of any advantage to be gained by doing so. The paper "Toward a Broadband Congestion Control Strategy" by C. A. Cooper et al. (IEEE Network: The Magazine of Computer Communications, vol. 4., no. 3, May 1990, New York) describes a similar technique and also considers the use of multiple thresholds. An ATM switching system switches cells having fifty-four (54) bytes split from various data, such as voice data, image data and other numerical data, and transmits in a multiplexed form the various data at different transmission speeds. Of the fifty-four (54) bytes in a cell, forty-eight (48) bytes form an information field and six (6) bytes form a header part. Of the four (4) bytes, i.e. thirty-two (32) bits, in the header part excluding two (2) bytes forming a tag, twenty-eight (28) bits indicate a VPI/VCI (virtual path identifier/virtual channel identifier), two (2) bits indicate a PT (payload type), one (1) indicates a CLP (cell loss priority) and one (1) bit is for a RES (reserve). A buffer in a multiplexer for these cells can experience an overflow when an unexpected number of cells flow in. Since overflowing cells are discarded, the communications quality is deteriorated. Although such a problem can be avoided by using a buffer having a larger capacity, a larger delay results, which is critical in transmitting voice data, image data, and so forth. Therefore, a buffer is required in which the occurrence of an overflow and its effect are minimized. The invention aims at avoiding a congestion state caused by a cell inflow over a band declared by a subscriber. According to a first aspect of the invention, there is provided a supervision control method in which a fixed length cell including a header field with an identified virtual connection, is received from a source, comprising: storing bandwidth information corresponding to the source of the fixed length cell, for transmission of the fixed length cell, the bandwidth information being stored at a memory address which is operatively accessed based on the identified virtual connection; monitoring a transmission rate on the identified virtual connection specified by a received fixed length cell; comparing the transmission rate with the bandwidth information stored at the memory address; and marking the received fixed length cell to indicate a higher cell loss priority when the monitored transmission rate of the received fixed length cell exceeds a rate corresponding to the bandwidth information. According to a second aspect of the invention, there is provided a supervision control apparatus in which a fixed length cell including a header field with an identified virtual connection, is received from a source, comprising: a memory (57) to store bandwidth information corresponding to the source of the fixed length cell, for transmission of the fixed length cell, the bandwidth information being stored at an address which is operatively accessed based on the identified virtual connection; a counter to monitor a transmission rate on the identified virtual connection specified by a received fixed length cell; a comparator, coupled to said memory and said cell counter, to compare the monitored transmission rate of said cell counter with the bandwidth information stored in said memory; and a judge circuit, coupled to said comparator, to mark the received fixed length cell to indicate a higher cell loss priority when the monitored transmission rate exceeds a rate corresponding to the bandwidth information. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block diagram of an ATM switching system; Figure 2 shows a cell configuration; Figure 3 is a block diagram of one (1) of the broadband remote switching units (BRSU) 7; Figure 4 is a block diagram of one embodiment of this invention; Figure 5A is a block diagram of parts pertinent to a first embodiment of this invention; Figure 5B illustrates an example of cell discarding; Figure 6 is a block diagram of a subscriber line trunk pertinent to a second embodiment of this invention; Figure 7 is a block diagram of the supervisor and the selection control processor pertinent to the second embodiment of this invention; and Figure 8 illustrates the input interface of a multiplexer. As shown in Figure 1, a plurality of terminals (TE) 10 are connected through respective terminal adapters (TA) 9 to one of plural network terminators (NT) 8 in an ATM switching system. The network terminators (NT) 8 are connected through respective interoffice trunks 6 to one (1) of plural broadband remote switching units (BRSU) 7. The broadband remote switching units (BRSU) 7 are connected to one (1) of a plurality of central offices (CO) 5. The interoffice trunks 6 interconnect the central offices (CO) 5. The interoffice trunks 6 and subscriber lines are high speed transmission paths, such as an optical transmission path e.g. comprising a SONET (Synchronous Optical Network), having bands of one hundred fifty-five mega-hertz (155 MHz), six hundred twenty-two mega-hertz (622 MHz) or higher. Figure 2 shows a cell configuration. As described earlier, voice data, image data, numerical data, etc. from the terminals (TE) 10 are split into cells having fifty-four (54) bytes. Because one (1) word is defined to comprise two (2) bytes, a cell has twenty-seven (27) words. As shown in Figure 2, of the fifty-four (54) bytes in a cell, forty-eight (48) bytes form an information field and six (6) bytes form a header part. Of the six (6) bytes, i.e. forty-eight (48) bits, in the header part, two (2) bytes, i.e. sixteen (16) bits, form a tag, twenty-eight (28) bits indicate a VPI/VCI (virtual path identifier/virtual channel identifier), two (2) bits indicate a PT (payload type), one (1) bit indicates a CLP (cell loss priority) and one (1) bit is for a RES (reserve). The network terminators (NT) 8 send the cells to the broadband remote switching units (BRSU) 7. The terminal adapters (TA) 9 receive cells from corresponding terminals (TE) 10 through the network terminators (NT) 8 and send the decelled voice data, image data, numerical data, etc. to the terminals (TE) 10. Figure 3 is a block diagram of one (1) of the broadband remote switching units (BRSU) 7. A multiplexer/demultiplexer (MDX) 12 multiplexes cells asynchronously inputted from subscribers through a buffer and transmitted over a plurality of subscriber line trunks 11, which are connected to respective subscriber lines. A concentrator switch (CSW) 13 switches the multiplexed cells. A multiplexer/demultiplexer (MDX) 14 demultiplexes the switched cells over to an appropriate one (for the predetermined central office) of a plurality of interoffice trunks 15, which form interoffice cell transmission paths. The multiplexer/demultiplexer (MDX) 14 multiplexes cells asynchronously inputted from correspondent subscribers through a buffer and transmitted over the interoffice trunks 15. The concentrator switch (CSW) 13 switches the multiplexed cells. The multiplexer/demultiplexer (MDX) 12 demultiplexes the switched cells over to an appropriate one (for the predetermined subscriber) of a plurality of subscriber line trunks 11. A local processor interface (LPIF) 16 connects a local processor (LPR) 17 with the concentrator switch (CSW) 13 and multiplexers/demultiplexers (MDXs) 12 and 14. When a larger number of cells flow into the multiplexer/demultiplexer (MDX) 12, its buffer causes some cells to overflow. Therefore, a virtual path is set at a call-up by having a subscriber declare his cell transmission bandwidth and by judging whether or not the cells can be multiplexed in the declared bandwidth. In this case, if this subscriber sends cells outside the declared bandwidth, cells from other subscribers who share the same buffer with this subscriber are also discarded. That is, if any subscriber sends cells outside the declared bandwidth, a large ill-effect occurs such that cells from other subscribers are also discarded. A supervision control system hereinafter defined prioritizes discarding of cells overflowing from a buffer by attaching a high cell-loss-priority sign to incoming cells outside the bandwidth declared by a subscriber, thereby minimizing ill-effects to other subscribers. As described, the supervision control apparatus for an ATM (Asynchronous Transmission Mode) cell switching system comprises a supervisor and a multiplexer. The supervisor further comprises a cell counter and a judge. The cell counter counts cells transmitted from a subscriber in a predetermined duration unit. The judge attaches a sign to the cells when the value counted by the cell counter exceeds a predetermined value. The multiplexer discards the cells to which the sign is attached when a buffer does not have enough capacity during a cell multiplexing. Figure 4 is a block diagram of this embodiment The supervision control apparatus for an ATM (Asynchronous Transmission Mode) cell switching system comprises a supervisor 23 and a multiplexer 24. A supervisor 23 further comprises a cell counter 21 and a judge 22. The cell counter 21 counts cells transmitted from a subscriber in a predetermined duration unit. The judge 22 attaches a sign to the cells when the value counted by the cell counter 21 exceeds a predetermined value. The multiplexer 24 discards the cells to which the sign is attached when a buffer does not have enough capacty during a cell multiplexation. That is, the cell counter 21 in the supervisor 23 counts cells flowing in from a subscriber in a predetermined duration unit. The judge 22 in the supervisor 23 judges whether or not the cells are flowing in outside the bandwidth declared by the subscriber and attaches a sign to a predetermined bit in the excess cells flowing in outside the declared bandwidth. The multiplexer 24 multiplexes the cells having the sign for their transmission when its buffer has sufficient capacity to handle them. However, the multiplexer 24 discards them with high priority, when neither the buffer in the multiplexer 24 itself nor the buffer in a concentrator switch, or in a demultiplexer, connected to the multiplexer 24 has sufficient capacity to handle the cells, thereby safeguarding the multiplexing and transmission of cells having no such sign inputted from other subscribers. Alternatively, the judge 22 in the supervisor 23 can set a first judging threshold and a second judging threshold. The first judging threshold corresponds to a bandwidth declared by a subscriber. The second judging threshold is set higher than the first judging threshold. The judge 22 attaches a sign to the inflowing cells from a subscriber in excess of the first judging threshold, and discards the inflowing cells from the subscriber in excess of the second judging threshold. That is, the judge 22 sets the first and second judging thresholds in correspondence with a bandwidth declared by a subscriber. The judge 22 attaches a sign to the excess inflowing cells over the first judging threshold by appropriately marking the CLP (cell loss priority) bit or the RES (reserve) bit of the cell, thereby prioritizing the discarding of such cells, when neither the buffer in the multiplexer 24 itself nor the buffer in a concentrator switch, or in a demultiplexer, connected to the multiplexer 24 has enough capacity to handle the cells. The judge 22 always discards the inflowing cells in excess of the second judging threshold, because they are likely to cause an overflow in the buffer. Embodiments of this invention are explained in further detail by referring to some of the attached drawings. First Embodiment Figure 5A is a block diagram of parts pertinent to a first embodiment of this invention. Subscriber line terminators 31-1 through 31-m each paired with the corresponding one of interfacers 32-1 through 32-m are equivalent to subscriber line trunks connecting subscribers to the ATM cell switching system. A multiplexer 34 receives cells from subscribers by way of the subscriber line terminnators 31-1 through 31-m and the interfacers 32-1 through 32-m. Supervisors 33-1 through 33-m respectively supervise cell inflows to the multiplexer 34 via the subscriber line terminators 31-1 through 31-m and the interfacers 32-1 through 32-m. When a subscriber calls up, he declares the bandwidth to be occupied. A processor 37 reads from a cell its VCI (virtual channel identifier) for specifying the switchers and the subscriber and its VPI (virtual path identifier) for specifying the paths among the switchers, and notifies the supervisors 33-1 through 33-m of the VCI/VPI and the maximum number of cells, set in correspondence with the subscriber declared bandwidth, passing through in a predetermined duration unit. The supervisors 33-1 through 33-m count the number of cells having particular VCIs/VPIs in the predetermined duration units, and attach a sign to the cells in excess of the maximum number of cells, set in correspondence with the subscriber declared bandwidth, passing through in a predetermined duration unit, for example, by marking the CLP (cell loss priority) bit in those cells. Figure 5B illustrates an example of cell discarding. Alternatively, the supervisors 33-1 through 33-m can attach a sign to the cells in excess of the maximum number of cells, set in correspondence with the subscriber declared bandwidth, passing through in a predetermined duration unit, by marking the RES (reserve) bit in those cells, as shown in Figure 5B. The multiplexer 34 has a buffer. It multiplexes cells from subscribers and supplies the multiplexed cells to a concentrator switch 35. However, it multiplexes cells having the sign marked in their CLP bit or in their RES bit only when the buffer has enough capacity, and discards them with high priority when the buffer does not have enough capacity. This ensures the multiplexing of cells from subscribers transmitted within the declared bandwidths. Also, even when the buffer in the multiplexer 34 has enough capacity to handle the cells, the buffer at an input terminal or at an output terminal of a device connected in a later stage, such as the concentrator switch 35 and a demultiplexer on the receiving side, discards the cells, if it does not have enough capacity. Also, the supervisors 33-1 through 33-m can attach a sign to the cells in excess of a first judging threshold equivalent to the maximum number of cells, set in correspondence with the subscriber declared bandwidth, passing through in a predetermimed duration unit, by marking either the CLP bit or the RES bit in those cells. The multiplexer 34 prioritizes the discarding of those cells having the sign, when its buffer lacks enough capacity. Other devices connected in later stages discard cells similarly, when their respective buffers lack enough capacity. When the number of cells passing through in a predetermined duration unit further increases and reaches a second judging threshold, the multiplexer 34 can be made to perform a control such that the excess cells over the second judging threshold are discarded regardless of the available capacity of the buffer in the multiplexer 34. The local processor interface 36 notifies the local processor 37 of the results of processing by the supervisors 33-1 through 33-m. Second Embodiment Figure 6 is a block diagram of a subscriber line trunk pertinent to a second embodiment of this invention. A subscriber line 41 such as a SONET based optical transmission path transmits cells in light signals. A light-electricity (electro-optic/opto-electric) converter (EO/OE) 42 converts the cells in light signals to cells in electric signals and supplies the converted cells through a terminator 43 and an interfacer 44 to a selection control processor 46. A local processor 52 notifies a supervisor 45 through a local processor interface 51 of a VCI/VPI and first and second judging thresholds set correspondingly to a subscriber declared bandwidth. An alternative arrangement can be made such that the processor notifies the supervisor 45 of a subscriber declared bandwidth and the supervisor 45 sets first and second judging thresholds accordingly. The supervisor 45 counts the number of cells having a particular VCI/VPI passing through the interfacer 44 in a predetermined duration unit for a comparison with the first and second judging thresholds. The supervisor 45 has the selection control processor 46 turn on e.g. the CLP bit in the header part of the cells exceeding the first judging threshold, so that discarding of those cells is prioritized when the buffer of a multiplexer 49, or the buffer of a device connected in a later stage, lacks enough capacity. Also, the supervisor 45 has the selection control processor 46 turn on e.g. the RES bit in the header part of the cells exceeding the second judging threshold, so that those cells are supplied to the multiplexer 49 as empty cells, thereby causing those cells to be discarded anyway regardless of the available capacity in the buffer of the multiplexer 49. This is because the multiplexer 49 does not send empty cells to a concentrator switch 50. Figure 7 is a block diagram of the supervisor 45 and the selection control processor 46 pertinent to the second embodiment of this invention. A buffer 55 delays cells in correspondence with the delays caused by respective supervisions. A memory 56 counts the number of cells passing through in a predetermined duration unit. Memories 57 and 58 respectively store first and second judging thresholds. A memory 59 measures an elapse of the predetermined duration unit. A memory 60 stores duration unit data. When the supervisor 45 commences its operations, a processor CC instructs selectors 65, 66 and 69 to select their "b" side inputs and to output them to respective address terminals of memories 57, 58 and 60, which receive data from the processor CC as respective data inputs. The processor CC supplies to "b" side inputs of selectors 65, 66 and 69 an address by which memories 57, 58 and 60 are accessed. The processor CC accesses memories 57, 58 and 60 to have them respectively store the first judging threshold, the second judging threshold and the duration unit data. When the supervisor 45 commences its operations, the processor CC also instructs selectors 67 and 68 to select their "a" side inputs and to output, so that memory 59 receives an address and data from the processor CC. Memory 59 stores as its initial data duration unit data similar to those stored in memory 60. The processor stores data to be stored in memories 57, 58, 59 and 60 at respective addresses corresponding to VPIs/VCIs. When the supervisor 45 commences its operations, memory 60 is reset and its stored data are initialized to zero (0). That is, the processor CC supplies the first and second judging thresholds set correspondingly to the subscriber declared bandwidth, the duration unit data indicating the supervision cycle and the addresses corresponding to the subscriber assigned VPIs/VCIs. Memory 57 stores the first judging threshold. Memory 58 stores the second judging threshold. Memory 60 stores the duration unit data. Although all the duration unit data can be set the same, they can be set in correspondence with the subscriber characteristics. This is because pass-through cells sent from a subscriber in a burst need to be counted over a comparatively long period of time, whereas those sent from another in a more or less constant speed can be counted over a relatively short period of time. Meanwhile, when the supervisor 45 commences its operation, selectors 63, 65 and 66 select their "a" side inputs, so that the memories 56, 57 and 58 are accessed by using the VPIs/VCIs in the header part of cells as addresses. Selector 64 has memory 56 count the number of cells passing through by having an incrementer 70 increment the value by one (1) in an address region of memory 56 corresponding to the VPIs/VCIs. Selector 68 has memory 59 store duration unit data stored in memory 60, each time the value stored in memory 59 becomes zero (0). A decrementer 71 decrements the value stored in memory 59 by one (1). When the value stored in memory 59 reaches zero (0), memory 56 clears its address regions corresponding to the VPIs/VCIs set in correspondence with the duration unit data. That is, memory 56 counts cells passing through in predetermined units in the address fields corresponding to their VPIs/VCIs. Then, memory 59 again stores duration unit data stored in memory 60, again. A counter 72 generates respective addresses for managing duration units e.g. up to the maximum value of the VPIs/VCIs. Although this is not shown in the drawings, the counter 72 receives a clock signal, accesses memory 59, each time its own value is incremented by one (1), by using the incremented value as the address, and decrements the value stored in memory 59 by one (1). At this time, selector 67 selects its "b" side input and selector 68 selects its "c" side input. A count cycle [from zero (0) to next zero (0)] in the counter 72 becomes the basis of the duration unit data stored in the memory 60. The values obtained by multiplying the count cycle by the duration unit data become duration units corresponding to the VPIs/VCIs. When the value stored in memory 59 accessed by value of the counter 72 becomes zero (0), selector 68 switches its selection from "c" side input to "b" side input, thereby loading the duration unit data in memory 60, again. The counting by the counter 72 enables the addresses corresponding to all the VPIs/VCIs to be specified, thereby realizing duration units to be managed. Ordinarily, selectors 63, 65, and 66 select their "a" side inputs, access the corresponding address each time a cell passes through, and perform the following operations. The count value of the cells passing through stored in memory 56 is supplied to the comparator 61 through the incrementer 70. The comparator 61 receives the first judging threshold stored in memory 57 and the second judging threshold stored in memory 58. When the number of cells passing through is less than the first and second thresholds, the comparator 61 does not control the selector 62 and cells passing through the buffer 55 also pass through selector 62 without any obstruction. Also, when the number of cells passing through is more than the first threshold but less than the second threshold, the excess cells over the first judging threshold are transmitted after selector 62 turns on the CLP bit in the header part of those cells. Finally, when the number of cells passing through is more than the second judging threshold, selector 62 converts the excess cells over the second judging threshold to empty cells, where all the bits are zero (0). The comparator 61 notifies the processor CC of the result of respective processings. Because selector 62 supplies cells to the multiplexer for cell multiplexing, cells with the CLP bit on are discarded when the buffer lacks the necessary capacity. Since cells from other subscribers are within their declared bandwidths, even if a subscriber sends cells in excess of his declared bandwidth, ill-effects on other subscribers are minimized. When the numbers of cells from other subscribers are small, i.e. when the buffer has enough capacity, the multiplexer multiplexes even the cells with their CLP bit on. Cells far exceeding the declared bandwidth are discarded, e.g. through an empty cell conversion, to avoid discarding of cells from other subscribers within their declared bandwidths. In the first and second embodiments of this invention, the supervisors 23, 33-1 through 33-m, and 45 output cells in which all the bits are zero (0) to multiplexers 24, 34 and 49, where these cells are actually discarded. However, this invention is not limited to such a configuration. Figure 8 illustrates the input interface of a multiplexer. As shown in Figure 8, the multiplexers 24, 34 and 49 can use an input interface such that a "Data Line" comprises sixteen (16) bits, a "Cell Frame" comprises one (1) bit, an "Enable" comprises one (1) bit and a "Parity" comprises one (1) bit. The "Data Line" is a signal line for transmitting the cell width. "Cell Frame" is a signal line specifying the head end of each cell by a pulse indicating a "Cell Slot". "Enable" is a signal line specifying the cells effectiveness. "Parity" is a data error scanning line. When the numbers of cells are more than the second threshold, the supervisors 23, 33-1 through 33-m and 45 turn off "Enable" without performing any data control. The multiplexers 23, 34 and 49 discard cells without multiplexing them when their "Enable" is off. This invention is not limited to those embodiments, but instead can be applied in various derivative forms. For instance, it goes without saying that counting of the number of cells passing through by memory 56 can be substituted by counting by an ordinary counter. Further, this invention can be applied also to any cell switching system, in addition to an ATM cell switching system. As explained earlier, in an ATM switching system its cell counter 21 in its supervisor 23 counts the cells inputted from respective subscribers. When the judge 22 discovers that the number of cells inputted from a particular subscriber exceeds his declared band, the judge 22 attaches a sign to the excess cells to be sent to the multiplexer 24 e.g. by turning on their CLP bit. When its buffer has enough capacity, the multiplexer 24 ordinarily multiplexes the cells. However, when its buffer lacks it, the multiplexer 24 prioritizes the discording of these cells. Thus, even if its buffer lacks enough capacity, the multiplexer 24 prioritizes the multiplexation of cells from other subscribers, as long as such cells are within their declared bandwidths. Hence, the distinct advantage is achieved of eliminating a possible ill-effect on others when a particular subscriber sends his cells outside his declared bandwidth, while maintaining the best communications quality by avoiding unnecessary cell discarding. Alternatively, the apparatus can be structured such that first and second judging thresholds are used for an even finer cell discarding control. That is, a sign is attached to excess cells over the first judging threshold e.g. by turning on the CLP bit, and excess cells over the second judging threshold are converted to empty cells and discarded. As a result, cells exceeding a declared bandwidth by a narrow margin can be multiplexed ordinarily depending on the availability of buffering capacity, while cells exceeding a declared bandwidth by a wide margin are discarded to minimize the ill-effect on other subscribers. Thus, this technique is effective in reduction of discarding cells within declared bandwidths and in preventing a deterioration in communications quality by supervising cells flowing in an ATM cell switching system.

Claims: A supervison control method in which a fixed length cell including a header field with an identified virtual connection, is received from a source, comprising: storing bandwidth information corresponding to the source of the fixed length cell, for transmission of the fixed length cell, the bandwidth information being stored at a memory address which is operatively accessed based on the identified virtual connection; monitoring a transmission rate on the identified virtual connection specified by a received fixed length cell; comparing the transmission rate with the bandwidth information stored at the memory address; and marking the received fixed length cell to indicate a higher cell loss priority when the monitored transmission rate of the received fixed length cell exceeds a rate corresponding to the bandwidth information. A supervision control method according to claim 1, wherein said monitoring comprises counting a number of the received fixed length cells having the same identified virtual connection in a predetermined duration unit. A supervision control method according to claim 1 or 2, further comprising discarding a received fixed length cell which has been marked to indicate a higher cell loss priority when a buffer has less capacity than an operative capacity for buffering the received fixed length cell. A supervision control apparatus in which a fixed length cell including a header field with an identified virtual connection, is received from a source, comprising a memory (57) to store bandwidth information corresponding to the source of the fixed length cell, for transmission of the fixed length cell, the bandwidth information being stored at an address which is operatively accessed based on the identified virtual connection; a counter to monitor a transmission rate on the identified virtual connection specified by a received fixed length cell; a comparator coupled to said memory and said counter, to compare the monitored transmission rate of said counter with the bandwidth information stored in said memory; and a judge circuit coupled to said comparator, to mark the received fixed length cell to indicate a higher cell loss priority when the monitored transmission rate exceeds a rate corresponding to the bandwidth information. A supervision control apparatus according to claim 4, wherein said counter counts a number of the received fixed length cells having the same identified virtual connection in a predetermined duration unit. A supervision control apparatus according to claim 4 or 5, further comprising a discard circuit to discard a received fixed length cell which has been marked to indicate a higher loss priority when a buffer has less capactiy than an operative capacity to store the received fixed length cell.

Inventors: AIHARA NAOKI, ASO YASUHIRO, KAKUMA SATOSHI, MURAYAMA MASAMI, YOSHIMURA SHUJI, AIHARA, NAOKI, ASO, YASUHIRO, KAKUMA, SATOSHI, MURAYAMA, MASAMI, YOSHIMURA, SHUJI, Aihara, Naoki, c/o Fujitsu Limited, Aso, Yasuhiro, c/o Fujitsu Limited, Kakuma, Satoshi, c/o Fujitsu Limited, Murayama, Masami, c/o Fujitsu Limited, Yoshimura, Shuji, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H04L12
IPC codes: H04M3, H04L12