Abstract:
There is disclosed an ATM layer device configured to realize the function of an ATM layer by selecting one from a plurality of physical layer devices realizing the function of a physical layer, and executing a transfer of a cell, which is a unit of transmission, between the ATM layer device and the selected physical layer device. Priority information indicating the previously set priority level of each of said plurality of physical layer devices is held, and the physical layer device which should execute the cell transfer is selected in a predetermined order in accordance with the priority information. Thus, drops in cell transmission rate and receiving buffer overflows in the physical layer device are eliminated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ATM (Asynchronous Transfer Mode) layer device for realizing the function of an ATM layer in an ATM transmission network. 
     2. Description of Related Art 
     In an ATM transmission network adopting the B-ISDN (Broadband Integrated Services Digital Network) standard and others, a protocol for transferring information comprises a plurality of layers. Respective layers have inherent functions, and one layer is realized by utilizing the function of a layer inferior to the first named layer by one layer. 
     Of the plurality of layers, a lower layer, which is the most remote layer from a user&#39;s side, includes a physical layer and an ATM layer. 
     The physical layer is a layer for providing a resource for transferring a cell, which is a unit of information transmission. Specifically, the physical layer generates a signal format suitable for a transmission medium, receives a signal, encodes, and/or generates and eliminates a transmission frame. Furthermore, the physical layer checks whether or not the information included in the cell is effective, and transfers the cell carrying the effective information (effective cell) to the ATM layer. 
     The ATM layer is a layer superior to the physical layer by one layer, and executes a multiplexing and a separation of cells. 
     An ATM layer device, which is a device realizing the function of the ATM layer, and a physical layer device, which is a device realizing the function of the physical layer, are connected through, for example, a UTOPIA interface (an interface standard, Universal Test and Operation PHY Interface for ATM). 
     FIG. 7 is a block diagram illustrating the connection between the ATM layer device and the physical layer devices. 
     As shown in FIG. 7, “n” physical layer devices  102   1  to  102 n (where “n” is a positive number) are connected to an ATM layer device  101  through a UTOPIA interface. The ATM layer device  101  selects one from the “n” physical layer devices  102   1  to  102 n, and transfers a cell between the ATM layer device and the selected physical layer device. Incidentally, the prior art ATM layer device gives an equal opportunity of selection to all the physical layer devices  102   1  to  102 n, since a priority order is not provided among the physical layer devices  102   1  to  102 n. 
     A system for equally selecting the physical layer devices  102   1  to  102 n includes, for example, a “round robin” scheduling. In this “round robin” scheduling system, as shown in FIG. 8, all the physical layer devices  102   1  to  102 n are selected in a predetermined order, as if a closed loop were depicted. 
     In the UTOPIA interface standard, on the other hand, since a transmission rate of the cell is not defined, a physical layer device having a high transmission rate and a physical layer device having a low transmission rate are connected to the ATM layer device in a mixed condition. 
     For example, consider a case in which a plurality of physical layer devices exists, each device having a low transmission rate and simultaneously receiving the cell, and in which one physical layer device having a high transmission rate exists and constantly receives the cell. 
     In this case, if the physical layer devices are selected in accordance with the round robin scheduling, after the ATM layer device receives the cell from the plurality of physical layer devices having the low transmission rate, the ATM layer device receives the cell from the physical layer device having the high transmission rate. 
     Accordingly, before the physical layer device having the high transmission rate transmits the cell to the ATM layer device, it receives a next cell, so that there is a possibility that a receiving buffer for temporarily storing the cells might overflow. 
     Even if the receiving buffer does not overflow, the actual transfer rate becomes smaller than the expected transfer rate because of the waiting time required for transferring the cell to the ATM layer device. 
     Accordingly, it is an object of the present invention to provide an ATM layer device which overcomes the above mentioned problem of the prior art. 
     Another object of the present invention is to provide an ATM layer device capable of preventing drops in the cell transfer rate and overflows of the receiving buffer provided in the physical layer device. 
     The above and other objects of the present invention are achieved in accordance with the present invention by an ATM layer device controlling method realizing the function of an ATM layer by selecting one from a plurality of physical layer devices realizing the function of a physical layer, and executing a transfer of a cell, which is a unit of transmission, between the ATM layer device and the selected physical layer device, the method including the steps of holding the priority information indicating the previously set priority level of each of the plurality of physical layer devices, and selecting the physical layer device that should execute the cell transfer, in a predetermined order in accordance with the priority information. 
     At this time, it is possible to output the address of the physical layer device which should execute the cell transfer, selected in accordance with the priority information, and to execute the cell transfer between the ATM layer device and the physical layer device designated by the address. Alternatively, it is also possible to output the address of the physical layer device of a candidate which should execute the cell transfer, in accordance with the priority information, and to respond to a cell transfer enable notify signal indicating that the cell transfer is possible, outputted from the physical layer device designated by the address, so as to output the address of the physical layer device that should execute the cell transfer. 
     Preferably, the priority is set such that the larger the transmission rate of the physical layer device, the higher the priority level of the physical layer device. 
     According to another aspect of the present invention, there is provided an ATM layer device configured to realize the function of an ATM layer by selecting one from a plurality of physical layer devices realizing the function of a physical layer, and executing a transfer of a cell, which is a unit of transmission, between the ATM layer device and the selected physical layer device, the ATM layer device including a priority information register holding the priority information indicating the previously set priority level of each of the plurality of physical layer devices, a selecting physical layer address output circuit for outputting the address of the physical layer device that should execute the cell transfer, in a predetermined order in accordance with the priority information, and a control circuit for executing the transfer of the cell between the ATM layer device and the physical layer device designated by the address. 
     In one embodiment, the ATM layer device further includes a polling physical layer address output circuit for outputting, in a predetermined order, the address of the physical layer device of a candidate that should execute the cell transfer, selected in accordance with the priority information, and wherein the selecting physical layer address output circuit responds to a cell transfer enable notify signal indicating that the cell transfer is possible, outputted from the physical layer device designated from the address, so as to output the address of the physical layer device that should execute the cell transfer. 
     Preferably, the selecting physical layer address output circuit can include a plurality of priority control registers for holding the information of the physical layer devices in units of different levels of priority, and the polling physical layer address output circuit can include a plurality of priority control registers for holding the information of the physical layer devices in units of different levels of priority. 
     Furthermore, it can be set such that the larger the transmission rate of the physical layer device, the higher the priority level of the physical layer device. 
     With the above mentioned arrangement, the ATM layer device in accordance with the present invention holds the priority information indicating the previously set priority level of each of a plurality of physical layer devices, and selects the physical layer device that should execute the cell transfer, in a predetermined order in accordance with the priority information. Therefore, the higher the priority level of the physical layer device, the larger the opportunity for selection of the physical layer device becomes, so that the data transfer amount per unit of time of a physical layer device having a high priority level can be increased. 
     The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the structure of an ATM layer device in accordance with the present invention; 
     FIG. 2 is a diagram illustrating one example of the content stored in the priority information register provided in the ATM layer device shown in FIG. 1; 
     FIGS. 3A,  3 B and  3 C are diagrams respectively illustrating the contents of the first and fourth priority control registers, the contents of the second and fifth priority control registers, and the contents of the third and sixth priority control registers, provided in the ATM layer device shown in FIG. 1; 
     FIG. 4 is a diagram illustrating one example of a method for selecting the physical layer device by the ATM layer device in accordance with the present invention; 
     FIG. 5 is a flowchart illustrating the processing procedures in the polling physical layer address output circuit provided in an ATM layer device in accordance with the present invention; 
     FIG. 6 is a flowchart illustrating the processing procedures in the selecting physical layer address output circuit provided in an ATM layer device in accordance with the present invention; 
     FIG. 7 is a block diagram illustrating a prior art connection between an ATM layer device and physical layer devices; and 
     FIG. 8 is a block diagram illustrating a prior art method for selecting a physical layer device by an ATM layer device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described with reference to the accompanying drawings. 
     The ATM layer device in accordance with the present invention holds priority information indicating a previously set priority level of respective physical layer devices and selects a physical layer device which should execute a cell transfer, in accordance with the priority information. 
     In the following discussion, only circuits which are provided in the ATM layer device and which realize the function of selecting the physical layer device will be described. However, as a matter of course, the ATM layer device includes means for realizing the function of the cell multiplexing and the function of the cell separation, but since these means do not have a close relation to the present invention, these means are omitted in the drawings, and explanation thereof will be omitted. 
     In addition, the connection between the ATM layer device and the physical layer vices is similar to that of the prior art (FIG.  7 ). 
     FIG. 1 is a block diagram illustrating the structure of the ATM layer device in accordance with the present invention. 
     As shown in FIG. 1, an ATM layer device  1  is connected to “n” physical layer devices  2   1  to  2 n and a host controller  3  for generating a control signal for rewriting the priority information of the physical layer devices  2   1  to  2 n. 
     The ATM layer device  1  outputs the address of the physical layer device ( 2   1  to  2 n) that should execute the cell transfer, and on the other hand, each of the physical layer devices  2   1  to  2 n outputs a cell transfer enable notify signal indicating whether or not the corresponding physical layer device can execute the cell transfer. 
     The ATM layer device  1  includes a priority information register  11  for holding the priority information of the respective physical layer devices  2   1  to  2 n, a selecting physical layer address output circuit  12  for outputting a first physical layer address, which is the address of the physical layer device that should execute the cell transfer (namely, the address of the selected physical layer device), a polling physical layer address output circuit  13  for outputting a second physical layer address, which is the address of the physical layer device that is a candidate for selection, a first selector  14  for selecting either of the first physical layer address and the second physical layer address in accordance with a first selection signal outputted from the selecting physical layer address output circuit  12 , and a second selector  15  for alternately outputting the physical layer address outputted from the first selector  14  and the data “1” (of the same bit number as that of the physical layer address) in accordance with a second selection signal supplied from a control circuit (not shown), in order to output the address signal defined in the UTOPIA interface. 
     The selecting physical layer address output circuit  12  includes a first priority control register  16   1  for holding the information of the physical layer devices having the highest priority, a second priority control register  16   2  for holding the information of the physical layer devices having a middle priority, and a third priority control register  16   3  for holding the information of the physical layer devices having the lowest priority. 
     Similarly, the polling physical layer address output circuit  13  includes a fourth priority control register  17   1  for holding the information of the physical layer devices having the highest priority, a fifth priority control register  17   2  for holding the information of the physical layer devices having a middle priority, and a sixth priority control register  17   3  for holding the information of the physical layer devices having the lowest priority. 
     FIG. 2 is a diagram illustrating one example of the content stored in the priority information register  11  shown in FIG.  1 . FIGS. 3A,  3 B and  3 C are diagrams illustrating the contents of the priority control registers shown in FIG.  1 . FIG. 3A is a diagram illustrating the contents of the first and fourth priority control registers  16   1  and  17   1 , FIG. 3B is a diagram illustrating the contents of the second and fifth priority control registers  16   2  and  17   2 , and FIG. 3C is a diagram illustrating the contents of the third and sixth priority control registers  16   3  and  17   3 . 
     As shown in FIG. 2, the priority information of each of the physical layer devices  2   1  to  2 n is stored in the priority information register  11  in units of two bits. 
     In addition, as shown in FIG. 3A, in each of the first priority control register  16   1  and the fourth priority control register  17   1 , the data “1” is written into the bit(s) corresponding to the physical layer device(s) having the highest priority. 
     Similarly, as shown in FIG. 3B, in each of the second priority control register  16   2  and the fifth priority control register  17   2 , the data “1” is written into the bit(s) corresponding to the physical layer device(s) having the middle priority. As shown in FIG. 3C, in each of the third priority control register  16   3  and the sixth priority control register  17   3 , the data “1” is written into the bit(s) corresponding to the physical layer device(s) having the lowest priority. 
     Incidentally, FIG.  2  and FIGS. 3A,  3 B and  3 C illustrate the examples of the contents stored in the priority information register  11 , the first to third priority control registers  16   1  to  16   3  and the fourth to sixth priority control registers  17   1  to  17   3  in the case having  31  physical layer devices. 
     The priority control registers are provided in accordance with the number of priority levels. For example, when the priority has two levels (namely, a high priority and a low priority), it is sufficient if each of the selecting physical layer address output circuit  12  and the polling physical layer address output circuit  13  includes two priority control registers. 
     With the above mentioned arrangement, the priority information register  11  is previously written with the priority information, as shown in FIG. 2, of the respective physical layer devices  2   1  to  2 n, by the host controller  3 . 
     The ATM layer device  1  first outputs from the polling physical layer address output circuit  13  the second physical layer address, which is the address of the physical layer device that is a candidate for selection. 
     At this time, the selecting physical layer address output circuit  12  outputs the first selection signal causing the first selector  14  to output the second physical layer address. The second selector  15  alternately outputs the second physical layer address outputted from the first selector  14  and the data “1” of the bit number equal to that of the second physical layer address, in accordance with the second selection signal, so as to transmit it to the selecting physical layer address output circuit  12  and the physical layer devices  2   1  to  2 n. 
     The physical layer device designated by the second physical layer address outputs the cell transfer enable notify signal, indicating whether or not the cell transfer is possible, to the selecting physical layer address output circuit  12 . 
     The selecting physical layer address output circuit  12  receives the second physical layer address outputted from the second selector  15  and the cell transfer enable notify signal outputted from the physical layer device designated by the second physical layer address, and outputs the address of the physical layer device that can execute the cell transfer (first physical layer address) and simultaneously outputs the first selection signal causing the first selector  14  to output the first physical layer address. 
     Next, the second selector  15  alternately outputs the first physical layer address outputted from the first selector  14  and the data “1” of the bit number equal to that of the first physical layer address, in accordance with the second selection signal, so as to transmit it to the selecting physical layer address output circuit  12  and the physical layer devices  2   1  to  2 n. 
     The physical layer device designated by the first physical layer address executes the cell tranfer between the physical layer device designated by the second physical layer address and a control circuit (not shown) provided in the ATM layer device. 
     For example, considering the case shown in FIG. 4, the  31  physical layer devices are divided into the physical layer devices PHY 0  and PHY 3  having the highest priority, the physical layer devices PHY 1  and PHY 2  having the middle priority and the physical layer devices PHY 4  to PHY 30  having the lowest priority. In this case, the physical layer devices are selected in the order determined by a priority-based hierarchical combination of a loop comprising the physical layer devices having the highest priority, another loop comprising the physical layer devices having the middle priority, and still another loop comprising the physical layer devices having the lowest priority, as in the named order of PHY 0 , PHY 3 , PHY 1 , PHY 0 , PHY 3 , PHY 2 , PHY 0 , PHY 3 , PHY 4 , PHY 0 , . . . 
     Next, the processing procedures in the polling physical layer address output circuit  13  and the processing procedures in the selecting physical layer address output circuit  12  will be described with reference to FIGS. 5 and 6. 
     FIG. 5 is a flowchart illustrating processing procedures in the polling physical layer address output circuit shown in FIG.  1 . FIG. 6 is a flowchart illustrating the processing procedures in the selecting physical layer address output circuit shown in FIG.  1 . 
     In the flowchart shown in FIG. 5, the polling physical layer address output circuit  13  first writes the priority information of the respective physical layer devices  2   1  to  2 n to the fourth to sixth priority control registers  17   1  to  17   3  on the basis of the content of the priority information register  11  (step S 1 ). 
     Next, the polling physical layer address output circuit  13  ascertains whether or not all the bits of the fourth priority control register  17   1  are “0” (step  2 ). If all the bits of the fourth priority control register  17   1  are not “0”, the polling physical layer address output circuit  13  outputs the address of the physical layer device corresponding to the least significant bit of the “1”-written bits in the fourth priority control register  17   1  (the second physical layer address) (step S 3 ). 
     Furthermore, the polling physical layer address output circuit  13  rewrites the bit of the fourth priority control register  17   1  corresponding to the physical layer device selected in step S 3 , to “0” (Step S 4 ), and returns to step S 2 , so as to repeat the processing of steps S 2  to S 4  until all the bits of the fourth priority control register  17   1  become “0”. 
     When all the bits of the fourth priority control register  17   1  are “0” in step S 2 , the polling physical layer address output circuit  13  writes again the priority information of the corresponding physical layer devices ( 2   1  and  2   4  in the example of FIG. 4) to the fourth priority control register  17   1  on the basis of the content of the priority information register  11  (step S 5 ). 
     After completion of step S 5 , the processing goes into step S 6 , in which the polling physical layer address output circuit  13  ascertains whether or not all the bits of the fifth priority control register  17   2  are “0”. If all the bits of the fifth priority control register  17   2  are not “0”, the polling physical layer address output circuit  13  outputs the address of the physical layer device corresponding to the least significant bit of the “1”—written bits in the fifth priority control register  17   2  (the second physical layer address) (step S 7 ). 
     Furthermore, the polling physical layer address output circuit  13  rewrites the bit of the fifth priority control register  17   2  corresponding to the physical layer device selected in step S 7 , to “0” (step S 8 ), and returns to step S 2 . 
     On the other hand, if all the bits of the fifth priority control register  17   2  are “0” in step S 6 , the polling physical layer address output circuit  13  writes again the priority information of the corresponding physical layer devices ( 2   2  and  2   3  in the example of FIG. 4) to the fifth priority control register  17   2  on the basis of the content of the priority information register  11  (step S 9 ). 
     After completion of step S 9 , the processing goes into step S 10 , in which the polling physical layer address output circuit  13  ascertains whether or not all the bits of the sixth priority control register  17   3  are “0”. If all the bits of the sixth priority control register  17   3  are not “0”, the polling physical layer address output circuit  13  outputs the address of the physical layer device corresponding to the least significant bit of the “1”—written bits in the sixth priority control register  17   3  (the second physical layer address) (step S 11 ). 
     Furthermore, the polling physical layer address output circuit  13  rewrites the bit of the sixth priority control register  17   3  corresponding to the physical layer device selected in step S 11 , to “0” (step S 12 ), and returns to step S 2 . 
     On the other hand, if all the bits of the sixth priority control register  17   3  are “0” in step S 10 , the polling physical layer address output circuit  13  writes again the priority information of the corresponding physical layer devices ( 2   5  to  2 n in the example of FIG. 4) to the sixth priority control register  173  on the basis of the content of the priority information register  11  (step S 13 ). Therefore, the processing returns to step S 2 , so that the processing of steps S 2  to S 13  is repeated. 
     With the above mentioned processing, the polling physical layer address output circuit outputs the physical layer address in the order determined by the priority-based hierarchical combination of a high priority loop comprising the physical layer devices having the highest priority, a middle priority loop comprising the physical layer devices having the middle priority, and a low priority loop comprising the physical layer devices having the lowest priority, as shown in FIG.  4 . 
     In the flow chart shown in FIG. 6, the selecting physical layer address output circuit  12  first writes the priority information of the respective physical layer devices  2   1  to  2 n into the first, second and third priority control registers  16   1  to  16   3  on the basis of the content of the priority information register  11  (step S 21 ). 
     Then, the selecting physical layer address output circuit  12  ascertains whether or not all the contents of the first priority control register.  16   1  are “0” (step S 22 ). When all the contents of the first priority control register  16   1  are not “0”, the selecting physical layer address output circuit  12  ascertains whether or not the physical layer device corresponding to the least significant bit of the “1”—written bits can execute the cell transfer (step S 23 ). Whether or not the cell transfer is executable is determined on the basis of the second physical layer address and the cell transfer enable notify signal outputted from the physical layer device designated by the second physical layer address. 
     When the result of the processing in step S 23  indicates that the cell transfer is possible, the selecting physical layer address output circuit  12  outputs the address of the corresponding physical layer device (first physical layer address) so that the cell transfer is executed between the ATM layer device and the corresponding physical layer device (step S 24 ). On the other hand, when the cell transfer is not possible or when the processing in step S 24  is completed, the selecting physical layer address output circuit  12  rewrites the least significant bit of the “1”—written bits in the first priority control register  16   1  to “0” (step S 25 ), and the processing returns to step S 22 , so that the processing of steps S 22  to S 25  is repeated until all the contents of the first priority control register  16   1  becomes “0”. 
     On the other hand, if all the bits of the first priority control register  16   1  are “0” in step S 22 , the selecting physical layer address output circuit  12  writes again the priority information of the corresponding physical layer devices ( 2   1  and  2   4  in the example of FIG. 4) to the first priority control register  16   1  on the basis of the content of the priority information register  11  (step S 26 ). 
     After completion of step S 26 , the processing goes into step S 27 , in which the selecting physical layer address output circuit  12  ascertains whether or not all the bits of the second priority control register  16   2  are “0”. If all the bits of the second priority control register  16   2  are not “0”, the selecting physical layer address output circuit  12  ascertains whether or not the physical layer device corresponding to the least significant bit of the “1”—written bits can execute the cell transfer (step S 28 ). when the result of the processing in the step S 28  indicates that the cell transfer is possible, the selecting physical layer address output circuit  12  outputs the address of the corresponding physical layer device (first physical layer address), so that the cell transfer is executed between the ATM layer device and the corresponding physical layer device (step S 29 ). On the other hand, when the cell transfer is not possible, or when the processing in step S 29  is completed, the selecting physical layer address output circuit  12  rewrites the least significant bit of the “1”—written bits in the second priority control register  16   2  to “0” (step S 30 ), and the processing returns to step S 22 . 
     On the other hand, if all the bits of the second priority control register  16   2  are “0” in step S 27 , the selecting physical layer address output circuit  12  writes again the priority information of the corresponding physical layer devices ( 22  and  23  in the example of FIG. 4) to the second priority control register  16   2  on the basis of the content of the priority information register  11  (step S 31 ). 
     After completion of step S 31 , the processing goes into step S 32 , in which the selecting physical layer address output circuit  12  ascertains whether or not all the bits of the third priority control register  16   3  are “0”. If all the bits of the third priority control register  16   3  are not “0”, the selecting physical layer address output circuit  12  ascertains whether or not the physical layer device corresponding to the least significant bit of the “1”—written bits can execute the cell transfer (step S 33 ). When the result of the processing in step S 33  indicates that the cell transfer is possible, the selecting physical layer address output circuit  12  outputs the address of the corresponding physical layer device (first physical layer address), so that the cell transfer is executed between the ATM layer device and the corresponding physical layer device (step S 34 ). On the other hand, when the cell transfer is not possible, or when the processing in step S 34  is completed, the selecting physical layer address output circuit  12  rewrites the least significant bit of the “1”—written bits in the third priority control register  16   3  to “0” (step S 35 ), and the processing returns to step S 22 . 
     On the other hand, if all the bits of the third priority control register  16   3  are “0” in step S 32 , the selecting physical layer address output circuit  12  writes again the priority information of the corresponding physical layer devices ( 2   5  to  2 n in the example of FIG. 4) to the third priority control register  16   3  on the basis of the content of the priority information register  11  (step S 36 ), and the processing returns to step S 22 , so that the processing of steps S 22  to S 36  is repeated. 
     With the above mentioned processing, the selecting physical layer address output circuit  12  outputs the physical layer address in the order determined by the priority-based hierarchical combination of a high priority loop comprising the physical layer devices having the highest priority, a middle priority loop comprising the physical layer devices having the middle priority, and a low priority loop comprising the physical layer devices having the lowest priority, as shown in FIG.  4 . 
     As seen from above, an ATM layer device in accordance with the present invention holds the priority information indicating the previously set priority level of each of a plurality of physical layer devices and selects the physical layer device that should execute the cell transfer in a predetermined order in accordance with the priority information. Therefore, the higher the priority level of the physical layer device, the larger the opportunity of the selection of the physical layer device becomes, so that the data transfer amount per unit of time, of the physical layer device having the high priority level, can be increased. 
     In particular, by giving a high priority to a physical layer device having a high transmission rate, it is possible to suppress the overflow of the receiving buffer in the physical layer device having the high transmission rate. 
     The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.