Abstract:
In serial transmission of a mixture of isochronous data and anisochronous data, communications thorough a transmission path with a high ratio of occurrence of transmission errors may disturb images during streaming reproduction and produce noise in sound. To solve this problem, the isochronous data that has not been successfully received is retransmitted using an anisochronous region. Thus, transmission error of the isochronous data can be reduced without impairment of isochronous characteristics of the data.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to communications control methods and, more specifically, to a method for controlling communications among a plurality of terminals coupled to each other forming a network for serial transmission of a mixture of isochronous data and anisochronous data.  
           [0003]    2. Description of the Background Art  
           [0004]    In recent years, computers, peripheral devices, digital video devices, and other components are often connected to each other in homes and offices to form a local area network. On the network of this type, a mixture of isochronous data recurring at periodic time intervals (for example, video and audio data for streaming reproduction) and anisochronous data (for example, burst-like communications data) is transmitted in serial.  
           [0005]    One example of the known standards for serial transmission of such mixture is IEEE 1394. In IEEE 1394, all components forming a network are under communications control as described below.  
           [0006]    [0006]FIG. 23 is a diagram showing an example of structure of a conventional local-area network  209  that complies with IEEE 1394. FIG. 24 is a schematic diagram showing a conventional communications control method according to IEEE 1394. An example of structure of a communications control cycle according to IEEE 1394 is shown in (A) of FIG. 24, and an example of communications control carried out on the network of FIG. 23 is shown in (B) of FIG. 24.  
           [0007]    In FIG. 23, the conventional network  209  includes a controller  210 , a digital video player  211 , a digital television  212 , a set-top box (hereinafter, STB)  213 , and a computer  214 . These components  210  to  214  are connected in serial (or in tree shape) to each other vie a cable that complies with IEEE 1394. On the network  209 , the controller  210  controls the other components  211  to  214 .  
           [0008]    On the above structured network  209 , assume herein that isochronous data “I1” is going to be transmitted from the digital video player  211  to the digital television  212 ; isochronous data “I2” is going to be transmitted from the STB  213  to the computer  214 ; and anisochronous data “N” is going to be transmitted from the computer  214  to the digital video player  211 .  
           [0009]    In IEEE 1394, as shown in (A) of FIG. 24, a control time is divided into predetermined cycles (for example, every 125 μs), and each cycle has a predetermined isochronous region of a predetermined time length (for example, 100 μs at maximum). The isochronous region is further divided into plural (two, in this example) regions. These two regions are respectively assigned to the components having isochronous data to be transmitted. In the example, one divided region is assigned, as a dedicated region (channel  1 ), to the digital television  212 , while the other is assigned, as a dedicated region (channel  2 ), to the STB  213 .  
           [0010]    As shown in (B) of FIG. 24, before start of transmission, the controller  210  reports, to each component, information  200  about the dedicated regions assigned to these components. The information  200  includes times when each isochronous data is to be transmitted. When transmission starts and enters into the cycle, the controller  210  first transmits a packet  201  indicating the start of the cycle to each component. On receiving the cycle start packet  201 , the components having the isochronous data to be transmitted (here, the digital video player  211  and the STB  213 ) transmit the isochronous data (I 2  and I 2 , respectively) using the dedicated region of their own (channel  1  and channel  2 , respectively).  
           [0011]    When the procedure exits the isochronous region, the controller  210  provides a transmission instruction  202  to the component having the anisochronous data (here, the computer  214 ). On receiving the instruction from the controller  210 , the computer  214  transmits the anisochronous data (N).  
           [0012]    Next, the destination of the anisochronous data (N) (here, the digital video player  211 ) receives the anisochronous data, and then returns a response packet  203  indicating whether the data has been successfully received or not, to the data originating terminal (the computer  214 ) and the controller  210 . On receiving the response packet  203  from the computer  214 , the controller  210  determines whether retransmission is required or not. In this example, the transmitted response packet  203  indicates that the data has been successfully received. Therefore, the controller  210  determines that retransmission is not required. Then, when the procedure exits the cycle and then enters into another, the controller transmits a packet indicating start of the next cycle to each terminal (such packet transmission procedure is not shown in the drawing), and then the procedure is repeated similarly thereafter.  
           [0013]    If the response packet  203  returned from the digital video player  211  indicates reception error, on the other hand, the controller  210  transmits, to the computer  214 , an instruction for retransmitting the anisochronous data (N). Then, the digital video player  211  returns response packets to the computer  214  and the controller  210 . If the response packet from the digital video player  211  indicates reception error, the controller  210  again instructs the computer  214  to retransmit the data (such retransmission procedure is not shown in the drawing).  
           [0014]    As such, according to IEEE 1394, the control time is divided into cycles, and each cycle has an isochronous region of a predetermined time length allocated. This isochronous region is further divided into dedicated regions to be assigned to the components having isochronous data. Therefore, each of these component can transmit the isochronous data one time per cycle. Consequently, the isochronous characteristics can be kept.  
           [0015]    On the other hand, the region other than the isochronous region in each cycle is an anisochronous region, where the components having anisochronous data to be transmitted are controlled (asynchronous control) so that they carry out transmission one after the other. Thus, serial transmission of mixed isochronous and the anisochronous data can be achieved.  
           [0016]    In IEEE 1394, even if reception error of the isochronous data occurs, retransmission control is not carried out. Instead, the ratio of error occurrence is reduced to less than a predetermined value by restricting the length of a cable connecting the components together to less than a predetermined length (4.5 m if conductor cable). With the ratio of error occurrence less than the predetermined value, quality deterioration in image and sound can be so suppressed as that the user cannot recognize the deterioration.  
           [0017]    In recent years, wireless connection between components has become more desired. Wireless connection dispense with time and trouble of wiring, and enables the user to use each component wherever he/she desires.  
           [0018]    In a wireless transmission path, however, transmission errors prone to occur more, compared with in a wired transmission path. The ratio of transmission error occurrence is significantly increased if the components are far spaced apart or an obstacle is located therebetween. As a result, it is highly possible that images may be disturbed during streaming reproduction and noise may be mixed in sound.  
         SUMMARY OF THE INVENTION  
         [0019]    Therefore, an object of the present invention is to provide a communications control method used in serial transmission of a mixture of isochronous data and anisochronous data for preventing image disturbances during streaming reproduction and noise mixed in sound even though a transmission path with a high ratio of occurrence of transmission errors is used for communications.  
           [0020]    The present invention has the following features to attain the object above.  
           [0021]    A first aspect of the present invention is directed to a method for controlling communications among a plurality of terminals coupled to each other to form a network in which a mixture of isochronous data and anisochronous data is serially transmitted, and the method includes:  
           [0022]    a step of dividing time into cycles, and providing an isochronous region and an anisochronous region for each cycle;  
           [0023]    a step of, after each terminal transmitting, for each cycle, the isochronous data using the isochronous region in a time division manner, determining by a destination terminal whether the isochronous data has been successfully received; and  
           [0024]    a step of, when there is any terminal that has not successfully received the isochronous data, instructing an originating terminal of the isochronous data to retransmit the isochronous data using the anisochronous region.  
           [0025]    In the above first aspect, the isochronous data that has not been successfully received is retransmitted using the anisochronous region. Therefore, transmission error of the isochronous data can be reduced without impairment of isochronous characteristics. As a result, image disturbances during streaming reproduction and noise mixed in sound can be prevented even if the transmission path between the terminals is the one with a high ratio of occurrence of transmission errors, such as a wireless transmission path.  
           [0026]    According to a second aspect, in the first aspect, each terminal transmits the isochronous data with an error detection code added thereto, and the destination terminal checks the error detection code after receiving the isochronous data, and  
           [0027]    the determination step is performed based on a check result.  
           [0028]    In the above second aspect, the destination terminal of the isochronous data transmits a result of checking the error detection code added to the isochronous data. Based on the result, whether the isochronous data has been successfully received can be determined.  
           [0029]    According to a third aspect, in the first aspect, each terminal transmits the isochronous data as divided into blocks, and  
           [0030]    the determination step is performed for each block, and the instructing step is performed for each block.  
           [0031]    In the above third aspect, the isochronous data is transmitted as divided into blocks. Therefore, when an error occurs in only part of the received data, only the block that has the error is retransmitted. Consequently, retransmission time is reduced. Thus, the possibility that retransmission of the isochronous data using the anisochronous region may consume time for transmitting the anisochronous data can be more prevented.  
           [0032]    If there are a plurality of terminals that have not successfully received the isochronous data, such control as indicated by the following fourth and fifth aspects is carried out.  
           [0033]    According to the fourth aspect, in the first aspect,  
           [0034]    in the instructing step, when there are the terminals that have not successfully received the isochronous data, the originating terminals of the isochronous data are sequentially instructed to retransmit the isochronous data using the anisochronous region in the time division manner.  
           [0035]    According to the fifth aspect, in the first aspect,  
           [0036]    in the instructing step, when there are the terminal that have not successfully received the isochronous data, transmission times are collectively reported to the originating terminals of the isochronous data so that the originating terminals retransmit the isochronous data using the anisochronous region in the time division manner.  
           [0037]    In the fourth and fifth aspects, the isochronous data can be retransmitted by each terminal using the anisochronous region in a time division manner.  
           [0038]    According to a sixth aspect, in the first aspect,  
           [0039]    each terminal modulates the isochronous data before transmission, and  
           [0040]    in the instructing step, the originating terminal is instructed to retransmit the isochronous data after changing a modulation scheme used in modulation.  
           [0041]    According to a seventh aspect, in the first aspect,  
           [0042]    each terminal encodes the isochronous data before transmission, and  
           [0043]    in the instructing step, the originating terminal is instructed to retransmit the isochronous data after changing a coding rate used in encoding.  
           [0044]    According to an eighth aspect, in the first aspect,  
           [0045]    each terminal encodes and modulates the isochronous data before transmission, and  
           [0046]    in the instructing step, the originating terminal is instructed to retransmit the isochronous data after changing a coding rate used in encoding and a modulation scheme used in modulation.  
           [0047]    In the above sixth to eighth aspects, the ratio of reoccurrence of transmission errors at retransmission can be more prevented. As a result, the number of times of retransmission is reduced, and thus the possibility that repetitive retransmission may consume time of transmitting the anisochronous data can be more prevented.  
           [0048]    According to a ninth aspect, in the first aspect,  
           [0049]    each terminal carries out multicast transmission of the isochronous data to the terminals that belong to respective specific groups,  
           [0050]    in the determination step, it is determined whether the isochronous data has been successfully received by all terminals that belong to the group destined to receive the isochronous data, and  
           [0051]    in the instructing step, when there is one or more terminals that have not successfully received the isochronous data in the group, the originating terminal is instructed to carry out multicast retransmission of the isochronous data to all of the terminals that belong to the group.  
           [0052]    In the above ninth aspect, even on a network where each terminal carries out multicast transmission, transmission errors of the isochronous data can be reduced without impairment of isochronous characteristics.  
           [0053]    According to a tenth aspect, in the first aspect,  
           [0054]    each terminal carries out broadcast transmission of the isochronous data to all of other terminals,  
           [0055]    in the determination step, it is determined whether the isochronous data has been successfully received by all terminals destined to receive the isochronous data, and  
           [0056]    in the instructing step, when there is one or more terminals that have not successfully received the isochronous data, the originating terminal is instructed to carry out broadcast retransmission of the isochronous data to all of the terminals.  
           [0057]    In the above tenth aspect, even on a network where each terminal carries out broadcast transmission, transmission errors of the isochronous data can be reduced without impairment of isochronous characteristics.  
           [0058]    According to an eleventh aspect, in the first aspect,  
           [0059]    the communications control method further includes the step of  
           [0060]    dividing the isochronous region into dedicated regions, assigning the dedicated regions to the terminals respectively, and reporting, to each terminal, information about the dedicated region before a start of a head of the cycle, thereby enabling transmission of the isochronous data for each cycle by each terminal using the isochronous region in the time division manner.  
           [0061]    In the above eleventh aspect, the dedicated region information is reported to each terminal before the start of the head cycle. By referring to the dedicated region information, each terminal transmits, for each cycle, the isochronous data at the time corresponding to the dedicated region assigned to itself.  
           [0062]    According to a twelfth aspect, in the first aspect,  
           [0063]    the communications control method further includes the step of  
           [0064]    dividing the isochronous region into dedicated regions, assigning the dedicated regions to the terminals respectively, and sequentially instructing, for each cycle at respective times corresponding to the dedicated region, the terminals to transmit the isochronous data, thereby enabling transmission of the isochronous data for each cycle by each terminal using the isochronous region in the time division manner.  
           [0065]    In the above twelfth aspect, the terminals are sequentially instructed, for each cycle, to carry out transmission at the time corresponding the dedicated region. In response to the instruction, each terminal transmits the isochronous data.  
           [0066]    According to a thirteenth aspect, in the first aspect,  
           [0067]    the communications control method further includes the step of  
           [0068]    giving an inquiry to the destination terminal about whether the isochronous data has been successfully received, wherein  
           [0069]    the determination step is carried out based on a response to an inquiry  
           [0070]    In the above thirteenth aspect, the destination terminal is inquired about whether reception has been succeeded.  
           [0071]    According to a fourteenth aspect, in the first aspect,  
           [0072]    the destination terminal spontaneously gives a response about whether the isochronous data has been successfully received, and  
           [0073]    the determination step is carried out based on the response.  
           [0074]    In the above fourteenth aspect, the destination terminal spontaneously reports whether reception has been succeeded.  
           [0075]    According to a fifteenth aspect, in the first aspect,  
           [0076]    the instructing step is carried out repetitively until the isochronous data is successfully received.  
           [0077]    In the above fifteenth aspect, retransmission is repeated until reception error is cleared.  
           [0078]    According to a sixteenth aspect, in the fifteenth aspect,  
           [0079]    the communications control method further includes the step of  
           [0080]    suspending retransmission of the isochronous data even though reception error is not cleared when a time consumed by one more retransmission presumably exceeds a predetermined maximum time allowed, for each cycle, to be consumed for retransmitting the isochronous data.  
           [0081]    In the above sixteenth aspect, the retransmission time of the isochronous data is limited to less than a predetermined value. Thus, the possibility that retransmission may impair the isochronous characteristics of the data or consume time for transmitting the anisochronous data is prevented.  
           [0082]    According to a seventeenth aspect, in the sixteenth aspect,  
           [0083]    the maximum time is less in value than a time length of the anisochronous region.  
           [0084]    In the above seventeenth aspect, retransmission of the isochronous data carried out in the isochronous region of the next cycle can be prevented. Therefore, impairment of the isochronous characteristics of the data can be prevented.  
           [0085]    According to an eighteenth aspect, in the seventeenth aspect,  
           [0086]    the maximum time is equal in value to a time length of the anisochronous region.  
           [0087]    In the above eighteenth aspect, the entire anisochronous region can be used for retransmission of the isochronous data. Thus, transmission error of the isochronous data can be most effectively reduced.  
           [0088]    According to a nineteenth aspect, in the seventeenth aspect,  
           [0089]    the communications control method further includes the step of  
           [0090]    retransmitting, in the anisochronous region of a next cycle, the isochronous data whose retransmission is suspended.  
           [0091]    In the above nineteenth aspect, suspended retransmission of the isochronous data is restarted after the procedure enters into the anisochronous region of the next cycle.  
           [0092]    According to a twentieth aspect, in the fifteenth aspect,  
           [0093]    the communications control method further includes the step of  
           [0094]    suspending retransmission of the isochronous data even though reception error is not cleared when the number of times of retransmission exceeds a predetermined maximum number of times of retransmission of the isochronous data.  
           [0095]    In the above twentieth aspect, infinite repetition of retransmission of the same isochronous data can be prevented.  
           [0096]    A twenty-first aspect of the present invention is directed to a communications control apparatus for controlling communications among a plurality of terminals coupled to each other to form a network in which a mixture of isochronous data and anisochronous data is serially transmitted, and the apparatus includes:  
           [0097]    a part for dividing time into cycles, and providing an isochronous region and an anisochronous region for each cycle;  
           [0098]    a part for, after each terminal transmitting, for each cycle, the isochronous data using the isochronous region in a time division manner, determining by a destination terminal whether the isochronous data has been successfully received; and  
           [0099]    a part for, when there is any terminal that has not successfully received the isochronous data, instructing an originating terminal of the isochronous data to retransmit the isochronous data using the anisochronous region.  
           [0100]    A twenty-second aspect of the present invention is directed to a computer-processable program having such a method as that according to the above first aspect written thereon.  
           [0101]    A twenty-third aspect of the present invention is directed to a recording medium having such a program as that according to the above twenty-second aspect stored thereon.  
           [0102]    These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0103]    [0103]FIG. 1 is a schematic diagram for demonstrating a communications control method according to a first embodiment of the present invention, (A) illustrating an example of structure of a communications control cycle and (B) illustrating an example of communications control carried out in a wireless local-are network  9  of FIG. 2;  
         [0104]    [0104]FIG. 2 is a diagram showing an example of structure of the network  9  to which the method of FIG. 1 is applied;  
         [0105]    [0105]FIG. 3 is a diagram showing an example of structure of a wireless communications module  20  provided in each component ( 10  to  14 ) of FIG. 2;  
         [0106]    [0106]FIG. 4 is a diagram showing the contents of program memory  24  of FIG. 3;  
         [0107]    [0107]FIG. 5 is a schematic diagram, in relation to (B) of FIG. 1, for demonstrating retransmission control carried out when isochronous data (block) has not been successfully received;  
         [0108]    [0108]FIG. 6 is a diagram, in relation to (B) of FIG. 1, showing the state where retransmission of isochronous data is suspended even if reception error is not solved;  
         [0109]    [0109]FIG. 7 is a flowchart showing the operation of a controller  10  (control station) of FIG. 2;  
         [0110]    [0110]FIG. 8 is a diagram showing one example of dedicated region information reported to each terminal in step S 5  of FIG. 7;  
         [0111]    [0111]FIG. 9 is a flowchart showing the operation of each terminal ( 11  to  14 ) of FIG. 2;  
         [0112]    [0112]FIG. 10 is a flowchart showing detailed transmission processing of isochronous data shown in step S 22  of FIG. 9;  
         [0113]    [0113]FIG. 11 is a flowchart showing detailed data reception processing shown in step S 26  of FIG. 9;  
         [0114]    [0114]FIG. 12 is a diagram showing an example of structure of a response packet transmitted in step S 63  of FIG. 11;  
         [0115]    [0115]FIG. 13 is a diagram showing the state where isochronous data whose retransmission is suspended based on the determination result in step S 8  of FIG. 7 is retransmitted at a next anisochronous region of the next cycle;  
         [0116]    [0116]FIG. 14 is a schematic diagram for demonstrating a communications control method according to a second embodiment of the present invention;  
         [0117]    [0117]FIG. 15 is a schematic diagram for demonstrating a communications control method according to a third embodiment of the present invention;  
         [0118]    [0118]FIG. 16 is a diagram showing the contents of a global polling packet  90  being multicast by the control station;  
         [0119]    [0119]FIG. 17 is a diagram showing the contents of a global retransmission request packet  95  being multicast by the control station;  
         [0120]    [0120]FIG. 18 is a schematic diagram showing a communications control method according to a fourth embodiment of the present invention;  
         [0121]    [0121]FIG. 19 is a schematic diagram showing a communications control method according to a fifth embodiment of the present invention, (A) illustrating an example of structure of a communications control cycle and (B) illustrating an example of communications control carried out on the network  9  of FIG. 2;  
         [0122]    [0122]FIG. 20 is a flowchart showing the operation of the control station  10  of FIG. 2 according to the fifth embodiment;  
         [0123]    [0123]FIG. 21 is a flowchart showing the detailed processing of transmitting isochronous data shown in step S 22  of FIG. 9, according to the fifth embodiment;  
         [0124]    [0124]FIG. 22 is a diagram, in relation to (B) of FIG. 19, showing a communications control method of spontaneous transmission of a response packet when each terminal receives isochronous data;  
         [0125]    [0125]FIG. 23 is a diagram showing an example of structure of a conventional local-area network that complies with IEEE 1394; and  
         [0126]    [0126]FIG. 24 is a schematic diagram for demonstrating a conventional communications control method according to IEEE 1394, (A) illustrating an example of structure of a communications control cycle according to IEEE 1394 and (B) illustrating an example of communications control carried out on a network  209  of FIG. 23. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0127]    [0127]FIG. 1 is a schematic diagram for demonstrating a communications control method according to a first embodiment of the present invention. FIG. 2 is a diagram illustrating an example of structure of a wireless local area network  9  to which the method of FIG. 1 is applied. In FIG. 2, the wireless local area network  9  includes a controller  10 , a digital video player  11 , a digital television  12 , a set-top box (hereinafter, STB)  13 , and a computer  14 . These components  10  to  14  are wirelessly coupled to each other. This network  9  is almost the same as the network  209  of FIG. 23 (refer to Background Art section), except that the ratio of occurrence of transmission errors is high because of wireless connection among the components  10  to  14 . On the network  9 , the controller  10  controls wireless communications among the components  11  to  14 .  
         [0128]    Each of the components  10  to  14  forming the wireless local area network  9  is provided with a wireless communications module. An example of structure of the wireless communications module is shown in FIG. 3. In FIG. 3, a wireless communications module  20  includes an antenna  21 , a modulation/demodulation circuit  22 , a processing circuit  23 , program memory  24 , and an input/output circuit  25 .  
         [0129]    The antenna  21  converts a signal into an electric wave for emission and also acquires an electric wave for conversion into a signal. The modulation/demodulation circuit  22  modulates/demodulates the signal. The input/output circuit  25  is connected to another input/output circuit (not shown) of any of the components  10  to  14  equipped with this module, transmitting and receiving signals with the component.  
         [0130]    The program memory  24  stores various programs. The processing circuit  23  includes a microcomputer that processes signals by following the programs stored in the program memory  24 . The contents of the program memory  24  are shown in FIG. 4.  
         [0131]    In FIG. 4, the program memory  24  stores a main program  30 , a control station program  31 , a terminal program  32 . The main program  30  describes the basic procedure for the module carrying out wireless communications. The control station program  31  describes the procedure for the module as a control station controlling the communications among the components. The terminal program  32  describes the procedure for the module as a terminal carrying out communications. These procedures described in the respective programs are computer-processable.  
         [0132]    The wireless communications module  20  equipped in the controller  10  is initially so set as that the control station program is first activated. The wireless communications module  20  equipped in any of the other components  11  to  14  is initially so set as that the terminal program is first activated. Note that, by changing the initial setting, it is possible to make the component such as the computer  14  or the STB  13  operate as a controller.  
         [0133]    On the above structured network  9 , assume herein that isochronous data “I1” is going to be transmitted from the digital video player  11  (first terminal) to the digital television  12  (second terminal) ; isochronous data “I2” is going to be transmitted from the STB  13  (third terminal) to the computer  14  (fourth terminal); and anisochronous data “N” is going to be transmitted from the computer  14  (fourth terminal) to the digital video player  11  (first terminal). Here, the controller (control station)  10  controls communications among the components (first to fourth terminals) so that a mixture of the isochronous data recurring at periodic time intervals and anisochronous data can be serially transmitted.  
         [0134]    Hereinafter, the controller  10  is referred to as “control station”, and the components  11  to  14  are referred to as “first to fourth terminals”, respectively. In this communications control, as shown in (A) of FIG. 1, time is divided into predetermined cycles, and each cycle is provided with an isochronous region and anisochronous regions. Then, the isochronous region is further divided into plural (two, in this example) dedicated regions. These regions are respectively assigned, as Ch.  1  and Ch.  2 , to the components having the isochronous data to be transmitted (first and third terminals)  
         [0135]    Here, the time length of the isochronous region is determined based on the amount of the isochronous data to be transmitted per unit time, and limited, however, to a predetermined value so that the transmission path is not totally occupied by the isochronous data. By way of example only, if one cycle is 125 μsec, the time length of the isochronous region is limited to not more than 100 μsec, for example.  
         [0136]    Furthermore, the isochronous region is provided so as to be located a predetermined time apart from the head of the cycle. Therefore, two regions preceding and following the isochronous region in the cycle are the anisochronous regions.  
         [0137]    Furthermore, the control station knows the amount of isochronous data to be transmitted from each terminal per unit time. Therefore, the control terminal divides the isochronous region into dedicated regions based on the amount of transmission from each terminal, and assigns these dedicated regions to the respective terminals.  
         [0138]    An example of structure of the communication control cycle is shown in (A) of FIG. 1. An example of communications control carried out on the network  9  of FIG. 2 is shown in (B) of FIG. 1.  
         [0139]    In the example of (A) of FIG. 1, each cycle has the isochronous region provided from 5 μsec away from the head of the cycle as the origin (0 μsec) to 85 μsec. Therefore, two regions from 0 to 5 μsec and from 80 to 125 μsec are the anisochronous regions. The isochronous region is divided into two dedicated regions, one (Ch.  1 ) from 5 to 25 μsec assigned to the first terminal and the other (Ch.  2 ) from 25 to 85 μsec to the third terminal.  
         [0140]    After such assignment of the dedicated regions, the control station reports, to each station before the start of data transmission, information  40  about the dedicated regions assigned the respective terminals. When data transmission is started and the procedure enters into the first cycle, the control station transmits a packet  41  indicating the start of the cycle to each terminal. The transmission of the cycle start packet  41  is carried out in the anisochronous region from 0 to 5 μsec.  
         [0141]    In response, the terminals (first and third terminals) having the isochronous data to be transmitted (I 1  and I 2 ) each transmit the isochronous data (I 1  and I 2 ) using the respective dedicated regions (channel  1  and channel  2 ). At this time, the third terminal divides the isochronous data I 2  into three blocks (I 2 - 1 , I 2 - 2 , and I 2 - 3 ) for transmission. The reason for such division will be described later.  
         [0142]    After the procedure exits the isochronous region, the control station first makes an inquiry about whether the transmitted isochronous data (I 1  and I 2 ) has been received without error. Specifically, the control station inquires the second and fourth terminals that are the destinations of I 1  and I 2  about whether the data have been successfully received or not. In other words, the control station first transmits an inquiry packet  42  to the second terminal, and then waits for a response packet  43  to be returned from the second terminal. Then, when the response packet  43  is returned from the second terminal, the control station then transmits an inquiry packet  44  to the fourth terminal, and waits for a response packet  45  to be returned from the fourth terminal. Then, when the response packet  45  is returned from the fourth terminal, the control station analyses the two response packets  43  and  45  from the second and fourth terminals to determine whether the isochronous data I 1  and I 2  have to be retransmitted or not. In the present example, both of the response packets indicate that the data has been successfully received. Therefore, the control station determines that no retransmission is required, and then the procedure goes to communications control for the anisochronous data.  
         [0143]    That is, the control station gives a transmission instruction  46  to the terminal having the anisochronous data to be transmitted, that is, the fourth terminal. On receiving the instruction  46  from the control station, the fourth terminal transmits the anisochronous data (N). The terminal that is the destination of the anisochronous data (first terminal) returns to the data originating terminal (fourth terminal) and the control station a response packet  47  indicating whether the data has been successfully received. In this example, the received response packet  47  indicates that the data has been successfully received. Therefore, the control station determines that no retransmission is required. Then, if any terminal having anisochronous data to be transmitted exists, the control station allows the terminal to carry out transmission, and if not, enters in a wait state.  
         [0144]    Then, when the procedure exits the first cycle and then enters into the next cycle, the control station transmits a packet indicating the start of the next cycle to each terminal, and then repeats the operation as describe above.  
         [0145]    In the example of (B) of FIG. 1, all of the isochronous data (blocks) transmitted have been successfully received. An example case where reception error occurs is shown in FIG. 5.  
         [0146]    [0146]FIG. 5 is a schematic diagram for demonstrating retransmission control carried out when the isochronous data (block) has not been successfully received. Shown in FIG. 5 is retransmission control carried out when the isochronous block I 2 - 3  transmitted from the third terminal has not been successfully received by the fourth terminal. In this case, on receiving an inquiry packet  50  from the control station, the fourth terminal returns a response packet  51  indicating that the isochronous block I 2 - 3  has not been successfully received. Based on the received response packet  51 , the control station determines that retransmission is required, and transmits an retransmission instruction  52  for retransmitting the isochronous block I 2 - 3  to the third terminal. In response, the third terminal retransmits the isochronous block I 2 - 3  to the fourth terminal.  
         [0147]    Then, the control station sends a packet  52  for inquiring the fourth terminal, which is the retransmission destination of the isochronous block I 2 - 3 , about whether the retransmitted data has been successfully received. The fourth terminal has received the retransmitted isochronous block I 2 - 3  without error, and therefore returns a response packet  54  indicating that as such. Based on the received response packet  54 , the control station determines that retransmission is not required. Then, the procedure goes to communications control of anisochronous data.  
         [0148]    As such, according to the present communications control method, when the isochronous data has not been successfully received, the isochronous data is retransmitted using the anisochronous data region. Therefore, transmission error of the isochronous data can be reduced without impairment of isochronous characteristics of the data.  
         [0149]    Also, the terminal divides the isochronous data into a plurality of blocks for transmission. Therefore, if an error occurs only in part of the data, only the block where the error occurred is transmitted, thereby reducing the time required for retransmission. Thus, it is possible to further reduce the possibility that retransmission of the isochronous data using the anisochronous region may consume time for transmission of the anisochronous data.  
         [0150]    In the example of FIG. 5, the isochronous data I 2  is divided into three blocks (I 2 - 1 , I 2 - 2 , I 2 - 3 ) for transmission. In general, the more the number of divisions, the less the possibility of consuming time for transmission of the anisochronous data. However, if the isochronous data is divided into a plurality of blocks, error detection has to be made for each block. Therefore, the more the number of divisions, the more the amount of processing for error detection. For this reason, it is preferable that the number of divisions is appropriately selected in consideration of the above mentioned possibility and the amount of processing for error detection.  
         [0151]    Retransmission of the anisochronous data that has not been successfully received is carried out in a similar manner to that of the conventional one. For example, when the response packet returned from the first terminal indicates that reception has been failed, the control station transmits an instruction for retransmission to the fourth terminal, which is the originating terminal of the anisochronous data (N). In response, the fourth terminal retransmits the anisochronous data. Then, the first terminal, which is the destination terminal of retransmission, returns a response packet to the fourth terminal and the control station. When the response packet indicates that reception has been failed, the control station again instructs the fourth terminal to retransmit the anisochronous data.  
         [0152]    As such, the communications control method divides time into cycles, and each cycle has an isochronous region and anisochronous regions. Then, communications control of the isochronous data is carried out in the isochronous region. In the anisochronous region, retransmission control of the isochronous data that has not been successfully received is first carried out, and then communications control of the anisochronous data is executed.  
         [0153]    Thus, a mixture of isochronous data and anisochronous data can be serially transmitted, and transmission error that may occur in the isochronous data can be reduced without impairment of the isochronous characteristics. Consequently, even if the rate of occurrence of transmission errors is high on the transmission path between the terminals, such as a wireless transmission path, disturbances in video during streaming reproduction and interruption in audio can be prevented.  
         [0154]    Note that, in the example of FIG. 5, the retransmitted isochronous data (block I 2 - 3 ) has been successfully received. However, if the retransmitted isochronous data has not again been successfully received, retransmission is repeated until reception error is cleared. Alternatively, a maximum value of time used for retransmission of the isochronous data per cycle may be predetermined. In this case, if one more retransmission operation causes the total retransmission time to exceed the maximum value, retransmission is suspended even though reception error is not cleared. Such case is illustrated in FIG. 6.  
         [0155]    In general, the maximum value of the retransmission time of the isochronous data per cycle is determined so as not to exceed the time length of the anisochronous region, thereby keeping the isochronous characteristics of the isochronous data.  
         [0156]    In FIG. 6, the above maximum value is set to a value equal to the time length of the anisochronous region (45 μsec, for example), thereby putting reduction in transmission error of the isochronous data on the highest priority. In the setting as shown in FIG. 6, however, anisochronous data may not possibly be transmitted at all. Therefore, in general, the maximum value is set to a value smaller than the time length of the anisochronous region (30 μsec, for example)  
         [0157]    Described next is the operation of the control station with reference to a flowchart. FIG. 7 is a flowchart showing the operation of the control station  10  of FIG. 2. In FIG. 7, the control station first divides time into cycles, and sets, in each cycle, an isochronous region and anisochronous regions (step S 1 ). Then, the maximum time used for retransmission of the isochronous data in each cycle is set (step S 2 ). This maximum value is set to a value that does not exceed the time length of the anisochronous region. Furthermore, the maximum number of times of retransmission indicating how may times retransmission of the same isochronous data can be allowed is set (step S 3 ). The maximum number of times of retransmission will be described in detail later.  
         [0158]    The control station then divides the isochronous region set in step S 1  into a plurality of dedicated regions, and assigns them to respective terminals (step S 4 ). The control station then reports information about the assigned dedicated region to each terminal before the start of transmission (step S 5 ). Described in the dedicated region information is timing of each terminal transmitting the isochronous data. An example of the dedicated region information is illustrated in FIG. 8  
         [0159]    As shown in FIG. 8, described in the dedicated region information packet are transmission times of respective isochronous data blocks.  
         [0160]    Following step S 5 , the control station reports the start of the cycle to each terminal (step S 6 ). In response, each terminal transmits its own isochronous data using the assigned dedicated region.  
         [0161]    Then, the control station determines whether the isochronous data transmitted from each terminal has been successfully received by the destination terminal (step S 7 ). If Yes, the procedure goes to step S 11 .  
         [0162]    If No in step S 7 , the control station further determines whether one more retransmission operation in the cycle causes the total retransmission time to exceed the maximum value set in step S 2  (step S 8 ). If Yes, the control station suspends retransmission, and goes to step S 1 l.  
         [0163]    If No in step S 8 , the control station determines whether the number of times of retransmission of the isochronous data that has not been successfully received reaches the maximum number of times of retransmission set in step S 3  (step S 9 ). If Yes, the procedure returns to step S 7 , and repeats the above processing.  
         [0164]    If No in step S 9 , the control station instructs the data originating terminal to retransmit the isochronous data that has not been successfully received (step S 10 ). In response, the data originating terminal retransmits the isochronous data using the anisochronous region The procedure then returns to step S 7 , and repeats the above processing.  
         [0165]    In step S 11 , the control station carries out transmission control on the anisochronous data until the procedure reaches at the end of the anisochronous region.  
         [0166]    In the next step S 12 , whether the current cycle is the last one or not is determined. If Yes, the operation of the control station ends.  
         [0167]    If No in step S 12 , the procedure returns to step S 6 , where the control station reports the start of the next cycle to each terminal, and then repeats the above processing.  
         [0168]    Described next is the operation of the terminal. FIG. 9 is a flowchart showing the operation of each of the terminals  11  to  14  shown in FIG. 2. In FIG. 9, the terminal first determines whether it has isochronous data to be transmitted (step S 21 ). If No, the procedure goes to step S 23 .  
         [0169]    If Yes in step S 21 , the terminal transmits the isochronous data (step S 22 ). Then, the terminal determines whether it has anisochronous data to be transmitted (step S 23 ) If No, the procedure goes to step S 25 .  
         [0170]    If Yes in step S 23 , the terminal transmits the anisochronous data (step S 24 ), and then the procedure goes to step S 25 . Note that the transmitted anisochronous data has an error detection code added thereto.  
         [0171]    In step S 25 , the terminal determines whether it has to receive any data. If Yes, the terminal executes data reception operation (step S 26 ), and then the procedure goes to step S 27 . If No in step S 25 , the procedure skips step S 26  to go to step S 27 .  
         [0172]    In step S 27 , whether to continue the operation is determined. If Yes, the procedure ends. If No, the procedure returns to step S 21 , and repeats the above processing.  
         [0173]    [0173]FIG. 10 is a flowchart showing the detailed processing of transmitting the isochronous data shown in step S 22  of FIG. 9. In FIG. 10, the terminal first receives a report about the dedicated region information from the control station (step S 41 ). Then, the terminal waits for a report indicating the start of the cycle (step S 42 ).  
         [0174]    Notified the start of the cycle, the terminal transmits the isochronous data using the assigned dedicated region (step S 43 ). That is, the terminal transmits the isochronous data at the time described in the dedicated region information received in step S 41 . Note that the transmitted isochronous data has an error detection code added thereto.  
         [0175]    Then, the terminal determines whether it has been instructed by the control station to retransmit the isochronous data (step S 44 ). If No, the procedure returns to the flow of FIG. 9, and goes to step S 23 .  
         [0176]    If Yes in step S 44 , the terminal retransmits the isochronous data using the anisochronous region (step S 45 ). Note that the retransmitted isochronous data has an error detection code added thereto. The procedure then returns to the flow of FIG. 9, and goes to step S 23 . These are the details of step S 22 .  
         [0177]    [0177]FIG. 11 is a flowchart showing the detailed processing of data reception shown in step S 26  of FIG. 9. In FIG. 11, the terminal first receives data destined thereto (step S 61 ). The terminal then checks whether the received data has any error or not (step S 62 ). This check can be done based on the error detection code added to the received data.  
         [0178]    The terminal then transmits a response packet including the check result obtained in step S 62  to the control station (step S 63 ). The procedure then returns to the flow of FIG. 9, and goes to step S 27 . An example structure of the response packet is shown in FIG. 12.  
         [0179]    As shown in FIG. 12, described in the response packet is a state of reception of the isochronous data (the presence or absence of error) for each block.  
         [0180]    Note that, the isochronous data whose retransmission is suspended based on the determination in step S 8  that further retransmission operation will exceed the maximum value is retransmitted in the anisochronous region of the next cycle. This is shown in FIG. 13. In FIG. 13, retransmission of the isochronous data I 1  is once suspended at the end of the cycle  61 , and then restarted after the procedure enters into the anisochronous region of the next cycle  62 .  
         [0181]    However, repetitive retransmission of the same isochronous data may consume time for retransmitting other isochronous data. To get around this problem, the number of times of retransmission of the same data is limited to not more than a predetermined number (two, for example). This is the maximum number of times of retransmission set in step S 3 . In the example of FIG. 13, even though the second retransmission using the anisochronous region of the next cycle  62  has not cleared the transmission error, the third retransmission is not performed because the number of retransmission of the same data is limited to not more than two.  
         [0182]    Here, to reduce the number of times of retransmission, when instructing the originating terminal to retransmit the isochronous data that has not been successfully received, the control station may also instruct the originating terminal to change the modulation scheme and/or the coding rate  
         [0183]    Specifically, before transmitting the isochronous data, each terminal encodes the data at a predetermined coding rate, and then modulates the encoded data in a predetermined scheme. If the modulation scheme used for transmission is changed at retransmission, the possibility of occurrence of transmission errors can be reduced, compared with the case where the same modulation scheme is used for retransmission. If the coding rate used for transmission is also changed at retransmission, the possibility of occurrence of transmission errors can be further reduced. Consequently, the number of times of retransmission can be reduced, and the possibility that repetitive retransmission may consume time for transmitting anisochronous data can be further reduced.  
         [0184]    In the first embodiment, each terminal transmits the isochronous data to a single terminal. Alternatively, each terminal may carry out multicast transmission, where each terminal transmits the isochronous data to a plurality of terminals of respective specific groups. Still alternatively, each terminal may carry out broadcast transmission, where each terminal transmits the isochronous data to all of the other terminals. Described in a second embodiment below is communications control when each terminal carries out multicast or broadcast transmission of isochronous data.  
         [0185]    (Second Embodiment)  
         [0186]    [0186]FIG. 14 is a schematic diagram for demonstrating a communications control method according to the second embodiment of the present invention. One example structure of a wireless local area network to which the method of FIG. 14 is applied is similar to that shown in FIG. 2. The structure of the wireless communications module  20  equipped in the control station and each terminal shown in FIG. 2 is similar to that shown in FIG. 3. The contents of the program memory  24  of FIG. 3 is similar to those shown in FIG. 4, except that the control station program  31  and the terminal program  32  are partly different from those according to the first embodiment.  
         [0187]    In the example of FIG. 14, ina single isochronous region, the first terminal carries out broadcast transmission of three isochronous blocks (I 1 - 1 , I 1 - 2 , and I 1 - 3 ) to the second to fourth terminals. Here, if many terminals other than those shown in the drawing exist and the second to fourth terminals belong to a single group, the first terminal is regarded as to carry out multicast transmission to the terminals that belong to the group.  
         [0188]    As shown in FIG. 14, the second terminal has successfully received the blocks I 1 - 2  and I 1 - 3 , but has not the block I 1 - 1 . The third terminal has successfully received the blocks I 1 - 1 , I 1 - 2 , and I 1 - 3 . The fourth terminal has successfully received the blocks I 1 - 1  and I 1 - 2 , but has not the block I 1 - 3 .  
         [0189]    In the anisochronous region, the control station sequentially transmits inquiry packets  70  to  72  for inquiring the destination stations (the second to fourth terminals) about whether they have successfully received the isochronous data. On sequentially receiving response packets  73  to  75  from the second to fourth terminals, the control station sequentially transmits, to the data originating station (the first terminal), packets  76  and  77  for instructing retransmission of the isochronous blocks that have not been successfully received (I 1 - 1  and I 1 - 3 ).  
         [0190]    In response, the first terminal carries out broadcast retransmission of the blocks I 1 - 1  and I 1 - 3  to the second to fourth terminals. Then, the control station sequentially transmits packets  78  and  79  for inquiring the second and fourth terminals, which have not successfully received the blocks I 1 - 1  and I 1 - 3 , respectively, about whether they have received the respective retransmitted blocks. Then, after the control station receives, from the second and fourth terminals, response packets  80  and  81  each indicating that the block has been successfully received, the retransmission processing ends.  
         [0191]    The operation of the control station is similar to that shown in FIG. 7. However, in the case of multicast transmission where each terminal transmits the isochronous data to a plurality of terminals that belong to respective specific groups, the control station determines in step S 7  whether the isochronous data has been successfully received by all terminals that belongs to one group. Then, if there is any terminal that has not successfully received the isochronous data in that group, multicast retransmission of the isochronous data to all terminals in the group is carried out.  
         [0192]    On the other hand, if each terminal carries out broadcast transmission of the isochronous data to all of the other terminals, the control station determines in step S 7  whether the isochronous data has been successfully received by all of the terminals. Then, if there is any terminal that has not successfully received the isochronous data, broadcast retransmission of the isochronous data to all terminals in that group is carried out.  
         [0193]    In the first embodiment, the control station sequentially inquires the destination terminals about whether reception has been succeeded. Such inquiries to these terminals can be made in a collective manner. Also, in the first embodiment, on receiving responses each indicating reception error from many terminals, the control station sequentially gives retransmission instructions to the respective terminals that have not successfully received the isochronous data. Such retransmission instructions can be made in a collective manner. Described in a third embodiment below is a communications control method capable of giving the respective terminals a global inquiry about whether reception has been succeeded and a global retransmission instruction.  
         [0194]    (Third Embodiment)  
         [0195]    [0195]FIG. 15 is a schematic diagram for demonstrating a communications control method according to the third embodiment of the present invention. An example of structure of a wireless local area network to which the method of FIG. 15 is applied is similar to that in FIG. 2 with a fifth terminal (another computer, for example) newly added thereto. The structure of the wireless communications module  20  equipped to the control station and each terminal of FIG. 2 is similar to that shown in FIG. 3. The contents of the program memory  24  of FIG. 3 is similar to those shown in FIG. 4, except that the control terminal program  31  and the terminal program  32  are partly different from those according to the first embodiment.  
         [0196]    [0196]FIG. 15 shows an example of retransmission control in which inquiries about whether reception has been succeeded and retransmission instructions are made in a collective manner. In FIG. 15, in the isochronous region, multicast transmission of two isochronous blocks (I 1 - 1  and I 1 - 2 ) is first carried out from the first terminal to the second and third terminals. Then, multicast transmission of one isochronous block (I 2 - 1 ) is carried out from the fourth terminal to the third and fifth terminals. In the anisochronous region, the control station first carries out multicast transmission of a packet  90  that includes inquiries to the second, third, and fifth terminals about whether reception has been succeeded. The contents of the global inquiry packet  90  is shown in FIG. 16.  
         [0197]    As shown in FIG. 16, the global inquiry packet  90  describes address of the terminals that should transmit a response packet indicating whether reception has been succeeded (that is, the second, third, and fifth terminals, which are the destination terminals of the isochronous blocks, “2”, “3”, and “5”, addresses of the terminals that should receive the response packet (that is, the first and fourth terminals, which are the originating terminals of the isochronous blocks), “1” and “4”, and times when the response packets should be transmitted, “t1” to “t4”.  
         [0198]    Referring back to FIG. 15, when receiving the global inquiry packet  90  as shown in FIG. 16, the second, third, and fifth terminals, which are the destination terminals of the isochronous blocks, transmit response packets  91  to  94 , at the respective specified times to the control station and the respective specified terminals.  
         [0199]    More specifically, in the example shown in FIGS. 15 and 16, the second terminal transmits, at the time “t1”, the response packet  91  indicating reception error of the isochronous block I 1 - 1  and successful reception of the isochronous block I 1 - 2  to the control station and the first terminal.  
         [0200]    Then, the third terminal transmits, at the time “t2”, the response packet  92  indicating successful reception of two isochronous blocks I 1 - 1  and I 1 - 2  to the control station and the first terminal. Furthermore, the third terminal transmits, at the time “t3”, the response packet  93  indicating reception error of the isochronous block I 2 - 1  to the control station and the fourth terminal.  
         [0201]    Then, the fifth terminal transmits, at the time “t4” the response packet  94  indicating successful reception of the isochronous block I 2 - 1  to the control station and the fourth terminal.  
         [0202]    When receiving the response packets  91  to  94  from the second, third, and fifth terminals, the control station carries out multicast transmission of a packet  95  for instructing, in a global manner, the originating terminals (first and fourth terminals) to retransmit the isochronous blocks (I 1 - 1  and I 2 - 1 ) that have not been successfully received. The contents of the global retransmission packet  95  are shown in FIG. 17.  
         [0203]    As shown in FIG. 17, the global retransmission instruction packet  95  describes addresses of the terminals that should execute retransmission (first and fourth terminals), “1” and “4”, and groups that should receive the isochronous data retransmitted by the terminals, “group 1” and “group 2”, and times when retransmission should be executed, “t5” and “t6”. Here, “group 1” includes the second and third terminals, and “group 2” includes the third and fifth terminals.  
         [0204]    Referring back to FIG. 15, when receiving the global retransmission instruction packet  95  as shown in FIG. 17, the first and fourth terminals carries out multicast retransmission of the respective isochronous blocks at the respective specific times to the terminals included in the specified group. That is, the first terminal carries out, at the time “t5”, multicast retransmission of the isochronous block I 1 - 1  to the second and third terminals included in “group 1”. The fourth terminal carries out, at the time “t6”, multicast retransmission of the isochronous block I 2 - 1  to the third and fifth terminals included in “group 2”.  
         [0205]    Then, the control station carries out multicast transmission of a global inquiry packet  96  for inquiring the second and third terminals, which are the destination terminals, whether reception has been succeeded. In response, the second terminal first transmits a response packet  97  indicating successful reception to the control station and the first terminal. Then, the third terminal transmits a response packet  98  indicating successful reception to the control station and the fourth terminal.  
         [0206]    In the first embodiment, the control station inquires each of the terminals, which are the destination terminals of the isochronous data, about whether reception has been succeeded. In response, each terminal transmits a response packet. Alternatively, each terminal may spontaneously transmit a response packet upon reception of the isochronous data. Described in a fourth embodiment below is a communication control method in which each terminal spontaneously transmits a response packet upon reception of the isochronous data.  
         [0207]    (Fourth Embodiment)  
         [0208]    [0208]FIG. 18 is a schematic diagram for demonstrating a communications control method according to the fourth embodiment of the present invention. An example structure of a wireless local area network to which the method of FIG. 18 is applied is similar to that shown in FIG. 2. The structure of the wireless communications module  20  equipped to the control station and each terminal of FIG. 2 is similar to that shown in FIG. 3. The contents of the program memory  24  is similar to those shown in FIG. 4, except that the control station program  31  and the terminal program  32  are partly different from those in the first embodiment.  
         [0209]    In FIG. 1, in the anisochronous region, the control station transmits the inquiry packets  42  and  44  to the second and fourth terminals, which are the destination terminals of the isochronous data I 1  and I 2 , respectively. In response to the inquiry, the second and fourth terminals transmit the response packets  43  and  45 , respectively.  
         [0210]    On the other hand, in FIG. 18, the destination terminal of the isochronous block (the fourth terminal) spontaneously transmits response packets  100  to  102  upon reception of the isochronous blocks (I 2 - 1 , I 2 - 2 , and I 2 - 3 ). Therefore, the control station does not have to make inquires in the anisochronous region about whether reception has been succeeded. Except that, FIG. 18 is similar to FIG. 1.  
         [0211]    In the first embodiment, the control station reports the dedicated region information to each terminal before the start of transmission. Each terminal transmits the isochronous data at the time described in the reported information. Alternatively, the control station may sequentially instruct the respective terminals of transmission and, in response, each terminals may transmit the isochronous data. Described in a fifth embodiment below is a communication control method of sequentially instructing the respective terminals to transmit the isochronous data.  
         [0212]    (Fifth Embodiment)  
         [0213]    [0213]FIG. 19 is a schematic diagram for demonstrating a communications control method according to the fifth embodiment of the present invention. An example of structure of a wireless local area network to which the method of FIG. 19 is applied is similar to that shown in FIG. 2. The structure of the wireless communications module  20  is similar to that in FIG. 3. The contents of the program memory  24  of FIG. 3 is similar to those shown in FIG. 4, except that the control station program  31  and the terminal program  32  are partly different from those according to the first embodiment.  
         [0214]    In FIG. 1, the control station reports the dedicated region information  40  to each terminal before the start of transmission. Each terminal executes transmission of the isochroous data at the time described in the reported information (refer to the first embodiment). On the other hand, in FIG. 19, the control station sequentially transmits transmission instructions to the respective terminals at the times corresponding to the respective dedicated regions. In response, the terminals transmit the isochronous block one after the other.  
         [0215]    More specifically, the control station transmits a transmission instruction  110  to the first terminal in timing corresponding to Ch.  1 . In response, the first terminal transmits an isochronous block I 1 . Then, the control station transmits transmission instructions  111 ,  112 , and  113  to the third terminal in timing corresponding to channel  2 . In response, the third terminal sequentially transmits isochronous blocks I 2 - 1 , I 2 - 2 , and I 2 - 3 . Except that, FIG. 19 is similar to FIG. 1  
         [0216]    The processing carried out when the isochronous data (block) has not been successfully received is similar to that shown in FIG. 5 (refer to the first embodiment).  
         [0217]    [0217]FIG. 20 is a flowchart showing the operation of the control station. The flowchart of FIG. 20 is similar to that of FIG. 7 (refer to the first embodiment), except the following point. Specifically, in FIG. 7, the control station reports the information about the dedicated region assigned in step S 4  to each terminal before the start of transmission (step S 5 ), and then reports the start of the cycle to each terminal (step S 6 ). On the other hand, in FIG. 20, after executing step S 4 , the control station skips step S 5  to execute step S 6 .  
         [0218]    After reporting the start of the cycle to each terminal in step S 6 , the control station sequentially instructs the respective terminals to transmit the isochronous data at the time corresponding to the dedicated region assigned in step S 4  (step S 101 ). In response to the instruction, each of the terminals sequentially transmits the isochronous data. Then, the procedure goes to step S 7 , and thereafter.  
         [0219]    The operation of each terminal is similar to that according to the first embodiment, which is shown in the flowchart of FIG. 9. The details on the data receiving processing shown in step S 26  of FIG. 9 are similar to those according to the first embodiment, which are shown in the flowchart of FIG. 10. However, the details on step S 22  are partly different from that according to the first embodiment, which are described below.  
         [0220]    [0220]FIG. 21 is a flowchart showing the details on the isochronous data transmission processing shown in step S 22  of FIG. 9. The flowchart of FIG. 21 is similar to that of FIG. 10 (refer to the first embodiment) except the following. That is, in FIG. 10, the terminal first receives a report of the dedicated region information from the control station (step S 41 ), and waits for a report of the start of the cycle (step S 42 ). Informed of the start of the cycle, the terminal transmits the isochronous data using the assigned dedicated region (step S 43 ).  
         [0221]    On the other hand, in FIG. 21, step S 41  is skipped. The terminal first executes step S 42 . Then, notified of the start of the cycle, the terminal enters in a wait state, waiting for a transmission instruction (step S 121 ). After receiving a transmission instruction from the control station, the terminal transmits the isochronous data (step S 122 ). Then, the procedure goes to step S 144  and thereafter.  
         [0222]    In the fifth embodiment, the control station inquires each destination terminal of the isochronous data about whether reception has been succeeded. In response, each terminal transmits a response packet. Alternatively, each terminal may spontaneously transmit a response packet upon receipt of the isochronous data. A communications control method used in such case is shown in FIG. 22.  
         [0223]    In FIG. 22, on receiving the isochronous data (I 2 - 1 , I 2 - 2 , and I 2 - 3 ), the fourth terminal transmits response packets  120 ,  121 , and  122 , respectively. Therefore, the control station does not have to make inquiries about whether reception has been succeeded in the anisochronous region.  
         [0224]    While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.