Patent Publication Number: US-2007121550-A1

Title: Wireless apparatus, monitor camera system using the wireless apparatus, and audio-video listening and viewing system using the wireless apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The teachings of Japanese Patent Application JP 2005-340236, filed Nov. 25, 2005, are entirely incorporated herein by reference, inclusive of the claims, specification, and drawings.  
     BACKGROUND OF THE INVENTION  
      The present invention relates to a wireless apparatus for wirelessly transmitting data which requires a real-time bandwidth guarantee, such as audio data or video data.  
      In recent years, equipment using a wireless LAN (Local Area Network) in place of a wired LAN has become widespread. In addition, the increased transmission speed of a wireless LAN has made it possible to employ an apparatus using the wireless LAN as means for performing high-quality and high-speed transmission of video data or the like.  
      A description will be given herein below to a wireless apparatus according to a conventional embodiment.  
       FIG. 17  shows a conventional wireless apparatus  101 . The wireless apparatus  101  has a transmission functional portion  101   a  and a reception functional portion  101   b , which respectively perform a transmission process and a reception process each by using an antenna  102 . The transmission functional portion  101   a  comprises: a time stamp add unit  106 ; a frame body assembly unit  105 ; and a wireless transmission unit  103 . The reception functional portion  101   b  comprises: a wireless reception unit  104 ; a digital filter  107 ; a frame body disassembly unit  109 ; and a packet read control unit  110 . In addition, the wireless apparatus  101  also comprises a wireless clock  111 .  
      Each of the wireless transmission unit  103  and the wireless reception unit  104  comprises an RF (Radio Frequency), a baseband, each of which is a physical layer, and a layer-2 processor for processing a data link layer, though they are not depicted.  
      A description will be given herein below to the operation of the wireless apparatus having a structure as described above. It will be easily understood that, when a wireless signal  117  is wirelessly transmitted, a plurality of wireless apparatus are used.  FIG. 17  illustrates the transmission of the wireless signal  117  between the wireless apparatus  101  and another wireless apparatus  101   x . It may be considered that the other wireless apparatus  101   x  has the same structure as the wireless apparatus  101  herein. In the following description, however, transmitting and receiving operations will be shown collectively using only the wireless apparatus  101  for simplified illustration. Specifically, the operation of transmitting the wireless signal  117 , which is performed by the transmission functional portion  101   a  of the wireless apparatus  101  via the antenna  102 , will be described simultaneously with the operation of receiving the wireless signal  117  which is intrinsically performed by the other wireless apparatus  101   x . However, instead of describing the receiving operation by showing the detailed structure of the other wireless apparatus  101   x , the receiving operation will be described on the assumption that it is performed by the reception functional portion  101   b  of the wireless apparatus  101  via the antenna  102 , not by the other wireless apparatus  101   x.    
      A packet input  112  to the wireless apparatus is inputted first to the time stamp add unit  106 . The time stamp add unit  106  generates a time-stamp-added packet  114  by adding a time stamp to the packet input  112  and outputs the time-stamp-added packet  114  to the frame body assembly unit  105 . At this stage, a transmission beacon signal  115  is inputted from the wireless transmission unit  103  to the time stamp add unit  106  so that the time stamp is added in accordance with the input timing of the packet input  112  based on the transmission beacon signal  115 . More specifically, the value of the transmission beacon signal  115  is held at the time at which the packet input  112  is inputted and the held value is added to the packet input  112 .  
      The frame body assembly unit  105  generates a frame body signal  116  for a wireless LAN from a single time-stamp-added packet  114  or from an assembly of a plurality of time-stamp-added packets  114  and outputs the frame body signal  116  for wireless LAN to the wireless transmission unit  103 . The wireless transmission unit  103  adds a header, which is needed in the wireless LAN, to the frame body signal  116 , further modulates the frame body signal  116  with the header, and transmits the modulated frame body signal  116  as the wireless signal  117  via the antenna  102 .  
      The wireless signal  117  is received by the other wireless apparatus  101   x . However, the description will be given instead to the substitute case where the wireless apparatus  101  has received the wireless signal  117 , as stated previously.  
      The wireless reception unit  104  that has received the wireless signal  117  via the antenna  102 outputs a reception beacon signal  118  to the digital filter  107 . At the same time, the wireless reception unit  104  checks the header needed in the wireless LAN after demodulation and recognizes the presence or absence of an error. When there is no error, the wireless reception unit  104  outputs a reception frame body signal  119  to the frame body disassembly unit  109 .  
      It is to be noted that the beacon signal is a control signal used between different wireless apparatus. Each of the wireless apparatus synchronizes based on the beacon signal and controls timing for transmission data.  
      On receiving the reception beacon signal  118 , the digital filter  107  generates a corrected reception beacon signal  121  by correcting fluctuations that have occurred in a wireless section and outputs the corrected reception beacon signal  121  to the packet read control unit  110 .  
      On receiving the corrected reception beacon signal  121 , the packet read control unit  110  synchronizes a reception time stamp timer (not shown) provided in the packet read control unit  110  with the corrected reception beacon signal  121 . The packet read control unit  110  further generates a packet read signal  122  showing timing for reading a packet from the result of a comparison between the reception time stamp timer and a reception time stamp  120  inputted from the frame body disassembly unit  109  and outputs the packet read signal  122  to the frame body disassembly unit  109 .  
      The frame body disassembly unit  109  disassembles the reception frame body signal  119 , which is either the single time-stamp-added packet  114  or the assembly of the plurality of time-stamp-added packets  114 , into the individual discrete time-stamp-added packets  114 . The frame disassembly unit  109  further outputs the time stamp added to each of the time-stamp-added packets  114  as the reception time stamp  120  to the packet read control unit  110 . On receiving the packet read signal  122  from the packet read control unit  110 , the frame body disassembly unit  109  generates a packet by removing the reception time stamp  120  from each of the time-stamp-added packets  114  and outputs the generated packet as a packet output  113 .  
      In the manner described above, the wireless apparatus  101 , which implements wireless transmission, performs data processing by using the wireless clock  111  at a fixed frequency. That is, each of the time stamp add unit  106 , the frame body assembly unit  105 , the wireless transmission unit  103 , the wireless reception unit  104 , the digital filter  107 , the packet read control unit  110 , and the frame body disassembly unit  109  operates based on the wireless clock  111 .  
      The foregoing description has been given thus far to the transmission and reception performed in the wireless apparatus  101 . Intrinsically, however, the wireless signal  117  transmitted from the wireless apparatus  101  is received by the other wireless apparatus  101   x . The other wireless apparatus  101   x  is provided with another fixed-frequency wireless clock  111   x  and controlled thereby. It follows therefore that the transmitting operation and the receiving operation are controlled by the different clocks.  
       FIG. 18  is a view for illustrating the operation of a wireless apparatus connected to a conventional wired network.  
       FIG. 18  shows the connection among a wireless LAN  202 , a wired network  212 , and a wireless LAN  217 . The wireless LAN  202  is composed of a wireless apparatus  1  (STA)  203  and a wireless apparatus  2  (AP)  207 . The wireless LAN  217  is composed of a wireless apparatus  3  (AP)  218  and a wireless apparatus  4  (STA)  221 . The STA represents a station. The AP represents an access point.  
      The operation of such a network will be described herein below.  
      Each of the wireless apparatus  1  (STA)  203 , the wireless apparatus  2  (AP)  207 , the wireless apparatus  3  (AP)  218 , and the wireless apparatus  4  (STA)  221  has the same structure as the wireless apparatus  101  shown in, e.g.,  FIG. 17 . Additionally, each of the wireless apparatus  2  (AP)  207  and the wireless apparatus  3  (AP)  218  has the function of communicating with the wired network  212 .  
      The wireless apparatus  1  (STA)  203  and the wireless apparatus  2  (AP)  207  are synchronized with each other via a wireless signal  206  including a beacon signal which is generated by the wireless apparatus  2  (AP)  207 .  
      A data clock  204  for processing an audio-video data input  201  in the wireless apparatus  1  (STA)  203  also operates based on the beacon signal mentioned above so that the audio-video data input  201  is formed into a packet based on the data clock  204 . The audio-video data input  201  in the form of a packet is transmitted to the wireless apparatus  2  (AP)  207  via the wireless signal  206 .  
      The wireless apparatus  2  (AP)  207  extracts the audio-video signal packet from the wireless signal  206  and outputs the extracted audio-video signal packet as wired data  209  to the wired apparatus  210 . The wired apparatus  210  processes the wired data  209  in accordance with a wired data format and further transmits the wired data  209  to the wired network  212  via a wired line  211 . The wired network  212  transmits the data received via the wired line  211  and further outputs the received data to a wired apparatus  215  via a wired line  214 . The wired apparatus  215  outputs the received data as wired data  216  to the wireless apparatus  3  (AP)  218 .  
      The wireless apparatus  3  (AP)  218  and wireless apparatus  4  (STA)  221  of the wireless LAN  217  are synchronized with each other via a wireless signal  220  including a beacon signal which is generated by the wireless apparatus  3  (AP)  218 .  
      The wireless apparatus  3  (AP)  218  transmits the wired data  216  received from the wired apparatus  215  to the wireless apparatus  4  (STA)  221  via the wireless signal  220 . The wireless apparatus  4  (STA)  221  decodes the data in the form of a packet from the wireless signal  220  based on a wireless clock  222  and a data clock  223  and outputs the decoded data as an audio-video data output  224 .  
      The wired network  212  is a network which guarantees a bandwidth synchronized by the wired clock  213 .  
      As examples of documents related to this technical field, there can be listed Japanese Unexamined Patent Publication Nos. 2005-39633 and 2003-60652, Japanese Examined Patent Publication No. HEI 6-18339, Japanese Patent No. 3160146, and the like.  
     SUMMARY OF THE INVENTION  
      However, the conventional wireless apparatus  101  shown in  FIG. 17  has the following problems.  
      The digital filter  107  of the receiver generates the corrected reception beacon signal  121  by correcting, based on the wireless clock of the receiver, the transmission beacon signal  115  generated based on the wireless clock of the transmitter. However, the wireless clock  111  provided in the conventional wireless apparatus  101 , which performs wireless communication, is an asynchronous clock. That is, the wireless clock of the transmitter and the wireless clock of the receiver are out of synchronization. As a result, fluctuations in clock width and fluctuations in the time accuracy range of the beacon signal occur in the corrected reception beacon signal  121  outputted from the digital filter  107 . The fluctuations are eventually transmitted to the packet output  113  and degrade the reliability of wireless communication and the like.  
      On the other hand, of the wireless apparatus connected to the conventional wired network shown in  FIG. 18  to transmit the audio-video data, the wireless apparatus  2  (AP)  207  of the wireless LAN connected to the wired network  212  and the beacon signal generated by the wireless apparatus  3  (AP)  218  are out of synchronization. In addition, the wireless clock  208  provided in the wireless apparatus  2  (AP)  207  and the wireless clock  219  provided in the wireless apparatus  3  (AP)  218  are also out of synchronization.  
      Accordingly, the wireless clock  222  and the data clock  223  each provided in the wireless apparatus  4  (STA)  221  as the receiver which reproduces an audio-video signal are not synchronized with the wireless clock  205  and the data clock  204  each provided in the wireless apparatus  1  (STA)  203 , respectively.  
      As a result, the packet interval (data bandwidth) is not guaranteed and there is the possibility that the audio-video data  224  with degraded characteristics is decoded. In other words, the mutually asynchronous clocks may cause the degradation of an image resulting from, e.g., reproduction with some image frames omitted, the freezing of an image, or the like.  
      It is therefore an object of the present invention to provide a wireless apparatus and a wireless method which allow the suppression of packet fluctuations and a real-time guarantee of the data bandwidth.  
      A wireless apparatus according to the present invention comprises: a wireless reception unit for receiving a wireless signal including a frame body signal having a packet with a time stamp added thereto and a beacon signal, extracting the frame body signal and the beacon signal from the wireless signal; a frame body disassembly unit for extracting the time stamp and the packet from the extracted frame body signal and outputting the extracted time stamp and the extracted packet; a packet read control unit for generating a packet read signal inputted to the frame body disassembly unit to control the outputting of the packet based on the extracted time stamp and outputting the packet read signal; and a wireless reference clock generation unit for generating a wireless clock and outputting the wireless clock to the wireless reception unit, the frame body disassembly unit, and to the packet read control unit, wherein the wireless reference clock generation unit selects either one of a free-running master clock and a clock which is synchronized with the beacon signal by using a PLL (Phase Locked Loop) and outputs the selected one as the wireless clock.  
      Since the wireless apparatus according to the present invention allows synchronization between the wireless clock thereof and that of another wireless apparatus which has transmitted the wireless signal received thereby, it becomes possible to suppress packet fluctuations and guarantee a data bandwidth. This allows real-time high-quality wireless communication, e.g., transfer of audio-video data, which will be described herein below to be performed.  
      The wireless reference clock generation unit provided in the wireless apparatus according to the present invention can generate the clock which is synchronized with the beacon signal included in the received wireless signal and can also select either one of the clock and a free-running masker clock and output the selected one. The beacon signal is generated based on another wireless clock provided in the other wireless apparatus which has transmitted the wireless signal. It follows therefore that the clock which is synchronized with the beacon signal is also synchronized with the other wireless clock provided in the other wireless apparatus.  
      The time stamp included in the received wireless signal is generated and added based on the other wireless clock in the other wireless apparatus that has transmitted the wireless signal. Accordingly, packet fluctuations can be suppressed by adding the time stamp using the wireless clock which is synchronized with the other wireless clock, i.e., by controlling the packet interval based on the difference between the time stamp values. The control of the packet interval based on the difference between the time stamp values indicates a control operation such that, when consideration is given to a first packet and a second packet received after the reception of the first packet, the difference between the time stamp values of the first and second packets is determined and, after the first packet is outputted, the second packet is outputted after the lapse of a time corresponding to the difference between the time stamp values.  
      In one aspect, the wireless apparatus according to the present invention preferably further comprises: a time stamp add unit for adding a transmission time stamp to a packet input in accordance with a transmission beacon signal and outputs the packet input as a time-stamp-added packet; a frame body assembly unit for generating a transmission frame body signal including the at least one time-stamp-added packet outputted from the time stamp add unit and outputting the transmission frame body signal; and a wireless transmission unit for generating the transmission beacon signal, outputting the transmission beacon signal to the time stamp add unit, and wirelessly transmitting the transmission beacon signal and the transmission frame body signal.  
      The arrangement allows the wireless apparatus according to the present invention to also perform the transmission of the wireless signal. By using the two wireless apparatus according to the present invention, transmission and reception for wireless communication with reduced packet fluctuations can be performed. The transmission time stamp, the transmission frame body signal, and the transmission beacon signal added or generated in the transmitter wireless apparatus are received as the time stamp, the frame body signal, and the beacon signal by the receiver wireless apparatus.  
      In still another aspect, the wireless apparatus according to the present invention preferably further comprises: a time stamp add unit which comprises a timer unit for a time stamp, adds a transmission time stamp to a packet input based on the timer unit for a time stamp, and outputs the packet input as a time-stamp-added packet; a frame body assembly unit for generating a transmission frame body signal including the at least one time-stamp-added packet outputted from the timer stamp add unit; and a wireless transmission unit for generating a transmission beacon signal and wirelessly transmitting the transmission beacon signal and the transmission frame body signal.  
      The timer unit for time stamp can perform counting based on the wireless clock which is higher in time accuracy than the transmission beacon signal generated in the wireless transmission unit. This allows the wireless apparatus to perform transmission and also perform packet control even in the case of using a bandwidth which requires the control of the packet interval shorter than the time interval between the transmission beacon signals. This also allows more reliable suppression of packet fluctuations.  
      In yet another aspect, the wireless apparatus according to the present invention preferably further comprises: means for detecting a change in a beacon period based on a beacon interval included in the beacon signal; and means for setting a parameter for the PLL in the wireless reference clock generation unit in response to the change in the beacon period, wherein the wireless reference clock generation unit generates the wireless clock in accordance with the parameter.  
      The arrangement allows the synchronization of the wireless clock with the transmitter wireless apparatus to be maintained even when the beacon period is changed and allows wireless communication with reduced packet fluctuations to be performed.  
      Likewise, the wireless apparatus according to the present invention preferably further comprises: means for changing a beacon period in accordance with a packet bandwidth; means for reporting the beacon period by using a beacon interval; means for detecting a change in the beacon period based on the beacon interval included in the beacon signal; and means for setting a parameter for the PLL in the wireless reference clock generation unit in response to the change in the beacon period, wherein the wireless reference clock generation unit generates the wireless clock in accordance with the parameter.  
      When the wireless signal is transmitted, the arrangement allows the beacon period to be changed in accordance with the difference between packet bandwidths and allows an improvement in communication efficiency. That is, when the packet bandwidth is large, the ratio of the transmission beacon signal to the time-stamp-added packet to be transmitted can be reduced by increasing the beacon period. Accordingly, a larger number of packets can be transmitted even by using the same packet bandwidth. In addition, since the change in beacon period is reported by using the beacon interval, the change in beacon period can be detected in the receiver wireless apparatus based on the beacon interval.  
      When the wireless signal is received, the synchronization of the wireless clock with the transmitter wireless apparatus can be maintained in accordance with the change in beacon period. Accordingly, wireless communication with reduced packet fluctuations can be performed.  
      In still another aspect, the wireless apparatus according to the present invention preferably further comprises: means for counting the beacon period based on the beacon signal; and means for detecting a beacon loss based on the beacon interval included in the beacon signal and on a count of the beacon period, wherein the wireless reference clock generation unit continues to output the same wireless clock as outputted before the detection of the beacon loss when the beacon loss is detected.  
      When the beacon loss is detected, i.e., when the beacon signal cannot be detected any more, the arrangement continued to output the same wireless clock as outputted before the beacon loss is detected. As a result, even when the beacon signal cannot be detected any more, a stable wireless clock can be generated without receiving serious influence therefrom. Normally, the communication status of the beacon signal is not monitored by using an ACK signal (receipt acknowledgment) so that, even when the beacon signal is lost, it cannot be acquired again. Therefore, the capability of generating a stable wireless clock even when the beacon signal is lost achieves a remarkable effect for communication with reduced packet fluctuations.  
      In yet another aspect, the wireless apparatus according to the present invention preferably further comprises: means for requesting switching of a wireless channel based on the beacon signal and on the detection of the beacon loss, wherein the means for detecting the beacon loss detects cancellation of the beacon loss after the switching of the wireless channel and the wireless reference clock generation unit outputs the master clock as the wireless clock when the wireless channel is switched and outputs the clock which is synchronized with the beacon signal as the wireless clock after the beacon loss is cancelled.  
      The arrangement allows a stable wireless clock to be outputted in accordance with the free-running master clock when the wireless channel is switched and also allows the wireless clock which is synchronized with the beacon signal to be outputted again after the switching of the wireless channel is completed. As a result, it becomes possible to reliably perform the switching of the wireless channel based on the beacon loss and also suppress packet fluctuations when the wireless channel is switched.  
      In still another aspect, the wireless apparatus according to the present invention preferably further comprises: means for generating a data clock for processing an audio-video signal formed into the packet, wherein the data clock is synchronized with the wireless clock.  
      By thus processing the audio-video signal based on the data clock which is higher in time accuracy than the beacon signal, fluctuations can be further suppressed than in the case where the processing is performed based on the beacon signal. As a result, when the audio-video signal is received and reproduced, the occurrence of the characteristic degradation thereof can be further suppressed.  
      In yet another aspect, the wireless apparatus according to the present invention preferably further comprises: means for generating a wireless clock which is synchronized with a wired network based on a wired synchronous phase reference signal in the wired network.  
      With the arrangement, even in the wireless apparatus connected to the wired network to communicate data such as audio-video data, packet fluctuations can be suppressed and the data bandwidth can be guaranteed. As a result, when the audio-video signal is communicated, the characteristic degradation of the audio-video signal during the reproduction thereof can be suppressed.  
      A monitor camera system according to the present invention comprises: a camera for acquiring audio-video data; a monitor apparatus for displaying the audio-video data;  
      and a wireless apparatus for transmitting the audio-video data from the camera to the monitor apparatus, wherein the wireless apparatus is the wireless apparatus according to any of the aspects of the present invention.  
      The monitor camera system according to the present invention allows real-time high-definition transmission of the audio-video data by using the wireless apparatus according to the present invention provided in each of the camera and the monitor apparatus. Therefore, the monitor camera system according to the present invention is useful as a system for monitoring an image and a sound at a remote location.  
      The monitor camera system according to the present invention may further comprise a recording apparatus for recording audio-video data transmitted from the camera via wireless communication in a recording medium such as a DVD (Digital Versatile Disc). This allows real-time high-definition recording of audio/video data which is wirelessly transmitted from a remote location.  
      An audio-video listening and viewing system according to the present invention comprises: a reproduction apparatus for reproducing audio-video data; a monitor apparatus for displaying the audio-video data; and a wireless apparatus for transmitting the audio-video data from the reproduction apparatus to the monitor apparatus, wherein the wireless apparatus is the wireless apparatus according to any of the aspects of the present invention.  
      With the audio-video listening and viewing system according to the present invention, it becomes possible to implement an audio-video listening and viewing system which allows real-time high-definition listening and viewing of the audio-video data even when it is wirelessly transmitted by using the wireless apparatus according to the present invention provided in each of the reproduction apparatus for reproducing the audio-video data from a recording medium such as a DVD and the monitor apparatus.  
      Thus, the wireless apparatus according to the present invention allows synchronization between the wireless clock thereof and that of a communication partner with which it performs wireless communication and thereby allows the suppression of packet fluctuations when packet interval control based on the difference between the time stamps is performed and the execution of high-quality data transmission. This can be implemented with a relatively small increase in the hardware scale of the PLL circuit for the wireless clock. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a view showing the structure of a wireless apparatus according to a first embodiment of the present invention;  
       FIG. 2  is a view showing a structure of a wireless reference clock generation unit in the wireless apparatus according to the first embodiment;  
       FIG. 3  is a flow chart illustrating the operation of the wireless apparatus according to the first embodiment;  
       FIG. 4  is a view illustrating the structure of a wireless clock in the wireless apparatus according to the first embodiment;  
       FIG. 5  is a view showing the structure of a wireless apparatus according to a second embodiment of the present invention;  
       FIG. 6  is a view showing the structure of a wireless apparatus according to a third embodiment of the present invention;  
       FIG. 7  is a view illustrating a beacon-period changing operation in the wireless apparatus according to the third embodiment;  
       FIG. 8  is a view showing a structure of the wireless reference clock generation unit in the wireless apparatus according to the third embodiment;  
       FIG. 9  is a view showing the structure of a wireless apparatus according to a fourth embodiment of the present invention;  
       FIG. 10  is a view illustrating a clock protecting operating during a beacon loss in the wireless apparatus according to the fourth embodiment;  
       FIG. 11  is a view showing the structure of a wireless apparatus according to a fifth embodiment of the present invention;  
       FIG. 12  is a view for illustrating the clock protecting operation during channel switching in the wireless apparatus according to the fifth embodiment;  
       FIG. 13  is a view showing the structure of a wireless apparatus according to a sixth embodiment of the present invention;  
       FIG. 14  is a view showing the structure of a wireless apparatus according to a seventh embodiment of the present invention;  
       FIG. 15  is a view showing the connection of the wireless apparatus according to the seventh embodiment to a wired network;  
       FIG. 16  is a view showing a monitor camera system which communicates audio-video data by using the wireless apparatus according to the seventh embodiment;  
       FIG. 17  is a view showing a structure of a conventional wireless apparatus; and  
       FIG. 18  is a view showing the connection of a conventional wireless apparatus to a wired network. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to the drawings, the individual embodiments of the present invention will be described herein below.  
     Embodiment 1  
       FIG. 1  schematically shows the structure of a wireless apparatus according to the first embodiment of the present invention. A wireless apparatus  0301  has a transmission functional portion  0301   a  and a reception functional portion  0301   b , which respectively perform a transmission process and a reception process each by using an antenna  0102 .  
      The transmission functional portion  0301   a  is the same as the transmission functional portion  101   a  in the conventional wireless apparatus  101  shown in  FIG. 17 .  
      Accordingly, the transmission functional portion  0301   a  comprises a time stamp add unit  0106 , a frame body assembly unit  0105 , and a wireless transmission unit  0103 . Compared with the conventional reception functional portion  101   b , the reception functional portion  0301   b  does not comprise the digital filter  107  but comprises a wireless reference clock generation unit. Accordingly, the reception functional portion  0301   b  comprises a wireless reference clock generator  0303  in addition to a wireless reception unit  0104 , a frame body disassembly unit  0109 , and a packet read control unit  0110 .  
      Each of the wireless transmission unit  0103  and the wireless reception unit  0104  comprises an RF (Radio Frequency), a baseband, each of which is a physical layer, and a layer-2 processor for processing a data link layer, though they are not depicted.  
      A description will be given herein below to a wireless method as the operation of the wireless apparatus  0301  according to the present embodiment having a structure as described above. For simplified illustration, transmitting and receiving operations according to the present embodiment will also be described collectively by using only the wireless apparatus  0301  depicted in detail, in the same manner as described in the prior art technology by using the conventional wireless apparatus  101 . That is, a wireless signal  0117  transmitted from the wireless apparatus  0301  is intrinsically received by another wireless apparatus  0301   x , while a wireless signal transmitted from the other wireless apparatus  0301   x  is also intrinsically received by the wireless apparatus  0301 . However, instead of describing the transmitting and receiving operations by depicting each of the wireless apparatus  0301  and the other wireless apparatus  0301   x  in detail, the transmission and reception will be described herein by using only the depiction of the wireless apparatus  0301 .  
      The transmitting operation by the transmission functional portion  0301   a  is the same as in the conventional wireless apparatus  101 . The transmitting operation will be described herein below.  
      A packet input  0112  to the wireless apparatus  0301  is inputted first to the time stamp add unit  0106 . The time stamp add unit  0106  generates a time-stamp-added packet  0114  by adding a time stamp to the packet input  0112  and outputs the time-stamp-added packet  0114  toward the frame body assembly unit  0105 . As a result, a transmission beacon signal  0115  is inputted from the wireless transmission unit  0103  to the time stamp add unit  0106 . Based on the transmission beacon signal  0115 , the time stamp is added in accordance with the input timing of the packet input  0112 . More specifically, the value of the transmission beacon signal  0115  is held at the time at which the packet input  0112  is inputted.  
      The frame body assembly unit  0105  generates a transmission frame body signal  0116  for a wireless LAN from a single time-stamp-added packet  0114  or an assembly of a plurality of time-stamp-added packets  0114  inputted from the time stamp add unit  0106  and outputs the transmission frame body signal  0116  to the wireless transmission unit  0103 . The wireless transmission unit  0103  adds a header needed in the wireless LAN to the transmission frame body signal  0116  inputted from the frame body assembly unit  0105 , further modulates the transmission frame body signal  0116  with the header, and transmits the modulated transmission frame body signal  0116  as the wireless signal  0117  via the antenna  0102 .  
      The wireless signal  0117  is received by the other wireless apparatus  0301   x .  
      However, the description will be given instead to the substitute case where the wireless apparatus  0301  has received the wireless signal  0117 , as stated previously.  
      The wireless reception unit  0104  that has received the wireless signal  0117  via the antenna  0102  extracts the beacon signal (transmission beacon signal  0115 ) and the frame body signal (transmission frame body signal  0116 ) from the wireless signal  0117  as a reception beacon signal  0118  and a reception frame body signal  0119 , respectively., and outputs them More specifically, the present embodiment outputs the reception fame body signal  0119  toward the fame body disassembly unit  0109 , similarly to the conventional embodiment, while outputting the reception beacon signal  0118  toward the wireless reference clock generation unit  0303 , unlike the conventional embodiment which has outputted the reception beacon signal toward the digital filter.  
      As will be described later in detail, the wireless reference clock generation unit  0303  is capable of outputting a wireless clock  0111 , which is synchronized with the wireless clock of the transmitter wireless apparatus by using a PLL circuit, based on the reception beacon signal  118  inputted from the wireless reception unit  0104 .  
      The wireless clock  0111  thus generated is used in each of the wireless transmission unit  0103  and the wireless reception unit  0104 , as will also be described later.  
      In addition, the wireless reference clock generation unit  0303  is also capable of generating a free-running clock and outputting it as the wireless clock  0111 , as will also be described later. The selection of which one is to be outputted as the wireless clock  0111  is performed in accordance with a reference voltage switch signal  0302 .  
      The frame body disassembly unit  0109  to which the reception frame body signal  0119  has been inputted disassembles the reception frame body signal  0119 , which is either the single time-stamp-added packet  0114  or the assembly of the plurality of time-stamp-added packets  0114 , into the individual discrete time-stamp-added packets  0114 . The frame body disassembly unit  0109  further outputs the time stamp added to each of the time-stamp-added packets  0114  as a reception time stamp  0120  to the packet read control unit  0110 . These operations are the same as in the conventional wireless apparatus  101 .  
      The packet read control unit  0110  that has received the reception time stamp  0120  generates a packet read signal  0122  and outputs it toward the frame body disassembly unit  0109 . In contrast to the conventional embodiment which has generated the packet read signal  122  based on the corrected beacon signal  121 , the present embodiment generates the packet read signal  0122  by using the wireless clock  0111  synchronized with the wireless clock of the transmitter in accordance with a time interval based on the differences between the individual reception time stamps. That is, when consideration is given to a first packet and to a second packet received after the reception of the first packet, a control operation is performed such that the difference between the respective time stamp values of the first and second packets is determined and, after the first packet is outputted, the second packet is outputted after the lapse of a time corresponding to the difference between the time stamp values. Since the respective wireless clocks of the transmitter wireless apparatus which has added the time stamp values and the receiver wireless apparatus are synchronized with each other, packet fluctuations can be suppressed reliably.  
      On receiving the packet read signal  0122 , the frame body disassembly unit  0109  generates a packet by removing the reception time stamp  0120  from each of the time-stamp-added packets  0114  and outputs the generated packet as a packet output  0113 .  
      Thus, the present embodiment performs the receiving process by using the wireless clock  0111  synchronized with the wireless clock of the transmitter wireless apparatus and thereby allows wireless communication with reduced packet fluctuations to be performed. Therefore, the present embodiment is particularly useful for real-time data communication.  
      The wireless apparatus  0301  according to the present embodiment comprises both the transmission functional portion  0301   a  and the reception functional portion  0301   b  to serve as a wireless apparatus capable of performing both transmission and reception.  
      In the case where it is unnecessary to transmit data which requires a bandwidth guarantee (such as audio-video data) but it is necessary to perform reception, the wireless apparatus is allowed to receive data with a bandwidth guarantee and suppress the characteristic degradation of the audio-video data during the reproduction thereof by comprising the reception functional portion  0301   b  and the antenna  0102 .  
      A description will be given next to the structure and operation of the wireless reference clock generation unit  0303  with reference to  FIG. 2 .  
      The reception beacon signal  0118  inputted to the wireless reference clock generation unit  0303  has the frequency thereof divided by a reference clock frequency divider  0401  and is outputted as a reference clock frequency-divided signal  0409 . The reference clock frequency-divided signal  0409  is compared with a generated clock frequency-divided signal  0410  in a phase comparator  0402  so that a phase comparison signal  0411  is outputted from the phase comparator  0402 . The phase comparison signal  0411  is inputted to a low pass filter  0403 , from which a gain signal  0412  is outputted. The gain signal  0412  is inputted to a voltage conversion circuit  0404 , from which a gain control voltage  0413  is outputted. The gain control voltage  0413  generated based on the gain signal  0412  is generated to control an oscillator  0406 .  
      Subsequently, the gain control voltage  0413  is inputted to a reference voltage switch (SEL)  0405 . To the reference voltage switch  0405 , an oscillator reference voltage  0416  has also been inputted so that either one of the gain control voltage  0413  and the oscillator reference voltage  0416  is selected in accordance with the reference voltage switch signal  0302  and the selected one is outputted as an oscillator control voltage  0414 .  
      The oscillator control voltage  0414  is inputted to the oscillator  0406  so that the oscillator  0406  generates a generated clock  0415  controlled by the oscillator control voltage  0414 .  
      The generated clock  0415  is inputted to a generated clock frequency divider  0407 , in which the frequency thereof is divided and the generated clock frequency-divided signal  0410  to be compared with the reference clock frequency-divided signal  0409  in the phase comparator  0402  is generated. At the same time, the generated clock  0415  is also inputted to a wireless clock generator  0408 , in which the frequency thereof is divided or multiplied and the wireless clock  0111  used in the wireless apparatus  0301  is generated.  
      The oscillator reference voltage  0416  inputted to the reference voltage switch  0405  is a control voltage for the oscillator  0406  when the wireless clock  0111  which does not allow synchronization with a communication partner (the wireless clock of the transmitter wireless apparatus) is used (i.e., when the wireless apparatus  0301  is the master). This provides a fixed voltage for generating a fixed frequency in accordance with the standard.  
      The reference voltage switch signal  0302  is a signal for selecting either one of the gain control voltage  0413  and the oscillator reference voltage  0416 . With the reference voltage switch signal  0302 , either one of the wireless clock  0111  which is synchronized with the communication partner and the wireless clock  0111  which is the master is selected.  
      In other words, the reference voltage switch signal  0302  allows the selection of whether the wireless apparatus  0301  is to be synchronized with the communication partner or not.  
      Thus, the wireless reference clock generation unit  0303  allows either one of the free-running master clock and the clock synchronized with the reception beacon signal to be selected and outputted as the wireless clock. Since the reception beacon signal  0118  has been generated based on the wireless clock of the transmitter wireless apparatus, it follows that the clock synchronized with the reception beacon signal  0118  is also synchronized with the wireless clock of the transmitter.  
       FIG. 3  is a view for illustrating a control method for the reference voltage switch signal  0302  for controlling the wireless reference clock generation unit  0303  provided in the wireless apparatus  0301  according to the present embodiment. After turning on the power source of the wireless apparatus  0301 , a master clock process  0501  is performed first to control the reference voltage switch signal  0302  such that the oscillator reference voltage  0416  is outputted as the oscillator control voltage  0414 .  
      Next, a beacon synchronization setting process  0502  determines whether or not synchronization with the reception beacon signal  0118  is allowed as a determination  0503  of beacon synchronization setting. As a result, when synchronization is not allowed, i.e., when the wireless signal  0111  serving as the master is used, a process  0508  of transmitting the beacon signal onto a wireless line is performed. On the other hand, when synchronization is allowed, a process  0504  of searching for the receiver of the beacon is performed.  
      A description will be given first to the case where the process  0508  of transmitting the beacon signal onto the wireless line is performed. In this case, the beacon signal is transmitted with the period of the beacon interval onto the wireless line via the antenna  0102 . At the same time, it is determined by a synchronization request detection  0509  whether or not there is a request for synchronization with the beacon signal. When the request for synchronization with the reception beacon is detected, the process  0504  of searching for the receiver of the beacon is performed. When there is no request for synchronization with the reception beacon, the whole flow returns to the process  508  of transmitting the beacon signal onto the wireless line and repeats the process.  
      Next, in the process  0504  of searching for the receiver of the beacon, a communication partner is detected in the determination  0505  of the receiver of the beacon. When a communication partner is not detected, the process  0504  is repeated. When a communication partner is detected, a process  0506  of selecting the oscillator control voltage is performed.  
      In the process  0506  of selecting the oscillator control voltage, the reference voltage switch signal  0302  is set such that the gain control voltage  0413  is selected and outputted as the oscillator control voltage  0414 . As a result, the wireless clock  0111  is synchronized with the reception beacon signal. Thereafter, a process  0507  of searching for a request to stop synchronization with the beacon signal is performed to detect whether or not there is a request to stop synchronization with the reception beacon. When there is a request to stop synchronization with the reception beacon, the whole flow returns to the master clock process  0501  and executes it.  
      The foregoing is the method for controlling the reference voltage switch signal  0302  for selecting either one of the clock synchronized with the reception beacon and the free-running mater clock.  
      A further description will be given to the wireless clock  0111  with reference to  FIG. 4 . The wireless clock  0111  is inputted to and used in each of the time stamp add unit  0106 , the frame body assembly unit  0105 , the frame body disassembly unit  0109 , and the packet read control unit  0110 , while it is also used in respective circuits composing the wireless transmission unit  0103  and the wireless reception unit  0104 . Specifically, the wireless clock  0111  is used also in a transmission layer-2 processor  1701 , a transmission baseband  1702 , and a transmission RF  1703  each composing the wireless transmission unit  0103  and in a reception layer-2 processor  1704 , a reception baseband  1705 , and a reception RF  1706  each composing the wireless reception unit  0104 . To these circuits, the individual clocks included in the wireless clock  0111 , i.e., a transmission layer-2 processor clock  1707 , a transmission baseband clock  1708 , a transmission RF clock  1709 , a reception layer-2 processor clock  1710 , a reception baseband clock  1711 , and a reception RF clock  1712  are inputted in this order and used for the processing of data.  
      In the manner described above, packet fluctuations between the different wireless apparatus resulting from the mutually asynchronous wireless clocks can be suppressed.  
     Embodiment 2  
      A description will be given next to a wireless apparatus according to the second embodiment of the present invention and to the operation thereof.  FIG. 5  diagrammatically shows the structure of a wireless apparatus  0601  according to the second embodiment. The wireless apparatus  0601  has a structure in which a timer unit  0602  for time stamp is further provided in addition to the components of the wireless apparatus  0301  according to the first embodiment shown in  FIG. 1 . Accordingly, a detailed description will be given herein below to the timer unit  0602  for time stamp, while omitting the detailed description of the components other than the timer unit  0602  for time stamp by retaining the same reference numerals as used in  FIG. 1 . In the present embodiment also, the transmitting and receiving operations will be described by using only the depiction of the wireless apparatus  0601 , while the depiction of the transmission functional portion  0301   a , the reception functional portion  0301   b , and the other wireless apparatus as the communication partner of the wireless apparatus  0601  is omitted.  
      The timer unit  0602  for time stamp generates a transmission time stamp  0603 . The transmission time stamp  0603  has a time stamp value as transmission time information based on the wireless clock  0111  of the transmitter.  
      The packet input  0112  is inputted to the time stamp add unit  0106  in which a time stamp is added thereto so that the time-stamp-added packet  0114  is outputted therefrom. In contrast to the first embodiment which has added the time stamp based on the transmission beacon signal  0115 , the present embodiment adds the time stamp based on the transmission time stamp  0603  generated by the timer unit  0602  for time stamp.  
      Except for that, the wireless apparatus  0601  according to the present embodiment operates in the same manner as in the first embodiment.  
      The transmission time stamp  0603  has the wireless accuracy of the wireless clock  0111  which is higher in time accuracy than the transmission beacon signal  0115 . In the case of adopting the IEEE 802.11, the time accuracy of the beacon timer counter for generating the transmission beacon signal  0115  is 1 μS (1 MHz), while the wireless clock  0111  used by the baseband for modulation, demodulation, a transmission/reception filter, and the like is 40 MHz. Accordingly, packet control can be performed with higher accuracy. Even in the case of using a bandwidth which is shorter in packet interval than when the time stamp is added by using the transmission beacon signal  0115 , packet control can be performed for necessary data. This allows more reliable suppression of packet fluctuations and a more reliable guarantee of a data bandwidth than in the first embodiment.  
     Embodiment 3  
      A description will be given next to a wireless apparatus according to the third embodiment of the present invention and to the operation thereof.  
       FIG. 6  shows a wireless apparatus  0701  according to the third embodiment. The wireless apparatus  0701  has a structure including components common to the wireless apparatus  0301  according to the first embodiment and further including several other components added thereto. Therefore,  FIG. 6  primarily shows a portion characteristic of the wireless apparatus  0701  according to the present embodiment, while omitting the depiction of a part thereof. The present embodiment will also describe the transmission and reception of a wireless signal by using only the wireless apparatus  0701  of  FIG. 6 , similarly to the first and second embodiments, though the transmission and reception are intrinsically performed in different wireless apparatus.  
      The wireless transmission unit  0103  has the function of receiving a transmission beacon period set signal  0702  inputted from an upper layer application which controls the wireless apparatus  0701  and changing a beacon period. The beacon period is changed in accordance with the packet bandwidth of a packet to be transmitted.  
      The wireless apparatus  0701  is also provided with a changed-beacon-period detection unit  0703  and a parameter set unit  0705 . A description will be given herein below to the characteristic operation of the wireless apparatus  0701 .  
      The wireless reception unit  0104  extracts the reception frame body signal  0119  and the reception beacon signal  0118  from the wireless signal  0117  received via the antenna  0102  in the same manner as in the first embodiment. The reception frame body signal  0119  is processed in the same manner as in the first embodiment. By contrast, the reception beacon signal  0118  is outputted not only toward the wireless reference clock generation unit  0303  in the same manner as in the first embodiment but also toward the changed-beacon-period detection unit  0703  in the present embodiment.  
      The reception beacon signal  0118  has a beacon interval added thereto so that the changed-beacon-period detection unit  0703  to which the reception beacon signal  0118  has been inputted detects the changed beacon period based on the beacon interval.  
      When the changed beacon period is detected, the changed-beacon-period detection unit  0703  outputs a changed-beacon-period detection signal  0706  and a reception beacon period value  0704 .  
      The reception beacon period value  0704  is inputted to the parameter set unit  0705 , in which a PLL set signal  0707  is generated. The PLL set signal  0707  is outputted toward the wireless reference clock generation unit  0303 . In the wireless reference clock generation unit  0303 , the wireless clock  0111  based on the reception beacon signal  0118 , on the changed-beacon-period detection signal  0706 , and on the PLL set signal  0707  is generated.  
      As will also be described later, the PLL set signal  0707  includes a reference-clock-frequency-division-ratio set signal  0801 , an order-coefficient set signal  0802 , a gain set signal  0803 , and a generated-clock-frequency-division-ratio set signal  0804 .  
      With such a structure, even when the beacon period is changed, the wireless clock  0111  according to the present embodiment can be synchronized with the transmitter wireless apparatus by generating the wireless clock synchronized with the wireless clock of the transmitter in response to the change in beacon period in the received wireless signal  0117 .  
       FIG. 7  shows the respective operations of individual signals and the like when the transmission-beacon-period set signal  0702  is changed from a first transmission beacon period set value to a second transmission beacon period set value. The individual signals and the like are the wireless signal  0117 , the reception beacon period value  0704 , the changed-beacon-period detection signal  0706 , the PLL set signal  0707 , and the oscillator control voltage  0414  (which will be described later).  
      In the case where the wireless signal  0117  is transmitted with the first transmission beacon period set value, a wireless beacon signal  0901  is outputted in each first beacon period  0903 , while a wireless data signal  0902  is outputted during the time period except for the time period in which the wireless beacon signal  0901  is outputted.  
      With a change in the amount of transmission data, the transmission-beacon-period set signal  0702  is changed from the first transmission beacon period set value to a second transmission beacon period set value in the transmitter wireless apparatus. As a result, the wireless beacon signal  0901  is outputted in each second transmission beacon period  0904  and the wireless data signal  0902  is outputted during the time period in which the wireless beacon signal  0901  is outputted.  
      Such a change in beacon period is detected in the changed-beacon-period detection unit  0703  and the changed-beacon-period detection unit  0703  outputs the changed-beacon-period detection signal  0706  as a result of the detection, as described above. Thereafter, the PLL set signal  0707 , the oscillator control voltage  0414 , and the like are set individually.  
       FIG. 8  shows a structure of the wireless reference clock generation unit  0303  used in the wireless apparatus  0701  according to the present embodiment. A detailed description of the components shown in  FIG. 8  which are the same as those of the wireless reference clock generation unit according to the first embodiment shown in  FIG. 2  will be omitted by retaining the same reference numerals as used in  FIG. 2 . The description will be given primarily to the differences between the wireless reference clock generation units shown in  FIGS. 2 and 8 .  FIG. 8  also shows the specific structure of the voltage conversion circuit  0404 .  
      To the wireless clock  0111  according to the present embodiment, the reception beacon signal  0118  and the reference voltage switch signal  0302  are inputted in the same manner as in the wireless reference clock generation unit according to the first embodiment.  
      In addition, the PLL set signal  0707 , i.e., the reference-clock-frequency-division-ratio set signal  0801 , the order-coefficient set signal  0802 , the gain set signal  0803 , and the generation-clock-frequency-division-ratio-set-signal  0804  are also inputted.  
      The reference-clock-frequency-division-ratio set signal  0801  for setting a frequency division ratio for dividing the frequency of the reception beacon signal  0118  is inputted to the reference clock frequency divider  0401 . As a result of frequency division in accordance with the reference-clock-frequency-division-ratio-set signal  0801 , the reference clock frequency-divided signal  0409  is outputted.  
      On receiving the reference clock frequency-divided signal  0409  and the generated clock frequency-divided signal  0410 , the phase comparator  0402  generates the phase comparison signal  0411  with a digital value and outputs the phase comparison signal  0411  toward the low pass filter  0403 .  
      The low pass filter  0403  receives the order-coefficient set signal  0802  in addition to the phase comparison signal  0411 . By the order-coefficient set signal  0802 , the characteristics (i.e., order and coefficient) of the low pass filter  0403  as a digital filter are set. In accordance with the set order and coefficient, the low pass filter  0403  outputs the gain signal  0412  toward the voltage conversion circuit  0404 .  
      To the voltage conversion circuit  0404 , the gain set signal  0803  in addition to the gain signal  0412  is inputted, thereby setting the gain of the gain signal  0412 . The voltage conversion circuit  0404  outputs the gain control voltage  0413  toward the reference voltage switch  0405 .  
      The voltage conversion circuit  0404  comprises: a gain control  0805 ; an adder  0808 ; a flip-flop  0810  (D); and a digital/analog converter  0811  (DA) to constitute a digital PLL.  
      The voltage conversion circuit  0404  converts a digital control voltage  0809  to an analog signal by using the digital/analog converter  0811 , thereby generating the gain control voltage  0413 . The digital control voltage  0809  is corrected based on the gain signal  0412  and the gain set signal  0803 . The correction process will be described herein below.  
      First, in the gain control  0805 , the gain of the gain signal  0412  is variable in accordance with the inputted gain set signal  0803 . By amplifying or attenuating the inputted gain signal  0412  in accordance with the gain set signal  0803 , a digital-control-voltage correction amount  0806  is outputted.  
      The adder  0808  adds up the control values for the oscillator  0406 . That is, the adder  0808  adds a previous digital control voltage value  0807  held in the flip-flop  0810  to the digital-control-voltage correction amount  0806  and outputs the sum as the digital control voltage  0809 .  
      Thereafter, the digital control voltage  0809  is converted to an analog signal by the digital/analog converter  0811 , which is outputted as the gain control voltage  0413  toward the reference voltage switch  0405 .  
      In the flip-flop  0810 , the value of the digital control voltage  0809  is held as the previous digital control voltage value  0807  with the timing with which the digital control voltage  0809  is converted to the analog signal.  
      The reference voltage switch  0405  and the oscillator  0406  are the same as in the first embodiment. The oscillator  0406  outputs the generated clock  0415  toward the generated clock frequency divider  0407  and toward the wireless clock generator  0408 .  
      To the generated clock frequency divider  0407 , the generated-clock-frequency-division-ratio set signal  0804  is inputted in addition to the generated clock  0415 . The generated clock frequency divider  0407  divides the frequency of the generated clock in accordance with the generated-clock-frequency-division-ratio set signal  0804  and outputs the generated clock frequency-divided signal  0410  toward the phase comparator  0402 .  
      The wireless clock generator  0408  outputs the wireless clock  0111  based on the generated clock  0415 .  
      When the wireless apparatus according to the present embodiment is used to perform wireless communication which allows the beacon period to be set in accordance with the data bandwidth, the structure described above allows programmable generation of the wireless clock  0111  in accordance with the beacon period of the wireless apparatus as the communication partner and allows synchronization with the wireless apparatus as the communication partner. It is also possible to increase accuracy by providing the timer unit  0602  for time stamp, in the same manner as in the second embodiment.  
     Embodiment 4  
      A description will be given next to a wireless apparatus according to the fourth embodiment of the present invention and to the operation thereof with reference to the drawings.  
       FIG. 9  shows a wireless apparatus  1001  according to the fourth embodiment. The wireless apparatus  1001  has a structure including components common to the wireless apparatus  0301  according to the third embodiment and further including several other components added thereto. Therefore,  FIG. 9  primarily shows a portion characteristic of the wireless apparatus  1001  according to the present embodiment, while omitting the depiction of a part thereof. The present embodiment will also describe both transmission and reception by using only the wireless apparatus  1001 , similarly to the first embodiment, though wireless communication is intrinsically performed through the reception of a wireless signal, which has been transmitted from the wireless apparatus  1001 , by another wireless apparatus (not shown).  
      The wireless apparatus  1001  is provided with a reception beacon period counter unit  1002  and with a beacon loss determination unit  1003  in addition to the components of the wireless apparatus  0701  according to the third embodiment. A description will be given herein below to the characteristic operation of the wireless apparatus  1001 .  
      The wireless reception unit  0104  extracts the reception frame body signal  0119  and the reception beacon signal  0118  from the wireless signal  0117  received via the antenna  0102  in the same manner as in the third embodiment. The reception frame body signal  0119  is processed in the same manner as in the third embodiment. The reception beacon signal  0118  is outputted toward the wireless reference clock generation unit  0303  in the same manner as in the third embodiment so that the wireless reference clock generation unit  0303  outputs the wireless clock  0111  based on the reception beacon signal  0118 .  
      In the present embodiment, however, the received reception beacon signal  0118  is outputted also to the reception beacon period counter unit  1002  and to the beacon loss determination unit  1003 .  
      The reception beacon period counter unit  1002  outputs a reception beacon period count value  1004  for measuring the reception beacon interval toward the beacon loss determination unit  1003 .  
      The beacon loss determination unit  1003  has the function of outputting a beacon loss detection signal  1005  toward the wireless reference clock generation unit  0303 . The beacon loss detection signal  1005  indicates the detection of a beacon loss.  
      The wireless reference clock generation unit  0303  outputs the wireless clock  0111  in response to the inputting of the beacon loss detection signal  1005 .  
      Such a structure allows the generation of a clock tolerable to the loss of the reception beacon signal  0118 , which will be further described herein below.  
       FIG. 10  shows the wireless signal  0117 , the reception beacon period value  0704 , the reception beacon period count value  1004 , the beacon loss detection signal  1005 , and the oscillator control voltage  0414  when the loss of the beacon signal has occurred.  
      The reception beacon period count value  1004  is a count value (Reception Beacon Count-Up Value  1107  in  FIG. 10 ) at which counting is started (Initialization  1104  in  FIG. 10 ) with the reception timing of the wireless beacon signal  1101 .  
      The beacon loss detection signal  1005  indicates the detection of a beacon loss based on the wireless beacon signal  1101  and on the reception beacon period count value  1004 . The beacon loss detection signal  1005  is generated as follows.  
      First, the beacon loss determination unit  1003  interprets the transmission beacon period  1102 , which is the interval of the wireless beacon signals  1101 , in accordance with the beacon interval included in the reception beacon signal  0118  and generates a reception beacon loss detection threshold  1103 . When the next wireless beacon signal  1101  cannot be received within the limits of the reception beacon loss detection threshold  1103  by using the reception beacon period count value  1004  which is initialized on receipt of the wireless beacon signal  1101 , the beacon loss detection signal  1005  is outputted on the assumption that the beacon is lost.  
      When the beacon loss detection signal  1005  is inputted to the wireless reference clock generation unit  0303 , the digital-control-voltage correction amount  0806  is set to  0  (zero) in the same voltage conversion circuit  0404  as used in the third embodiment shown in  FIG. 8 . This allows retention and stable output of the digital control voltage  0809  (First Oscillator Control Voltage in  FIG. 10 ) generated based on the reception beacon signal  0118  before the beacon is lost. As a result, the wireless clock  0111  is also supplied stably.  
      Next, when the wireless apparatus  1001  receives the wireless beacon signal  1101  with a correct beacon interval during the detection of the beacon loss, i.e., in Section  1105  in which Previous Oscillator Control Voltage Value is Held in  FIG. 10 , the correction of the digital control voltage  0809  is resumed. That is, the correction of the digital control voltage  0809  based on the digital-control-voltage correction amount  0806  is performed and the corrected digital control voltage  0809  is converted to an analog signal, which is outputted as the gain control voltage  0413 . As a result, it becomes possible to resume the synchronization with the output-side wireless apparatus.  
      With such a structure, the present embodiment can implement a wireless apparatus and a wireless method each of which has reduced packet fluctuations and allows stable reception of the wireless signal to be performed even when the reception beacon signal is lost. In the present embodiment also, accuracy can be increased by providing the timer unit  0602  for time stamp in the same manner as in the second embodiment.  
     Embodiment 5  
      A description will be given next to a wireless apparatus according to the fifth embodiment of the present invention and to the operation thereof with reference to the drawings.  
       FIG. 11  shows a wireless apparatus  1201  according to the fifth embodiment. The wireless apparatus  1201  has a structure including components common to the wireless apparatus  1001  according to the fourth embodiment and further including several other components added thereto. Therefore,  FIG. 11  primarily shows a portion characteristic of the wireless apparatus  1201  according to the present embodiment, while omitting the depiction of a part thereof. The present embodiment will describe both transmission and reception by using only the wireless apparatus  1201  of  FIG. 11 , similarly to the first embodiment.  
      The wireless apparatus  1201  of  FIG. 11  is provided with a beacon protection unit  1202  in addition to the components of the wireless apparatus  1001  according to the fourth embodiment. The beacon protection unit  1202  outputs a channel switch request signal  1203 . A description will be given herein below to the characteristic operation of the wireless apparatus  1201 .  
      In the wireless apparatus  1201  according to the present embodiment, the channel switch request signal  1203 , which will be described later, is inputted to the wireless transmission unit  0103 . In the same manner as in the wireless apparatus  1001  according to the fourth embodiment, the reception beacon signal counter unit  1002  outputs the reception beacon signal count value  1004  based on the reception beacon signal  0118 , while the beacon loss determination unit  1003  outputs the beacon loss detection signal  1005 .  
      In the present embodiment, however, the reception beacon signal  0118  and the beacon loss detection signal  1005  are also inputted to the beacon protection unit  1202 . The beacon protection unit  1202  outputs the channel switch request signal  1203  toward the wireless reference clock generation unit  0303  and toward the wireless transmission unit  0103  based on the detection status of the beacon loss detection signal  1005 .  
      With the structure described above, the wireless apparatus  1201  according to the present embodiment is allowed to perform stable clock generation even when channel switching is performed, which will be described herein below.  
       FIG. 12  shows the wireless signal  0117 , the channel switch request signal  1203 , and the oscillator control voltage  0414  when channel switching is performed.  
      On detecting that conditions for detection (Detection of Channel Switch Request  1303  in  FIG. 12 ) are satisfied for the wireless beacon signal  1301  included in the wireless signal  0117 , the beacon protection unit  1202  outputs the channel switch request signal  1203  (Channel Switch Request  1306  in  FIG. 12 ). The conditions for detection include, e.g., no detection of a beacon within a set time period, consecutive occurrences of the beacon loss  1302  within the set time period, and consecutive occurrences of the status in which the beacon loss  1302  is detected once or more times within the set time period.  
      On receiving the channel switch request signal  1203 , the wireless transmission unit  0103  issues a channel switch request to the wireless apparatus (not shown) which has outputted a beacon signal on a wireless line and performs channel switching between the individual wireless apparatus currently in communication. At this time, when the wireless clock  0111  used in the wireless apparatus  1201  is synchronized with the reception beacon signal  0118 , the reception of the reception beacon signal  0118  becomes unstable. Accordingly, it may be assumed that the wireless clock  0111  also becomes unstable.  
      To prevent this, a clock based on a stable free-running internal clock is used as the wireless clock  0111  when channel switching is performed (in Section  1307  in which Channel Switching and Protection of Oscillator during Switching are Performed). This can be implemented by causing the same wireless reference clock generation unit  0303  as shown in  FIG. 8  to output the oscillator control voltage  0414  based on the oscillator reference voltage  0416 . For this purpose, the reference voltage switch signal  0302  is controlled such that the oscillator reference voltage  0416  is selected in the reference voltage switch  0405 .  
      The oscillator control voltage  0414  before channel switching is performed is the gain control voltage  0413  generated based on the reception beacon signal  0118 . However, when the beacon is lost, the oscillator control voltage  0414  before channel switching may also be the gain control voltage  0413  which has been held from before the beacon loss instead of the gain control voltage  0413  generated based on the reception beacon signal  118 , as described in the fourth embodiment.  
      Next, when the statuses which do not satisfy the conditions for detecting the channel switch request consecutively occur within the set time period after channel switching is performed (within the time period of Detection of Channel Switch Cancellation  1305  of  FIG. 12 ), the channel switch request  1306  is cancelled. That is, the beacon protection unit  1202  cancels the channel switch request using the channel switch request signal  1203 .  
      When the channel switch request signal  1203  is cancelled, the wireless reference clock generation unit  0303  generates the wireless clock  0111  by performing switching from the clock based on the free-running internal clock to the wireless clock synchronized with the transmitter wireless apparatus. For this purpose, the wireless reference clock generation unit  0303  performs switching from the oscillator reference voltage  0416  to the gain control voltage  0413  (see  FIG. 8  and the like.).  
      When the channel switch request signal  1203  is not cancelled even after channel switching is performed (i.e., when the conditions for detecting the channel switch request remain satisfied), switching to another channel is performed and the same process is continued.  
      In the manner described above, the wireless apparatus  1201  according to the present embodiment can generate a stable wireless clock even when channel switching is performed as a result of the loss of the reception beacon signal  0118 . In the present embodiment also, accuracy can be increased by providing the timer unit  0602  for time stamp in the same manner as in the second embodiment.  
     Embodiment 6  
      A description will be given next to a wireless apparatus according to the sixth embodiment of the present invention and to the operation thereof with reference to the drawings.  
       FIG. 13  schematically shows the structure of a wireless apparatus  1401  according to the present embodiment. In addition to the structure of the conventional wireless apparatus comprising a data clock generation unit  1407  for generating an audio-video clock, the wireless apparatus  1401  also has the capability of synchronizing the data clock generation unit  1407  with the wireless clock  0111 .  FIG. 13  shows that, compared with the wireless apparatus  0601  according to the second embodiment shown in  FIG. 5 , the wireless apparatus  1401  according to the present embodiment further comprises the data clock generation unit  1407  and also depicts an audio-video encoder unit  1404 , a transmission protocol processing unit  1405 , a reception protocol processing unit  1406 , and an audio-video data decoder unit  1408 . The wireless apparatus  1401  may also include the components described in the third, fourth, and fifth embodiments, though they are not depicted.  
      The description will be given herein below to the wireless apparatus  1401 .  
      First, the operation of the transmitter will be described. On receiving an audio-video data input  1402 , the audio-video encoder unit  1404  encodes the audio-video data input  1402  by using a data clock  1409  and outputs the encoded audio-video data input  1402  as transmission audio-video packet data  1410  toward the time stamp add unit  0106 .  
      In contrast to the conventional embodiment in which the data clock of the transmitter has been locked to the transmission beacon signal, the data clock  1409  according to the present embodiment is generated based on the wireless clock  0111 .  
      On receiving the transmission audio-video packet data  1410 , the time stamp add unit  0106  adds the transmission time stamp  0603  to the transmission audio-video packet data  1410  in accordance with the input timing of the transmission audio-video packet data  1410  and outputs it as the time-stamp-added packet  0114  toward the frame body assembly unit  0105 . At this time, the transmission time stamp  0603  is generated based on the data clock  1409 .  
      On receiving the time-stamp-added packet  0114 , the frame body assembly unit  0105  generates transmission data  1411  from the single time-stamp-added packet  0114  or from an assembly of the plurality of time-stamp-added packets  0114  in accordance with a data format for an upper wireless layer (e.g., TCP/IP, UDP/IP, or the like) and outputs the transmission data  1411  toward a transmission protocol processing unit  1405 .  
      On receiving the transmission data  1411 , the transmission protocol processing unit  1405  adds a header to the transmission data  1411  in accordance with the protocol and outputs the transmission data  1411  with the header as the transmission frame body signal  0116  toward the wireless transmission unit  0103 . On receiving the transmission frame body signal  0116 , the wireless transmission unit  0103  outputs it as the wireless signal  0117  via the antenna.  
      The receiving operation will be described next. The wireless signal  0117  outputted from the wireless apparatus  1401  is received by the other wireless apparatus (not shown). However, the description will be given herein on the assumption that the receiving operation is performed in the wireless apparatus  1401 , in the same manner as in the other embodiments.  
      The wireless reception unit  0104  receives the wireless signal  0117  via the antenna  0102 . The wireless reception unit  0104  extracts the reception frame body signal  0119  and the reception beacon signal  0118  from the wireless signal  0117  and outputs them. The wireless reference clock generation unit  0303  receives the reception beacon signal  0118  and outputs the wireless clock  0111  synchronized with the wireless clock of the transmitter wireless apparatus in the same manner as in the first embodiment.  
      On receiving the reception frame body signal  0119  from the wireless reception unit  0104 , the reception protocol processing unit  1406  checks the header in accordance with the upper layer protocol. When an error is not found herein, the reception protocol processing unit  1406  removes the header in accordance with the upper layer protocol and outputs the reception frame body signal  0119  without the header as reception data  1412  toward the frame body disassembly unit  0109 .  
      The frame body disassembly unit  0109  disassembles the reception data  1412 , which is either the single time-stamp-added packet  0114  or the assembly of the plurality of time-stamp-added packets  0114 , into the individual discrete time-stamp-added packets  0114 . The frame body disassembly unit  0109  further outputs each of the time stamps added at the transmitter as the reception time stamp  0120  toward the packet read control unit  0110 . Such an operation of the frame body assembly unit  0105  is the same as in the prior art technology. However, the characteristic feature of the present embodiment is that the data clock  1409  of the receiver is generated based on the wireless clock  0111 . The wireless clock  0111  is synchronized with the wireless clock of the transmitter.  
      The packet read control unit  0110  outputs the packet read signal  0122  toward the frame body disassembly unit  0109  based on the difference between the previously inputted reception time stamp  0120  and the newly inputted reception time stamp  0120 .  
      On receiving the packet read signal  0122 , the frame body disassembly unit  0109  generates reception audio-video packet data  1413  by removing the reception time stamp  0120  from each of the time-stamp-added packets  0114  and outputs the audio-video packet data  1413  toward the audio-video decoder unit  1408 .  
      The audio-video decoder unit  1408  decodes the transmission audio-video packet data  1410  inputted thereto in accordance with the data clock  1409  and outputs the decoded transmission audio-video packet data  1410  as audio-video packet data  1403 .  
      In this manner, the wireless apparatus  1401  according to the present embodiment can implement real-time high-quality transmission of audio-video data.  
      As stated previously, the beacon signal does not return a receipt acknowledgment signal (ACK). Accordingly, in a situation in which the beacon signal cannot be received, the conventional wireless apparatus cannot properly reproduce the clock so that the characteristic degradation of the audio-video data occurs.  
      By contrast, the wireless apparatus  1401  according to the present embodiment synchronizes the data clock for processing the audio-video data with the wireless clock, thereby allowing the suppression of the characteristic degradation of the audio-video data during the reproduction thereof and allowing real-time high-quality transmission and reproduction of audio data. Packet fluctuations are suppressed by synchronizing the respective wireless clocks of the transmitter wireless apparatus and the receiver wireless apparatus with each other in the same manner as in the other embodiments of the present invention.  
     Embodiment 7  
       FIG. 14  shows the structure of the characteristic portion of a wireless apparatus  1501  according to the seventh embodiment of the present invention. Compared with the wireless apparatus according to the other embodiments of the present invention, the wireless apparatus  1501  further has a structure for synchronization with a wired network.  
      Specifically, the wireless reference clock generation unit  0303  provided in the wireless apparatus  1501  comprises a reference clock switch  1509  in addition to the wireless reference clock generation unit shown in, e.g.,  FIG. 2 . To the frame body assembly unit  0105  and to the frame body disassembly unit  0109 , a wired clock  1503  is further inputted. A description will be given herein below to the characteristic operation of the wireless apparatus  1501 .  
      In the same manner as in the prior art technology, a wired data input  1502  synchronized with the wired clock  1503  is inputted to the frame body assembly unit  1505 . The frame body assembly unit  1505  generates a transmission frame body signal  1507  in accordance with a wireless LAN format and outputs the transmission frame body signal  1507  toward the wireless transmission unit  0103 . The wireless transmission unit  0103  adds a header to the transmission frame body signal  1507 , modulates the transmission frame body signal  1507  with the header, and outputs the modulate transmission frame body signal  1507  as the wireless signal  0117  via the antenna  0102 .  
      Although the wireless signal  0117  is received by another wireless apparatus not shown, the description will also be given herein by using an operation when the wireless apparatus  1501  performs reception as a substitute, in the same manner as in the other embodiments.  
      The wireless reception unit  0104  receives the wireless signal  0117  via the antenna  0102  and performs an error check needed in the wireless LAN after demodulation, in the same manner as in the conventional embodiment. If there is no error, the wireless reception unit  0104  outputs the reception frame body signal  1508  toward the frame body disassembly unit  1506 . At the same time, the wireless reception unit  0104  outputs the reception beacon signal  0118  toward the wireless reference clock generation unit  0303 .  
      The frame body disassembly unit  1506  disassembles the reception frame body signal  1508  inputted thereto and outputs it.  
      However, when the wireless apparatus  1501  is used as an access point, the wireless clock  0111  synchronized with a wired synchronous phase reference signal  1510  composed of a wired clock, a wired frame signal, and the like inputted from a wired apparatus is used. To generate the wired clock  0111  which is synchronized with such a wired network, the wireless reference clock generation unit  0303  is provided with the reference clock switch  1509 . The reference clock switch  1509  has the function of selecting either one of the reception beacon signal  0118  and the wired synchronous phase reference signal  1510  which is synchronous with the wired network and outputting the selected one as a reference phase synchronization signal  1512  toward the reference clock frequency divider  0401 . As for the subsequent operation of the wireless reference clock generation unit  0303 , it is the same as in the other embodiments, e.g., in the wireless reference clock generation unit shown in  FIG. 2  or  8 .  
      This allows selective use of an arbitrary one of the wireless clock  0111  synchronized with the reception beacon signal  0118  and the wireless clock  0111  composed of a free-running master clock, in the same manner as in the other embodiments. In addition, it is also possible in the present embodiment to selectively use the wireless clock  0111  synchronized with the wired synchronous phase reference signal  1510 .  
       FIG. 15  shows the connection between the wireless LAN using the wireless apparatus  1501  according to the present embodiment and a wired network  0212 , which is the same as in the conventional structure shown in  FIG. 18 . However, the wireless apparatus  1501  is used as each of a wireless apparatus  2  (AP)  0207  and a wireless apparatus  3  (AP)  0218  as access points, while the wireless apparatus  1401  according to the sixth embodiment is used as each of a wireless apparatus  1  (STA)  0203  and a wireless apparatus  4  (STA)  0221  as stations.  
      In this manner, the network shown in  FIG. 15  can operate by using the synchronized clock. Specifically, the wireless apparatus  1  (STA)  0203  and the wireless apparatus  2  (AP)  0207  are synchronized with each other and the wireless apparatus  3  (AP)  0218  and the wireless apparatus  4  (AP)  0221  are synchronized with each other. In addition, the wireless LAN  0202 , the wired network  0212 , and the wireless LAN  0217  are synchronized with each other. As a result, it becomes possible to suppress packet fluctuations with regard to data transmission in the network shown in  FIG. 15 .  
      This allows real-time transmission of audio-video data between the wireless LAN stations each connected to the wired network  0212 , while suppressing the characteristic degradation thereof. It will be easily understood that a wireless apparatus having all the components and all the functions described in the first to seventh embodiments can be implemented.  
       FIG. 16  shows a monitor camera system connected to a wired network to transmit audio-video data by using the wireless apparatus having the structure according to the present embodiment. The monitor camera system generates an audio-video data input  0201  by using a camera  1801  and displays an audio-video data output  0224  by using a monitor apparatus  1802  in the network shown in  FIG. 15   
      In  FIG. 16 , the camera  1801  which senses a sound, a moving image, or a still image comprises the same wireless apparatus  1  (STA)  0203  as the wireless apparatus according to the first or present embodiment and transmits an audio-video signal resulting from the sensing to the wireless apparatus  2  (AP)  0207  via the wireless apparatus  1  (STA)  0203 .  
      The wireless apparatus  2  (AP)  0207  has at least the function of the wireless apparatus according to the present embodiment. On receiving the audio-video data via the wireless signal  0206 , the wireless apparatus  2  (AP)  0207  transmits the audio-video data to a wired apparatus  0215  on the wired network  0212 . The wired apparatus  0215  further transmits the audio-video data to the wireless apparatus  3  (AP)  0218  connected thereto.  
      The wireless apparatus  3  (AP)  0218  transmits the audio-video data received via the wired apparatus  0215  by using a wireless signal  0220 . The transmitted wireless signal  0220  is received by the wireless apparatus  4  (STA)  0221  having at least the function of the wireless apparatus according to the first embodiment so that the audio-video data is decoded and displayed on the monitor  1802  comprising the wireless apparatus  4  (STA)  0221 . If an audio-video recording apparatus (not shown) is provided in addition to the monitor apparatus  1802 , the audio-video data can be recorded on a recording medium such as DVD (Digital Versatile Disc) or hard disc.  
      The wireless apparatus used in the monitor camera system described above implements real-time high-definition transmission of the audio-video data so that the monitor camera system is useful as a monitor camera system for monitoring a remote location which requires security.  
      By replacing the camera  1801  with an audio-video reproduction apparatus for reproducing audio-video data from a DVD or the like, it becomes possible to provide an audio-video listening and viewing system which allows real-time high-definition listening and viewing of the audio-video data even when the transmission of audio-video data is performed by wireless communication between the audio-video reproduction apparatus and the monitor apparatus.  
      There are cases where audio-video data can be transmitted directly between the camera  1801  and the monitor  1802  without intervention of the wired network  0212  by wireless communication. Specifically, there are cases where the audio-video data obtained by using the camera  1801  can be transmitted from the wireless apparatus  1  (STA)  0203  to the wireless apparatus  4  (STA)  0221 . In such a case also, real-time high-definition transmission of the audio-video data can be performed by using the wireless apparatus according to any of the embodiments of the present invention.  
      As described above, the wireless apparatus according to the present invention can perform transmission with reduced packet fluctuations by synchronizing the wireless clock thereof with that of a communication partner with which it performs wireless communication. Therefore, the wireless apparatus is useful as a wireless apparatus which performs real-time high-quality transmission of audio data, video data, or the like.