Abstract:
A method and apparatus for communicating between devices by selectively using a plurality of communication methods including a centralized control method, in which each communication apparatus sends and receives data under control of a control apparatus, and a distributed control method, in which each communication apparatus sends and receives data in an autonomous and distributing manner. The method and apparatus include detecting radio frequency interference, and switching between the centralized control method and the distributed control method according to a result of detecting the radio frequency interference.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a communication method and a communication apparatus useful in the case where radio frequency interference occurs and to a program that causes a computer to perform the communication method. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, a wireless local area network (LAN) system standardized as Institute of Electrical and Electronics Engineers (IEEE) 802.11 has a higher speed due to the introduction of IEEE 802.11b, IEEE 802.11g, and IEEE 802.11n, which exceeds 100 Mbps. 
       Furthermore, in order to cope with stream data transmission, IEEE 802.11e, which supports a Quality of Service (QoS) technique, has also been standardized. 
       [0005]    The increase in speed and the relatively inexpensive cost to establish wireless LANs, the number of wireless LAN apparatuses installed and used in homes has increased. As a result of the increase in number of installed apparatuses has increased, so has the problem of potential radio frequency interference occurring between the apparatuses. 
         [0006]    In the IEEE 802.11b and IEEE 802.11g wireless LAN systems, a wireless radio frequency band of the Industrial Scientific and Medical (ISM) band is used, which can also be used by a wireless apparatus other than a wireless LAN apparatus, i.e., wireless non-LAN apparatus. Accordingly, a radio frequency band used by a wireless non-LAN apparatus may overlap with a radio frequency band used by a wireless LAN apparatus, thus causing radio frequency interference. 
         [0007]    In order to solve radio frequency interference occurring between wireless LAN apparatuses and between a wireless LAN apparatus and a wireless non-LAN apparatus, various methods have been proposed (see, for example, Japanese Patent Application Laid-Open No. 2002-158667 and Japanese Patent Application Laid-Open No. 2004-336387). 
         [0008]    Radio frequency interference becomes especially problematic when performing a band-control type communication used in a stream data transmission. For example, in the case of a centralized control method in which an access control is performed based on polling from an access point to secure a band, if radio frequency interference occurs during sending and receiving of data, data communication may be delayed, and accordingly, a desired data communication rate may not be secured. As a result, a buffer underrun may occur in a receiving apparatus or a buffer overrun may occur in a sending apparatus. 
         [0009]    U.S. Patent Application Publication No. US 2003/0125087 A1 (Japanese Patent Application Laid-Open No. 2003-198564), Japanese Patent Application Laid-Open No. 2000-253017, and Japanese Patent Application Laid-Open No. 08-274788 discuss methods for switching between a centralized control method and a distributed control method. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention addresses strengthening a solution for a problem of data delay occurring when radio frequency interference occurs during a communication. 
         [0011]    According to an aspect of the present invention, a method for communicating with another communication apparatus by selectively using a plurality of communication methods including a centralized control method, in which each communication apparatus sends and receives data under control of a control apparatus, and a distributed control method, in which each communication apparatus sends and receives data in an autonomous and distributed manner, includes detecting radio frequency interference, and switching between the centralized control method and the distributed control method according to a result of the detection. 
         [0012]    According to another aspect of the present invention, a communication apparatus configured to communicate with another communication apparatus by selectively using a plurality of communication methods including a centralized control method, in which each communication apparatus sends and receives data under control of a control apparatus, and a distributed control method, in which each communication apparatus sends and receives data in an autonomous and distributed manner, includes a detection unit configured to detect radio frequency interference, and a switching unit configured to switch between the centralized control method and the distributed control method according to a result of the detection by the detection unit. 
         [0013]    According to yet another aspect of the present invention, a computer-readable program for controlling a communication apparatus configured to communicate with another communication apparatus by selectively using a plurality of communication methods including a centralized control method, in which each communication apparatus sends and receives data under control of a control apparatus, and a distributed control method, in which each communication apparatus sends and receives data in an autonomous and distributed manner, causes a computer to perform operations including detecting radio frequency interference, and switching between the centralized control method and the distributed control method according to a result of the detection. 
         [0014]    Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are incorporates in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principle of the invention. 
           [0016]      FIG. 1  illustrates a form of utilizing a wireless LAN according to a first exemplary embodiment of the present invention. 
           [0017]      FIG. 2  illustrates an exemplary configuration of a QoS compatible access point (QAP 1 ) according to the first exemplary embodiment of the present invention. 
           [0018]      FIG. 3  illustrates an exemplary state of using a radio frequency band in a case where a Hybrid Coordination Function Controlled Channel Access (HCCA) method is used. 
           [0019]      FIG. 4  illustrates an exemplary radio frequency band in a case where radio frequency interference occurs. 
           [0020]      FIG. 5  is a flow chart illustrating an operation of the QAP 1  according to the first exemplary embodiment of the present invention. 
           [0021]      FIG. 6  illustrates an exemplary radio frequency band in a case where the first exemplary embodiment is applied. 
           [0022]      FIG. 7  illustrates a form of utilizing a wireless LAN according to a second exemplary embodiment of the present invention. 
           [0023]      FIG. 8  is a flow chart illustrating an operation of the QAP 1  according to the second exemplary embodiment of the present invention. 
           [0024]      FIG. 9  illustrates an exemplary configuration of the QAP 1  according to the second exemplary embodiment of the present invention. 
           [0025]      FIG. 10  illustrates a form of utilizing a wireless LAN in which a hidden node problem occurs. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    Various exemplary embodiments of the present invention will now be described in detail with reference to the drawings. It should be noted that the relative arrangement of the components, the numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present invention unless it is specifically stated otherwise. 
       First Exemplary Embodiment 
       [0027]      FIG. 1  illustrates a form of utilizing a wireless LAN according to a first exemplary embodiment of the present invention. 
         [0028]    In the first exemplary embodiment, a wireless LAN is utilized in a user&#39;s house  104 . In the user&#39;s house  104 , an IEEE 802.11e-compliant QoS compatible access point (hereinafter referred to as “QAP 1 ”)  101  is installed. In addition, a personal computer  102 , which is an IEEE 802.11e-compliant QoS compatible wireless LAN terminal apparatus (hereinafter referred to as “QSTA 1 ”), is also installed. A television set  103  is connected to the QAP 1   101  via a video cable. 
         [0029]    The QSTA 1   102  includes a wireless LAN unit. Thus, the QSTA 1   102  functions as a wireless LAN terminal that wirelessly sends video data stored in a hard disk. 
         [0030]    The QAP 1   101  includes a function for managing an access by a wireless LAN terminal apparatus installed in a surrounding area. Furthermore, the QAP 1   101  includes a decoding unit that decodes radio video data. Thus, the QAP 1   101  decodes received video data and sends the decoded video data to the television set  103  as a video signal. 
         [0031]    Both the QAP 1   101  and the QSTA 1   102  are wireless LAN apparatuses compliant with the IEEE 802.11g and IEEE 802.11e standards. Thus, the QAP 1   101  and the QSTA 1   102  can communicate with each other via a wireless LAN. 
         [0032]    Note that in the IEEE 802.11e standard, two access control methods are defined, namely, an Enhanced Distributed Channel Access (EDCA) method and a Hybrid Coordination Function Controlled Channel Access (HCCA) system. 
         [0033]    The EDCA method uses an autonomous distributed control, which is an expansion of a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method. More specifically, the EDCA method is an access control method in which in sending data by each wireless LAN terminal apparatus, a time taken for carrier sensing before sending data is set variable according to a priority level of data, and high priority data has a higher chance of being sent. 
         [0034]    The HCCA method is a centralized-control access control method using polling. More specifically, in the HCCA method, a QoS access point (QAP) performs scheduling considering a priority level of each wireless LAN terminal apparatus (QSTA) and sends a polling frame to a QSTA. Each QSTA reads a permitted channel use time from the received polling frame, and sends data based on the read channel use time. Thus, the QSTA can secure a parameter for a specified band and a delay time to implement the QoS. 
         [0035]    Turning back to  FIG. 1 , in a neighboring house  107 , an NSTA 1   105  and an NSTA 2   106 , which are wireless non-LAN apparatuses are utilized. 
         [0036]    The NSTA 1   105  and the NSTA 2   106  respectively utilize the same band as a radio frequency band used by the QAP 1   101  and the QSTA 1   102 , which are wireless LAN apparatuses. In addition, a transmission power of the NSTA 1   105  and the NSTA 2   106  is strong enough to reach the QAP 1   101 . Accordingly, the wireless communication between the NSTA 1   105  and the NSTA 2   106  may cause radio frequency interference against the QAP 1   101  and the QSTA 1   102 . 
         [0037]      FIG. 2  illustrates an exemplary configuration of the QAP 1   101 . The QAP 1   101  includes a radio frequency (RF) unit  201 , a wireless communication unit  202 , a control unit  203  that controls the entire QAP 1   101 , a memory  204 , a video decoding unit  205 , and a LAN communication unit  206 . 
         [0038]    The RF unit  201  performs processing for sending and receiving a radio wave in an operating radio frequency band. The RF unit  201  transfers a received signal to the wireless communication unit  202  and externally outputs a signal transferred from the wireless communication unit  202  as a radio wave. Furthermore, the RF unit  201 , when detecting a radio wave, outputs a wireless power detection signal  207  to the control unit  203 . 
         [0039]    The wireless communication unit  202  performs processing for analyzing and assembling a media access control (MAC) frame for a wireless LAN signal. In addition, the wireless communication unit  202 , when detecting data that cannot be recognized to be IEEE 802.11g-compliant, i.e., a wireless non-LAN signal, from a signal from the RF unit  201 , outputs a wireless non-LAN apparatus detection signal  208  to the control unit  203 . 
         [0040]    Accordingly, the control unit  203  can, when receiving the wireless power detection signal  207  from the RF unit  201 , determine which of a wireless LAN signal and a wireless non-LAN signal is received according to whether the control unit  203  receives the wireless non-LAN apparatus detection signal  208  from the wireless communication unit  202 . 
         [0041]    The memory  204  is used by a processing program for the control unit  203  as a work area. In addition, the memory  204  is used as a buffer area for communication data between the wireless communication unit  202  and the control unit  203 , and between the LAN communication unit  206  and the control unit  203 . 
         [0042]    Data received by the RF unit  201  is frame-analyzed by the wireless communication unit  202 . If the data received by the RF unit  201  is a wireless LAN signal, the data portion remaining after header information is removed from the data is transferred to and stored in a receiving buffer area in the memory  204 . 
         [0043]    The LAN communication unit  206  performs processing for analyzing and assembling a MAC frame for a LAN signal. When a LAN signal is received, the LAN communication unit  206  frame-analyzes the received LAN signal and transfers the data portion to a receiving buffer area in the memory  204 . 
         [0044]    The receiving buffer area in the memory  204  is reserved independently for each of received data from the wireless communication unit  202  and received data from the LAN communication unit  206 . The size of the receiving buffer area to be reserved is determined according to the size of a frame body in the MAC frame to be received and a number of the MAC frames for the received data received during a time in which the control unit  203  makes a determination as to the transfer processing. 
         [0045]    The control unit  203  determines a sending destination of the received data based on the header information analyzed by the wireless communication unit  202  or the LAN communication unit  206 . 
         [0046]    In the case of data addressed to its own apparatus (the QAP 1   101 ), the control unit  203  reads the received data from the memory  204  and transfers the read data to the video decoding unit  205 . Then, the video decoding unit  205  decodes the transferred data into a video signal, and sends the video signal to the television set  103  via a cable. Thus, a video signal is reproduced and displayed on a display of the television set  103 . 
         [0047]    When the sending destination is another terminal apparatus (QSTA) connected to the QAP 1   101 , the control unit  203  reads the received data from the memory  204  and transfers the read data to the wireless communication unit  202 . Then, the wireless communication unit  202  reconstructs the transferred data into a wireless LAN frame, and then sends the frame via the RF unit  201 . 
         [0048]    When the sending destination is neither the QAP 1   101  nor another terminal apparatus connected to the QAP 1   101 , the control unit  203  reads the received data from the memory  204  and sends the read data to the LAN communication unit  206 . Then, the LAN communication unit  206  reconstructs the transferred data into a wired LAN frame, and then sends the frame to the LAN. 
         [0049]    In the above-described configuration, when a user operates the QSTA 1   102  and the television set  103  to send MPEG-2-coded video data from the QSTA 1   102  using a wireless LAN, the video data received by the QAP 1   101  is decoded into a video signal and is output to the television set  103 . Then, the decoded video signal is displayed on a display screen of the television set  103 . Note that because the data amount of the MPEG-2-coded video data is very large, the data is transferred in a stream transfer method in which the data is reproduced while being transferred instead of a transfer method in which the data is transferred in a unit of one file. 
         [0050]      FIG. 3  illustrates a state of using a radio frequency band used in sending and receiving of data by each terminal apparatus in a form of utilization described above. Here, data is transmitted using the IEEE 802.11e-compliant HCCA method. 
         [0051]    Referring to  FIG. 3 , beacon signals  301 ,  311 , and  313  are sent by the QAP 1   101 . QoS CF-Poll signals  302 ,  312 , and  314  are polling signals sent by the QAP 1   101  to the QSTA 1   102  to provide the QSTA 1   102  with an access authority. A QoS Data+CF-Ack signal  303  includes data sent by the QSTA 1   102  to the QAP 1   101  and a response command signal sent responsive to the QoS CF-Poll signal. QoS Data signals  305 ,  307 , and  309  are data sent by the QSTA 1   102  to the QAP 1   101 . Ack signals  304 ,  306 ,  308 , and  310  are data sent by the QAP 1   101  as an acknowledgment to the sent data from the QSTA 1   102 . 
         [0052]    The QAP 1   101  notifies information related to a network identifier (subsystem identification (SSID)) and a polling period previously scheduled by the QAP 1   101  together with the beacon signals  301 ,  311 , and  313 . In addition, the QAP 1   101  sends the QoS CF-Poll signals  302 ,  312 , and  314  to the QSTA 1   102  to notify a timing of sending data by the QSTA 1   102 . 
         [0053]    Before sending the beacon signals  301 ,  311 , and  313  and the QoS CF-Poll signals  302 ,  312 , and  314 , the QAP 1   101  scans the operating frequency channel for a specific length of time, and after confirming that the channel is not currently used, sends the beacon signals  301 ,  311 , and  313  and the QoS CF-Poll signals  302 ,  312 , and  314 . Accordingly, when a wireless LAN apparatus compliant to IEEE 802.11g and IEEE 802.11e is present in the same area, the QAP 1   101  adjusts the timing of sending data so that no conflict occurs between them. 
         [0054]    Furthermore, each of the QoS CF-Poll signals  302 ,  312 , and  314  includes a parameter “Network Allocation Vector” (NAV) that indicates a period in which the QSTA 1   102  can continuously send data. The QSTA 1   102  is permitted to continuously send data during the period NAV. NAV 1 , NAV 2 , and NAV 3  in  FIG. 3  respectively indicate NAV periods notified using the QoS CF-Poll signals  302 ,  312 , and  314 . 
         [0055]    The QAP 1   101 , using a Hybrid Coordinator (HC) function defined in the IEEE 802.11e standard, computes a necessary period NAV based on a data rate of video data to be transmitted, a communication rate at which two-way communication is available in a wireless LAN, and a beacon period, so as to perform scheduling. 
         [0056]    The QSTA 1   102 , after receiving the QoS CF-Poll signal  302 , determines that a sending authority is given to the QSTA 1   102  and thus sends data using the QoS Data+CF-Ack signal  303 . The QAP 1   101 , after receiving the QoS Data+CF-Ack signal  303 , in order to notify the QSTA 1   102  that the QAP 1   101  has received data, sends the Ack signal  304  to the QSTA 1   102 . The QSTA 1   102 , after receiving the Ack signal  304 , sends data using the QoS Data signal  305 . After that, the QSTA 1   102  sends data (the QoS Data signals  307  and  309 ) until the period NAV ends. In response to the data, the QAP 1   101  sends the Ack signals  306  and  308 . 
         [0057]    An operation performed when radio frequency interference occurs during an operation of the QAP 1   101  in a conventional method will now be described with reference to  FIG. 4 . 
         [0058]    As described above, the NSTA 1   105  and the NSTA 2   106  perform a wireless communication in the same radio frequency band as the radio frequency band used by the QAP 1   101 , in a method different from that of the IEEE 802.11 wireless LAN. Accordingly, as illustrated in  FIG. 4 , the NSTA 1   105  and the NSTA 2   106  send data at unique timings  405 ,  406 ,  407 , and  408 , regardless of whether data is sent by the QAP 1   101  or the QSTA 1   102 . 
         [0059]    Then, data  405 , which is sent by the NSTA 1   105 , and data  406 , which is sent by the NSTA 2   106 , collide with the data sent by the QSTA 1   102 . Thus, an interference occurs and, accordingly, the QAP 1   102  cannot receive normal wireless data. As a result, the second half of the period NAV 2  is suspended. 
         [0060]    In this regard, the QAP 1   101  newly schedules a period NAV 2   a  in a period available before the next beacon  410  and sends a QoS CF-Poll signal  409  to provide the QSTA 1   102  with a sending authority. If the period NAV 2   a  is too short for a period for transferring residual data, which had been scheduled to be sent in the period NAV 2 , then the QAP 1   101  again sends a QoS CF-Poll signal  411  after the beacon  410  to provide the QSTA 1   102  with a period NAV 2   b  and allows the QSTA 1   102  to send the residual data. 
         [0061]    With the above-described processing performed, a delay occurs in a transfer of data in the period NAV 3 , which has been scheduled immediately after the beacon  410 . As a result, an underrun in a receiving buffer of the QAP 1   101  occurs. Thus, a video signal to be reproduced on the television set  103  may either stop or be delayed. 
         [0062]      FIG. 5  is a flow chart illustrating an operation of the QAP 1   101  according to the present embodiment, which is performed to prevent the defects described with reference to  FIG. 4 . 
         [0063]    First, in step S 1 , the QAP 1   101  determines whether the wireless power detection signal  207  is output by the RF unit  201 . If no wireless power detection signal is detected in step S 1  (No in step S 1 ), the QAP 1   101  ends this routine. If the wireless power detection signal  207  is detected in step S 1  (Yes in step S 1 ), then in step S 2 , the QAP 1   101  determines whether the wireless non-LAN apparatus detection signal  208  is present. If no wireless non-LAN apparatus detection signal is detected in step S 2  (No in step S 2 ), the QAP 1   101  ends this routine. If the wireless non-LAN apparatus detection signal  208  is detected in step S 2  (Yes in step S 2 ), the QAP 1   101  determines that radio frequency interference caused by a wireless non-LAN apparatus is detected. 
         [0064]    When radio frequency interference caused by a wireless non-LAN apparatus is thus detected, the processing branches according to whether the QAP 1   101  is performing a communication in a centralized control mode (HCCA) (step S 3 ). Note that in the case where the QAP 1   101  is connected with a plurality of terminal apparatuses, the QAP 1   101  determines that a communication is performed in the centralized control mode as long as the QAP 1   101  is in communication with any one of the plural terminal apparatuses in the centralized control mode. 
         [0065]    If it is determined in step S 3  that the QAP 1   101  is not communicating with another terminal apparatus in the centralized control mode (No in step S 3 ), that is, if the QAP 1   101  is communicating with another terminal apparatus in the distributed control mode (EDCA), then the processing advances to step S 8 . 
         [0066]    If it is determined that the QAP 1   101  is communicating with another apparatus in the centralized control mode (Yes in step S 3 ), then in step S 4 , the QAP 1   101  performs a confirmation of a use status (carrier sensing) with respect to a radio frequency channel different from an operating radio frequency channel. 
         [0067]    For example, an IEEE 802.11g-compliant radio frequency currently available in Japan is within a band ranging from 2,400 MHz to 2483.5 MHz and a band ranging from 2,471 MHz to 2,497 MHz. A bandwidth of 26 MHz is occupied per each radio frequency channel. Accordingly, four frequency channels, at maximum, can be simultaneously used without interfering with one another. 
         [0068]    If, as a result of the carrier sensing, it is determined that an available radio frequency channel is present (Yes in step S 5 ), then in step S 6 , the QAP 1   101  changes the radio frequency channel used by the QAP 1   101  and the QSTA 1   102  connected to the QAP 1   101  to the available radio frequency channel, and then the QAP 1   101  ends the processing. 
         [0069]    If it is determined that no radio frequency channel is available (No in step S 5 ), then in step S 7 , the QAP 1   101  shifts from the centralized control mode (HCCA) to the distributed control mode (EDCA). Note that if a plurality of terminal apparatuses is connected to the QAP 1   101 , all the terminal apparatuses are switched to the distributed control mode (EDCA). 
         [0070]    In the distributed control mode (EDCA), because the QSTA 1   102  performs carrier sensing on the operating radio frequency channel and sends data if the channel is not used, the bandwidth is not secured, unlike the case of the centralized control mode (HCCA). Accordingly, the QAP 1   101  operates so that data can be sent and received as much as possible when a chance of sending data is provided to the QSTA 1   102 . 
         [0071]    In step S 8 , the QAP 1   101  increases the size of the receiving buffer for the wireless communication unit  202  in the memory  204  so that receiving can be continuously performed even when a large amount of data is sent from the QSTA 1   102  in one sending operation. 
         [0072]    In step S 9 , the QAP 1   101  determines whether the communication between the QAP 1   101  and the QSTA 1   102  is a traffic streaming, that is, whether a Traffic Specification (TSPEC) parameter, which is defined in the IEEE 802.11e standard, is set. 
         [0073]    If it is determined in step S 9  that the TSPEC parameter is set (Yes in step S 9 ), then in step S 10 , the QAP 1   101  operates to change a value for a “Peak Data Rate” in the TSPEC parameter to a larger value. More specifically, the QAP 1   101  consults with the QSTA 1   102  for a value to which the “Peak Data Rate” can be changed, and the QAP 1   101  issues a notification to the QSTA 1   102  so that the value for the “Peak Data Rate” is changed to a maximum value. After receiving the notification, the QSTA 1   102  thereafter sends the data at the “Peak Data Rate” thus set. Accordingly, a large amount of data can thereafter be sent at once. 
         [0074]    If it is determined that no TSPEC parameter is set in step S 9  (No in step S 9 ), the QAP 1   101  ends this routine. 
         [0075]      FIG. 6  illustrates a state of use of a radio frequency channel by the QAP 1   101  and the QSTA 1   102  in the case where radio frequency interference is detected and the above-described operation is performed. 
         [0076]    If, as a result of the carrier sensing of the operating radio frequency channel, it is determined that the channel is not currently used, the QSTA 1   102 , which is shifted to the distributed control mode (EDCA), sends video data at a newly set “Peak Data rate”. The QAP 1   101  performs receiving of video data up to a maximum capacity for storing data in the receiving buffer, the size of which has been increased. Because the rate of the MPEG-2 used for decoding by the video decoding unit  205  is the same as that in the centralized control mode (HCCA), a larger amount of video data than in the case of the centralized control mode (HCCA) is stored in the receiving buffer. Furthermore, the video decoding unit  205  serially decodes the received video data and outputs the decoded video data to the television set  103 . 
         [0077]    In addition, in periods  603 ,  604 ,  605 , and  606 , in which a data communication is performed between the NSTA 1   105  and the NSTA 2   106 , which are wireless non-LAN apparatuses, no communication between the QAP 1   101  and the QSTA 1   102  is performed. During this period, the QAP 1   101  reads the received data stored in the receiving buffer and allows the video decoding unit  205  to decode the data and to output the video signal to the television set  103 . 
         [0078]    If no data communication between the NSTA 1   105  and the NSTA 2   106  is detected, a video data communication between the QAP 1   101  and the QSTA 1   102  is resumed in the distributed control mode (EDCA). 
         [0079]    According to the present embodiment, if radio frequency interference caused by a wireless non-LAN apparatus is detected during a period in which a streaming data communication is performed in the centralized control mode, the QAP 1   101  shifts to the distributed control mode so that data is adaptively sent and received when an available radio frequency channel is present. Thus, a delay in a streaming data communication can be reduced to a minimum. In addition, a receiving buffer is increased and the speed of data transfer is increased as much as possible. Accordingly, a large amount of data can be sent and received at once when a communication time is available, without causing an overrun of the receiving buffer. 
       Second Exemplary Embodiment 
       [0080]    A second exemplary embodiment of the present invention will now be described. In the first exemplary embodiment, video data is sent from a QoS compatible wireless LAN terminal apparatus (QSTA) to a QoS compatible access point (QAP) that is connected to a television set via a cable. In the second exemplary embodiment, video data is sent from an apparatus that is connected to a QAP via a cable to a QSTA. 
         [0081]      FIG. 7  illustrates a form of utilizing a wireless LAN according to the second exemplary embodiment. 
         [0082]    In the second exemplary embodiment, a wireless LAN is utilized in a user&#39;s house  704 . In the user&#39;s house  704 , an IEEE 802.11e-compliant QoS compatible access point  701  (hereinafter referred to as “QAP 2 ”) is installed. In addition, a television set  702 , which is an IEEE 802.11e-compliant QoS compatible wireless LAN terminal apparatus (hereinafter referred to as “QSTA 2 ”), is also installed. A hard disk drive (HDD) recorder  703  is connected to the QAP 2   701  via a video cable. 
         [0083]    The QSTA 2   702  includes a wireless LAN unit. Thus, the QSTA 2   702  converts video data received via the wireless LAN into a video signal using an MPEG-2 decoding unit installed therein, and displays the video data on a television screen. 
         [0084]    The QAP 2   701  includes a function for managing an access by a wireless LAN terminal apparatus installed in a surrounding area. Furthermore, the QAP 2   701  includes a decoding unit that decodes radio video data. Thus, the QAP 2   701  decodes video data received from the HDD player  703  via a cable and sends the decoded video data to the QSTA 2   702  via the wireless LAN. 
         [0085]    Both the QAP 2   701  and the QSTA 2   702  are wireless LAN apparatuses compliant with the IEEE 802.11g and IEEE 802.11e standards. The QAP 2   701  and the QSTA 2   702  can communicate with each other via a wireless LAN. 
         [0086]    In a neighboring house  707 , an NSTA 3   705  and an NSTA 4   706 , which are wireless non-LAN apparatuses, are utilized. 
         [0087]    The NSTA 3   705  and the NSTA 4   706  respectively use the same band as a radio frequency band used by the QAP 2   701  and the QSTA 2   702 , which are wireless LAN apparatuses. In addition, a transmission power of the NSTA 3   705  and the NSTA 4   706  is strong enough to reach the QAP 2   701 . Accordingly, the wireless communication between the NSTA 3   705  and the NSTA 4   706  may cause radio frequency interference against the QAP 2   701  and the QSTA 2   702 . 
         [0088]      FIG. 9  illustrates an exemplary configuration of the QAP 2   701  according to the present embodiment. As compared to the example illustrated in  FIG. 2 , the QAP 2   701  includes a video coding unit  905  instead of the video decoding unit  205 . The other portions and units are similar to those illustrated in  FIG. 2  and as such have the same reference numerals as in  FIG. 2 . 
         [0089]    The video coding unit  905  encodes a video signal input from the HDD recorder  703  into video data and temporarily stores the coded video data in a sending buffer area in the memory  204 . 
         [0090]    In sending the video data to the QSTA 2   702 , the control unit  203  transfers the video data stored in the sending buffer area in the memory  204  to the wireless communication unit  202 . The wireless communication unit  202  adds header information to the transferred data to compose a wireless LAN frame and sends the frame to the QSTA 2   702  via the RF unit  201 . 
         [0091]      FIG. 8  is a flow chart illustrating an operation of the QAP 2   701  according to the present embodiment. As compared to the example illustrated in  FIG. 5 , all of the steps of  FIG. 8  are similar to those in  FIG. 5 , except for step S 8  (S 8 ′ in  FIG. 8 ), and as such have the same step numbers as  FIG. 5 . 
         [0092]    In step S 8 ′, the QAP 2   701  requests the QSTA 2   702  to increase the size of the receiving buffer. After receiving the request, the QSTA 2   702  increases the size of the receiving buffer. After that, when a chance of sending data is given to the QAP 2   701 , a large amount of data can be sent and received between the QAP 2   701  and the QSTA 2   702 . 
         [0093]    According to the present embodiment, even in the case of transmitting stream data from a QAP to a QSTA, an effect similar to that in the first exemplary embodiment can be achieved. 
       Third Exemplary Embodiment 
       [0094]    A third exemplary embodiment of the present invention will now be described. In the first and second exemplary embodiments, the present invention is applied to an access point (QAP). However, the present invention can also be applied to a communication terminal apparatus (QSTA). 
         [0095]    Furthermore, in the first and second exemplary embodiments, a wireless apparatus that may cause radio frequency interference is a wireless non-LAN apparatus. However, the present invention can also be applied to a case where interference occurs between wireless LAN apparatuses when a so-called “hidden node problem” occurs, as illustrated in  FIG. 10 . 
         [0096]    Referring to  FIG. 10 , a QAP 4   1101  and a QAP 5   1104 , which are access points, are installed in a user&#39;s house  1103  and a neighboring house  1106 , respectively. In addition, a QSTA 4   1102  and a QSTA 5   1105 , which are wireless LAN terminal apparatuses, are installed in the user&#39;s house  1103  and the neighboring house  1106 , respectively. The QSTA 4   1102  can wirelessly communicate with the QAP 4   1101  in the HCCA mode. The QSTA 5   1105  can wirelessly communicate with the QAP 5   1104  in the HCCA mode. 
         [0097]    A radio wave from the QAP 4   1101  can reach an area  1108 . In this state, the radio wave from the QAP 4   1101  can be received by the QSTA 4   1102 , but cannot be received by the QAP 5   1104 . In addition, a radio wave from the QAP 5   1104  can reach an area  1107 . In this state, the radio wave from the QAP 5   1104  can be received by the QSTA 4   1102 , but cannot be received by the QAP 4   1101 . 
         [0098]    Accordingly, between the QAP 4   1101  and the QAP 5   1104 , which are respectively installed at a position at which a radio wave from the other apparatus cannot reach each apparatus, each radio wave cannot be detected. Thus, both the QAP 4   1101  and the QAP 5   1104  asynchronously send a radio wave. As a result, the timing of sending of a signal from the QAP 4   1101  and the timing of sending of a signal from the QAP 5   1104  may overlap each other, thus causing radio frequency interference. 
         [0099]    In this regard, if the QSTA 4   1102  detects that a polling signal sent from the QAP 4   1101  cannot be correctly received due to interference caused by a polling signal or a downlink data signal from the QAP 5   1104 , the QSTA 4   1102  performs a method of preventing or reducing interference as described in the above-described exemplary embodiments. 
         [0100]    According to the present embodiment, even when interference occurs between wireless LAN apparatuses due to a hidden node problem, an effect similar to that in the above-described embodiments can be achieved. 
         [0101]    The detection of a hidden terminal is not limited to a detection at the timing of sending data. That is, the configuration can be arranged to detect that no sending authority can be given at the sending timing for which a band is previously reserved. 
       Other Exemplary Embodiments 
       [0102]    In the above-described embodiments, the detection of radio frequency interference is performed using an RF unit of a wireless LAN apparatus. However, the present invention is not limited to this configuration. That is, the detection of radio frequency interference can be performed by independently providing a receiving unit that receives data in the same radio frequency band. In addition, the configuration can be arranged such that two systems of an RF unit and a wireless communication unit are provided, and one of the two systems is provided as a receiving unit dedicated for searching a radio frequency channel. In this case, channel searching processing is performed in parallel to the sending and receiving processing. Accordingly, the processing can be performed in a shorter length of time. 
         [0103]    Both the receiving buffer and the sending buffer can be provided as a memory independently provided for communication, instead of a shared use with the work memory. 
         [0104]    In the above-described embodiments, when the HCCA mode is used at the time when radio frequency interference is detected, the mode always shifts to the EDCA mode. However, the configuration can be arranged such that it is determined whether a periodicity in a stream communication using the HCCA mode can be maintained and, if a periodicity in a stream communication using the HCCA mode cannot be maintained, the mode shifts to the EDCA mode. 
         [0105]    In addition, the configuration can be arranged such that, after performing a communication for a given length of time in the EDCA mode, the mode shifts back to the HCCA mode. Thus, after radio frequency interference is addressed, a streaming data communication can be periodically performed in the HCCA mode. 
         [0106]    Furthermore, in the above-described embodiments, a television set or an HDD player is connected to a QAP via a cable. However, the QAP can include a function as a television set or a function as an HDD player. 
         [0107]    Moreover, the wireless LAN is not limited to the IEEE 802.11g standard and can conform to any wireless standard (e.g., IEEE 802.11b). Alternatively, the wireless LAN can be configured to use another radio frequency band such as that in IEEE 802.11a. 
         [0108]    While the above-described embodiments describe a wireless LAN, the present invention can be applied to a different wireless communication method that would enable practice of the present invention. In addition, in the above-described embodiments, a personal computer, a television set, and an HDD player are used. However, the present invention can be applied to other apparatuses that would enable practice of the present invention, such as a digital video camera. 
         [0109]    The present invention can also be achieved by providing a system or an apparatus with a storage medium storing program code of software implementing the functions of the embodiments and by reading and executing the program code stored in the storage medium with a computer of the system or the apparatus (a central processing unit (CPU) or a micro processing unit (MPU)). In this case, the program code itself, which is read from the storage medium, implements the functions of the embodiments described above, and accordingly, the storage medium storing the program code constitutes the present invention. 
         [0110]    As the storage medium for supplying such program code, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, -a read-only memory compact disc (CD-ROM), a recordable compact disc (CD-R), a rewritable compact disc (CD-RW), a magnetic tape, a nonvolatile memory card, a read-only memory (ROM), and a digital versatile disc (DVD), for example, can be used. 
         [0111]    In addition, the functions according to the embodiments described above can be implemented not only by executing the program code read by the computer, but also implemented by the processing in which an operating system (OS) or the like carries out a part of or the whole of the actual processing based on an instruction given by the program code. 
         [0112]    Further, in another aspect of the embodiment of the present invention, after the program code read from the storage medium is written in a memory provided in a function expansion board inserted in a computer or a function expansion unit connected to the computer, a CPU and the like provided in the function expansion board or the function expansion unit carries out a part of or the whole of the processing to implement the functions of the embodiments described above. 
         [0113]    As described above, according to the embodiments of the present invention, when communication using the centralized control method is interfered with by radio frequency interference, the operating mode of a communication apparatus is switched to the distributed control method to adaptively send data at timings at which a radio frequency band is available. Accordingly, an underflow of scheduled receiving of data can be prevented or reduced. Furthermore, when a variance in the amount of flow of data increases due to irregular interferences occurring due to radio frequency interference, a capacity of a receiving buffer in the wireless communication apparatus is enlarged. Thus, communication can be continued while preventing an overflow of data in the wireless communication apparatus. 
         [0114]    As described above, according to the embodiments of the present invention, the operating mode is switched from the centralized control method to the distributed control method according to a result of detection of radio frequency interference, and an amount of data that can be temporarily stored is controlled. Thus, a data delay and a buffer overflow in the communication apparatus can be reduced. 
         [0115]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions. 
         [0116]    This application claims priority from Japanese Patent Application No. 2006-080907 filed Mar. 23, 2006, which is hereby incorporated by reference herein in its entirety.