Patent Publication Number: US-2010124171-A1

Title: Wireless station, fault detecting method, and computer readable medium

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2008-293489, filed on Nov. 17, 2008; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless station, a fault detecting method, and a computer readable medium. 
     2. Related Art 
     A problem associated with a wireless system compliant with IEEE802.11 standard is that a radiowave interference fault occurs and degrades communication capability of wireless terminals. One of major causes of radiowave interference is the problem of hidden terminals. 
     A wireless terminal compliant with IEEE802.11 standard has the ability of Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) for refraining from transmission of wireless signals when it has detected a wireless signal transmitted from other wireless terminal so as to prevent interference of wireless signals with each other. The function of detecting a wireless signal is called carrier sensing. On the other hand, when wireless terminals are unable to detect each other by carrier sensing being separated by an obstruction such as a wall, one of the terminals cannot detect a wireless signal transmitted from the other terminal by carrier sensing and starts transmission of a wireless signal, which leads to interference of their wireless signals. This problem is generally called a hidden terminal problem. As errors occur in interfering frames and the frames are discarded, communication capability of wireless terminals degrades. 
     JP-A 06-232870 (Kokai) describes a method for detecting wireless terminals that are in hidden terminal relation. This method has wireless terminals attempt to directly transmit and receive test frames to/from each other and determines that they cannot detect each other by carrier sensing and are in hidden terminal relation if transmission and reception fail. 
     As mentioned above, the conventional method determines whether there is a hidden terminal relation or not based on direct transmission and reception of test frames between wireless terminals. 
     However, in infrastructure mode of IEEE802.11 standard, a wireless terminal is permitted to communicate only with a wireless base station with which it has a connection relation and the conventional method that involves direct transmission and reception of test frames between wireless terminals cannot be applied. Assuming that one of wireless terminals attempts transmission and reception of test frames according to the conventional method, the method always determines that the wireless terminals are in hidden terminal relation because the other wireless terminal does not receive the test frames and ignores them. 
     Also, carrier sensing according to IEEE802.11 standard bases its determination only on presence of electromagnetic wave and does not involve demodulation of data that has been modulated into wireless signals. In general, depending on reception signal strength of data, there can be a condition in which presence of electromagnetic wave is detected and carrier sensing succeeds but demodulation of data fails. If demodulation of test frames fails when transmission and reception of test frames are attempted in such a condition according to the conventional method, an incorrect result of determination is returned that indicates wireless terminals are in hidden terminal relation even if they are in a normal relation in which carrier sensing succeeds. 
     As described, the conventional method has a problem in accuracy of determination of a hidden terminal relation. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provide a wireless station that performs wireless communication with first and second wireless terminals based on a carrier sensing scheme, comprising: a measuring unit configured to communicate with the first and second wireless terminals to measure first and second communication delays of the first and second wireless terminals; a first determining unit configured to determine first and second transmission rates of the first and second wireless terminals; a second determining unit configured to determine first and second timings of transmission for a response request frame to each of the first and second wireless terminals and first and second response frame lengths of response frames to be returned from the first and second wireless terminals, based on the first and second transmission rates, the first and second communication delays, and a range of an initial backoff time for carrier sensing; a transmitting unit configured to transmit a response request frame that requests returning of response frames having the first and second response frame length at the first and second transmission rates, to the first and second wireless terminals at the first and second transmission timings; a receiving unit configured to receive the response frames returned from the first and second wireless terminals; and a detecting unit configured to detect that the first and second wireless terminals are in hidden terminal relation upon detecting a collision of the response frames, wherein the second determining unit determines the first and second response frame lengths and the first and second transmission timings so that the response frames returned from the first and second wireless terminals collide with each other given that the first and second wireless terminals are in the hidden terminal relation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an illustrative configuration of a communication system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing an illustrative configuration of base station A in the communication system; 
         FIG. 3  shows a flow of processing for the base station A to transmit and receive ICMP Echo frames for measuring communication delay of wireless terminals H 1  and H 2 ; 
         FIG. 4  illustrates how the frame length of a response frame is determined; 
         FIG. 5  illustrates that frame collision does not occur between wireless terminals that are not in a hidden terminal relation because carrier sensing effectively functions; 
         FIG. 6  shows a flow of processing for the base station A to transmit and receive ICMP Echo frames to and from wireless terminals H 1  and H 2 ; 
         FIG. 7  is a flowchart illustrating a flow of operations of the base station; 
         FIG. 8  shows a flow of processing for the base station A to transmit and receive ICMP Echo frames to and from wireless terminals H 2  and H 3 ; 
         FIG. 9  shows the data frame format of IEEE802.11 standard; 
         FIG. 10  illustrates how to determine the frame length of a response frame when a response request frame is transmitted by multicast; and 
         FIG. 11  shows a flow of processing for the base station A to transmit and receive ICMP Echo frames to and from wireless terminals H 1  and H 2  by multicast. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to drawings. 
       FIG. 1  shows an exemplary configuration of a communication system according to an embodiment of the present invention. 
     In  FIG. 1 , reference numeral  1  denotes a network and “A” is a base station (or an access point or AP) connected to the network  1 . The base station A corresponds to a wireless station of the present invention, for example. Reference numeral  2  denotes a wireless link, and H 1 , H 2  and H 3  are wireless terminals (STAtions or STAs) connected to the wireless link  2 . “B” is a wall that obstructs radiowave transmitted by the wireless terminals H 2  and H 3  so that radiowave does not reach the other terminal. It means that the wireless terminals H 2  and H 3  are in a hidden terminal situation with respect to each other, being in a relation that gives rise to a hidden terminal fault which causes frames transmitted by them to collide with each other. The base station A and wireless terminals H 1 , H 2 , and H 3  perform communication according to a carrier sensing scheme. Although only three wireless terminals are connected to the base station A in  FIG. 1 , four or more wireless terminals may connect to the base station A. 
       FIG. 2  shows a configuration of the base station A according to the present embodiment. 
     As shown in  FIG. 2 , base station A includes an information acquiring unit (acquiring unit)  11  for obtaining information on physical interface specifications of wireless terminals H 1 , H 2  and H 3  (transmission rates here); a communication delay measuring unit (measuring unit)  12  for measuring communication delay of the wireless terminals H 1 , H 2  and H 3 ; a response request frame transmitting unit (first determining unit, second determining unit, transmitting unit, third determining unit)  13  for generating a response request frame based on the transmission rates, communication delay, and the range of initial backoff time of test objects (two selected from wireless terminals H 1  to H 3 ) and transmitting the response request frame to the test objects; a fault detecting unit (detecting unit)  14  for determining whether there is a hidden terminal fault or not based on presence/absence of collision of response frames which are returned from the test objects in response to the response request frame; a wireless communication unit (receiving unit, transmitting unit)  15  for communicating with wireless terminals H 1 , H 2  and H 3 ; and a wired or wireless communication unit  16  which communicates with the network  1 . The information acquiring unit  11 , communication delay measuring unit  12 , and response request frame transmitting unit  13  perform their respective processing via the wireless communication unit  15 . 
     While the components  11 ,  12 ,  13 ,  14 ,  15  and  16  are described as being configured as hardware, it is not limitation and they may provide functions as software (program) executed in the base station A. Incidentally, the term “unit” may be replaced with “device” or “section” and so on. 
     Hereinafter, firstly, description will be given about a flow of processing for determining at the base station A whether wireless terminals H 1  and H 2  as test targets are in a relation that causes a hidden terminal fault (i.e., a hidden terminal relation) (a first embodiment). The flow of processing by the base station A is shown in the flowchart of  FIG. 7 . This processing is performed in response to input from a user of an instruction for starting a test with wireless terminals H 1  and H 2  specified (see  FIG. 1 ), for example. 
     The information acquiring unit  11  of the base station A obtains the transmission rates of the wireless terminals H 1  and H 2  as physical interface specifications used by the terminals for wireless communication (S 21 ). The transmission rates may be obtained by the base station A defining a frame for making an inquiry to the wireless terminals H 1  and H 2 , or by the base station A storing transmission rates that were used for frames received from the wireless terminals H 1  and H 2  and making reference to such transmission rates. 
     Next, the communication delay measuring unit  12  measures communication delay between the base station A and each of the wireless terminals H 1  and H 2  (S 22 ). Communication delay may be measured by transmitting an ICMP Echo Request from base station A to each of the wireless terminals H 1 , H 2  and measuring Round Trip Time (RU) until reception of an ICMP Echo Reply returned from them, for example. 
     Here, measurement may be performed separately for each terminal and a terminal as the target of measurement may be instructed to disable its backoff function. This allows communication delay to be measured without including backoff time. Also, to improve accuracy of measurement, measurement may be performed a number of times and an average value may be calculated. 
     In addition, when transmission error occurred in an ICMP Echo Reply and retransmission took place in MAC layer, the result of measurement for the retransmission may be excluded from measurement because it can cause disturbance. 
     Here, communication delay is measured for the following reason. In general, wireless terminals have differing performance in network processing and it causes a difference in input/output processing time from reception of an ICMP Echo Request to transmission of an ICMP Echo Replay onto a wireless link. If the frame length of a response frame and timing of transmitting a response request frame are determined by the response request frame transmitting unit  13 , which is discussed later, without considering such a difference in processing time when the difference is large, it leads to degradation of the accuracy of the fault detecting unit  14 . It is therefore important to actually measure communication delay by using ICMP Echo. 
       FIG. 3  shows a flow of processing for transmitting and receiving ICMP Echo frames for the base station A to measure communication delay of the wireless terminals H 1  and H 2 . 
     The base station A transmits an ICMP Echo Request frame to wireless terminal H 1  (A 1 ), and receives an ICMP Echo Reply frame from wireless terminal H 1  (A 2 ). It is assumed here that a measured communication delay of wireless terminal H 1  is 100 μsec. The base station A similarly transmits an ICMP Echo Request frame to wireless terminal H 2  (A 3 ), and receives an ICMP Echo Reply frame from wireless terminal H 2  (A 4 ). It is assumed here that a measured communication delay of wireless terminal H 2  is 200 μsec. In this way, the base station A transmits and receives ICMP Echo frames by unicast when measuring communication delay. This is because if wireless terminals as targets of measurement are in hidden terminal relation, interference occurs in response frames and correct measurement cannot be performed. 
     Next, the response request frame transmitting unit  13  determines a response frame length and timing of transmitting a response request frame that ensure collision of response frames for each of the wireless terminals H 1  and H 2  based on their transmission rates and communication delay as well as the range of the initial backoff time (S 23 ). The response frame length and timing of transmission are determined such that response frames collide with each other assuming that the wireless terminals H 1  and H 2  are in hidden terminal relation. The response request frame transmitting unit  13  transmits a response request frame that asks for returning of a response frame of the determined frame length at the transmission rates to the wireless terminals H 1  and H 2  at the timings of transmission for them (S 23 ). Hereinafter, step S 23  will be described in greater detail. 
     Assume first that the frame length of the response frame can be explicitly specified by base station A, which transmits a response request. For example, ICMP Echo Request and ICMP Echo Reply can be used for a response request frame and a response frame, respectively. This is because the base station A can also indirectly control the payload length of an ICMP Echo Reply by controlling the payload length of an ICMP Echo Request because there is an established rule that an ICMP Echo Reply should be returned with a payload length equal to that of an ICMP Echo Request. It is assumed that ICMP Echos are transmitted and received by unicast. 
     Here, assume that the base station A and wireless terminals H 1 , H 2  use IEEE802.11a standard and they all have a transmission rate of 54 Mbps. Also, assume that communication delay of wireless terminals H 1  and H 2  is 100 μsec and 200 μsec, respectively (communication delay is measured with their backoff function disabled here). The base station A determines timing of transmission to the wireless terminals H 1  and H 2  such that a response request frame is first transmitted to the wireless terminal H 2  and a response request frame is then transmitted to wireless terminal H 1  after elapse of 100 μsec so that the wireless terminals H 1  and H 2  simultaneously transmit a response frame on the assumption that they have the same backoff time. 
     In IEEE802.11a standard, the initial backoff time for a wireless terminal to perform carrier sensing before transmitting data is randomly determined within a range of 0 to 135 μsec and this time newly emerges as a communication delay. Therefore, to ensure that response frames from the wireless terminals H 1  and H 2  collide with each other, it is necessary to determine such a frame length that makes the response frames occupy a wireless link for at least 135 μsec (the minimum value) or longer. 
     For example, as shown in  FIG. 4(A) , even if response request frames are transmitted to the wireless terminals H 1  and H 2  at their respective timings given that the transmission rate is 54 Mbps and the response frame length is 8 bytes (occupation time is 32 μsec), response frames from the wireless terminals H 1  and H 2  do not collide with each other when the backoff time of wireless terminal H 1  is 0 μsec and that of wireless terminal H 2  is 135 μsec. 
     On the other hand, as shown in  FIG. 4(B) , when the response frame length is set to a sufficiently large value, e.g., 1472 bytes, the time for which response frames occupy the wireless link is 248 μsec and frames can be caused to collide. While  FIG. 4(B)  shows an example of setting the response request frame length to a sufficiently large value of 1472 bytes, the response request frame length can be of course set to a smaller value as long as response frames occupy the wireless link exceeding the initial backoff time. Because efficiency of frequency utilization generally lowers with increase in traffic for system control and management, such as test packets, it is important to reduce such traffic as much as possible. While the response frame lengths for wireless terminals H 1  and H 2  are set to the same value in this example, they do not have to be the same value and may be any values that ensure collision of response frames. 
     Here, if communication delay is measured without disabling backoff function (i.e., being effective), the frame length may be determined such that response frames occupy the wireless link for at least a time period twice the length of the initial backoff time, for example. 
     Also, while in the description above the frame length and timing of transmission are determined using transmission rates obtained from the wireless terminals H 1  and H 2 , transmission rates may also be specified by a base station separately for the wireless terminals. In such a case, a transmission rate which should be used by a wireless terminal for a response is specified in a response request frame, and the wireless terminal returns a response frame at the specified transmission rate. Incidentally, when transmission rates obtained from wireless terminals H 1  and H 2  are used, a transmission rate need not be explicitly specified when a response request frame is transmitted. 
     In addition, while in the description above a response request frame is first transmitted to wireless terminal H 2  which has a communication delay of 200 μsec and thereafter a response request frame is transmitted to wireless terminal H 1  which has a communication delay of 100 μsec upon elapse of 100 μsec, the present invention is not limited thereto and timing of transmission to wireless terminals H 1  and H 2  may be determined by an arbitrary method. In such a case, the difference in transmission timing and the difference in communication delay should be reflected in determination of frame length of a response frame. For example, when the wireless terminals have the same communication delay time, the difference in transmission timing should be reflected in the frame length of response frames. In short, such a frame length and timing of transmission should be determined that cause response frames from wireless terminals H 1  and H 2  to collide with each other given that they are in hidden terminal relation. 
     Next, the fault detecting unit  14  determines whether response frames have been received from wireless terminals H 1  and H 2  (whether there has been a collision of response frames) (S 25 ). Here, carrier sensing of wireless terminals H 1  and H 2  effectively functions because the terminals are not in a hidden terminal situation. Therefore, each of the wireless terminals H 1  and H 2  avoids transmission of a response frame while the other one transmits a response frame and accordingly collision of frames does not occur. That is, the fault detecting unit  14  does not detect a collision of response frames from the wireless terminals H 1  and H 2 . The fault detecting unit  14  therefore determines that the wireless terminals H 1  and H 2  are not in hidden terminal relation (i.e., that a hidden terminal fault is not occurring) (S 26 ). 
     For instance, when the backoff time of wireless terminal H 1  is 0 μsec and that of wireless terminal H 2  is 135 μsec as shown in  FIG. 5 , wireless terminal H 1  first starts transmission of a response frame, but wireless terminal H 2  detects it by carrier sensing and thus avoids transmission of a response frame during transmission by the wireless terminal H 1 . After detecting absence of radiowave transmitted from the wireless terminal H 1 , the wireless terminal H 2  makes sure that no radiowave is being detected by carrier sensing during DIFS and backoff time (135 μsec), and then transmits a response frame. Consequently, response frames from the wireless terminals H 1  and H 2  are normally received, and the fault detecting unit  14  determines that wireless terminals H 1  and H 2  are not in hidden terminal relation (that no hidden terminal fault is occurring) (S 26 ). 
       FIG. 6  shows a flow of processing for the base station A to transmit and receive ICMP Echo frames to and from wireless terminals H 1  and H 2  in the above-described example. It is understood that no occurrence of a hidden terminal fault is correctly detected when wireless terminals H 1  and H 2  that are not in a hidden terminal situation are test objects. 
     Hereinafter, description will be given about a flow of processing for determining at the base station A whether wireless terminals H 2  and H 3  as test objects are in a relation that causes a hidden terminal fault (a second embodiment). The flow of processing by base station A is shown in the flowchart of  FIG. 7 . This processing is performed in response to input of an instruction to start a test from the user specifying wireless terminals H 2  and H 3  (see  FIG. 1 ), for example. 
     First, the information acquiring unit  11  of the base station A obtains transmission rates of wireless terminals H 2  and H 3  as physical interface specifications used by the terminals for wireless communication (S 21 ). 
     Next, the communication delay measuring unit  12  measures communication delay between the base station A and each of the wireless terminals H 2  and H 3  (S 22 ). The flow of transmission and reception of ICMP Echo frames relating to measurement of communication delay is similar to the one shown in  FIG. 3  described in the first embodiment. 
     Next, a response frame length and timing of transmitting a response request frame that ensure collision of response frames are determined for each of the wireless terminals H 2  and H 3  based on their transmission rates and communication delay as well as the range of the initial backoff time (S 23 ). Then, the response request frame transmitting unit  13  transmits a response request frame that asks for returning of a response frame of the determined response frame length to the wireless terminals H 2  and H 3  at the determined timings of transmission to them (S 23 ). 
     It is assumed here that the base station A, and wireless terminals H 2  and H 3  use IEEE802.11a standard and have a transmission rate of 54 Mbps. It is also assumed that communication delay of wireless terminals H 2  and H 3  is 100 and 200 μsec, respectively. The base station A determines timing of transmission such that a response request frame is first transmitted to wireless terminal H 3  and a response request frame is subsequently transmitted to wireless terminal H 2  after elapse of 100 μsec so that the terminals simultaneously transmit a response frame assuming that they have the same backoff time, for example. 
     As mentioned earlier, in IEEE802.11a standard, the initial backoff time for a wireless terminal to perform carrier sensing before transmitting data is randomly determined within a range of 0 to 135 μsec, which newly emerges as a communication delay. Therefore, to ensure that response frames from the wireless terminals H 2  and H 3  collide with each other, it is necessary to determine such a frame length that makes response frames occupy a wireless link for at least a time period of 135 μsec (the minimum value) or longer. As the wireless terminals H 2  and H 3  both have a transmission rate of 54 Mbps, if response frame length is set to 1472 bytes, for example, response frames occupy a wireless link for a time period of 248 μsec and the frames can be caused to collide with each other (see  FIG. 4(B) ). 
     Next, the fault detecting unit  14  determines whether response frames have been received from wireless terminals H 2  and H 3  (whether there has been a collision of response frames) (S 25 ). As the wireless terminals H 2  and H 3  are in a hidden terminal situation and carrier sensing does not function, the wireless terminal H 3  does not avoid transmission of a response frame and frame collision occurs (see  FIG. 4(B) ). The fault detecting unit  14  therefore detects a collision of response frames from wireless terminals H 2  and H 3  and determines that a hidden terminal fault is occurring between the wireless terminals H 2  and H 3  (S 27 ). 
     Collision of frames can be detected by a method that determines that there has been a collision if frame reception is not detected within a certain time period after transmission of a response request frame, for example. According to IEEE802.11 standard, retransmission of a frame takes place when frame error occurred due to interference or the like. As Retry field in a header portion of a retransmitted frame is set to “1”, whether a received frame is a retransmitted frame or not can be determined by referencing the value in Retry field of the frame, and it may be determined that a collision has occurred if it is a retransmitted frame. The data frame format of IEEE802.11 standard is shown in  FIG. 9 . 
       FIG. 8  shows a flow of processing for the base station A to transmit and receive ICMP Echo frames to and from wireless terminals H 2  and H 3  in the second embodiment described above. Note that indication of initial backoff time is omitted here. It is understood that no occurrence of a hidden terminal fault is correctly detected when wireless terminals H 2  and H 3  which are in a hidden terminal situation are test objects. 
     Hereinafter, processing for transmitting ICMP Echo Request, which is sent by the fault detecting unit  14 , by multicast instead of unicast (a third embodiment) will be described. 
     Multicast refers to a communication mode in which data is transmitted to a particular group consisting of multiple wireless terminals, as opposed to unicast that transmits data to a single wireless terminal. For example, when the base station A transmits a frame having a particular multicast address, only a particular group assigned the particular multicast address (e.g. a pair of wireless terminals H 1  and H 2  or a pair of H 2  and H 3 ) performs reception of the frame. 
     Hereinafter, processing performed by the base station A will be described that checks whether wireless terminals H 2  and H 3  as test objects are in a relation that causes a hidden terminal fault. A flow of processing performed by base station A is shown in the flowchart of  FIG. 7 . 
     The information acquiring unit  11  of the base station A obtains the transmission rates of wireless terminals H 2  and H 3  as physical interface specifications used by the terminals for wireless communication (S 21 ). 
     Next, the communication delay measuring unit  12  measures communication delay between the base station A and each of wireless terminals H 2  and H 3  (S 22 ). It is assumed that communication delay is measured by transmitting and receiving ICMP Echo by unicast. The flow of transmitting and receiving ICMP Echo frames relating to measurement of communication delay is similar to the one shown in  FIG. 3  which was described in the first embodiment. 
     Then, the response request frame transmitting unit  13  determines a frame length common to wireless terminals H 2  and H 3  that ensures their response frames collide with each other based on the transmission rates and communication delay of wireless terminals H 2  and H 3 , and the range of initial backoff time (S 23 ). Such a frame length corresponds to a common response frame length of the present invention, for example. The response request frame is transmitted to the wireless terminals H 2  and H 3  at the same time because it is transmitted by multicast. 
     Here, assume that base station A and wireless terminals H 2  and H 3  use IEEE802.11a standard and have a transmission rate of 54 Mbps. It is also assumed that communication delay of wireless terminals H 2  and H 3  is 100 μsec and 200 μsec, respectively (communication delay is measured with their backoff function disabled). In addition, as mentioned earlier, IEEE802.11a standard randomly determines the initial backoff time in which a wireless terminal performs carrier sensing before transmitting data within a range of 0 to 135 μsec, which newly emerges as a communication delay. In this situation, the minimum value of time required before the wireless terminal H 2  transmits a response frame to wireless link  2  is 100 μsec+0 μsec=100 μsec, and the maximum value of time required before wireless terminal H 3  transmits a response frame to wireless link  2  is 200 μsec+135 μsec=335 μsec. Therefore, to ensure collision of response frames from wireless terminals H 2  and H 3 , it is required to determine such a frame length that makes the response frames occupy the wireless link for at least 335 μsec−100 μsec=235 μsec. Since transmission rate is set to 54 Mbps here, if response frame length is set to 1472 bytes, for example, the response frames occupy the wireless link for 248 μsec and the frames can be caused to collide. This is illustrated in  FIG. 10(B) . On the other hand, if response frame length is set to 8 bytes, for example, time of occupation becomes sufficiently shorter than 235 μsec and frame collision cannot be caused, as shown in  FIG. 10(A) . While the above description assumes a case where communication delay is measured with the backoff function of the wireless terminals disabled, if backoff function is enabled at the time of measurement, the time for which response frames occupy the wireless link can be at least: 235 μsec+the length of the initial backoff time, i.e., 135 μsec=370 μsec, for example. 
     When the difference in communication delay of wireless terminals H 2  and H 3  is large, a wireless link occupation time sufficient for causing collision of frames might not be secured just by making the response request frame length large since the upper limit on frame length is limited. In such a case, the time for which response frames occupy the wireless link can be further increased to cause a collision by decreasing the transmission rates of wireless terminals H 2  and H 3 . The transmission rates can be decreased by explicitly specifying transmission rates by the base station A by indicating transmission rates that should be used by wireless terminals H 2  and H 3  in the payload of response request frames sent from the base station A to wireless terminals H 2  and H 3 , for example. 
     Next, the response request frame transmitting unit  13  sets response request frame length to 1472 bytes, and sends a response request frame with a multicast address to which the wireless terminals H 2  and H 3  belong (S 24 ). In other words, the response request frame is transmitted to the wireless terminals H 2  and H 3  at the same time. Although a multicast address is illustrated here, an ICMP Echo Request may be instead transmitted by broadcast when two wireless terminals are connected to the base station A. 
     Next, the fault detecting unit  14  determines whether there has been a collision of response frames from wireless terminals H 2  and H 3  (S 25 ). Because wireless terminals H 2  and H 3  are in a hidden terminal situation and cannot detect radiowave from each other by carrier sensing, wireless terminal H 3  does not avoid transmission of a response frame and collision of frames occurs ( FIG. 10(B) ). Therefore, a collision of response frames from wireless terminals H 2  and H 3  is detected, and the fault detecting unit  14  determines that a hidden terminal fault is occurring between wireless terminals H 2  and H 3  (S 27 ). 
     As mentioned above, collision of frames can be detected by a method that determines that there has been a collision if frame reception is not detected within a certain time period, for example. Also, according to IEEE802.11 standard, retransmission of a frame takes place when frame error occurred due to interference or the like. As Retry field in a header portion of a retransmitted frame is set to “1”, whether a received frame is a retransmitted frame or not can be determined by referencing the value in Retry field of the frame, and it may be determined that collision has occurred if it is a retransmitted frame. 
     A flow of processing for base station A to transmit and receive ICMP Echo frames to and from wireless terminals H 2  and H 3  in the third embodiment described above is shown in  FIG. 11 . Note that indication of initial backoff time is omitted. As can be seen from  FIG. 11 , occurrence of a hidden terminal fault between test objects can be detected using a multicast response request frame when wireless terminals H 2  and H 3  which are in a hidden terminal situation are test objects. 
     As has been described, according to the embodiments of the present invention, it is possible to detect wireless terminals that are in hidden terminal relation with high accuracy in a wireless LAN system. That is, unlike the method adopted by the conventional technique of JP-A 06-232870 (Kokai) that determines presence/absence of a hidden terminal relation by wireless terminals attempting direct transmission and reception of a response request frame therebetween, the embodiments of the present invention determine whether there is hidden terminal relation or not by transmitting a response request frame from a base station to wireless terminals to see whether interference actually occurs. The conventional method has the problem of decreased accuracy of determination, whereas the present method directly judges the essence of whether a pair of wireless terminals actually causes interference or not and has an advantage of the ability to reliably detect presence or absence of a hidden terminal relation. In addition, the present method may be reused without requiring modification to existing hardware or software and can be implemented and realized at a low cost because it does not violate infrastructure mode specifications of the IEEE802.11 standard that does not permit direct communication between wireless terminals.