Abstract:
Disclosed is a method of selecting a communication path having wider communication bands with respect to wireless communication apparatuses capable of performing direct communication between the wireless communication apparatuses without routing through a relay apparatus. A first wireless communication apparatus determines the capability of the relay apparatus to determine a threshold and compares the received signal strength indicator of a packet transmitted from a second wireless communication apparatus to the first wireless communication apparatus by direct communication with the threshold to determine a communication path for a packet to be transmitted to the second wireless communication apparatus.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of selecting a communication path for communication between wireless communication apparatuses, and particularly to a method of determining whether or not the wireless communication apparatuses should communicate directly. 
     BACKGROUND ART 
     Of various types of networks in which information terminals are connected to one another, wireless communication has advantages over wired communication. Advantages thereof include excellent terminal portability and placement flexibility, and a lightweight body achievable due to elimination of wires. Thus, the wireless communication apparatuses are now used not only for data transmission used with personal computers (Hereinafter referred to as PC), but also for various purposes such as video or audio transmission used with home electrical appliances and online competitive games used with game machines. 
     An example of the wireless communication, which is commonly used, is a wireless Local Area Network (LAN) system. In general, a topology called an infrastructure mode is used in the wireless LAN system. In the infrastructure mode, a station which is a client machine is connected to an access point which is a host machine. For example, in the case where there are a server PC and a digital TV which are wirelessly connected to an access point and when content stored in the server PC is to be viewed by the digital TV placed in a different room, the server PC once transmits the content data for the digital TV to the access point, and the access point transmits the received data to the digital TV. As described above, in the infrastructure mode, data is transmitted between apparatuses via the access point and thus communication bands cannot be used efficiently, which poses a problem of degradation in communication performance. 
     To address this problem, there is a technique which performs direct communication between two stations under the same access point so that the communication bands can be used efficiently, thereby allowing an accelerated transmission.  FIG. 5  shows a concept of a placement of the access point and stations, and packet transmission. In the direct communication technique disclosed in PTL  1 , first, a first station  101  and a second station  102 , which are the stations transmit data, exchange a control packet through transmission paths  112  and  113  via an access point  103 . When the exchange of the control packet is finished, the first station  101  is capable of directly sending a data packet to the second station  102  through a transmission path  111 . This allows the efficient use of communication bands, thereby allowing the accelerated and stabilized communication. 
     CITATION LIST 
     Patent Literature 
     Japanese Unexamined Patent Application Publication No. 2007-104600 
     SUMMARY OF INVENTION 
     Technical Problem 
     The direct communication technique is effective in the efficient use of communication bands and the acceleration of communication. However, depending on the placement of the terminals, there is a case that communication via the access point  103  offers wider communication bands and more stable transmission than direct communication. For example, as shown in  FIG. 6 , when plural walls  104  are on the transmission path  111  for direct communication, a packet communication through the transmission path  111  cannot obtain a sufficient signal strength, and thus communication via the access point  103  is more preferable. On contrast, there is a case that wider communication bands and a higher effective rate can be obtained by direct communication despite a weak signal strength by direct communication, depending on the capability of the access point  103  which relays the packet. For this reason, a function is required which determines whether to perform direct communication or communication via the access point. 
     Solution To Problem 
     To provide a solution to the above problem, the method of selecting a wireless communication path according to an aspect of the present invention is characterized by determining the capability of the access point to determine a threshold and comparing the threshold with a received signal strength indicator of a packet received by direct communication to determine communication path for the packet to be sent to a station which is a communication partner. 
     The capability of the access point is determined based on information included in a control packet. 
     In addition, the method of selecting a wireless communication path according to an aspect of the present invention is characterized by comparing the received signal strength indicator of a packet received by direct communication with the received signal strength indicator of a packet received from the access point to determine a communication path for a packet to be sent to the station which is a communication partner. 
     Advantageous Effects of Invention 
     The present invention makes it possible to accurately determine merits and demerits on performance of direct communication and that of communication via an access point, thereby allowing a fast and stable packet transmission between stations. In addition, the present invention is easy to implement because no change is required to a protocol or a control packet of the existing wireless LAN system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration example of a wireless communication apparatus according to an embodiment of the present invention. 
         FIG. 2  is a process flow chart according to the embodiment of the present invention. 
         FIG. 3  is a table for use in determining a threshold for determining whether or not direct communication is available according to the embodiment of the present invention. 
         FIG. 4  is a graph showing a relationship between an RSSI and an effective rate when direct communication is performed between a first station and a second station according to the embodiment of the present invention. 
         FIG. 5  is a conceptual diagram of a placement of an access point and the stations, and packet transmission. 
         FIG. 6  is a conceptual diagram of a placement of the access point and the stations, and packet transmission, in the environment that there are walls. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With reference to  FIG. 1  to  FIG. 5 , a method of selecting a wireless communication path according to an embodiment of the present invention is described. 
       FIG. 1  is a block diagram showing a configuration example of the wireless communication apparatus according to this embodiment,  FIG. 2  is a flow chart showing a process of the method of selecting a wireless communication path,  FIG. 3  is a table for use in determining a threshold, and  FIG. 4  is a graph showing a relationship between the RSSI and the effective rate when direct communication is performed between the first station and the second station. 
     The wireless communication system according to the embodiment includes, in the same manner as in  FIG. 5 , a first station  101 , a second station  102 , and an access point  103 . In the embodiment, a process of selecting a communication path at the first station  101  when the first station  101  transmits a data packet to the second station  102  in  FIG. 5 . 
     First, the configuration of the first station  101 , which corresponds to a wireless communication apparatus according to the present invention, is described with reference to  FIG. 1 . Here, the configuration of the second station  102  is the same as that of the first station  101 , and the description shall be omitted. 
     In  FIG. 1 , a wireless signal sending and receiving unit  122  sends and receives a wireless signal  121 . A direct communication setup unit  123  sets up direct communication with a wireless communication apparatus which is a communication partner. An access point capability determining unit  124  determines the capability of the access point. A threshold determining unit  125  determines a threshold based on information input from the access point capability determining unit  124  with reference to a threshold table storage unit  126 . The threshold table storage unit  126  stores plural thresholds beforehand. A path selecting unit  127  selects a wireless communication path based on information input from the threshold determining unit  125  and the wireless signal sending and receiving unit  122 , and notifies the wireless communication path to the wireless signal sending and receiving unit  122 . 
     Direct communication setup unit  123  (i) determines whether or not the second station  102 , which is the destination of the data packet, corresponds to direct communication between stations, and when the correspondence to direct communication is confirmed, (ii) exchanges a packet for setting up direct communication with the second station  102  via the wireless signal sending and receiving unit  122  (S 01 ). It is to be noted that how to determine the correspondence to direct communication is not limited, since it is not the nature of the present invention. For example, it is determined that the second station  102  is capable of direct communication when the first station  101  notifies a query on whether or not direct communication is available to the second station  102  and a response is received from the second station  102 . This packet exchange may be performed via the access point  103  or directly between the stations. Furthermore, the information on setup to be exchanged is not limited in this embodiment. When this process is finished, a secure direct communication is available, for example, between the first station  101  and the second station  102 . 
     Next, the access point capability determining unit  124  determines the maximum physical rate to which the access point  103  corresponds, as the capability of the access point  103  which relays the packet to the second station  102  (S 02 ). In a wireless LAN complying with IEEE 802.11 family which is standardized by the Institute of Electrical and Electronics Engineers, Inc. (Hereinafter referred to as IEEE), the access point  103  constantly exchanges control packets such as beacon, probe, association and reassociation, with the first station  101  and the second station  102  which are the terminals under the access point  103 . Inside these packets are information related to the maximum physical rate such as a physical rate, bandwidths and guard interval length to which the terminal which has sent the control packet corresponds. It is sufficient for the access point capability determining unit  124  to refer to the information when determining the maximum physical rate to which the access point  103  corresponds. 
     Then, the threshold determining unit  125  determines a threshold in a parameter for determining whether transmitting the data packet to the second station  102  by a direct communication or via the access point  103  (S 03 ). In this embodiment, the parameter for use in determination is a Received Signal Strength Indicator (Hereinafter referred to as RSSI) of the packet when direct communication is performed between the first station  101  and the second station  102 . The threshold determining unit  125  determines a threshold by collating (i) the table  126 T shown in  FIG. 3 , which is stored in the threshold table storage unit  126  and used for determining a threshold for determining whether or not direct communication is available, and (ii) the maximum physical rate to which the access point  103  corresponds and which is determined by the access point capability determining unit  124  in S 02 . For example, when the maximum physical rate to which the access point  103  corresponds is 300 Mbps, the threshold is set to −78 dBm. 
     The table  126 T for use in determining thresholds is created in the following manner.  FIG. 4  is a graph showing a relationship between the RSSI of the packet and the effective rate; that is, the band of the transmission path  111  when direct communication is performed by the first station  101  and the second station  102 . It is assumed here that the maximum physical rate of both of the first station  101  and the second station  102  are 300 Mbps. Furthermore, in  FIG. 4 , the effective rate does not increase even when RSSI is greater than −60 dBm. This phenomenon is mainly caused by the performance of the central processing unit (Hereinafter referred to as CPU) of the first station  101  and the second station  102 , and is seen in many wireless communication systems. Here, when the maximum physical rate of the access point  103  is also 300 Mbps, it can be said that the effective rate of the transmission paths  112  and  113  is 90 Mbps at most, as is the case with the transmission path  111 . However, in a wireless LAN system, it is impossible for the transmission paths  112  and  113  to transmit concurrently due to the carrier sense function. Therefore, when the first station  101  and the second station  102  communicate through the transmission paths  112  and  113  via the access point  103 , the effective rate during communication decreases by half. Taking overhead such as transmission waiting time being further added to this into consideration, the maximum effective rate of communication between the first station  101  and the second station  102  via the access point  103  is 36 Mbps which is approximately 80% of half of 90 Mbps. More specifically, as long as the effective rate of direct communication exceeds 36 Mbps, the effective rate of direct communication is always greater than communication via the access point  103 . Therefore, according to  FIG. 4 , the threshold is set to the RSSI of −78 dBm which offers the effective rate of 36 Mbps of by direct communication. Furthermore, a case that the maximum physical rate to which the access point  103  corresponds is 150 Mbps shall be considered. When the physical rate to which the access point  103  corresponds is 150 Mbps and the first station  101  and the second station  102  under the access point  103  communicate with the access point  103 , the physical rates of the stations are limited to the maximum of 150 Mbps, even though the maximum physical rates to which the stations correspond are 300 Mbps. The effective rate of packet transmission under the physical rate of 150 Mbps is approximately 70 Mbps. More specifically, since the effective rates of the transmission paths  112  and  113  are 70 Mbps, based on the same idea described above, the maximum effective rate for communication between the first station  101  and the second station  102  via the access point  103  is 28 Mbps which is approximately 80% of half of 70 Mbps. In this case, according to  FIG. 4 , the threshold is set to the RSSI of −81 dBm which offers the effective rate of 28 Mbps by direct communication. In addition, assuming cases where the maximum physical rate to which the access point  103  corresponds is (i) 11 Mbps and the effective rate per transmission path is 6 Mbps and (ii) 54 Mbps and the effective rate per transmission path is 30 Mbps, the thresholds for each of the cases can be set to −89 dBm and −86 dBm, in the same manner as the above. These thresholds are coordinated and shown in the table  126 T in  FIG. 3 . 
     After a threshold is determined in S 03 , the path selecting unit  127  performs the process of selecting a communication path. First, the path selecting unit  127  (i) compares the threshold determined in S 03  with the RSSI of the packet to be transmitted through the transmission path  111 , that is, by direct communication (Hereinafter refereed to as RSSI_DIRECT) and (ii) determines the status of the transmission path  111  for direct communication (S 04 ). When it is immediately after setting up direct communication and the value of RSSI_DIRECT has not been obtained, the path selecting unit  127  sets the RSSI_DIRECT to the default value of 0 and prepares for the first packet to be transmitted by direct communication. The path selecting unit  127  determines, when the RSSI_DIRECT is equal to or greater than the threshold, that the effective rate of direct communication is greater than that of communication via the access point  103  and determines to directly send the packet for the second station  102  to the second station  102  (S 07 ). While, when the RSSI_DIRECT is smaller than the threshold, the path selecting unit  127  proceeds to the next determining process S 05 , since there is a possibility that the effective rate of communication via the access point  103  is greater. 
     The path selecting unit  127  compares the RSSI of the packet to be transmitted through the transmission path  112  (Hereinafter refereed to as RSSI_AP_STA 1 ) with RSSI_DIRECT (S 05 ). As described above, when the packet is transmitted from the first station  101  to the second station  102  via the access point  103 , it is impossible to concurrently transmit the packet over the transmission paths  112  and  113 . Furthermore, communication via the access point  103  offers better characteristics than direct communication only when the band of the transmission path  112  is greater than that of the transmission path  111 . Thus, this condition is determined in S 05 . The packet for use in detecting RSSI_AP_STA 1  is not necessarily be the packet transmitted from the second station  102  via the access point  103  and may be a control packet sent from the access point  103 . The path selecting unit  127  determines, when the RSSI_DIRECT is greater than the RSSI_AP_STA 1 , that the effective rate of direct communication is greater than that of communication via the access point  103  and determines to directly send the packet for the second station  102  to the second station  102  (S 07 ). While, when the RSSI_AP_STA 1  is greater than the RSSI_DIRECT, the path selecting unit  127  determines that the effective rate of communication via the access point  103  is greater than that of direct communication and determines to send the packet for the second station  102  to the access point  103  (S 06 ). The path selecting unit  127  notifies the determined communication path to the wireless signal sending and receiving unit  122 . 
     The above process of selecting a path is repeated until the end of communication (S 08 ). The set of the process may be performed for each of the packets or at regular time intervals. 
     The performance or the thresholds raised in this embodiment is an example and does not limit the scope of the present invention. 
     As described above, such a configuration allows to easily and accurately select an appropriate communication path from direct communication and communication via the access point, and to transmit data in a fast and stable manner. 
     [Industrial Applicability] 
     The method of selecting a wireless communication path according to the present invention can be easily mounted to a device mounted with a wireless LAN system which is capable of direct communication between terminals, and is especially effective for use with storage devices or AV stream compatible devices which requires stable data transmission. 
     [Reference Signs List] 
     
         
           101  First station 
           102  Second station 
           103  Access point 
           104  Wall 
           111  Transmission path for the case where the first station transmits a packet to the second station by direct communication 
           112  Transmission path for the case where the first station transmits a packet to the access point 
           113  Transmission path for the case where the access point transmits a packet to the second station 
           121  Wireless signal 
           122  Wireless signal sending and receiving unit 
           123  Direct communication setup unit 
           124  Access point capability determining unit 
           125  Threshold determining unit 
           126  Threshold table storage unit 
           126 T Table for use in determining the threshold which is stored in the threshold table storage unit and which is used for determining whether or not direct communication is available 
           127  Path selecting unit