Patent Publication Number: US-2015063229-A1

Title: Communication apparatus, communication-apparatus control method, information processing apparatus, information processing system, and recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-180583, filed on Aug. 30, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a communication apparatus, a communication-apparatus control method, a communication-apparatus control program, an information processing apparatus, and an information processing system. 
     BACKGROUND 
     The transmission rate has been improving splendidly by recent development of wireless technology. By standards, such as Institute-of-Electrical-and-Electronic-Engineers Inc. (IEEE) 802.11ac and IEEE 802.11ad, the transmission rate of several gigabits per second (Gbps) can be realized. Therefore, application of IEEE802.11ac or IEEE802.11ad standard to the wireless communications between servers is expected. 
     Communication between nodes is performed through access points. The node refers to a communication device in general such as a server and a client. Moreover, the access point refers to a device that performs mediation in wireless communications such as connection between nodes. 
     As a communication scheme between nodes, there is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme that avoids access contention. In the CSMA/CA scheme, a node performs data transmission to a node of a communication destination, after confirming that the communication path between access points is vacant for a predetermined period of time or more continuously. In the explanation below, the communication path may be referred to as “channel”. In the CSMA/CA scheme, collision is avoided by changing the timing of transmission of frames for each node after detection of the channel being in the idle state by carrier sense. 
     In the IEEE802.11 standard, an inter frame space (IFS) is defined as a signal transmission interval before transmission of a signal. IFS is a period until it is determined that a channel has shifted to an idle state in a node when a radio wave is no longer detected in the channel that used to be busy. For IFS, three kinds of IFS, short IFS (SIFS), point coordination function (PCF) IFS (PIFS), and distributed coordination function (DCF) IFS (DIFS) are defined. 
     SIFS is IFS that is used for an acknowledgement (ACK), which is a signal for response, and the like and has the highest priority. When the DIFS period has passed, each node can perform data transmission to an access point after the back-off time that is assigned to each node passes. To the back-off time, a value generated from the random number table for each node is set. 
     For example, when a node performs data transmission, the channel is to be in a busy state. Therefore, when the back-off time of one node has passed and the node has performed data transmission to an access point, the other nodes wait for the DIFS period after confirming that the channel becomes in the idle state, and then start counting the back-off time. That is, at a certain point of time, the node that has the shortest back-off time is to be the node that performs data transmission first after the point of time. When one node performs data transmission to an access point, the other nodes carry over the back-off time until the next count. Upon receiving the data, the access point sends an acknowledgement to the node of the source of transmission. The node that has performed the data transmission and received the acknowledgement from the access point resets the back-off time. 
     For example, a back-off time can be calculated by multiplying a random number within a contention window (CW) and a slot time. The random number within the CW is a value that is generated at random with the value of the CW as the upper limit. Moreover, the slot time is a fixed period of time. Each node has a value of the same CW and the slot time. 
     As described, in the CSMA/CA scheme, access contention among nodes is avoided by setting a back-off time that is generated from a random number table for each node. 
     The following techniques have been proposed as a method of avoiding access contention in wireless communications. For example, there is a related technique in which different back-off values are given to respective nodes from an access point. Moreover, there is a related technique in which an offset value is added to values of random number within a CW to calculate a back-off time, and the back-off time is notified to each node (Japanese Laid-open Patent Publication No. 2012-178694 and Japanese Laid-open Patent Publication No. 2005-64795). 
     However, a back-off time is calculated using a random number value within the CW, without specifying the number of nodes connected to an access point. In this point, when the number of nodes connected to an access point is fixed, access contention can be avoided, in some cases, in a period shorter than the back-off time calculated using the random number value within the CW. That is, when a back-off time is calculated using a random number value within the CW, there is a possibility that a node is forced to have needless standby time depending on a value of the calculated back-off time if the number of nodes connected to an access point is fixed. 
     Moreover, when a back-off time is calculated using a random number value within the CW, it can occur that an identical back-off time is set to two or more nodes. In that case, access contention occurs. 
     When the related technique in which various back-off values are set for nodes from an access point is applied, it returns to the method of using a random-number value within the CW when successive data is not present, and therefore, reduction of the standby time and suppression of occurrence of access contention are difficult. 
     Moreover, even if the related technique in which an offset value is added to random number values within the CW to notify a back-off time is applied, reduction of the standby time and suppression of occurrence of access contention are difficult. 
     SUMMARY 
     According to an aspect of the embodiments, a communication apparatus includes: a memory; and a processor coupled to the memory. The processor executes a process including: measuring elapsed time of a set standby time; determining whether a communication path to a transmission destination of data is available; transmitting data when the standby time passes in a state in which the communication path is available after it is determined that the communication path is available at the determining; setting, as the standby time, an initial value that is generated using a predetermined time as a unit at start of data transmission at the transmitting, and that is a different value from an initial value of another communication apparatus that communicates with the transmission destination; and resetting, as the standby time, a resetting value that is generated using the predetermined time as a unit when the standby time has passed. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram depicting an example of an information processing system; 
         FIG. 2  is a configuration diagram depicting the details of a server; 
         FIG. 3  is a block diagram depicting details of a wireless module; 
         FIG. 4  is a table indicating an example of information stored in a setting register according to a first embodiment; 
         FIG. 5  is a sequence diagram indicating communication when all servers have a transmission data in the first embodiment; 
         FIG. 6  is a sequence diagram indicating communication when a server that has no transmission data is included in the first embodiment; 
         FIG. 7  is a flowchart of a wireless-communication processing in a wireless module according to the first embodiment; 
         FIG. 8  is a configuration diagram illustrating another example of the information processing system; 
         FIG. 9  is a sequence diagram indicating communication in an information processing system according to a second embodiment; 
         FIG. 10  is a table indicating an example of information stored in a setting register according to a third embodiment; 
         FIG. 11  is a sequence diagram indicating communication in an information processing system according to the third embodiment; and 
         FIG. 12  is a sequence diagram indicating communication in an information processing system according to a fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments will be explained with reference to accompanying drawings. Note that the communication apparatus, the communication-apparatus control method, the communication-apparatus control program, the information processing apparatus, and the information processing system disclosed in the present application are not limited to the following embodiments. 
     [a] First Embodiment 
       FIG. 1  is a configuration diagram depicting an example of an information processing system. In an information processing system according to the present embodiment, more than one information processing apparatus (hereinafter “server”)  10  and an access point (AP)  20  are mounted in a rack 1. It is possible that a large number of the servers  10  are mounted in the rack 1. In such a case, when the servers  10  are connected using a local area network (LAN) cables and the like, a lot of cables are to be arranged, and the management of the cables becomes complicated. Therefore, wiring can be simplified by connecting the servers  10  in the rack 1 to a network using a wireless LAN. 
     Although three units of the servers  10  are illustrated in  FIG. 1  as an example of the configuration, the number of the servers  10  is not particularly limited. Moreover, although three open spaces for mounting the server  10  are illustrated in  FIG. 1 , the number of those is also not particularly limited. 
     Moreover, although one unit of the access point  20  is illustrated in  FIG. 1 , it is not limited thereto. For example, when the number of the servers  10  is more than the number that can be managed by a single unit of the access point  20 , the servers  10  may be divided into groups, and a different unit of the access point  20  to be connected may be assigned to each group. In the following explanation, a group to which one unit of the access point  20  and multiple units of the servers  10  belong is explained, and when more than one group is present, similar processing is performed in each group. 
     The server  10  and the access point  20  perform mutual data communication using wireless communications. 
     The server  10  communicates with another information processing apparatus, such as another unit of the servers  10  and an external information processing apparatus, through the access point  20 . Specifically, the server  10  transmits data to another information processing apparatus by transmitting the data to be transmitted to the information processing apparatus, to the access point  20 . Moreover, the server  10  receives data transmitted from another information processing apparatus from the access point  20 . 
       FIG. 2  is a configuration diagram depicting the details of the server. The server  10  includes central processing units (CPU)  11 , memories  12 , a storage  13 , and a crossbar switch  14 . Furthermore, the server  10  includes wireless modules  100  and antennas  15  each of which corresponds to each of the wireless modules  100 . 
     The memories  12  and the storage  13  are connected to the CPU  11  through a bus. The CPU  11  can write and read data to and from the memories  12  and the storage  13 . The CPU  11  executes a program and the like stored in the storage  13  by developing in the memory  12 . 
     The CPUs  11  are connected to the wireless modules  100 , respectively, through the crossbar switch  14 . The CPU  11  transmits data to be transmitted to another information processing apparatus, to the wireless module  100  through the crossbar switch  14 . Moreover, the CPU  11  receives data from another information processing apparatus from the wireless module  100  through the crossbar switch  14 . 
     The wireless module  100  outputs data received from the crossbar switch  14  to the access point  20  through the antenna  15 . Furthermore, the wireless module  100  receives data transmitted by another information processing apparatuses from the access point  20 . The wireless module  100  then transmits the received data to the CPU  11  through the crossbar switch  14 . 
       FIG. 3  is a block diagram indicating details of the wireless module. As illustrated in  FIG. 3 , the wireless module  100  includes a transmitting unit  101 , a receiving unit  102 , a back-off-time setting unit  103 , a determining unit  104 , a packet analyzing unit  105 , and an interface (I/F)  106 . 
     The I/F  106  functions an interface when the wireless module  100  transmits and receives data to and from the crossbar switch  14 . The I/F  106  receives data from the crossbar switch  14 . The I/F  106  then transmits the received data to the back-off-time setting unit  103 . Moreover, the I/F  106  receives data transmitted from another information processing apparatus from the packet analyzing unit  105 . Subsequently, the I/F  106  transmits the received data to the crossbar switch  14 . 
     The transmitting unit  101  receives input of data to be transmitted to another information processing apparatuses from the I/F  106 . Upon receiving notification that a back-off time has passed from an elapsed-time measuring unit  133  described later, the transmitting unit  101  transmits the data received from the I/F  106  to the access point  20  through the antenna  15 . 
     The receiving unit  102  receives data transmitted from another information processing apparatus from the access point  20  through the antenna  15 . The data transmitted from the other information processing apparatus includes ACK and the like also. The receiving unit  102  outputs the data received from the other information processing apparatus to the packet analyzing unit  105 . 
     Moreover, the receiving unit  102  receives a beacon from the access point  20 . The receiving unit  102  outputs the beacon to the packet analyzing unit  105 . 
     Furthermore, the receiving unit  102  performs signal detection in a communication path (channel) to the access point  20 . In the following explanation, the channel between the access point  20  and the wireless module  100  is simply called “channel”. The receiving unit  102  then outputs a result of the signal detection in the channel to the determining unit  104 . 
     The packet analyzing unit  105  receives a beacon and data from the receiving unit  102 . The packet analyzing unit  105  analyzes the received signal. 
     When the received signal is a beacon, the packet analyzing unit  105  notifies the reception of a beacon to an initial-value setting unit  132  described later. 
     When the received signal is ACK, the packet analyzing unit  105  notifies the reception of ACK to a resetting unit  134  described later. 
     The determining unit  104  stores a DIFS period that is a period to determine that it has shifted to an idle state from a point of time when a signal is no longer detected in a channel. 
     The determining unit  104  receives a result of signal detection in a channel from the receiving unit  102 . When it shifts from a state in which a signal is detected to a state in which no signal is detected, the determining unit  104  starts measuring time. When the state in which no signal is detected continues for the period of DIFS after start of measurement of time, the determining unit  104  determines that the channel has shifted to the idle state. In the following explanation, the state of a channel in which a signal is detected is described as “busy”. The state that is determined by the determining unit  104  that it has shifted to the idle state corresponds to one example of “communication path is available”. 
     Determining that it has shifted to the idle state, the determining unit  104  outputs notification that the channel is in the idle state to the elapsed-time measuring unit  133  in the back-off-time setting unit  103 . Subsequently, the determining unit  104  forwards the result of signal detection in the channel received from the receiving unit  102  to the elapsed-time measuring unit  133 . 
     On the other hand, when detection of a signal is notified before the period of DIFS passes after measurement of time is started, the determining unit  104  determines that the channel has shifted to the busy state, ends the measurement of time, and waits in standby until the channel shifts again to the state in which no signal is detected. 
     The back-off-time setting unit  103  includes a setting register  131 , the initial-value setting unit  132 , the elapsed-time measuring unit  133 , and the resetting unit  134 . The back-off-time setting unit  103  is implemented by a field-programmable gate array (FPGA), for example. 
     The setting register  131  is a data holding circuit, such as a flip-flop. In the setting register  131 , an initial value and a resetting value of the back-off time are stored in advance. The back-off time is standby time from when a channel has become the idle state until the wireless module  100  starts data transmission. 
     The setting register  131  holds information as illustrated in  FIG. 4  as the initial value and the resetting value of the back-off time, for example.  FIG. 4  is a table indicating an example of information stored in the setting register according to a first embodiment. A column of bit in a table  150  in  FIG. 4  indicates each bit of the setting register  131 . Moreover, a column of field in the table  150  indicates information on data stored in the corresponding bit. Furthermore, a column of explanation in the table  150  is for explaining what kind of data each of the stored data is. 
     In the present embodiment, the setting register  131  has a space of 32 bits. As illustrated in  FIG. 4 , bit  8  to bit  31  of the setting register  131  are secured as a reserved space, and is not used. 
     Bit  4  to bit  7  in the setting register  131  are used to store the resetting values. For example, as described in the explanation in the table  150 , when bit  4  to bit  7  are “0001”, the resetting value is to be a value obtained by multiplying the slot time by 1. Moreover, when bit  4  to bit  7  are “0010”, the resetting value is to be a value obtained by multiplying the slot time by 2. 
     The “slot time” is the smallest unit of the back-off time, and is time requested for transmitting one frame, for example. That is, setting the back-off time to a value obtained by multiplying the slot time by n means that the transmitting unit  101  waits transmission of data in standby until n pieces of the slot time passes from the time when the channel has become idle. 
     Furthermore, bit  0  to bit  3  in the setting register  131  are used to store the initial values. For example, as described in the explanation in the table  150 , when bit  0  to bit  3  are “0001”, the initial value is to be a value obtained by multiplying the slot time by 1. Moreover, when bit  0  to bit  3  are “0010”, the initial value is to be a value obtained by multiplying the slot time by 2. 
     While in the present embodiment, information of the resetting values and the initial values are both stored using space of four bits in the setting register  131 , if larger values are to be stored, it is only requested to expand a region to be used in the setting register  131 . 
     In the server  10  according to the present embodiment, a different initial value is assigned to each of the wireless modules  100  of the server  10 . For example, when there are three units of the servers  10 , and three units of the wireless modules  100  are equipped in each of the servers  10 , either one out of 1×slot time to 9×slot time is assigned to each of the wireless modules  100  so as to differ from each other. Particularly, as for assignment of the initial values, it is preferable to assign the initial values to the respective servers  10  so that the slot time increases sequentially by 1 each from the assignable shortest time. For example, when the assignable shortest time is 1×slot time, 1×slot time is assigned to one of the servers  10 , and back-off times with the slot time increased sequentially by one each are assigned to the other servers  10 . 
     As described, by assigning the shortest time, the standby time after the period of DIFS passes can be reduced. However, if a certain amount of standby time is permissible, the initial values of the respective servers  10  could only differ from each other. 
     Moreover, in the server  10  according to the present embodiment, the same resetting value is assigned to any of the wireless modules  100 . The resetting value in the present embodiment matches with a value obtained by multiplying the total number of the wireless modules  100  connected to the access point  20  by the slot time. For example, when the number of the wireless modules  100  connected to the access point  20  is n, it is preferable that the resetting value of the back-off time be n×slot time. For example, when three units of the servers  10  are connected to the access point  20  and three units of the wireless modules  100  are equipped in each of the servers  10 , it is preferable that the resetting value be a value obtained by multiplying the slot time by 9. 
     By thus setting a value that is obtained by multiplying the number of the wireless modules  100  by the slot time as the resetting value, the standby time following the first communication can be the shortest when values increased sequentially by one slot time each from the shortest time are assigned to the wireless modules  100  as the initial values thereof. 
     However, if a certain amount of standby time is permissible, the initial values can be values other than these. For example, the shortest initial value is set to 1×slot time or more, and a value to which a predetermined number of slot times are added may be assigned to the wireless module  100  after that. Moreover, if it is not necessary to make the standby time shortest, the resetting values of the back-off time may also be other values as long as the values are longer than the remaining time of the back-off time of the other node at the time when the back-off time is reset in each node. 
     The initial-value setting unit  132  receives, from the packet analyzing unit  105 , notification that a beacon is received. Upon receiving the notification of reception of a beacon, the initial-value setting unit  132  refers to the bit indicating the initial value of the setting register  131  to acquire the initial value. The initial-value setting unit  132  then sets the acquired initial value to the elapsed-time measuring unit  133  as the back-off time. 
     The resetting unit  134  determines the presence or absence of transmission data, by receiving the same data as data that is transmitted to the transmitting unit  101  by the I/F  106 . 
     When transmission data has not been received by the time back-off time has passed after the channel had shifted to the idle state, the resetting unit  134  acquires a resetting value from the setting register  131 . Subsequently, the resetting unit  134  resets the acquired resetting value to the elapsed-time measuring unit  133  as the back-off time. 
     On the other hand, when transmission data occurs before the back-off time passes after the channel has shifted to the idle state, the resetting unit  134  waits for notification of reception of ACK from the packet analyzing unit  105 . When the notification of reception of ACK is received from the packet analyzing unit  105 , the resetting unit  134  acquires a resetting value from the setting register  131 . Subsequently, the resetting unit  134  resets the acquired resetting value to the elapsed-time measuring unit  133  as the back-off time. 
     The elapsed-time measuring unit  133  has a counter. Receiving the input of the initial value of the back-off time from the initial-value setting unit  132 , the elapsed-time measuring unit  133  sets the initial value at the counter. 
     Subsequently, the elapsed-time measuring unit  133  receives notification that the channel is in the idle state from the determining unit  104 . The elapsed-time measuring unit  133  counts down until the counter to which the initial value has been set becomes zero. When the counter becomes zero, the elapsed-time measuring unit  133  notifies the transmitting unit  101  of elapse of the back-off time. Moreover, the elapsed-time measuring unit  133  notifies the resetting unit  134  of elapse of the back-off time. 
     Thereafter, receiving input of a resetting value of the back-off time from the resetting unit  134 , the elapsed-time measuring unit  133  sets the counter to the resetting value. Upon receiving the notification that the channel is in the idle state from the determining unit  104 , the elapsed-time measuring unit  133  starts counting down the resetting value. The elapsed-time measuring unit  133  counts down until the counter to which the resetting value is set becomes zero. When the counter becomes zero, the elapsed-time measuring unit  133  notifies the transmitting unit  101  of elapse of the back-off time. 
     Subsequently, receiving input of a resetting value of the back-off time from the resetting unit  134 , the elapsed-time measuring unit  133  sets again the counter to the resetting value. The elapsed-time measuring unit  133  repeats setting of resetting values, counting time, and notifying elapse of the back-off time described above. 
     However, the elapsed-time measuring unit  133  stops counting down, when notification that the channel has become busy is received from the determining unit  104  during the countdown. While maintaining the back-off time before the stop of countdown, the elapsed-time measuring unit  133  waits in standby until notification that the channel is in the idle state is received from the determining unit  104 . Subsequently, the elapsed-time measuring unit  133  resumes countdown of the back-off time starting from the value before the stop of countdown. 
     Avoidance of contention of communication by the back-off time in the wireless module  100  according to the present embodiment is explained. 
     As described above, in the present embodiment, the initial values are assigned such that the respective wireless modules  100  have different initial values from each other. Therefore, the initial values set by the elapsed-time measuring unit  133  at first differ from each other in the respective wireless modules  100 . In other words, the back-off times of the wireless modules  100  are not finished in the same timing with the initial values. Accordingly, contention does not occur in the communication using the initial values of back-off time. Particularly, in the present embodiment, the initial values of back-off time is assigned such that the minimum value of 1×slot time is assigned to one of the servers  10  as the initial value of the back-off time, and initial values obtained by increasing the slot time sequentially by one each are assigned to the other servers  10 . In this case, the server  10  to which 1×slot time is assigned as the initial value performs communication first, and the other servers  10  carries over the initial values under countdown. 
     Furthermore, in the present embodiment, the resetting value is the number of the wireless module  100 ×slot time. Accordingly, the back-off times of the respective wireless modules  100  always differ from each other, and contention of communication of the wireless modules  100  does not occur. 
     The transmitting unit  101  receives data to be transmitted to another information processing apparatus from the I/F  106 . Upon receiving notification of elapse of the back-off time from the elapsed-time measuring unit  133 , the transmitting unit  101  transmits the data received from the I/F  106  to the access point  20 . If there is no data to transmit when the notification of elapse of the back-off time is received, the transmitting unit  101  waits in standby without performing data transmission. 
     Next, an overall flow of communication in the information processing system according to the present embodiment is explained with reference to  FIG. 5  and  FIG. 6 .  FIG. 5  is a sequence diagram indicating communication when all of the servers have transmission data in the first embodiment.  FIG. 6  is a sequence diagram indicating communication when a server having no transmission data is included in the first embodiment. In this example, explanation is given in a case in which servers  10 A to  10 C are present as the servers  10 . Moreover, one unit of the wireless module  100  is mounted in each of the servers  10 A to  10 C. 
     A case in which all the servers have transmission data is explained with reference to  FIG. 5 . The initial value of the back-off time of the server  10 A is 1×slot time. Furthermore, the initial value of the back-off time of the server  10 B is 2×slot time. Moreover, the initial value of the back-off time of the server  10 C is 3×slot time. Furthermore, the resetting value of the back-off time of the servers  10 A to  10 C is 3×slot time. 
     The servers  10 A to  10 C wait in standby for the DIFS period after the channel has shifted from the busy state. If the channel does not become busy while waiting in standby, the servers  10 A to  10 C set the initial value of the back-off time in the respective elapsed-time measuring units  133 . Specifically, the initial-value setting unit  132  of the server  10 A sets the initial value of the back-off time expressed by a period  201  in the elapsed-time measuring unit  133 . One small rectangle indicated by the period  201  expresses one slot time. The initial-value setting unit  132  of the server  10 B sets the initial value of the back-off time expressed by a period  202  in the elapsed-time measuring unit  133 . The initial-value setting unit  132  of the server  10 C sets the initial value of the back-off time expressed by a period  203  in the elapsed-time measuring unit  133 . 
     In this state, the back-off time of the server  10 A is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 A determines that the back-off time has passed, the transmitting unit  101  of the server  10 A transmits data  204  to the access point  20 . At this time, a shaded area of the period  202  and a shaded area of the period  203  are carried over to following measurement of elapsed time as the back-off times of the servers  10 B and  10 C, respectively. A period expressed in shade with diagonal lines in each of the back-off times in  FIG. 5  is a period to be carried over to next measurement of elapsed time. That is, the elapsed-time measuring unit  133  of the server  10 B carries over 1×slot time to next measurement of elapsed time. Moreover, the elapsed-time measuring unit  133  of the server  10 C carries over 2×slot time to next measurement of elapsed time. 
     The access point  20  receives the data  204 . Subsequently, the access point  20  transmits ACK  205  to the server  10 A, if the channel does not become busy after waiting for the SIFS period in standby. The SIFS period is set short compared with the DIFS period. Accordingly, the access point  20  can return a response of ACK before the server  10 B or  10 C transmits data. 
     The receiving unit  102  of the server  10 A receives ACK  205  from the access point  20 . The packet analyzing unit  105  of the server  10 A notifies the resetting unit  134  of reception of ACK  205 . The resetting unit  134  of the server  10 A resets a period  206  of 3×slot time, which is the resetting value, to the elapsed-time measuring unit  133 . A period having a range indicated by a brace in  FIG. 5  is a resetting period. 
     In this case, the elapsed-time measuring unit  133  of the server  10 B sets a period  207  that is a carried over period, 1×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 C sets a period  208  that is a carried over period, 2×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 B is the shortest. Therefore, when it is determined that the back-off time has passed by the elapsed-time measuring unit  133  of the server  10 B, the transmitting unit  101  of the server  10 B transmits data  209  to the access point  20 . At this time, a shaded area of the period  206  and a shaded area of the period  208  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A and  10 C, respectively. 
     The access point  20  receives the data  209 . Subsequently, the access point  20  transmits ACK  210  to the server  10 B, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 B receives ACK  210  from the access point  20 . The packet analyzing unit  105  of the server  10 B then notifies the resetting unit  134  of reception of ACK  210 . The resetting unit  134  of the server  10 B resets a period  212  of 3×slot time, which is the resetting value, to the elapsed-time measuring unit  133 . 
     In this case, the elapsed-time measuring unit  133  of the server  10 A sets a period  211  that is a carried over period, 2×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 C sets a period  213  that is a carried over period, 1×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 C is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 C determines that the back-off time has passed, the transmitting unit  101  of the server  10 C transmits data  214  to the access point  20 . At this time, a shaded area of the period  211  and a shaded area of the period  212  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A and  10 B, respectively. 
     The access point  20  receives the data  214 . Subsequently, the access point  20  transmits ACK  215  to the server  10 C, if the channel does not become busy after waiting for the SIFS period in standby. 
     Next, a case in which a server having no transmission data is included is explained with reference to  FIG. 6 . In this case also, the initial values and the resetting values of the servers  10 A to  10 C are the same as the case in  FIG. 5 . 
     The servers  10 A to  10 B wait for the DIFS period in standby after the channel has shifted from the busy state. If the channel does not become busy while waiting in standby, the servers  10 A to  10 B set periods  221 ,  222  and  223  that are the initial values of the back-off time in the respective elapsed-time measuring units  133 . 
     In this state, the back-off time of the server  10 A is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 A determines that the back-off time has passed, the transmitting unit  101  of the server  10 A transmits data  224  to the access point  20 . At this time, a shaded area of the period  222  and a shaded area of the period  223  are carried over to following measurement of elapsed time as the back-off times of the servers  10 B and  10 C, respectively. 
     The access point  20  receives the data  224 . Subsequently, the access point  20  transmits ACK  225  to the server  10 A, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 A receives ACK  225  from the access point  20 . The packet analyzing unit  105  of the server  10 A notifies the resetting unit  134  of reception of ACK  225 . The resetting unit  134  of the server  10 A resets a period  226  of 3×slot time, which is the resetting value, to the elapsed-time measuring unit  133 . 
     In this case, the elapsed-time measuring unit  133  of the server  10 B sets a period  227  of 1×slot time that is a carried over period as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 C sets a period  229  that is a carried over period, 2×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 B is the shortest. Accordingly, the elapsed-time measuring unit  133  of the server  10 B determines that the back-off time has passed first. However, the server  10 B has no transmission data. Therefore, the resetting unit  134  of the server  10 B resets a period  228  of 3×slot time, which is the resetting value, in the elapsed-time measuring unit  133 . 
     In this case, because there is no data transmission from the server  10 B, the channel does not become busy. Therefore, the elapsed-time measuring unit  133  of the servers  10 C and  10 A continue measurement of elapse of the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 B also continues measurement of elapse of the reset back-off time. 
     In this case, because the second shortest back-off time is one of the server  10 C, the elapsed-time measuring unit  133  of the server  10 C determines that the back-off time has passed next. The transmitting unit  101  of the server  10 C then transmits data  230  to the access point  20 . At this time, a shaded area of the period  226  and a shaded area of the period  228  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A and  10 B, respectively. 
     The access point  20  receives the data  230 . Subsequently, the access point  20  transmits ACK  231  to the server  10 C, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 C receives ACK  231  from the access point  20 . The packet analyzing unit  105  of the server  10 C notifies the resetting unit  134  reception of ACK  231 . The resetting unit  134  of the server  10 C resets a period  234  of 3×slot time, which is the resetting value, to the elapsed-time measuring unit  133 . 
     In this case, the elapsed-time measuring unit  133  of the server  10 A sets a period  232  that is a carried over period, 1×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 B sets a period  233  that is a carried over period, 2×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 A is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 A determines that the back-off time has passed, the transmitting unit  101  of the server  10 A transmits data  235  to the access point  20 . At this time, a shaded area of the period  233  and a shaded area of the period  234  are carried over to following measurement of elapsed time as the back-off times of the servers  10 B and  10 C, respectively. 
     The access point  20  receives the data  235 . Subsequently, the access point  20  transmits ACK  236  to the server  10 A, if the channel does not become busy after waiting for the SIFS period in standby. 
     Next, a flow of the wireless-communication processing in the wireless module  100  according to the present embodiment is explained with reference to  FIG. 7 .  FIG. 7  is a flowchart of a wireless-communication processing in the wireless module according to the first embodiment. 
     An initial value and a setting value are set in the setting register  131  (step S 1 ). 
     When a channel shifts to a state in which no signal is detected from a busy state and then the SIFS period passes without the channel shifting to the busy state, the initial-value setting unit  132  acquires an initial value of the back-off time from the setting register  131 . The initial-value setting unit  132  sets the initial value in the elapsed-time measuring unit  133  as the back-off time (step S 2 ). 
     The determining unit  104  determines whether the channel is an idle state (step S 3 ). When the channel is not in an idle state (step S 3 : NO), it is waited in standby until the channel becomes idle. 
     On the other hand, when the channel is in an idle state (step S 3 : YES), the determining unit  104  determines whether the DIFS period has passed (step S 4 ). When the DIFS period has not passed (step S 4 : NO), the determining unit  104  goes back to step S 3 . 
     On the other hand, when the DIFS period has passed (step S 4 : YES), the determining unit  104  notifies the elapsed-time measuring unit  133  of elapse of the DIFS period. In response to the notification of elapse of the DIFS period, the elapsed-time measuring unit  133  performs subtraction on the back-off time to measure elapsed time of the back-off time (step S 5 ). 
     The elapsed-time measuring unit  133  determines whether the back-off time has passed (step S 6 ). When the back-off time has not passed (step S 6 : NO), the wireless module  100  goes back to step S 3 . 
     On the other hand, when the back-off time has passed (step S 6 : YES), the resetting unit  134  determines whether transmission data is present (step S 7 ). When no transmission data is present (step S 7 : NO), the resetting unit  134  proceeds to step S 10 . 
     On the other hand, when transmission data is present (step S 7 : YES), the transmitting unit  101  transmits data to the access point  20  (step S 8 ). 
     Subsequently, the resetting unit  134  determines whether ACK is received (step S 9 ). When ACK is not received (step S 9 : NO), the resetting unit  134  waits in standby until ACK is received. 
     When no transmission data is present (step S 7 : NO), or when ACK is received (step S 9 : YES), the resetting unit  134  acquires a resetting value of back-off time from the setting register  131 . The resetting unit  134  then sets the resetting value in the elapsed-time measuring unit  133  as back-off time (step S 10 ). 
     The back-off-time setting unit  103  determines whether communication is completed (step S 11 ). When the communication is not completed (step S 11 : NO), the back-off-time setting unit  103  goes back to step S 3 . 
     On the other hand, when communication is completed (step S 11 : YES), the wireless module  100  ends the wireless communication processing. 
     As explained above, in the information processing apparatus according to the present embodiment, setting one slot time as the minimum value, and values obtained by increasing therefrom by one slot time each are assigned to respective wireless modules as the initial values of back-off time. Furthermore, the total number of the wireless modules is assigned to each of the wireless modules as a resetting value. Accordingly, the information processing apparatus according to the present embodiment can reduce standby time after a channel becomes idle, and can suppress occurrence of contention in communication. 
     Moreover, in the present embodiment, initial values and resetting values are selected so that standby time is to be the shortest. However, when there is allowance in standby time, the initial values could only be set so as not to overlap among the wireless modules, and the resetting values could only take a value longer than remaining back-off times of other wireless modules when resetting is performed in any of the wireless modules. Also when the initial values and the resetting values are thus set, occurrence of contention of communication can be suppressed. 
     [b] Second Embodiment 
       FIG. 8  is a configuration diagram illustrating another example of the information processing system. An information processing system according to the present embodiment differs in that each of the servers  10  is managed by an administration server 2 from the first embodiment. The server  10  according to the present embodiment is also indicated by the block diagram in  FIG. 2 . Moreover, the wireless module  100  according to the present embodiment is also indicated by the block diagram in  FIG. 3 . In the following, explanation of operation of respective components similar to that of the first embodiment is omitted. 
     In the information processing system according to the present embodiment, more than one unit of the server  10 , the access point  20 , and a network switch  30  are mounted in the rack 1. Moreover, the information processing system according to the present embodiment includes the administration server 2. 
     The administration server 2 is connected to the network switch  30 . Moreover, the access point  20  is connected to the network switch  30 . 
     The administration server 2 communicates with the servers  10  through the access point  20  and the network switch  30 . 
     The administration server 2 notifies initial values and resetting values of the back-off time to the servers  10  through the network switch  30  and the access point  20 . The administration server 2 controls the setting register  131  of the server  10  to store the initial values and setting values of the back-off time. In the following explanation, setting of the initial values and the resetting values by the administration server 2 may be collectively called “back-off time setting”. The administration server 2 performs back-off time setting for each of the servers  10 . For example, the administration server 2 performs back-off time setting for one of the servers  10 , and thereafter, in response to reception of ACK replied from the server  10 , performs back-off time setting for a next one of the servers  10 . 
     Although the administration server 2 is arranged externally from the rack 1 in the present embodiment, the administration server 2 may be mounted in the rack 1. 
     The access point  20  transmits a beacon to each of the servers  10  at appropriate timing, after the back-off time is set in all of the servers  10  connected to the access point  20 . 
     The receiving unit  102  of each of the servers  10  receives a back-off time setting from the administration server 2. The receiving unit  102  transmits the back-off time setting to the CPU  11  through the packet analyzing unit  105 , the I/F  106 , and the crossbar switch  14 . 
     The CPU  11  acquires the initial value and the resetting value that are designated by the back-off time setting, to store in the setting register  131 . Furthermore, the CPU  11  instructs the transmitting unit  101  to transmit ACK. 
     The transmitting unit  101  receives instruction to transmit ACK from the CPU  11 . The transmitting unit  101  then returns ACK to the administration server 2, after waiting for the SIFS period in standby. 
     Subsequently, each of the servers  10  starts data transmission processing after receiving a beacon. 
     Next, an overall flow of communication in the information processing system according to the present embodiment is explained with reference to  FIG. 9 .  FIG. 9  is a sequence diagram indicating communication in the information processing system according to the second embodiment. Here, a case in which the servers  10 A to  10 C are present as the servers  10  is explained. Moreover, one unit of the wireless module  100  is mounted in each of the servers  10 A to  10 C. 
     When operation of the information system is started, the administration server 2 transmits a back-off time setting  241  to the server  10 A. The back-off time setting  241  indicates that the initial value of back-off time is 1×slot time, and the resetting value is 3×slot time. 
     The server  10 A receives the back-off time setting  241  from the administration server 2, and stores in the setting register  131 . Specifically, the server  10 A receives a back-off time setting at the receiving unit  102 . The receiving unit  102  transmits the back-off time setting  241  to the CPU  11  through the packet analyzing unit  105 , the I/F  106 , and the crossbar switch  14 . The CPU  11  acquires the initial value and the resetting value that are designated in the back-off time setting, to store in the setting register  131 . The transmitting unit  101  of the server  10 A returns ACK  242  to the administration server 2, after waiting for the SIFS period in standby. 
     The administration server 2 waits for the SIFS period in standby after receiving ACK  242 , and then transmits a back-off time setting  243  to the server  10 B. The back-off time setting  243  indicates that the initial value of back-off time is 2×slot time, and the resetting value is 3×slot time. 
     The server  10 B receives the back-off time setting  243  from the administration server 2, and stores in the setting register  131 . The server  10 B returns ACK  244  to the administration server 2 after waiting for the SIFS period in standby. 
     The administration server 2 waits for the SIFS period in standby after receiving ACK  244 , and then transmits a back-off time setting  245  to the server  10 C. The back-off time setting  245  indicates that the initial value of back-off time is 3×slot time, and the resetting value is 3×slot time. 
     The server  10 C receives the back-off time setting  245  from the administration server 2, and stores in the setting register  131 . The server  10 C returns ACK  246  to the administration server 2 after waiting for the SIFS period in standby. 
     After setting of the back-off time by the administration server 2 is completed in all of the servers  10 A to  10 C, the access point  20  waits for the DIFS period in standby, and then transmits a beacon  247  to each of the servers  10 A to  10 C at appropriate timing. Upon receiving the beacon  247  from the access point  20 , the servers  10 A to  10 C wait for the DIFS period in standby after detecting that the channel is not busy, and then perform wireless communication processing confirming that the channel is in an idle state. 
     Because the remainder of the wireless communication processing indicated in  FIG. 9  is the same as that of the first embodiment, explanation thereof is omitted. Although a case in which all of the servers  10  have transmission data similarly to  FIG. 5  is indicated in  FIG. 9 , this is one example of communication. For example, when the server  10  having no transmission data is included, the wireless communication processing after the servers  10 A to  10 C receive the beacon  247  in  FIG. 9  is as the wireless communication processing indicated in  FIG. 6 . 
     As explained above, the information processing apparatus according to the present embodiment acquires an initial value and a resetting value of the back-off time from an administration unit, and performs communication using the initial value and the resetting value acquired. With such a configuration that the administration unit manages initial values and resetting values of respective nodes in a consolidated manner, when the number of the information processing apparatuses or the wireless modules, that is the number of nodes is changed, the administration unit can change an initial value and a resetting value promptly according to the change in the number of nodes. That is, also when change in the number of nodes occurs, reduction of standby time after a channel shifts to an idle state, and suppression of access contention can be appropriately achieved. 
     [c] Third Embodiment 
     Next, a third embodiment is explained. The servers  10  and the wireless modules  100  that are the information processing apparatuses according to the present embodiment change a traffic amount according to a transmission rate of each, and in that point, differ from those in the first embodiment. The server  10  according to the present embodiment is also indicated by the block diagram in  FIG. 2 . Moreover, the wireless module  100  according to the present embodiment is also indicated by the block diagram in  FIG. 3 . In the following explanation, explanation of respective components that perform similar operation as that of the first embodiment is omitted. 
     The setting register  131  according to the present embodiment holds information as illustrated in  FIG. 10  as initial values and resetting values of back-off time, for example.  FIG. 10  is a table indicating an example of the information stored in a setting register according to the third embodiment. The column of bit in a table  151  in  FIG. 10  indicates each bit of the setting register  131 . Moreover, the column of field in the table  151  indicates information on data stored in the corresponding bit. Furthermore, the column of explanation in the table  151  is for explaining what kind of data each of the stored data is. The table  151  is described taking a case in which eight units of the wireless modules  100  are connected to the access point  20  as an example. 
     Bit  24  to bit  31  of the setting register  131  are left as a reserved space. Moreover, in bit  21  to bit  23  of the setting register  131 , the transmission rate of node #8 is stored. Furthermore, in bit  18  to bit  20  of the setting register  131 , the transmission rate of node #7 is stored. Moreover, in bit  15  to bit  17  of the setting register  131 , the transmission rate of node #6 is stored. Furthermore, in bit  12  to bit  14  of the setting register  131 , the transmission rate of node #5 is stored. Moreover, in bit  9  to bit  11  of the setting register  131 , the transmission rate of node #4 is stored. Furthermore, in bit  6  to bit  8  of the setting register  131 , the transmission rate of node #3 is stored. Moreover, in bit  3  to bit  5  of the setting register  131 , the transmission rate of node #2 is stored. Furthermore, in bit  0  to bit  2  of the setting register  131 , the transmission rate of node #1 is stored. 
     Nodes #1 to #8 indicate the wireless modules  100  that have the respective numerics as node numbers. The node number is assigned to each of the wireless modules  100  in advance. 
     The transmission rate is indicated by three bits of the setting register  131  as indicated in  FIG. 10  in the present embodiment. For example, values from 1 to 7 can be expressed as a transmission rate such that when a bit string is 001, the transmission rate is 1, and when a bit string 010, the transmission rate is 2, or the like. The transmission rate is a rate that expresses a value of a transmission rate in ratio. For example, when the transmission rate is 2, communication is performed in an amount twice as much as a case in which the transmission rate is 1. Moreover, when the transmission rate is 7, communication is performed in an amount seven times as much as a case in which the transmission rate is 1. In the present embodiment, it is set such that the transmission rate of node #1 is 1. 
     The wireless module  100  to which I (I=1, 2, 3, 4, . . . ) is assigned as the node number is explained as “node #I.” 
     The initial-value setting unit  132  acquires the transmission rate corresponding to each node number from the setting register  131 . When I=1, that is, in the case of node #1, the initial-value setting unit  132  sets an initial value to 1×slot time. Moreover, when I is 2 or larger, the initial-value setting unit  132  of node #I divides the initial value of the node #(I−1) by slot time, and sets a value obtained by multiplying a sum of a result of division and the transmission rate of node #(I−1) by the slot time, to the initial value. That is, the initial value I of node #I is expressed by following equation (1). t, 21   
     Upon receiving a beacon from the access point  20 , the initial-value setting unit  132  of each of the wireless modules  100  sets the determined initial value in the elapsed-time measuring unit  133  as the back-off time. 
     The resetting unit  134  acquires the transmission rate corresponding to each node number from the setting register  131 . Next, the resetting unit  134  of node #I sums the transmission rates of the respective nodes, subtracts a value obtained by subtracting 1 from the transmission rate of the node #I of itself from the sum, and sets a value obtained by multiplying the result of subtraction by the slot time, to the resetting value. That is, a resetting value I of node #I is expressed by following equation (2). 
     
       
         
           
             
               
                 
                   
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     When the back-off time has passed, the resetting unit  134  determines whether transmission data is present. When no transmission data is present, the resetting unit  134  immediately performs following resetting processing of the back-off time. On the other hand, when transmission data is present, the resetting unit  134  waits for reception of ACK from the access point  20 , and performs the following resetting processing of the back-off time. 
     The resetting processing performed by the resetting unit  134  is explained. The resetting unit  134  determines whether communication is performed successively for the number of times according to the transmission rate of the node of itself. The number of times according to the transmission rate of the node of itself is, for example, a value of the transmission rate of the node of itself. For example, when the transmission rate of the node of itself is 3, the number of times according to the transmission rate of the node of itself is three times. That is, in this case, the resetting unit  134  determines whether communication is performed three time successively. 
     When communication is not performed successively for the number of times according to the transmission rate of the node of itself, the resetting unit  134  sets 1×slot time in the elapsed-time measuring unit  133 . That is, the resetting unit  134  repeats setting 1×slot time in the elapsed-time measuring unit  133  until communication is performed for the number of times corresponding to the value of the transmission rate, that is for the number of times corresponding to a value obtained by subtracting 1 from the value of the transmission rate. For example, when the transmission rate is 3, the resetting unit  134  sets 1×slot time twice in the elapsed-time measuring unit  133  after the initial value of the back-off time has passed and communication is performed. 
     When communication is performed successively for the number of times corresponding to the value of the transmission rate, the resetting unit  134  sets the resetting value calculated using equation (2) in the elapsed-time measuring unit  133  as the back-off time. 
     Next, an overall flow of communication in the information processing system according to the present embodiment is explained with reference to  FIG. 11 .  FIG. 11  is a sequence diagram indicating communication in the information processing system according to the third embodiment. Here, a case in which servers  10 A to  10 D are present as the servers  10  is explained. Moreover, one unit of the wireless module  100  is mounted in each of the servers  10 A to  10 D. That is, because the servers  10 A to  10 D and the wireless modules  100  are arranged in one-to-one correspondence, each of the servers  10 A to  10 D is explained as one node. Moreover, a case in which all of the servers  10 A to  10 D have transmission data is explained here. 
     The node number of the server  10 A is 1, the node number of the server  10 B is 2, the node number of the server  10 C is 3, and the node number of the server  10 D is 4. 
     The transmission rate of the servers  10 A and  10 B is 1, the transmission rate of the server  10 C is 2, and the transmission rate of the server  10 D is 3. 
     Because the server  10 A is node #1, the initial-value setting unit  132  of the server  10 A sets the initial value to 1×slot time based on equation (1). 
     Furthermore, the initial-value setting unit  132  of the server  10 B calculates an initial value based on the equation (1) by adding the transmission rate (=1) of node #1 to a value (=1) obtained by dividing the initial value of node #1 by the slot time, and multiplying the result of addition by the slot time. That is, the initial-value setting unit  132  of the server  10 B sets 2×slot time to the initial value. 
     Moreover, the initial-value setting unit  132  of the server  10 C calculates an initial value based on the equation (1) by adding the transmission rate (=1) of node #2 to a value (=2) obtained by dividing the initial value of node #2 by the slot time, and multiplying the result of addition by the slot time. That is, the initial-value setting unit  132  of the server  10 C sets 3×slot time to the initial value. 
     Furthermore, the initial-value setting unit  132  of the server  10 D calculates an initial value based on the equation (1) by adding the transmission rate (=2) of node #3 to a value (=3) obtained by dividing the initial value of node #3 by the slot time, and multiplying the result of addition by the slot time. That is, the initial-value setting unit  132  of the server  10 D sets 5×slot time to the initial value. 
     The servers  10 A to  10 D wait for the DIFS period in standby after the channel has shifted from the busy state. If the channel does not become busy while waiting in standby, the servers  10 A to  10 D set the initial value of the back-off time in the respective elapsed-time measuring units  133 . 
     Specifically, the initial-value setting unit  132  of the server  10 A sets the initial value of the back-off time expressed by a period  251 , that is 1×slot time, in the elapsed-time measuring unit  133  as the back-off time. The initial-value setting unit  132  of the server  10 B sets the initial value of the back-off time expressed by a period  252 , that is 2×slot time, in the elapsed-time measuring unit  133  as the back-off time. The initial-value setting unit  132  of the server  10 C sets the initial value of the back-off time expressed by a period  253 , that is 3×slot time, in the elapsed-time measuring unit  133  as the back-off time. The initial-value setting unit  132  of the server  10 D sets the initial value of the back-off time expressed by a period  254 , that is 5×slot time, in the elapsed-time measuring unit  133  as the back-off time. 
     In this state, the back-off time of the server  10 A is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 A determines that the back-off time has passed, the transmitting unit  101  of the server  10 A transmits data  255  to the access point  20 . At this time, shaded areas of the periods  252 ,  253 , and  254  are carried over to following measurement of elapsed time as the back-off times of the servers  10 B and  10 D, respectively. That is, the elapsed-time measuring unit  133  of the server  10 B carries over 1×slot time to next measurement of elapsed time. Moreover, the elapsed-time measuring unit  133  of the server  10 C carries over 2×slot time to next measurement of elapsed time. Furthermore, the elapsed-time measuring unit  133  of the server  10 D carries over 4×slot time to next measurement of elapsed time. 
     The access point  20  receives the data  255 . Subsequently, the access point  20  transmits ACK  256  to the server  10 A, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 A receives ACK  256  from the access point  20 . The packet analyzing unit  105  of the server  10 A notifies the resetting unit  134  of reception of ACK  256 . 
     The resetting unit  134  of server  10 A acquires the transmission rate of each node from the setting register  131 , and calculates a resetting value using equation (2). Specifically, the resetting unit  134  acquires a sum of the transmission rates of the respective node. In this example, the sum of the transmission rates of the respective nodes is 7. Next, the resetting unit  134  subtracts a value obtained by subtracting 1 from the transmission rate of the node of itself from the sum of the transmission rates. In this example, because the transmission rate of the server  10 A is 1, a value that is obtained by subtracting the value obtained by subtracting 1 from the transmission rate of the node of itself from the sum of the transmission rates is 7. Subsequently, the resetting unit  134  calculates a resetting value by multiplying the calculated value by the slot time. That is, the resetting unit  134  of the server  10 A calculates 7×slot time as the resetting value. 
     The resetting unit  134  of the server  10 A resets a period  257  of 7×slot time, which is the resetting value, in the elapsed-time measuring unit  133  as the back-off time. 
     In this case, the elapsed-time measuring unit  133  of the server  10 B sets a period  258  that is the carried over period of 1×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 C sets a period  259  that is the carried over period of 2×slot time, as the back-off time. Furthermore, the elapsed-time measuring unit  133  of the server  10 D sets a period  260  that is the carried over period of 4×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 B is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 B determines that the back-off time has passed, the transmitting unit  101  of the server  10 B transmits data  261  to the access point  20 . At this time, shaded areas of the periods  257 ,  259 , and  260  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A,  10 C and  10 D, respectively. 
     The access point  20  receives the data  261 . Subsequently, the access point  20  transmits ACK  262  to the server  10 B, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 B receives ACK  262  from the access point  20 . The packet analyzing unit  105  of the server  10 B notifies the resetting unit  134  of reception of ACK  262 . 
     The resetting unit  134  of the server  10 B acquires the transmission rate of each node from the setting register  131 , and calculates a resetting value using equation (2). Specifically, the resetting unit  134  subtracts a value obtained by subtracting 1 from the transmission rate of the node of itself from the sum of the transmission rates. In this example, because the transmission rate of the server  10 B is 1, a value that is obtained by subtracting the value obtained by subtracting 1 from the transmission rate of the node of itself from the sum of the transmission rates is 7. Subsequently, the resetting unit  134  calculates a resetting value by multiplying the calculated value by the slot time. That is, the resetting unit  134  of the server  10 B calculates 7×slot time as the resetting value. 
     The resetting unit  134  of the server  10 B resets a period  264  of 7×slot time, which is the resetting value, in the elapsed-time measuring unit  133  as the back-off time. 
     In this case, the elapsed-time measuring unit  133  of the server  10 A sets a period  263  that is the carried over period of 6×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 C sets a period  265  that is the carried over period of 1×slot time, as the back-off time. Furthermore, the elapsed-time measuring unit  133  of the server  10 D sets a period  266  that is the carried over period of 3×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 C is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 C determines that the back-off time has passed, the transmitting unit  101  of the server  10 C transmits data  267  to the access point  20 . At this time, shaded areas of the periods  263 ,  264 ,  266  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A,  10 B and  10 D, respectively. 
     The access point  20  receives the data  267 . Subsequently, the access point  20  transmits ACK  268  to the server  10 C, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 C receives ACK  268  from the access point  20 . The packet analyzing unit  105  of the server  10 C notifies the resetting unit  134  of reception of ACK  268 . 
     The resetting unit  134  of the server  10 C receives the notification of reception of ACK  268  from the packet analyzing unit  105 . In this case, because the transmission rate of the node of itself is 2, the resetting unit  134  of the server  10 C sets a period  271  of 1×slot time in the elapsed-time measuring unit  133  as the back-off time, for the second data transmission. 
     In this case, the elapsed-time measuring unit  133  of the server  10 A sets a period  269  that is the carried over period of 5×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 B sets a period  270  that is the carried over period of 6×slot time, as the back-off time. Furthermore, the elapsed-time measuring unit  133  of the server  10 D sets a period  272  that is the carried over period of 2×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 C is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 C determines that the back-off time has passed, the transmitting unit  101  of the server  10 C transmits data  273  to the access point  20 . At this time, shaded areas of the periods  269 ,  270 , and  272  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A,  10 B and  10 D, respectively. 
     The access point  20  receives the data  273 . Subsequently, the access point  20  transmits ACK  274  to the server  10 C, if the channel does not become busy after waiting for the SIFS period in standby. 
     The receiving unit  102  of the server  10 C receives ACK  274  from the access point  20 . The packet analyzing unit  105  of the server  10 C notifies the resetting unit  134  of reception of ACK  274 . 
     Because the transmission rate of the node of itself is 2 and data transmission is performed twice successively, the resetting unit  134  of the server  10 C acquires the transmission rate of each node from the setting register  131 , and calculates a resetting value using equation (2). Specifically, the resetting unit  134  subtracts a value obtained by subtracting 1 from the transmission rate of the node of itself from a sum of the transmission rates. In this example, because the transmission rate of the server  10 C is 2, a value that is obtained by subtracting the value obtained by subtracting 1 from the transmission rate of the node of itself from the sum of the transmission rates is 6. Subsequently, the resetting unit  134  calculates a resetting value by multiplying the calculated value by the slot time. That is, the resetting unit  134  of the server  10 C calculates 6×slot time as the resetting value. 
     The resetting unit  134  of the server  10 C resets a period  277  of 6×slot time, which is the resetting value, in the elapsed-time measuring unit  133  as the back-off time. 
     In this case, the elapsed-time measuring unit  133  of the server  10 A sets a period  275  that is the carried over period of 4×slot time, as the back-off time. Moreover, the elapsed-time measuring unit  133  of the server  10 B sets a period  276  that is the carried over period of 5×slot time, as the back-off time. Furthermore, the elapsed-time measuring unit  133  of the server  10 D sets a period  278  that is the carried over period of 1×slot time, as the back-off time. 
     In this state, the back-off time of the server  10 D is the shortest. Therefore, when the elapsed-time measuring unit  133  of the server  10 D determines that the back-off time has passed, the transmitting unit  101  of the server  10 D transmits data  279  to the access point  20 . At this time, shaded areas of the periods  275 ,  276 , and  277  are carried over to following measurement of elapsed time as the back-off times of the servers  10 A to  10 C, respectively. 
     The access point  20  receives the data  279 . Subsequently, the access point  20  transmits ACK  280  to the server  10 C, if the channel does not become busy after waiting for the SIFS period in standby. Thereafter, the wireless communication processing explained herein is to be repeated. 
     While in  FIG. 11 , a case in which all of the server  10 A to  10 D have transmission data is illustrated, when the server  10  having no transmission data is included, the transmitting unit  101  does not perform data transmission. In this case, the resetting unit  134  repeats setting 1×slot time as the back-off time until the number of times of communication reaches the number of times corresponding to the value of the transmission rate, and then resets the calculated resetting value in the elapsed-time measuring unit  133 , without waiting for reception of ACK. Moreover, when transmission data is generated while performing the resetting processing without waiting for reception of ACK, the resetting unit  134  shifts to processing in which the resetting processing is performed waiting until ACK is received. 
     As explained above, the information processing apparatus according to the present embodiment sets a transmission rate for each node, and determines an initial value and a resetting value of the back-off time according to the transmission rate of each of the information processing apparatuses such that contention of communication does not occur. Accordingly, even when transmission rates differ for respective nodes, standby time after a channel shifts to an idle state can be reduced, and occurrence of contention of communication can be suppressed. 
     [d] Fourth Embodiment 
     Next, an information processing apparatus according to a fourth embodiment is explained. The information processing system according to the present embodiment differs from that of the third embodiment in that each of the servers  10  and the wireless modules  100  are managed by the administration server 2. The information processing system according to the present embodiment is expressed by a configuration diagram in  FIG. 8 . Moreover, the server  10  according to the present embodiment is also indicated by the block diagram in  FIG. 2 . Furthermore, the wireless module  100  according to the present embodiment is also indicated by the block diagram in  FIG. 3 . In the following explanation, explanation of operation of respective components similar to that of the first embodiment is omitted. 
     The administration server 2 stores transmission rates of each of the wireless modules  100 . The administration server 2 makes the setting register  131  of each of the wireless modules  100  store information on the transmission rates of the respective wireless modules  100  through the network switch  30  and the access point  20 . The setting register  131  stores the information on the transmission rates of the respective wireless modules  100 , for example, in a state as illustrated  FIG. 10 . 
     Furthermore, after transmitting the information on the transmission rates to each of the wireless modules  100  and waiting for the SIFS period in standby, the administration server 2 notifies a node number to each of the wireless modules  100 . Specifically, the administration server 2 repeats processing of notifying a node number to one of the wireless module  100 , receiving ACK from the wireless module  100  to which the node number is notified after the SIFS period passes, and notifying a node number to a next one of the wireless module  100 . 
     Although the administration server 2 is arranged externally from the rack 1 in the present embodiment, the administration server 2 may be mounted in the rack 1. 
     The access point  20  transmits a beacon to each of the wireless modules  100  at appropriate timing, after a back-off time is set in all the wireless modules  100  connected to the access point  20 . 
     Subsequently, each of the wireless modules  100  starts data transmission processing after receiving a beacon. Thereafter, each of the wireless modules  100  calculates initial values and resetting values according to the transmission rates of the respective nodes, similarly to the third embodiment, and performs data transmission. 
     Next, an overall flow of communication in the information processing system according to the present embodiment is explained with reference to  FIG. 12 .  FIG. 12  is a sequence diagram indicating communication in the information processing system according to the fourth embodiment. Here, a case in which servers  10 A to  10 D are present as the servers  10  is explained. Moreover, one unit of the wireless module  100  is mounted in each of the servers  10 A to  10 D. 
     When operation of the information system is started, the administration server 2 transmits transmission rate information  300  including information on the transmission rate of each node to each of the servers  10 A to  10 D. The transmission rate information  300  indicates that the transmission rate of node #1 and node #2 is 1, the transmission rate of node #3 is 2, and the transmission rate of node #4 is 3. 
     The administration server 2 waits for the SIFS period in standby after transmitting the transmission rate information  300 , and if the channel does not become busy while waiting in standby, transmits node number information  301  to the server  10 A. In this example, the node number of the server  10 A is 1. 
     The receiving unit  102  of the server  10 A receives a signal including the node number information  301 . The receiving unit  102  outputs the received signal to the packet analyzing unit  105 . The packet analyzing unit  105  analyzes the signal and notifies the node number information  301  to the initial-value setting unit  132  and the resetting unit  134 . Subsequently, the packet analyzing unit  105  notifies the node number to the CPU  11  through the I/F  106  and the crossbar switch  14 . In response to reception of the signal, the CPU  11  instructs the transmitting unit  101  to transmit ACK through the crossbar switch  14  and the I/F  106 . The transmitting unit  101  transmits ACK  302  to the administration server 2 through the access point  20  and the network switch  30  after the SIFS period passes. 
     Upon receiving ACK  302 , the administration server 2 waits for the SIFS period in standby, and if the channel does not become busy while waiting in standby, transmits node number information  303  to the server  10 B. In this example, the node number of the server  10 B is 2. 
     The receiving unit  102  of the server  10 B receives a signal including node number information  301 . Thereafter, the server  10 B performs the same processing as the server  10 A. The transmitting unit  101  transmits ACK  304  to the administration server 2 through the access point  20  and the network switch  30  after the SIFS period passes. 
     Upon receiving ACK  304 , the administration server 2 waits for the SIFS period in standby, and if the channel does not become busy while waiting in standby, transmits node number information  305  to the server  10 C. In this example, the node number of the server  10 C is 3. 
     The receiving unit  102  of the server  10 C receives a signal including the node number information  305 . Thereafter, the server  10 C performs the same processing as the server  10 A. The transmitting unit  101  transmits ACK  306  to the administration server 2 through the access point  20  and the network switch  30  after the SIFS period passes. 
     Upon receiving ACK  306 , the administration server 2 waits for the SIFS period in standby, and if the channel does not become busy while waiting in standby, transmits node number information  307  to the server  10 D. In this example, the node number of the server  10 D is 4. 
     The receiving unit  102  of the server  10 D receives a signal including the node number information  307 . Thereafter, the server  10 D performs the same processing as the server  10 A. The transmitting unit  101  transmits ACK  308  to the administration server 2 through the access point  20  and the network switch  30  after the SIFS period passes. 
     The access point  20  waits for elapse of the DIFS period, after the notification of the node numbers to the servers  10 A to  10 D by the administration server 2 is completed, and after ACK  308  is transmitted from the server  10 D. The access point  20  transmits a beacon  309  to each of the servers  10 A to  10 D at appropriate timing after the DIFS period passes. Upon receiving the beacon  309  from the access point  20 , the servers  10 A to  10 D wait for the DIFS period in standby after detecting that the channel is not busy, and then perform wireless communication processing confirming that the channel is in an idle state. Because the remainder of the wireless communication processing indicated in  FIG. 12  is the same as that of the third embodiment, explanation thereof is omitted. 
     As explained above, the information processing apparatus according to the present embodiment acquires a transmission rate of each node from the administration unit, and performs communication using an initial value and a resetting value of the back-off time that are calculated using the acquired transmission rate. With such a configuration that the administration unit manages transmission rates of respective nodes in a consolidated manner, even if the transmission rates differ for the respective nodes, when the number of nodes is changed, the administration unit can change an initial value and a resetting value promptly according to the change. That is, even when change in the number of nodes occurs in a state in which the transmission rates of the respective nodes differ from each other, reduction of the standby time after a channel shifts to an idle state, and suppression of access contention can be appropriately achieved. 
     According to one embodiment of a communication apparatus, the communication-apparatus control method, a communication-apparatus control program, an information processing apparatus, and an information processing system, there is an effect that occurrence of access contention can be suppressed while reducing a standby time. 
     All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.