Abstract:
There are provided an apparatus and a method for establishing a contention window in a WiMedia wireless network. The contention window establishing apparatus includes: a parsing unit extracting the number of devices that intend to transmit data from a beacon received during a beacon period and an access category (AC) of each device; a calculation unit calculating a transmission probability of each access category (AC) on the basis of the extracted number of devices; and a contention window establishing unit establishing a contention window for each access category (AC) on the basis of the calculated transmission probability, to reduce contentions among the devices and decrease data transmission latency time, thereby maximizing efficiency in data transmission latency time.

Description:
RELATED APPLICATIONS 
     The present application is a continuation of application Ser. No. 13/092,497, filed Apr. 22, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety, which claims the priority of Korean Patent Application No. 10-2010-0093883 filed on Sep. 28, 2010, in the Korean Intellectual Property Office, the disclosure of which is also incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to establishing a contention window, and more particularly, to an apparatus and a method for establishing a contention window in a WiMedia wireless network able to reduce data transmission latency time and maximize data transmission efficiency by establishing an optimal contention window for each access category (AC) on the basis of the number of devices and access categories (AC) acquired from a beacon which has been received during a beacon period (BP), in a prioritized contention access (PCA) method of a WiMedia wireless network. 
     2. Description of the Related Art 
     In general, a beacon group is a wireless local network configured to have only WiMedia devices without a coordinator controlling the devices. All of the devices in the network need to transmit beacons during a beacon period (BP) and other devices also need to transmit the beacons. Through these beacons, the WiMedia devices may detect all devices adjacent thereto and be synchronized with the adjacent devices. 
     Meanwhile, the transmitting and receiving of data between devices in WiMedia network is performed by a distributed reservation protocol (DRP) method and a prioritized contention access (PCA) method. 
     The DRP method is a non-contention method in which, when a data transmitting device requests a reservation, a data receiving device determines request permission on the basis of its own state and a reservation state within the beacon group. 
     In contrast, the PCA method is a contention method in which a media acquiring device that transmits data through the contentions of devices during a transmission opportunity (TXOP) period. Since the PCA method is the contention method, the contentions of the devices included in the beacon group rapidly concentrated at a starting point of the PCA period. In this case, in the PCA method according to the related art, a back-off process is performed by random contention windows from the minimum window (CWmin) to the maximum window (CWmax). As a result, a predetermined bandwidth may not be efficiently used. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides an apparatus and a method for establishing a contention window capable of maximizing the data transmission efficiency by reducing contentions among devices and decreasing data transmission latency time. 
     According to an aspect of the present invention, there is provided an apparatus for establishing a contention window in a WiMedia wireless network, the apparatus including: a parsing unit extracting the number of devices that intend to transmit data from a beacon received during a beacon period and an access category (AC) of each device; a calculation unit calculating a transmission probability of each access category (AC) on the basis of the extracted number of devices; and a contention window establishing unit establishing a contention window for each access category (AC) on the basis of the calculated transmission probability. 
     The access category (AC) may be included in a reserved field of an information element (IE) in the beacon. 
     The parsing unit may calculate the transmission probability of each access category (AC) by the following equation:
 
 P   BE   =P   BK =1/θ,  P   VI   =β/θ, P   VO =α/θ, where
     P BE  represents a transmission probability of Best-Effort (BE),   P BK  represents a transmission probability of Background (BK),   P VI  represents a transmission probability of Video (VI), P VO  represents a transmission probability of Voice (VO), θ is expressed by α×N VO +β×N VI +N BE +N BK , N VO  represents the number of devices that intend to transmit VO, N VI  represents the number of devices that intend to transmit VI, N BE  represents the number of devices that intend to transmit BE, N BK  represents the number of devices that intend to transmit BK, α is 4, and β is 2.   

     Further, the contention window establishing unit may established the contention window for each access category (CA) by the following equation: CW VO =(2θ/α)−2, CW VI =(2θ/β)−2, CW BE =CW BK =2θ−2, where CW VO  represents a contention window value of VO, CW VI  represents a contention window value of VI, CW BE  represents a contention window value of BE, CW BK  represents a contention window value of BK, θ is expressed by α×N VO +β×N VI +N BE +N BE , N VO  represents the number of devices that intend to transmit VO, N VI  represents the number of devices that intend to transmit VI, N BE  represents the number of devices that intend to transmit BE, N BK  represents the number of devices that intend to transmit BK, α is 4, and β is 2. 
     According to another aspect of the present invention, there is provided a method for establishing a contention window in a WiMedia wireless network, the method including: (a) extracting the number of devices that intend to transmit data from beacon received during a beacon period and an access category (AC) of each device; (b) calculating a transmission probability of each access category (AC) on the basis of the extracted number of devices; and (C) establishing a contention window for each access category (AC) on the basis of the calculated transmission probability. 
     The access category (AC) may be included in a reserved field of an information element (IE) in the beacon. 
     In step (b), the transmission probability of each access category (AC) may be calculated by the following equation: P BE =P BK =1/θ, P VI =β/θ, P VO =α/θ, where 
     P BE  represents a transmission probability of Best-Effort (BE), P BK  represents a transmission probability of Background (BK), P VI  represents a transmission probability of Video (VI), P VO  represents a transmission probability of Voice (VO), θ is expressed by α×N VO +β×N VI +N BE +N BK , N VO  represents the number of devices that intend to transmit VO, N VI  represents the number of devices that intend to transmit VI, N BE  represents the number of devices that intend to transmit BE, N BK  represents the number of devices that intend to transmit BK, α is 4, and β is 2. 
     In step (c), the contention window is established for each access category (AC) by the following equation:
 
CW VO =(2θ/α)−2, CW VI =(2θ/β)−2, CW BE =CW BK =2θ−2,
 
where CW VO  represents a contention window value of VO, CW VI  represents a contention window value of VI, CW BE  represents a contention window value of BE, CW BK  represents a contention window value of BK, θ is expressed by α×N VO +β×N VI +N BE +N BK , N VO  represents the number of devices that intend to transmit VO, N VI  represents the number of devices that intend to transmit VI, N BE  represents the number of devices that intend to transmit BE, N BK  represents the number of devices that intend to transmit BK, α is 4, and β is 2.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a configuration diagram of an apparatus for establishing a contention window according to an exemplary embodiment of the present invention; 
         FIG. 2  is a diagram showing an IE including an access category (AC) according to an exemplary embodiment of the present invention; 
         FIG. 3  is a conceptual diagram of a method for establishing a contention window according to an exemplary embodiment of the present invention; 
         FIG. 4  is a graph comparing the performance of a network when establishing a contention window by a general PCA method with the performance of a network when establishing a contention window according to the present invention; and 
         FIG. 5  is a flowchart describing a method for establishing a contention window according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like reference numerals refer to like elements in the accompanying drawings. 
       FIG. 1  is a configuration diagram of an apparatus  100  for establishing a contention window according to an exemplary embodiment of the present invention and may include a beacon receiving unit  110 , a parsing unit  120 , a calculation unit  130 , and a contention window establishing unit  140 . Meanwhile,  FIG. 2  is a diagram showing an information element (IE) including an access category (AC) according to an exemplary embodiment of the present invention. In addition,  FIG. 3  is a conceptual diagram of a method for establishing a contention window according to an exemplary embodiment of the present invention.  FIG. 4  is a graph comparing the performance of a network when establishing a contention window by a general PCA method and the performance of a network when establishing a contention window according to the present invention. Hereinafter, the present invention will be referred to as a distributed and dynamic adaptation scheme (DDAS). 
     Referring to  FIG. 1 , the beacon receiving unit  110  receives beacons from other devices that exist in a WiMedia network and transmits the received beacons to the parsing unit  120 . 
     The parsing unit  120  analyzes the beacons transmitted from the beacon receiving unit  1120  and thereafter, extracts the number of devices and the access category (AC) of the corresponding device. The number of devices and the access category (AC) of the corresponding device that are extracted are transmitted to the calculation unit  130 . The access category (AC) may include Best-Effort (BE), Background (BK), Video (VI), and Voice (VO). 
     In general, for WiMedia communication to occur, all devices existing in a network should transmit beacons for a beacon period and other devices should receive the beacons. Numbers from 0 are sequentially granted to beacon slots included in the beacons. The devices scan the beacons and select a beacon slot, other than one of the beacon slots occupied by adjacent devices, and transmit the selected beacon slot by including its own communication mechanism in the information element (IE). Each device finds adjacent devices on the basis of the received IE and is synchronized with the adjacent devices. The used IE, as an IE having MAC capabilities, is transmitted while being included in the beacon for the general WiMedia communication. According to the present invention, the IE included in the beacon may include the access category (AC). That is, a reserved field of the IE including the access category (AC) is transmitted so as to use the existing resource efficiently. 
     The IE will be described in detail with reference to  FIG. 2 . 
     Referring to  FIG. 2 , the IE includes an element ID and a length, a MAC capability bitmap, and the like and in addition, has a reserved field to be used afterwards. According to the present invention, 4 bits of the reserved field are allocated as a region  200  for the access category (AC) and this region including the access category (AC) may be transmitted to other devices. 
     Referring back to  FIG. 1 , the calculation unit  130  calculates a transmission probability of the access category (AC) on the basis of the number of devices and the access category (AC) of the corresponding device that are received from the parsing unit  120 . The calculated transmission probability of the access category (AC) is transmitted to the contention window establishing unit  140 . 
     In detail, the calculation unit  130  calculates the transmission probability of each access category (AC) by Equation 1.
 
 P   BE   =P   BK =1/θ,  P   VI   =β/θ, P   VO =α/θ  [Equation 1]
 
     Herein, θ is α×N VO +β×N VI +N BE +N BK , N VO  is the number of devices of which the access category (AC) is VO, N VI  is the number of devices of which the category (AC) is VI, N BE  is the number of devices of which the category (AC) is BE, N BK  is the number of devices of which the category (AC) is BK, α may be 4, and β may be 2. 
     Hereinafter, a process of deriving Equation 1 will be described. 
     The access category (AC) applied to the PAC method of the WiMedia MAC may be divided into Voice (VO), Video (VI), Best-Effort (BE), and Background (BK). When the transmission probabilities of the access categories are P VO , P VI , P BE , P BK , a relationship of P VO &gt;P VI &gt;P BE =P BK  may be established according to the priority among the access categories (AC). The reason is that a contention window having a small value can be expressed as a high transmission probability because the transmission probability is influenced by a value of a contention window. Accordingly, a relationship between P VO  and P VI  may be established as shown in Equation 2 on the basis of P BE  which is a low priority.
 
 P   VO   =α×P   BE   , P   VI   =β×P   BE   [Equation 2]
 
     Herein, α and β are constants determined by the contention window using the PCA method and a condition of α&gt;β and β&gt;0.1 should be satisfied. Further, α=4 and β=2 are determined by considering the relationship among contention window values (i.e., Cwmin[VO]=4, Cwmax[VO]=256, Cwmin[VI]=8, Cwmax[VI]=512, Cwmin[BE, BK]=16, and Cwmax[BE, BK]=1023) defined in the existing PCA method. 
     Next, Equation 3 may be considered in order to acquire an optimum transmission probability.
 
 N   VO   ×P   VO   +N   VI   ×P   VI   +N   BE   ×P   BE   +N   BK   ×P   BK =1  [Equation 3]
 
     Herein, N VO , N VI , N BE , and N BK  are the numbers of devices of which the access categories are VO, VI, BE, and BK. 
     Equation 4 may be acquired from Equation 2 and Equation 3.
 
(α× N   VO   +β×N   VI   +N   BE   +N   BK )× P   BE =1  [Equation 4]
 
     Thereafter, by considering Equation 2 while assuming that α×N VO +β×N VI +N BE +N BK  is θ, Equation 1 may be acquired and the transmission probability of each access category (AC) may be calculated by Equation 1. 
     Meanwhile, the contention window establishing unit  140  establishes the contention window for each access category on the basis of the transmission probability of each access category (AC) transmitted from the calculation unit  130 . 
     In detail, the contention window establishing unit  140  may establish the contention window by Equation 5. 
     
       
         
           
             
               
                 
                   CWi 
                   = 
                   
                     ⌊ 
                     
                       
                         2 
                         Pi 
                       
                       - 
                       2 
                     
                     ⌋ 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     Herein, CW represents the contention window value, Pi represents the transmission probability for each access category acquired from Equation 1, and i may be BE, BK, VI, and VO. 
     Last, when the contention window value is expressed for each access category (AC) from Equation 5 and Equation 1, the contention window value may be expressed as shown in Equation 6.
 
CW VO =(2θ/α)−2, CW VI =(2θ/β)−2, CW BE =CW BK =2θ−2  [Equation 6]
 
     Herein, CW VO  represents a contention window value of VO, CW VI  represents a contention window value of VI, CW BE  represents a contention window value of BE, CW BK  represents a contention window value of BK, θ is expressed by α×N VO +β×N VI +N BE +N BK , N VO  represents the number of devices transmitting VO, N VI  represents the number of devices transmitting VI, N BE  represents the number of devices transmitting BE, N BK  represents the number of devices transmitting BK, α may be 4, and β may be 2. 
       FIG. 3  is a diagram for describing the contention window establishing method according to the exemplary embodiment of the present invention. 
     As shown in  FIG. 3 , for the WiMedia communication to occur, all devices that exist in the network transmit the beacons during a beacon period  301  from a beacon period start time  300  and other devices should receive the beacons. Beacon slot numbers (BSN)  302  from 0 are sequentially granted to beacon slots included in the beacon, and each of the devices scans the beacon and selects a beacon slot, other than one of those beacon slots occupied by adjacent devices, and transmits the selected beacon slot by including its own communication mechanism in the information element (IE). Thereafter, when it is detected that a busy medium  303  is terminated, a standby time AIFS[VO], AIFS[VI], or AIFS[BE, BK] as long as a predetermined time for each access category elapses according to a defer access  304  and thereafter, a backoff process is performed according to a value set in the contention window. Herein, in the backoff process, a terminal that defers data transmission until media are not being used does not start transmitting data just when AIFS is ended, but starts transmitting the data after it waits for a predetermined time according to the value set in the contention window. 
     According to the present invention, as shown in reference numeral  305  (and Equation 5), since the optimum contention window is established for each access category on the basis of the transmission probability considering the number of devices, contentions among the devices may be reduced and data transmission latency time may be decreased so as to maximize efficiency in data transmission as compared with the existing PCA method for establishing random contention windows from Cwmin to Cwmax. 
       FIG. 4  is a graph comparing the performance of a network in establishing a contention window by a general PCA method with the performance of a network in establishing a contention window according to the present invention and in  FIG. 4 , a horizontal axis represents the number of devices and a vertical axis represents transmission latency. 
     As shown in  FIG. 4 , assuming that devices of the same number exist in the WiMedia network, transmission latency, by the DDAS method of the present invention, may be remarkably decreased in respects to each of the access categories (VO, VI, BE, and BK) as compared with the existing PCA method. 
       FIG. 5  is a flowchart describing a method for establishing a contention window according to an exemplary embodiment of the present invention. For simplicity of the present invention, a duplicated description with the matters described above with reference to  FIGS. 1 to 4  will be omitted. 
     In step S 500 , a beacon receiving unit  110  receives beacons from other devices that exist in a WiMedia network and transmits the received beacons to a parsing unit  120 . 
     In step S 501 , the parsing unit  120  analyzes the beacons transmitted from the beacon receiving unit  110  and thereafter, extracts the number of devices and an access category (AC) of the corresponding device. The number of devices and the access category (AC) of the corresponding device that are extracted are transmitted to a calculation unit  130 . The access category (AC) may include Best-Effort (BE), Background (BK), Video (VI), and Voice (VO). 
     In step S 502 , the calculation unit  130  calculates a transmission probability of the access category (AC) on the basis of the number of devices and the access category (AC) of the corresponding device that are received from the parsing unit  120 . The calculated transmission probability of the access category (AC) is transmitted to a contention window establishing unit  140 . 
     Last, in step S 503 , the contention window establishing unit  140  establishes the contention window for each access category on the basis of the transmission probability of each access category (AC) transmitted from the calculation unit  130 . Thereafter, a backoff process is performed according to the established contention window. 
     As set forth above, it is possible to maximize the efficiency of data transmission by reducing contentions among devices and decreasing data transmission latency time by establishing an optimal contention window for each access category (AC) on the basis of the number of devices and an access category (AC) acquired from a beacon received during a beacon period (BP). 
     While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims. Accordingly, the scope of the present invention will be determined by the appended claims.