Abstract:
The present invention relates to a method for providing traffic differentiation in a wireless LAN, comprising the step of defining a plurality of access categories each associated with a priority value for the traffic it relates to. According to the present invention, said at least one access category relates to downlink traffic and at least one access category relates to uplink traffic.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method for providing traffic differentiation in a wireless LAN environment and to a corresponding wireless LAN station.  
         [0002]     802.11e standard related to wireless LANs provides for traffic differentiation in wireless LAN environment using the EDCA (Enhanced Distributed Channel Access) mechanism. EDCA mechanism relays on the defining of virtual queues inside a wireless LAN station (i.e. user terminal or access point). Each queue competes against the others to access the transmission medium. For controlling the transmission opportunity, each virtual queue is associated a so called access category consisting in a set of three parameters defined in IEEE 802.11e standard: first parameter is Contention Window (CW) parameter, second parameter is the Arbitration Inter Frame Space (AIFS) parameter, third parameter is the Transmission Opportunity Limit (TXOP_Limit) parameter. Setting these three parameters adequately enables it to define higher or lower priorities for the different virtual queues to access the transmission medium.  
         [0003]     Usually, the traffic differentiation is performed depending on the traffic type. For example, preferably, voice traffic may constitute one access category with a high transmission priority to ensure that real time constraints are respected, video traffic may constitute a second access category with a lower priority and data traffic a third category with a still lower priority so that best effort is applied to this type of traffic.  
         [0004]     The table below shows an example for the AIFS and CW values chosen for defining three access categories for three different types of traffic.  
                                           Access   Arbitration Inter   Contention           Category   Frame Space   Window   Type of Traffic       (AC)   (AIFS)   (CWmin:CWmax)   (TT)                   AC0   50 μs    3:31   Voice       AC1   50 μs   15:63   Video       AC2   50 μs    31:255   Data (best                   effort)                  
 
         [0005]     A drawback is that the traffic differentiation based on traffic type only does not provide an efficient traffic differentiation for achieving an optimal radio resource utilization and guarantying an acceptable quality of service.  
         [0006]     A particular object of the present invention is to provide a method of traffic differentiation in a wireless LAN environment providing for fair priorities to the different traffics.  
         [0007]     Another object of the invention is to provide a corresponding wireless LAN station.  
       SUMMARY OF THE INVENTION  
       [0008]     These objects, and others that appear below, are achieved by a method for providing traffic differentiation according to claim  1 , a wireless LAN station according to claim  6 .  
         [0009]     According to the present invention, the method consists in performing a traffic differentiation depending on the traffic direction (i.e. downlink or uplink traffic). In a 802.11 WLAN infrastructure, uplink traffic is usually defined as the traffic going from a user terminal to an access point and downlink traffic the traffic going from an access point to a user terminal.  
         [0010]     The method according to the present invention presents the advantage to reduce the unfairness between the uplink and the downlink direction. Indeed, it has been observed by simulation of the prior art methods that downlink traffic experiences higher delay and jitter and more generally worse performance than the uplink traffic inside an identical traffic type since downlink traffic is served by a single instance of the access protocol while in uplink there are as many protocol instances as the number of user terminals.  
         [0011]     The method according to the present invention further presents the advantage to provide a better use of the radio resources available in an 802.11 enabled wireless LAN resulting in a cost reduction and in a higher client satisfaction.  
         [0012]     Further advantageous features of the invention are defined in the dependent claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment given by way of non-limiting illustrations, and from the accompanying drawings, in which:  
         [0014]      FIG. 1  shows a wireless LAN system providing traffic differentiation according to the present invention;  
         [0015]      FIG. 2  shows an implementation of the method according to the present invention;  
         [0016]      FIG. 3  shows an implementation of a wireless LAN station according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  shows a wireless LAN system providing traffic differentiation according to the present invention. Wireless LAN system comprises a wireless LAN terminal  11  and a wireless LAN access point  12 .  
         [0018]     Additional non represented wireless LAN terminals and access points may also be part of the wireless LAN system.  
         [0019]     Wireless LAN terminal  11  and access point  12  exchange traffic of different types (e.g. voice, video, data). According to the present invention, the wireless LAN terminal and the access point differentiate between traffic sent in the uplink and traffic sent in the downlink. Access category AC 0  corresponds to downlink voice while access category AC 1  corresponds to uplink voice. Access category AC 2  corresponds to video and access category AC 3  corresponds to data. It will be understood by a person skilled in the art that this configuration corresponds to a non binding but advantageous example. It could be envisaged to define more than four access categories and to differentiate also video and data traffic type in uplink and downlink.  
         [0020]     According to the present invention, it is advantageous to associate different priorities to the different access categories in order to ensure a fair transmission of the different traffic.  
         [0021]     The priority of the different access categories is set with the help of three parameters related to an access category i.e. contention window (CW) parameter, Arbitration Inter Frame Space (AIFS) parameter, and Transmission Opportunity Limit (TXOP_Limit) parameter. Contention window parameter comprises a minimum limit CWmin and a maximum limit CWmax for the contention window size.  
         [0022]     Preferably, the contention window parameter, the Arbitration Inter Frame Space parameter, and the Transmission Opportunity Limit parameter are optimized using simulation tools in order to provide the best appropriate fairness for the different traffics.  
         [0023]     For example, when four access categories are defined comprising an access category for downlink voice with the highest priority, an access category for uplink voice with a lower priority, an access category for video with a still lower priority and an access category for data with a still lower priority the CW and AIFS parameters are preferably set as given in the next table.  
                                               Arbitration                   Inter Frame   Contention       Access Category   Space   Window   Type of Traffic       (AC)   (AIFS)   (CWmin:CWmax)   (TT)                   AC0   30 μs    3:255   Downlink Voice       AC1   50 μs    7:511   Uplink Voice       AC2   50 μs    31:1023   Downlink Data       AC3   70 μs    63:4095   Uplink Data                  
 
         [0024]     Preferably, the indication related to the access category of the transmitted traffic is contained in a header field of the frames transporting the traffic.  
         [0025]     Preferably, the field indicating the access category is a 2-bit field, enabling it to define four access categories as already foreseen in the IEEE 802.11e standard.  
         [0026]     Alternatively, as foreseen in the 802.11e Draft the access category is identified by a 4-bit value called TID (Traffic Identifier), the TID value is reported in the header field of the frames transporting traffic (4-bits) instead of the access category itself.  
         [0027]     Nevertheless, it will be understood by a person skilled in the art that this field may be larger if there is a need for defining additional access categories.  
         [0028]      FIG. 2  shows an implementation of the method according to the present invention. In an exemplary implementation, the method according to the present invention comprises following steps: 
        Step  21  consists in checking if the traffic to be transmitted is voice video or data traffic;     Step  22  consists in checking in case of voice traffic if this traffic is downlink or uplink traffic (i.e. if the traffic is sent by a WLAN terminal the traffic is uplink voice traffic and if the traffic is sent by a WLAN access point the traffic is downlink traffic);     Step  23  consists in granting an higher probability to downlink traffic than to uplink traffic in that a corresponding field of the header frames transporting the traffic is set to the corresponding access category.          
         [0032]      FIG. 3  shows an implementation of a wireless LAN station according to the present invention.  
         [0033]     Wireless LAN station comprises as many virtual queues  31  as defined access categories.  
         [0034]     In a preferred embodiment of the present invention, four access categories AC 0 , . . . , AC 3  are defined.  
         [0035]     Additionally the virtual queues are connected to a scheduler  32  which controls the access of the different virtual queues to the transmission medium according to the priority associated to the different virtual queues.  
         [0036]     According to the present invention, wireless LAN station comprises either one virtual queue AC 0  exclusively dedicated to downlink traffic or one virtual queue AC 1  exclusively dedicated to uplink traffic.  
         [0037]     In a preferred embodiment of the present invention, AC 0  is dedicated to voice downlink traffic and AC 1  to voice uplink traffic. Voice downlink traffic being granted an higher priority that voice uplink traffic.  
         [0038]     These two different virtual queues, although located on different types of wireless stations (user terminal or access point), grant prioritized access thanks to the different parameters associated to them.  
         [0039]     It will be understood by those skilled in the art that the wireless LAN station according to the present invention may be a wireless LAN terminal as well as a wireless LAN access point.