Patent Publication Number: US-8125969-B2

Title: Method and apparatus for adjusting EDCA channel access parameters

Description:
FIELD 
     This disclosed technology relates generally to Wireless Local Area Networks (WLANs) and more particularly to accessing communications channels in WLANs. 
     BACKGROUND 
     In a Wireless Local Area Network (WLAN) an Access Point (AP) can communicate with each of a plurality of member stations, over a communications channel. Each member station may be, for example, a cellular telephone, a portable computer, or any other wireless receiver and transmitter unit. 
     As defined by IEEE 802.11 WLAN standards, there are two main communications-channel access-methods by which a member station communicates with an Access Point (AP) in a Wireless Local Area Network (WLAN). One of these methods is scheduled access and the other is contention-based access. In a scheduled access approach, an Access Point (AP) polls each of its member stations and each member station can only transmit over a communications channel after being polled. Since only one member station communicates over a communications channel at a time, there can be no conflict between member stations, and therefore contention resolution algorithms are not needed. Thus in a scheduled access method, the overall efficiency is high and the Quality of Service (QoS) that each member station receives is guaranteed by a polling schedule. Unfortunately, the majority of WLAN hardware available today does not support scheduled access. 
     The majority of WLAN hardware today implement a contention-based method, in particular the Carrier Sense Multiple Access Collision Avoidance (CSMA-CA) method. In this contention based approach, a member station is not polled, but rather when the member station wants to transmit to the Access Point over the communications channel, it first senses whether the channel is available. If the channel is determined to be busy, the particular member station will wait for a waiting time period before attempting to transmit again. The waiting time period includes an Arbitration Inter-Frame Space (AIFS) followed by a Contention Window (CW). Only if the communications channel remains idle during the whole waiting time period may the particular member station begin its transmission over the communications channel to the Access Point. 
     The use of a waiting time period in a contention based approach reduces the possibility of transmission collisions: two member stations transmitting over a communications channel at the same time. However, the waiting time period results in delays in packet transmission from the member stations to the Access Point (AP). 
     The waiting time period in the contention based approach of CSMA-CA is determined by several parameters. The Arbitration Inter-Frame Space (AIFS) is determined by a configuration parameter called Arbitration Inter-Frame Space Number (AIFSN), which specifies the length of the AIFS in terms of time slots. The Contention Window (CW) for a particular transmission is determined randomly between 0 and an upper bound. The initial upper bound is a parameter called minimum CW, or CWmin. Every time a transmission fails, the upper bound is doubled until it reaches the maximum upper bound, which is the called CWmax. 
     The parameters AIFSN, CWmin, and CWmax have been set differently for different Quality of Service (QoS) classes. Setting theses parameters lower results in shorter wait times and a greater likelihood of access (i.e. a higher access priority) to a communications channel. For example, a voice data quality of service class might have lower AIFSN, CWmin, and CWmax values than a video quality of service class, resulting in a greater likelihood of transmission of voice data then video data over a communications channel. Setting these channel access parameters differently for different data traffic or communication classes is known as Enhanced Distributed Channel Access (EDCA) and the parameters AIFSN, CWmin, and CWmax can be called EDCA parameters. 
     EDCA offers no quality of service (QoS) guarantee to individual traffic streams. It only offers statistically higher channel access probability to communication traffic of higher Quality of service (QoS) classes. Traffic streams within the same service class still rely on contention based resolution methods to compete for channel access. Channel access is not efficient because of the time spent resolving contentions. As the number of traffic streams increases, such as with an increase in the number of member stations in a Wireless Local Area Network (WLAN), the number of traffic streams belonging to the same class will increase resulting in more contentions. In addition, since traffic streams of the same Quality of Service (QoS) class are treated the same, they have the same opportunity of transmission, which may not be desirable. 
     U.S. published patent application no. 2006/0187840 A1 to Cuffaro et al., discloses a method of modifying contention windows and Arbitration Inter-Frame Space (AIFS) for low priority traffic when congestion is detected on a wireless medium. A contention window associated with low priority traffic is incremented and a processor updates an EDCA parameter set included in frames transmitted by a transmitter. The size of the AIFS can be increased in parallel with the increase in the contention window. 
     SUMMARY 
     The disclosed technology, in one embodiment, provides a contention-based communications channel access method, which emulates scheduled access by dynamically updating Enhanced Distribution Channel Access (EDCA) parameters for groups of member stations in a Wireless Local Area Network (WLAN). 
     A Wireless Local Area Network (WLAN) may include a Basic Service Set (BSS) comprised of at least a first group of member stations and a second group of member stations. In one embodiment, either or both groups of member stations may have only a single member station. A method in accordance with an embodiment of the present disclosed technology, may further include setting EDCA parameters for the first group of member stations so that the first group of member stations has a first level of access priority to a communications channel during a first channel access throttling time period. The method may further include setting EDCA parameters for the second group of member stations so that the second group of member stations has a second level of access priority to the communications channel during the first channel access throttling time period. 
     During a second channel access throttling time period EDCA parameters for the second group of member stations may be set so that the second group of member stations has a third level of access priority to the communications channel. The EDCA parameters for the first group of member stations may be set during the second channel access throttling time period so that the first group of member stations has a fourth level of access priority to the communications channel. The first level of access priority may be higher than the second level of access priority, and the third level of access priority may be higher than the fourth level of access priority. 
     The first level of access priority may be equal to the third level of access priority, while the second level of access priority may be equal to the fourth level of access priority. The method may further include repeating the first channel access throttling time period and the second channel access throttling time period. 
     The Basic Service Set (BSS) may be comprised of a plurality of further groups of member stations. A group of member stations may be given a higher level of access priority while the other groups of member stations are given a lower level of access priority, during a particular channel access throttling time period. In this manner, each group of member stations is given a channel access throttling time period during which it will be much easier for the particular group of member stations to transmit over the communications channel than it will be for other groups of member stations. The EDCA parameters, which may be modified, may include an Arbitration Interframe Space Number, a minimum contention window, and a maximum contention window. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a high-level schematic of a Wireless Local Area Network (WLAN) with a single access point; 
         FIG. 2  shows a flow chart of a method in accordance with an embodiment of the present disclosed technology; 
         FIG. 3  shows a message or signal timing diagram; and 
         FIG. 4  shows a high level block diagram of a computer. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In one or more embodiments of the disclosed technology, a contention based communications channel access method is provided which emulates scheduled access by dynamically updating Enhanced Distribution Channel Access (EDCA) parameters for groups of member stations in a Wireless Local Area Network (WLAN). An access point (AP) performs an operation called “channel access throttling” (CAT), which increases or decreases the communications channel access probability of groups of member stations by adjusting their EDCA channel access parameters. 
       FIG. 1  shows a high-level schematic of a Wireless Local Area Network (WLAN)  100  with a single access point (AP)  104 . The WLAN  100  further includes member stations STA 1   106 , STA 2   108 , STA 3   110 , and STA 4   112 . The member stations  106 ,  108 ,  110 , and  112  are part of a Wireless Local Area Network cell called a Basic Service Set (BSS)  102 . The access point (AP)  104  communicates with each of the member stations  106 ,  108 ,  110 , and  112  over a wireless communications channel. The access point (AP)  104  communicates to a packet data network, details not shown, through a distribution system DS  114 . 
       FIG. 2  shows a flow chart  200  of a method in accordance with an embodiment of the present disclosed technology. The method of  FIG. 2  may be executed by the access point (AP)  104 . The access point (AP)  104  may be a computer, such as a computer  400  shown in  FIG. 4 . 
     At step  202  of the flow chart  200 , the access point (AP)  104  may set EDCA parameters for a specific group of member stations to high priority access. For example, the AP  104  may set EDCA parameters for a group of member stations which may include member station STA 1   106  and member station STA 2   108 , to high priority access. In step  204 , the access point (AP)  104  may set EDCA parameters for groups of member stations other than the specific group of member stations to low priority access. In this example, the AP  104  may set EDCA parameters for STA 3   110  and STA 4   112  to low priority access. 
     The AP  104  may then leave the EDCA parameters unchanged for a channel access throttling time period. After the channel access throttling time period has passed, the AP  104  may then select a different group of member stations to be the specific group of member stations. In this example, the AP  104  may next select a group of member stations including STA 3   110  and STA 4   112  to be the specific group of member stations. The method then loops back to step  202  and the EDCA parameters for the new specific group of member stations (STA 3   110  and STA 4   112 ) is set to high priority access while the groups other than the new specific group (STA 1   106 , and STA 2   108 ) is set to low priority access at step  204 . The method of  FIG. 2  can be repeated for a plurality of groups of member stations, and after each group has had the opportunity to have high priority access, while other member stations have low priority access, the method of  FIG. 2  can be cycled through again starting with the first group of member stations. The order in which each group of member stations receives higher priority access can also be changed. 
     The AP  104  may set EDCA parameters by including a special Information Element (IE) called the EDCA parameter set in its beacon messages or signals and in several other management messages or signals. The IE describes detailed configurations for EDCA channel related parameters such as Contention Window (CW) bounds for each Quality of Service (QoS) class. EDCA channel related parameters enclosed in the EDCA parameter set IE are applied to all member stations of the BSS. There is no field for including intended receivers for the intended receivers of the EDCA channel related parameters in the IE either. All member stations, such as stations STA 1   106 , STA 2   108 , STA 3   110 , and STA 4   112  shown in  FIG. 1 , receive the beacon message or signal and in response change their EDCA channel access parameters according to the IE in the beacon message or signal or other management messages or signals. The details of the Information Element (IE) are explained in the IEEE 802.11 standard. 
     The access point (AP)  104  may issue or transmit a special EDCA parameter adjustment message or signal over a communications channel to the member stations STA 1   106 , STA 2   108 , STA 3   110 , and STA 4   112  of the WLAN  100  in  FIG. 1 . The AP  104  may issue or transmit the special EDCA parameter adjustment message or signal in a manner similar to a polling message or signal in scheduled access. Inside of these EDCA adjustment messages the AP  104  may enclose special EDCA parameter configurations for groups of member stations or individual member stations to differentiate channel access priorities among them and give one particular member station or particular group of member stations elevated channel access priority relative to the rest of the member stations. As a result, the particular or specified member station or group of member stations will have preferred channel access priority and likely succeed in accessing the communications channel. Such a special EDCA parameter adjustment message or signal may be transmitted whenever an AP, such as AP  104  needs to change channel access priority for its member stations. In an alternative embodiment the special EDCA parameter adjustment message may carry special time elements describing the time interval that the enclosed EDCA parameters should be applied by the specified member stations. Yet in another alternative embodiment the same information transmitted by the special EDCA parameter adjustment message can be embedded in the beacon messages and transmitted as a special IE. 
     Each specific group of member stations referred to in step  202  of  FIG. 2  can include only one member station, such as only one of member stations STA 1   106 , STA 2   108 , STA 3   110 , and STA  4   112 . The EDCA parameters may include an Arbitration Inter-Frame Space Number (AIFSN), a minimum contention window (CWmin), and a maximum contention window (CWmax) for a member station or a group of member stations. One or more of these EDCA parameters may be set to give higher priority to a specific group of member stations. The Arbitration Inter-Frame Space Number (AIFSN), a minimum contention window (CWmin), and a maximum contention window (CWmax) are lowered to give a shorter waiting time and therefore higher priority for a particular member station or group of member stations. The Arbitration Inter-Frame Space Number (AIFSN), a minimum contention window (CWmin), and a maximum contention window (CWmax) are raised to give a longer waiting time and therefore lower priority for a particular member station or group of member stations. 
     The channel access priorities can be rotated among groups of member stations or among individual member stations by the access point (AP)  104 . The access point (AP)  104  can compute, construct, and execute a schedule for EDCA parameter rotation that balances the Quality of Service (QoS) requirements of all member stations. 
     When a channel access throttling time period ends, the member stations roll back their EDCA parameters to what they were prior to the beginning of a channel access throttling period (i.e. the default EDCA parameters), unless the EDCA parameters are set in accordance with a new channel access throttling time period. A period that a member station or group of member stations channel access priority is raised can be called a channel access throttling (CAT) high period, and a period that a member station or group of member stations channel access priority is lowered can be called a channel access throttling (CAT) low period. 
       FIG. 3  shows an exemplary timing diagram  300  of signals or messages transmitted from access point (AP)  104 , graphed versus time (t). The AP  104  starts transmitting a beacon signal or message over a wireless communications channel, at time t 0  and stops transmitting the beacon message or signal at time t 1 . The level of the bar shown for beacon message or signal  302  may indicate an amplitude of the beacon message or signal. 
     To start a channel access throttling (CAT) period, the AP  104  transmits a CAT Frame (CF) signal or message  304  from time t 2  to time t 3 . The CF signal or message  304  is transmitted from the AP  104  out onto a wireless communications channel to all of the member stations, including STA 1   106 , STA 2   108 , STA 3   110 , and STA 4   112 , in the Basic Service Set  102  of the WLAN  100  shown in  FIG. 1 . In this example, the CF message or signal  304 , when received by the STA 1   106 , causes the STA 1   106  to change its EDCA parameters (such as AIFSN, and Contention Window time periods CWmin, and CWmax) from default values to higher values, which will increase the waiting time period for STA 1   106  and result in lower access priority for STA 1   106 . The period of time from t 3  to t 4 , in this example, is called a CAT low period (lower access priority) for STA 1   106  because the EDCA parameters are set high to cause a lower access priority for STA 1   106 , than STA 1   106  would have with its default EDCA parameters. The time t 4  is the start of the next beacon message or signal  306 , and the time t 5  is the end of the beacon signal or message  306 . The difference between the start times of two consecutive beacon signals or messages is called the beacon interval. 
     To those skilled in the art, various uses of a special EDCA parameter adjustment message and a beacon embedded special EDCA parameter adjustment IE, combined with an EDCA parameter set IE of the IEEE 802.11 standard, can be derived to accomplish different goals. For example, an AP, such as AP  104 , may use an EDCA parameter set IE to establish default EDCA parameters for all member stations, such as member stations  106 ,  108 ,  110 , and  112  of a BSS, such as BSS  102 . Then the particular AP, such as AP  104 , may use a beacon embedded special EDCA parameter adjustment IE to schedule CAT periods that repeatedly appear in each beacon interval. Then the particular AP, such as AP  104 , may also use special EDCA parameter adjustment messages to schedule additional CAT periods when necessary. The special EDCA parameter adjustment messages may be addressed to one member station, such as one of  106 ,  108 ,  110 , and  112 , if the enclosed EDCA parameter settings only applies to that particular member station, or addressed to multiple member stations in ways that are considered as known art. The special EDCA parameter adjustment messages may even be addressed to a non-existent member station, in which case effectively the particular AP, such as AP  104 , creates “vacant” time slots, which can be used by entities external to the particular BSS, such as BSS  102 . Such entities can be members of other co-located BSSes, or even by non-Wi-Fi (trademarked) devices using other CSMA type of channel access technologies such as ZigBee (IEEE 802.15.4). A “Wi-Fi” device is a wireless local area network (WLAN) product that is based on the Institute of Electrical and Electronics Engineers&#39; (IEEE) 802.11 standards. “Wi-Fi” is a registered trademark. “ZigBee” is the name of a specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE 802.15.4-2006 standard for wireless personal area networks (WPANs), such as wireless headphones connecting with cell phones via short-range radio. The ZigBee technology is intended to be simpler and less expensive than other WPANs, such as Bluetooth. ZigBee is targeted at radio-frequency (RF) applications that require a low data rate, long battery life, and secure networking. 
     In the disclosed technology, the initial beacon message or signal  302  may set default EDCA parameters, which may be differentiated by Quality of Service (QoS). The time period when channel access throttling is not used for any member stations, but rather default EDCA parameters are used, may be called a default time period. Currently there are four different classes of Quality of Service (QoS): voice, video, background, and best effort. However, currently an initial beacon message or signal of the prior art does not differentiate EDCA parameters among member stations or groups of member stations. 
     The AP  104  may use the CAT frame (CF) signal or message  304  to instruct a member station or group of member stations to set its EDCA parameters to values different from default values. For example, the AP  104  may transmit or issue a command or CF  304  for the member station STA 1   106  to set its EDCA parameters to CWmin=CWmax=1023, and Arbitration Interframe Space Number (AIFSN) to 7, and a channel access throttling time period using these EDCA parameters may begin at time t 3  and last until the end of the beacon interval at time t 4 . Because the default EDCA parameters for STA 1   106 , in this example, are lower than the channel access throttling time period EDCA parameters, this configuration defines a CAT low period (low priority access) for STA 1   106 . During the STA 1   106  CAT low period from t 3  to t 4  other member stations, such as member stations  108 ,  110 , and  112 , have higher channel access priority than member station STA 1   106 . Thus, this CF signal, message, or command  304  effectively “polls” all other member stations ( 108 ,  110 , and  112 ), other than member station STA 1   106 . The transmission opportunity window given to the “polled” member stations in this example is from immediately after the command message or signal CF  304  (time t 3 ) until the beginning of the next beacon interval (time t 4 ). 
     The AP  104  may use the CAT command message or signal in various ways. For example, the AP  104  may raise the EDCA parameters (thus raising the waiting time period) for certain member stations to implicitly poll other member stations. The AP  104  may alternatively lower the EDCA parameters (thus lowering the waiting time period) for certain member stations to explicitly poll these member stations. The AP  104  may also raise the EDCA parameters for some member stations while at the same time lowering the EDCA parameters for other member stations to enhance the priority difference between polled member stations and other member stations. The AP  104  may also schedule recurrent channel access throttling (CAT) operations. The AP  104  may also differentiate the channel access priorities that different classes of either the “polled” members or not “polled” members get by assigning different parameters to them. 
     A high-level block diagram of a computer  400  is illustrated in  FIG. 4 . The access point  104 , or any of the member stations  106 ,  108 ,  110 , and  112 , may include a computer similar to or identical to the computer  400 . Computer  400  contains a processor  404 , which controls the overall operation of the computer  400  by executing computer program instructions, which define such operation. The computer program instructions may be stored in a storage device  410  or other computer readable medium (e.g., magnetic disk, CD ROM, etc.), and loaded into a memory  406  when execution of the computer program instructions is desired. Thus, the method steps of  FIG. 2  can be defined by the computer program instructions stored in the memory  406  and/or storage device  410  and controlled by the processor  404  executing the computer program instructions. For example, the computer program instructions can be implemented as computer executable code programmed by one skilled in the art to perform an algorithm defined by the method steps of  FIG. 2 . Accordingly, by executing the computer program instructions, the processor  404  executes an algorithm defined by the method steps of  FIG. 2 . The computer  400  also includes one or more network interfaces  308  for communicating with other devices via a network. The computer  400  also includes input/output devices  402  that enable user interaction with the computer  400  (e.g., display, keyboard, mouse, speakers, buttons, etc.) One skilled in the art will recognize that an implementation of an actual computer could contain other components as well, and that  FIG. 4  is a high level representation of some of the components of such a computer for illustrative purposes. 
     The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.