Abstract:
A method of ameliorating the hidden node problem in wireless local area networks employing power control is disclosed. The illustrative embodiments function in a variety of ways that have a common theme: while the Data Frames are transmitted at lesser potency, at least one of the control frames—Request-to-Send, Clear-to-Send, and Acknowledgement—associated with the Data Frame are sent at a greater potency. This causes at least one of the “loud” control frames to be heard and decoded by all of the potentially contending stations. And because the control frames carry duration information for the virtual carrier sense mechanism, their reception suppresses transmissions by potentially-contending stations that cannot sense the “quiet” Data Frames.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of: 
     1. U.S. Provisional Application No. 60/383,750, filed on May 28, 2002, entitled “Method of Optimizing Transmit Power for EDCF Based Wireless Networks,” 
     which is also incorporated by reference. 
     The following U.S. patent applications are incorporated by reference: 
     2. U.S. patent application Ser. No. 10,377,323, filed on Feb. 28, 2003, entitled “Embedding Class of Service Information in MAC Control Frames,” and 
     3. U.S. patent application Ser. No. 10/353,391, filed on Jan. 29, 2003, entitled “Direct Link Protocol in Wireless Area Networks.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to a technique for power management in networks that communicate via a shared-communications channel. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  depicts a schematic diagram of an IEEE 802.11-compliant wireless local area network, which comprises: station  101 - 1 , station  101 - 2 , which is an access point, and station  101 - 3 . The communications between station  101 - 1 , station  101 - 2 , and station  101 - 3  occur within a shared-communications channel, and, therefore, a medium access control protocol is used to allocate usage of the channel among the stations. 
     In accordance with the IEEE 802.11 standard, one medium access control protocol used by the stations is carrier sense multiple access. In accordance with carrier sense multiple access, a station desiring to transmit a frame first listens to the channel and transmits only when it fails to sense another transmission. 
     For the purposes of this specification, the “potency” of a transmitted frame is defined as the effective spatial reach of the transmitted frame. As is well-known to those skilled in the art, the potency of a frame can be adjusted by the transmitter and is affected by the energy per bit at which the frame is transmitted. When, as in  FIG. 1 , each station is within the transmission range of every other station, carrier sense multiple access works well. In contrast, when every station is not within transmission range of every other station, as in  FIG. 2 , carrier sense multiple access might not work as well. For example, when station  201 - 1  transmits a Frame, station  201 - 3  will not sense it, and, therefore, might begin a transmission that prevents station  201 - 2  from correctly receiving either transmission. This is known as the “hidden” node problem. 
     The IEEE 802.11 standard addresses the hidden node problem with a mechanism known as Request-to-Send/Clear-to-Send. The message flow associated with the Request-to-Send/Clear-to-Send mechanism is depicted in  FIG. 3 . 
     In accordance with the Request-to-Send/Clear-to-Send mechanism, station  201 - 1  sends a Request-to-Send Frame at time t 0  to all of the stations within its transmission range (i.e., station  201 - 2 ). The Request-to-Send Frame contains a duration value that extends through the duration of the Clear-to-Send Frame and any Data and Acknowledgement Frames that station  201 - 1  expects will be transmitted as part of its request. All of the stations within the transmission range of station  201 - 1  receive and decode the Request-to-Send Frame to recover the value in the duration field. The value in the duration field is then used to populate a timer, called the Network Allocation Vector, which indicates how long those stations are to refrain from transmitting, regardless of whether they sense a transmission in the channel or not. 
     In response to the receipt of the Request-to-Send Frame, station  201 - 2  transmits a Clear-to-Send Frame at time t 2  to all of the stations within its transmission range (i.e., station  201 - 1  and station  201 - 3 ). The Clear-to-Send Frame contains a duration value that extends through the duration of any Data and Acknowledgement Frames that station  201 - 1  desires to transmit. All of the stations within the transmission range of station  201 - 2  receive and decode the Request-to-Send Frame to recover the value in the duration field. The value in the duration field is then used to populate their Network Allocation Vector. 
     In this way, the Request-to-Send/Clear-to-Send mechanism addresses the hidden node problem by ensuring that station  201 - 3  will not transmit while station  201 - 1  is transmitting its Data Frame to station  201 - 2 . 
     SUMMARY OF THE INVENTION 
     Some IEEE 802.11 compliant stations transmit their frames at a fixed level of potency. In contrast, some IEEE 802.11 compliant stations (e.g., 802.11(h) compliant stations, etc.) can adjust the potency of their transmitted frames. The stations that can adjust the potency of their transmitted frames are advantageous because they can conserve energy in contrast to stations that cannot adjust the potency of their transmitted frames. The conservation of energy is particularly advantageous for battery-powered stations such as notebook computers, personal digital assistants, and digital cameras. 
     In general, the stations that can adjust the potency of their transmitted frames must balance two competing goals:
         (1) the potency must be sufficient to ensure that the intended recipient of the frame can receive the frame, and   (2) the potency should be as small as possible so as to conserve as much energy as possible.       

     An unintended and disadvantageous consequence of having a station decrease the potency of its transmitted frames is that it increases the likelihood that a hidden node might exist. In other words, as a station reduces the potency of its transmitted frames, it increases the likelihood that its transmissions will not be sensed by another station, and, therefore, becomes a hidden node. 
     To overcome this problem, the illustrative embodiment transmits Data Frames with a different level of potency that one or more of the medium access control (“MAC”) control frames Request-to-Send, Clear-to-Send, and Acknowledgement Frames associated with the Data Frame. In particular, while the Data Frames are transmitted with lesser potency, one or more of the medium access control (“MAC”) control frames Request-to-Send, Clear-to-Send, and Acknowledgement Frames associated with the Data Frame are transmitted with greater potency. 
     In accordance with the illustrative embodiment of the present invention, the potency of a transmitted frame is affected by: 
     i. the energy per bit of the frame, or 
     ii. the length of the frame, or 
     iii. any combination of i and ii. 
     In particular, frames with fewer bits are more potent than frames with more bits because the probability of receiving a frame with a bit error increases with the number of bits in the frame. 
     Furthermore, in accordance with the illustrative embodiment of the present invention, the energy per bit of a frame is affected by: 
     i. the radiated average power level, or 
     ii. the bit rate, or 
     iii. the coding rate, or 
     iv. any combination of i, ii, and iii. 
     It will be clear to those skilled in the art how each of these factors affects the energy per bit of a frame and how each of these factors affects the rate at which the transmitter consumes energy. 
     Even though the illustrative embodiments cause some or all of the control frames to be transmitted with greater potency than they might otherwise be, many of the embodiments will still consume, on average, less energy than stations that transmit both data and control frames at a fixed level of potency. 
     Some embodiments of the present invention are useful when an access point relays Data Frames between the source and destination stations, and some embodiments are useful when the access point does not relay Data Frames (e.g., when the stations communicate directly in accordance with the direct link protocol, etc.). U.S. patent application Ser. No. 10/353,391, entitled “Direct Link Protocol in Wireless Area Networks,” teaches a direct link protocol. 
     The illustrative embodiment of the present invention comprises: wirelessly receiving a first Data Frame via a shared-communications channel, wherein said first Data Frame was transmitted with a first potency; and wirelessly transmitting a first Acknowledgement Frame into said shared-communications channel at a second potency, wherein said first Acknowledgement Frame is transmitted in response to the receipt of said first Data Frame; wherein said second potency is higher than said first potency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of a local area network in the prior art in which there is no “hidden” node problem. 
         FIG. 2  depicts a schematic diagram of a local area network in the prior art in which there is a hidden node problem. 
         FIG. 3  depicts the message flows associated with the Request-to-Send/Clear-to-Send mechanism for addressing the hidden node problem in  FIG. 2 . 
         FIG. 4  depicts a schematic diagram of a local area network in accordance with the illustrative embodiments of the present invention. 
         FIG. 5  depicts a block diagram of the salient components in a station in accordance with the illustrative embodiments of the present invention. 
         FIG. 6  depicts the message flows associated with the first illustrative embodiment of the present invention. 
         FIG. 7  depicts the message flows associated with the second illustrative embodiment of the present invention. 
         FIG. 8  depicts the message flows associated with the third illustrative embodiment of the present invention. 
         FIG. 9  depicts the message flows associated with the fourth illustrative embodiment of the present invention. 
         FIG. 10  depicts the message flows associated with the fifth illustrative embodiment of the present invention. 
         FIG. 11  depicts the message flows associated with the sixth illustrative embodiment of the present invention. 
         FIG. 12  depicts the message flows associated with the seventh illustrative embodiment of the present invention. 
         FIG. 13  depicts the message flows associated with the eighth illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  depicts a schematic diagram of local area network  400  in accordance with the illustrative embodiments of the present invention. Local area network  400  comprises a plurality of stations, station  401 - 1  through  401 - 3 , that communicate wirelessly via a shared-communications channel. In accordance with the illustrative embodiment, all of the stations in the network operate in compliance with the IEEE 802.11 standard. 
       FIG. 5  depicts a block diagram of the salient components of station  401 - i , for i=1 to 4, in accordance with illustrative embodiments of the present invention. Station  401 - i  is one station in an IEEE 802.11-compliant wireless local area network, and, therefore all of the Frames are transmitted by all of the stations in the network in compliance with the IEEE 802.11 standard. It will be clear to those skilled in the art, however, after reading this disclosure, how to make and use embodiments of the present invention that operate in a non-IEEE 802.11 compliant network. 
     Throughout the course of each of the illustrative embodiments, stations  401 - 1  through  401 - 4  are deemed to be stationery and the radio frequency environment stable. It will be clear to those skilled in the art, after reading this disclosure, how to make and use embodiments of the present invention that operate in a network in which one or more of the stations move during the course of an atomic operation or in which the radio frequency environment changes during the course of an atomic operation or both. 
     Station  401 - i  comprises: processor  406 , host interface  402 , transmitter  403 , receiver  404 , and memory  405 , interconnected as shown. Station  401 - i  is fabricated on one or more integrated circuits and interfaces with a host computer (not shown) and an antenna (not shown) in well-known fashion. 
     Processor  406  is a general-purpose processor that is capable of executing instructions stored in memory  405 , of reading data from and writing data into memory  405 , and of executing the tasks described below and with respect to  FIGS. 6 through 13 . In some alternative embodiments of the present invention, processor  406  is a special-purpose processor. In either case, it will be clear to those skilled in the art, after reading this disclosure, how to make and use processor  406 . 
     Host interface  402  is a circuit that is capable of receiving data and instructions from a host computer (not shown) and of relaying them to processor  406 . Furthermore, host interface  402  is capable of receiving data and instructions from processor  406  and relaying them to the host computer. It will be clear to those skilled in the art how to make and use host interface  402 . 
     Transmitter  403  is a hybrid analog and digital circuit that is capable of receiving frames from processor  406  and of transmitting them into the shared-communications channel at times in accordance with IEEE 802.11. It will be clear to those skilled in the art, after reading this disclosure, how to make and use transmitter  403 . 
     Receiver  404  is a hybrid analog and digital circuit that is capable of receiving frames from the shared-communications channel and relaying them to processor  406 . It will be clear to those skilled in the art, after reading this disclosure, how to make and use receiver  404 . 
     Memory  405  is a non-volatile random-access memory that stored instructions and data For processor  406 . It will be clear to those skilled in the art how to make and use memory  405 . 
       FIGS. 6 through 13  depict message flows in accordance with various embodiments of the present invention. In each figure, the formatting of the text of the name of a Frame indicates how the Frame is transmitted. The styles of text and their meaning is described in Table 1. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Character of Frames Transmitted in Accordance with the 
               
               
                 Illustrative Embodiments of the Present Invention 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 
                   Frame 
                 
                 italics denotes a Frame that is transmitted to the access point at 
               
               
                   
                 lesser potency. 
               
               
                 
                   Frame 
                 
                 bold denotes a Frame that is transmitted at greater potency. 
               
               
                 Frame 
                 no emphasis denotes a Frame that is transmitted to the desire 
               
               
                   
                 peer station at lesser potency. 
               
               
                 
                   Frame 
                 
                 underline denotes a Frame that is transmitted from one peer to 
               
               
                   
                 another peer station without being relayed by an access point 
               
               
                   
                 (i.e., as part of a direct link stream). 
               
               
                   
               
             
          
         
       
     
     In all of the embodiments, the Request-to-Send, Clear-to-Send, Data, and Acknowledgement Frames comprise a value equal to the remainder of the atomic operation. This enables all of the stations that hear any of these frames to set their Network Allocation Vectors so that even if they can&#39;t physically sense the following frames (i.e., are hidden from station  401 - 1 ), their virtual carrier sense mechanism will prevent their transmitting onto them. 
     In accordance with the illustrative embodiments, the Request-to-Send and Clear-to-Send Frames are sent at the same low bit rate, and the Data and Acknowledgement Frames are generally sent the highest possible bit rate that results in an acceptable probability of being received correctly. It will be clear to those skilled in the art how to make and use embodiments of the present invention in which the bit rates of the various frames are sent at different bit rates than in the illustrative embodiments. 
       FIG. 6  depicts the message flows in the first illustrative embodiment of the present invention. In accordance with the first illustrative embodiment of the present invention, station  401 - 2  (the access point) is involved in the transmission and reception of all of the Request-to-Send, Clear-to-Send, Data, and Acknowledgement Frames. 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 2  at a first potency. The Request-to-Send Frame is received by station  401 - 2  at time t 1 . 
     At time t 2 , station  401 - 2  transmits a Clear-to-Send Frame at a second potency. The second potency is higher than the first potency. The Clear-to-Send Frame is received at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 2  at a third potency. The Data Frame is received by station  401 - 2  at time t 5 . The third potency is equal to the first potency and less than the second potency. 
     At time t 6 , station  401 - 2  re-transmits the Data Frame to station  401 - 3  at a fourth potency. The Data Frame is received by station  401 - 3  at time t 7 . 
     At time t 8 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fifth potency, in response to the receipt of the Data Frame. The fifth potency is equal to the fourth potency and less than the second potency. The Acknowledgement Frame is received by station  401 - 2  at time t 9 . 
     At time t 10 , station  401 - 2  re-transmits the Acknowledgement Frame to station  401 - 1  at a sixth potency. The sixth potency is equal to the first and third potency and less than the second potency. The Acknowledgement Frame is received by station  401 - 1 , in response to the receipt of the Data Frame, at time t 11 . 
     After time t 11 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 7  depicts the message flows in the second illustrative embodiment of the present invention. In accordance with the second illustrative embodiment of the present invention, station  401 - 2  (the access point) is involved in the transmission and reception of all of the Request-to-Send, Clear-to-Send, Data, and Acknowledgement Frames. 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 2  at a first potency. The Request-to-Send Frame is received by station  401 - 2  at time t 1 . 
     At time t 2 , station  401 - 2  transmits a Clear-to-Send Frame at a second potency. The second potency is higher than the first potency. The Clear-to-Send Frame is received at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 2  at a third potency. The Data Frame is received by station  401 - 2  at time t 5 . The third potency is equal to the first potency and less than the second potency. 
     At time t 6 , station  401 - 2  re-transmits the Data Frame to station  401 - 3  at a fourth potency. The Data Frame is received by station  401 - 3  at time t 7 . 
     At time t 8 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fifth potency, in response to the receipt of the Data Frame The fifth potency is higher than the fourth potency and equal to the second potency. The Acknowledgement Frame is received by station  401 - 2  at time t 9 . 
     At time t 10 , station  401 - 2  re-transmits the Acknowledgement Frame to station  401 - 1  at a sixth potency. The sixth potency is higher than the first and third potency and equal to the second potency. The Acknowledgement Frame is received by station  401 - 1 , in response to the receipt of the Data Frame, at time t 11 . 
     After time t 11 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 8  depicts the message flows in the third illustrative embodiment of the present invention. In accordance with the third illustrative embodiment of the present invention, station  401 - 2  (the access point) is involved in the transmission and reception of all of the Data and Acknowledgement Frames. In accordance with the third illustrative embodiment, there are no transmitted Request-to-Send Frames or Clear-to-Send Frames. This is particularly useful when the first Data Frame of a contention free burst is a short frame because both the Data Frames and the Acknowledgement Frames convey the duration information for the remainder of the burst. 
     At time t 0 , station  401 - 1  transmits a Data Frame to station  401 - 2  at a first potency. The Data Frame is received by station  401 - 2  at time t 1 . 
     At time t 2 , station  401 - 2  re-transmits the Data Frame to station  401 - 3  at a second potency. The Data Frame is received by station  401 - 3  at time t 3 . 
     At time t 4 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a third potency, in response to the receipt of the Data Frame The third potency is higher than the second potency. The Acknowledgement Frame is received by station  401 - 2  at time t 5 . 
     At time t 6 , station  401 - 2  re-transmits the Acknowledgement Frame to station  401 - 1  at a fourth potency. The fourth potency is higher than the first potency. The Acknowledgement Frame is received by station  401 - 1 , in response to the transmission of the Data Frame, at time t 7 . 
     After time t 8 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 9  depicts the message flows in the fourth illustrative embodiment of the present invention. In accordance with the fourth illustrative embodiment, station  401 - 1  and station  401 - 3  communicate directly and without station  401 - 2 . 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 3  at a first potency. The Request-to-Send Frame is received by station  401 - 3  at time t 1 . 
     At time t 2 , station  401 - 3  transmits a Clear-to-Send Frame at a second potency. The Clear-to-Send Frame is received by station  401 - 1  at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 3  at a third potency. The Data Frame is received by station  401 - 3  at time t 5 . The third potency is lower than to the first potency. 
     At time t 6 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fourth potency, in response to the receipt of the Data Frame. The Acknowledgement Frame is received by station  401 - 3  at time t 7 . The fourth potency is less than the first potency and the second potency and equal to the third potency. 
     After time t 11 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 10  depicts the message flows in the fifth illustrative embodiment of the present invention. In accordance with the fifth illustrative embodiment, station  401 - 1  and station  401 - 3  communicate directly and without station  401 - 2 . 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 3  at a first potency. The Request-to-Send Frame is received by station  401 - 3  at time t 1 . 
     At time t 2 , station  401 - 3  transmits a Clear-to-Send Frame at a second potency. The second potency is lower than the first potency. The Clear-to-Send Frame is received by station  401 - 1  at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 3  at a third potency. The Data Frame is received by station  401 - 3  at time t 5 . The third potency is lower than to the first potency and equal to the second potency. 
     At time t 6 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fourth potency, in response to the receipt of the Data Frame. The Acknowledgement Frame is received by station  401 - 3  at time t 7 . The fourth potency is less than the first potency and equal to the second and third potency. 
     After time t 11 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 11  depicts the message flows in a sixth illustrative embodiment of the present invention. In accordance with the sixth illustrative embodiment, station  401 - 1  and station  401 - 3  communicate directly and without station  401 - 2 . 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 3  at a first potency. The Request-to-Send Frame is received by station  401 - 3  at time t 1 . 
     At time t 2 , station  401 - 3  transmits a Clear-to-Send Frame at a second potency. The second potency is higher than the first signal to noise ratio. The Clear-to-Send Frame is received by station  401 - 1  at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 3  at a third potency. The Data Frame is received by station  401 - 3  at time t 5 . The third potency is equal to the first potency and lower than the second potency. 
     At time t 6 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fourth potency, in response to the receipt of the Data Frame. The Acknowledgement Frame is received by station  401 - 3  at time t 7 . The fourth potency is less than the second potency and equal to the first and third potencies. 
     After time t 11 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 12  depicts the message flows in a seventh illustrative embodiment of the present invention. In accordance with the fourth illustrative embodiment, station  401 - 2  is involved in the Request-to-Send and Clear-to-Send Frame flow, but stations  401 - 1  and  401 - 3  transmit the Data and Acknowledgement Frames directly and without station  401 - 2 . 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 2  at a first potency. The Request-to-Send Frame is received by station  401 - 2  at time t 1 . 
     At time t 2 , station  401 - 2  transmits a Clear-to-Send Frame at a second signal-to-noise. The second potency is equal to the first potency. The Clear-to-Send Frame is received by station  401 - 1  and station  401 - 2  at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 3  at a third potency, which is received by station  401 - 3  at time t 5 . The third potency is lower than the first potency. 
     At time t 6 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fourth potency, in response to the receipt of the Data Frame. The fourth potency is equal to the third potency. The Acknowledgement Frame is received by station  401 - 3  at time t 7  in response to the transmission of the Data Frame. 
     After time t 7 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
       FIG. 13  depicts the message flows in the eighth illustrative embodiment of the present invention. In accordance with the first illustrative embodiment of the present invention, station  401 - 2  (the access point) is all involved in the transmission and reception of the Request-to-Send, Clear-to-Send, data, and Acknowledgement Frames. First, station  401 - 1  transmits a Data Frame to station  401 - 3  via station  401 - 2 , and then station  401 - 3  transmits a Data Frame to station  401 - 1 . The point of this illustrative embodiment is to show the symmetry associated with the transmission of Data Frames. 
     At time t 0 , station  401 - 1  transmits a Request-to-Send Frame to station  401 - 2  at a first potency. The Request-to-Send Frame is received by station  401 - 2  at time t 1 . 
     At time t 2 , station  401 - 2  transmits a Clear-to-Send Frame at a second potency. The Clear-to-Send Frame is received at time t 3 . 
     At time t 4 , station  401 - 1  transmits a Data Frame to station  401 - 2  at a third potency, which is received by station  401 - 2  at time t 5 . The third potency is less than the first potency. 
     At time t 6 , station  401 - 2  re-transmits the Data Frame to station  401 - 3  at a fourth potency The fourth potency is less than the second potency. The Data Frame is received by station  401 - 3  at time t 7 . 
     At time t 8 , station  401 - 3  transmits an Acknowledgement Frame to station  401 - 2  at a fifth potency, in response to the receipt of the Data Frame. The Acknowledgement Frame is received by station  401 - 2  at time t 9 . The fifth potency is higher than the fourth potency. 
     At time t 10 , station  401 - 2  re-transmits the Acknowledgement Frame to station  401 - 1  at a sixth potency. The Acknowledgement Frame is received by station  401 - 1  at time t 11 . The sixth potency is higher than the third potency. 
     After time t 11 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
     At time t 12 , station  401 - 3  transmits a Request-to-Send Frame to station  401 - 2  at a seventh potency. The Request-to-Send Frame is received by station  401 - 2  at time t 13 . The seventh potency equals the fifth potency. 
     At time t 14 , station  401 - 2  transmits a Clear-to-Send Frame at a eighth potency. The eighth potency is equal to the second potency. The Clear-to-Send Frame is received at time t 15 . 
     At time t 16 , station  401 - 3  transmits a Data Frame to station  401 - 2  at a ninth potency. The Data Frame is received by station  401 - 2  at time t 17 . The ninth potency is equal to the fourth potency and less than the fifth and seventh potencies. 
     At time t 18 , station  401 - 2  re-transmits the Data Frame to station  401 - 1  at a tenth potency. The Data Frame is received by station  401 - 1  at time t 19 . The tenth potency is equal to the third potency and less than the first and sixth potencies. 
     At time t 20 , station  401 - 1  transmits an Acknowledgement Frame to station  401 - 2  at an eleventh potency, in response to the receipt of the Data Frame. The Acknowledgement Frame is received by station  401 - 2  at time t 21 . The eleventh potency is higher than the third and tenth potencies and equal to the first and sixth potencies. 
     At time t 21 , station  401 - 2  re-transmits the Acknowledgement Frame to station  401 - 3  at a twelfth potency. The Acknowledgement Frame is received by station  401 - 1  at time t 22 . The twelfth potency is higher than the fourth and ninth potencies and equal to the fifth and seventh potencies. 
     After time t 22 , the network allocation vector in all of the stations that received any frame in the process will have expired, and, therefore contention for the shared-communications channel can resume in well-known fashion unless more Data Frames are to be transmitted as part of a contention free burst. In the case of a contention free burst, the second and subsequent Data Frames are protected when stations  401 - 1 ,  401 - 2 , and  401 - 3  transmit a frame containing the duration of the remainder of the burst. It will be clear to those skilled in the art how to make and use embodiments of the present invention that accommodate contention free bursts. 
     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the present invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.