Abstract:
A technique is disclosed for re-enabling transmission protection at enhanced stations while in the presence of high, enhanced modulation traffic conditions once protection has been disabled. Transmission protection allows enhanced modulation stations to co-exist with legacy modulation stations on a shared-communications channel. The problem being solved is when transmission protection is set to inactive and the Orthogonal Frequency Division Multiplexing (i.e., enhanced modulation) traffic load is high, legacy traffic is likely to collide repeatedly with Orthogonal Frequency Division Multiplexing transmissions, with the result that the access point does not notice that a legacy station has become active again. Consequently, the access point does not activate transmission protection. The present invention addresses the problem by defining access point mechanisms that are capable of re-enabling transmission protection, once protection has been disabled, in the presence of high, enhanced modulation traffic conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Serial No. 60/439,697, filed Jan. 13, 2003, entitled “Dynamic OFDM Protection Methods for Access Points,” which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to wireless local area networks. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  depicts a schematic diagram of a portion of wireless local area network  100  in the prior art. Local area network  100  comprises stations  101 ,  102 - 1 , and  102 - 2 . Stations  101 ,  102 - 1 , and  102 - 2  use a shared-communications channel to communicate among themselves and only one of the terminals can transmit into the channel at a time. When two terminals transmit into the channel at the same time, the result is a cacophony and both transmissions are garbled. 
     Therefore, a technique called Carrier Sense Multiple Access is used by the stations to coordinate when each of them transmits. In accordance with this technique, each radio listens to the shared-communications channel and waits to transmit until the channel is quiet (i.e., no other stations are transmitting). Carrier Sense Multiple Access is similar to the way in which each person in a group of polite people waits to speak until the person speaking is finished. 
     Station  101  can transmit and receive using:
         i. a first modulation scheme.       In contrast, stations  202 - 1  and  202 - 2  can transmit and receive using:
       i. the first modulation scheme, and   ii. a second modulation scheme.
 
Because stations  102 - 1  and  102 - 2  can use a superset of the modulations schemes available to station  101 , they are called “enhanced” stations. In contrast, station  101  is called a “legacy” station.
   
       

     The modulation schemes available to a legacy station are called “legacy modulation schemes” and the modulation schemes available to an enhanced station are called “enhanced modulation schemes.” 
     Any two stations that need to communicate must do so in accordance with a modulation scheme that is available to both of them. Therefore, two enhanced stations can communicate with any of the enhanced modulation schemes, but any communication involving a legacy station must use a legacy modulation scheme. 
     Stations  102 - 1  and  102 - 2  communicate with each other using the second modulation scheme when possible because it enhances communication throughput in comparison to the first modulation scheme. One effect of using the second scheme, however, is that station  101  cannot detect when stations  102 - 1  and  102 - 2  are communicating (i.e., stations  102 - 1  and  102 - 2  are essentially invisible to station  101  when stations  102 - 1  and  102 - 2  are communicating using the second modulation scheme). This can cause station  101  to transmit when stations  102 - 1  and  102 - 2  are communicating, which causes all of the transmissions to be garbled. 
     To address this problem, a method called “transmission protection” is known in the prior art to prevent legacy stations from transmitting while transmissions using the second modulation scheme are in progress. 
     In accordance with transmission protection, an enhanced station that is about to transmit a frame using the second modulation scheme first transmits a short frame using the first modulation scheme. This short frame is detectable by the legacy stations in the area. 
     A duration field in the short frame contains a value that indicates how long the legacy terminals should refrain from transmitting, and the field is populated with a duration that is long enough to cover the length of time for transmissions of frames using the second modulation scheme. The duration information inside the Request-to-Send or Clear-to-Send frame activates a virtual carrier sense mechanism in the legacy stations, which will not transmit, as a result, during the protected, subsequent second transmission. 
     A mechanism is also known in the prior art for notifying all of the enhanced stations in the network when to use and when not to use transmission protection. In accordance with this mechanism, one enhanced station has the capability to activate and deactivate transmission protection in the other enhanced stations by signaling whether or not protection must be used. 
     If transmission protection is disabled and there is a heavy traffic load using the second modulation scheme, transmissions from legacy stations will repeatedly collide with transmissions using the second modulation scheme and there might be an unreasonable delay before an enhanced station notices that a legacy station is trying to transmit. As a result, there might be a delay in activating transmission protection in the enhanced terminals. 
     Therefore, the need exists for a technique for reasonably activating transmission protection in enhanced terminals. 
     SUMMARY OF THE INVENTION 
     The present invention addresses this problem by defining access point mechanisms that are capable of re-enabling transmission protection, once protection has been disabled, in the presence of heavy, enhanced modulation (e.g., Orthogonal Frequency Division Multiplexing, etc.) traffic. 
     In accordance with the first illustrative embodiment of the present invention, when a legacy communication station is in a power save state, the access point periodically transmits a Null frame to that station. When the station “awakens” (i.e., has exited the power save state) and responds with an Acknowledgement frame, the access point re-enables transmission protection. 
     In accordance with the second illustrative embodiment of the present invention, when a legacy station becomes active and sends a message to the access point, the access point immediately activates transmission protection by sending a Probe-Response frame, comprising a protection status field that is set to active, to the enhanced stations. This is a different use of the Probe-Response frame than in the prior art because the frame is broadcast proactively, as opposed to being unicast in response to a previously-sent Probe-Request frame. 
     In accordance with the third illustrative embodiment of the present invention, the access point intermittently switches between enabling transmission protection and disabling transmission protection at enhanced stations. The intervals during which protection is activated facilitate the transmission of any pending legacy modulation frame. 
     In accordance with the fourth illustrative embodiment of the present invention, the access point repeatedly transmits an enhanced modulation frame that inhibits transmissions by enhanced stations for a portion of the time period between repeated enhanced modulation frame transmissions. This gives legacy stations the opportunity to transmit as needed because they do not detect the enhanced inhibiting frame. 
     The illustrative embodiment comprises: determining a power save status of a first station wherein said first station communicates via a shared-communications channel in accordance with a first modulation scheme; and enabling transmission protection at a second station via said shared-communications channel wherein said enabling is dependent on said power save status. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of a portion of wireless local area network  100  in the prior art. 
         FIG. 2  depicts a schematic diagram of a portion of local area network  200  in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a block diagram of the salient components of enhanced station  202 - 2  in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts a message flow diagram in accordance with the first illustrative embodiment of the present invention. 
         FIG. 5  depicts a message flow diagram in accordance with the second illustrative embodiment of the present invention. 
         FIG. 6  depicts a message flow diagram in accordance with the third illustrative embodiment of the present invention. 
         FIG. 7  depicts a message flow diagram in accordance with the third illustrative embodiment of the present invention wherein enhanced station  202 - 2  receives a legacy modulation frame from legacy station  201  during a protected period. 
         FIG. 8  depicts a message flow diagram in accordance with the third illustrative embodiment of the present invention wherein enhanced station  202 - 2  receives a legacy modulation frame from legacy station  201  during an unprotected period. 
         FIG. 9  depicts a message flow diagram in accordance with the fourth illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  depicts a schematic diagram of wireless local area network  200  in accordance with the illustrative embodiment of the present invention. Local area network  200  operates in accordance with the IEEE 802.11 set of protocols and comprises communication stations  201 ,  202 - 1 , and  202 - 2 , and hosts  203 - 1  and  203 - 2 , interconnected as shown. 
     Station  201  transmits and receives using:
         i. Complementary Code Keying, and   ii. Barker modulation.
 
In contrast, stations  202 - 1  and  202 - 2  transmit and receive using:
   i. Complementary Code Keying,   ii. Barker modulation, and   iii. Orthogonal Frequency Division Multiplexing.
 
Because stations  202 - 1  and  202 - 2  can use a superset of the modulations schemes available to station  201 , they are called “enhanced” stations whereas station  201  is called a “legacy” station.
       

     The modulation schemes available to a legacy station are called “legacy modulation schemes” and the modulation schemes available to an enhanced station are called “enhanced modulation schemes.” Any two stations that need to communicate must do so in accordance with a modulation scheme that is available to both of them. Therefore, two enhanced stations can communicate with any of the enhanced modulation schemes, but any communication involving a legacy station must use a legacy modulation scheme. 
     For the purposes of this specification, a frame that is transmitted using a legacy modulation scheme is referred to as “legacy modulation frame,” and a frame that is transmitted using an enhanced modulation scheme is referred to as “enhanced modulation frame.” 
     It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that operate in accordance with other protocols and modulation schemes. Furthermore, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that use a wireline or tangible shared-communications channel. And still furthermore, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention with any number of legacy stations and any number of enhanced stations. 
     Enhanced station  202 - 2  is an access point and, as such, enables stations  201  and  201 - 1  within local area network  200  to communicate with devices in other communications networks. Because station  202 - 2  is an access point, stations  201  and  202 - 1  communicate with each other through enhanced station  202 - 2 , because enhanced station  202 - 2  coordinates the communications on local area network  200 . The salient details of enhanced station  202 - 2  are described below and with respect to  FIG. 3 . 
     Legacy station  201  comprises the radio that enables host  203 - 1  to communicate via the shared-communications channel. Legacy station  201  is capable of receiving a data block from host  203 - 1  and transmitting over the shared-communications channel one or more data frame comprising the payload portion of the data block. Legacy station  201  is also capable of receiving one or more data frames from the shared communications channel and sending to host  203 - 1  a data block comprising payload portion of the data frames. It will be clear to those skilled in the art how to make and use legacy station  201  and host  203 - 1 . 
     Enhanced station  202 - 1  and host  203 - 2  have a relationship similar to that described for legacy station  201  and host  203 - 1 . It will be clear to those skilled in the art how to make and use stations  202 - 1 , and hosts  203 - 1  and  203 - 2 . 
       FIG. 3  depicts a block diagram of the salient components of enhanced station  202 - 2 , the access point, in accordance with the illustrative embodiment of the present invention. Enhanced station  202 - 2  comprises: receiver  301 , processor  302 , memory  303 , and transmitter  304 , interconnected as shown. 
     Receiver  301  is a circuit that is capable of receiving frames from another station via the shared-communications channel, in well-known fashion, and of forwarding them to processor  302 . It will be clear to those skilled in the art, after reading this specification, how to make and use receiver  301 . 
     Processor  302  is a general-purpose processor that is capable of performing the tasks described below and with respect to  FIGS. 4 through 9 . It will be clear to those skilled in the art, after reading this specification, how to make and use processor  302 . 
     Memory  303  is capable of storing programs and data used by processor  302 . It will be clear to those skilled in the art how to make and use memory  303 . 
     Transmitter  304  is a circuit that is capable of receiving frames from processor  302 , in well-known fashion, and of transmitting them to another station via the shared-communications channel in accordance with an enhanced modulation scheme. It will be clear to those skilled in the art, after reading this specification, how to make and use transmitter  304 . 
       FIG. 4  depicts a message flow diagram in accordance with the first illustrative embodiment of the present invention. When the message flow diagram in  FIG. 4  begins, transmission protection is disabled for the enhanced station in local area network  200 . 
     Enhanced station  202 - 2  (i.e., the access point) tracks the power save status of the legacy stations (e.g., legacy station  201 , etc.) present in local area network  200 . The power save state is defined as a state in which the transmitter (or both the transmitter and receiver) of a station is powered down, and the power save status is an indication of whether or not a legacy station is in the power save state. 
     With message  401 , enhanced station  202 - 2  transmits to legacy station  201  a Null frame to determine the power save status of legacy station  201 . A Null frame, as defined in the IEEE 802.11 protocols, is a frame with an empty payload, which is answered with an Acknowledgement frame by the receiving station and then discarded. In some alternative embodiments of the present invention, message  401  can be a Request-to-Send frame or a data frame instead. When legacy station  201  is in the power save state, then enhanced station  202 - 2  does not receive a response, as is the case for message  401 . Enhanced station  202 - 2  then attempts to determine the power save status of other legacy stations in the network that it coordinates. 
     With message  402 , enhanced station  202 - 2  transmits to legacy station  201  another Null frame as part of continually tracking the power save status of legacy stations,. When legacy station  201  is not in the power save state, legacy terminal  201  responds with message  403 , which is an acknowledgement that legacy terminal  201  has received message  402 . Message  403  is an Acknowledgement frame or a Clear-to-Send frame depending, as will be clear to those skilled in the art, on whether message  402  is a Null frame, a Request-to-Send frame, or a data frame. 
     When station  202 - 2  receives message  403  enhanced station  202 - 2  immediately activates transmission protection for itself and broadcasts message  404  to activate transmission protection in the other enhanced stations in the network. Message  404  is a management frame, which can be for example, a Beacon frame or a Probe-Response frame with protection status set to active. The protection status can be represented by the Use_Protection bit for IEEE 802.11 protocols, for example. It will be clear to those skilled in the art how to broadcast a management frame to stations in local area network  200 . 
     In some alternative embodiments of the present invention, station  202 - 2  broadcasts the management frame using a legacy modulation scheme. In yet some other alternative embodiments of the present invention, station  202 - 2  first transmits a control frame (e.g., Request_to_Send, Clear_to_Send, etc.) using a legacy modulation scheme, specifying a duration that covers a management frame that station  202 - 2  subsequently broadcasts using either an enhanced or legacy modulation scheme. 
       FIG. 5  depicts a message flow diagram in accordance with the second illustrative embodiment of the present invention. When the message flow diagram in  FIG. 5  begins, transmission protection is disabled for the enhanced station in local area network  200 . 
       FIG. 5  represents a scenario in which enhanced station  202 - 2  (i.e., the access point) detects a legacy station transmission, but cannot wait until the next beacon frame transmission to update the enhanced stations&#39; transmission protection. 
     With message  501 , legacy station  201  transmits a frame to enhanced station  202 - 2  when transmission protection is inactive, which message is indicative that a legacy station has become active. 
     As a result of the receipt of message  501 , enhanced station  202 - 2  immediately activates transmission protection for its own transmissions. Enhanced station  202 - 2  then with message  502  enables transmission protection for all enhanced stations in the network. 
     With message  502  enhanced station  202 - 2  broadcasts a Probe-Response frame to all stations, including enhanced station  202 - 1  with the protection status set to active. The protection status can be represented by the Use_Protection bit for IEEE 802.11 protocols, for example. The second illustrative embodiment of the present invention uses the Probe-Response frame proactively and with a broadcast address, as opposed to using the Probe-Response as a unicast response, as is known in the art, to an IEEE 802.11 Probe Request message. 
     In some alternative embodiments of the present invention, station  202 - 2  broadcasts the Probe-Response frame using a legacy modulation scheme. In yet some other alternative embodiments of the present invention, station  202 - 2  first transmits a control frame (e.g., Request_to_Send, Clear_to_Send, etc.) using a legacy modulation scheme, that specifies a duration that covers a Probe-Response frame that station  202 - 2  subsequently broadcasts using either an enhanced or legacy modulation scheme. 
       FIG. 6  depicts a message flow diagram in accordance with the third illustrative embodiment of the present invention. When the message flow diagram in  FIG. 6  begins, transmission protection is disabled for the enhanced station in local area network  200 . 
     In accordance with the third illustrative embodiment, enhanced station  202 - 2  (i.e., the access point) alternately switches between enabling transmission protection and disabling transmission protection for both its own transmissions and for all of the enhanced stations in local area network. The intervals during which protection is activated facilitate the transmission of any pending frames from legacy station  201 . 
     With message  601 , enhanced station  202 - 2  broadcasts a management frame indicating active protection status. The management frame can be a Beacon frame or a Probe-Response frame with the protection status field set to active, which directs the enhanced stations in local area network  200  to enable transmission protection during a first time period. 
     In some alternative embodiments of the present invention, station  202 - 2  broadcasts the management frame using a legacy modulation scheme. In yet some other embodiments of the present invention, station  202 - 2  first transmits a control frame (e.g., Request_to_Send, Clear_to_Send, etc.) using a legacy modulation scheme that specifies a duration that covers a management frame that station  202 - 2  subsequently broadcasts using either an enhanced or legacy modulation scheme. 
     With message  602 , enhanced station  202 - 2  broadcasts a management frame indicating inactive protection status. The management frame can be a Beacon frame or a Probe-Response frame with the protection status field set to inactive, which directs the enhanced stations in local area network  200  to disable transmission protection during a second time period. 
     Messages  603  and  604  are the same as messages  601  and  602 , respectively. Enhanced station  202 - 2  alternately enables and disables transmission protection for enhanced station  202 - 1  and other enhanced stations with respect to time. In some embodiments, this enabling and disabling is periodic with respect to time, so that the length of the third time period depicted in  FIG. 6  is the same length as the first time period, and the length of the fourth time period is the same length as the second time period, and so on. In some other embodiments of the present invention, alternately enabling and disabling transmission protection can be sporadic with respect to time. 
     In still other embodiments, the enabling and disabling of transmission protection can be performed with respect to management frames transmitted. For example, enhanced station  202 - 2  can alternately enable transmission protection in two successive management frames and disable transmission protection in a third management frame. It will be clear to those skilled in the art, after reading this specification, that any number of frames in which transmission protection is enabled can be followed by any number of frames in which transmission protection is disabled. This pattern can be periodic or sporadic with respect to frames transmitted by enhanced station  202 - 2 . 
       FIG. 7  depicts a message flow diagram in accordance with the third illustrative embodiment of the present invention wherein enhanced station  202 - 2  receives a legacy modulation frame from legacy station  201  during a protected period. When the message flow diagram in  FIG. 7  begins, transmission protection is disabled for the enhanced station in local area network  200 . 
     In accordance with the third illustrative embodiment, enhanced station  202 - 2  (i.e., the access point) alternately switches between enabling transmission protection and disabling transmission protection for both its own transmissions and for enhanced station  202 - 1 &#39;s transmissions. The intervals during which protection is activated facilitate the transmission of any pending frames from legacy station  201 . 
     With message  701 , enhanced station  202 - 2  broadcasts a management frame indicating active protection status. The management frame can be a Beacon frame or a Probe-Response frame with the protection status field set to active, thereby directing enhanced station  202 - 1 , as well as other enhanced stations, to enable transmission protection for its own transmissions during the first time period of length T 1 . 
     With message  702 , enhanced station  202 - 2  broadcasts a management frame indicating inactive protection status. The management frame can be a Beacon frame or a Probe-Response frame with the protection status field set to inactive, thereby directing enhanced station  202 - 1 , as well as other enhanced stations, to disable transmission protection for its own transmissions during the second time period. 
     With message  703 , enhanced station  202 - 2  broadcasts a management frame indicating active protection status for the third time period. 
     With message  704 , enhanced station  202 - 2  receives a legacy modulation frame from legacy station  201 . In accordance with the illustrative embodiment of the present invention, enhanced station  202 - 2  extends the protected period to length T 2  as a result of having detected the legacy modulation frame, message  704 . Length T 2 , for example, can be greater than length T 1 , the length of the protected period that follows message  701 . 
     With message  705 , enhanced station  202 - 2  resumes alternately enabling and disabling protection by broadcasting a management frame indicating inactive protection status for the fourth time period. 
       FIG. 8  depicts a message flow diagram in accordance with the third illustrative embodiment of the present invention wherein enhanced station  202 - 2  receives a legacy modulation frame from legacy station  201  during an unprotected period. Transmission protection is initially enabled for enhanced station  202 - 1  and other enhanced stations. 
     In accordance with the third illustrative embodiment, enhanced station  202 - 2  (i.e., the access point) alternately switches between disabling transmission protection and enabling transmission protection for both its own transmissions and for enhanced station  202 - 1 &#39;s transmissions. The intervals during which protection is activated facilitate the transmission of any pending frames from legacy station  201 . 
     With message  801 , enhanced station  202 - 2  broadcasts a management frame indicating inactive protection status. The management frame can be a Beacon frame or a Probe-Response frame with the protection status field set to inactive, thereby directing enhanced station  202 - 1 , as well as other enhanced stations, to disable transmission protection for its own transmissions during the first time period of length T 1 . 
     With message  802 , enhanced station  202 - 2  broadcasts a management frame indicating active protection status. The management frame can be a Beacon frame or a Probe-Response frame with the protection status field set to active, thereby directing enhanced station  202 - 1 , as well as other enhanced stations, to enable transmission protection for its own transmissions during the second time period of length T 2 . 
     With message  803 , enhanced station  202 - 2  broadcasts a management frame indicating inactive protection status for the third time period. 
     With message  804 , enhanced station  202 - 2  receives a legacy modulation frame from legacy station  201 . In accordance with the illustrative embodiment of the present invention, enhanced station  202 - 2  reduces the length T 3  of the unprotected third time period as a result of having detected the legacy modulation frame of message  804 . Length T 3 , for example, can be made shorter than length T 1 , the length of the protected period that follows message  801 . 
     With message  805 , enhanced station  202 - 2  effectively ends the unprotected period by broadcasting a management frame indicating active protection status. The length T 4  of the protected fourth period is made greater than the length of T 2 , the length of the previous protected period (i.e., the second time period). 
     With message  806 , enhanced station  202 - 2  resumes alternately enabling and disabling protection for set lengths (e.g., T 5  is made equal to T 1 , etc.) by broadcasting a management frame indicating inactive protection status for the fifth time period. 
       FIG. 9  depicts a message flow diagram in accordance with the fourth illustrative embodiment of the present invention. Transmission protection is initially disabled for enhanced station  202 - 1  and other enhanced stations. 
     With message  901 , enhanced station  202 - 2  (i.e., the access point) transmits a frame using the Orthogonal Frequency Division Multiplexing (i.e., enhanced) modulation scheme to enhanced station  202 - 1 . This Orthogonal Frequency Division Multiplexing frame comprises a duration field with a value that is used by all stations that receive the frame to set their network allocation vectors (NAV) and refrain from transmitting, in well-known fashion. During the time interval corresponding to the duration field value (depicted as being K time units in length), enhanced stations, including enhanced station  202 - 1 , refrain from transmitting any frames and are muted. 
     Meanwhile, legacy station  201  is unable to detect this frame, because legacy station  201 , as well as other legacy stations, is unable to detect Orthogonal Frequency Division Multiplexing modulations, and as a result will not set its network allocation vector. Legacy station  201 , therefore, has an opportunity to transmit while the enhanced stations are muted. 
     With message  902 , legacy station  201  transmits a frame to enhanced station  202 - 2  using a legacy modulation scheme (e.g., Barker, Complementary Code Keying, etc.). 
     At time  903 , the time interval ends for enhanced stations to be muted. 
     With message  904 , enhanced station  202 - 1  is allowed to transmit a frame to enhanced station  202 - 2 . 
     With message  905 , the muting cycle begins again, starting with enhanced station  202 - 2  transmitting a frame using the Orthogonal Frequency Division Multiplexing modulation scheme to enhanced station  202 - 1 . This Orthogonal Frequency Division Multiplexing frame comprises a duration field with a value that is used by all stations that receive the frame to set their network allocation vectors (NAV) and refrain from transmitting, in well-known fashion. During the time interval corresponding to the duration field value (depicted as being K time units in length), enhanced station  202 - 1 , noting the duration field value, refrains from transmitting any frames. 
     At time  906 , the time interval ends for enhanced stations to be muted. 
     The illustrative embodiment of the present invention takes advantage of the fact that when enhanced station  202 - 2  transmits a frame comprising a duration value using an enhanced modulation scheme, the legacy stations (e.g., legacy station  201 , etc.) are unable to detect the frame and, as a result, continue to access the shared-communications channel as needed. This is in contrast to earlier techniques disclosed outside of this specification in which a station transmits a frame comprising a duration value to mute both enhanced and legacy stations. 
     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 invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.