CROSS-REFERENCE TO RELATED  APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/469,263, entitled “Variable-Width Modulation Schemes Featuring Backward-Compatible Local Area Network Operation,” filed on May 9, 2003, which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to 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 legacy stations  101 - 1  through  101 -K, wherein K is a positive integer, and enhanced stations  102 - 1  through  102 -L, wherein L is a positive integer. Legacy stations  101 - 1  through  101 -K and enhanced stations  102 - 1  through  102 -L use shared-communications medium  103  to communicate among themselves. Shared-communications medium  103  comprises multiple shared-communications channels. Only one of the stations can transmit into a given channel at a time, although one of the stations can transmit into a first channel while another station transmits into a second channel. 
     When two stations transmit into a given channel at the same time, the result is a cacophony and both transmissions are garbled. Prior art techniques, such as Carrier Sense Multiple Access, are used by the stations to coordinate when each of them transmits over a single shared-communications channel. 
     Legacy stations  101 - 1  through  101 -K can transmit and receive using:
     i. a modulation scheme that uses one shared-communications channel at a time (i.e., a single-channel modulation scheme).
 
In contrast, enhanced stations  102 - 1  through  102 -L can transmit and receive using:
   i. a modulation scheme that uses one shared-communications channel at a time (i.e., a single-channel modulation scheme), and   ii. a modulation scheme that uses multiple shared-communications channels simultaneously (i.e., a multi-channel modulation scheme).   

     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 each other by using a multi-channel modulation scheme, but any communication involving a legacy station must use a single-channel modulation scheme. 
     Enhanced stations  102 - 1  and  102 - 2 , for example, communicate with each other by using the multi-channel modulation scheme when possible because it enhances communication throughput in comparison to the single-channel modulation scheme. One effect of using the multi-channel modulation scheme, however, is that legacy station  101 - 1 , for example, cannot detect when enhanced stations  102 - 1  and  102 - 2  are communicating (i.e., enhanced stations  102 - 1  and  102 - 2  are essentially undetectable by legacy station  101 - 1  when enhanced stations  102 - 1  and  102 - 2  are communicating by using the multi-channel modulation scheme). This can cause legacy station  101 - 1  to transmit when enhanced stations  102 - 1  and  102 - 2  are communicating, which causes all of the transmissions to be garbled. 
     Even if enhanced station  102 - 1  successfully transmits the data block to enhanced station  102 - 2 , enhanced station  102 - 2  might subsequently have difficulty transmitting an acknowledgment of the data block back to enhanced station  102 - 1 . The difficulty in transmitting the acknowledgment is attributed to the same reason of enhanced nodes being undetectable that was described earlier. In the event of an unsuccessful acknowledgment transmission, enhanced station  102 - 1  has to re-transmit the data block in its entirety. 
     What is needed is a way to improve data transmission over multiple shared-communications channels without some of the performance disadvantages in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique that improves the efficiency of transmitting data over multiple shared-communications channels without some of the costs and disadvantages associated with techniques in the prior art. In particular, and in accordance with the illustrative embodiment of the present invention, a data frame is simultaneously transmitted across a plurality of channels, but the acknowledgment frame to the data frame is not. Instead, a plurality of redundant acknowledgment frames are generated and each one is transmitted independently and within each of the channels used to transmit the data frame. 
     For example, a single data frame is transmitted across two channels, and a redundant acknowledgment frame is transmitted in each of the two channels in legacy mode. Although a single acknowledgment frame could be transmitted across the two channels, the transmission of redundant acknowledgment frames in each channel is advantageous because it increases the likelihood that the acknowledgment will be received. The likelihood that the acknowledgment will be received is increased in accordance with the present invention because the acknowledgment will be received even when one of the channels is unusable, whereas the disruption of either channel would prevent the reception of an acknowledgment frame that spans two channels. 
     When the likelihood that the acknowledgment will be received is increased, the number of unnecessary data retransmissions is reduced, which saves channel bandwidth. 
     The station receiving the acknowledgment, in some embodiments, can take action based on (i) the contents of the received acknowledgment frame or frames and (ii) the channel or channels over which the acknowledgment was received. For example, depending on the indications provided in a received acknowledgment and the channel over which the acknowledgment was received, the station might decide to re-transmit some of the data block or the entire data block over one or more shared-communications channels, or the station might decide that a re-transmission is unnecessary. 
     In some embodiments of the present invention, before transmitting the data frame, a station transmits a control frame (e.g., a clear_to_send frame, etc.) into each communications channel to ensure that the communications channel is available. The multiple control frames specify a duration value that protects the subsequent transmission over multiple channels. 
     An illustrative embodiment of the present invention comprises: receiving a first portion of a data block on a first shared-communications channel; receiving a second portion of the data block on a second shared-communications channel; transmitting a first acknowledgment frame into the second shared-communications channel only, wherein the first acknowledgment frame indicates receipt of the first portion of the data block; and transmitting a second acknowledgment frame into the first shared-communications channel only, wherein the second acknowledgment frame indicates receipt of the second portion of the data block. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of wireless local area network  100  in the prior art. 
         FIG. 2  depicts a schematic diagram of a portion of network  200  in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a block diagram of the salient components of station  203 -j in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts a flowchart of the salient tasks performed in accordance with the first illustrative embodiment of the present invention. 
         FIG. 5  depicts a message flow diagram in accordance with the first illustrative embodiment of the present invention. 
         FIG. 6  depicts a flowchart of the salient tasks performed in accordance with the second illustrative embodiment of the present invention. 
         FIG. 7  depicts a message flow diagram in accordance with the second illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  depicts a schematic diagram of network  200  in accordance with the illustrative embodiment of the present invention. Network  200  operates in accordance with the IEEE 802.11 set of protocols and comprises legacy stations  201 - 1  through  201 -M, wherein M is a positive integer; legacy host computers  202 - 1  through  202 -M; enhanced stations  203 - 1  through  203 -N, wherein N is a positive integer; host computers  204 - 1  through  204 -N; and wireless communications medium  205 , interconnected as shown. Wireless communications medium  205  comprises shared-communications channels  206 - 1  through  206 -P. 
     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. 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 communications medium. 
     Legacy station  201 -i, for i=1 through M, comprises the radios that enable legacy host computer  202 -i to communicate via communications medium  205  by using a single shared-communications channel  206 -k at a time, wherein k is a value between 1 and P, inclusive. Legacy station  201 -i is capable of receiving data blocks from legacy host computer  202 -i and transmitting over communications medium  205  data frames comprising the data received from legacy host computer  202 -i. Legacy station  201 -i is also capable of receiving data frames from shared communications channel  205  and sending to legacy host computer  202 -i data blocks comprising data from the data frames. It will be clear to those skilled in the art how to make and use legacy station  201 -i. 
     Legacy host computer  202 -i is capable of generating data blocks and transmitting those data blocks to legacy station  201 -i. Legacy host computer  202 -i is also capable of receiving data blocks from legacy station  201 -i and of processing and using the data contained within those data blocks. Legacy host computer  202 -i can be, for example, a desktop or a laptop computer that uses network  200  to communicate with other hosts and devices. It will be clear to those skilled in the art how to make and use legacy host computer  202 -i. 
     Enhanced station  203 -j (or, concisely, “station”  203 -j), for j=1 through N, comprises the radios that enable host computer  204 j to communicate via communications medium  205  by using one or more of shared-communications channels  206 - 1  through  206 -P at a time. Station  203 -j is capable of receiving data blocks from host computer  204 -j and transmitting over communications medium  205  data frames comprising the data received from host computer  204 -j. Station  203 -j is also capable of receiving data frames from shared communications channel  205  and sending to host computer  204 -j data blocks comprising data from the data frames. It will be clear to those skilled in the art, after reading this specification, how to make and use station  203 -j. The salient details for station  203 -j are described below and with respect to  FIG. 3 . 
     Host computer  204 -j is capable of generating data blocks and transmitting those data blocks to station  203 -j. Host computer  204 -j is also capable of receiving data blocks from station  203 -j and of processing and using the data contained within those data blocks. Host computer  204 -j can be, for example, a desktop or a laptop computer that uses network  200  to communicate with other hosts and devices. It will be clear to those skilled in the art how to make and use host computer  204 -j. 
     In some embodiments, station  203 j, wherein j is equal to a value between 1 and N, inclusive, is an access point as is known in the art. Station  203 j, when functioning as an access point, enables other stations  203 - 1  through  203 -N within network  200  to communicate with devices in other communications networks. Furthermore, stations  203 - 1  through  203 -N communicate with each other through the access point, because the access point can be used to coordinate communications on network  200 . 
     Communications medium  205  comprises one or more shared-communications channels  206 -k, for k=1 through P. Shared-communications channel  206 -k exists in a variety of forms. For example, each shared-communications channel  206 -k can correspond to a given block of frequency spectrum out of multiple blocks, contiguous or otherwise. It will be clear to those skilled in the art how to allocate one or more shared-communications channels  206 -k, for k=1 through P. 
       FIG. 3  depicts a block diagram of the salient components of station  203 -j in accordance with the illustrative embodiment of the present invention. Station  203 -j comprises receiver  301 -j, processor  302 -j, memory  303 -j, and transmitter  304 -j, interconnected as shown. 
     Receiver  301 -j is a circuit that is capable of receiving frames from one or more of shared-communications channels  206 - 1  through  206 -P, in well-known fashion, and of forwarding them to processor  302 -j. It will be clear to those skilled in the art how to make and use receiver  301 -j. 
     Processor  302 -j is a general-purpose processor that is capable of performing the tasks described below and with respect to  FIGS. 4 through 7 . It will be clear to those skilled in the art, after reading this specification, how to make and use processor  302 -j. 
     Memory  303 -j is capable of storing programs and data used by processor  302 -j. It will be clear to those skilled in the art how to make and use memory  303 -j. 
     Transmitter  304 -j is a circuit that is capable of receiving frames from processor  302 -j, in well-known fashion, and of transmitting them on one or more of shared-communications channels  206 - 1  through  206 -P. It will be clear to those skilled in the art how to make and use transmitter  304 -j. 
       FIG. 4  depicts a flowchart in accordance with the first illustrative embodiment of the present invention. In the example depicted, station  203 - 1  exchanges frames with station  203 - 2 . It will be clear to those skilled in the art, after reading this specification, how to make and use stations that exchange frames in the manner depicted. 
     For pedagogical purposes, two shared-communications channels are used throughout the examples in this specification. It will be clear, however, to those skilled in the art, after reading this specification, how to apply the illustrative embodiment of the present invention to two or more shared-communications channels. 
     At task  401 , in some embodiments, station  203 - 1  transmits a first control frame on shared-communications channel  206 - 1  in well-known fashion. In some embodiments, the first control frame is a request_to_send frame or a clear_to_send frame, as is known in the art. It will be clear to those skilled in the art how to make and use a control frame. 
     Station  203 - 1 , in some embodiments, also transmits a second control frame on shared-communications channel  206 - 2 . In some embodiments, the second control frame is a request_to_send frame or a clear_to_send frame, as is known in the art. 
     Station  203 - 1 , in some embodiments, transmits the first and second control frame at substantially the same time. Transmitting the first and second control frame at substantially the same time and with the duration fields set to an effective value has the effect of protecting a pending data block transmission that utilizes both shared-communications channel  206 - 1  and  206 - 2 . It will be clear to those skilled in the art how to set the duration field to a value that is effective in covering the time interval of a subsequent data transmission. In this specification, control frames transmitted in this fashion are referred to as providing “stacked protection.” 
     At task  402 , station  203 - 1  transmits to station  203 - 2  a first portion of a data block on shared-communications channel  206 - 1 , in well-known fashion. 
     At task  403 , station  203 - 1  transmits to station  203 - 2  a second portion of the data block on shared-communications channel  206 - 2 , in well-known fashion. In some embodiments, the first portion corresponds to the part of a multi-channel-modulated signal that utilizes shared-communications channel  206 - 1 , while the second portion corresponds to the part of the same multi-channel-modulated signal that utilizes shared-communications channel  206 - 2 . Stated differently, the first portion and the second portion constitute the same modulated signal. In other embodiments, the first portion corresponds to a first collection of bits constituting a data block, while the second portion corresponds to a second collection of bits constituting the same data block. It will be clear to those skilled in the art how to allot portions of a data block across multiple shared-communications channels in other ways. 
     Station  203 - 1 , in some embodiments, transmits the first portion and the second portion of the data block at substantially the same time. 
     At task  404 , station  203 - 2  receives the first portion of the data block. 
     At task  405 , station  203 - 2  receives the second portion of the data block. 
     At task  406 , station  203 - 2  transmits to station  203 - 1  a first acknowledgment frame on shared-communications channel  206 - 2 . The first acknowledgment frame comprises an indication of having received (i) the first portion of the data block, (ii) the second portion of the data block, or (iii) both. In other embodiments, the first acknowledgment frame comprises an indication of having received either (i) all of the transmitted portions of the data block or (ii) not all of the transmitted portions of the data block. It will be clear to those skilled in the art how to provide the indication in the first acknowledgment frame. 
     At task  407 , station  203 - 2  transmits to station  203 - 1  a second acknowledgment frame on shared-communications channel  206 - 1 . The second acknowledgment frame comprises an indication of having received (i) the first portion of the data block, (ii) the second portion of the data block, or (iii) both. It will be clear to those skilled in the art how to provide the indication in the second acknowledgment frame. 
     Station  203 - 2 , in some embodiments, transmits the first acknowledgment frame and second acknowledgment frame at substantially the same time. 
     At task  408 , station  203 - 1  receives the first acknowledgment frame. 
     At task  409 , station  203 - 1  receives the second acknowledgment frame. 
     In some embodiments, station  203 - 1  transmits an additional portion of the data block after receiving one or both of the first and second acknowledgment frames. Station  203 - 1 , in some embodiments, transmits the additional portion of the data block on one shared-communications channel  206 -k (e.g., shared-communications channel  206 - 1 , etc.). In other embodiments, station  203 - 1  transmits the additional portion of the data block on more than one shared-communications channel  206 -k. 
       FIG. 5  depicts a message flow diagram in accordance with the first illustrative embodiment of the present invention. In the two examples depicted, station  203 - 1  exchanges frames with station  203 - 2 . It will be clear to those skilled in the art, after reading this specification, how to make and use stations that exchange frames in the manner depicted. 
     In the first example, station  203 - 1  transmits clear_to_send control frames  501  and  502  in well-known fashion. Each of control frames  501  and  502  specify a duration value as is known in the art that protects the pending transmission of a data block on shared-communications channel  206 - 1  and concurrently on shared-communications channel  206 - 2 . 
     Station  203 - 1  then transmits to station  203 - 2  data block  503  using both shared-communications channels  206 - 1  and  206 - 2 . 
     Upon receiving data block  503 , station  203 - 2  transmits to station  203 - 1  acknowledgment frames  504  and  505  in well-known fashion. Acknowledgment frames  504  and  505  each comprise an indication of the channel or channels on which data block  503  was received. 
     In the second example, station  203 - 1 , an access point in this example, transmits poll frames  506  and  507  in well-known fashion. Each of poll frames  506  and  507  specify a duration value as is known in the art that protects the pending transmission of a data block on shared-communications channel  206 - 1  and concurrently on shared-communications channel  206 - 2 . 
     Station  203 - 1  then transmits to station  203 - 2  data block  508  using both shared-communications channels  206 - 1  and  206 - 2 . 
     Upon receiving data block  508 , station  203 - 2  transmits to station  203 - 1  acknowledgment frames  509  and  510  in well-known fashion. Acknowledgment frames  509  and  510  each comprise an indication of the channel or channels on which data block  508  was received. 
       FIG. 6  depicts a flowchart in accordance with the second illustrative embodiment of the present invention. In the example depicted, station  203 - 3  processes frames in accordance with the illustrative embodiment. 
     At task  601 , station  203 - 3  receives an acknowledgment after having transmitted a data block via both shared-communications channels  206 - 1  and  206 - 2 . The acknowledgment indicates receipt of at least one portion of the data block on (i) shared-communications channel  206 - 1 , (ii) shared-communications channel  206 - 2 , or (iii) both. 
     At task  602 , station  203 - 3  evaluates the received acknowledgment. Station  203 - 3  checks if the acknowledgment was received on both channels or one channel. Station  203 - 3  checks each acknowledgment frame received for an indication of the channel or channels over which the data block was successfully received. 
     At task  603 , station  203 - 3  re-transmits a portion of the data block. Station  203 - 3  re-transmits on one of or both shared-communications channels  206 - 1  and  206 - 2 . In accordance with the illustrative embodiment of the present invention, station  203 - 3  chooses the channel or channels over which to re-transmit based on the evaluation performed at task  602 . 
       FIG. 7  depicts a message flow diagram in accordance with the second illustrative embodiment of the present invention. In the two examples depicted, station  203 - 3  exchanges frames with station  203 - 4  in accordance with the illustrative embodiment. 
     In the first example depicted, station  203 - 3  transmits data block  701  to station  203 - 4  in well-known fashion. Data block  701  comprises a first portion that is transmitted on shared-communications channel  206 - 1  and a second portion that is transmitted on shared-communications channel  206 - 2 . 
     Station  203 - 4  receives the first portion of data block  701  on shared-communications channel  206 - 1 . As part of the example, station  203 - 4  does not receive the second portion of data block  701 . Station  203 - 4  transmits to station  203 - 3  first acknowledgment frame  702  indicating that has received the first portion of data block  701 . 
     Acknowledgment frame  702  comprises fields that are used to indicate receipt of the corresponding portions of data. For instance, the fields can indicate the particular portion of data received, the shared-communications channel over which the portion was received, etc. 
     Station  203 - 3  then receives first acknowledgment frame  702  on shared-communications channel  206 - 1  from station  203 - 4 . Station  203 - 3  determines that it only received an acknowledgment frame (i.e., acknowledgment frame  702 ) on shared-communications channel  206 - 1  and that received acknowledgment frame  702  indicated that only the first portion of data block  701  was received by station  203 - 4  and on shared-communications channel  206 - 1 . 
     Consequently, station  203 - 3  re-transmits the second portion of data block  701 , in the form of data block  703 , on shared-communications channel  206 - 1 . Station  203 - 3  bases this decision on the contents of received acknowledgment  702  and on the shared-communications channel on which acknowledgment  702  was received (i.e., shared-communications channel  206 - 1 ). 
     In the second example depicted, station  203 - 3  transmits data block  704  to station  203 - 4  in well-known fashion. Data block  704  comprises a first portion that is transmitted on shared-communications channel  206 - 1  and a second portion that is transmitted on shared-communications channel  206 - 2 . 
     The difference of the second example from the first example is that station  203 - 4  receives the second portion of data block  704  on shared-communications channel  206 - 2 . Also, station  203 - 4  does not receive the first portion of data block  704 . Station  203 - 4  transmits to station  203 - 3  first acknowledgment frame  705  indicating that it has received the second portion of data block  704 . 
     Station  203 - 3  then receives first acknowledgment frame  705  on shared-communications channel  206 - 1  from station  203 - 4 . Station  203 - 3  determines that it only received an acknowledgment frame (i.e., acknowledgment frame  705 ) on shared-communications channel  206 - 1  and that received acknowledgment frame  705  indicated that only the second portion of data block  704  was received by station  203 - 4  and on shared-communications channel  206 - 2 . 
     Consequently, station  203 - 3  re-transmits the first portion of data block  704 , in the form of data block  706 , using both shared-communications channels  206 - 1  and  206 - 2 . Station  203 - 3  bases this decision on the contents of received acknowledgment  705  and on the shared-communications channel on which the acknowledgment was received (i.e., shared-communications channel  206 - 1 ). 
     It will be clear to those skilled in the art that other outcomes are possible, with respect to re-transmitting portions of data blocks based on the contents of an acknowledgment, the shared-communications channel on which the acknowledgment was received, and other conditions. 
     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. For example, in this Specification, numerous specific details are provided in order provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
     Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.
A technique is disclosed that improves the efficiency of transmitting data over multiple shared-communications channels without some of the costs and disadvantages associated with techniques in the prior art. In particular, and in accordance with the illustrative embodiment of the present invention, a data frame is simultaneously transmitted across a plurality of channels, but the acknowledgment frame to the data frame is not. Instead, a plurality of redundant acknowledgment frames are generated and each one is transmitted independently and within each of the channels used to transmit the data frame.