Abstract:
An apparatus and method for informing a coordinator of the particular characteristics of a periodic traffic source are disclosed. A station that generates a periodic traffic stream encodes the temporal period and temporal offset of the traffic stream within a quality-of-service (QoS) traffic specification, and transmits the traffic specification with a poll request. The coordinator, upon receiving a polling request, processes the associated traffic specification and, via appropriate decoding logic, determines whether the requesting station generates periodic traffic, and if so, the temporal period and temporal offset of the traffic stream. The coordinator subsequently can establish, based on the temporal period and temporal offset, a polling schedule that minimizes the delay between (i) the station generating a frame, and (ii) the station transmitting the frame (and thus the destination receiving the frame).

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional patent application Serial No. 60/433,604, filed Dec. 16, 2002, entitled “Poll Scheduling and Power Saving,” (Attorney Docket: 3655-0184P), which is also incorporated by reference.  
         [0002]    The following patent application is incorporated by reference: U.S. patent application Ser. No. 60/______, filed on Sep. 29, 2003, Attorney Docket 630-039, entitled “Exploratory Polling of Periodic Traffic Sources.” 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention relates to telecommunications in general, and, more particularly, to local area networks.  
         BACKGROUND OF THE INVENTION  
         [0004]    [0004]FIG. 1 depicts a schematic diagram of wireless local-area network  100  in the prior art, which comprises: access point  101 , stations  102 - 1  through  102 -N, wherein N is a positive integer, and hosts  103 - 1  through  103 -N, interconnected as shown. Each station  102 -i, wherein i is a positive integer in the set {1, . . . N}, enables host  103 -i (a device such as a notebook computer, personal digital assistant [PDA], tablet PC, etc.) to communicate wirelessly with other hosts in local-area network  100  via access point  101 .  
           [0005]    Access point  101  and stations  102 - 1  through  102 -N transmit blocks of data called frames. A frame typically comprises a data portion, referred to as a data payload, and a control portion, referred to as a header. Frames transmitted from a station  102 -i to access point  101  are referred to as uplink frames, and frames transmitted from access point  101  to a station  102 -i are referred to as downlink frames. A series of frames transmitted from a station  102 -i to access point  101  is referred to as an uplink traffic stream, and a series of frames transmitted from access point  101  to a station  102 -i is referred to as a downlink traffic stream.  
           [0006]    Access point  101  and stations  102 - 1  through  102 -N transmit frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). Consequently, local-area networks typically employ protocols for ensuring that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit one or more frames.  
           [0007]    Such protocols can be classified into two types: contention-based protocols, and contention-free protocols. In a contention-based protocol, stations  102 - 1  through  102 -N and access point  101  compete to gain exclusive access to the shared-communications channel, just as, for example, several children might fight to grab a telephone to make a call.  
           [0008]    In a contention-free protocol, in contrast, a coordinator (e.g., access point  101 , etc.) grants access to the shared-communications channel to one station at a time. An analogy for contention-free protocols is a parent (i.e., the coordinator) granting each of several children a limited amount of time on the telephone to talk, one at a time. One technique in which a coordinator can grant access to the shared-communications channel is polling. In protocols that employ polling, stations submit a polling request (also referred to as a reservation request) to the coordinator, and the coordinator grants stations exclusive access to the shared-communications channel sequentially in accordance with a polling schedule. A polling schedule has a temporal period (e.g., 5 seconds, etc.) and continually loops back to the beginning of the schedule after its completion. Since stations transmit only in response to a poll from the coordinator, polling-based protocols can provide contention-free access to the shared-communications channel.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention enables a station that transmits periodic traffic (e.g., a station that transmits a frame every 25 milliseconds, etc.) to inform a coordinator of the particular characteristics of the periodic traffic. In particular, in the illustrative embodiment a station:  
           [0010]    (i) encodes  
           [0011]    a temporal period that specifies the periodicity of the station&#39;s traffic stream (e.g., 25 milliseconds, etc.), and  
           [0012]    a temporal offset that specifies the phase of the periodic traffic stream with respect to a particular reference (e.g., an IEEE 802.11 beacon, etc.)  
           [0013]    in a traffic specification (e.g., an IEEE 802.11e TSPEC, etc.) that specifies QoS-related information, and  
           [0014]    (ii) transmits the encoded traffic specification along with its poll request to the coordinator.  
           [0015]    When a coordinator receives a polling request, the coordinator processes the associated traffic specification and, via appropriate decoding logic, determines whether the requesting station generates periodic traffic, and if so, the temporal period and temporal offset of the traffic stream. The coordinator then establishes a polling schedule so that the station is polled as soon as possible after generating a frame, thereby minimizing the delay between (i) the station generating a frame, and (ii) the station transmitting the frame (and thus the destination receiving the frame). This is especially advantageous in real-time communications such as voice calls and instant messaging.  
           [0016]    The illustrative embodiment comprises: populating a first field of a traffic specification with a function of one of a temporal period and a temporal offset, wherein the temporal period and the temporal offset are for a plurality of expected future transmissions; populating a second field of the traffic specification with the value of the first field; and transmitting a polling request with the traffic specification. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 depicts a schematic diagram of an exemplary wireless local-area network  100  in the prior art.  
         [0018]    [0018]FIG. 2 depicts a schematic diagram of a portion of local-area network  200  in accordance with the illustrative embodiment of the present invention.  
         [0019]    [0019]FIG. 3 depicts a block diagram of the salient components of access point  201 , as shown in FIG. 2, in accordance with the illustrative embodiment of the present invention.  
         [0020]    [0020]FIG. 4 depicts a block diagram of the salient components of station  202 -i, as shown in FIG. 2, in accordance with the illustrative embodiment of the present invention.  
         [0021]    [0021]FIG. 5 depicts an event loop of the salient tasks performed by a station  202 -i that transmits periodic traffic, in accordance with the illustrative embodiment of the present invention.  
         [0022]    [0022]FIG. 6 depicts a flowchart of the salient tasks performed by access point  201  in establishing a polling schedule, in accordance with the illustrative embodiment of the present invention.  
         [0023]    [0023]FIG. 7 depicts a flowchart of the salient tasks performed by access point  201  in establishing a downlink transmission schedule, in accordance with the illustrative embodiment of the present invention.  
         [0024]    [0024]FIG. 8 depicts a flowchart of the salient tasks performed by access point  201  in combining a polling schedule and a transmission schedule into a composite schedule, in accordance with the illustrative embodiment of the present invention.  
         [0025]    [0025]FIG. 9 depicts an event loop for access point  201  for processing a composite schedule, in accordance with the illustrative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]    [0026]FIG. 2 depicts a schematic diagram of local-area network  200  in accordance with the illustrative embodiment of the present invention. Local-area network  200  comprises access point  201 , stations  202 - 1  through  202 -N, wherein i is a positive integer in the set {1, . . . N}, and hosts  203 - 1  through  203 -N, interconnected as shown.  
         [0027]    As shown in FIG. 2, station  202 -i enables host  203 -i to communicate wirelessly with other hosts in local-area network  200  via access point  201 .  
         [0028]    Host  203 -i is a device (e.g., a computer, a personal digital assistant, a printer, etc.) that is capable of generating and transmitting data to station  202 -i. Host  203 i is also capable of receiving, processing, and using the data received from station  202 -i. It will be clear to those skilled in the art how to make and use host  203 -i.  
         [0029]    Station  202 -i is capable of receiving data from host  203 -i and of transmitting that data over a shared-communications channel to access point  201 . Station  202 -i is also capable of receiving frames from the shared communications channel and of sending that data to host  203 -i. The salient details of station  202 -i are described below and with respect to FIGS. 4 and 5.  
         [0030]    Access point  201  receives frames from stations  202 - 1  through  202 -N in accordance with a polling schedule and transmits frames to  202 - 1  through  202 -N in accordance with a transmission schedule. The salient details of access point  201  are described below and with respect to FIGS. 3 and 6 through  10 .  
         [0031]    [0031]FIG. 3 depicts a block diagram of the salient components of access point  201  in accordance with the illustrative embodiment of the present invention. Access point  201  comprises receiver  301 , processor  302 , memory  303 , and transmitter  304 , interconnected as shown.  
         [0032]    Receiver  301  is a circuit that is capable of receiving frames from shared communications channel  203 , in well-known fashion, and of forwarding them to processor  302 . It will be clear to those skilled in the art how to make and use receiver  301 .  
         [0033]    Processor  302  is a general-purpose processor that is capable of executing instructions stored in memory  303 , of reading data from and writing data into memory  303 , and of executing the tasks described below and with respect to FIGS. 6 through 10. In some alternative embodiments of the present invention, processor  302  is a special-purpose processor (e.g., a network processor, etc.). In either case, it will be clear to those skilled in the art, after reading this disclosure, how to make and use processor  302 .  
         [0034]    Memory  303  is capable of storing programs and data used by processor  302 , as is well-known in the art, and might be any combination of random-access memory (RAM), flash memory, disk drive, etc. It will be clear to those skilled in the art, after reading this specification, how to make and use memory  303 .  
         [0035]    Transmitter  304  is a circuit that is capable of receiving frames from processor  302 , in well-known fashion, and of transmitting them on shared communications channel  203 . It will be clear to those skilled in the art how to make and use transmitter  304 .  
         [0036]    [0036]FIG. 4 depicts a block diagram of the salient components of station  202 -i, in accordance with the illustrative embodiment of the present invention. Station  202 -i comprises receiver  401 , processor  402 , memory  403 , and transmitter  404 , interconnected as shown.  
         [0037]    Receiver  401  is a circuit that is capable of receiving frames from shared communications channel  203 , in well-known fashion, and of forwarding them to processor  402 . It will be clear to those skilled in the art how to make and use receiver  401 .  
         [0038]    Processor  402  is a general-purpose processor that is capable of executing instructions stored in memory  403 , of reading data from and writing data into memory  403 , and of executing the tasks described below and with respect to FIG. 6. In some alternative embodiments of the present invention, processor  402  is a special-purpose processor (e.g., a network processor, etc.). In either case, it will be clear to those skilled in the art, after reading this disclosure, how to make and use processor  402 .  
         [0039]    Memory  403  is capable of storing programs and data used by processor  402 , as is well-known in the art, and might be any combination of random-access memory (RAM), flash memory, disk drive, etc. It will be clear to those skilled in the art, after reading this specification, how to make and use memory  403 .  
         [0040]    Transmitter  404  is a circuit that is capable of receiving frames from processor  402 , in well-known fashion, and of transmitting them on shared communications channel  203 . It will be clear to those skilled in the art how to make and use transmitter  404 .  
         [0041]    In the illustrative embodiment of the present invention, access point  201  and stations  202 - 1  through  202 -N support at least one IEEE 802.11 protocol. In some other embodiments access point  201  and stations  202 - 1  through  202 -N might support other protocols in lieu of, or in addition to, one or more IEEE 802.11 protocols. Furthermore, in some embodiments local-area network  200  might comprise an alternative shared-communications channel (for example, wired instead of wireless). In all such cases, it will be clear to those skilled in the art after reading this specification how to make and use access point  201  and stations  202 - 1  through  202 -N.  
         [0042]    [0042]FIG. 5 depicts an event loop of the salient tasks performed by a station  202 -i that transmits periodic traffic, for i=1 to N, in accordance with the illustrative embodiment of the present invention.  
         [0043]    At task  510 , station  202 -i transmits a polling request that specifies (i) the temporal period of expected future transmissions (e.g., 100 milliseconds, 3 seconds, etc.), and (ii) a temporal offset with respect to a particular reference (e.g., access point  201 &#39;s IEEE 802.11 beacon, etc.) The polling request thus informs access point  201  of the period of station  202 -i&#39;s future traffic stream, and the phase of the traffic stream with respect to the reference.  
         [0044]    As is well-known in the art, in local-area networks that operate in accordance with IEEE 802.11e, a version of 802.11 that supports quality-of-service (QoS), station  202 -i transmits a polling request to access point  201  in combination with a traffic specification (TSPEC) that characterizes, via a plurality of fields, traffic generated by station  202 -i. In some embodiments, an IEEE 802.11e-compliant station  202 -i might encode one or both of the temporal period and temporal offset in the traffic specification via one or more TSPEC fields. In one such encoding, station  202 -i populates both the TSPEC Minimum Service Interval and Maximum Service Interval fields with the temporal period. In accordance with this encoding, access point  201 , upon receipt of a polling request in which the associated traffic specification has a Minimum Service Interval field and a Maximum Service Interval field with the same value, recognizes that station  202 -i generates periodic traffic with a temporal period equal to this value. (A method by which access point  201  ascertains the temporal offset when a polling request specifies only the temporal period is disclosed in co-pending U.S. patent application “Exploratory Polling For Periodic Traffic Sources,” [Attorney Docket: 630-039us].)  
         [0045]    In another exemplary encoding, station  202 -i populates the Maximum Service Interval field with the temporal period, and the Minimum Service Interval field with the sum of the temporal period and the temporal offset. In accordance with this encoding, access point  201 , upon receipt of a polling request in which the associated traffic specification has a Minimum Service Interval field value that is greater than the Maximum Service Interval field value, deduces that station  202 -i generates periodic traffic with a temporal period equal to the Maximum Service Interval field value, and a temporal offset equal to the difference between the Minimum Service Interval and Maximum Service Interval field values.  
         [0046]    As will be appreciated by those skilled in the art, at task  510  station  202 -i can encode one or both of the temporal period and temporal offset as arbitrary functions of one or more traffic specification fields, and at task  610 , described below, access point  201  can retrieve the temporal period and temporal offset from the traffic specification via the appropriate decoding logic.  
         [0047]    At task  520 , station  202 -i queues, in well-known fashion, one or more frames in accordance with the temporal period and temporal offset specified at task  510 .  
         [0048]    At task  530 , station  202 -i receives a poll from access point  201  in well-known fashion.  
         [0049]    At task  540 , station  202 -i transmits the frame(s) queued at task  520  in accordance with the appropriate protocol (e.g., an IEEE 802.11 protocol, etc.). After task  540  has been completed, execution continues back at task  520 .  
         [0050]    [0050]FIG. 6 depicts a flowchart of the salient tasks performed by access point  201  in establishing a polling schedule, in accordance with the illustrative embodiment of the present invention.  
         [0051]    At task  610 , access point  201  receives a polling request from station  202 -i that specifies a temporal offset φ and period π, where i is a positive integer less than or equal to N. As described above, in an IEEE 802.11e network access point  201  might obtain φ and π from a traffic specification associated with the polling request in some embodiments.  
         [0052]    At task  620 , access point  201  checks whether a polling schedule P already exists. (P is a schedule for polls to all stations in local-area network  200 ; i.e., P can be thought of as the union of a plurality of polling schedules {P 1 , P 2 , . . . , P N }, where P i  is a polling schedule for station  202 -i. If polling schedule P already exists, execution proceeds to task  640 , otherwise execution proceeds to task  630 .  
         [0053]    At task  630 , access point  201  creates a new polling schedule P with one or more polls sent to station  202 -i in accordance with temporal offset φ and period π. Polling schedule P repeats continually, and thus in some embodiments it is particularly convenient to set the duration of schedule P to a value divisible by period π. After completion of task  630 , execution of the method of FIG. 6 terminates.  
         [0054]    At task  640 , access point  201  adds one or more polls of station  202 -i to polling schedule P in accordance with temporal offset φ and period π. In accordance with the illustrative embodiment, task  640  comprises, if necessary, adjusting the temporal period of schedule P accordingly. For example, if a poll that occurs every 6 seconds is to be added to a polling schedule that has a temporal period of 4, then the new polling schedule should have a temporal period of 12, comprising (i) three successive instances of the previous polling schedule, and (ii) two instances of the added poll.  
         [0055]    Formally, if polling schedule P previously included polls to a set of stations S and previously had a temporal period Q, then the temporal period of the new polling schedule P should be increased, if necessary, to a value Q′ such that for all stations  202 -n ∈ S, Q′ is divisible by π n , where π n  is the temporal period of station  202 -n. (In other words, Q′ is the least common multiple of the temporal periods of every station in polling schedule P.)  
         [0056]    As another example, when:  
         [0057]    the previous polling schedule P has  
         [0058]    (i) a temporal period of 6.0 seconds, and  
         [0059]    (ii) a single poll to station  202 -j at time 5.0 (i.e., π Y =6.0 and φ Y =5.0), wherein j is a positive integer such that j≦N and j≠i;  
         [0060]    and  
         [0061]    station  202 -i, which has temporal period π i =4.0 and offset φ i =2.0, is added to polling schedule P;  
         [0062]    then  
         [0063]    the new polling schedule will have  
         [0064]    (i) a temporal period of 12.0 seconds,  
         [0065]    (ii) polls to station  202 -j at times 5.0 and 11.0, and  
         [0066]    (iii) polls of station  202 -i at times 2.0, 6.0, and 10.0.  
         [0067]    As will be appreciated by those skilled in the art, the above method of constructing a polling schedule might result in simultaneous polls of two or more stations. As described above, since only one station can be polled at a time, the illustrative embodiment employs polling events in the polling schedule, where each polling event specifies a list of one or more stations to be polled. A description of how the illustrative embodiment processes the lists associated with polling events is disclosed below and with respect to FIG. 7 and FIG. 8.  
         [0068]    After completion of task  640 , execution of the method of FIG. 6 terminates.  
         [0069]    [0069]FIG. 7 depicts a flowchart of the salient tasks performed by access point  201  in establishing a transmission schedule, in accordance with the illustrative embodiment of the present invention. The transmission schedule specifies when access point  201  transmits buffered downlink traffic to stations  202 -i in wireless local-area network  200 , for i=1 to N.  
         [0070]    At task  710 , access point  201  monitors the arrival times of downlink frames received by access point  201  and transmitted from access point  201  to station  202 -i. The monitoring of task  710  occurs during a time interval τ that is sufficiently long to serve as an “observation period” for characterizing downlink traffic to station  202 -i. It will be clear to those skilled in the art how to choose a suitable value for τ.  
         [0071]    At task  720 , access point  201  determines, in well-known fashion, whether downlink frames for station  202 -i arrive in accordance with a regular temporal period, based on the observation period of task  710 . If the determination is affirmative, execution proceeds to task  730 , otherwise, the method of FIG. 7 terminates.  
         [0072]    At task  730 , access point  201  determines the temporal period π and temporal offset Φ of downlink frames for station  202 -i, where offset Φ is relative to the 802.11 beacon transmitted by access point  201 . It is well-known in the art how to determine the period and offset (i.e., “phase”) of a periodic traffic stream.  
         [0073]    At task  740 , access point  201  checks whether a transmission schedule T already exists. If none exists, execution proceeds to task  750 , otherwise execution continues at task  760 .  
         [0074]    At task  750 , a new transmission schedule T is created with downlink frames for station  202 -s transmitted in accordance with temporal offset Φ and temporal period π.  
         [0075]    At task  760 , existing transmission schedule T is augmented with the transmission of downlink frames to station  202 -i in accordance with temporal offset Φ and temporal period π. As will be appreciated by those skilled in the art, it is possible that a downlink transmission to station  202 -i occurs at the same time as another downlink transmission already in schedule T. Consequently, the illustrative embodiment maintains a list of one or more stations at each “transmission event” that indicates to which station(s) downlink frames should be transmitted. As is the case for polling schedule P, downlink frames are transmitted sequentially to the stations in the list, beginning at the time specified in transmission schedule T. The illustrative embodiment employs a mechanism disclosed below in the description of FIG. 9 for ensuring “fairness” with respect to the order in which downlink frames are transmitted to stations in a list.  
         [0076]    At optional task  770 , any collisions between the new transmission schedule T and polling schedule P (i.e., a transmission in schedule T and a poll in schedule P that occur simultaneously) are overcome by suitably adjusting (i.e., via a slight time shift) the appropriate newly-added transmission of schedule T. In some embodiments, one or more tasks or methods might be performed in lieu of task  770  for avoiding collisions between schedules P and T. (For example, the illustrative embodiment employs (i) the method of FIG. 8, disclosed below, for combining polling schedule P and transmission schedule T into a composite schedule, and (ii) the event loop mechanism of FIG. 9, disclosed below, for rotating between simultaneous polls and transmissions in round-robin fashion, in lieu of task  770 .  
         [0077]    [0077]FIG. 8 depicts a flowchart of the salient tasks performed by access point  201  in combining a polling schedule and a transmission schedule into a composite schedule, in accordance with the illustrative embodiment of the present invention.  
         [0078]    At task  810 , index variable i is initialized to 1.  
         [0079]    At task  820 , variable C, which is used to store the composite schedule, is initialized to transmission schedule T.  
         [0080]    At task  825 , for each combination of station and time in composite schedule C, an associated transmission flag is set to true, and an associated poll flag is set to false. In the illustrative embodiment, the transmission and poll flags are stored in composite schedule C; however, it will be clear to those skilled in the art that in some other embodiments these flags might be stored in a separate data structure.  
         [0081]    At task  830 , variable t is set to the time of the i th  poll of polling schedule P, and variable s is set to the station polled in the i th  poll of polling schedule P.  
         [0082]    At task  840 , access point  201  checks whether composite schedule C has a transmission at time t (obtained from transmission schedule T at task  820 ). If so, execution proceeds to task  850 , otherwise execution continues at task  880 .  
         [0083]    At task  850 , access point  201  checks whether station s is already in the list of stations at time t in composite schedule C. If so, execution proceeds to task  870 , otherwise, execution proceeds to task  860 .  
         [0084]    At task  860 , station s is added to the list of stations at time t in composite schedule C. The associated poll flag for station s at time t is set to true, and the associated transmission flag is set to false. After the completion of task  860 , execution continues at task  880 .  
         [0085]    At task  870 , the associated poll flag for station s at time t is set to true. After the completion of task  870 , execution continues at task  880 .  
         [0086]    At task  880 , access point  201  checks whether the poll at time t is the last poll in polling schedule P. If so, execution continues at task  890 , otherwise the method of FIG. 8 terminates.  
         [0087]    At task  890 , index variable i is incremented by 1. After the completion of task  890 , execution continues back at task  830 .  
         [0088]    [0088]FIG. 9 depicts an infinite event loop for access point  201  for processing composite schedule C, in accordance with the illustrative embodiment of the present invention. When the event loop is first started, execution begins at task  910 .  
         [0089]    At task  910 , variable E is set to the next event in composite schedule C. If the event loop has just been started, the next event is the first event of schedule C.  
         [0090]    At task  920 , variable L is set to the list of stations associated with event E.  
         [0091]    At task  930 , index variable i is initialized to 1.  
         [0092]    At task  940 , access point  201  checks whether the transmission and poll flags for the i th  station in list L (i.e., L[i]) are both true, indicating that a combined transmission/poll is the appropriate action. If so, execution proceeds to task  950 , otherwise execution continues at task  960 .  
         [0093]    At task  950 , access point  201  transmits a downlink frame with a “piggybacked” poll to station L[i] in well-known fashion. After the completion of task  950 , execution proceeds to task  985 .  
         [0094]    At task  960 , access point  201  checks whether the transmission and poll flags for L[i] are true and false, respectively, indicating that a downlink transmission is the appropriate action. If so, execution proceeds to task  970 , otherwise execution continues at task  980 .  
         [0095]    At task  970 , access point  201  transmits a downlink frame to station L[i] in well-known fashion. After the completion of task  970 , execution proceeds to task  985 .  
         [0096]    At task  980 , access point  201  transmits a poll to station L[i] in well-known fashion. After the completion of task  980 , execution proceeds to task  985 .  
         [0097]    At task  985 , access point  201  checks whether variable i is equal to the size of list L, indicating that all stations in the list have been processed in accordance with tasks  940  through  980 . If so, execution proceeds to task  990 , otherwise execution proceeds to task  995 .  
         [0098]    At task  990 , list L is rotated one position so that the first station in the list becomes the last, the second station in the list becomes the first, the third station becomes the second, etc. This establishes a new order for list L in preparation for processing event E in the next iteration of schedule C. After the completion of task  990 , execution continues back at task  910  for the next invocation of the event loop.  
         [0099]    At task  995 , index variable i is incremented by 1. After the completion of task  995 , execution continues back at task  940  for processing the next station in list L.  
         [0100]    Although the illustrative embodiment of the present invention is disclosed in the context of IEEE 802.11 local-area networks, it will be clear to those skilled in the art after reading this specification how to make and use embodiments of the present invention for other kinds of networks and network protocols.  
         [0101]    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.