Patent Publication Number: US-8526604-B2

Title: Enabling wireless clients for low-power operation when clients require control messages from external sources for communication

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     Embodiments of the present disclosure relate generally to wireless communications, and more specifically to techniques for enabling wireless clients for low-power operation when the clients require control messages from external sources for communication. 
     2. Related Art 
     A wireless network generally includes two or more devices (“wireless devices”) that communicate with each other over a wireless medium. A wireless local area network (WLAN) designed to operate according to IEEE 802.11 standard(s) is an example of a wireless network. Wireless devices (also termed wireless clients) of a wireless network may communicate with a wired network via an access point (AP). Wireless clients may be either mobile devices or be fixed (non-mobile). A wireless client (client) may transmit data to and receive data from other clients in the wireless network either directly or via the AP. A client may also transmit to and receive data from wireless devices outside the wireless network via the AP. 
     In addition to data (representing information of interest such as text files, images etc), a client may also receive other types of messages, such as control messages, from (or via) the AP of the wireless network. Control messages generally refer to messages that specify operational parameters that enable clients (and the AP, if present) to operate correctly or in a desired manner in the wireless network. Control messages may often be updated (and transmitted to the wireless clients) by the AP at appropriate time instances. Clients typically need to receive the updated control messages and perform corresponding actions specified or required by the control messages to ensure proper operation. 
     Clients are often operated in ‘power-save’ modes that permit reduction in power consumption. According to one example technique, a client may be powered-ON (and thus fully operational) only intermittently or periodically, while remaining in a power-OFF state in the remaining durations. One problem faced while operating in such power-save modes is that a client may be in a power-OFF state when control messages are transmitted by the AP. Not receiving one or more control messages may potentially disrupt normal operation (in powered-ON mode) of the client in the wireless network. One known technique used to address the problem noted above is to reduce the power-OFF durations of the client. Such an approach, however, may translate to a reduction in power savings in the client, and therefore may not desirable. 
     Several embodiments of the present disclosure are directed to enabling wireless clients for low-power operation when clients require control messages from external sources for communication. 
    
    
     
       BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS 
       Example embodiments of the present invention will be described with reference to the accompanying drawings briefly described below. 
         FIG. 1  is a block diagram of an example environment in which several features of the present invention can be implemented. 
         FIG. 2  is a diagram illustrating example waveforms representing transmission of beacon frames and control messages by a wireless station, and power-ON/power-OFF sequences of another wireless station, in an embodiment. 
         FIG. 3  is a flowchart illustrating the manner in which a wireless station, operating in a power-save mode, ensures receipt of control messages transmitted from another wireless station, in an embodiment. 
         FIG. 4  is a state transition diagram of a wireless station operating in a power-save mode, in an embodiment. 
         FIG. 5  is a diagram containing example waveforms used to illustrate the operation of a wireless station, in an embodiment. 
         FIG. 6  is a block diagram of the internal details of a wireless station in an embodiment. 
     
    
    
     The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION 
     1. Overview 
     According to an aspect of the present invention, a wireless station (A) is operated in a power-save mode, in which the station is alternately in power-ON and power-OFF states to reduce power consumption. Wireless station (A) computes at least some future time instances at which another wireless station (B) is expected to start transmitting control messages. Wireless station (A) is ensured to be in the power-ON state in corresponding time intervals encompassing durations of at least some of such future transmissions of control messages by wireless station (B), and is thereby enabled to receive the control messages. 
     In an embodiment, the control messages correspond to group key message updates in which values of a decryption key are transmitted. In the embodiment, wireless station (A) is a wireless client, and wireless station (B) is an access point. Wireless stations (A) and (B) operate in a wireless network consistent with IEEE 802.11 specifications, and communication between wireless stations (A) and (B) is encrypted. 
     Several aspects of the invention are described below with reference to examples for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One skilled in the relevant arts, however, will readily recognize that the invention can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the features of the invention. 
     2. Example Environment 
       FIG. 1  is a block diagram illustrating an example environment in which several features of the present invention can be implemented. The example environment is shown containing only representative systems for illustration. However, real-world environments may contain many more systems/components as will be apparent to one skilled in the relevant arts. Further, in the description below, the components and the environment are described as operating consistent with IEEE 802.11 standard(s), merely for illustration. Implementations in other environments are also contemplated to be within the scope and spirit of various aspects of the present invention. 
     System  100  is shown containing client devices (clients)  110 A- 110 E, access point (AP)  110 F, wired network  130 , wired network backbone  140  and wireless network manager  150 . Block  110  represents a basic service set (BSS) consistent with the 802.11 standard. Other environments may include more than one BSS, with the BSSs being interconnected to form an extended service set (ESS) consistent with IEEE 802.11 standards. While the description below is provided with respect to an infrastructure BSS, several embodiments of the present disclosure can be implemented in an independent BSS (IBSS) as well. As is well-known in the relevant arts, an IBSS is an ad-hoc network and does not contain access points. The term “wireless station” is used herein to refer to both a wireless client as well as an access point. 
     AP  110 F is connected by a wired medium ( 141 ) to wired network backbone  140  and thus to wired network  130 . Each of clients  110 A- 110 E may communicate with AP  110 F (as well as with each other) wirelessly according to any of the family of IEEE 802.11 protocols (including as specified in IEEE 802.11a, 802.11b, 802.11g and 802.11n) and thereby with wired network  130 . Wired network  130  may represent the internet, also known as the World Wide Web. One or more of clients  110 A- 110 E may correspond, for example, to a laptop computer, smart phone, or a wireless sensor. 
     Wireless network manager  150  transmits configuration and control messages to AP  110 F. Some of the configuration and control messages may be meant for clients  110 A- 110 E. Accordingly, AP  110 F forwards the corresponding configuration and control messages meant for the clients, either as unicast messages (directed to a specific one of clients  110 A- 110 E) or as broadcast messages. Although shown separate from AP  110 F, the features of wireless network manager may instead be integrated within AP  110 F in some embodiments. 
     Wireless network manager  150  may additionally be designed to operate as a controller of BSS  110 , and issue network commands to and receive data from one or more of clients  110 A- 110 E, and may thus operate to provide desired features such as building or plant automation, based on the specific environment in which the components of  FIG. 1  are deployed. The data received from clients  110 A- 110 E may represent measured values of desired parameters such as temperature, pressure, humidity, etc. In other embodiments, clients  110 A- 110 E may be deployed for purposes other than for providing features such as plant automation. For example, one or more of clients may represent a computing device such as a laptop, and may transfer data with other devices in BSS  110  or wired network  130  based on the requirements of the user of the laptop. 
     One or more of clients  110 A- 110 E may be designed to operate in a ‘power-save’ mode. For example, in the context of IEEE 802.11 operation, a client (assumed to be client  110 A herein for simplicity) may operate in the standard Power Save Poll Mode (PSPM, or power-save mode, in general). Upon joining BSS  110 , client  110 A communicates to AP  110 F that it (client  110 A) is to operate in PSPM. In PSPM, client  110 A periodically “wakes up” (i.e., powers-ON for full functionality) from a power-OFF state to transmit data to, or receive data from, AP  110 F or the other clients of BSS  110 . 
     Waveform  210  of  FIG. 2  is an example waveform illustrating periodic power-ON and power-OFF sequences of client  110 A in PSPM. Waveform  220  represents an example of periodic beacon frames transmitted by AP  110 F. Waveform  230  represents an example sequence of periodic control messages transmitted by AP  110 F. Waveform  240  represents an example of transmission of an asynchronous control message in a sequence of otherwise periodic control messages transmitted by AP  110 F. Interval t 20 -t 23  represents the interval between successive power-ON (wake) states, and is termed the ‘listen interval’. Interval t 20 -t 22  is the duration for which client  110 A is in the power-ON state in each listen interval, and may be different for different listen intervals depending on the volume of data to be transmitted or received or other considerations. Upon joining BSS  110 , client  110 A communicates the listen interval to AP  110 F. The listen interval is typically a multiple of a beacon frame interval/period. 
     Beacon frames refer to a type of management frame specified by the IEEE 802.11 standard, and are periodically transmitted by AP  110 F. Beacon frames generally specify information about the corresponding wireless network (BSS  110  in  FIG. 1 ). Beacon frames are transmitted periodically to announce the presence of a Wireless LAN network. Beacon frames are transmitted by the Access Point (AP) in an infrastructure BSS. In IBSS networks, beacon frame generation is distributed among the stations in the IBSS. Some of the information contained in beacon frames includes timestamp (for synchronization of time among all the stations in a BSS), beacon frame interval (time interval between beacon frames), capability information (specifying capabilities of the wireless network), supported data rates, etc. 
     Client  110 A synchronizes its local clock with respect to the clock of the AP, based on the time stamp contained in a beacon frame. Client  110 A, when operating in power-save mode, sets its listen interval equal to some multiple (K) of the beacon frame interval, and aligns its power-ON durations (such as in interval t 20 -t 22  in  FIG. 2 ) with the beacon frames. 
     In  FIG. 2 , waveform  220  represents an example of periodic beacon frames transmitted by AP  110 F. Specifically, durations such as t 20 -t 21  (in general, logic-high durations of waveform  220 ) represent beacon frame transmissions. In  FIG. 2 , the listen interval of client  110 A is shown as equaling six beacon frame intervals of beacon frames transmitted by AP  110 F. However, the specific listen interval, and thus the multiple (K) may be set based on specific considerations. For example, if very low-power operation is required for client  110 A, and data exchange between client  110 A and other stations in BSS  110  are relatively infrequent, a large value (e.g., 100) may be set for K. 
     Control messages transmitted by AP  110 F are typically periodic, the period equaling some multiple (M) of the beacon interval. Control messages are synchronized with beacon messages. Waveform  230  in  FIG. 2  represents an example of periodic control messages transmitted by AP  110 F. Specifically, durations such as t 20 -t 21  (in general, logic high durations of waveform  230 ) represent control message transmissions. The logic-high durations of respective waveforms  210 ,  220  and  230  are merely meant for illustration. The specific lengths of each of the corresponding transmissions may be system, network or device-specific. 
     In  FIG. 2 , the multiple M equals five. In general, however, other values of M may be used. Control messages transmitted by AP  110 F may be generated by wireless network manager  150 , and provided to AP  110 F via path  141  ( FIG. 1 ). In  FIG. 2 , waveform  230  is shown separate from waveform  220  merely for clarity. In practice, control messages are advertised within beacon frames transmitted by AP  110 F. Multicast control messages are transmitted immediately after they are advertised in the beacon frame. AP  110 F intimates to a corresponding client the presence of unicast control messages, and the corresponding unicast control messages are fetched by the client. 
     It may be observed from  FIG. 2  that, excepting for the first control message transmission ( 233 ), client  110 A is in a power-OFF state during transmissions (or intimation of presence by AP  110 F) of the other control messages. In general the periodicities and/or the alignment of the listen intervals of client  110 A and transmissions (or intimation of presence by AP  110 F) of control messages may be such that client  110 A may not be in a power-ON state to receive one or more transmitted control messages. As a result, the proper operation of client  110 A, and potentially of BSS  110 , may not be ensured. In particular, the IEEE 802.11 standard (or the corresponding amendments to the standard) stipulates that some control messages (e.g., Group Key Update messages) be acknowledged by the corresponding client(s). Non-acknowledgement of a control message may potentially result in client  110 A being dissociated from BSS  110 , thereby requiring re-establishment of the association with BSS  110 , which in turn may be wasteful of power in client  110 A. 
     It is noted here that AP  110 F may store (i.e., buffer) control messages and transmit such buffered control messages to the corresponding destination client when such client sends the fetch request (PS-POLL frame in IEEE 802.11) to AP  110 F. In the context of IEEE 802.11, AP  110 F informs client  110 A about such buffered control messages (as well as application-level data, if any are undelivered) using a traffic indication message (TIM) within a beacon frame. The broadcast or multicast messages for client  110 A are indicated in the beacon every DTIM (Delivery Traffic Indication Message) interval. After a DTIM interval, access point  110 F transmits the buffered control messages (and data, if any are present) to client  110 A. 
     In practice, the storage capacity in AP  110 F may be limited, and AP  110 F may not be able to store more than a certain number of such un-acknowledged control messages and data. As a result, at least in some implementations, one or more control messages may still never be received by client  110 A. 
     When BSS  110  is operated in a “secure mode”, data and messages exchanged between the wireless stations of BSS  110  are encrypted. A decryption key (or decryption keys) required for decrypting the encrypted messages may be generated by wireless network manager  150 , and provided to AP  110 F. AP  110 F may then unicast the decryption key(s) to each of clients  110 A- 110 E. 
     Examples of such secure-mode operation are Wi-Fi Protected Access (WPA Personal/Enterprise) and Wi-Fi Protected Access II (WPA2 Personal/Enterprise), which are security protocols developed by the WiFi Alliance, and as defined in the IEEE 802.11i amendment to the IEEE 802.11 standard. Consistent with the WPA (Personal/Enterprise) and WPA2 (Personal/Enterprise) protocols, the decryption key is updated periodically. Typically, wireless network manager  150  periodically updates the value of the decryption key, and provides the updated values to AP  110 F. AP  110 F broadcasts the updated decryption key (termed the group key since the same decryption key is used by all clients in the BSS) at the corresponding time instances. 
     Thus, the logic-high durations of waveform  230  represent durations in which corresponding updated values of the decryption key(s) are available at AP  110 F for transmission by AP  110 F. Thus, for example, a first value of the decryption key is indicated as available in transmission marked  233  in  FIG. 2 , a next (updated) value of the decryption key is transmitted in transmission marked  234 , and so on. Period (P) of the updates is marked in  FIG. 2 . It may be observed that client  110 A is powered-OFF during transmissions marked  234 ,  235 ,  236 ,  237  and  238 . 
     Non-receipt of the latest value of the decryption key implies that client  110 A cannot decrypt messages received from AP  110 F (or the other clients in BSS  110 ). Further, inability to decrypt messages received from AP  110 F may result in non-acknowledgement by client  110 A of such messages, thereby potentially resulting in client  110 A being dissociated with BSS  110 , as noted above. 
     It is also noted that, although typically the control messages (for example, the group-key updates) are sent periodically, certain asynchronous events may result in some control messages being generated and transmitted asynchronously (i.e., not in keeping with the update interval), as shown by waveform  240 . As shown there, transmission shown numbered  241  occurs asynchronously, i.e., out of sequence with respect to the periodic transmissions that occurred previously in time. Subsequent transmissions may be periodic with the same period as before. 
     One technique to minimize the probability of missing receipt of transmitted control messages is to reduce the power-OFF durations (or equivalently the listen intervals) of client  110 A. Doing so, however, may result in increased power consumption in client  110 A, which may not be desirable. The manner in which the problems noted above are overcome in embodiments of the present disclosure is described in detail next. 
     3. Technique 
       FIG. 3  is a flowchart illustrating the manner in which a wireless station, operating in a power-save mode, ensures receipt of control messages transmitted from another wireless station, in an embodiment of the present invention. The flowchart is described with respect to the environment of  FIG. 1 , and in relation to client  110 A in particular, merely for illustration. However, various features described herein can be implemented in other environments (for example, in ad-hoc networks not requiring an access point) and using other components, as will be apparent to one skilled in the relevant arts by reading the disclosure provided herein. 
     Further, the steps in the flowchart are described in a specific sequence merely for illustration. Alternative embodiments using a different sequence of steps can also be implemented without departing from the scope and spirit of several aspects of the present invention, as will be apparent to one skilled in the relevant arts by reading the disclosure provided herein. The flowchart starts in step  301 , in which control passes immediately to step  310 . 
     In step  310 , client  110 A computes at least some future time instances at which another wireless station is expected to start transmissions of corresponding control messages. For example, in the environment of  FIG. 1 , client  110 A may compute future time instances of transmissions of control messages by AP  110 F. Future time instances refer to time instances later in time than the current time. Control then passes to step  320 . 
     In step  320 , client  110 A ensures that it (client  110 A) is in the power-ON state in corresponding time intervals encompassing start and end of each transmission of the corresponding control messages. As a result, client  110 A is able to receive the transmitted control messages. Control then passes to step  399 , in which the flowchart ends. 
     While the flowchart of  FIG. 3  is noted as ending in step  399  (and after step  320 ), the operations of the flowchart may be performed multiple desired points in time based on requirements. For example, an asynchronous event may cause AP  110 F to reset its group key message update starting point (base) and/or the group key update period. For example, the starting point of the ‘new’ group key message update period may be reset as indicated by transmission  241  of waveform  240  of  FIG. 2 . In such circumstances, steps  310  and  320  may be performed repeatedly. 
     The operation of steps  310  and  320  noted above are illustrated next with respect to example embodiments. In particular, the following description is provided in the context of receiving group key message updates. However, similar or corresponding techniques can be adopted in other contexts or types of control messages. 
     4. Embodiment 
     In an embodiment, based on whether sufficient information regarding a reference point (base) and the period of group key update messages is known or not, client  110 A operates in one of three states.  FIG. 4  is a state transition diagram of client  110 A, and shows the three states, namely “UNKNOWN” ( 410 ), “LEARN” ( 420 ) and “KNOWN” ( 430 ). 
     Immediately following association with BSS  110 , and if the base of the group key update messages is not known to client  110 A, client  110  enters the UNKNOWN state ( 410 ). In this state, period (P) may either be known or not known to client  110 A. An example of when (P) is known a priori to client  110 A is when the period (P) of the group key update messages is provided at the time of association of client  110 A with BSS  110 , or if (P) is hardcoded (or provisioned) in client  110 A. 
     In state “UNKNOWN” ( 410 ), client  110 A remains in a “radio-learn mode”, i.e., client  110 A is either continuously in the power-ON state or powers-ON sufficiently frequently such that one group key update message is received. When powering-ON frequently (rather than continuously be in the power-ON state), client  110 A may power-ON once every ‘R’ beacon frame transmissions from AP  110 F, with R being much smaller than the interval (B) for which control messages are buffered by AP  110 F. Interval (B) may be stored in client  110 A prior to deployment based on known parameters of BSS  110 . In an embodiment, R equals thirty. However, in other embodiments other values for R may be used. For example, a value of R of one may be used, with client  110 A waking up (i.e., powering-ON) for every beacon frame transmission. In yet another embodiment, R may be progressively reduced to the value one, based on the length of time spent in states “Unknown” or “Learn”. 
     On receiving the group key update message for the first time, the ‘base’ of the periodic group key update messages is available to client  110 A. Client  110 A then changes to state “KNOWN” ( 430 ) if period (P) is already known, as indicated by arrow  413 . If the period (P) is not yet known, client  110 A transitions to state “LEARN” ( 420 ) as indicated by arrow  412 . 
     In “LEARN” state  420 , the base of the periodic series represented by the periodic updates of the group key messages is known, but he period (P) is not known. Client  110 A continues in the “radio-learn” mode (noted above) till at least a desired number of following and successive group key update messages are received. Client  110 A computes the difference in the time stamps of successive pairs of group key update messages received to obtain several estimates of the period (P). Client  110 A then transitions to state “KNOWN”. When the desired number of group key update message is X or more (wherein X may equal three), if at least a threshold number of estimates of (P) are all equal (within a tolerance range), client  110 A transitions to state KNOWN, as indicated by arrow  423 . 
     However, if different values for (P) are obtained for at least two of the threshold number of estimates, client  110 A remains in state “LEARN” till period P is reliably determined (based on receipt of following successive group key update messages). When the desired number (X) is two, only a single computation of (P) is possible, and client  110 A may directly transition to the KNOWN state. If client  110 A is de-authenticated from the network (i.e., dissociated from BSS  110 ), client  110 A transitions to state UNKNOWN. Such de-authentication may occur due to non-receipt of one or more group key update messages, and may occur, for example, due to a reset of the group key update periodic series, such as illustrated with respect to waveform  240  of  FIG. 2 . 
     In “KNOWN” state  430 , both the base of the periodic series represented by the periodic updates of the group key messages a well as the period (P) is known to client  110 A. In the “KNOWN” State, client  110 A ensures that it is in a power-ON state for receiving each group key update message occurring at intervals of P. If client  110 A is de-authenticated from the network (i.e., dissociated from BSS  110 ), client  110 A transitions to state UNKNOWN. Such de-authentication may occur due to non-receipt of one or more group key update messages that may occur, for example, due to a reset of the group key update periodic series, such as illustrated with respect to waveform  240  of  FIG. 2 . Client  110 A also transitions to state UNKNOWN if all applications executing in client  110 A disconnect from the corresponding stations or nodes in BSS  110  or wired network  130 , or if client  110 A misses receiving a threshold number of group key update messages from the network. Client  110 A may miss receiving one or more group key update messages if a reset of the group key update periodic series has occurred. 
       FIG. 5  illustrates example waveforms showing the operation of client  110 A in the three states noted above. The arrows in sequence (or waveform)  550  represent transmissions of beacon frames by AP  110 F. Beacon frames that contain group key update messages (or control messages in general) are represented by taller arrows in waveform  550 , examples being arrows at time instances t 501  and t 502 . Arrows in waveform  560  represent wake-up (i.e., power-ON) durations of client  110 A. Client  110 A is in the power-OFF state in the time interval between any two successive arrows, which also represents the listen interval of client  110 A. For simplicity, the transmission durations of AP  110 F and power-ON durations of client  110 A are shown as being infinitesimal (being the ‘time interval’ represented by an arrow itself). Actual transmission and power-ON durations have non-zero values. The specific values of listen intervals, group key update periods, etc., of  FIG. 5  are provided merely to illustrate the manner in which client  110 A operates to obtain at least some group key update messages, and real-world values for such parameters may be different from those shown in  FIG. 5 . The corresponding states of operation (UNKNOWN, LEARN and KNOWN) are also indicated in  FIG. 5  for the example. 
     Client  110 A is assumed to have associated with BSS  110  immediately prior to time instance t 500 . Corresponding to the description provided with respect to  FIG. 5 , client  110 A enters the UNKNOWN state at (or immediately prior to) t 500 , and is in a radio-learn mode. In  FIG. 5 , listen intervals of client  110 A in the UNKNOWN state are assumed to equal the period of beacon frame transmission from AP  110 F. However, larger listen intervals or continuous listening (without power-OFF, i.e., maintaining client  110 F in power-ON state) can instead be used. 
     At time instance t 501 , client  110 F receives the first group key update message, thus obtaining the base (or reference point) of the periodic series represented by the group key update messages. Client  110 A transitions to the LEARN state at (or immediately after) t 501 . 
     In the LEARN state, client  110 A continues in the radio-learn mode, receiving the following three group key messages at t 502 , t 503  and t 504 . Client  110 A computes period (P) of the group key update messages, and transitions to the KNOWN state at or immediately after t 504 . 
     In the KNOWN state, client  110 A increases the length of the listen interval to reduce power consumption. In the example of  FIG. 5 , the listen interval is shown as being increased to equal four beacon frame periods starting from t 504 . Hence, client  110 A wakes-up at t 505 , t 507  and t 509 . In addition, client  110 A also wakes-up at time instances t 506 , t 508  and t 511  to receive group key message updates, based on the knowledge of the base and period (P) of the group key update messages, computed at or prior to t 504 . Time instances t 506 , t 508  and t 511  are examples of ‘future time instances’ with respect to t 504  (or a time interval occurring slightly earlier than t 504 ), when client  110 A performs the corresponding computations noted above and ‘decides’ to wake up at the ‘future time instances’. AP  110 F broadcasts group key update messages at t 506  and t 508 , and client  110 A receives the corresponding messages. 
     However, AP  110 F broadcasts a group key update message at t 510 , the earlier periodic series of group key updates being reset at t 510 . Client  110 A is, however, in the power-OFF state at t 510 , and misses the message broadcast at t 510 . Client expects to receive a group key update message at t 511 , but does not receive one at t 511  due to the group key update period having been reset. Hence, at or slightly after t 511 , client  110 A transitions to the UNKNOWN state. 
     Thus, in the UNKNOWN state starting at t 511 , client  110 A wakes-up every beacon frame period and receives a group key update message at t 512 , thus obtaining the (new) base of the group key update messages. Client  110 A transitions to the LEARN state at or immediately after t 512 . 
     In the LEARN state, client  110 A continues to wake-up to receive every beacon frame, thereby also receiving the three group key update messages at t 513 , t 514  and t 515 . Client  110 A (re-)computes period (P) of the group key update messages, and transitions to the KNOWN state at or immediately after t 515 . In the KNOWN state, again client  110 A increases the length of the listen interval to reduce power consumption, and wakes-up only once (t 516  is shown in  FIG. 5 ) every four beacon frame periods. In addition, client  110 A wakes up to receive group key update messages (t 517  is shown in  FIG. 5 ) based on the base and the period P of the group key update messages. The wake-up duration may also ‘encompass’ the corresponding group key update message transmission durations, i.e., client  110 A may wake up slightly earlier than the start of the corresponding beacon frame (or at least the group key message in the beacon frame), and power-OFF after the end of the beacon frame. Thus, in steady-state, wake-up durations of client  110 A are aligned to both the listen period as well as the group key update period. 
     Although, not illustrated in  FIG. 5 , client  110 A may transition from/to the corresponding one of the three states based on the corresponding events as described above with reference to  FIG. 5 . 
     Client  110 A designed to operate as described in detail above may provide the benefits of relatively low power consumption, while still being able to receive most group key update messages (or in general, control messages). In the event of non-receipt of an ‘expected’ control message due to asynchronous events or reset of the control message periodic series (e.g., as a t 511  in  FIG. 5 ), client  110 A is designed to automatically re-align its wake-up durations so as to be able to be in the power-ON state to receive future control message updates. Thus, the need for buffering of such messages in AP  110 A may be reduced, as also the probability of being disconnected or dissociated with BSS  110  due to non-acknowledgement of control messages. Each of the other clients  110 B- 110 E may also be implemented similar to client  110 A, and thereby provide similar benefits. 
     The details of a client  110 A, in an embodiment, are described next. 
     5. Wireless Station 
       FIG. 6  is a block diagram of the internal details of a wireless station in an embodiment. Wireless station  600  may correspond to clients  110 A- 110 E, and with corresponding modifications to AP  110 F. Wireless station  600  is shown containing processing block  610 , flash memory  620 , random access memory (RAM)  630 , real-time clock (RTC)  640 , battery  645 , non-volatile memory  650 , sensor block  660 , transmit block  670 , receive block  680 , switch  690  and antenna  695 . The whole of wireless station  600  may be implemented as a system-on-chip (SoC), except for battery  645 . Alternatively, the blocks of  FIG. 6  may be implemented on separate integrated circuits (IC). 
     Again, the components/blocks of sensor device  600  are shown merely by way of illustration. However, wireless station  600  may contain more or fewer components/blocks. Further, although not shown in  FIG. 6 , all blocks of wireless station  600  may be connected automatically to an auxiliary power source (such as battery  645 ) in the event of failure of main power source (not shown). 
     Sensor block  660  may contain one or more sensors, as well as corresponding signal conditioning circuitry, and provides on path  661  measurements/values of physical quantities such as temperature, pressure, etc., sensed via wired path  662  or wireless path  663 . 
     Antenna  695  operates to receive from and transmit to a wireless medium, corresponding wireless signals containing data. Switch  690  may be controlled by processing block  610  (connection not shown) to connect antenna  695  either to receive block  680  via path  698 , or to transmit block  670  via path  679 , depending on whether wireless station  600  is to receive or transmit. 
     Transmit block  670  receives data to be transmitted on path  671  from processing block  610 , generates a modulated radio frequency (RF) signal according to IEEE 802.11 standards, and transmits the RF signal via switch  690  and antenna  695 . Receive block  680  receives an RF signal bearing data via switch  690  and antenna  695 , demodulates the RF signal, and provides the extracted data to processing block  610  on path  681 . 
     RTC  640  operates as a clock, and provides the ‘current’ time to processing block  610  on path  641 . RTC  640  may be backed-up by battery  645  (in addition to the normal source of power, not shown in the Figure). RTC  640  may also contain memory to store critical information received from processing block  610 . Although not shown as such in  FIG. 6 , battery  645  may also be used as back-up power to one or more of the other components/blocks of station  600 . Thus, for example, the power supply to flash memory  620  may be automatically switched (by corresponding circuitry not shown) to battery  645  in case of failure of the main power source (not shown). 
     Flash memory  620  represents an example memory, which contains memory locations organized as blocks. A block represents a set of memory locations (typically contiguous in terms of memory address), which are to be all erased before data can be rewritten into any location. Flash memory  620  may be used to store data obtained from sensor block  660  via processing block  610 . 
     Non-volatile memory  650  is a non-transitory machine readable medium, and stores instructions, which when executed by processing block  610 , causes wireless station  600  to provide several desired features. For example, in the context of wireless sensor networks used for building or plant automation, processing block  610  may process and transmit measurement data such as temperature, pressure etc., obtained from sensor block  660 . In addition, the instructions may be designed to enable wireless client to operate consistent with the description provided above with respect to client  110 A. Thus, the instructions enable wireless station  600  to align its wake-up durations with control messages and thus to receive most of such control messages. Further, the instructions enable wireless station to  600  to be set in power-ON and power-OFF (or at least standby mode). In some embodiment, flash memory  620  may store the instructions for processing block  610 . 
     Processing block  610  (or processor in general) may contain multiple processing units internally, with each processing unit potentially being designed for a specific task. Alternatively, processing block  610  may contain only a single general-purpose processing unit. 
     RAM  630  and non-volatile memory  650  (which may be implemented in the form of read-only memory/ROM) constitute computer program products or machine (or computer) readable medium, which are means for providing instructions to processing block  610 . Thus, such medium can be in the form of removable (floppy, CDs, tape, etc.) or non-removable (hard drive, etc.) medium. Processing block  610  may retrieve the instructions (via corresponding paths  651  and  631 ), and execute the instructions to provide several features of the present invention (related to management of configuration data), as described below. It should be appreciated that the processors can retrieve the instructions from any randomly accessible storage units (e.g., RAM  630  or flash memory  620 ) and execute the instructions to provide the features described above. 
     The instructions thus executed by processing block  610 , enable client  110 A (or wireless station  600  in general) to receive control messages according to several aspects of the present invention. 
     6. Conclusion 
     References throughout this specification to “one embodiment”, “an embodiment”, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents.