Abstract:
In one embodiment, a method of transmitting data from a transmitter in a wireless local area network (WLAN), where the transmitter has a timer and a modem. The transmitter periodically transmits a transmission signal that includes a timestamp field that includes a timestamp for synchronizing a receiver timer in a receiver of the WLAN. The timestamp, which represents a value within the count sequence of the transmitter timer, accounts for delays in the modem.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuation of U.S. patent application Ser. No. 11/331,919, filed on Jan. 13, 2006 now U.S. Pat. No. 7,289,578 issued on Oct. 30, 2007, which is a continuation of U.S. patent application Ser. No. 10/681,267, filed on Oct. 9, 2003 now U.S. Pat. No. 7,010,058 issued on Mar. 7, 2006, which is a divisional of U.S. patent application Ser. No. 10/092,295, filed on Mar. 7, 2002 now U.S. Pat. No. 6,707,867 issued on Mar. 16, 2004, which is a continuation of U.S. patent application Ser. No. 08/155,661, filed on Nov. 22, 1993, now abandoned, which claimed foreign priority from British patent application 9304622.5, filed on Mar. 6, 1993. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to wireless local area network apparatus. 
   2. Description of the Related Art 
   A wireless local area network commonly comprises a plurality of communication stations located in a Basic Service Area (BSA). The stations can send and receive communication signals via a base station and, in this manner, the base station receives the signals from a station in the BSA and re-transmits the signals to the intended recipient station. 
   The BSA can be provided as one of a plurality of BSAs which together form an Extended Service Area. In this case, the base station of each BSA may comprise an access point for a backbone infrastructure for connecting the BSAs for allowing communication between stations in different BSAs within the Extended Service Area. 
   Communication between stations, whether by way of a base station or otherwise, can require synchronization between a transmitter of one station or an access point and a receiver of another station. Disadvantageously, accurate synchronization between a transmitter and a receiver in a BSA cannot be readily achieved due, in particular, to operational limitations such as transmission and reception delays and delays in accessing the wireless medium. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide wireless local area network apparatus having improved synchronization between the transmitters and the receivers in the network. 
   According to certain embodiments of the present invention, there is provided wireless local area network apparatus comprising transmitter means and receiver means, characterized in that said transmitter means includes transmitter timer means for controlling periodic generation of transmission signals, said receiver means includes receiver timer means, and said transmitter means has means for including transmitter timer data in said signals for synchronizing said receiver timer means with said transmitter timer means, said transmitter timer data representing the state of said transmitter timer means at the time of transmission of the signal in which it is included. 
   The wireless local area network apparatus of certain embodiments of the present invention is particularly advantageous for power management applications in which low power portable wireless stations are employed in the BSA. The stations periodically switch between a low power consumption state, in which their transceivers are de-energized, and a high power consumption state, in which their transceivers are energized, and can thereby receive periodic signals transmitted from some other station. The synchronization between the signals transmitted from some other station and the switching of the power-consumption state of the receiver stations is advantageously achieved by the apparatus of the present invention. The improved synchronization of the present invention allows for operation of the stations in a wireless local area network with reduced power-consumption, which is particularly important for stations having an on-board power supply. 
   The apparatus of certain embodiments of the present invention can be advantageously employed to control other timing relationships between a transmitter and a receiver in a wireless local area network. For example, in so-called frequency-hopping devices, the transmission frequency employed by a transmitter is periodically changed and so a receiver has to adapt to this change in communication-signal frequency. The apparatus of certain embodiments of the present invention allows for accurate synchronization between the operational changes in the transmitter and receiver during such frequency hopping. 
   In one embodiment, the present invention is a receiver for a wireless local area network (WLAN). The receiver comprises a radio modem adapted to receive, from a transmitter of the WLAN, a transmission signal containing a time stamp value; a first register adapted to receive the transmission signal from which the time stamp value is retrieved; a timer adapted to initiate a count sequence based on the time stamp value and generate a timer control signal at the completion of the count sequence; and a controller adapted to control operations of the receiver based on the timer control signal from the timer. 
   In another embodiment, the present invention is a method for a receiver in a WLAN. A transmission signal containing a time stamp value is received from a transmitter of the WLAN. The time stamp value is retrieved from the transmission signal, and a count sequence is initiated based on the time stamp value. A timer control signal is generated at the completion of the count sequence, and operations of the receiver are controlled based on the timer control signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the invention is described further hereinafter, with reference to the accompanying drawings in which: 
       FIG. 1  shows a wireless local area network which forms part of an extended service area; 
       FIG. 2  is a block diagram of a transmitter for use in apparatus embodying the present invention; 
       FIG. 3  shows the structure of a Traffic Indication Message constructed in the transmitter of  FIG. 2 ; 
       FIG. 4  is a flow diagram of the operation of the transmitter of  FIG. 2 ; 
       FIG. 5  is a block diagram of a receiver for use in apparatus embodying the present invention; 
       FIG. 6  is a flow diagram of the operation of the receiver of  FIG. 5 ; and 
       FIG. 7  is a timing diagram illustrating operation of the transmitter of  FIG. 2  and the receiver of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   As mentioned above, the apparatus of the present invention can be used in a power management system for a wireless local area network. 
   Such a local area network is shown in  FIG. 1  and comprises a basic service area (BSA)  10  having six mobile stations  12 . 1 - 12 . 6  located therein. In the illustrated embodiment each of the stations  12 . 1 - 12 . 6  is powered by an on-board d.c. supply (not shown) although some of the stations could be supplied by connection to an a.c. source. An access point  14  is also located in the BSA  10  and is typically connected to an a.c. power supply (not shown) and is connected to a backbone structure  18  linking the access point  14  to access points of other BSAs (not shown). The stations  12 . 1 - 12 . 6  communicate with each other via the access point  14 . Thus, a communication signal from one station  12 . 1  to another station  12 . 2  will not be received directly by the station  12 . 2  but will first be received by the access point  14  and then transmitted to the station  12 . 2 . 
   In order to reduce the power consumption of the stations  12 . 1 - 12 . 6 , and thereby increase the operational life-time before the on-board d.c. power supply needs to be recharged or replaced, the stations  12 . 1 - 12 . 6  are operated in a power-save-mode in which their transceivers are periodically de-energized and the station is then in a so-called doze state. In order to operate the station  12 . 1 - 12 . 6  in a power-save-mode without losing any transmitted data packets, a data packet that is intended for a station that is in a doze state is buffered in the access point  14  until such time as the station wakes-up from its doze state into a so-called awake state and energizes its transceiver to receive the buffered data. 
   Traffic Indication Message (TIM) packets are transmitted at regular intervals from the access point  14  and indicate for which stations  12 . 1 - 12 . 6  in the BSA  10  data packets are buffered in the access point  14 . The transceivers in the stations  12 . 1 - 12 . 6  are periodically energized at regular intervals such that the stations  12 . 1 - 12 . 6  wake up from a doze state to receive the TIM packets transmitted by the access point  14 . If a TIM packet received indicates that a data packet is buffered in the access point  14  for one of the stations  12 . 1 - 12 . 6 , the transceiver of that station either waits to receive the data packet which is arranged to automatically follow the TIM packet, or the station transmits a poll packet to the access point  14  to request that the data packet be transmitted. In both of the above situations, the transceiver in the station remains in an energized state once it has received a TIM packet indicating that data is buffered for that station. Once the data packet has been received, the station returns to a doze state until it awakes to receive another TIM packet. 
   Accordingly, with the exception of the periodic waking to receive the TIM packets, a station  12 . 1 - 12 . 6  remains in a power saving doze state unless a TIM packet indicates a data packet is buffered for that station. In this manner, the power consumption of each station  12 . 1 - 12 . 6  is reduced and the operational life-time, i.e. the time before recharging or replacement of the d.c. power source is necessary, of the station is increased. The improved synchronization provided by the present invention provides for improved synchronization between the access point  14  and the stations  12 . 1 - 12 . 6  operating in a power-save mode so as to achieve advantageously reduced power consumption in the stations  12 . 1 - 12 . 6 . 
   Further power consumption reductions can be achieved by operation of the stations  12 . 1 - 12 . 6  in a so-called extended-power-save mode. The improved synchronization provided by the present invention advantageously supports operation of the stations  12 . 1 - 12 . 6  in the extended-power-save mode. In this mode, the station is controlled to wake up from a doze state to receive only every xth TIM packet transmitted by the access point  14 . For example, if x=150 then the station awakes to receive only every 150th TIM packet transmitted by the access point  14  and so the station remains in a doze state for a longer period than if it wakes to receive every TIM packet transmitted by the access point  14 . Power consumption in the station is thereby further reduced. Since, in the above example, a station awakes only every xth TIM packets, accurate synchronization between the access point  14  and the station is required so that the station wakes up at an appropriate time to receive every 150th TIM packet. The present invention provides for such accurate synchronization. 
   It should be noted that although the access point  14  may have a data packet buffered therein to transmit to a station operating in an extended-power-save mode, the data packet remains buffered in the access point  14  until the station  12  wakes up upon receipt of the xth TIM packet after which the station will poll the access point  14  to transmit the buffered packet and so data is not lost. 
   The energization of the transceivers in the stations  12 . 1 - 12 . 6  and in the access point  14  can be controlled by timers which include crystal oscillators. Synchronization between the timers in the stations  12 . 1 - 12 . 6  and the access point  14  is achieved by apparatus embodying the present invention and an indication of the reduced power consumption of a station having such a timer and operating in an extended-power-save mode is given below in which: 
   The time interval between successive TIM packets transmitted from the access point  14  is 200 msec; the station&#39;s transceiver has a power-up delay of 1 msec; the timing drift of the oscillator in the station is 100 micro sec/sec; the timing drift of the oscillator in the access point  14  is 100 micro sec/sec; the TIM packet medium access delay is between 0 and 5 msec; and the station is required to wake up to receive every 150th TIM packet from the access point  14 . 
   Using the above values as examples:
         The station doze interval=150×200 msec=30 sec   The maximum drive of each oscillator in the doze interval=100 micro sec/secx30=3 msec   The maximum drift for both oscillators therefore=6 msec       

   Thus, in view of the station&#39;s 1 msec power-up delay, the station should wake up 7 msec before the expected TIM packet to compensate for the oscillator drift and the power-up delay. 
   With a TIM access delay of 5 msec as an example, the period during which the station is in an awake state to receive a TIM packet is between 1 msec (when there is no crystal drift and the TIM access delay is 0 msec) and 1 msec+6 msec+5 msec=12 msec (when the total crystal drift is experienced and the TIM interval delay is 5 msec). 
   Assuming that the TIM packet has a duration of 0.5 msec, the average duration of the awake state of the station is 1+6/2+5/2+0.5=7 msec. 
   Thus, in this example, the station will be in an awake state, i.e., with its transceiver energized, for, on average, only 7 msec every 30 sec which provides for a particularly advantageous power consumption reduction. 
   By way of comparison, and assuming the same values as above, if the station wakes-up at every TIM, thereby requiring an average “on-time” of 1+5/2=3.5 msec per 200 msec TIM interval, the station is then awake for 525 msec every 30 sec. 
     FIG. 2  illustrates a transmitter  20  for use in the access point  14 . The transmitter  20  includes a modulo n counter  22  which, in operation, is free running and synchronized with a similar modulo n counter  58  in a station&#39;s receiver (see  FIG. 5 ). 
   The modulo n counter  22  functions as a timer and when the count value reaches n, a TIM function generator  24  is triggered by way of an interrupt signal  25  indicating that the next TIM packet should be constructed, and transmitted by way of a radio modem  26 . 
   The TIM packet  28  is constructed in a transmitter buffer  30  and an example of a TIM packet is illustrated in  FIG. 3 . The TIM packet comprises a wireless medium access (WMAC) header and a data field format. The WMAC header includes, amongst other fields, a Type field that identifies the packet as a TIM packet. 
   The data field format includes:
         A TIME STAMP FIELD in which is loaded a so-called time stamp of the value of the modulo n counter in the transmitter  20  at the time of transmission of the TIM;   A TIMER INTERVAL FIELD which indicates the value of n of the modulo n counter in the transmitter  20 ;   A TRAFFIC PENDING FIELD which indicates for which stations data packets are buffered; and   A TRAFFIC BROADCAST PENDING FIELD which indicates the number of outstanding broadcast data packets buffered for the stations.       

   Referring again to  FIG. 2 , once the TIM packet  28  has been constructed, it is delivered to a multiplexer  32  where the time stamp, and cyclic redundancy check (CRC) data from a CRC generator  34 , are loaded into the TIM packet  28 . A WMAC control  36  controls access to the medium via the modem  26  so that the TIM packet  28  is not transmitted from the access point  14  immediately upon generation of the interrupt signal  25 . The WMAC control  36  follows a medium access protocol such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). According to the CSMA/CA protocol, the energy level on the wireless medium is sensed by the modem  26  to determine if there is any existing network activity, and if the sensed energy level is above a threshold value, a medium busy signal  40  is delivered from the modem  26  to the WMAC Control  36 . If no medium busy is issued, so the medium is sensed “free,” the WMAC control  36  turns on the transmitter of the modem  26  by issuing a request to send (RTS) signal. The modem  26  will then start to send a training sequence and will issue a clear-to-send signal (CTS) once the training sequence is complete. The modem  26  then sends the serialized data that arrives from the buffer via the multiplexer  32  and a shift register  44 . If the medium is sensed as “busy,” the WMAC control  36  waits until the medium becomes free and then generates a random backoff delay after which the medium is again sensed. If the medium is sensed as “free” at this point then the control  36  follows the RTS, CTS procedure above. 
   When accessing the medium and once the training sequence has ended, the modem  26  provides the CTS  42  and the TIM packet stored in the buffer  30  is loaded into the shift register  44  via the multiplexer  32 . Once transmission of the header has started, the time stamp is loaded from the timer  22  into the shift register  44  via the multiplexer  32  and under the control of a transmit control circuit  43  in the WMAC control  36 . The transmit control circuit  43  also controls the start of the transmission of the header. As mentioned above, the modulo n counter  22  in the access point  14  of transmitter  20  is free running and so by the time the CSMA/CA protocol has been completed, and particularly if a medium busy signal  40  was received by the WMAC control  36 , the counter  22  is already into its next count sequence, i.e. at a value between 0 and n, by the time that the clear-to-send signal  42  is received by the WMAC control  36 . At a predetermined time relative to the clear-to-send signal  42 , which predetermined time is an accurate estimation of the exact time at which the TIM packet will be transmitted having regard to delays in the modem  26 , the so-called “time stamp” i.e. the value of the modulo n counter  22  at that predetermined time, will be loaded in the TIM packet  28  stored in the buffer  30 . The TIM packet  28  is loaded into a shift register  44  upon generation of a load signal  46  from the WMAC control  36 , and then transmitted by way of the modem  26 . 
     FIG. 4  further illustrates the operation of the transmitter  20  outlined above. 
     FIG. 5  illustrates a receiver  48  of one of the stations  12 . 1 - 12 . 6  in the BSA which is arranged to receive a TIM packet  28  and a data packet (not shown) from the access point  14 . 
   The operation of the receiver  48  is outlined below and further illustrated in  FIG. 6 . 
   Energization of the receiver  48  is controlled by a modulo n counter  58  which functions as a timer to wake up the station  12 . 1  from a doze state to receive the TIM packet  28  transmitted from the access point  14 . 
   The TIM packet  28  is received by a receiver modem  50  and its time stamp value retrieved from the TIM TIME STAMP FIELD ( FIG. 3 ). The retrieved time stamp is delivered by way of a shift register  52  to a counter register  54  which commences a modulo n count starting from the point between 0 and n which corresponds to the time stamp value. The counter register  54  continues its modulo n count with the same clock signal  56  that controls the modulo n counter  58 . This modulo n count is stored in the counter register  54  until the TIM packet  28  is completely received and the CRC data checked. If the CRC is correct, the modulo n count is loaded from the counter register  54  into the modulo n counter  58 . The use of the counter register  54  is particularly advantageous in that it allows TIM packets of different lengths to be received. This arises since the modulo n count sequence, that commences at the time stamp value, is buffered in the register  54  while the TIM packet  28  is processed completely. The counter register  54  maintains the cyclic modulo n count for as long as is necessary to process the TIM packet. 
   If all the TIM packets are of the same known length, then a TIM-packet-processing compensation factor could be applied to the time stamp value to allow for the known time taken to process the TIM packet of known length. The compensated time stamp value would then be loaded directly into the modulo n counter  58  and so the intermediate counter register  54  would not be required. 
   Referring again to the embodiment illustrated in  FIG. 5 , a delay compensation value  60  is added to the modulo n count by an adder  62  as the count is transferred from the counter register  54  to the modulo n counter  58 . The compensation value  60  compensates for the propagation delay of the receiver  48  and the transmitter  20 . Once the compensated modulo n counter value is transferred from the counter register  54  to the counter  58 , the counter  58  is then accurately synchronized with the modulo n counter  22  in the transmitter ( FIG. 2 ). 
   Once the modulo n counters  22 ,  58  in the station  12 . 1  and the access point  14  are accurately synchronized, the counter  58  provides the station  12 . 1  with an accurate indication of the time at which the counter  22  in the access point  14  reaches its n value and generates a TIM packet for transmission. Since the counter  22  in the access point  14  remains free-running, and the counter  58  in the station  12 . 1  is accurately synchronized with the counter  22 , the station  12 . 1  can be controlled to accurately wake up in time to receive only every xth TIM packet without requiring the station  12 . 1  to wake up unnecessarily early as would be required to assure receipt of the TIM packet if accurate synchronization between the counters  22 ,  58  was not available. The reduction in the need for early wake up of the station  12 . 1  advantageously reduces the power consumption of the station  12 . 1 . 
   It should be noted that each station  12 . 1 - 12 . 6  in the BSA  10  can operate with different doze intervals. For example one of the stations  12 . 1  can be controlled to wake up every 150 TIM packets while another station  12 . 2  wakes up every 200 TIM packets. Each time the station  12 . 1  wakes up to receive a TIM packet, the modulo n counter  58  is reset by the time stamp retrieved from the TIM packet so that continued accurate synchronization can be achieved. 
     FIG. 7  is a timing diagram that further illustrates the improved synchronization of the present invention as provided in a power management application. The access point  14  activity indicates the transmission of the first five TIM packets  64 - 72 , and the last TIM packet  73 , of a one hundred and fifty TIM packet series and the first five TIM generation signals  74 - 82  generated each time the modulo n counter  22  in the access point  14  reaches its value n. As shown, the transmission of the first TIM packet  64  is delayed due to a medium busy signal obtained from the CSMA/CA protocol. The first TIM packet  64  is therefore actually transmitted m counts of modulo n counter  22  into the first count sequence  74 - 76 . The station  12 . 1  has previously been synchronized to wake up at  84  to receive the first TIM packet  64 . The TIM packet  64  carries a time stamp value m representing the value of the modulo n counter  22  in the access point  14  at the actual time of transmission of the TIM packet  64 . As described above, the station  12 . 1  retrieves the time stamp from the TIM packet  64  and loads it into its own modulo n counter  58  which then commences its count sequence at value m. As shown in  FIG. 7 , the two modulo n counters  22 ,  58  remain in syncnronization as they cyclically count up to value n. This synchronization readily allows the station  12 . 1  to remain in a doze state until its modulo n counter  58  indicates that the 150th TIM packet  73  is to be generated in, and transmitted from, the access point  14 , and the station  12 . 1  wakes up at  85 . Only a minor amount of compensation is necessary to allow for the possible modem delay of the transmitter  20  and receiver  48 . 
   If a time stamp value of the access point counter  22  is not taken and instead the station counter  58  is reset to 0 by the actual receipt of the TIM packet  64 , the late arrival of the TIM packet  64  due to the CSMA/CA delay leads to unsynchronized operation of the counters  22 ,  58  because when the access point counter  22  has reached a value m, the station counter  58  is being reset to 0 by receipt of the TIM PACKET  64 . The station counter  58  has therefore just recorded a TIM interval of n+m counts and if the station is then controlled to remain in a doze state until 150 TIM packets have been transmitted, i.e. until after 150 TIM intervals, the station erroneously dozes for 150×(m+n) intervals instead of 150×n intervals and further power consuming compensatory steps are necessary which disadvantageously reduces the power saved by energizing the station receiver only every 150 TIM packets. 
   Thus, by including a time stamp representing the state of the access point counter  22  at the exact time of transmission of the TIM packet, the power saving benefit of energizing the station only every 150 TIM packets can be increased. 
   The above describes a preferred embodiment of the integration of the synchronization function in the medium-access-control function. Other forms, in which the reference point in time, where the “time stamp” is sampled, is available to both the transmitter and the receiver, can utilize the start of the frame or the actual location of the time stamp field. 
   The invention is riot restricted to the details of the foregoing power-management embodiment. For example, the apparatus of the present invention can be employed to provide synchronization of frequency channel selection in frequency-hopping devices. In such devices the base station, for example the access point, switches communication operating frequency at a precise moment, and it is required that the other stations in the network are synchronized so as to switch their operating frequency to the new frequency at that moment. In, accordance with a further advantage provided by the invention, the access point does not need to transmit a separate frequency-hop signal each time the communication operating frequency is required to change but can include a timing signal for two or more successive frequency-hops which can therefore be delivered to the stations at intervals that are longer than the intervals between the required frequency-hops. Accordingly, the stations can operate in an extended-sleep-mode wherein each xth TIM packet that is received also includes timing information indicating when the station should switch its communication operating frequency. Thus, providing frequency change logic ( 86  in  FIG. 5 ) remains operational during the extended sleep period, the required frequency hop, or hops, can occur during the sleep period so that when the station next wakes up, it is still operating with the same communication frequency as the access point. Advantageously, the synchronized timing control of a frequency hopping device can be combined with the power management function of such a device so that the frequency-change logic  86  and a station wake-up control  88  are controlled by the same timing source  58 .