Abstract:
The present invention allows a single access point (“AP”;  100 ) to support a plurality of devices ( 102, 104 ) operating on different frequency bands. During a first period of time, the AP initiates a contention free period at a first frequency. The AP switches from the first frequency to a second frequency, and communicates with devices operating at the second frequency. Periodically during the first period of time, the AP temporarily ceases communication with devices operating at the second frequency to initiate a contention free period at the second frequency, switch from the second frequency to the first frequency, initiate another contention free period at the first frequency, and switch from the first frequency back to the second frequency.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is related to U.S. application Ser. No. 10/325,384 filed Dec. 20, 2002, titled “Method of Communicating with a Plurality of Sets of Users” by Ekl et al. (attorney docket no. CM05380H) commonly owned together with this application by Motorola, Inc. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to a method for supporting a plurality of subscribers operating on different frequency bands using a single access point.  
       BACKGROUND OF THE INVENTION  
       [0003]     Current wireless systems, most notably 802.11 wireless local area network (“WLAN”) systems, operate in half-duplex mode on a single frequency. That is, a subscriber in a wireless system either transmits or receives at any given time, but not both simultaneously. Further, the subscriber typically operates on a single frequency. Once a subscriber is on a frequency, it stays on that frequency.  
         [0004]     A problem with today&#39;s wireless systems is that they only serve one group, where a group is a set of subscribers with common characteristics (e.g., operating on a common frequency band). In the broad case, groups can be large classes of subscribers. Examples of two different groups are city workers and public safety users (e.g., police personnel, fire personnel, or the like). In a technical configuration of the system, and specifically in the case of a WLAN access point (“AP”), the common set of technical characteristics includes frequency, service set identifier (“SSID”), and associations.  
         [0005]     To serve multiple groups with existing technology requires multiple APs, which is typically not economically feasible. Further, multiple APs in close proximity exhibit interference issues, which non-deterministically decreases the throughput to both APs (in the best case scenario), and may make the APs completely unusable (in the worst case scenario).  
         [0006]     Thus, there exists a need for a single AP to support a plurality of subscribers operating on different frequency bands. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]     A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which:  
         [0008]      FIG. 1  illustrates an example of a system topology having a single access point supporting both a first set of subscribers operating at a first frequency and a second set of subscribers operating at a second frequency in accordance with the present invention; and  
         [0009]      FIG. 2  illustrates an example of channel activity as seen by the access point during dual frequency band operation in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements.  
         [0011]     As illustrated in  FIG. 1 , the present invention allows one physical access point (“AP”)  100  to support a plurality of devices/subscribers  102 ,  104  operating on different frequency bands by timesharing between a plurality of frequency bands. For simplicity sake, the following discussion assumes that the AP  100  is timesharing between two different frequency bands (e.g., 4.9 GHz and 5.0 GHz); in other words, the AP  100  operates in the first frequency band for some predetermined amount of time, then operates in the second frequency band for some predetermined amount of time, reverts back to the first frequency band, etc. As a result, the AP  100  maintains states in both frequency bands. It should be noted, however, that the amount of time the AP  100  operates in the first frequency band is not necessarily equal to the amount of time the AP operates in the second frequency band. Thus, the present invention saves the customer from buying two separate APs for two separate frequency bands requiring two separate power amplifiers. Further, the present invention mitigates interference between the two frequency bands. Details of the present invention are illustrated below for exemplary purposes only, and are not intended to limit the scope of the present invention.  
         [0012]     For the sake of terminology, the present invention refers to the frequency band in which the AP  100  is currently receiving and/or transmitting information as the active frequency band, and the frequency band in which the AP  100  is not currently receiving and/or transmitting information as the inactive frequency band. Upon startup, a duty cycle of M:N beacon intervals between the active and inactive frequency bands are chosen, where both M and N are integer numbers. As an example, the active frequency band for one frequency will last “M” beacon intervals; the active frequency band for the other frequency will last “N” beacon intervals. The length of one beacon interval can be arbitrarily long, thus accounting for most any circumstance. Preferably, an average timeshare between the two frequency bands is maintained to allow information exchanges that occur on frequency band-switching boundaries to complete the transaction; average timesharing is illustrated further in the example below.  
         [0013]     If either M or N of the duty cycle is not 1, the AP  100  swiftly transitions to the inactive frequency band and transmits a beacon to keep subscribers operating on the inactive frequency band from possibly migrating to another AP  100  (i.e., disassociating from its current AP). The AP  100  does this by quickly creating a contention free period (“CFP”) on the first frequency band (i.e., the AP  100  signals all subscribers  102  operating on the first frequency band to stay quite and transmit only if polled by the AP  100 ), transitioning to the second frequency band, transmitting a CFP-indicating beacon on the second frequency band, transitioning back to the first frequency band, and transmitting a CF-End on the first frequency band (i.e., the AP  100  signals all subscribers  102  operating on the first frequency band that the CFP has ended and initiates a distributed coordinated function (“DCF”) mode; the DCF mode allows any subscriber in the frequency band to transmit inbound to the AP  100  without having to be polled by the AP  100 ).  
         [0014]     Every beacon has a traffic indication message (“TIM”) which is a delivery traffic indication message (“DTIM”) to insure that at least one beacon is present in each beacon interval to deliver traffic to subscribers in power savings mode. The TIM and the DTIM signals subscribers to stay awake and receive information because outbound traffic has been buffered and awaits delivery. The TIM and the DTIM are typically known to those individuals skilled in the art and are familiar with the 802.11 standard, and thus will not be discussed in greater detail.  
         [0015]     In order for the DTIM to approximate a regular periodicity (if not periodic, the subscriber is always listening for the DTIM which is counterproductive to the subscriber), and because every CFP signal is initiated by a DTIM, each beacon is a DTIM.  
         [0016]     The last beacon in every active frequency band signals the CFP and therefore this beacon must be a DTIM; however this DTIM only signals a CFP and does not signal subscribers that traffic is pending delivery. Therefore, at least one other beacon in the active frequency band must be a DTIM. Therefore, to insure DTIM periodicity, every beacon must be a DTIM.  
         [0017]     The DTIM period is therefore one for all frequency bands. If multicast messages are pending for sleeping subscribers, these subscribers can now receive them after each beacon (except the last) in every active period. After every beacon (except the last) and after all subscriber traffic has completed, the AP terminates the CFP with a CF-End. At the end of the active time, the AP  100  signals a CFP (which is a DTIM—however no pending multicast traffic will be indicated in the TIM), transitions to the other frequency band and creates a CFP with a DTIM at the beginning of the newly started active frequency band.  
         [0018]     While the subscribers are in an inactive frequency band, its best for these subscribers to suspend inbound traffic. If the subscribers do not suspend inbound traffic, the subscribers will transmit inbound to the AP  100 , the AP  100  will not hear the subscribers (because it is “off” frequency) and therefore the subscribers will receive no acknowledgement (“ACK”) from the AP  100 . Failure to receive an ACK from the AP  100  will result in the subscriber counting a failed attempt against that inbound packet. With enough failed attempts, the subscriber will drop the packet. Preferably, during the inactive frequency band, because the AP  100  cannot hear any transmissions from subscribers operating in the inactive frequency band, the subscribers should be “deactivated” so as to prevent dropped packets. In addition to preventing subscribers from transmitting inbound to the AP  100 , deactivating the subscribers operating in the inactive frequency band also prevents the binary exponential back-off from occurring which unnecessarily delays information exchanging. As a result, the AP  100  creating a CFP in the inactive frequency band suspends subscribers from transmitting inbound traffic, thus minimizing the number of subscribers dropping packets, or even, disassociating form the AP  100 . If the subscribers are reliably signaled that a CFP is in progress, they will not transmit inbound unless polled by the AP  100 . In this situation, the AP  100  will not poll and has no intention of polling these subscribers because the AP  100  is “off frequency”.  
         [0019]     The beacon interval should be picked to be sufficiently long (e.g., 50 ms) to make power savings a benefit since the subscriber must listen and interpret the beacon, and then enter a sleep mode until the next beacon. Transmitting two beacons, CF-END, and switching frequencies is an overhead of less than 2 ms that is less than 3% for 50 ms beacon intervals. Therefore there can still be a 95% energy savings in the subscriber.  
         [0020]     Let us now refer to an example of the channel activity as seen by the AP  100  dual frequency band operation in accordance with the present invention as illustrated in  FIG. 2 . The AP  100  defines the timing for the entire basic service set by transmitting beacons according to a beacon interval attribute within the AP  100 . If the medium is determined by the carrier-sense mechanism to be unavailable, the AP  100  delays the actual transmission of a beacon according to the basic medium access rules. It should be noted that though the transmission of a beacon might be delayed because of carrier sense multiple access deferrals, subsequent beacons should be scheduled at the nominal beacon interval. Assume for this example that a beacon interval is 50 TUs (i.e., 50 ms). Also assume that there is a 2:4 duty cycle between 5.0 GHz and 4.9 GHz, respectively.  
         [0021]     In general, between target beacon intervals 1 through 5, and between 7.5 through 11, 4.9 GHz is the active frequency band and 5.0 GHz is the inactive frequency band; between target beacon intervals 5 through 7.5, and between 11 through 12, 5.0 GHz is the active frequency band and 4.9 GHz is the inactive frequency band. Let us now look at specific details of this example and how the AP  100  supports a plurality of frequency bands.  
         [0022]     During target beacon interval 1, the AP  100  sends an announcement to the subscribers  102  operating at 5.0 GHz that the channel is going to a CFP. Upon hearing this announcement, all subscribers  102  operating at 5.0 GHz are waiting to be polled so they stay quite. In accordance with the present invention, the AP  100  never had the intention of polling the subscribers  102  operating at 5.0 GHz at this time, but rather sent the announcement in order to keep the subscribers  102  operating at 5.0 GHz quiet since the AP  100  will not be listening to the frequency. The AP  100  then toggles frequencies to 4.9 GHz, thus making the 4.9 GHz band the active frequency band.  
         [0023]     Once operating on the 4.9 GHz band, the AP  100  sends a CFP message to the subscribers  104  operating at 4.9 GHz. Upon hearing this announcement, all subscribers  104  operating at 4.9 GHz also wait to be polled so they stay quite. The AP  100  then sends multicast traffic to the subscribers  104  operating at 4.9 GHz, if any. Once all the multicast traffic has been transmitted to the subscribers  104  operating at 4.9 GHz, the AP  100  sends a CF-End message to the subscribers  104  operating at 4.9 GHz, after which, the DCF mode begins (i.e., the air interface becomes contentious for the subscribers  104  operating at 4.9 GHz).  
         [0024]     Because every beacon is a DTIM and the target beacon intervals have a fairly regular period regardless of when the frequency is active or inactive, subscribers in the active frequency band wake up for at least a portion of every target beacon interval. Therefore, the subscribers in the active frequency band are awake for the multicast traffic that has been buffered for them for several target beacon intervals. After delivering the multicast traffic to the subscribers in the active frequency band, the frequency band resumes to DCF mode by signaling the CF-end message.  
         [0025]     During target beacon interval 2, the AP  100  signals a CFP at 4.9 GHz to keep subscribers  104  operating at 4.9 GHz from attempting to futilely transmit inbound. After signaling the CFP at 4.9 GHz, the AP  100  transitions to 5.0 GHz and transmits a beacon at 5.0 GHz to keep a fairly periodic train of beacons in the frequency band as well as redundantly signal the CFP at 5.0 GHz. The AP  100  transitions back to 4.9 GHz and signals a CF-End to resume DCF traffic at 4.9 GHz.  
         [0026]     The DCF traffic that follows target beacon interval 2 delays the subsequent beacons scheduled for transmission during target beacon interval 3; the DCF data/ACK sequence had not yet completed before the third target beacon interval time is scheduled. The 802.11 standard allows for delays in beacon transmissions as long as the average rate approaches a beacon interval. Thus, upon completion of the DCF mode, the AP  100  repeats the signaling as it occurred in target beacon interval 2: the AP  100  signals a CFP at 4.9 GHz, transitions to 5.0 GHz and transmits a beacon at 5.0 GHz, and transitions back to 4.9 GHz and signals a CF-End. Even though there is no remaining DCF traffic from the subscribers  104  operating at 4.9 GHz during target beacon interval 3, the AP  100  remains in this mode until target beacon interval 5.  
         [0027]     At target beacon interval 5, the AP  100  signals to the subscribers  104  operating at 4.9 GHz that a CFP is about to begin without the intention of ever polling. The AP  100  switches to 5.0 GHz, transmits another CFP, transmits multicast traffic to the subscribers  102  operating at 5.0 GHz and then transmits a CF-End. In target beacon interval 5, there is no DCF traffic from the subscribers  102  operating at 5.0 GHz, so the scheduled signaling for target beacon interval 6 starts on time.  
         [0028]     During target beacon interval 6, the AP  100  signals a CFP at 5.0 GHz to keep subscribers  102  operating at 5.0 GHz from attempting to futilely transmit inbound. After signaling the CFP at 5.0 GHz, the AP  100  transitions to 4.9 GHz and transmits a beacon at 4.9 GHz to keep a fairly periodic train of beacons in the frequency band as well as redundantly signal the CFP at 4.9 GHz. The AP  100  transitions back to 5.0 GHz and signals a CF-End to enter a DCF mode at 5.0 GHz. The DCF traffic at 5.0 GHz, however, delays the scheduled signaling for target beacon interval 7. The signaling in the remaining target beacon intervals repeats for the next duty cycle.  
         [0029]     Thus, in accordance with the present invention, a beacon preferably initiates each active frequency band, thus hastening the time it takes a subscriber to associate with an AP  100 ; polling is also possible because each beacon can signal a CFP. The AP  100  buffers multicast traffic for power-savings in the subscribers and delivers information per the 802.11 standard. There are multiple opportunities at signaling the CFP in the inactive frequency band increasing reliability. Multiple opportunities exist for DTIM/TIM functions to run per the standard.  
         [0030]     While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. For example, it is possible to design the system in accordance with the present invention with a very rigid switch with little allowance for variability in the switching period. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.  
         [0031]     Moreover, the term “a” or “an”, as used herein, are defined as one or more than one. The term “plurality”, as used herein, is defined as two or more than two. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and/or “having”, as used herein, are defined as comprising (i.e., open language).