Abstract:
Within a radio access network ( 11   1   , 11   2 ) an access point ( 18   1   , 18   2   , 18   3  and  18   4 ) selects a channel by first randomly choosing a channel from those available for use. The Access Point then monitors the channel for a random interval to determine if the channel is presently carrying traffic. The Access Point will select the channel unless the channel is in use by another AP. If so, then the AP will select another channel and commence monitoring for a random interval in order to determine whether the channel is available. The AP will continue to choose channels until a suitable channel is found, or all available channels have been exhausted.

Description:
TECHNICAL FIELD 
   This invention relates to a technique for automatically selecting channels in a Radio Access Network, such as a wireless Local Area Network (LAN). 
   BACKGROUND ART 
   Advances in the field of wireless LAN technology has led to the availability of relatively inexpensive wireless LAN equipment, which, in turn, has resulted in the emergence of publicly accessible wireless LANs (e.g., “hot spots”) at rest stops, cafes, libraries and similar public facilities. Presently, wireless LANs offer users access to a private data network, such as a Corporate Intranet, or a public data network such as the Internet. The relatively low cost to implement and operate a wireless LAN, as well as the available high bandwidth (usually in excess of 10 Megabits/second) makes the wireless LAN an ideal access mechanism through which a mobile terminal user can exchange packets with an external source. 
   Within the wireless LAN, there exists one or more Access Points (APs) each typically comprising a transceiver for exchanging radio frequency signals with a mobile terminal user. Each AP communicates with the mobile terminal user on a particular channel. In most radio technologies, such as the radio technology embodied in the IEEE 802.11b standard, adjacent channels tend to overlap. For this reason, geographically adjacent APs within the wireless LAN try to assign non-overlapping channels to avoid interference. Often such channel assignments require manual intervention or specific protocol modifications. Unfortunately, this approach doesn&#39;t work well when geographically adjacent APs belong to wireless LANs operated by separate entities that lack the incentive to cooperate to reduce radio frequency interference. 
   Thus, there is need for a technique for automatically selecting channels in a wireless LAN to reduce interference without any modifications to the underlying communication protocol. 
   BRIEF SUMMARY OF THE INVENTION 
   Briefly, in accordance with present principles, a method is provided for selecting a radio channel by an access point (AP) within a radio access network. The method commences upon the AP initially choosing a channel randomly from among a plurality of available radio channels. The AP then monitors the selected channel for a random interval to determine if any traffic is present. If the channel is presently free (i.e., no use by another AP), then the AP claims the selected channel to carry traffic. Otherwise, if the channel already carries traffic, the AP chooses another channel. Thereafter, the AP monitors the newly selected channel during a random interval to determine the availability of the newly chosen channel. In practice, the AP repeats the process until it finds an available channel found, or it scans all the available channels. 

   
     BRIEF SUMMARY OF THE DRAWINGS 
       FIG. 1  depicts a block schematic diagram of a communication network that includes a plurality of radio access networks; and 
       FIG. 2  depicts a flow chart illustrating the steps of the method of present principles for selecting a traffic-carrying channel within the communications system of FIG.  1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a block schematic diagram of a communications system  10  that includes at least one, and preferably, a plurality of radio access networks, illustratively depicted by radio access networks  11   1  and  11   2 . The radio access networks  11   1  and  11   2  each enable at least one user, and preferably a plurality of users (e.g., users  12   1 ,  12   2 , and  12   3 ) to access an external data network  14  such as the Internet or the like. In a preferred embodiment, the user  12   1  utilizes a lap top computer while the user  12   2  utilizes a Personal Data Assistant and the user  12   3  utilizes a wired communications appliance. Other users (not shown) could employ other types of wired or wireless communication appliances. 
   Each of the radio access networks  11   1  and  11   2  includes at least one, and preferably, a plurality of access points (APs), illustratively illustrated by APs  18   1 - 18   4 , via which each of the users  12   1 ,  12   2  and  12   3  accesses a wireless Local Area Network (LAN)  20  within each access network. In the illustrated embodiment, each AP, such as AP  18   1 , includes a wireless transceiver (not shown) for exchanging radio frequency signals with a radio transceiver (not shown) within a communications appliance employed by a user, such one of users  12   1  and  12   2 . One or more of the APs  18   1 - 18   4  could also include a wired access mechanism by which a user, such as user  12   3 , can access the network via a wired communications appliance. Each of the APs  18   1 - 18   4  in each of the radio access networks  11   1  and  11   2  employs one or more well-known wireless or wired data exchange protocols, such as the “HiperLan 2” or IEEE 802.11 protocol. Indeed, different APs can employ different wireless protocols to accommodate users whose communications appliances use different protocols. 
   Each of the access points  18   1 - 18   3  communicates with a corresponding one of users  12   1  and  12   2  over a radio frequency link by choosing a particular radio channel over which to send and receive data. With most radio technologies, such as the IEEE 802.11b standard, adjacent channels exhibit some degree of overlap. Thus, radio interference will occur when geographically adjacent APs transmit on adjacent channels. In the past, the problem of adjacent channel interference has been overcome by manual intervention, or the adoption of specific protocol modifications to force the selection of non-adjacent channels. These approaches incur the disadvantage of requiring either full-time administrative control over geographically adjacent APs or requiring them to implement specific protocol modifications. Typically, the ability to exert control over adjacent APs does not exist when such adjacent APs belong to different network entities. 
     FIG. 2  illustrates in flow chart form the steps of a method in accordance with the present principles for selecting a radio frequency channel for an AP, such one of the APs  18   1 - 18   4  within one of the access networks  11   1  and  11   2 . The channel selection method of  FIG. 2  commences upon execution of step  100  during which the AP sets the following parameters: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               PARAMETER 
               DESCRIPTION 
             
             
                 
                 
             
           
           
             
                 
               current_channel 
               The current channel selected by the AP 
             
             
                 
               channel_min 
               The lowest channel number available 
             
             
                 
               channel_max 
               The highest channel number available 
             
             
                 
               channel_step 
               The separation between selected channels 
             
             
                 
               time_min 
               The minimum time for channel monitoring 
             
             
                 
               time_max 
               The maximum time for channel monitoring 
             
             
                 
                 
             
           
        
       
     
   
   Following step  100 , the AP makes a check during step  102  to determine whether the last channel used by the AP still remains available. If so, then AP selects the last channel used as the current channel during step  104 . Checking the availability of the previously used channel increases efficiency. Often times, the AP can re-use the last channel before another AP claims the channel. Should the AP find the last channel unavailable, then the AP will select a channel at random from the available channel list during step  106 . 
   Following either of steps  104  and  106 , the AP executes step  108  during which the AP sets the channel-monitoring interval scan_time to a random value lying between time_min and time_max. Thereafter, the AP monitors the channel during step  110 . The AP monitors the channel by listening during the interval scan_time for the presence of traffic (if any) carried by another access point (AP). During step  112 , the AP checks whether the monitoring performed during step  110  uncovered use of the channel by another AP. Upon finding the channel free of traffic during step  112 , the AP claims the channel for use during step  114  and the channel selection process ends (step  116 ). 
   If the AP finds the channel in use during step  112 , the AP then makes a check during step  118  to determine whether other channels remain available for selection, taking into account the separation between selected channels prescribed by the current value of channel_step. Thus, for example, if the channel_step=2, then after selecting channel #1, the channel #3 becomes the next channel available for selection. Depending on the value of the current_channel and the number of channels available, the AP could find that additional channels still remain available. If so, the AP executes step  120  selects a next new channel in accordance with the relationship:
 
new channel=current_channel+channel_step
 
before proceeding to re-execute step  108 . Otherwise, upon detecting exhaustion of the available channels during step  118 , the AP resets the value of current_channel during step  122  in accordance with the relationship:
 
channel_step=floor(channel_step/2)
 
In other words, during step  122 , the AP resets the value of channel_step to the lowest integer value of one-half the previous value of channel_step. Thus, if the value of channel_step prior to step  122  was 3 for example, then following step  122 , the new value would be one. After step  122 , program execution branches to step  120 .
 
   To better understand the above-described channel selection method, consider the following three examples. 
   EXAMPLE 1 
   Assume that the APs  18   1  and  18   2  in the radio access network  11   1  of  FIG. 1  use channels #1 and #6, respectively, while AP  18   4  remains off line. Further assume that AP  18   3  just commenced operation and is the only AP implementing the channel selection method of the present principles. In implementing the channel selection method, assume that AP  18   3  employs the following values: channel_min=1, channel_max=11, channel_step=5, time_min=500, and time_max=1000, with the time_min and time_max measured in milliseconds. Additionally, assume that AP  18   3  did not store any information about the previously used channel. 
   In accordance with the channel selection method of the present principles, the AP  18   3  will first select a current channel in accordance with the relationship
 
current_channel=random(1, 6, 11)
 
   For present purposes, assume that AP  18   3  selected channel #6. Next, the AP  18   3  selects the value for scan_time in accordance with the relationship:
 
scan_time=random(500, 1000)
 
   For present purposes, assume that AP  18   3  selected the value of scan_time as 600 milliseconds. Accordingly, the AP  18   3  will listen to channel #6 for that interval. Since the AP  18   2  is currently using channel #6 in this example, the AP  18   3  will detect use of this channel. Therefore, the AP  18   3  will select a new channel in accordance with the relationship:
 
new channel=current_channel+channel_step
 
   In the present example, with current_channel=6 and channel_step=5, the AP  18   3  will select channel #11. The AP  18   3  now establishes a new value for the parameter scan_time, say 750 ms and then listens to channel #11 for that interval. Assuming that this channel carries no other traffic, the AP  18   3  will now claim channel #11 for use. 
   EXAMPLE 2 
   Assume that the APs  18   1 ,  18   2 , and  18   3  within the radio access network  11   1  use channels #1, #6, and #11, respectively. Further assume that AP  18   4  in the same radio access network now commences operation and implements the channel selection method according to present principles. In implementing the channel selection method, assume that the AP  18   4  selects the following parameter values: channel_min=1, channel_max=11, channel_step=5, time_min=500, time_min=1000. Additionally assume that AP  18   4  did not store information about the previously used channel. 
   Using the channel selection method of the present principles, the AP  18   4  will first select a channel in accordance with the relationship
 
current_channel=random(1, 6, 11)
 
   Assume that AP  18   4  selects channel #6 as in the previous example. Next, the AP  18   4  selects the value for the parameter scan_time in accordance with the relationship:
 
scan_time=random(500,1000)
 
   Assume that AP  18   4  selected a value of 660 milliseconds for scan_time so AP  18   4  will listen to channel #6 for 660 ms for that interval. With channel #6 in use by AP  18   2  in this example, the AP  18   4  will find channel #6 occupied. Thus, the AP  18   4  selects a new channel in accordance with the relationship: 
    new channel=current_channel+channel_step 
   In the current example, the AP  18   4  will now select channel #11. 
   The AP  18   4  next establishes a new value for scan_time, say 550 ms and then listens to channel #11 for that interval. Since channel #11 remains in use by AP  18   3  in this example, the AP  18   4  will find channel #11 occupied as well. 
   Having found channel #11 busy, the AP  18   4  selects another channel, and in this example, will now pick channel #1 due to wrap around. Having selected channel #1, the AP  18   4  selects a new value for scan_time, say to 800 milliseconds and will listen to channel #1 for that interval. Since channel #1 remains in use by AP  18   1  in this example, the AP  18   4  will find channel #1occupied as well. 
   All possible channels for selection have been exhausted in this example when channel_step=5. Therefore the AP  18   4  will reduce the value of the parameter channel_step in accordance with the relationship:
 
channel_step=└channel_step/2┘
 
   In the current example, the new value of channel_step becomes 2. After reducing the value of channel_step to 2, the AP  18   4  now selects another channel using the previously described relationship. In this example the AP  18   4  now selects channel #3. Thereafter, the AP  18   4  selects a new value for scan_time, say 730 milliseconds and now listens to channel  3  during that interval. Upon finding channel #3 free of traffic, the AP  18   4  now claims this channel for use. Note that in this case, interference cannot be completely avoided because all interference-free channels are already in use. 
   EXAMPLE 3 
   This example describes scenario when two APs, such as APs  18   2  and  18   3  within the radio access network  11   1 , both startup simultaneously, while another AP, such as AP  18   1  in the same network, currently uses channel #1. Assume that both APs  18   2  and AP  18   3  utilize the following parameter values: channel_min=1, channel_max=11, channel_step=5, time_min=500, time_min=1000. Also, assume that neither AP  18   2  nor AP  18   3  store information about the channel each previously used. 
   Using the method of the present principles, both of the APs  18   2  and  18   3  will each select a current channel as follows in accordance with the relationships: 
   For AP  18   2  
 
current_channel=random(1, 6, 11)
 
   Assume AP  18   2  has randomly selected channel #6. 
   For AP  18   3  
 
current_channel=random(1,6,11)
 
   Also assume AP  18   3  selects channel #6. If both AP  18   2  and AP  18   3  were to actually claim the same channel, severe interference would result. However, as will become apparent from the discussion hereinafter, the channel selection method of the present principles will prevent both of the APs  18   2  and  18   3  from claiming the same channel. 
   After each of the APs  18   2  and  18   3  initially selects a channel, each AP establishes a value for the parameter scan_time as follows: 
   For AP  18   2  
 
scan_time=random(500,1000) (say 660)
 
For AP  18   3  
 
scan_time=random(500,1000) (say 820)
 
   Next, the AP  18   2  listens to channel #6 for 660 ms while AP  18   3  also listens to the same channel for 820 milliseconds. Assuming that no traffic otherwise exists, then AP  18   2  will find the channel free during the period it listened and thus claim the channel for use. Since the parameter scan_time established by AP  18   3  is longer than the parameter scan_time established by the AP  18   2 , the AP  18   3  will listen longer and will ultimately detect use of channel #6 by the AP  18   2  during the time interval between 660 and 820 ms. 
   Having found channel #6 occupied, the AP  18   3  now must choose another channel and does so in the manner previously described. In the current example, assume AP  18   3  selects channel #11. The AP  18   3  now selects a new value for scan_time, say 530 milliseconds The AP  18   3  now listens to newly selected channel #11 for 530 ms, and upon finding it free, now claims the channel for use. 
   The foregoing describes a technique for selecting a radio channel by an access point in a radio access network to minimize adjacent channel interference.