Patent Publication Number: US-8532072-B2

Title: System and method for adaptively setting the probability of generating a beacon broadcast in a wireless network

Description:
TECHNICAL FIELD 
     The disclosure relates generally to synchronizing nodes in a wireless mesh network. 
     LIMITED COPYRIGHT WAIVER 
     A portion of the disclosure of this patent document contains material to which the claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction by any person of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office file or records, but reserves all other rights whatsoever. 
     BACKGROUND 
     To conserve power, it is desirable for the nodes of a wireless network to cycle between sleeping and waking phases. During the sleeping phase, a node shuts down its communication and other major functions and draws very little current, thereby saving power. During the waking phase, a node powers its transceiver and exchanges messages with other nodes in the network. One problem faced in such networks is how to maintain agreement, or to synchronize, the clocks of different network nodes so that they wake at approximately the same time. 
     In the past, networks have addressed the problem of synchronization is a number of ways. Some networks have relied, directly or indirectly, on an external time reference such as the internet. The internet ultimately uses the Internet Time Service (ITS), provided by NIST and USNO, where servers respond to requests using the Network Time Protocol (NTP) by supplying time of day responses with accuracy and precision traceable to centralized government standards. The problem with this approach is that it depends on an authority, standard, or reference which is external and therefore may be unavailable to a wireless network due to physical isolation or interference. 
     Other networks have relied on a local time reference authority, such as a dedicated node. The dedicated node periodically generates a synchronizing message to be used as a time reference by all the other nodes in the network. The problem with this approach is that it depends on a dedicated node which, by design, creates the potential for a single point of failure. If the dedicated node were to fail, the entire network would lose synchronization. Eventually, the loss of synchronization due to accumulated clock drift would cause the nodes to lose communication with each other, causing the network to fail. 
     What is needed is a system and method for synchronizing nodes that addresses these issues and others that will become apparent while reading this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a schematic representation of a plurality of nodes that make up a wireless network according to an example embodiment; 
         FIG. 1   b  is a schematic representation of a failed beacon node in a wireless network according to an embodiment; 
         FIG. 1   c  is a schematic representation of a plurality of nodes that make up a wireless network after the result depicted in  FIG. 1   b  according to an example embodiment; 
         FIG. 1   d  is a schematic representation of a plurality of nodes that make up a wireless network after the result depicted in  FIG. 1   b  according to an example embodiment; 
         FIG. 2   a  is a schematic representation of a plurality of nodes that make up a wireless network according to an example embodiment; 
         FIG. 2   b  is a schematic representation of a plurality of nodes that make up a wireless network after the passage of time of the wireless network depicted in  FIG. 2   a  according to an example embodiment; 
         FIG. 2   c  is a schematic representation of a plurality of nodes that make up a wireless network after the passage of time of the wireless network depicted in  FIG. 2   a  according to an example embodiment; 
         FIG. 3  illustrates a nomination-election process; 
         FIG. 4  represents a route-discovery table with a series of route-discovery entries lodged therein; and 
         FIG. 5  is a schematic diagram illustrating a medium having an instruction set for replacing beacon nodes according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As we noted above, it can be a problem to rely on an external time reference, since the external time reference may be or may become unavailable. We instead use an internal time reference, designating a single node to act as the time authority for a network. This node is tasked with broadcasting a time synchronization message, a beacon, throughout the network during the waking phase. 
     In the past, such an approach resulted in a single point of failure. To avoid this problem, we teach that more than one node in the network should have the ability to detect and replace a failed beacon node. Should the designated node fail, then the other nodes in the network select a replacement through a nomination-election process. 
     In one such approach, only one node at a time acts as the beacon of time synchronization messages. The synch message is broadcast during the waking cycle. The synch message contains a field which describes the time offset until the start of the next waking cycle. As the synch message is relayed, the relaying node reduces the time offset value in the message to compensate for the time which was lost during reception and re-transmission. 
     In such an embodiment, nodes that are not the beacon node receive beacon signals and use the beacon signals to adjust settings such as sleeping configuration parameters. Further, the receiving node relays the messages received from the beacon to neighbor nodes. Information regarding neighbor nodes may be stored in a neighbor table (NT). 
     In some embodiments, several types of beaconed messages may be conveyed by the beacon node during the waking interval. One beaconed message is clock synchronization. Another beaconed message is a status inquiry regarding node activity. For example, a node may drop out of the wireless network for any number of reasons, and network status may be adjusted depending on the number of active nodes. The beacon node can also transmit messages containing its address, or a guard interval used by the receiving node to delay a certain time before accepting a synchronization message. 
     The nomination-election process is invoked when the designated beacon node fails. First, nodes in the network detect the absence of synch messages. One or more nodes nominate themselves as the new beacon nodes. In one embodiment, if a node experiences a configured number of consecutive cycles without receiving a time synchronization message, then, by a configured probability, it nominates itself to act as a beacon during the next waking cycle. 
     Because more than one node may nominate itself to act as a time synch generator at the same time, it is necessary to reduce the number to a single node. The system uses an election process to select between nodes nominated for the beacon. In one such election process, a beaconing node removes itself as a beacon candidate should it receive a synch message from a competing node which outranks it. Nodes may, for instance, be ranked by MAC address, the value of which is unique for every node. The election criteria should be such that every node is ranked in a unique and ordinal manner. For example, one could decide by which node has the superior rank according to a preconfigured value which is unique to each node. 
     In one embodiment, nodes that receive sync messages from more than one beacon node update their configuration parameters with the latest sync message from the highest ranking node. In some embodiments, the MAC address of the node is used to establish the highest ranking node (i.e., the beacon node with the lowest, or highest, MAC address is the highest ranking node). In other embodiments, the highest ranking node may be a function of how close the node is to the receiving node, or how close the node is to the center of the wireless network. 
     In one such embodiment, if a node received a sync message during the waking interval, an update to the time clock is deferred until just before the node goes to sleep. This allows the node to select between sync messages in the event more than one was received during that wake interval. 
     In one embodiment, the probability that a node will nominate itself is a parameter configured by someone such as the system administrator. In one such embodiment the probability selected is in inverse proportion to the size of the network. This reduces the number of nodes which will compete for the role of beacon, thereby reducing congestion, and improving the speed with which a single beacon node will emerge as beacon. 
     The use of a threshold that provides a fixed probability of nomination can be limiting. If the fixed probability of nominating oneself is high in a large network, then many nodes will nominate themselves at the same time. This will create congestion, and potentially require several cycles to resolve the issue of who should be elected as the sole beaconing node. If the fixed probability is low in a network of few nodes, then it may require several cycles before a node nominates itself. In this case the network runs the risk of losing synchronization, thereby losing communication with each other, and the network will fail. This scenario becomes even more of a problem when the network size changes over time (e.g., when subnets join or leave the network over time) 
     To overcome this limitation, in one adaptive embodiment, the probability for nominating oneself is initially set to a fixed percentage. Each node keeps a count of the number of different sources of beacon messages it receives during a cycle. If the number is high, the node reduces its probability for nominating itself. If the number is one, the probability is left unchanged. If the number is zero after a fixed number of cycles, a node increases its probability by a fixed amount or rate, up to a maximum of 100%. 
     In one embodiment, each node in the wireless network wakes with some probability that it will issue a beacon signal containing a clock sync message. If a node hears too many sync messages during a waking interval, it adjusts its probability of emitting a sync message down by a factor of 2 by bumping up its nomination probability P by one. If it fails to hear any sync messages for a few successive periods, it adjusts its probability of emitting a sync message up by a factor of 2 by adjusting P down by one. In such one embodiment, the initial value of P is the smallest whole number that is greater than log 2 (the number of nodes in the network). 
     An advantage of such an adaptive approach is that the nomination probability will automatically adjust itself in proportion to the number of nodes in the network. This will reduce the potential for congestion in the RF medium if too many nodes were to nominate themselves, or improve the rate of response to the failure of a beacon if any node was hesitant to nominate itself. 
     One advantage of the described nomination-election approach is that time authority within such a network is robust by design. No central authority is used to establish an external time reference. At the same time, no dedicated node is used to establish the time reference. 
     The solution is robust in that the network can automatically replace a failed beacon node. The time synchronization function, therefore, has no single point of failure. 
       FIG. 1   a  is a schematic representation of a wireless network  100  according to one aspect of the present invention. In  FIG. 1 , wireless network  100  includes a plurality of wireless nodes. Several nodes are depicted, including a beacon node  110 . The remaining nodes, including nodes  112 ,  114 , and  116 , are referred to as common nodes. In the example shown beacon node  110  is depicted in heavy outline. For the purposes of this description, a “beacon node” is a node in a wireless network that provides a beacon signal to other nodes in network  100 . An assignment as beacon node may be temporary and of indeterminate duration. A “common node” is any other node in wireless network  100 . Each common node is designed to have the potential to become the beacon node if necessary. 
     In the following, the common nodes  112  and  116  may be referred to as “interior nodes  112  and  116 ” since they each are located within the geographic interior of the wireless network  100 . The common node  114  may be referred to as an “exterior node  114 ” since it is located on exterior of the geographic limitations of the wireless network  100 . 
     A node that is currently the beacon node synchronizes all other nodes in the mesh network for cyclic sleep and wake intervals. In one embodiment, the beacon node synchronizes for specific tasks such as network maintenance functions or to compensate for latency. 
     As noted above, any node in network  100  may take over as the beacon node for synchronization purposes. In one embodiment, a given node may lose its position as beacon node when a difficulty arises in wireless network  100  that causes the beacon node to fail or to lose efficiency such that the wireless network fails or loses useful efficiency. Consequently, common nodes in the wireless network have the capability of self-nomination to assert their interest in becoming the beacon node. 
     In one embodiment, a common node determines if it should nominate itself as the new beacon node as a function of a self-nomination calculation performed by the common node. In one embodiment, the self-nomination calculation is a function of a probability parameter that is set for each common node. In one such embodiment, the probability parameter is a function of the geographic location in the network; that is, the closer the node is to the center of the network, the higher the probability that the node should assert itself as the new beacon node. 
     As noted above, each common node in wireless network  100  is capable of generating the beacon signal used to synchronize a cyclic function. Each node looks to receive the beacon signal at predetermined intervals. If a node fails to receive the beacon signal within the predetermined interval, it determines if it should assert itself as the new beacon node. In one embodiment, each node that fails to receive the beacon signal within the predetermined interval calculates a random number. If the random number is below a given threshold number, the node nominates itself as the new beacon node at the start of the next waking cycle. If the random number is greater than the given threshold number, the node waits a cycle before determining if it should try to nominate itself again. 
     Methods for generating random numbers are known and conventional. Random number generation may be carried out by a number of conventional methods. An example method of determining whether to generate a beacon signal at a given common node is given in pseudo code below. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 def EmitSynch( ) 
               
               
                 if random( ) &lt; THRESHOLD then 
               
               
                    TransmitSynch( Time offset until next call to Periodic_Function ) 
               
               
                 def Periodic_Function( ) 
               
               
                 EmitSynch( ) 
               
               
                 Function2( ) 
               
               
                 Function3( ) 
               
               
                 ... 
               
               
                 schedule a call to Periodic_Function in PERIODIC seconds 
               
               
                 def ProcessIncomingMessages( ) 
               
               
                 if there is a message to process 
               
               
                  if the message has not been seen before (using source and sequence 
               
               
                  number) 
               
               
                  switch(message.type){ 
               
               
                   case Synch: 
               
               
                   reschedule call to Periodic_Function using time offset in payload 
               
               
                   of message 
               
               
                   update time offset in payload of message 
               
               
                   relay message 
               
               
                   ... 
               
               
                   } 
               
               
                 def Main( ) 
               
               
                 schedule a call to Periodic_Function in PERIODIC seconds 
               
               
                 do forever 
               
               
                    ProcessIncomingMessages( ) 
               
               
                 def ProcessSynch(msg) 
               
               
                 increase synch_count 
               
               
                 if not synched_this_cycle or msg.Source&gt;syncher 
               
               
                    // synch using msg 
               
               
                    probability_of_beaconing = 0 
               
               
                    synched_this_cycle = true 
               
               
                    syncher = msg.Source 
               
               
                    Next_WakingEpoch = now + msg.WakingEpochOffset 
               
               
                    Next_SleepTime = msg.SleepTime 
               
               
                    Next_WakeTime = msg.WakeTime 
               
               
                    msg.WakingEpochOffset = Next_WakingEpoch − now 
               
               
                    relay the msg 
               
               
                 def EndWakeCycle( ) 
               
               
                 if not synched_this_cycle 
               
               
                    increase the probability_of_beaconing, but not more than 
               
               
                    100 percent 
               
               
                 if we received multiple beacons during the cycle 
               
               
                    decrease the probability_of_beaconing, but not less than 0 percent 
               
               
                 WakeTime = Next_WakeTime 
               
               
                 SleepTime = Next_SleepTime 
               
               
                 WakingEpoch = Next_WakingEpoch 
               
               
                 Def StartWakeCycle( ) 
               
               
                 Syncher = 0 
               
               
                 Synch_count = 0 
               
               
                 synched_this_cycle = false 
               
               
                 if a random roll meets the probability_of_beaconing 
               
               
                    generate a beacon 
               
               
                    synched_this_cycle = true 
               
               
                    syncher = this node’s address 
               
               
                    synch_count = 1 
               
               
                 def doWake( ) 
               
               
                    StartWakeCycle( ) 
               
               
                    for WakeTime 
               
               
                       if a msg has been received 
               
               
                       switch(msg.type) 
               
               
                       case SYNCH:ProcessSynch(msg) 
               
               
                       ... 
               
               
                       } 
               
               
                    EndWakeCycle( ) 
               
               
                 def doSleep( ) 
               
               
                 sleep the node for SleepTime 
               
               
                 def Main( ) 
               
               
                 do Forever 
               
               
                    doWake( ) 
               
               
                    doSleep( ) 
               
               
                   
               
            
           
         
       
     
     The beacon signal from the beacon node informs neighbor nodes the extent of the time until the next cyclic function starts. Neighbor nodes update the message to compensate for latency, and relay the message to their neighbor nodes until the beacon propagates throughout the wireless network. In one embodiment, where a node receives multiple beacon signals during the same cycle, then it uses the last received beacon as the authority for setting the time offset until the start of the next cycle. 
     In one embodiment, the initial threshold value for transmitting a beacon signal in the absence of other beacon signals is chosen to limit the amount of beacon signal traffic as a function of network size. In one such embodiment, the initial threshold value for the generated random number is adjusted in inverse proportion to the size of the wireless network. Consequently in this embodiment, the probability value is adjusted downward as the size of the wireless network is increased. This adjustment of beacon-generating probability is done to maintain an adequate, without being excessive, rate of beaconing. 
     In one embodiment, as nodes receive more synchronization messages, they are designed to reduce the frequency of transmitting synchronization messages. 
     In one embodiment, each node keeps a count of the number of different sources of beacon messages it receives during a cycle. If the number is high, the node reduces its probability for nominating itself by a fixed amount or rate. If the number is one, the probability is left unchanged. If the number is zero after a fixed number of cycles, a node increases its probability by a fixed amount or rate, up to a maximum of 100%. The probability of a node nominating itself as a candidate for replacing the beacon node is based, therefore, on a dynamic self-adjusting probability, rather than a constant hard-coded, or manually configured value. 
     In one embodiment, the probability that any node nominates itself is set to zero. With a probability of zero, each node in the network will wait for a period of time to see if a beacon is already established. The network will, therefore, quietly awaken. The nodes will adjust their probabilities upward as needed to a number reflective of the number of nodes in the network. 
     In one embodiment, the probability that any node nominates itself is set to 100%. With a probability of 100%, each node in the network will nominate itself to be the beacon. After an initial flurry of activity, the nodes will adjust their probabilities downward to a number reflective of the number of nodes in the network. 
     In an embodiment, the initial threshold value for transmitting a beacon signal in the absence of other beacon signals is chosen to prefer some common nodes over other common nodes. For instance, in one embodiment, the probability parameter is a function of the geographic location in the network; that is, the closer the node is to the center of the network, the higher the probability that the node should assert itself as the new beacon node. 
     One method of calculating the geographical location of the common node is to compare a tally of the total number of node hops between the common node and all other nodes. The node having the lowest tally approximately the most centrally located node. It should, therefore, receive a preference to be the next beacon node. 
     In one embodiment, each node modifies its probability for nominating itself adaptively as a function of its geographic location In one such embodiments, nodes toward the center of the network increase their probability of nomination faster than those farther from the center in the absence of beacon signals, while decreasing their probability slower in the presence of beacon signals. In one such embodiment, nodes must keep aware of their position in the network as the network adds or loses nodes. 
     Returning to  FIG. 1   a , beacon node  110  has just succeeded in transforming from an interior common node by one of the methods described above. Beacon node  110  continues to transmit the beacon signal until it fails or is replaced. 
       FIG. 1   b  is a failed beacon node in a wireless network  101  according to an embodiment. As depicted in  FIG. 1   a , a beacon node  110  was present but it has failed for any number of reasons. For example, the beacon node  110  may have been a mobile device that has been removed from the vicinity of the wireless network  100  ( FIG. 1   a ) such that communication has been lost from the wireless network  101  ( FIG. 1   b ). The beacon node  110  ( FIG. 1   a ) by way of example may have suffered a power failure such as a depleted battery that was the power source such that communication has been lost. The beacon node  110  ( FIG. 1   a ) by way of example may have been blocked in signal strength by an object such as a door opening or closing or such as a vehicle that has moved into the main path of broadcasting for the former beacon node, such that communication has been lost. 
       FIG. 1   b  is a schematic representation of the combination of network  100  from  FIG. 1   a  with another network to form wireless network  101 . Several nodes are depicted, including the former beacon node  110  and common nodes  112 ,  114 ,  116 ,  118 ,  120 ,  122 ,  124 ,  126 ,  128  and  130 , although all unmarked nodes may be referred to as common nodes. 
     As depicted in  FIG. 1   b , the wireless network  101  exists without a beacon node. When no beacon is received after a period of time, each common node generates a random number that is compared to a predefined initial threshold value in order to determine whether that node should transmit a beacon signal. Each node that transmits a beacon signal in a given cycle is nominated to become the new beacon node. 
     As noted above, in one embodiment, each node modifies its threshold to adapt to changing conditions. In one such embodiment, each nodded keeps a count of the number of different sources of beacon messages it receives during a cycle. If the number is high, the node reduces its probability for nominating itself by a fixed amount or rate. If the number is one, the probability is left unchanged. If the number is zero after a fixed number of cycles, a node increases its probability by a fixed amount or rate, up to a maximum of 100%. The probability of a node nominating itself as a candidate for replacing the beacon node is based, therefore, on a dynamic self-adjusting probability, rather than a constant hard-coded, or manually configured value. 
     If two or more common nodes are nominated, an election process must choose between them. In one embodiment, the election method, consisting of some programmed criteria, removes a nominated beacon node if it receives a synch message from a competing node which outranks it. The election criteria should be such that every node is ranked in a unique and ordinal manner. For example, by which node has the greater (or lesser) MAC address, the value of which is unique per node. Or by which node has the superior rank according to a preconfigured value which is unique to each node. 
     In one embodiment, therefore, when one nominated common node receives a beacon signal from a second nominated common node it compares one or more parameters associated with the transmitter node against the same parameters at the nominated node that received the beacon signal. For example, the node that received the beacon signal could compare its MAC address with the MAC address of the transmitting node. If the receiving node compared to the transmitter node has the higher MAC address, it continues to send out the beacon signal. If, however, the receiving node compared to the transmitter node has the lower MAC address, it stops sending out the beacon signal. This continues until only a single beacon node remains. 
     In one embodiment, when we select a new beacon node, we want to favor nodes close to the center of network  100  while disfavoring nodes farther away from the center of network  100 . In one such embodiment, the node that received the beacon signal could compare its total node-hop tally with the node-hop tally of the transmitting node. If the receiving node compared to the transmitter node has the lower total number of node hops between itself and all other nodes, it continues to send out the beacon signal. If, however, the receiving node compared to the transmitter node does not have the lower total number of node hops between itself and all other nodes, it stops sending out the beacon signal. This continues until only a single beacon node remains. 
     The network is, therefore, synchronized by an emergent behavior of the mob of nodes, rather than by that of a selected or predetermined node, or a centralized time authority, or a time reference external to the network. 
       FIG. 1   c  is a schematic representation of a plurality of nodes that make up a wireless network  102  after the result depicted in  FIG. 1   b  according to an example embodiment. The wireless network  102  is represented as a more recent state of the wireless network  101  depicted in  FIG. 1   b . As a result of each common node having participated in a beacon node election process, the former common node  116   122  ( FIG. 1   b ) has been elected to be the new beacon node  117   122  ( FIG. 1   c ). 
       FIG. 1   d  is a schematic representation of a plurality of nodes that make up a wireless network  103  after the result depicted in  FIG. 1   b  according to an example embodiment.  FIG. 1   d  represents an alternative embodiment after the event depicted in  FIG. 1   b . The wireless network  103  is represented as a more recent state of the wireless network  101  depicted in  FIG. 1   b . As a result of each common node having participated in a beacon node election process, the former common node  114  ( FIG. 1   b ) has been elected to be the new beacon node  115  ( FIG. 1   d ). As illustrated by way of example, although the physical and geographical limitations of the common node  114  ( FIG. 1   b ) have not significantly changed, the former common node  114  ( FIG. 1   b ) has been elected to become the new beacon node  115  ( FIG. 1   d ). As illustrated according to an embodiment, the new beacon node  115  is an exterior beacon node  115 . 
       FIG. 1   d  is a schematic representation of a plurality of nodes that make up wireless networks  104  and  105  after the network  102  depicted in  FIG. 1   c  splits into two different networks. When that happens, node  122  may no longer be the best beacon node for network  104  and the beacon duties migrate to node  110 , in this example.  FIG. 1   d  represents an alternative embodiment after the event depicted in  FIG. 1   b . As a result of each common node having participated in a beacon node election process, the former common node  110  has been elected to be the new beacon node for network  104 , while the former common node  130  has been elected to be the new beacon node for network  105 . 
       FIG. 2   a  is a schematic representation of a plurality of nodes that make up a wireless network  200  according to an example embodiment. Several nodes are depicted including a beacon node  210  and some common nodes  212 ,  214 , and  216  are also depicted, although all unmarked nodes may be referred to as common nodes. The beacon node  210  is depicted in heavy outline. 
     In one embodiment, when we select a new beacon node, we want to favor nodes close to the center of network  100  while disfavoring nodes farther away from the center of network  100 . 
     In an embodiment, a cyclic function for the wireless network  200  has initiated an election to find a useful beacon node. The election may result in the beacon node  210  losing beacon node status and a new beacon node being elected. 
     As the cyclic function is initiated to either retain the current beacon node  210  or to select a useful new beacon node, each common node as well as the beacon node  210  initiate a self-nomination process according to an embodiment. In this embodiment, the self-nomination process may use a metric that weights an interior node heavier toward election than an exterior node.  FIG. 2   a  may represent the result of an election wherein the beacon node  210  has become the new beacon node in part by virtue of its geographic location. For example, an election may have considered all nodes including the interior common nodes  210  and  212 , the election may have also considered all nodes including the exterior common nodes  214  and  216 . 
     In one embodiment, a first metric in the election includes the geographical type of each node, such that the exterior common nodes  214  and  216  are disqualified. In an embodiment after all exterior common nodes are disqualified, the remaining common nodes are interior nodes such as the nodes  210  and  212 . 
     In an embodiment, there remain only the common nodes including the interior common nodes  210  and  212 , and a random number generation algorithm is used to elect the node  210  to be the new beacon node  210  over the other interior common nodes including the interior common node  212 . 
     In an embodiment, a geographical centrality metric is used to select among all common nodes. For example, an evaluation may be made of a tally of the total number of node hops that must be accomplished for communication of each node in the wireless network with every other node. In an example embodiment, a node-hop tally is tabulated for each node. An exterior node, e.g., the exterior node  214  must accomplish at least two node hops to reach an interior node, e.g. the interior node  210 . And the exterior node  216  must accomplish one node hop to reach the interior node  210 . After all node hops are tallied for all nodes, a most central interior node may have a node-hop tally that is less than all other node-hop tallies. 
     In an embodiment where more than one node may have the same lowest node-hop tally, the metric then weights these nodes by use of a random number generation algorithm. Thereafter among these nodes, the node with the highest random number, for example, is elected to be the new beacon node. Other methods for breaking ties can be used as well. 
       FIG. 2   b  is a schematic representation of a plurality of nodes that make up a wireless network  201  after the passage of time of the wireless network  200  depicted in  FIG. 2   a  according to an example embodiment. Several nodes have joined the wireless network  200  depicted in  FIG. 2   a  to become the wireless network  201  depicted in  FIG. 2   b . Whereas the common node  214  depicted in  FIG. 2   a  was an exterior node, the common node  214  has become an interior node due to the accretion of several other nodes. The beacon node  210  as was depicted in  FIG. 2   a  is depicted in heavy dashed lines to indicate an election for a new beacon node. As depicted, the interior nodes  210 ,  212 , and  214  are all depicted in heavy dashed lines to indicate an election among the three as self-nominated nodes to become the new beacon node. By use of the node-hop tally method embodiment, each of these three interior nodes may be part of the selection process to become the beacon node. 
     According to the several embodiments set forth in this disclosure, any one of the indicated nodes  210 ,  212 , and  214  may become the new beacon node. 
     In an embodiment, the frequency of reevaluation of the entire wireless network  210  may be inversely proportional to the size of the wireless network  201 . In other words, the probability of any common node becoming the new beacon node is adjusted downwards as the size of the size of the wireless network  201  increases. Consequently, where each node may use the random-number generation method for self-nomination, as the size of the wireless network  201  has increased, each node has a relatively higher threshold to overcome to become a self-nominated node. 
     In an embodiment, a self-nomination event is being carried out as depicted in  FIG. 2   b . A node-hop tally method has resulted in three nodes  210 ,  212 , and  214  being tied or otherwise indeterminate as the three most useful nodes that could become the new beacon node. 
       FIG. 2   c  is a schematic representation of a plurality of nodes that make up a wireless network  202  after the passage of time of the wireless network  201  depicted in  FIG. 2   c  according to an example embodiment. The node  212  has been elected the new beacon node  212 . 
       FIG. 3  is a diagram of a method flow  300  for electing a beacon node according to an example embodiment. 
     At  310 , the method originates such as when a wireless node has awakened from a sleep-wake cycle. After the origination of the method embodiment, the method proceeds to  320 . 
     At  320 , the method includes executing a query whether it is appropriate to nominate itself as a beacon node. A beacon node election time may be appropriate where the beacon node has failed for instance. Beacon node failure may include removal of the mobile platform that was the beacon node or a reduction or blockage in quality of the beacon signal as discussed above. 
     A beacon node nomination time may be initiated for reasons other than beacon node failure. For example, a metric to trigger a beacon node nomination time may be that the beacon node has a high volume of traffic and that the excessive traffic could cause latency and delay in the beacon node being able to efficiently execute the beaconing process to all other nodes. Other burdens on a beacon node may be a full or near capacity routing table (RT), or a full or near capacity neighbor table (NT). Consequently, comparison between a current beacon node and a neighbor node may reveal the neighbor nodes may have a lesser traffic load and that the lower traffic would make the node better able to operate usefully as the beacon node. 
     Where there is no useful reason for a beacon node nomination to be carried out, the method may continue by returning to START at  310  including the beacon node executing various functions including the beaconing process. 
     At  330  where the query results in “true” that a beacon node should be nominated, the method includes determining if each node should self-nominate. In one embodiment, as noted above, a random number generator is used on each node in the wireless network to determine whether the node should self-nominate. In one embodiment, if the random number generated on a particular node is below a certain threshold number, the node self-nominates and, at the beginning of the next cycle, sends out a beacon signal. For example, if the threshold is set at 0.6 and the random number generator generates a number between 0 and 1, the node will send out a beacon signal in those conditions approximately 60% of the time. If the node determines it should self-nominate, control moves to  340  and a beacon signal is generated at the appropriate time. Control then moves to  350 . 
     At  350 , each self-nominated node looks to see if it received a beacon signal from another node. If not, control moves to  370  and the probability of nominating threshold is set higher (increasing the likelihood that the node will nominate itself in the future). Then control moves to  320  and the node remains the beacon node. 
     If, however, at  350  a self-nominated node received a beacon signal from another one other node, control moves to  360 . 
     Finally, at  350  a self-nominated node received a beacon signal from two or more other nodes, control moves to  380  and the probability of nominating threshold is set lower (decreasing the likelihood that the node will nominate itself in the future). 
     At  360 , each node looks at any beacon signals it has received from other nodes and determines if those nodes have a higher priority for becoming the beacon node. If any of the nodes have a higher priority, control moves to  320  and the node stops sending out beacon signals. If, however, at  350  the node determines that none of the other nodes from which it received a beacon signal have a higher priority, control moves to  340  and a new beacon signal is generated at the appropriate time. The process is repeated until a single beacon node remains. 
     As noted above, a node may have a higher priority for becoming a beacon node doe to its place in the network, due to its MAC address or link quality, or for whatever other reason selected by the network designer. 
     The network executing the method above is synchronized by an emergent behavior of the mob of nodes, rather than by that of a selected or predetermined node, or a centralized time authority, or a time reference external to the network. 
     This beaconing process occurs as well when new networks are established. In one embodiment, a new sleeper node awakens without being preconfigured for a network. A newly powered up node assumes default values for wake and sleep intervals, then stays awake for a whole wake and sleep interval in order to listen for a sync message broadcast. If the node does not detect any beacon signal during that first wake/sleep cycle, it becomes a “loner node” and sends its own beacon signal. A loner node has a high probability of sending out a beacon signal. 
     If a reply is received as a result of the loner node sending the beacon signal, the loner node makes a decision. The loner node may initiate a command for the node that sent the response to join the loner node in order to form a network. The loner node may also initiate a request to join the node that sent the response. Consequently, a loner node will either join an existing subnet, or it will join to another loner node to form a new network. In an embodiment where the loner node may join a second loner node, the new subnet of two previous loner nodes will attempt to seek out and join to another subnet. In an embodiment where the loner node may join a second loner node, the new subnet of two previous loner nodes then may attempt to aggregate itself with other subnets to make larger networks. 
       FIG. 4  is a representation of a route-discovery table (RDT)  400  that may be used to lodge link quality data etc. for modifying a pure random-number election of the new beacon node according to an example embodiment. The RDT  400  includes a plurality of entries  450 . In an embodiment, each entry  450  includes information detailing a route discovery in process between the given node and at least the neighbor nodes to the given node. In one such embodiment, each entry  450  includes the following information: 
     Succ—address of successor node, neighbor of this node that should be used to relay a message beyond the given node. 
     TalHops—tally of the minimum total number of relays between this node and all other nodes. 
     In one embodiment, a network layer (NWK) may include a mesh network such as XMesh. A medium-access control (MAC) layer may include a standard such as IEEE 802.15.4, established and periodically updated by the WPAN™ Task Group 4 known in industry. The physical layer (PHY) may be frequent-hopping spread-spectrum (FHSS) radio communication. In one such embodiment, the NWK and MAC layers are used to estimate link quality for a given node as a function of the quality of its component links. The quality of a link between two nodes may be determined as a function of factors such as link count and the signal strength in each link. 
       FIG. 5  is a schematic diagram illustrating a medium having an instruction set that results in election of a new beacon node according to an example embodiment. A machine-readable medium  600  includes any type of storage medium such as a disk drive or a solid state memory device, or the like. A machine-readable medium  600  includes instructions within an instruction set  650 . The instruction set  650 , when executed by a machine such as an information handling system or a processor, cause the machine to perform operations that include electing a new beacon node. 
     In an example embodiment of a machine-readable medium  600  that includes the instruction set  950 , the instructions, when executed by a machine, cause the machine to perform operations such as modifying a random number node election based upon a link-quality metric or a node-burden metric. 
     Thus, methods and a machine-readable medium including instructions for electing a new beacon node have been described. Although the various methods for electing a new beacon node have been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader embodiment of the disclosed subject matter. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that achieve the same purpose, structure, or function may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the example embodiments of the invention described herein. It is intended that this invention be limited only by the claims, and the full scope of equivalents thereof.