Abstract:
A low power consumption protocol for low power communication devices attached to an asynchronous network is described. In this protocol, a communication device is used as a high communication duty cycle Mediation Device (MD), thus permitting other communication devices to use a low communication duty cycle framing structure. The MD functions as a storage and retrieval service for messages between two devices when one device is not able to communicate. When the previously unavailable device becomes available, it can check in with the MD to retrieve any missed messages and respond to these messages accordingly. In a communication network, each of the low power communication devices can be configured to behave as MD&#39;s for a small amount of time. Sharing this responsibility among all communication devices in the network allows each device to maintain an low average communication duty cycle. This technique is applicable to a low power, low cost, zero-configuring, self-organizing, asynchronous network.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is related to the applications entitled “System for Spread Spectrum Communication” (Docket No. CM003351J), “A Protocol for a Self-Organizing Network Using a Logical Spanning Tree Backbone” (Docket No. CM03403J), and “System for Code Division Multi-Access Communication” (Docket No. CM03333J) all filed on the same date as the present invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to communication networks and more specifically to the use of multiple access protocols in asynchronous communication networks.  
         BACKGROUND OF THE INVENTION  
         [0003]    In communication networks, asynchronous transmission of information is often the technique used when communicating information between one or more communication devices within a communication network. Asynchronous transmission is often used when low power devices make up the network. These low power devices can use a low communication duty cycle frame structure in order to minimize the amount of power used while not actively communicating with other network devices, but the use of a low communication duty cycle frame structure often implies that device availability is reduced. In wireless communication networks, a fundamental challenge is maintaining high availability communications while using low power wireless communication devices.  
           [0004]    The neuRFon™ device by Motorola is an example of a low power, low cost, small size, and simple wireless device. The neuRFon™ device network is a zero-configuring, self-organizing, asynchronous network containing multiple neuRFon ™ devices. For this network, power consumption and cost are two major concerns.  
           [0005]    To lower the power consumption, the average communication duty cycle of all the devices in the described network has to be decreased to a minimum. The average communication duty cycle refers to the fraction of time that the wireless device is able to send and receive messages. For the described asynchronous network, the average communication duty cycle may be set so low that the infrequent communications between a transmitter and a destined receiver become a problem. For example, device A may attempt to contact device B, but device B may be not be able to receive messages due to its low average communication duty cycle. This will prevent device A from establishing contact.  
           [0006]    A representative low average communication duty cycle frame structure, in which the above problem is illustrated, is shown in FIG. 1. Using this frame structure, a low average communication duty cycle device uses 1 ms to warm up, 1 ms to transmit and receive messages from other devices in its group, and is asleep for the remaining 998 ms of the 1 second cycle. This gives a communication duty cycle of about 0.1%, which is very power efficient.  
           [0007]    The problem with this approach is illustrated in FIG. 2. FIG. 2 shows a small network comprising several low power, low average communication duty cycle devices, where each device is represented as a small dot. Referring again to FIG. 2, device A tries to talk to device B. If the low average communication duty cycle frame structure in FIG. 1 is assumed, both A and B devices are able to communicate only 0.1% of the time. If we further make the reasonable assumption that device A and device B don&#39;t each have access to the other&#39;s time schedule, the probability of device A to establish communication with device B is approximately 0.1%, which is too small for most applications.  
           [0008]    To reduce the cost, low cost crystal and/or Micro Electro-Mechanical Systems (MEMS) may be utilized as the frequency generator for the wireless device. The issue with these technologies is their inherent poor frequency stability that further makes synchronization difficult. Removing the requirement for highly accurate frequency synchronization would allow the cost of such devices to be kept low. Such a protocol would enable asynchronous networks employing these low power, low communication duty cycle devices to be built at low cost and low power consumption.  
           [0009]    An invention that introduces a new protocol enabling low power, low communication duty cycle devices to communicate with each other, while at the same time not requiring highly accurate frequency synchronization would be able to address the issues described above.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The novel features believed characteristic of the invention are set forth in the claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 is a low communication duty cycle frame structure, according to the prior art.  
         [0012]    [0012]FIG. 2 is a low power communication device network, according to the prior art.  
         [0013]    [0013]FIG. 3 is a block diagram illustrating the internal functionality of a low power device, according to the present invention.  
         [0014]    [0014]FIG. 4 is a network containing multiple low power devices and a single dedicated Mediation Device (MD), according to a first embodiment of the present invention.  
         [0015]    [0015]FIG. 5 illustrates the internal functionality of a dedicated MD, according to the first embodiment of the present invention.  
         [0016]    [0016]FIG. 6 is a low communication duty cycle frame structure for a low power device, according to the present invention.  
         [0017]    [0017]FIG. 7 is a high communication duty cycle frame structure for a dedicated MD, according to the first embodiment of the present invention.  
         [0018]    [0018]FIG. 8 is a flowchart for dedicated MD operation, according to the first embodiment of the present invention.  
         [0019]    [0019]FIG. 9 is a timing diagram for communication between two low power devices and a dedicated MD when a query occurs later than the communication request, according to the first embodiment of the present invention.  
         [0020]    [0020]FIG. 10 is a flowchart for communication between two low power devices and a dedicated MD when the query occurs later than the communication request, according to the first embodiment of the present invention.  
         [0021]    [0021]FIG. 11 is a timing diagram for communication between two low power devices and a dedicated MD when the query occurs earlier than the communication request, according to the first embodiment of the present invention.  
         [0022]    [0022]FIG. 12 is a flowchart for communication between two low power devices and a dedicated MD when the query occurs earlier than the communication request, according to the first embodiment of the present invention.  
         [0023]    [0023]FIG. 13 is a network containing multiple low power devices and an additional low power device functioning as a MD, according to a second embodiment of the present invention.  
         [0024]    [0024]FIG. 14 is the internal functionality of a low power device operable as a MD, according to the second embodiment of the present invention.  
         [0025]    [0025]FIG. 15 illustrates a low communication duty cycle frame structure for a low power device functioning as a MD, according to the second embodiment of the present invention.  
         [0026]    [0026]FIG. 16 is a timing diagram for communication between two low power devices and a third low power device acting as MD when the query occurs later than the communication request, according to the second embodiment of the present invention.  
         [0027]    [0027]FIG. 17 is a flowchart of the communication between two low power devices and a third low power device acting as MD when the query occurs later than the communication request, according to the second embodiment of the present invention.  
         [0028]    [0028]FIG. 18 is a timing diagram for communication between two low power devices and a third low power device acting as MD when the query occurs earlier than the communication request, according to the second embodiment of the present invention.  
         [0029]    [0029]FIG. 19 is a flowchart for the communication between two low power devices and a third low power device acting as MD when the query occurs earlier than the communication request, according to the second embodiment of the present invention.  
         [0030]    [0030]FIG. 20 is a timing diagram illustrating a collision between two low power devices acting as MD when a collision avoidance strategy is not used, according to the second embodiment of the present invention.  
         [0031]    [0031]FIG. 21 is a timing diagram illustrating the collision avoidance strategy between two low power devices acting as MD, according to the second embodiment of the present invention.  
         [0032]    [0032]FIG. 22 is the collision avoidance strategy flowchart, according to the second embodiment of the present invention.  
         [0033]    [0033]FIG. 23 is a timing diagram for a multiple access scheme for a small network of low power devices, wherein each device is operable as a MD, according to a third embodiment of the present invention.  
         [0034]    [0034]FIG. 24 is a low communication duty cycle frame structure for a multiple access scheme for a small network of low power devices, wherein each device is operable as a MD, according to the third embodiment of the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0035]    While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawing.  
         [0036]    Therefore, according to the present invention, a multiple access protocol and structure for communication devices in an asynchronous network is described. This multiple access protocol is applied to the situation in which there are multiple low power communication devices in an asynchronous communication network. These low power communication devices may be neuRFon™ devices, or any similar device capable of low power, low communication duty cycle, and low cost information transmission. Referring now to FIG. 3, a system level block diagram  300  of the internal operation of a low power communication device is shown. Incoming messages  310  are received by a message receiver  330  of the communication device, and are prepared for processing by device processor  340 . Device processor  340  interacts with a controller  350 , timing  360 , and storage  370  in order to allow the communication device to receive and process incoming messages  310  from other communication devices in the network. The timing block  360  allows for synchronization between the communication device and other communication devices in the network, as well as providing a timing reference for internal device functionality. Device transmitter  380  receives messages from device processor  340 , prepares messages for transmission, and transmits outgoing messages  320  to other communication devices in the network. One will recognize that the functional blocks illustrated in the system level block diagram  300  of FIG. 3 may be modified or combined without departing from the spirit and scope of a low power communication device that sends messages, receives messages, and processes messages.  
         [0037]    In order to keep the power consumption at a minimum, and yet still achieve reliable communication, a mediation device (MD) is introduced. The MD acts as a mediator between communication devices within the network, and is capable of recording and playing back message related information. This is most useful when two communication devices are unable to establish contact. More than one MD may also be used in a large network, where each MD mediates for a group of low-power, low communication duty cycle communication devices. The MD has a relatively high communication duty cycle as compared to the low-power communication devices in the group and is thus able to store and forward messages between two or more low power communication devices in the asynchronous network. Note that the MD functionality may actually be a feature of the low power communication devices in the group. In this case, each low power communication device may be operable as a MD within the asynchronous network, with a low power communication device of the network being randomly chosen to temporarily operate as a MD of the group. This allows the overall network to remain a low power, low cost asynchronous network and each low power wireless device to remain a low communication duty cycle device, except when serving as a MD. Since a low power wireless device is a MD only occasionally, the average communication duty cycle of each low power wireless device can remain low. Distributing MD functionality across all the communication devices in the network also allows the communication devices to use energy mining in order to increase the average communication duty cycle of the communication devices in the network.  
         [0038]    During normal asynchronous network operation, each communication device except those serving as MD has a low communication duty cycle frame structure. The communication duty cycle of the MD may be adapted to the design parameters of the network, so that changing the parameters of the MD has an impact on the availability of each communication device within communication range of the MD.  
         [0039]    The present invention discloses a method and structure for a low power consumption protocol for low power wireless devices attached to an asynchronous network. Referring to FIG. 4, a simplified representation of a network  400  containing a plethora  440  of low power communication devices  410  and a dedicated MD  430  is shown, according to a first embodiment of the present invention. Each of the low power communication devices  410  of network  400  has a low communication duty cycle and must therefore rely upon dedicated MD  430  for reliable communication. Dedicated MD  430  has a high communication duty cycle in comparison to the non-MD low power devices  410 , and can store messages destined for any of low power wireless devices  410  within the group  440 .  
         [0040]    Referring now to FIG. 5, a system level block diagram  500  of the internal operation of a dedicated MD device  430  is shown. The dedicated MD device  430  is capable of sending and receiving several types of messages, including the source communication device id, the destination communication device id, the message, time of desired communication, message replay requests, control words, and device status messages. Incoming messages  510  are received by a MD message receiver  530  of the dedicated MD device  430 , and are prepared for processing by device processor  540 . MD Device processor  540  interacts with a controller  550 , timing  560 , and storage  575  in order to allow the MD  430  to receive, store, replay and process incoming messages  510  from other communication devices in the network. In addition to non-specific memory, the storage block  575  also includes message playback  580  and message record  570  memory, so that MD processor  540  can mediate requests from communication devices within the network to record messages, playback messages, and save communication device contact information. The timing block  560  allows for synchronization between the MD  430  and other communication devices in the network, as well as provides a timing reference for internal device functionality. Device transmitter  590  receives messages from device processor  540 , prepares messages for transmission, and transmits outgoing messages  520  to other communication devices in the network. One skilled in the art will recognize that the functional blocks illustrated in the system level block diagram  500  of FIG. 5 may be modified or combined without departing from the spirit and scope of a low power communication device that sends messages, receives messages, and processes messages.  
         [0041]    Referring now to FIG. 6, a representative low communication duty cycle frame structure  600  characteristic of the low power, low communication duty cycle devices of the network is shown. A single frame  670  of low communication duty cycle frame structure  600  contains a warm-up block  610 , a communication block  620 , and a sleep block  650 . Warm-up block  610  occurs first chronologically, and is a very small percentage of the overall frame duration. Warm-up block  610  is followed by communication block  620 . Communication block  620  is also a small percentage of the overall frame duration. After communication block  620  ends, sleep block  650  begins. Sleep block  650  is a relatively high percentage of the overall frame duration in order to preserve the low communication duty-cycle characteristic of the device. Exemplary values are 1 ms for warm-up block  610 , 1 ms for communication block  620 , and 998 ms for sleep block  650 , although these values can be changed significantly without departing from the invention. At the end of frame  670 , the three-block cycle  610 ,  620 ,  650  is repeated for the next frame. This block order is preserved for all frames in the low communication duty cycle frame structure  400 . In the preferred first embodiment, communication block  620  may be further subdivided into a transmit period and a receive period, although it will be clear to one of skill in the art that the communication devices in the network can have multiple transmit and receive periods within communication block  620 . Under normal operation of the first preferred embodiment, the receive period occurs first, followed by a transmit period of equal duration. Under certain situations, such as synchronization between communication devices, the transmit period may occur before the receive period.  
         [0042]    Referring now to FIG. 7, a representative high communication duty cycle frame structure  700  is shown. This high communication duty cycle frame  700  is representative of the dedicated MD communication duty cycle frame, according to the first embodiment. A single frame  725  of low communication duty cycle frame structure  700  contains a warm-up block  710  and a communication block  720 . Warm-up block  710  occurs first chronologically, and is a very small percentage of the overall frame duration. Warm-up block  710  is followed by communication block  720 . Communication block  720  is a relatively very large percentage of the overall frame duration. At the end of frame  725 , sleep cycle  750  begins, which can last for several frames. Exemplary values are 1 ms for warm-up block  710 , 2 seconds for communication block  720 , and 100 seconds for sleep cycle  750 . At the end of sleep cycle  750 , warm-up block  710  and communication block  720  are repeated for the next frame. This block order is preserved for all frames in the high communication duty cycle frame structure  700 . In the preferred first embodiment, communication block  720  may be further subdivided into a transmit period and a receive period, although it will be clear to one of ordinary skill in the art that the communication devices in the network can have multiple transmit and receive periods within communication block  720 . Under normal operation of the first preferred embodiment, the receive period occurs first, followed by the transmit period of equal duration. It should also be noted that the length of the transmit period of communication block  720  should be long enough for the high communication duty cycle device to receive two complete transmit cycles of a low duty cycle device. This will ensure that a dedicated MD will be able to receive a transmission from any communication device within communication range.  
         [0043]    Referring now to FIG. 8, a flow diagram  800  for applying the dedicated MD to communication between a communication device A, and a communication device B, according to the first preferred embodiment, is shown. In block  820 , device A attempts to communicate with device B. If B is reachable, then decision block  830  evaluates to No and device A establishes communications with device B as shown in block  860 . Device A and device B then return to normal operation, as shown in termination block  870 . If the answer in decision block  830  is Yes, then device B is not available, and device A communicates contact information with the dedicated MD as shown in block  840 . In block  850 , dedicated MD replays the contact information for device B when device B is able to communicate. Device B uses this contact information to synchronize communications with device A. Device A is now able to communicate with device B, as shown in block  860 . After device A communicates with device B, both devices return to normal operation, as shown in block  870 . Note that device A can fail to reach device B when device B is in sleep mode, in transmit mode, out of range, or other similar reasons.  
         [0044]    Referring now to FIG. 9, a more detailed timing diagram is shown for the case described in FIG. 8, according to the first embodiment. As in FIG. 8, device A  910  attempts to communicate with device B  920 , but is not successful, so dedicated MD  930  is used to mediate between device A  910  and device B  920  until device B  920  is able to communicate. Device A  910  is a low duty cycle device with a communication period containing a receive slot  965  and a transmit slot  970 . Device B  920  is also a low duty cycle device with a communication period containing receive slot  985  and transmit slot  990 . Dedicated MD  930  contains a communication period that is much longer than either communication period of device A  910  or device B  920 . The communication period of dedicated MD  930  similarly contains a receive period  975  and a transmit period  980 . Referring also to FIG. 10, the flowchart for the timing diagram of FIG. 9 is shown. The flow in block  1020  of FIG. 10, as well as the timing diagram of FIG. 9, assume that device A  910  has not been successful in communicating directly with device B  920 . So, there is a need for device A  910  to use the dedicated MD  930  to store a communication request with device B  920 . As shown in block  1030 , device A  910  sends a communication request  935  to dedicated MD  930  during the transmit slot  970  of the communication period of device A  910 . Communication request  935  is received by dedicated MD  930  during the receive slot  975  of the communication period of dedicated MD  930 . Also during the receive period  975 , device B  920  enters its communication period. Upon waking, device B  920  sends a query  940  using transmit slot  990  to dedicated MD  930  to check for messages. Note that both device A  910  and device B  920  are able to communicate with dedicated MD  930  during a single receive slot  975 . The duration of receive slot  975  and transmit slot  980  of dedicated MD  930  should be chosen large enough that dedicated MD  930  is able to receive both communication request and query messages. Since MD  930  received communication request  935  from device A  910  prior to the query  940  from device B  920 , during the transmission slot  980  of the communication period of dedicated MD  930  dedicated MD  930  first sends an acknowledgement  945  to device A  910  during receive slot  965  in the next communication period of device A  910 , as indicated in block  1040 . Dedicated MD  930  then sends a replay message  950  to device B  920  during the same transmit slot  980  of the communication period of dedicated MD  930 , as indicated in block  1050 . In the first preferred embodiment, the communication period of each communication device in the network occurs with a fixed period, so that dedicated MD  930  is able to sync with device B  920  without device B  920  sending synchronization information. However, other implementations of the second embodiment could require each communication device in the network to transmit explicit synchronization information to dedicated MD  930 , so that the query message would contain synchronization information. Device B  920  now has enough information to enable communications with device A  910  without the use of dedicated MD  930 . In order to synchronize with device A, device B uses timing information provided in the communication request  935  of device A  910 , and swaps communication slots so that the transmit slot  990  precedes the receive slot  985  of device B  920 . As shown in block  1060 , device B  920  then sends an acknowledgement  955  to device A during the receive slot  965  of the communication period of device A. As shown in block  1070 , device A  910  is now able to communicate a message  960  to device B  920  during the transmit slot  970  of device A  910  and the receive slot  985  of device B  920 . Once the communication between device A  910  and device B  920  is complete, device A  910  and device B  920  return to normal operation, as shown in block  1080 .  
         [0045]    The discussion above referring to FIG. 9 and FIG. 10, illustrated the sequence of communication operations and timing flow for the case in which the query from device B  920  is later than the communication request from device A  910 . It is also possible for the query of device B  920  to precede the communication request of device A  910 . This scenario is illustrated in the timing diagram of FIG. 11, and the flowchart of FIG. 12. The sequence of operations shown in FIG. 12 is identical to those in FIG. 9, except that device A  910  first attempts to communicate a communication request  1150  with dedicated MD  930  during a time that dedicated MD  930  is unavailable, as in block  1230 . Since the dedicated MD  930  is not available, device A  910  has to wait until a communication period in which dedicated MD  930  has an active receive period  975 . This delay means that device B  920  sends query  940  to the receive slot  975  of dedicated MD  930  prior to the communication request  935  of device A  910 , as in block  1240 . It is important to note that since both the transmit slot  975  and receive slot  980  of dedicated MD  930  are as long as two transmit periods of device A  910  or device B  920 , during the transmit slot of dedicated MD  930 , dedicated MD is able to replay message  950  to device B  920  and acknowledge  945  device A  910 . This allows the handshake between device A  910  and device B  920  to proceed as in block  1060 - 1080 . So, the ordering of the communication request  935  and the query  940  within a single receive slot  975  and the ordering of replay  950  and acknowledgement  945  within a single transmit slot  980  of dedicated MD  930  does not affect establishment of communications between device A  910  and device B  920 .  
         [0046]    Instead of using dedicated MD  930 , the functionality of a MD may be coupled to each of the plethora  440  of low power communication devices. Referring now to FIG. 13, a network  1300  containing a plethora of low power communication devices is shown, according to a second embodiment of the present invention. Each low power communication device of the low power communication devices can function as MD. In FIG. 13, low power communication device  1330  is functioning as a MD. Note that low power communication device  1330  is not a dedicated MD  930 . The low power communication device of the low power communication devices that will function as MD is selected at random.  
         [0047]    There are several approaches that may be used to select the next MD. The MD could be selected at random when the low power communications device acting as MD is not able to act as MD any longer. If each MD uses a randomly generated initial phase offset t 0 , then the distribution of MD functionality across the low power communication devices within the network should be uniform. This selection process will prevent collisions between two low power communications devices attempting to concurrently act as MD, but it requires coordination amongst the low power communication devices within the network. A second approach, and the one used in the second preferred embodiment, is to let each low power wireless device randomly determine when it will act as MD. In the case of two low power communication devices acting as MD, a collision avoidance strategy will be used to ensure only one MD is within communication range of a low power communication device.  
         [0048]    Referring now to FIG. 14, a system level block diagram  1400  of the internal operation of a low power communication device with MD functionality is shown according to the second embodiment of the invention. The low power communication device  1400  is capable of supporting specialized functionality for sending and receiving several types of MD messages, including the source communication device id, the destination communication device id, the message, time of desired communication, message replay requests, control words, and device status messages. This specialized functionality is in addition to the normal operational mode representative of the plethora of low power communication devices in the network. Incoming messages  1410  are received by message receiver  1415  of communication device  1400 , and are prepared for processing by message processor  1425 . Message processor  1425  contains further MD processing functionality  1430  that interacts with the MD functionality of a MD controller  1440 , MD timing  1450 , and MD memory  1460  in order to allow the communication device  1400  to receive, store, replay and process incoming messages  1405  from other communication devices in the network while acting as MD. Note that the communication device  1400  also contains the functionality illustrated in FIG. 5, but with additional MD functionality shown in dashed boxes. The MD memory  1460  of storage block  1455  also includes message playback  1470  and message record  1465  memory, so that MD processor  1430  can mediate requests from communication devices within the network to record messages, playback messages, and save communication device contact information. The MD timing block  1450  allows for synchronization between the communication device  1400  and other communication devices in the network, when communication device  1400  is acting as MD. Device transmitter  1420  receives messages from device processor  1425 , prepares messages for transmission, and transmits outgoing messages  1410  to other communication devices in the network. One skilled in the art will recognize that the functional blocks illustrated in the system level block diagram  1400  of FIG. 14 may be modified or combined without departing from the spirit and scope of a low power communication device that sends messages, receives messages, and processes messages. In particular, it should be noted that the MD functionality shown in FIG. 14 may be further combined or isolated from the non-MD operation of communication device  1400 , so long as the device  1400  is operable as a MD to the network.  
         [0049]    Referring now to FIG. 15, a representative low communication duty cycle frame structure  1500  of low power communication device  1400  functioning as MD is shown. Low communication duty cycle frame structure  1500  contains several cycles of operation in which each cycle contains a plethora of frames that are repeated multiple times. A first frame of the plethora of frames of low communication duty cycle frame structure  1500  contains a random delay t 0  block  1535 , a warm-up block  1505 , and a communication block  1510 . The random delay has duration t 0   1535 , where t 0  is between 0 and the duration of a single transmit or receive period. This delay randomizes the start time of the communication devices in the network, so that the probability of multiple devices to function as MD concurrently is reduced. Warm-up block  1505  occurs next chronologically, and is a very small percentage of the overall frame duration. Warm-up block  1505  is followed by communication block  1510 . Note that communication block  1510  and warm-up block  1505  directly follow the random delay t 0   1535 . However, it is also feasible for communication block  1510  and warm-up block  1505  to be delayed a random amount during the start of the next cycle of operation. In other words, the timing of communication block  1510  within the sequence of cycles is not constrained to be periodic. Communication block  1510  is a large percentage of the overall frame duration, and further comprises a receive slot  1525  and a transmission slot  1530 . In the second preferred embodiment, the one receive slot  1525  precedes the one transmit slot  1530 , although the order could be switched. Also, one of ordinary skill in the art will recognize that communication block  1510  could contain several transmit and receive slots in various arrangements. Also, although transmit slot  1530  occurs directly before or directly after receive slot  1525 , the hardware contained in the transmitter and receiver will require some time to switch between the transmit and receive modes. The duration of the switching time can be dependent on the switching speed of the hardware in the communication devices, or it could be determined by a user specified parameter in the communication device processor.  
         [0050]    After communication block  1510  ends, a second frame having a duration roughly one half of communication block  1510  begins. Second frame contains one sleep block  1515 . Sleep block  1515  is a very high percentage of the overall frame duration. At the end of the second frame the low power communication device  1400  resumes low communication duty cycle operation starting with a third frame of the plethora of frames. The low communication duty cycle frame structure  1500  is shown in FIG. 6. Low communication duty cycle operation occurs for several frames of the plethora of frames of low communication duty cycle frame structure  1500 . The duration of the first cycle of low communication duty cycle frame structure  1500  is a random number that is generated by the communication device  1400  at the start of the delay block  1535 .  
         [0051]    At the conclusion of the first cycle, the entire framing sequence just described is repeated until low power communication device  1400  stops functioning as MD. The decision to stop functioning as MD is made solely by the low power communication device  1400  in the second preferred embodiment, although it is also possible to coordinate the role of MD among several low power communication devices. Note that low power communication device  1400  contains the ability to generate and store random or pseudo-random numbers. These numbers could be generated by MD processor  1430 , and stored in MD memory  1460 .  
         [0052]    Referring now to FIG. 16, a detailed timing diagram for a device A  1610  attempting to send a communication request to a device B  1630  using a device C  1620  acting as MD, according to the second embodiment, is shown. As in FIG. 9, device A  1610  first attempts to communicate with device B  1630  prior to a query request  1660  of device B  1630 . The communication sequence shown in FIG. 16 for the second embodiment is identical to the communication sequence shown in FIG. 9 for the first embodiment. A difference between the FIG. 9 and FIG. 16 is that device C  1630  is a low power communication device acting as MD. Device A  1610  is a low duty cycle device with a communication period containing a receive slot  1665  and a transmit slot  1670 . Device B  1620  is also a low duty cycle device with a communication period containing receive slot  1685  and transmit slot  1690 . Device C  1630  contains a communication period that is much longer than either communication period of device A  1610  or device B  1620 . The communication period of device C  1630  similarly contains a receive period  1675  and a transmit period  1680 . Referring also to FIG. 17, the flowchart for the timing diagram of FIG. 16 is shown. The flow in block  1730  of FIG. 17, as well as the timing diagram of FIG. 16, assume that device A  1610  has not been successful in communicating directly with device B  1620 . So, there is a need for device A  1610  to use the device C  1630  to store a communication request with device B  1620 . As shown in block  1730 , device A  1610  sends a communication request  1635  to device C  1630  during the transmit slot  1670  of the communication period of device A  1610 . Communication request  1635  is received by device C  1630  during the receive slot  1675  of the communication period of device C  1630 . Also during the receive period  1675 , device B  1620  enters its communication period. Upon waking, device B  1620  sends a query  1640  using transmit slot  1690  to device C  1630  to check for messages. Note that both device A  1610  and device B  1620  are able to communicate with device C  1630  during a single receive slot  1675 . The duration of receive slot  1675  and transmit slot  1680  of device C  1630  should be chosen to be large enough that device C  1630  is able to receive both communication requests and query messages. Since MD  1630  received communication request  1635  from device A  1610  prior to the query  1640  from device B  1620 , during the transmission slot  1680  of the communication period of device C  1630 , device C  1630  first sends an acknowledgement  1645  to device A  1610  during receive slot  1665  in the next communication period of device A  1610 , as indicated in block  1750 . Device C  1630  then sends a replay message  1650  to device B during the same transmit slot  1680  of the communication period of device C  1630 , as indicated in block  1760 .  
         [0053]    Device B  1620  now has enough information to enable communications with device A  1610  without the use of device C  1630 . In order to synchronize with device A, device B uses timing information provided in the communication request  1635  of device A  1610 , and swaps communication slots so that the transmit slot  1690  precedes the receive slot  1685  of device B  1620 . As shown in block  1780 , device B  1620  then sends an acknowledgement  1655  to device A during the receive slot  1665  of the communication period of device A. As shown in block  1790 , device A  1610  is now able to communicate a message  1660  to device B  1620  during the transmit slot  1670  of device A  1610  and the receive slot  1685  of device B  1620 . Once the communication between device A  1610  and device B  1620  is complete, device A  1610  sleeps for a randomly generated duration ta  1661  and device B  1620  sleeps for a randomly generated duration tb  1662 . Device A  1610  and device B  1620  then return to normal operation, as shown in block  1795 .  
         [0054]    The discussion above referring to FIG. 16 and FIG. 17, illustrated the sequence of communication operations and timing flow for the case in which the query from device B  1620  is later than the communication request from device A  1610 , according to the second embodiment. It is also possible for the query of device B  1620  to precede the communication request of device A  1610 . This scenario is illustrated in the timing diagram of FIG. 18, and the flowchart of FIG. 19. The sequence of operations shown in FIG. 18 is similar to those in FIG. 11, except that device C  1630  is a low power communication device functioning as a MD, rather than the dedicated MD  930  shown in FIG. 11. Since the device C  1630  is not available, the first communication request  1850  of device A  1610  is not received as shown in block  1930 , and device A  1610  has to wait until a communication period in which device C  1630  has an active receive period  1675 , as shown in block  1940 . This delay means that device B  1620  sends query  1640  to the receive slot  1675  of device C  1630  prior to the communication request  1635  of device A  1610 , as in block  1950 . It is important to note that since both the transmit slot  1675  and receive slot  1680  of device C  1630  are as long as two transmit periods of device A  1610  or device B  1620 , during the transmit slot of device C  1630 , device C is able to replay message  1650  to device B  1620  and acknowledge  1645  device A  1610 . This allows the handshake between device A  1610  and device B  1620  to proceed as in block  1985 - 1990 . So, the ordering of the communication request  1635  and the query  1640  within a single receive slot  1675  and the ordering of replay  1650  and acknowledgement  1645  within a single transmit slot  1680  of device C  1630  does not affect establishment of communications between device A  1610  and device B  1620 . Once the communication between device A  1610  and device B  1620  is complete, device A  1610  sleeps for a randomly generated duration ta  1661  and device B  1620  sleeps for a randomly generated duration tb  1662 .  
         [0055]    For the second embodiment of the present invention, it is possible for two communication devices within the network to independently decide to function as MD&#39;s at the same time. If this occurs, then it is possible that the transmit periods of the two devices of the two devices overlap and a collision results. This situation is illustrated in FIG. 20. Device A  2010  first starts to function as MD. A short time later, a second device B starts to function as MD. When transmission slot  2040  of device A  2010  overlaps transmission slot  2060  of device B  2020 , the two transmitters interfere and communications capability is degraded. Note that the issue of collision is only a problem when the transmitters in the communication devices within the network occupy overlapping frequency bands and the communicated bit rate/bandwidth is high. If the transmitter data is modulated using a spreading waveform, then the communication devices could occupy the same frequency band and collisions could be less of an issue. According to the preferred second embodiment, each of the communication devices are single-channel devices using the same transmission frequency, so that the collision issue must be addressed.  
         [0056]    Referring now to the timing diagram of FIG. 21, and the flowchart of FIG. 22, a collision avoidance strategy is described according to the second embodiment of the present invention. Device A  2105  is randomly selected as MD prior to device B  2115 . At the start of the MD mode of device A  2105 , device A transmits announcement message  2120  (block  2210 ) in a very short duration transmission slot  2122 , and then switches to receive slot  2125  of communication period  2160  (block  2220 ). Device B  2115  is then randomly selected as MD, and device B  2115  sends out announcement message  2120  (block  2230 ) in a very short duration transmission slot  2140  prior to switching to receive slot  2145  of communication period  2160 . Device A receives the announcement message  2120  of device B  2115  (block  2240 ) during receive slot  2125 , and generates alarm message  2130  during it&#39;s next transmit period  2135  (block  2250 ). Device B  2115  receives this alarm message  2130  during receive period  2145 , and immediately stops acting as MD (block  2260 ). Device B  2115  then sleeps a random amount of time t 2  to prevent future collisions and resumes operation as a normal low power communication device (block  2270 ). Instead of sleeping for t 2  seconds, device B  2115  could wait until the end of the MD communication period of device A  2105 . At this time, device B  2115  functions as a MD, since device A  2105  is not in communication period  2160 .  
         [0057]    According to the second preferred embodiment, the random initial offset t 0   1535  is between 0 and 1 sleep period  1545 , the amount of delay ta  1661  and tb  1662  are between 0 and 1 sleep period  1515 , and the cycle time t 1   1560  follows the inequality, 0.5T&lt;t 1 &lt;1.5T, where T is the average frequency of the communication period of a communication device acting as MD.  
         [0058]    The second embodiment of the present invention presents a protocol and structure for a network of low power communication devices to improve the reliability of inter-device communication through the use of a Mediation Device (MD). One issue with this approach is the reliability when the number of communication devices in the network is small. As an example, if the network contains only five communication devices, and the MD communication cycle of each is 1000 seconds (t 1 ), then the average latency will be roughly 200 seconds. One way to reduce the latency is to constrain all nodes to communicate within a specified time window.  
         [0059]    Referring to FIG. 23, a multiple access scheme in which N communication devices  2310  are in the network, according to a third embodiment of the present invention. Each communication device of the N communication devices is constrained to receive and transmit information within a window  2320  of duration Tw seconds. Duration Tw is selected from the size of the network as determined by the number of communication devices in the network.  
         [0060]    For example, for a network with five communication devices operating within a t 1  period of 1000 seconds, the average latency is 200 seconds. If the communication cycle t 1  is reduced to 300 seconds, the average latency is reduced by a factor of three. If the duration Tw of window  2320  is smaller than the communication period of the communication device, the duration of the communication period must be reduced to fit within window  2320 . Reducing the duration of the communication period increases the likelihood of communication devices moving out of communication range without the knowledge of communication device acting as MD. Also, new communication devices entering the network will not be recognized unless their communication period lies within the window  2320 .  
         [0061]    One solution to this is illustrated in FIG. 24, according to the third embodiment of the present invention. The timing diagram  2400  of FIG. 24 shows a communication device which has a shortened communication period so that the communication device is able to communicate within window  2420 . In order to check for new communication devices, or recognize the loss of existing communication devices, a long communication period  2410  is occasionally used when the communication device is acting as MD. This long communication period  2410  is followed by the low power communication duty cycle  2430  structure of FIG. 6, and additional short MD communication periods  2420 . With the exception of the long communication periods  2410 , the communication device has essentially the same low communication duty cycle frame structure illustrated in FIG. 15 of the second embodiment.  
         [0062]    As an example of the third embodiment, a long communication period  2410  of 2 seconds, a short communication period  2420  of 200 ms, and the normal low communication duty cycle mode with a communication period of 2 ms is used. The frequency of the long communication period  2410  is 1500 seconds, the frequency of the short communication period is 300 seconds, and the frequency of the normal communication period is 1 second. This leads to a duty cycle of about 0.4%.  
         [0063]    It is noted that the communication device described herein may be a NeuRFon® device or any suitable communication device having similar operating characteristics.  
         [0064]    While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, one of ordinary skill in the art will recognize that low power devices may by physically connected or wireless, without departing from the spirit and scope of the invention. In the first, second, and third preferred embodiments, single-channel communication devices are contained in each of the first, second and third preferred embodiments. Also, for each preferred embodiment, the transmit slot and receive slot of a communication device of the plurality of communication devices do not overlap. For all three embodiments, the duty cycle is defined as the fraction of time that the communication device is in either a transmit slot or a receive slot. Also for all three embodiments, the communication device is assumed to be a low communication duty cycle, low power, wireless device, but, again, this need not be the case.  
         [0065]    While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.