Abstract:
Methods and apparatus for forming and maintaining an ad hoc mesh network suitable for reliable voice and data communications in underground, industrial and other hazardous environments. This invention includes support for cooperative negotiation of dedicated transmission bandwidth for all fixed network nodes, determination of network time synchronization without a pre-determined time master, discovery and merging of adjacent ad hoc mesh networks, and routing techniques for voice, data and network status packets which react instantaneously to network topology changes. Furthermore this invention provides a reliable communication network for mobile nodes carried by personnel and sensor nodes that are fixed or mobile that supports voice, data and tracking/situation awareness. A current application for this technology is a coal mine communication system with an operations center to dispatch, monitor and control coal mine operation including communication and location of mine personnel, and environmental conditions in the mine.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a data protocol to implement a wireless voice and data communication system that for use in underground and hazardous areas for dispatch, remote supervision, and tracking of personnel, as well as, monitoring, asset control, and management of wireless sensors and equipment. More specifically the protocol enables the creation of a wireless ad hoc mesh network and facilitates the transport of communication packets for voice, data, text and network operations supporting normal and emergency operation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recent events have indicated the need for reliable communication systems during emergencies in underground and hazardous work areas such as coal mines. During a mine disaster the current voice and data communication systems fail or shutdown so the conditions of personnel, environment and equipment is unknown which complicate recovery efforts. Past mining accidents have demonstrated that current communication systems are not sufficient to provide the support required to effectively handle evacuation and rescue operations. The 2006 MINER Act amends the Federal Mine safety and Health Act of 1977 stating underground coal mine operators must provide for post accident communication between underground and surface personnel via a wireless two-way medium within three years. Robust and reliable mine communications are critical for both mining operations and in the event of a mine emergency. The National Institute for Occupational Safety and Health (NIOSH) released a solicitation in late 2006 for an underground communication system that is highly reliable and provides in-mine and mine-to-surface voice and data communications to evaluate wireless mesh network technology as part of an underground communications system. 
         [0003]    Prior art mesh communications systems for use in hazardous environments have used the standard Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless protocol to transfer status and multimedia information as well as centrally generated and controlled routing tables to control packet latency in a multi-cast application with a changing network topography. The IEEE802.11 wireless protocol supports an infrastructure mode in which mobile nodes are connected to a coordinating master referred to as the Access Point (AP) and an ad hoc mode in which mobile nodes form peer-to-peer connections with no preconfigured network topology. The original 802.11 protocol had very limited Quality of Service (QoS) mechanisms. A Point Coordination Function (PCF) was specified in which each AP sends a poll to each client one at a time to provide a contention-free transmission opportunity. This media access control (MAC) method has not been implemented in many APs and is bandwidth inefficient since it always requires a poll packet be sent to the client for each high priority packet to be sent to the AP. The 802.11e amendment was incorporated into the 802.11 standard in 2007. It specifies two additional mandatory QoS MAC methods. The first method is the Enhanced Distributed Channel Access (EDCA) method which specifies traffic priority levels and assigned transmission opportunities for each priority level. This method does protect against collisions (and subsequent loss) of high-priority packets due to low-priority packets, however it does not protect against collisions of packets with the same priority from multiple clients. The second QoS method introduced by 802.11e is an infrastructure-only method referred to as Hybrid Coordinator Function Controlled Channel Access (HCCA). This QoS method is similar to the PCF method with the exceptions of the timing of the contention-free transmission opportunities is not limited to certain inter-beacon time slots and the AP is able to poll a client for a particular Traffic Class (TC) to enable session-aware QoS. This HCCA method suffers from the same bandwidth inefficiency as PCF. In addition to the lack of an efficient method to guarantee transmission opportunities for high priority packets between a client and an AP (infrastructure mode) or between two clients (ad hoc mode), the 802.11 solutions do not provide the additional necessary capability to forward said packet to the next destination in the network in an efficient and contention-free manner. Therefore, what is needed is a method and apparatus for establishing dedicated transmission opportunities for each fixed node in a given cluster to forward packets to the next node in the cluster as well as contention-based transmission opportunities for fixed nodes and mobile nodes to join the cluster. 
         [0004]    Furthermore, the efficient routing of packets across a mesh network is a vital component in determining the packet latency through the network. Prior art mesh network communication systems for hazardous environments have utilized routing tables generated from a centralized location and periodically propagated across the network using high priority MAC message mechanisms, resulting in a delay in routes being modified as conditions change. There is also a need in the art for an intelligent de-centralized flood routing technique to ensure mobile units can seamlessly roam throughout the network. 
       SUMMARY OF INVENTION 
       [0005]    This invention relates to a computer-implemented method for providing a protocol enabling reliable communications between elements of an ad hoc mesh network which include at least fixed mesh nodes, mobile mesh radio nodes and gateway nodes. One of the fixed mesh nodes is designated a time master node, and an intelligent de-centralized flood routing technique is used to ensure that units containing mobile mesh radio nodes can seamlessly roam throughout the network. Each fixed mesh node transmits over a pair of bonded, time-synchronized channels, one of which is a voice channel while the other is a data channel. 
         [0006]    The protocol includes at least one super-frame time interval. If there is more than one super-frame time interval, they are all of equal duration and synchronized with the time master node. There are at least two time slots within the primary level of each super-frame, one of which is assigned to be a first level contention access period time slot and the second of which is assigned to be a period during which fixed mesh nodes may transmit network packets to neighboring fixed mesh nodes and to any mobile mesh radio nodes with which they are associated. The protocol has multiple levels of time slots, each of which is assigned to a specified type of activity to account for the various functions to be provided by the protocol. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]    The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the invention with reference to the drawings, in which 
           [0008]      FIG. 1  illustrates the concept of a wireless voice and data ad hoc mesh network deployment in a logical design block diagram view according to an embodiment of the present invention. 
           [0009]      FIG. 2  illustrates an example of a cluster of Fixed Mesh Nodes, depicting the parent-child relationships as well as the transceiver operation during a typical protocol super-frame. 
           [0010]      FIG. 3  illustrates the partitioning of a protocol super-frame into first level time slots as defined by this invention. 
           [0011]      FIG. 4  illustrates the partitioning of a first level FMN-to-FMN time slot into second level time slots as defined by this invention. 
           [0012]      FIG. 5  illustrates the partitioning of a first level Data Channel Contention Access Period (CAP) time slot into second and third level time slots as defined by this invention. 
           [0013]      FIG. 6  illustrates the partitioning of a first level Voice Channel Contention Access Period (CAP) time slot into second and third level time slots as defined by this invention. 
           [0014]      FIG. 7  illustrates the partitioning of a generic first level frame into second and third level frames as defined by this invention. 
           [0015]      FIG. 8  illustrates the partitioning of a third level Frame Control Field frame into fourth level frames as defined by this invention. 
           [0016]      FIG. 9  illustrates the partitioning of a third level Extended Header frame into fourth level frames as defined by this invention. 
           [0017]      FIG. 10  illustrates the partitioning of a generic fourth level Extended Header frame into fifth level frames as defined by this invention. 
           [0018]      FIG. 11  illustrates the partitioning of a fourth level Slot Info Extended Header frame into fifth level and sixth level frames as defined by this invention. 
           [0019]      FIG. 12  illustrates the partitioning of a sixth level Cycle Slot #/Foster Rotation frame of a fourth level Slot Info Extended Header frame into seventh level frames as defined by this invention. 
           [0020]      FIG. 13  illustrates the partitioning of a fourth level Piggyback Acknowledge (ACK) Extended Header frame into fifth level and sixth level frames as defined by this invention. 
           [0021]      FIG. 14  illustrates the partitioning of a fourth level Piggyback Grant Extended Header frame into fifth level and sixth level frames as defined by this invention. 
           [0022]      FIG. 15  illustrates the partitioning of a fourth level Future Grant Extended Header frame into fifth and sixth level frames as defined by this invention. 
           [0023]      FIG. 16  illustrates the partitioning of a first level Single Data Frame Packet into second level, third level and fourth level frames as defined by this invention. 
           [0024]      FIG. 17  illustrates the partitioning of a first level Concatenated Data Frame Packet into second level, third level and fourth level frames as defined by this invention. 
           [0025]      FIG. 18  illustrates the partitioning of a first level SYNC MAC frame into second level, third level and fourth level frames as defined by this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    This invention helps to provide reliable voice and data communications with personnel and sensors within a hazardous area and also with a remote operation center during normal operations as well as during an event that requires shut down of normal operations. The invention allows rescue teams to determine personnel status, where personnel are located, and environment conditions such as water, toxic gases, and oxygen availability in the hazardous area. The wireless voice/data network protocol of the present invention comprises a number of intercommunicating blocks which automatically form a Wireless Mesh Network (WMN)  100  that can reform when links are removed or blocked. 
         [0027]    The system and method of the present invention provide a voice/data network protocol to supply Operations Center staff with event detection information, voice and text communications, and personnel location information, as illustrated in  FIG. 1 . In a preferred embodiment, four types of nodes are provided in the network: 
         [0028]    Fixed Mesh Node FMN  102  is a stationary dual-transceiver mesh radio unit operates on the WMN  100 . Multiple units operate together to form the semi-static infrastructure for the WMN  100 . Each FMN  102  has the capability to coordinate individual clusters within the wireless mesh network WMN  100  and route data through the network between mobile nodes and to a Gateway Node (GWN)  104 . An FMN  102  can also communicate through a wired backbone headend  106  with a wired backbone  107 , such as a leaky feeder system, as well as form the core links for WMN  100 . 
         [0029]    GWN  104  supports the transfer of information between WMN  100  and the infrastructure for a wide area network (WAN). GWN  104  is a highly modular design that is normally implemented as a fixed device but could be a mobile device. GWN  104  includes all of the functions of a full function FMN  102 . 
         [0030]    Mobile Mesh Radio (MMR)  101  is a portable device carried by personnel that allows them to have voice and data communication with a Network Operations Center  105  and/or other personnel equipped with an MMR  101 . MMR  101  can also be a relay link between another MMR  101  and an FMN  102 , or between a sensor mesh node (SMN)  103  and an FMN  102 . 
         [0031]    SMN  103  connects data from various types of sensors such as light, noxious gas, acoustic, temperature, oxygen, and imaging sensors into WMN  100  by communicating through one or more FMNs  102  or one or more MMRs  101  to forward information from a sensor to network operations center (NOC)  105 . 
         [0032]    Other units may also be added to WMN  100 . For example, a Data Mesh Node may be used to communicate with equipment operating in a mine or hazardous area. Thus, use of this invention is not limited to the types of nodes specifically mentioned. 
       1. DEFINITIONS 
       [0033]    The following terms are used throughout this description with the following meanings: 
         [0034]    Node—This term is used to denote a Fixed Mesh Node (FMN)  102 , Gateway Node (GWN)  104 , Mobile Mesh Radio (MMR)  101  or Sensor Mesh Node (SMN)  103 . 
         [0035]    Time Master—This term refers to the node which is providing the protocol super-frame timing to all the other nodes in a cluster. This node may have direct children nodes and if it does it may have inherited children through these children. Its direct children of the Time Master will directly synchronize to the time master&#39;s slot timing while the inherited children will synchronize to their respective parents slot timing thus inheriting the time master&#39;s timing with some offset due to propagation delay. For example, in  FIG. 2 , Node  1  is the Time Master; Node  2  is the only direct child of Node  1  while Nodes  3  and  4  are the inherited children of Node  1  through Node  2 . 
         [0036]    Cluster—A cluster is a group of nodes which all have the same time master. Nodes in a cluster do not necessarily (and often will not) have direct RF connectivity to all the other nodes in the cluster. Information packets between clusters pass through a pair of gateways if a gateway exists in each cluster.  FIG. 2  illustrates a cluster of FMN&#39;s containing nodes  1 ,  2 ,  3  and  4 . 
         [0037]    Parent Node—A parent node is a node which is providing a time reference to a child node. In  FIG. 2 , a parent-child relationship is illustrated by the lines towards which indicator  205  points in which the arrowhead for each of the lines points towards that node&#39;s parent. Thus, node  1  is the parent of node  2  while node  2  is the parent of node  3  and node  4 . The numbering system for the nodes is described in further detail below. 
         [0038]    Child Node—A child node is a node which is basing its time reference on a parent node. The only node in a cluster which is not a child to some parent is the time master. 
         [0039]    Peer Node—A peer node is a node which has the same time reference as a given node but is not a child or parent of that node. In  FIG. 2 , a peer relationship is illustrated by a line without arrowheads, such as line  210 . 
         [0040]    Observing Node—A node which is examining the frequency spectrum for adjacent nodes, parent node, children nodes, peer nodes, and known nodes. 
         [0041]    Known Node—A node is said to be a known node to an observing node if its presence is known by the observing node but it is not in the same cluster as the observing node. 
         [0042]    Neighbor—A node is considered to be a neighbor of another node if it is the parent, a child or a peer to that node. Known nodes are not considered to be neighbors. 
         [0043]    Orphan Node—An orphan node is a node which is currently operating in a dedicated search mode. In this mode it will continually search for any node with a higher time master ranking than it has itself. 
         [0044]    Orphan Cluster—An orphan cluster is a cluster which does not contain a gateway. 
         [0045]    Wireless Mesh Network (WMN)  100 —Refers to the mesh network in its entirety, including FMNs  102 , GWNs  104 , MMRs  101  and SMNs  103   
       2. FUNCTIONAL OVERVIEW 
       [0046]    The mesh protocol provides a means to form and maintain the fixed network (FMNs  102  and GWNs  104 ) and provides access into the network for mobile nodes (MMRs  101  and SMNs  103 ). The fundamental purpose of this network is to transmit application layer packets between various points in the system with the highest reliability and lowest latency which can be achieved given a certain system cost. 
         [0047]    Since a GWN  104  operates as a mesh network member in a manner which is indistinguishable from an FMN  102  on the mesh radio side, the term FMN  102  will be used in the remainder of this description when referencing functionality common to both FMN  102  and GWN  104 . 
       3. FREQUENCY AND SLOT ASSIGNMENT 
       [0048]    An important aspect of this invention is the ability to form a mesh network which can provide dedicated bandwidth for each FMN  102  within a cluster. This dedicated bandwidth is essential in the efficient routing of voice communications which is very sensitive to any system latency. When an FMN  102  attempts to join an existing cluster of FMN&#39;s  102  it sends an Association Request to the FMN  102  which it has determined to be its best parent. The potential parent responds with an Association Response indicating whether it accepts or rejects the joining FMN  102  as a child. In the case of the former, the parent FMN  102  provides a channel and slot assignment on which the joining FMN  102  is to operate. In this way the bandwidth within a cluster can be managed such that, within a given RF collision domain, each cluster member has a unique RF channel/time slot combination for it to transmit its FMN-to-FMN messages in time slots  310  within super-frame  305 , shown in  FIG. 3 . 
       4. MESH OPERATION 
       [0049]      FIG. 2  illustrates the operation of a small mesh consisting of 4 nodes. For purposes of this description each node is referenced by its ID, i.e. Node  1  is the node with an ID of 1. Each node has a frequency pair associated with it, one for the primary channel and one for the secondary channel. This frequency pair is “bonded” since there is a fixed relationship between them and if one is known, the other can be easily derived. For example, Node  3  in  FIG. 2  operates with a primary frequency of f 7  and a secondary frequency of f 7 ′. Each node learns the frequency pair associated with each of its neighbors during cluster formation. 
         [0050]    In addition each node has a first level FMN-to-FMN time slot  310  associated with it. For example Node  4  in  FIG. 2  has time slot  0  associated with it. The time slot for each member of a cluster is assigned during cluster formation. 
         [0051]    As illustrated in  FIG. 2 , during a typical super-frame, each node will tune its primary and secondary transceiver frequencies to its assigned frequencies during its assigned first level FMN-to-FMN time slot  310  and transmit any packets it has queued for transmission on that channel as described previously. In addition, during a typical super-frame, for each first level FMN-to-FMN time slot  310  which is assigned to a neighbor, a node will tune its transceivers to the frequencies associated with that neighbor and attempt to receive packets from that neighbor. Also during a typical super-frame, each node will tune its transceivers to its assigned frequencies during the first level contention access period (CAP) time slot  315 , shown in  FIG. 3  and explained further below, attempting to receive packets from transmitting mobile nodes or other FMNs  102  which are attempting to join the cluster. Finally, during any first level FMN-to-FMN time slot  310  which is not currently assigned to a neighbor, a node will use both transceivers to scan for other clusters which might provide an improved time metric. 
         [0052]    Exceptions to this typical super-frame operation are made in order to allow nodes to discover other nodes associated with other clusters when these other nodes are operating in time slots which overlap the time slots assigned to discovering node or the discovering node&#39;s neighbors. These exceptions are known as puncturing events. A node will periodically puncture its own first level FMN-to-FMN time slot  310 , as well as its neighbors&#39; assigned time slot(s) and the first level CAP time slot  315 . During a puncturing event, the node performing the puncturing will replace the typical operation described in the preceding paragraph with a scan operation. The puncturing schedule for the two transceivers for a given node is independent. 
       5. ROUTING 
       [0053]    In order to support multicast communications, seamless mobility for mobile nodes such as MMRs  101  and SMNs  103  and redundant communications links, the routing of voice packets through the mesh network must be dynamic. This invention accomplishes this using an intelligent flood routing technique. 
         [0054]    Mobile nodes transmit and receive with the FMN  102  which provides the best communications path at that given time; this FMN  102  is considered to be the mobile node&#39;s parent at that time. As a mobile node travels through the network its position relative to a given parent changes and therefore the RF characteristics change. When the quality of the link between the mobile node and its parent FMN  102  has degraded such that it is below a programmable threshold of acceptable operation, the mobile node seeks another parent which can provide a higher quality communication link. This results in the entry point of the voice and data packets into the network changing with time. 
         [0055]    In addition to mobility, the occasional removal of one or more FMNs  102  from the network is an operational reality. This can occur due to maintenance action, failure of a unit or loss of line of sight between units due to obstructions or cave-in. Each of these scenarios can cause a modification to the current route which packets are taking through the network. Since the intelligent flood routing algorithm used by the current invention does not attempt to establish any static or semi-static routes for communication packets from a given source to a given destination, it is able to immediately react to each of these scenarios with minimal or no loss of data. 
         [0056]    Flood routing is understood to those skilled in the art to be a technique by which a router forwards packets received on all interfaces except the one on which said packet was received. This is known to artisans as a technique which can be used to add multicast support to a mesh network. However, this technique can result in large amounts of redundant traffic since there are many possibilities for circular routing loops. The present invention eliminates circular loops by storing a history of data packet sequence numbers and voice packet timestamps. As each data packet is forwarded, its sequence number is recorded in memory along with a timestamp indicating the current system time. If another data packet is received with this same sequence number before a programmable timeout period, this packet is discarded. However if the sequence number of the data packet does not match any stored sequence number of recently forwarded packets, the packet is forwarded and the timestamp associated with that sequence number is replaced with the current time. 
         [0057]    A similar procedure is used for filtering duplicate voice packets, however timestamps are used instead of sequence numbers to eliminate the possibility of forwarding a voice packet associated with a given voice stream which is significantly older than a previously forwarded voice packets associated with the same voice stream. Each voice packet contains two timestamps, one which indicates the time at which the first voice packet of the associated voice stream was transmitted into the network and one which indicates the time at which this specific packet of the stream was transmitted into the network. As each voice packet is received by a FMN  102  it will only be forwarded if no other voice packet was recently forwarded with the same voice stream origination time and a packet origination time which is greater than or equal to the packet origination time in packet under consideration. In a preferred embodiment of this invention the definition of “recently” is controlled by a configurable timeout parameter. 
       6. SUPER-FRAME FORMAT 
       [0058]    All transmissions on the mesh network are strictly time synchronized to increase the likelihood of successful transmission of any given packet. Each FMN  102  utilizes a pair of bonded channels on which it will transmit.  FIG. 3  illustrates the super-frame timing used on both of these channels. The timing of the super-frames  305  on each channel is synchronized to prevent an FMN  102  from attempting to receive on one channel while it is transmitting on another channel. The timing of the super-frame  305  for all the nodes in a cluster is synchronized with the time master for that cluster providing the time reference. 
         [0059]    In one embodiment of this invention each super-frame  305  is 60 ms in duration and is subdivided into six 10 ms first level time slots. The first five first level 10 ms time slots  310  are dedicated as first level FMN-to-FMN time slots and the sixth 10 ms first level time slot  315  is dedicated as the first level Contention Access Period (CAP) time slot. The purpose of the first level FMN-to-FMN time slot  310  is to provide a guaranteed time period during which a FMN  102  can transmit network packets to its neighboring FMN(s)  102  and to any mobile units (MMRs  101  or SMNs  103 ) which may be using it as their current parent. 
         [0060]    The first level FMN-to-FMN time slot  310  is sub-divided into six second level time slots as illustrated in  FIG. 4 . Each of these second level time slots has an assigned operation which will be executed by the FMN which is assigned the specific time slot. The second level Pre-guard time slots  440  allows for clock drift between network components, the assigned FMN does not transmit or receive during this second level time slot. The duration of the second level Pre-guard time slots  440  is fixed for a given network based on the clock accuracy of the components within the network. The second level PA Ramp time slot  445  is a time slot set aside to allow the Power Amplifier (PA) component of the FMN to reach full output power. The assigned FMN turns on its transmitter circuitry at the beginning of this second level time slot and then delays for the duration of the second level time slot without transmitting. The duration of the second level PA Ramp time slot  445  is fixed for a given network. The third second level time slot is the second level Primary time slot  450 . The assigned FMN uses the second level Primary time slot  450  to transmit queued network packets to its listening neighbors. Typically, a single first level Concatenated Data Frame  1700 , as shown in  FIG. 17 , is used to transfer all the network packets. The duration of the second level Primary time slot is variable depending on the number of network frames in the assigned FMN&#39;s  102  outgoing queue. The second level Primary time slot  450  is followed by an second level Idle time slot  460  during which the FMN  102  can power down its transmitter to conserve power. The duration of the second level Idle time slot  460  is fixed for a given FMN  102 ; it may be adjusted during network deployment to provide a method to trade-off network formation/re-formation time with FMN power consumption. The second level Idle time slot  460  is followed by the second level Sync time slot  465  during which the assigned FMN  102  will transmit first level PHY frames  701  containing third level Sync frames  1800 . These Sync frames  1800  are used to aid scanning FMNs  102  or mobile nodes to find and synchronize with the network. The number of Sync frames  1800  to be sent in a given second level Sync time slot  465  may vary between subsequent super-frames depending on the durations of the second level Idle time slot  460  and the second level Primary time slot  450 . The second level Post-guard time slot  455  follows the second level Sync time slot  465 . The FMN  102  uses the second level Post-guard time slot  455  to turn off its transmitter and switch to receive mode for the next first level time slot  310  or  315 . 
         [0061]    The purpose of the first level CAP time slot  315  is to provide an opportunity for other FMNs  102  to transmit Child Association request frames to a potential parent FMN  102  and to provide an opportunity for mobile nodes to transmit frames containing network packets into WMN  100 . The exact format for first level CAP time slot  315  depends on whether that particular bonded channel is dedicated to voice or data. For a given FMN  102 , one of the bonded channels is designated as the data channel due to the first level CAP time slot  315  being formatted into second level CAP data time slots  505 , as illustrated in  FIG. 5 , and the other channel in the pair is designated as the voice channel due to the first level CAP time slot  315  being formatted into second level CAP voice time slots  605 , as illustrated in  FIG. 6 . 
         [0062]    As  FIG. 5  also illustrates, the second level CAP data time slots are further comprised of a third level TX On Guard time slot  510 , a third level data Physical Layer (PHY) frame time slot  515 , a third level data Idle time slot  520  and a third level data TX Off Guard time slot  525 . Third level data TX On Guard time slot  510  and third level data TX Off Guard time slot  525  instruct the radio transmitting in the CAP slot time to turn its transmit circuitry on and off, respectively. Third level data Idle time slot  520  is the period of time after the transmitting radio has transmitted the last bit of the variable length third level data frame time slot  515  and before the beginning of third level data TX Off Guard time slot  525 . 
         [0063]    Similarly  FIG. 6  illustrates, the second level CAP voice time slots  605  are further comprised of a third level voice TX On Guard time slot  610 , a third level voice Physical Layer (PHY) frame time slot  615 , a third level voice Idle time slot  620  and a third level voice TX Off Guard time slot  620 . Third level voice TX On Guard voice time slot  610  and third level voice TX Off Guard time slot  625  instruct the radio transmitting in the CAP slot time to turn its transmit circuitry on and off, respectively [0034]  FIG. 7  illustrates the general format of a first level PHY frame  701 . This frame format applies to both the third level Data Physical Layer (PHY) frame time slot  515  and the third level Voice Physical Layer (PHY) frame time slot  615 . A first level PHY frame  701  is further comprised of a second level PHY header frame  705  and a second level MAC frame  710 . The second level PHY header frame  705  further consists of a third level Preamble frame  715 , a third level Sync frame  720  and a third level Length frame  725 . 
         [0064]    The third level Preamble frame  715  is encoded as a set of alternating 1 and 0 bits. The purpose of this third level frame is to provide a means for receiving radios to reliably obtain bit synchronization on the incoming bit stream. 
         [0065]    The third level Sync frame  720  consists of a unique bit pattern which is preconfigured into all devices in WMN  100 . This third level frame is used by a receiving radio to obtain slot synchronization. 
         [0066]    The third level Length frame  725  indicates to the receiver the number of bytes to expect in the second level MAC frame  710 . 
         [0067]    The second level MAC frame  710  contains several third level frames which provide control and error correction functions. These third level frames include a third level Frame Control frame  730 , a third level Sequence Number frame  735 , a third level Destination Address frame  740 , a third level Source Address frame  745 , an third level Extended Header frame  750 , a third level MAC Payload frame  755  and a third level a cyclic redundancy check (CRC) frame  760 . 
         [0068]    The third level Frame Control frame  730  provides several fourth level frames which inform the receiving radio of the purpose and format of the remainder of second level MAC frame  710 . The format and purpose of these fourth level frames are discussed in a subsequent section. 
         [0069]    Third level Sequence Number frame  735  allows the acknowledgement of individual MAC packets and the filtering of duplicate packets. The value of this third level frame is set by the transmitter of the packet. Whenever a unicast acknowledgement is sent to indicate the status of the reception of this packet the value of the third level Sequence Number frame  735  in the Acknowledgement is set to this value. 
         [0070]    Third level destination address frame  740  specifies the network address of the intended recipient of this packet. The length of this third level frame is determined by fourth level Destination Mode frame  845 , shown in  FIG. 8 , of third level frame control frame  730 . In the preferred embodiment of this invention certain Destination Address values are reserved to indicate a packet is a broadcast packet and should therefore be processed by all receiving nodes. 
         [0071]    Third level Source Address frame  745  specifies the network address of the transmitting unit. The length of this third level frame is determined by fourth level Source Mode frame  855 , shown in  FIG. 8 , of third level Frame Control frame  730 . 
         [0072]    The purpose of third level Extended Header(s) frame  750  is to provide a mechanism by which the sender can dynamically expand the MAC header with additional network information when needed. The format of this third level frame is described in a subsequent section. 
         [0073]    The purpose and content of third level MAC Payload frame  755  is context dependent. The types of MAC packets which are supported on the WMN  100  are described in a subsequent section. 
         [0074]    Third level CRC frame  760  provides the receiver with a method for detecting bits errors which may have occurred in the demodulation of the data bits in other third level frames  730 ,  735 ,  740 ,  745 ,  750  and  755 . This third level frame is 16 bits in length and is calculated using the standard CCITT CRC16 parameters. 
       Frame Control Field 
       [0075]    The third level Frame Control frame  730  consists of several fourth level frames which inform the receiving radio of the purpose and format of the remainder of the second level MAC frame  710 .  FIG. 8  illustrates the format of the third level Frame Control frame  730 . The fourth level frames within the third level Frame Control frame are shown in  FIG. 8  and comprise the fourth level Frame Type frame  805 , the fourth level Security Enabled frame  810 , the fourth level Frame Pending frame  815 , the fourth level ACK Request frame  820 , the fourth level Intra-personal area network (Intra-PAN) frame  825 , the fourth level Extended Header indication flag frame  830 , the fourth level Scan frame  835 , the fourth level Retry frame  840 , the fourth level Destination Mode frame  845 , the fourth level Power Control frame  850  and the fourth level Source Mode frame  855 . 
         [0076]    The fourth level Frame Type frame  805  indicates the purpose of this second level MAC frame  701  and therefore indicates the format of the third level MAC Payload frame  755 . The three fourth level MAC frame types used on this system are Single Data Frames, Concatenated Data Frames and Sync frames. 
         [0077]    The fourth level Security Enabled frame  810  indicates whether or not MAC layer security is enabled. 
         [0078]    The fourth level Frame Pending frame  815  indicates whether the sending unit has additional frames to send to the same destination address. 
         [0079]    The fourth level ACK Request frame  820  indicates whether the sending unit is requesting a MAC layer acknowledgement of this frame. 
         [0080]    The fourth level Intra-PAN frame  825  is reserved for future intra-PAN communications. 
         [0081]    The fourth level Extended Header indication flag frame  830  is a flag which, when set to true, indicates whether or not one or more fourth level extended header frames follows the third level Source Address frame. 
         [0082]    The fourth level Scan Notify frame  835  indicates whether or not the transmitter will be performing a scan operation in the next time slot or will be performing a normal transmission in the time slot. 
         [0083]    The fourth level Retry frame  840  indicates whether or not the first level frame is a first transmission of the contained information or a retry of the transmission of the information. 
         [0084]    The fourth level Destination Mode frame  845  indicates number of bits in the third level Destination Address frame  740 . 
         [0085]    The fourth level Power Control frame  850  is used to indicate the power level at which the transmitter is transmitting the packet. 
         [0086]    The fourth level Source Mode frame  855  indicates the number of bits in the third level Source Address frame  745 . 
       Extended Headers 
       [0087]    The third level Extended Header frame  750  of the second level MAC frame  710  may consist of zero or more fourth level Extended Header frames  905  as illustrated in  FIG. 9 . The presence or absence of the first fourth level Extended Header frame  905  is indicated by fourth level Extended Header indication frame  830  of third level Frame Control frame  730 . The presence or absence of each subsequent fourth level Extended Header  905  is indicated by the fifth level Next Extended Header frame  1005  in the header of the preceding fourth level Extended Header  905  frame. There are four different types of Extended Header frames  905 , each of which has a different format. 
         [0088]    As illustrated in  FIG. 10 , each fourth level Extended Header frame  905  consists of four generic elements. These elements are a fifth level Next Extended Header frame  1005 , a fifth level Extended Header Length frame  1010 , a fifth level Extended Header Type frame  1015  and a fifth level Extended Header Data frame  1020 . 
         [0089]    The fifth level Next Extended Header frame  1005  indicates the presence or absence of an additional fourth level Extended Header frame  905  following this fourth level Extended Header frame  905 . 
         [0090]    The fifth level Extended Header Length field  1010  indicates the number of octets in the fifth level Extended Header Data frame  1020 . 
         [0091]    The fifth level Extended Header Type field  1015  indicates the purpose of this particular fourth level Extended Header frame  905  and the subsequent format of the fifth level Extended Header Data frame  1020 . There are five possible types of fourth level Extended Header frames which are Slot Info headers, Piggyback ACK headers, Piggyback Grant headers, Power Control headers and Future Grant headers. 
         [0092]    The format of the first type of fourth level Extended Header frame  905  incorporates a fifth level Slot Info Extended Header frame  1100 , as illustrated in  FIG. 11 . The contents of the fifth level Slot Info Extended Header Data frame  1100  consists of six sixth level frames: the ACK Map frame  1105 , the Channel frame  1110 , the Cycle Slot #/Foster Rotation frame  1115 , the Long Slot Number frame  1120 , the Time Master frame  1125  and the Time Hops frame  1130 . 
         [0093]    The sixth level ACK Map frame  1105  is a bit mapped frame used to acknowledge multi-cast packets which have been received by the FMN  102  transmitting the Slot Info Extended Header. 
         [0094]    The sixth level Channel frame  1110  indicates the logical channel on which this first level frame is being transmitted. This value, in combination with the slot and super-frame numbers can be used to synchronize to the transmitting unit hopping sequence if frequency hopping is being used. 
         [0095]    The sixth level Cycle Slot #/Foster Rotation frame  1115  consists of three seventh level frames as illustrated in  FIG. 12 . The first seventh level frame is the parent slot number frame  1205  which advertises whether the transmitting unit is a fostering MMR  101  or an FMN  102  and if it is the latter, this field indicates the slot number of the fostering MMRs  101  parent FMN  102 . The second seventh level frame is the Foster Rotation Number frame  1210  which advertises the future position of the voice and data CAP slots for a MMR which is being fostered. The final seventh level frame is the Slot Rotation Cycle frame  1215  which is used by a fostering MMR  101  to advertise which slot rotation schedule it is currently employing. 
         [0096]    The sixth level Long Slot Number frame  1120  of a Slot Info Extended Header indicates the current value of the 16 bit slot number. 
         [0097]    The sixth level Time Master frame  1125  of a Slot Info Extended Header indicates the ID of time master of the cluster to which the transmitting unit is a member. 
         [0098]    The sixth level Time Hops frame  1130  indicates the number of hops to the Time Master of the cluster. 
         [0099]      FIG. 13  illustrates the format of the second type of fourth level Extended Header frame  905 . This type of frame incorporates a fifth level Piggyback ACK Extended Header Data frame  1300  which is used by the transmitting unit to acknowledge a previously received unicast packet. A sixth level Sequence Number frame  1305  indicates the sequence number of the packet being acknowledged. A sixth level Address frame  1310  indicates the Source ID which was received in the packet being acknowledged. The sixth level receive signal strength indication (RSSI) frame  1315  indicates the receive signal strength at which the packet being acknowledged was received. 
         [0100]      FIG. 14  illustrates the format of the third type of fourth level Extended Header frame  905 . This type of frame incorporates a fifth level Piggyback Grant Extended Header frame  1400  which is used by the transmitting unit to indicate which nodes are allowed to transmit in the next first level CAP time slot  315 . This arrangement provides a method by which bandwidth can be reserved for a unit which has multiple packets to send. Each of the four sixth level Address frames  1405 ,  1410 ,  1415  and  1420  will specify either a unicast address if the corresponding second level upstream time slot is reserved for a unicast transmission from a particular unit or to the special broadcast address if the corresponding second level CAP time slot is to be used as a CAP slot. In alternative embodiments, there may be more than four sixth level Address frames. 
         [0101]    The fourth type of fourth level Extended Header frame  905  incorporates a fifth level Power Control Extended header frame which is not illustrated. This type of frame is used to communicate the current power setting of the transmitter. This is necessary since the power control algorithm may cause a unit to transmit at less than its nominal power setting and this information is needed to allow the receiving units to normalize the received signal strength readings associated with this unit. Since the receive power levels are used for position triangulation. 
         [0102]      FIG. 15  illustrates the format of the fifth type of fourth level Extended Header frame  905 . This type of frame incorporates a fifth level Future Grant Extended Header frame  1500  which is used to reserve second level CAP time slots  315  for the upstream two slots in the future. Its primary purpose is to prevent an FMN  102  from transmitting in the next second level CAP time slot and missing its guaranteed grant coming up in the second level time slot after that. Each of the four sixth level Address frames  1505 ,  1510 ,  1515  and  1520  will specify either a unicast address if the corresponding second level CAP time slot is reserved for a unicast transmission from a particular unit or to the special broadcast address if the corresponding second level CAP time slot is to be used as a CAP slot. In alternative embodiments, there may be more than four sixth level Address frames. 
       MAC Packet Types 
       [0103]    There are three possible formats for the third level MAC payload frame  755  used on WMN  100 . As mentioned above, the specific type of MAC packet is indicated in the fourth level Frame Type frame  805 . These three third level frame types are Single Data Frames, Concatenated Data Frames and Sync Frames. 
         [0104]      FIG. 16  illustrates the format of a first level Single Data frame  1600 . The purpose of a first level Single Data frame  1600  is to transfer one and only one network packet. For this type of frame the third level MAC Payload frame  755  consists entirely of a single fourth level network packet frame  1605 . The third level Length field  725  will indicate the length of the fourth level network packet frame  1605 . The other third level fields of the first level Single Data frame  1600  will be used as described in the preceding sections. 
         [0105]      FIG. 17  illustrates the format of a first level Concatenated Data frame  1700 . This frame type is used to transfer zero, one or more fourth level network packet frames  1605  between units. For this type of frame, the third level MAC Payload frame  755  consists of zero, one or more Length/Network Packet pairs. Each of these pairs will consist of a fourth level Length frame  1705  and a variable length fourth level Network Packet frame  1605  The fourth level Length frame  1705  indicates the number of octets in the corresponding fourth level Network Packet frame  1605 . The fourth level Network Packet frame  1605  contains the actual application layer information being transferred. The third level PHY Length frame  725  will indicate the number of octets in all of the fourth level Length/Network Packet pairs. The other third level frames of this type of first level packet frame are used as described in the preceding sections. 
         [0106]      FIG. 18  illustrates the format of first level Sync frame  1800 . This type of frame may be sent during a first level FMN-to-FMN time slot  310  to facilitate adjacent FMNs  102  and mobile nodes in finding the network. The third level MAC Payload frame  755  for this first level frame type  1800  consists of a fourth level Time Master frame  1805 , a fourth level Slot Number frame  1810  and a fourth level Channel Number frame  1815 . The value of the fourth level Time Master frame  1805  is set to the network ID of the current time master of the transmitting unit. The value of the fourth level Slot Number frame  1810  is set to the current slot number for the cluster to which the transmitting unit belongs. The value of the fourth level Channel Number frame  1815  is set to the logical channel number on which the SYNC frame is being transmitted. 
         [0107]    In alternative embodiments, more than one of any of the disclosed types of time slots and frames may be incorporated into the communications protocol formed with this invention. 
         [0108]    In an emergency situation the need for reliable communications is paramount. For example, in a hazardous environment, such as underground, direct communications between two points is often made unreliable or impossible by destroyed infrastructure and/or blockage of line-of-sight by equipment and/or debris. By design, this invention creates a method by which the ad hoc mesh nodes automatically form and re-form in clusters to provide reliable network communications. This characteristic facilitates the natural creation of clusters of communication around FMN&#39;s  102  to provide an infrastructure and provides a method by which isolated pockets (clusters) of FMN&#39;s  102  can quickly reconnect with the main infrastructure. This invention further creates a protocol that provides a reliable wireless network formed by ad hoc mesh nodes. The communication protocol supports the creation and maintenance of a reliable network  100  including fixed and mobile nodes. This protocol provides methods by which mobile devices can communicate with other mobile devices and with Network Operations Center  105  while roaming freely through the network formed by the Fixed Mesh Nodes (FMN)  102 . The protocol supports voice, data and text communications in emergency and non-emergency modes. In addition this protocol provides methods by which each infrastructure node and mobile node can be monitored and managed at remote Network Operations Center  105 . 
         [0109]    While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teaching of the present invention to a particular situation without departing from its central scope. Therefore it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the appended claims.