Abstract:
A cluster tree network formed by self-organization of a number of nodes. The method of self-organization includes processes for cluster formation, cluster network maintenance, intra-cluster communication. In the cluster formation process, each node discovers if any neighboring node is a cluster head or if any node is already a member of a cluster (thus making it a networked node), and if a cluster head or a networked node is discovered, each node establishes a communication link with the cluster head or the networked node. If no cluster head or networked node is discovered, the node itself becomes a cluster head. The network is maintained by each node periodically broadcasting a HELLO message to neighboring nodes, receiving responses to the HELLO message and updating a neighbor list in accordance with responses to the HELLO message. Multi-cluster networks are also provided using the processes of inter-cluster network formation, inter-cluster network maintenance, and inter-cluster communication. The resulting network has one or more clusters of nodes, each with a cluster head and a number of member nodes, each assigned a node identifier by the cluster head. In a multi-cluster network, a designated device assigns identifies to each cluster head in the network. Border nodes, which are members of at least two clusters, act as routers connecting the clusters and relaying information packets between the clusters.

Description:
PRIORITY DATA 
     This application claims the benefit under Title 35, United States Code Section 119(e), to U.S. provisional application Ser. No. 60/285,165 filed Apr. 20, 2001. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 09/803322 filed Mar. 9, 2001 titled “A Multiple Access Protocol and Structure for Communication Devices in an Asynchronous Network”, Now U.S. Pat. No. 6,816,493, and to pending U.S. patent application Ser. No. 10/022935 filed Dec. 18, 2001 titled “A Multiple Access Protocol and Structure for Communication Devices in an Asynchronous Network” and currently pending, which are hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field of communication networks. More particularly, the invention relates to a protocol and structure for a self-organizing network. 
     BACKGROUND OF THE INVENTION 
     There are many applications for wireless communication networks, such as wireless sensors, industrial control and monitoring, intelligent agriculture, asset and inventory tracking, and security. The manual configuration of such networks can be time consuming and expensive. There is therefore a need for a communication protocol that produces an ad hoc, self-organizing network; that is, a network with a random topology in which the network organization and maintenance occur without human intervention. 
     SUMMARY OF THE INVENTION 
     The present invention relates generally to self-organizing communication networks and in particular to structures and protocols for the operation of cluster tree self-organizing networks. Objects and features of the invention will become apparent to those of ordinary skill in the art upon consideration of the following detailed description of the invention. 
     The Cluster Tree Protocol of the present invention is a protocol of the logical link and network layers for a wireless ad-hoc network. In one embodiment, the protocol uses link-state packets to form either a single cluster network, or a potentially larger cluster tree network. The network is basically self-organized, and supports network redundancy to attain a degree of fault resistance and self-repair. 
     Nodes select a cluster head and form a cluster according to the self-organized manner. In the cluster formation process the cluster head assigns a unique node ID to each member node. 
     Self-developed clusters connect to each other using a Designated Device. The Designated Device is a special node that has high computing ability and large memory space; in most applications it is also the gateway between the network and the Internet. The Designated Device assigns a unique cluster ID to each cluster. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as the 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 drawing(s), wherein: 
         FIG. 1  is a diagrammatic representation of a cluster head selection process of the invention. 
         FIG. 2  is a diagrammatic representation of a link setup process between a cluster head and a member node in accordance with the invention. 
         FIG. 3  is a diagrammatic representation of a single-hop cluster structure in accordance with the invention. 
         FIG. 4  is a diagrammatic representation of a multi-hop cluster setup procedure in accordance with the invention. 
         FIG. 5  is a diagrammatic representation of a multi-hop cluster structure in accordance with the invention. 
         FIG. 6  is a diagrammatic representation of a process for updating a neighbor list in accordance with the invention. 
         FIG. 7  is a diagrammatic representation an exemplary network. 
         FIG. 8  is a neighbor list of a node in cluster border of the network shown in  FIG. 7 . 
         FIG. 9  is a diagrammatic representation an exemplary network. 
         FIG. 10  is a link-state report corresponding to the network in  FIG. 9 . 
         FIG. 11  is a diagrammatic representation an exemplary network. 
         FIG. 12  is a topology update table corresponding to the network in  FIG. 11 . 
         FIG. 13  is a diagrammatic representation of an exemplary network with a failed node. 
         FIG. 14  is a modified link-state report table for the network shown in  FIG. 13 . 
         FIG. 15  is a diagrammatic representation of the network in  FIG. 13  following a first stage of link recovery. 
         FIG. 16  is a topology update table for the network shown in  FIG. 15 . 
         FIG. 17  is a diagrammatic representation of the network in  FIG. 13  following a second stage of link recovery. 
         FIG. 18  is a link-state report table for the network shown in  FIG. 17 . 
         FIG. 19  is a topology update table for the network shown in  FIG. 17 . 
         FIG. 20  is a diagrammatic representation of multiple access control using RTS/CTS messages. 
         FIG. 21  is a flow diagram showing data packet forwarding flow. 
         FIG. 22  is an interaction diagram of a first example of cluster ID assignment. 
         FIG. 23  is a diagrammatic representation of a network corresponding to  FIG. 22 . 
         FIG. 24  is an interaction diagram of a second example of cluster ID assignment. 
         FIG. 25  is a diagrammatic representation of a network corresponding to  FIG. 24 . 
         FIG. 26  is an interaction diagram of a third example of cluster ID assignment. 
         FIG. 27  is a diagrammatic representation of a network corresponding to  FIG. 26 . 
         FIG. 28  is an interaction diagram of a fourth example of cluster ID assignment. 
         FIG. 29  is a diagrammatic representation of a network corresponding to  FIG. 28 . 
         FIG. 30  is an interaction diagram of an exemplary network. 
         FIG. 31  is a network link-state report corresponding to the network shown in  FIG. 30 . 
         FIG. 32  is a diagrammatic representation of an exemplary network. 
         FIG. 33  is a network topology update table corresponding to the network shown in  FIG. 32 . 
         FIG. 34  is a diagrammatic representation of an exemplary network illustrating network redundancy. 
         FIG. 35  is a modified network link-state report corresponding to the network shown in  FIG. 34 . 
         FIG. 36  is a modified network topology update table corresponding to the network shown in  FIG. 34 . 
         FIG. 37  is a diagrammatic representation of an exemplary multi-cluster network illustrating border nodes. 
         FIG. 38  shows the structure of an exemplary HELLO message. 
         FIG. 39  shows the structure of an exemplary CONNECTION REQUEST message. 
         FIG. 40  shows the structure of an exemplary CONNECTION RESPONSE message. 
         FIG. 41  shows the structure of an exemplary NODE ID REQUEST message. 
         FIG. 42  shows the structure of an exemplary NODE ID RESPONSE message. 
         FIG. 43  shows the structure of an exemplary DISCONNECTION REQUEST message. 
         FIG. 44  shows the structure of an exemplary DISCONNECTION RESPONSE message. 
         FIG. 45  shows the structure of an exemplary LINK-STATE REPORT message. 
         FIG. 46  shows the structure of an exemplary TOPOLOGY UPDATE 
         FIG. 47  shows the structure of an exemplary NETWORK CONNECTION REQUEST message. 
         FIG. 48  shows the structure of an exemplary NETWORK CONNECTION RESPONSE message. 
         FIG. 49  shows the structure of an exemplary CLUSTER ID REQUEST message. 
         FIG. 50  shows the structure of an exemplary CLUSTER ID RESPONSE message. 
         FIG. 51  shows the structure of an exemplary NETWORK DISCONNECTION REQUEST message. 
         FIG. 52  shows the structure of an exemplary NETWORK DISCONNECTION RESPONSE message. 
         FIG. 53  shows the structure of an exemplary NETWORK LINK-STATE REPORT message. 
         FIG. 54  shows the structure of an exemplary NETWORK TOPOLOGY UPDATE message. 
         FIG. 55  shows the structure of an exemplary REQUEST TO SEND (RTS) message. 
         FIG. 56  shows the structure of an exemplary CLEAR TO SEND (CTS) message. 
         FIG. 57  shows the structure of an exemplary ACKNOWLEDGEMENT (ACK) for Intra Cluster Communication. 
         FIG. 58  shows the structure of an exemplary ACKNOWLEDGEMENT (ACK) for Inter Cluster Communication. 
         FIG. 59  shows the structure of an exemplary Intra Cluster DATA frame. 
         FIG. 60  shows the structure of an exemplary Inter Cluster DATA frame. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one or more specific embodiments, with the understanding that the present disclosure is to be considered as exemplary 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 drawings. 
     The Cluster Tree Protocol of the present invention is a protocol of the logical link and network layers for a wireless ad-hoc network. In one embodiment, the protocol uses link-state packets to form either a single cluster network, or a potentially larger cluster tree network. The network is basically self-organized, and supports network redundancy to attain a degree of fault resistance and self-repair. 
     Nodes select a cluster head and form a cluster according to the self-organized manner that will be described below. In the cluster formation process the cluster head assigns a unique node ID to each member node. 
     Self-developed clusters connect to each other using the Designated Device. The Designated Device is a special node that has high computing ability and large memory space; in some applications it is also the gateway between the network and the Internet. The Designated Device assigns a unique cluster ID to each cluster. 
     In the preferred embodiment, a network is made of one or more clusters, each cluster having a cluster head and a number of member nodes. The formation and operation of a single cluster is described first. Multi-cluster networks are described later. Each node is directed by a computer program stored in a memory, an application specific integrated circuit, a digital signal processor or an equivalent device. Each node has an input for receiving data and an output for transmitting data. 
     Single Cluster Network: Cluster Formation Process 
     The Cluster formation process begins with the selection of the cluster head, the first node in the cluster. After a cluster head is selected, the cluster head expands links with other member nodes to form a cluster. 
     One example of selecting a cluster head is illustrated in  FIG. 1 . After a node turns on, it operates as a regular network node, and listens and searches for a HELLO message from other nodes. (A HELLO message is a simple broadcast message identifying the transmitting node.) If the node does not receive any HELLO messages for a first period of time, e.g., 1–30 seconds, it then operates as a cluster head and sends out a HELLO message to its neighbors. The new cluster head waits for responses from neighboring nodes for a second period of time, e.g., 2–60 seconds. If no connection requests are received, the node turns back to operation as a regular network node and listens again. 
     Other methods to select a cluster head are possible. The cluster head can be selected based on stored/calculated parameters of each node, like transmission range, power capacity, computing ability or location information. After a node is selected as a cluster head (CH), it broadcasts a periodic HELLO message that contains a part of the cluster head MAC (Multiple Access Control) address and a node ID (0 for example) that indicates the cluster head. This is shown in  FIG. 2 . Referring now to  FIG. 2 , the nodes that receive this HELLO message send a CONNECTION REQUEST message to the cluster head. When the cluster head receives the CONNECTION REQUEST, it responds to the node with a CONNECTION RESPONSE message that contains a node ID for the node. The node ID should preferably be unique within a cluster and the cluster head has the responsibility to assign and manage unique node IDs to its member nodes. The node that is assigned a node ID replies with an ACK (acknowledge) message to the cluster head. After every message exchange is finished, both nodes set each other as parent or child. Each node maintains a neighbor list, which includes a list of parent and child nodes. Specifically, the cluster head denotes the newly added node as a child in its neighbor list and the new node denotes the cluster head as a parent. The link between the cluster head and the member node is established at this moment. 
     If all nodes are located in the range of the cluster head, the topology of connection becomes a star, as shown in  FIG. 3 , and every member node is connected to the cluster head with one hop. In the preferred embodiment, the maximum number of nodes in a cluster is 254 including the cluster head. If node addresses with N-bits are used the maximum number of nodes is 2 N −2. The administrator or the manufacturer may limit the node feature to supporting only single hop cluster. 
     A cluster can expand into a multi-hop structure when each node supports multiple connections. Although network delay increases, the coverage within one cluster can increase. The multi hop cluster setup procedure is described in  FIG. 4 . After node B has established a link with the cluster head, it starts to relay HELLO messages from the cluster head. When node C gets the message from node B, it sends a CONNECTION REQUEST message to node B. Node B requests a new node ID to the cluster head for node C. When node B receives a new node ID from the cluster head, it sends a CONNECTION RESPONSE message to node C. Then node C receives it and answers with an ACK message. After this message exchange, node C sets node B as its parent, node B sets node C as its child, and the cluster head sets node C as node B&#39;s child. Node C then starts to relay HELLO messages to announce itself to its neighborhood. 
     When a node receives several HELLO messages from different nodes, there are many different ways to select the Hello message to which to respond. In a preferred embodiment, the node responds to the earliest HELLO message. In another embodiment, it responds to the strongest HELLO message. The path to the cluster head might not be ideal at this time. The route to the cluster head will optimize in a later process. 
     This expansion process can continue until the cluster head runs out of node IDs. The maximum hop count may also be limited to reduce maximum network delay. 
     When the cluster head has run out of node IDs or the cluster has reached some other defined limit, the cluster head should reject connection requests from new nodes. To reject the connection request, the temporary NID (NID  254  for example) is used in the destination NID field of the CONNECTION RESPONSE message or the new NID field of the NODE ID RESPONSE message. 
     When a requester node receives a NODE ID RESPONSE message with NID  254 , it sends a CONNECTION RESPONSE message with NID  254  to the new node. 
     If a new node has received a CONNECTION RESPONSE with NID  254 , it stores the cluster ID and stop sending a CONNECTION REQUEST message to the node belonging to the same cluster for a while. 
     An example of a multi-hop cluster structure is shown in  FIG. 5 . 
     Single Cluster Network: Network Maintenance 
     The cluster head periodically broadcasts HELLO messages to its member nodes. When these member nodes receive the HELLO message from the cluster head, they also send HELLO messages to announce themselves to their neighbors. Every node records their neighbor nodes in their neighbor list. The entry of the neighbor list is updated by the periodic HELLO message. If a node entry doesn&#39;t update until certain timeout limit, it should be eliminated. This process is shown in  FIG. 6 . 
     The member nodes can talk directly with the neighbor nodes. If a node wants to communicate with a node outside of its range, it asks the cluster head or the parent node to relay the message to the destination. 
     A node may receive a HELLO message from a node that belongs to different cluster. In that case, the node adds the cluster ID (CID) of the transmitting node in the neighbor list. An exemplary network is shown in  FIG. 7 . The corresponding neighbor list for node  2  is shown in  FIG. 8 . 
     Every node has to report its link state to the cluster head. A member node periodically sends a LINK-STATE REPORT message that contain its neighbors node ID list to the cluster head. The frequency of Link-State Report message will be determined by application requirements and stability.  FIG. 9  shows an exemplary network. A table of the link-state reports sent by each node is shown in  FIG. 10 . 
     Based on the LINK-STATE REPORT message the cluster head periodically calculates the shortest path between itself and member nodes and informs it to the members by TOPOLOGY UPDATE message. An example of a TOPOLOGY UPDATE report for the network shown in  FIG. 11  is shown in  FIG. 12 . 
     The cluster head should choose the route with the smallest hop count. If there are several routes with the same hop count, the cluster head should choose the route that has the smallest node ID as the parent node or some similar arbitration rule. 
     If a member node receives the TOPOLOGY UPDATE message that the different parent node is linked to the node, it changes the parent node as indicated in the message. The member node also records its child nodes and the nodes below it in the tree at this time. The nodes within a cluster basically communicate with other node through the parent node except the case where they communicate with their neighbor nodes directly. The cycle of the Topology Update depends on the Link-State Report cycle. 
     If a member node has trouble and becomes unable to communicate, the tree route of the cluster would be reconfigured. In the cluster show in  FIG. 13 , the node  2  has trouble and stops communication. A modified table of corresponding link-state reports is shown in  FIG. 14 . Since the nodes  2 ,  7 ,  8  and  10  cannot send the LINK-STATE REPORT, the cluster head calculates a new route from other link-state information. By the first TOPOLOGY UPDATE message, the node  7  establishes a new connection with the node  3 , as shown in  FIG. 15 . The corresponding topology update report is shown in  FIG. 16 . In the next cycle of TOPOLOGY REPORT and UPDATE, the nodes  8  and  10  are instructed to connect to node  7 . The resulting network is shown in  FIG. 17 . The corresponding link-state report is shown in  FIG. 18  and the corresponding topology update is shown in  FIG. 19 . 
     When the cluster head has trouble, the distribution of HELLO messages is stopped and all member nodes know that they have lost the cluster head. The member nodes lose their node ID and connections with the parent/children nodes. The cluster is then reconfigured in the same way as the cluster formation process. 
     Single Cluster Network: Intra Cluster Communication. 
     There are many options in Multiple Access Control. One is CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance); another is pure ALOHA (where messages are sent at any time and then resent if the message is not received). In the CSMA/CA option, RTS (Request To Send)/CTS (Clear To Send) messages are used. Referring now to  FIG. 20 , when a node wants to send a packet to other node, it sends RTS at first and then waits for CTS. After receiving RTS, the receiving node sends a CTS frame to acknowledge the right for the sending node to send data frame. This procedure reduces the chance of collision by hidden nodes. 
     A node receiving an error-free frame can send an ACK frame to the sending node to acknowledge the successful reception of the frame. 
     When a node wants to send a packet to other node, i.e. it wants to unicast a message, it sets its node ID in the source NID field of the packet and its destination node ID in destination NID field. If a node isn&#39;t sending to one of its neighbors, and if the destination node is below the source in the tree, the source node sets its child node ID in the receiving NID field and asks its child node to forward to the destination. If the source isn&#39;t sending to one of its neighbors, and if the destination node isn&#39;t below the source branch, the source sets its parent node ID in the receiving NID field and sends the packet to its parent. Each intermediate node should relay the packet toward the destination node as it updates receiving and transmitting NID fields. 
     The packet is routed along the tree topology except for the last one hop. If the destination node is below the sender node in the tree structure, the packet is forwarded along the branch to the destination. Otherwise, the packet goes up along the tree structure and looks for the destination. If the intermediate node has the destination node in its neighbor list, the packet is routed apart from the tree route. 
     When a node receives a unicast message, the receiving node should respond to the transmitting node with an ACK message. The detail of packet forwarding process is described in  FIG. 21 . Referring to the flow chart in  FIG. 21 , the receiving node receives a packet at block  120 . At decision block  122  a check is made to determine if the Cluster Head ID matches that of the cluster. If the Cluster Head ID is that of a different cluster, the packet is discarded at block  124 . If the Cluster Head ID is that of the present cluster, flow continues to decision block  126 . At decision block  126 , the frame type is checked. If the frame type does not indicate that the packet contains data, the packet is passed to a different process at block  128 . If the frame type indicates that the packet contains data, flow continues to decision block  130 , where a check is made to determine if the Node ID is that of the present node. If the ID is that of another node, flow continues to block  124  and the packet is discarded. If the ID indicates that this is a broadcast message, flow continues to block  132  where the packet is accepted. At decision block  134  the Source Node ID is checked. If the source Node ID is that of the parent node, the packet is forwarded at block  136 , otherwise no further action is taken, as indicated by block  138 . Returning to decision block  130 , if the receiving node ID is that of the receiving node, flow continues to decision block  140  and the Destination Device ID is checked. If the Destination Device ID matches the receiving node ID, the packet is accepted at block  142  and an acknowledgement (ACK) message is sent at block  144 . If the Destination Device ID does not match the receiving node ID, the RNID field in the packet is updated at block  146 , the packet is forwarded at block  148  and an ACK message is sent at block  150 . 
     The broadcast message within a cluster is sent by the cluster head and forwarded by all member nodes. The receiving node shouldn&#39;t respond to the broadcast message with ACK message. A member node should forward the broadcast message that is sent by its parent to avoid forwarding the same packet more than once. 
     Large packets may be sent in several parts, in accordance with a packet fragmentation rule. 
     Inter Cluster Network 
     The preferred embodiment of multi-cluster network formation and the subsequent communication between clusters is now described. 
     To form a multi-cluster network, a Designated Device is needed in the network. The Designated Device assumes an important role in the network. It has the responsibility to assign a unique cluster ID to each cluster head. This cluster ID, combined with the node ID that the cluster head assigns to each node within a cluster, forms a logical address and is used to route packets. Another role of the Designated Device is to calculate the shortest route from the cluster to Designated Device and inform it to all nodes within the network. 
     Inter Cluster Network: Network Formation Process 
     Each node is unique due to the combination of the cluster ID (CID) and the node ID (NID). The NID is assigned by each cluster head (CH) and the Designated Device (DD) assigns a unique CID to each cluster in early stage of multi-cluster network formation. 
     Referring now to the interaction diagram shown in  FIG. 22 , when the DD joins the network, it acts as the cluster head of cluster  0  and starts to send HELLO message to the neighborhood. If a CH has received this message, it sends a CONNECTION REQUEST message and joins the cluster  0 . After that, the CH requests a CID to the DD. In this case, the CH is a border node that has two logical addresses. One is for a member node of the cluster  0  and the other is for a cluster head. When the CH gets a new CID, it informs to its member nodes by sending a HELLO message. The corresponding network is shown in  FIG. 23 . 
     Referring to  FIG. 24 , if a member node has received the HELLO message from the DD, it adds CID  0  in its neighbor list and reports to its CH. The reported CH selects the member node as a border node to its parent cluster and sends a NETWORK CONNECTION REQUEST message to the member node to set up a connection with the DD. The border node requests a connection and joins the cluster  0  as its member node. Then it sends a CID REQUEST message to the DD. After the CID RESPONSE message arrival, the border node sends a NETWORK CONNECTION RESPONSE message that contains a new CID to the CH. When the CH gets a new CID, it informs its member nodes by the HELLO message. The corresponding network is shown in  FIG. 25 . 
     The clusters not bordering cluster  0  use intermediate clusters to get a CID. Two cases can be thought as the same as above. One case, shown in the interaction diagram in  FIG. 26  and the network in  FIG. 27 , is where the CH becomes the border node to its parent cluster. The other case, shown in the interaction diagram of  FIG. 28  and the corresponding network in  FIG. 29 , is where the CH names a member node as the border to its parent cluster. In both cases, the process is triggered by the HELLO message that contains a CID from 1 to 253 instead of the HELLO from the DD. 
     Each member node of the cluster records its parent cluster, child/lower clusters and the border node IDs associated with both the parent and child clusters. The DD stores the whole tree structure of the clusters. 
     Inter Cluster Network: Network Maintenance 
     Although the clusters form an initial tree topology in the CID assignment procedure, it may not be the optimal tree structure and the tree structure may change due to the failure of nodes. The clusters use the cluster link-state information to calculate the optimized route and periodically update their topology for the network redundancy. 
     Every cluster reports its link-state information to the DD. The cluster head periodically sends a NETWORK LINK-STATE REPORT message that contains its neighbor cluster CID list to the DD. An exemplary network is shown in  FIG. 30  and the corresponding link-state reports are shown in  FIG. 31 . 
     Based on the NETWORK LINK-STATE REPORT message, the DD periodically calculates the optimized tree route and sends a NETWORK TOPOLOGY UPDATE message to inform up-to-date route from the DD to the clusters. An exemplary network is shown in  FIG. 32  and the corresponding network topology updates are shown in  FIG. 33 . The DD chooses the route with the smallest hop count. If there are several routes with the same hop count, the DD should choose the cluster that has the smallest CID as the parent cluster, or some other functional rule for arbitrating ties. 
     If a cluster head receives the NETWORK TOPOLOGY UPDATE message and determines that a different parent cluster is linked to the cluster, it changes the parent cluster as indicated in the message. All nodes within the cluster should memorize its parent cluster, child/lower clusters and the border nodes&#39; NID at this time. 
     When a failure has occurred in the network, the cluster may have to find an alternative route to the DD. This feature is achieved by using the messages explained above. 
     In the example network shown in  FIG. 34 , a problem has occurred in cluster  1 . The NETWORK LINK-STATE REPORT messages, shown in  FIG. 35 , from cluster  1  and  3  fail to arrive at the DD. The link-sate report from cluster  3  fails to arrive because it was linked to the DD via the failed cluster. The link-state report from cluster  2  no longer indicates a link to cluster  1 . The DD broadcasts a new NETWORK TOPLOGY UPDATE message, shown in  FIG. 36 , and indicates cluster  3  to switch the parent to cluster  4 . 
     A backup Designated Device (BDD) can be prepared to prevent network down time due to the DD&#39;s trouble. One example is that a BDD is connected to the DD by wired or wireless network and periodically duplicate the list of cluster ID and network link-state information from the DD. The BDD takes over the DD role as soon as it detects the DD&#39;s failure. Other solutions may be possible to realize the BDD. 
     Inter Cluster Communication 
     Inter cluster communication is realized by routing. The border nodes act as routers that connect clusters and relay packets between clusters. An exemplary multi-cluster network with border nodes is shown in  FIG. 37 . 
     Every node knows its parent cluster, child/lower cluster and the border node ID. When a node sends a unicast message (a message to a specific node), receiving nodes can decide where they should send/forward the packet. When a border node receives a packet, it examines the destination address, then forwards to the next border node in the adjacent cluster or to the destination node within the cluster. 
     Only the DD can broadcast a message by sending it to all nodes within its network. The message is forwarded along the tree route of clusters. The border node should forward the broadcast packet from the parent cluster to the child cluster. 
     An exemplary implementation of the network of the present invention is described in more detail below 
     Address Scheme 
     An exemplary address scheme is described below. 
     Each node is assigned a 16 bit logical address that consists of a cluster ID (CID) and a node ID (NID). 
     Cluster ID 
     The Designated Device assigns a unique 8-bit cluster ID to the cluster. CID  255  means all clusters and is used for broadcast message. 
                                                         TABLE 1                   Cluster ID                Binary   Decimal   CID Function                            00000000   0   Designated Device (DD)           00000001   1   Regular Cluster           |   |           11111101   253           11111110   254   Temporary Cluster ID           11111111   255   Broadcast                        
Node ID
 
     The cluster head assigns a unique 8-bit node ID to its member nodes. The cluster head uses NID  0 . NID  255  is for broadcast and  254  for temporary use. 
                                                         TABLE 2                   Node ID                Binary   Decimal   NID Function                            00000000   0   Cluster Head (CH)           00000001   1   Member node           |   |           11111101   253           11111110   254   Temporary node ID           11111111   255   Broadcast                        
Frame Structure
 
     One embodiment of the different types of packets that are used for communication within and clusters is described below. 
     Frame Type 
     A 6-bit field is defined for the frame type. The first two bits define the category of the function and the next four bits indicate the detail functions. 
                                             TABLE 3                   Frame Type            Frame Type                (bit 1, bit 2)   (bit 3, 4, 5, 6)   Frame Function               Intra Cluster   0000   HELLO       Management   0001   CONNECTION REQUEST       Frame       00   0010   CONNECTION RESPONSE           0011   NODE ID REQUEST           0100   NODE ID RESPONSE           0101   DISCONNECTION REQUEST           0110   DISCONNECTION RESPONSE           0111   LINK-STATE REPORT           1000   TOPOLOGY UPDATE           1001–1111   Reserved       Inter Cluster   0000   NETWORK CONNECTION REQUEST       Management   0001   NETWORK CONNECTION RESPONSE       Frame       01   0010   CLUSTER ID REQUEST           0011   CLUSTER ID RESPONSE           0100   NETWORK DISCONNECTION               REQUEST           0101   NETWORK DISCONNECTION               RESPONSE           0110   NETWORK LINK-STATE REPORT           0111   NETWORK TOPOLOGY UPDATE           1000–1111   Reserved       Control   0000   REQUEST TO SEND (RTS)       Frame       10   0001   CLEAR TO SEND (CTS)           0010   ACKNOWLEDGEMENT (ACK) for               Intra Cluster           0011   ACKNOWLEDGEMENT (ACK) for               Inter Cluster           0100–1111   Reserved       Data Frame   0000   INTRA CLUSTER DATA       11   0001   INTRA CLUSTER DATA with ACK           0010   INTER CLUSTER DATA           0011   INTER CLUSTER DATA with ACK           0100–1111   Reserved                    
Management Frames
 
Intra Cluster Management Frames
 
     The structure of the HELLO message is shown in  FIG. 38 . Referring to  FIG. 38 , CH DID denotes the Cluster Head Device ID, which is a part of cluster head MAC address. This field is used to determine whether the transmitting node belongs to the same node cluster. TNID denotes the Transmitting Node ID: the node ID of source/intermediate node that sends the packet. TCID denotes the Transmitting Cluster ID, i.e. the cluster ID of transmitter. Before assignment of CID, the cluster head uses temporary CID  254 . 
     The structure of the CONNECTION REQUEST message is shown in  FIG. 39 . Referring to  FIG. 39 , CH DID denotes the Cluster Head Device ID which is a part of the cluster head MAC address that the new node wants to join. Dst NID denotes the Destination Node ID, i.e. the node ID that the new node requests a connection and Src DID denotes the Source Device ID: a part of the source node MAC. 
     The structure of the CONNECTION RESPONSE message is shown in  FIG. 40 . Referring to  FIG. 40 , CH DID denotes the Cluster Head Device ID. Src NID denotes the Source Node ID, i.e. the node ID that is requested the connection by the new node. Dst DID is the Destination Device ID, and is a copy of Src DID field of CONNECTION REQUEST message. New NID denotes the New Node ID, which is the new node ID that is assigned to the requester node. When the requested node rejects the request, it puts  254  in this field. 
       FIG. 41  shows the structure of the NODE ID REQUEST message. Referring to  FIG. 41 , CH DID denotes the Cluster Head Device ID and RNID denotes the Receiving Node ID, i.e. the node ID of destination/intermediate node that should receive the packet. Src NID denotes the Source Node ID, i.e. the node ID that is requesting the connection for the new node. New Node DID denotes the New Node Device ID. This is a copy of Src DID field of the CONNECTION REQUEST message 
     The structure of the NODE ID RESPONSE is shown in  FIG. 42 . Referring to  FIG. 42 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID, Dst NID denotes the Destination Node ID and New Node DID denotes the New Node Device ID. The New Node DID is a copy of New Node DID field of the CLUSTER ID REQUEST message. New NID denotes the New Node ID, i.e. the node ID that is assigned to the new node. When the cluster head rejects the request, it puts the ID  254  in this field. 
       FIG. 43  shows the structure of the DISCONNECTION REQUEST message. Referring to  FIG. 43 , CH DID denotes the Cluster Head Device ID and Src NID denotes the Source Node ID (the node ID of requesting node). 
       FIG. 44  shows the structure of the DISCONNECTION RESPONSE message. Referring to  FIG. 44 , CH DID denotes the Cluster Head Device ID and Dst NID denotes the Destination Node ID. 
       FIG. 45  shows the structure of the LINK-STATE REPORT message. Referring to  FIG. 45 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID, and Src NID denotes the Source Node ID. Length  1  denotes the number of NID fields and Length  2  denotes the number of CID fields. NID #n is the identifier of neighbor node #n. CID #m is the identifier of neighbor cluster #m. 
       FIG. 46  shows the structure of the TOPOLOGY UPDATE message. Referring to  FIG. 46 , CH DID denotes the Cluster Head Device ID, Length  1  denotes the number of NID fields and Length  2  denotes the number of CID fields. NID #n is the identifier of member node #n. Parent NID is the Parent Node ID, that is the parent node ID for the member node #n named in the previous field. CID #m is the identifier for neighbor Cluster #m. Border NID is the Border Node ID: the border node ID for the cluster #m named in the previous field. 
     Inter Cluster Management Frames 
       FIG. 47  shows the structure of the NETWORK CONNECTION REQUEST message. Referring to  FIG. 47 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID and Dst NID denotes the Destination Node ID. CID denotes the cluster ID that the border node should set up a connection with. 
       FIG. 48  shows the structure of the NETWORK CONNECTION RESPONSE message. Referring to  FIG. 48 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID and Src NID is the Source Node ID, that is the node ID of the border node. New CID is the New Cluster ID that is assigned to the cluster head by the Designated Device. 
       FIG. 49  shows the structure of the CLUSTER ID REQUEST message. Referring to  FIG. 49 , CH DID denotes the Cluster Head Device ID, RNID is the Receiving Node ID and Src CID is the Source Cluster ID, that is the cluster ID of the border node. Src NID is the Source Node ID. 
       FIG. 50  shows the structure of the CLUSTER ID RESPONSE message. Referring to  FIG. 50 , CH DID denotes the Cluster Head Device ID. RNID denotes the Receiving Node ID, that is the node ID of destination/intermediate node that should receive the packet. Dst CID is the Destination Cluster ID, i.e. the cluster ID of the border node that requested a new CID. Dst NID is the Destination Node ID, i.e. the node ID of the border node that requested a new CID. New CID is the New Cluster ID that is assigned by the Designated Device. 
       FIG. 51  shows the structure of the NETWORK DISCONNECTION REQUEST message. Referring to  FIG. 51 , CH DID denotes the Cluster Head Device ID. RNID denotes the Receiving Node ID and Dst NID denotes the Destination Node ID. CID is the cluster ID that the border node should disconnect. 
       FIG. 52  shows the structure of the NETWORK DISCONNECTION RESPONSE message. Referring to  FIG. 52 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID, Src NID denotes the Source Node ID and CID denotes the cluster ID that the border node has disconnected with. 
       FIG. 53  shows the structure of the NETWORK LINK-STATE REPORT message. Referring to  FIG. 53 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID and Src NID denotes the Source Node ID. Length  1  denotes the number of fields for CIDs and CID #n denotes the identifier of the neighbor cluster. 
       FIG. 54  shows the structure of the NETWORK TOPOLOGY UPDATE message. Referring to  FIG. 54 , CH DID denotes the Cluster Head Device ID, Length  1  denotes the number of fields for CIDs and their Parent CIDs. CID #n denotes the identifier of the cluster ID that exists in the network. Parent CID is the Parent Cluster ID for the cluster #n named in previous field. 
     Control Frames 
       FIG. 55  shows the structure of the RTS message. Referring to  FIG. 55 , CH DID denotes the Cluster Head Device ID. The value of the Duration field is the amount of time the sending node needs to transmit the data frame, one CTS frame, one ACK frame and three inter-frame space intervals. RNID denotes the Receiving Node ID and TNID denotes the Transmitting Node ID. 
       FIG. 56  shows the structure of the CTS message. Referring to  FIG. 56 , CH DID denotes the Cluster Head Device ID. Duration is the duration of previous RTS frame minus the time required to transmit the CTS frame and an inter-frame space interval. RNID denotes the Receiving Node ID and TNID denotes the Transmitting Node ID. 
       FIG. 57  shows the structure of the ACK message for Intra Cluster Communication. Referring to  FIG. 57 , CH DID denotes the Cluster Head Device ID and RNID denotes Receiving Node ID, that is the node ID of destination/intermediate node that should receive the packet. Dst NID denotes the Destination Node ID and Src NID denotes the Source Node ID. 
       FIG. 58  shows the structure of the ACK message for Inter Cluster Communication. Referring to  FIG. 58 , CH DID denotes Cluster Head Device ID, RNID denotes the Receiving Node ID, Dst CID denotes the Destination Cluster ID.and Dst NID denotes the Destination Node ID. Src CID denotes Source Cluster ID and Src NID denotes the Source Node ID. 
     Data Frames. 
       FIG. 59  shows the structure of an Intra Cluster Data Frame. CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID (the node ID of destination/intermediate node that should receive the packet) and Dst NID denotes the Destination Node ID. Src NID is the Source Node ID and Payload denotes the Data itself. 
     The Intra Cluster Data Frame with ACK has the same frame structure as Intra Cluster Data Frame except the Frame Type field. 
       FIG. 60  shows an Inter Cluster Data Frame. Referring to  FIG. 60 , CH DID denotes the Cluster Head Device ID, RNID denotes the Receiving Node ID (the node ID of destination/intermediate node that should receive the packet), Dst CID denotes the Destination Cluster ID and Dst NID denotes the node ID of the destination node. Src CID denotes the node ID of the source node, Src NID denotes the Source Node ID and Payload denotes the Data itself. 
     The Inter Cluster Data Frame with ACK has the same frame structure as Inter Cluster Data Frame except the Frame Type field. 
     Those of ordinary skill in the art will recognize that the present invention has been described in terms of exemplary embodiments based upon use of a particular message set. However, the invention should not be so limited, since the present invention could be implemented functionally equivalent messages. 
     The nodes themselves may comprise a variety of hardware components including as special purpose hardware and/or dedicated processors. Similarly, general purpose computers, microprocessor based computers, digital signal processors, microcontrollers, dedicated processors, custom circuits, ASICS and/or dedicated hard wired logic may be used to construct alternative equivalent embodiments of the present invention. 
     Each node is directed by a computer program. Those ordinarily skilled in the art will appreciate that the program steps and associated data used to implement the embodiments described above can be implemented using disc storage as well as other forms of storage, such as, for example, Read Only Memory (ROM) devices, Random Access Memory (RAM) devices, optical storage elements, magnetic storage elements, magneto-optical storage elements, flash memory, core memory and/or other equivalent storage technologies without departing from the present invention. Such alternative storage devices should be considered equivalents. 
     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.