Abstract:
A wireless mesh network routes messages between a host computer and a plurality of field devices. The mesh network is synchronized to a global regular active schedule that defines active periods when messages can be transmitted or received by nodes of the network, and inactive periods when messages cannot be transmitted or received. Based upon messages to be sent by the host computer to selected field devices, the network is controlled to selectively maintain active those nodes required to route messages to the selected field devices. Those required nodes are maintained in an active state as long as communication with the selected field devices continues, while other nodes are allowed to return to a low power inactive state. When communication between the host computer and the selected field devices is no longer required, the entire network is allowed to enter the low power inactive state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from a co-pending application entitled LOW POWER WIRELESS NETWORKS OF FIELD DEVICES, Ser. No. 60/758,167, filed on Jan. 11, 2006, which is incorporated by reference. 
     Reference is also made to co-pending applications filed on even date with this application: CONTROL OF FIELD DEVICE ON LOW POWER WIRELESS NETWORKS, Ser. No. 11/652,393; CONTROL SYSTEM WITH WIRELESS ADDRESS DOMAIN TO FIELD DEVICE ADDRESS DOMAIN TRANSLATION, Ser. No. 11/652400; CONTROL SYSTEM WITH PREDICTIVE FIELD DEVICE RESPONSE TIME OVER A WIRELESS NETWORK, Ser. No. 11/652,392; VISUAL MAPPING OF FIELD DEVICE MESSAGE ROUTES IN A WIRELESS MESH NETWORK, Ser. No. 11/652,398; SELECTIVE ACTIVATION OF FIELD DEVICES IN LOW POWER WIRELESS MESH NETWORKS, Ser. No. 11/652,395; and CONTROL SYSTEM WITH WIRELESS MESSAGES CONTAINING MESSAGE SEQUENCE INFORMATION, Ser. No. 11/652,401, which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to wireless networks. In particular, the invention relates to a wireless mesh network in which process control messages are communicated between a host and field devices at nodes of the wireless mesh network. 
     In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. Often, such control systems have a centralized control room with a host computer that communicates with field devices that are separated or geographically removed from the control room. 
     Generally, each field device includes a transducer, which may generate an output signal based on a physical input or generate a physical output based on an input signal. Types of transducers used in field devices include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow sensors, positioners, actuators, solenoids, indicators, and the like. Traditionally, analog field devices have been connected to the process subsystem and the control room by two-wire twisted-pair current loops, with each device connected to the control room by a single two-wire twisted pair loop. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. An analog field device that performs an action under the control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the ports of the process subsystem under the control of the controller. 
     While historically field devices were capable of performing only one function, more recently hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) superimposes a digital carrier signal on the current loop signal. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions. 
     Another approach uses a digital communication bus to connect multiple field devices to the host in the control room. Examples of digital communication protocols used with field devices connected to a digital bus include Foundation Fieldbus, Profibus, Modbus, and DeviceNet. Two way digital communication of messages between a host computer and multiple field devices can be provided over the same two-wire path that supplies power to the field devices. 
     Typically, remote applications have been added to a control system by running very long homerun cables from the control room to the remote application. If the remote application is, for example, a half of a mile away, the costs involved in running such a long cable can be high. If multiple homerun cables have to be run to the remote application, the costs become even higher. Wireless communication offers a desirable alternative, and wireless mesh networks have been proposed for use in industrial process control systems. However, to minimize costs, it is also desirable to maintain existing control systems and communication protocols, to reduce the costs associated with changing existing systems to accommodate the wireless communication. 
     In wireless mesh network systems designed for low power sensor/actuator-based applications, many devices in the network must be powered by long-life batteries or by low power energy-scavenging power sources. Power outlets, such as 120VAC utilities, are typically not located nearby or may not be allowed into the hazardous areas where the instrumentation (sensors) and actuators must be located without incurring great installation expense. The need for low installation cost drives the need for battery-powered devices communicating as part of a wireless mesh network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a well functioning wireless device. Batteries are expected to last more than 5 years and preferably as long as the life of the product. 
     In a true wireless mesh network, each node must be capable of routing messages for itself as well as other nodes in the mesh network. The concept of messages hopping from node to node through the network is beneficial because lower power RF radios can be used, and yet the mesh network can span a significant physical area delivering messages from one end to the other. High power radios are not needed in a mesh network, in contrast a point-to-point system which employs remote nodes talking directly to a centralized base-station. 
     A mesh network protocol allows for the formation of alternate paths for messaging between nodes and between nodes and a data collector, or a bridge or gateway to some higher level higher-speed data bus. Having alternate, redundant paths for wireless messages enhances data reliability by ensuring there is at least one alternate path for messages to flow even if another path gets blocked or degrades due to environmental influences or due to interference. 
     Some mesh network protocols are deterministically routed such that every node has an assigned parent and at least one alternate parent. In the hierarchy of the mesh network, much as in a human family, parents have children, children have grandchildren, and so on. Each node relays the messages for their descendants through the network to some final destination such as a gateway. The parenting nodes may be battery-powered or limited-energy powered devices. The more descendants a node has, the more traffic it must route, which in turn directly increases its own power consumption and diminishes its battery life. 
     In order to save power, some protocols limit the amount of traffic any node can handle during any period of time by only turning on the radios of the nodes for limited amounts of time to listen for messages. Thus, to reduce average power, the protocol may allow duty-cycling of the radios between On and Off states. Some protocols use a global duty cycle to save power such that the entire network is On and Off at the same time. Other protocols (e.g. TDMA-based) use a local duty cycle where only the communicating pair of nodes that are linked together are scheduled to turn On and Off in a synchronized fashion at predetermined times. Typically, the link is pre-determined by assigning the pair of nodes a specific time slot for communications, an RF frequency channel to be used by the radios, who is to be receiving (Rx), and who is to be transmitting (Tx) at that moment in time. 
     Some protocols employ the concept of assigning links to nodes on a regular repetitive schedule and thereby enable regular delivery of updates and messages from devices in the network. Some advanced TMDA-based protocols may employ the concept of multiple active schedules, these multiple schedules running all at the same time or with certain schedules activated/deactivated by a global network controller as the need arises. For example, slow active schedules link nodes sending messages with longer periods of time (long cycle time) between messages to achieve low power consumption. Fast active schedules link nodes sending messages more rapidly for better throughput and lower latency, but result in higher power consumption in the nodes. With protocols that allow multiple active schedules, some schedules could be optimized for upstream traffic, others for downstream traffic and yet others for network management functions such as device joining and configuration. Globally activating/deactivating various schedules throughout the entire network in order to meet different needs at different times provides a modicum of flexibility for achieving advantageous trade-offs between power consumption and low latency, but applies the same schedule to all nodes and thus does not provide local optimization. 
     In a synchronized system, nodes will have to wait to transmit until their next predetermined On time before they can pass messages. Waiting increases latency, which can be very detrimental in many applications if not bounded and managed properly. If the pair of nodes that are linked together are not synchronized properly, they will not succeed in passing messages because the radios will be On at the wrong time or in the wrong mode (Rx or Tx) at the wrong time. If the only active schedule has a long cycle time, the time between scheduled links will be long and latency will suffer. If a fast schedule is activated, the time between scheduled links will be short but battery life will be measurably reduced over time. 
     Some protocols allow running a slow schedule in the background and globally activating/deactivating an additional fast schedule. Since it takes time to globally activate a fast schedule throughout the entire network and get confirmation back from all nodes that they have heard the global command, the network or sub-network remains in the less responsive mode during the transition time. Furthermore, with a globally activated fast schedule, power is wasted in all the parenting nodes in the network, even those whose descendants will not benefit from the fast schedule. These unappreciative parent nodes must listen more often on the global fast active schedule (i.e. turn their radios On to Rx more often); even though their descendants have nothing extra to send that a regular active schedule would not suffice in that portion of the network. 
     Some protocols may limit the number of descendants a node can have, thereby reducing the load the node must support. Other protocols may employ a combination of all of these measures to reduce average power consumption. All of these power-saving measures have the effect of reducing the availability of the nodes in the network to do the work of passing messages, thereby increasing the latency of messages delivered through the network. Duty-cycling the radio increases latency. Hopping messages from node to node increases latency. Increasing hop depth (hop count) by limiting the number of descendants increases latency. Running a slow active schedule (long cycle period) increases latency. Even globally activating a fast active schedule takes time. It is likely that the value of information diminishes with time, so the longer the latency, the less valuable the information may be. 
     BRIEF SUMMARY OF THE INVENTION 
     A host computer of a control system interacts with field devices through a wireless mesh network. Based upon messages from the host computer that are addressed to selected field devices, the network determines which nodes are required to be active so that messages can be routed to those selected field devices. When the network goes to an active state, the nodes required for communication with the selected field devices remain on while the remaining nodes are allowed to return to the inactive state. After communication between the host computer and selected field devices has ceased, the entire network is returned to an inactive state. Determination of the nodes that must be selectively maintained active can be based upon the addresses of the selected field devices and the communication topology of the wireless mesh network, or can be determined dynamically by those nodes that are actively participating in transmitting and receiving messages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a control system in which a wireless mesh network routes wireless messages between a host and field devices. 
         FIG. 2  is a block diagram of a portion of the control system of  FIG. 1 , including a host computer, a gateway node, and a wireless node with a field device. 
         FIG. 3  is a diagram illustrating the format of wireless messages transmitted by the wireless network. 
         FIG. 4  shows the format of a control message from a host to a field device based upon a control system protocol. 
         FIG. 5  shows one embodiment of the control message as modified to form the payload of the wireless message shown in  FIG. 3 . 
         FIG. 6  shows another embodiment of the control message as modified with a trailer to form the payload of the wireless message shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows control system  10 , which includes host computer  12 , high-speed network  14 , and wireless mesh network  16 , which includes gateway  18  and wireless nodes  20 ,  22 ,  24 ,  26 ,  28 , and  30 . Gateway  18  interfaces mesh network  16  with host computer  12  over high-speed network  14 . Messages may be transmitted from host computer  12  to gateway  18  over network  14 , and are then transmitted to a selected node of mesh network  16  over one of several different paths. Similarly, messages from individual nodes of mesh network  16  are routed through mesh network  16  from node-to-node over one of several paths until they arrive at gateway  18  and are then transmitted to host  12  over high-speed network  14 . 
     Control system  10  can make use of field devices that have been designed for and used in wired distributed control systems, as well as field devices that are specially designed as wireless transmitters for use in wireless mesh networks. Nodes  20 ,  22 ,  24 ,  26 ,  28 , and  30  show examples of wireless nodes that include conventional field devices. 
     Wireless node  20  includes radio  32 , wireless device router (WDR)  34 , and field devices FD 1  and FD 2 . Node  20  is an example of a node having one unique wireless address and two unique field device addresses Nodes  22 ,  24 ,  26 , and  28  are each examples showing nodes having one unique wireless address and one unique field device address. Node  22  includes radio  36 , WDR  38 , and field device FD 3 . Similarly, field device  24  includes radio  40 , WDR  42 , and field device FD 4 ; node  26  includes radio  44 , WDR  46 , and field device FD 5 ; and node  28  includes radio  48 , WDR  50 , and field device FD 6 . 
     Node  30  has one unique wireless address and three unique field device addresses. It includes radio  52 , WDR  54 , and field devices FD 7 , FD 8 , and FD 9 . 
     Wireless network  16  is preferably a low power network in which many of the nodes are powered by long life batteries or low power energy scavenging power sources. Communication over wireless network  16  may be provided according to a mesh network configuration, in which messages are transmitted from node-to-node through network  16 . This allows the use of lower power RF radios, while allowing network  16  to span a significant physical area to deliver messages from one end of the network to the other. 
     In a low power wireless network that includes field devices, power can be conserved by placing the entire network and the field devices into a low power (Off or asleep) state. The network switches to a high power (On or active) state so that the host computer can interact with field devices. For example, a global duty cycle for the wireless network can be established that defines when all nodes are turned On to receive and transmit messages. 
     When the wireless network is activated, however, it is wasteful to activate all field devices if only a subset of the field devices is going to be utilized during that On or active period of the wireless network. Power used to activate field devices that will not be involved in communication wastes energy available at the nodes, which can affect the battery life. 
     In addition, if only a limited number of field devices will be involved in communication, at least some of the nodes of the wireless network will not be needed, since they are not in likely communication paths through the wireless network between the field devices and the host computer. Maintaining the radio On to receive messages, when none will be received, wastes energy and affects battery life. 
     Control system  10  can micro-manage turning On and turning Off of field devices and turning On and turning Off of wireless nodes, so that only those nodes and field devices necessary for communication taking place need to remain at full power. At the same time, control system  10  can ensure that those field devices and nodes that are required to be at full power remain in the On state while the desired communication with host computer  12  takes place. 
     In control system  10 , there are circumstances when host computer  12  may need to communicate for an extended period of time with a particular field device. For example, at start up of control system  10 , host computer  12  may do discovery, to detect the presence of each field device and to obtain all stored parameters and configuration data from each field device. During this process, multiple messages will be sent between host computer  12  and each individual field device FD 1 -FD 9 . Another example is when host computer  12  needs to configure one of the field devices FD 1 -FD 9 . The amount of configuration data that needs to transferred results in multiple messages between host computer  12  and the particular field device being configured. 
     In either of these cases, it would be inefficient to turn On all of the field devices FD 1 -FD 9  when wireless network  16  turns On, when only one field device may be involved in the communication. Control system  10  addresses this issue by maintaining all of the field devices in an asleep or Off state until a control message is received from host computer  12  addressed to the particular field device. At that time, power is provided by the wireless device router (WDR) at that node to the addressed field device. For example, in response to receiving the control message from host computer  12  addressed to field device FD 3 , WDR  38  of node  22  turns On power to field device FD 3 . 
     In the case of wireless nodes having more than one field device, turning On one of the field devices may require that all of the field devices at that node be turned On. For example, if field devices FD 1  and FD 2  at node  20  share a common power and communication bus with WDR  34 , both field devices FD 1  and FD 2  will turn On when power is applied to the bus. 
     Once a field device has been powered On, it is desirable to keep that device in a full power state until host computer  12  is done communicating with that field device. Even if wireless network  16  is cycling On and Off according to a scheduled duty cycle, it is desirable to maintain the field device that is communicating with host computer  12  in a full powered state as long as active communication is continuing. Depending upon the type of field device, it may take only a few seconds to as many as 60 seconds for the field device to reach a full powered state in response to a control message from host computer  12 . 
     When a control message is received from host computer  12  requiring that the addressed field device be turned On, the control message can include a command to maintain the field device in a full powered On state for a particular period of time specified by host computer  12  as being necessary to complete the intended communication. Alternatively, the command that the field device be maintained in the On state until interaction with host computer  12  has halted. This can be determined by the wireless device router associated with the field device, which receives the control messages from host computer  12  and routes them to the field device, and also receives responses from the field device that are sent back to host computer  12 . When a period of message inactivity has occurred, the wireless device router automatically turns Off the field device. 
     By individually controlling the power state of individual field devices FD 1 -FD 9 , control system  10  reduces overall power consumption of wireless network  16 , and in particular power consumption at individual nodes  20 - 30  of network  16 . By returning the field device to a low power state only after communication with host computer  12  has halted, responsiveness between control computer  12  and the particular field device is enhanced. Undesirable transitions of the field device between full power (On) and low power (Off) states are avoided. 
     Another way in which power can be conserved at nodes  20 - 30  of wireless network  16  is by allowing those nodes that will not be participating in communication to go into a low power (Off) state while those nodes that are actively participating in communication remain in an extended high power (On) state so that host computer  12  can complete its communication with a selected field device. 
     In a wireless mesh network, messages typically travel from node to node. Alternate, redundant paths for wireless messages will typically exist. When a message is directed to a particular field device within wireless mesh network  16 , several nodes may be involved in receiving and transmitting the message on to the ultimate destination. For example, consider a message intended for field device FD 7  at node  30 . The path of the wireless message to node  30  may pass from gateway  18  through nodes  20  and  22  to node  30 . Alternatively, the message may pass through node  26  to node  30 , or through nodes  24  and  28  to node  30 . A similar return path may exist for the response message from field device FD 7  that is sent from node  30  to gateway  18  and then to host computer  12 . If the communication between host computer  12  and field device FD 7  takes place on a path from gateway  18  through node  26  to node  30 , and back along that same path, then the other nodes  20 ,  22 ,  24 , and  28  are not needed as long as the communication will only involve host computer  12  and field device FD 7 . 
     Gateway  18  receives the messages that host computer  12  wants sent over wireless network  16 . When a high power (On) state of wireless network  16  occurs, gateway  18  can send a message to each node that will be involved in receiving and transmitting the messages from host computer  12  and instruct those nodes to remain On for a specified period of time, or until the communication ends. Gateway  18  can identify the nodes that will be involved by maintaining information on signal routing paths within network  16 . Gateway  18  can periodically interrogate each node to determine the links that node has established with neighboring nodes to transmit and receive messages. Based upon that information, the likely path or paths of the messages from host computer  12  can be identified by gateway  18 , and used to provide instructions to the required nodes. Those nodes that do not receive a message from gateway  18  instructing them to stay On will automatically turn Off at the end of the normal high power (On) state in the communication duty cycle. The remaining devices, which have been instructed to remain On, will remain in a high power (On) state as long as host computer  12  is continuing to communicate with at least one field device. 
     Alternatively, gateway  18  can provide messages to each of the nodes that will not be actively involved in planned communications instructing those nodes to turn Off. Any node that does not receive an instruction to turn Off will remain On. This approach, however, can result in a node remaining On, even though it is not involved in communication, simply because it did not receive the message to turn Off. 
     Another way to way to manage which nodes remain On and which turn Off requires that any device that has received and transmitted a message during the normal high power (On) portion of the communication duty cycle to remain On until it either receives a message from gateway  18  instructing it to turn Off, or until a period of time has elapsed without any further message being received or transmitted by that node. In this way, network  16  dynamically configures itself to maintain On the nodes that are necessary to maintain so that messages can be routed to and from target field devices. Those nodes that are not involved will automatically turn Off at the end of the high power (On) portion of the duty cycle. 
     Allowing the communication to continue with an extended On state involving only those nodes actively involved in communication means latency can be reduced and communication improved, without permanently causing wireless network  16  to remain in a On state. When communication ceases, the nodes that have been involved in the extended On state will be resynchronized with the normal Off/On communication duty cycle of wireless network  16 . 
     In a wired control system, interaction between the host computer and the field devices occurs using well known control messages according to a control message protocol such as HART, Fieldbus, Profibus, or the like. Field devices capable of use in wired control systems (such as field devices FD 1 -FD 9  shown in  FIG. 1 ) make use of control messages according to one of the known control message protocols. Wireless nodes  20 - 30 , which are part of wireless network  16 , cannot directly exchange these well known control messages with host computer  12  because the wireless communication over network  16  occurs according to a wireless protocol that is general purpose in nature. 
     Rather than require host computer  12  and field devices FD 1 -FD 9  to communicate using wireless protocol, a method can be provided to allow sending and receiving well known field device control messages between host computer  12  and field devices FD 1 -FD 9  over wireless network  16 . The well known field device control messages are embedded into the general purpose wireless protocol so that the control messages can be exchanged between host computer  12  and field devices FD 1 -FD 9  to achieve control of an interaction with field devices FD 1 -FD 9 . As a result, wireless network  16  and its wireless communication protocol is essentially transparent to host computer  12  and field devices FD 1 -FD 9 . In the following description, the HART protocol will be used as an example of a known control message protocol, although the invention is applicable to other control message protocols (e.g. Foundation Fieldbus, Profibus, etc.) as well. 
     A similar issue relates to the addresses used by host computer  12  to direct messages to field devices FD 1 -FD 9 . In wired systems, the host computer addresses each field device with a unique field device address. The address is defined as part of the particular communication protocol being used, and typically forms a part of control messages sent by the host computer to the field devices. 
     When a wireless network, such as network  16  shown in  FIG. 1  is used to route messages from the host computer to field devices, the field device addresses used by the host computer are not compatible with the wireless addresses used by the communication protocol of the wireless network. In addition, there can be multiple field devices associated with a single wireless node, as illustrated by wireless nodes  20  and  30  in  FIG. 1 . Wireless node  20  includes two field devices, FD 1  and FD 2 , while wireless node  30  is associated with three field devices, FD 7 -FD 9 . 
     One way to deal with addresses is to require host computer  12  to use wireless addresses rather than field device addresses. This approach, however, requires host computer  12  to be programmed differently depending upon whether it is communicating over wired communication links with field devices, or whether it is communicating at least in part over a wireless network. In addition, there remains the issue of multiple field devices, which will typically have different purposes, and which need to be addressed individually. 
     An alternative approach uses gateway  18  to translate field device addresses provided by host computer  16  into corresponding wireless addresses. A wireless message is sent to the wireless address, and also includes a field device address so that the node receiving the message can direct the message to the appropriate field device. By translating field device addressees to corresponding wireless addresses, host computer  12  can function in its native field address domain when interacting with field devices. The presence of wireless network  16  is transparent to host computer  12  and field devices FD 1 -FD 9 . 
     Still another issue caused by the use of wireless network  16  to communicate between host computer  12  and field devices FD 1 -FD 9  is the unavailability of field devices because of power conservation. In a wired control system, the host computer interacts with field devices as if they were available on demand. The assumption is that the field devices are always powered up and available. 
     In a low power wireless network, this is not the case. To conserve power, field devices in a low power wireless network are unavailable, or asleep, most of the time. Periodically, the wireless network goes into an active state during which messages can be communicated to and from the field devices. After a period of time, the wireless network again goes into a low power sleep state. 
     If the host computer attempts to communicate during a period when the wireless network is in a sleep state, or when a particular field device is in a low power sleep state, the failure of the field device to respond immediately can be interpreted by the host computer as a communication failure. The host computer does not determine the particular route that messages take through the wireless network, and does not control the power up and power down cycles for wireless communication. As a result, the host computer can interpret a lack of response of field devices as a device failure, when the lack of response is an inherent result of the way that communication takes place within a low power wireless network. 
     In order to make the presence of wireless network  16  transparent to host computer  12 , gateway  18  decouples transmission of field device messages between host computer  12  and wireless network  16 . Gateway  18  determines the current state of wireless network  16  and tracks its power cycles. In addition, it maintains information on the response times required for a field device to be turned on and then be ready to provide a response message to a control message from host computer  12 . 
     When a message is provided by host computer  12  to gateway  18 , a determination of an expected response time is made based upon the field device address. That expected response time is provided to host computer  12 , so that host computer  12  will not treat the absence of a response message prior to the expected response time elapsing as a communication failure. As a result, host computer  12  is allowed to treat field devices FD 1 -FD 9  as if they were available on demand, when in fact wireless network  16  and field devices FD 1 -FD 9  are not available on demand. 
       FIG. 2  shows a block diagram of a portion of the control system  10  shown in  FIG. 1 .  FIG. 2 , host computer  12 , high-speed network  14 , gateway  18 , and wireless node  22  are shown. 
     In  FIG. 2 , host computer  12  is a distributed control system host running application programs to facilitate sending messages to field devices FD 1 -FD 9 , and receiving and analyzing data contained in messages from field devices FD 1 -FD 9 . Host computer  12  may use, for example, AMS™ Device Manager as an application program to allow users to monitor and interact with field devices FD 1 -FD 9 . 
     Host computer  12  communicates with gateway  18  using messages in extendable markup language (XML) format. Control messages intended for field devices FD 1 -FD 9  are presented according to the HART protocol, and are communicated to gateway  18  in XML format. 
     In the embodiment shown in  FIG. 2 , gateway  18  includes gateway interface  60 , mesh manager  62 , and radio  64 . Gateway interface  60  receives the XML document from host computer  12 , extracts the HART control message, and modifies the control message into a format to be embedded in a wireless message that will be transmitted over wireless network  16 . 
     Mesh manager  62  forms the wireless message with the HART control message embedded, and with the wireless address of the node corresponding to the field device to which the HART message is directed. Mesh manager  62  may be maintaining, for example, a lookup table that correlates each field device address with the wireless address of the node at which the field device corresponding to that field device address is located. In this example, the field device of interest is device FD 3  located at wireless node  22 . The wireless message according to the wireless protocol includes the wireless node address, which is used to route the wireless message through network  16 . The field device address is contained in the HART message embedded within the wireless message, and is not used for routing the wireless message through network  16 . Instead, the field device address is used once the wireless message has reached the intended node. 
     Mesh manager  62  causes radio  64  to transmit the wireless message, so that it will be transmitted by one or multiple hops within network  16  to node  22 . For example, the message to node  22  may be transmitted from gateway  18  to node  20  and then to node  22 , or alternatively from gateway  18  to node  26  and then to node  22 . Other routes are also possible in network  16 . 
     Gateway interface  60  and mesh manager  62  also interact with host computer  12  to manage the delivery of control messages to field devices as if wireless network  16  were powered on even though it may be powered Off (i.e. sleep mode). Mesh manager  60  determines the correct powered state of wireless network  16 . It also calculates the time of the power cycles in order to determine the future time when wireless network  16  will change state from power On to Off, or from power Off to On. Response time can be affected if a message is sent while power is on to the wireless network, but a response will not occur until the next power on cycle. Still another factor is the start-up time of the field device. Mesh manager  62  or gateway interface  60  may maintain a data base with start-up times for the various field devices. By knowing field device address, an expected start-up time can be determined. 
     Based upon the current power state of wireless network  16 , the amount of time before wireless network will change state, the field device&#39;s start-up time, expected network message routing time, and the potential for a response to occur in the next power on cycle rather than the current cycle, estimated times required for the message to be delivered to the field device and for the response message to return to gateway  18  can be calculated. That information can then be provided to host computer  12 . Since host computer  12  will not expect a response prior to the estimated response time, the failure to receive a message prior to that time will not be treated by host computer  12  as a communication failure or field device failure. 
     Based upon the factors affecting response time, gateway  18  may also determine the best strategy to attempt communication with the field device given the known power cycle of wireless network  16 . For example, if a power cycle is about to change from On to Off, a better strategy may be to wait until the beginning of the next power on cycle to begin routing the message through wireless network  16 . 
     As shown in  FIG. 2 , wireless node  22  includes radio  36 , wireless device router (WDR)  38 , and field device FD 3 . In this particular example, field device FD 3  is a standard HART field device, which communicates field data using the HART control message protocol. Field device FD 3  is powered On and Off by, and communicates directly with, WDR  38 . 
     The wireless message transmitted over network  16  is received at radio  36  of wireless node  22 . The wireless message is checked by WDR  38  to see whether it is addressed to node  22 . Since node  22  is the destination address, the wireless message is opened, and the embedded HART message is extracted. WDR  38  determines that the HART message is intended for field device FD 3  based upon the field device address contained in the embedded HART message. 
     For power saving reasons, WDR  38  may be maintaining field device FD 3  in sleep mode until some action is required. Upon receiving the HART message contained within the wireless message, WDR  38  takes steps to start up field device FD 3 . This may be a matter of only a few seconds, or may be, for example, a delay on the order of 30 to 60 seconds. When field device FD 3  is ready to receive the HART message and act upon it, WDR  38  transmits the HART control message to field device FD 3 . 
     The message received by field device FD 3  may require providing a message in response that includes measurement data or other status information. Field device FD 3  takes the necessary action to gather the measurement data or generate the status information, generates a response message in the HART control format, and transmits the message to WDR  38 . The HART response message is then modified and embedded into a wireless response message according to the wireless protocol, and addressed to gateway  18 . WDR  38  provides the wireless response message to radio  36  for transmission onto wireless network  16 . The wireless response message is then transmitted in one or multiple hops to gateway  18 , where the HART response message is extracted from the wireless response message, is formatted in XML, and is transmitted over high-speed network  14  to host computer  12 . 
       FIG. 3  shows a diagram of a typical wireless message sent over the wireless network shown in  FIGS. 1 and 2 . Wireless message  70  includes wireless protocol bits  72 , payload  74 , and wireless protocol bits  76 . Protocol bits  72  and  76  are required for proper routing of wireless message  70  through mesh network  16  to the desired destination. Payload  74  represents the substance of the control message being transmitted. In the present invention, the control message (in the control message protocol used by both host computer  12  and field devices FD 1 -FD 9 ) is embedded within wireless message  70  as payload  74 . 
       FIG. 4  shows the format of control message  80  as generated by host computer  12 . In this particular example, control message  80  is configured using the HART protocol. Control message  80  includes preamble  82 , delimiter  84 , field device address  86 , command  88 , byte count  90 , data  92 , and check byte  94 . Control message  80  is modified at gateway interface  60  and then embedded into wireless message  70  as payload  74 . 
       FIG. 5  shows the format of payload  74  formed from control message  80 . To produce payload  74 , interface  60  removes physical layer overhead from control message  80  and adds sequence information. 
     As shown by a comparison of  FIGS. 4 and 5 , the first difference between payload  74  and control message  80  is that preamble  82  has been removed. Since the control message will be sent over the network using the wireless protocol, the use of a preamble is unnecessary. Removal of preamble  82  improves efficiency of network  16  by eliminating unnecessary information. 
     The second difference between payload  74  and control message  80  is the addition of message ID  96 , which is a two-byte number that follows data  92 , and precedes check byte  94 . The removal of preamble  82  and the addition of message ID  96  also requires that check byte  94  be recalculated. 
     The purpose of message ID  96  is for stale message rejection. This allows the receiver of a message to reject out of order messages. Wireless mesh network  16  is designed such that messages can take multiple paths to get to their destination. The message is passed from one node to another, and it is possible that the message may be delayed at a particular node. This could be caused by interference or poor signal quality. If a message is delayed long enough, host  12  may issue a retry and/or a new message. In that case, it is possible that one or more messages may arrive at the destination node before the delayed message is delivered. When the delayed control message is delivered, message ID  96  can be used to accept or reject the control message. 
       FIG. 6  shows a second embodiment of the format of payload  74 , in which trailer function code  98  and trailer payload (or message ID)  96  form trailer frame  100 , which is appended to the control message formed by delimiter  84 , field device address  86 , command  88 , byte count  90 , data  92  and check byte  94 . Trailer  100  is not included in check byte  94 , and instead depends on the wireless network protocol layers for data integrity and reliability. 
     Trailer  100  contains function code  98  and payload  96  (which includes the message ID, if any). Function code  98  is an unsigned byte which defines the content of trailer  100 . Undefined payload bytes such as additional padding bytes will be ignored. Use of trailer  100  only applies to messages between gateway  18  and wireless field devices FD 1 -FD 9 . Table 1 shows an example of function codes defined for trailer  100 : 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Function 
                   
                 Payload Length and 
               
               
                 Code 
                 Meaning 
                 Description 
               
               
                   
               
             
             
               
                 0 
                 No Message ID 
                 0-2 bytes (optional padding) 
               
               
                 1 
                 Force Accept 
                 2 bytes - message ID 
               
               
                 2 
                 Clear Force Accept 
                 2 bytes - message ID 
               
               
                   
                 With Force 
                   
               
               
                 3 
                 Normal Message ID 
                 2 bytes - message ID 
               
               
                   
               
             
          
         
       
     
     Function codes 0-3 are used with reference to a message ID. Message IDs are used for stale message rejection on wireless mesh network  16 . This allows the receiver of a message to reject out of order messages. Additionally, message IDs can be used by gateway  18  to determine whether published data has arrived out of order. 
     Rules for generating the Message ID are as follows: 
     The message ID enumerates a message sequence from a sender to a receiver. It is a two byte unsigned value which must be unique and increasing by one with each new message ID. 
     A new message ID should be generated for every request/response transaction. Retries of a request from a sender to a receiver may re-use a message ID provided that there is no more than one request outstanding from a sender to a receiver. After receiving a valid request message with a valid message ID, the field device must echo back the received message ID with the response. 
     A new message ID should be generated for every publish message from a device. Publish message IDs are generated independently of request/response message IDs. 
     Rules for validating the Message ID are as follows: 
     The receiver must implement a window for validating message IDs so that the validity comparison survives a rollover of the message ID counter. As an example, any messages within a window of 256 previous IDs could be ignored as out of order by the WDR/field device. But, if message ID is safely outside the window the receiver should accept the message. Any accepted message will cause the message ID to be cached as the last valid received message ID. 
     After a restart, a receiver may accept the first message ID it receives or else it must initialize its validity-checking in whatever manner the device application sees fit. A guideline for this initialization would be for a device to always accept new stateless requests without requiring a device publish to first reach the gateway. 
     The receiver of a published message with an invalid (out of order) ID may either use or reject the message, depending on the receiver&#39;s application. 
     Rules for interpreting function codes are as follows: 
     A sender can send a message without a message ID by either omitting trailer  100  or by specifying NO MESSAGE ID as the function code. If a response is generated and the WDR/field device supports trailers, the return function code should be set to “NO MESSAGE ID”. 
     If a message ID is provided, it must be accepted if the function code is set to FORCE ACCEPT or CLEAR FORCE ACCEPT WITH FORCE. A message with a function code of NORMAL ID will be subject to potential discard via the message ID validation rules. 
     If gateway  18  has reset, it should make its first request using the FORCE ACCEPT function code. The will force the receiving field device to accept the request and the attached message ID. This relieves gateway  18  of needing to learn the value of the device&#39;s valid message ID counter. Gateway  18  should stop using FORCE ACCEPT once it has received a valid response message with the matching message ID. 
     Gateway  18  should honor the CLEAR FORCE ACCEPT WITH FORCE function code as a valid message ID, but a WDR/field device should not send CLEAR FORCE ACCEPT WITH FORCE to gateway  18 . 
     If a WDR/field device in the system has reset, it should send publish messages with the command set to FORCE ACCEPT. This will force gateway  18  to accept the published data. 
     If gateway  18  sees the FORCE ACCEPT function code, it may issue a CLEAR FORCE ACCEPT WITH FORCE in a subsequent message along with a valid message ID. 
     On receipt of CLEAR FORCE ACCEPT WITH FORCE, the WDR/field device should clear the force accept condition and always accept the message ID provided. 
     The use of embedded control messages (in a control message protocol) within wireless messages (in a wireless protocol) enables the host computer of a distributed control system to interact with field devices through a wireless communication network. Control messages can be exchanged between the host computer and the field devices using known control message formats, such as HART, Fieldbus, or the like, without having to be modified by either the host computer or the field devices to accommodate transmission of the control messages over the wireless network. The control message is embedded within the wireless communication protocol such that the substance of the control message exchanged between the host computer and the field device is unmodified as a result of having passed through the wireless network. 
     Control messages that are too large to be routed through the wireless communication protocol can be broken into parts and sent as multiple parts. Each part is embedded in a wireless message, and the multiple parts can be reassembled into the original control message as the multiple parts exit the wireless network. By use of a message ID in the embedded control message, the multiple parts can be reassembled in proper order, even though individual wireless messages having embedded parts of the original control message may take different paths through the wireless network. 
     The translation of field device addresses to corresponding wireless addresses allows host  12  to function in its native field device address domain, while interacting with field devices within the wireless address domain. The use of wireless network  16  to route messages to and from the field devices is transparent to host  12 . The address translation and inclusion of both the wireless address and the field device address in the wireless message allows multiple field devices associated with a single node (i.e. a single wireless address) to be addressed individually. 
     Although embedding the field device address in the payload of the wireless message as part of the control message is simple and effective, the field device address could be contained separately in the payload or elsewhere in the wireless message, if desired. 
     The presence of wireless network  16  is also made transparent to host computer  12  by decoupling the transmission of messages to field devices between host computer  12  and wireless network  16 . Gateway  18  monitors the state of wireless network  16 , and factors that can affect the response time to a message. By providing an estimated response time to messages being sent by host computer  12 , gateway  18  allows host computer  12  to treat what field devices FD 1 -FD 9  and wireless network  16  as if they were available on demand, even though network  16  and field devices FD 1 -FD 9  are often in a low power sleep state. 
     By micro-managing the On/Off status of individual field devices and individual nodes, only those field devices and nodes that are required for a particular communication with the host remain On until the communication is complete. This reduces power consumption by nodes and field devices that are not involved in the communication, and makes the communication with the host more efficient since the nodes and field devices do not cycle On and Off in the midst of the communication with the host. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, control system  10  is illustrated with six nodes and nine field devices, but other configurations with fewer or greater numbers of nodes and field devices are equally applicable.