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A Study of Process Data Transmission Scheduling in Wireless
Mesh Networks
Jianping Song, Song Han
Aloysius K. Mok
Dept. of Computer Sciences
University of Texas at Austin
Austin, TX 78712
Deji Chen, Mark Nixon
Mike Lucas
Emerson Process Management
12301 Research Blvd. Building III
Austin, TX 78759
Wireless mesh network, Scheduling, WirelessHART
The combination of technology advances in wireless technology, low power microprocessors,
integrated circuits, measurement and control techniques, and energy storage has enabled the
development of low-cost, low-power, devices. Networking wireless devices together into a sensor
network that is able to meet the reliability, latency, real-time, and performance requirements of process
automation applications, while at the same time taking into consideration the practical limits on
available resources, is a challenging task. Missing or delay of the process data by the network can
severely degrade overall control performance. Wireless signal strength varies in time; wireless nodes
come and go; data is routed dynamically on different paths; and batteries have a finite amount of
energy. To meet these demands network management techniques must be able to form the overall
network topology using information reported by the devices and the network itself and then schedule
and optimize the overall network for latency, power, and throughput. We present an approach for
forming the mesh network, allocating network resources, scheduling, and grooming the network. We
base this approach on the emerging WirelessHART™ standard. WirelessHART is a secure networking
technology operating in the 2.4GHz ISM radio band. WirelessHART utilizes IEEE 802.15.4
compatible DSSS radios with channel hopping on a packet by packet basis. WirelessHART
communication is arbitrated using the WirelessHART Network to schedule link activity. A given
WirelessHART Network communication slot may be dedicated to communication between a network
pair or a slot may support Slotted-Aloha shared communication access.
Distributed with permission of author(s) by ISA 2007
Presented at ISA EXPO Technical Conference;
The combination of technology advances in wireless technology, low power microprocessors,
integrated circuits, measurement and control techniques, and energy storage has enabled the
development of low-cost, low-power, devices. Networking these devices through wireless networks
allows for monitoring, control, and maintenance through multi-hop routing, frequency hopping, and
time and path diversity.
Managing latency while at the same time conserving power is the dominant problem facing wireless
sensor networks today. To minimize latency communications need to be organized so that packets are
not delayed en-route from source to destination. To minimize the energy usage devices should be kept
in a low-power mode as much as possible – this state is referred to as sleep-mode in this paper.
Deployments must be easy to set up; and once deployed devices must run unattended for long periods
of time, usually many years.
At the communication distances typical in sensor networks listening for information on the radio
channel costs about the same as data transmission. Furthermore, the energy cost for a device in idle
mode is approximately the same as for a device that is in receive mode. Thus, the biggest single action
to save power is to turn the radio and the device itself off during idle times, i.e., put the device to sleep.
Turning the device off implies advanced knowledge about when the device will be idle. The approach
taken by WirelessHART is to configure the device with knowledge about when it should wake up,
perform some function, and go back to sleep – this configuration is performed by the network manager
and is called scheduling.
In WirelessHART communications are precisely scheduled using an approach referred to as Time
Division Multiple Access (TDMA). The vast majority of communications are directed along graph
routes. Scheduling is performed by a centralized network manager which uses overall network routing
information in combination with communication requirements that devices and applications have
provided. The schedule is subdivided into slots and transferred from the network manager to individual
devices; devices are only provided with the slots for which they have transmit or receive requirements.
The network manager continuously adapts the overall network graph and network schedule to changes
in network topology and communication demand.
The key ideas behind this paper are described in the write-up on the WirelessHART™ Network
Manager. The Network Manager communicates Network Management Commands through the
Network Layer to the network of WirelessHART Devices. The scope of the Network Manager is
illustrated below in Figure 1.
The Network Manager initializes and maintains network communication parameter values. The
Network Manager provides mechanisms for devices joining and leaving the network. It is also
responsible for managing dedicated and shared network resources. The network layer provides an
interface through which network management functions can be invoked. The Network Manager is also
responsible for collecting and maintaining diagnostics about the overall health of the network and
reporting to host-based applications.
Distributed with permission of author(s) by ISA 2007
Presented at ISA EXPO Technical Conference;
This paper describes the approach taken by WirelessHART to manage the overall network topology
and perform scheduling. The discussion begins with an overview of the WirelessHART Architecture,
presents details on routing and scheduling. While WirelessHART standard specifies the rules for
network management, it does not spell out how to create route graph, generate network schedule, etc.,
so that the mesh network can support the latency, throughput, and other requirements of the application
running on top of it. We spend the rest of the paper detailing one of our approaches. We give an
example that illustrates how routing and scheduling can be performed.
Command, Data, &
Block Transfers
Adds HART Specific Commands or
Network Management Information
Application Layer
Network Layer
Adds Routing Information
Network Layer
Logical Link Control
Media Access Control
Adds Addressing and
Frame Information
Data Link Layer/
MAC Layer
(Physical Layer Services)
Adds Preambles for
Transceiver Synchronization
Physical Link
(RF Channel)
Figure 1. Network Management Scope
Many of the desirable features of the WirelessHART Network such as self-healing, self-organization,
and redundant routing are achieved through the establishment and updating of a network
communication schedule. The Network Manager is responsible for the creation of this schedule and the
associated connections. It is also responsible for the distribution of this schedule to the individual
network devices. This scheduling function may be broken into the following phases:
1. Support devices joining the network. As part of this the Network Manager is responsible for
authenticating and orchestrating the join process.
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2. Establishment of routes. As part of this the Network Manager is responsible for the creation of
routes that can be used by plant automation hosts, gateways, other devices, and the Network
Manager itself to perform communications with the application layer in network devices.
3. Schedule communications. As part of this the Network Manager is responsible for the
establishment of Superframes and Slots that the user layer application of a network device may
use to transfer process data, alerts, diagnostics and other traffic to the gateway for access by the
plant automation host. The Superframes also include slots for network management and the join
4. Scheduling control functions. For network devices that are actuators, interlocks, or any device
that affects the process, the Network Manager is responsible for the establishments of Routes,
Superframes, and Slots that the plant automation host may use to send setpoints and outputs to
the user layer application in field devices.
5. Adapting the network. The Network Manager will continually adapt the network. The Network
Manager continually collects data from devices on the health of connections and traffic patterns
and uses this information to adjust routing and scheduling.
The effectiveness of the overall network ultimately boils down to a combination of routing and
scheduling. The services provided in the protocol stack allow network communications to be
established in many ways.
The WirelessHART Network supports a wide variety of devices from many manufactures. To the
Wireless HART Network these devices can be classified as one of a small number of network types.
Figure 2 illustrates the basic network device types.
A Network Device is any device directly connected to the WirelessHART Network. A Network Device
transmits and receives WirelessHART packets and performs the basic functions necessary to support
network formation and maintenance. Network device types include Field Devices, Router Devices,
Gateway Devices, Adapters, and Handheld Devices. All Network Devices must support routing.
A Field Device is connected to and characterizes or controls the Process. They are a producer and
consumer of WirelessHART packets and must be capable of routing packets on behalf of other
Network Devices.
A Router Device is a Network Device that forwards packets from one Network Device to another.
A Gateway Device is an access point that connects the WirelessHART Network to a plant automation
network, allowing data to flow between the two networks. The Gateway Device provides host
applications access to the Network Devices. A Gateway Device can be used to convert from one
protocol to another, as go-between two or more networks that use the same protocol, or to convert
commands and data from one format to another.
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A Handheld Device is used in the installation, control, monitoring, and maintenance of Network
Devices. Handheld Devices are portable equipment operated by the plant personnel.
Plant Automation
Application Host
Plant Automation
Figure 2. Example of a WirelessHART Network
The core network functions are shown in Figure 3 below.
In the model shown in Figure 3, the Network Manager contains one overall Network Schedule. This
schedule is further broken down into Superframes and time slots – time slots are associated with links.
The Network Manager also contains a list of all devices in the network. It also contains the overall
network topology including a complete graph of the network and portions of the graph that have been
installed into each device. The Network Manager generates route and connection information using
information that it receives from the Network Devices. The Graph of the network is built from the list
of Network Devices, their reported Neighbors, and a prioritization exercise.
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The Network Manager is also responsible for generating and maintaining all of the route information
for the network. The Network Manager uses this route information to generate a complete graph
leading from each network device back to the Network Manager. There may also be special purpose
routes which are used to send commands and other settings from the Gateways to Network Devices.
Finally, there are broadcast routes which are used to send broadcast messages from the Network
Manager through the Gateways to all of the Network Devices.
Security Manager
Host Application
Network Manager
-TopologyType {Mesh, Star}
-Session Keys
-Join Keys
-Network Key
Network Schedule
-Destination Address
Connection Table
Network Device
Figure 3. Network Manager
The Network Manager is responsible for adapting the network to changing conditions and for
scheduling communication resources. As devices join and leave the network, the Network Manager
updates its internal model of the WirelessHART Network and uses this information to generate the
schedule and routes. Network performance and diagnostic information is also used by the Network
Manager to adapt the overall network to changes in topology and communication requirements. Once
the overall schedule has been generated, the schedule is transferred through a series of commands from
the Network Manager to the Network Devices.
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The Security Manager works with the Network Manager to secure the WirelessHART Network from
adversarial threats to its operation. The Security Manager generates and manages the cryptographic
material used by the network. It is responsible for the generation, storage, and management of keys.
As part of its system functions, the Network Manager collects network performance and diagnostic
information. This information is accessible during run-time making it possible to view and analyze the
behavior of the overall network. If problems are detected, reconfiguration of the network is performed
while the network is operating. Network diagnostic information can be accessed through HART
The WirelessHART Network maintains very high reliability through the use of several mechanisms
including multiple paths to network devices, multiple RF channels, and multiple communication tries.
If improved reliability is required, more paths can be inserted by adding additional gateways and field
devices. Additional devices improve path diversity. Additional gateway access points, and devices in
general, increase throughput, reduce latency, and can be used to route around potential interferers.
All communication on the WirelessHART Network is time-synchronized. The basic unit of measure is
a time slot which is a unit of fixed time duration commonly shared by all Network Devices in a
network. The duration of a time slot is sufficient to send or receive one packet per channel and an
accompanying acknowledgement, including guard-band times for network-wide synchronization. The
per-channel qualification indicates that more than one communication can occur in the same time slot.
Precise time synchronization is critical to the operation of networks based on time division
multiplexing. Since all communication happens in time slots, the Network Devices must have the same
notion of when each time slot begins and ends, with minimal variation. The WirelessHART protocol
defines mechanisms for time synchronization. In a WirelessHART Network, time propagates outwards
from the Gateway.
The Network Managers’ two most important functions are to setup and manage all routes used
throughout the WirelessHART Network and to allocate communication resources. The allocation of
communication resources is referred to as scheduling. The key components of the Network Manager
are the Network Schedule, the collection of Network Devices, the collection of Neighbor Tables, the
collection of Connection Tables, and the collection of Routes. The Network Manager also maintains
associations to the Security Manager and Host Applications. The Network Manager and Security
Manager are shown in Figure 3.
There are two methods of routing packets in a WirelessHART Network—graph routing and source
routing. When using graph routing, a Network Device sends packets with a Graph ID in the network
layer header along a set of paths to the destination. All Network Devices on the way to the destination
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must be pre-configured with graph information that specifies the neighbors to which the packets may
be forwarded. In a properly configured network, all devices will have at least two devices in the Graph
through which they may send packets.
With source routing, pre-configuration of the forwarding devices is not necessary. To send a packet to
its destination, the source Network Device includes in the network layer header an ordered list of
devices through which the packet must travel. As the packet is routed, each routing device utilizes the
next Network Device address from the packet to determine the next hop to use. Since packets may go
to a destination without explicit setup of intermediate devices, source routing requires knowledge of
the network topology.
The Network Manager contains a complete list of Routes, Connections, and Network Devices. When
devices are initially added to the network, the Network Manager stores all Neighbor entries including
signal strength information as reported from each Network Device. The Network Manager uses this
information to build a complete Network Graph – the Network Graph is a not a complete map of the
network – a large number of possible (but suboptimal) links have been removed. The Network Graph is
put together optimizing several properties including reliability, hop count, reporting rates, power usage,
and overall traffic flow. A key part of the topology is the list of Connections that connect devices
Every graph in a network is associated with a unique Graph Id. To send a packet on a graph, the source
Network Device includes a Graph Id in the packet’s network header. The packet travels along the paths
corresponding to the Graph Id until it reaches its destination, or is discarded. In order to be able to
route packets along a graph, a device needs to be configured with a Connection table. The Connection
table contains entries that include the Graph Id and neighbor address. Redundant paths may be setup by
having more than one neighbor associated with the same Graph Id. Using Graph Routing, a device
routing a packet must perform a lookup in the connection table by Graph Id, and send the packet to any
of the listed neighbors. Once any neighbor acknowledges receipt of the packet (MAC level
acknowledgement), the routing device may release it and remove the packet from its transmit buffer. If
an acknowledgement is not received, the device will attempt to retransmit the packet at its next
available opportunity.
Managing latency while at the same time conserving power is the dominant problem facing wireless
sensor networks today. To minimize latency communications need to be organized so that packets are
not delayed en-route from source to destination. To minimize the energy usage devices should be
placed into sleep mode as much as possible. Power consumption limits the utility of sensor networks,
which must operate unattended on the order of years. Replacing batteries is a laborious task and
extremely difficult in some environments. Conserving energy is therefore critical for prolonging the
lifetime of the sensor network. Meeting both latency and power consumption requirements can be
addressed through scheduling.
WirelessHART takes the approach that applications and devices specify their communication
requirements and a centralized network manager in-turn allocates communication resources. The
Distributed with permission of author(s) by ISA 2007
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centralized network manager then distributes the schedule to each of the effected devices. When
devices are not scheduled to transmit or receive they can be put into sleep mode.
Each WirelessHART Network contains exactly one overall schedule that is created and managed by the
Network Manager. The schedule is subdivided into Superframes. Each Superframe is further
subdivided into frame relative links that repeat as the Superframe cycles.
Requests for communication resources are directed to the network manager through network
management commands. For example, an application may request a device to schedule a burst mode
communication every second. The device in-turn would make a request to the network manager for the
communication resources that it needs to meet this request. This kind of network-centric approach
allows a sensor network to collect and process data in a very energy-efficient manner.
WirelessHART Scheduling provides adaptive schedules. Advertising for new devices continues after
the initial schedule is built. As new devices arrive, or bandwidth demands change, devices can request
more slots or release some. Services allow the network manager and devices to manage overall
communication bandwidth.
Key points covered by the WirelessHART scheduler include:
1. Scheduling reduces contention and increases fairness. This is one reason why network
scheduling yields such high end-to-end packet reception.
2. WirelessHART scheduling is extremely well suited to data gathering sensor network
applications. The approach is energy efficient and provides substantial power savings over
alternative approaches.
3. Broadcast: A huge practical problem for sensor networks is the need to broadcast. Broadcast
messages are transferred along broadcast routes. Using broadcast routing improves the overall
effectiveness of the network.
4. Time Sync: Time synchronization is critical to WirelessHART for two reasons. First, the entire
network is dependent on a common understanding of an absolute slot number. Second, time
synchronization is vital to sensor networks in general because it is needed to correlate sensor
5. Latency Optimizations: To maximize the efficiency of WirelessHART networks the scheduler
allocates slots in sequence from the source to the destination.
6. Schedule Traffic Flows: With a schedule WirelessHART can reserve bandwidth for the flow
and have fair bandwidth allocation from source to sink.
7. Schedules must be Adaptive: The topology of a sensor network changes over time. This is due
to varying communication demand, changes in connectivity, new devices joining and leaving
the network, mobility, and failures. Schedules need to be adaptive to be robust.
A Superframe is a collection of links assigned to time slots repeating in time. The number of slots in a
given Superframe (Superframe size) determines how often each slot repeats, thus setting a
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communication schedule for devices that use the slots. When a Superframe is created, it is associated
with a Graph Id. The Network Manager uses this association to help it allocate Slots and configure
Links. In runtime the device determines how a Link will be used.
Every new Superframe instance in time is called a Superframe cycle. Figure 4 shows how devices may
communicate in a simple three slot Superframe. Devices A and B communicate during slot 0, devices
B and C communicate during slot 1, and slot 2 is not being used. Every three slots, the link schedule
Cycle N
Cycle N+1
Cycle N+2
Figure 4. Example of a Three-slot Superframe
The size of Superframes should follow a harmonic chain, i.e., all periods should divide into each other.
Examples of harmonic chains are 1, 2, 4, 8, 16, …, and 3, 6, 12, 24, and as well as any other period that
conforms to the expression abn.
A given WirelessHART Network may contain several concurrent Superframes of different sizes. A
Superframe is a product of both channels and time slots. Multiple Superframes may be used to define a
different communication schedule for various groups of devices or to run the entire network at different
duty cycles. Superframe 0 shall be used to schedule all Network Manager’s requests. Additional
Superframes may also be allocated for different communication rates, QoS requirements, alarms &
events, and HART commands issued through host applications.
In this section a simple scheduling example will be presented to illustrate our approach to schedule a
mesh network according to the WirelessHART standard. The example covers the same network shown
in Figure 2 above. The WirelessHART Network consists of 1 Gateway, 6 Field Devices (D), 2 Routers
(R), and 1 Handheld (H). The Gateway is identified as (V) and the Gateway Network Access Point is
identified as (G). The burst mode rate of the devices is:
Burst Mode Rate (seconds)
The topology is shown below in Figure 5. It shows two paths from node V to node R.
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Figure 5. Example Four Network Device WirelessHART Network
The first step that the scheduler goes through is to create the overall network graph. The Network
Manager uses the following rules:
1. If possible the preferred path to the gateway should be a single hop.
2. Route through powered devices if they are available.
3. Try to minimize the number of hops.
4. Use signal strength to select between the best paths to neighbors.
5. Use a combination of weighted signal strengths to select between alternative routes.
6. Try to keep the number of hops to 3 or less.
7. Prune the number of neighbors to 4 or less.
The Network Manager then needs to generate graphs to cover the following:
a) An overall network graph describing paths from each network device to the virtual gateway.
b) A broadcast graph from the virtual gateway downward to each network device.
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c) Graphs from the virtual gateway down to each device.
These graphs are shown for device 3 in the drawing below:
Figure 6. Routing Graphs of the Example
Device 3 is identified by a unique ID. The first information shown for device 3 are its neighbors. The
signal strength for neighbors is shown in the neighbor table. This information is reported by device 3 to
the network manager. The next piece of information shows the upstream graph for device 3 – in this
case device has a single connection to the single gateway network access point. The next piece of
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information shown is the broadcast graph. In this case device 3 broadcasts to 2 other device – devices 5
and 10. The last piece of information expanded in the table above is the downward graph to device 5.
Dividing the overall Network Management task into two major parts is important – the first part allows
the Network Manager to generate the overall network graphs; the second part allows the Network
Manager to generate schedule information.
The strategy that the scheduler follows is summarized below:
1. Maximum number of concurrent active channels is 16.
2. No devices can be scheduled to listen twice in the same slot.
3. More than one device can transmit to the same device (e.g. A broadcast link and dedicated links
to each of the listening devices can coexist.)
4. On a multi-hop path, early hop must be scheduled first
5. The supported scan rates will be defined as 2n where ‘n’ is positive or negative integer values
e.g. scan rate selections of 1 sec, 2 sec, 4 sec, 8sec, 16 sec, and 32 sec.
6. Network Management and Burst Mode communications should not exceed 30% of the
available communication bandwidth (100 slots/sec max).
7. The final schedule (not counting the Gateway spec) should have 50% free slots (i.e. allocated
for retries, listens).
8. Starting from slot 0, the devices are assigned to channel offsets.
9. Allocate slots starting with the fastest to the slowest scan rate.
10. From the furthest end device, allocate one link for each en-route Network Device to the
Gateway. Allocate a 2nd dedicated slot to handle a retry.
11. Each transmission is also scheduled with another path (if one is available).
And some additional rules:
1. The Network Management Superframe has priority over data Superframes.
2. The network management Superframe should be (6000 slots).
3. Advertisement (note, Advertisement slots are slots that devices use to allow devices wishing to
become part of the network to join through).
4. Traverse the graph by breath-first search, starting from the gateway, number the devices as N0,
N1, … Nn
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5. Every device needs to have a slot for a keep-alive message (If a Network Device has not sent a
packet to its parent within this interval, it shall send a KEEP-ALIVE packet). The keep alive
timer is 60 seconds (every 6000 slots). This should be shared receives on the parent side.
6. For join requests, from the furthest devices, allocate one link for each en-route Network Device
to the Gateway (No redundancy provided).
7. For join responses, traverse the graph by breath-first search, allocate one link for each en-route
Network Device from the Gateway to end Network Device.
8. Share the Network Management links with join requests and responses.
As was described above slots are allocated in frames and then combined into an overall network
schedule. The schedule is maintained by the Network Manager and distributed to each device as a set
of slots. The diagram below shows the overall schedule for the network example being used here:
Figure 7 Overall Schedule of the Example
The top portion of the drawing shows the overall slot allocations subdivided into channels. Blacklisting
a channel would remove that channel from the overall schedule. The bottom portion of the schedule
shows the transmit slots and receive slots for each device.
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Over time the network topology will change – for example a barrier could be placed between device 3
and device 5. When this happens the Network Manager will restructure the network – this is illustrated
in the diagram below.
Figure 8. Network Restructuring of the Example
In this paper we presented an approach for scheduling sensor networks. We based our approach on the
work that has been done by the WirelessHART specification team. We summarized information behind
the WirelessHART network and then showed a working example of scheduling a WirelessHART
network. We showed how dividing the problem into two parts, routing and scheduling, simplifies the
approach. We further showed how dividing the schedule up into Superframes simplifies scheduling.
We believe that a network-centric approach provides a solid foundation to address the dual relationship
between power usage and latency. We believe that the supporting network services address many of the
scenarios that are likely to arise in real plant settings. One example that was shown was the ability of
WirelessHART to route around physical obstructions. Another implied feature was frequency hopping.
Moving forward, we believe that the WirelessHART network schedule shown should be enhanced to
optimize power usage and latency. As part of this, research should be done in the areas of handling
service requests, network load balancing, network congestion and flow control, latency optimization,
and power utilization.
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7. ISO 7498-1 Information Processing Systems - OSI Reference Model - The Basic Model
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