U.S. patent application number 13/444350 was filed with the patent office on 2012-08-02 for suspending transmissions in a wireless network.
This patent application is currently assigned to HART COMMUNICATION FOUNDATION. Invention is credited to Tomas P. Lennvall, Mark J. Nixon, Robin S. Pramanik, Wallace A. Pratt, JR., Eric D. Rotvold.
Application Number | 20120196636 13/444350 |
Document ID | / |
Family ID | 39939431 |
Filed Date | 2012-08-02 |
United States Patent
Application |
20120196636 |
Kind Code |
A1 |
Pratt, JR.; Wallace A. ; et
al. |
August 2, 2012 |
SUSPENDING TRANSMISSIONS IN A WIRELESS NETWORK
Abstract
A method of increasing operational safety of a wireless
communication network operating in a process control environment
and having a plurality of network devices includes detecting a
first triggering condition associated with an increased operational
risk due to a wireless transmissions by at least one of the
plurality of network devices, sending a suspend command to at least
some of the plurality of network devices, suspending transmissions
at the at least some of the plurality of network devices; and
resuming transmissions at the at least some of the plurality of
network devices upon occurrence of a second triggering
condition.
Inventors: |
Pratt, JR.; Wallace A.;
(Pflugerville, TX) ; Nixon; Mark J.; (Round Rock,
TX) ; Rotvold; Eric D.; (West St. Paul, MN) ;
Pramanik; Robin S.; (Karlsruhe, DE) ; Lennvall; Tomas
P.; (Vasteras, SE) |
Assignee: |
HART COMMUNICATION
FOUNDATION
Austin
TX
|
Family ID: |
39939431 |
Appl. No.: |
13/444350 |
Filed: |
April 11, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12101011 |
Apr 10, 2008 |
8169974 |
|
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13444350 |
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60911795 |
Apr 13, 2007 |
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Current U.S.
Class: |
455/509 ;
455/517 |
Current CPC
Class: |
H04W 80/00 20130101;
H04W 56/002 20130101; H04W 24/00 20130101; G01D 21/00 20130101;
H04W 56/0015 20130101 |
Class at
Publication: |
455/509 ;
455/517 |
International
Class: |
H04W 72/04 20090101
H04W072/04; H04W 4/06 20090101 H04W004/06 |
Claims
1. A method of temporarily suspending communications in a wireless
communication network operating in a process control environment
and having a plurality of network devices, comprising: detecting a
first triggering condition that requires radio silence in a certain
geographical area of the process control environment, the first
triggering condition corresponding to a planned event in the
process control environment; and sending, via the wireless
communication network, a suspend command to at least some of the
plurality of network devices corresponding to the certain
geographical area of the process control environment to cause
wireless transmissions to be suspended at the at least some of the
plurality of network devices.
2. The method of claim 1, wherein detecting the first triggering
condition corresponding to the planned event in the process control
environment comprises detecting the first triggering condition
corresponding to at least one of: an event having an elevated
safety risk, a planned controlled explosion, a test, a measurement,
or an operator request.
3. The method of claim 1, wherein detecting the first triggering
condition includes one of: receiving a suspension request from an
external host operating outside the wireless communication network;
or receiving a suspension request from a handheld device operating
in the wireless communication network as one of the plurality of
network devices.
4. The method of claim 1, wherein sending the suspend command to
the at least some of the plurality of network devices includes
broadcasting the suspend command to each of the plurality of
network devices.
5. The method of claim 1, further comprising sending, after a
suspension period, a resume command to the at least some of the
plurality of network devices to cause the at least some of the
plurality of network devices to resume wireless transmissions.
6. The method of claim 5, wherein sending the resume command to the
at least some of the plurality of network devices comprises
broadcasting the resume command to the at least some of the
plurality of network devices.
7. The method of claim 1, wherein sending the suspend command
includes: specifying, in the suspend command, a suspend time at
which a recipient network device must suspend wireless
transmissions; and specifying, in the suspend command, one of: a
resume time at which the recipient network device is allowed to
resume wireless transmissions; and a quiet time interval during
which the recipient network device is not allowed to resume
wireless transmissions.
8. The method of claim 7, further comprising maintaining an
absolute slot number counter indicative of a number of
communication timeslots of a predetermined duration scheduled since
a formation of the wireless communication network, wherein each of
the plurality of network devices communicates with at least one
other of the plurality of network devices according to a
communication schedule having a plurality of concurrent
superframes, each superframe of the concurrent superframes defined
as a repeating sequence of at least one of the communication
timeslots; and wherein specifying the suspend time includes
specifying a first absolute slot number counter value, and
specifying one of the resume time or the quiet time interval
includes specifying a second absolute slot number counter
value.
9. The method of claim 1, further comprising pre-configuring each
of the at least some of the plurality of network devices with a
quiet time interval during which the each of the at least some of
the plurality of network devices is not allowed to resume
communications following a reception of the suspend command.
10. The method of claim 1, further comprising defining a
communication schedule having a plurality of links, each link
specifying a communication timeslot of a predetermined duration, at
least one sender, and at least one receiver, wherein the at least
one sender and the at least one receiver are included in the
plurality of network devices and wherein each of plurality of
network devices communicates according to the communication
schedule; and wherein sending the suspend command to the at least
some of the plurality of network devices comprises sending the
suspend command to the at least some of the plurality of network
devices according to the communication schedule.
11. The method of claim 10, wherein defining a communication
schedule includes allocating timeslots to each of the plurality of
network devices according to a respective rate at which the each of
the at least some of the plurality of network devices transmits
process control or measurement data.
12. A method of temporarily suspending communications in a wireless
communication network operating in a process control environment
and having a plurality of network devices, comprising: receiving,
at a particular network device included in the plurality of network
devices, a suspend command corresponding to a planned event in the
process control environment; suspending, based on the reception of
the suspend command, wireless transmissions by the particular
network device; and resuming wireless transmissions by the
particular network device upon an occurrence of a triggering
condition.
13. The method of claim 12, wherein receiving the suspend command
corresponding to the planned event comprises receiving the suspend
command corresponding to at least one of: an event having an
elevated safety risk, a planned controlled explosion, a test, a
measurement, or an operator request.
14. The method of claim 12, wherein resuming wireless transmissions
upon the occurrence of the triggering condition comprises resuming
wireless transmissions upon a reception of a resume command.
15. The method of claim 12, wherein at least one of: receiving the
suspend command comprises receiving a transmission suspend time;
receiving the suspend command comprises receiving a suspend command
including a transmission resume time and wherein the triggering
condition corresponds to the transmission resume time; or receiving
the suspend command comprises receiving a suspend command including
a quiet time interval and wherein the triggering condition
corresponds to an elapsing of the quiet time interval.
16. The method of claim 15, further comprising maintaining an
absolute slot number counter indicative of a number of
communication timeslots of a predetermined duration scheduled since
a formation of the wireless communication network, wherein each of
the plurality of network devices communicates with at least one
other of the plurality of network devices according to a
communication schedule having a plurality of concurrent
superframes, each superframe of the concurrent superframes defined
as a repeating sequence of at least one of the communication
timeslots; and wherein receiving the transmission suspend time
comprises receiving a first absolute slot number counter value and
receiving the transmission resume time comprises receiving a second
absolute slot number counter value.
17. The method of claim 12, wherein the particular network device
is configured with a quiet time interval, and wherein resuming
wireless transmissions upon the occurrence of the triggering
condition comprises resuming wireless transmissions upon an
elapsing of the quiet time interval.
18. The method of claim 12, wherein receiving the suspend command
comprises receiving the suspend command via the wireless
communication network.
19. The method of claim 12, wherein receiving the suspend command
comprises receiving the suspend command according to a
communication schedule having a plurality of links, each link
specifying a communication timeslot of a predetermined duration, at
least one sender, and at least one receiver; wherein the at least
one sender and the at least one receiver are included in the
plurality of network devices; and wherein each of plurality of
network devices communicates according to the communication
schedule.
20. The method of claim 12, wherein receiving the suspend command
according to the communication schedule includes receiving the
suspend command via one or more timeslots allocated to the
particular network device according to a rate at which the network
device transmits process control or measurement data.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of U.S. application Ser. No.
12/101,011, entitled "Suspending Transmissions in a Wireless
Network" filed Apr. 10, 2008 (Attorney Docket No. 31244/42874),
which claims benefit of the United States Provisional Application
Serial No. 60/911,795, entitled "Routing, Scheduling, Reliable and
Secure Operations in a Wireless Communication Protocol" filed Apr.
13, 2007 (Attorney Docket No. 31244/42509P), the disclosures of
which are hereby expressly incorporated herein by reference.
FIELD OF TECHNOLOGY
[0002] The present invention relates generally to wireless
communications and, more particularly, to temporarily suspending
communications in a wireless network for safety and other
purposes.
BACKGROUND
[0003] It is known to use standardized communication protocols in
the process control industry to enable devices made by different
manufacturers to communicate with one another in an easy to use and
implement manner. One such well known communication standard used
in the process control industry is the Highway Addressable Remote
Transmitter (HART) Communication Foundation protocol, referred to
generally as the HART protocol. Generally speaking, the HART
protocol supports a combined digital and analog signal on a
dedicated wire or set of wires, in which on-line process signals
(such as control signals, sensor measurements, etc.) are provided
as an analog current signal (e.g., ranging from 4 to 20 milliamps)
and in which other signals, such as device data, requests for
device data, configuration data, alarm and event data, etc., are
provided as digital signals superimposed or multiplexed onto the
same wire or set of wires as the analog signal. However, the HART
protocol currently requires the use of dedicated, hardwired
communication lines, resulting in significant wiring needs within a
process plant.
[0004] There has been a move, in the past number of years, to
incorporate wireless technology into various industries including,
in some limited manner, the process control industry. However,
there are significant hurdles in the process control industry that
limit the full scale incorporation, acceptance and use of wireless
technology. In particular, the process control industry requires a
completely reliable process control network because loss of signals
can result in the loss of control of a plant, leading to
catastrophic consequences, including explosions, the release of
deadly chemicals or gases, etc. For example, Tapperson et al., U.S.
Pat. No. 6,236,334 discloses the use of a wireless communications
in the process control industry as a secondary or backup
communication path or for use in sending non-critical or redundant
communication signals. Moreover, there have been many advances in
the use of wireless communication systems in general that may be
applicable to the process control industry, but which have not yet
been applied to the process control industry in a manner that
allows or provides a reliable, and in some instances completely
wireless, communication network within a process plant. U.S. Patent
Application Publication Numbers 2005/0213612, 2006/0029060 and
2006/0029061 for example disclose various aspects of wireless
communication technology related to a general wireless
communication system.
[0005] One factor significantly inhibiting the development and
application of wireless communications in the process control
industry is the difficulty of retrofitting legacy devices for the
use with wireless communication networks. In some cases, devices
cannot be retrofitted at all and need to be replaced with newer,
wireless-ready models. Moreover, many of the supporting
installations are similarly rendered obsolete by a transition to
wireless communications. In other words, wireless networks cannot
easily extend wired networks. An additional challenge particularly
pertinent to the process control industry is the high cost of the
existing wired installations and the understandable reluctance of
the operators to completely replace the wired infrastructure with a
wireless infrastructure. Meanwhile, wireless networks typically
require stationary antennas or access points to transmit and
receive radio signals and may therefore require an expensive
infrastructure which makes the transition to wireless
communications less desirable. Thus, while some operators may
recognize the advantages of a wireless approach to process
measurement and control, many may be unwilling to dismantle the
existing installations, decommission the wired devices which may be
fully operational, and purchase wireless devices.
[0006] Another factor contributing to the slower than expected
proliferation of wireless standards in the process control industry
is the impact on a user, such as a technician or an operator of a
process control system. During operation of a typical process
control system, users may remotely access individual devices for
the purposes of configuring, monitoring, and controlling various
functions of the devices. For example, to enable access and
exchange of information over the HART protocol, devices are
assigned unique addresses according to a predefined addressing
scheme. Users and the software applications developed for operators
and technicians in the process control industry have come to rely
on an efficient addressing scheme which cannot be supported by the
available wireless standards. Thus, a transition to a wireless
standard in a process control industry is widely expected to entail
adopting a new addressing scheme, updating the corresponding
software applications and providing additional training to the
personnel.
[0007] Additionally, some of the existing wireless standards, such
as the IEEE 802.11(x) WLAN, for example, do not satisfy all of the
demands of the process control industry. For example, devices
communicate both process and control data which may typically have
different propagation delay constraints. In general, some of the
critical data exchanged in the process control industry may require
efficient, reliable and timely delivery which cannot always be
guaranteed by the existing wireless protocols. Moreover, because
some of the modules used in the process control industry are used
to control very sensitive and potentially dangerous process
activities, wireless standards suitable for this industry need to
provide redundancy in communication paths not readily available in
the known wireless networks. Finally, some process control devices
may be sensitive to high power radio signals and may require radio
transmissions to be limited or held at a well controlled power
level. Meanwhile, the available wireless standards typically rely
on antennas or access points which transmit relatively strong
signals to cover large geographic areas.
[0008] Similar to wired communication protocols, wireless
communication protocols are expected to provide efficient, reliable
and secure methods of exchanging information. Of course, much of
the methodology developed to address these concerns on wired
networks does not apply to wireless communications because of the
shared and open nature of the medium. Further, in addition to the
typical objectives behind a wired communication protocol, wireless
protocols face other requirements with respect to the issues of
interference and co-existence of several networks that use the same
part of the radio frequency spectrum. To complicate matters, some
wireless networks operate in the part of the spectrum that is
unlicensed, or open to the public. Therefore, protocols servicing
such networks must be capable of detecting and resolving issues
related to frequency (channel) contention, radio resource sharing
and negotiation, etc.
[0009] In the process control industry, developers of wireless
communication protocols face additional challenges, such as
achieving backward compatibility with wired devices, supporting
previous wired versions of a protocol, providing transition
services to devices retrofitted with wireless communicators, and
providing routing techniques which can ensure both reliability and
efficiency. Meanwhile, there remains a wide number of process
control applications in which there are few, if any, in-place
measurements. Currently these applications rely on observed
measurements (e.g. water level is rising) or inspection (e.g.
period maintenance of air conditioning unit, pump, fan, etc.) to
discover abnormal situations. In order to take action, operators
frequently require face-to-face discussions. Many of these
applications could be greatly simplified if measurement and control
devices were utilized. However, current measurement devices usually
require power, communications infrastructure, configuration, and
support infrastructure which simply is not available.
[0010] In another aspect, the process control industry requires
that the communication protocol servicing a particular process
control network be able to accommodate field devices with different
data transmission requirements, priorities, and power capabilities.
In particular, some process control systems may include measurement
devices that frequently (such as several times per second) report
measurements to a centralized controller or to another field
device. Meanwhile, another device in the same system may report
measurements, alarms, or other data only once per hour. However,
both devices may require that the respective measurement reports
propagate to a destination host, such as a controller, a
workstation, or a peer field device, with as little overhead in
time and bandwidth as possible.
[0011] Further, some applications of wireless communications and
especially certain uses of wireless devices in the process control
industry may require additional caution because of sensitivity of
some equipment to electromagnetic waves. Thus, engineers and
operators who wish to instrument a potentially explosive process
control environment such as an oil refinery with wireless devices
face yet another additional challenge of ensuring safety without
comprising the reliability and efficiency of communications.
SUMMARY
[0012] A wireless mesh network for use in, for example, process
control plants includes a plurality of network devices
communicating according to a network schedule defined as a set of
concurrent overlapping superframes. When a certain event occurs, a
network manager running in or outside the wireless mesh network
temporarily suspends communications between at least some of the
network devices by propagating a predefined suspend command through
the wireless mesh network. The interval during which the network
suspends communications may be referred to as "quiet time." In some
embodiments, the network manager suspends all communications
between each pair of network devices and, in this sense,
temporarily suspends the entire wireless mesh network. In another
embodiment, an application running outside the wireless mesh
network triggers the decision of the network manager to suspend a
portion or the whole of the wireless mesh network by reporting an
event to the network manager via a gateway device connecting the
wireless mesh network to an external network. In one particular
embodiment, the event which triggers the suspension of wireless
communications is a planned controlled explosion requiring radio
silence.
[0013] In some embodiments, the network manager specifies, as a
part of the suspend command, an amount of time during which some or
all of the network devices must suspend wireless transmissions. In
other embodiments, network devices participating in the wireless
mesh network are preconfigured with the amount of time during which
the wireless transmissions must be suspended.
[0014] In one embodiment, a network device immediately suspends
transmissions upon receiving the suspend command from the network
manager, and the network manager does not expect to receive an
acknowledgement for the suspend command. In other embodiments, the
suspend command specifies both the time at which the network device
must suspend wireless communications and the time at which the
network must resume communications. In at least some of these
embodiments, the network device may properly acknowledge the
suspend command.
[0015] In yet another embodiment, a network device may suspend
transmissions but continue to listen to incoming data packets after
receiving the suspend command. In this embodiment, neither the
suspend command nor the network device configuration may specify
the resume time or the duration of a quiet period. Instead, the
network manager may propagate the suspend command through the
network and, upon occurrence of a certain condition, may propagate
a wake-up command to resume communications through the suspended
network. Because the network devices may continue to listen to data
packets according to the original schedule, the wireless network
may quickly and efficiently resume operation after a period of
quiet time.
[0016] In another aspect, each superframe defined in the wireless
mesh network includes several communication timeslots of a
predetermined duration and each superframe repeats immediately as a
new superframe cycle after the occurrence of all communication
timeslots in the previous superframe cycle. The timeslots within
each superframe cycle are sequentially numbered relative to the
beginning of the cycle. The wireless mesh network additionally
maintains an Absolute Slot Number (ASN) indicative of a number of
timeslots scheduled since the time of formation of the wireless
network. In at least some of the embodiments, the network devices
efficiently re-synchronize communications upon the expiration of
suspend time by aligning the respective ASN values with a global
ASN value maintained by the network manager. Further, the suspend
command in some of the embodiments specifies the time to suspend
the network, the time to resume the network, or both in ASN
units.
DETAILED DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a block diagram that illustrates a system
utilizing a WirelessHART network to provide wireless communication
between field devices and router devices, which are connected to a
plant automation network via a gateway device.
[0018] FIG. 2 is a schematic representation of the layers of a
WirelessHART protocol implemented in accordance with one of the
embodiments discussed herein.
[0019] FIG. 3 is a block diagram that illustrates segments of a
communication timeslot defined in accordance with one of the
embodiments discussed herein.
[0020] FIG. 4 is a block diagram that illustrates an exemplary
association of timeslots of a three-slot superframe with several
communicating devices.
[0021] FIG. 5 schematically illustrates association of a timeslot
of an exemplary superframe with several communication channels.
[0022] FIG. 6 is a block diagram that schematically illustrates an
exemplary superframe definition including several concurrent
superframes of different length.
[0023] FIG. 7 is another block diagram that schematically
illustrates several concurrent superframes of different length in
relation to an absolute slot number counter.
[0024] FIG. 8 illustrates an example state machine which a network
device may execute when operating in the wireless network of FIG.
1.
DETAILED DESCRIPTION
[0025] FIG. 1 illustrates an exemplary network 10 in which the
method for suspending transmissions described herein may be used.
In particular, the network 10 may include a plant automation
network 12 connected to a wireless communication network 14. The
plant automation network 12 may include one or more stationary
workstations 16 and one or more portable workstations 18 connected
over a communication backbone 20 which may be implemented using
Ethernet, RS-485, Profibus DP, or using other suitable
communication hardware and protocol. The workstations and other
equipment forming the plant automation network 12 may provide
various control and supervisory functions to plant personnel,
including access to devices in the wireless network 14. The plant
automation network 12 and the wireless network 14 may be connected
via a gateway device 22. More specifically, the gateway device 22
may be connected to the backbone 20 in a wired manner and may
communicate with the plant automation network 12 using any suitable
(e.g., known) communication protocol. The gateway device 22, which
may be implemented in any other desired manner (e.g., as a
standalone device, a card insertable into an expansion slot of the
host workstations 16 or 18, as a part of the input/output (TO)
subsystem of a PLC-based or DCS-based system, etc.), may provide
applications that are running on the network 12 with access to
various devices of the wireless network 14. In addition to protocol
and command conversion, the gateway device 22 may provide
synchronized clocking used by time slots and superframes (sets of
communication time slots spaced equally in time) of a scheduling
scheme associated with a wireless protocol (referred to herein as a
WirelessHART protocol) implemented in the network 14.
[0026] In some configurations, the network 10 may include more than
one gateway device 22 to improve the efficiency and reliability of
the network 10. In particular, multiple gateway devices 22 may
provide additional bandwidth for the communication between the
wireless network 14 and the plant automation network 12, as well as
the outside world. On the other hand, the gateway 22 device may
request bandwidth from the appropriate network service according to
the gateway communication needs within the wireless network 14. A
network manager software module 27, which may reside in the gateway
device 22, may further reassess the necessary bandwidth while the
system is operational. For example, the gateway device 22 may
receive a request from a host residing outside of the wireless
network 14 to retrieve a large amount of data. The gateway device
22 may then request the network manager 27 to allocate additional
bandwidth to accommodate this transaction. For example, the gateway
device 22 may issue an appropriate service request. The gateway
device 22 may then request the network manager 27 to release the
bandwidth upon completion of the transaction.
[0027] In general, the network manager 27 may be responsible for
adapting the wireless network 14 to changing conditions and for
scheduling communication resources. As network devices join and
leave the network, the network manager 27 may update its internal
model of the wireless network 14 and use this information to
generate communication schedules and communication routes.
Additionally, the network manager 27 may consider the overall
performance of the wireless network 14 as well as the diagnostic
information to adapt the wireless network 14 to changes in topology
and communication requirements. Once the network manager 27 has
generated the overall communication schedule, all or respective
parts of the overall communication schedule may be transferred
through a series of commands from the network manager 27 to the
network devices.
[0028] To further increase bandwidth and improve reliability, the
gateway device 22 may be functionally divided into a virtual
gateway 24 and one or more network access points 25, which may be
separate physical devices in wired communication with the gateway
device 22. However, while FIG. 1 illustrates a wired connection 26
between the physically separate gateway device 22 and the access
points 25, it will be understood that the elements 22-26 may also
be provided as an integral device. Because the network access
points 25 may be physically separated from the gateway device 22,
the access points 25 may be strategically placed in several
different locations with respect to the network 14. In addition to
increasing the bandwidth, multiple access points 25 can increase
the overall reliability of the network 14 by compensating for a
potentially poor signal quality at one access point 25 using the
other access point 25. Having multiple access points 25 also
provides redundancy in case of a failure at one or more of the
access points 25.
[0029] In addition to allocating bandwidth and otherwise bridging
the networks 12 and 14, the gateway device 22 may perform one or
more managerial functions in the wireless network 14. As
illustrated in FIG. 1, a network manager software module 27 and a
security manager software module 28 may be stored in and executed
in the gateway device 22. Alternatively, the network manager 27
and/or the security manager 28 may run on one of the hosts 16 or 18
in the plant automation network 12. For example, the network
manager 27 may run on the host 16 and the security manager 28 may
run on the host 18. The network manager 27 may be responsible for
configuration of the network 14, scheduling communication between
wireless devices, managing routing tables associated with the
wireless devices, monitoring the overall health of the wireless
network 14, reporting the health of the wireless network 14 to the
workstations 16 and 18, as well as other administrative and
supervisory functions. Although a single active network manager 27
may be sufficient in the wireless network 14, redundant network
managers 27 may be similarly supported to safeguard the wireless
network 14 against unexpected equipment failures. Meanwhile, the
security manager 28 may be responsible for protecting the wireless
network 14 from malicious or accidental intrusions by unauthorized
devices. To this end, the security manager 28 may manage
authentication codes, verify authorization information supplied by
devices attempting to join the wireless network 14, update
temporary security data such as expiring secret keys, and perform
other security functions.
[0030] With continued reference to FIG. 1, the wireless network 14
may include one or more field devices 30-36. In general, process
control systems, like those used in chemical, petroleum or other
process plants, include such field devices as valves, valve
positioners, switches, sensors (e.g., temperature, pressure and
flow rate sensors), pumps, fans, etc. Field devices perform
physical control functions within the process such as opening or
closing valves or take measurements of process parameters. In the
wireless communication network 14, field devices 30-36 are
producers and consumers of wireless communication packets.
[0031] The devices 30-36 may communicate using a wireless
communication protocol that provides the functionality of a similar
wired network, with similar or improved operational performance. In
particular, this protocol may enable the system to perform process
data monitoring, critical data monitoring (with the more stringent
performance requirements), calibration, device status and
diagnostic monitoring, field device troubleshooting, commissioning,
and supervisory process control. The applications performing these
functions, however, typically require that the protocol supported
by the wireless network 14 provide fast updates when necessary,
move large amounts of data when required, and support network
devices which join the wireless network 14, even if only
temporarily for commissioning and maintenance work.
[0032] In one embodiment, the wireless protocol supporting network
devices 30-36 of the wireless network 14 is an extension of the
known wired HART protocol, a widely accepted industry standard,
which maintains the simple workflow and practices of the wired
environment. In this sense, the network devices 30-36 may be
considered WirelessHART devices. The same tools used for wired HART
devices may be easily adapted to wireless devices 30-36 with a
simple addition of new device description files. In this manner,
the wireless protocol may leverage the experience and knowledge
gained using the wired HART protocol to minimize training and
simplify maintenance and support. Generally speaking, it may be
convenient to adapt a protocol for wireless use so that most
applications running on a device do not "notice" the transition
from a wired network to a wireless network. Clearly, such
transparency greatly reduces the cost of upgrading networks and,
more generally, reduces the cost associated with developing and
supporting devices that may be used with such networks. Some of the
additional benefits of a wireless extension of the well-known HART
protocol include access to measurements that were difficult or
expensive to obtain with wired devices and the ability to configure
and operate instruments from system software that can be installed
on laptops, handhelds, workstations, etc. Another benefit is the
ability to send diagnostic alerts from wireless devices back
through the communication infrastructure to a centrally located
diagnostic center. For example, every heat exchanger in a process
plant could be fitted with a WirelessHART device and the end user
and supplier could be alerted when a heat exchanger detects a
problem. Yet another benefit is the ability to monitor conditions
that present serious health and safety problems. For example, a
WirelessHART device could be placed in flood zones on roads and be
used to alert authorities and drivers about water levels. Other
benefits include access to a wide range of diagnostics alerts and
the ability to store trended as well as calculated values at the
WirelessHART devices so that, when communications to the device are
established, the values can be transferred to a host. In this
manner, the WirelessHART protocol can provide a platform that
enables host applications to have wireless access to existing
HART-enabled field devices and the WirelessHART protocol can
support the deployment of battery operated, wireless only
HART-enabled field devices. The WirelessHART protocol may be used
to establish a wireless communication standard for process
applications and may further extend the application of HART
communications and the benefits that this protocol provides to the
process control industry by enhancing the basic HART technology to
support wireless process automation applications.
[0033] Referring again to FIG. 1, the field devices 30-36 may be
WirelessHART field devices, each provided as an integral unit and
supporting all layers of the WirelessHART protocol stack. For
example, in the network 14, the field device 30 may be a
WirelessHART flow meter, the field devices 32 may be WirelessHART
pressure sensors, the field device 34 may be a WirelessHART valve
positioner, and the field device 36 may a WirelessHART pressure
sensor. Importantly, the wireless devices 30-36 may support all of
the HART features that users have come to expect from the wired
HART protocol. As one of ordinary skill in the art will appreciate,
one of the core strengths of the HART protocol is its rigorous
interoperability requirements. In some embodiments, all
WirelessHART equipment includes core mandatory capabilities in
order to allow equivalent device types (made by different
manufacturers, for example) to be interchanged without compromising
system operation. Furthermore, the WirelessHART protocol is
backward compatible to HART core technology such as the device
description language (DDL). In the preferred embodiment, all of the
WirelessHART devices should support the DDL, which ensures that end
users immediately have the tools to begin utilizing the
WirelessHART protocol.
[0034] If desired, the network 14 may include non-wireless devices.
For example, a field device 38 of FIG. 1 may be a legacy 4-20 mA
device and a field device 40 may be a traditional wired HART
device. To communicate within the network 14, the field devices 38
and 40 may be connected to the WirelessHART network 14 via a
WirelessHART adapter (WHA) 50. Additionally, the WHA 50 may support
other communication protocols such as Foundation.RTM. Fieldbus,
PROFIBUS, DevicesNet, etc. In these embodiments, the WHA 50
supports protocol translation on a lower layer of the protocol
stack. Additionally, it is contemplated that a single WHA 50 may
also function as a multiplexer and may support multiple HART or
non-HART devices.
[0035] Plant personnel may additionally use handheld devices for
installation, control, monitoring, and maintenance of network
devices. Generally speaking, handheld devices are portable
equipment that can connect directly to the wireless network 14 or
through the gateway devices 22 as a host on the plant automation
network 12. As illustrated in FIG. 1, a WirelessHART-connected
handheld device 55 may communicate directly with the wireless
network 14. When operating with a formed wireless network 14, the
handheld device 55 may join the network 14 as just another
WirelessHART field device. When operating with a target network
device that is not connected to a WirelessHART network, the
handheld device 55 may operate as a combination of the gateway
device 22 and the network manager 27 by forming its own wireless
network with the target network device.
[0036] A plant automation network-connected handheld device (not
shown) may be used to connect to the plant automation network 12
through known networking technology, such as Wi-Fi. This device
communicates with the network devices 30-40 through the gateway
device 22 in the same fashion as external plant automation servers
(not shown) or the workstations 16 and 18 communicate with the
devices 30-40.
[0037] Additionally, the wireless network 14 may include a router
device 60 which is a network device that forwards packets from one
network device to another network device. A network device that is
acting as a router device uses internal routing tables to conduct
routing, i.e., to decide to which network device a particular
packet should be sent. Standalone routers such as the router 60 may
not be required in those embodiments where all of the devices on
the wireless network 14 support routing. However, it may be
beneficial (e.g. to extend the network, or to save the power of a
field device in the network) to add one or more dedicated routers
60 to the network 14.
[0038] All of the devices directly connected to the wireless
network 14 may be referred to as network devices. In particular,
the wireless field devices 30-36, the adapters 50, the routers 60,
the gateway devices 22, the access points 25, and the wireless
handheld device 55 are, for the purposes of routing and scheduling,
network devices, each of which forms a node of the wireless network
14. In order to provide a very robust and an easily expandable
wireless network, all of the devices in a network may support
routing and each network device may be globally identified by a
substantially unique address, such as a HART address, for example.
The network manager 27 may contain a complete list of network
devices and may assign each device a short, network unique 16-bit
nickname. Additionally, each network device may store information
related to update rates, connection sessions, and device resources.
In short, each network device maintains up-to-date information
related to routing and scheduling within the wireless network 14.
The network manager 27 may communicate this information to network
devices whenever new devices join the network or whenever the
network manager 27 detects or originates a change in topology or
scheduling of the wireless network 14.
[0039] Further, each network device may store and maintain a list
of neighbor devices that the network device has identified during
listening operations. Generally speaking, a neighbor of a network
device is another network device of any type potentially capable of
establishing a connection with the network device in accordance
with the standards imposed by a corresponding network. In case of
the WirelessHART network 14, the connection is a direct wireless
connection. However, it will be appreciated that a neighboring
device may also be a network device connected to the particular
device in a wired manner. As will be discussed later, network
devices promote their discovery by other network devices through
advertisement, or special messages sent out during designated
periods of time. Network devices operatively connected to the
wireless network 14 have one or more neighbors which they may
choose according to the strength of the advertising signal or to
some other principle.
[0040] In the example illustrated in FIG. 1, each of a pair of
network devices that are connected by a direct wireless connection
65 recognizes the other as a neighbor. Thus, network devices of the
wireless network 14 may form a large number of inter-device
connections 65. The possibility and desirability of establishing a
direct wireless connection 65 between two network devices is
determined by several factors, such as the physical distance
between the nodes, obstacles between the nodes (devices), signal
strength at each of the two nodes, etc. Further, two or more direct
wireless connections 65 may be used to form communication paths
between nodes that cannot form a direct wireless connection 65. For
example, the direct wireless connection 65 between the WirelessHART
hand-held device 55 and WirelessHART device 36 along with the
direct wireless connection 65 between the WirelessHART device 36
the router 60 form a communication path between the devices 55 and
60.
[0041] Each wireless connection 65 is characterized by a large set
of parameters related to the frequency of transmission, the method
of access to a radio resource, etc. One of ordinary skill in the
art will recognize that, in general, wireless communication
protocols may operate on designated frequencies, such as the ones
assigned by the Federal Communications Commission (FCC) in the
United States, or in the unlicensed part of the radio spectrum
(e.g., 2.4 GHz). While the system and method discussed herein may
be applied to a wireless network operating on any designated
frequency or range of frequencies, the example embodiment discussed
below relates to the wireless network 14 operating in the
unlicensed, or shared part of the radio spectrum. In accordance
with this embodiment, the wireless network 14 may be easily
activated and adjusted to operate in a particular unlicensed
frequency range as needed.
[0042] One of the core requirements for a wireless network protocol
using an unlicensed frequency band is the minimally disruptive
coexistence with other equipment utilizing the same band.
Coexistence generally defines the ability of one system to perform
a task in a shared environment in which other systems can similarly
perform their tasks while conforming to the same set of rules or to
a different (and possibly unknown) set of rules. One requirement of
coexistence in a wireless environment is the ability of the
protocol to maintain communication while interference is present in
the environment. Another requirement is that the protocol should
cause as little interference and disruption as possible with
respect to other communication systems.
[0043] In other words, the problem of coexistence of a wireless
system with the surrounding wireless environment has two general
aspects. The first aspect of coexistence is the manner in which the
system affects other systems. For example, an operator or developer
of the particular system may ask what impact the transmitted signal
of one transmitter has on other radio system operating in proximity
to the particular system. More specifically, the operator may ask
whether the transmitter disrupts communication of some other
wireless device every time the transmitter turns on or whether the
transmitter spends excessive time on the air effectively "hogging"
the bandwidth. Ideally, each transmitter should be a "silent
neighbor" that no other transmitter notices. While this ideal
characteristic is rarely, if ever, attainable, a wireless system
that creates a coexistence environment in which other wireless
communication systems may operate reasonably well may be called a
"good neighbor." The second aspect of coexistence of a wireless
system is the ability of the system to operate reasonably well in
the presence of other systems or wireless signal sources. In
particular, the robustness of a wireless system may depend on how
well the wireless system prevents interference at the receivers, on
whether the receivers easily overload due to proximate sources of
RF energy, on how well the receivers tolerate an occasional bit
loss, and similar factors. In some industries, including the
process control industry, there are a number of important potential
applications in which the loss of data is frequently not allowable.
A wireless system capable of providing reliable communications in a
noisy or dynamic radio environment may be called a "tolerant
neighbor."
[0044] Effective coexistence (i.e., being a good neighbor and a
tolerant neighbor) relies in part on effectively employing three
aspects of freedom: time, frequency and distance. Communication can
be successful when it occurs 1) at a time when the interference
source (or other communication system) is quiet; 2) at a different
frequency than the interference signal; or 3) at a location
sufficiently removed from the interference source. While a single
one of these factors could be used to provide a communication
scheme in the shared part of the radio spectrum, a combination of
two or all three of these factors can provide a high degree of
reliability, security and speed.
[0045] Still referring to FIG. 1, the network manager 27 or another
application or service running on the network 14 or 12 may define a
master network schedule 67 for the wireless communication network
14 in view of the factors discussed above. The master network
schedule 67 may specify the allocation of resources such as time
segments and radio frequencies to the network devices 25 and 30-55.
In particular, the master network schedule 67 may specify when each
of the network devices 25 and 30-55 transmits process data, routes
data on behalf of other network devices, listens to management data
propagated from the network manager 27, and transmits advertisement
data for the benefit of devices wishing to join the wireless
network 14. To allocate the radio resources in an efficient manner,
the network manager 27 may define and update the master network
schedule 67 in view of the topology of the wireless network 14.
More specifically, the network manager 27 may allocate the
available resources to each of the nodes of the wireless network 14
(i.e., wireless devices 30-36, 50, and 60) according to the direct
wireless connections 65 identified at each node. In this sense, the
network manager 27 may define and maintain the network schedule 67
in view of both the transmission requirements and of the routing
possibilities at each node.
[0046] The master network schedule 67 may partition the available
radio sources into individual communication channels, and further
measure transmission and reception opportunities on each channel in
such units as Time Division Multiple Access (TDMA) communication
timeslots, for example. In particular, the wireless network 14 may
operate within a certain frequency band which, in most cases, may
be safely associated with several distinct carrier frequencies, so
that communications at one frequency may occur at the same time as
communications at another frequency within the band. One of
ordinary skill in the art will appreciate that carrier frequencies
in a typical application (e.g., public radio) are sufficiently
spaced apart to prevent interference between the adjacent carrier
frequencies. For example, in the 2.4 GHz band, IEEE assigns
frequency 2.455 to channel number 21 and frequency 2.460 to channel
number 22, thus allowing the spacing of 5 KHz between two adjacent
segments of the 2.4 GHz band. The master network schedule 67 may
thus associate each communication channel with a distinct carrier
frequency, which may be the center frequency in a particular
segment of the band.
[0047] Meanwhile, as typically used in the industries utilizing
TDMA technology, the term "timeslot" refers to a segment of a
specific duration into which a larger period of time is divided to
provide a controlled method of sharing. For example, a second may
be divided into 10 equal 100 millisecond timeslots. Although the
master network schedule 67 preferably allocates resources as
timeslots of a single fixed duration, it is also possible to vary
the duration of the timeslots, provided that each relevant node of
the wireless network 14 is properly notified of the change. To
continue with the example definition of ten 100-millisecond
timeslots, two devices may exchange data every second, with one
device transmitting during the first 100 ms period of each second
(i.e., the first timeslot), the other device transmitting during
the fourth 100 ms period of each second (i.e., the fourth
timeslot), and with the remaining timeslots being unoccupied. Thus,
a node on the wireless network 14 may identify the scheduled
transmission or reception opportunity by the frequency of
transmission and the timeslot during which the corresponding device
may transmit or receive data.
[0048] To properly synchronize the network devices 25A-B and 30-50
with the master network schedule 67, the network manager 27 may
maintain a counter 68 to keep track of a number of timeslots
scheduled since the formation of the wireless network 14, i.e.,
since a first network device initiated the process of forming the
wireless network 14. As indicated above, the first network device
may be the gateway device 22, for example. The number of timeslots
elapsed since the beginning of the wireless network 14 is referred
to herein as the Absolute Slot Number ("ASN"), in contrast to a
relative slot number of a timeslot in a particular superframe. The
network manager 27 may initialize the ASN counter 68 to zero at the
time of formation of the wireless network 14 and increment
consequently increment the ASN counter 68 by one with each
occurrence of a new timeslot. Additionally, each of the network
devices 25A-B and 30-50 may similarly maintain a local copy of the
ASN counter 68 and periodically synchronize the local copy with the
master ASN counter 68 maintained by the network manager 27.
[0049] As discussed in greater detail below, some or all of the
network devices 25A-B and 30-50 may sometimes suspend wireless
transmission and, optionally, reception in response to receiving a
suspend command from the network manager 27. The suspend command
may be triggered by an event reported by a user operating the
workstation 16, by a command from the handheld device 55, by an
automated task running in the plant automation network 12, or
internally by one of the network devices 25A-B and 30-50 which may
report a certain high-severity alarm to the network manager 27,
thereby triggering a temporary or permanent emergency shutdown of
wireless communications. The network devices 25A-B and 30-50 may
rely the ASN value to determine the period of time during which
wireless transmissions are not allowed and further for
re-synchronizing with the wireless network 14 when the suspension
expires or is explicitly lifted by an operator command.
[0050] As part of defining an efficient and reliable network
schedule 67, the network manager 27 may logically organize
timeslots into cyclically repeating sets, or superframes. As used
herein, a superframe may be more precisely understood as a series
of equal superframe cycles, each superframe cycle corresponding to
a logical grouping of several adjacent time slots forming a
contiguous segment of time. The number of time slots in a given
superframe defines the length of the superframe and determines how
often each time slot repeats. In other words, the length of a
superframe, multiplied by the duration of a single timeslot,
specifies the duration of a superframe cycle. Additionally, the
timeslots within each frame cycle may be sequentially numbered for
convenience. To take one specific example, the network manager 27
may fix the duration of a timeslot at 10 milliseconds and may
define a superframe of length 100 to generate a 1-second frame
cycle (i.e., 10 milliseconds multiplied by 100). In a zero-based
numbering scheme, this example superframe may include timeslots
numbered 0, 1, . . . 99.
[0051] The network manager 27 may reduce latency and otherwise
optimize data transmissions by including multiple concurrent
superframes of different sizes in the network schedule 67.
Moreover, some or all of the superframes of the network schedule 67
may span multiple channels, or carrier frequencies. Thus, the
master network schedule 67 may specify the association between each
timeslot of each superframe and one of the available channels.
[0052] Thus, the master network schedule 67 may correspond to an
aggregation of individual device schedules. For example, a network
device, such as the valve positioner 34, may have an individual
device schedule 69A. The device schedule 69A may include only the
information relevant to the corresponding network device 34.
Similarly, the router device 60 may have an individual device
schedule 69B. Accordingly, the network device 34 may transmit and
receive data according to the device schedule 69A without knowing
the schedules of other network devices such as the schedule 69B of
the device 60. To this end, the network manager 27 may manage both
the overall network schedule 67 and each of the individual device
schedules 69 (e.g., 69A and 69B) and communicate the individual
device schedules 69 to the corresponding devices when necessary. In
other embodiments, the individual network devices 25 and 35-50 may
at least partially define or negotiate the device schedules 69 and
report these schedules to the network manager 27. According to this
embodiment, the network manager 27 may assemble the network
schedule 67 from the received device schedules 69 while checking
for resource contention and resolving potential conflicts.
[0053] The communication protocol supporting the wireless network
14 generally described above is referred to herein as the
WirelessHART protocol 70, and the operation of this protocol is
discussed in more detail with respect to FIG. 2. As will be
understood, each of the direct wireless connections 65 may transfer
data according to the physical and logical requirements of the
WirelessHART protocol 70. Meanwhile, the WirelessHART protocol 70
may efficiently support communications within timeslots and on the
carrier frequencies associated with the superframes defined by the
device-specific schedules 69.
[0054] FIG. 2 schematically illustrates the layers of one example
embodiment of the WirelessHART protocol 70, approximately aligned
with the layers of the well-known ISO/OSI 7-layer model for
communications protocols. By way of comparison, FIG. 2 additionally
illustrates the layers of the existing "wired" HART protocol 72. It
will be appreciated that the WirelessHART protocol 70 need not
necessarily have a wired counterpart. However, as will be discussed
in detail below, the WirelessHART protocol 70 can significantly
improve the convenience of its implementation by sharing one or
more upper layers of the protocol stack with an existing protocol.
As indicated above, the WirelessHART protocol 70 may provide the
same or greater degree of reliability and security as the wired
protocol 72 servicing a similar network. At the same time, by
eliminating the need to install wires, the WirelessHART protocol 70
may offer several important advantages, such as the reduction of
cost associated with installing network devices, for example. It
will be also appreciated that although FIG. 2 presents the
WirelessHART protocol 70 as a wireless counterpart of the HART
protocol 72, this particular correspondence is provided herein by
way of example only. In other possible embodiments, one or more
layers of the WirelessHART protocol 70 may correspond to other
protocols or, as mentioned above, the WirelessHART protocol 70 may
not share even the uppermost application layer with any of the
existing protocols.
[0055] As illustrated in FIG. 2, the wireless expansion of HART
technology may add at least one new physical layer (e.g., the IEEE
802.15.4 radio standard) and two data-link layers (e.g., wired and
wireless mesh) to the known HART implementation. In general, the
WirelessHART protocol 70 may be a secure, wireless mesh networking
technology operating in the 2.4 GHz ISM radio band (block 74). In
one embodiment, the WirelessHART protocol 70 may utilize IEEE
802.15.4b compatible direct sequence spread spectrum (DSSS) radios
with channel hopping on a transaction by transaction basis. This
WirelessHART communication may be arbitrated using TDMA to schedule
link activity (block 76). As such, all communications are
preferably performed within a designated time slot. One or more
source and one or more destination devices may be scheduled to
communicate in a given slot, and each slot may be dedicated to
communication from a single source device, or the source devices
may be scheduled to communicate using a CSMA/CA-like shared
communication access mode. Source devices may send messages to one
ore more specific target devices or may broadcast messages to all
of the destination devices assigned to a slot.
[0056] Because the WirelessHART protocol described herein allows
deployment of mesh topologies, a significant network layer 78 may
be specified as well. In particular, the network layer 78 may
enable establishing direct wireless connections 65 between
individual devices and routing data between a particular node of
the wireless network 14 (e.g., the device 34) and the gateway 22
via one or more intermediate hops. In some embodiments, pairs of
network devices 25A-B and 30-50 may establish communication paths
including one or several hops while in other embodiments, all data
may travel either upstream to the gateway device 22 or downstream
from the gateway device 22 to a particular node.
[0057] To enhance reliability, the WirelessHART protocol 70 may
combine TDMA with a method of associating multiple radio
frequencies with a single communication resource, e.g., channel
hopping. Channel hopping provides frequency diversity which
minimizes interference and reduces multi-path fading effects. In
particular, the data link 76 may create an association between a
single superframe and multiple carrier frequencies which the data
link layer 76 cycles through in a controlled and predefined manner.
For example, the available frequency band of a particular instance
of the WirelessHART network 14 may have carrier frequencies
F.sub.1, F.sub.2, . . . F.sub.n. A relative frame R of a superframe
S may be scheduled to occur at a frequency F.sub.1 in the cycle
C.sub.n, at a frequency F.sub.5 in the following cycle C.sub.n+1,
at a frequency F.sub.2 in the cycle C.sub.n+2, and so on. The
network manager 27 may configure the relevant network devices with
this information so that the network devices communicating in the
superframe S may adjust the frequency of transmission or reception
according to the current cycle of the superframe S.
[0058] The data link layer 76 of the WirelessHART protocol 70 may
offer an additional feature of channel blacklisting, which
restricts the use of certain channels in the radio band by the
network devices. The network manager 27 may blacklist a radio
channel in response to detecting excessive interference or other
problems on the channel. Further, operators or network
administrators may blacklist channels in order to protect a
wireless service that uses a fixed portion of the radio band that
would otherwise be shared with the WirelessHART network 14. In some
embodiments, the WirelessHART protocol 70 controls blacklisting on
a superframe basis so that each superframe has a separate blacklist
of prohibited channels.
[0059] In one embodiment, the network manager 27 is responsible for
allocating, assigning, and adjusting time slot resources associated
with the data link layer 76. If a single instance of the network
manager 27 supports multiple WirelessHART networks 14, the network
manager 27 may create an overall schedule for each instance of the
WirelessHART network 14. The schedule may be organized into
superframes containing time slots numbered relative to the start of
the superframe.
[0060] The WirelessHART protocol 70 may further define links or
link objects in order to logically unite scheduling and routing. In
particular, a link may be associated with a specific network
device, a specific superframe, a relative slot number, one or more
link options (transmit, receive, shared), and a link type (normal,
advertising, discovery). As illustrated in FIG. 2, the data link
layer 76 may be frequency-agile. More specifically, a channel
offset may be used to calculate the specific radio frequency used
to perform communications. The network manager 27 may define a set
of links in view of the communication requirements at each network
device. Each network device may then be configured with the defined
set of links. The defined set of links may determine when the
network device needs to wake up, and whether the network device
should transmit, receive, or both transmit/receive upon waking
up.
[0061] With continued reference to FIG. 2, the transport layer 80
of the WirelessHART protocol 70 allows efficient, best-effort
communication and reliable, end-to-end acknowledged communications.
As one skilled in the art will recognize, best-effort
communications allow devices to send data packets without an
end-to-end acknowledgement and no guarantee of data ordering at the
destination device. User Datagram Protocol (UDP) is one well-known
example of this communication strategy. In the process control
industry, this method may be useful for publishing process data. In
particular, because devices propagate process data periodically,
end-to-end acknowledgements and retries have limited utility,
especially considering that new data is generated on a regular
basis. In contrast, reliable communications allow devices to send
acknowledgement packets. In addition to guaranteeing data delivery,
the transport layer 80 may order packets sent between network
devices. This approach may be preferable for request/response
traffic or when transmitting event notifications. When the reliable
mode of the transport layer 80 is used, the communication may
become synchronous.
[0062] Reliable transactions may be modeled as a master issuing a
request packet and one or more slaves replying with a response
packet. For example, the master may generate a certain request and
can broadcast the request to the entire network. In some
embodiments, the network manager 27 may use reliable broadcast to
tell each network device in the WirelessHART network 14 to activate
a new superframe. Alternatively, a field device such as the sensor
30 may generate a packet and propagate the request to another field
device such as to the portable HART communicator 55. As another
example, an alarm or event generated by the 34 field device may be
transmitted as a request directed to the gateway device 22. In
response to successfully receiving this request, the gateway device
22 may generate a response packet and send the response packet to
the device 34, acknowledging receipt of the alarm or event
notification.
[0063] Referring again to FIG. 2, the session layer 82 may provide
session-based communications between network devices. End-to-end
communications may be managed on the network layer by sessions. A
network device may have more than one session defined for a given
peer network device. If desired, almost all network devices may
have at least two sessions established with the network manager 27:
one for pairwise communication and one for network broadcast
communication from the network manager 27. Further, all network
devices may have a gateway session key. The sessions may be
distinguished by the network device addresses assigned to them.
Each network device may keep track of security information
(encryption keys, nonce counters) and transport information
(reliable transport sequence numbers, retry counters, etc.) for
each session in which the device participates.
[0064] Finally, both the WirelessHART protocol 70 and the wired
HART protocol 72 may support a common HART application layer 84.
The application layer of the WirelessHART protocol 70 may
additionally include a sub-layer 86 supporting auto-segmented
transfer of large data sets. By sharing the application layer 84,
the protocols 70 and 72 allow for a common encapsulation of HART
commands and data and eliminate the need for protocol translation
in the uppermost layer of the protocol stack.
[0065] FIGS. 3-6 provide a more detailed illustration of channel
and timeslot resource allocation supported by the data link layer
76 and the network layer 78 of the WirelessHART protocol 70. As
discussed above in reference to FIG. 1, the network manager 27 may
manage the definition of one or more superframes and may associate
individual timeslots within each of the defined superframes with
one of the available channels (e.g., carrier frequencies). By way
of one specific example, FIG. 3 illustrates a possible
communication scheme within an individual timeslot, while FIG. 4
illustrates an example data exchange between several devices using
the timeslots of a certain superframe. Next, FIG. 5 illustrates a
possible association between an example timeslot and several
available channels, and FIG. 6 is a schematic representation of
several concurrent superframes which include the timeslots
illustrated in FIGS. 3-5.
[0066] Referring specifically to FIG. 3, two or mode network
devices may exchange data in a timeslot 100, which may be a
dedicated timeslot shared by one transmitting device and one
receiving device or a shared timeslot having more than one
transmitter and/or one or more receivers. In either case, the
timeslot 100 may have a transmit schedule 102 and a receive
schedule 104. In other words, one or more transmitting devices may
communicate within the timeslot 100 according to the transmit
timeslot schedule 102 while one or more receiving devices may
communicate within the timeslot 100 according to the receive
timeslot schedule 104. Of course, the timeslot schedules 102 and
104 are substantially precisely synchronized and begin at the same
relative time 106. Over the course of the timeslot 100, a
transmitting network device sends a predetermined amount of data
over a communication channel such as a carrier radio frequency. In
some cases, the transmitting network device may also expect to
receive a positive or negative acknowledgement within the same
timeslot 100.
[0067] Thus, as illustrated in FIG. 3, the transmit timeslot
schedule 102 may include a transmit segment 110 for transmitting
outbound data, preceded by a pre-transmission segment 112, and may
include a receive segment 122 for receiving an acknowledgement for
the data transmitted during the segment 110. The transmit segment
110 may be separated from the receive segment 122 by a transition
segment 116, during which the corresponding network device may
adjust the hardware settings, for example. Meanwhile, the receive
schedule 104 may include segments for performing functions
complementary to those carried out in the segments 112-122, as
discussed below.
[0068] In particular, the transmitting device may send out the
entire packet or stream segment associated with a capacity of the
timeslot 100 during the segment 110. As mentioned above, the
network schedule 67 may include shared timeslots which do not
exclusively belong to an individual device schedule 69 of one of
the network devices 25 and 30-55. For example, a shared timeslot
may have a dedicated receiver such as the gateway device 22 but no
single dedicated transmitter. When necessary, one of the network
devices 25-60 may transmit unscheduled information, such as a
request for additional bandwidth, over the shared timeslot. In
these cases, the potentially transmitting device may check whether
the shared timeslot is available by performing Clear Channel
Assessment (CCA) in a pre-transmission segment 112. In particular,
the transmitting network device may listen to signals propagated
over the communication channel associated with the timeslot 100 for
the duration of the pre-transmission segment 112 to confirm that no
other network device is attempting to use the timeslot 100.
[0069] On the receiving end of the timeslot 100, the receiving
device may receive the entire packet associated with the timeslot
100 within a packet receive segment 114. As illustrated in FIG. 3,
the packet receive segment 114 may begin at an earlier point in
time than the transmit segment 110. Next, the transmit timeslot
schedule 102 requires that the transmitting device transition the
radio mode in a transition segment 116. Similarly, the receive
timeslot schedule 104 includes a transition segment 118. However,
the segment 116 may be shorter than the segment 118 because the
transmitting device may start listening for acknowledgement data
early to avoid missing a beginning of an acknowledgement.
[0070] Still further, the transmit schedule 102 may include an
acknowledgement receive segment 122 during which the transmitting
device receives an acknowledgement transmitted during an
acknowledgement transmit segment 124 associated with the receive
schedule 104. The transmitting device may delete the packet
transmitted during the transmit segment 110 from an associated
transmit queue upon receiving a positive acknowledgement. On the
other hand, the transmitting device may attempt to re-transmit the
packet in the next scheduled dedicated timeslot or in the next
available shared timeslot if no acknowledgement arrives or if the
acknowledgement is negative.
[0071] Several timeslots 100 discussed above may be organized into
a superframe 140, as schematically illustrated in FIG. 4. In
particular, the superframe 140 may include a (typically) infinite
series of superframe cycles 150-154, each cycle including a set if
timeslots, illustrated in FIG. 4 as a timeslot 142 with a relative
timeslot number 0 (TS0), a timeslot 144 with a relative timeslot
number 1 (TS1), and a timeslot 146 with a relative timeslot number
2 (TS2). Accordingly, the size of the superframe 140 of FIG. 4 is
three timeslots. In other words, each of the timeslots 142-146 of
the superframe 140 repeats in time at an interval of two
intermediate timeslots. Thus, for a 10 millisecond timeslot, the
interval between the end of a timeslot with a particular relative
slot number and the beginning of a next timeslot with the same
relative slot number is 20 milliseconds. Conceptually, the
timeslots 142-146 may be further grouped into superframe cycles
150-154. As illustrated in FIG. 4, each superframe cycle
corresponds to a new instance of a sequence of timeslots
142-146.
[0072] The master network schedule 67 may associate transmission
and reception opportunities of some of the network devices
participating in the wireless network 14 with particular timeslots
of the superframe 140. Referring again to FIG. 4, a network
fragment 160 schematically illustrates a partial communication
scheme implemented between the network devices 34, 60, and 36 of
FIG. 1. To simplify the illustration of the superframe 140, the
network devices 34, 60, and 36 are additionally designed in FIG. 4
as nodes A, B, and C, respectively. Thus, according to FIG. 4, the
node A transmits data to the node B which, in turn, transmits data
to the node C. As discussed above, each of the nodes A-C includes a
device schedule 69A-C, which specifies the timeslots and channels
(e.g., radio carrier frequencies) for transmitting and receiving
data at the corresponding device. The master network schedule 67
may include part of all of the data information stored in the
individual device schedules 69A-C. More specifically, the network
manager 27 may maintain the master network schedule 67 as an
aggregate of the schedules associated with each of the network
devices 25A-B and 30-50, including the device schedules 69A-C.
[0073] In this example, the duration of the timeslot 100 (FIG. 3)
may be 10 milliseconds and the network device A may report data to
the device C every 30 milliseconds. Accordingly, the network
manager 27 may set the length of the superframe 140 at three
timeslots specifically in view of the update rate of the network
device A. Further, the network manager 27 may assign the timeslot
142 with a relative number 0 (TS0) to the network devices A and B,
with the device A as the transmitter and the device B as the
receiver. The network manager 27 may further allocate the next
available timeslot 144, having the relative slot number 1 (TS1), to
be associated with the transmission from the device B to the device
C. Meanwhile, the timeslot 146 remains unassigned. In this manner,
the superframe 140 provides a scheme according to which the network
manager 27 may allocate resources in the network fragment 160 for
the transmission of data from the device A to the device C in view
of the available wireless connections between the devices A, B, and
C.
[0074] In the example illustrated in FIG. 4, the network device at
node A may store information related to the timeslot 142 as part of
its device schedule 69A. Similarly, the network device at node B
may store information related to the timeslots 142 (receive) and
144 (transmit) as part of its device schedule 69B. Finally, the
network device C may store information related to the timeslot 144
in the device schedule 69C. In at least some of the embodiments,
the network manager 27 stores information about the entire
superframe 140, including an indication that the timeslot 146 is
available.
[0075] Importantly, the superframe 140 need not be restricted to a
single radio frequency or other single communication channel. In
other words, the individual timeslots 142-146 defining the
superframe 140 may be associated with different radio frequencies
on a permanent or floating basis. Moreover, the frequencies used by
the various devices need not always be adjacent in the
electromagnetic spectrum. In one embodiment, for example, the
timeslot 142 of each of the superframe cycles 150-154 may be
associated with a carrier frequency F.sub.1 and the timeslot 144 of
each of the superframe cycles 150-154 may be associated with a
carrier frequency F.sub.2, with the frequencies F.sub.1 and F.sub.2
being adjacent or non-adjacent in the electromagnetic spectrum.
[0076] In another embodiment, at least some of the timeslots
142-146 may move about the allocated frequency band in a predefined
manner. FIG. 5 illustrates an example association of the timeslot
144 of FIG. 4 with channels 172-179 (corresponding to frequency
sub-bands F.sub.1-F.sub.5) in the available frequency band 170. In
particular, each of the channels 172-179 may correspond to one of
the center frequencies F.sub.1, F.sub.2, . . . F.sub.5 which
preferably differ from their respective neighbors by the same
offset. The channels 172-179 preferably form a continuous section
of the spectrum covering the entire available frequency band 170,
although the channels 172-179 need be contiguous or form a
continuous band in all embodiments. The superframe 140 may use at
least a portion of the frequency band 170, so that one or more of
the timeslots 142-146 are scheduled on different carrier
frequencies in at least two consecutive cycles.
[0077] As illustrated in FIG. 5, the timeslot 144 may use the
channel 176 (frequency F.sub.3) during the frame cycle 150, may use
the channel 174 (frequency F.sub.4) during the frame cycle 152, and
may use the channel 178 (frequency F.sub.2) during the frame cycle
154. The timeslot 144 may then "return" to the channel 176 in the
next superframe cycle 150A, which may similar to the cycle 150.
Each of the specific associations of the timeslot 144 with one of
the channels 172-179 is illustrated as a timeslot/channel tuple
144A-C. For example, the tuple 144A specifies the timeslot 2
scheduled, in the cycle 150, on the channel 176 associated with the
center frequency F.sub.3. Similarly, the tuple 144B specifies the
timeslot 2 scheduled, in the cycle 152, on the channel 174
associated with the center frequency F.sub.4. Meanwhile, the
channel 172 associated with the center frequency F.sub.5 may not be
assigned to the timeslot 2 during any of the cycles 150-152.
However, a different timeslot of the superframe 140 such as the
timeslot 146, for example, may be associated with the channel 172
during one or more of the cycles 150-152.
[0078] In this example, the frequency assignment associated with
the superframe cycle 150 may repeat immediately following the cycle
154 (illustrated as a cycle 150A in the FIG. 5), and the timeslot
144 may again correspond to the tuple 144A after two cycles of the
superframe 140. Thus, the timeslot 144 may regularly cycle through
the channels 176, 174, and 178. It will be appreciated that the
timeslot 144 may similarly cycle through a greater or smaller
number of channels irrespective of the length of the superframe
140, provided, of course, that enough channels are available in the
frequency band 170. The association of a single timeslot with
multiple channels during different superframe cycles, discussed
above with respect to FIG. 5 and referred to herein as "channel
hopping," significantly increases the reliability of the wireless
network 14. In particular, channel hopping reduces the probability
that a pair of devices, scheduled to communicate in a particular
timeslot of a certain superframe, fail to transmit and receive data
when a certain channel is jammed or otherwise unavailable. Thus,
for example, the failure of the channel 174 prevents the devices
using the timeslot 144 from communicating in the frame cycle 152
but not during the frame cycles 150 or 154.
[0079] Referring again to FIG. 4, the device schedules 69B and 69C
may include the information regarding each of the tuples 144A-C
discussed above in reference to FIG. 5. In particular, each of the
device schedules 69B and 69C may store an assignment of the
timeslot 144 to one of the channels 172-179 within each of the
cycles 150-152. The master network schedule 67 (FIG. 1) may
similarly include this information. Meanwhile, the device schedule
69A need not necessarily include the information related to the
timeslot 144 because the corresponding node A (the device 34) does
not communicate during the timeslot 144 of the superframe 140. In
operation, the devices 60 and 36 corresponding to the nodes B and C
may prepare for data transmission and reception, respectively, at
the beginning of each timeslot 144. To determine whether the
timeslot 144 currently corresponds to the tuple 144A, 144B, or
144C, the devices 60 and 36 may apply a locally stored copy of the
ASN counter 68 to determine whether the timeslot 144 is currently
in the frame cycle 150, 152, or 154.
[0080] In the process of defining the network schedule 67, the
network manager 27 may define multiple concurrent superframes in
view of the update rates of the network devices 25 and 35-50. As
illustrated in FIG. 6, the network schedule 67 may include the
superframe 140 of length three as well superframes 190 and 192. The
superframe 190 may be a five-slot superframe and the superframe 192
may be a four-slot superframe, although the different superframes
may have a different number of slots and various different
superframes may have the same number of slots. As illustrated in
FIG. 6, the superframes need not necessarily align with respect to
the relative slot numbers. In particular, at a particular time 194,
the superframe 190 may schedule the timeslot with the relative
number two (TS2) while the superframes 140 and 192 may schedule the
timeslots with the relative number one (TS1). Preferably, the
superframes 140, 190, and 192 are time-synchronized so that each
transition to a new timeslot within each of these superframes
occurs at the same time.
[0081] Each of the superframes 140, 190 and 192 may be primarily
associated with, or "belong to" an individual one of or a subset of
the network devices 25A-B and 30-50. For example, the superframe
140 illustrated in FIG. 4 may belong to the node A (i.e., the
network device 34), and the length of the superframe 140 may be
advantageously selected so that the node A sends out measurement
data to the node B during the timeslot 142 (TS0) once during each
of the cycles 150-154. In case the wireless network 14 defines 10
millisecond timeslot, the node A sends data to the node B once
every 30 milliseconds. If, however, the node A is reconfigured to
report measurements once every 50 milliseconds, the network manager
27, alone or in cooperation with the node A, may reconfigure the
frame 140 to have a length of five timeslots instead. In other
words, the length of each superframe may reflect a particular
transmission requirement of a particular network device 25A-B or
30-50.
[0082] On the other hand, more than one network device 25A-B or
30-50 may use a superframe for transmitting or receiving data.
Referring again to FIG. 4, the node B (the network device 60) may
regularly transmit data to the node C (the network device 36) in
the timeslot 144 of the superframe 140, although the superframe 140
may be primarily associated with the node A. Thus, different
timeslots of a particular superframe may be used by different
network devices to originate, route, or receive data. In a sense,
the timeslots of each superframe may be understood as a resource
allocated to different devices, with a particular priority assigned
to the device that "owns" the superframe. Further, it will be
appreciated that each network device may participate in multiple
superframes. For example, the network device 34 in FIG. 4 may route
data on behalf of other network devices (e.g., the network device
32 illustrated in FIG. 1), in addition to propagating its own data
via the router device 60. Preferably, a device participating in
multiple superframes does not schedule simultaneous communications
in different superframes. While only three superframes are
illustrated in FIG. 6, the wireless network 14 of FIG. 1 may
include any number of superframes, with each of the different
superframes having any desired or useful length based on the types
and frequencies of communication being performed in or between
particular devices and set of devices.
[0083] As indicated above, the ASN counter 68 (see FIG. 1) may
reflect the total number of timeslots consecutively scheduled since
the activation of the wireless network 14. In other words, only
those timeslots which occur following another timeslot affect the
ASN count, and the number of concurrently scheduled superframes has
no impact on the ASN value. To further outline the operation of the
ASN counter 68, FIG. 7 illustrates a schedule 250 including several
concurrent superframes 252-256 created at or after a network start
time 260. The superframe 252 may be a four-timeslot superframe in
which the relative slot numbers iterate from zero to three.
Similarly, the superframe 254 may similarly start at the network
start time 260 but include eight timeslots numbered zero through
seven. On the other hand, the superframe 256 may be created at a
later time when a new network device joins the wireless network 14,
for example, or when the network manager 27 allocates temporary
resources for a special purpose such as to accommodate a block mode
transfer. The values which the network manager 27 may assign to the
ASN counter 68 during the operation of the network schedule 250 are
generally indicated as a sequence 270. It will be noted that the
value of the ASN counter 68 increases with every new timeslot
irrespective of a superframe with which the timeslot is
associated.
[0084] Referring back to FIG. 1, each of the network devices 25A-B
and 30-50 may maintain a local copy of the ASN counter 68. During
operation of the wireless network 14, the gateway device 22 may
propagate the current value of the ASN counter 68 to each network
device 25A-B or 30-50 for network synchronization. Every network
device 25A-B or 30-50 may then compare a local copy of the ASN
counter to the value reported in a data packet sent by the gateway
device 22 and, if necessary, update the local copy to match the
value of the ASN counter adjusted according to a propagation delay
of the message. For example, the network schedule 67 may specify
that the network node 32 receives a certain type of a data packet,
originated by the gateway device 22 and associated with a
particular superframe, in a third timeslot following the timeslot
in which the gateway device 22 transmits the packet to a neighbor
device. The network node 32 may accordingly check whether the
current ASN value stored by the network node 32 is indeed the value
of ASN included in the data packet plus three (i.e., the number of
timeslots scheduled since the gateway device 22 sent out the data
packet).
[0085] It will be further noted that by propagating ASN information
along multiple paths to each network device 25A-B and 30-50 (FIG.
1), the wireless network 14 ensures that as some of the direct
wireless connections 65 encounter obstacles or fail for other
reasons, the network device 25A-B and 30-50 typically have at least
one more access to synchronization information, thus increasing the
stability of the wireless network 14 and improving its overall
resilience.
[0086] Additionally or alternatively, the network devices 25A-B and
30-50 also use the ASN value included in a data packet for
ascertaining an age of the data packet. For example, a destination
network node may receive a data packet, subtract the ASN inserted
into the data packet at the originating network node from the local
copy of the ASN value, and calculate the age of the data packet by
multiplying the difference in the number of timeslots by the
duration of an individual timeslot. It will be noted that by
relying on the ASN value included in data packet, the wireless
network 14 may enforce time-to-live (TTL) requirements, perform
network diagnostics, collect delivery delay statistics, etc.
[0087] In some embodiments, every message between a pair of
neighbor devices may include the ASN value in a Network Protocol
Data Unit (NPDU). If the wireless network 14 uses the WirelessHART
protocol 70 schematically illustrated in FIG. 2, each frame
associated with the layer 78 may include the ASN value to ensure
that the neighbors sharing a direct wireless connection 65 are
properly synschronized. In one particular embodiment, each network
device 25A-B or 30-50 may include only a portion of the ASN value
in an NPDU frame to reduce the amount of data transmitted at the
level of the network layer protocol. More specifically, the
wireless network 14 may maintain a 32-bit ASN value but the
corresponding ASN snippet may include only the lower 16 bits of the
ASN value. It will be appreciated that because a typical message is
delivered within a seconds or even milliseconds, several lower bits
of the ASN value may be sufficient to measure the TTL value. Of
course, other embodiments may use an even smaller snippet.
[0088] Further, the network devices 25A-B and 30-50 may use the ASN
value to determine a current timeslot in a particular superframe.
In some embodiments, these devices may apply the following function
to calculate a relative slot number within a superframe: relative
slot number=ASN % (length of the superframe), where the symbol "%"
represents the modulo division function. A network device 25A-B or
30-50 may use this formula to construct an ordered list of the
timeslots that are about to occur in the relevant superframes. It
will be noted that in some embodiments, each new superframe of a
certain length may start at such a time as to fit an integer number
of superframes of this length between this time and the start time
of the network. Referring again to FIG. 7, for example, the
superframe 256 may have eight timeslots and may accordingly start a
timeslot 0, 8, 16, . . . , 8n, where n is an integer. In other
embodiments, new superframes may not start at an ASN value equal to
a multiple of the superframe length, and the joining device may add
an additional offset to a result of applying the formula above.
[0089] In another embodiment, the devices attempting to join the
wireless network 14 may use the ASN value to properly synchronize
with the activate network schedule 67. In particular, each active
network device 25A-B and 30-50 may periodically sent out
advertisement packets which the potential new neighbors of these
devices may process to determine whether one or more new direct
wireless connections 65 may be formed between the joining device
and one more of the advertising devices. In addition to evaluating
the strength and, optionally, the quality of a signal associated
with each advertising (potential) neighbor, the joining device may
consider a number of other factors when processing advertisement
packets. For example, each advertisement packet may include a
network identity field which the joining device may compare to the
network identity with which the joining device has been previously
provisioned. This process may ensure that the joining device joins
the correct network if several similar wireless networks 14 operate
within a short distance from each other or if there is some overlap
between the geographical areas covered by these networks.
[0090] Referring to FIG. 8, a state diagram 300 illustrates some of
the representative states associated with a network protocol layer
of a network device participating in the wireless network 14. The
network device may enter the state 302 immediately upon power-up
and remain in the idle state 302 until receiving a command to
initiate a join sequence. In the state 302, the network device may
not be provisioned to communicate with any other devices of the
wireless network 14. In at least some of the embodiments, an
operator may provision or otherwise communicate with the network
device via a maintenance port.
[0091] Referring to FIG. 8, a state diagram 300 illustrates some of
the representative states associated with a network protocol layer
of a network device participating in the wireless network 14. It
will be noted that the state diagram 300 corresponds to a
particular embodiment of a network device 25A-B or 30-50 and that
in other embodiments the corresponding state machine may combine
certain states illustrated in FIG. 8 together or, conversely,
implement more states responsive to fewer or more transition
events. Further, one of ordinary skill in the art will appreciate
that a network device 25A-B or 30-50 may implement multiple
concurrent state machines associated with different operational
modes, sessions, network layers, etc., and that some contemplated
embodiments may associate some of the operations illustrated in the
state diagram 300 with other protocol layers, for example.
[0092] As illustrated in FIG. 8, the network device may enter the
state 302 immediately upon power-up and remain in the idle state
302 until receiving a command to initiate a join sequence. In the
state 302, the network device may not be provisioned to communicate
with any other devices of the wireless network 14. In at least some
of the embodiments, an operator may provision the network device
with one or more security keys, network identity, and or other
configuration data via a maintenance port.
[0093] In a joining state 304, the network device may begin to
listen for advertisement messages and attempt to locate the
wireless network 14 by comparing the network identity reported in
the advertisement messages to the network identity value with which
the network device has been provisioned. In particular, a join
procedure may begin with the network device selecting a particular
radio channel and starting to listen for advertisement packets.
This mode of operation may be called a promiscuous mode. If the
network device does not receive an advertisement packet within a
certain amount time (e.g., four timeslots, one second, etc.), the
join procedure may select a different radio channel for another
iteration. If, on the other hand, the network device receives an
advertisement packet, the join procedure may process the packet and
either accept the advertisement or return to the promiscuous mode
and listen for additional advertisement packets. In an alternate
embodiment, the join procedure may accumulate a certain number of
advertisement packets prior to selecting the best candidate from
the accumulated set.
[0094] In evaluating advertisement packets, the network device may
consider several factors such as the strength of a signal
transmitted by a potential neighbor device, for example. Because it
may not be desirable to define a link that has poor signal quality,
or because a network device may have several choices with respect
to selecting one or more neighbors, it may be desirable to consider
the signal strength as one of the factors in defining routes
through graph or source routing as well as in defining schedules.
More specifically, the network device may calculate a received
signal strength indication (RSSI) which is indicative of the
energy, not quality of the signal. To arrive at the RSSI value, the
network device may measure received signal level (RSL) which may be
expressed in decibels of the detected signal. Alternatively, the
network device may choose the traditional approach of measuring the
quality, and not the strength, of the signal. It some embodiments,
the network device may report signal strength measurements to the
network manager 27 and may then wait for to receive superframe,
graph, and link configuration from the network manager 27. In these
embodiments, the network manager 27 may further consider such
factors during graph and schedule formation as the projected number
of hops, projected amount of traffic traveling through each node,
power capability at each node, and the resulting latency for each
type of traffic, etc.
[0095] By processing advertisement messages, the network device may
also synchronize with the wireless network 14 and update a local
copy of the ASN counter 68. Once synchronization is complete, the
network device may forward a request for admission into the
wireless network 14 to the network manager 27. To this end, the
network device may extract the information regarding an available
join session from one or more advertisement packets.
[0096] Next, the network manager 27 may also perform one or more
authentication procedures to ensure that the network device is
properly authorized to participate in the wireless network 14. With
continued reference to FIG. 8, the network device may operate in a
quarantined state 306 until the network manager 27 or an external
application fully approves the newly joined network device. The
network device may perform limited functions in the wireless
network 14 while in the quarantined state 306. For example, the
network device may not be allowed to forward data packets
originated by peer network devices 25A-B or 30-50 until allowed to
transition to an operational state 308.
[0097] In the operational state 308, the network device may fully
participate in all network operations such as interacting with the
gateway 22 to provide access to an external application to various
operational parameters of the network device, negotiate bandwidth
for publishing scheduled process data and/or unscheduled data in a
block transfer mode, and sending out advertisement packets to
invite new wireless devices to join the wireless network 14, for
example. As indicated above, the network device may also be allowed
to route data between peer network devices 25A-B or 30-50 in the
operational state 308.
[0098] At some during the operation of the wireless network 14, the
gateway device 22 may receive an indication from the outside
network that there may be an explosive, radio-sensitive device in
the vicinity of one or more wireless network devices 25A-B or
30-50. The gateway device 22 may forward this indication to the
network manager 27 which, in turn, may broadcast a suspend message
requesting suspension of all communications in the wireless network
14. Alternatively, the gateway device 22 may support a suspend
command on the wired interface connecting the gateway device 22 to
the plant automation network 12. A properly authorized operator may
use the workstation 16, for example, to suspend the wireless
network 14 by directing a certain command to the network manager 27
directly or via the gateway device 22.
[0099] In some embodiments, the suspend command may be a broadcast
message carrying the same information from the network manager 27
or the gateway device 22 to every network device 25A-B and 30-50.
The broadcast suspend command may specify, for example, the time at
which the communications should stop and, optionally, the time at
which the communications should resume. Alternatively, the suspend
command may specify the time at which the wireless communications
should stop and a duration of quiet time during which the
communications are not allowed to resume. As yet another
alternative, each network device 25A-B or 30-50 may start a timer
upon receiving the suspend command and resume communications when
the time expires. For example, each network device 25A-B or 30-50
may be preconfigured to suspend communications for five seconds
after receiving the suspend command. Of course, some of the network
device 25A-B or 30-50 may also be configured with different timeout
values depending on the device type, for example. As yet another
alternative, the timeout value may be proportional to the update
rate of the fastest device in the wireless network 14. It will be
noted that the suspend command in these embodiments need not
specify the time at which the communications should resume.
[0100] Some embodiments of the wireless network 14 may also use the
suspend command as a unicast or a multicast message. For example,
an external host such as the workstation 16 may send to the network
manager 27 a reporting specifying a condition which may require
radio silence specific to a certain geographical area. In some
embodiments, the network manager 27 may be aware of the topology of
the wireless network 14 not only schematically (e.g., maintaining a
graph based on signal strength, etc.) but also spatially, i.e., in
terms of at least approximate physical locations of the network
devices 25A-B or 30-50. In these embodiments, the network manager
27 may be able to determine which of the network devices 25A-B and
30-50, if any, are proximate to the geographical area specified in
the condition report. The network manager 27 may then suspend
communications in a particular portion of the wireless network 14
by sending unicast or multicast suspend commands to the relevant
network devices 25A-B and 30-50.
[0101] Referring again to FIG. 8, the network device implementing
the state machine 300 may transition to a suspended state 310. In
this state, the network device may at least suspend radio
transmissions. In some embodiments, the network device may also
stop listening to incoming data to preserve battery life. However,
it is contemplated that in at least one possible implementation,
the suspension of transmissions in the wireless network 14 may be
indefinite, and the network devices 25A-B and 30-50 may not
transmit data until receiving a wake-up command to cancel the
suspend command. In this case, the network devices 25A-B and 30-50
may either continue to listen to incoming data at the scheduled
rate or at a reduced rate, such as by waking up once every second,
for example. In the particular embodiment illustrated in FIG. 8,
the network device may start a suspend timer upon a transition into
the suspend state 310. If desired, the suspend timer may be the
difference between the transmission resume time specified in the
suspend message and the current time or, alternatively, a
predefined value with which the network device has been provisioned
via the maintenance port.
[0102] In some embodiments of the wireless network 14, a network
device 25A-B and 30-50 may be allowed to forward the suspend
command prior to aborting transmissions in compliance with the
suspend command. Referring back to FIG. 1, for example, the network
device 32 may receive a suspend command broadcast from the network
manager 27 via the gateway 22 and forward the suspend command to
the network device 60 prior to suspending transmissions at the
network device 32. In this particular embodiment, the gateway
device 22 may not expect to receive an acknowledgement for the
suspend command from each command recipient. On the other hand, if
the network manager 27 explicitly specifies the time at which the
communications should stop as part of the suspend command, each
network device 25A-B and 30-50 may properly respond to the suspend
command with a positive or a negative acknowledgement.
[0103] Referring still to FIG. 8, the network device may transition
from the suspended state 310 to a re-synching state 312 when the
suspend timeout expires. As discussed above, the transition to the
state 312 could be triggered by an explicit command in some of the
alternative embodiments. In the re-synching state 312, the network
device may determine the start of a timeslot at the relative time
106 (see FIG. 3) and, once the timeslot timing is determined, the
network device may calculate relative slot numbers in or more
superframes in which the network device participates. To this end,
the network device may apply the modulo division formula presented
above, i.e., the network device may calculate the relevant relative
slot numbers based on the ASN value. The states 304-312 may
therefore include a continual update of the ASN count based on an
internal clock of the individual network device. In this sense, the
network schedule may advance by the same number of timeslots
irrespective of whether one or more network devices 25A-B or 30-50
are in the suspended state 310.
[0104] Further with respect to ASN, the suspend command may specify
the time at which the communications should stop and/or the time at
which the communications should resume in ASN units to reduce the
amount of data transmitted in the wireless network 14 and to
provide a discrete time scale. Accordingly, each of the network
devices 25A-B or 30-50 may measure timeout intervals in ASN units
to simplify time-related calculations.
[0105] In general, it will be appreciated that a human operator or,
in some automated decisions, the network manager 27 may suspend the
wireless network 14 in response to detecting conditions other than
a blast operation discussed in the example above. For example, an
operator may require radio silence because of an unrelated test or
due to a highly sensitive measurement performed near one or more
network devices 25A-B or 30-50. In short, the suspend command may
be used for a variety of purposes and, because of the ability of
the wireless network 14 to efficiently resynchronize
communications, may have a very limited and controlled impact on
the process control environment.
[0106] In another aspect, it will be noted that an operator such as
a plant technician may use the handheld device 55 to connect to the
wireless network 14 and send a suspension request to the network
manager 27 which may process the suspension request, optionally
verify the operator's authorization with respect to triggering a
temporary period of radio silence, and propagate the suspend
command through the wireless network 14 in the manner discussed
above. Moreover, the operator may use a wired handheld device (not
shown) to connect to a wireless adapter such as the WirelessHART
adapter 50 and thereby access the wireless network 14. Of course,
as discussed above, the operator may similarly issue the same or
similar suspension request from outside the wireless network
14.
[0107] Although the forgoing text sets forth a detailed description
of numerous different embodiments, it should be understood that the
scope of the patent is defined by the words of the claims set forth
at the end of this patent. The detailed description is to be
construed as exemplary only and does not describe every possible
embodiment because describing every possible embodiment would be
impractical, if not impossible. Numerous alternative embodiments
could be implemented, using either current technology or technology
developed after the filing date of this patent, which would still
fall within the scope of the claims.
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