Co-Existence of WirelessHART with other Wireless

Co-Existence of WirelessHART with other Wireless
Co-Existence of WirelessHART
with other Wireless Technologies
Co-Existence of WirelessHART with other Wireless Technologies
HCF_LIT-122
Rev. 1.0
Date of Publication: March 9, 2010
Contributors: Gareth Johnston, ABB; Jim Cobb, Emerson Process Management; Eric Rotvold, Emerson
Process Management; Rojiv Singhal, Cisco Systems
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approval please contact the HART Communication Foundation (HCF) at the address shown below.
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Co-Existence of WirelessHART with other Wireless Technologies
Synopsys:
The IEEE 802.15.4 radios specified by the WirelessHART specification use
the same 2.4 GHz Industrial, Scientific, and Medical (ISM) non-licensed
frequency band as several other wireless technologies. There have been
several studies on how these technologies may interfere with one another
however; this research has focused on the use of a single wireless channel
of operation. WirelessHART utilizes new advancements such as channel
hopping and mesh networking to ensure co-existence with other wireless
networks.
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Co-Existence of WirelessHART with other Wireless Technologies
Co-Existence
Co-existence is defined as “The ability of one system to perform a task in a given shared environment
where other systems have an ability to perform their task and may or may not be using the same set of
rules.”i Successful co-existence is measured by the reliability of each network to deliver its messages to
the desired destination. Therefore, each network must be able to accomplish its objective while not
disrupting the ability of another network to complete its objective.
Problems can occur when two or more packets of information are transmitted at the same time and
frequency such that they “collide” in the same physical space. If networks aren’t designed to minimize or
avoid these occurrences, unreliable communications will result.
There are several techniques that can be used to minimize network interference:
 Channel hopping – changing the frequency channel
 Time Division Multiplexing – varying the time of communications
 Power Modulation – low power transmission
 Direct Sequence Spread Spectrum
 Mesh networking supports large physical space with low power instruments
 Blacklisting and channel assessment
In the data link layer of the WirelessHART protocol, packet acknowledgment with automatic retry assures
data is not lost if interference does happen to occur.
Channel Hopping
As specified by IEEE802.15.4, the 2.4 GHZ ISM frequency band is divided into 16 non-overlapping
frequency channels. WirelessHART instruments use a pseudo-random channel hopping sequence to
reduce the chance of interference with other networks, such as IEEE802.11b/g (Wi-Fi) which operates in
the same ISM frequency band.
The figure below shows the potential of overlap between IEEE802.11g and IEEE802.15.4 radios. Since the
802.11b/g channels are wider, there are only 3 non-overlapping channels for Wi-Fi networks. A given
IEEE802.11 Wi-Fi access point will only use 1 of the 3 non-overlapping channels and will only broadcast
periodically, so the channel is not in continuous use. Pseudo-random channel hopping inherent to
WirelessHART instruments ensures that they do not fix on using a channel being used by an
IEEE802.11b/g network for any lengthy period of time. Together with the other techniques listed above,
the probability of interference is minimal either way.
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Co-Existence of WirelessHART with other Wireless Technologies
35
Outdoor Mesh
802.11 b/g
EIRP (4W)
30
Tx power [dBm]
25
Channel 1
Channel 6
Channel 11
20
Indoor 802.11 b/g
EIRP (100mW)
15
802.15.4
EIRP (1-10mW)
10
5
0
-5
-10
-15
-20
-25
2400
2412
2437
Frequency [MHz]
2462
2483.5
802.11 (US)
802.15.4
Time Division Multiplexing
A WirelessHART network utilizes Time Division Multiple Access (TDMA) technology to ensure that only
one instrument is talking on a channel at any given time. This prevents message collisions within the
WirelessHART network. A network is provided with an overall schedule which is divided into 10 msec
timeslots. At any time, only one pair of instruments are communicating on the same frequency channel,
however, it is possible that more than one pair of instruments can communicate at the same time using
different channels. In most cases, only one pair of instruments is communicating in a given timeslot so
the WirelessHART network will not monopolize the frequency spectrum that is shared with other wireless
networks.
Power Modulation
The IEEE802.15.4 radios were chosen because they are relatively low power instruments suited to
wireless process control applications, as well as being readily available at a reasonable cost. The radios
are used with 10dB amplifiers to allow communication of up to 200m to the next instrument, which in
turn can serve as a router to pass the message along. In cases where the full distance is not required,
WirelessHART instruments can transmit at a lower power to reduce the chance of interfering with other
networks in the ISM frequency band. The lower transmit power of WirelessHART instruments also means
that any chance of interfering with a IEEE802.11b/g Wi-Fi network is small.
Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum (DSSS) technology provides about 8dB of additional gain utilizing
unique coding algorithms. The transmission is spread over the entire frequency of the selected 802.15.4
channel. Devices with the correct decoding information can receive the data while others see the
transmissions as white noise and disregard it. This allows multiple overlapping radio signals to be
received and understood only by other devices in their own networks.
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Co-Existence of WirelessHART with other Wireless Technologies
Mesh Networking
The use of mesh networking technology complements the use of the low power IEEE802.15.4 radios.
With mesh networking, instruments do not need to have a direct transmission path to the network
gateway. It is only required that any instrument be able to communicate to any other instrument in the
mesh network. Each WirelessHART field instrument is capable of routing the message of other
instruments along a route that will ensure the message is received at its ultimate gateway destination.
Mesh networks also provide path redundancy and thus achieve better reliability than if each instrument
were required to have a direct line of sight path to the gateway. The mesh network can adapt to
changing communication and other environmental conditions to find a reliable communication path to
the gateway.
Physical separation of >1m between WirelessHART instruments and Wi-Fi access points is all that is
required for peaceful coexistence between the two types of wireless networks.
Blacklisting and Channel Assessment
In conjunction with channel hopping the WirelessHART network can be configured to avoid specific
channels that are highly utilized by other networks and therefore likely to provide interference. However
because most networks are not loaded continuously this is rarely required.
To further avoid any conflict with other neighboring networks a WirelessHART instrument listens to the
frequency channel prior to transmitting data. If other transmissions are detect the WirelessHART
instrument will back off and attempt the communication in another timeslot on a different frequency.
Summary
The WirelessHART technology was designed specifically to work in the 2.4GHz ISM band in an
environment where other wireless networks are expected to be found. Using techniques such as channel
hopping, time division multiple access, low power, mesh networking, and direct sequence spread
spectrum coding allow a WirelessHART network to maintain high data reliability and at the same time
minimize, if not eliminate, any effect it has on other overlapping networks.
i
IEEE Std 802.15.2-2003, Part 15.2: Coexistence of Wireless Personal Area Networks with Other Wireless
Instruments Operating in Unlicensed Frequency Bands, 28 August 2003.
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