Wireless, serial and Ethernet link for environment project
It is not often that a project demonstrates just how far industrial
telemetry has developed from the early days of radio datacomms. This project
- the remote wireless sensing of an extensive wetlands area for ecology research
- makes use of the best characteristics of wireless serial-to-Ethernet technology
and shows clearly how its use could be advantageous in other wide-area applications.
By Tim Cutler
Elkhorn Slough, located on the southern
California coast of the United States,
commands a great deal of interest among
coastal management scientists. It's a beautiful
area encompassing over 1400 acres,
much of it wetlands that form a breeding
area for sea life and migratory birds. It is
also the site of a power plant and an active
fishing harbour, and is surrounded by commercial
As such, Elkhorn Slough sustains life in
a delicate balance that has been watched
over for decades by marine scientists.
Monitoring has consisted of regularly testing
the makeup and quality of the water to
determine whether industrial and agricultural
processes are becoming problematic
for natural life. This has required periodically
visiting various locations in the slough
by boat, collecting water samples and
transporting those samples to a laboratory
While this activity provided continuing
measurement of water quality and alerted
coastal managers to states of imbalance,
the data was of limited use. Because sampling
was sporadic, it could not reflect the
impact of such short-term events as a
heavy rain and the associated draining
into the Slough of nutrient-rich surface
water from farmland. And if a severe
imbalance was discovered, days could be
lost in the effort to redress the situation. In
all cases, scientists monitoring the area
lacked clear visibility into the ecosystem's
The Monterey Bay Area Research
Institute (MBARI), an alliance of ocean
scientists and engineers, undertook to
improve on this situation in 2004. The scientists
knew that if they could create an
automated system that sampled not only
the chemistry of the water but also such
properties as its temperature and current
speed, and obtain these samples in real-time
from various locations simultaneously,
they could better understand the area's biological,
geological and chemical changes
while pinpointing the causes of those
changes. They could then make better informed
decisions regarding the management
of Elkhorn Slough.
this end, MBARI established the Land/Ocean Biogeochemical Observatory (LOBO),
with the goal of determining the potential that sensor networks might play in
managing this coastal resource and, if the potential proved real, to create
such a system. There were no ready-made models to follow; this would be a pioneering
effort in which high-resolution data would need to make its way from buoys placed
throughout wetlands, without benefit of electricity, all the way to a shore-side
Ethernet network miles away and eventually to the Internet for near real-time
The LOBO team's first objective was to design the remote instrumentation that
would collect the data. Sensors would be needed to monitor such water properties
as salinity, temperature and current velocity, and essential nutrients including
nitrate, ammonium, and phosphate ions. Sampling from these sensors needed to
be high-resolution, especially for the nutrient sensors, to ensure precise and
reliable identification. At the same time, the sensors had to consume as little
power as possible as electricity was unavailable throughout the slough. As this
meant that the sensors would need to be battery-powered, they would need to
be serial rather than Ethernet devices.
The team selected off-the-shelf instruments for all but the nitrate sensor,
which had been developed in-house by MBARI specifically for marine sensing.
The entire group of sensors were capable of being neatly packed within a single
capsule, to be placed on a mooring for off-shore deployment.
In planning the placement of the sensor moorings, the team identified four
locations to be monitored. This placement called for four nodes throughout the
slough, with an overall end-to-end distance exceeding four miles.
Just as the location of these sensors throughout the wetlands made supply
of electric power problematic, the environment and the distance conspired to
make data cabling impractical. Wireless networking became a requirement, with
radio placed within each mooring to relay the sensor data.
The wireless network
investigation of industrial radios to use with the sensors was guided by five
primary requirements, as follows:
Power consumption. As was the case with the sensors, the radios would
have to be battery powered due to lack of electricity. The team sought to
identify the most power-miserly radios that would also satisfy other radio
Interference resistance. Because of the amount of industry in the
immediate area largely from the power plant - there was a known
high level of 802.11 traffic that could potentially interfere with
radio transmission - any radio selected for the project would need
to be highly resistant to interference and jamming.
Transmission range. The sensor moorings were to be spread
throughout the slough in an area exceeding 6km in total length.
While end-to-end transmission was not a requirement, all of the
radios would need to transmit their data to a single collection
point. The team estimated that the most remote radio in the network
would need to support transmission over a line-of-sight distance
of at least 4.5km.
Serial-to-Ethernet conversion. As the data collection began with
serial sensors but would ultimately be delivered to an Ethernet network
for Web presentation, the network would need to offer serialto-
Ethernet data conversion.
Near real-time delivery. To support the precise pinpointing of biogeochemical
processes in the slough, the network would need to deliver data from all moorings
in near real-time so that data could be aggregated for an accurate view into
After extensive investigation, the wireless hardware selected by the LOBO team
comprised three closely related devices manufactured by Cirronet.
Each mooring would include a 2.4GHz wireless OEM module
(WIT2410), paired with a whipless antenna, to log and provide
telemetry for instrument data. Of special interest was the module's
low power consumption (less than 50µA in sleep mode, 40mA typical
operation), which would help conserve battery power, and its
frequency hopping spread spectrum modulation characteristic
which minimises data loss caused by jamming and interference
while maximising bandwidth sharing. The radio/antenna combination
provides reliable transmission at ranges of up to 8km, which
was more than adequate for the distance to be covered in Elkhorn
The Wireless Access Point
A 2.4GHz serial network access point (SNAP2410) would
receive transmissions from the OEM modules (up to 62 remote
radios are supported). The LOBO team used the terminal's built-in
serial-to-Ethernet conversion facility so eliminating the need for a
separate serial-to-10/100BaseT Ethernet conversion device. The
SNAP allows limited-intelligence, legacy serial devices to appear as
nodes on an Ethernet network while doing away with the need for
those devices to handle the TCP/IP protocol directly. Each node
can have either an individual IP address or a unique port number
under the SNAP's IP address. SNAPs are compatible with standard
socket-based server applications, which communicate with
the serial devices via standard WinSock routines.
The SNAP server mode was put into service to allow seamless
data collection; the feature eliminated the need to poll remote
radios so reducing battery duty at the radio buoys. The mooring
radios would need to power up only for brief transmissions. The
buoy controllers have internal timers that wake them up every hour
to send updated data. Data freshness is a key requirement of this
application as many elements in the water can change concentrations
rapidly. The buoys timers are independent of each other. The
timer wakes up the controller which takes a reading, wakes up the
radio and sends the data. There is no requirement that the radios
be polled thus they can remain asleep until the prescribed time to
send data. Although MBARI is not changing the sampling period
remotely for this application, there is no reason preventing this
from being done.
The SNAP2410 is built around the WIT2410 module, ensuring the highest degree
of transmission compatibility.
Networking multiple types of radios. Each SNAP2410 hands off its encapsulated
data to a SEM2410, which forwards it over-air to another SEM2410 attached to
an Ethernet network. Note that there are actually three wireless networks involved
in this application: Two networks facilitate communications between each SNAP2410
and its associated WIT2410s, and a separate network links the SEM2410s. The
64 hopping patterns included in the underlying FHSS technology enable the networks
to coexist without interfering with each other.
The end product: plotting data is delivered over the Internet
Wireless Ethernet bridge
Once encapsulated as Ethernet datagrams, the sensor data would
be backhauled to the MBARI LAN by a 2.4GHz wireless Ethernet
bridge (SEM2410) which is also built around the WIT2410 module.
The SEM2410 would deliver the IP data to a shore-side Unix
machine, where a custom MBARI-designed server application
would present the data over the Internet. The delivery of information
via standard web browsers was extremely attractive, as it
would make data available in near real-time to scientists wherever
they might be located. MBARI's mission involves educational outreach,
and making data accessible over the Internet was important
to supporting that mission as well.
The only part of this plan that required modification during
implementation involved the SNAP2410 data collection point. All
four moorings were intended to transmit their data to a single
SNAP access point, but one mooring location made line-of-site
impractical. The solution was to place a separate, intermediate
SNAP2410, configured to provide a repeating function.
For added assurance of successful data transmission, MBARI
commissioned a special ZMODEM file transfer implementation
that governs the delivery of sensor data from the moorings to the
MBARI server. While the radios' built-in ARQ (automatic repeatrequest)
mode provides valuable error correction at the link layer,
ZMODEM provides application-level correction, including crash
recovery that restarts an interrupted transmission at the point at
which transmission was dropped.
How it all works
The MBARI LOBO application demonstrates clearly how an integrated
wireless network can capture raw serial data, transmit it
over-air for several kilometres, and deliver the data as fully formatted
Ethernet ready for a near real-time monitoring application. The
process is completely self-contained and automated, and the principles
apply to a wide variety of industrial needs.
In this particular application, an MBARI node controller wakes
up each mooring's radio when it is time to transmit sensor data.
Each transmission is extremely brief, done at a relatively fast data
rate (nearly 1.5Mbps), and this is the only time the radio requires
significant power from the mooring's lithium batteries. Each radio
reverts to sleep mode immediately after transmitting its data.
Serial data streams are transmitted over-air from the moorings to
the wireless access point; though they are sending unformatted serial
data, the moorings appear to the application as nodes on the
Ethernet network. Each SNAP encapsulates unformatted serial
data into Ethernet datagrams.
Once the Ethernet data reaches the final shore-side SEM2410, it
is passed to a Unix server where an MBARI-designed application
makes the sensor readings immediately available to authorised
users everywhere, with a browser interface that lets scientists select
how to plot the data.
The LOBO project, which has completely met its goals, validated
the use of automated sensing in the study of coastal biogeochemistry.
By simultaneously monitoring the slough's hydrological
and nutrient chemical cycles, scientists are now able to evaluate the
effects of human activities on those cycles at the time and place at
which they occur. Precise measurement of such activity at the
boundary of land and ocean has long been a major scientific challenge,
and LOBO met that challenge with great success. Scientists
involved in the project expect that their counterparts in numerous
oceanographic organisations will have the same need for this pioneering
system, which represents a tremendous improvement over
the old system of retrieving samples by boat for delayed analysis at
a laboratory, with no means to investigate underlying dynamics.
The workings of the LOBO network also point the way to solutions
in virtually any industrial application where serial data must
move seamlessly to an IP network - including PLC and sensor
monitoring, factory floor automation and SCADA - where wired
connections are impractical, using the exact same components as
those deployed successfully at Elkhorn Slough.
MBARI Technical Contact, Luke Coletti firstname.lastname@example.org www.mbari.org/lobo
Tim Cutler is vice-president, marketing, Cirronet Inc