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 farmland.

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 for analysis.

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 dynamics.

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.

To 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 access.

The sensors

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

The 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 requirements.

• 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 biogeochemical processes.

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 Slough.

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 coletti@mbari.org www.mbari.org/lobo

Tim Cutler is vice-president, marketing, Cirronet Inc