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Sensing problems at the network periphery

Posted: 10 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:seismograph? sensor-driven analytics? Ethernet?

Almost two millennia ago, the first seismograph known to man was built by Zhang Heng during the Han Dynasty. It could indicate the general direction of an earthquake's epicenter by dropping a bronze ball from one of eight bronze dragon's heads into the mouth of one of eight bronze toads waiting below.

Granted, this wasn't a lot of data. The device could tell you that an earthquake had occurred, and it could provide some rather vague information about where it might have happened. It wasn't much, but it was a start. Sensors and their accompanying recording devices have been growing steadily more sophisticated ever since.

In the modern world, sensors are used in ways that Zhang Heng could never have imagined, recording everything from exhaust gas temperatures to the electrical activity in a human heart. And they don't just collect data anymore. They're becoming intelligent. Embedded microcontrollers and built-in software are turning them into thinking, communicating, and active nodes of intelligent networks, systems that demand localized intelligence and real time, sensor-driven analytics. You don't just connect them anymore; they communicate with you.

The network periphery
Earlier generations of sensors reported their data via basic analog or digital connections. These days they're becoming Ethernet-aware, with all of the accompanying advantages. Their use of universal Ethernet communications protocols enables them to work with off-the-shelf technology, and their ability to communicate via Ethernet means that they can be placed just about anywhere.

But Ethernet technology has its roots in safe, climate-controlled office environments. The IT world thought in terms of structured cabling systems, mature protocol and transport standards, and hardware vendors who would provide standardized products with near-seamless interoperability. Who knew, back then, that networking would outgrow its tame, office-based beginnings and move out into the real world?

Today networks must function reliably in increasingly harsh terrainon factory floors, on gas and oil pipelines, in industries ranging from mining to transportation. And sensors tend to live way out on the network periphery, where conditions are the very worst. Design engineers must know what to do about that.

Getting to the Ethernet
Connecting sensors to Ethernet can be problematic in real world scenarios. The first issue is distance. Copper wire-based Ethernet has a practical range limitation of 100m. That's adequate for a network in an office or a small building, but it won't do the job when you need to monitor the turbines on a wind farm or the chlorine levels at a water treatment plant. To be useful outside the office, Ethernet must function at far greater ranges.

One answer is a device called an Ethernet extender (figure). Ethernet extenders use DSL technology to create a long-distance Ethernet bridge over virtually any available copper pair. There's a drop in bandwidth as the range increases, but you can reliably extend Ethernet over thousands of feet while maintaining a quite serviceable connection rate of several Mbit/s. Better yet, Ethernet extenders give the system designer the freedom to use existing wiring infrastructure, like any telephone cabling or legacy coaxial cable that may be present. As the labor and materials involved in cable installation are often the most expensive element in setting up a network, the flexibility provided by Ethernet extenders can represent an enormous savings.

Figure: Ethernet extenders enable data to travel up to 6200 feet on copper wire.

Fiber optics
A second option is fiber optic cable. There are two kinds. The cheaper option, multimode fiber optic cable, uses LED light and can carry data several miles. Its long-range cousin, single-mode fiber optic cable, transmits with a laser rather than an LED. It's more expensive than multimode cable, as are the associated transceivers and receivers. But it has the ability to transmit data over great distances. Telephone and cable companies use it for their long haul applications.

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