Facility uses free space optics for wireless feeds

Perched on the rooftops of several Austin Radiological Association sites are four optical wireless linkheads. These combination transmitter/receivers busily beam radiology data using infrared lasers at speeds up to 100 Mb per second. The optical bridges represent the association's foray into free space optics.

This new approach uses "line-of- sight" optical technology. Whereas fiber-optic technology transmits light data through fiber lines, free space optics sends light data from a transmitter to a receiver through the air using lasers.

The radiology practice, which supports 14 imaging centers throughout central Texas, has implemented free space optics products developed by LightPointe, a company based in San Diego.

"I consider free space optics as one of many viable options for providing high-bandwidth wide area network/campus network connectivity, especially for areas where more traditional options are either nonexistent or prohibitively expensive," said Dr. Paul Chang, director of radiology informatics at the University of Pittsburgh Medical Center.

Chang is a frequent speaker on wireless technology.

High-bandwidth "last mile" networking technology, which connects the enterprise to the high-speed, high-bandwidth metro fiber backbone, can be extremely expensive. Providing fiber connections over that last mile may involve high costs to lay new fiber down to the enterprise or expensive licensing fees for traditional radio-frequency wireless technologies. Cost considerations were one important reason the ARA chose free space optics as its last mile networking technology.

The choice gave them the same bandwidth as their typical carrier circuit with a quick return on investment, according to chief information officer R. Todd Thomas.

"We went with wireless because it saves us so much money," said ARA network engineer Robert Berken. "A standard Ethernet circuit will cost about $2000 a month, basically a recurring cost until death do us part. If you go for a superdeluxe model of free space optics, it may set you back about $30,000-that's it."

The large upfront cost covers the technology's hardware and does not include facilities costs such as power to the equipment. These expenses, however, are the same for leased line networks, he said.

Improved security was another factor tipping the balance in favor of free space optics for the ARA. RF technology has inherent security issues that don't exist with free space optics, according to Berken. Various flavors of the popular 802.11 series of RF technology share bandwidth with other wireless devices such as wireless phones and Bluetooth-enabled wireless devices. In addition to a degradation in performance if the frequency becomes too crowded, security can be a significant problem with these shared-spectrum technologies.

Chang has previously said that for the three 802.11 technologies-a, b, and g-security has often been an afterthought. Free space optics does not share bandwidth with any other technologies or broadcast information that would leave it vulnerable to hackers.

ROAD TO A ROBUST NETWORK

Like other wireless technologies, free space optics may have to clear up questions of robustness before it can win additional converts.

"I would like to see if it's robust in areas that aren't mission-critical and see its use become a bit more mainstream," said Dr. Gary J. Wendt, vice chair of informatics at the University of Wisconsin.

Wendt believes that wireless technology still isn't ready for prime time, primarily because of bandwidth concerns. Average data transmission speeds under 100 Mb more commonly experienced with wireless technologies may be fine for text data and compressed images, but for CT exams with 3000+ images, radiologists need much larger pipes, he said.

"HIPAA we can deal with, less productivity we can't," Wendt said.

Another issue that free space optics must deal with is roof access. If a hospital doesn't own the building it resides in, it may not be free to place optical linkheads on the roof. Building residents must seek permission from owners, and in some cases, building owners may charge a fee for placing objects on the roof.

Requesting permission wasn't that difficult for Austin Radiological Association, a high-paying large tenant, according to Berken. In its case, the building owner did not charge a fee but simply made stipulations regarding possible damage to the roof.

SIGHT UNSEEN

Because free space optics is a line-of-sight technology, it is prey to interference problems. Anything that can get in the way of the laser beam for a long enough period of time can disrupt data transmission.

Physical objects such as cranes or even birds flying through the data path can block the laser. Typically, disturbances don't last long enough to fully disrupt connectivity, and the transmitters in use at the ARA use multibeam technology to get around obstructions.

The proximity of the association's buildings allowed its members to easily see the other buildings they needed to connect, a necessary condition for the successful implementation of line-of-sight technology. This may not always be the case, however.

"The line-of-sight nature of free space optics is both an advantage and a disadvantage," Chang said. "It can simplify security matters considerably, but it can also limit utility in areas where line-of-sight is problematic. For example, here in western Pennsylvania, the hilly topography makes line-of-sight implementation difficult without a lot of repeaters."

Another complication is the fact that buildings are not completely rigid. All buildings have a certain amount of sway, and if this sway goes beyond an acceptable range, free space optics transmitters and receivers can become misaligned.

"The linkheads we use have an autotracking technology to keep them aligned," Thomas said.

Perhaps the technology's biggest Achilles heel is weather.

"We haven't started using optics yet. In the Midwest, there is the winter weather, especially snowstorms, to contend with," Wendt said.

Fog may be the biggest enemy. The multitude of tiny water particles suspended in the air work together to scatter, absorb, and reflect the laser beam.

"We've already run into this problem," Berken said. "I'm actually surprised at how little it takes. Given our longitude and latitude, fog is a slight concern. We're not San Francisco."

Redundancy options

This is where network redundancy steps in. Currently, the Austin Radiological Association uses T1 lines to back up the free space optics circuits at one of its facilities. Unfortunately, this stopgap measure does not have nearly the bandwidth of free space optics. Going from 100 Mb/sec to 3 Mb/sec is a big hit, according to Berken.

The association is still actively discussing different redundancy options.

"At one clinic we use T1s, at another we are thinking about wireless, at another we are thinking about using the hospital's network," Thomas said.

Another issue with network redundancy is the need for spare parts. There are no physical lines to maintain, but there are physical parts, such as power supplies for the optical linkheads, that may break down.

The issue of redundancy is complex. While facilities may save $2000 a month by not using a leased line, if the system is down for eight hours due to broken parts, this washes away any savings, Berken said.

While the linkhead parts have not broken down in the six months the facility has been using them, the organization does have an advanced maintenance policy to get replacement parts shipped next-day to the facility in case of equipment failure. And it is discussing having a spare set on hand to cope with any possible delays in parts shipment, according to Berken.

"Free space optics can certainly be used effectively and securely for medical imaging. However, one must carefully understand the performance requirements with respect to bandwidth and latency that imaging demands," Chang said.