July 22, 2004Laser LinksIn places where fiber can�t reach, whether due to physical or economical constraints, you might resort to an 802.11a, b, or g link. Then again, distance, bandwidth, or security requirements may rule out Wi-Fi. Canon�s Canobeam optical transceivers take the RF out of wireless networking, overcoming many of Wi-Fi�s limitations. The Canobeam is essentially a laser. Using free space optics, a pair of Canobeam DT-130 transceivers is able to securely pump data via a beam of light at gigabit speeds across distances of as far as 0.75 miles. The DT-130s also boast active tracking that can sustain a link even if the transceivers move slightly, such as when mounted atop two swaying skyscrapers. In the lab, I set up the DT-130s about 30 feet apart, running from the SC-SX multimode fiber port of each transceiver to a single layer-3 switch. Using their integrated scopes, I was able to quickly aim each unit and establish a link between them. In practice, I found operation to be quite binary: either there is a link, or there isn�t; signal strength does not necessarily affect throughput. I also found that active tracking works well when the motion is smooth and steady; rough motion will cause link failure. The farther away the other transceiver, the better the tracking works. One unfortunate facet of the DT-130s is its layer-1 characteristics. When fiber is run from a switch to a transceiver and the transceiver is powered on, no link is established. Until an optical link is achieved, the fiber remains dark. This shortcoming means that fluctuating optical links could cause deeper network problems. For example, should OSPF or another routing protocol be running across the link, twitches to the optical link could cause frequent route recalculations and increased load on the routers and switches. In a layer-3 switching environment, spanning-tree will have similar problems. Integrating switching and 802.1q trunking capabilities would remove the need for the separate copper management interface and provide fiber link stabilization. Tunable parameters within the DT-130, such as a configurable hold-down timer, can also be used to prevent rapid link flapping. After setup, I pushed billions of packets through the DT-130s. I also ran raw TCP and UDP (User Datagram Protocol) throughput tests using wildly different packet and window sizes. For all intents and purposes, the link performed like a gigabit fiber link. I measured raw TCP throughput at a consistent 920Mbps. Another test included flood pinging between the servers on either side of the transceivers and measuring the packet loss created by interruptions in the light path. When cutting straight through the air, the link is quite stable. But even the karate-chop movement of a hand through the light path is enough to trigger packet loss. Each transceiver has an array of management options. The rear panel includes a DB9 serial console connector, a 10Base-T management port, an SX or LX SC fiber port, and a rich set of status indicators, including separate lights for fiber and optical links, as well as an LED optical signal strength meter. SNMP data is provided by the management port, permitting the transceivers to be integrated into most network management systems. Unfortunately, as the port is 10Base-T, a copper run must be made to the transceiver location. Due to the 300-foot distance limit on copper, this may not be possible in some installation scenarios. After the tests in the lab, it was time to head outside. I placed one transceiver out in the elements, linked to the other through a window. Link status and throughput tests were then conducted anew, with identical results to the previous tests. Rain did not bother the link, but fog caused some problems. The rule of thumb seems to be that below 50-percent visibility, the optical link will suffer. A backup path, such as an 802.11b RF link, may be required for mission-critical applications to maintain connectivity during low-visibility periods. There is room for improvement, but Canon has done well with the DT-130. True gigabit speeds are definitely achievable, and the transceivers performed well even when exposed to the elements. With a strict reliance on a continuous line-of-sight connection, the Canobeam will prove problematic for some applications, but it will provide solid connectivity for many others.
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