It has the potential to be a classic cartoon: an electrician with a run of co-ax coming in on the left that needs to link to a run of fiber going out on the right. No amount of black tape will link the two. What is needed is a magic box.

Regularly, fiber networks must be connected to other media or a new device has to be linked to its distribution frame. Sometimes the situation requires tying a fiber backbone to Ethernet, fast Ethernet, or token ring. It could necessitate multi-mode to singlemode fiber links. Another common challenge these days is upgrading an existing network to Gigabit Ethernet.

The magic box, in many cases, is a media converter. According to Dave Stenger, director of product strategy at Transition Networks, Minneapolis, Minn.(www.transition.com), media converters are small devices that take in signals from one media type and send them out on another. In an otherwise “impossible” situation, they are the vital tools used to link the fiber portion of the network to the copper.

Consider the following scenario: A device is connected to the distribution frame with twisted pair horizontal cabling. The two distribution frames between the device and the hub, however, are connected by singlemode fiber optic cabling. Alternatives to enable the two devices to communicate include:

* Laying new cabling between the device and the hub. New interface cards may also need to be added, but many popular network interface cards (NIC) are either not available with fiber options or expensive. Cost and time required for cabling, installation challenges, and NIC cards usually prohibit this solution.

* Replacing the chassis, modular devices, or stand-alone hubs in order to provide like interfaces between the new device and the hub. Few stackable routing and switching products support fiber cabling and their cost is prohibitive.

Media converters can provide an “invisible” link between twisted pair horizontal cable and the fiber backbone without requiring new cable or replacement of expensive equipment. For example, Stenger said that, in a Fast Ethernet environment, a 100BASE-TX to 100BASE-FX Media Converter connects a 100BASE-TX twisted pair device to a 100BASE-FX compliant single mode fiber port that has either an ST or SC fiber optic connector.

Many Fast Ethernet systems are being upscaled to Gigabit Ethernet. Using a Gigabit Ethernet Media Converter provides simple conversion from one or more 1000BASE-SX ports on a Gigabit Ethernet switch to 1000BASE-LX for use in campus area networks or other applications requiring the distance advantages of singlemode fiber. It is only necessary to convert those ports required for backbone connection.

In a typical scenario, the end-user has gone to fiber in the backbone and deployed FDDI or Fast Ethernet as the protocol. Yet the users want faster speeds—perhaps moving data from a Sun work station or other high-speed graphic workstations from one office to another. Nobody wants to pull new cable, but everyone is aware of the distance limitations of different types of cable.

Multi-mode components typically offer 260-meter (1000Base-DX) or 550-meter (1000Base-LX) distances over multi-mode fiber, Stenger said. The LX specification assumes use of a mode-conditioning patch cable.

Why? Because Gigabit Ethernet, in both the DX and LX versions, is using lasers. With DX, VCSELs (vertical cavity surface-emitting lasers) are used; in the LX case, FP (Fabry-Perot) lasers are used. A laser has a much smaller spot size than a typical LED. This changes the way the light propagates down the core of a fiber. In a typical multi-mode application, the light floods down the fiber following as many as 5,000 or 6,000 different paths. Lasers, however, are very precise. Perhaps only a couple of light paths will appear at the far end. At Gigabit speeds, these two signals often are distinct enough to be misinterpreted as two separate data pulses, Stenger explained.

The solution is a mode conditioning patch cable—a one-meter-long strand of singlemode fiber. At the far end there is a fusion splice to multi-mode. Instead of the fibers being perfectly aligned, they are offset slightly. So, instead of the singlemode fiber launching its light into the exact center of the fiber, it is launched slightly off to the side. The result is that more modes are excited as the light bounces off the walls of the fiber, creating more modes and making the LX work efficiently.

Given that most networks have a singlemode fiber backbone in place, it is possible to use a couple of the pairs in the backbone for transport. The next step is to pull the NICs from the machines. Instead of using Fast Ethernet fiber, they use Gigabit Ethernet fiber and a media converter.

Transition has a multi-mode-to-singlemode media converter called the F-SM-MM-02, which allows connecting a singlemode fiber to a 1300nm multi-mode fiber. The F-SM-MM-05 links single mode to 850nm multi-mode fiber. The converter for 1.25 gbps networks allows integration of Gigabit Ethernet on a port-by-port basis. Gigabit Ethernet can be extended over single mode fiber up to 3,000 meters.

The cost of installing media converters typically ranges from several hundred dollars to $2,000, depending on the converter’s actual function. The alternative is multiple hubs and new fiber cabling at a cost of between 100 and 150 times that of the converters.

HARLER, a freelance writer, is based in Strongsville, Ohio. He can be reached at (440) 238-4556 or charler@compuserve.com.