Patrick Gorrell, AIDCO Inc.
The components used for an air-blown fiber (abf) system compare favorably to standard fiber equipment. In terms of the physical-space requirements and overall costs, high-density fiber-optic crossconnect units and mechanical splice enclosures used in abf installations typically compare well to standard fiber termination and distribution units that require connectorization of the fiber.
Another advantage of these abf components is that they require only a single-ended patch cord or "tail" to make a connection. It makes sense to fabricate a tail by building a standard double-ended patch cord of twice the desired length and then cutting it in half after it has been tested end-to-end. But watch out. This simple act could be a management nightmare. Patch cords are often provided with the transmit/receive color-coding and any corresponding labeling rolled from one end of the cable to the other. That is, the strand supporting transmission on one end becomes the receiver strand on the other end, and vice versa. When ordering patch cords that are intended to be used as tails, order them in a straight-through configuration.
In most local area network fiber-optic backbone applications, signal attenuation (or loss) is a source of major concern and the primary factor in limiting transmission distances in the design. Surprisingly, this is a very important factor in planning fiber-to-the-desk (fttd) applications, despite the comparatively short distances allowed by the telecommunications systems bulletin tsb-72 standard of the Telecommunications Industry Association (Arlington, VA). However, in this case, instead of worrying about having too much loss (as in the case of backbone cabling), the designer must ensure that he or she has enough loss in the horizontal cabling to avoid a different problem called saturation. Saturation can easily occur when a transmitter with slightly too much power output is connected to a receiver that is a bit oversensitive, via a short section of fiber-optic cabling. In cases like this, there may simply not be enough loss to permit the reliable transfer of data.
The first step in providing a solution to saturation problems is to try to avoid them in the first place. Designers of fttd systems should obtain specifications and compare transmitter power output and receiver sensitivity data for compatibility before selecting these devices. If this cannot be done and a saturation problem exists, the alternative is to add signal attenuators. These handy devices are available in a wide variety of attenuation values and are readily incorporated into most high-density crossconnect cabinets, mechanical-splice units, or traditional termination units.
Patrick Gorrell, registered communications distribution designer (rcdd), is chief system designer for aidco Inc. (Chino, CA).
