802.11 Wireless Network Site Surveying and Installation

326 Chapter 14: Outdoor Bridge Deployments

Interference Study

Although the unlicensed spectrum offers the benefit of no licensing fees, users pay a

penalty in terms of interference. There are no restrictions on the types of devices that

operate in these bands, provided that they all conform to a common set of rules. Although

the 5-GHz band is less crowded than the 2.4-GHz band today, over time the 5-GHz band

will likely become equally crowded with more and more interference-causing devices.

The Industrial, Scientific, and Medical (ISM) frequencies can contain emissions from

microwave ovens, heaters, plywood laminators, medical diathermy, and other noncom-

munication devices. Although most of these types of devices usually pose no threats of

interference to bridge links (because they are low-power, indoor devices), the engineer must

be aware that the possibility exists of some industrial high-power system (such as a

10-kilowatt industrial oven next door) that wipes out any attempted communications use of

that band. Equipment operating in this type of environment is subject to FCC rules and

regulations as well.

Amateur radio operators are also licensed to use parts of the bands in which bridge products

are designed to be used. Although not many amateurs use these bands, a few full-time (on

the air continually) point-to-point amateur microwave links operate in this band.

There is also the possibility of interference from other data systems in the same bands. In

any case, it is necessary to do an interference analysis using a spectrum analyzer to make

sure you have an interference-free radio link.

Installing Bridges

Bridges typically fall into one of three general design categories: single-piece outdoor

devices, single-piece indoor devices, or two-piece indoor/outdoor devices. Some systems

have the entire bridge designed to withstand outdoor installations. This also permits the

antenna to be attached directly to the bridge, reducing cable loss and increasing possible

range. The downside to this type of device is that if the bridge happens to fail, it might mean

climbing a tower or other structure for replacement, which is particularly difficult in bad

weather.

The second bridge design is not intended to withstand weather, and must be mounted in-

doors, or at least in some type of controlled environment. This has the advantage of physical

access to the bridge devices, as well as reduced cost (no need for weatherproof enclosures,

temperature-stability circuits, and so on). However, in these cases, some type of RF coax

cable is almost always required between the bridge and the antenna, increasing installation

cost slightly and reducing overall path capabilities.

The third design style lies between these two. It splits the bridge into two devices. One is

the indoor digital portion, referred to as the indoor unit (IDU). The radio section, or outdoor

unit (ODU), gets mounted outdoors with the antenna. Actually, this type of device has the