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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 indoors, 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 professional installer to climb the tower.) Will the mounting structure be strong enough to prevent movement or oscillation in the wind?

Is there a source of electrical power (assuming power is needed) for power tools such as drills test equipment, or for the bridge (if required)?

Are there other RF systems in the same vicinity? Some systems can be licensed to run very high power, and close proximity to such might be hazardous. Verify with the site owner as to other systems located at the site. When in doubt, employ or seek assistance from a professional installer.

Lightning Protection

Lightning is caused by the buildup of electrical potential between cloud and ground, between clouds, or between clouds and the surrounding air. During thunderstorms, static electricity builds up within the clouds. A positive charge builds in the upper part of the cloud, while a large negative charge builds in the lower portion. When the difference between the positive and negative charges becomes large enough, the electrical charge jumps from one area to another, creating a lightning bolt. Most lightning bolts actually occur from one cloud to another, but the difference of potential can also occur between a cloud and the earth, or items that are located on the earth.

One step in preventing lightning strikes is to prevent static energy from building up on the antennas and supporting structures. Metallic objects that are not grounded can build up electrical charges and attract lightning strikes. Providing a good ground path will assist in bleeding off this energy.

However, most damage to equipment does not come from direct lightning strikes, but rather from nearby strikes. As a lightning strike occurs, a very large current moves from one location to another. This current can cause electrical energy to be coupled (inductive coupling) into nearby conductors (such as coax cable, electrical wires, and antennas). This inductive coupling can be great enough to cause severe damage to any device on the attached cables (see Figure 14-10).

The first step in protecting against potential lightning damage is to ground all mounting structures and devices. Second, ground all cables through appropriate means. If using coaxial cable, place a lightning arrestor near the cable entrance to the building, or between the building entrance and the bridge itself. Placing the arrestor at the antenna does not provide protection to the cable on the bridge side of the arrestor (see Figure 14-11).

For outdoor devices, the manufacturer usually provides some type of grounding block. The Cisco BR1400 recommends grounding in a manner shown in Figure 14-12.

For the most effective grounding, use a heavy-gauge wire and keep ground wire as short as possible. The use of a good ground lessens the chance of damage because of a nearby strike and/or helps to bleed off any static charges that might build up on the cable. The National Electrical Code handbook recommends a #6 copper wire for grounding.

Suitable grounds include (but are not limited to) the following:

Ground rod buried into the earth

Electrical panel ground

Building structural steel such as I beams (providing the building has a good ground)

Professional grounding systems that may already be installed

Metal air-conditioner units (attached to the building), provided they are grounded

Metal radio tower (assuming the tower is grounded)

Note

Some towers, especially AM radio towers, are not grounded because the tower is actually isolated from ground and is used as the antenna itself. This is known as a hot tower, and you must isolate the bridge and all grounds from this type of tower.

If you are working in an older building, sometimes you can use a cold water pipe (if metallic, and not plastic) for a ground connection. Make certain the water system has the proper bypassing on meters, hot-water tanks, and so on, in accordance with the local electrical codes.

There is no known or guaranteed protection from a direct lightning strike. A direct hit will almost always damage the device. This can also cause repercussions to the network itself. Because the bridge is usually attached to a switch or router on the network, it is possible for the energy surge to move through the bridge (usually causing catastrophic failure) and affect the switch or router.

One way to protect the network is to use a length of fiber-optic cable to isolate, from a DC voltage point of view, the network and the bridge (see Figure 14-13). Because fiber is a glass material, it does not conduct electricity and would stop any surge from reaching the network. If you use two Ethernet-to-fiber converters, make certain the converters are powered from different AC circuits to prevent the electrical surges from following that path.

Indoor Testing Before Installation: Understanding Maximum Operational Receive Level

Calculating Distances for Outdoor RF Links" sidebar earlier in this chapter, for every doubling of this distance an extra 6 dB of loss occurs. Using this, you could calculate the minimum distance you need to provide the necessary attenuation. That means that a distance of between 64 and 128 wavelengths would be required. Because the wavelength of a 2.4-GHz signal is approximately 4.7 inches and at 5.8 GHz a wavelength is approximately 1.9 inches, this would result in distances of 50 and 20 feet respectively for 128 wavelengths. Of course, this might not be practical in a lab, based on the size of the facility. Therefore, adding in attenuation via RF attenuators between the radio and antenna might be necessary.

Aligning the Antenna

When first setting up the system, align the antennas first, using known direction and LoS. A compass and GPS is an ideal way to start here. Most systems offer some type of receive signal strength indication (RSSI) measurements for antenna alignment. Using these utilities, make very minor adjustments until the RSSI peaks. Some systems have a slight delay in reporting the RSSI, so make minor adjustments slowly.

Weatherproofing the Connectors

After you have aligned the antennas and configured everything physically, it is time to weatherproof the connectors. Failure to weatherproof the coaxial cables and antenna connectors can result in failure over time because of corrosion or water ingress. Weatherproofing your connectors on the nice warm day you install the bridge can prevent the need to troubleshoot the link in the dead of winter.

Use a good electrical joint compound on the connector threads and grounding points because it serves as a water repellent and anti-seizing thread lubricant. Teflon or silicon grease is a suitable compound.

For sealing, one of the most common, inexpensive products is called Coax-Seal, a form of moldable plastic from Universal Electronics (http://www.universal-radio.com/catalog/cable/1194l). To completely cover the connectors, wrap in a spiral direction up the connector (opposite of the way the water would flow).

Many installers use a layer of high-quality electrical tape, such 3M Scotch Super 88 or 88T PVC electrical tape, to weatherproof the connectors. The 88T PVC electrical tape has a better temperature range. A thin layer of electrical tape followed by Coax-Seal and then another layer of tape is sometimes used so that the weatherproofing can be undone easily using a simple utility knife.

Warning

Avoid using poor-quality electrical tape or other forms of weatherproofing such as rubber silicones, RTV, or liquid rubber-type spray-on coatings. These types of sealants can contain acetone or other chemicals that can eat the rubbers or gaskets found in some connectors, or cause connector corrosion. These types of products might also break down in ultraviolet light (sunlight), destroying the sealing properties. If you need to use something like this for whatever reason, always cover it with a good-quality electrical tape first.

If the connection will be underground, use tape layers and Coax-Seal and then apply a rubber coating such as Plastic Dip spray-on or dip coating over the final tape layers.

One word of warning: If an RF cable has suffered water intrusion into the connector, it is very likely the water has found its way into the cable itself. The braid and shield of coax can act like a wick and pull water far up into the cable itself. This changes the velocity factor of the cable, affecting the cable impedance. This in turn will increase losses in the cable. If there has been water intrusion, you should replace the entire length of cable.

Parallel Bridge Links for Increased Throughput

In some installations, you might want to install two parallel bridge wireless links between two buildings to increase the throughput.

Systems such as this require that the links operate on nonadjacent, nonoverlapping RF channels. This "stacking" of bridges can provide higher bandwidth, redundancy, and load balancing. This is possible, but minimum physical separation criteria must be followed when installing the antennas so that mutual interference between the systems does not influence performance. (See the sidebar "Receiver Desensitization" in Chapter 5, "Selecting the WLAN Architecture and Hardware").

Figure 14-14 shows a conceptual diagram.

Sufficient isolation must exist between redundant links, and you can create such by physically separating the antennas. When using 802.11a or 8902.11g products, this is even more important because of the large amount of energy in the OFDM sideband. Another way to add isolation is to change antenna polarization, which can add up to 20 dB more isolation. You can use an RSSI reading to verify isolation.

Also note that the bridge needs to work in an integrated system environment, wherein the attached switches or routers use aggregation protocols such as Fast EtherChannel (FEC) and Port Aggregation Protocol (PagP). FEC and PagP are used to provide up to 100 Mbps of combined bandwidth. In particular, if the bridges provide 802.1d spanning tree, one link might be shut down if the switched network is not correctly designed and configured.