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 advantages of the outdoor units for range, but keeps at least part of the system (usually the CPU and digital components) indoors. However, this approach is typically the most expensive, and in many cases requires special cabling between the IDU and ODU.

It is a good idea to configure the bridges and verify RF connectivity before installing them. By doing so, you can document any potential configuration problems you might encounter. In addition, you might save time by knowing the devices can link and are configured and working correctly before going on site.

At the survey of the site where the bridges will be mounted, address the following issues:

Is the mounting location structurally sound, and will it hold the weight of the bridge? (Some of the outdoor units can be heavy.)

If the mounting location is on a rooftop, will the roof itself not affect the Fresnel zone? (Moving the devices closer to the edge of the building might assist in this effort.)

Is there a good source of electrical or earth ground available (for grounding the antenna structure)?

Is there access to route the cables inside the building or will holes need to be drilled?

Will additional resources, such as a bucket truck or the services of others (some sites require union workers or the use of licensed electricians) be required?

If a tower is used, will it handle the extra wind load and weight of the bridge and/or antennas? (When using a tower, it is recommended to use a 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).

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.

Figure 14-13. Using Fiber-Optic Cable for Protection

Indoor Testing Before Installation: Understanding Maximum Operational Receive Level

When performing testing in a lab or indoor environment and the bridge antennas happen to be located in close proximity, you could encounter throughput issues because of receiver overload. When performing an indoor test, it is recommended that when possible to set the RF devices to operate at their lowest power setting and use the lowest-gain antenna possible. Physical separation of the antennas is also required.

Warning

Under no circumstance should two RF devices be directly connected from antenna port to antenna port without the use of a proper attenuator. Should it be required to connect directly (because of interference or some other issue), keep in mind that the maximum survivable receive level is typically 0 dBm or less for most receivers. Exceeding a product's limit can cause damage to the receiver. A maximum receiver level of 50 dBm is a very good level for most systems when performing configuration and lab testing. To determine the suitable attenuation, just take the transmitter power and subtract 50 from it. If the transmitter is +20 dBm, you would need 70 dB of attenuation (+20 minus 50 equals 70.)

To determine how much attenuation is needed when using antennas in lab testing, just add the transmitter power to the antenna gain and then add the gain of the other receiving antenna. This assumes the antennas are focused at each other. Suppose, for example, the transmitter is set to +20 dBm. The antenna on the transmitter is 13.5 dBi, and the antenna on the other bridge is 21 dBi. This provides a total of 54.5 dBm maximum possible power (20 + 13.5 + 21 = 54.5). To obtain a 50-dBm signal level at the receiver, approximately 100 dB of attenuation (54.5 dB [50] = 104.5) is required.

The conversion from electrical to radiated energy for an antenna provides approximately a 22-dB loss, when measured at the first wavelength from the antenna. As explained in the "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/1194.html). 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.

Figure 14-14. Parallel Links for Increased Throughput

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.

Installing Bridges