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Cabling, Connectors, and Bulkhead Extenders
When you are installing a wireless system, especially one with an external antenna, a number of things within and outside your control can affect the systems performance. Once you take the items outside your control (for example, weather, line of site, and so on) into account, the design of a wireless system shifts to items within your control. We have already looked at antenna choices; therefore, the next item to examine is how the signal gets to and from the antenna. This communications occurs over cabling and through connectors and bulkheads. Each of which we examine in the course of this section.
It may seem strange that wiring can have an impact on a wireless system, but choosing the wrong cabling could mean the difference between the success or failure of your wireless system. Specifically, the cabling being referred to is the cabling between the AP and the antenna. This cabling carries both the signals from the AP or bridge to the antenna and from the antenna to the AP or bridge.
The cabling that is used in most installations for this purpose is coaxial, or coax, cable (see Figure 9.8). Coaxial cable comes in many different varieties and sizes, but all of these different types share a common construction. In the center of a coaxial cable is a single conductor. This conductor may be solid, stranded, or in some rare instance, a tube, and is usually made of copper. Surrounding this conductor is a dielectric material that acts as an insulator. One common dielectric that is used for this purpose is solid or foam-based polyethylene. On top of this dielectric, a shielding layer is added. This layer can be awire braid, a foil wrap, a metal tube, or a combination of these items. Though a metal tube provides the best overall performance, the overall flexibility of the cable suffers. In many instances, a foil wrap is used in conjunction with a wire braid to allow for cable flexibility as well as good protection. Finally, an outer jacket is added to the cable. A common material for this jacket is Polyvinyl Chloride (PVC). This jacket protects the outer most conductor in the cable. The signal on a coaxial cable travels on the center conductor of the coaxial cable. The outer conductor, whether that be a wire braid, foil, or both, acts a shield from outside interference as well as a ground for the cable. The dielectric compound that separates the two parts acts as an insulator and ensures that the center conductor stays in or very close to the center of the cable. This combination of the outer conductive shielding and insulating material allows the cable to carry signals with minimal interference and distortion.
The impedance of coaxial cable can range from 35 to 185 ohms, however, the most common values are 50, 75, and 93 ohms. For use with the Cisco wireless devices, you should use 50-ohm cable. This is because the Cisco wireless devices are manufactured with 50 ohm components, and for most efficient energy transfer, all parts of a system; transmitter, cabling, and receiver should have the same impedance values.
RG-58 and RG-8 Cabling
Though the origin of the acronym RG can not be determined for certain, the general belief is that it was derived from U.S. military terminology and stands for "Radio Grade." This is because, the basis of the RG grading values is U.S. military specifications, specifically MIL-C-17. From these general specification, cable manufacturing companies produce multiple different variations of these cables that have different performance characteristics. Many characteristics can change from cable to cable, including attenuation, shielding type and quality, dielectric type and quality, flexibility of the cable, bend radius, center conductor material, shielding material, and outer cover material. All of these characteristics have an impact on the overall cable performance.
The two most common cable types used for Cisco wireless systems are RG-58 and RG-8. Both RG-58 and RG-8 are have 50-ohm impedance values, matching the impedance that is found on the AP or bridge. Though very similar, the major difference between RG-58 and RG-8 is the center conductor size. The RG-8 center conductor is almost twice as large as the RG-58 center conductor is. Because of this size difference, RG-8 cabling has better transmission qualities for the frequency range that the APs and bridges use. Therefore, for longer runs or runs that need to have a higher quality cable, RG-8 is preferred. However, in some situations, the performances of standard RG-58 or RG-8 cabling will still not meet the installation requirements. In these cases, you should look at the possibility of using specially designed low-loss or ultra low-loss coaxial cabling, 9913 Cabling 9913 cabling is a low-loss coaxial cable specifically designed and manufactured by Belden cable. Due to its success, many other cable manufacturing companies manufacture their cable to the Belden 9913 specifications. This cable will perform substantially better than a normal RG-58 or RG-8 cable, but the RG-58 or RG-8 cable will cost less. However, you can easily justify the additional cost if you require high performance or have a long run to the antenna. The cable itself is a 50-ohm RG-8 coaxial cable and comes in two separate varieties, the 9913 and the newly released 9913F. Though both of these cables are low-loss, and use the same outer conductor design, there are some differences in how they are constructed. The 9913 cable by Belden (See Figure 9.9) has a single copper conductor that is 9.5 AWG (American Wire Gauge). The dielectric that is used is a semisolid polyethylene in a helical construction. Due to this construction technique, the cable has numerous air pockets. In an outdoor environment, in the event of a faulty termination or cable slice, there is a possibility for water to collect in these pockets rendering the cable useless. Therefore, when using this cable outside, you should be careful to keep the cable watertight. Finally, the outer coating of the 9913 is PVC.
By contrast, the 9913F (see Figure 9.10), also manufactured by Belden has a stranded center conductor made up of 19 tightly wound cables to give an overall conductor size of 10 AWG. The dielectric used in the 9913F is a nitrogen gas-injected foam polyethylene. This process creates a solid dielectric layer with minimal difference from the 9913 cable in attenuation loss at higher frequencies. The solid dielectric also helps the cable stand up better in wet conditions. The final difference between the 9913 and the 9913F is that the outer sheath on the 9913F is made out of Belflex, which was created by Belden for added ruggedness and flexibility.
Designing & Planning
Transmission and Transmission Media Terminology
In the course of this chapter, as well as in the book, we have used some terms in describing the characteristics of transmission media that were not necessarily explained. Therefore, we wanted to take this opportunity to further explain what some of these terms mean. For this discussion, we focus primarily on the terms that affect cabling and connectors.
The first of these terms that you hear quite often is decibels, or dB. The dB scale is used to measure the power of a signal and is logarithmic in nature. In general, every 3 dB increase in signal strength doubles the power of a signal. For example, increasing the signal strength from 10 dB to 13 dB doubles the power of the signal. This can then be applied to the transmit power ratings associate with the Cisco wireless devices. For example, a 100 milliwatt transmit power setting translates to a 20 dBm (m standing for milliwatt) signal. A 50-milliwatt transmit power setting translates to a 17-dBm signal. Moreover, the progression continues with the 20, 5, 2, and 1 milliwatt corresponding to 13, 7, 3 and 0 dBm respectively. You may also hear dB referred to as dBi. The "i" in this case refers to comparing the signal to a theoretical isotope that radiates energy equally in all directions. For example, an antenna that is rated at 6 dBi will enhance the signal strength by fourfold.
The impedance of a cable has been described as the AC equivalent to resistance. The specific impedance of any cable is determined at the time of manufacturing. You can obtain the impedance value of a cable or connector by examining the voltage and current characteristics of the cable or conductor over the operating frequency range of the cable. This information is then put into a formula that determines the overall impedance of the cable. As was mentioned in the cabling section, impedance should match across all components used in a system.
Another term that is used quite often is attenuation. This is nothing more than the reduction of the amplitude of the electrical signal. Attenuation is affected not only by material type, but also by length of cable as well as the frequency at which the signal is transmitted. All things being equal, a lower frequency signal will have a lower attenuation of the length of a cable but will also be able to transmit less data. Obviously, in your installations the lower the attenuation the better.
Finally, we discuss the term Voltage Standing Wave Ratio (VSWR). Due to irregularities in cabling and connectors, the signal on a cable will be reflected back onto itself. These reflections cause dips and peaks in the amplitude of the signal. VSWR is simply a measure of the ratio of peak to dip voltage. If there were no reflections in the cable the VSWR would be 1:1, however, not many devices are perfect, so when looking at devices, one with a lower VSWR ratio has better transmission qualities.
Along with the cabling, one of the items that can have the largest impact on the quality of the signal that the bridge or AP receives is the connector that is used. Connectors are used to interface the cabling with the AP or bridge as well as the antenna or bulkhead. As was previously discussed, the primary type of cabling used to connect the AP or bridge to the antenna is coaxial. Therefore, for the purpose of this book, we discuss only coaxial connectors. Because coaxial cable is used for numerous applications, you can find a wide variety of connectors, coming in different shapes, sizes, and containing different characteristics. As was the case with the cabling, you need to choose a connector that matches the impedance of the system being installed and that is capable of handling the power and frequency range of this system. When selecting a connector, you also need to ensure that you use one that is appropriate for the environment in which it will be installed. For example, you should not use a connector rated for indoor use in an exterior installation. Some of the more popular types of coaxial conductors are BNC, F, N, and TNC.
As the introduction stated, there are many different varieties of coaxial connectors, however, the Cisco APs, bridges, and accessories use primarily the RP-TNC connector (see Figure 9.11). You may have noticed that the RP-TNC connector was not in the list of popular types. This is because FCC regulations (part 15.203) state that all wireless devices with removable antenna are required to have "nonstandard" connectors. The meaning of nonstandard has been debated, however for our case, it means that Cisco APs, bridges, and antennas come with RP-TNC connectors.
At first glance, the design of the RP-TNC connector looks exactly like a TNC connector. This is because it was based on the TNC design. The TNC connector was first made in the 1950s as an improvement upon the Bayonet-Neill-Concelman (BNC) connector. The TNC connector is usually a little over .5 inches in diameter and has a threaded connection. The threads allow for a consistent fit that will not be easily compromised by movement or vibration. The TNC as well as the RP-TNC connector can handle frequencies up 11 GHz, well within the range used by Cisco wireless devices. The difference between a normal TNC connector and a RP-TNC connector comes in where the female and male contacts are located. Specifically, in a TNC connector, the male contact is in the plug connector and the female connector is in the jack. In the RP-TNC, the contacts are reversed. In this manner, it is assured that equipment not suited for wireless use can not be accidentally connected to an AP or bridge.
Bulkhead extenders are cables that have a normal connector, such as an RP-TNC, on one end and a bulkhead connector on the other. Because we have already discussed normal connectors, we focus on the bulkhead connector at this point. A bulkhead is nothing more than a mounting style of connector. Primarily bulkheads are inserted through a premade panel or precut hole and secured by a nut screwed onto the end of the connector. By installing a bulkhead, you are able to attach a cable or antenna pigtail to a secure point that will not move around and ensure a watertight fit around the connector. You can use bulkhead extenders to easily move a bulkhead connector to another location, such as onto another panel or from the inside to the outside of an NEMA enclosure. This methodology allows for a watertight seal around the cable that can easily be relocated if necessary. This seal is crucial in environments where sensitive electronic equipment is installed in locations that are outside the normal operating specifications of the equipment. The main bulkhead extender that Cisco sells for use with its APs and bridges is a 60-inch extender (see Figure 9.12). This bulkhead extender is made from RG-58 cable with RP-TNC connectors. The jack side of the RP-TNC connector has the bulkhead connector on it.