WiFoo..The.Secrets.of.Wireless.Hacking [Electronic resources] نسخه متنی

WLAN Equipment

This section describes various tools needed in a WLAN, including the following:

• Access points

• Client devices

• Connectors

• Antennas

• Cables

• Attenuators

• Physical measuring devices

• RF analyzers

• Two-way radios

• Outdoor tools

• Battery packs

• Digital camera

• Mounting hardware

Note

Before surveying, it is important to understand what the customer applications will require for bandwidth and range. This may make a difference in what technology will work best for the site.

You have a wide variety of WLAN products from which to choose. The customer's network design should have the technology defined (or recommended) as well as the product models. The survey engineer must verify that these will, in fact, work for this site. The survey should not only provide the location, antenna style, configuration, and so on, it should also actually verify that the customer's design will result in adequate performance. If you have any doubts, raise them with your customer (and document them in the site survey report).

Access Points

Many users believe that for a survey, any access point (AP) will work and that they all work similarly. Such a belief strays very far from the truth. As you learned in Chapter 5, "Selecting the WLAN Architecture and Hardware," APs vary drastically with regard to technology, style, capability, and performance. To complete a successful survey and installation, you must use the AP that is intended to be installed. As with any radio technology, the performance from one radio device to another can have drastic differences. This is even true of different radio models from the same manufacturer.

A comparison of two of the more popular APs in the enterprise industry todaythe Oronoco AP2500 with an 802.11b radio and a Cisco AP1100 with an 802.11b radioshows the variations that can occur between different products. The Oronoco has a transmitter power of +15 dbm (30 mW), whereas the Cisco AP1100 has +20 dbm (100 mW). That is a 5-dB difference. The difference of transmitter power between these two devices can make as much as a 30-percent variation in cell coverage.

Another difference that occurs quite often is variation in receiver performance. The Cisco AP1100 has a receiver sensitivity of 85 dBm when operating at 11 Mbps. Yet the 802.11b radio in the Oronoco AP has a receiver sensitivity of only 82 dBm. Although 3 dB may not seem like much, it can have a dramatic effect on the fringe areas.

Figure 7-1 shows various APs that you can use for site surveys. Notice the variations between antenna styles, capabilities (attached versus remote), and physical size.

Looking at two APs from the same company, the Cisco AP1100 and AP1200 both have the same radio and the same radio specifications. Because of the antenna variations, however, they are miles apart (figuratively speaking) on coverage performance. The AP1100 has an internal dipole antenna, and unlike the AP1200 it has no options of adding external antennas and no way to improve coverage performance by changing antennas. The same drawback is true of the Avaya AP5 AP.

Another common issue that comes up when trying to select an AP for a survey is the ability to use various antennas. Antennas must be certified with the radio in many regulatory domains. Some vendors have a limited number of available antennas. The Enterasys Roamabout R2 with an 802.11b radio, for example, offers an external antenna capability, but does not enable you to use external diversity antennas. In addition, the selection of certified antennas is very limited.

Be aware that some of the centralized intelligence architecture products require you to have the controller or switch along with the AP; otherwise, the AP will not operate. This can be quite a cumbersome solution for surveying.

Surveying with one model AP and installing another can create a major problem in the system when it is installed. Bottom line, and it cannot be stressed enough, survey with the same product that will be installed!

Client Devices

It is also of utmost importance to know and understand the client that will be used in the site. Although it is not practical to survey with every client that will be used, it should be noted in the survey report what clients will be used, what the radio transmitter and receiver specifications are, and what type of antenna is used on the device.

For example, on the pre-site survey report, you see that a warehouse may use various clients, including a standard laptop, bar code scanners, and Voice over IP (VoIP) phones. The warehouse manager and a few of the employees who need network access in both the warehouse and the office will use the laptop. The radio card used in this device is the Cisco PCM350. These radios provide a transmitter power of 100 mW, and a receiver sensitivity of 85 dBm when running 11 Mbps (802.11b). They also use a built-in antenna that has a unity gain (0 dBm).

The bar code scanners may be mounted to the lift trucks. These devices use the Symbol radio that has a receiver sensitivity of 84 dBm. Again, the scanners have a transmitter power of 100 mW, and use a 5-dBi antenna mounted to the roof of the vehicle.

Also in the warehouse is a Symbol Netvision 802.11b VoIP phone, with an output power of 60 mW, a receiver sensitivity of 82 dBm, and an embedded antenna that, as a best guess (it is not documented), has a gain of 2 dB or less (based on size).

So which client do you choose to survey with? Unfortunately there is no simple answer. However, you can make certain assumptions. First, the device that has the minimum receiver sensitivity should be viewed. From this list, the phone, with a sensitivity of 82 dBm, probably has the worst-case receiver. It is down 3 dB from the Cisco client card. Because the phone antenna is not exactly known, assume it is no more than 1 dB, giving the receiver a total receive performance of 83 dBm. Still, it has the lowest performance of the three devices cited on the presurvey report.

For the transmitter side of the clients, you also need to consider the antenna as well as transmitter power. In this case, the lift-truck system has a 5-dBi gain antenna, so it will by far have the best performance. The PCMCIA card in the laptop has 100 mW with a 0-dBi antenna, and the phone has a 60-mW radio with a minimal-gain antenna. So again, the phone has the worst-case specifications.

The dilemma is that the phone may not have a good site survey tool. Certain client devices can adjust transmitter power, so as a solution you can set this to an appropriate setting, comparable to the lowest-performing transmitter device.

Although it is easy to compensate for transmitter power differences, simulating receiver sensitivity is not quite as easy. To compensate for the difference in the client receiver, you can usually adjust the AP transmitter power (assuming the transmitter power is programmable) to effectively reduce the signal to the receiver, emulating a lower receiver performance.

Figure 7-2 shows several different devices that you might use in any given site and will have to evaluate for overall RF performances. Included in these may be bar code scanners, VoIP phones, PCI cards, laptops with built-in radio devices, or specialty RF devices.

To summarize, follow these steps when selecting client survey devices:

Note

Many 802.11b and 802.11g devices on the market only offer 15 dBm (30 mW) of power, whereas others offer 200 mW. The 802.11a product lines vary even more widely. Be certain to do the proper research before going on site to perform a survey.

Connectors

Most of the connectorized APs use unique connectors and vary from vendor to vendor. As a survey engineer, it might prove helpful to build some very short adaptor cables, allowing the use of one style coax and antenna connector with any of the APs you might encounter for surveys. In this manner, you can carry a single set of antennas and accessories but still connect to any of the various APs that the customer may specify. Figure 7-3 shows connector and adaptor cable examples you might use in site surveying.

Antennas

Omnidirectional antennas are, by far, the most common, and every kit should contain a typical omnidirectional dipole antenna. This is like a universal soldier, and can be used in many different locations. In many cases, you will need a pair of these for supporting the diversity capabilities of the AP. Appendix B, "Antenna Radiation Patterns," shows examples of different antennas.

A good understanding of these antennas and their particular type of radiation patterns will help in selecting the proper antennas.

Another very common and useful antenna is the diverse ceiling-mountable omnidirectional antenna. You can attach this antenna below the ceiling and locate the cable routed to the AP above the ceiling. This setup enables you to hide the AP, enables coverage similar to a dipole antenna, and supports diversity. An added advantage of this antenna is its ease of deployment. Installation takes about 30 seconds; you snap the antenna onto the ceiling crossbar.

For some applications, you might have to use a slightly higher-gain omnidirectional antenna. For this, you have several choices. When selecting which omni to use, remember the issue of radiation pattern. When mounting one omni in the ceiling in an inverted fashion (like a mast-mount antenna, hanging down from the building truss), it may have a different vertical pattern than an antenna that is designed to be mounted in that fashion. Such an antenna is designed to be mounted to a mast, with the feed line at the bottom. The radiation pattern has more energy below the horizontal line than above it; and if you invert this antenna, you will have more energy radiating into the ceiling than at the floor. An omni antenna designed for ceiling mountings is virtually the same antenna with a different mounting design, and it does have the same radiation pattern. When mounted to the ceiling, however, it radiates more energy downward, as designed.

An omni antenna is almost never intended to be mounted in a horizontal (sideways) position because the radiation pattern would radiate in a vertical pattern rather than a horizontal pattern.

Sometimes a higher-gain omni is needed, such as the 5-dBi antennas, but the environment will not permit (because of aesthetics) the ceiling-mount or mast-mount antennas. A few antennas are available that offer a "stealthy" look. The pillar-mount antenna provides a diversity antenna with a look that blends into the surrounding walls.

Directional antennas are often used for coverage of outdoor sites, but they can also prove very useful in covering indoor areas. One of the most useful is a lower-gain patch or panel antenna. There are quite a few different versions out there, but they are all similar in performance if the gain is similar.

One advantage of a panel or patch antenna is that they can be mounted to a wall, with the cables exiting through the wall, or up behind the ceiling. And the antenna itself can be painted the same color as the wall. (Just be certain the paint has no metallic properties.)

For certain applications, such as a very long corridor, a warehouse with long narrow racks, or even certain manufacturing areas, a Yagi can be used. It is not uncommon to use directional antennas to "fill" a particular area with RF. An airplane hangar is one site where this has been done. The hangar ceiling is extremely high, and the antennas must be placed above the planes. The Yagis are mounted in the ceiling and pointed down to provide coverage of the area below it.

No single antenna is perfect for all applications. A variety of antennas are included in most site survey kits because they are needed at certain times. (See Figure 7-4.) The customer in many cases dictates antenna choice and placement. A customer might not want the antenna to be visible or to be located in a high-traffic area. By carrying a variety of antennas, you will be prepared for any situation. To summarize, the recommended minimum collection of antennas should include (but is not limited to) antennas similar to the following:

• 2.2-dBi dipole antenna rubber duck

• 2.2-dBi diversity omni ceiling-mount antenna

• 2.0-dBi ceiling-tile antenna

• 5.2-dBi mast-mount antenna

• 5.2-dBi omni ceiling mount

• 5.2-dBi pillar-mount diversity omni antenna

• 6.0-dBi diversity patch wall-mount antenna

• 8.5-dBi hemispherical patch antenna

• 10.0-dBi Yagi mast-mount antenna

• 13.5-dBi Yagi mast-mount antenna

If you plan on using diversity antennas, you need to carry two of every antenna unless the antenna is especially made to support diversity and contains two antennas.

Do not use a different antenna and attempt to "guesstimate" the coverage. You are performing the site survey to take the guesswork out of the installation.

Cables

Additional antenna cable introduces loss, and therefore should be avoided whenever possible. In some situations, however, it is not possible to locate the AP within reach of the antenna's attached cable. In these cases, you can use coax cable to extend the reach.

As RF energy is carried between the antenna and the radio equipment through a coaxial cable, it introduces signal loss for both the transmitter and receiver. To reduce signal loss, minimize the cable length and use only a very low-loss antenna cable to connect radio devices to antennas.

It is not necessary to carry various cables in a survey kit. If you anticipate using a cable for the actual install, the survey should note what cable is to be used (type and length), and the survey engineer will simulate the cable loss by using an attenuator. This enables the engineer to simulate virtually any cable type and length. It is recommended, however, to carry the information for typical coax cables that would be specified. This way the engineer can calculate the losses and program the attenuator with the proper simulation loss.

Figure 7-5 shows the LMR-400 and LMR-600 cables. Table 7-1 describes the features of each.

It might prove helpful to carry a single 10- or 20-foot (3- or 6-meter) cable in the kit for surveys in locations where the AP cannot be located with the antenna, even for the survey itself. Locations such as extremely wet areas (processing plants) or extremely cold areas (freezers) might require the survey AP be located outside the environment and the antenna located inside the area. In this event, you might need a cable to complete the survey.

Attenuators

Always perform surveys using the equipment that will eventually be installed. This can sometimes be difficult with splitters, lightning arrestors, and extension cables.

Instead of carrying one of every length of cable, lightning arrestors, splitters, and other accessories, some engineers outfit the site survey kit with an attenuator. The attenuator enables you to inject varying amounts of loss without needing the actual accessories.

The attenuator must have the proper RF specifications. Many attenuators are only good up to 1 or 2 GHz, and you must use one that has a specification high enough to cover the band that you will be using. An attenuator good for up to 2.7 GHz is fine for a 2.4-GHz survey, but it will not work in a 5-GHz system. An attenuator with a 6-GHz rating could be used at either 2.4 or 5 GHz. Figure 7-6 shows a variable attenuator that can be used to simulate various losses in the antenna system or cables.

Attenuators are expensive. As frequency of the device goes up, so does the price. This is one area where you cannot cut costs.

Physical Measuring Devices

One tool that is often overlooked is a measuring devicesomething that can measure long-distance runs of hundreds or even thousands of feet or meters. In many cases, you need to add measurements to your drawings for room size, building size, or even parking lot size. Or you might need to measure the distance a cable will have to run between and AP and a switch (keeping in mind whether that will meet the intended cable-length limitation). If you guess the Category (Cat) 5 cable run to be 300 feet (91 meters) and it turns out to be 380 feet (116 meters), the customer will be very dissatisfied. For some measurements, a tape measure might prove impractical and difficult to use (especially for one person). Your kit also should include a measuring wheel similar to the one shown in Figure 7-7.

Many survey engineers find that some of the more advanced equipment, such as laser measuring devices and range finders, can save time and effort. For measuring vertical distances, these can prove extremely helpful, especially when doing a survey alone. An alternative method is to use a rope marked in 10-foot (3-meter) increments so that you can accurately judge distances floor to ceiling.

RF Analyzers

In today's world of communications, RF is everywhere, in every possible environment. Therefore, survey engineers must look at the RF environment for other radio devices and interference. Failure to do so can result in a system that is installed but never works.

Looking at the RF spectrum requires a specialized receiver or a spectrum analyzer. The next section examines different tools for analyzing the RF environment.

Spectrum Analyzer

The spectrum analyzer is the best and most useful tool for RF spectrum analyses. It not only provides a look at the spectrum in which the WLAN system will operate, it can also determine the frequency of other signals, the duration they are transmitting, the strength of the signal, and with some experience the engineer may even be able to determine the type of modulation in use, leading to an understanding of the type of device being used. By using a directional antenna, you can use a spectrum analyzer to locate a signal source.

Spectrum analyzers have several drawbacks as a site survey tool. The first drawback is cost. For an analyzer that supports the 5-GHz band, the cost could reach well above $10,000! Second, most spectrum analyzers require AC power. A few models are offered with battery packs, but the duration of operation is limited based on the battery power.

Portable Analyzer Tools

Portable analyzer tools such as the Berkley Valtronics Grasshopper can prove very useful and cost-effective (see Figure 7-8). The Grasshopper is a handheld, wireless receiver designed specifically for sweeping and optimizing WLANs. It can measure coverage of 802.11b systems, enabling the user to measure and determine the AP and received signal strength identification (RSSI) signal levels (which helps the user locate the APs throughout a building). It can also detect and differentiate from narrowband multipath interferences (such as from microwave ovens and frequency-hopping systems) and features a built-in display, keypad, and removable battery pack for true portability.

Some WLAN products enable you to sniff, or listen, to the RF, acting like a spectrum sniffer listening to frequencies the gear is designed to operate in. This is a convenient tool, but it is not as effective as a good spectrum analyzer. One such WLAN product is the Cisco Aironet AP1200 AP. It offers a feature called carrier test. It provides a dBm level of the highest signal received on any of the available 802.11 channels. The AirMagnet wireless sniffer product also offers a feature to sniff the air for other RF energy. Although these tools are very useful, they do not compare to the versatility of a spectrum analyzer in the hands of an experienced engineer.

Two-Way Radios

In some situations, two engineers will conduct a survey together. In such cases, a communication link between them can help to reduce time. Family Radio Service walkie-talkies are very inexpensive and typically require no license (at least in the U.S.). Before using them in a customer site, however, always inquire as to any company regulations about radios. Some sites may have areas where high-power two-way radios are not permitted.

Outdoor Tools

If the survey is for outdoor links, you might need other equipment. Tools to determine necessary antenna height, waterproof-test enclosures (for the equipment during testing), and Global Positioning System (GPS) receivers are just a few tools that can prove very helpful. Chapter 14, "Outdoor Bridge Deployments," covers some of the tools used in outdoor site surveys.

Battery Packs

APs require power to operate. However, power will not always be available nearby while you conduct a site survey. A solution that enables you to power the AP without running long cables is the best method for surveying. A good survey battery pack will last for at least eight hours, enabling the engineer to survey all day without having to recharge.

Because power requirements vary from one vendor AP to another, a single power pack is not something that is universal. Some battery packs are available with various power offerings (such as the TerraWave pack shown in Figure 7-9).