239 Chapter 9 –Troubleshooting Wireless LAN Installations If you do co-locate three access points in this manner, it is recommended that you implement the co-location using the same manufacturer's hardware for all three access points. It has been noted in many lab scenarios that using differing vendors' equipment for co-location has a negative effect on throughput of one or more of the access points. This negative effect could be simply due to differing output power and proximity between access points, but could be related to many other factors as well. Solutions for Co-location Throughput Problems As a wireless LAN installer or administrator, you really have two choices when considering access point co-location. You can accept the degraded throughput, or you can attempt a workaround. Accepting the fact that your users will not have 5 Mbps of actual throughput to the network backbone on each access point may be an acceptable scenario. First, however, you must make sure that the users connecting to the network in this situation can still be productive and that they do not actually require the full 5 Mbps of throughput. The last thing you want to be responsible for as a wireless LAN administrator is a network that does not allow the users to do their jobs or achieve the connections that they require. An administrator's second option in this case is to attempt a workaround. Below, we describe some of the alternatives to co-location problems. Use Two Access Points One option, which is the easiest, is to use channels 1 and 11 with only 2 access points, as illustrated in Figure 9.11. Using only these two channels will ensure that you have no overlap between channels regardless of proximity between systems, and therefore, no detrimental effect on the throughput of each access point. By way of comparison, two access points operating at the maximum capacity of 5.5 Mbps (about the best that you can expect by any access point), give you a total capacity of 11 Mbps of aggregate throughput, whereas three access points operating at approximately 4 Mbps each (degraded from the maximum due to actual channel overlap) on average yields only 12 Mbps of aggregate throughput. For an additional 1 Mbps of throughput, an administrator would have to spend the extra money to buy another access point, the time and labor to install it, and the continued burden of managing it. FIGURE 9.11 Using two access points instead of three 2.401 GHz 2.473 GHz Channel 1 Channel 6 Channel 11 f P Remove this access point allowing more channel separation between access points for greater throughput CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 9 –Troubleshooting Wireless LAN Installations 240 In certain instances, the extra 1 Mbps of bandwidth might still be advantageous, but in a small environment, it might not be practical. Don't forget that this scenario applies only to access points located in the same physical space serving the same client base, but using different, non-overlapping channels. This configuration does not apply to channel reuse, where cells on different non-overlapping channels are alternately spread throughout an area to avoid co-channel interference. Use 802.11a Equipment As a second option, you could use 802.11a compliant equipment operating in the 5 GHz UNII bands. The 5 GHz UNII bands, which are each wider than the 2.4 GHz ISM band, have three usable bands, and each band allows for four non-overlapping channels. By using a mixture of 802.11b and 802.11a equipment, more systems can be co-located in the same space without fear of interference between systems. With two (or three) co- located 802.11b systems and up to 8 co-located 802.11a systems, there is the potential for an incredible amount of throughput in the same physical space. The reason that we specify 8 instead of 12 co-located access points with 802.11a is that only the lower and middle bands (with 4 non-overlapping channels each) are specified for indoor use. Therefore, indoors, where most access points are placed, there's normally only the potential for up to 8 access points using 802.11a compliant devices. Issues with 802.11a Equipment 802.11a equipment is now available from only a few vendors, and is more expensive than equipment that uses the 2.4 GHz frequency band. However, the 5 GHz band has the advantage of many more non-overlapping channels than the 2.4 GHz band (8 vs. 3), allowing you to implement many more co-located access points. You must keep in mind that while the 2.4 GHz band allows for less expensive gear, the 2.4 GHz band is much more crowded, which means you are more likely to encounter interference from other nearby wireless LANs. Remember that 802.11a devices and 802.11b devices are incompatible. These devices do not see, hear, or communicate with one another because they utilize different frequency bands and different modulation techniques. Summary Why do "non-overlapping" channels overlap? There could be many answers to this question; however, it seems that the greatest cause is access points being located too close together. By separating the access points by a greater distance, the overlap between theoretically non-overlapping channels is reduced. Watching this configuration on a spectrum analyzer, you can see that for close-quarters co-location, there needs to be a channel separation larger than 3 MHz; however, since that is what we, as administrators, have to work with, we have to find a workaround. We can either physically separate the radios by a further distance or we can use channels further than 3 MHz apart (hence the suggestion of using channels 1 & 11 only for close- quarters co-location). It also seems that co-location of different vendors' equipment makes a difference as well. Using the same vendor's equipment for close-quarters co- CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 241 Chapter 9 –Troubleshooting Wireless LAN Installations location has less severe overlapping than does using multiple vendors' equipment. Whether this phenomenon is due to inaccuracies in the radios, or just due to each vendor's implementation of hardware around the radio, is unknown. Idiosyncrasies like non-overlapping channels overlapping one will not be tested on the CWNA exam. For the exam it is important to know the theory of how co-channel throughput is theoretically supposed to work. Types of Interference Due to the unpredictable behavioral tendencies of RF technology, you must take into account many kinds of RF interference during implementation and management of a wireless LAN. Narrowband, all-band, RF signal degradation, and adjacent and co- channel interference are the most common sources of RF interference that occur during implementation of a wireless LAN. In this section, we will discuss these types of interference, how they affect the wireless LAN, how to locate them, and in some cases how to work around them. Narrowband Narrowband RF is basically the opposite of spread spectrum technology. Narrowband signals, depending on output power, frequency width in the spectrum, and consistency, can intermittently interrupt or even disrupt the RF signals emitted from a spread spectrum device such as an access point. However, as its name suggests, narrowband signals do not disrupt RF signals across the entire RF band. Thus, if the narrowband signal is primarily disrupting the RF signals in channel 3, then you could, for example, use Channel 11, where you may not experience any interference at all. It is also likely that only a small portion of any given channel might be disrupted by narrowband interference. Typically, only a single carrier frequency (a 1 MHz increment in an 802.11b 22 MHz channel) would be disrupted due to narrowband interference. Given this type of interference, spread spectrum technologies will usually work around this problem without any additional administration or configuration. FIGURE 9.12 Picture of a handheld digital spectrum analyzer showing a narrowband signal CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 9 –Troubleshooting Wireless LAN Installations 242 To identify narrowband interference, you will need a spectrum analyzer, shown above in Figure 9.12. Spectrum analyzers are used to locate and measure narrowband RF signals, among other things. There are even handheld, digital spectrum analyzers available that cost approximately $3,000. That may seem like quite a bit of money to locate a narrowband interference source, but if that source is disabling your network, it might be well worth it. As an alternative, some wireless LAN vendors have implemented a software spectrum analyzer into their client driver software. This software uses a FHSS PCMCIA card to scan the useable portion of the 2.4 GHz ISM band for RF signals. The software graphically displays all RF signals between 2.400 GHz and 2.4835 GHz, which gives the administrator a way of "seeing" the RF that is present in a given area. An example of the visual aid provided by such a spectrum analyzer is shown in Figure 9.13. FIGURE 9.13 Screenshot of a spectrum analyzer showing narrowband interference In order to remedy a narrowband RF interference problem, you must first find where the interference originates by using the spectrum analyzer. As you walk closer to the source of the RF signal, the RF signal on the display of your spectrum analyzer grows in amplitude (size). When the RF signal peaks on the screen, you have located its source. At this point, you can remove the source, shield it, or use your knowledge as a wireless network administrator to configure your wireless LAN to efficiently deal with the narrowband interference. Of course, there are several options within this last category, such as changing channels, changing spread spectrum technologies (DSSS to FHSS or 802.11b to 802.11a), and others that we will discuss in later sections. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 243 Chapter 9 –Troubleshooting Wireless LAN Installations All-band Interference All-band interference is any signal that interferes with the RF band from one end of the radio spectrum to the other. All-band interference doesn't refer to interference only across the 2.4 GHz ISM band, but rather is the term used in any case where interference covers the entire range you're trying to use, regardless of frequency. Technologies like Bluetooth (which hops across the entire 2.4 GHz ISM band many times per second) can, and usually do, significantly interfere with 802.11 RF signals. Bluetooth is considered all-band interference for an 802.11 wireless network. In Figure 9.14 a sample screen shot of a spectrum analyzer recording all-band interference is shown. FIGURE 9.14 Screenshot of a software spectrum analyzer showing all-band interference A possible source of all-band interference that can be found in homes and offices is a microwave oven. Older, high-power microwave ovens can leak as much as one watt of power into the RF spectrum. One watt is not much leakage for a 1000-watt microwave oven, but considering the fact that one watt is many times as much power as is emitted from a typical access point, you can see what a significant impact it might have. It is not a given that a microwave oven will emit power across the entire 2.4 GHz band, but it is possible, depending on the type and condition of the microwave oven. A spectrum analyzer can detect this kind of problem. When all-band interference is present, the best solution is to change to a different technology, such as moving from 802.11b (which uses the 2.4 GHz ISM band) to 802.11a (which uses the 5 GHz UNII bands). If changing technologies is not feasible due to cost or implementation problems, the next best solution is to find the source of the all-band interference and remove it from service, if possible. Finding the source of all-band CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 9 –Troubleshooting Wireless LAN Installations 244 interference is more difficult than finding the source of narrowband interference because you're not watching a single signal on the spectrum analyzer. Instead, you are looking at a range of signals, all with varying amplitudes. You will most likely need a highly directional antenna in order to locate the all-band interference source. Weather Severely adverse weather conditions can affect the performance of a wireless LAN. In general, common weather occurrences like rain, hail, snow, or fog do not have an adverse affect on wireless LANs. However, extreme occurrences of wind, fog, and perhaps smog can cause degradation or even downtime of your wireless LAN. A radome can be used to protect an antenna from the elements. If used, radomes must have a drain hole for condensation drainage. Yagi antennas without radomes are vulnerable to rain, as the raindrops will accumulate on the elements and detune the performance. The droplets actually make each element look longer than it really is. Ice accumulation on exposed elements can cause the same detuning effect as rain; however, it stays around longer. Radomes may also protect an antenna from falling objects such as ice falling from an overhead tree. 2.4 GHz signals may be attenuated by up to 0.05 dB/km (0.08 dB/mile) by torrential rain (4 inches/hr). Thick fog produces up to 0.02 dB/km (0.03 dB/mile) attenuation. At 5.8 GHz, torrential rain may produce up to 0.5 dB/km (0.8 dB/mile) attenuation, and thick fog up to 0.07 dB/km (0.11 dB/mile). Even though rain itself does not cause major propagation problems, rain will collect on the leaves of trees and will produce attenuation until it evaporates. Wind Wind does not affect radio waves or an RF signal, but it can affect the positioning of outdoor antennas. For example, consider a wireless point-to-point link that connects two buildings that are 12 miles apart. Taking into account the curvature of the Earth (Earth bulge), and having only a five-degree vertical and horizontal beam width on each antenna, the positioning of each antenna would have to be exact. A strong wind could easily move one or both antennas enough to completely degrade the signal between the two antennas. This effect is called "antenna wind loading", and is illustrated in Figure 9.15. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 245 Chapter 9 –Troubleshooting Wireless LAN Installations FIGURE 9.15 Antenna Wind Loading on Point-to-point networks No Wind Beam arrives at receiver Beam misses receiver Wind moves antenna Other similarly extreme weather occurrences like tornadoes or hurricanes must also be considered. If you are implementing a wireless LAN in a geographic location where hurricanes or tornadoes occur frequently, you should certainly take that into account when setting up any type of outdoor wireless LAN. In such weather conditions, securing antennas, cables, and the like are all very important. Stratification When very thick fog or even smog settles (such as in a valley), the air within this fog becomes very still and begins to separate into layers. It is not the fog itself that causes the diffraction of RF signals, but the stratification of the air within the fog. When the RF signal goes through these layers, it is bent in the same fashion as visible light is bent as it moves from air into water. Lightning Lightning can affect wireless LANs in two ways. First, lightning can strike either a wireless LAN component such as an antenna or it may strike a nearby object. Lightning strikes of nearby objects can damage your wireless LAN components as if these components are not protected by a lightning arrestor. A second way that lightning affects wireless LANs is by charging the air through which the RF waves must travel after striking an object lying between the transmitter and receiver. The affect of lightning is similar to the way that the Aurora Borealis Northern Lights provide problems for RF television and radio transmissions. Adjacent Channel and Co-Channel Interference Having a solid understanding of channel use with wireless LANs is imperative for any good wireless LAN administrator. As a wireless LAN consultant, you will undoubtedly CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 9 –Troubleshooting Wireless LAN Installations 246 find many wireless networks that have many access points, all of them configured for the same channel. In these types of situations, a discussion with the network administrator that installed the access points will divulge that he or she thought it was necessary for all access points and clients to be on the same channel throughout the network in order for the wireless LAN to work properly. This configuration is very common, and often incorrect. This section will build on your knowledge of how channels are used; explaining how multiple access points using various channels can have a detrimental impact on a network. Adjacent Channel Interference Adjacent channels are those channels within the RF band being used that are, in essence, side-by-side. For example, channel 1 is adjacent to channel 2, which is adjacent to channel 3, and so on. These adjacent channels overlap each other because each channel is 22 MHz wide and their center frequencies are only 5 MHz apart. Adjacent channel interference happens when two or more access points using overlapping channels are located near enough to each other that their coverage cells physically overlap. Adjacent channel interference can severely degrade throughput in a wireless LAN. It is especially important to pay attention to adjacent channel interference when co- locating access points in an attempt to achieve higher throughput in a given area. Co- located access points on non-overlapping channels can experience adjacent channel interference if there is not enough separation between the channels being used, as illustrated in Figure 9.16. FIGURE 9.16 Adjacent channel Interference Channel 1 Channel 3 Adjacent Channel Interference 2.401 GHz f P In order to find the problem of adjacent channel interference, a spectrum analyzer will be needed. The spectrum analyzer will show you a picture of how the channels being used overlap each other. Using the spectrum analyzer in the same physical area as the access points will show the channels overlapping each other. There are only two solutions for a problem with adjacent channel interference. The first is to move access points on adjacent channels far enough away from each other that their cells do not overlap, or turn the power down on each access point enough to where the cells do not overlap. The second solution is to use only channels that have no overlap CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 247 Chapter 9 –Troubleshooting Wireless LAN Installations whatsoever. For example, using channels 1 & 11 in a DSSS system would accomplish this task. Co-channel Interference Co-channel interference can have the same effects as adjacent channel interference, but is an altogether different set of circumstances. Co-channel interference as seen by a spectrum analyzer is illustrated in Figure 9.17 while how a network configuration would produce this problem is shown in Figure 9.18. FIGURE 9.17 Co-channel Interference Ch1/Ch1 Co-channel Interference f P 2.401 GHz FIGURE 9.18 Co-channel Interference in a network Co-channel Interference Physical configuration Channel 1 Channel 1 To illustrate co-channel interference, assume a 3-story building, with a wireless LAN on each floor, with the wireless LANs each using channel 1. The access points’ signal ranges, or cells, would likely overlap in this situation. Because each access point is on CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 9 –Troubleshooting Wireless LAN Installations 248 the same channel, they will interfere with one another. This type of interference is known as co-channel interference. In order to troubleshoot co-channel interference, a wireless network sniffer will be needed. The sniffer will be able to show packets coming from each of the wireless LANs using any particular channel. Additionally, it will show the signal strength of each wireless LAN's packets, giving you an idea of just how much one wireless LAN is interfering with the others. The two solutions for co-channel interference are, first, the use of a different, non- overlapping channel for each of the wireless LANs, and second, moving the wireless LANs far enough apart that the access points’ cells do not overlap. These solutions are the same remedy as for adjacent channel interference. In situations where seamless roaming is required, a technique called channel reuse is used in order to alleviate adjacent and co-channel interference while allowing users to roam through adjacent cells. Channel reuse is the side-by-side locating of non-overlapping cells to form a mesh of coverage where no cell on a given channel touches another cell on that channel. Figure 9.19 illustrates channel reuse. FIGURE 9.19 Channel reuse Channel 1 Channel 1 Channel 1 Channel 11Channel 11 Channel 6 Range Considerations When considering how to position wireless LAN hardware, the communication range of the units must be taken into account. Generally, three things will affect the range of an RF link: transmission power, antenna type and location, and environment. The maximum communication range of a wireless LAN link is reached when, at some distance, the link begins to become unstable, but is not lost. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. [...]... advantage of 5 GHz (80 2.11a) equipment over 80 2.11b equipment? A The lower 5 GHz UNII band is wider than the 2.4 GHz ISM band B The 80 2.11a equipment is less expensive than 80 2.11b C The 5 GHz UNII bands allows for more non-overlapping channels than the 2.4 GHz ISM band D 80 2.11a equipment is backwards compatible with 80 2.11g equipment CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc 255 Chapter... casual eavesdroppers from tinkering with or using your wireless LAN CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc Chapter 10 – Wireless LAN Security 2 68 MAC Address Filtering Wireless LANs can filter based on the MAC addresses of client stations Almost all access points (even very inexpensive ones) have MAC filter functionality The network administrator can compile, distribute, and maintain... information Physical security Inventory and audits Using advanced solutions Public networks Identify how and where the following security precautions are used to secure a wireless LAN WEP Cell sizing Monitoring User authentication Wireless DMZ CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc Chapter 10 – Wireless LAN Security 260 Wireless LANs are not inherently secure; however, if you do not take any... users into four distinct groups of 25 CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc Chapter 10 – Wireless LAN Security 264 If a WEP key were compromised, it would mean changing 25 stations and an access point or two instead of the entire network Another reason for multiple WEP keys is in case there is a mix of 64-bit and 1 28- bit cards on the network Since an administrator might want to use as... significantly more expensive than 80 2.11b equipment and is not compatible with 80 2.11b or 80 2.11g equipment in any capacity The UNII bands (all three of them) allow for a larger useable portion than does the 2.4 GHz ISM band, yielding a maximum of 4 non-overlapping DSSS channels CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc CHAPTER CHAPTER Wireless LAN Security CWNA Exam Objectives Covered:... does not have to actually connect to an access point to listen to packets traversing the wireless segment Wireless LAN sniffers or custom applications are typically used to gather information about the wireless network from a distance with a CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc 271 Chapter 10 – Wireless LAN Security directional antenna, as illustrated in Figure 10.6 This method of... gain valuable information from or to gain access to your network, jamming is a technique that would be used to simply CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc 273 Chapter 10 – Wireless LAN Security shut down your wireless network Similar to saboteurs arranging an overwhelming denial of service (DoS) attack aimed at web servers, so a wireless LAN can be shut down by an overwhelming RF signal... implemented as a basic security mechanism, but network administrators should first be aware of WEP’s weaknesses and how to compensate for them The administrator should also be aware of the fact that each vendor’s use of WEP can and may be different, hindering the use of multiple vendor hardware CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc Chapter 10 – Wireless LAN Security 266 Advanced Encryption... Moving the hidden node(s) CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc Chapter 9 –Troubleshooting Wireless LAN Installations 254 16 How is the threshold set when using RTS/CTS in "On with Threshold" mode on a wireless LAN? A Automatically by the access points only B Manually by the user of the hidden node C Manually on the clients and access points by the wireless LAN administrator D Automatically... securing a wireless LAN in some instances, they are still susceptible to the following intrusions: Theft of a PC card that is in the MAC filter of an access point Sniffing the wireless LAN and then spoofing with the MAC address after business hours CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc 269 Chapter 10 – Wireless LAN Security MAC filters are great for home and small office networks where . channel use with wireless LANs is imperative for any good wireless LAN administrator. As a wireless LAN consultant, you will undoubtedly CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc (DSSS to FHSS or 80 2.11b to 80 2.11a), and others that we will discuss in later sections. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 243 Chapter 9 –Troubleshooting Wireless LAN Installations. Figure 9.15. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 245 Chapter 9 –Troubleshooting Wireless LAN Installations FIGURE 9.15 Antenna Wind Loading on Point-to-point networks No