Installation Test 195 8.3.5 Pitfalls The site survey process attempts to characterize a complicated phenomenon (indoor RF propagation) using a relatively small number of data points, and is therefore subject to a number of potential issues. These should be kept in mind when performing the survey and interpreting the results. Firstly, the placements of test APs signifi cantly affect the quality of the results. As previously noted, initial placements are based on installer guesswork, experience, and instinct. Repeating the site survey for different test AP placements can be very burdensome, thus if an initial placement is barely adequate or “tweakable” there is frequently no effort put into changing the placements and redoing the survey. This hit-or-miss approach defi nitely does not provide an optimal solution – for example, the output of the site survey may indicate that many more APs are required than originally expected. Secondly, the survey process takes a long time and a great deal of manual effort. This produces signifi cant possibilities for error, as well as problems created by installers taking shortcuts or skipping measurements. Another issue is that the site survey is usually a one-time snapshot of conditions. (It is quite laborious doing a single site survey; requiring an installer to do several over the course of a day or a week is quite unreasonable!) However, the actual indoor RF environment changes on an hour-by-hour and day-by-day basis, according to workfl ow patterns and changes in the surroundings. Thus a considerable amount of margin has to be built into the results in order to deal with the variations. Also, it is diffi cult to convert coverage and signal strength measurements made during the site survey process into true capacity and mutual interference fi gures; the installer or tool has to estimate these fi gures based on empirical data supplied by the AP vendor as well as experience. This is because, as noted above, the test APs used as signal sources are only emitting beacons, not handling actual traffi c. Beacons arrive at a slow rate (10 per second) and fi xed bit rate (1 Mb/s), unlike regular data traffi c which may produce thousands of packets per second at a variety of bit rates. Therefore, interference with the actual data traffi c may not be found during the site survey, but can manifest itself later, when the network “goes live”. (Some tools – e.g., AirMagnet Survey – can run data traffi c to the test APs.) To some extent, the above issues can be mitigated by a three-step process: 1. First, performing a comprehensive site survey to get a rough idea of the “lay of the land”. 2. Second, over provisioning the system by some factor, to provide reserves of channel capacity and transmit power that can be used to overcome undetected interference and shadowing effects. Ch08-H7986.indd 195Ch08-H7986.indd 195 6/28/07 10:20:07 AM6/28/07 10:20:07 AM Chapter 8 196 3. Third, enabling automatic RF management functions in the WLAN controllers and switches to dynamically set channels and transmit power, thereby utilizing the reserve capacity to maintain continuous availability and high performance. The last step is possible as a result of the much more capable and powerful RF management functions available in enterprise-class WLAN controllers today. Such controllers are capable of automatically and continuously receiving, analyzing, and integrating signal, noise, and interference measurements from their connected APs; forming an assessment of channel conditions and interference caused to or by nearby devices; and setting AP channels and power to maintain the desired traffi c rates while minimizing mutual interference. In some cases, the WLAN controllers are even capable of instructing the client laptops and handhelds to increase or reduce power in order to minimize the effects of interference. 8.4 Propagation Analysis and Prediction A (potentially) much more accurate method of determining coverage, bandwidth and other parameters uses complex RF propagation modeling software to simulate and analyze an indoor RF environment, and predict the signal strength contours at all points within the environment. From the signal strength contours and the characteristics of the equipment to be installed, the path loss, throughput, error rate, etc. can be deduced. Once the necessary amount of building data has been gathered and input to the software program, this is a far faster method of determining optimum AP placement, as it does not involve trial placements of actual APs followed by tedious walking around. 8.4.1 Indoor Wireless Propagation Propagation of RF signals is basically identical to the propagation of light, with the signifi cant exception that the wavelength of interest is much larger; thus metallic objects smaller than a few centimeters in size are effectively “invisible” to RF energy produced by WLANs at 2.4 and 5 GHz, and materials that are opaque to light allow RF to pass through them. Further, the propagation medium does not change in relative density (in terms of the dielectric constant ε) very much over the short distances involved in indoor environments, and so refraction is not usually a factor. With these exceptions in mind, the familiar optical principles of straight-line propagation, refl ection, diffraction, etc. apply. Four key effects control RF propagation in an indoor environment: 1. Attenuation (absorption): Walls, partitions, fl oors, ceilings, and other non-metallic objects – including humans! – attenuate radio waves passing through them. In extreme cases, virtually all of the RF energy may be absorbed, in which case the region behind the object is in an RF shadow. Ch08-H7986.indd 196Ch08-H7986.indd 196 6/28/07 10:20:07 AM6/28/07 10:20:07 AM Installation Test 197 2. Refl ection: Large metallic objects, with dimensions substantially greater than one wavelength, refl ect RF energy impinging on them according to the standard principle for optical waves (i.e., the angle of refl ection is equal to the angle of incidence.) Refl ection from metallic objects also causes RF shadows. 3. Interference: If two or more waves arrive at the same point in space but take different paths, and hence have different path lengths, then constructive and destructive interference occurs. In the case of RF, this is usually referred to as fast fading. 4. Diffraction: Large metallic objects with distinct edges, such as metal sheets or furniture, cause diffraction at their edges, and enable propagation into areas that would otherwise be in RF shadows. The following fi gure illustrates the various mechanisms underlying RF propagation in an indoor environment. See Figure 3.4 as well. Reflection from metallic objects Diffraction around metallic edges Attenuation when passing through non-metallic objects Reflection from surfaces behind receiver RX TX Multipath Signal paths with multiple reflections Direct ray Figure 8.5: Indoor Propagation It is convenient to express the path between transmitter and receiver, which has a particular set of properties that affect signals passing from the former to the latter, as an RF “channel” (in the same sense as a waterway). These properties are determined by the propagation effects imposed on transmitted signals before they get to the receiver. As the indoor environment is very complex and not easy to calculate exactly, statistical methods are usually used to model the channel and estimate its effects upon RF signals. The channel is referred to either as Ricean or Rayleigh, depending on the statistical distribution of amplitudes in the signals arriving from the transmitter at various points in the environment. In empirical terms, a Ricean channel generally has a strong line-of-sight component (i.e., the bulk of the RF energy propagates in a straight line from transmitter to receiver). A Rayleigh channel, on the other hand, has the bulk of the energy arriving along non-line-of-sight paths. Ch08-H7986.indd 197Ch08-H7986.indd 197 6/28/07 10:20:07 AM6/28/07 10:20:07 AM Chapter 8 198 For relatively low data rate PHY layers such as 802.11a, 802.11b, and 802.11g, the distinction between Ricean and Rayleigh channels is not very important. However, for 802.11n, this makes a signifi cant difference, as we will see in the next chapter. 8.4.2 Propagation Models A propagation model is the term given to a statistical model of a channel between any two points, in terms of Ricean or Rayleigh statistics. Due to the complexity of the indoor environment, however, these models are frequently implemented as computer programs rather than equations. Two kinds of propagation models have been generally used: parametric models, which express the channel properties in the frequency domain, and ray-tracing models, which operate in the spatial domain. The most common modeling and simulation approach used for the indoor environments that WLANs are concerned with is ray-tracing, as this approach is best able to deal with the complexity of the environment. 8.4.3 Propagation Simulation Propagation simulation originally focused on implementing models (usually parametric) for satellite and cellular communications, but now extends to indoor propagation – usually ray-tracing, as described previously. Such propagation simulation is fairly complex because the indoor environment is full of artifacts (walls, ceilings, doors, furniture) that affect RF propagation. However, the use of powerful computers makes it possible to simulate the propagation accurately within quite large indoor areas. Ray-tracing, borrowed from computer graphics, is the principal means of performing indoor RF propagation simulation today. The ray-tracing method is conceptually very simple. The features of the environment (doors, walls, etc.) are represented to scale on a grid within a computer, resembling an architectural fl oorplan, but referencing the RF properties of the various elements. A simulated RF “source” is placed at some desired location. “Rays” are then drawn in all directions from the RF source, representing electromagnetic waves propagating linearly outwards from the source with a given signal strength. Where the rays strike elements of the environment, the laws of propagation (i.e., refl ection, attenuation, diffraction, etc.) are applied to determine the magnitude and phase of the resulting transmitted and refl ected rays. If two or more rays intersect, then interference calculations are made to determine the resultant signal strength. The process is carried out until some desired degree of resolution is reached; plotting the signal strength at each point on the fl oorplan then gives the simulated propagation of RF from the simulated source. Using the principle of superposition, the procedure can be repeated for any number of sources at different locations until a complete picture of the RF signal strengths within the environment is obtained. Figure 8.6 shows a simplifi ed view of this process. The ray-tracing method is computationally intensive but is very powerful. If the dimensions and RF properties of the objects within the environment are known, as well as the Ch08-H7986.indd 198Ch08-H7986.indd 198 6/28/07 10:20:08 AM6/28/07 10:20:08 AM Installation Test 199 properties of the source, then the RF fi eld strength can be plotted very accurately at any point. Experimental comparisons between the ray-tracing method and actual propagation measurements show very good correlation, and it is now the de facto method for indoor propagation simulation. 8.4.4 The Prediction Process With ray-tracing simulation, it is possible to bypass the manual site survey process and directly predict the coverage and throughput available from a given AP placement. This type of prediction process is as follows: • The building fl oorplan and material characteristics (i.e., the RF properties of walls, furniture, etc.) are entered into the simulator. • The RF characteristics – transmit power, antenna radiation pattern, etc. – of the equipment (APs) are also entered. • A set of candidate AP placements are made on the fl oorplan. • The simulator then takes over, performs a ray-tracing simulation, and plots the coverage (in terms of signal strength contours) on the fl oorplan. Once the signal strength is known, the simulator may even deduce the available throughput at various points based on the characteristics of some selected WLAN receiver. • The coverage and throughput contours are manually inspected. If the coverage is unacceptable, the AP placements can be changed and the simulation re-run immediately. The prediction process is far faster than the manual site-survey, provided that the building and equipment characteristics are known in advance. Further, it is possible to perform many Simulated TX Diffracted ray Reflected ray Attenuated ray “Rays” generated in all directions Represent floorplan to scale on a grid Set RF properties of walls, doors, furniture, objects Place simulated RF source at a location on floorplan Draw rays from source in all directions (360 degrees) Solve RF propagation equations for interference Rays intersect metallic object? Solve RF propagation equations for reflection, diffraction Intersection point becomes new “virtual source” Rays intersect non-metallic objects? Solve RF propagation equations for attenuation Calculate amplitudes at all points along rays Figure 8.6: Ray-Tracing Simulation Process (Simplifi ed) Ch08-H7986.indd 199Ch08-H7986.indd 199 6/28/07 10:20:08 AM6/28/07 10:20:08 AM Chapter 8 200 “what-if” scenarios and arrive at an optimal placement. Obviously, this is a much simpler and less labor-intensive process than the traditional site survey – if accurate and complete data on the building is available. Several commercial SW packages, such as LANPlanner from Motorola Inc., implement this process. The more sophisticated packages support various features, such as automatic entry of fl oorplans from AutoCAD drawings (i.e., DXF format fi les), a large materials database with RF properties of common building materials, a fl oorplan editor to allow users to place furniture and other metallic objects, and a database of APs with properties. One extension to the above process is to perform a prediction of coverage based on known data, and then to refi ne the predictions with actual measured data. This is effectively a blending of the propagation modeling and the site survey processes. The fl oorplan and materials are entered fi rst, the propagation is modeled, and initial predictions of coverage made. An actual AP or signal source is then placed at a target location and a special receiver is used to record signal strengths at some points around the coverage area. These data points are compared with the predicted values from the simulation, and the differences between measurements and prediction are used to refi ne the propagation modeling and compensate for errors. This allows a more accurate result, but without all the manual labor of the site survey. Tools such as InFielder from Motorola Inc. assist here. 8.4.5 Modeling Equipment Characteristics As the purpose of the installation process is to determine the optimal placement of APs, only the APs really need to be characterized. (While the RF characteristics of the client cards play a signifi cant role in the actual end-user experience, the installer has little control over this; all he or she can do is to place the APs at optimal locations to assure the desired signal strength and coverage.) For the purposes of propagation modeling, APs can be characterized by three factors: 1. The total radiated power: This is the transmit power integrated over three dimensions (i.e., the total power output of the transmitter minus the power lost in the antenna and cabling). 2. The total isotropic sensitivity: This is the sensitivity of the AP as integrated over three dimensions (i.e., the sensitivity of receiver divided by the effi ciency of the antenna and cabling). 3. The antenna radiation pattern. If these three factors are known, then the coverage (receive and transmit) of the AP can be predicted using the ray-tracing simulation process. Unfortunately, most vendors do not publish any of the above characteristics. However, they can be approximated; further, for most purposes it is only necessary to model the transmit Ch08-H7986.indd 200Ch08-H7986.indd 200 6/28/07 10:20:09 AM6/28/07 10:20:09 AM Installation Test 201 characteristics of the APs. The receive coverage is assumed to be about equal to the transmit coverage, which is true for most well-designed APs. In addition, the total radiated power can be approximated as being equal to the transmit power of the AP (this assumes losses in the radiating system are negligible, which in most cases is true). This leaves the antenna radiation pattern as the unknown factor. If standard vertical antennas are used on the APs, then the antenna radiation pattern can be assumed to be the typical doughnut shape of a vertical dipole. On the other hand, if a directional antenna such as a patch is used, then the radiation pattern is no longer a doughnut, but has lobes (regions of higher signal strength) and nulls (regions of lower signal strength) in various radial directions. These lobes and nulls can be predicted, with a bit of diffi culty, from known antenna radiation patterns. These two can be plugged into the propagation modeling software, and the resulting coverage contours plotted. Fortunately for the installer using commercial propagation modeling software, these characteristics have already been incorporated into the software for many commonly available APs. All that the installer needs to do is to select the appropriate AP from a list and orient it on the on-screen fl oorplan as desired. The software will then consult its database of equipment properties and obtain the information necessary. 8.4.6 Limitations and Caveats The propagation modeling process can produce results that are very close to reality, but the biggest limitation is the need for complete and accurate entry of environmental data. Without complete knowledge of the indoor space, producing a truly accurate picture of the RF channel is diffi cult or impossible. “Complete” here is to be taken literally; every large metallic object needs to be input (heating ducts, elevator shafts, cubicle walls, etc.) and the RF properties of every wall, door, and window must be entered as well. However, the architectural drawings are often not available, or are not in a form that is readily acceptable to the software. (A stack of blueprints makes for a laborious and tedious process of conversion into a vector drawing, such as with AutoCAD.) Further, even if such drawings were available, the actual building very frequently diverges from the architectural drawings, thanks to changes and architectural license taken during the construction process. Further, the materials composing the fl oors, walls, and ceilings are often not known; even if they are, the RF properties of the materials may not be known. Details such as the furniture play a signifi cant role in the propagation, but these materials and dimensions are even less well known than the walls and partitions. Another limitation is that the surroundings can play a substantial role in the RF propagation, but is typically diffi cult or impossible to model. For example, large glass windows are transparent to RF; a concrete wall just outside the windows can therefore refl ect RF back into the indoor space, substantially changing the fi eld strength pattern. Predicting interference from neighboring areas is particularly diffi cult. Ch08-H7986.indd 201Ch08-H7986.indd 201 6/28/07 10:20:09 AM6/28/07 10:20:09 AM Chapter 8 202 Finally, as has been noted above, the characteristics of the equipment (APs, etc.) are not straightforward to include, as they are not usually available from the vendor and not easily measured without complex equipment. Apart from the variations in equipment RF characteristics due manufacturing tolerances, there is also an impact due to cabling (e.g., the angle at which cables are run to and from the APs) and the proximity of surrounding metallic objects, which will alter the radiation pattern of the APs. All of these effects make propagation modeling considerably less accurate than would normally be expected. Fortunately, the level of accuracy needed for arriving at a workable placement of APs is relatively low; with a moderate safety margin, it is possible to obtain fairly good results even in the absence of all the comprehensive information regarding the indoor space. The ability of enterprise WLAN controllers to “manage” the RF environment also simplifi es the task; small errors in the modeling process can be masked by changing the transmit power of selected APs up or down to compensate. 8.5 Maintenance and Monitoring Wired enterprise LANs require continuous monitoring and maintenance for proper operation; WLANs in the enterprise are not exempt from this requirement either. However, WLANs have a further complexity in that they are subject to changes in the surroundings and the interior physical environment, which makes monitoring even more important. Some examples of changes in the indoor environment that could signifi cantly alter the operation of a WLAN are: • new interferers (e.g., a newly installed but leaky microwave oven); • malicious intrusion from outside; • changes in propagation conditions causing coverage loss, such as metallic furniture being moved; • the installation of neighboring WLANs, causing an increase in the channel congestion; • an increase in the number of clients; unlike wired LANs, where the number of clients is limited to the number of physical ports, a WLAN can see arbitrary increases in client counts as users bring in laptops and handheld devices. Such changes can cause signifi cant adverse impact on the operation of the WLAN as originally installed, and the WLAN confi guration may have to be modifi ed to cope with these changes and restore the same level of service formerly provided to the users. 8.5.1 Monitoring and Maintenance Tools As mentioned previously, two kinds of tools are utilized for WLAN monitoring and maintenance. Firstly, the APs (and WLAN controllers) themselves contain quite extensive Ch08-H7986.indd 202Ch08-H7986.indd 202 6/28/07 10:20:09 AM6/28/07 10:20:09 AM Installation Test 203 built-in statistics and data gathering facilities, that function even as the APs are operating to support clients. In addition, several vendors offer dedicated diagnostic tools specifi cally designed to address issues in enterprise WLANs. In many respects these are complementary approaches; the built-in tools within the WLAN infrastructure can alert the IT staff to problems, and the dedicated tools can be used to localize and diagnose these problems and verify solutions. The built-in monitoring capabilities within virtually all enterprise-class APs represent the simplest and cheapest way of performing continuous monitoring of installed WLANs. Considerable passive surveillance and monitoring functions can be performed using these facilities, which can track the level of interference and noise surrounding each AP, scan channels to fi nd WLAN devices in the neighborhood, monitor signals received from neighboring APs and clients belonging to the same WLAN, monitor signals from APs and clients that are not part of the same WLAN (sometimes referred to as “rogues”), and detect malicious attacks or intrusion attempts. If problems are suspected with clients, the APs can perform simple RF tests on the clients by exchanging packets with them and tracking the results. WLAN controller-based systems are particularly effective at monitoring, as they can integrate information received from multiple APs and report it up to the management console as a network-wide report. Further, these monitoring functions can integrate into large, widely used enterprise network management platforms (such as OpenView from Hewlett Packard) and provide the IT staff with a picture of the wired and wireless network as a unifi ed whole. The advantages of having the monitoring functions built into APs are: • low cost • simplicity • reduced cabling and infrastructure complexity • less management overhead (less devices to manage and maintain) • easier setup • information can be shared between network management and network monitoring. The sharing of information between network management and network monitoring is a powerful argument in favor of building monitoring functions directly into the WLAN infrastructure. For example, clients can be identifi ed as legitimate by the WLAN controller based on the security credentials negotiated when they connect, and this information can be used to automatically screen out valid clients when checking for rogues and intruders. This can greatly reduce the burden on the IT staff. Dedicated diagnostic tools typically comprise the same equipment as used in site surveys: laptops with “sniffer” software, spectrum analyzers, handheld PDA-based signal monitors, Ch08-H7986.indd 203Ch08-H7986.indd 203 6/28/07 10:20:10 AM6/28/07 10:20:10 AM Chapter 8 204 etc. When a problem is detected, these tools are used to localize and identify the nature of the problem, and diagnose the root cause. For example, a sniffer can be used to scan for intrusion attempts or denial-of-service attacks, or WLANs that have started up in adjacent offi ces or buildings. In some cases a “mini-site-survey” can be performed using the tools, to systematically locate and diagnose the issue. (It is useful to have the results of the original site survey available for comparison, so that large changes in properties can be quickly identifi ed.) 8.5.2 Active Monitoring Companies such as AirMagnet also provide dedicated monitoring functions using a hardware monitoring architecture. In these products, wireless monitoring “sensors” are deployed around the WLAN coverage area, and connected to the wired infrastructure; these devices are independent of the APs and WLAN controllers, and are installed and operated as a separate subsystem. The sensors can pick up and track all the WLAN signals in their surrounding area; sampling techniques allow them to track multiple channels concurrently (though not simultaneously, unless special radios are used). The sensors then feed information to a management server that aggregates and consolidates all the information, after which a management console can be used by the IT staff or network administrator to inspect and analyze the data. The sensors can operate in remote offi ces as well as locally, thus enabling an entire corporate-wide network to be managed as a unit. Such a distributed system can monitor for many problems: • malfunctioning or misconfi gured APs and clients, • rogue APs or clients, • excessive noise and interference levels, • malicious attacks and intrusions, • APs or clients suffering excessive traffi c loss due to weak signals, • mutual interference between WLAN devices, • excessive collision levels and channel overload. Active monitoring offers several advantages when compared to building similar functionality into the APs themselves. The sensors are dedicated, and hence can monitor continuously (an AP cannot monitor when it is transmitting, and vice versa). Also, they can switch rapidly from channel to channel, or even monitor multiple channels concurrently; an AP must stay on one channel or risk dropping all its associated clients. These systems can detect a much larger range of issues, as they typically use special radios backed by powerful analysis software. The sensors can be placed in known problem areas, thus eliminating the need to choose between the best sensing locations vs. the optimum AP placements. Also, converting all APs into Ch08-H7986.indd 204Ch08-H7986.indd 204 6/28/07 10:20:10 AM6/28/07 10:20:10 AM [...]... models were verified by extensive experimentation and data gathering While the models were originally intended for simulation, design, and testing of competing 802.11n PHY proposals, the channel models are also very useful for the testing and performance measurement of actual 802.11n devices and systems 9. 2.3 IEEE 802.11n Operational Modes: Antennas, Bandwidth, and Coding IEEE 802.11n is characterized by... industry and academic research work, and were specialized to deal 2 19 Chapter 9 with indoor MIMO propagation; they are based on the cluster models developed by Saleh and Valenzuela The six models are labeled A through F, and are described in the following table: Model A Represents An ideal free-space environment with no scattering and simple flat fading B Residential (relatively small floor area and few... the 802.11n standard, published in March 2006 As of this writing the draft standard has been revised twice and is slowly making its way through the IEEE standards process, which involves numerous cycles of review, voting, and modification The final 802.11n standard is not expected to be ratified until 2008 at the earliest; however, in the mean time, vendors are already creating chipsets and systems based... interference and greater effective range (distance between the transmitter and receiver) However, these techniques are particularly difficult to test, as they are both complex and highly sensitive to the RF environment in which they are deployed This chapter covers some of the special needs and approaches for testing MIMO devices and systems Note that as much of this technology is just being developed, and the... frequency ranges and channel bandwidths in the WLAN bands are defined by regulatory bodies and would hence be more-or-less the same as for standard 802.11a/g PHYs, but 5–10 times more bits had to be squeezed in somehow After a good deal of preliminary work, the Study Group demonstrated that modern technology could yield a cost-effective and high-performance PHY capable of the desired level of bandwidth As... wireless LANs (WLANs) is still very much in its infancy, many of the test techniques and approaches are still under research and development This chapter should, however, arm the prospective test engineer with enough background to get a start on the MIMO testing process Before diving into test techniques, however, we will take a reasonably detailed look at what MIMO is and how it works 9. 1 What is MIMO?... over a narrower bandwidth As shown in the figure above, this form of channel sounder is conceptually quite simple, comprising an RF pulse generator that modulates (switches on and off ) a carrier wave, a pair of antennas, an amplifier, and an envelope detector However, it is susceptible to interference (because the receiver must be very wideband) and it is difficult to construct a wideband system that... inherently a spread-spectrum device, and therefore has the noise and interference rejection benefits of spread-spectrum technology 9. 1.4 MIMO Channel Capacity The standard formula for calculating the capacity of a radio (or any information) channel is Shannon’s limit on channel capacity: C ϭ B ϫ log2(1 ϩ SNR) where C is the capacity in bits/second, B is the bandwidth in Hz, and SNR is the ratio of the signal... transmit and receive) is M times the capacity of a SISO system operating at the same channel bandwidth and SNR This assumes a rich scattering environment In general, a MIMO system is characterized by the number of transmit and receive antennas used, as these directly indicate the channel capacity improvement that can be obtained Thus a 4 ϫ 4 MIMO system has 4 transmit and 4 receive antennas, and can... automatically to reserve capacity and power, which can then be used to overcome unexpected issues or changes in the environment 206 CHAPTER 9 Testing MIMO Systems The IEEE 802.11n draft standard (scheduled to be ratified in 2008) uses advanced Multiple Input Multiple Output (MIMO) radio techniques, using two or more simultaneously active antennas combined with two or more transmitter and receiver channels MIMO . understand how MIMO works and its effects on both WLAN equipment and test procedures. CHAPTER 9 Ch 09- H 798 6.indd 207Ch 09- H 798 6.indd 207 6/28/07 11:24:03 AM6/28/07 11:24:03 AM Chapter 9 208 9. 1.1. ed) Ch08-H 798 6.indd 199 Ch08-H 798 6.indd 199 6/28/07 10:20:08 AM6/28/07 10:20:08 AM Chapter 8 200 “what-if” scenarios and arrive at an optimal placement. Obviously, this is a much simpler and less. dimensions and RF properties of the objects within the environment are known, as well as the Ch08-H 798 6.indd 198 Ch08-H 798 6.indd 198 6/28/07 10:20:08 AM6/28/07 10:20:08 AM Installation Test 199 properties