A coverage-oriented WLAN is designed to provide maximum WLAN coverage with the least amount of APs. (A typical coverage-oriented network provides a user to AP ratio of 25 to 1.) Some characteristics of a coverage-oriented deployment include the following:

In coverage-oriented deployments, the typical applications have low packet rates and low bandwidth requirements. Because of these meager demands on the WLAN, the users can expect effective throughput in these deployments to be quite high.

This setup allows more users to leverage the WLAN while still maintaining adequate performance.

These types of deployments might be common in warehouse environments or retail environments where WLANs are mission critical for inventory control and just-in-time purchasing and where the IT staff is in a central site, with no local support staff to troubleshoot coverage issues.

Also, it might be common for small or medium branch offices to opt for such a deployment as an alternative to installing wired Ethernet. In these cases, the branch offices experience a significant amount of relocation and Category 5 cabling expenses are minimized. A simple-to-deploy WLAN provides basic network connectivity such as file and printer sharing.

Figure 8-2 illustrates the example floor plan from Figure 8-1 with a coverage-oriented WLAN deployment.

Figure 8-2. Typical Office Floor Plan with a Coverage-Oriented WLAN

Figure 8-2 provides WLAN coverage with roughly 25 to 30 users per AP. To provide complete coverage for the floor, the plan uses 14 APs. Note that the deployment provides coverage to all areas, including storage closets, restrooms, and stairwells where coverage might not be normally required. It is reasonable to assume that other possible deployment configurations use fewer APs, but the configuration in Figure 8-2 is used for the sake of simplicity and illustration.

A capacity-oriented WLAN is designed to provide maximum throughput and packet rate for each client in a BSS. Capacity-oriented cell sizes are smaller than coverage-oriented cells, requiring a higher AP density. Capacity-oriented deployments are required for areas that have the following characteristics:

Figure 8-3 illustrates the same floor plan with a capacity-oriented deployment. Note that there are more than twice as many APs in the capacity-oriented deployment (30 APs versus 14 APs).

Figure 8-3. Typical Office Floor Plan with a Capacity-Oriented WLAN

The coverage area each AP provides is much smaller than in Figure 8-2; in fact, the cells are nearly half as small. Each AP provides coverage for roughly 12 users, and the entire deployment requires 30 APs as opposed to 14 APs in the coverage-oriented deployment.

A large number of deployments begin by providing coverage in common areas, such as conference rooms. The usual result from such a deployment is that users near the conference room leverage the WLAN while those who are farther away do without. These deployments are the seed to full-coverage deployments after the WLAN has been in place for some time. The user demand for WLANs usually forces the IT department to fully deploy at some point, so it might be useful to prepare for full deployments even when starting with a partial deployment.

It makes sense to perform an initial site survey for a complete coverage deployment. Figure 8-4 shows a partial deployment given the same coverage-oriented site survey from Figure 8-2. The deployed APs provide coverage to the common areas and conference rooms in the floor plan. The black dots represent the locations of APs for future deployment that will not interfere with the existing APs. Note that the deployed APs are in the same places as in Figure 8-2. This setup allows future deployments to occur without the need for additional costly site surveys.

Figure 8-4. Partial AP Deployment

As the site survey engineer, you face many challenges, and you need to understand and evaluate them up front before you even start performing the physical site survey. For example, if you don't understand the wired infrastructure in a warehouse, you might find that you are placing APs in locations that exceed the 100 meter (m) Category 5 100BASE-T cable lengths. The result might be that you spend hours performing measurements, only to discover that you need to start over at the beginning. Many of the obstacles that you face can be specific to your industry, as in the previous warehousing example, where the vastness of the facility can require unique solutions. Although this section discusses the types of challenges you face and how to look for them, Chapter 10, “WLAN Design Considerations,” places those challenges into specific industries.

In addition to considering the size of the facility, you also need to be aware of how the wireless propagation environment can change over time. Working off the previous warehousing example, the environment provides different propagation characteristics with fully stocked shelves rather than empty shelves. You also need to pay attention to objects that tend to reflect microwave energy, causing multipath, as introduced in Chapter 7, “Radio Frequency Essentials,” or objects that absorb the radio waves, making propagation itself challenging. Shelves stocked with energy-absorbing paper products, for example, tend to limit the overall propagation of radio signals, while also providing obstructions that can create coverage shadows in much the same way that they can shadow light. In the opposite extreme, with fully exposed empty metal shelves, free propagation occurs along with potentially harmful multipath. Your survey needs to consider both extremes and the specific needs of the network under these conditions. To cover a deployment that will vary from empty shelves to fully stocked over time, you likely want to employ smaller or more directional coverage areas that put the RF energy right where it is needed from locations that will never be obstructed. At the same time, you want to minimize the amount of energy that bleeds into other areas when shelves are empty, either by lowering the transmit power settings or by utilizing directional antennas. In all cases, you need to take careful signal-strength measurements during the site survey to ensure a robust design.

In a retail environment, the demands on the network can change from light usage to heavy usage as store managers use a small number of 802.11 phones during business hours, and then large numbers of employees flood the wireless network during nightly inventory sessions using barcode scanners. These scanners might require the transmission of many small transaction packets that need to arrive at their destination within a certain amount of time. In contrast, a streaming video display of specials might just require very high through-put. In the former case, it is critical that you achieve 100 percent coverage wherever the scanners might be operating. Because there is little data to share, it is satisfactory to only provide lower data rate coverage in some areas. The video case will likely require high data rate coverage, but it will only be at a fixed location or at a subset of the locations where scanner coverage is required.

You might not be able to control some sources of interference, such as some cordless phones, microwave ovens, and even other WLAN networks. Non-802.11–based cordless phones represent the most common source of harmful interference to WLAN networks. These phones are often designed to operate in either of the 2.4 GHz or 5.8 GHz ISM bands, utilizing either frequency-hopping or direct-sequence technology. The degree of degradation, if any at all, depends upon the numbers and types of phones in use. Some consumer-grade 2.4 GHz phones might clutter the entire band, whereas others are more spectrum friendly. If only a single phone is in use, as in the home, the best guidance is to place the base station away from APs or locations where client activity is anticipated. An office environment with many phones might raise the overall noise floor, thereby inhibiting WLAN transmissions. The best guidance is to either use an 802.11-compliant phone system or keep your data and phone networks in different frequency bands.

The other interference concern, which often appears in healthcare applications, is mission-critical equipment to which your network could be a harmful source of interference. Because the 2.4 GHz ISM band was allocated for medical devices, some hospitals might have units operating in this band. In general, WLANs operate at much lower transmit power levels than medical devices, and you might be able to place them upon channels where medical devices are not located or at power levels where they will not interfere. First and foremost, you need to understand where your medical devices are located and what their characteristics are. WLAN equipment that is compliant with the requirements of the International Electrotechnical Commission (IEC) 601-1.2 meets the industry limits. This compliance does not ensure that interference will not occur, although it might reduce the chances. For complete assurance, if your facility does use mission-critical equipment in the 2.4 GHz band, you might find that the easiest design choice is to use 802.11a in a non-ISM band such as the U-NII-1 band.

The physical environment itself might provide other challenges if it requires your equipment to operate under extreme temperature, high or low humidity, or wet conditions. You might find that you need to provide coverage inside a refrigerated room in a warehouse, for example. In this case, it might be necessary to place the AP inside a heated enclosure if the required operating temperature falls outside the range specified by the manufacturer. Similarly, if you are creating outdoor hotspots and do not want to run a long RF cable from an indoor location for the AP to the outdoor antenna location, you might place the AP in a properly rated NEMA enclosure to protect it from the rain and elements.

If you need to provide coverage on multiple floors of a facility such as an apartment building, you also need to remember that your first-floor network might interfere with your second-floor network and vice versa, depending upon the design and construction of your building. This potential will lengthen the site survey process because it will be necessary not only to measure the signal strength on the desired floor, but also to measure it on the floor above and the floor below.

In retail or education environments, you might have special aesthetic requirements to hide the network infrastructure, either to protect it from vandalism or to meet specific corporate requirements. Under these circumstances, it is common to place plenum-rated APs above ceiling panels, if that space is available, and then place the antenna in the ceiling or wall itself. Because they are small and flat, patch antennas often work well in these circumstances.

In addition to these challenges, you need to consider the characteristics of the end-user clients. This consideration includes such factors as the types of client devices that your network will need to support, the nature of the workforce using the clients, and the type of applications they are running over the WLAN. With stationary PCs in an office environment, there will be very little roaming, so coverage can be more focused with less overlap. Even if laptops are in use, it might not be necessary to provide high data rate coverage away from desks and conference rooms. If your firm is an engineering firm that runs applications with high-bandwidth applications, such as CAD tools, you need to maximize capacity with much smaller cells and fewer users per cell. In contrast, if your end users are running session-based applications on barcode scanners, they might have minimal bandwidth requirements but require coverage that allows for seamless roaming with support for connectivity at all locations. With voice, your network will be filled with many small data packets, so you must understand the capacity of your WLAN infrastructure devices with respect to voice. In these examples, there can be no holes in your coverage. Some specialized client devices might not even support higher data rates. You must also take into account this possibility. From all these questions, your most basic need is to understand the throughput and packet-forwarding requirements on your network, on a global scale and in localized special requirements.

Wi-Fi CERTIFIED

The Wi-Fi CERTIFIED on your client devices ensures a base level of interoperability between devices from different vendors. It is imperative that both your access points and client devices have this logo. The Wi-Fi Alliance, as discussed in Chapter 7, “Radio Frequency Essentials,” performs interoperability testing so you can safely deploy without fear of interoperability concerns.

As indicated earlier, it is not enough to understand these wireless-driven requirements, but you must also have a thorough understanding of the LAN infrastructure to which you are interfacing. One of the most important questions is, “What is the topology of your network?” You need to understand the networking equipment, hubs, switches, and routers that you are connecting to and where it is located. You must also understand what types of media interfaces your LAN can provide. It is most likely unshielded twisted-pair (UTP), but it might also be fiber. Remember that 100BASE-T Ethernet can only run 100 m over Category 5 UTP. If Dynamic Host Configuration Protocol (DHCP) is in use, where is the server and what lease times are in use? A WLAN network of clients might have very different LAN client network requirements.

You need to know whether cabling is routed through plenum spaces and whether your infrastructure equipment needs to be capable of being deployed there. Most buildings have firewalls, and the National Electric Code specifies procedures for penetrating them. You should measure cable lengths with straight segments and 90 degree turns; provide for service loops for the cabling contractor to deal with unforeseen issues; and, if antenna cables are in use, minimize their length and be aware of the losses they introduce at your operating frequency.

With your WLAN challenges, requirements, and goals, it is time to gather the tools you need to conduct your site survey:

After you gather all the necessary tools, it is time to conduct the survey. Remember, as indicated at the beginning of this section, the questions you need to answer during your site survey are

The answers should be detailed enough that someone else could perform the install.

Considering the two extremes of a coverage-oriented approach and a capacity-oriented approach, your physical survey follows slightly different steps. With both, you should set channel assignments to avoid overlap and do each survey on the channel that will be used. You should also survey at the minimum desired data rate. In the multifloor case, remember that the floors above and below are part of the extremities, and you need to make measurements there as well. Do all measurements on the channel you intend to use—or you might get bitten by unknown interference. If cells seem smaller than expected, try changing channels because there might be interference issues.

With the coverage-oriented approach, choose one of the edges of your desired coverage area and place an AP there. Now, walk toward the center of the desired coverage area, until you locate the boundary of desired coverage. Move the AP to that location and survey the coverage. At this point, you have two options:

With either of these approaches, you need to define your edges to allow for the appropriate level of overlap. Figure 8-5 shows an example of the extremity approach, and Figure 8-6 shows the AP-to-AP approach. In each figure, the channel in use is indicated in the center of each range circle. The lighter-shade APs indicate the initial locations used to determine the center point for the AP. The arrows show how the positioning of one AP determines the next AP's location.

Figure 8-5. Coverage-Oriented Extremity Plus Hole-Filling Survey

Figure 8-6. Coverage-Oriented AP-to-AP Survey

With a capacity-oriented approach, determine the number of users you want to attach to each AP and the user density, and from that, determine the desired cell radius. Use the same techniques as before, but adjust the power until you achieve the desired size cell. You should most likely use the extremity approach when you have isolated pockets of users, but you might use the other approach when the user density is fixed throughout the environment.

Depending upon the complexity of the site and the requirements, you might experience a lot of trial and error. You need to be creative at times because there might be many solutions to the puzzle.

Chapter 4, “802.11 Wireless LAN Security,” covers security in depth and delves into how 802.1x is ingrained in the WiFi Protected Access (WPA) specification and the forthcoming 802.11i standard for WLAN security. The solution requires a AAA server to provide user-based authentication. The AAA server is probably located in a secured data center, several router hops away for the network edge. With the advent of Layer 3, wire rate switching, you can measure the network latency between the network edge and the data center in single digit milliseconds, if not microseconds.

802.1x-based deployments become more complicated when distributed across a WAN link. WAN links are generally lower bandwidth than LAN connections, and as a result, congestion can occur on these links. Congestion can have a significant impact on 802.1x-based authentication in that dropped RADIUS packets can cause client-station authentications to time out and fail, as well as potentially impact roaming, as depicted in Figure 8-7.

Figure 8-7. WAN Link Congestion Impacting 802.1x Authentication in Remote Sites

You can alleviate the problem in a couple of ways:

Local Authentication at Remote Sites

Local authentication at remote sites might seem like the obvious answer to the problem, but by no means is it a panacea. Deploying AAA servers at remote sites has issues:

• Expense—. Large remote-site deployments require at least one server per site.

• Manageability

  • Authentication servers could number into the thousands, depending on the deployment.

  • The replication of user databases to large numbers of remote sites can be problematic.

  • Administrator access can be an issue if remote-site administrators need to constantly access the central server.

Some vendors, such as Cisco Systems, incorporate WAN survivability into their APs to let customers avoid the expense and to manage the headaches associated with local AAA servers, as shown in Figure 8-8.

Figure 8-8. Local Authentication Service on the AP

Although the solution is not perfect, it enables administrators to confidently deploy WLANs in remote sites while maintaining a single authentication database to manage.

Network management, and in particular WLAN management, is a topic that requires a book unto itself. This section highlights some key concepts to consider during deployment.

In any type of network,

Many large networks can have as many as a thousand managed devices. In a typical large enterprise WLAN deployment, it is not uncommon to see nearly three times that many APs. WLANs can have a major impact on how you manage your network and also challenge the existing tools you use. To have a WLAN that performs as reliably as a wired LAN and minimize the management complexity, you need a management solution that includes WLAN management.

Many early adopters of WLANs have been plagued by the management burden of WLANs. Most cost-effective management packages hardly scale to several thousand devices without requiring multiple management stations, and none offer RF-specific management functionality. This absence left these deployments with poor-performing WLAN deployments and forced administrators to develop their own toolkits for effectively managing WLANs. Today, however, most laptops ship with 802.11 network interface cards (NICs) as standard equipment, and users are starting to rely on WLANs and demand the same level of availability as wired networks.

Many WLAN management solutions provide wired-like management services: Simple Network Management Protocol (SNMP) polling, fault monitoring, trap collection, configuration distribution, firmware distribution, and so on. No available solutions give the administrator insight into the radio network itself. WLAN performance varies widely with each implementation. Materials in the walls and the location of external interferers such as microwave ovens can impact the performance of WLANs, and the introduction of Bluetooth devices, ad-hoc clients, and neighbors using WLANs can degrade the performance of a WLAN to the point of making it useless.

Radio management gives the administrator clear visibility into all these issues and, depending on the solution implementation, can automate the control of radio parameters such as frequency/channel selection and client/AP transmit power to adapt to harsh RF environments.

Seek solutions that provide this functionality because it will make your job as an administrator much easier. RF networking is such a drastic departure from wired networking that without years of experience, finding a management tool that performs complex site survey calculations, path loss, interference detection, and possible location services certainly makes sense.