Deploying the WLAN

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Now you're ready for an actual WLAN deployment, but—take it easy, start with a trial or pilot program. If possible, borrow the necessary components from the proposed vendor(s), if not, make a small initial purchase for the trial run. Choose a test group consisting of both experienced wireless users (e.g. they have a WLAN at home) and neophytes. (For more details on planning and implementing a pilot project, see Chapter 14.)

A well-run trial or pilot program will reveal design flaws and possible incompatibilities within the overall networking environment. While you should use the test program for normal daily business processes, also introduce intentional failures so you can see how end-users and IT staff react when the network goes down.

Once everyone's satisfied that the design functions well, the components are compatible, and a WLAN is beneficial to the organization's bottom line, it's time to train the IT support staff. The training program should include support for the wireless computing devices, support of the end-users, and competency in the use of any new applications that may be needed for optimal network management, maintenance and security.

The most efficient way to rollout an enterprise WLAN system is to do it in phases, one department or locale at time. This allows the deployment team to deal more easily with problems that might crop up along the way.

Check List

The final step before deployment is to establish a check list. Use the check list to determine that everything is ready for deployment. Items that should be on the check list include:

• Determine that you have the right amount and the right kind of wireless adapters to equip all mobile participants and, if necessary, create auto-installers to install all the WLAN drivers, VPN software (if any), etc., in participants' computing devices.

• Make sure to quality-test the drivers and software in the computing devices as an integrated solution set (not only as individual units) prior to deployment.

• Consider what kind of security will be implemented, e.g. WEP, installing switches to route traffic from access points to a network demilitarized zone in only one direction, using RADIUS servers for secondary authentication, VPNs, etc.

• Determine whether a VLAN architecture is necessary, especially if the WLAN is expected to support roaming or bandwidth-intensive applications.

• Decide what type of network management tools will be used to manage the network after it is operational.

• Decide the end-user support to be provided, including training and ongoing help desk support.

• Equip the deployment team with the necessary tools to deploy the equipment, e.g. blueprint, photos taken during the site survey, flashlight, etc.

Access Point Placement and Power

An access point's functionality can be equated to a media-independent, multi-port bridge that provides bridging from a wired media to a wireless media. For the best access point placement, management, and performance, the designer, installer and IT staff need to understand basic AP functionality and configuration options. Most APs include features for different interface connections and network management, and all provide MAC bridging between their interfaces.

APs monitor traffic from their interfaces and, based on frame address, forward the frames to the proper destination. APs track the frames' sources and destinations to provide intelligent bridging as mobile computing devices roam or network topologies change. APs also handle broadcast and multicast message initiations, and respond to mobile computing devices' association requests.

An access point's radio frequency signals may have to pass through ceilings, floors, walls, and other objects—all of which can cause signal degradation. In a typical corporate office, education, or healthcare installation, the access points will be mounted at ceiling height, whereas in distribution centers, manufacturing plants, warehouses and other facilities with high ceilings, the access points should be mounted in a location that is no more than 15 to 25 feet from the floor. Of course, mounting access points in such locations can create the additional problem of getting power to the unit. If your installation requires such AP placements, you should consider purchasing units that utilize PoE (i.e. can access power over the Category 5e Ethernet cable). And in fact, in all but the smallest WLAN installations, using PoE can drastically reduce the cost and complication of installation.

Unusual access point placement can jeopardize proper signal distribution, whereas creative antenna placement can provide a solution. For example, if an AP needs to be placed above ceiling panels, its antenna will need to be positioned below the tiles. Such a configuration requires that the access points used should (1) have remote antenna capability, and (2) be plenum-rated, since many regions legally require that devices installed above a ceiling have a metal casing that meets the specific fire code requirements of your area. Outdoor situations bring up another host of challenges, such as how to provide power and bring cabling to the devices, and how to harden the device(s) to inclement weather.

The best way to determine optimal access point placement is the site survey. Of course you must use the same model of antennae you intend to use in the final deployment stage. Using different models produces different wave propagation patterns. This is the only way to determine how a building's construction materials or campus's topographical features will block or absorb signals.

Too Much of a Good Thing

While you must install enough access points to support the end-users and their applications, too many access points can be as bad as having too few. When more than one access point sends at the same signal strength to a specific location, an accessing client can become confused when forced to constantly evaluate which access point it should utilize. However, if only one access point sends at a strong signal and another is far enough away that there is a 20-decibel or so difference, there is no problem, the accessing client knows to go for the stronger signal.

Once an organization determines where to place the access points, the rest is easy. In fact, installing and configuring the access points isn't much harder than just hooking up the wires and turning them on. Windows 2000 and XP, as well as many handheld operating systems, are designed to automatically locate Wi-Fi signals.

The Importance of Antennae

The antenna is a significant component in your wireless network. Understanding antenna technology can make a difference not only in the overall performance, but also in the total cost of deploying and maintaining a wireless system. The right antenna technology can mean the difference between meeting your organization's specific wireless connectivity needs in the most cost effective manner possible, or incurring added costs from installing more access points than are truly necessary.

The antenna directs radio frequency from the radio to the coverage area. Different antennae produce different coverage patterns, and thus need to be selected and positioned according to a specific site's coverage requirements. Base antenna selection upon regulatory requirements, size and shape of the area requiring coverage, antenna mounting options, and aesthetics.

While the size of the coverage area is the most important determining factor for antenna selection and placement, it isn't the sole criterion. Building construction, ceiling height, internal obstructions, available mounting locations, and physical appearance all must be considered. External considerations, such as public locations that prohibit the use of larger antennae, and other areas (e.g. executive offices), may require creative antenna placement so that they are unobtrusive and blend well with the surroundings.

Note 

Access points designed for the 802.11a U-NII 1 indoor band are required to have an antenna that is permanently attached. Other APs have antennae that can be placed remotely via an antenna cable, but even those antennae can't be mounted more than a few feet from the AP's position, because the coax cable used for RF has a high signal loss.

Antennae deliver flexibility and robustness to any WLAN, but for some reason the complexity of antenna technology, placement, etc., is hardly referenced in most WLAN documentation. Nevertheless, antennae can optimize certain applications, e.g. building-to-building bridging. Furthermore, because wireless is a dynamic medium, by using high or low gain antennae, it is possible to alter how signals propagate. For example, by understanding how antennae work, it is possible for a designer to focus an RF pattern and energy down a long narrow hallway, avoiding wasted energy and/or multipath interference.

Antenna radiation patterns are affected by polarization, free space loss, and propagation in solids. The transmission loss between a transmitting and a receiving antenna is a function of the antenna gain, the distance, and the frequency. For best performance, the transmitting and receiving antennae must have similar polarization alignments. Additional "free space loss" occurs because of signal spreading: as a signal radiates outward from an antenna, its radiated power is spread across an expanding spherical surface, with the power level inversely proportional to the distance from the source antenna. This antenna radiation must pass by and through solid objects, so it will be subject to losses from reflection and absorption. For example, oxygen atoms in the atmosphere cause a prominent peak in the attenuation effect of the atmosphere. Clandestine inter-satellite communications are performed on this frequency so that the signals will not reach earth. The atmosphere's attenuation almost vanishes at 94 GHz (the W-band), which is why many radar systems operate around this frequency.

Water, however, absorbs signals above 2 GHz. Fog, rain, the leaves of trees, people, etc. can rob energy from a microwave signal. The reason microwave ovens use the 2.4 GHz range is that 2.4 GHz penetrates food very well, but since water molecules cannot vibrate as fast as the microwaves push them, the molecules absorb the microwave energy and release it as heat. And that dear reader is how a microwave oven delivers piping hot food for our dining pleasure.

Reflections from objects (metal objects in particular) give rise to multipath distortion (fading). Some paths converge and become constructive (adding to signal strength) or destructive (fading). The selection of an antenna's azimuth pattern is driven by the shape of the coverage area and the location of the items within that area. Elevation pattern shapes are controlled to keep the maximum response at or slightly below the horizon for best far and near field coverage. The dimensions and height of a communication sector will determine an antenna's azimuth and elevation beamwidth requirements. These can be derived from the established beam area formula for the approximate gain needed:

G(dBi) = 10 log 10 / 29,000 / antenna azimuth * the antenna elevation

Spacing in excess of 0.75 wavelengths from a large conducting surface leads to deep nulls. And, antennae placed close to the ceiling will be prone to diffraction loss if they can't clear the doorjambs.

Radio Propagation

The WLAN designer's job is to deploy a WLAN that provides the best performance possible given the constraints of cost and power. The variables a WLAN designer considers include:

• Radio design variables, such as RF Tx power, antenna gain (Tx/Rx), receiver noise figure, required data rate, required packet error rate, E_b/N_o (waveform PSK, CCK).

• External link variables, such as propagation conditions (range, multipath, environment), interference from outside sources (light fixtures, cordless phones, microwave ovens), and co-channel interference.

Some variables are under the control of the designer and others are not. The external link variables can be managed by careful site analysis, planning, and installation. Access point placement and antenna pointing can help, as can better design and installation of the station antennae along with antenna diversity. The final key to good performance is equalization in the receiver processor.

Note 

This might be a good time to review the detailed discussion on propagation provided in Chapter 7

If you've ever experienced bad reception on a car radio when driving through a downtown area or through terrain with hills and valleys, you can appreciate the importance of setting up a WLAN so that it can provide all users with adequate coverage. Appropriately placed access points, paired with the correct antennae positioned relative to environmental obstructions or competing radio signals, are mandatory for good coverage and data throughput.

To that end, a site survey helps to define the wireless network's coverage and bandwidth performance at different locations within a cell, and to indicate where the "fall back" rates will occur. It also allows the designer to determine the exact number of access points needed and their optimal placement, and whether a special antenna system will be required.

Security

Does the deployment team know the WLAN's vulnerabilities, and/or how to test for them? Do they know what else the wireless network is doing other than talking to its clients? Do they know what is being sent out over the airwaves? If not, it's not too difficult to find an expert who can.

Wireless security experts can perform vulnerability studies that estimate how long it would take someone to break into a network, how they would do it, and what type of data is vulnerable. They can figure out ways to maximize internal wireless coverage without allowing too much data to seep outside the corporate facilities. An expert also can suggest security tools that can be used to reduce the risk of network penetration, as well as other means to make a network more secure.

Furthermore, a wireless security expert can perform an equipment audit that looks for rogue employee devices that make networks more vulnerable to attack.

There are a number of security measures that are easy to implement and that can help to ensure a WLAN's protection. They are discussed in detail in Chapter 17.

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