Highly-directional Antennas

As their name would suggest, highly-directional antennas emit the most narrow signal beam of any antenna type and have the greatest gain of these three groups of antennas. Highly-directional antennas are typically concave, dish-shaped devices, as can be seen in Figures 5.10 and 5.11. These antennas are ideal for long distance, point-to-point wireless links. Some models are referred to as parabolic dishes because they resemble small satellite dishes. Others are called grid antennas due to their perforated design for resistance to wind loading.




Usage
High-gain antennas do not have a coverage area that client devices can use. These antennas are used for point-to-point communication links, and can transmit at distances up to 25 miles. Potential uses of highly directional antennas might be to connect two buildings that are miles away from each other but have no obstructions in their path. Additionally, these antennas can be aimed directly at each other within a building in order to "blast" through an obstruction. This setup would be used in order to get network connectivity to places that cannot be wired and where normal wireless networks will not work.

Semi-Directional Antennas

Semi-directional antennas come in many different styles and shapes. Some semidirectional antennas types frequently used with wireless LANs are Patch, Panel, and Yagi (pronounced “YAH-gee”) antennas. All of these antennas are generally flat and designed for wall mounting. Each type has different coverage characteristics. Figure 5.7 shows some examples of semi-directional antennas.


These antennas direct the energy from the transmitter significantly more in one particular direction rather than the uniform, circular pattern that is common with the omnidirectional antenna. Semi-directional antennas often radiate in a hemispherical or cylindrical coverage pattern as can be seen in Figure 5.8.


Usage
Semi-directional antennas are ideally suited for short and medium range bridging. For
example, two office buildings that are across the street from one another and need to
share a network connection would be a good scenario in which to implement semidirectional antennas. In a large indoor space, if the transmitter must be located in the corner or at the end of a building, a corridor, or a large room, a semi-directional antenna would be a good choice to provide the proper coverage. Figure 5.9 illustrates a link between two buildings using semi-directional antennas.


Many times, during an indoor site survey, engineers will constantly be thinking of how to best locate omni-directional antennas. In some cases, semi-directional antennas provide such long-range coverage that they may eliminate the need for multiple access points in a building. For example, in a long hallway, several access points with omni antennas may be used or perhaps only one or two access points with properly placed semi-directional antennas - saving the customer a significant amount of money. In some cases, semidirectional antennas have back and side lobes that, if used effectively, may further reduce the need for additional access points.

RF Antennas

An RF antenna is a device used to convert high frequency (RF) signals on a transmission line (a cable or waveguide) into propagated waves in the air. The electrical fields emitted from antennas are called beams or lobes. There are three generic categories of RF antennas:

• Omni-directional
• Semi-directional
• Highly-directional

Each category has multiple types of antennas, each having different RF characteristics and appropriate uses. As the gain of an antenna goes up, the coverage area narrows so that high-gain antennas offer longer coverage areas than low-gain antennas at the same input power level. There are many types of antenna mounts, each suited to fit a particular need. After studying this section, you will understand which antenna and mount best meets your needs and why.

Omni-directional (Dipole) Antennas

The most common wireless LAN antenna is the Dipole antenna. Simple to design, the dipole antenna is standard equipment on most access points. The dipole is an omnidirectional antenna, because it radiates its energy equally in all directions around its axis. Directional antennas concentrate their energy into a cone, known as a "beam." The dipole has a radiating element just one inch long that performs an equivalent function to the "rabbit ears" antennas on television sets. The dipole antennas used with wireless LANs are much smaller because wireless LAN frequencies are in the 2.4 GHz microwave spectrum instead of the 100 MHz TV spectrum. As the frequency gets higher, the wavelength and the antennas become smaller.

Figure 5.1 shows that the dipole's radiant energy is concentrated into a region that looks like a doughnut, with the dipole vertically through the "hole" of the "doughnut." The signal from an omni-directional antenna radiates in a 360-degree horizontal beam. If an antenna radiates in all directions equally (forming a sphere), it is called an isotropic radiator. The sun is a good example of an isotropic radiator. We cannot make an isotropic radiator, which is the theoretical reference for antennas, but rather, practical antennas all have some type of gain over that of an isotropic radiator. The higher the gain, the more we horizontally squeeze our doughnut until it starts looking like a pancake, as is the case with very high gain antennas.

The dipole radiates equally in all directions around its axis, but does not radiate along the length of the wire itself - hence the doughnut pattern. Notice the side view of a dipole radiator as it radiates waves in Figure 5.2. This figure also illustrates that dipole antennas form a "figure 8" in their radiation pattern if viewed standing beside a perpendicular antenna.

If a dipole antenna is placed in the center of a single floor of a multistory building, most of its energy will be radiated along the length of that floor, with some significant fraction sent to the floors above and below the access point. Figure 5.3 shows examples of some different types of omni-directional antennas. Figure 5.4 shows a two-dimensional example of the top view and side view of a dipole antenna.


High-gain omni-directional antennas offer more horizontal coverage area, but the vertical coverage area is reduced, as can be seen in Figure 5.5. This characteristic can be an important consideration when mounting a high-gain omni antenna indoors on the ceiling. If the ceiling is too high, the coverage area may not reach the floor, where the users are located.

Usage
Omni-directional antennas are used when coverage in all directions around the horizontal axis of the antenna is required. Omni-directional antennas are most effective where large coverage areas are needed around a central point. For example, placing an omnidirectional antenna in the middle of a large, open room would provide good coverage. Omni-directional antennas are commonly used for point-to-multipoint designs with a hub-n-spoke topology (See Figure 5.6). Used outdoors, an omni-directional antenna should be placed on top of a structure (such as a building) in the middle of the coverage area. For example, on a college campus the antenna might be placed in the center of the campus for the greatest coverage area. When used indoors, the antenna should be placed in the middle of the building or desired coverage area, near the ceiling, for optimum coverage. Omni-directional antennas emit a large coverage area in a circular pattern and are suitable for warehouses or tradeshows where coverage is usually from one corner of the building to the other.

Enterprise Wireless Gateways

An enterprise wireless gateway is a device that can provide specialized authentication and connectivity for wireless clients. Enterprise wireless gateways are appropriate for large-scale wireless LAN environments providing a multitude of manageable wireless LAN services such as rate limiting, Quality of Service (QoS), and profile management.

It is important that an enterprise wireless gateway device needs to have a powerful CPU and fast Ethernet interfaces because it may be supporting many access points, all of which send traffic to and through the enterprise wireless gateway. Enterprise wireless gateway units usually support a variety of WLAN and WPAN technologies such as 802.11 standard devices, Bluetooth, HomeRF, and more. Enterprise wireless gateways support SNMP and allow enterprise-wide simultaneous upgrades of user profiles. These devices can be configured for hot fail-over (when installed in pairs), support of RADIUS, LDAP, Windows NT authentication databases, and data encryption using Industry standard VPN tunnel types. Figure 4.18 shows an example of an enterprise wireless gateway, while Figure 4.19 illustrates where it is used on a wireless LAN.


Authentication technologies incorporated into enterprise wireless gateways are often built into the more advanced levels of access points. For example, VPN and 802.1x/EAP connectivity are supported in many brands of enterprise level access points.

Enterprise wireless gateways do have features, such as Role-Based Access Control
(RBAC), that are not found in any access points. RBAC allows an administrator to assign a certain level of wireless network access to a particular job position in the company. If the person doing that job is replaced, the new person automatically gains the same network rights as the replaced person. Having the ability to limit a wireless user's access to corporate resources, as part of the "role", can be a useful security feature.

Class of service is typically supported, and an administrator can assign levels of service to a particular user or role. For example, a guest account might be able to use only 500 kbps on the wireless network whereas an administrator might be allowed 2 Mbps connectivity.

In some cases, Mobile IP is supported by the enterprise wireless gateway, allowing a user to roam across a layer 3 boundary. User roaming may even be defined as part of an enterprise wireless gateway policy, allowing the user to roam only where the administrator allows. Some enterprise wireless gateways support packet queuing and prioritization, user tracking, and even time/date controls to specify when users may access the wireless network.


MAC spoofing prevention and complete session logging are also supported and aid greatly in securing the wireless LAN. There are many more features that vary significantly between manufacturers. Enterprise wireless gateways are so comprehensive that we highly recommend that the administrator take the manufacturer's training class before making a purchase so that the deployment of the enterprise wireless gateway will go more smoothly.

Consultants finding themselves in a situation of having to provide a security solution for a wireless LAN deployment with many access points that do not support advanced security features might find enterprise wireless gateways to be a good solution. Enterprise wireless gateways are expensive, but considering the number of management and security solutions they provide, usually worth the expense.


Configuration and Management
Enterprise wireless gateways are installed in the main the data path on the wired LAN segment just past the access point(s) as seen in Figure 4.19. Enterprise wireless gateways are configured through console ports (using CLI), telnet, internal HTTP or HTTPS servers, etc. Centralized management of only a few devices is one big advantage of using enterprise wireless gateways. An administrator, from a single console, can easily manage a large wireless deployment using only a few central devices instead of a very large number of access points.

Enterprise wireless gateways are normally upgraded through use of TFTP in the same fashion as many switches and routers on the market today. Configuration backups can often be automated so that the administrator won't have to spend additional management time backing up or recovering from lost configuration files. Enterprise wireless gateways are mostly manufactured as rack-mountable 1U or 2U devices that can fit into your existing data center design.