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.

Wireless Residential Gateways

A wireless residential gateway is a device designed to connect a small number of wireless nodes to a single device for Layer 2 (wired and wireless) and Layer 3 connectivity to the Internet or to another network. Manufacturers have begun combining the roles of access points and gateways into a single device. Wireless residential gateways usually include a built-in hub or switch as well as a fully configurable, Wi-Fi compliant access point. The WAN port on a wireless residential gateway is the Internet-facing Ethernet port that may be connected to the Internet through one of the following:

Cable modem
xDSL modem
Analog modem
Satellite modem


Common Options
Because wireless residential gateways are becoming increasingly popular in homes of
telecommuters and in small businesses, manufacturers have begun adding more features
to these devices to aid in productivity and security. Common options that most wireless
residential gateways include are:

Point-to-Point Protocol over Ethernet (PPPoE)
Network Address Translation (NAT)
Port Address Translation (PAT)
Ethernet switching
Virtual Servers
Print Serving
Fail-over routing
Virtual Private Networks (VPNs)
Dynamic Host Configuration Protocol (DHCP) Server and Client
Configurable Firewall

This diverse array of functionality allows home and small office users to afford an all-inone single device solution that is easily configurable and meets most business needs. Residential gateways have been around for quite some time, but recently, with the extreme popularity of 802.11b compliant wireless devices, wireless was added as a feature. Wireless residential gateways have all of the expected SOHO-class access point configuration selections such as WEP, MAC filters, channel selection, and SSID.


Configuration and Management
Configuring and installing wireless residential gateways generally consists of browsing to the built-in HTTP server via one of the built-in Ethernet ports and changing the userconfigurable settings to meet your particular needs. This configuration may include changing ISP, LAN, or VPN settings. Configuration and monitoring are done in similar fashion through the browser interface. Some wireless residential gateways units support console, telnet, and USB connectivity for management and configuration. The text-based menus typically provided by the console port and telnet sessions are less user-friendly than the browser interface, but adequate for configuration. Statistics that can be monitored may include items such as up-time, dynamic IP addresses, VPN connectivity, and associated clients. These settings are usually well marked or explained for the nontechnical home or home office user.

When you choose to install a wireless residential gateway at your home or business, be
aware that your ISP will not provide technical support for getting your unit connected to the Internet unless they specifically state that they will. ISPs will usually only support the hardware that you have purchased from them or that they have installed. This lack of service can be especially frustrating to the non-technical user who must configure the correct IP addresses and settings in the gateway unit to get Internet access. Your best source of support for installing these devices is the manual provided with the device or someone who has already successfully installed similar units and can provide free guidance. Wireless residential gateways are so common now that many individuals that consider themselves non-technical have gained significant experience installing and configuring them.

Wireless LAN Client Devices

The term “client devices” will, for purposes of this discussion, cover several wireless LAN devices that an access point recognizes as a client on a network. These devices include:

PCMCIA & Compact Flash Cards
Ethernet & Serial Converters
USB Adapters
PCI & ISA Adapters

Wireless LAN clients are end-user nodes such as desktop, laptop, or PDA computers that need wireless connectivity into the wireless network infrastructure. The wireless LAN client devices listed above provide connectivity for wireless LAN clients. It is important to understand that manufacturers only make radio cards in two physical formats, and those are PCMCIA and Compact Flash (CF). All radio cards are built (by the manufacturers) into these card formats and then connected to adapters such as PCI, ISA, USB, etc.

PCMCIA & Compact Flash Cards
The most common component on any wireless network is the PCMCIA card. More commonly known as “PC cards”, these devices are used in notebook (laptop) computers and PDAs. The PC card is the component that provides the connection between a client device and the network. The PC card serves as a modular radio in access points, bridges, workgroup bridges, USB adapters, PCI & ISA adapters, and even print servers. The following figure shows an example of a PCMCIA card.
Antennas on PC cards vary with each manufacturer. You might notice that several
manufacturers use the same antenna while others use radically different models. Some
are small and flat such as the one shown in the above figure, while others are detachable and connected to the PC card via a short cable. Some PC cards are shipped with multiple antennas and even accessories for mounting detachable antennas to the laptop or desktop case with Velcro.

Wireless Ethernet & Serial Converters
Ethernet and serial converters are used with any device having Ethernet or legacy 9-pin
serial ports for the purpose of converting those network connections into wireless LAN
connections. When you use a wireless Ethernet converter, you are externally connecting
a wireless LAN radio to that device with a category 5 (Cat5) cable. A common use of
wireless Ethernet converters is connection of an Ethernet-based print server to a wireless network.

Serial devices are considered legacy devices and are rarely used with personal computers. Serial converters are typically used on old equipment that uses legacy serial for network connectivity such as terminals, telemetry equipment, and serial printers. Many times manufacturers will sell a client device that includes both a serial and Ethernet converter in the same enclosure.

These Ethernet and serial converter devices do not normally include the PC card radio.
Instead, the PC card must be purchased separately and installed in the PCMCIA slot in
the converter enclosure. Ethernet converters in particular allow administrators to convert a large number of wired nodes to wireless in a short period of time.

Configuration of Ethernet and serial converters varies. In most cases, console access is provided via a 9-pin legacy serial port. The above figure shows an example of an Ethernet and serial converter.


USB Adapters
USB clients are becoming very popular due to their simple connectivity. USB client
devices support plug–n-play, and require no additional power other than what is delivered through the USB port on the computer. Some USB clients utilize modular, easily removable radio cards and others have a fixed internal card that cannot be removed without opening the case. When purchasing a USB client device, be sure you understand whether or not the USB adapter includes the PC card radio. In cases of a USB adapter that requires a PC card, it is recommended, although not always required, that you use the same vendor’s equipment for both the adapter and the PC card. Figure 4.14 shows an example of a USB client.


PCI & ISA Adapters

Wireless PCI and ISA are installed inside a desktop or server computer. Wireless PCI
devices are plug–n–play compatible, but may also only come as an “empty” PCI card and
require a PC card to be inserted into the PCMCIA slot once the PCI card is installed into the computer. Wireless ISA cards will likely not be plug-n-play compatible and will require manual configuration both via a software utility and in the operating system. Since the operating system cannot configure ISA devices that aren’t plug-n-play compatible, the administrator must make sure the adapter’s setting and those of the operating system match. Manufacturers typically have separate drivers for the PCI or ISA adapters and the PC card that will be inserted into each. As with USB adapters, it is recommended that you use the same vendor’s equipment for the PCI/ISA adapters and the PC card. The above figure shows an example of a PCI adapter with a PC card inserted.

Wireless Workgroup Bridges

Similar to and often confused with wireless bridges are wireless workgroup bridges (WGB). The biggest difference between a bridge and a workgroup bridge is that the workgroup bridge is a client device. A wireless workgroup bridge is capable of aggregating multiple wired LAN client devices into one collective wireless LAN client.

In the association table on an access point, a workgroup bridge will appear in the table as a single client device. The MAC addresses of devices behind the workgroup bridge will not be seen on the access point. Workgroup bridges are especially useful in environments with mobile classrooms, mobile offices, or even remote campus buildings where a small group of users need access into the main network. Bridges can be used for this type of functionality, but if an access point rather than a bridge is in place at the central site, then using a workgroup bridge prevents the administrator from having to buy an additional bridge for the central site.


In an indoor environment in which a group of users is physically separated from the main body of network users, a workgroup bridge can be ideal for connecting the entire group back into the main network wirelessly. Additionally, workgroup bridges may have protocol filtering capabilities allowing the administrator to control traffic across the wireless link.

Common Options
Because the wireless workgroup bridge is a type of bridge, many of the options that you will find in a bridge – MAC and protocol filtering, fixed or detachable antennas, variable power output, and varied types of wired connectivity – are also found in a workgroup bridge. There is a limit to the number of stations that may use the workgroup bridge from the wired segment. This number ranges between 8 and 128 depending on the manufacturer. Use of more than about 30 clients over the wireless segment is likely to cause throughput to drop to a point at which users might feel that the wireless link is simply too slow to adequately perform their job tasks.

Configuration and Management
The methods used to access, configure, and manage a wireless workgroup bridge are similar to those of a wireless bridge: console, telnet, HTTP, SNMP support, or custom configuration and management software. Workgroup bridges are configured for a default IP address from the manufacturer, but can be changed either by accessing the unit via console port, web browser, telnet, or custom software application. The administrator can reset the device to factory defaults by using the hardware reset button on the device.

Wireless Bridges

A wireless bridge provides connectivity between two wired LAN segments, and is used
in point-to-point or point-to-multipoint configurations. A wireless bridge is a half-duplex device capable of layer 2 wireless connectivity only. The above figure shows an example of a wireless bridge, while the following figure illustrates where a wireless bridge is used on a wireless LAN.


Wireless Bridge Modes
Wireless bridges communicate with other wireless bridges in one of four modes:

Root Mode
Non-root Mode
Access Point Mode
Repeater Mode

Root Mode
One bridge in each group of bridges must be set as the root bridge. A root bridge can
only communicate with non-root bridges and other client devices and cannot associate
with another root bridge. The above figure illustrates a root bridge communicating with nonroot bridges.

Non-root Mode
Wireless bridges in non-root mode attach, wirelessly, to wireless bridges that are in root mode. Some manufacturers’ wireless bridges support client connectivity to non-root
mode bridges while in bridging mode. This mode is actually a special mode where the
bridge is acting as both an access point and as a bridge simultaneously. When using the
Spanning Tree Protocol, all non-root bridges must have connectivity to the root bridge.

Access Point Mode
Some manufacturers give the administrator the ability to have clients connect to bridges, which is actually just giving the bridge access point functionality. In many cases, the bridge has an “access point” mode that converts the bridge into an access point.

Repeater Mode
Wireless bridges can also be configured as repeaters, as shown in the above figure. In repeater configuration, a bridge will be positioned between two other bridges for the purpose of extending the length of the wireless bridged segment. While using a wireless bridge in this configuration has the advantage of extending the link, it has the disadvantage of decreased throughput due to having to repeat all frames using the same half duplex radio. Repeater bridges are non-root bridges, and many times the wired port will be disabled while the bridge is in repeater mode.


Common Options
The hardware and software options of a wireless bridge are similar to those of an access
point, and for many of the same purposes:

Fixed or Detachable Antennas
Advanced Filtering Capabilities
Removable (Modular) Radio Cards
Variable Output Power
Varied Types of Wired Connectivity

Access Points

Second only to the basic wireless PC card, the access point, or “AP”, is probably the most common wireless LAN device with which you will work as a wireless LAN administrator. As its name suggests, the access point provides clients with a point of access into a network. An access point is a half-duplex device with intelligence equivalent to that of a sophisticated Ethernet switch.

Access Point Modes
Access points communicate with their wireless clients, with the wired network, and with other access points. There are three modes in which an access point can be configured:

Root Mode
Repeater Mode
Bridge Mode

Root Mode
Root Mode is used when the access point is connected to a wired backbone through its wired (usually Ethernet) interface. Most access points that support modes other than root mode come configured in root mode by default. When an access point is connected to the wired segment through its Ethernet port, it will normally be configured for root mode. When in root mode, access points that are connected to the same wired distribution system can talk to each other over the wired segment. Access points talk to each other to coordinate roaming functionality such as re-association. Wireless clients can communicate with other wireless clients that are located in different cells through their respective access points across the wired segment, as shown in above.

Bridge Mode
In bridge mode, access points act exactly like wireless bridges, which will be discussed later in this chapter. In fact, they become wireless bridges while configured in this manner. Only a small number of access points on the market have bridge functionality, which typically adds significant cost to the equipment. We will explain shortly how wireless bridges function, but you can see from above figure that clients do not associate to bridges, but rather, bridges are used to link two or more wired segments together wirelessly.

Repeater Mode
In repeater mode, access points have the ability to provide a wireless upstream link into the wired network rather than the normal wired link. As you can see in the following figure, one access point serves as the root access point and the other serves as a wireless repeater. The access point in repeater mode connects to clients as an access point and connects to the upstream root access point as a client itself. Using an access point in repeater mode is not suggested unless absolutely necessary because cells around each access point in this scenario must overlap by a minimum of 50%. This configuration drastically reduces the range at which clients can connect to the repeater access point. Additionally, the repeater access point is communicating with the clients as well as the upstream access point over the wireless link, reducing throughput on the wireless segment. Users attached to the repeater access point will likely experience low throughput and high latencies in this scenario. It is typical for the wired Ethernet port to be disabled while in repeater mode.

Applications of Wireless LANs

Access Role
Wireless LANs are deployed in an access layer role, meaning that they are used as an entry point into a wired network. The following figure illustrates mobile clients gaining access to a wired network through a connection device (access point).


Network Extension
Wireless LANs can be easily implemented to provide seamless connectivity to remote areas within a building, as illustrated by the floor plan image in the following. Because little wiring is necessary to install a wireless LAN, the costs of hiring installers and purchasing Ethernet cable might be completely eliminated.


Building-to-Building Connectivity
There are two different types of building-to-building connectivity. The first is called point-to-point (PTP), and the second is called point-to-multipoint (PTMP). Point-to-point links are wireless connections between only two buildings, as illustrated in the following figure. PTP connections almost always use semi-directional or highly-directional antennas at each end of the link.


Last Mile Data Delivery
"Last mile" refers to the communication infrastructure—wired or wireless—that exists between the central office of the telecommunications company (telco) or cable company and the end user. Currently the telcos and cable companies own their last mile infrastructure, but with the broadening interest in wireless technology, WISPs are now creating their own wireless last mile delivery service, as illustrated in the following figure.

Mobility
As an access layer solution, wireless LANs cannot replace wired LANs in terms of data rates (100BT at 100Mbps versus IEEE 802.11a at 54Mbps). What wireless LANs do offer is an increase in mobility (as can be seen in the following figure) as the trade off for speed and quality of service.

Radio Frequency Behaviors

Gain
Gain, illustrated in the following figure, is the term used to describe an increase in an RF signal's amplitude. Gain is usually an active process; meaning that an external power source, such as an RF amplifier, is used to amplify the signal or a high-gain antenna is used to focus the beamwidth of a signal to increase its signal amplitude.


Loss
Loss describes a decrease in signal strength (the following figure). Many things can cause RF signal loss, both while the signal is still in the cable as a high frequency AC electrical signal and when the signal is propagated as radio waves through the air by the antenna.

Reflection
Reflection, as illustrated in the following figure, occurs when a propagating electromagnetic wave impinges upon an object that has very large dimensions when compared to the wavelength of the propagating wave.



Refraction
Refraction describes the bending of a radio wave as it passes through a medium of different density. As an RF wave passes into a denser medium (like a pool of cold air lying in a valley) the wave will be bent such that its direction changes.

Principles of Antennas

Line of Sight (LOS)
The LOS is an apparently straight line because light waves are subject to changes in direction due to refraction, diffraction, and reflection in the same way as RF frequencies. The following figure illustrates LOS.


Fresnel Zone
A consideration when planning or troubleshooting an RF link is the Fresnel Zone. The Fresnel Zone occupies a series of concentric ellipsoid-shaped areas around the LOS path, as can be seen in the following figure.


Intentional Radiator
As defined by the Federal Communication Commission (FCC), an intentional radiator is an RF device that is specifically designed to generate and radiate RF signals. In terms of hardware, an intentional radiator will include the RF device and all cabling and connectors up to, but not including, the antenna, as illustrated in Figure below.