Wireless LAN Accessories

When the time comes to connect all of your wireless LAN devices together, you will need to purchase the appropriate cables and accessories that will maximize your throughput, minimize your signal loss, and, most importantly, allow you to make the connections correctly. This section will discuss the different types of accessories and where they fit into a wireless LAN design. The following types of accessories are discussed in this section:

• RF Amplifiers
• RF Attenuators
• Lightning Arrestors
• RF Connectors
• RF Cables
• RF Splitters

Each of these devices is important to building a successful wireless LAN. Some items are used more than others, and some items are mandatory whereas others are optional. It is likely that an administrator will have to install and use all of these items multiple times while implementing and managing a wireless LAN.


RF Amplifiers
As its name suggests, an RF amplifier is used to amplify, or increase the amplitude of, an RF signal, which is measured in +dB. An amplifier will be used when compensating for the loss incurred by the RF signal, either due to the distance between antennas or the length of cable from a wireless infrastructure device to its antenna. Most RF amplifiers used with wireless LANs are powered using DC voltage fed onto the RF cable with a DC injector near the RF signal source (such as the access point or bridge).

Sometimes this DC voltage used to power RF amplifiers is called "phantom voltage" because the RF amplifier seems to magically power up. This DC injector is powered using AC voltage from a wall outlet, so it might be located in a wiring closet. In this scenario, the RF cable carries both the high frequency RF signal and the DC voltage necessary to power the in-line amplifier, which, in turn, boosts the RF signal amplitude. Figure 5.20 shows an example of an RF amplifier (left), and how an RF amplifier is mounted on a pole (right) between the access point and its antenna.

RF amplifiers come in two types: unidirectional and bi-directional. Unidirectional amplifiers compensate for the signal loss incurred over long RF cables by increasing the signal level before it is injected into the transmitting antenna. Bi-directional amplifiers boost the effective sensitivity of the receiving antenna by amplifying the received signal before it is fed into the access point, bridge, or client device.


RF Attenuators

An RF attenuator is a device that causes precisely measured loss (in –dB) in an RF signal. While an amplifier will increase the RF signal, an attenuator will decrease it. Why would you need or want to decrease your RF signal? Consider the case where an access point has a fixed output of 100mW, and the only antenna available is an omni-directional antenna with +20 dBi gain. Using this equipment together would violate FCC rules for power output, so an attenuator could be added to decrease the RF signal down to 30mW before it entered the antenna. This configuration would put the power output within FCC parameters. Figure 5.22 shows examples of fixed-loss RF attenuators with BNC connectors (left) and SMA connectors (right). Figure 5.23 shows an example of an RF step attenuator.

Power over Ethernet (PoE) Devices

Power over Ethernet (PoE) is a method of delivering DC voltage to an access point, wireless bridge, or wireless workgroup bridge over the Cat5 Ethernet cable for the purpose of powering the unit. PoE is used when AC power receptacles are not available where wireless LAN infrastructure devices are to be installed. The Ethernet cable is used to carry both the power and the data to the units.

Consider a warehouse where the access points need to be installed in the ceiling of the building. The labor costs that would be incurred to install electrical outlets throughout the ceiling of the building to power the access points would be considerable. Hiring an electrician to do this type of work would be very expensive and time consuming. Remember that Ethernet cables can only carry data reliably for 100 meters and, for any distance more than 100 meters, PoE is not a viable solution. The following figure illustrates how a PoE device would provide power to an access point.


Common PoE Options
PoE devices are available in several types.

• Single-port DC voltage injectors
• Multi-port DC voltage injectors
• Ethernet switches designed to inject DC voltage on each port on a given pair of pins

Single-port DC Voltage Injectors

Access points and bridges that specify mandatory use of PoE include single-port DC voltage injectors for the purpose of powering the unit. See Figure 5.17 below for an example of a single-port DC voltage injector. These single-port injectors are acceptable when used with a small number of wireless infrastructure devices, but quickly become a burden, cluttering wiring closets, when building medium or large wireless networks.


Multi-port DC Voltage Injectors

Several manufacturers offer multi-port injectors including 4, 6, or 12-port models. These models may be more economical or convenient for installations where many devices are to be powered through the Cat5 cable originating in a single wiring closet or from a single switch. Multi-port DC voltage injectors typically operate in exactly the same manner as their single-port counterparts. See Figure 5.18 for an example of a multi-port PoE injector. A multi-port DC voltage injector looks like an Ethernet switch with twice as many ports. A multi-port DC voltage injector is a pass-through device to which you connect the Ethernet switch (or hub) to the input port, and then connect the PoE client device to the output device, both via Cat5 cable. The PoE injector connects to an AC power source in the wiring closet. These multi-port injectors are appropriate for mediumsized wireless network installations where up to 50 access points are required, but in large enterprise rollouts, even the most dense multi-port DC voltage injectors combined with Ethernet hubs or switches can become cluttered when installed in a wiring closet.


Active Ethernet Switches

The next step up for large enterprise installations of access points is the implementation of active Ethernet switches. These devices incorporate DC voltage injection into the Ethernet switch itself allowing for large numbers of PoE devices without any additional hardware in the network. See Figure 5.19 for an example of an Active Ethernet switch. Wiring closets will not have any additional hardware other than the Ethernet switches that would already be there for a non-PoE network. Several manufacturers make these switches in many different configurations (number of ports). In many Active Ethernet switches, the switch can auto-sense PoE client devices on the network. If the switch does not detect a PoE device on the line, the DC voltage is switched off for that port. As you can see from the picture, an Active Ethernet switch looks no different from an ordinary Ethernet switch. The only difference is the added internal functionality of supplying DC voltage to each port.

RF Antenna Concepts

There are several concepts that are essential knowledge when implementing solutions that require RF antennas. Among those that will be described are:

• Polarization
• Gain
• Beamwidth
• Free Space Path Loss

The above list is by no means a comprehensive list of all RF antenna concepts, but rather a set of must-have fundamentals that allow an administrator to understand how wireless LAN equipment functions over the wireless medium. A solid understanding of basic antenna functionality is the key to moving forward in learning more advanced RF concepts.

Knowing where to place antennas, how to position them, how much power they are radiating, the distance that radiated power is likely to travel, and how much of that power can be picked up by receivers is, many times, the most complex part of an administrator's job.


Polarization
A radio wave is actually made of up two fields, one electric and one magnetic. These two fields are on planes perpendicular to each other, as shown in the following figure.


The sum of the two fields is called the electro-magnetic field. Energy is transferred back and forth from one field to the other, in the process known as "oscillation." The plane that is parallel with the antenna element is referred to as the "E-plane" whereas the plane that is perpendicular to the antenna element is referred to as the "H-plane." We are interested primarily in the electric field since its position and direction with reference to the Earth's surface (the ground) determines wave polarization.

Polarization is the physical orientation of the antenna in a horizontal or vertical position. The electric field is parallel to the radiating elements (the antenna element is the metal part of the antenna that is doing the radiating) so, if the antenna is vertical, then the polarization is vertical.

• Horizontal polarization - the electric field is parallel to the ground
• Vertical polarization - the electric field is perpendicular to the ground

Vertical polarization, which is typically used in wireless LANs, is perpendicular to the Earth’s plane. Notice the dual antennas sticking up vertically from most any access point - these antennas are vertically polarized in that position. Horizontal polarization is parallel to the Earth. In the foolowing figure illustrates the effects polarization can have when antennas are not aligned correctly. Antennas that are not polarized in the same way are not able to communicate with each other effectively.


Gain
Antenna gain is specified in dBi, which means decibels referenced to an isotropic radiator. An isotropic radiator is a sphere that radiates power equally in all directions simultaneously. We haven't the ability to make an isotropic radiator, but instead we can make omni-directional antennas such as a dipole that radiates power in a 360-degree horizontal fashion, but not 360 degrees vertically. RF signal radiation in this fashion gives us a doughnut pattern. The more we horizontally squeeze this doughnut, the flatter it becomes, forming more of a pancake shape when the gain is very high. Antennas have passive gain, which means they do not increase the power that is input into them, but rather shape the radiation field to lengthen or shorten the distance the propagated wave will travel. The higher the antenna gain, the farther the wave will travel, concentrating its output wave more tightly so that more of the power is delivered to the destination (the receiving antenna) at long distances. As was shown in the figure, the coverage has been squeezed vertically so that the coverage pattern is elongated, reaching further.


Beamwidth
As we've discussed previously, narrowing, or focusing antenna beams increases the antenna’s gain (measured in dBi). An antenna’s beamwidth means just what it sounds like: the “width” of the RF signal beam that the antenna transmits. The following figure illustrates the term
beamwidth.

There are two vectors to consider when discussing an antenna’s beamwidths: the vertical and the horizontal. The vertical beamwidth is measure in degrees and is perpendicular to the Earth's surface. The horizontal beamwidth is measured in degrees and is parallel to the Earth's surface. Beamwidth is important for you to know because each type of antenna has different beamwidth specifications. The chart below can be used as a quick reference guide for beamwidths.


Free Space Path Loss
Free Space Path Loss (or just Path Loss) refers to the loss incurred by an RF signal due largely to "signal dispersion" which is a natural broadening of the wave front. The wider the wave front, the less power can be induced into the receiving antenna. As the transmitted signal traverses the atmosphere, its power level decreases at a rate inversely proportional to the distance traveled and proportional to the wavelength of the signal. The power level becomes a very important factor when considering link viability.
The Path Loss equation is one of the foundations of link budget calculations. Path Loss represents the single greatest source of loss in a wireless system. Below is the formula
for Path Loss.

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.

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.