Wireless Network Components
Wireless networks are a little less complex than their wired counterparts. They require fewer components to operate properly. There are two main devices that can be found in a small wireless
network: a wireless access point and a wireless NIC. In order to understand proper wireless network installation, you should understand the basics of these two components.
Wireless Access Points (WAPs)
For a majority of wired networks, there is a central component, like a hub or a switch, that connects the nodes together and allows them to communicate. Wireless networks are similar in that they have a component that connects all wireless devices together. That device is known as a wireless access point (WAP). Its function is to operate as a hub of sorts for the wireless devices.
It has at least one antenna (sometimes two for better reception) and a port to connect the wireless AP to a wired network. Figure 6.4 shows an example of a wireless access point.
FIGURE 6 . 4 A wireless access point
One way of thinking of a WAP is as a bridge between the wireless clients and the wired network. In fact, an WAP can be used as a wireless bridge (depending on the settings) to bridge two
wired network segments together.
NOTE:
In addition to the stand-alone WAP, there is a WAP that includes a built-in router that can be used to connect both wired and wireless clients to the Internet. This device is usually known as a wireless router. Wireless routers usually act as Network Address Translation (NAT) servers by using the one ISP-provided global IP address to multiplex multiple local IP addresses (often handed out to inside clients by the wireless router from a pool in the 192.168.x.x range). Therefore, the subscriber need not change their service with the ISP in order to increase the number of devices that can simultaneously access the Internet.
Wireless NIC
Every station that wants to connect to a wireless network will need a wireless network interface card (NIC). In most respects, a wireless NIC does the same job as a traditional NIC, but instead
of having a socket to plug some cable into, the wireless NIC will have a radio antenna. In addition to the different types of wireless networking (discussed in the next section), wireless NICs
(like other NICs) can also differ in which type of connection they use to connect to the host computer. Figure 6.5 shows an example of a wireless NIC.
FIGURE 6 . 5 A wireless NIC
NOTE:
There are wireless adapters that are not NICs. For example, Linksys makes an external USB wireless adapter for notebooks. It is not a NIC because it isn’t an expansion card (the C in NIC), so they are generally referred to as “adapters.” Additionally, NICs also come in the form of PC cards, generally for laptops, notjust conventional expansion cards.
Wireless antennas act as both transmitters and receivers. There are two broad classes of antennas on the market, omni directional (Omni, or point-to-multipoint) and directional (Yagi or point-topoint). As a general rule, Yagi antennas have greater range than Omni antennas of equivalent gain because Yagis focus all their power in a single direction whereas Omnis must disperse the same
power in all directions at once. The drawback of using a directional antenna, though, is that more care must be taken to align communication points, generally making Yagi a good choice only for point-to-point bridging of access points. Most WAPs use Omnis because clients and other APs could be in any direction at any given moment. A non-networking example of an Omni antenna
is the FM antenna on your automobile. The orientation of your car does not affect the reception of the signal. The television aerials that some of us are old enough to remember rotating into a specific
direction for a certain channel (how many of you labeled your set-top antenna dial for the actual TV stations you could receive?) are examples of Yagi antennas. Omnis and Yagis are both rated according to their signal gain with respect to an actual or theoretical
laboratory reference antenna. These ratings are relative indicators of the corresponding production antenna’s range. Range is also affected by the bit rate of the underlying technology,
with higher bit rates extending shorter distances. Remember, a Yagi will always have a longer range than an equivalently rated Omni, but the straight-line Yagi will be limited in coverage area.
Manufacturers rate these antennas in units of decibel isotropic (dBi) or decibel dipole (dBd), based on the type of reference antenna (isotropic or dipole) of equivalent frequency operation
used to rate the production antenna. A positive value for either unit of measure represents a gain in signal strength with respect to the reference antenna. Webster’s defines isotropic as “exhibiting
properties (as velocity of light transmission) with the same values when measured along axes in all directions.” Isotropic antennas are not able to be produced in reality, but their properties
can be engineered from antenna theory for reference purposes.
As a practical example, consider Cisco Systems’s series of Aironet Access Point (indoor) and Bridge (outdoor) antennas. Table 6.1 illustrates the effect gain ratings and attempted bit rates
have on range limitations.
TABLE 6 . 1 Wireless Antenna Types and Ranges
The rule of thumb is that antennas operating with frequencies below 1GHz are measured in dBd while those operating above 1GHz are measured in dBi. As this is not always the case, you
may find the need to compare the strength of one antenna, measured in dBd, with another, measured in numerically equivalent dBi, in order to determine which is stronger. That’s why it’s
important to know that a particular numerical magnitude of dBd is more powerful than the same numerical magnitude of dBi. The good news is that the relationship between the two is linear,
making the conversion quite simple. At the same operating frequency, a dipole antenna has about 2.2dB gain over a 0dBi theoretical isotropic antenna. Therefore, you can easily convert
from dBd to dBi by adding 2.2 to the dBd rating. Conversely, subtract 2.2 from the dBi rating to produce the equivalent dBd rating. Taking into account what you’ve learned about the difference between Omni and Yagi antennas and the difference between dBd and dBi gain ratings, you should be able to compare the relative range of transmission of one antenna with respect to another based on a combination of these characteristics. By way of example, the following four antenna ratings are given in relative order from greatest to least range:
7dBi Yagi (longer range than 7dBi Omni)
4.8dBd Omni (equivalent to a 7dBi Omni)
4.8dBi Omni (equivalent to a 2.6dBd Omni)
