Cisco 802.11 Wireless Networking Quick Reference

Cisco offers a number of antennas, and each caters to a different need, a different function, and a different range. The following models are basic types of antennas that you are likely to encounter in your Wi-Fi adventures:

• Omnidirectional antennas The signal radiates out in a circle from the antenna. That is, a client to the left of the antenna can receive the same signal as a client the same distance away on the right.

• Directional antennas The signal is focused into a tighter beam and projected a greater distance than the omnidirectional antenna. Although range is increased, the antenna serves only clients in a given direction. Examples of directional antennas include the Yagi antenna, patch antenna, and parabolic dish.

• Diversity antennas Two antennas reduce multipath distortion. The radio receives a signal on both antennas; however, it transmits on the one with the best signal reception.

Note

Multipath distortion is covered in greater depth later in this chapter.

Antenna Tech Primer

Before looking at the models of antennas Cisco offers, it is helpful to understand what the statistics next to each device mean.

Beam Width

This describes how the signal radiates from an antenna and is expressed in degrees horizontally and vertically. For example, an omnidirectional antenna radiates its signal 360 degrees horizontally. It depends on the model; it might have as much coverage area as 75 degrees vertically. As such, it doesn't radiate its signal in a complete sphere around the antenna. Rather, it's more of a donut shape. This is illustrated in Figure 2-1.

Gain

Gain is a measurement of increase in power, measured in decibels (dB). The decibel scale is logarithmic, and it is used to demonstrate the ratio of one power value to another.

The decibel communicates the amount of signal loss or gain within a system. The decibel is somewhat confusing because it is not a concrete measurement, such as meters, kilometers, or kilograms. The decibel is not a measure of signal strength; rather, it's a ratio between two power levels.

When power is lost or gained in a system, it does not occur in regular, fixed amounts. Instead, power is lost by varying increments, such as one half, one quarter, and so on. To determine how much loss has occurred, you cannot just add and p. These amounts must be multiplied. Decibels make this process simpler to understand. Loss and gain are easier to calculate in a system in which you simply add the decibels. Following is the equation for how decibels are calculated:

dB = 10 log (P₀/P₁)

Where P₀ is the antenna used for comparison and P₁ is the antenna that's evaluated.

For example, if your signal gains 3dB, then it has doubled in power. If it gains 6dB, then the power has quadrupled. If it has lost 3dB, then the power is cut in half.

Although the decibel is not a measure of signal strength, the decibel milliwatt (dBm) is. It's easy to confuse dB and dBm. A dBm is the signal power in relation to 1 milliwatt. For example, a signal power of 0dBm is 1 milliwatt. Because of logarithms, a signal power of 3dBm is 2 milliwatts.

Because the decibel is a ratio of two signal powers, you need a reference point to talk about antenna gain. When you look at antenna statistics, note the dBi abbreviation. In this case, an antenna rating is to the gain of an isotropic antenna. An isotropic antenna does not really exist. It's a theoretical antenna that radiates its signal in a perfect sphere around itself.

dBi is a comparison of the antenna against this theoretical antenna. The isotropic antenna has a power rating of 0dB (that is, there is no gain and no loss when the antenna is compared against itself).

Standard dipole antennas (which are common on Cisco gear) are typically omnidirectional. Because the omnidirectional antenna "concentrates" its signal in the donut shape around itself, it has an increase in gain over the isotropic antenna. As such, an antenna such as the AIR-ANT4941 has a 2dBi gain in comparison to our theoretical antenna.

Note

If you see the notation dBd, the antenna is compared to a dipole antenna. For example, our AIR-ANT4941 antenna would have a 0dBd gain.

To determine the dBi rating of an antenna with a dBd rating, simply add 2.14. For instance, in the case of a 3dBd omnidirectional antenna, such as the AIR-ANT-2506, you simply add 2.14 to arrive at the correct dBi rating of 5.14. This is sometimes rounded up to 5.2dBi.

Note

A dipole antenna is an antenna that contains two different elements. An example of a dipole antenna is the rabbit ear antenna on a television set, although a dipole does not necessarily have to be in a v shape.

Cisco 2.4-GHz Antennas

Although APs typically come with their own antennas, you can easily upgrade, depending on your particular organization and its needs. To serve 802.11b and 802.11g devices, Cisco offers a number of different antennas. Table 2-1 compares these antennas, stacking their type, beam width, gain, and ranges up against each other.

Cisco 5-GHz Bridge Antennas

The 5-GHz, 802.11a devices operate at a higher frequency than 802.11b and 802.11g devices. As with the 2.4-GHz devices, Cisco offers a broad range of antennas for unique needs. Table 2-2 compares these models.

Cisco 2.4-GHz Bridge Antennas

The bridges that work in the 2.4-GHz bands (like the Cisco 1300) have their own line of antennas. Table 2-3 compares these antennas.