Figure 25-3: (Bulletproof web design) Distance of obstructive objects from the
Figure 25-3: Distance of obstructive objects from the wireless signal is called the clearance. Distance Although the actual distances that antennas can send and receive data varies greatly based on different factors, you can achieve distances of many miles with outdoor antennas. For example, two Orinoco 24 dBi directional parabolic grid antenna can theoretically achieve distances of up to 52 miles at an 11 Mbps transmission speed with a 180-meter clearance. Reduce that transmission rate to 1 Mbps and you can achieve distances of up to 149 miles with a 1200-meter clearance. Less expensive equipment, such as the Orinoco 14 dBi directional antenna (about $149), can achieve distances of up to 5.3 miles at 11 Mbps with a 13-meter clearance. Cable factor The distances that transmissions travel on the cables between the wireless cards and the antennas can impact the antenna you need. The shorter the cables, the greater the distances and speeds you will be able to achieve on your antenna. The power of an antenna is rated in terms of gain. Gain is measured in decibels, based on a theoretic isotropic radiator (or dBi). Higher gains offer opportunities for reaching greater distances at greater speeds. However, the ability of the antenna to focus that power (directional versus omnidirectional), greatly changes the speeds and distances that can be achieved. Installing Wireless Linux Software If you did a Laptop or an “Everything” installation of Red Hat Linux on your computer, the software packages needed to create your wireless LAN may already be installed. Drivers and modules needed to support PCMCIA cards and wireless cards should already be built into your Linux system. Besides the wireless drivers, the following software packages contain tools for configuring and working with your wireless LAN cards in Red Hat Linux: Kernel-pcmcia-cs Contains commands and configuration files to support PCMCIA cards. Included in the package is the wireless script (for starting the wireless LAN interface) and the wireless.opts file (sets default options for each type of wireless card). Wireless-tools Contains commands for setting extensions for your wireless LAN interface. Commands include iwconfig (for configuring your wireless interface) and iwlist (for listing wireless statistics).
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