From a paper I wrote, might be of use (let me know if you want sections I,II and IV on how to hack into WLANs.
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III. PHYSICAL WIRELESS CONFIGURATION
Requirements for wireless systems vary based on organizational size, geographical spread, security requirements and many other factors. The first step to implementing a wireless system is to gain a keen understanding of radio wave behavior, and to know the different ways that waves propagate.
Omni-directional antennas have a 360-degree cover pattern. The waves that they emit radiate outwards in a doughnut-shaped pattern with the antenna at the center of the doughnut hole. Waves are transmitted uniformly in every direction, though wind up being reflected from ceilings and floors, resulting in a squished-doughnut propagation pattern.
<<Fig. 1>>. Omni-directional and directional antenna radiation patterns.
Directional antennas transmit data in one direction, almost like the beam of a headlight. Beams can be focused to provide longer, more directional coverage. As with omni-directional antennas, 802.11 signals can penetrate through brick and concrete. Windows and doors allow much more signal leakage.
Most access points use omni-directional antennas, though some allow replacement antennas to be swapped in. Most stations do not allow replacement antennas, though are easily modifiable with a screw-driver and a soldering iron. It is important to note that radio equipment from vendors may not be modified due to FCC regulations. Within the US, Equivalently Isotropically Radiated Power is restricted by the FCC to four watts maximum, with additional restrictions depending on the type of antenna.
After considering different antennas and frequencies, it is a good idea to construct a map of the current network infrastructure and list the requirements that the wireless system will address. Carefully consider the following questions:
• Who needs wireless access? All employees? Inventory clerks? Business executives?
• What existing resources should each group be allowed to access?
• How many members need this access? What bandwidth will they require?
• Are there main gathering points that will need to support a larger load? Will users be mobile, such as in a warehouse or emergency response zones?
• What classification of materials is needed for each zone?
• Do any zones need access to the Internet?
Most large-scale business networks using 802.11b rely on non overlapping channels. The 802.11b standard is broken up into 11 separate sub-channels. Each of these channels slightly overlap each other to allow users to load-balance across multiple channels, and achieve 11Mbps throughput on each channel. Channels 1, 6 and 11 allow the greatest range of non-overlapping frequency, and provide users with the greatest range of useful signal space. Because of this, many companies with 100s of users in a close area tune their access points to use these three sub channels, and space them in a hexagonal pattern over their target area. It is important to note that many experienced wireless intruders hoping to access business networks often attempt to gain access to WLANs on these frequencies first. It is important to note that different countries have different allowances for channel and frequency use.
Some other general radio frequency issues:
• Radio data rates decrease with range
• Range usually increases proportional to antenna height
• Open areas provide better radio range
• Steel pillars, concrete, microwave ovens and water are barriers to radio waves
• In sitting areas, try to point client antennas at the access point
• Conduct surveys during normal business hours
• Implement wireless security procedures on access points
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