## Environmental Interference

Physical and environmental conditions affect the performance of wireless LAN equipment. Wireless LAN sites should not be excessively hot, cold, humid, or dusty. The area should also be relatively free of electronic equipment that generates background Radio Frequency (RF) noise, which can interfere with wireless LANs.

### Interference from Equipment and Materials

Some of the equipment and materials that can interfere with wireless LAN transmissions include:

- Equipment that produces radio waves in the 2.4 or 5.2 GHz range
- Existing wireless LANs
- Cordless telephones or cordless telephone stations except for cellular telephones
- Microwave ovens closer than three meters (ten feet) from the antenna
- Radio frequency identification tags
- Ceiling-mounted sprinkler heads closer than 60 centimeters (two feet) to the antenna
- Outdoor broadcast television used by mobile television cameras
- Uninterruptible power supply (UPS) devices
- Pine trees with needles near wavelength or half wavelength
- Large heat-producing machines or chambers
- Fluorescent lights
- Overhead cranes
- Overhead conveyors
- Elevator shafts
- Heavy-duty motors
- Lead paint
- Plants and trees
- Power surge protectors
- Refrigerators
- Sulfur plasma lighting
- Transformers

Some of the items above apply to 802.11b networks but not 802.11a networks, and vice versa.

Relative Attenuation of RF Obstacles
Obstruction | Degree of Attenuation | Example
--- | --- | ---
Open Space | None | Cafeteria, courtyard
Wood | Low | Inner wall, office partition, door, floor
Plaster | Low | Inner wall (old plaster lower than new plaster)
Synthetic Materials | Low | Office partition
Cinder block | Low | Inner wall, outer wall
Asbestos | Low | Ceiling
Glass | Low | Non-tinted window
Wire Mesh in Glass | Medium | Door, partition
Metal Tinted Glass | Medium | Tinted window
Human Body | Medium | Large group of people
Water | Medium | Damp wood, aquarium, organic inventory
Bricks | Medium | Inner wall, outer wall, floor
Marble | Medium | Inner wall, outer wall, floor
Ceramic (Metal Content or Backing) | High | Ceramic tile, ceiling, floor
Paper | High | Roll or stack of paper stock
Concrete | High | Floor, outer wall, support pillar
Bulletproof Glass | High | Security booth
Silvering | Very High | Mirror
Metal | Very High | Desk, office partition, reinforced concrete, elevator shaft, filing cabinet, sprinkler system, ventilator

### Electrical Power

The right kind of power supply should be available for access points. The following are electrical installation alternatives listed in order of most to least desirable.

1. Dedicated circuit with a Uninterrupted Power Supply (UPS) that also acts as a filter and surge suppressor.
2. Non-dedicated circuit with a UPS.
3. Non-dedicated circuit with a surge suppressor.

Configurations without a UPS are not recommended. A dedicated circuit is preferable to a non-dedicated circuit. Non-dedicated circuits have open receptacles; the load and type of use cannot be predicted at the time of installation. Although the current draw of the access points is minimal, other devices on the circuit can affect the access points.

If it is absolutely necessary to have a non-dedicated circuit, it is recommended that the circuit not support the following:

- Non-network devices, such as lighting, powered directly from the circuit.
- Devices with components that produce heat such as space heaters, laser printers, heat guns, soldering irons, or photocopiers.
- Devices that often cause sudden sharp surges in the power line contain medium or large motors, such as electric staplers, refrigerators, floor cleaning equipment, air conditioners, fans, or any single device drawing more than 20 percent of the rated value the circuit.
- Any combination of devices drawing more than 60 percent of the rated value of the circuit.

In all cases, the power to the access points must be unswitched and available 24 hours per day. It is recommended that the power never be provided from an energy management system.

### Size and Shape of the Site

The size of the site determines what equipment you need and where to place it.

- A small site requires fewer than 10 access points; two are usually sufficient.
- A medium site requires between 10 and 20 access points.
- A large site requires 20 to 100 access points and possibly some directional antennas.

The shape of the site is also a concern. RF transmissions propagate differently in a narrow space, such as a hallway, than in a large open area, such as an office filled with cubicles. Open doorways might also affect RF propagation.

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