When designing and deploying a WLAN, you will always be concerned about both coverage and capacity. Various factors can affect the coverage range of a wireless cell, and just as many factors can affect the aggregate throughput in an 802.11 WLAN.
The following variables can affect the range of a WLAN:
- Transmission power rates The original transmission amplitude (power) will have an impact on the range of an RF cell. An access point transmitting at 30 mW will have a larger coverage zone than an access point transmitting a 1 mW assuming that the same antenna is used.
- Antenna gain Antennas are passive gain devices that focus the original signal. An access point transmitting at 30 mW with a 6 dBi antenna will have greater range than it would if it used only a 3 dBi antenna.
- Antenna type Antennas have different coverage patterns. Using the right antenna will give the greatest coverage and reduce multipath and nearby interference.
- Wavelength Higher frequency signals have a smaller wavelength property and will attenuate faster than a lower frequency signal with a larger wavelength. 2.4 GHz access points have greater range than 5 GHz access points.
- Free space path loss In any RF environment, free space path loss (FSPL) attenuates the signal as a function of distance and frequency.
- Physical environment Walls and other obstacles will attenuate an RF signal due to absorption and other RF propagation behaviors. A building with concrete walls will require more access points than a building with drywall because concrete is denser and attenuates the signal faster than drywall.
Proper WLAN design must take into account both coverage and capacity. The above-mentioned variables all affect range so therefore also affect coverage.
Capacity performance considerations are equally as important as range considerations. Please remember that 802.11 data rates are considered bandwidth and not throughput.
The following are among the many variables that can affect the throughput of a WLAN:
- Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) The medium access method that uses interframe spacing, physical carrier sense, virtual carrier sense and the random backoff timer creates overhead and consumes bandwidth. The overhead due to medium contention usually is 50 percent or greater.
- Encryption Extra overhead is added to the body of an 802.11 data frame whenever encryption is implemented.
WEP/RC4 encryption adds an extra 8 bytes of overhead per frame, TKIP/RC4 encryption adds an extra 20 bytes of overhead per frame, and CCMP/AES encryption adds an extra 16 bytes of overhead per frame. Layer 3 VPNs often use DES or 3DES encryption, both of which consume significant bandwidth.
- Application use Different types of applications will have variant affects in bandwidth consumption. VoWiFi and data collection scanning typically do not require a lot of bandwidth. Other applications that require file transfers or database access often are more bandwidth intensive.
- Number of clients Remember that the WLAN is a shared medium. All throughput is aggregate and all available bandwidth is shared.
- Interference All types of interference can cause frames to become corrupted. If frames are corrupted, they will need to be retransmitted and throughput will be affected.
Weather
When deploying a wireless mesh network outdoors or perhaps an outdoor bridge link, a WLAN administrator must take into account the adverse affect of weather conditions. The following three weather conditions must be considered:
- Lightning - Direct and indirect lightning strikes can damage WLAN equipment. Lightning arrestors should be used for protection against transient currents. Solutions such as lightning rods or copper/fiber transceivers may offer protection against lightning strikes.
- Wind - Due to the long distances and narrow beamwidths, highly directional antennas are susceptible to movement or shifting caused by wind. Even slight movement of a highly directional antenna can cause the RF beam to be aimed away from the receiving antenna, interrupting the communications.
In high-wind environments, a grid antenna will typically remain more stabile than a parabolic dish. Other mounting options may be necessary to stabilize the antennas from movement.
- Water - Conditions such as rain, snow, and fog present two unique challenges. First, all outdoor equipment must be protected from damage from exposure to water. Water damage is often a serious problem with cabling and connectors.
Connectors should be protected with drip loops and coax seal to prevent water damage. Cables and connectors should be checked on a regular basis for damage.
A radome should be used to protect antennas from water damage. Outdoor bridges, access points, and mesh routers should be protected from the weather elements using appropriate
National Electrical Manufacturers Association (NEMA) enclosure units. Precipitation can also cause an RF signal to attenuate. A torrential downpour can attenuate a signal as much as .08 dB per mile (.05 dB per kilometer) in both the 2.4 GHz and 5 GHz frequency ranges.
Over long-distance bridge links, a system operating margin (SOM) of 20 dB is usually recommended to compensate for attenuation due to rain or fog or snow.
- Air stratification - A change in air temperature at high altitudes is known as air stratification (layering). Changes in air temperature can cause refraction.
Bending of RF signals over longdistance point-to-point links can cause misalignment and performance issues. K-factor calculations may be necessary to compensate for refraction over long-distance links.
- UV/sun - UV rays and ambient heat from rooftops can damage cables over time unless proper cable types are used.
