For areas where user traffic is high (such as office spaces, schools, retail stores, and hospitals), Cisco recommends that you place the access point out of sight and place unobtrusive antennas below the ceiling. Separation for non−diversity antennas should not exceed 18 inches.
Interference and Multipath Distortion
The throughput performance of the WLAN network is affected by unusable signals. WLAN interference can be generated by microwave ovens, 2.4 GHz cordless phones, Bluetooth devices, or other electronic equipment operating in the 2.4 GHz band. Interference also typically comes from other access points and client devices that belong in the WLAN but that are far enough away so that their signal is weakened or has become corrupted. Access points that are not part of the network infrastructure can also cause WLAN interference and are identified as rogue access points.
Interference and multipath distortion cause the transmitted signal to fluctuate. Interference decreases the signal−to−noise ratio (SNR) for a particular data rate. Packet retry counts go up in an area where interference and/or multipath distortion are high. Interference is also referred to as noise level or noise floor. The strength of the received signal from its associated access point must be high enough above the receiver's noise level to be decoded correctly. This level of strength is referred to as the signal−to−noise ratio, or SNR. The ideal SNR for the Vocera Badge is 25 dB. For example, if the noise floor is 95 decibels per milliwatt (dBm) and the received signal at the phone is 70 dBm, then the signal−to−noise ratio is 25 dB. (See Figure 25.)
Figure 25Signal−to−Noise Ratio (SNR)
When you change the type and location of the antenna, it can reduce multipath distortion and interference. Antenna gain adds to the system gain and can reduce interference if the interfering transmitter is not directly in front of the directional antenna.
