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Installation Guide

2.3.4 Multi-path and Signal Distortion

Multipath distortion is a form of RF interference that occurs when a radio signal has more than one

path between the transmitter and the receiver causing multiple signal wave fronts to be heard at the

receiver. This is typically caused by the radio signal reflecting off physical barriers such as metal

walls, ceilings and other structures and is a very common problem in factories and storage

environments. Multiple converging wave fronts may be received as either an attenuated or amplified

signal by the receiver. In some instances, if the signals arrive exactly out of phase, the result is a

complete cancellation of any signal.

Multipath can cause severe network throughput degradation because of high error rates and packet

retries. This in turn can lead to severe voice quality issues with Avaya wireless telephones. Correctly

locating antennas and choosing the right type of antenna can help reduce the effects of multipath and

interference.

Diversity antennas should be used to help improve performance in a multipath environment. A

diversity solution uses two antennas for each radio, and will send and receive signals on the antenna

which is receiving the best signal. This greatly increases the probability that both the AP and the client

will receive a better signal quality in multipath environments. Most access points support receive

diversity in that they receive on the antenna that is getting the best signal, but some also support full

transmit diversity where the transmission is made on the same antenna that was last used to receive a

signal from that specific client. Avaya recommends the use of APs supporting both receive and full

transmit diversity where multipath is an issue in order to provide optimal voice quality.

Access point antennas should not be placed near a metal roof, wall, beam or other metal obstruction

in a multipath environment, as this will amplify the reflection effects. Additionally, antennas should be

positioned so that they have a line of site to most of the clients that they service. Additional

instructions from the wireless network infrastructure vendor should be followed with regard to antenna

selection and placement to provide correct diversity operation.

2.3.5 Site Surveys

A wireless site survey is highly recommended for any wireless network deployment. However it is

especially critical when voice is an application on the network and is essential for large or complex

facilities. A site survey can ensure that the wireless network is optimally designed and configured to

support voice by confirming RF coverage, cell overlap, channel allocation/reuse, packet transmission

quality and other deployment considerations. While many tools exist that allow customers to perform

their own assessment, Avaya recommends a professional site survey to ensure optimum coverage

and minimum interference.

To verify coverage of an installed Wi-Fi network, Avaya wireless telephones offer a site-survey mode

that can be used to ensure the AP locations and configurations are both correct and adequate. This

mode detects the strongest AP signals and displays the signal strength along with the AP channel

assignments. The site survey mode may be used to detect areas with poor coverage or interfering

channels; check for rogue APs' confirm the Extended Service Set Identification (ESSID) and data

rates of each AP; and detect AP configuration problems. With Avaya wireless telephones, the entire

coverage area must be checked to ensure that at least one access point's output meets the signal

strength requirements summarized in Section 2.1.2 of this document. If the site-survey mode

indicates that two APs are using the same channel, then at least one other different AP must be

indicated at 10 dBm stronger than those APs, to avoid channel conflicts.

After a site survey is complete, coverage issues can be resolved by adding and/or relocating APs.

Overlap issues may be resolved by reassigning channels or by relocating some access points. When

adjustments are made to the WLAN configuration, an additional site survey should be performed to

ensure that the changes are satisfactory and have not had adverse impact in other areas of coverage.

The maximum number of simultaneous telephone calls an AP can support is determined by dividing

Octiober 2008