TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
29
4. Factors Affecting RF
4. Factors Affecting RF Propagation
There are many factors that can affect RF propagation and many of these factors concern
the physical characteristics of the site such as its layout, design, floor plan and materials
used in the site construction. Structural changes to the physical environment such as the
moving of furniture and removing and setting up walls can cause permanent changes to the
RF environment.
Many sites are also subject to dynamic elements t
han can adversely affect RF propagation.
The RF propagation changes as people and objects move through coverages area. Even
people leaving doors open, using cordless phones and Bluetooth devices and turning on all
the microwave ovens at lunch time can all cause interference and increased noise, even
though temporarily, in the RF environment.
RF signal propagation may also be affe
cted by other APs and people roaming through the
coverage area. Wireless APs and clients have therefore to constantly adapt to the changing
RF environment, for example, by dropping the connection rate when an environment
suddenly becomes noisier and restoring the connection rate when the environment quietens
down.
When deploying dense WLAN networks, network admi
nistrators must therefore pay careful
consideration to:
• The deployment of APs, particularly where there are
many APs deployed in a relatively
small area. Dense deployment of APs can cause RF interference with neighboring APs
residing on the same or neighboring channels.
• RF interference and impedance from structural and temporary changes to the physical
environment.
4.1 Jitter, Latency and Packet Loss
When the RF environment is compromised, the fundamental requirement of any system
that transmits digitized voice signals over a WiFi network in the form of data packets may
also be compromised. This is the requirement to preserve of the isochronous nature of the
voice packet streams. Asynchronous pollution of the RF environment causes distortion to
the voice quality and can occur when:
• The latency in voice packets exceeds certain limits, usu
ally greater than 150ms in one
direction or 300ms for both directions, between the receipt of each packet
• Delay variations from packet to packet, that
is, jitter, that exceed 20ms
• Packet loss that starts to exceed 3%
Because voice signals have to travel across many ne
twork infrastructure components,
packet loss, latency, and jitter can combine to reduce the signal quality especially when the
network becomes congested with traffic. For additional information, always check the
Application Partner Program, Ascom Inter-Operability Reports for specific recommendations
applicable to the particular vendor equipment.
4.1.1 Signal Loss
RF loss is a decrease in the signal amplitude. There is
always a clear and predictable loss as a
radio signal propagates through the air, even if the airspace is completely unobstructed. The
atmosphere causes the modulated signal to attenuate exponentially as the signal
propagates farther away from the antenna. Signal loss can occur when metal, concrete,
walls, or floors block transmission.
The physical site characteristics can conside
rably exacerbate signal loss because of
obstructions in the airspace such as walls, doors and ceilings. The material used to
manufacture and construct such obstructions can also affect the degree of signal loss.
Therefore, the signal must have enough power to reach the desired distance at a signal level
acceptable that the receiver needs.
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