Voice Transmission Over A Vowifi System; Ap Scanning; Active Scanning; Passive Scanning - ASCOM Myco 3 Troubleshooting Manual

The Handset as a WLAN Wireless Client
approach to troubleshooting network issues. It enables the support engineer to adopt either a top down
approach or a bottom up approach to troubleshooting and reconciling network issues.
With top-down troubleshooting, the support engineer starts with the application layer or user services level.
Bottom-up troubleshooting involves checking the device for physical problems such as no power, a bad
cable or other physical problems. A positive response would confirm that network connectivity had been
established, in which case it is safe to conclude that connectivity up to the Network layer was successful.
The focus can now be turned to troubleshooting the upper four layers of the OSI.
4.2

Voice Transmission over a VoWiFi System

VoWiFi requires that digitized voice signals are sent over a Wi-Fi network in the form of data packets as
defined in layer 4 of the TCP/IP protocol stack. This basic requirement places a substantial overhead on a
WLAN. If a WLAN is unable to support the VoWiFi requirement consistently, or only able to provide sporadic
and irregular support, then noticeable and unacceptable levels of deterioration to the quality of the voice
service occur.
For example, if voice data packets are unable to arrive at their destinations at regular time intervals,
typically every 20 ms, distortions in the conversation are likely to occur. The VoWiFi system must also keep
packet loss, delay, and jitter within required limits to avoid a loss or reduction in the voice service.
As a voice signal travels through the VoWiFi infrastructure of APs, Ethernet switches, routers, and gateways,
packet loss, delay, and jitter can add up to reduce the signal quality, especially when the network gets
congested with traffic.
4.3

AP Scanning

When a handset is powered on or on the move, it needs to associate with an AP to be able to connect to
the wired LAN. The scanning process checks the air for the available APs to associate with and, based on
this information, the handset creates a list of candidate APs. The handset will then try to associate with the
AP with the strongest RF signal.
4.3.1

Active Scanning

Active scanning occurs when a handset actively seeks to associate with an AP and ultimately connect to a
network by transmitting 802.11 probe requests frames to APs on each of the channels the handset is
configured to use. The probe request frames contain the SSID of the network that the handset wishes to
connect to, and all APs that are configured with the same SSID return a probe response to the handset.
If the handset receives probe responses from more than one AP, it can use a number of criteria to decide
which AP too associate with. Although these criteria are vendor specific, a handset will usually select the AP
with the strongest RF signal.
4.3.2

Passive Scanning

When the handset performs passive scanning it listens (on each channel it is configured to use and at
specific intervals) for beacon frames transmitted by APs. When passively scanning for beacons, the handset
transmits no frames itself. When associated with an AP, the handset will indicate that for this AP it will go
into the power save mode while continue doing passive scanning on other channels.
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TROUBLESHOOTING GUIDE
TD 93297EN / 19 December 2019 / Ver. B
Ascom Myco 3