TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
12
3. The VoWiFi Handset as
used to describe the path over which applications communicate. Most application level
protocols are associated with one or more port number.
The SIP protocol is an Application Layer protocol designed to be independent of the
under
lying Transport Layer.
3.1.1 Troubleshooting the Layers
Understanding the layers in the TCP/IP protocol stack
and how they communicate with each
other is important because it provides the support engineer with a consistent, structured
and methodological 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 e
ngineer starts with the layer 4, the
application layer or user services level.
Bottom-up troubleshooting involves checking the device for physical problems such as no
po
wer, 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.
3.2 Voice Transmission over a VoWiFi System
Voice over WiFi (VoWiFi) requires that digitized voice signals are sent over a WiFi 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 un
able 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.
3.3 Handset Startup Procedure
The handset is started in the following way:
1 The user presses the On/off button (En
d key).
2 The handset loads the boot code, which then loads the latest installed firmware.
3 The handset loads licenses.
4 The handset actively scans for the configured SSID on all configured channels and
c
hooses the best AP to associate with and authenticate to.
5 The handset is identified to the network by it’s I
P address. If the handset Dynamic
Host Configuration Protocol (DHCP) mode is “Enabled”, the handset is dynamically
assigned an IP address, otherwise the statically configured IP address is used.
6 The handset sends an ARP request for resolving t
he MAC addresses of the IP PBX and
the Unite server.
7 The handset registers with the IP PBX and Unite server, and logs on to and
synchronizes with the
se devices.
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