TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
6
2. WLAN Overview
2. WLAN Overview
A WLAN enables various devices, or WLAN clients, to communicate across RF channels
through Access Points (APs). APs provide RF coverage throughout the site covered by the
WLAN and this enables users, and their devices, to move around the site without being
disconnected from the network. Each AP is identified by a hard-coded MAC address.
Initially, WLANs were designed to allow users to send and recei
ve data from devices such as
laptop computers but increasingly WLANs are being required to support different and more
demanding types of traffic such as voice and multimedia. A VoWiFi system is illustrated in
figure 1:
Figure 1. VoWiFi Overview
IMS3 / Unite CM
LAN
Access Point
VoWiFi Handsets
IP-PBX
Presence
Management
System
Location Server
Unite System
Controller
optional equipment
required equipment
2.1 The Problem of Legacy WLANs
Legacy WLAN systems were designed for transmitting data packets; support for voice and
other multimedia was never envisaged when these systems were designed, deployed and
commissioned. So many traditional cell-based WLAN topologies proved unsuitable for
handling channelization issues, AP-to-AP handoffs, and the unpredictably of system
bandwidth that are introduced when VoWiFi is implemented. Voice is very susceptible to
such issues being addressed satisfactorily. A WLAN that does not support VoWiFi but is
being upgraded to do so therefore requires very careful planning, design, provisioning and
hardware deployment to get the level of performance to adequately support voice. These
problems are multiplied when different traffic types such as voice, video and data all
contend for the same airspace. Adding voice to a WLAN may require radical changes of the
APs placement and the amount of APs needed. If this is not considered feasible to undertake
in a current installation VoWiFi may never perform as expected.
It follows that if a legacy WLAN is to support VoWi
Fi then
an assessment of how suitable the
WLAN is to support VoWiFi must be made. The number and deployment of additional
devices involved in the WiFi solution must be carefully assessed. For example, the
assessment should consider the adequacy of the current deployment in meeting cell
boundary requirements because adequate cell overlap is a fundamental requirement for a
voice system. The assessment should conclude with a readiness report documenting the
suitability of the WLAN for both the wired and the wireless part of the network. The
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