Ap Scanning; Active Scanning; Passive Scanning; Configured Channels And Scanning Intervals - ASCOM i62 Troubleshooting Manual

Troubleshooting Guide
Ascom i62 VoWiFi Handset
8 The handset displays the idle screen with signal strength indicator, the current time,
battery status, date, user name, user number and license type. In this state, the
handset is described as being in idle mode.
3.4

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 linked list of candidate
APs. The handset will then usually try and associate with the AP with the strongest RF
signal.
3.4.1

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. A handset with a null SSID, that
is, it is not currently configured with a SSID, may also transmits probe requests. If an AP
exists with no security settings at all, the handset may be able to associate with that AP.
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.
If no suitable AP is selected the "No Network" message is displayed in the handset after
about 5 seconds. For each subsequent 5 second intervals, the handset continues to perform
actives probe for suitable APs using alternate broadcast and unicast probe requests. The
process will not be interrupted until the handset either successfully associates with an AP or
the battery runs out.
3.4.2

Passive Scanning

When a handset becomes associated with an AP but is not used to make a call, it will
normally go into power save mode and start to perform passive, as opposed to active
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.
Beacon frames are similar to the probe response frames transmitted by an APs when a
handset is actively scanning, except that they contain additional information about traffic
pending for the handset. For additional information about how passive scanning works
when the handset is conserving power, see the section
3.4.3

Configured Channels and Scanning Intervals

When a handset is started from power off mode the procedure to find an AP to associate
with depends on the parameter and protocol settings in the handset.
One of the default protocols that are used by the handset is 802.11d. This protocol
transports regulatory domain information based on country information stored in the AP
which is sent out in the beacon.
The handset needs to know this information before it can start to scan for APs so it can
adapt to the country rules for channels to use, power level, etcetera. It therefore needs to
use passive scanning to grab a beacon that contains that information. Thereafter the
handset can start to use active probing, where it will send out a probe request packet with
its configured SSID on each configured channel for the configured band. It will stay around
20ms on each channel waiting for replies.
28 June 2012 / Ver. A
3. The VoWiFi Handset as
TD 92685EN
3.5 Power Management on page 14.
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