TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
18
3. The VoWiFi Handset as
increasingly stronger signal as the handset is carried towards it. As a user moves away from
an AP of decreasing received signal strength and towards one of increasing signal strength,
it is predictable enough to be able to apply a roaming algorithm based on the measurement
of received signal strength at fixed time intervals, for example, every 4 seconds. The
algorithm then decides where and when a handset disconnects from one AP and connects to
a different AP.
An additional factor that must be included in an
y ro
aming algorithm is to provision for
sudden, large and often unpredictable reductions in received signal strength due to some
dynamic event that is introduced into the physical environment, such as the closing of a
heavily clad steel door or the handset user enters an elevator. These events can occur
between the scans performed at the fixed time intervals and cause additional scans to be
performed. The trigger for an additional scan is when a 6dBm or greater received signal
strength reduction is detected since the last scan.
To ensure that unnecessary extra scans are not
initiate
d between scheduled measurement
periods, the reduction in received signal strengths is calculated accordingly:
reduction in dBm = 75% of scheduled AP RSSI measurement + 25% of beacon RSSI.
Note: O
nly beacons are used for RSSI calculations in the ha
ndset when the handset is not
roaming. When roaming, probe responses are also used in the RSSI calculations.
For example, consider a doorway between an asso
ciated AP and a handset, with a metal
door in the open position. if the door was momentarily closed and then opened, there might
be a sudden 16bB reduction in received signal strength and a rapid loss in communication.
However, this would be largely restored once the door was opened again and make an extra
scan unnecessary and a waste of network resources. However, if the door was left closed,
the loss in received signal strength would be sustained and probably worsen as the user
moved away from the AP. An extra scan would therefore be justified.
Roaming is implemented differently
depending on whether the
handset is in idle mode or in
a call. Different roaming algorithms apply to these modes.
3.6.1 Idle Mode
In idle mode, it is not so important to consider received sign
al strength and roaming as voice
quality is not an issue. The following roaming algorithms are deemed sufficient for ensuring
the handset is able to perform TIM discovery as the user moves between AP:
RSSI Scans Roams
> -70dBm No scanning. n/a
< -70dBm and > -73dBm Every 4secs. If a candidate AP offers an improvement of
6dBm or more
< -73dBm Every 4secs. If a candidate AP offers an improvement of
3dBm or more
Additional Scan in Idle Mode
An additional scan is triggered in idle mode when scanning is being perform
ed and when a
reduction of >= 6dBm is detected in a beacon being sent by the AP at the regular beacon
interval.
3.6.2 Call Mode
In call mode, received signal strength is very i
m
portant for ensuring voice quality,
particularly if the user is moving around the site. The following roaming requirements are
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