ASCOM I62 - Page 19

TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
16
3. The VoWiFi Handset as
On association, the handset negotiates with the AP and trigger-enables all four Enhanced
Distributed Channel Access (EDCA) categories. These are:
Category 802.11e Priority
Voice 6 or 7
Video 4 or 5
Best Effort 0 and 3
Background 1 to 2
For additional information about U-APSD and h
o
w to inspect and troubleshoot U-APSD
setup, see section 7.2 U-APSD Troubleshooting on pa
ge 55.
U-APSD Flow Example
The example is based on a voice call with bi-direction
al flow and symmetric codec speech
frames.
A call is established between two parties. In this the parties are assumed to
be a VoWiFi
handset and a fixed line phone that resides somewhere on the LAN.
1 In the AP, a voice packet from the fixed phone is received from the wired LAN. The
pa
cket is destined to the handset and the AP intends to transmit the packet on the
RF interface immediately.
However, the destination, that is, the handset
for t
he packet is in PS-mode and since
the AP keeps track of all associated stations PS management mode, this is recognized
and the packet is buffered in the AP.
2 After a short while, the handset transmits a voice packet, as is done eve
r
y 20 ms with
default settings. This packet is transmitted within the EDCA Access Category “Voice”,
which means that it is highly prioritized.
3 The AP receives the packet from the handse
t, forwards the packet to the wired L
AN
and recognizes that the sending station has a buffered packet. The AP releases the
packet and transmits the packet immediately to the VoWiFi handset that now is
awaiting a packet.
The procedure when an AP transmits a buffered packet due to a trigger frame is
calle
d an “Unsched
uled Service Period”