Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
14
3 Wired LAN/Backbone Requirements
There are several things to consider when designing a network for VoWiFi:
In order to achieve optimal performance for VoWiFi, t
he wireless infrastructure should be
connected to a switched network (that is, there are no hubs or repeaters).
In a switched network the transmission delay should not be an issu
e, but if voice traffic is
routed, a significant transmission delay could be added.
If the transmission delay is too long an echo will appear in the voice path im
pacting the
systems voice quality. The transmission delay will also add to the speech delay.
Jitter in voice packages will also add to the sp
eech delay since the portable will adjust the
jitter buffer size.
See also section 11 Known Problems on page 34.
3.1 Quality of Service (QoS) Recommendations
To be able to provide voice grade communication over WLAN, the use of WMM or 802.11e is
a necessity. These standards define the mapping of priorities on the WLAN to priorities on
the wired LAN using either Layer 2 (CoS, Class of Service) or Layer 3 priorities Differentiated
Services Code Point (DSCP). Traffic shaping in the switches should be avoided and instead
the use of packet-based priority by the STAs should be used. Each packet will be prioritized,
according to the standards mentioned above, depending on the packet type.
Priority is primarily needed for wireless prioritiz
a
tion and secondarily for wired LAN
prioritization.
The User Priority (UP) or DSCP value of the frame will
determine what Access Category will
handle the frame.
Four Access Categories (ACs) are defined in the WMM spec
ification:
•AC_BK (background)
• AC_BE (best effort)
• AC_VI (video)
•AC_VO (voice)
WMM maps the User Priority used in the 802.11 frames to a corresponding pri
ority on the
wired LAN 802.3 frame.
• Layer 2 priority uses the 802.1p priority field in
the 802.1Q VLAN tag, on the wired side
of the AP/controller.
• Recommended value for 802.1p priority for voice is 6.
For both the wired and wireless side of the AP or controller:
• Recommended value for the DSCP value is 46 (EF, Expedite
d Forwarding) for RTP frames.
• SIP signalling DSCP value (0x1A (26), Assured Forwarding 31 for both VoWiFi Handset
types).
For fu
rther information regarding the infrastructure, see Ascom Inte
roperability Reports for
respective system.
3.1.1 IEEE 802.11 Priority Field
The 802.11 User Priority is sent using the 2 bit QoS Control Field in the 802.
11 MAC header.