Specifications

16 Wired LAN/Backbone Requirements
Battery Lifetime
Since the number of charging cycles needed are dependent on the power consumption, the life-
time of the battery is highly dependent of the settings used. A poor network setup with no power
save functionality will decrease the lifetime dramatically.
Wired LAN/Backbone Requirements
There are several things to consider when designing a network for VoWiFi:
In order to achieve optimal performance for VoWiFi, the wireless infrastructure should be connect-
ed to a switched network (that is, there are no hubs or repeaters).
In a switched network the transmission delay should not be an issue, 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 impacting the systems
voice quality. The transmission delay will also add to the speech delay.
Jitter in voice packages will also add to the speech delay since the portable will adjust the jitter
buffer size.
See also section <Blue>Chapter , “Known Problems”.
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 stan
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dards mentioned above, depending on the packet type.
Priority is primarily needed for wireless prioritization and secondarily for wired LAN prioritization.
The User Priority (UP) or DSCP value of the frame will determine what Access Category will han-
dle the frame.
Four Access Categories (ACs) are defined in the WMM specification:
• 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 priority 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, Expedited Forwarding) for RTP frames.
• SIP signalling DSCP value (0x1A (26), Assured Forwarding 31 for both handset types).
For further information regarding the infrastructure, see Ascom Interoperability Reports for respec-
tive system.