Voice over Wireless LAN Solution Guide v1.0 December 2005
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; WLAN access points must be SVP or View compatible as tested by
SpectraLink Corp. Nortel requires all WLAN networks that carry voice be SVP
enabled to receive NETS/GNTS support. For SVP compatible APs, see
http://www.spectralink.com/service/manuals_config.html.
; Enable SVP in the APs. SVP must be enabled in all APs that carry voice traffic.
Not all AP vendors use SVP terminology. Cisco 350/1100/1200 series AP, for
instance, refers to SVP compatibility as Protocol 119 support. The SpectraLink
web site provides AP settings used in SVP compatibility testing. For AP
configuration manuals, see
http://www.spectralink.com/service/manuals_config.html.
; Set admissions limit in 2245 Wireless IP Telephony Manager. The value you
choose will prevent voice handsets from overloading an AP. The recommended
setting is 7. Admissions Limit higher than 7 may severely limit bandwidth to data
users when voice traffic is high. To increase bandwidth for data, lower the
admissions limit so that fewer voice terminals will consume AP capacity and
resources. WLAN performance studies with 802.11b radios have demonstrated
that the admissions limit should not exceed 10.
; Handsets require relative signal strength (RSSI) of –70 dB or better for high
QoS. When RSSI drops below –70 dB, handsets will attempt to roam to an AP
with higher RSSI.
; Up to three APs can occupy the same area, as 802.11b provides three non-
overlapping channels. Handsets require like channels between adjacent APs to
have 15 to 20 dB of separation to achieve good QoS and avoid the “ping pong”
effect of rapid and repeated roaming between APs impacting QoS during
constant handover.
; WLAN infrastructure must be configured for high performance, with delay
between the 221x handset and the 2245 less than 100 ms, less than 1 percent
packet loss and less than 30 ms jitter. WLAN networks that previously supported
only data applications may not meet these performance criteria and consequently
may not be suitable for voice services.
; RF Co-channel interference reduces both the capacity and reach of WLAN
networks. Plan coverage areas and scan using site surveys to insure that rogue
APs are not present. Co-channel interference may also be created by fluorescent
light, microwave, 2.4 GHz analog or digital telephones, Bluetooth adapters, and
2.4 GHz frequency-hopping applications such as first-generation AP or DECT 2.4
GHz wireless.
; Building construction can impact RF. Metal floors, metal walls, or metal
ceilings may create RF signal reflections, and create a scenario known as
multipath, creating problems in the voice packet stream.
; Handsets have built-in Site Survey mode that will show actual RSSI from the
four strongest APs at any present location. Site survey mode can be used to