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192 Appendix B Troubleshooting WLAN IP Telephony installations
3.
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, and
1200 series APs, for instance, refer to SVP compatibility as Protocol 119
support. The SpectraLink Web site provides AP settings used in SVP
compatibility testing. To download the AP configuration manuals, go to
w
ww.spectralink.com.
4.
Configure the admissions limit in the 2245 Wireless IP telephony
manager. The value you chose limits high-priority clients such as voice
terminals from overloading an AP. The Nortel-recommended value is
7. An admissions limit higher than 7 can 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 handover to
the AP. WLAN performance studies with 802.11b radios show that the
admissions limit must not exceed 10.
5.
Handsets require a relative signal strength (RSSI) of -70dB or better
for high QoS. When the RSSI drops below -70dB, handsets attempt to
handover to an AP with a higher RSSI.
6.
Up to three APs can occupy the same area because 802.11b provides
three nonoverlapping channels. Handsets require like-channels,
between adjacent APs, to have 15-20dB of separation to achieve good
QoS and to avoid ping-pong between APs, which impacts QoS by
creating constant handover.
7.
WLAN infrastructure must be configured for high performance with delay
between 221x handset and 2245 less than 100 ms, less than 1% packet
loss and less than 30 ms jitter. WLAN networks that previously only
supported data applications sometimes cannot meet this performance
criteria and consequently are not be suitable for voice services.
8.
RF cochannel interference reduces both the capacity and reach of WLAN
networks. Use site surveys to plan coverage areas and scan them to
insure that Rogue APs are not present. Cochannel interference can also
be created by florescent light, microwave ovens, 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.
9.
Building construction can impact RF. Metal floors, metal walls, and metal
ceilings can create RF signal reflections, and create a scenario known
as multipath, which creates interference to the voice packet stream.
10.
Handsets have a built-in Site Survey mode that shows the actual RSSI
from the four strongest APs at any current location. Use Site Survey
mode to determine holes in coverage that can create dropped calls or
poor voice QoS.
11.
Poor voice QoS received in handsets is caused, 70% of the time, by
problems in the infrastructure, such as missing SVP enabled, poor RSSI
coverage, cochannel interference, Ethernet duplex mismatch, excessive
retransmission of packets, or other RF interference.
Nortel Communication Server 1000
WLAN IP Telephony Installation and Commissioning
NN43001-504 01.02 Standard
Release 5.0 15 June 2007
Copyright © 2004-2007, Nortel Networks
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