Avaya Communication Server 1000 Installation And Commissioning Manual page 228

Troubleshooting WLAN IP Telephony installations
1. why you need it, see the SpectraLink Voice Priority White Paper available from
http://www.polycom.com.
2. WLAN Access points must be SVP- or View-compatible as tested by SpectraLink
Corp. Avaya requires all WLAN networks that carry voice be SVP-enabled or use
WMM to receive NETS and GNTS support. For more information about SVP
Compatible APs, go to http://www.polycom.com.
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 http://www.polycom.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 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
228 Avaya WLAN IP Telephony Installation and Commissioning November 2010