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Quality of Ser vice checklist for voice over WLAN applications 195
— Enable the voice VLAN across the network between the AP, to
the SVP, to the Signaling Server, to the Call Manager and all
components in between.
— Assign all devices in this VLAN to one subnet.
– Ideally, there is no routing between any network devices used
for voice communication.
– If routing is necessary, keep it in the back-end, between the SVP
and PBX Call Server. Keep routing short and quick.
There are other access point parameters necessary for the proper operation
of voice communications. Often, these are not adjustable. By specifying
IP protocol 119 the following parameters are enabled on the voice path
in the access point:
•
Voice packet retry limit of three
If a voice packet is not ACKed, it is retried three times and then dropped.
This parameter must be enabled in the code to be able to control the
number of packet retries and limit them to three. There are access
points that identify IP protocol 119 (SpectraLink voice packets), to limit
packet retries to three and other access points simply identify voice
transmissions to limit retries.
•
Round Robin Queuing
During voice packet retry transmissions, the AP can also transmit
other voice packets in the queue. This ability prevents other wireless
telephones that use the AP from having their voice communications held
up while retries occur. This causes all other wireless telephones to have
poor audio while one wireless telephone is in a retry state.
These parameters can be invoked by specifying the voice profile or IP
protocol 119 < 10 ms latency. Disable the following parameters:
•
Dynamic Transmit Power at the Control (DTPC) for wireless clients
•
AP Dynamic TX Power (or configure to On Demand Only)
•
AP Dynamic Channel Select (or configure to On Demand Only)
•
Intrusion Detection System (IDS) involvement
IDS causes the AP to go off channel and listen for other wireless clients
and APs. This causes degraded voice quality.
•
Load balancing at the AP for clients and network
•
Interference detection and avoidance
•
Coverage hole detection and correction
•
Client Holdoff Time
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
.