Challenges of integrating voice applications 35
Power adjustments and variable capacity
A WLAN has dynamic mechanisms in place for adjusting channels, adjusting power, and filling coverage holes, all in response to changes in the Radio Frequency (RF) environment. All of these mechanisms present challenges to the engineering of voice networks.
Dynamic adjustments work well for guaranteeing minimum coverage and connectivity of devices, particularly data devices. Voice requires more planned engineering.
Usually, the number of calls per area (square foot) and calls per AP determines the number of APs required to support the voice applications and devices. Power adjustments affect these parameters. If an AP increases power, it provides coverage for a larger area, meaning a greater call demand for the AP. Doubling the power of an AP can quadruple its coverage area, which means up to four times as much call demand as originally engineered. That increased coverage area also has substantial portions of lower data rate coverage. In addition, the added cochannel interference to other cells using the same channel degrades their call capacity. The net effect is that a network previously tuned for voice is now less capable of meeting the demands of voice than it was before the dynamic power adjustment.
Automatic RF changes do not always have a negative impact on
When VoWLAN drives the engineering of the network both in scale and capacity, sometimes automatic RF features create more challenges than they resolve.
Quality of Service
802.11is a shared media technology, but only one device can use the media at a time. The AP abides by this rule as well.
Because the transmitting device cannot detect collisions, 802.11 uses a statistical mechanism to reduce the possibility of collisions when two devices are ready to transmit at the same time. After the medium becomes available, the mechanism requires the devices to wait a random amount of time before starting transmission. Because of this simple mechanism, a nonvoice device is as equally as likely to be allowed to transmit as a voice device is.
Nortel Communication Server 1000
WLAN IP Telephony Installation and Commissioning
Release 5.0 15 June 2007
