Challenges of integrating voice applications 35Power adjustments and variable capacityA WLAN has dynamic mechanisms in place for adjusting channels,adjusting power, and filling coverage holes, all in response to changes inthe Radio Frequency (RF) environment. All of these mechanisms presentchallenges to the engineering of voice networks.Dynamic adjustments work well for guaranteeing minimum coverage andconnectivity of devices, particularly data devices. Voice requires moreplanned engineering.Usually, the number of calls per area (square foot) and calls per APdetermines the number of APs required to support the voice applicationsand devices. Power adjustments affect these parameters. If an AP increasespower, it provides coverage for a larger area, meaning a greater call demandfor 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 datarate coverage. In addition, the added cochannel interference to other cellsusing the same channel degrades their call capacity. The net effect is thata network previously tuned for voice is now less capable of meeting thedemands of voice than it was before the dynamic power adjustment.Automatic RF changes do not always have a negative impact onvoice-engineered networks. Admission control techniques help with theoversubscription problems related to increasing cell sizes dynamically. Holefilling, after an AP failure occurs, also provides substantial value to a voicesolution.When VoWLAN drives the engineering of the network both in scale andcapacity, sometimes automatic RF features create more challenges thanthey resolve.Quality of Service802.11 is a shared media technology, but only one device can use themedia at a time. The AP abides by this rule as well.Because the transmitting device cannot detect collisions, 802.11 uses astatistical mechanism to reduce the possibility of collisions when two devicesare ready to transmit at the same time. After the medium becomes available,the mechanism requires the devices to wait a random amount of time beforestarting transmission. Because of this simple mechanism, a nonvoicedevice is as equally as likely to be allowed to transmit as a voice device is.Nortel Communication Server 1000WLAN IP Telephony Installation and CommissioningNN43001-504 01.02 StandardRelease 5.0 15 June 2007Copyright © 2004-2007, Nortel Networks.
