Quality Of Service - Avaya Communication Server 1000 Installation And Commissioning Manual

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 voice-engineered networks.
Admission control techniques help with the oversubscription problems related to increasing
cell sizes dynamically. Hole filling, after an AP failure occurs, also provides substantial value
to a voice solution.
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

802.11 is 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.
For example, if a data device does seize the medium, it can send a 1500-byte frame at the
lowest data rate (if it is far away from the AP), and further delay voice frames. In addition,
several data devices contending for the medium can each, in turn, send large frames before
the voice device gained access to the medium.
Without a way to give preferential transmission opportunities to voice devices, supporting voice
applications is a tremendous challenge on 802.11 WLANs. SpectraLink Voice Priority (SVP)
has evolved into a de facto standard for Quality of Service (QoS) and serves as a model to
illustrate the functions that a successful QoS mechanism can implement.
The 802.11e standard ultimately resolves QoS issues, but the delays in the standard create a
number of additional implementation-specific challenges. Wi-Fi Multimedia (WMM) is a step
toward full 802.11e compliance for voice and multimedia, but it is not a solution. Because it
is a step, QoS feature evolution must progress towards better and more solid standards-
based QoS capabilities.
WMM refines 802.11 to give statistical preference to certain classes over other classes. It is
fully backward-compatible to legacy non-WMM devices, which function just like WMM best-
effort class devices.
Avaya WLAN IP Telephony Installation and Commissioning
Challenges of integrating voice applications
November 2010 45