Voice over Wireless LAN Solution Guide v1.0 December 2005
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1.1.3 Power adjustments and variable capacity The WLAN market has matured to the point that most vendor product solutions have dynamic
mechanisms in place for adjusting channels, adjusting power, and filling coverage holes, all in
response to changes in the Radio Frequency (RF) environment. Although the robustness of the
mechanisms and features varies, all pose the same basic challenge to engineering voice
networks.
Dynamic adjustments work well for guaranteeing minimum coverage and connectivity of devices,
particularly data devices. Voice requires more deterministic engineering, though. Generally, 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. Yet power adjustments affect these parameters, for
better or for worse. If an AP increases power, it provides coverage for a larger area, meaning a
greater call demand per AP. Doubling the power of an AP may quadruple its coverage footprint,
which means up to four times as much call demand as originally engineered. As described in the
previous section, that increased footprint will also have substantial portions of lower data rate
coverage. In addition, the added co-channel interference to other cells using the same channel
will degrade 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.
This is not to imply that auto-RF changes always have a negative impact on voice engineered
networks. Admission control techniques do help with the oversubscription problems related to
increasing cell sizes dynamically. Hole filling in case of AP failure also provides substantial value
to a voice solution. As a general statement, when VoWLAN is driving the engineering of the
network both in scale and capacity, sometimes auto-RF features create more challenges than
they resolve.
1.1.4 Quality of Service (QoS) 802.11 is a shared media technology. Only one device can use the media at a time. The AP
abides by this rule as well. Because collisions are impossible to detect by the transmitting device,
802.11 uses a statistical mechanism to reduce the possibility of collisions when two devices are
ready to transmit at the same time. When the medium becomes available, the mechanism
requires devices to wait a random amount of time before starting transmission. Because of this
simple mechanism, a non-voice device is equally likely to be allowed to transmit as a voice
device. If, for example, a data device does seize the medium, it could send a 1500 byte frame at
the lowest data rate (if it was far away from the AP), thus further delaying voice frames. In
addition, several data devices contending for the medium could 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 as opposed to data devices, supporting voice
applications is a tremendous challenge on 802.11 WLANs.
SpectraLink Voice Priority (SVP) became the initial step towards QoS during the time when there
was no ratified standard for QoS, and evolved into a de facto standard for QoS, even though
SpectraLink handsets are the only terminals that support SVP. SVP does not solve all QoS
problems, but does serve as a model to illustrate the functions that a successful QoS mechanism
should implement.
The recently ratified 802.11e standard will ultimately resolve these QoS issues, but the delays in
the standard have created a number of additional implementation-specific challenges. Wi-Fi
Multimedia (WMM) is a step along the path to full 802.11e compliance for voice and multimedia,
not a solution, and because of this, QoS feature evolution will be marked by a progression
towards better and more solid standards-based QoS capabilities. WMM essentially refines the
existing statistical nature of 802.11 to give statistical preference to certain classes over other
classes. WMM is not a deterministic method of QoS. Because of this, it is full backward
compatible to legacy non-WMM devices, which function just like WMM best-effort class devices.