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The first published IEEE 802.11 WLAN standard [1] did not have any provision
for supporting QoS over the wireless medium. This problem was subsequently
addressed by the draft IEEE 802.11e standard [2]. In addition, an industry body
known as Wi-Fi Alliance, formed for certification of IEEE 802.11 standards,
published its own interoperability specification called Wireless Multimedia
(WMM) [3], which is based on an earlier draft version of IEEE 802.11e. QoS
support in IEEE 802.11e comes in two flavors:
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Prioritized QoS: This allows classification of WLAN traffic into different
categories based on their priorities. Higher priority traffic is given
preferred access to the WLAN over lower priority traffic. This is achieved by a
channel access function known as Enhanced Distributed Coordination Function
(EDCF), or Enhanced Distributed Channel Access (EDCA). Prioritized QoS only
provides a statistical guarantee. Voice and network control traffic are given
the highest priority followed by video, best-effort data, and background data.
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Parameterized QoS: Support for parameterized QoS is provided using a
centralized Hybrid Coordination Function (HCF) at the WLAN AP. HCF allows the
wireless medium to be alternately used for contention-based access (using EDCF)
and contention-free access (using HCF controlled channel access or HCCA). In
the HCCA scheme, the AP grants opportunities for the WLAN Stations (STAs) to
transmit by polling them based on their traffic requirements.
Admission control can be used with both the above schemes to limit the type and
mix of calls on the AP. Parameterized QoS is optional and is not widely
supported by AP and WLAN client vendors. This paper refers to only the EDCA
scheme of IEEE 802.11e/WMM.
To achieve priority-based packet processing, a VoIP client needs to mark the
VoIP packets that they send out with a pre-configured DSCP value. This will
ensure that VoIP packets are processed with high priority according to the
WMM/IEEE 802.11e specification implemented by the MAC layers of the WLAN client
and the AP.
If the receiver is also a WLAN client associated with an AP, IP routing and
Layer 2 switching will deliver the VoIP packet to the AP. The AP identifies the
packet as a VoIP packet from its DSCP marking. Additional classification based
on IEEE 802.11e Traffic Classification (TCLAS) may be performed by the AP in
its WLAN MAC layer to classify the packet to a particular Traffic Stream (TS)
for an associated WLAN client. The VoIP packet is then scheduled for delivery
to the WLAN client by the AP's MAC layer.
Priority-based packet processing using EDCA mechanisms definitely helps
real-time traffic like VoIP, but this is not adequate to provide hard
guarantees. For example, an EDCA-based WLAN cannot support an infinite number
of VoIP calls even though VoIP traffic can be treated with higher priority.
Based on channel conditions, existing load on the AP, etc. there is a limit on
how many calls can be supported. Once this limit is reached, MAC collisions,
retransmissions, etc. cause delays to all the VoIP calls, thereby degrading the
call quality. Hence, call admission control is needed. Once the AP enforces a
limit on the number of VoIP calls that can be supported as high-priority calls,
all subsequent requests must be rejected in order to maintain the
delay/bandwidth guarantees expected for the already admitted calls. More
details on how prioritization and admission control work in WLANs are available
in IEEE 802.11e/WMM.
Our simulations show that both priority packet processing and call admission
control are essential for providing call capacity and bandwidth guarantees to
VoIP calls. Figure 3 shows the improvement in VoIP capacity with EDCF, over
plain IEEE 802.11 WLANs, for a single AP. The effect of admission control on
the quality of the VoIP calls can be seen in Figure 4, where the quality of
admitted calls does not go down when traffic exceeds the capacity limit. Calls
that are denied admission are treated as best effort, so that admitted calls
continue to get the reserved QoS.
Figure 3: Comparison of performance of VoIP call in DCF vs. EDCF
click image for larger view
Figure 4: Effect of call admission control on the performance of VoIP calls
click image for larger view
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