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Page 5 | How To: Configure the WLAN with QoS alliedtelesis.com
How Wi-Fi Multimedia works
In the IEEE 802.11 standard, the wireless nodes access the medium using the
“listen-before-talk” algorithm CSMA/CA-based
1
Distributed Coordination Function
(DCF). The wireless node transmits a frame once its checks indicate that the
medium is free for a period of distributed inter-frame space (DIFS) and a random
back-off time has elapsed. Otherwise, it defers the frame delivery. Note that
contention reduces the available bandwidth, because nodes must back off before
retrying to communicate.
DCF offers an efficient mechanism for transferring bursty traffic, but all nodes use
the same access method, consequently a real time data source will face
continuous contention with a large data transfer happening at the same time.
WMM provides two features that resolve contention and enable QoS:
Enhanced Distributed Channel Access (EDCA)
Hybrid Controlled Channel Access (HCCA)
2
EDCA gives precedence to higher-priority traffic by means of a shorter arbitration
interframe space (AIFS) and contention windows (CW). The back off time is the
sum of predefined AIFS and a random value from 0 to CW, which is doubled at
each collision till the limit (CW
max
) is reached. Figure 1depicts the timing. In
addition, transmit opportunity (TXOP) bounds the time allocated to the node for
data delivery.
HCCA is a centralized access control facility coordinated by the access point,
which acts as point coordinator (PC). The PC collects the status of registered
stations (via polling) and provides them the permission for channel occupancy
and assigns, for example, the TXOP, the transmit start time and duration.
1 Carrier Sense Multiple Access with Collision Avoidance.
2 HCCA facility is not delivered by current software release.