User's Manual

Page 6 | How To: Configure the WLAN with QoS alliedtelesis.com

busy medium

SIFS

PIFS

DIFS

AIFS = DIFS + 2 slots

slot time

voice

video

best-effort

background

AIFS = DIFS + 2 slots

AIFS = DIFS + 3 slots

AIFS = DIFS + 7 slots

CW = 0~3 slots

CW = 0~7 slots

CW = 0~15 slots

backoff slots

contention window

minimum wait (AIFS) random backoff (CW)

transmission opportunity

Interframe spaces:

SIFS: short interframe space

PIFS: PCF interframe space - used by HCCA facility

DIFS: DCF interframe space

AISF: arbitration interframe space - used by QoS facility

EIFS: extended interframe space - used to extend the DIFS in case no ACK is received in time

Note, the node delivering data waits for DIFS before sending next frame and, within that interval, the receiver replies

the ACKnowledgement frame to confirm the reception succeeded.

It may happen the receiver delays the replay, due to the signal propagation on long distance link or additional time is

required by receiver for decoding and error recovery; in that case, EIFS is included giving the opportunity to return the

ACK.

EIFS (optional)

Figure 1 – WMM access timing

WMM requires that the clients running multimedia applications assign the

appropriate priority level to the generated data traffic and that network

infrastructure consistently applies QoS policies.

WMM derives four Access Categories from IEEE 802.1D Class of Service (CoS)

as depicted by Table 2.

Within the wireless node, the WMM manages four independent transmit queues,

one per access category, and dispatches to them incoming packets depending on

the assigned Layer 3 differentiated service field (TOS/DSCP). Packet forwarding

will happen basing on transmit opportunities.

Table 2 - WMM Access Categories and IEEE 802.1d tag mapping

Access