Filter
Top Products
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
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
Category
Priority
Description
IEEE 802.1D
Traffic Types
Voice
Highest
Low-latency and toll voice quality
6, 7
Video
Highest -1
SDTV and HDTV streaming
4, 5
Best-effort
Lower + 1
Traffic less sensitive of latency, but
affected by delays
0, 3
Background
Lower
Low priority traffic without throughput
constraint (i.e. file transfer)
1, 2