Finally, let's review Traffic Streams.
There are 2 different concepts: logical layer (admission) and completely new
media access mechanism HCCA (Hybrid Coordinator Channel Access).
Idea behind HCCA is similar to PCF (Point Coordinator Function) from old
802.11 standard. Hybrid Coordinator (HC), which have to be AP, decides who
will talk when.
Technically it is implemented as following:
All time divided into contention based (EDCA TXOP's) and contention free CAP
(Controlled AccessPhase). When HC want to start CAP, it simply starts it
after PIFS interval of idle media. PIFS is shorter then any interval between
EDCA frame exchange sequences, thus HC will preempt any EDCA flow at his
will. Then, HC can either transmit frames, or it may send Contention Free
Poll (CFP) frame to some STA. This frame gives to the STA "polled" TXOP, in
contrast with EDCA TXOP. CFP frame provides TXOP duration.
Now, admission control. For HCCA it is the only mechanism. But, admission
control may be activated also for 2 high priority EDCA categories.
For EDCA it mean the following: one can't use these categories as it was
described in EDCA, instead, TID should be obtained. For EDCA this mechanism
used to control bandwidth allocation. But remember, EDCA is contention based
policy, without any guarantee.
TID's 8-15 dedicates to TS (Traffic Streams). In order to obtain TID, STA
should send request to AP. Corresponded frame called ADDTS, it contain
traffic specification: requested bandwidth, delay, periodicity, frame size
etc. AP may accept or decline this request. It may also come with counter
proposal with different parameters.
TS may be up-stream, down-stream, or bi-direction
If TS accepted by AP, AP provides schedule information. This information
consist of Service Interval (SI) - interval between successive Service
Periods (SP), during which AP serves this TS, and Start Time.
Start time have the following meaning: STA may request exact timing in ADDTS
request. In this case, it will be served at precise times StartTime+n*SI.
This used for power save: STA may identify itself as power down, but wake up
for its stream. Obvious usage is .11 phones.
Here, important QoS related part is:
AP admin decides upon admission control policy. For "legacy stations", that
don't speak TS (stations implementing WME spec), it may provide only 2 low
priority access categories. These STA will be severely impacted in
performance.
STA have to know TGE in order to get real performance. And, STA need to know
how to identify streams (RSVP like).
One additional feature: DLP (Direct Link Protocol). Unrelated to anything
else. Idea is to let 2 stations to talk directly. Link establishment is
through AP, but later they send frames directly, reducing media time 2x. This
is important for applications like streaming of high definition TV, for which
"usual" way when AP forward frames, is unacceptable.
P.S. I did not covered many other stuff which is not related directly to QoS.
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