wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Wireless Local Area Networks 48-7
48.4.4 HCF Controlled Channel Access
Contention-free medium access in 802.11e [IEE05e] is realized with HCF Controlled Channel Access
(HCCA). HCCA is the replacement of the previously defined PCF and used for controlled channel
access. The time between two consecutive beacon frames is called a superframe. It is divided into an
optional contention-free period (CFP) and a contention period (CP). During the CFP, the hybrid coordinator
(HC) controls the access to the channel by polling its associated stations with QoS requirements.
However, the HC is also allowed to initiate a controlled access phase (CAP) during the CP after detecting
that the channel is idle for a time interval longer than a PIFS (PIFS = SIFS + aSlotTime), and whenever
there is a need to transfer time-critical data. An example for a superframe is shown in Figure 48.3.
A polled station is granted a transmission opportunity (TXOP) allowing the station to occupy the channel
for a time period equal to the TXOP value. This concept elevates HCCA with greater flexibility than its
predecessor, although the time for generating CAPs is limited to a maximum duration in order to leave
space for stations operating under EDCA.
A very important concept for QoS support in 802.11e is the admission control. Whenever a station
wants to associate with a certain BSS, it has to specify its requirements during a TSPEC negotiation
as was introduced in the integrated services architecture [IET97]. The negotiation is done with a traffic
specification element (TSPEC) that may contain parameters related to the time-critical traffic flow,
such as the mean data rate or the delay bound: They are exchanged between the QAP and the QSTAs to
establish a traffic stream (TS). The admission or the rejection of the new TS depends on the adherence
of its requirements. Every station can have up to eight different TSs with different QoS requirements.
After the negotiation, the polling schedule for the stations is calculated by the HC based on the previously
defined requirements. The ability of the HC to perform the scheduling depends on several mandatory
parameters, such as the Nominal MSDU Size, the Mean Data Rate, the delay bound, etc. Although
the applied algorithm for scheduling and admission control has been completely left open to the implementer,
i.e., it can be adapted to the needs of specific applications and kinds of traffic flows, a sample
scheduler can be found in Annex K of the 802.11e [IEE05e].
48.4.5 Direct Link Protocol and Block ACK
In order to decrease the protocol overhead, the 802.11e [IEE05e] also defines a way to directly communicate
with other clients in infrastructure networks and to acknowledge more than one frame with a block
acknowledgment. The direct link setup (DLS) enables two stations belonging to the same BSS to communicate
directly to each other without directing the frames through the AP. Before the transmission starts,
the direct link has to be set up via a request to the AP that is forwarded to the intended other station.
Contention free period (CFP)
IEEE 802.11e superframe
Contention period (CP)
Controlled access phase (CAP)
QAP
QSTA 1...n
B
C C
eacon
FPoII1
C
C
CF FPoII3
A A FĒnd
FPoII4
CF
B
Poll
EDCA
A Poll A A EDCA eacon
CK CK Data to
CK CK CK Data
STA 9
STA 7
Data
STA
1
2
+
Data
Data
STA
2
+
ACK
Data
STA
3
A
CK
Data
STA
4
5
+
Data
Data
STA
5
+
ACK
EDCA
Data
STA 8
A
CK
t
PIFS SIFS SIFS SIFS SIFS AIFS[AC] SIFS PIFS SIFS SIFS SIFS
+Backoff
FIGURE 48.3
Example of an 802.11e superframe.
© 2011 by Taylor and Francis Group, LLC