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

48-6 Industrial Communication Systems
The PCF uses a virtual carrier-sense mechanism aided by an access priority mechanism. The PCF
distributes information using Beacon management frames to gain control of the medium by setting the
NAV in the stations. In addition, all frame transmissions under the PCF may use a Point Interframe
Space (PIFS) that is smaller than the DIFS for frames transmitted via the DCF. This means that pointcoordinated
traffic has priority in accessing the medium over stations in overlapping networks operating
in the DCF mode. Such an access priority may be utilized to realize a contention-free (CF) access
method. The PC controls the frame transmissions of the stations, so as to eliminate contention for a
limited period of time. Although the ability of providing support for collision-free transmissions would
be very beneficial to industrial communication, especially when handling time-constrained traffic, the
PCF is implemented only in very few hardware devices, as it is not included in the Wi-Fi Alliance’s
[Wi09] interoperability standard.
48.4.3 Enhanced Distributed Channel Access
The EDCA is an extension of the DCF and defines eight different priority levels. The priorities are
mapped into four access categories (AC) in compliance with the IEEE 802.1D standard [IEE04], thereby
providing differentiated and distributed channel access. Within a wireless station with QoS support
(QSTA) and the Access Point (AP) with QoS support (QAP), every AC is represented as an independent
transmission queue. Every single queue contends for the medium separately and has different parameter
sets for accessing the channel.
The parameter set consists of the arbitration interframe space number (AIFSN), the two bounding
values for the contention window CWmin[AC] and CWmax[AC], and the maximum allowed transmission
time for one station TXOP limit . The AIFSN depends on the AC and is used to derive the arbitration
interframe space (AIFS) with Equation 48.1, where aSlotTime and SIFS are determined by the used PHY
layer (e.g., 9 and 16.μs for 802.11g).
AIFS[ AC] = AIFSN[AC]*
aSlotTime + SIFS
(48.1)
Similar to the DCF, a station always has to wait an AIFS before it can contend for the medium.
The backoff procedure is also similar to the DCF, except for different values for CWmin and CWmax.
The contention for the medium with different priorities is shown in Figure 48.2. As a result, the EDCA
parameter sets of the four ACs cause different waiting times, i.e., a decreased waiting time for high
priority frames and a longer waiting time for low priority frames. Therefore, the probability of the successful
data transmission of high priority traffic is increased.
AIFS [BK_AC]
Contention window
QSTA 1
Data
AIFS [BE_AC]
AIFS
[VI_AC]
AIFS
[VO_AC]
PIFS
SIFS
Contention
window
Contention window
Contention window
802.11g
aSlotTime =
SIFS =
PIFS =
e.g. AIFS [VO_AC] =
9 μs
16 μs
25 μs
34 μs
QSTA 2
SIFS
Ack
Slot time
Defer Access
QSTA2 starts to transmit
FIGURE 48.2
Channel access with different priorities.
© 2011 by Taylor and Francis Group, LLC