Adaptive Packet
Reservation Protocols
童曉儒 副教授
國立屏東科技大學 資管系
OUTLINE
• An Overview of IEEE 802.11
• Legacy 802.11：
– DCF ( Distributed coordination function )
– PCF ( Point coordination function )
• QoS Support Mechanisms of 802.11e：
– EDCF ( Enhanced DCF )
– HCF controlled channel access
2
AN OVERVIEW OF IEEE 802.11
• IEEE 802.11 advantages：
• Broadband bandwidth capability
• Low deployment cost
• Internet services access anytime,
anywhere
• Mobility and connectivity
3
• IEEE 802.11 disadvantages：
– Best effort services
– No build in QoS modification of
existing standards required
– Shared medium
4
Table 1. The family of IEEE 802.11 standards
5
Figure 1. The IEEE 802 family and its relation to the OSI model
6
Figure 2. PHY components
• PLCP ( Physical Layer Convergence Procedure )
• PMD ( Physical Medium Dependent )
7
Types of Networks
• Independent BSS (IBSS)
– IBSSs are sometimes referred to as ad hoc BSSs
or ad hoc networks.
8
• Infrastructure BSS
– An infrastructure BSS is defined by the distance
from the access point.
– Access points in infrastructure networks are in a
position to assist with stations attempting to save
power.
9
MAC Access Modes
• Access to the wireless medium is controlled by
coordination functions.
• Ethernet-like CSMA/CA access is provided by the
distributed coordination function (DCF).
• If contention-free service is required, it can be
provided by the point coordination function (PCF),
which is built on top of the DCF.
• Contention-free services are provided only in
infrastructure networks.
10
Figure 3. MAC coordination functions
11
Interframe Spacing
• Varying interframe spacings create different
priority levels for different types of traffic.
• The high-priority traffic doesn't have to wait
as long after the medium has become idle.
• To assist with interoperability between
different data rates, the interframe space is a
fixed amount of time, independent of the
transmission speed.
12
• Short interframe space (SIFS)
– The SIFS is used for the highest-priority
transmissions, such as RTS/CTS frames and
positive acknowledgments.
• PCF interframe space (PIFS)
– The PIFS is used by the PCF during contentionfree operation.
– Stations with data to transmit in the contentionfree period can transmit after the PIFS has
elapsed and preempt any contention-based traffic.
13
• DCF interframe space (DIFS)
– The DIFS is the minimum medium idle time for
contention-based services.
– Stations may have immediate access to the
medium if it has been free for a period longer than
the DIFS.
• Extended interframe space (EIFS)
– It is not a fixed interval.
– It is used only when there is an error in frame
transmission.
14
Figure 4. Interframe spacing relationships
15
Figure 5. Interframe spacing and priority
16
Table 2. IFS and priority
IFS
802.11a
802.11b/g
purpose
priority
1
SIFS
16μs
10μs
RTS、CTS、
ACK
PIFS
25μs
30μs
PCF
2
DIFS
34μs
50μs
DCF
3
EIFS
43μs
70μs
retransmission
4
17
Distributed Coordination
Function (DCF)
• The DCF is the basis of the standard
CSMA/CA access mechanism.
• Like Ethernet, it first checks to see that the
radio link is clear before transmitting.
• To avoid collisions, stations use a random
backoff after each frame, with the first
transmitter seizing the channel.
18
• In some circumstances, the DCF may use the
CTS/RTS clearing technique to further reduce
the possibility of collisions.
• Most traffic uses the DCF, which provides a
standard Ethernet-like contention-based
service.
• The DCF allows multiple independent
stations to interact without central control, and
thus may be used in either IBSS networks or
in infrastructure networks.
19
Figure 6. RTS/CTS clearing
20
Backoff with the DCF
• A period called the contention window or backoff
window follows the DIFS.
• This window is divided into slots.
• Stations pick a random slot and wait for that slot
before attempting to access the medium; all slots are
equally likely selections.
• When several stations are attempting to transmit, the
station that picks the first slot (the station with the
lowest random number) wins.
21
Figure 7. Diagrammatic representation of DCF access
method
22
• As in Ethernet, the backoff time is selected
from a larger range each time a transmission
fails.
• Contention window sizes are always 1 less
than a power of 2 (e.g., 31, 63, 127,255).
• Each time the retry counter increases, the
contention window moves to the next greatest
power of two.
23
• When the contention window reaches its
maximum size, it remains there until it can be
reset.
• The contention window is reset to its
minimum size when frames are transmitted
successfully, or the associated retry counter
is reached, and the frame is discarded.
24
Figure 8. DSSS contention window size
25
Table 3. CW range
Initial attempt
1st retransmission
2st retransmission
3st retransmission
4st retransmission
5st retransmission
Above 5st retransmission
802.11a
802.11b/g
0 ~ 15
0 ~ 31
0 ~ 63
0 ~ 127
0 ~ 255
0 ~ 511
0 ~ 1023
0 ~ 31
0 ~ 63
0 ~ 127
0 ~ 255
0 ~ 511
0 ~ 1023
0 ~ 1023
26
Figure 9. Timing of the 802.11 DCF
27
Point Coordination
Function (PCF)
• To support applications that require near realtime service, the 802.11 standard includes a
second coordination function to provide a
different way of accessing the wireless
medium.
• The PCF allows an 802.11 network to provide
an enforced "fair" access to the medium.
28
• Point coordination provides contention-free
services.
• Special stations called point coordinators
are used to ensure that the medium is
provided without contention.
• Point coordinators reside in access points, so
the PCF is restricted to infrastructure
networks.
29
• To gain priority over standard contentionbased services, the PCF allows stations to
transmit frames after a shorter interval.
• The PCF is not widely implemented.
• The PCF is an optional part of the 802.11
specification; products are not required to
implement it.
30
• Contention-free service is not provided fulltime.
• Periods of contention-free service arbitrated
by the point coordinator alternate with the
standard DCF-based service.
• When the PCF is used, time on the medium is
divided into the contention-free period (CFP)
and the contention period (CP).
31
Figure 10. Diagrammatic representation of PCF
access method
32
• The contention period must be long enough
for the transfer of at least one maximum-size
frame and its associated acknowledgment.
• Alternating periods of contention-free service
and contention-based service repeat at
regular intervals, which are called the
contention-free repetition interval.
33
Reserving the medium during
the contention-free period
• At the beginning of the contention-free period,
the access point transmits a Beacon frame.
• One component of the beacon
announcement is the maximum duration of
the contention free period, CFPMaxDuration.
• All stations receiving the Beacon set the NAV
to the maximum duration to lock out DCFbased access to the wireless medium.
34
The polling list
• After the access point has gained control of
the wireless medium, it polls any associated
stations on a polling list for data
transmissions.
•
• During the contention-free period, stations
may transmit only if the access point solicits
the transmission with a polling frame.
35
• Contention-free polling frames are often
abbreviated CF-Poll.
• Each CF-Poll is a license to transmit one
frame.
• Multiple frames can be transmitted only if the
access point sends multiple poll requests.
36
• Generally, all transmissions during the
contention-free period are separated by only
the short interframe space (SIFS).
• To ensure that the point coordinator retains
control of the medium, it may send to the next
station on its polling list if no response is
received after an elapsed PCF interframe
space (PIFS).
37
Transmissions from the
Access Point
• Time in the contention-free period is precious,
so acknowledgments, polling, and data
transfer may be combined to improve
efficiency.
38
• Several different frame types can be used in
the contention free period：
–
–
–
–
–
–
–
–
–
–
Data
CF-Ack
CF-Poll
Data+CF-Ack
Data+CF-Poll
CF-ACK+CF-Poll
Data+CF-ACK+CF-Poll
CF-End
CF-End+CF-Ack
Any Management
39
Contention-Free Period
Duration
• The minimum length of the contention period
is the time required to transmit and
acknowledge one maximum-size frame.
• It is possible for contention-based service to
overrun the end of the contention period,
however.
• When contention-based service runs past the
expected beginning of the contention-free
period, the contention-free period is
foreshortened, as in Figure 11.
40
Figure 11. CFP foreshortening
41
Table 2. Common tunable parameters in 802.11
42
QoS Support Mechanisms
of 802.11e
• In order to support QoS approaches in 802.11,
TGe has defined a new mechanism called
HCF( Hybrid Coordination Function ).
• This mechanism is backwardly compatible with
legacy DCF and PCF.
• It has both polling based and contention based
channel access mechanisms in a single channel
access protocol.
43
• HCF ( Hybrid Coordination Function )
– EDCF ( Enhanced DCF )
– HCF controlled channel access
• In HCF there may still be two phases of
operations within the superframes, the CFP
and CP, which alternate over time.
• The EDCF is used in the CP only, while the
HCF controlled channel access method is
use in both phases, which makes this new
coordination function hybrid.
44
EDCF ( Enhanced DCF )
• EDCF is a contention-based channel access
scheme.
• EDCF provides differentiated service, distributed
access to the wireless medium for 8 delivery
priorities.
• EDCF access channel on each ESTA uses at
most 8 prioritized output queues, one for each
delivery priority, called Traffic Categories (TCs).
45
• The CWmin and CWmax parameters can be
set differently for different traffic categories,
such as, a high priority TC with small values
of CWmin and CWmax.
• Instead of using a DIFS, as a minimum
specified idle duration time as defined in DCF,
a new kind of interframe space called
Arbitration Interframe Space (AIFS) is used.
46
• A big difference from the legacy DCF is that
After any unsuccessful transmission attempt
a new contention window is calculated with
the help of the persistence factor PF[TCi] and
another uniformly distributed backoff counter
out of this new, enlarged CW is drawn, to
reduce the probability of a new collision.
47
• Whereas in legacy 802.11, CW is
always doubled after any unsuccessful
transmission (i.e., equivalent to a PF=2).
EDCF, uses the PF to increase the CW
different for each TCi：
48
Figure 12. Diagrammatic representation of EDCF
access method
49
• A single station may implement up to eight
transmission queues realized as virtual
stations inside a station, with QoS parameters
that determine their priorities.
• If the counters of two or more parallel TCs in
a single station reach zero at the same time,
a scheduler inside the station avoids the
virtual collision.
50
Figure 13. Virtual backoff of eight traffic categories
51
HCF
controlled channel access
• The HCF controlled channel access uses a
hybrid coordinator (HC) which manages the
allocation of the wireless medium data
transfer bandwidth.
• The HC may allocate TXOPs to itself to
initiate MSDU Deliveries whenever it wants,
however, only after detecting the channel as
being idle for PIFS, which is shorter than
DIFS.
52
• During CP, each TXOP begins either when
the medium is determined to be available
under the EDCF rules, i.e., after AIFS plus
backoff time, or when the station receives a
special poll frame, the QoS CF-Poll, from the
HC.
• The QoS CF-Poll from the HC can be sent
after a PIFS idle period without any backoff.
• Therefore the HC can issue polled TXOPs in
the CP using its prioritized medium access.
53
• During the CFP, the starting time and
maximum duration of each TXOP is specified
by the HC, again using the QoS CF-Poll
frames.
• Stations will not attempt to get medium
access on its own during the CFP, so only the
HC can grant TXOPs by sending QoS CFPoll frames.
• The CFP ends after the time announced in
the beacon frame or by a CF-End frame from
the HC.
54
Figure 14. A typical 802.11e superframe.
The concept relies on TXOPs.
Polled-TXOPs may be located in CP and CFP
55
LIST OF ABBREVIATIONS
•
•
•
•
•
•
•
•
•
•
ACK
AIFS
AP
CA
CDF
CFP
CF-Poll
CF-End
CP
CSMA
Acknowledgement
Arbitration Inter Frame Space (802.11e)
Access Point
Collision Avoidance
Complementary Cumulative Distribution Function
Contention Free Period
Contention Free – Poll
Contention Free – End
Contention Period
Carrier Sense Multiple Access
56
•
•
•
•
•
•
•
•
•
•
•
•
•
CW
CWmax
CWmin
DCF
EDCF
HC
HCF
IEEE
ISM
LRE
MAC
MSDU
NAV
Contention Window
Contention Window Maximum
Contention Window Minimum
Distributed Coordination Function
Enhanced DCF (802.11e)
Hybrid Coordinator (802.11e)
Hybrid Coordination Function (802.11e)
Institute of Electrical and Electronics Engineers
Industrial, Science, Medical
Limited Relative Error
Medium Access Control
MAC Service Data Unit
Network Allocation Vector
57
•
•
•
•
•
•
•
•
•
•
•
•
•
PC
Point Coordinator
PCF
Point Coordination Function
PF
Persistence Factor (802.11e)
PHY mode
Physical Layer mode, coding and modulation scheme
PIFS
PCF Inter Frame Space
(Q)BSS (QoS-supporting) Basic Service Set (802.11e)
QoS
Quality of Service
RTS/CTS Request to Send/Clear to Send
SIFS
Short Inter Frame Space
TBTT
Target Beacon Transmission Time
TC
Traffic Category (802.11e)
TXOP
Transmission Opportunity (802.11e)
WLAN
Wireless Local Area Network
58
REFERENCES
• M. Gast, 802.11 Wireless Networks: the Definitive Guide,
O’Reilly, 2002.
• H. Zhu, M. Li, I. Chlamtac, and B. Prabhakaran, “A survey of
quality of service in IEEE 802.11 networks,” IEEE Trans.
Wireless Commun., vol. 11, no. 4, Aug. 2004, pp. 6 – 14.
• F. Mico, P. Cuenca, and L. Orozco-Barbosa, “QoS in IEEE
802.11 wireless LAN: current research activities,” IEEE CCECE
’04, vol. 1, 2-5, May 2004, pp. 447 – 452, vol.1.
• A. Grilo and M. Nunes, "Performance evaluation of IEEE
802.11e," in Proc. IEEE PIMRC'02, Sept. 2002, pp. 511-517.
59
• P. Garg, R. Doshi, R. Greene, M. Baker, M. Malek, and X.
Cheng. “Using IEEE 802.11e MAC for QoS over Wireless,”
Technical report, Computer Science Dept., Standford University,
2002.
• S. Mangold, S. Choi, P. May, O. Klein, G. Hiertz, L. Stibor. "IEEE
802.11e Wireless LAN for Quality of Service ," In Proceedings of
the European Wireless, volume 1, pages 32-39, Florence, Italy,
February 2002.
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END