2 O UTLINE 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
3 A N O VERVIEW OF IEEE 802.11 IEEE 802.11 advantages ： Broadband bandwidth capability Low deployment cost Internet services access anytime, anywhere Mobility and connectivity
4 IEEE 802.11 disadvantages ： –Best effort services –No build in QoS modification of existing standards required –Shared medium
8 Types of Networks Independent BSS (IBSS) –IBSSs are sometimes referred to as ad hoc BSSs or ad hoc networks.
9 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.
10 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.
12 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.
13 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 contention- free operation. –Stations with data to transmit in the contention- free period can transmit after the PIFS has elapsed and preempt any contention-based traffic.
14 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.
18 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.
19 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.
21 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.
22 Figure 7. Diagrammatic representation of DCF access method
23 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.
24 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.
28 Point Coordination Function (PCF) To support applications that require near real- time 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.
29 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.
30 To gain priority over standard contention- based 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.
31 Contention-free service is not provided full- time. 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).
32 Figure 10. Diagrammatic representation of PCF access method
33 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.
34 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 DCF- based access to the wireless medium.
35 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.
36 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.
37 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).
38 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.
39 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
40 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.
42 Table 2. Common tunable parameters in 802.11
43 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.
44 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.
45 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).
46 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.
47 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.
48 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 ：
49 Figure 12. Diagrammatic representation of EDCF access method
50 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.
51 Figure 13. Virtual backoff of eight traffic categories
52 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.
53 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.
54 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 CF- Poll frames. The CFP ends after the time announced in the beacon frame or by a CF-End frame from the HC.
55 Figure 14. A typical 802.11e superframe. The concept relies on TXOPs. Polled-TXOPs may be located in CP and CFP
56 LIST OF ABBREVIATIONS ACK Acknowledgement AIFS Arbitration Inter Frame Space (802.11e) AP Access Point CA Collision Avoidance CDF Complementary Cumulative Distribution Function CFP Contention Free Period CF-Poll Contention Free – Poll CF-End Contention Free – End CP Contention Period CSMA Carrier Sense Multiple Access
57 CW Contention Window CWmax Contention Window Maximum CWmin Contention Window Minimum DCF Distributed Coordination Function EDCF Enhanced DCF (802.11e) HCHybrid Coordinator (802.11e) HCF Hybrid Coordination Function (802.11e) IEEEInstitute of Electrical and Electronics Engineers ISM Industrial, Science, Medical LRE Limited Relative Error MAC Medium Access Control MSDU MAC Service Data Unit NAV Network Allocation Vector
58 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
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