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.All stations that hear the RTSdefer access to the medium until the NAV elapses.RTS frames are not necessarily heard by every station in the network.Therefore, therecipient of the intended transmission responds with a CTS that includes a shorter NAV.This NAV prevents other stations from accessing the medium until the transmissioncompletes.After the sequence completes, the medium can be used by any station afterdistributed interframe space (DIFS), which is depicted by the contention windowbeginning at the right side of the figure. RTS/CTS exchanges may be useful in crowded areas with multiple overlappingnetworks.Every station on the same physical channel receives the NAV and defersaccess appropriately, even if the stations are configured to be on different networks.3.2.2 Interframe SpacingAs with traditional Ethernet, the interframe spacing plays a large role in coordinatingaccess to the transmission medium.802.11 uses four different interframe spaces.Threeare used to determine medium access; the relationship between them is shown in Figure3-6.Figure 3-6.Interframe spacing relationshipsWe've already seen that as part of the collision avoidance built into the 802.11 MAC,stations delay transmission until the medium becomes idle.Varying interframe spacingscreate different priority levels for different types of traffic.The logic behind this issimple: high-priority traffic doesn't have to wait as long after the medium has becomeidle.Therefore, if there is any high-priority traffic waiting, it grabs the network beforelow-priority frames have a chance to try.To assist with interoperability between differentdata rates, the interframe space is a fixed amount of time, independent of the transmissionspeed.(This is only one of the many problems caused by having different physical layersuse the same radio resources, which are different modulation techniques.) Differentphysical layers, however, can specify different interframe space times.Short interframe space (SIFS)The SIFS is used for the highest-priority transmissions, such as RTS/CTS framesand positive acknowledgments.High-priority transmissions can begin once theSIFS has elapsed.Once these high-priority transmissions begin, the mediumbecomes busy, so frames transmitted after the SIFS has elapsed have priority overframes that can be transmitted only after longer intervals.PCF interframe space (PIFS)The PIFS, sometimes erroneously called the priority interframe space, is used bythe PCF during contention-free operation.Stations with data to transmit in thecontention-free period can transmit after the PIFS has elapsed and preempt anycontention-based traffic. 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 periodlonger than the DIFS.Extended interframe space (EIFS)The EIFS is not illustrated in Figure 3-6 because it is not a fixed interval.It isused only when there is an error in frame transmission.3.2.2.1 Interframe spacing and priorityAtomic operations start like regular transmissions: they must wait for the DIFS beforethey can begin.However, the second and any subsequent steps in an atomic operationtake place using the SIFS, rather than during the DIFS.This means that the second (andsubsequent) parts of an atomic operation will grab the medium before another type offrame can be transmitted.By using the SIFS and the NAV, stations can seize the mediumfor as long as necessary.In Figure 3-5, for example, the short interframe space is used between the different unitsof the atomic exchange.After the sender gains access to the medium, the receiver replieswith a CTS after the SIFS.Any stations that might attempt to access the medium at theconclusion of the RTS would wait for one DIFS interval.Partway through the DIFSinterval, though, the SIFS interval elapses, and the CTS is transmitted.3.3 Contention-Based Access Using the DCFMost traffic uses the DCF, which provides a standard Ethernet-like contention-basedservice.The DCF allows multiple independent stations to interact without central control,and thus may be used in either IBSS networks or in infrastructure networks.Before attempting to transmit, each station checks whether the medium is idle.If themedium is not idle, stations defer to each other and employ an orderly exponentialbackoff algorithm to avoid collisions.In distilling the 802.11 MAC rules, there is a basic set of rules that are always used, andadditional rules may be applied depending on the circumstances.Two basic rules apply toall transmissions using the DCF:1.If the medium has been idle for longer than the DIFS, transmission can beginimmediately.Carrier sensing is performed using both a physical medium-dependent method and the virtual (NAV) method. a.If the previous frame was received without errors, the medium must befree for at least the DIFS.b.If the previous transmission contained errors, the medium must be free forthe amount of the EIFS.2.If the medium is busy, the station must wait for the channel to become idle.802.11 refers to the wait as access deferral.If access is deferred, the station waitsfor the medium to become idle for the DIFS and prepares for the exponentialbackoff procedure.Additional rules may apply in certain situations.Many of these rules depend on theparticular situation "on the wire" and are specific to the results of previous transmissions.1.Error recovery is the responsibility of the station sending a frame.Senders expectacknowledgments for each transmitted frame and are responsible for retrying thetransmission until it is successful.a.Positive acknowledgments are the only indication of success.Atomicexchanges must complete in their entirety to be successful [ Pobierz całość w formacie PDF ]