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.No limit is placed on the number offragments, but the total frame length must be shorter than any constraint placed on theexchange by the PHY.Fragmentation is controlled by the fragmentation threshold parameter in the MAC.Mostnetwork card drivers allow you to configure this parameter.Any frames larger than thefragmentation threshold are fragmented in an implementation-dependent way.Networkadministrators can change the fragmentation threshold to tune network behavior.Higherfragmentation thresholds mean that frames are delivered with less overhead, but the costto a lost or damaged frame is much higher because more data must be discarded andretransmitted.Low fragmentation thresholds have much higher overhead, but they offerincreased robustness in the face of hostile conditions.3.7.2.3 RTS/CTSTo guarantee reservation of the medium and uninterrupted data transmission, a stationcan use the RTS/CTS exchange.Figure 3-17 shows this process.The RTS/CTS exchangeacts exactly like the initial exchange in the fragmentation case, except that the RTS framedoes not carry data.The NAV in the RTS allows the CTS to complete, and the CTS isused to reserve access for the data frame.Figure 3-17.RTS/CTS lockoutRTS/CTS can be used for all frame exchanges, none of them, or something in between.Like fragmentation, RTS/CTS behavior is controlled by a threshold set in the driversoftware.Frames larger than the threshold are preceded by an RTS/CTS exchange toclear the medium, while smaller frames are simply transmitted.3.7.2.4 RTS/CTS with fragmentation In practice, the RTS/CTS exchange is often combined with fragmentation (Figure 3-18).Fragmented frames are usually quite long and thus benefit from the use of the RTS/CTSprocedure to ensure exclusive access to the medium, free from contention from hiddennodes.Some vendors set the default fragmentation threshold to be identical to the defaultRTS/CTS threshold.Figure 3-18.RTS/CTS with fragmentation3.7.3 Power-Saving SequencesThe most power-hungry components in RF systems are the amplifiers used to boost asignal immediately prior to transmission and to boost the received signal to an intelligiblelevel immediately after its reception.802.11 stations can maximize battery life byshutting down the radio transceiver and sleeping periodically.During sleeping periods,access points buffer any unicast frames for sleeping stations.These frames are announcedby subsequent Beacon frames.To retrieve buffered frames, newly awakened stations usePS-Poll frames.3.7.3.1 Immediate responseAccess points can respond immediately to the PS-Poll.After a short interframe space, anaccess point may transmit the frame.Figure 3-19 shows an implied NAV as a result ofthe PS-Poll frame.The PS-Poll frame contains an Association ID in the Duration/ID fieldso that the access point can determine which frames were buffered for the mobile station.However, the MAC specification requires all stations receiving a PS-Poll to update theNAV with an implied value equal to a short interframe space and one ACK.Although theNAV is too short for the data frame, the access point acquires that the medium and allstations defer access for the entire data frame.At the conclusion of the data frame, theNAV is updated to reflect the value in the header of the data frame.Figure 3-19.Immediate PS-Poll response If the buffered frame is large, it may require fragmentation.Figure 3-20 illustrates animmediate PS-Poll response requiring fragmentation.Like all other stations, access pointstypically have a configurable fragmentation threshold.Figure 3-20.Immediate PS-Poll response with fragmentation3.7.3.2 Deferred responseInstead of an immediate response, access points can also respond with a simpleacknowledgment.This is called a deferred response because the access pointacknowledges the request for the buffered frame but does not act on it immediately.Astation requesting a frame with a PS-Poll must stay awake until it is delivered.Undercontention-based service, however, the access point can deliver a frame at any point.Astation cannot return to a low-power mode until it receives a Beacon frame in which itsbit in the traffic indication map (TIM) is clear.Figure 3-21 illustrates this process.In this figure, the station has recently changed from alow-power mode to an active mode, and it notes that the access point has buffered framesfor it.It transmits a PS-Poll to the access point to retrieve the buffered frames.However,the access point may choose to defer its response by transmitting only an ACK.At thispoint, the access point has acknowledged the station's request for buffered frames andpromised to deliver them at some point in the future.The station must wait in activemode, perhaps through several atomic frame exchanges, before the access point deliversthe data.A buffered frame may be subject to fragmentation, although Figure 3-21 doesnot illustrate this case.Figure 3-21.Deferred PS-Poll response example After receiving a data frame, the station must remain awake until the next Beacon istransmitted.Beacon frames only note whether frames are buffered for a station and haveno way of indicating the number of frames.Once the station receives a Beacon frameindicating that no more traffic is buffered, it can conclude that it has received the lastbuffered frame and return to a low-power mode. Chapter 4.802.11 Framing in DetailChapter 3 presented the basic frame structure and the fields that comprise it, but it did notgo into detail about the different frame types.Ethernet framing is a simple matter: add apreamble, some addressing information, and tack on a frame check at the end.802
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