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.Therefore, this bit is set to 0.4.2.2 Request to Send (RTS)RTS frames are used to gain control of the medium for the transmission of "large"frames, in which "large" is defined by the RTS threshold in the network card driver.Access to the medium can be reserved only for unicast frames; broadcast and multicastframes are simply transmitted.The format of the RTS frame is shown in Figure 4-13.Like all control frames, the RTS frame is all header.No data is transmitted in the body,and the FCS immediately follows the header.Figure 4-13.RTS frameFour fields make up the MAC header of an RTS:Frame ControlThere is nothing special about the Frame Control field.The frame subtype is setto 1011 to indicate an RTS frame, but otherwise, it has all the same fields as othercontrol frames.(The most significant bits in the 802.11 specification come at theend of fields, so bit 7 is the most significant bit in the subtype field.)Duration An RTS frame attempts to reserve the medium for an entire frame exchange, sothe sender of an RTS frame calculates the time needed for the frame exchangesequence after the RTS frame ends.The entire exchange, which is depicted inFigure 4-14, requires three SIFS periods, the duration of one CTS, the final ACK,plus the time needed to transmit the frame or first fragment.(Fragmentation burstsuse subsequent fragments to update the Duration field.) The number ofmicroseconds required for the transmission is calculated and placed in theDuration field.If the result is fractional, it is rounded up to the next microsecond.Figure 4-14.Duration field in RTS frameAddress 1: Receiver AddressThe address of the station that is the intended recipient of the large frame.Address 2: Transmitter AddressThe address of the sender of the RTS frame.4.2.3 Clear to Send (CTS)The CTS frame answers the RTS frame.Its format is shown in Figure 4-15.Figure 4-15.CTS frameThree fields make up the MAC header of a CTS frame:Frame ControlThe frame subtype is set to 1100 to indicate a CTS frame. DurationThe sender of a CTS frame uses the duration from the RTS frame as the basis forits duration calculation.RTS frames reserve the medium for the entire RTS-CTS-frame-ACK exchange.By the time the CTS frame is transmitted, though, only thepending frame or fragment and its acknowledgment remain.The sender of a CTSframe subtracts the time required for the CTS frame and the short interframespace that preceded the CTS from the duration in the RTS frame, and places theresult of that calculation in the Duration field.Figure 4-16 illustrates therelationship between the CTS duration and the RTS duration.Figure 4-16.CTS durationAddress 1: Receiver AddressThe receiver of a CTS frame is the transmitter of the previous RTS frame, so theMAC copies the transmitter address of the RTS frame into the receiver address ofthe CTS frame.4.2.4 Acknowledgment (ACK)ACK frames are used to send the positive acknowledgments required by the MAC andare used with any data transmission, including plain transmissions; frames preceded byan RTS/CTS handshake; and fragmented frames (see Figure 4-17).Figure 4-17.ACK frameThree fields make up the MAC header of an ACK frame:Frame Control The frame subtype is set to 1101 to indicate an ACK frame.DurationThe duration may be set in one of two ways, depending on the position of theACK within the frame exchange.ACKs for complete data frames and finalfragments in a fragment burst set the duration to 0.The data sender indicates theend of a data transmission by setting the More Fragments bit in the Frame Controlheader to 0.If the More Fragments bit is 0, the transmission is complete, and thereis no need to extend control over the radio channel for additional transmissions.Thus, the duration is set to 0.If the More Fragments bit is 1, a fragment burst is in progress.The Duration fieldis used like the Duration field in the CTS frame.The time required to transmit theACK and its short interframe space is subtracted from the duration in the mostrecent fragment (Figure 4-18).The duration calculation in nonfinal ACK framesis similar to the CTS duration calculation.In fact, the 802.11 specification refersto the duration setting in the ACK frames as a virtual CTS.Figure 4-18.Duration in non-final ACK framesAddress 1: Receiver AddressThe receiver address is copied from the transmitter of the frame beingacknowledged.Technically, it is copied from the Address 2 field of the framebeing acknowledged.Acknowledgments are transmitted in response to directeddata frames, management frames, and PS-Poll frames.4.2.5 Power-Save Poll (PS-Poll)When a mobile station wakes from a power-saving mode, it transmits a PS-Poll frame tothe access point to retrieve any frames buffered while it was in power-saving mode.Theformat of the PS-Poll frame is shown in Figure 4-19.Figure 4-19.PS-Poll frame Four fields make up the MAC header of a PS-Poll frame:Frame ControlThe frame subtype is set to 1010 to indicate a PS-Poll frame.Association ID (AID)Instead of a Duration field, the PS-Poll frame uses the third and fourth bytes in theMAC header for the association ID.This is a numeric value assigned by theaccess point to identify the association.Including this ID in the frame allows theaccess point to find any frames buffered for the now-awakened mobile station.Address 1: BSSIDThis field contains the BSSID of the BSS created by the access point that thesender is currently associated with.Address 2: Transmitter AddressThis is the address of the sender of the PS-Poll frame.The PS-Poll frame does not include duration information to update the NAV.However,all stations receiving a PS-Poll frame update the NAV by the short interframe space plusthe amount of time required to transmit an ACK [ Pobierz całość w formacie PDF ] |
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