he NAV value of each packet in a sequence is also set to capture station A the medium for the entire remaining sequence of packets. Once all the necessary parts of an exchange are complete, the medium may then become free for the Access distributed interframe space, at which point stations Point waiting to transmit frames randomly select a time slot and start transmission if no carrier has been detected. Because collisions cannot be detected, the opposite approach is taken: 802.11 depends on positive acknowl- station B edgement of a successful transmission through the receipt of an ACK packet by the transmitting station. Figure 5.1-5 Stations A and B are hidden from one another, Failure to receive an ACK, whether due to poor signal though both are visible to the access point. strength, collisions with other stations, or interference, is Wireless local area networks CHAPTER 5.1 contention slots CLIENT ACCESS POINT CLIENT RTS reservation: 3x(SIFS) + CTS + DATA ACK ACCESS POINT CTS: 2x(SIFS) + DATA + ACK optional Figure 5.1-6 Example of packet exchange under CSMA/CA. considered to be indicative of a collision; the transmitting to be resent. Like the RTS/CTS exchange, a station station will wait until the medium is again free, choose transmitting a packet sets the NAV value to reserve the a random time slot within a larger possible window (the medium for the whole exchange consisting of all the Ethernet backoff mechanism), and attempt to transmit fragments of the packet and an ACK for each fragment. the packet again. An important consequence of the MAC is that an Two additional optional provisions can be taken to 802.11 radio is never transmitting and receiving simul- improve performance when, for whatever reason, taneously: it is a half-duplex system in radio terms. packets are lost frequently. First, the sending station This choice simplifies the design of the radio front end (typically a client rather than an access point) can pre- by eliminating the need to distinguish between the cede its data transmission with 