802.11 Wireless Networks: The Definitive Guide (O'Reilly by Matthew Gast

By Matthew Gast

This can be a nice booklet for wireless method engineers to appreciate 802.11 a\b\g PHY and MAC. The publication not just talks approximately theoretical stuff but additionally offers sensible elements of wireless networking.

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Extra resources for 802.11 Wireless Networks: The Definitive Guide (O'Reilly Networking)

Sample text

1 Error Recovery with the DCF Error detection and correction is up to the station that begins an atomic frame exchange. When an error is detected, the station with data must resend the frame. Errors must be detected by the sending station. In some cases, the sender can infer frame loss by the lack of a positive acknowledgment from the receiver. Retry counters are incremented when frames are retransmitted. Each frame or fragment has a single retry counter associated with it. Stations have two retry counters: the short retry count and the long retry count.

This window is divided into slots. Slot length is mediumdependent; higher-speed physical layers use shorter slot times. 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. As in Ethernet, the backoff time is selected from a larger range each time a transmission fails. Figure 3-7 illustrates the growth of the contention window as the number of transmissions increases, using the numbers from the direct-sequence spread-spectrum (DSSS) physical layer.

If either carrier-sensing function indicates that the medium is busy, the MAC reports this to higher layers. Physical carrier-sensing functions are provided by the physical layer in question and depend on the medium and modulation used. It is difficult (or, more to the point, expensive) to build physical carrier-sensing hardware for RF-based media, because transceivers can transmit and receive simultaneously only if they incorporate expensive electronics. Furthermore, with hidden nodes potentially lurking everywhere, physical carrier-sensing cannot provide all the necessary information.

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