 remains free after this time has elapsed. The contention window parameter Cw is Device A Data packet Medium busy Medium free Medium free Device B attempts to send Carrier Random sensing Data packet backoff Medium busy Medium free Medium busy Device C attempts to send Carrier Random Device C computes a longer backoff time than device B sensing backoff Backoff counter is suspended when medium goes busy Figure 6-4: 802.11 CSMA/CA Chapter Six specified in terms of a multiple of a slot time that is 20 us for 802.11b or 9 us for 802.11a/g networks. The backoff time is randomised SO that, if many stations are waiting, they will not all try again at the same time - one will have a shorter backoff and will succeed in starting its transmission. If a station has to make repeated attempts to transmit a packet, the computed backoff period is doubled with each new attempt, up to a maximum value Cw max defined for each station. This ensures that, when many stations are competing for access, individual attempts are spaced out more widely to minimise repeated collisions. If another station is sensed transmitting before the end of the DIFS period, this is because a short IFS (SIFS) can be used by a station that is waiting either to transmit certain control frames (CTS or ACK - see Figure 6-5) or to continue the transmission of parts of a data packet that has been fragmented to improve transmission reliability. Device A Random backoff Destination A Device B Random backoff Random backoff Start of contention period Start of contention period Figure 6-5: DCF Transmission Timing CSMA/CA is a simple media access protocol that works efficiently if there is no interference and if the data being transmitted across the network is not time critical. In the presence of interference, network throughput can be dramatically reduced as stations continually backoff to avoid collisions or wait for the medium to become idle. CSMA/CA is a contention-based protocol, since all stations have to compete for access. With the exception of the SIFS mechani