to transmit at the same time. Furthermore, as we've al- expects to be acknowledged by the receiver, will provide ready seen, received signals are tiny. Provisions to sepa- a NAV value in the header of the frame. All stations that rate a tiny received signal from a large transmitted signal receive the frame note the value of the NAV and defer for are possible if the two are at differing frequencies, but the additional required time even if they can't detect any this requires bandwidth that was not available in the signal during that time. limited 2.4-GHz ISM band. It is difficult and relatively Requirements on the timing between frames-inter- expensive to reliably receive a tiny transmitted signal frame spaces-are used to enable flexible access control while simultaneously transmitting at the same frequency, of the medium using the carrier sense mechanisms. Once SO collision detection is not practical for a WLAN station. a transmission has begun, the transmitting station has Thus, in a wireless implementation, CSMA/CD cannot captured the medium from all other stations that can be used, because there is no way to confidently associate hear its transmissions using the physical carrier sense the absence of a carrier with a free medium or reliably mechanism. A station wishing to send a new packet must detect all collisions. first ensure that both the physical carrier sensing mech- To get around this difficulty, 802.11 stations use anism and the NAV indicate the medium to be free for carrier-sense multiple access with collision avoidance a time equal to the distributed interframe space (DIFS). (CSMA/CA, Figure 5.1-6). All stations listen to the The distributed interframe space is relatively long, SO current radio channel before transmitting; if a signal is that stations that are completing exchanges already in progress, which are permitted to transmit after a short interframe space (SIFS), can capture the medium to transmit their response packets before any new station is allowed to contend for the right to transmit. T