a request to send (RTS) powerful transmitted signal and a tiny received signal. packet. This sort of packet informs the access point and The MAC layer defined by the original 802.11 stan- any other stations in range that a station would like to dard is used almost unchanged for the enhanced versions send a packet and provides the length of the packet to be (802.1 a, and g) of the standard and thus is worth sent. The access point responds after the short inter- a moment's reflection. The MAC, although not as simple frame space with a clear to send (CTS) packet, whose as the original Ethernet MAC, is nevertheless a fairly NAV reserves the medium for the time required for the basic object. We have not discussed the point-co- remainder of the data transmission and ACK. The ordination function because it is rarely used. Absent this, advantage of the scheme is that one can hope that all the MAC provides no central coordination of contending associated stations can hear the access point CTS packet stations and no guarantees of performance except for the even if they are not able to detect the RTS packet. An continuity of an ongoing packet exchange. exchange like this is depicted schematically in Figure 5.1-6. The exchange begins with the client's RTS packet, which 5.1.3.3 802. 11 Classic direct-sequence reserves the NAV for the whole of the envisioned ex- change (the length of the data packet, three short inter- frame spaces, a CTS frame, and an ACK). The access To maximize the probability that multiple users could point responds with clearance, reserving the medium for share the unlicensed bands without unduly interfering the remainder of the exchange with its NAV setting. All with each other, the FCC placed restrictions on com- stations then defer until the end of the exchange. After munications systems that could operate there. A funda- a distributed interframe space has passed without any mental part of the requirement was that the systems not more transmissions, a station waiting to send data can use transmit all t