r example, acknowledgement transmissions and RTS and CTS transmissions, (see COMMUNICATION below), do not use it. Instead, they access the channel MEDIUM after an interval called SIFS (short interframe space) following the transmission to which they are responding. Figure 5.2-4 Open system interconnection reference model. SIFS is shorter than DIFS, SO other stations waiting to Short-range wireless applications and technologies CHAPTER 5.2 APPLICATIONS LOGICAL LINK CONTROL PROFILES ETHERNET BLUETOOTH PHY IEEE 802.11,a,b,g IEEE 802.3 IEEE 802.15.1 Figure 5.2-5 Data link and physical layers (PHY). transmit cannot interfere since they have to wait a longer to access the channel again. This RTS/CTS procedure is time, after the previous transmission, and by then the also effective when not all stations on the network have channel is already occupied. compatible modulation facilities for high rate communi- In waiting for a channel to become idle, a transmission cation and one station may not be able to detect the contender doesn't have to listen continuously. When one transmission length field of another. RTS and CTS trans- hears another station access the channel, it can interpret missions are always sent at a basic rate that is common to the frame length field that is transmitted on every frame. all participants in the network. After taking into account the time of the acknowledge- The PCS is an optional access method that uses ment transmission that replies to a data transmission, the a master-slave procedure for polling network members. time that the channel will become idle is known even An AP station assumes the role of master and distributes without physically sensing it. This is called a virtual car- timing and priority information through beacon man- rier sense mechanism. agement transmissions, thus creating a contention free The procedure shown in Figure 5.2-6 may not work access method. One use of the PCS is for voice com- well under some circumstances. For example, if several munications, which must use regu