ait until the channel is idle. In order to and defragmentation of packets originating in upper minimize the possibility of collisions when more than one layers station wants to transmit at the same time, each station Protection against transmission error waits a random time-period, called a back off inter- Control of access to the physical medium val, before transmitting, after the channel goes idle. Security services-authentication and encryption. Figure 5.2-6 shows how this method works. An important attribute of any communications network is The figure shows activity on a channel as it appears to the method of access to the medium. Specification 802.11 a station that is attempting to transmit. The station may prescribes two possibilities: DCF (distributed coordination start to transmit if the channel is idle for a period of at function) and PCF (point coordination function). least a duration of DIFS (distributed coordination func- tion interframe space) since the end of any other trans- mission (section 1 of the figure). However, if the channel LOGICAL PEER-TO-PEER LINKS-PROTOCOLS is busy, as shown in section 2 of the figure, it must defer access and enter a back off procedure. The station waits APPLICATION APPLICATION until the channel is idle, and then waits an additional period of DIFS. Now it computes a time-period called PRESENTATION PRESENTATION a back off window that equals a pseudo-random number multiplied by constant called the "slot time." As long as SESSION SESSION the channel is idle, as it is in section 3 of the figure, the TRANSPORT TRANSPORT station may transmit its frame at the end of the back off window, section 4. During every slot time of the back off NETWORK NETWORK window the station senses the channel, and if it is busy, the counter that holds the remaining time of the back off DATA LINK DATA LINK window is frozen until the channel becomes idle and the back off counter resumes counting down. PHYSICAL PHYSICAL Actually, the back off procedure is not used for every access of the channel. Fo