sence of an ACK from the receiver), a ran- dom backoff number is drawn and the process repeats. With every backoff, the random counter value is doubled, that is, increasing as 16, 32, 64, etc. This random-access process, referred to as CSMA/CA, is illustrated in Fig. 8.5. Most wireless systems operating in unlicensed spectrum employ CSMA as the basis for channel access. In this case, the most commonly used medium access mechanism is distrib- uted coordination function (DCF), which is based on CSMA/CA. The DCF concept is illus- trated in Fig. 8.5. A transmitting node senses the channel, and if the channel is idle for a certain time duration, that is, the DCF interframe space (DIFS), the node starts to transmit; otherwise, it continues to monitor the channel until it becomes idle for a time duration equal to DIFS. At this time, the node generates a random backoff timer, uniformly distributed within a contention window (CW). The random backoff helps avoid potential collisions, which may happen when two or more nodes are simultaneously waiting for the channel to be cleared. The backoff timer is decremented as long as the channel is idle but remains fro- zen when a transmission is detected and is reactivated after the DIFS period of time as soon as the channel is free. A node refrains from transmission until its backoff timer expires. Note that the DCF mechanism tries to ensure that only one transmission is present in a channel at any time, and each node has a fair share of the channel. The channel use for each node at a particular time is not guaranteed. Thus there is no deterministic schedule transmission, reflecting the random and contentious nature of communication in the unli- censed spectrum. Reliable services and efficient resource usage are typically hard to achieve in unlicensed operation. This property is very different from that of operation in a licensed spectrum. 858 Chapter 8 Device A Random Medium busy Medium free backoff Device B (attempts to send) Carrier sensing Medium busy Medium free Medium busy De