 any access point that detects the probe request, it sends a probe response to the device. Once the client and the access point go through authentication and association procedure, the client can send or receive data. Unlike cellular standards, IEEE 802.11 does not support dedicated control/traffic channels and it does not have the MAC scheduling functionality in previous generations of Wi-Fi. However, IEEE 802.11a adds an OFDMA scheduling capa- bility and more granular resource allocation scheme. The stations are allowed to transmit at any time as long as the medium is not occupied by transmissions from other stations. To determine whether the medium is free and to overcome the hidden-node problem, the sta- tion transmits a short request to send (RTS) frame containing source address, destination address, and the duration of upcoming data transmission. Other stations located around the transmitting station may receive the RTS burst and check if the RTS is meant for them. If the RTS is meant for a station and the medium is free, the receiving device would transmit Operation in Unlicensed and Shared Spectrum Device Passive scan AP Device Active scan Beacon signal Probe request Decode beacon Detect probe Initial sync Probe response Authentication request Request to send (RTS) Challenge phrase Clear to send (CTS) Encrypted challenge phrase Encrypted challenge phrase Decode beacon Contention Association request Association response Figure 8.8 Scanning and association procedures [30]. a CTS frame containing the duration of the transaction. At this time, other devices in the vicinity of the communicating stations know that the medium will be occupied for certain time duration and set their NAV counters, which is a MAC-level contention control timer accordingly SO that it would not try sensing the medium and transmitting during that period (see Fig. 8.9). When a packet arrives at the MAC layer from higher layers, a sequence number is assigned to it, and if the packet length is larger than a single MAC frame, it is segmen