ain access to the medium and to ensure collision avoidance with other contenders wishing to access the shared medium. The stations either passively scan the beacons transmitted by nearby access points or actively scan neighboring access points by transmitting a probe signal. In IEEE 802.11, there are two options for medium access. The first is a centralized control scheme that is referred to as point coordination function (PCF), and the second is contention-based approach known as DCF. The PCF mode supports time-sensitive traffic Application layer Layers 3-7 data Presentation layer Data link layer MSDU is encapsulated in the MPDU Session layer (layer 2) Transport layer MAC header and trailer are added or removed MPDU = PSDU, the PSDU is encapsulated in the PPDU Network layer Physical layer Preamble and PHY header IEEE 802.2 Logical Link Control (LLC) Data link-layer (layer 1) are added/removed IEEE 802.11 MAC Header 0100010001110111000111 IEEE 802.11 PLCP Header Data is transmitted as bits PhysicaHayer RF/PHY Medium (DSSS, OFDM, etc.) Figure 8.6 OSI and IEEE 802.1 11 protocol relationships [ 30]. Table 8.2: Evolution of IEEE 802.11 air-interface technologies [30]. Frequency Bandwidth Transmission Highest Order Spatial Peak Data Rate Standard (GHz) (MHz) Scheme Modulation Coding Rate Streams (Mbps) 802.11 Convolutional coding with coding rates 802.11b DSSS/CCK 1/2, 2/3, 3/4, and 5/6 802.11a 64QAM 802.11g OFDM, 64QAM DSSS/CCK 802.11n 2.4 and 5 20, 40 OFDM/SDM 64QAM 6.5-600 802.11ac 20, 40, 80, OFDM/SDM/ 256QAM 433 (80 MHz and MU-MIMO one spatial stream) 6933 (160 MHz and 8 spatial streams) 802.11ad 256QAM Up to 6912 802.11ax 2.4 and 5 20, 40, 80, OFDMA/ 1024QAM Convolutional coding and LDPC with 600.4 (80 MHz and SDM/MU- coding rates 1/2, 2/3, 3/4, and 5/6 one spatial stream) 9607.8 (160 MHz and 8 spatial streams) 862 Chapter 8 flows where the access points periodically send beacon frames to communicate network management and identification, which is specific to that Wi-Fi node. Between the sending of these frames,