MAC layer

MAC layer IEEE 802.11 defines two MAC sublayers the distributed coordination function (DCF)and point coordination function (PCF) as shown in Figures. MAC layers in IEEE802.11 standard PCF MAC optional DCF MAC Physical layer PCF is an optional and complex access method that can be implemented in aninfrastructure network (not in ad hoc network DCP is similar to CSMA/CA CSMA/CAwith some additional control features.). Wireless LANS cannot increment CSMA/CD for three reasons. 1: Collision detection implies that the station must be able to find data and receive collision signals at the same time. This implies costly stations and increasedbandwidth requirements. 2: Collision may not be detected because of the hidden terminal problem. A terminal may be hidden from another in a wireless environment (due to naturalobstacles such as mountains or artificial obstacles such as buildings). Supposestations A and B both have data to sink to station C. Station B is hidden fromA, so if there is a collision near B, A will not hear. This does not happen inwired LANS because all stations are connected by wire and collision is heard byall stations. 3: The distance between stations in wireless LANS can be great. Signal taking could prevent station at one end from hearing a collision at other end. RAMF Exchange Time Exchange of data and control frames in time. 1. Before finding a frame, the source stations sense the medium by checking theenergy level at the carrier frequency. (a) The channel uses a persistence strategy with backoff until the channel is idle.

(b) After the station is found idle, the station waits for a period of the called thedistributed interframe spous (DIFS): Then the station finds a control frame called therequest to send (RTS). 2. After receiving the RTS and waiting for short period of time, called the shortinterframe space (SIFS), the destination station finds a control frame called the clearto find (CTS) to the source station. This control frame indicates that the destinationstation is ready to receive data. 3. The source station finds data after waiting for an amount of time equal to SIFS.[Source Interframe Space (SIRS)]