Today, we discuss further Wireless LANs: Media Access Control. The Media Access Control sublayer is responsible for the channel access procedure, protocol data units addressing, frame formatting, error control and the fragmentation and reassembly of the service data units. The sublayer also provides options to support a security service through authentication and privacy mechanisms. The Media Access Control management services are also defined to support roaming within an extended service set and to assist stations in power management. The IEEE 802.11 Media Access Control Protocol is specified in terms of coordination functions that determine when a station in a basic service set is allowed to transmit, and when it may be able to receive protocol data units over the wireless medium. It defines two coordination functions: the distributed coordination function and the point coordination function. So distributed coordination function is required for all stations which provide support for asynchronous data transfer on a best-effort basis. Under this function, the transmission medium operates in the contention mode exclusively, requiring all stations to contend for the channel for each packet transmitted. The point coordination function is optional, which may be implemented by the access point to support a connection oriented time-bounded transfer of service data units. Under this function, the medium can alternate between the contention period and contention-free period. During contention-free periods, the media usage is controlled by the access point. No need for stations to contend for channel access. The distributed coordination function is based on carrier-sense multiple access with collision avoidance (CSMA-CA) protocol, which we learned in the last lesson. The figure shows the basic CSMA-CA operations. All stations are obligated to remain quiet for a certain minimum period after a transmission has been completed called interframe space IFS. The length of the interframe space depends on the type of frame that the station is about to transmit. High-priority frames must only wait as a short IFS period before they contend for the channel. Frame types that are used in short IFS include acknowledgements, clear-to-send, data frames of segmented service data units, and a few more. The point of coordination function interframe space (PIFS) is intermediate in duration and the use of by the point of coordination function to gain quality access to the medium at the start of a contention-free period. The DCF interframe space (DIFS) is used by the distributed coordination function to transmit data. If channel is still idle after DIFS period, ready station can transmit an initial protocol data unit. If a channel becomes busy before DIFS, the station must schedule backoff time for the reattempt. A station that completes a frame transmission is not allowed to transmit immediately. In IEEE 802.11, carrier sensing is performed as opposed to the air interface referred to the physical carrier sensing and as the MAC sublayer referred as virtual carrier sensing. The channel is busy if either sensing is busy. Virtual carrier sensing is used by a source station to inform all other stations in the basic service set. How long the channel will be utilized for the successful transmission of a protocol data unit? The source station also set as a duration field in the MAC header of the data frames. Or in a request send and it clear to send the control frames. All stations adjust network allocation vector to indicate when the channel will become idle. In practice, stations can choose to never use the RTS and the CTS, particularly if the data from site is small and as the communication media is lightly loaded. The figure shows a time in diagram that illustrates the successful transmission of a datagram. When the data frame is transmitted, the duration field of the frame lets all stations in the basic service set know how long the medium will be building. All stations here in the data frame adjust their network allocation vector based on the duration field of value, which includes a shot IFS and the acknowledgements following the data frame. Stations can choose to use RTS and CTS whenever the service data unit exceeds the value of RTS threshold. Or to always use RTS and CTS. The figure shows that timing diagram illustrates the transmission sequence of the frames. We can see the overhead of RTS and CTS frame transmissions impose additional delay. While channels cannot handle very long transmissions due to they are relatively large error rates, large surface data units may look higher fragmentation to increase transmission reliability. The collision avoidance portion of CSMA-CA is performed through a random backoff procedure. The point of coordination function is optional capability that can be used to provide connection oriented contention-free services but in neighboring porting stations to transmit without a contention for the channel. The PCF function is performed by the point coordinator in the access point. We're done with the details here for the point coordination access.
