DCF, PCF, NAV vector with its importance:

• The IEEE 802.11 standard supports two access methods: a mandatory Distributed Coordination Function (DCF) method which is available in ad hoc and infrastructure configuration.
• Point-Coordinated Function (PCF) is optional, which is available in certain infrastructure environments and can provide time-bounded services. The Access point polls terminals according to a list.
• DCF is the functional access method used to support asynchronous data transfer on the best effort basis. All the stations must support DCF. DCF employs the carrier sensing CS mechanism.
• DCF is a carrier sensing multiple access with collision avoidance (CSMA/CA) protocol and uses a Network Allocation vector (NAV) to implement collision avoidance.
• DCF is suitable for multi-hop ad hoc networking, it optionally uses RTS / CTS exchange to avoid hidden terminal problem. Any node overhearing a CTS cannot transmit for the duration of the transfer and uses ARQ to provide reliability.
• In order to minimize the possibility of collisions a random backoff mechanism is used to randomize moments at which medium is tried to be accessed.
• 802.11 Distributed Coordination Function (DCF) is a protocol which uses carrier sensing along with a four way handshake to maximize the throughput while preventing packet collisions.
• A packet collision is defined as any case where a node is receiving more than one packet at a time, resulting in neither packet being correctly received. The 802.11 protocol is the most well known and widely used wireless networking protocol in real applications.
• 802.11 is a carrier sensing multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol using a direct sequence spread spectrum (DSSS) physical interface.
• The DCF protocol is enhanced further by provision of a virtual CS indication called Net Allocation Vector (NAV) which is based on duration information transferred in special RTS / CTS frames before the data exchange.
• NAV allows stations to avoid transmissions in time intervals in which the medium is surely busy. The handshaking usage allows increasing the network performance.
• The collisions of short information packets and reduction of an unprofitable hidden stations influence can increase the throughput.
• Network allocation vector (NAV) is used to inform other nodes how long the current node will need the channel. Any nodes overhearing the NAV know that they have no need of sensing the channel for the time indicated.
• Since idle sensing of the channel is one of the biggest uses of energy in a network, the NAV reduces the amount of idle sensing required at any nodes which can overhear it, thus saving energy at all nodes in the network.
• To provide fairness, each node which is transmitting first performs a random countdown, where the length of the countdown is within the length of the contention window.
• During the countdown, if the node senses that another node is transmitting, it will pause its countdown and continue at that same number after the other transmission is finished.
• When the countdown reaches zero, the node will sense the channel and, if the channel is still free, transmit the RTS. The range of values which can be chosen for the random backoff time is referred to as the contention window.
• The size of the window is very important and can change based on network conditions. If the window is too small, there is an increased chance that two nodes will attempt to transmit at the same time.
• If the window is too large, the nodes may be idly waiting for a long time before transmitting. The window size can increase by a factor of 2 if a transmission fails (i.e. a collision or a node which doesn’t respond to an RTS).