How to Send a Frame

Station A wants to send a frame so Both the physical and virtual carrier need to be free.

1.NAV : If a frame (RTS, CTS, Data, Ack) is heard, NAV is set to the duration in that frame. Stations can sense the media after NAV expires.

2.Backoff time : Client need to listen and choose a random time and begin the countdown process(time called as Backoff time) before sending any frame.

3.Slot Time : The speed at which the countdown occurs is called a slottime and is different for 802.11a, b, and g.

 

—Step-1 Station A selects the random timer value of 29.

—Step-2 Station A starts counting at 29, 28, 27, 26, and so on. While Station A is counting down, it is also listening for whether anyone else is sending a frame.

—Step-3 When the timer is at 18, Station B sends a frame, having a duration value in the header of 45.

—Step-4 The duration of 45 that is in the header of the frame sent by Station B is called a network allocation vector (NAV) and is a reservation of the medium that includes the amount of time to send its frame, wait for the SIFS, and then receive an ACK from the AP.

—Step-5 Station A adds 45 to the 18 that is left and continues counting down, 63, 62, 61, and so on. The total time that Station A waits before sending is called the contention window.

—Step-6 After the timer on Station A reaches 0, it can send its frame. At this point, the medium should be clear.

 

—If Station A sends but fails, it resets the backoff timer to a new random number and counts down again. The backoff timer gets larger as the frames fail in transmission. For example, the initial timer can be any number between 0 and 31. After the first failure, it jumps to any number between 0 and 127. It doubles for the next failure, then again, then again.

— This entire process is known as the distributed coordination function (DCF). This simply means that each station is responsible for coordinating the sending of its data.