Chen's Learning Note

With fragmentation the MSDU would be broken into smaller sections and each section encapsulated in an MPDU . Each MPDU is sent in a separate PPDU with the preamble of each PPDU providing a new  channel estimate. (PHY Layer) MSDU fragmented condition:  when its length exceeds a threshold . Each fragment contains an even number of bytes and all fragments are the same size , except the last fragment, which may be smaller. The timing relation between A-MSDU/A-MPDU/PPDU :

1.What is Short Inter-frame Space (SIFS)? Ans : the small time interval between the data frame and its acknowledgment . Used for the highest priority transmissions enabling stations with this type of information to access the radio link first. 2. What is the content of SIFS ? Ans :This value is fixed per PHY and is calculated in such a way that transmitting station will be able to switch back to receive mode and be capable of decoding the incoming packets. A SIFS duration is a constant value and it depends on the amendments . 3.What is  DCF inter-frame space( DIFS) ? Ans : The DIFS is used by stations operating under the DCF method to transmit data frames and management.(Read more : DCF Method ) 4.How to use DIFS? Ans : A station using the DCF is allowed to transmit if it determines that the medium is idle for the duration of the DIFS, or if it determines that the medium is idle for the duration of the DIFS + the remaining back-off time following the recept...

The random back-off count is selected as a pseudo-random integer drawn from a uniform distribution over the interval [0,CW] , where CW, an integer value, is the contention window. Update procedure of CW value 1. CW parameter takes the initial value CWmin . 2. Every unsuccessful MPDU transmit , for example each time an ACK response is not received for a data frame. CW parameter doubles . 3. Every successful MPDU transmit, CW parameter reset . 4. If CW parameter increase due to unsuccessful MPDU transmit and reach the value CWmax , CW parameter keep CWmax until reset . Random back-off procedure 1. As shown in the figure above, STA3 which select the smallest back-off count win the contention and transmit first. 2. STA2 and STA4  suspend their back-off and resume DIFS after the medium goes to idle . 3. STA4, which select the second smallest back-off count win the 2nd contention. 4. And it goes STA2 and finally STA1 transmit.