The IEEE 80211 Medium Access Control (MAC)

Fig.1 CSMA/CA: direct access if medium free for > DIFS, else defer & backoff Fig.2 CSMA/CA + ACK: receiver sends ACK immediately if CRC okay

Fragm entation and Reassembly

-Fragmentation: why?

?It allows higher-level packets and management frames to go through the channel broken in smaller pieces.

?It improves reliability and increasing throughput when interference

exists by reducing percentage of data corrupted by interference.

-Fragmentation: when?

?If packet’s length exceeds fragmentation threshold.

-Fragmentation: how?

?All fragments: same frame sequence number, ascending fragment

number and a frame control(accounts for # of fragments).

?All the fragmets within a frame sent in a fragmentation burst.

Fig.3

Fragmentation