Mac-Level Acknowledgments; Fragmentation And Reassembly - Black Box LW0050A Manual

The following diagrams show an exchange between stations A and B, and the NAV
setting of their neighbors:
G3
RTS
Src
Dest
Other
Figure E-2. Transaction Between Stations A and B.
The NAV State is combined with the physical carrier sense to indicate the busy
state of the medium.
E.3.3 MAC-L A
EVEL
As mentioned earlier in this document, the MAC layer performs Collision
Detection by expecting the reception of an acknowledge to any transmitted
fragment. (Packets that have more than one destination, such as Multicasts, are not
acknowledged.)
E.3.4 F
RAGMENTATION AND
Typical LAN protocols use packets several hundred bytes long (the longest
Ethernet packet could be up to 1518 bytes long).
There are several reasons why it is preferable to use smaller packets in a wireless
LAN environment:
• Because of the higher Bit Error Rate of a radio link, the probability of a
packet's getting corrupted increases with the packet size.
• In case of packet corruption (due to either collision or noise), the smaller the
packet, the less overhead it causes to retransmit it.
• On a Frequency Hopping system, the medium is interrupted periodically for
hopping (in our case every 20 milliseconds), so, the smaller the packet, the
smaller the chance that the transmission will be postponed after dwell time.
Data
G1 G1
CTS
NAV (RTS)
NAV (CTS)
Defer Access
CKNOWLEDGMENTS
R
EASSEMBLY
APPENDIX E: IEEE 802.11 Technical Tutorial
G1
Ack
G3
Backoff After Defer
G1=SIFS
G3 =DIFS
CW=Contention Window
CW
Next MPDU
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