User's Manual

Appendix
BreezeNET PRO.11 Series 8-25 User’s Guide
8.5.3.4 Fragmentation and Reassembly
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:
• Due to the higher Bit Error Rate of a radio link, the probability of a
packet getting corrupted increases with the packet size.
• In case of packet corruption (either due to 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.
However, it doesn’t make sense to introduce a new LAN protocol that
cannot deal with packets 1518 bytes long which are used on Ethernet, so the
committee decided to solve the problem by adding a simple fragmentation/
reassembly mechanism at the MAC Layer.
The mechanism is a simple Send-and-Wait algorithm, where the
transmitting station is not allowed to transmit a new fragment until one of
the following happens:
1. Receives an ACK for the said fragment, or
2. Decides that the fragment was retransmitted too many times and drops
the whole frame.
It should be noted that the standard does allow the station to transmit to a
different address between retransmissions of a given fragment. This is
particularly useful when an AP has several outstanding packets to different
destinations and one of them does not respond.
The following diagram shows a frame (MSDU) being divided to several
fragments (MPDUs):