Black Box LW0050A Manual page 132

PRO 11 SERIES WIRELESS ETHERNET
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. It receives an ACK for the said fragment, or
2. It 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 Access Point 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):
MAC
Frame Body
HDR
Fragment 0
130
C
C
MAC
R
Frame Body R
HDR
C
C
Fragment 1
Figure E-3. Frame Fragmentation.
MSDU
MAC
Frame Body
HDR
Fragment 2
C
MAC
Frame Body
R
HDR
C
Fragment 3
C
R
C