738
21 Wireless Local Area Networks
busy (or becomes busy during the DIFS interval) the station defers using the
expo-
nential backoff algorithm.
This scheme implies that, except in cases of very high network congestion,
no packets will be lost because retransmission occurs each time a packet is not
acknowledged. This entails that all packets sent will reach their destination in
sequence.
The 802.11 MAC layer provides for two other robustness features:
cycle
redundancy check
(
CRC
)
checksum
and
packet fragmentation
. Each packet has a
CRC checksum calculated and attached to ensure that the data was not corrupted
in transmit. This is different from the Ethernet, where higher-level protocols such
as TCP handle error checking.
Packet fragmentation allows large packets to be segmented into smaller
units when sent over the medium. This is useful in very congested environments
or when interference is a factor, since large packets have a better chance of being
corrupted. This technique reduces the need for retransmission in many cases and
improves overall wireless network performance. The MAC layer is responsible for
reassembling fragments received, rendering the process transparent to higher-level
protocols. The following are some of the reasons it is preferable to use smaller
packets in a WLAN environment:
Due to higher BER of a radio link, the probability of a packet getting
corrupted increases with packet size.
In case of corrupted packets (either due to collision or interference), smaller
packets cause less overhead.
On an FHSS system the medium is interrupted periodically for hopping.
With smaller packets the chance that the transmission will be postponed
after dwell time is reduced.
A simple
send-and-wait
algorithm is used at the MAC sublayer. In this mech-
anism the transmitting station is not allowed to transmit a new packet until one
of the following happens:
Receives an ACK for the packet, or
Decides that packet was retransmitted too many times and drops the whole
frame.
