11
system” mode. When the “shared key” option is chosen, the “Encryption” tab of
the Utility program must be properly filled out.
Fragmentation Threshold:
In the presence of hostile RF environment, such as
interference, frames longer than this threshold numbers in bytes are divided prior
to transmission into one or more fragments equal in length to the fragmentation
threshold. The default value is set at 2436 bytes, the maximum 802.11 data
frame size, such that no frames are ever fragmented.
RTS Threshold:
To minimize the potential packet collision associated with hidden
nodes in a wireless network, 80211 standard has the option to complete a
“Request to Send(RTS)” and “Clear to Send(CTS)” two-frame exchange prior to
sending the real data. This obviously reduces the throughput of the real data.
Since the probability of packet collision increases with the size of the packets
transmitted, an optimum trade-off between data-throughput and data-integrity
may be reached by turning on the two-frame exchange only for data packets
exceeding a certain size. The number entered in this field is that threshold packet
size in Bytes. For example, if “500” is entered, data packets with sizes less than
500 bytes are transmitted without being preceded by the RTS-CTS exchange
and thereby taking a small risk of getting corrupted by packet collisions. If “2346”
(the maximum 802.11 data frame size) or a larger number is entered, then every
data packet is transmitted without being preceded by the RTS-CTS exchange
and thereby maximizing the data throughput.
Channel:
Click on the arrow and highlight the desired frequency (channel) for the
radio network. A minimum of 20MHz separation from the frequency of an
adjacent AP is required.
Beacon Interval:
An Access Point periodically sends out “Beacon” packets to
broadcast information for the Stations already associated with the AP or for
Stations yet to associate with it. This field specifies how frequently the beacon
packet is sent. The default value is 100 mSec. For a relatively “fixed” network,
where Stations roaming in and out is infrequent, the interval can be long to
minimize the overhead usage of the bandwidth.
DTIM Period:
An Access Point generates a
Traffic Indication Map
(TIM) to
identify the Stations that have data buffered inside the AP. The TIM is included in
the Beacon sent out by the AP. A Station determines that data packets are
buffered for it by the AP and takes appropriate actions by listening to the beacon
and interpreting a TIM. A special delivery TIM (DTIM) is used to signal that
broadcast/multicast frames buffered at the AP will be sent immediately following
the beacon that contains the DTIM. This is necessary only when the AP detects
that at least one of the Stations associated to it is in the Power Save (PS) mode.
DTIM period is measured in terms of beacon intervals. The default setting is 10
beacon intervals. This means one out every 10 beacons sent by the AP contains
DTIM. If prior knowledge is available that Stations in a particular network rarely
uses the PS mode, a high value for DTIM Period should be entered to minimize
the overhead usage of the bandwidth.
