TRENDnet 12dBi User Manual page 17

TRENDnet User's Guide
Short Slot : By default, it's "Enable" for reducing the slot time from the standard 20
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microseconds to the 9 microsecond short slot time
Slot time is the amount of time a device waits after a collision before retransmitting
a packet. Reducing the slot time decreases the overall back-off, which increases
throughput. Back-off, which is a multiple of the slot time, is the random length of
time a station waits before sending a packet on the LAN. For a sender and receiver
own right of the channel the shorter slot time help manage shorter wait time to re-
transmit from collision because of hidden wireless clients or other causes. When
collision sources can be removed sooner and other senders attempting to send are
listening the channel(CSMA/CA) the owner of the channel should continue
ownership and finish their transmission and release the channel. Then, following
ownership of the channel will be sooner for the new pair due to shorter slot time.
However, when long duration of existing collision sources and shorter slot time exist
the owners might experience subsequent collisions. When adjustment to longer
slot time can't improve performance then RTS/CTS could supplement and help
improve performance.
Extra Slot Time : Slot time is in the range of 1~255 and set in unit of microsecond.
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The default value is 9 microsecond.
ACK Timeout : ACK timeout is in the range of 1~255 and set in unit of microsecond.
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The default value is 32 microsecond.
All data transmission in 802.11b/g request an "Acknowledgement" (ACK) send by
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receiving radio. The transmitter will resend the original packet if correspondent ACK
failed to arrive within specific time interval, also refer to as "ACK Timeout".
ACK Timeout is adjustable due to the fact that distance between two radio links
may vary in different deployment. ACK Timeout makes significant influence in
performance of long distance radio link. If ACK Timeout is set too short, transmitter
will start to "Resend" packet before ACK is received, and throughputs become low
due to excessively high re-transmission.
ACK Timeout is best determined by distance between the radios, data rate of
average environment. The Timeout value is calculated based on round-trip time of
packet with a little tolerance, So, if experiencing re-transmissions or poor
performance the ACK Timeout could be made longer to accommodate.
Beacon Interval : Beacon Interval is in the range of 20~1024 and set in unit of
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millisecond. The default value is 100 msec.
Access Point (AP) in IEEE 802.11 will send out a special approximated 50-byte
frame, called "Beacon". Beacon is broadcast to all the stations, provides the basic
information of AP such as SSID, channel, encryption keys, signal strength, time
stamp, support data rate.
All the radio stations received beacon recognizes the existence of such AP, and may
proceed next actions if the information from AP matches the requirement. Beacon
is sent on a periodic basis, the time interval can be adjusted.
By increasing the beacon interval, you can reduce the number of beacons and
associated overhead, but that will likely delay the association and roaming process
because stations scanning for available access points may miss the beacons. You
can decrease the beacon interval, which increases the rate of beacons. This will
make the association and roaming process very responsive; however, the network
will incur additional overhead and throughput will go down.
DTIM Interval : The DTIM interval is in the range of 1~255. The default is 1.
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DTIM is defined as Delivery Traffic Indication Message. It is used to notify the
wireless stations, which support power saving mode, when to wake up to receive
multicast frame. DTIM is necessary and critical in wireless environment as a
mechanism to fulfill power-saving synchronization.
A DTIM interval is a count of the number of beacon frames that must occur before
the access point sends the buffered multicast frames. For instance, if DTIM Interval
is set to 3, then the Wi-Fi clients will expect to receive a multicast frame after
receiving three Beacon frame. The higher DTIM interval will help power saving and
possibly decrease wireless throughput in multicast applications.
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