User's Manual

-56-
ü Minimizing the number of walls and ceilings.
ü Position the WLAN antenna for best reception.
ü Keep WLAN devices away from other electrical devices, eg: microwaves, monitors, electric
motors, … etc.
ü Add additional WLAN Access Points if necessary.
5.8 What are the Open System and Shared Key authentications?
IEEE 802.11 supports two subtypes of network authentication services: open system and shared
key. Under open system authentication, any wireless station can request authentication. The station
that needs to authenticate with another wireless station sends an authentication management frame
that contains the identity of the sending station. The receiving station then returns a frame that
indicates whether it recognizes the sending station. Under shared key authentication, each wireless
station is assumed to have received a secret shared key over a secure channel that is independent
from the 802.11 wireless network communications channel.
5.9 What is WEP?
An optional IEEE 802.11 function that offers frame transmission privacy similar to a wired network.
The Wired Equivalent Privacy generates secret shared encryption keys that both source and
destination stations can use to alert frame bits to avoid disclosure to eavesdroppers.
WEP relies on a secret key that is shared between a mobile station (e.g. a laptop with a wireless
Ethernet card) and an access point (i.e. a base station). The secret key is used to encrypt packets
before they are transmitted, and an integrity check is used to ensure that packets are not modified
in transit.
5.10 What is Fragment Threshold?
The proposed protocol uses the frame fragmentation mechanism defined in IEEE 802.11 to achieve
parallel transmissions. A large data frame is fragmented into several fragments each of size equal to
fragment threshold. By tuning the fragment threshold value, we can get varying fragment sizes. The
determination of an efficient fragment threshold is an important issue in this scheme. If the fragment
threshold is small, the overlap part of the master and parallel transmissions is large. This means the
spatial reuse ratio of parallel transmissions is high. In contrast, with a large fragment threshold, the
overlap is small and the spatial reuse ratio is low. However high fragment threshold leads to low
fragment overhead. Hence there is a trade-off between spatial re-use and fragment overhead.
Fragment threshold is the maximum packet size used for fragmentation. Packets larger than the
size programmed in this field will be fragmented.
If you find that your corrupted packets or asymmetric packet reception (all send packets, for
example). You may want to try lowering your fragmentation threshold. This will cause packets to be
broken into smaller fragments. These small fragments, if corrupted, can be resent faster than a
larger fragment. Fragmentation increases overhead, so you'll want to keep this value as close to the
maximum value as possible.
5.11 What is RTS (Request To Send) Threshold?
The RTS threshold is the packet size at which packet transmission is governed by the RTS/CTS
transaction. The IEEE 802.11-1997 standard allows for short packets to be transmitted without RTS/
CTS transactions. Each station can have a different RTS threshold. RTS/CTS is used when the data
packet size exceeds the defined RTS threshold. With the CSMA/CA transmission mechanism, the
transmitting station sends out an RTS packet to the receiving station, and waits for the receiving
station to send back a CTS (Clear to Send) packet before sending the actual packet data.