Fragmentation Threshold; Preamble Type - ZyXEL Communications ZYWALL USG 20 Manual

Note: Enabling the RTS Threshold causes redundant network overhead that could
negatively affect the throughput performance instead of providing a remedy.

Fragmentation Threshold

A Fragmentation Threshold is the maximum data fragment size (between 256
and 2432 bytes) that can be sent in the wireless network before the AP will
fragment the packet into smaller data frames.
A large Fragmentation Threshold is recommended for networks not prone to
interference while you should set a smaller threshold for busy networks or
networks that are prone to interference.
If the Fragmentation Threshold value is smaller than the RTS/CTS value (see
previously) you set then the RTS (Request To Send)/CTS (Clear to Send)
handshake will never occur as data frames will be fragmented before they reach
RTS/CTS size.

Preamble Type

Preamble is used to signal that data is coming to the receiver. Short and long refer
to the length of the synchronization field in a packet.
Short preamble increases performance as less time sending preamble means
more time for sending data. All IEEE 802.11 compliant wireless adapters support
long preamble, but not all support short preamble.
Use long preamble if you are unsure what preamble mode other wireless devices
on the network support, and to provide more reliable communications in busy
wireless networks.
Use short preamble if you are sure all wireless devices on the network support it,
and to provide more efficient communications.
Use the dynamic setting to automatically use short preamble when all wireless
devices on the network support it, otherwise the ZyWALL uses long preamble.
Note: The wireless devices MUST use the same preamble mode in order to
communicate.
IEEE 802.11g Wireless LAN
IEEE 802.11g is fully compatible with the IEEE 802.11b standard. This means an
IEEE 802.11b adapter can interface directly with an IEEE 802.11g access point
ZyWALL USG 20/20W User's Guide
Appendix C Wireless LANs
807